T&X-V-- 3#0

Current status of converter steelmaking

Oghbasilasie Haile Holappa Lauri

RECEIVED OCT 1 5 1998OSTI

Espoo 1995

MASTERdistribution of rm document is unlimited

Helsinki University of Technology f\@>Faculty of Process Engineering and Materials Science Department of Materials Science and Rock EngineeringLaboratory of Metallurgy __________________RGpOft TK.K-V-B110

Helsinki University of Technology Faculty of Process Engineering and Materials Science Department of Materials Science and Rock Engineering Laboratory of Metallurgy

Current status of converter steelmaking

Oghbasilasie Haile Holappa Lauri

Report TKK-V-B110(1995)

Key words: Steelmaking, converter process, top blowing, bottom blowing,combined blowing, post combustion

Vuorimiehentie 2 K FIN-02150 Espoo, Finland

ISSN 0785-5168 ISBN 951-22-2845-9

DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

CONTENTS

ABSTRACT..........................................................................................................................3

1. INTRODUCTION............................................................................................................ 4

2. FUNDAMENTALS OF CONVERTER PROCESS.........................................................9

2.1. Hydrodynamics of Process................................................................................102.1.1. Top Blowing...................................................................................................172.1.2. Bottom Blowing..............................................................................................192.1.3. Combined Blowing......................................................................................... 212.2. Principles of Chemical Reactions in Converter.................................................232.2.1. Oxidation of Carbon and other Elements in the Melts................................... 232.2.2. Slag Formation................................................................................................26

3. SPECIAL FEATURES OF MODERN PROCESS.......................................................... 27

3.1. Post Combustion.................................................................................................273.2. Injection (Solids).................................................................................................293.3. Measurements, Instrumentation..........................................................................323.4. Process Control.......................... 343.5. Hot Metal Chemistry.......................................................................................... 36

4. FUTURE ASPECTS.........................................................................................................42

4.1. Post Combustion.................................................................................................444.2. Increased Scrap Melting..................................................................................... 454.3. Continuous Process.............................................................................................45

CONCLUSIONS.................................................................................................................. 48

REFERENCES.....................................................................................................................50

2

ABSTRACT

This literature work is mainly focusing on the mechanisms of modern converter steelmaking and related with the evaluation of converter technology applied during the last decades and further to the future.

The history of steelmaking has been briefly reviewed from bloomeries and early-steelmaking processes to the progress of modern converter process. The pneumatic converter processes were developed in the 1850's and thereafter the basis for the rapid growth of steel industries was established for the next 100 years.

The world production of steel has not continuously grown but fluctuating quite much. It reached 723 Mt in 1994. The production is believed to grow the forecast for the year 2003 being approximately 800 Mt. Electric arc furnace production is estimated to reach 280 Mt by 2003, and BOFIOH will reach 520 Mt by 2003.

The current status of the converter steelmaking process is briefly described both on its theoretical bases and practical technological progresses. Developments which significantly improve the process are briefly discussed. Several more recent developments such as combined oxygen blowing process, increased scrap melting, post combustion and hot metal pretreatment are discussed.

The future progress will be in further develoment of these process characteristics as well as in eventual emerging of the continuous converter process.

3

1. INTRODUCTION

The history of an ordinary converter process goes back to the last century when the industrial revolution really took place and the need for steel mass production was quite evident. The steelmaking processes in use in the beginning of the 19* century were small scale, manual methods like puddling, crucible steelmaking, Lancashire hearth, Vallon hearth, etc. They were proper to produce small amounts of special steel but not suited for real mass production /!/.

Henry Bessemer was a professional inventor who made 117 patents in different fields of technology though most of them in iron and steelmaking manufacturing. In the 1850’s he worked trying to develop a gun with rotating projectiles but he had material problems because cast iron was brittle, wrought iron was too soft and crucible steel being good in quality but ten times more costly than cast iron /!/. He made experiments with a cupola and a reverberatory furnace and occasionally observed that the iron melting in intimate contact with air blow was completely decarburized by the action of air. So he stated to test blowpipes and soon also blowing air through the bottom of a vessel into the molten metal (Fig. 1).

Fig. 1 Bessemer’s first, from outside heated "Converter'* (left). First industrially tested Converter (right) IU

4

Bessemer patented his converter process in 1855. In the first experiment the reaction was quite violent, throwing slag and metal splashes, the flame being like a volcano melting the cast iron plate above the reactor and the roof being in danger to set on fire.

The first ingot made in August 1856 had the analyses:

C = 0.8 %, Si ~ 0, S = 0.162 %, P = 0.42 %, Mn ~ 0, As ~ 0 but the quality was bad due tohigh P content.

In 1857 a Swedish steel plant owner G.F.Goransson visited Bessemer and bought rights for his patent. He also shipped a converter from England to Sweden. This cylindrical furnace did not succeed but the own construction by Goransson was then a success.

When Bessemer heard about this in England trials were started again and Bessemer then designed a pear shaped tillable converter which became common all around.

The invention of the converter process which could produce steel directly from liquid BE hot metal in 15-20 min/charge started the new era of steel mass production with remarkably decreased production costs. As a drawback was only that low phosphorus iron ore was necessary to get good quality steel.

The Bessemer process, developed in 1856, was the first of these processes to achieve commercial success. This process converted molten pig iron into steel by oxidizing the carbon and silicon iron. Air was introduced, under pressure, into the bottom of the relatively large vessel. Through a series of holes in the bottom of the vessel, the air was forced upward through the bath to carry out the oxidation process.

The result was a dramatic increase in the growth of the steel industry. Prior to the introduction of the Bessemer process, world steel production stood at less than 50,000 metric tons per year. Less than 35 years later, in 1890, world steel production exceeded 10,000,000 metric tons per year. More than 80 % of the steel was produced, at that time, by the Bessemer process 15/. The world steel production from 1870 to 1993 is shown in Table 1, Fig. 2 shows world steel production in percentage by different processes since 1995.

The open-hearth process, which succeeded the Bessemer process to a great extent and was the biggest steelmaking method during the first half of this century, should not be considered as a pneumatic steelmaking process as originally conceived. However, as tonnage oxygen became available at reasonable cost after world war II, many open-hearth furnaces were fitted with roof mounted oxygen lances to utilize this new decarburizing method. Those few open- hearth furnaces still remaining in the world today are also being retrofitted with submerged tuyeres for the injection of oxygen directly into the molten metal bath to further increase the rate of carbon removal /5/.

5

The invention of converter process which could produce steel directly from liquid BF hot metal in 15—20 min/charge, started the new era of steel mass production with remarkably decreased production costs. As a drawback, it was necessary to have a low phosphorus iron ore in order to get good quality of the steel.

10 0 10 0 LO 0 IO 0 1010 0 0 1^ CO CO O' O'0 0 o> O' O' O' O' O' O'

Year

Fig.2 Percentage of World Steel Production by different processes /based on ref. 1,2,3,4and calculated data/

The idea of oxygen instead of air had been proposed already by Bessemer, but pure 0% was that time a very "rare" material and was produced only in laboratories. Karl von Linde developed his method for gaseous 0% production already in 1896 but in industrial scale it came first in 1920's whereafter its price became reasonable to be used in steelmaking. Oxygen enriched air (30-32 % O2) could be used in converter. With higher O2 content bottom wear became a problem due to high local temperature and iron oxide rich conditions in the tuyere zone. This was eliminated by diluting gas with endothermic components, H2O or CO2, even

6

Table 1. World Crude Steel Production Million metric tons 16,3,1, SI

Year Production Year Production Year Production Year Production1870 9.8 1920 72.5 1946 111.6 1972 630.21875 11.1 1921 45.2 1947 136.0 1973 696.41880 14.6 1922 68.8 1948 155.3 1974 703.51885 15.7 1923 78.3 1949 160.0 1975 643.51890 23.7 1924 78.5 1950 189.8 1976 675.11895 25.3 1925 90.4 1951 211.3 1977 675.31900 28.3 1926 93.4 1952 211.8 1978 716.51901 31.0 1927 101.8 1953 235.1 1979 746.71902 34.5 1928 110.0 1954 224.4 1980 716.31903 36.1 1929 120.8 1955 270.5 1981 707.11904 36.3 1930 95.1 1956 283.9 1982 644.91905 44.9 1931 69.6 1957 293.0 1983 663.41906 51.2 1932 50.7 1958 271.7 1984 710.11907 53.0 1933 68.0 1959 306.6 1985 718.881908 41.4 1934 82.4 1960 347.1 1986 713.5871909 54.2 1935 99.5 1961 347.2 1987 735.5101910 60.3 1936 124.3 1962 353.6 1988 780.0921911 60.5 1937 135.7 1963 379.2 1989 785.9481912 72.8 1938 110.0 1964 429.9 1990 770.1411913 76.4 1939 137.1 1965 452.4 1991 736.1551914 60.4 1940 140.6 1966 471.3 1992 723.5971915 66.6 1941 153.8 1967 493.3 1993 730.5311916 78.2 1942 151.4 1968 523.9 1994 723.6841917 82.0 1943 159.6 1969 570.91918 77.2 1944 151.2 1970 595.31919 58.5 1945 113.1 1971 582.3

50 % CC>2 + 50 % C>2 was possible. This resulted in the lower N2 content in the steel but in poor scrap melting capacity again. O2 enrichment was studied in the 1930's e.g. in Maximilianshiitte in Germany. Prof. Robert and Prof. Schwartz made experiments with oxygen in Switzerland in the 1930's and 1940's. In 1948 Diirrer wit Dr. Hellbrugge made experiments with top blowing at von Roll in Gerlafingen in 2.7 t pilot which seemed very successful. This was followed by campaigns in Linz by Voest company in 1949 in 15 t scale. The first industrial

7

LD-converter was commercialized in Linz with two 30 t converters in Nov. 1952 and the following year in Donawitz.

