research on pickling hot rolled commercial pure titanium ... · after hot rolled on the hot...

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Research on Pickling Hot Rolled Commercial Pure Titanium

Strip on the Continuous Pickling Line of Special Steel

Authors: Wang Chuang (BaoSteel Special Steel Business Unit, Engineer)

Hao Renguan (BaoSteel Special Steel Business Unit, Director)

Wang Wei (BaoSteel Special Steel Research Institute, Engineer)

Xu Dalong (BaoSteel stainless Steel Business Unit, Enigeer)

Feng Huang (BaoSteel Special Steel Business Unit, Master)

Li Chengguo (BaoSteel Special Steel Business Unit, Engineer)

Abstract

After hot rolled on the hot continuous rolling

mill and annealed in the continuous

annealing line, the commercial pure titanium

(CP-Ti) strip (3.8 mm x 980 mm) is covered

with scale layer, mainly TiO2, on its surface.

This scale layer must be removed by acid

pickling before cold rolling.

This paper describes the acid pickling

process for CP Titanium strip on the

continuous pickling line of Special Steel. In

order to treat CP-Ti on the pickling line of

special steel without stopping the line, we

used a special method (a patent of BaoSteel)

to connect CP-Ti strip and special steel strip

without welding them together directly. The

pickling process for hot rolled CP-Ti is

different from those for special steel pickled

on the same pickling line. We used sulfuric

acid as pre pickling and two mixed acid

(HNO3 + HF) section as final pickling. The

scale breaker has as well been applied before

pickling. The roughness of the pickled CP-Ti

strip surface is about Ra 2.3 – 2.8 μm and

the surface quality meets the requirements of

NO.1, according to ASTM.

Key words: Commercial pure titanium,

Special steel, Strip, Pickling

1. Introduction

Titanium has good corrosion resistance and

high specific strength. It is often used in

construction, large motorcycle muffler and

other civilian areas, which means that the

surface of titanium must meet the quality

requirements mentioned above. During the

hot rolling and annealing process of titanium

and titanium alloy, a very compacted oxide

layer with excellent corrosion resistance will

be generated on the surface of titanium and

titanium alloy, mainly composed of rutile

structure of TiO2[1]

. For subsequent

processing, this oxide layer must be

removed.

The quality of the titanium surface can be

controlled by using a variety of methods,

such as machining methods, blasting or

chemical methods like pickling. The most

common method is to pickle it in HNO3-HF.

However, there is a relatively small demand

for production for titanium and titanium

alloy plate. It is not very economic to design

a specific pickling line for titanium. Because

the pickling process of CP-Ti is similar to

special stainless steel products, this research

aims to realize pickling CP-Ti on the

pickling line for special stainless steel by

adjusting the pickling parameters.

2. Research Schedule

Research is divided into two steps. First, we

simulated the pickling process of CP-Ti on

the special steel pickling line in the

2

laboratory and then we practiced it on the

actual production line.

For the hot rolled special stainless steel,

usually a two-stage acid-pickling method is

adopted in the actual production line. That is

sulfuric acid as pre pickling and mixed acid

(HNO3 + HF) section as final pickling. In

order to make it compatible, we chose the

same pickling method for CP-Ti strip in

laboratory. That is sulfuric acid plus mixed

acid.

The second step is to pickle CP-Ti strip on

the continuous pickling line of special

stainless steel. The pickling parameters are

designed according to the best results of the

laboratory pickling tests.

3 Laboratory Experiment

3.1 Testing Material and sample

preparation

Test material is cut from annealed hot rolled

CP-Ti coils (3.8 mm x 980mm x C). The

main chemical compositions are listed in

Table 1.

Table 1 Titanium plate and the main chemical

ingredients (mass.%)

C N O H Fe Ti

0.03 0.008 0.06 0.0014 0.05 the

rest

The CP-Ti plates are cut into 50 mm x 30

mm x 3.8 mm size specimens for laboratory

tests and evaluation of the pickling process.

3.2 Sulfuric Acid Pickling

For the sulfuric acid pickling process, the

current density, concentration, temperature

and pickling time are the main influence

factors. In order to make it compatible with

the special stainless steel pickling process,

conventional sulfuric acid process

parameters of typical stainless steel special

were selected, with only pickling time

changed. Three different acid pickling times

were tested in the experiment, shown in

table 2.

