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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
keVFull Scale 6172 cts Cursor: 20.280 (0 cts)
Spectrum 2
Ti
Ti Ti
Be
Ti
0 2 4 6 8 10 12 14 16 18 20
keVFull Scale 4083 cts Cursor: 20.220 (0 cts)
Spectrum 1
Ti
Ti
Be
Ti
Ti
0 2 4 6 8 10 12 14 16 18 20
keVFull Scale 4116 cts Cursor: 20.220 (0 cts)
Spectrum 2
3#
VCV
Ti
V
Ti
Ti
V
Ti
0 2 4 6 8 10 12 14 16 18 20
keVFull Scale 6388 cts Cursor: 20.310 (0 cts)
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.
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.
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
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
0 2 4 6 8 10 12 14 16 18 20keVFull Scale 6388 cts Cursor: 20.310 (0 cts)
Spectrum 1
TiTi Ti
Be
Ti
0 2 4 6 8 10 12 14 16 18 20keVFull Scale 4083 cts Cursor: 20.220 (0 cts)
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.
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