Oxygen top blowing through a watercooled lance avoided the problem of local hot spot and lining wear characteristic for bottom blowing. LD or BOF became popular very fast and in the 1960's almost all new installations were based on this new process. It took the position of the earlier Bessemer and Thomas processes as well as Siemens-Martin or open-hearth process using hot metal and scrap. In LD process the nitrogen problem was solved. It had a high scrap melting capacity, high productivity and relatively good economy compared with other processes. The only drawback in the 1950-60's was that in its normal mode it was limited to low phosphorus hot metal (Pmax 0-1-0.3 %). For high P (Thomas) hot metal special solutions were developed using lime powder injection and two slag practice (LD-AC, OLP). Also top blown rotary converter (Kaldo) in Sweden for high P hot metal. These special modifications of the top blown converter have, however, gradually disappeared because of good availability of low phosphorus iron ore which has given better economy compared with the use of modesdc high P ores and those special processes /!/.

OBM or Q-BOP emerged in the early 1970's. Some plants have installed bottom blown converters but it has not by any means threatened the domination of LD-process. However, its metallurgical benefits became evident and it led to development of combined blowing converters. Top blowing converters were provided with gas blowing/stirring devices through the bottom, mostly with inert gas N2 and Ar in some cases also with oxygen. On the other hand bottom blowing converters have been equipped with top oxygen lance in order to intensify decarburization and to be able to control the oxidation state of slag /!/.

Bottom stirring technology was developed in the late Seventies and early Eighties for the application to BOF converters. Since the mid Eighties, the bottom injection-stirring process has spread from Europe and Japan around the world. The technology improves charge blowing behaviour, enhances yields, improves the end product and cuts production cost. In addition, the injection-stirring system permits extensive automation of the blowing process, thanks to the significantly less violent blowing behaviour during the main decarburization phase /9/.

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2. FUNDAMENTALS OF CONVERTER PROCESS

The technology of steelmaking has evolved through the ages. The ancient men of the Iron Age started to produce iron by heating together lumps of charcoal and iron ore in a pit. Scientific and technology advancements then led to the invention of crucible furnace, cupolas, open hearth furnace, electric furnace, Bessemer converter, oxygen steelmaking converters, smelting reduction furnaces, ladle furnaces, plasma furnaces, etc. However, unlike other advanced industries, the inventions in the steel industry have been implemented at a slower rate primarily due to different reasons /10/.

World-wide oxygen steelmaking capacity now totals 725 Mt which shows a slight increasing. While the change in capacity has been minor, the most noticeable change has been the increased use of various forms of combined blowing. Plants everywhere have installed new or improved forms of combined blowing /16/.

The fundamental features of the LD process are:

i) High iron losses in the slag and high oxygen content in the bath.ii) High dust losses in the waste gases.iii) High CO% content in the waste gases.

- iv) Difficulties in making very low carbon steel.

These are some of the basic limitations of the LD process. However, the process has certain advantages which are:

i) Simplicity in the methodology of oxygen introduction in the form of a separate water cooled lance which is not an integral part of the converter

ii) Ability to control the extent of oxygen transfer to the metal and slag by adjusting the lance height.

In between ironmaking and steelmaking there is a narrow but important area of pretreatment of hot metal. The objective of this pretreatment is to control the concentration of dissolved impurities like silicon, phosphorus and sulphur within desired limits before feeding hot metal to the steelmaking shop. In the future, and in view of the pace of development of new smelting reduction processes, the intermediate area of hot metal pretreatment may merge with ironmaking. On the other hand, if the continuous steelmaking processes (from ore to steel) become a viable alternative then both iromaking and steelmaking processes may merge into a single discipline /10/.

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2.1. Hydrodynamics of Process

After charging the scrap and hot metal, the converter is placed into the blowing position and the lance is lowered. Oxygen, at a pressure of 7.5 to 2.5 10 bar, is then blown on to the iron bath. The exit velocity of oxygen is about 1.5 to 2,5 times the velocity of sound. Generally, ignition occurs immediately after the start of the oxygen blow. However, ignition can be delayed by as much as minutes, especially when purchase scrap is used.

During the initial phases of blowing, the amount of slag is still small, the slag is pushed towards the converter walls by the gas reflected from the surface and the metal droplets are sprayed mostly within the space of the converter. Eventually the amount of slag increases because of the oxidation of the elements and the addition and dissolution of the lime. The sprayed metal droplets can become trapped for longer periods by the slag. The system formed consists of three or more phases (liquid metal droplets, gas bubbles, and solid material dispersed in the liquid slag).

The nature of the circulation of the liquid metal bath in the LD process has been frequently discussed. While some have concluded that the bath moves downwards at the axis and upwards near the walls of the converter, others concluded the opposite low direction. The factors that may cause bath circulation are essentially as follows /11/:

(i) The impact impulse of the jet,(ii) The gas reflected from the impact depression,(iii) The formation and rise of CO bubbles in the bath,(iv) Temperature gradients

Due to friction, the reflected gas jet which causes splashing of liquid droplets produces an outward radial flow from the axis to the converter wall. This flow must be compensated for by corresponding flow in the opposite direction so that a circulation pattern as shown in Fig. 3 and 4 results. Although opinions differ regarding the relative amount of carbon removed in the hot spot zone region and from the splashed droplets, it can be safely assumed that part of the carbon monoxide is formed in the hot spot zone. Circulation of the bath can originate in this manner, as shown in Fig. 5, which is based on concepts by Walker and Anderson. Here too, it is assumed that an upward flow along the axis and a downward flow at the walls occurin/.

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downward

«--- flow lines of jet—- flow in the bath

stagnation area

---flow lines of jetflow In the both

downwardI *

1

f j secondary

Fig. 3 Motion of the bath and splashing by Fig. 4 Motion of the bath and splashing by single-hole nozzle 1121 multi-hole nozzle /12/.

Bottom blowing of pure oxygen gas together with fluxing agents into the bath of OBM/Q- BOP yields many metallurgical advantages over the conventional LD process. Typical advantages involve decreased disequilibrium which gives a higher metallic yield and improved homogeneity of temperature and chemistry in the bath which brings up a higher hitting rate of aiming carbon and temperature.

Fig-5 Circulation pattern induced in the metal by rising carbon monoxide bubbles /12/

it

These preferable aspects of OBM/Q-BOP have stimulated metallurgists to modify or change the conventional LD process to attain a higher metallic yield with quiescent blowing. Since a salient feature of Q—BOP is its intensive stirring of the bath, enhanced stirring is a prerequisite to improve LD process. This is simply realized by the installation of submerged gas jet, hence various combined blowing processes have come into use.

To obtain fundamental aspects of combined blowing process, theoretical and experimental studies have been made with particular emphases on stirring intensity and partial pressure of CO gas in the vessel, both of which play major roles to describe oxidizing refining process.

Time, x, required for complete mixing of the bath for the top blowing is several times longer than that for bottom blowing, decreasing with increasing rate, Q, of gas injection from top lance as shown in Fig. 6 /13/.

Fig. 6 Comparison between top and bottom blowing of time required for complete mixing of bath /13/

Fig. 7 Capacity coefficient for mass transfer in bottom, combined, and top-blowing as determined in water model experiments /13/

12

The rate of mass transfer as shown in Fig. 7 is much larger for bottom blowing, indicating that the partition of solutes between the slag and the metal is closer to the equilibrium /13/.

The mixing time of a process was determined in the experiment made by Nakanishi, Saito et. al. /13/. A copper wire bundle was added into each heat, and metal samples were taken at a given time from three different positions.

calculation

experiment. mixing lime

Time after addition of Cu tracer (min)Qb/(Qb*Qt)

Fig. 8 Typical examples of tracer dispersion Fig. 9 Comparison of calculated and curves obtained by plotting the data from measured mixing time of the bath /13/.many independent heats 1131.

