U.S. patent application number 10/714987 was filed with the patent office on 2004-05-20 for ferritic stainless steel sheet for fuel tank and fuel pipe and method for making the same.
This patent application is currently assigned to JFE Steel Corporation, a corporation of Japan. Invention is credited to Fukuda, Kunio, Katoh, Yasushi, Miyazaki, Atsushi, Muraki, Mineo, Ozaki, Yoshihiro, Yazawa, Yoshihiro.
Application Number | 20040094240 10/714987 |
Document ID | / |
Family ID | 26606419 |
Filed Date | 2004-05-20 |
United States Patent
Application |
20040094240 |
Kind Code |
A1 |
Yazawa, Yoshihiro ; et
al. |
May 20, 2004 |
Ferritic stainless steel sheet for fuel tank and fuel pipe and
method for making the same
Abstract
A ferritic stainless steel sheet for fuel tanks and fuel pipes
comprises, by mass percent, about 0.1% or less of C; about 1.0% or
less of Si; about 1.5% or less of Mn; about 0.06% or less of P;
about 0.03% or less of S; about 1.0% or less of Al; about 11% to
about 20% Cr; about 2.0% or less of Ni; about 0.5% to about 3.0%
Mo; about 0.02% to about 1.0% V; about 0.04% or less of N; at least
one of about 0.01% to about 0.8% Nb and about 0.01% to about 1.0%
Ti; and the balance being Fe and incidental impurities. The
ferritic stainless steel sheet is produced by rough-rolling a slab
having the above composition; hot-rolling the rough-rolled sheet
under a linear pressure of at least about 3.5 MN/m at a final pass
in the finish rolling; cold-rolling the hot-rolled sheet at a gross
reduction rate of at least about 75%; and annealing the cold-rolled
sheet. The cold-rolling step includes one rolling stage or at least
two rolling stages including intermediate annealing.
Inventors: |
Yazawa, Yoshihiro; (Chiba,
JP) ; Muraki, Mineo; (Chiba, JP) ; Ozaki,
Yoshihiro; (Chiba, JP) ; Fukuda, Kunio;
(Chiba, JP) ; Miyazaki, Atsushi; (Chiba, JP)
; Katoh, Yasushi; (Chiba, JP) |
Correspondence
Address: |
IP DEPARTMENT OF PIPER RUDNICK LLP
3400 TWO LOGAN SQUARE
18TH AND ARCH STREETS
PHILADELPHIA
PA
19103
US
|
Assignee: |
JFE Steel Corporation, a
corporation of Japan
Tokyo
JP
|
Family ID: |
26606419 |
Appl. No.: |
10/714987 |
Filed: |
November 17, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10714987 |
Nov 17, 2003 |
|
|
|
10016543 |
Dec 11, 2001 |
|
|
|
Current U.S.
Class: |
148/325 ;
148/610; 420/63; 420/68 |
Current CPC
Class: |
C22C 38/46 20130101;
C10M 105/24 20130101; C21D 8/0205 20130101; C10N 2050/023 20200501;
C10M 2207/1253 20130101; C10M 2209/0845 20130101; C10M 107/28
20130101; C21D 8/0236 20130101; C21D 8/0226 20130101; C22C 38/001
20130101; C22C 38/48 20130101; C10M 111/04 20130101; C22C 38/44
20130101; C22C 38/50 20130101; C22C 38/004 20130101; C10N 2010/04
20130101; C21D 8/0278 20130101; C10M 107/04 20130101; C10M
2205/0225 20130101 |
Class at
Publication: |
148/325 ;
148/610; 420/063; 420/068 |
International
Class: |
C22C 038/50; C22C
038/28 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 22, 2000 |
JP |
2000-391108 |
Apr 4, 2001 |
JP |
2001-105483 |
Claims
What is claimed is:
1. A ferritic stainless steel sheet for fuel tanks and fuel pipes
comprising, by mass percent: about 0.1% or less of C; about 1.0% or
less of Si; about 1.5% or less of Mn; about 0.06% or less of P;
about 0.03% or less of S; about 1.0% or less of Al; about 11% to
about 20% Cr; about 2.0% or less of Ni; about 0.5% to about 3.0%
Mo; about 0.02% to about 1.0% V; about 0.04% or less of N; at least
one of about 0.01% to about 0.8% Nb and about 0.01% to about 1.0%
Ti; and the balance being Fe and incidental impurities.
2. The ferritic stainless steel sheet according to claim 1, wherein
the ferritic stainless steel sheet has a ridging height of about 50
.mu.m or less at a 25% deformation in uniaxial stretching.
3. The ferritic stainless steel sheet according to claim 1, wherein
a lubricant coat comprising an acrylic resin, calcium stearate, and
polyethylene wax is coated and baked on at least one surface of the
ferritic stainless steel sheet in a coating amount of about 0.5
g/m.sup.2 to about 4.0 g/m.sup.2.
4. The ferritic stainless steel sheet according to claim 2, wherein
a lubricant coat comprising an acrylic resin, calcium stearate, and
polyethylene wax is coated and baked on at least one surface of the
ferritic stainless steel sheet in a coating amount of about 0.5
g/m.sup.2 to about 4.0 g/m.sup.2.
5. A fuel tank comprising the ferritic stainless steel sheet
according to claim 1.
6. A fuel pipe comprising the ferritic stainless steel sheet
according to claim 1.
7. The ferritic stainless steel sheet according to claim 1, wherein
the ferritic stainless steel sheet has an r-value of at least about
1.5.
