U.S. patent application number 17/312693 was filed with the patent office on 2022-02-03 for cr-based stainless steel having excellent hydrogen embrittlement resistance.
This patent application is currently assigned to NIPPON STEEL Stainless Steel Corporation. The applicant listed for this patent is NIPPON STEEL Stainless Steel Corporation. Invention is credited to Masaharu HATANO, Yuuichi TAMURA.
Application Number | 20220033944 17/312693 |
Document ID | / |
Family ID | |
Filed Date | 2022-02-03 |
United States Patent
Application |
20220033944 |
Kind Code |
A1 |
HATANO; Masaharu ; et
al. |
February 3, 2022 |
CR-BASED STAINLESS STEEL HAVING EXCELLENT HYDROGEN EMBRITTLEMENT
RESISTANCE
Abstract
A Cr-based stainless steel sheet includes: 0.020 mass % or less
of C; 1.00 mass % or less of Si; 1.00 mass % or less of Mn; 0.040
mass % or less of P; 0.0030 mass % or less of S; 10.0 to 18.0 mass
% of Cr; 0.020 mass % or less of N; 0.10 mass % or less of Al; and
one or both of 0.5 mass % or less of Nb and 0.5 mass % or less of
Ti; in which a texture in a sheet surface satisfies (i) and (ii)
below. (i) In the sheet surface, an area ratio of crystal grains
({211}.+-.10-degree-oriented grains) whose orientation difference
between a normal direction of the surface and a {211}-plane
orientation is 10 degrees or less is less than 30%. (ii) For the
{211}.+-.10-degree-oriented grains, a length in a rolling direction
and a length in a sheet width direction are each less than 0.15 mm
on average.
Inventors: |
HATANO; Masaharu; (Tokyo,
JP) ; TAMURA; Yuuichi; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL Stainless Steel Corporation |
Tokyo |
|
JP |
|
|
Assignee: |
NIPPON STEEL Stainless Steel
Corporation
Tokyo
JP
|
Appl. No.: |
17/312693 |
Filed: |
December 18, 2019 |
PCT Filed: |
December 18, 2019 |
PCT NO: |
PCT/JP2019/049717 |
371 Date: |
June 10, 2021 |
International
Class: |
C22C 38/54 20060101
C22C038/54; C22C 38/50 20060101 C22C038/50; C22C 38/48 20060101
C22C038/48; C22C 38/46 20060101 C22C038/46; C22C 38/44 20060101
C22C038/44; C22C 38/42 20060101 C22C038/42; C22C 38/30 20060101
C22C038/30; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; C21D 9/46 20060101 C21D009/46; C21D 8/02 20060101
C21D008/02; C21D 6/00 20060101 C21D006/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 21, 2018 |
JP |
2018-239243 |
Claims
1. A Cr-based stainless steel sheet comprising: 0.020 mass % or
less of C; 1.00 mass % or less of Si; 1.00 mass % or less of Mn;
0.040 mass % or less of P; 0.0030 mass % or less of S; 10.0 to 18.0
mass % of Cr; 0.020 mass % or less of N; 0.10 mass % or less of Al;
one or both of 0.5 mass % or less of Nb and 0.5 mass % or less of
Ti; 0 to 0.3 mass % of Sn; 0 to 0.005 mass % of B; 0 to 1 mass % of
Ni; 0 to 1 mass % of Cu; 0 to 1 mass % of Mo; 0.2 mass % or less of
Sb; 0 to 0.5 mass % of V; 0 to 0.5 mass % of W; 0 to 0.5 mass % of
Zr; 0 to 0.5 mass % of Co; 0 to 0.005 mass % of Mg; 0 to 0.005 mass
% of Ca; 0 to 0.020 mass % of Ga; 0 to 0.1 mass % of La; 0 to 0.1
mass % of Y; 0 to 0.1 mass % of Hf; 0 to 0.1 mass % of REM; and a
balance consisting of Fe and impurities, wherein a texture in a
sheet surface of the Cr-based stainless steel sheet satisfies (i)
and (ii) below, (i) in the sheet surface, an area ratio of crystal
grains (hereinafter referred to as "{211}.+-.10-degree-oriented
grains") whose orientation difference between a normal direction of
a surface of the steel sheet and a {211}-plane orientation is 10
degrees or less is less than 30%, and (ii) for the
{211}.+-.10-degree-oriented grains defined in (i), a length in a
rolling direction and a length in a sheet width direction are each
less than 0.15 mm on average.
2. The Cr-based stainless steel sheet according to claim 1, further
comprising 0.001 to 0.3 mass % of Sn and 0.005 mass % or less of B,
wherein the Cr-based stainless steel sheet satisfies Formula (1)
below, Si+0.5 Mn+10 P+5 Nb+2 Ti<2.00 Formula (1), where in the
formula, symbols of elements indicate contents (mass %) of the
respective elements.
3. The Cr-based stainless steel sheet according to claim 1, further
comprising one or more selected from 1 mass % or less of Ni, 1 mass
% or less of Cu, 1 mass % or less of Mo, 0.2 mass % or less of Sb,
0.5 mass % or less of V, 0.5 mass % or less of W, 0.5 mass % or
less of Zr, 0.5 mass % or less of Co, 0.005 mass % or less of Mg,
0.005 mass % or less of Ca, 0.020 mass % or less of Ga, 0.1 mass %
or less of La, 0.1 mass % or less of Y, 0.1 mass % or less of Hf,
and 0.1 mass % or less of REM.
4. The Cr-based stainless steel sheet according to claim 1, wherein
the Cr-based stainless steel sheet is used for metals for high
pressure hydrogen gaseous equipment.
Description
TECHNICAL FIELD
[0001] The present invention relates to a Cr-based stainless steel
sheet with excellent hydrogen embrittlement resistance,
specifically to a Cr-based stainless steel sheet suitable for
metals for high pressure hydrogen gaseous equipment.
BACKGROUND ART
[0002] In recent years, it has been strongly desired to reduce
generation of greenhouse gases mainly including carbon dioxide in
order to cope with extreme weather that is partly due to global
warming. As part of this move, development of automobiles and
transport equipment that use a fuel cell as a power source has been
in progress. The fuel cell, which generates electric power by using
hydrogen as fuel, does not generate carbon dioxide and achieves
high energy conversion efficiency, thereby being regarded as a
promising power source.
