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journa l homepage: www.e lsev ier .com/ locate / jmatprotec

emi-solid rolling process of steel strips

ong Renbo ∗, Kang Yonglin, Zhao Aiminepartment of Material Processing and Controlling, School of Material Science and Engineering, University of Science and Technologyeijing, Xueyuan Road 30, Beijing 100083, China

r t i c l e i n f o

rticle history:

eceived 20 April 2006

eceived in revised form

0 May 2007

ccepted 30 June 2007

eywords:

emi-solid

teel

olling

trip

on-dendritic

a b s t r a c t

Semi-solid metal forming (SSF) is recognized as a new forming technology, which has been

paid more and more attention by the researchers all over the world. The semi-solid rolling

process (Rheo-rolling) combines the fabrication of slurry with continuously rolling as an

important neoteric means to achieve near-shape forming. In this paper, an original device of

semi-solid rolling has been designed and manufactured to prepare the spring steel–60Si2Mn

and stainless steel–1Cr18Ni9Ti. The semi-solid slurry is successfully fabricated by electro-

magnetic stirring and rolled into the strips with the thickness of 3–5 mm, the width of

70 mm and the length of 1000 mm. The effects of the stirring parameters, including stirring

power and time, on the solid fraction and microstructure of steel, have been investigated

through electromagnetic stirring process. The experimental results show that the semi-solid

slurry with the solid phase diameter of 50–150 �m and the solid fraction (fs) of 5–65% can be

obtained by electromagnetic stirring. The highest ultimate tensile strength and elongation

obtained for 60Si2Mn strip with 60% solid fraction are 860 MPa and 13.4%, respectively. For

1Cr18Ni9Ti strip with 60% solid fraction, the highest ultimate tensile strength and elonga-

tion are 1022 MPa and 40.6%, respectively. With increasing the solid fraction, the properties

of semi-solid rolled products become better. The method of grooved rolling can ameliorate

the stress state at the edge of rolled strip, which is beneficial to improve the homogenization

at present, it is mainly used to produce the low melting point

of microstructure.

. Introduction

emi-solid metal forming (SSF), first studied at MIT in thearly 1970s, has been recognized as a new forming tech-ology, which is different from the present metal formingethods. In this technique, the alloy is heated to temper-

tures at which the solid and liquid phases coexist and ishen subjected to a forming process. The semi-solid slurryith a non-dendritic microstructure exhibits distinct rheolog-

cal behavior, and can be cut with a knife (Flemings, 2000).his technology provides several potential advantages over

he conventional casting, extrusing and rolling, etc., such asurther homogeneous refinement of crystal grains, low tem-erature of forming, long mould life, reduction of porosity,

∗ Corresponding author. Tel.: +86 10 6233 3174; fax: +86 10 6233 4743.E-mail address: songrb@263.net (R. Song).

924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2007.06.079

© 2007 Elsevier B.V. All rights reserved.

excellent mechanical properties and near net-shaped forming(Flemings, 2000; Kopp and Horst, 2002; Kang et al., 1997; Zoquiet al., 2002; Liu et al., 2001). Basic research on semi-solid metalforming (SSF) has been put into operation and a number ofSSF techniques have been widely applied in industry. Severalcompanies in Europe, Japan and the US have started to man-ufacture in this technique, producing millions of componentsannually for motor industry (Chen et al., 2002; Margarido andRobert, 2003; Chiarmetta, 1998; Jung et al., 2001; Iwasaku et al.,1998; Nohn et al., 2000). In the application of SSF technique,

alloys such as Al-base, Zn-base and Mg-base alloys (Watari etal., 2004; Haga, 2001; Cook et al., 1995; Ichikawa et al., 2002;Tsuchiya et al., 2003; Robert et al., 2002; Song et al., 2002; Kang

n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 291–299

Fig. 1 – Schematic diagram of semi-solid steels preparationand rheo-rolling. 1: Electromagnetic stirring; 2, tundish; 3,liquid steel; 4, bung; 5, semi-solid slurry; 6, stirring

292 j o u r n a l o f m a t e r i a l s p r o c e s s i

et al., 2001; Seidl et al., 2002; Meuser and Bleck, 2001; Choi andPark, 1998; Seo and Kang, 2005), but the high melting pointalloys, for example iron and steel as the most widely use-ful metal are not extensively studied and applied. Their SSFtechnique has a great difference from that of the low melt-ing point alloys, for example: (1) the region size of liquid andsolid phases coexisting for many steel alloys is smaller; (2) thetemperature of two phases coexisting region is higher; (3) thecontrol precision of semi-solid slurry temperature and solidfraction are difficultly ensured; (4) at the high temperature,the conveying and temperature maintaining of slurry are hardto reach, etc. In the foregoing reasons, the research on semi-solid steel materials has made less progress since 1990s (Seoand Kang, 2005; Di, 2005; Bao et al., 2003; Haghayeghi et al.,2005). Few investigations were made in the semi-solid formingof steels, such as spring steel and stainless steel (Haghayeghiet al., 2005; Li et al., 2005).