8. A method for making a ferritic stainless steel sheet for fuel
tanks and fuel pipes, comprising the steps of: rough-rolling a slab
comprising, by mass percent, about 0.1% or less of C, about 1.0% or
less of Si, about 1.5% or less of Mn, about 0.6% or less of P,
about 0.03% or less of S, about 1.0% or less of Al, about 11% to
about 20% Cr, about 2.0% or less of Ni, about 0.5% to about 3.0%
Mo, about 0.02% to about 1.0% V, about 0.04% or less of N, at least
one of about 0.01% to about 0.8% Nb and about 0.01% to about 1.0%
Ti, and the balance being Fe and incidental impurities; hot-rolling
the rough-rolled sheet under a linear pressure of at least about
3.5 MN/m at a final pass in the finish rolling; cold-rolling the
hot-rolled sheet at a gross reduction rate of at least about 75%,
the cold-rolling including one rolling stage or at least two
rolling stages including intermediate annealing; and annealing the
cold-rolled sheet.
9. The method according to claim 8, wherein the hot-rolled sheet is
subjected to hot-rolled sheet annealing according to the following
equations, cold roiling, and finish annealing:
900.ltoreq.T+20t1,150 and t.ltoreq.10 wherein T is annealing
temperature (.degree. C) and t is holding time (minutes).
10. The method according to claim 8, wherein a lubricant coat
comprising an acrylic resin, calcium stearate, and polyethylene wax
is coated and baked on at least one surface of the hot-rolled or
annealed hot-rolled sheet in a coating amount of about 0.5
g/m.sup.2 to about 4.0 g/m.sup.2.
11. The method according to claim 9, wherein a lubricant coat
comprising an acrylic resin, calcium stearate, and polyethylene wax
is coated and baked on at least one surface of the hot-rolled or
annealed hot-rolled sheet in a coating amount of about 0.5
g/m.sup.2 to about 4.0 g/m.sup.2.
12. A fuel tank comprising a ferritic stainless steel sheet made
from the method according to claim 8.
13. A fuel pipe comprising a ferritic stainless steel sheet made
from the method according to claim 8.
Description
BACKGROUND
[0001] 1. Field of the Invention
[0002] This invention relates to ferritic stainless steel sheets
suitable for containers and piping elements for organic fuels such
as gasoline, methanol and the like. In particular, the invention
relates to a ferritic stainless steel sheet which can be readily
shaped into fuel tanks and fuel pipes and which is resistant to
organic fuels, particularly deteriorated gasoline containing
organic acids produced in the ambient environment. The invention
also relates to a method for making the ferritic stainless steel
sheet.
[0003] 2. Description of the Related Art
[0004] Automobile fuel tanks are generally manufactured by plating
surfaces of a soft steel sheet with a lead alloy and shaping and
welding the terne coated steel sheet. The continued use of
lead-containing materials, however, tends to be severely limited
with the increasing sensitivity to environmental issues.
[0005] Several substitutes for the terne coated steel sheet have
been developed. Unfortunately, the substitutes have the following
problems. Al--Si plating materials as lead-free plating materials
are unreliable in weldability and long-term corrosion resistance
and, thus, are used only in restricted fields. Although resinous
materials have been tried for uses in fuel tanks, industrial use of
the resinous materials which are inevitably permeable to fuel is
limited under circumstances such as regulations against fuel
transpiration and recycling. Also, the use of austenitic stainless
steels, which requires no lining treatments, has been attempted.
Although the austenitic stainless steels exhibit superior
processability and higher corrosion resistance compared with the
ferritic stainless steels, the austenitic stainless steels are
expensive for fuel tanks and have the possibility of stress
corrosion cracking (SCC). Thus, the austenitic stainless steels
have not yet been used in practice.
[0006] In contrast, the ferritic stainless steels not containing
nickel are advantageous in material costs compared with the
austenitic stainless steels, but do not exhibit satisfactory
corrosion resistance to so-called "deteriorated gasoline"
containing organic acids such as formic acid and acetic acid which
are formed in the ambient environment. Furthermore, the ferritic
stainless steels do not exhibit sufficient processability to deep
drawing for forming fuel tanks having complicated shapes and to
expanding and bending of the pipes for forming expanded fuel pipes
and bent fuel pipes.
[0007] Japanese Unexamined Patent Publication Nos. 6-136485 and
6-158221 disclose double-layer steel sheets each including a
corrosion-resistant steel layer and a low-carbon or
ultra-low-carbon steel layer having excellent processability to
achieve both corrosion resistance and processability. However, the
double-layer steel sheets exhibit less adaptability to mass
production.
SUMMARY OF THE INVENTION
[0008] The invention provides a ferritic stainless steel sheet
which exhibits superior processability and high corrosion
resistance to deteriorated gasoline and is useful for automobile
fuel tanks and fuel pipes. In particular, the ferritic stainless
steel of the invention has a thickness in the range of about 0.4 to
about 1.0 mm and superior deep drawing processability, namely, an
r-value of at least about 1.50 and preferably at least about
1.90.
[0009] The r-value in the invention represents a mean plastic
strain ratio determined by equation (1) according to Japanese
Industrial Standard (JIS) Z2254: 1 r = r 0 + 2 r 45 + r 90 4
[0010] wherein,
[0011] r.sub.0 is a plastic strain ratio measured using a test
piece which is sampled in parallel to the rolling direction of the
sheet;
[0012] r.sub.45 is a plastic strain ratio measured using a test
piece which is sampled at 45.degree. to the rolling direction of
the sheet; and
[0013] r.sub.90 is a plastic strain ratio measured using a test
piece which is sampled at 90.degree. to the rolling direction of
the sheet.