[0003] In the fuel cell using hydrogen as fuel and equipment such
as a hydrogen station supplying hydrogen thereto, components are
exposed to a hydrogen gaseous environment. In metals exposed to a
hydrogen gaseous environment, a phenomenon that mechanical
properties such as tensile strength, elongation and drawability
decrease due to hydrogen penetrating into the metals is known. This
phenomenon is referred to as hydrogen embrittlement. In
consideration of the issue of hydrogen embrittlement, Japan
Automobile Research Institute, Technical Standard JARIS001
stipulates that austenitic stainless steel SUS316L and an aluminum
alloy 6061-T6 should be used for a 35-MPa high-pressure hydrogen
tank for automobiles and KHKS0128 stipulates that the austenitic
stainless steel SUS316L and the aluminum alloy 6061-T6 should be
used for a 70-MPa high-pressure hydrogen tank for automobiles.
[0004] Exemplified Standards for High Pressure Gas Safety Act
stipulate that materials of austenitic stainless steel sheets
(SUS316 and SUS316L) stipulated in JIS G 4304 and JIS G 4305 in
which an Ni equivalent (Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si +12.6C) is
increased (for instance, the Ni equivalent 28.5) should be used for
hydrogen infrastructure equipment with a pressure in a range from
20 MPa to 82 MPa. An operating temperature is in a range from -45
degrees C. to 250 degrees C. For the austenitic stainless steel,
for instance, Patent Literature 1 and Patent Literature 2 disclose
stainless steel exemplified by SUS316L whose strength is enhanced
and whose economic efficiency is improved through a reduction in
Mo, which is expensive.
[0005] The Safety Regulations for General High-Pressure Gas was
amended in 2016 to abolish material restrictions for hydrogen
equipment with a pressure of 20 MPa or less. In the wake of easing
of the restrictions, there has been an increasingly growing need
for using a stainless steel sheet having high economic efficiency
even in a high pressure hydrogen gas and also a demand for
evaluating various steel materials for hydrogen embrittlement
resistance in the high pressure hydrogen gas. Ferritic and
martensitic stainless steel sheets (hereinafter, collectively
referred to as a "Cr-based stainless steel sheet") contain almost
no Ni (a rare metal) and thus have more excellent economic
efficiency than that of the austenitic stainless steel sheets.
[0006] Conventionally, for instance, Non-Patent Literature 1
discloses hydrogen embrittlement properties of all steel materials
including stainless steel that are evaluated in a high pressure
hydrogen gas at room temperature. Typical austenitic stainless
steel SUS304 and the Cr-based stainless steel are reported to be
prone to hydrogen embrittlement. Accordingly, use of SUS316L and
SUS316 is also typically recommended in the high pressure hydrogen
gas at a pressure of about 20 MPa. Further, the Cr-based stainless
steel having a body-centered cubic structure decreases in toughness
at a low temperature (lower than or equal to a room temperature)
(i.e., low-temperature embrittlement) as compared with the
austenitic stainless steel having a face-centered cubic
structure.
[0007] A material coated with Al or an Al alloy having excellent
hydrogen embrittlement resistance also has been devised in order to
obtain more materials usable in the high pressure hydrogen gaseous
environment. Patent Literature 3 discloses a pressure tank for high
pressure hydrogen gas and high pressure hydrogen gaseous piping,
which are coated with Al or an Al alloy. Examples thereof concern
formation of a film on the austenitic stainless steel and duplex
stainless steel including an austenitic phase but do not disclose
formation of a film on a steel material that is prone to hydrogen
embrittlement (e.g., the Cr-based stainless steel) and hydrogen
penetration properties thereof.
[0008] Patent Literature 4 discloses a substrate for hydrogen
equipment obtained by: hot-dipping a steel material that is by
itself prone to hydrogen embrittlement with the use of an Al-Si
alloy added with Si in an amount in a range from 1 to 5%; and
forming a hydrogen-impermeable film. The steel material for the
substrate is set as carbon steel, low alloy steel or the Cr-based
stainless steel, which prevents hydrogen embrittlement and keeps a
manufacturing cost low. However, Examples thereof are limited to
SUS304, SUS630 (15Cr-4Ni-3Cu) and SCM435 (low alloy steel).
Hydrogen embrittlement properties and use of the Cr-based stainless
steel sheet having high economic efficiency are also not
disclosed.
CITATION LIST
Patent Literature(s)
[0009] Patent Literature 1: JP 2014-114471 A
[0010] Patent Literature 2: JP 2016-183412 A
[0011] Patent Literature 3: JP 2004-324800 A
[0012] Patent Literature 4: WO 2015-098981
Non-Patent Literature(s)
[0013] Non-Patent Literature 1: PVP2007-26820
[0014] Non-Patent Literature 2: Michihiko Nagumo, "Fundamentals of
Hydrogen Embrittlement", Uchida Rokakuho (December 2008)
SUMMARY OF THE INVENTION
Problem(s) to be Solved by the Invention
[0015] The stainless steel described in Patent Literatures 1 to 4
is only austenitic stainless steel, duplex stainless steel and
SUS630 (precipitation hardening type). In addition, the Cr-based
stainless steel disclosed in Non-Patent Literature 1 is prone to
hydrogen embrittlement and thus does not have hydrogen
embrittlement resistance for use in the high pressure hydrogen gas.
The Cr-based stainless steel also has a problem of the
low-temperature embrittlement.
[0016] In light of the above circumstances, an object of the
invention is to provide a Cr-based stainless steel sheet with
excellent hydrogen embrittlement resistance, the Cr-based stainless
steel sheet having hydrogen embrittlement resistance for use in a
high pressure hydrogen gas and thus being suitable for metals for
high pressure hydrogen gaseous equipment. In addition thereto, an
object thereof is to achieve low-temperature embrittlement
resistance together with hydrogen embrittlement resistance.