Different methods have been used to fabricate steelswith non-dendritic structures, such as electromagnetic stir-ring, partial re-melting and ultra-refining using inoculants(Tsuchiya et al., 2003; Li et al., 2005). The present workused the electromagnetic stirring method to obtain semi-solidslurry. At the same time, spring steel–60Si2Mn and stainlesssteel–1Cr18Ni9Ti were rolled into thick strips in the semi-solidstate with different solid fractions. It is aimed at studying themicrostructure and properties of the strips to establish thefeasibility of semi-solid rolling for the production of the steelstrips. According to the present research work, it has beenshown that semi-solid rolling process combines the castingand hot rolling into a single step for near net-shape produc-tion, compared with the conventional hot-rolled metallurgicalprocess. Besides being such a cost-effective process, semi-solid rolling process possesses irregular crystal grains such asrosette-type primary crystals in the microstructures becauseof sufficient agitation during solidification (Flemings, 2000).The overall homogenization of the macrostructures in thewhole part of steel ingot can be achieved. Therefore, compar-ing to the twin-roll strip casting process (Watari et al., 2004;Haga, 2001; Cook et al., 1995; Ichikawa et al., 2002; Di, 2005;Bao et al., 2003), they have beneficial effects on microstructureof product such as reducing segregation, improving inclusionsize distribution and refining microstructural and texturalhomogeneity (Tsuchiya et al., 2003; Robert et al., 2002; Songet al., 2002; Kang et al., 2001; Seidl et al., 2002; Meuser andBleck, 2001; Choi and Park, 1998; Seo and Kang, 2005; Di, 2005;Bao et al., 2003).

2. Experimental procedures

The spring steel–60Si2Mn and stainless steel–1Cr18Ni9Ti areemployed as experimental materials. Their chemical compo-sitions of the specimens are shown in Table 1. The reasons

Table 1 – The chemical compositions of semi-solid 60Si2Mn an

C Si Mn

60Si2Mn 0.61 1.83 0.701Cr18Ni9Ti 0.10 0.10 2.10

crucible; 7, water-cooling roller.

for selecting them are that they have the more commonusage and a wider freezing range than other steels. Thedifference between the liquidus and solidus temperaturesare 70 ◦C for 60Si2Mn and 40 ◦C for 1Cr18Ni9Ti, respectively.Semi-solid temperature of 60Si2Mn used in this investigationlies between 1400 ◦C and 1470 ◦C, and the temperature for1Cr18Ni9Ti is between 1390 ◦C and 1430 ◦C. Semi-solid rollingprocess was performed using the experimental device of semi-solid rolling system, which is shown in Fig. 1. The deviceconsists of induction melting furnace, electromagnetic stir-ring, tundish, bung and rolls, etc.

After melting, the molten steel is transferred to the tundish(shown in Fig. 1) where the semi-solid slurries are fabricated byelectromagnetic stirring (EMS), which is a method of obtainingthe spheroidal microstructure (Kopp and Horst, 2002; Meuserand Bleck, 2001). EMS, on the other hand, due in part to itshigh production rate, has become the main method of produc-

ing semi-solid slurry commercially. EMS also avoids contact ofmolten metal with stirrers, and in some cases the crucible, andmay be easier to implement for high temperature alloys. Forthese above reasons, we decide to use the EMS to fabricate the

d 1Cr18Ni9Ti (wt.%)

Cr Ni Ti Fe

Balance17.43 10.63 0.59

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nttaaectstmrflt

3

3s

T6cacwt

j o u r n a l o f m a t e r i a l s p r o c e s s i n g

emi-solid steel slurry with various solid fractions, fs, obtainedy changing the stirring parameter, such as stirring power andime. The temperature of semi-solid slurry is measured by ahermocouple inserted within stirring crucible. The temper-ture accuracy in this study is ±5 ◦C. When the temperaturef the semi-solid slurry reaches the temperature of requiredolid fraction, the rolling test is performed. The vertical twintrip rolling mill is used in the present study. Roller is made ofodular cast iron and cooled by the circulating water throughhe holes inside the rollers. After the strip exits the roll gap,he strip is usually water-cooled. A slot-type ceramic nozzle issed to achieve a uniform flow across the width. As semi-solidetal feeds through two rotating rolls, there is a formation

f solidified shell near the contact region between slurry andolls. At the same time, its thickness is reduced by the rollingction of the rotating rolls.