Table 2 Sulfuric Acid Laboratory Testing

Parameters for CP-Ti

Sample

No.

concentr

ation

g/l

Temp.

℃

Time

min

1# 100-350 70-85 1.10

2# 100-350 70-85 4-6

3# 100-350 70-85 5-8

3.3 Mixed Acid Pickling

The main factors of mixed acid pickling are

solution ratio, temperature, metal ion content

and the pickling time. As in the sulfuric acid

electrolyte pickling, we chose the same

mixed acid process parameters except

pickling time, see table 3. In order to make

acid fresh, we replace the acid after the

treatment of a group of samples.

Table 3 Acid Wash titanium mixed with laboratory

test parameters

Sample

No.

HNO3

g/l

HF

g/l

Temp.

℃

Me

g/l

Time

min

1# 100-2

00

20-

40 54 <55 4.40

2# 100-2

00

20-

40 54 <55 8.80

3# 100-2

00

20-

40 54 <55 17.60

3.4 Laboratory Test Results and Analysis

In the sulfuric acid pickling process, huge

amounts of bubbles were produced on the

surface of the sample.

After 1.1 min pickling, the surface color of

sample 1# turned lighter. After 4.4 min

pickling, the surface color of sample 2# and

3# were silver-white and the color of the

local and shear cut area was mauve. Oxide

layer damage effect can be observed

3

obviously.

In the mixed acid (HNO3 + HF) solution,

the reaction of sample 1# was not very

impetuous. Only a small amount of bubbles

were produced on the surface. After 4

minutes pickling, the acid temperature rose

about 2 - 5 ℃ with a small amount of

precipitation. The reaction of samples 2# and

3# in the mixed acid was impetuous. After

pickled sample 3# 17.6 minutes, the acid

temperature rose about 10 - 20 ℃. For

sample 2# after 8 minutes treatment, acid

temperature rose about 5 - 10 ℃.

Figure 1 consists of the SEM and EDS

pictures of three group samples after pickled

in H2SO4 and mixed acid (HNO3 + HF).

Fig 1 shows that sample 1# with short

pickling time remained residual oxide on its

surface, which means under pickling or

insufficient pickling. There was no oxide

residue on sample 3#, but the metal matrix

was little over pickling. For sample 2#, the

surface was smooth and the EDS

composition analysis showed no residual

oxide and the surface pickled fully enough.

Fig 1 The SEM and EDS pictures of pickled

samples

4 Pickle CP-Ti Strip on the Pickling Line

4.1 Pickling Process Design

According to the laboratory experiment

results, the pickling process of sample 2#

was better. Therefore this paper took the

pickling process of sample 2# as the base for

pickling commercially pure titanium on

1#

Zn ZnZnCF

Zn

S

S

Na

Ti

Ti

O

Ti

Ti

0 2 4 6 8 10 12 14 16 18 20

keVFull Scale 6795 cts Cursor: 20.280 (0 cts)

Spectrum 1

Zn ZnZnC

FZn

S

S

Na

Ti

Ti

O

Ti

Ti

0 2 4 6 8 10 12 14 16 18 20


Spectrum 2

Ti

Ti Ti

Be

Ti

0 2 4 6 8 10 12 14 16 18 20


Spectrum 1

Ti

Ti

Be

Ti

Ti

0 2 4 6 8 10 12 14 16 18 20


Spectrum 2

3#

VCV

Ti

V

Ti

Ti

V

Ti

0 2 4 6 8 10 12 14 16 18 20


Spectrum 1

2#

4

special stainless production line. That is the

sulfuric acid electrolyte as pre-pickling and

the two-stage mixed acid (HNO3 + HF) as

chemical pickling.

The special stainless steel pickling line is a

continuous pickling line with pickling

section length 140 m and it has an automatic

MIG welding machine at the entrance.

According to the result of sample 2#

pickling tests in the laboratory, the line

speed should be set to 140 / (4.4 + 8.8) =

12.7 m/min. Taking into account the

difference between laboratory and

production line, the process speed of CP-Ti

on the stainless steel pickling line is set to

10-30 m/ min. The parameters of pickling

CP-Ti on the stainless pickling line are listed

in table 4.

Table 4 Parameters of Pickling CP-Ti on

Pickling Line

(a) PrePickling section – H2SO4

concen.

g/l

Temp.

℃

Me

g/l

200-300 75-85 -

(b) First mixed acid pickling section

HNO3

g/l

HF

g/l

Temp.