Means for Solving the Problem(s)
[0017] In order to achieve the above objects, the invention adopts
a configuration below. [0018] [1] A Cr-based stainless steel sheet
including: 0.020 mass % or less of C; 1.00 mass % or less of Si;
1.00 mass % or less of Mn, 0.040 mass % or less of P; 0.0030 mass %
or less of S, 10.0 to 18.0 mass % of Cr; 0.020 mass % or less of N;
0.10 mass % or less of Al; one or both of 0.5 mass % or less of Nb
and 0.5 mass % or less of Ti, 0 to 0.3 mass % of Sn; 0 to 0.005
mass % of B; 0 to 1 mass % of Ni, 0 to 1 mass % of Cu; 0 to 1 mass
% of Mo; 0.2 mass % or less of Sb; 0 to 0.5 mass % of V; 0 to 0.5
mass % of W; 0 to 0.5 mass % of Zr, 0 to 0.5 mass % of Co; 0 to
0.005 mass % of Mg; 0 to 0.005 mass % of Ca; 0 to 0.020 mass % of
Ga; 0 to 0.1 mass % of La; 0 to 0.1 mass % of Y; 0 to 0.1 mass % of
Hf; 0 to 0.1 mass % of REM; and a balance consisting of Fe and
impurities, in which a texture in a sheet surface of the Cr-based
stainless steel sheet satisfies (i) and (ii) below, [0019] (i) in
the sheet surface, an area ratio of crystal grains (hereinafter
referred to as "{211}.+-.10-degree-oriented grains") whose
orientation difference between a normal direction of a surface of
the steel sheet and a {211}-plane orientation is 10 degrees or less
is less than 30%, and [0020] (ii) for the
{211}.+-.10-degree-oriented grains defined in (i), a length in a
rolling direction and a length in a sheet width direction are each
less than 0.15 mm on average. [0021] [2] The Cr-based stainless
steel sheet according to the invention, further including 0.001 to
0.3 mass % of Sn and 0.005 mass % or less of B, in which the
Cr-based stainless steel sheet satisfies Formula (1) below,
[0021] Si+0.5 Mn+10 P+5 Nb+2 Ti<2.00 Formula (1),
[0022] where in the formula, symbols of elements indicate contents
(mass %) of the respective elements. [0023] [3] The Cr-based
stainless steel sheet according to the invention, further including
one or more selected from 1 mass % or less of Ni, 1 mass % or less
of Cu, 1 mass % or less of Mo, 0.2 mass % or less of Sb, 0.5 mass %
or less of V, 0.5 mass % or less of W, 0.5 mass % or less of Zr,
0.5 mass % or less of Co, 0.005 mass % or less of Mg, 0.005 mass %
or less of Ca, 0.020 mass % or less of Ga, 0.1 mass % or less of
La, 0.1 mass % or less of Y, 0.1 mass % or less of Hf, and 0.1 mass
% or less of REM. [0024] [4] The Cr-based stainless steel sheet
according to the invention, in which the Cr-based stainless steel
sheet is used for metals for high pressure hydrogen gaseous
equipment.
[0025] According to the invention, a Cr-based stainless steel sheet
having excellent low-temperature toughness together with excellent
hydrogen embrittlement resistance can be provided. Further, the
Cr-based stainless steel sheet according to the invention can be
suitably used for metals for the high pressure hydrogen gaseous
equipment.
DESCRIPTION OF EMBODIMENT(S)
[0026] In order to achieve the above objects, the inventors have
intensely studied an influence of alloy elements and a texture on
hydrogen embrittlement resistance and low-temperature embrittlement
resistance in a Cr-based stainless steel sheet, thereby obtaining
the following novel findings to arrive at the invention.
[0027] (a) As described above, properties required for metals for
high pressure hydrogen gaseous equipment are hydrogen embrittlement
resistance and low-temperature embrittlement resistance. For a
Cr-based stainless steel sheet, a hydrogen amount to penetrate into
a steel material from a high pressure hydrogen gas reduces because
of a crystal structure thereof as compared with that of an
austenitic stainless steel sheet. However, a Cr-based stainless
steel sheet having hydrogen embrittlement resistance suitable for
use in a high pressure hydrogen gas has not been obtained.
According to Non-Patent Literature 2, hydrogen embrittlement is
characterized as a decrease in mechanical properties (strength,
elongation and drawability) which plastic deformation involves.
Accordingly, hydrogen embrittlement is a phenomenon that a fracture
of the metals progresses due to an interaction between hydrogen
penetrating into the steel material from the high pressure hydrogen
gas and plastic deformation. From recent research results,
Hydrogen-Enhanced Strain-Induced Vacancy Theory, which states that
the interaction between hydrogen and plastic deformation promotes
generation of vacancy-type lattice defects in the steel to progress
the fracture, is considered as reliable in explaining a mechanism
of hydrogen embrittlement (Non-Patent Literature 2). Accordingly,
in order to achieve the Cr-based stainless steel sheet suitable for
use in the high pressure hydrogen gas, there is a need for reducing
the interaction between hydrogen and plastic deformation to the
extent possible. In particular, since Cr has high hydrogen trapping
capability, the Cr amount is reduced to 18% or less in the
invention. Further, the inventors have found that added amounts of
Si, Mn, P, Ti and Nb are preferably controlled to be in respective
predetermined ranges.
[0028] (b) The inventors further have found that when a slow strain
rate tensile test is performed in the high pressure hydrogen gas, a
crystal orientation influences generation of cracks caused by the
interaction between hydrogen and plastic deformation. When hydrogen
embrittlement becomes apparent, the cracks frequently are generated
or develop from an inside of crystal grains. It has been found that
the cracks in the crystal grains are often generated in a rolling
texture in a form of {211}-oriented grains (crystal grains where a
{211}-plane orientation is in a normal direction of a surface of
the steel sheet), not in a recrystallization texture in a form of
{111}-oriented grains (crystal grains where a {111}-plane
orientation is in the normal direction of the surface of the steel
sheet). From these facts, it is presumed that strains are likely to
be introduced to and accumulated in the {211}-oriented grains due
to the interaction between hydrogen and plastic deformation. It is
speculated that the generation of the vacancy-type lattice defects
is activated, so that the {211}-oriented grains act as sites of
crack generation. In order to suppress hydrogen embrittlement that
progresses in such a mechanism, it is effective not only to adjust
respective ranges of the above alloy elements but also to reduce an
area ratio and a size of the {211}-oriented grains, and thresholds
of the area ratio and the size have been found.
[0029] (c) Hydrogen penetrating into the steel material from the
high pressure hydrogen gas transfers along a grain boundary acting
as a main diffusion path. Addition of a minute amount of Sn and B,
which are grain boundary segregation elements, forms a diffusion
barrier at the grain boundary with hydrogen, thereby reducing the
interaction between hydrogen and plastic deformation. In a
conventional Cr-based stainless steel, impurity elements such as P
and S segregate at the grain boundary, thereby tending to promote
low-temperature embrittlement. Accordingly, the inventors have
focused on the addition of a minute amount of Sn and B and have
found that these elements are contained in respective predetermined
ranges to suppress an adverse effect of P, S and the like, thereby
likely achieving both hydrogen embrittlement resistance and
low-temperature embrittlement resistance.
[0030] A summary of the invention achieved on the basis of the
above findings (a) to (c) is as follows.