[0014] An r-value of less than about 1.50 precludes deep drawing
into a complicated fuel tank shape and bending into a complicated
bent pipe shape and exhibits high impact brittleness (secondary
processing brittleness) even if the sheet is capable of
processing.
[0015] The invention also provides a ferritic stainless steel
having a surface ridging height of about 50 .mu.m or less at 25%
deformation in uniaxial stretching. Ridges formed during processing
of steel sheets for automobile fuel tanks are not necessarily so
small because these tanks are produced by press forming of the
sheet. According to our investigations, however, ridges cause
cracking of the sheet during severe press forming processes which
are used in the production of fuel tanks. Hence, the ridging height
must be small. The ridges generated in the sheeting process vary
the state of contact of the unprocessed steel sheet piece with the
press die and results in "gnawing" or "galling" due to a local
deficiency of lubricant oil film. The gnawing also causes cracking
along the ridges.
[0016] According to our further investigations, a steel sheet
exhibiting superior press formability suitable for processing of
fuel tanks having complicated shapes has a surface ridging height
of about 50 .mu.m or less at a 25% deformation in uniaxial
stretching. Herein, the ridges on the steel sheet generated during
processing are evaluated by the height of the ridges in a direction
perpendicular to the stretching direction when the steel is
stretched in the rolling direction.
[0017] The invention also solves a problem in the art known in the
case of severe forming of a ferritic stainless steel into fuel
tanks and fuel pipes and in the case of lubricant-free press
forming. That is, the invention provides a ferritic stainless steel
by a lubricant-free process exhibiting superior deep drawability
and requires no lubrication steps for treating the sheet with
lubricant oil.
[0018] We discovered that a predetermined amount of a lubricant
coat primarily containing an acrylic resin which is applied on the
surfaces of a ferritic stainless steel sheet decreases the dynamic
friction coefficient between the steel sheet and the press die,
thus preventing "gnawing" and being capable of processing into
articles having further complicated shapes.
[0019] We intensively investigated the effects of the composition
of ferritic stainless steel sheets and the method for making the
same on the corrosion resistance in deteriorated gasoline and the
r-value of the ferritic stainless steel sheet and found that the
corrosion resistance to the deteriorated gasoline is remarkably
improved by adding appropriate amounts of Mo and V to the steel
sheets.
[0020] Since the addition of Mo precludes processability, we
further investigated the r-value as a reference of processability
of Mo-containing steel sheets and found that a high r-value is
achieved by a specified method.
[0021] Furthermore, we found that optimized annealing conditions
for hot-rolled ferritic stainless steel sheets minimize the ridging
height, provide superior press formability, and that the
application of a lubricant coat on the steel sheet surfaces
improves sliding performance in forming, decreases the dynamic
friction coefficient between the steel sheet and the press die, and
facilitates forming of articles having further complicated
shapes.
[0022] According to an aspect of the invention, a ferritic
stainless steel sheet for fuel tanks and fuel pipes comprises, by
mass percent, about 0.1% or less of C; about 1.0% or less of Si;
about 1.5% or less of Mn; about 0.06% or less of P; about 0.03% or
less of S; about 1.0% or less of Al; about 11% to about 20% Cr;
about 2.0% or less of Ni; about 0.5% to about 3.0% Mo; about 0.02%
to about 1.0% V; about 0.04% or less of N; at least one of about
0.01% to about 0.8% Nb and about 0.01% to about 1.0% Ti; and the
balance being Fe and incidental impurities.
[0023] Preferably, the ferritic stainless steel sheet has a ridging
height of about 50 .mu.m or less at a 25% deformation in uniaxial
stretching.
[0024] Preferably, a lubricant coat comprising an acrylic resin,
calcium stearate, and polyethylene wax is coated by baking on the
surfaces of the ferritic stainless steel sheet in a coating amount
of about 0.5 g/m.sup.2 to 4.0 g/m.sup.2.
[0025] According to another aspect of the invention, a method for
making a ferritic stainless steel sheet for fuel tanks and fuel
pipes, comprises the steps of rough-rolling a slab comprising, by
mass percent, about 0.1% or less of C, about 1.0% or less of Si,
about 1.5% or less of Mn, about 0.06% or less of P, about 0.03% or
less of S, about 1.0% or less of Al, about 11% to about 20% Cr,
about 2.0% or less of Ni, about 0.5% to about 3.0% Mo, about 0.02%
to about 1.0% V, about 0.04% or less of N, at least one of about
0.01% to about 0.8% Nb and about 0.01% to about 1.0% Ti, and the
balance being Fe and incidental impurities; hot-rolling the
rough-rolled sheet under a linear pressure of at least about 3.5
MN/m at a final pass in the finish rolling; cold-rolling the
hot-rolled sheet at a gross reduction rate of at least about 75%,
the cold-rolling step including one rolling stage or at least two
rolling stages including intermediate annealing; and annealing the
cold-rolled sheet.
[0026] Preferably, the hot-rolled sheet is subjected to hot-rolled
sheet annealing according to the following equations:
900.ltoreq.T+20t.ltoreq.1,150 and t.ltoreq.10
[0027] wherein T is the annealing temperature (.degree. C) and t is
the holding time (minutes).