A specific concentration of tracer is calculated for each heat as (Cut - Cu0)/(Cuf - Cu0), where Cu represents copper concentration (wt %), and suffixes, o, t, and f, respectively, denote initial (before addition of tracer), the time when the samples are taken, and final (after complete mixing). Thus obtained specific concentration is plotted against sampling time as shown in Fig. 8, where the mixing time for a given process is clearly determined. The mixing time obtained in this way decreases as shown in Fig. 9 with the increase of the fraction bottom blown gas (Qb/(Qb + Qt)> where QB is the flow rate of bottom blown gas and QT is

13

the flow rate of top blown gas) from about 45 sec for LD to about 20 sec for K—BOP (40 % bottom blown) and to about 15 sec for Q-BOP (100 % bottom blown) /13/.

0.5Qb/(Qb*Qt)

Fig. 10 Relation between energy dissipation rate and the ratio, Qb/(Qb+Qt)> *n combined blowing /13/.

Fig.ll Relation between time for perfect mixing of bath and the ratio, QbAQb + Qt), in combined blowing 1131

Fig. 10 shows that a small amount of bottom blown gas provides a remarkably larger mixing energy to the bath than that of the same amount of gas blown from the top lance. It can be seen in Fig. 11, that if 20 % of the total gas is blown from the bottom, mixing time is markedly shortened and it is not affected by the change of the height of the top lance /13/.

As the jet passes through the converter atmosphere it carries some of the ambient medium along with it. The jet impinging on the bath surface will thus be a mixture of oxygen and the oxide of carbon /14/.

14

Calculations reported in the literature show that in the case of a 100 t vessel, the oxygen content on the axis of a LD jet at the steel bath level could be as low as 60 % when relatively high lance height (1.8 m) and soft blowing (1.8-2 Nm^/min/t of steel) conditions are used. At

any given distance from the nozzle, the concentration of oxygen decreases and the concentration of CO increases as the radical distance from the jet axis is increasing /14/.

r in cm. h = Bath depthr = Radial distance

Fig. 12 Flow pattern in liquid caused by impinging jet top figure: M = 14700 Dyne, h = 17 cm. Bottom Figure: M = 8650 dyne, h = 8 cm. Number inside the diagrams: Liquid Velocities in cm/s /14/.

15

Once the jet strikes the bath surface, the kinetic energy of the jet is partially transferred to the bath. The bath starts circulating and oxidation of the impurities begins. It has been conclusively proved that the bath circulates upwards on the vessel axis and radially outwards on the surface (Fig. 12) /14/.

With all the lancing conditions and types of lance investigated the vector diagrams gave the same form of movement by the liquid descending currents at the periphery and ascending currents in the center. With a rate of flow of air in the model of 0.71 m^/min, the jet of gas

penetrated the full depth of the liquid, and air flowed across the bottom to form an inverted

Sc57e~" ‘'—■‘0-39 cm/s

k^LoUisyis*___ Entry of blown gasx' |

150 100 50 (DISTANCE FROM AXIS OF MODEL,mm

Fig. 13 Vectors of liquid movement speeds during lancing. Three jet lance, a = 30° lancing rate 0.36 Nm-tymin /15/

funnel. However, the liquid still moved in the same manner. As an example, Fig. 13 is a vector diagram for the movement of the liquid during lancing with a three—jet lance and a = 30° the rate flow of gas from the nozzles being 75.5 m/s /15/.

There are various categories of blowing from the classical top blowing (100 % oxygen from the top) to the classical bottom blowing (100 % oxygen from the bottom). A direct consequence of the percentage of oxygen blow from the top and oxygen/other gases from the bottom, is the distinct variation in mixing time a measure of the time it takes to homogenize the bath.

16

LD - KG

NK - CB

-OTB

LD - AB

BSC - BAPL D-OB K-BOP

LD-HC

Flow rate of bottom gas,Nm3/min. t liquid steel

OBH

Fig. 14 Effect of the basic gas flow rate on the time for complete bath mixing /15/

Fig. 14 shows how the hybrid processes lie in between the two limits of top and bottom blowing. From this fundamental standpoint of mixing time, some hybrid processes are closer to the top blowing process while some others resemble the bottom blowing /14/.

2.1.1. Top Blowing

In the top-blowing process oxygen is blown with the help of water-cooled lance inserted through the mouth of the vessel (Fig. 15). The lance is fitted with a de Laval nozzle at its tip so as to deliver oxygen at supersonic velocity.

The LD vessel is lined with high-quality basic refractories like tar-bonded dolomites or carbon-magnesite. Due to the fact that basic refractories are used and that oxygen is blown for the refining the metal, the LD process also is called the basic oxygen process (BOP) or basic oxygen furnace process (EOF).

The sequence of operations for steelmaking in LD is as follows. The LD vessel is tilted by about 30 — 40°. Scrap is charged into the vessel (or oxygen steelmaking converter) with the

17

help of a charging bucket. Hot metal is then poured on the scrap. The vessel is then tilted back to its normal upright position for blowing oxygen. An oxygen lance is gradually lowered up to a specified distance from the liquid metal surface (bath surface) and blowing is started simultaneously (while lowering the lance). Within the first 3-6 minutes all the lime is added through the mechanized hoppers (or bunkers) to flux the oxides formed by the oxidation of silicon, iron and manganese. Iron ore and spar (Cal?2) may also be added. The lime silicate slag thus formed essentially contains CaO, SiC>2, MnO, P2O5 and FeO. After specified periods the lance is gradually lowered to the last predetermined position. Carbon is oxidized to CO and in minor amount to CO2. The waste gases contain approximately 90 % CO and 10 % CO2. Gas recovery hood is placed on the top of the vessel to facilitate the collection of unburnt gases. Total blowing time may vary from 17 to 22 minutes. At the end of the blow the lance is raised. Refined steel is tapped into a ladle and slag is taken out by tilting the converter. Only in the first heat the lining refractories are preheated; there after, once the first charge has been tapped, the lining has a heat content produced by the process itself to keep it going during subsequent heats.

Lance cooling water

Waste gases+ flue dust

Molten pig iron Fluxes+ore

Scrap Heat losses

MoltensteelInfiltrate

air

Molten slag'

Systemboundary

Fig. 15 A typical open system in steelmaking

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The typical end point composition of steel is 0.04-0.06 wt % carbon, 0.2 wt % manganese, 0.02 wt % phosphorus, 0.015 wt % sulphur, the contents being a function of input compo­sition of scrap and hot metal /10/.

2.1.2. Bottom Blowing

As the name suggests, the essential difference from the oxygen top-blowing process (or LD process) is that all the required oxygen gas is introduced through the bottom of the vessel with the help of tuyeres. Lime may be injected in fine powder form along with oxygen (to fasten the dissolution of lime and hence the formation of a well-mixed homogeneous slag). The oxygen bottom blowing process is also called the OBM, Q-BOP or LWS process depending upon the types of tuyere design used for injecting oxygen through the bottom. Tuyeres are shrouded with natural gas or oil in order to protect them from the intense heat generated by oxidation reactions at the tip of the tuyere. The letter 'Q' in Q-BOP stands for 'Quiescent'; compared with the top blowing process, the noise generated and bath eruptions in the Q-BOP are much less. A schematic diagram of the OBM/Q-BOP process is also shown in Fig. 16.

oxygen-

compressed air

nitrogen

3^ ■«—hydro­carbon

nitrogen and/or argon

Fig. 16 Schematic diagram of OBM/Q-BOP process Z10/

The AOD (Argon Oxygen Decarburization) process is, strictly speaking, a side blow converter but is sometimes considered as a bottom-blowing process. It is used only to produce stainless and special alloy steels /10/.

19

Bottom blowing steelmaking on the other hand results in:

i) More efficient conversion of the oxygen blow directly into the bath into the agitation power which is at least 10 times more than top blowing. As a result, there are no tempe­rature and concentration gradients that can cause slopping and the oxygen utilization is so efficient that very little oxygen is available for post combustion above the bath.

ii) The absence of an emulsion: This makes it necessary to inject the lime as a fine powder along with oxygen even for low phosphorus hot metal, thereby altering in the reaction mechanism of slag metal reactions which proceed more efficiently.

These features help bottom blowing steelmaking converters to operate with /16/:

(i) Lower slag iron content and better yield.(ii) Better phosphorus and sulphur partition coefficients because of use of powdered limes as

well as better turbulence.(iii) Higher manganese content and lower oxygen content in the bath and hence better

ferroalloy recovery.(iv) Ability to produce low carbon steels without overoxidation of metal and slag.(v) High hydrogen content of the steel arising out of the hydrocarbon.(vi) Limited ability to melt scrap because the lower oxidation levels and very limited post

combustion.

Bottom blowing has certain inherent features namely /16/:

(i) For bottom blowing lime and oxygen, a highly complex gas solid injection apparatus is required.

(ii) The heat balance around each tuyere is critical and too much or too little coolant compared with the oxygen blow at any instant can prove to be detrimental to the tuyeres.

(iii) The bottom tuyere area constitutes the heart of the process and the life of the bottom tuyere can become an additional limiting factor in converter operation, particularly when increased scrap is used in the charge.