[0031] A Cr-based stainless steel sheet according to an exemplary
embodiment is a Cr-based stainless steel sheet with excellent
hydrogen embrittlement resistance and low-temperature embrittlement
resistance, the Cr-based stainless steel sheet including: 0.020
mass % or less of C; 1.00 mass % or less of Si; 1.00 mass % or less
of Mn; 0.040 mass % or less of P; 0.0030 mass % or less of S, 10.0
to 18.0 mass % of Cr; 0.020 mass % or less of N; 0.10 mass % or
less of Al; one or both of 0.5 mass % or less of Nb and 0.5 mass %
or less of Ti; and a balance consisting of Fe and impurities, in
which a texture in a sheet surface of the chromium stainless sheet
satisfies (i) and (ii) below. [0032] (i) In the sheet surface, an
area ratio of crystal grains ({211}.+-.10-degree-oriented grains)
whose orientation difference between a normal direction of a
surface of the steel sheet and a {211}-plane orientation is 10
degrees or less is less than 30%. [0033] (ii) For the
{211}.+-.10-degree-oriented grains defined in (i), a length in a
rolling direction and a length in a sheet width direction are each
less than 0.15 mm on average.
[0034] It is preferable that the Cr-based stainless steel sheet
according to the exemplary embodiment further contains 0.001 to 0.3
mass % of Sn and 0.005 mass % or less of B, and satisfies Formula
(1) below.
Si+0.5 Mn+10 P+5 Nb+2 Ti<2.00 Formula (1)
[0035] In the above formula, symbols of elements indicate contents
(mass %) of the respective elements.
[0036] The Cr-based stainless steel sheet according to the
exemplary embodiment may further contain one or more selected from
1 mass % or less of Ni, 1 mass % or less of Cu, 1 mass % or less of
Mo, 0.2 mass % or less of Sb, 0.5 mass % or less of V, 0.5 mass %
or less of W, 0.5 mass % or less of Zr, 0.5 mass % or less of Co,
0.005 mass % or less of Mg, 0.005 mass % or less of Ca, 0.020 mass
% or less of Ga, 0.1 mass % or less of La, 0.1 mass % or less of Y,
0.1 mass % or less of Hf, and 0.1 mass % or less of REM.
[0037] It is preferable that the Cr-based stainless steel sheet
according to the exemplary embodiment is used for metals for high
pressure hydrogen gaseous equipment.
[0038] Elements of the invention will be described in detail below.
It should be noted that an indication "%" for contents of elements
means "mass %".
[0039] C is 0.020% or less.
[0040] C increases work-hardening of steel through
solid-dissolution thereof and precipitation of carbides to
deteriorate hydrogen embrittlement resistance. C further decreases
toughness to deteriorate low-temperature embrittlement
resistance.
[0041] Accordingly, the C content is preferably as small as
possible and thus has an upper limit of 0.020% or less. However, a
reduction in the C content requires a complex refining process,
thereby resulting in an increase in cost. Accordingly, the C
content is preferably 0.001% or more. In view of the refining cost
in addition thereto, the C content is preferably in a range from
0.003 to 0.015%, more preferably in a range from 0.003 to
0.010%.
[0042] Si is 1.00% or less.
[0043] Si is an effective deoxidizing element. However, excessive
addition of Si increases solid solution strengthening and
work-hardening, thereby resulting in a reduction in hydrogen
embrittlement resistance and low-temperature embrittlement
resistance. Accordingly, an upper limit of the Si content is set at
1.00% or less. In order to ensure deoxidation capability, a lower
limit of the Si content is preferably 0.01% or more. In view of
productivity and properties, the Si content is preferably in a
range from 0.05 to 0.50% and may be in a range from 0.05 to
0.30%.
[0044] Mn is 1.00% or less.
[0045] Mn is an effective deoxidizing element and also an element
effective in achieving low-temperature embrittlement resistance
through improvement in toughness by fixing S. However, excessive
addition of Mn increases work-hardening, thereby resulting in a
reduction in hydrogen embrittlement resistance and low-temperature
toughness. Accordingly, an upper limit of the Mn content is set at
1.00% or less. In order to ensure a deoxidation effect and a fixing
effect for S, a lower limit of the Mn content is preferably 0.01%
or more. In view of effects and productivity, the Mn content is
preferably in a range from 0.05 to 0.50% and may be in a range from
0.05 to 0.30%.
[0046] P is 0.040% or less.
[0047] P is an element reducing low-temperature embrittlement
resistance through grain boundary segregation. Accordingly, the P
content is preferably as small as possible, and thus has an upper
limit of 0.040% or less. However, an excessive reduction in the P
content results in an increase in a refining cost. Accordingly, a
lower limit of the P content is preferably 0.005% or more. In view
of production cost and properties, the P content is more preferably
in a range from 0.010 to 0.030% and may be in a range from 0.010 to
0.020%.
[0048] S is 0.0030% or less.
[0049] S deteriorates low-temperature embrittlement resistance
through grain boundary segregation and formation of sulfides in
steel. Accordingly, the S content is preferably as small as
possible, and thus has an upper limit of 0.0030% or less. However,
an excessive reduction in the S content results in an increase in
material and refining costs. Accordingly, a lower limit of the S
content is preferably 0.0001% or more. In view of production cost
and properties, the S content is more preferably in a range from
0.0002 to 0.0015% and may be in a range from 0.0002 to 0.0008%.
[0050] Cr is 10.0 to 18.0%.
[0051] Cr is a basic element in the Cr-based stainless steel
according to the exemplary embodiment and is also an essential
element for maintaining hydrogen embrittlement resistance and
low-temperature embrittlement resistance in addition to corrosion
resistance of the steel. In order to obtain the above properties on
the assumption that the steel according to the exemplary embodiment
is used in a high pressure hydrogen gas, a lower limit of the Cr
content is set at 10.0% or more. In order to achieve both hydrogen
embrittlement resistance and low-temperature embrittlement
resistance, an upper limit of the Cr content is set at 18.0% or
less.
[0052] More than 18.0% of Cr, which has high hydrogen trapping
capability, increases a hydrogen amount to penetrate into the steel
from a high pressure hydrogen gaseous environment to deteriorate
hydrogen embrittlement resistance and sometimes causes the texture
to fall outside the preferable ranges of the invention. The Cr
content may be more preferably 11.0 or more and less than 17.0% or
in a range from 12.0 to 15.0%.
[0053] N is 0.020% or less.
[0054] Similarly to C, N increases work-hardening of steel through
solid dissolution thereof and precipitation of carbides to
deteriorate hydrogen embrittlement resistance.