[0028] Preferably, a lubricant coat comprising an acrylic resin,
calcium stearate, and polyethylene wax is coated by baking on the
surfaces of the hot-rolled or annealed hot-rolled sheet in a
coating amount of about 0.5 g/m.sup.2 to about 4.0 g/m.sup.2.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a graph illustrating the effects of the Mo and V
contents in ferritic stainless steel sheets on the corrosion
resistance in the deteriorated gasoline;
[0030] FIG. 2 is a graph illustrating the effects of the linear
pressure at the final pass in the finish rolling and the gross
cold-rolling reduction rate on the r-value of the final product;
and
[0031] FIG. 3 is a graph illustrating the effects of the hot-rolled
sheet annealing condition on the ridging height.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0032] Reasons for limitation of the composition and process
conditions of the ferritic stainless steel sheet according to the
invention will now be described. The content of each component is
represented by mass percent (hereinafter merely referred to as
percent or %).
[0033] C: about 0.1% or less
[0034] Although a required amount of carbon (C) is added to
strengthen grain boundaries and to enhance brittle resistance to
secondary processing, excess carbon precipitates at grain
boundaries as carbides which adversely affect brittle resistance to
secondary processing and corrosion resistance at grain boundaries.
Since these adverse affects are noticeable at a C content exceeding
about 0.1%, the C content is limited to be about 0.1% or less. The
C content is preferably in the range of more than about 0.002% to
about 0.008% in view of an improvement in brittle resistance in
secondary processing.
[0035] Si: about 1.0% or less
[0036] Silicon (Si) contributes to improved oxidation and corrosion
resistance and, thus, improved corrosion resistance on the outer
and inner surfaces of a fuel tank. In order to achieve such
effects, the Si content is preferably about 0.2% or more. However,
a Si content exceeding about 1.0% causes the embrittlement of the
steel sheet and the deterioration of brittle resistance in
secondary processing at the weld. Thus, the Si content is about
1.0% or less and preferably about 0.75% or less.
[0037] Mn: about 1.5% or less
[0038] Manganese (Mn) improves oxidation resistance. Although about
0.5% or more of Mn is preferably used to achieve such an effect, an
excess amount of Mn causes the deterioration of toughness of the
steel sheet and the deterioration of brittle resistance in the
secondary processing at the weld. Thus, the Mn content is about
1.5% or less and preferably about 1.30% or less.
[0039] P: about 0.06% or less
[0040] Phosphorus (P) readily precipitating at grain boundaries
decreases the strength at the grain boundaries after severe
processing such as deep drawing for making fuel tanks. Thus, the P
content is preferably as low as possible to improve brittle
resistance in secondary processing (resistance to cracking by
slight impact after severe processing). Since a significantly low P
content results in an increase in production cost of steel-making
process, the P content is about 0.06% or less and more preferably
about 0.03% or less.
[0041] S: about 0.03% or less
[0042] Although sulfur (S) precludes corrosion resistance of the
stainless steel, about 0.03% is allowable as the upper limit in
view of desulflrization cost in of steel-making process.
Preferably, the S content is about 0.01% or less which can be fixed
by Mn and Ti:
[0043] Al: about 1.0% or less
[0044] Although aluminum (Al) is an essential element as a
deoxidizer in the steel-making process, an excess amount of
aluminum causes deterioration of surface appearance and corrosion
resistance due to inclusions. Thus, the Al content is limited to be
about 1.0% or less and preferably about 0.50% or less.
[0045] Cr: about 11% to 6.20%
[0046] At least about 11% chromium (Cr) must be contained in the
steel to achieve sufficient brittle and corrosion resistance. On
the other hand, a Cr content exceeding about 20% results in the
deterioration of processability due to increased strength and
decreased ductility even if the revalue is high. Thus, the Cr
content is in the range of about 11% to about 20%. Preferably, the
Cr content is about 14% or more and more preferably in the range of
about 14% to about about 18%, in view of corrosion resistance at
the weld.
[0047] Ni: about 2.0% or less
[0048] At least about 0.2% nickel (Ni) is preferably contained to
improve the corrosion resistance of the stainless steel. An amount
exceeding about 2.0% nickel causes hardening of the steel and
stress corrosion cracking due to the formation of an austenite
phase. Thus, the Ni content is about 2.0% or less and preferably in
the range of about 0.2% to about 0.8%.
[0049] Mo: about 0.5% to 3.0%
[0050] Molybdenum (Mo), as well as vanadium (V), is effective in an
improvement in corrosion resistance to deteriorated gasoline.
Atleast about 0.5% Mo is required to achieve superior corrosion
resistance to deteriorated gasoline. However, a Mo content
exceeding about 3.0% results in deterioration of processability due
to precipitation formed during annealing. Thus, the Mo content is
in the range of about 0.5% to about 3.0% and preferably about 0.7%
to about 1.6%.
[0051] V: about 0.02% to 1.0%
[0052] Vanadium (V) is effective in an improvement in corrosion
resistance to deteriorated gasoline by a combination with
molybdenum (Mo). Such an improvement is observed at a V content of
at least about 0.02%. However, a V content exceeding about 1.0%
results in the deterioration of processability due to precipitation
during annealing. Thus, the V content is in the range of about
0.02% to about 1.0% and preferably about 0.05% to about 0.3%.
[0053] The relationships between the Mo and V contents and the
corrosion resistance to deteriorated gasoline will now be
described. FIG. 1 is a graph illustrating the relationships between
the Mo and V contents in ferritic stainless steel sheets and the
corrosion resistance. The ferritic stainless steel sheets contains
about 0.003% to about 0.005% C, about 0.07% to about 0.13% Si,
about 0.15% to about 0.35% Mn, about 0.02% to about 0.06% P, about
0.01% to about 0.03% S, about 14.5% to about 18.2% Cr, about 0.2%
to about 1.0% Ni, about 0.02% to about 0.04% Al, about 0.001% to
about 0.45% Nb, about 0.3% to about 0.5% Ti, and about 0.004% to
about 0.011% N, and the corrosion resistance is measured in a
deteriorated gasoline containing 800 ppm. of formic acid for 120
hours. In the graph, the symbol .largecircle. represents that the
appearance after the corrosion resistance test in the deteriorated
gasoline does not change, and the symbol .circle-solid. represents
that the surface red rust is observed.