(iv) The flexibility provided by the top lance is lost

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2.1.3. Combined Blowing

A large number of the processes have been patented depending upon the amount of gas injected from the top and bottom and also the type of tuyere and the gas (or gas mixtures) injected.

Considering the advantages of the top and bottom blowing methods, it has been proposed to use the combined blowing process, i.e. to blow a converter from the top and through the bottom (Fig. 18). The metal in converter is blown from the top with oxygen and through the bottom, with oxygen or argon (sometimes with nitrogen). A number of modifications of this process have been suggested and named respectively LD-OB (LD + oxygen bottom blowing), LD-AB (LD + argon bottom blowing), and LBE (lance-bubbling equilibrium). In the LBE process, the bath is top-blown with oxygen through a lance and at the same time, with a small amount of an inert gas (argon or nitrogen) which is supplied through gas- permeable elements (bricks) arranged in the converter bottom. This ensures intensive stirring of the metal and slag, so that an equilibrium is easily achieved. In 1994, more than 100 combined blowing converters were in operation in the world.

Top blowing with oxygen makes it possible to form, rather quickly and easily, an active fluid ferruginous-lime slag in the bath and to after bum partially CO in the converter vessel, which in turn allows a greater proportion of solid scrap to be used in the charge. Bottom blowing ensures intensive agitation of the bath, accelerates the melting process, decreases the loss of iron to slag and the oxidation degree of the metal. Thus, the combined process makes full use of the advantage of the top and bottom blowing.

top blowing lance

or side tuyereslagformers

hydro­carbonoxygen

N2and/or

Ar

Fig. 18 Combined-blowing technique /10/

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Upon reconstructing top-blown converter to combined blowing, the followingmodifications of the combined process can be used /!?/:

(i) Part of the oxygen (up to 20 %) is blown through the bottom and the remaining portion, through the top.

(ii) The metal is bottom-blown with argon, with oxygen top blowing at the same time or after the oxygen blowing.

(iii) As in Item 2, but with nitrogen instead of argon.(iv) Blowing-in of powdered lime (through the top or bottom) in addition to any of the above

variants.

As compared to the top-blown process, combined blowing offers the following advantages/17/:

(i) increased yield of final metal (owing to better agitation and lower loss of iron to slag and melting dust),

(ii) decreased use of ferro-alloys (due to lower oxidation of metal and alloying elements Mn),

(iii) decreased consumption of auxiliary materials (due to more intensive agitation at the bath and, as consequence, quicker formation of slag),

(iv) improved quality of steel, since inert gas blowing at the end of the procedure diminishes the concentration of gases in the metal, and

(v) lower consumption of oxygen (less oxygen is used for oxidation of iron).

The following disadvantages of the combined blowing have been reported.

(i) converter equipment for the combined-blown process is more complicated, which increases the cost of the shop, but this is more than compensated by the advantages mentioned above /!?/.

(ii) high cost of argon gas which is in many cases tried to replace at least partly with nitrogen(iii) availability of bottom stirring nozzles or bricks is often less than 100 % due to more

severe wear of the bricks as compared with the other converter lining.

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2.2. Principles of Chemical Reactions in Converter

2.2.1. Oxidation of Carbon and other Elements in the Melts

When the bath is blown with oxygen, or inert gases the furnace space contains gas bubbles (0%, N2, CO, etc.) which have passed through the bath. With a low intensity of blowing, gas bubbles simply pierce the bulk of the metal; with a higher intensity of blowing, metal droplets may turn out to be suspended in the gas flow /17/.

The carbon dissolved in the metal is mainly oxidized to carbon monoxide. At lower carbon concentrations, however, the reaction [C]+2[0] = C02(g) must be considered in addition to the main reaction [C] + [O] = CO(g). According to the calculated data, with 0.2 % [C] present in the metal the contribution of the reaction that forms CC>2(g) is only 0.5 %, but increasing up to 10 % when the carbon content in the metal is decreased to 0.03 %.

The equilibrium constant of the main decarburization reaction, Kc can be written as /17/:

P Pg - rCO _________ICO_______ Q)a[c] • a\a\ y c [C]Y o [o]

The activity coefficients Yc and Yo have been determined quite accurately. Since, however, the melt in the final stage of the steelmaking process contain little carbon (below 1 %) and a very low concentration of oxygen (below 0.1 %), it may be assumed that both Yc and Yo are close to unity. Accordingly:

Kc = Pco/[C]-[0] (2)

Kc = [c]-[o] at Pco ~ 1 (3)

[C][0\ = l/Kc=m (4)

The quantity logKc= A/T+B has been determined by various reseachers. When

analyzing the data of various researchers, it may be noticed that (a) various authors give the thermal effect with different signs and (b) the magnitude dependence is essentially not high. The value for the carbon-oxgygen product was first obtained by Wacher and Hamilton (USA) at the normal atmospheric pressure at 1620°C/10/:

23

m = [C] • [0] = 0.0025 (5)

Later the value for carbon-oxygen product = 0.0020 was determined by Chipman /18/. Thermodynamic values of essential reaction in the oxygen converter process are collected intable 2.

Table 2. Gibbs energies for the most important reactions in the converter process

.Reactions AG°; [J/moIe]

[C]+[0] = COfe) -25 000 - 37.90T (6)

[Cl+(FeO)= Fem + COM 115 000 - 98.18T (7)rCl+l/209M = COm -142 000 - 40.79T (8)rsil+2[01 = (SiO?) -542 165 + 202.83T (9)rSil+0?M = (SiO?) -775 851 + 198.04T GO)rSil+2(FeO) = (SiO?) + 2Fem -29 991 + 98.03T (11)[Mnl+rOl = (MnO) -244 521 + 108.78T2 (12)rMnl+l/20?M = (MnO) -361 464 + 106.39T (13)[Mnl+(FeO) = (MnO) + Fern -123 516 + 56.40T (14)[PI + 2.5[O] = (PO? s)fin slflcA -326520 + 162.883T (15)P + 2.5 (FeO) = (PO? 4) + 2.5Fem 6280 + 17.908T (16)

It is commonly assumed that in the steelmaking slags the initial product of the reaction of SiC>2 with lime is 2CaO-SiC>2 /IT/:

(Si02) + 2(CaO) = (2Ca0-Si02) (17)

Manganese is oxidized readily, forming manganese oxide, MnO, at high temperatures. The oxidation of manganese can occur by its interaction with the oxygen dissolved in metal (12) /19/.

Dephosphorization is one of the basic and most important unit processes in steelmaking as phosphorus is a detrimental element for most steels. Both oxidizing dephosphorization and reducing dephosphorization are feasible in principle. Under practical conditions, however, the oxidizing dephosphorization is much more feasible than reducing dephosphorization especially for ordinary steels. In fact, the oxidizing dephosphorization is now the most commonly used way to remove phosphorus form the iron and steel melts /20/.

T24

Considering the various criteria for evaluating the de-P power of a slag, the phosphorus distribution ratio Lp = (P)/[P] is of much greater practical interest than the others. But there is no explicit relationship between Lp and slag composition, i.e. Lp can not be presented as an explicit function of slag composition. Regarding to the ionic structure of slags the dephosphorization reaction should be written /35/:

[p]+5/2io]+y2(°2-)={poi-) (is)

It is difficult to determine activities of ions, so simpler reaction modes have to be used /35/:

7[P]+5(FeO) = (P2Os) + Fe(19)

Sof

Sulphur transfer takes place through the following reaction /10/:

(20)

[S] + °2(g) — S02(g)(direct oxidation at the gas-metal interface in jet impact zone)

[s]+(o2-)=(sz-)+[o](partitioning of sulphur at the slag-metal interface).

(21)

(22)

It is found that approximately 15-25% of dissolved sulphur is directly oxidized (21) into the gaseous phase due to the turbulent and oxidizing conditions existing in the jet impact zoneno/.

A certain quantity of sulphur present in the metal can be oxidized during the heat and removed to the gaseous phase. The oxidization of sulphur may occur by the following reactions /17/:

between sulphur and dissolved oxygen of the metal:[S] + 2[0] = S02(g) (23)

between sulphur and iron oxides of the slag:(S) + 2(FeO) = 802(g) + 2Fcq (24)

on the slag surface, with the oxidizing gaseous phase:

A,

25

(S) + 0%(g) = S02(g) (25)

These reactions cannot proceed deeply. For the reaction in the metal: [S] + 2[0] = SO2, the equilibrium constant can be found as /17/:

log K = log Pso2 = 294 [S)[0]2 T

-2.8 (26)

2.2.2. Slag Formation

The process of the slag formation takes up a certain length of time. The processes of oxidation of impurities (silica, manganese, phosphorus, etc.) in the charge into corresponding oxides occur at a definite rate. A certain spell of time is required to heat up and melt the charge (scrap, iron ore, lime, etc.) /17/.