[0055] Further, N decreases toughness to deteriorate
low-temperature embrittlement resistance. Accordingly, the N
content is preferably as small as possible and thus has an upper
limit of 0.020% or less. However, a reduction in the N content
requires a complex refining process, thereby resulting in an
increase in cost. Accordingly, the N content is preferably 0.001%
or more. In view of properties and production cost, the N content
is preferably in a range from 0.005 to 0.015%.
[0056] Al is 0.10% or less.
[0057] Al is a highly effective deoxidizing element. However, Al
decreases toughness of steel to deteriorate low-temperature
embrittlement resistance and sometimes causes the texture to fall
outside the preferable ranges of the invention. Accordingly, an
upper limit of the Al content is set at 0.10% or less. In view of a
deoxidation effect, a lower limit of the Al content is preferably
0.005% or more. In view of properties and productivity, the Al
content is preferably in a range from 0.01 to 0.07% and may be in a
range from 0.01 to 0.05%.
[0058] One or both of 0.5% or less of Nb and 0.5% or less of Ti
[0059] Nb and Ti segregate at a grain boundary to suppress grain
boundary segregation of P and S, thereby improving low-temperature
embrittlement resistance. Further, Nb and Ti are also likely to
improve hydrogen embrittlement resistance by preventing
work-hardening of steel by functioning as stabilizing elements for
fixing C, N, P and S. Nb and Ti both exhibit these two functions
and thus are elements effective in improving hydrogen embrittlement
resistance and low-temperature embrittlement resistance, which are
the object of the invention. When Nb and Ti are contained, the Nb
and Ti contents are each preferably 0.01% or more in order to
exhibit the effects. However, excessive addition of Nb and Ti
increases work-hardening, thereby resulting in a reduction in
hydrogen embrittlement resistance or an increase in alloy cost.
Further, the excessive addition of Nb and Ti decreases toughness
and sometimes causes the texture to fall outside the preferable
ranges of the invention. Accordingly, an upper limit of each of the
Nb and Ti contents is set at 0.5% or less. In view of the effect of
improving the properties and the alloy cost, a total content of one
or both of Nb and Ti is preferably in a range from 0.05 to 0.5%.The
total content of one or both of Nb and Ti is more preferably in a
range from 0.08 to 0.4% and may be 0.1 to 0.3%.
[0060] The Sn and B contents are further preferably in respective
ranges below.
[0061] Sn is 0.001 to 0.3%.
[0062] Sn is an effective element for improving hydrogen
embrittlement resistance and low-temperature embrittlement
resistance, which are the object of the invention. Sn, which is a
grain boundary segregation element, forms a diffusion barrier at a
grain boundary with hydrogen, thereby reducing an interaction
between hydrogen and plastic deformation. Further, Sn suppresses
segregation of P and S at a grain boundary to alleviate an adverse
effect of low-temperature embrittlement resistance. The Sn content
being in a predetermined range is likely to achieve both hydrogen
embrittlement resistance and low-temperature embrittlement
resistance. Accordingly, the Sn content is preferably in a range
from 0.001 to 0.5% in the invention. 0.001% or more of Sn is to be
contained to exhibit the above effects, thereby improving hydrogen
embrittlement resistance. However, excessive addition of Sn
increases a Sn concentration at a grain boundary to reduce
low-temperature embrittlement resistance and productivity.
Accordingly, an upper limit of the Sn content is set at 0.5% or
less. The Sn content is preferably in a range from 0.005 to 0.3%
and may be in a range from 0.010 to 0.2%.
[0063] B is 0.005% or less.
[0064] B is a grain boundary segregation element and is also an
element for improving hydrogen embrittlement resistance and
low-temperature embrittlement resistance similarly to Sn.
Accordingly, it is effective to contain B in the Cr-based stainless
steel according to the exemplary embodiment. In order to improve
hydrogen embrittlement resistance properties, the B content is
preferably 0.0003% or more in the invention. However, excessive
addition of B results in a reduction in elongation or productivity.
Accordingly, an upper limit of the B content is set at 0.005% or
less. The B content is preferably in a range from 0.0005 to 0.002%
and may be in a range from 0.001 to 0.002%.
[0065] It is preferable that Si, Mn, P, Nb and Ti are contained in
the above respective ranges and further satisfy Formula (1) below
in order to improve hydrogen embrittlement resistance and
low-temperature embrittlement resistance, which are the target of
the invention.
Si+0.5 Mn+10 P+5 Nb+2 Ti<2.00 Formula (1)
[0066] In the above formula, symbols of elements indicate contents
(mass %) of the respective elements.
[0067] In order to improve the above properties (the object of the
invention), it is preferable that the left side of Formula (1) is
less than 2.00 and a lower limit thereof is 0.05 in terms of
properties and productivity. The left side of Formula (1) is
preferably in a range from 0.35 to 1.80, more preferably in a range
from 0.50 to 1.50.
[0068] In addition to the above elements, Fe and impurities are
contained. However, elements described below can be optionally
contained in addition to the above elements as long as effects
achieved by the technical features of the invention are not
impaired. Reasons for limitation of the content thereof will be
described below. A lower limit of each of the elements described
below is 0%.
[0069] Ni is 1% or less.
[0070] Cu is 1% or less.
[0071] Mo is 1% or less.
[0072] Ni, Cu and Mo are elements effective in improving corrosion
resistance. Ni and Cu are also effective in improving
low-temperature toughness. In order to exhibit these effects, Ni,
Cu and Mo may be each contained at 0.05% or more. Excessive
addition thereof increases solid solution strengthening and
work-hardening of stainless steel, resulting in a reduction in
hydrogen embrittlement resistance. Accordingly, an upper limit of
each of the Ni, Cu and Mo contents is set at 1% or less.
[0073] The Ni, Cu and Mo contents are each more preferably in a
range from 0.1% to 0.8%, further preferably in a range from 0.2% to
0.5%.
[0074] Sb is 0.2% or less.
[0075] V is 0.5% or less.
[0076] W is 0.5% or less.
[0077] Zr is 0.5% or less.
[0078] Co is 0.5% or less.