℃

Me

g/l

150-200 25-35 54-60 <45

(c) Second mixed acid pickling section

HNO3

g/l

HF

g/l

Temp.

℃

Me

g/l

150-200 25 -35 54-60 <45

The special stainless steel pickling line is a

continuous pickling line (CPL). We should

weld the CP-Ti strip end and stainless steel

strip end together by the MIG welder. But

the weld seam between CP-Ti and stainless

steel is not strong enough to pass through the

pickling line without breaking. We designed

a special connecting method, as shown in

Fig.2. This method is a patent of Baosteel.

The patent number is CN200920069095.7

Fig 2 CP-Ti – Stainless steel connecting

method diagram

By using this special connecting plate, CP-Ti

strip and stainless steel strip can be linked

together to pass through the special stainless

steel pickling line without stopping.

4.2 Field trial results and analysis

Totally, two CP-Ti coils were pickled on the

CPL and the specifications are 3.5 mm x 980

mm x C. During pickling, the reaction

between titanium matrix and the HF was

very impetuous, which resulted in a rising of

temperature of the mixed acid solution. The

real pickling process parameters collected

from CPL are all in the range of setting

points.

After pickling, the surface quality of middle

section of titanium coil is better. While

visible surface defect "tiger striae" can be

seen across the width direction on the strip

ends (about 20 m long), shown in Figure 3.

Figure 3 "tiger striae" on the ends of the

titanium coil

Fig 4 shows the macrography, SEM picture

and EDS result of CP-Ti pickled on CPL. (a)

and (b) are the upper and lower surface

appearance of titanium strip obtained from

image scanners. White parts of the original

5

sample were presented as reflective blue.

Figure (a) shows the "tiger striae" defect

with a roughness surface which is

perpendicular to the rolling direction. Figure

(b) has the smooth, bright and clean surface

but there are still few obvious scratches.

Figure (c), (d) are SEM pictures of figure (a),

(b). Figure (c) local spectrum analysis (EDS)

shows that the tiger striae is part of oxides.

Figure (d) shows the grain boundary clearly,

the surface quality is good without oxide

residue.

The surface roughness of the CP-Ti strip

pickled on CPL is shown in Table 5.

Table 5 the surface roughness of CP-Ti strip

Drive side

(Ra, μm)

Central

(Ra, μm)

Operating side

(Ra, μm)

2.387 2.727 2.372

3.724 2.505 3.162

The average roughness of the drive side,

central and the operation side, was Ra 3.01

um, Ra 2.62 um and Ra 2.77 um respectively,

which meets the ASTM NO.1 surface

roughness requirements.

Fig 4 The Macrography, SEM Picture and

EDS result of CP-Ti pickled on CPL

5 Conclusions

(1) A series of laboratory and on-site

experiments shows that it is feasible to

pickle commercial pure titanium on the

pickling line of special stainless steel by

using "sulfuric acid pickling+ mixed acid

pickling" parameters. It can switch freely

without re-threading.

(2) After proper adjustments of the process

of "sulfuric acid + HNO3/HF", a good

descaled treatment for annealed hot-rolled

commercial pure titanium can be realized.

References

[1] Huang Deming. Research on Oxidation

Behavior of CP Titanium TA2 [J]. IRON

STEEL VANADIUM TITANIUM, 2010, 31

(1): 11-16.

Pickling CP-Ti on the Continuous Pickling Line of

Special Steel

content

1. Brief introduction 2. Preparing of hot rolled CP-Ti3. Laboratory pickling test4. Pickling process and result in site 5. Conclusions

1. Brief Introduction

Recently Baosteel has invested in special steel business unit which aims to produce high-temperature alloy,Nickle-based alloy, special stainless alloy,precision alloy and pure titanium.

In order to achieve the above object,we have built up HCAL,HCPL,6-high mill,20-high mill,CGL, BAL ,VAF, CCL ,SPM, STL and other related production line in cold rolling mill plant.

It is possible to pickle the first four alloys in the HCPL continuously. But for the pure titanium,the pickling time and process parameters are quite different. This paper describes the acid pickling process for CP titanium strip on the continuous pickling line of special steel. We made several experiments in the laboratory by using different pickling parameters. In the end ,we made a trial on line which lead to a satisfied result.