[0054] FIG. 1 shows that samples containing both Mo and V and
having a Mo content of about 0.5% or more and a V content of about
0.02% or more exhibit high corrosion resistance in the deteriorated
gasoline.
[0055] N: about 0.04% or less
[0056] Although nitrogen (N) strengthens grain boundaries which
improves brittle resistance in secondary processing for making
tanks and the like, an excess amount of nitrogen precipitates at
the grain boundaries as nitrides which adversely affects corrosion
resistance. Thus, the N content is about 0.04% or less and
preferably about 0.020% or less.
[0057] Nb: about 0.01% to about 0.8% and Ti: about 0.01% to about
1.0%
[0058] Niobium (Nb) and titanium (Ti) fix carbon and nitrogen in a
solid-solution state as compounds to increase the revalue. The
content of each element to fix carbon and nitrogen is about 0.01%
or more. These elements may be contained alone or in combination. A
Nb content exceeding about 0.8% causes remarkable deterioration of
toughness, and a Ti content exceeding about 1.0% causes
deterioration of the surface appearance and toughness. Preferably,
the Nb content is in the range of about 0.05% to about 0.4% and the
Ti content is in the range of about 0.05% to about 0.40%.
[0059] The ferritic stainless steel sheet of the invention may
further contain about 0.3% or less of cobalt (Co) and about 0.01%
or less of boron (B) to improve brittle resistance in secondary
processing. Moreover, the ferritic stainless steel sheet may
contain the following incidental impurities: about 0.5% or less of
zirconium (Zr), about 0.1% or less of calcium (Ca), about 0.3% or
less of tantalum (Ta), about 0.3% or less of tungsten (W), abouit
1% or less of copper (Cu), and about 0.3% or less of tin (Sn), as
long as the steel sheet exhibits the above-described
advantages.
[0060] The ferritic stainless steel sheet according to the
invention may be produced by a known method which is generally
employed in production of ferritic stainless steel sheets. However,
conditions for hot rolling and cold rolling are partly changed, as
described below. In steel malking, preferably, steel containing the
above essential components and auxiliary components added according
to demand is produced in a converter or electric furnace and the
steel is subjected to secondary refinement by vacuum oxygen
decarbonization (VOD). The molten steel may be subjected to any
known casting process and preferably a continuous casting process
in view of productivity and quality. The steel material obtained by
the continuous casting process is heated to a temperature between
about 1,000.degree. C. and about 1,250.degree. C. and hot-rolled to
form a hot-rolled steel sheet having a desired thickness.
[0061] The linear pressure at the final pass in the hot rolling is
at least about 3.5 MN/m to continuously produce a steel sheet
having a high revalue. The linear pressure represents a pressure
during rolling divided by the sheet width. A larger linear pressure
is considered to continuously obtain a high r-value because strain
is accumulated in the steel sheet. A large linear pressure is
achieved by any combination of a decrease in hot rolling
temperature, high-alloy formulation, an increase in hot rolling
speed, and an increase in roller diameter.
[0062] The resulting hot-rolled sheet is, if necessary and
preferably, subjected to continuous annealing (hot-rolled sheet
annealing) at a temperature in the range of about 900.degree. C. to
about 1,100.degree. C., pickling, and cold rolling to form a
cold-rolled sheet. The cold rolling step may include at least two
cold rolling stages including an intermediate annealing for
production procedure reasons, if necessary. In order to produce a
steel sheet having a high r-value, the above-described linear
pressure at the final pass in the hot rolling must be secured and
the gross reduction rate in the cold rolling step including one
cold rolling stage or two cold rolling stages must be at least
about 75% and more preferably at least about 82%.
[0063] The cold-rolled sheet is preferably subjected to continuous
annealing (cold-rolled sheet annealing) at a temperature in the
range of about 800.degree. C. to about 1,100.degree. C. and
pickling to form a cold-rolled annealed sheet as the final product.
The cold-rolled annealed sheet may be subjected to slight rolling
to adjust the shape and quality of the steel sheet according to the
usage.
[0064] FIG. 2 is a graph illustrating the effects of the linear
pressure at the final pass in the finish hot rolling of slabs and
the gross reduction rate of the subsequent cold rolling on the
r-value of the final product in which the slab contains about
0.003% to about 0.005% C, about 0.07% to about 0.13% Si, about
0.15% to about 0.35% Mn, about 0.02%. to about, 0.06% P, about
0.01% to about 0.03% S, about 14.5% to about 18.2% Cr, about 0.2%
to about 1.0% Ni, about 0.5% to about 1.6% Mo, about 0.02% to about
0.43% V, about 0.02% to about 0.04% Al, about 0.001% to about 0.45%
Nb, about 0.3% to about 0.5% Ti, about 0.004% to about 0.011% N,
and the balance substantially being Fe.
[0065] FIG. 2 shows that a high revalue is always achieved at a
linear pressure at the hot-rolling final pass of at least about 3.5
MN/m and a gross cold-roling reduction rate of at least about 75%
in high-alloy steels containing at least about 0.5% Mo.
[0066] The method for making the steel sheet according to the
invention will now be described. The steel sheet according to the
invention is produced by a known method employed in production of
ferritic stainless steel sheets, but the production conditions are
partly modified. That is, the cold-rolled annealed steel sheet is
produced through steel making, hot rolling, annealing, pickling,
cold rolling and finish annealing.