[0079] Sb, V, W, Zr and Co are elements effective in improving
corrosion resistance and also improving low-temperature
embrittlement resistance by suppressing grain boundary segregation
of P and S, and thus are contained as required. In particular, Sb
is a strong grain boundary segregation element and has an effect of
blocking grain boundary segregation of impurity elements such as P
and S similarly to Sn and B. When Sb, V, W, Zr and Co are
contained, the Sb, V, W, Zr and Co contents are each preferably
0.01% or more in order to exhibit the effects. Since excessive
addition thereof reduces productivity and low-temperature
embrittlement resistance, the Sb content is set at 0.2% or less and
V, W, Zr and Co are each set at 0.5% or less. The
[0080] Sb content is more preferably in a range from 0.02 to 0.15%,
further preferably in a range from 0.02 to 0.1%.The V, W, Zr and Co
contents are each more preferably in a range from 0.02 to 0.3%,
further preferably in a range from 0.02 to 0.2%.
[0081] Mg is 0.005% or less.
[0082] Mg acts as a deoxidizer by forming an Mg oxide with Al in
molten steel and also as a crystallization nucleus of TiN. Since
TiN becomes a solidification nucleus of a ferrite phase in a
solidification process, Mg facilitates crystallization of TiN to
finely generate the ferrite phase in solidification. Refinement of
a solidified structure also can improve low-temperature
embrittlement resistance. When Mg is contained, the Mg content is
preferably 0.0001% or more in order to exhibit the effects.
However, since more than 0.005% of Mg deteriorates productivity and
corrosion resistance, an upper limit of the Mg content is set at
0.005% or less. The Mg content is preferably in a range from 0.0003
to 0.002%, further preferably in a range from 0.0003 to 0.001%.
[0083] Ca is 0.005% or less.
[0084] Ga is 0.020% or less.
[0085] Ca and Ga are elements for improving cleanliness of steel
and are contained as required in order to suppress an increase in
work-hardening and thereby increase hydrogen embrittlement
resistance. When Ca and Ga are contained, the Ca and Ga contents
are each preferably 0.0003% or more in order to exhibit the
effects. However, excessive addition of Ca and Ga results in a
reduction in productivity and corrosion resistance. Accordingly, an
upper limit of the Ca content is set at 0.005% or less and an upper
limit of the Ga content is set at 0.020% or less. It is preferable
that the Ca content is in a range from 0.0003 to 0.0030% and the Ga
content is in a range from 0.0030 to 0.015%.
[0086] La is 0.1% or less.
[0087] Y is 0.1% or less.
[0088] Hf is 0.1% or less.
[0089] REM is 0.1% or less.
[0090] La, Y, Hf and REM are elements for improving cleanliness of
steel similarly to Ca and Ga and may be contained as required in
order to suppress an increase in work-hardening and thereby
increase hydrogen embrittlement resistance. When La, Y, Hf and REM
are contained, the La, Y, Hf and REM contents are each preferably
0.001% or more in order to exhibit the effects. However, excessive
addition of La, Y, Hf and REM results in an increase in alloy cost
and a reduction in productivity. Accordingly, an upper limit of the
La, Y, Hf and REM contents are each set at 0.1% or less. The La, Y,
Hf and REM contents are each preferably in a range from 0.001 to
0.05%, further preferably in a range from 0.001 to 0.03%.
[0091] Specifically, REM (rare-earth elements) collectively refers
to two elements of scandium (Sc) and yttrium (Y); and fourteen
elements (lanthanoid) from cerium (Ce) to lutetium (Lu) in the
periodic table. These elements may be contained alone or may be
contained in a form of a mixture.
[0092] It should be noted that impurities contained in the balance
mean components mixed from ores or scraps used as a material or due
to a manufacturing environment in industrially manufacturing steel,
which are acceptable as long as the objects of the invention can be
achieved. 0.1% or less of Ta, 0.01% or less of Bi, 0.05% of Zn and
0.0005% or less of H may be contained as required. The Cr-based
stainless steel according to the exemplary embodiment contains
ferrite crystal grains and further may contain martensite crystal
grains.
[0093] Next, the texture of the Cr-based stainless steel sheet
according to the exemplary embodiment will be described. The
texture in the sheet surface of the chromium stainless sheet
according to the exemplary embodiment satisfies (i) and (ii) below.
[0094] (i) In the sheet surface, an area ratio of crystal grains
({211}.+-.10-degree-oriented grains) whose orientation difference
between a normal direction of the surface of the steel sheet and a
{211}-plane orientation is 10 degrees or less is less than 30%.
[0095] (ii) For the {211}.+-.10-degree-oriented grains defined in
(i), a length in a rolling direction and a length in a sheet width
direction are each less than 0.15 mm on average.
[0096] The {211}-plane orientation refers to a normal direction of
the {211}-plane.
[0097] The {211} orientation is referred to as an a-fiber, which is
a rolling texture aggregated through cold-rolling. According to the
invention, it has been found that it is effective to control, in
the sheet surface, the area ratio and a size of the
{211}.+-.10-degree-oriented grains, which are frequently sites of
crack generation, in order to improve hydrogen embrittlement
resistance. The area ratio of the {211}.+-.10-degree-oriented
grains is set at less than 30% and an abundance ratio of a
recrystallization texture in a form of a {111} orientation is
increased in the sheet surface, thereby contributing to improving
hydrogen embrittlement resistance. The area ratio of the
{211}.+-.10-degree-oriented grains is preferably in a range from 5
to 20%, more preferably in a range from 3 to 15% in terms of
hydrogen embrittlement resistance and productivity.
[0098] In the sheet surface, the size of the
{211}.+-.10-degree-oriented grains is set such that the length in
the rolling direction and the length in the sheet width direction
(rolling vertical direction) are each less than 0.15 mm on average.
A reduction in the size of the {211}.+-.10-degree-oriented grains
reduces introduction of strains to and accumulation of the strains
in the {211}.+-.10-degree-oriented grains, thereby contributing to
improving hydrogen embrittlement resistance. The size of the
{211}.+-.10-degree-oriented grains is preferably less than 0.10 mm,
more preferably less than 0.07 mm in terms of hydrogen
embrittlement resistance and productivity.
[0099] In the invention, the "sheet surface" refers to regions
reaching at most t/8 of a thickness t of the steel sheet, i.e.,
regions on respective two sides of the steel sheet reaching at most
a thickness 1/8t from the respective surfaces of the steel sheet in
a surface direction. In the sheet surface, the
{211}.+-.10-degree-oriented grains refer to crystal grains having a
crystal orientation whose orientation difference between the normal
direction of the surface of the steel sheet and the {211}-plane
orientation is 10 degrees or less.