Since the removal of phosphorus is ensured by the reaction in which CaO participates, the final result depends on the activity of CaO in slag (on the slag basicity determined for all slag components). Noting that a nearly equilibrium state is quite quickly established between the phosphorus concentrations in the metal and slag, it is also possible to produce metal low in

, phosphorus by increasing the amount of slag. A large bulk of slag, however, requires additional heat for melting it and involves additional losses of iron (for the same concentration of iron oxides in slag, a greater bulk of slag naturally involves a greater loss of iron to it).

The intensity of the formation of slag of the desired composition has a strong effect on the rate of phosphorus oxidation. The sooner a new free-running ferruginous-limy slag is formed, the earlier the intensive oxidation of phosphorus will start. If fluid ferruginous-lime slag prepared in a separate plant is poured into a steelmaking plant with molten metal, the process of dephosphorization will start at the moment when the fluid slag contacts molten metal.

The bottom stirring process assists the slagging reactions of silicon, manganese and iron in approaching equilibria during the main decarburization phase. These equilibria are not quite reached during oxygen blowing, however. They achieved only once refining has been completed and the after-stirring phase has started. Dissolution of lime is accelerated by the injection of inert gas. Addition of fluorspar to liquefy the slag is therefore no longer necessary. Omission of fluorspar significantly enhances the durability of the converter's refractory lining and is contributory factor in increasing the service-life of the refractory wear lining /9/.

26

3. SPECIAL FEATURES OF MODERN PROCESS

The fundamental change which has occurred in the world steelmaking scenario during the last few decades has been the gradual increase in the percentage of steel made by the oxygen steelmaking process. Oxygen steelmaking was unknown in early 1950's contributed less than 5 % to world production in 1960, the share went up to 40 % in 1970 and in 1992 it went up to 60 % /15/. Paralel to the growth of the converter steel tonnage the process itself has been developed in many detailes which are discussed here in the following.

3.1. Post Combustion

Steelmakers have continued strenuous efforts for the development of hot metal pretreatment process in light of producing high-purity steel with the minimum amount of slag. In recent years, a significant progress has been made in the hot metal pretreatment, leading to the substantial reduction in the manufacturing cost. Since the hot metal pretreatment involves some drawbacks, such as the reduction in silicon as a heat source and the increase in heat loss due to the division of refining process, the heat sources in the converter have become insufficient, being required the operation of high hot metal ratio. It has also become necessary to develop a method for the maximum utilization of low-cost scrap which will be produced in a large quantity as an iron source in the near future /21/.

If the technology for the addition of coal, coke or other fuels is applied to, increases in sulphur and nitrogen contents in the steel can not be avoided. On the other hand, there is no fear of such increases in the technology for the promotion of post combustion in the converter. Accordingly, this technology is important not only for the addition of clean fuel but also, for minimizing the consumption of coal or coke in the case that the addition of such a fuel becomes necessary /21/.

The post combustion ratio in the converter is dependent upon the balance between the oxidation of CO produced to CO% and the reduction of CO2 to CO. It is considered that the post combustion ratio depends on the results of complicated heterogeneous reactions in a nonsteady state. Under the assumption that the main reactions of post combustion are the formation of CO% by the reaction between the oxygen jet and CO in the atmosphere and the formation of CO by the reaction between CO% produced as described above and carbon contained in the steel, the post combustion ratio was studied on the basis of the reaction model shows in Fig. 18.

27

Lance nozzle

CO entrained by Og jet

CO? getting out of jet

CO by de- carburization reaction

Region of COg getting out

TSupersonic jet core region

Free jet region

Fig. 9. Schematic diagram of post combustion.

Fig. 18 Schematic diagram of post combustion /21/

The velocity of oxygen jet with increasing distance from the outlet of the lance nozzle changes from the supersonic jet core region to the free jet region through the transition jet region. In this model a jet of C02 is formed by the reaction between the oxygen free jet region and CO produced by the decarburization reaction, being entrained from the atmosphere into the oxygen free jet region. See in the following reactions /21/:

CO(g) + l/202(g) -* C02(g) (27)co2(g) 4- [q -> 2CO(g) (28)co2(g) + Fe(l) -> (FeO) + CO(g) (29)(FeO) + [C] -» CO(g) + Fe(1) (30)

Carbon dioxide at the surface layer of jet with the velocity below a certain critical value is entrained into and dissipated with the flow of CO produced from the fire spot, and the rest of C02 which is not dissipated and the oxygen jet reacts with carbon in the steel bath to form CO. It is assumed that the amount of C02 dissipated is proportional to the flow rate with the velocity below a certain critical value in the free jet region, and that the post combustion ratio is proportional to the ratio of the amount of C02 dissipated to the amount of CO produced by the decarburization reaction /21/.

The post combustion model described above is consisted of the following three assump­tions:

28

(i) Carbon monoxide entrained into the oxygen free jet region is mixed up to the center of jet and the reaction CO(g) + l/2C>2(g) -> CO%(g) proceeds at a sufficiently high rate.

(ii) The rate of reaction 'CO%(g) + [C] -> 2CO(g) (including the reactions CO%(g) + Fcq -4 (FeO) + CO(g) and (FeO) +[C] —> CO(g) + Fe^p is sufficiently high and the total amount of C02(g) reaching the steel bath is consumed by the decarburization reaction in the same manner as oxygen.

(iii) Carbon dioxide is dissipated from the surface layer of the free jet region where the velocity is small.

3.2. Injection (Solids)

Combined blowing with oxygen and lime powder provides major metallurgical, operating and cost benefits together with the ability to employ higher scrap rates. Operating costs of the K-OBM process are estimated to be $8/ton lower than with the BOF process. Addition of submerged coal injection results in the KMS process with higher scrap rate capability 1221.

e- 800

Pz

oV 600

400

200-

DOWN

NK-CBWITHOUTLIMEINJECTION

12)LD-STB LD-OBK-OBM

0 0.01 0.02 0.04 0J 6,2 0.4 j.O 2.0 4j)6.0 GAS FLOW RATE FROM BOTTOM (Nm3/min-t)

Fig. 19 Oxygen content at turndown 1221

The oxygen content of steel produced with the K-OBM process without lime injection is higher than with K-OBM, KMS and OBM/Q-BOP processes (Fig. 19). However, it is lower than obtained with processes with low bottom-blowing rates. These differences are even higher if the carbon contents are below 0.05 %. With the K—OBM process with lime powder

29

injection, 0.005 % carbon is achieved with 0.05 % oxygen and approximately 22 % Fe (oxidized) in the slag under operating conditions.

The use of relatively small amounts of oxygen to improve bath agitation is more effective than an inert gas: i.e., up to approximately 10 % of the total oxygen is introduced through small tuyeres in the OBM/Q-OBP process (K-OBM without lime powder injection). Advantages over inert gas stirring systems are the use of relatively small amounts of oxygen coupled with a wide turndown control ratio of 4 to 1. These results, however, are not as those obtained with lime injection, particularly with regard to dephosphorizadon, desulfurization, post combustion and liquid steel yield. The advantage is the improvement of LD operating results and costs together with low conversion costs 1221.

Table 3. Typical KMS heat performance with 20 % post combustion 1221

ChargeScrap 545 kg/tonne Mn 0.40 %Hot metal 545 kg/tonne P 0.10 %C 4.25% S 0.02 %Si 0.75 % Temperature 1330°CFlux, oxygen and coal consumptionLime powder 50 kg/tonne Oxygen 120 Nm3/t

Dolomitic lime 8 kg/tonne Anthracite 85 kg/tTapping conditions steel

C 0.04 % Fe (oxide) 12%Mn 0.18 % CaO 47%P 0.006 % SiO? 19%S 0.036 % MnO 2.6%Temperature 1640 % P?05 1.1 %Slag 12 kg/tonne MgO 5.5%

GasCO 51.7 % N? 8.3%CO? 12.4 % Volume 285 Nm3/t

H?. 7.9% Energy content 2.1 GJ/tHgO 19.7%

Post combustion is particularly significant in the allothermic KMS process in which coal is injected with oxygen and lime. For example, in replacing 1 tonne of hot metal by 1 additional

tonne of scrap (1.9 GJ/t) in the charge, the consumption of lignite coke is reduced from 410 kg without post combustion to 220 kg with 20 % post combustion, and to approximately 160 kg with 40 % post combustion. The performance of a typical KMS heat with a 50 % scrap charge and 20 % post combustion is summarized in Table 3.

A scrap rate of 545 kg/t of liquid steel is possible with a total blowing time of approximately 30 min during which 120 Nm^ of 0%, 85 kg of anthracite and 58 kg lime per t of liquid steel are added.

An increase in the level of post combustion from 20 to 25 % would result in a reduction of the coal consumption of 15 kg/t and reduction in blowing time of 3 min. With 25 % post combustion without coal injection (K-OPM), the scrap rate would be 265 kg/t of liquid steel.