[0067] Steel having the above composition is produced in a
converter or electric furnace and the melt subjected to secondary
refinement by VOD. The molten steel may be subjected to any known
casting process and, preferably, a continuous casting process in
view of productivity and quality. The steel material obtained by
the continuous casting process is heated to a temperature between
about 1,000.degree. C. and about 1,250.degree. C. and hot-rolled to
form a hot-rolled steel sheet having a desired thickness.
[0068] The hot-rolled sheet is annealed. Annealing conditions are
essential for continuous production of steel sheets having low
ridging height and superior press formability. The annealing
temperature T (.degree. C) and the holding time t (minutes) are
determined so as to satisfy the relationship
900.ltoreq.T+20t.ltoreq.1,150. Continuous heating furnaces are
generally used in industrial facilities. The holding time t is
preferably about 10 minutes or less in view of productivity and
controllability.
[0069] FIG. 3 is a graph illustrating the effects of the hot-roled
sheet annealing condition on the ridging height of a ferritic
stainless steel sheet containing about 0.003% to about 0.005% C,
about 0.07% to about 0.13% Si, about 0.15% to about 0.35% Mn, about
0.02% to about 0.06% P, about 0.01% to about 0.03% S, about 14.5%
to about 18.2% Cr, about 0.2% to about 1.0% Ni, about 0.5% to about
1.6% Mo, about 0.04% to about 0.43% V, about 0.02% to about 0.04%
Al, about 0.001% to about 0.45% Nb, about 0.3% to about 0.5% Ti,
about 0.004% to about 0.011% N. and the balance being Fe. FIG. 3
suggests that a combination of an annealing temperature T and a
holding time t satisfying the relationship
900.ltoreq.T+20t.ltoreq.1,150 can achieve a ridging height of about
50 .mu.m or less.
[0070] Cold rolling is performed at a gross rolling reduction rate
of about 84%, a finish annealing temperature of about 900.degree.
C., and a holding time of about 60 seconds.
[0071] After annealing, the hot-rolled steel sheet is subjected to
pickling and cold rolling to produce a cold-rolled sheet. This cold
rolling step may include two or more cold rolling stages including
intermediate annealing for production procedure reasons, if
necessary. Preferably, the gross rolling reduction rate during the
cold rolling is at least about 75%. The cold-rolled sheet is
preferably subjected to (continuous) finish annealing at a
temperature between about 800.degree. C. and about 1,100.degree. C.
and pickling to produce a cold-rolled annealed sheet as a final
product. The cold-rolled annealed sheet may be subjected to slight
rolling to adjust the shape and quality of the steel sheet
according to usage.
[0072] In order to omit lubricant vinyl or oil in severe processing
for complicated shapes and press forming, a lubricant coat is
preferably applied to the surfaces of the steel sheet in a coating
amount of about 0.5 g/m.sup.2 to about 4.0 g/m.sup.2. The lubricant
coat in the invention contains about 3 to about 20 percent by
volume of calcium stearate and about 3 to about 20 percent by
volume of polyethylene wax.
[0073] The applied lubricant coat improves sliding performance of
the steel sheet and facilitates deep drawing into complicated
shapes. Preferably, the lubricant coat is a removable type which
can be readily removed with alkali. If the steel sheet containing
the remaining lubricant coat is subjected to spot welding or seam
welding, sensitive weld portions cause noticeable deterioration of
corrosion resistance.
[0074] According to press forming testing, at least about 0.5
g/m.sup.2 of lubricant coat must be applied to ensure the
improvement in sliding performance. At a coating amount exceeding
about 4.0 g/m.sup.2, the effect of the lubricant coat is no longer
enhanced. Furthermore, the steel sheet having such a high amount of
lubricant coat amount is not suitable for seam welding or spot
welding because the lubricant coat precludes electrical conduction
in the welding process and causes excessive sensitivity at the
welding portion. The coating amount of the lubricant coat on the
steel sheet is preferably about 1.0 to about 2.5 g/m.sup.2 in view
of compatibility between weldability and processability. The
lubricant coat may be applied to one side or preferably two sides
of the stainless steel.
[0075] The thickness of the steel sheet made by the above
production steps is preferably at least about 0.4 mm to ensure that
sufficient strength is imparted to a tank filled with fuel.
However, excess thickness results in a decrease in cold rolling
reduction rate and r-value, thereby precluding press formability
and pipe expansion. Hence, the maximum thickness is preferably
about 1.0 mm. The resulting steel sheet according to the invention
has an r-value of at least about 1.50 or at least about 1.90 under
optimized production conditions. Thus, the steel sheet according to
the invention exhibits high corrosion resistance and high toughness
after the steel sheet is shaped into a fuel tank or a pipe. Fuel
pipes made of the steel sheet according to the invention may be
welded by any known welding method such as arc welding including
tungsten inert gas (TIG) welding, metal inert gas (MIG) welding,
and ERW; electric resistance welding; and laser welding.
EXAMPLES
Example 1
[0076] Steel slabs having the compositions shown in Table 1 were
heated to 1,120.degree. C., and hot-rolled to form hot-rolled
sheets having a thickness in the range of 4.0 to 5.5 mm. Each
hot-rolled sheet was continuously annealed (hot-rolled annealing)
and then cold-rolled. The resulting cold-rolled sheet was
continuously annealed (cold-rolled annealing) and subjected to
pickling to remove scales. Test steel sheets were thereby
prepared.