[0100] The texture can be analyzed using electron backscatter
diffraction (hereinafter, EBSD). EBSD rapidly measures and analyzes
the crystal orientation of each of the crystal grains in a
microregion of a sample surface. Regarding a crystal orientation
group contributing to hydrogen embrittlement resistance, the area
ratio and the grain size of the {211}.+-.10-degree-oriented grains
can be quantified by displaying a crystal orientation map of the
crystal orientation group divided into the
{211}.+-.10-degree-oriented grains and other regions in the sheet
surface. For instance, in a plane reaching at most t/8 of the
thickness t of the steel sheet from the surface of the steel sheet
and being parallel to the surface of the steel sheet, a measurement
region (sheet width direction: 850 .mu.m, rolling direction: 2250
.mu.m) is subjected to EBSD measurement at a magnification of 100
to display the crystal orientation map of the crystal grains (i.e.,
the {211}.+-.10-degree-oriented grains) whose orientation
difference between the normal direction of the plane parallel to
the surface of the steel sheet and the {211}-plane orientation is
10 degrees or less, whereby the area ratio and the grain size (in
the rolling direction and the sheet width direction) thereof can be
quantified. When regions reaching at most t/8 of the thickness t of
the steel sheet from the surface of the steel sheet are defined as
an inspection surface, the texture in the sheet surface can be
reproducibly evaluated.
[0101] The hydrogen embrittlement resistance is evaluated by a slow
strain rate tensile test for which a relatively slow strain rate is
used. The strain rate is preferably 10.sup.-5/s. When the strain
rate is relatively large, i.e., 10.sup.-4/s or more, penetration
and diffusion of hydrogen into the steel do not progress, sometimes
resulting in a reduction in hydrogen embrittlement of the steel. In
contrast, when the strain rate is small, i.e., 10.sup.-6/s,
excessive test time is required and the effect of the strain rate
to hydrogen embrittlement properties is saturated. The hydrogen
embrittlement resistance is evaluated for tensile strength and
fracture elongation in the slow strain rate tensile test. It is
more favorable that a value of the tensile strength and the
fracture elongation in the high pressure hydrogen gas is more
unlikely to be lowered as compared with the value thereof in an
atmosphere or in an inert gas. Here, a value obtained by dividing
the tensile strength in the high pressure hydrogen gas by the
tensile strength in the atmosphere or in the inert gas is referred
to as a "relative tensile strength". A value obtained by dividing
the fracture elongation in the high pressure hydrogen gas by the
fracture elongation in the atmosphere or in the inert gas is
referred to as a "relative elongation". For the Cr-based stainless
steel sheet according to the exemplary embodiment, it is preferable
that the relative tensile strength is 0.98 or more and the relative
elongation is 0.75 or more. It is more preferable that the relative
tensile strength is in a range from 0.98 to 1.05 and the relative
elongation is a range from 0.85 to 1.05.
[0102] The low-temperature embrittlement resistance is evaluated by
a Charpy impact test according to JIS Z 2242. For instance, a
2-mm-thick test piece having a V-notch shape is used to measure
absorption energy. The low-temperature embrittlement resistance is
evaluated in terms of an energy transition temperature according to
Annex D of JIS. A lower energy transition temperature is more
favorable. The energy transition temperature refers to a
temperature corresponding to a half value of the absorption energy
at a temperature at which a fracture rate due to a ductile fracture
is 100%.The energy transition temperature is preferably -10 degrees
or less in terms of use of the Cr-based stainless steel sheet
according to the exemplary embodiment for outdoor and on-vehicle
hydrogen equipment. The energy transition temperature is more
preferably -40 degrees or less in terms of the use thereof in a
cold region.
[0103] Next, a manufacturing method of the Cr-based stainless steel
sheet according to the exemplary embodiment will be described.
[0104] When the Cr-based stainless steel sheet according to the
exemplary embodiment satisfies the above chemical composition, the
hydrogen embrittlement resistance and low-temperature embrittlement
resistance (the object of the invention) sometimes can be ensured
even if typical process conditions such as casting, hot-rolling and
cold-rolling are used for manufacturing thereof.
[0105] It is preferable that the Cr-based stainless steel sheet
according to the exemplary embodiment satisfies the above chemical
composition and manufactured by the following method in order to
improve the hydrogen embrittlement resistance by forming the
texture of the invention.
[0106] It is preferable that the steel having the above chemical
composition is hot-rolled, annealed after the hot-rolling at a
temperature of 900 degrees C. or less, subsequently cold-rolled at
a rolling reduction rate of 40% or more, and is subjected to finish
annealing at a temperature of more than 900 degrees C. The heat
treatment after the hot-rolling (annealing after the hot-rolling)
is preferably performed at a temperature of 900 degrees C. or less,
more preferably in a temperature range from 700 to 900 degrees C.
in order to suppress a growth of the {211}-oriented grains, which
are generated at the hot-rolling stage.
[0107] The cold-rolling may be performed by a reversible 20-stage
Sendzimir rolling mill, a 6 or 12-stage rolling mill, ora tandem
rolling mill configured to continuously roll a plurality of passes.
In order to form the texture of the invention, a larger work roll
diameter is preferable. Accordingly, the work roll diameter is
preferable 200 mm or more. Rolling with such a large-diameter roll
is preferably performed in a primary cold-rolling (initial
cold-rolling in a case where plural times of cold-rolling are
repeatedly performed). The cold-rolling grows the recrystallization
texture in a form of the {111}-oriented grains to reduce the area
ratio of the rolling texture in a form of the
{211}.+-.10-degree-oriented grains, thereby being effective in
forming the target texture of the invention. The cold-rolling is
preferably performed at a rolling reduction rate of 40% or more.
When the cold-rolling rate is less than 40%, the area ratio and the
size of the {211}.+-.10-degree-oriented grains in the
recrystallization texture are likely to increase, sometimes
resulting in a reduction in the hydrogen embrittlement resistance.
The rolling reduction rate is preferably in a range from 40 to 90%,
more preferably in a range from 50 to 80% in terms of the hydrogen
embrittlement resistance and the productivity.
[0108] The finish annealing after the cold-rolling is preferably
performed through the heat treatment at a temperature of more than
900 degrees C. in order to reduce the area ratio and the size of
the {211}-oriented grains by growing the {111}-oriented grains.
Since an excessive temperature rise increases the size of the
{211}.+-.10-degree-oriented grains through the crystal grain
growth, an upper limit of a finish annealing temperature is
preferably 1050 degrees C. An atmosphere for the finish annealing
is not particularly defined but preferably an atmospheric air, an
LNG fuel atmosphere and a BA atmosphere.