/ CHEMICALLY / /BOUND HEAT/ / COAL GAS /

ENERGY NOT CONVERTED

F= CARBON =9 FROMShot metal8 SCRAR COAL

COAL GAS

= SLAG

STEELSENSIBLE HEAT HOT METAL, SCRAP

ENERGY INPUT ENERGY OUTPUT1 = CRACK-AND FE-L0SS 2= HEAT LOSS3= PROTECTIVE GAS (CH4)AND H2 4= OXIDATION Si.Mn.P, Fe

Fig. 20 Energy balance for KMS heat: 50 % scrap, 20 % post combustion with anthracite injection 1221

The energy balance of the KBS heat summarized earlier in Table 3 shows that off-gas contains a considerable amount of chemically bound heat (2.3 GJ/t) (Fig. 20). The energy input (4.8 GJ/t of liquid steel) by hot metal, scrap, tuyere protective gas and, in particular by

31

anthracite, is shown in the left column. The sensible heat is relatively small, approximately 0.8 GJ/t/22/.

3.3. Measurements, Instrumentation

Every control or expert system needs real-time information from the process they control. Therefore direct measurement of the steel composition and temperature have been very high on the list of three wishes of metallurgists. In addition, some of the indirect measurements can give as valuable information presuming that they have a proper consistency.

ThermometerOptical fiber (80 m) /

Spectrometer

Lance

Converter

Fig. 21 Arrangement for direct analysis of manganese from steel melt /23/

Direct measurements of certain elements on trial basis have given promising results. Manganese contents have been analyzed by Nippon Steel by processing the emission spectrum of the hot spot detected with optical fiber inside the oxygen lance. (Fig. 21)

Analytsis precision of 10 % was estimated when Mn contents vary between 0.3 and 0.8 % Mn. The research was working on a similar measurement system for chromium and nickel. For phosphorus, sulphur and vanadium the method was unsuitable due to the low temperature of the hot spot /23/.

Table 4 Instrumentation used for BOF refining control and the utilized information 723/

Object Instrumentation Utilization of information

Steel temperature

Sublance system

Batch measurement

before and after blow-

end (sublance-like

equipment applied to

measurement during

tapping)

Dynamic control of end-point

carbon and temperature

Determination of alloying amounts

in the ladle

Estimation of end-point phosphorus

and manganese contents

Steel carbon content

Steel free oxygen

content

Metal sampling

Slag foaming or

Slag formation

Sound wave

Continuous

measurement

during blowing

Prediction and control of sloping

Control of slag formation

(Modification of flux adding,

oxygen blowing condition, etc.)

Estimation of end-point phosphorus

and manganese contents

Micro .wave

Lance vibration

Vessel vibration

Direct observation

(Optical fiber)

Residual oxygen

amount in the furnace

Mass analysis of

waste gas content

Continuous

measurement during

blowing

Control of slag formation

(Modification of flux adding,

oxygen blowing condition, etc.)

Bath level Sublance system Batch measurement

before blowing

Modification of lance height set-

valueMicrowave

Condition of furnace

lining

Laser beam Batch measurement

during idling time Control of lining wearingInfrared rays

(photo)

Thermocouple

(Temperature and

conduction check)

Continuous

measurement during

blowing (especially

around tuyere)

33

Another example is utilization of laser-induced emission spectral analysis (LIESA) to measure directly the carbon content from the steel melt via a narrow channel through the converter side wall. The carbon detection limit was 100 ppm and it was proven that the system can withstand the extreme conditions of a steel furnace.

Along with the development of these rather new measurements the existing measuring methods such as determination of carbon, dissolved oxygen and temperature with a single sublance probe have gained an excellent consistency. Laser techniques for lining wear control has become so fast and reliable that the measurement of a lining profile takes only a few minutes causing no disturbance in the production and allowing to optimize the campaign length.

At Raahe steel works a laser device developed by Rautaruukki New Technology is used. The LR-2000 is light, fast (total time for a full profile in five minutes, critical areas less), accurate and it does not need any fixed place to stand every time.

In most of the measurements the accuracy relies on the proper signal processing and this field has a great potential as the computers and their software develop. If one concentrates only on the new sophisticated methods there is a danger of forgetting how important the consistency of pressure, flow rate and positioning measurements are. The diagnostic maintenance measurements to prevent unexpected interruptions in the process, should also be mentioned. Table 4 shows some of the instrumentation used in connection with converters and also the utilized information that the measurements give /23/.

3.4. Process Control

Better refining control, higher productivity and lower labour costs have formed the basis for the development of BOF blowing control. On one hand there is an attempt to perform end-point control of also other indices than carbon and temperature by combining various new sensor techniques and control logics developed in recent years. On the other hand steelmakers aim at totally automated operation where direct tapping and 100 % sublance availability allow "one-touch-one-man operation". As an example Fig. 22 and 23 show the development in blowing control, basic concept of BOF refining control. These measures have allowed the converters to be operated by only two men at the working place 1231.

34V

1960 1970 1980 1990 .-1------- -------- L

Sublancedynamiccontrol

1st stage

, • Fundamental 2nd stageresearch

• Introduction of on-line equipment

One touchoperation f Fully automatic') ^blowing J

&

Directtapping

3rd stage

■ Development

Staticcontrol

EOFprocess computer

1 ISublance

4th stage

- Establishment (C, temp.)

\5th stagey

1—V■ Labor saving ' New control system

(for Mn, P|t| f

Combined blown Slopping prediction BQF and control

Comprehensive blowing control according to exhaust gas analysis

Fig. 22 The development in blowing control at Nippon Steel Nagoya works 1231

The refining control of combined blown BOF as mentioned is a combination of different techniques for different elements. End-point carbon and temperature is given very consistently by sublance sensoring connected with rapid in site analysis.

Refining process

Direct tapping operation

Combined blowing BOF

|Fully automatic^; |BOF operation if

Improvement of simultaneous [C]& temp, hitting rate

;Sublance dynamic control; technology

.aw\.v4.v.x > ak ?:

:Labor saving & automation § technology |

.••.•£>Xw&"X'X-A-.WA<svfr.w«'\vv.*iv»w>/X'Xsvw^,wvAvv.

^Approach to end-point control -of [Mn] & IP] .........------ ------ -

Hot metal pretreatment

Secondary refining

Divided refining process

Fig. 23 Basic control of the refining control at Nippon Steel Nagoya works /23/

35

Phosphorus content is calculated from the oxygen balance given by mass spectrometer analysis of waste gases. From the oxygen balance the oxygen potential of the slag is calculated and hence phosphorus content controlled to meet the aimed value.

This can also be supported by rapid in site analysis of the preceding slag. In addition rapid in site analysis of the intermediate sublance sample can be used for prosphorus control together with the carbon and temperature control. The significance of the phosphorus control decreases if a separate hot metal refining can be used. Fig. 24 shows arrangement for phosphorus refinement /23/.

Blow-ond TappingBlowing

ActionAction

Static control model .

Blowing . sequence

Sampling by sublance

[Cl, temp, aimed at blow end

the preceding heatSlag sampling of

Sublance measurement MCI. Temp.)___________

Dynamic control model

ijjlPl analysis in really Stirne at the site gg

{Slag analysis in real! itime at the she

• Refining condition• Hot metal, scrap

condition

End-point [PI control by the analysis in real time at the she

Conventional static & sublance dynamic control ([CL temp.)

[PL temp. & slag composition aimed at blow end

i Refining control modelsggj1 of [PJat blow end 1 (static, dynamic)

Fig. 24 Flow chart of the phosphorus control in the basic oxygen converter 723/

One big step towards the perfection of the LD converter was the adoption of the sublance system which made it possible to control the turn down carbon, temperature and phosphorus very accurately /23/.

3.5. Hot Metal Chemistry

In the late 1960's methods to significantly improve blast furnace performance, with regards to coke rate, were developed which resulted in higher sulphur contents in the hot metal. In addition, demands for lower sulphur steel led to the need for hot metal desulphurization.

36

Reagents, such as CaO, CaC%, Mg, Mg-CaO, Al-CaO and dozens of others, have been used successfully. As the demand for steels low in phosphorus grew and lower silicon contents for OSM were desired, desiliconization and dephosphorization processes were developed. The reagents used in these processes included Na2COg, CaO-FeOx and CaO-0%. With low sulphur, silicon and phosphorus hot metal contents, slag minimization processes were developed. These processes are easier to control with regards to chemistry and operations, such as the elimination of slopping /24/.

O 5 10 15 20 25 30 35Consumption of reagent

(Kg/Tl

' E QOS

V. 0.06

V ••

O 2 8 10 12 M 16CaO consumption (kg/t).

Fig. 25 Desiliconization using mill- Fig. 26 Dephosphorization using Fe203~CaO scale and sinter dust for different -CaF% with initial phosphorus of about 0.14 initial Si content /24/. % /24/.

Recent work in hot metal treatments have been very extensive and it is impossible to review it all satisfactorily. The research has been primarily concerned with desiliconization and dephosphorization. Desiliconization is usually carried out in the blast furnace runner using millscale and sinter dust. Typical results are shown in Fig. 25. Dephosphorization is carried out in the ladle using CaO - O2 or CaO — FG2O3, typical results are shown in Fig. 26 for mixture of 55 — 70 % millscale, 20 - 30 % CaO and 10 - 15 % CaF2 and with an initial phosphorus content about 0.14 %. Dephosphorization can occur after desiliconization.