[0077] Table 2 shows process conditions, such as linear pressure of
the final pass in the hot rolling, gross rolling reduction rate in
the cold rolling, and annealing temperature.
[0078] The r-value of each test steel sheet was measured according
to JIS-Z2254. The steel sheet was subjected to cylindrical deep
drawing at a punch diameter of 33 mm and a blank diameter of 70 mm
and cracking was visually observed. The deep drawn sample was
immersed in deteriorated gasoline containing 1,200 ppm of formic
acid and 400 ppm of acetic acid for 5 days for corrosion testing.
In "Corrosion resistance to deteriorated gasoline" in Table 2,
letter "A" represents a change in weight of 0.1 g/m.sup.2 or less
and no red rust in appearance observation, and letter "B"
represents cases other than "A".
[0079] Table 2 also includes the results of other tests. Table 2
shows that the steel sheets according to the invention exhibit
superior processability and high corrosion resistance to
deteriorated gasoline.
1TABLE 1 Steel Composition (mass %) No. C Si Mn P S Al Cr Ni V Mo
Nb Ti N Remarks 1 0.004 0.10 0.18 0.04 0.01 0.04 18.2 0.2 0.06 1.2
0.002 0.300 0.010 EX 2 0.004 0.10 0.18 0.04 0.01 0.04 18.2 0.2 0.01
1.2 0.002 0.300 0.010 CE 3 0.011 0.14 0.28 0.03 0.02 0.03 17.9 0.3
0.72 0.7 0.300 0.200 0.010 EX 4 0.006 0.26 0.22 0.02 0.01 0.02 14.8
0.7 0.18 1.6 0.045 0.010 0.007 EX 5 0.007 0.24 0.25 0.05 0.02 0.08
11.2 0.4 0.05 2.1 0.05 0.350 0.009 EX 6 0.004 0.35 0.10 0.03 0.01
0.15 15.5 0.8 0.08 0.4 0.04 0.01 0.006 CE 7 0.015 0.45 0.40 0.04
0.02 0.02 17.3 0.4 0.52 0.8 0.004 0.004 0.005 CE EX: Example
according to the invention CE: Comparative Example
[0080]
2 TABLE 2 Gross Inter- Cold- cold mediated rolled Hot rolling
Hot-rolled annealing rolling annealing annealing Sheet Steel Final
pass linear FDT Temp. reduction Temp. Time Temp. Time No. No.
pressure (MN/m) (.degree. C.) (.degree. C.) Time (s) rate (%)
(.degree. C.) (s) (.degree. C.) (s) A 1 5.8 780 980 60 84 -- -- 920
60 B 1 3.6 800 930 150 85 900 60 960 40 C 1 4.2 820 910 100 76 900
60 890 75 D 1 4.8 750 870 300 94 810 120 1000 94 E 1 4.4 810 930
200 80 850 150 930 120 F 1 4.4 810 930 200 80 850 150 930 120 G 1
3.9 790 910 150 82 -- -- 960 200 H 1 3.4 790 960 80 84 880 250 960
150 I 1 3.8 820 930 80 74 880 150 980 80 J 1 3.8 830 940 120 77 920
100 950 120 K 2 3.2 870 980 60 84 -- -- 920 60 L 2 5.8 820 930 100
84 900 120 940 160 M 3 5.4 780 980 60 84 -- -- 920 60 N 4 7.1 760
1020 60 87 990 60 970 60 O 5 3.8 740 880 90 84 800 150 940 120 P 6
4.2 810 950 60 83 850 120 950 80 Q 7 4.4 820 930 120 80 900 120 940
80 Corrosion Low resis- pressure Final Cracking tance rolling sheet
during to dete- Sheet Steel reduction thick- r- deep riorated Re-
No. No. rate (%) ness (mm) value drawing gasoline marks A 1 -- 0.8
2.01 Not observed A EX B 1 -- 0.8 1.90 Not observed A EX C 1 -- 1.0
1.61 Not observed A EX D 1 -- 0.4 2.80 Not observed A EX E 1 -- 1.0
1.80 Not observed A EX F 1 3 1.0 1.81 Not observed A EX G 1 -- 1.0
1.90 Not observed A EX H 1 -- 0.7 1.40 Observed A CE I 1 -- 1.0
1.20 Observed A CE J 1 -- 1.1 1.33 Observed A EX K 2 -- 0.8 1.41
Observed B CE L 2 -- 0.8 1.60 Not observed B CE M 3 -- 0.8 1.60 Not
observed A EX N 4 -- 0.6 1.91 Not observed A EX O 5 -- 0.7 2.00 Not
observed A EX P 6 -- 0.8 1.93 Not observed B CE Q 7 -- 1.0 1.20
Observed A CE EX: Example according to the invention CE:
Comparative Example
Example 2
[0081] Steel slabs having the compositions shown in Table 3 were
heated to 1,120.degree. C., and hot-rolled at a final hot-rolling
temperature of 780.degree. C. to form hot-rolled sheets having a
thickness of 5.0 mm. Each hot-rolled sheet was annealed under the
conditions shown in Table 4, subjected to pickling for descaling,
and then cold-rolled into a thickness of 0.8 mm. The gross
reduction rate in the, cold rolling step was 84%. The resulting
cold-rolled sheet was finish-annealed at 900.degree. C. or more and
subjected to pickling to remove scales. Test steel sheets were
thereby prepared.