In this experiment, the roll speed is 25 mm/s. The thick-ess of the rolled strip is 3–5 mm, the width of 70 mm andhe length of 1000 mm. In order to examine the microstruc-ural evolution of specimens during rolling, the rolled productsre quickly quenched in cool water immediately without anydditional heat treatment, and then they are polished andtched. The etching reagent for 60Si2Mn and 1Cr18Ni9Ti washrysolepic acid and nitro-muriatic acid, respectively, andheir microstructures were observed by use of optical micro-cope. Tests of tensile strength are performed to measurehe mechanical properties of the rolled products. The speci-

ens were cut from the midsection of the strip parallel to theolling direction. Tensile properties were measured by usingat tensile specimens with 25 mm gauge length, 3 mm gaugehickness and 4 mm gauge width.

. Results and discussion

.1. Microstructures of semi-solid slurry and rolledtrip

he as-casted microstructures at the same magnification of0Si2Mn and 1Cr18Ni9Ti are shown in Fig. 2, showing typi-al interdendritic structure. Fig. 3 shows the microstructures

t the same magnification of semi-solid slurry–60Si2Mn fabri-ated by EMS under different stirring power and stirring timeith the temperature of 1440 ◦C. It can be observed that as

he stirring power and stirring time increase, the grain size

Fig. 2 – Microstructures of the as-casted spec

h n o l o g y 1 9 8 ( 2 0 0 8 ) 291–299 293

decreases and the grain shape becomes more non-dendritic.At stirring time of 60 s and stirring power of 0.8 kW, the spher-ical primary grains nearly cannot be obtained, and only thefine, equiaxed dendrites can be observed (see Fig. 3a). Thesolid fraction is only about 5% under this condition. Fromthe above phenomena, it can be inferred that in smaller stir-ring power and shorter time the shearing stress produced byelectromagnetic stirring appears to have no enough efficiencyin breaking dendrites and introducing finer globules. Withthe stirring power and time increasing, the solid fraction ofthe slurry continues increasing and the spherical grains aremore and more. The grain size is also decreased. At stirringtime of 360 s and stirring power of 1.2 kW, a homogeneousmicrostructure together with fine and well-distributed glob-ules is highlighted (see Fig. 3h). The most globules’ size is seenat 80 �m. The measured solid phase fraction (fs) in this sampleis 60%. Fig. 4 shows the microstructures at the same magnifi-cation of semi-solid slurry–1Cr18Ni9Ti under different stirringpower and stirring time with the temperature of 1420 ◦C. Thesame behavior as for 60Si2Mn could be observed. At stirringtime of 365 s and stirring power of 1.4 kW, most globules are at50 �m. The solid phase fraction (fs) is 65% (see Fig. 4h).

From the above experiments, the primary grains willbecome spherical or nearly spherical under the electromag-netic stirring condition, which may be related with thefollowing reasons. Under the condition of electromagneticstirring, intense convection occurs in liquid, resulting in thechange of growth condition of crystals. The primary grainswill not grow in dendrite form, but grow in non-dendriticform, similar to sphere shape (Ichikawa et al., 2002; Tsuchiyaet al., 2003; Robert et al., 2002; Song et al., 2002). Comparedwith Fig. 2, the developed dendrite arms disappeared and thenon-dendritic structure formed, composed of spherical pri-mary solid phases and fine, equiaxed dendrites. A model ofmechanical fracture of dendrites and a theory of dendrites’re-melting and fracture can be used to explain the forma-tion of the above microstructure (Meuser and Bleck, 2001; Choiand Park, 1998; Seo and Kang, 2005; Di, 2005; Bao et al., 2003;Haghayeghi et al., 2005). The dendrite arms break off at theroots due to shear force, or the dendrite arms melt off attheir roots due to heat fluctuation (Liu et al., 2001; Jung et

al., 2001). The larger the stirring power is, the more vigorousthe slurry motion and the stronger the temperature fluctua-tion of the primary solid phase will be, and it is much morepossible for the dendrite arms to re-melt on their roots. The

imens: (a) 60Si2Mn and (b) 1Cr18Ni9Ti.