37

Mn Y

ield

%Pretreated hot metal leads to improved steelmaking, to higher Mn yield (Fig. 27) and post combustion (Fig. 28), and decreased hydrogen and nitrogen contents and blowing times 1241.

Hog Minimized Blowing

Carbon Content at Turn-Down %

rt qtf esknfcieKinder betnwtalHerod hot nmolPrr-traeHd hot metal

P.C.R.Ore*01 :Ore cdeutoted tiy theoretical static modelP.C. R. ; Post Combustion Ratio

C/.C02>C4C01+R4C02) XtOO

Where C4CO): CO content of LO gas in the furnace

CXCOzfcCOz content of LO gas in the furnace

Fig. 27 Improved Mn yield with slag Fig. 28 Improveed post combustion ratios minimization /24/. (PCR) indicated by increased coolant /24/.

In order to reduced the cost of the steelmaking process and to meet the demand for ultra- low sulfur and phosphorous steel, active research has been carried out recently on the external treatment of hot metal, substantially hot metal is step wisely desiliconized, phosphorized and desulfurized.

Concerning the steelmaking process accompanied with hot metal pretreatment, the following two methods are considered /25/:

(1) Method in which, after desiliconization and desulfurization treatments, blowing with minimum slag is performed for decarburization in the converter.

(2) Method in which, after desiliconization, desulfurization and supplemental dephosphorizadon treatments, decarburization and final dephosphorization are performed in the converter.

The effectiveness of the K—BOF in the above mentioned two methods have been examined. In the LD and general combined-blown converters, addition of lumpy lime on the bath surface of externally desiliconized hot metal (Si: 0.01 to 0.1 %, P: 0.12 to 0.15 %),

38

makes slag formation difficult, whereas in the K-BOP, this drawback is eliminated by the injection of small amount of powdered lime as shown in Table 5. Furthermore, method (2) coupled with the K-BOP enables cutting down of the overall lime consumption by more than 5 kg/t/25/.

Table 5 Example of blowing performance of desiliconized hot metal 1251

Hot 0 Si- Mn P • S Temp. H.R.Metal (%) 4.65 > 0.07 0.36 0.137 0.018 ' .1240‘C 96-. 5

Flux material Lime (F/I) •10.6 kg/t

and oxygen Dolomite 3.4 kgItconsumption Ore 27.8 kg It

(K-BOP) Qz 45.7 Nm3/t

Metal C Mn P S Temp.

Blow 0.088 0.23. 0.019 . 0.011 1 632*C

end Slag T.Fe CaO SiOz MnO P2Os MgO S

(%) 14.7 46.7 10.9 5.9 5.61 • 8.8 0.122

To suppress the dissolution of MgO out of furnace refractories, calcined dolomite is commonly added to the slag. The externally desiliconized hot metal reduces the consumption of calcined dolomite required (Fig. 29). Fig. 30 shows an example of the typical behavior of the elements during the blow.

It is also found that since the total iron (T.Fe) in the slag of the K—BOP is lower than that of the LD, the K—BOP requires less calcined dolomite. When hot metal containing 0.2 - 0.3 % Si is externally pre-dephosphorized, the K—BOP can easily blow low phosphorus iron ([%P] = 0.01 %) with a lime consumption of 20 kg/t/25/.

The most important cause of poor steel cleanliness is oxygen in the converter slag. On the other hand, highly oxygenated slag is essential to dephosphorization in the converter. Thus dephosphorization stands in a reciprocal relationship to cleanliness which is adversely affected by high oxygen content. Removing the dephosphorizing burden from the converter therefore makes possible a remarkable reduction in the negative effects of converter slag.

Other problems arise when high and low phosphorus blowing are practiced successively in the same vessel. After high phosphorus blowing, a large quantity of phosphorus will remain in the vessel as a result of slag adhering to the converter walls, even if all free slag is removed. If the same vessel is next used to blow dephosphorized hot metal, the contaminated

39

lining will cause phosphorus pick-up by the melt. The fact that the amount of phosphorus contamination varies widely and is unpredictable makes stable operation of the converter all

aLDo K-BOP Si-less blowing • K-BOP ordinary blow­

ing (Si/0.4 -0.5%)1 630*C

K-BOP K-BOPa K-BOP/ordinary

' blowing /LD

blowing

5 10 15 20 25 30 35Amount of dolomitic lime(kg/t-steel)

■8 • 0Blowing time in pin

-0.1

§|

o a

Fig. 29 Influence of burnt dolomitic lime Fig. 30 The changes in bath compositionaddition on MgO balance /25/ during the blow in a basic oxygen

steelmaking converter /12/

, the more difficult under these conditions. Fig. 31 illustrates this problem. When pretreated hot metal is blown after blowing with high P hot metal, phosphorus pick-up from the furnace lining does not decrease to a negligible level until about the tenth heat, at which point the vessel can be considered clean. From this, it can be understood that the dephosphorizing burden cannot be effectively eliminated unless total pretreatment is adopted /26/.

Number of heal

Fig. 31 Change in P pick-up from furnace since the change to pretreated hot metal in stead of conventional hot metal 1261

40

The following is a summary of the main results expected from the adoption of total hotmetal pretreatment /26/:

(1) Improvement of steel quality(i) Reduction of (T.Fe) in the converter slag and prevention of the excessive oxidation of

the melt(ii) Improvement in the quality of ladle slag as a result of reduced P in the converter slag(iii) Reduction and stabilization of tapping temperatures with use of the catch-carbon

method and recovery techniques for in-fumace Mn and Cr(iv) Establishment of a reliable mass-production technique for refining ultra-low P steels

(2) Shorter and stable leading times(i) Improvement of the blowing control system as a result of simplification and

standardization of the converter blowing operation(ii) Enhancement of converter process capabilities, including improved hit accuracy of

chemistry control, improved hit accuracy of target tapping weight due to more consistent yield rates.

(3) Reduction in Refining Costs(i) Reduction in converter refractory costs (as a result of (l)-(i), (l)-(iii), and (2)-(i))(ii) Reduction in the consumption of slag-making submaterials and increased steel yield(iii) Reduction in consumption of Fe-Mn alloys (as a result of (1)—(iii))(iv) Reduction of converter slag as a sintering material, made possible by reduced slag P

content

41

4. FUTURE ASPECTS

A new report published by MBPS (Europe) Ltd, 'World crude steel production and consumption to the year 2000', provides a positive outlook for the steel scene for the remainder of the decade /27/.

The decline in the world steel production will be reversed in 1995, MBPS believes. The second half of the 1990's will be a period of increasing steel demand and production. It is forecast that the crude steel production will push through the 800 Mt barrier in 1999 outperforming the previous peak year in 1989. Steady but sustained growing the steel output is expected for the remainder of the decade.

The production and consumption forecasts are given in Table 6 and the world crude steel production by process is shown in Fig. 32.

Table 6. Forecasts of production and consumption of steel worldwide Hit

AreaConsumption of finished steel, Mt Production of crude steel, Mt1993 2000 1993 2000

Western Europe 133.0 135.0 158.5 172.0Central andEastern Europe

72.0 73.0 125.4 127.0

North America 101.4 105.0 102.5 103.0Latin America 27.0 33.0 43.4 50.0Middle East/Africa 22.5 29.0 21.5 26.0People's Republicof China

103.5 115.0 89.5 113.0

Japan 75.0 88.0 99.6 103.0Other Asia 95.2 130.0 78.5 100.0Oceania 5.7 7.0 8.6 11.0World total 615.3 715.0 727.5 805.0

Crude steel production is forecast to rise to 145 Mt in the EU by 1998 before declining at the end of the decade. The output in 1997—98 will be broadly similar to the peak year in 1989. In Eastern Europe many countries have reported increases in steel output in 1994. Production is forecast to increase by 17 Mt in this geographic area between 1994 and 2000.

42

MBPS comments that the crude steel production in North America in 1994 is close to the peak performance in this cycle. Output is forecast to drop to 96 Mt before rising again to a high level in the year 2000.

800

700| 600o

5002- 400S 3002| 200

100

0

Fig.32 World crude steel production by process /Based on Ref. 1,5,24,27 and calcu­lated data/

Asian crude steel production is forecast to increase by 50 Mt between 1994 and the first year of the next century (an increase of approximately 19 % overall). The high output will come in most countries as they try to satisfy the increasing domestic demand for steel. Australia output of steel is expected to expand to meet some of this demand /27/.