[0082] Tensile test pieces were prepared from each steel sheet such
that the stretching direction corresponded to the rolling
direction. One of the test pieces was deformed by 25% by uniaxial
stretching. The height of ridges generated on the surface of the
deformed steel sheet was measured in the direction perpendicular to
the stretching direction. Another test piece was subjected to a
bulging test with a 100-mm diameter spherical punch and a
commercially available lubricant oil in which the bulged height
when a crack was formed was measured, as press formability. Another
test piece was prepared from each steel sheet and immersed in a
deteriorated gasoline containing 1,200 ppm of formic acid and 400
ppm of acetic acid for 5 days for corrosion testing. In "Corrosion
resistance to deteriorated gasoline" in Table 2, letter "A"
represents a change in weight of 0.1 g/m.sup.2 or less and no red
rust in appearance observation, and "B" represents cases other than
"A".
[0083] Table 4 also includes the results of these tests.
[0084] Table 4 shows that each sheet according to the invention has
a small ridging height and thus exhibits superior
processability.
3TABLE 3 Steel Composition (mass %) No. C Si Mn P S Al Cr Ni Mo V
Nb Ti N A 0.004 0.70 0.18 0.04 0.01 0.04 18.2 0.2 2.1 0.06 0.60
0.30 0.010 B 0.011 0.14 1.20 0.03 0.02 0.03 17.9 0.3 0.7 0.72 0.30
0.20 0.010 C 0.006 0.26 0.22 0.02 0.007 0.02 14.8 1.4 1.6 0.80
0.045 0.003 0.007 D 0.080 0.10 0.28 0.04 0.01 0.40 11.8 0.7 1.2
0.18 0.002 0.05 0.020 E 0.004 0.70 0.19 0.03 0.01 0.03 18.3 0.2 0.4
0.07 0.60 0.31 0.010 F 0.005 0.69 0.18 0.04 0.01 0.03 18.2 0.2 1.3
0.003 0.50 0.30 0.010
[0085]
4 TABLE 4 Hot-rolled Corrosion annealing Ridging Bulged resistance
to Sheet Steel Temp. Time height height *) deteriorated No. No.
(.degree. C.) (m) (.mu.m) (mm) Evaluation gasoline Remarks 1 A 1100
0.5 40 38 H A EX 2 A 1000 0.5 38 43 H A EX 3 A 900 0.5 39 37 H A EX
4 A 750 0.5 48 33 M A EX 5 B 1000 2.0 33 46 H A EX 6 B 900 2.0 32
49 H A EX 7 B 1000 3.0 29 49 H A EX 8 B 750 2.0 47 34 M A EX 9 C
850 4.0 24 51 H A EX 10 C 800 6.0 35 43 H A EX 11 C 950 6.5 36 49 H
A EX 12 C 1100 5.0 59 26 L A CE 13 D 850 7.0 45 44 H A EX 14 D 800
8.0 42 46 H A EX 15 D 850 9.5 46 39 H A EX 16 D 800 9.5 45 38 H A
EX 17 D 1150 8.5 54 24 L A CE 18 D 1000 8.5 61 22 L A CE 19 E 1000
0.5 40 42 H B CE 20 F 900 0.5 38 36 H B CE *) Evaluation standard
H: bulged height .gtoreq. 35 .mu.m M: 35 .mu.m > bulged height
.gtoreq. 30 .mu.m L: 30 .mu.m > bulged height
Example 3
[0086] Cold-rolled steel sheets A (thickness: 0.8 mm) shown in
Table 2 in EXAMPLE 1 were washed with an alkaline solution, and
various amounts of lubricant coat containing an acrylic resin as a
main component, 5 percent by volume of calcium stearate, and 5
percent by volume of polyethylene wax were applied to these steel
sheets. Each sheet, was baked at 80.+-.5.degree. C. for 15 seconds.
The spot weldability and sliding performance of test pieces
prepared from each sheet were examined. The results are shown in
Table 5.
[0087] In the sliding performance testing, a test piece with a
length of 300 mm and a width of 10 mm was disposed between flat
dies with a contact area with the test piece of 200 mm.sup.2 under
an area pressure of 8 kgf/mm.sup.2 and a dynamic friction
coefficient (.mu.) was determined by a pulling-out force (F). The
spot weldability was evaluated by a nugget diameter at the welded
portion of two test pieces with a thickness of 0.8 mm which were
welded using a chromium-copper alloy (diameter 16 mm) and a R type
electrode (radius=40 mm) at a current of 5 kA under a pressure of 2
KN. A nugget diameter of 3{square root}t or less was evaluated as
unsatisfactory welding performance (B) and a nugget diameter
exceeding 3{square root}t was evaluated as satisfactory welding
performance (A) wherein t means the sheet thickness.
[0088] According to the results, at least about 0.5 g/m.sup.2 of
lubricant coat must be applied to improve the sliding performance.
However, at a coating amount exceeding about 4.0 g/m.sup.2, the
improvement in sliding performance is saturated and weldability
precluded due to poor electrical conductivity during the spot
welding.
5TABLE 5 Sliding test Coating amount (Dynamic friction Weldabiity
(g/m.sup.2) coefficient: .mu.) (Nugget diameter) 0.2 0.265 A 0.4
0.166 A 0.5 0.102 A 0.8 0.101 A 1.5 0.099 A 2.2 0.097 A 2.8 0.097 A
3.8 0.098 A 4.2 0.097 B 5.0 0.097 B B: .ltoreq.3{square root}t, A
> 3{square root}t t: thickness
[0089] As described above, the ferritic stainless steel sheet
according to the invention exhibits superior processability and
high corrosion resistance to deteriorated gasoline. Thus,
containers and piping elements produced using this steel sheet can
be safely used in severe environments, for example, in the presence
of deteriorated gasoline or methanol.
* * * * *