2. Preparing of hot rolled CP-Ti

• Ingot (VAR) • Heavy plate

hot rolled titanium coils: 3.8X990XC

#1 Coil #2 Coil

Thickness Width Thickness Width

3.711 992 3.922 990

3.728 989 3.888 981

3.632 989 3.876 985

3.619 987 3.902 985

3.492 995 3.805 989

3. Laboratory Test

• Laboratory TestSpecimens Size: 3.8 mm x 30 mm x 50 mmChemical Composition of CP-Ti:(mass %)

C N O H Fe Ti

0.03 0.008 0.06 0.0014 0.05

• Sulfuric Acid Pre-Pickling

Sample No. concentrationg/l

Temp.℃

Timemin

1# 100-350 70-85 1.10

2# 100-350 70-85 4-6

3# 100-350 70-85 5-8

• Mixed Acid Pickling

Sample No.

HNO3g/l

HFg/l

Temp.℃

Meg/l

Timemin

1# 100-200 20-40 54 <55 4.40

2# 100-200 20-40 54 <55 8.80

3# 100-200 20-40 54 <55 17.60

SEM Pictures• 1.1min +4.4min 4.4min +8.8min 4.4min+17.6min

1# 2# 3#

EDS Pictures

Zn ZnZnCF

Zn

SS

NaTiTiO

Ti

Ti

0 2 4 6 8 10 12 14 16 18 20keVFull Scale 6795 cts Cursor: 20.280 (0 cts)

Spectrum 1

VCVTiVTi

TiV

Ti


Spectrum 1

TiTi Ti

Be

Ti


Spectrum 1

1#

2#

3#

samplePickling time(min)

Surface condition

Metal matrix

1# 1.1+4.4 residual oxide no errosion

2# 4.4+8.8 no oxide remained

no errosion

3# 4.4+17.6 no oxide remained

errosion with over pickling

Laboratory test result

4. Pickling process & result in site

• Pickling CP-Ti on stainless steel pickling lineThe pickling process were based on the pickling parameters of sample

2# in laboratory.• Connection of CP-Ti and stainless steel

By using this special connecting plate, CP-Ti strip and stainless steel strip can be linked together to pass through the special stainless steel pickling line without stopping.

Patent No.: CN200920069095.7

• Pre-set pickling parameter(a) PrePickling section – H2SO4

concen.(g/l) Temp.(℃) Me(g/l)

200-300 75-85 -

(b) First mixed acid pickling section

HNO3

g/l

HF

g/l

Temp.

℃

Me

g/l

150-200 25-35 54-60 <45

(c) Second mixed acid pickling section

HNO3

g/l

HF

g/l

Temp.

℃

Me

g/l

150-200 25 -35 54-60 <45

• Surface qualityAfter pickling, the

surface quality ofmiddle section oftitanium coil is quitegood. But at the stripends(about 20 m long),visible surface defect"tiger striae" can beseen across the widthdirection

(a) (b)

1cm

R R

(c) (d)

70μm

Ti、Fe、O

Ti

FeFeTi

Cu

FeTi

TiO

0 2 4 6 8Full Scale 238 cts Cursor: 3.688 (9 cts)

Ti

Ti

Ti

0 2 4 6 8Full Scale 392 cts Cursor: 3.688 (12 cts)

Figure (a) is sample from topend area which shows "tigerstriae" defect with a roughnesssurface .Figure (b) is sample frommiddle section which has thesmooth, bright and cleansurface but there are still fewobvious scratches.Figure (c), (d) are SEMpictures of figure (a), (b).Figure (c) local spectrumanalysis (EDS) shows that thetiger striae are part of oxides.Figure (d) shows the grainboundary clearly, the surfacequality is good without oxideresidue.

• Surface roughness of pickled CP-Ti strip

Drive side

(Ra, μm)

Central

(Ra, μm)

Operating side

(Ra, μm)

2.387 2.727 2.372

3.724 2.505 3.162

5. Conclusions

• Conclusions(1) A series of laboratory and on-site experiments

shows that it is feasible to pickle commercial pure titanium on the pickling line of special stainless steel by using "sulfuric acid pickling+ mixed acid pickling" parameters. It can switch freely without re-threading.

(2) After proper adjustments of the process of "sulfuric acid + HNO3/HF", a good descaled treatment for annealing hot-rolled commercial pure titanium can be realized.

Thank You Very Much

research on pickling hot rolled commercial pure titanium ... · after hot rolled on the hot...

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