294 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 291–299

Fig. 3 – Microstructures of semi-solid 60Si2Mn at different stirring power and time: (a) 0.8 kW, 60 s; (b) 0.8 kW, 80 s; (c)0.8 kW, 100 s; (d) 0.8 kW, 120 s; (e) 1.2 kW, 120 s; (f) 1.2 kW, 180 s; (g) 1.2 kW, 260 s; (h) 1.2 kW, 360 s.

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 291–299 295

F stir0 80 s;

ftgtli

ig. 4 – Microstructures of semi-solid 1Cr18Ni9Ti at different.58 kW, 150 s; (d) 0.8 kW, 120 s; (e) 0.8 kW, 180 s; (f) 1.4 kW, 1

ragments of the dendrites are brought into the inner part ofhe melt by convection, and become nuclei of new grains. The

rains ripen into non-dendritic spheroids in accordance withhermo-mechanical laws also by Ostwald ripening or Coa-escence effect (Haghayeghi et al., 2005). If the stirring times long enough, the primary solid phase will become more

ring power and time: (a) 0.58 kW, 80 s; (b) 0.58 kW, 120 s; (c)(g) 1.4 kW, 230 s; (h) 1.4 kW, 365 s.

spherical, which has been proved by the microstructures inFigs. 3 and 4.

Concerning the semi-solid processing, solid particle mor-phology, especially the solid fraction in the slurry has apronounced effect on flow behavior of slurry and on themicrostructure of the final product. Figs. 5 and 6 show

296 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 291–299

rolle

Fig. 5 – Cross-sectional optical micrographs of the

the cross-sectional microstructures of the rolled strips for60Si2Mn and 1Cr18Ni9Ti. They are subjected to reduction of60% in the semi-solid state with various solid fractions, fs. Inthe case of solid fraction with lower value, such as fs = 20% or30%, the fine, equiaxed dendrites transforming from the liq-uid phases through the thickness of strip are observed in theupper and lower surfaces. The fine globular solid particles, i.e.the white grain-shaped portions, aggregate at the center of thestrip. As the microstructure photograph shows, the distribu-

tion of solid and liquid phases is not uniform at the surfaceand center areas of the specimen. It showed the macroseg-regation of the liquid–solid phase took place because of thedifferent flow behavior between the liquid and solid phase

Table 2 – The mechanical properties of semi-solid rolled strip fo

Solid fraction fs (%)

Yield strength (MPa)

60Si2Mn40 45750 46260 473

1Cr18Ni9Ti40 50350 49660 489

d strips-60Si2Mn with various solid fractions, fs.

during the rolling deformation (Liu et al., 2001; Chen et al.,2002; Margarido and Robert, 2003; Chiarmetta, 1998; Jung etal., 2001; Iwasaku et al., 1998; Nohn et al., 2000; Tsuchiya etal., 2003; Robert et al., 2002; Song et al., 2002; Kang et al., 2001;Seidl et al., 2002; Meuser and Bleck, 2001). The liquid phaseplays an important part in the lubrication of the movement ofsolid grain. If the grain shape is globular, the liquid phase atthe boundaries of the grain leads to the smooth movement ofthe grain. However, if the grain shape is not globular, the adja-

cent grains stick together and agglomerate due to insufficientlubrication and, in the long run, the liquid phase drains outfrom the grain network, which process is called macrosegre-gation of the liquid–solid phase (Li et al., 2005). In the case that

r 60Si2Mn and 1Cr18Ni9Ti

Mechanical properties

Ultimate tensile strength (MPa) Elongation (%)

512 8.9575 10.6860 13.4

617 16.9665 27.4

1022 40.6

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 291–299 297

olled

gtpW6sFflllrtd(

3

Tsrsh4tuwt

and 13.4%, respectively. For 1Cr18Ni9Ti with 60% solid frac-tion, the highest ultimate tensile strength and elongation are1022 MPa and 40.6%, respectively.