Economic Associates, Inc. has developed and is updating a world steel production forecast through the year 2003. The metallics required to feed this growing production can be met by three major sources hot metal, scrap and scrap substitutes [i.e., direct reduced iron (DRI) and hot briquetted iron (HBI)]. Forecasting steel production by various routes enables one to estimate these metallic requirements. Table 7 is a world metallic balance for the year 1993, 1998 and 2003. The aforementioned problems facing the integrated producers will enable only a small increase in hot metal production to 2003, according to Table 7. There also will be constraints on scrap supplies. Production of home scrap, which is generated within the steel mill, will decline as casting and rolling yields continue to improve. By the year 2000, the United States and most other steel industries will be at essentially 100 percent continuous casting. Home scrap generation in the primary end falls from the traditional 18 percent to

to o LO O to O LO o LO oLO o <3 r--» r-~ CO CO O' O' oo o O' O O' O' O' O' O' o

CM

year

43

about 2 percent due to continuous casting. Add to this the impact of efficiency improvements in the rolling mill and the result is a drop in home scrap generation.

Table 7. World Metallics Requirements (million metric tons) /28/

Increase1993 1998 • 2003 1993-2003

Crude steel production 725 774 800 10.3EAF 225 255 280 24.4BOF/OH 500 519 520 4.0Steelmaking metallics requirements 841 907 949 12.8Hot metal 407 421 420 3.2Scrap 408 448 479 17.4Home scrap 113 115 108 -4.4Prompt scrap 87 93 98 12.6Other* 208 240 273 31.3Scrap substitutes 25 38 50 100.0

*Other (primarily obsolete scrap) is defined so that nonsteel requirements for scrap (e.g., foundries) and

availability are assumed to be in balance.

Prompt scrap, which is generated by the steel consumer, is expected to increase only marginally until 2003 as a result of efficiency improvements by consumers /28/.

4.1. Post Combustion

In the EOF steelmaking process that uses the heat of carbon oxidation, the heat of reaction from C to CO is mainly used. The heat of reaction from CO to C02, however, is much greater than that from C to CO as given by Eqs. (32) and (33). If the secondary combustion (post combustion) step from CO to CO2 is utilized, the amount of scrap melting can be increased without prolonging the tap-to-tap time.

[C]+l/202(g) = CO(g) + 9,209 kJ/kg C (31)CO(g) + l/202(g) = C02(g) + 23,567 kJ/kg C (32)

In the normal EOF operation, the ratio of CO burned to C02 (the post combustion ratio) is 5 - 10 %. It has been confirmed that the amount of scrap melting can be increased by

44

increasing the post combustion ratio. For example, the scrap ratio can be increased by 3.4 % by increasing the post combustion ratio by 10 % /29/.

In view of the importance of grasping the post combustion behavior during EOF refining, Nippon Steel worked to clarify the mechanism of post combustion. The post combustion mechanism was explained in the chapter 3.1. /29/.

Functionally, it is doubtful if converters will change very much during the rest of this decade. Gas collection, secondary lances and slag stoppers are standard, as are arrangements for bottom stirring and oxygen injection if called for. Post combustion is being developed and used more widely. It allows more scrap melting and thus reduces hot metal demand as well as saving energy if the works gas balance is not upset/30/.

4.2. Increased Scrap Melting

Recycling of used materials is becoming a world wide trend, as symbolized by the issues of the global environment. The hot heel EOF scrap melting process will draw attention as a new process that can solve the iron source problem while making effective use of the structure of existing steel plants.

As an integrated steelmaker, Nippon Steel has pioneered effective scrap utilization processes, as represented by the new hot heel EOF scrap melting process, and has moved to address future changes in the iron source structure. The authors intend to push ahead with studies for an early commercialization of new technologies through the activities of the Environmentally Friendly Advanced Steelmaking Process Forum that has been researching on the effective utilization of low-grade scrap 1291.

4.3. Continuous Process

The American Iron and Steel Institute (AISI) and the US Department of Energy (DOE), steelmaking process, the technical program, and its organization and structure have been described in detail eleswhere/31/.

Fig. 33 shows the AISI direct steelmaking. For steelmaking experiments, the vessel was divided into two zones by installing a refractory dam. Two of the oxygen lances were located in the ironmaking zone along with the raw materials feed chutes. The steelmaking zone was located at the end of the vessel directly under the offgas system. The single oxygen lance in the steelmaking zone was modified for the projected flow rates required for decarbonization. Provisions were made for manual feeding of the raw materials into the steelmaking zone.

The concept was to operate each zone of the vessel relatively as independently. The ironmaking zone was operated as steadily as possible, maintaining carbon saturation (excess

45

char) and thermal control using the control strategies employed in-previous experimentation. Carbon-saturated iron produced in the ironmaking zone flowed through a hole in the bottom of the dam into the steelmaking zone. There the metal was decarbonized by the oxygen lance, and low carbon metal was tapped from the steelmaking zone /34/.

topofthevemilcover U—'Prr1 tj ' solid ' solid off-gas port

A#

bottom ofhcriz. vessel

steel

Fig. 33 AISI direct steelmaking process /based in ref. 34/

According to the AISI press release, the results of the direct steelmaking project show a gap between the actual results obtained at the pilot plant and the original goals for productivity and fuel rate. This gap is estimated to be about 40 percent in the case of high volatile coals. However, AISI is optimistic about the results and says the gap can be closed by a combination of the following technology:

(i) Improved distribution of oxygen through the application of side-blown tuyeres(ii) Better distribution of the raw materials charge into the vessel(iii) Use of newly developed sensors to measure foam height, and to observe char distribution

and behavior within the pressurized vessel(iv) Cooperative information exchanges with other smelting programs

46

To complete the commitment to study the refining of hot metal directly to steel, an eight- person task force was assembled to study both batch and continuous processes for the refining hot metal to steel. The task force examined trough processes, such as COSMOS, WORCRA and iron carbide, post-hearth refining coupled to the smelter, electrical arc furnace processes, the IRSID continuous steelmaking process, and the energy optimizing furnace. A final report, Evaluation of Steelmaking Process, was published in January 1994.

The report concluded, "There is insufficient technical or economic incentive to replace a working BOF with any of these processes to refine hot metal. The IRSID process could be developed with a pilot plant associated with the AISI smelter".

The task force also considered a fully continuous process from ironmaking to casting of the product. The group determined, "The capital and operating cost savings that could be achieved beyond those for direct ironmaking and continuous refining are relatively small. A fully continuous process should not be considered until direct ironmaking and continuous refining are perfected /31/.

47

CONCLUSIONS

The main functions of the converter process today are dephosphorization and decarburization, as well as adjustment of desired tapping temperature.

The history of ordinary converter process goes to the last century when the industrial revolution really took place and the need for steel mass production was quite evident. The steelmaking processes in use in the beginning of the 19^ century were small scale, manual methods like puddling, crucible steemaking, Lancashire hearth, Vallon hearth, etc. They were proper to produce small amounts of special steel but not suited for real mass. Production, the pneumatic converter processes were developed in the 1850's and thereafter the basis for rapid growth of steel industries was established for the next 100 years.

When looking at steel production via different processes in the beginning of this century most of the steel was done in converters (acid or basic) and open hearth furnaces. Smaller part special steels were still made by old methods like puddling. Electrical steelmaking was just in the beginning of its career. In the 1950's most steelmaking was done in open hearths, both acid and basic, converter process had remained its position, and electric steelmaking started to increase. The breakthrough of BOF process changed the situation dramatically, open hearths were rapidly discarded and the new generation basic oxygen converter process took the dominating position. Scrap based steelmaking has at the same time moved to electric furnaces.

The world production of steel has been fluctuating quite much e.g it fell down due to the collapse of the former USSR but is now again continuously growing. The world production was 723 Mt in 1994 and is forecast to exceed 800 Mt in the year 2000.

Oxygen steelmaking has advanced significantly in its approximately thirty years of existence and became the dominant steelmaking process almost twenty years ago. Oxygen steelmaking will continue to be the major steelmaking process for the foreseeable future. Oxygen converter process for steelmaking is a very fast, chemical high temperature process, where many different physical and chemical phenomena are occurred simultaneously (Fig. 34). In practice, it is either very difficult or almost impossible to get information about the process during the blow for example with the help of process measurements. The lack of insufficient real-time information about the compositions of liquid metal and slag leads often to such measures like additional blows or additional coolings, which consume a lot of energy and raw materials. Major developments have included bottom and combined blowing, hot metal pretreatments, increased scrap melting and combustion, see Fig. 34.

48

slag-metal-carbon oxideeraulsion,1"

splashingofmetal droplets;;..

linn

slagO

•iironmeli1300/1350 ->1650/1700°c

refractory lininj

post combustionco(g)+men (g}= co^(g)CO(g)4- l&OFeOx}^-C02{g) 4-M)

heat flow to the lining and. environment

-scrapmelting?'

inert gas bottom j Stimng

I - primary oxidation zone, oxygen Jet -iron melt impinging surfacesurface temperature > 2000°C ?

II - secondary reaction zone =droplet-slag/droplet-gas Interface

III - ternary reaction zone = slag-metal interface

Fig. 34 Scheme on EOF and its main features /33/

49

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