Fig. 6 – Cross-sectional optical micrographs of the r

rain is globular; the effective liquid fraction is much higherhan the nominal liquid fraction (1 − fs). Relatively small liquidools were also located inside the grains (Kang et al., 2001).ith increasing the solid fraction, under the level of 50% or

0%, the deformation of solid particles takes place, and thehape of solid phase is changed from roundness to ellipse (seeig. 7). Because of the network formation of solid particles, theuidity of liquid phases greatly decreases, which results in the

iquid phase surrounding the solid particles. Some entrappediquid phases between the solid phases flow together with sur-ounded solid phases during the deformation (see Fig. 7). Athe same time, the degree of macrosegregation also decreasesue to the uniform distribution of the liquid and solid phases

see Fig. 5).

.2. Mechanical properties of rolled strip

able 2 lists the mechanical properties of rolled strip of semi-olid 60Si2Mn and 1Cr18Ni9Ti with various solid fractions atoom temperature. The yield strength of the as-rheo-rolledpecimens is similar whatever the value of solid fraction isigh or lower. The value of yield strength for 60Si2Mn is about65 MPa, and for 1Cr18Ni9Ti is 500 MPa. However, the strip with

he solid fraction of 60% is seen to have much higher val-es of tensile strength and elongation compared to the stripsith the solid fraction of 40% and 50%. It is well known that

he strength of a metallic material increases as the crystal

strips-1Cr16Ni9Ti with various solid fractions, fs.

grain becomes finer according to the Hall–Petch relationship.In the present study, a fine, homogeneous microstructure wasachieved in the strips with the solid fraction of 60% by semi-solid rolling methods. Therefore, the results show that theincrease of the solid fraction enhances homogeneous defor-mation, thereby increasing strength as well as ductility (Junget al., 2001). In addition, the more globular the solid particleshape is, the higher the value of tensile strength and elonga-tion is. The highest ultimate tensile strength and elongationobtained for 60Si2Mn with solid fraction of 60% are 860 MPa

Fig. 7 – Microstructure of entrapped liquid phases betweenthe solid phases (fs = 58%).

n g t

298 j o u r n a l o f m a t e r i a l s p r o c e s s i

3.3. Flow behavior of semi-solid slurry at differentshapes of roller

In semi-solid rolling process, it is necessary to properly con-trol the flow behavior of semi-solid slurries for high-qualityproducts. There are a number of parameters that affect theflow behavior of semi-solid slurries such as viscosity, reduc-tion, shape of roller, roll velocity, deformation temperature,etc. In the present study, the effects of shape of roller on flowbehavior were investigated. Fig. 8 shows the microstructural

difference at the same magnification of rolled strips at theedge of the specimen under two types of passes for 1Cr18Ni9Ti.In the plain barrel pass, the semi-solid slurry receives theacting force from the roller in the thickness direction and is

Fig. 8 – Microstructures of semi-solid rolled strips at different sh

e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 291–299

not restricted in the width direction during rolling. The liq-uid and solid phases are detached and a number of liquidphases flow to the edge during deformation, which results inthe heterogeneous distribution of the liquid and solid phases(see Fig. 8a). In order to decrease the above phenomena, theshape of roller was redesigned with a box groove pass (seeFig. 8b). The characteristic of the box groove pass is that theroller has a closed pass to optimize the semi-solid slurry flowpattern. The closed pass prevents the lateral spreading of thestrip during rolling, indicating that the stress state is changed

at the edge of the specimen. The flow rate of liquid phasedecreases near the edge, which results in the uniform distri-bution of the solid and liquid phases (see Fig. 8b) (Song et al.,2002).

apes of roller: (a) plain barrel pass and (b) box groove pass.

t e c

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. Conclusions

n practical case, the electromagnetic stirring technique is aiable process to obtain a semi-solid steel material with glob-lar primary phase. With stirring power and time increasing,he solid fraction increases and the grain size decreases.

The semi-solid rolling process is applied to produce steeltrip. The microstructure of rolled strip produced by this pro-ess consists of the spherical solid particles and the fine,quiaxed dendrites, which is different from the conventionalendrite structure.

The macrosegregation is observed in the specimen due tohe separation of solid and liquid phases during rolling defor-

ation. The macrosegregation and the mechanical propertiesf specimen have a close relationship with the solid fraction.

In order to improve the quality of the rolled strip, the boxroove pass is used. By changing the stress state at the edge ofhe specimen, the homogeneity of microstructure increases.

cknowledgments

his work was supported by the National Natural Scienceoundation of China (grant nos. 59995440 and 50504002). Theroject was also sponsored by the Scientific Research foun-ation for the Returned Overseas Chinese Scholars, Stateducation Ministry.

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