[0109] A soaking time for the heat treatment (finish annealing) is
preferably in a range from 10 seconds to 10 minutes. The soaking
time is preferably 10 seconds or more in order to soften a material
for the cold-rolling. When the soaking time is 10 minutes or less,
the texture effective for the hydrogen embrittlement resistance can
be ensured by suppressing the growth of the
{211}.+-.10-degree-oriented grains to reduce the size of the
crystal grains.
EXAMPLE(S)
[0110] Example(s) of the invention will be described below.
TABLE-US-00001 TABLE 1 Chemical Composition (Mass %), Balance: Fe
and Impurities C Si Mn P S Cr N Al Nb Ti Sn B Formula (1) Others A
0.019 0.93 0.01 0.016 0.001 12.1 0.018 0.011 0.13 0.14 0.00 0.00
2.03 B 0.006 0.08 0.07 0.038 0.0005 11.6 0.008 0.089 0.00 0.46 0.00
0.00 1.42 Mg: 0.002, Ca: 0.0012, Ga: 0.0025 C 0.013 0.05 0.04 0.024
0.0011 17.7 0.011 0.045 0.14 0.09 0.18 0.0016 1.19 Cu: 0.18, Mo:
0.2, Sb: 0.02, V: 0.1, Ni: 0.21 D 0.011 0.02 0.05 0.003 0.0028 10.6
0.004 0.055 0.37 0.00 0.00 0.00 1.93 W: 0.2, Co: 0.3, Hf: 0.02,
REM: 0.02 E 0.006 0.11 0.09 0.020 0.0008 13.9 0.012 0.042 0.13 0.08
0.06 0.0006 1.17 F 0.001 0.06 0.04 0.011 0.0001 13.4 0.008 0.028
0.01 0.07 0.00 0.00 0.38 G 0.005 0.09 0.11 0.016 0.0021 16.4 0.012
0.038 0.03 0.16 0.00 0.0011 0.78 H 0.012 0.11 0.96 0.023 0.0005
12.2 0.009 0.057 0.05 0.25 0.01 0.00 1.57 La: 0.06, Y: 0.03, Zr:
0.07 I 0.023 0.25 0.21 0.018 0.0011 13.1 0.015 0.065 0.16 0.12 0.00
0.00 1.58 J 0.011 1.03 0.31 0.019 0.0006 13.8 0.011 0.045 0.09 0.03
0.00 0.00 1.89 K 0.012 0.21 1.02 0.016 0.0007 15.2 0.009 0.031 0.14
0.11 0.00 0.00 1.80 L 0.011 0.18 0.15 0.043 0.0008 14.6 0.008 0.075
0.19 0.12 0.00 0.00 1.88 M 0.009 0.17 0.21 0.018 0.0032 11.8 0.013
0.066 0.13 0.21 0.00 0.00 1.53 N 0.008 0.13 0.31 0.019 0.0011 18.2
0.007 0.041 0.13 0.22 0.00 0.00 1.57 O 0.007 0.19 0.15 0.017 0.0021
11.9 0.022 0.038 0.14 0.23 0.00 0.00 1.60 P 0.009 0.26 0.26 0.021
0.0007 14.1 0.009 0.108 0.13 0.24 0.00 0.00 1.73 Q 0.011 0.21 0.31
0.018 0.0012 14.6 0.013 0.038 0.02 0.52 0.00 0.00 1.69 (Note) Nb,
Ti, Sn, B: 0.00, not added. Underlined values are shown to be
outside the respective ranges of the invention.
[0111] Cr-based stainless steel having chemical compositions shown
in Table 1 was manufactured through melting. For the Nb, Ti, Sn and
B contents in Table 1, the entry "0.0" means that these elements
are not added.
[0112] The Cr-based stainless steel was hot-rolled by being heated
to a heating temperature in a range from 1150 to 1250 degrees C. to
manufacture a 5.0-mm-thick hot-rolled steel sheet. The hot-rolled
steel sheet was annealed after the hot-rolling in a temperature
range from 700 to 900 degrees C. and, subsequent to being pickled,
was cold-rolled in a thickness range from 1.5 to 2.5 mm to provide
a cold-rolled steel sheet. Conditions for the cold-rolling are
shown in Table 2. The cold-rolling was performed by a Sendzimir
rolling mill and a tandem rolling mill having respective different
work roll diameters. The former used a small diameter roll (60 mm)
(indicated as "S" in Table 2) and the latter used a large diameter
roll (200 mm) (indicated as "L" in Table 2). The cold-rolled steel
sheet was subjected to finish annealing in a temperature range from
920 to 1020 degrees C. and pickling to manufacture a Cr-based
stainless steel sheet.
[0113] A texture was analyzed using EBSD. A crystal orientation
group contributing to hydrogen embrittlement resistance was
quantified by displaying a crystal orientation map of the crystal
orientation group divided into {211}.+-.10-degree-oriented grains
and other regions in the sheet surface. In other words, in a plane
in a range of t/8 of a thickness t of the steel sheet from a
surface of the steel sheet and parallel to the surface of the steel
sheet, a measurement region (sheet width direction: 850 .mu.m,
rolling direction: 2250 .mu.m) was subjected to EBSD measurement at
a magnification of 100, displayed the crystal orientation map of
the crystal grains (i.e., the {211}.+-.10-degree-oriented grains)
whose orientation difference between a normal direction of the
plane parallel to the surface of the steel sheet and the
{211}-plane orientation was 10 degrees or less and also displayed a
grain boundary, so that an area ratio and average grain size (in
the rolling direction and the sheet width direction) of the crystal
grains were measured. Notations shown in the column of "Size" of
the {211}.+-.10-degree-oriented grains in Table 2 mean respective
sizes in the "rolling direction/sheet width direction". For some
comparatives, measurement results at a thickness center (t/2) were
also shown for reference. A site having a difference in a crystal
orientation of 15 degrees or more was defined as a grain
boundary.
TABLE-US-00002 TABLE 2 Cold Rolling {211} .+-. 10-Degree- Low-
Condition Oriented Grain Hydrogen Hydrogen Temperature Rolling Area
Size Gaseous Embrittlement Embrittlement Roll Reduction Ratio (Note
2) Pressure Resistance Resistance No Steel (Note 1) % % mm MPa
(Note 3) (Note 4) Remarks 1 A L 50 26 0.14/0.13 20 B B Example of
the 2 B S 60 13 0.09/0.08 20 A A Invention 3 C S 70 18 0.09/0.07 20
A B 4 D L 50 23 0.13/0.12 20 B B 5 E S 60 17 0.08/0.07 45 B A 6 L
60 10 0.06/0.05 45 A A 7 F S 50 22 0.12/0.11 20 B B 8 L 50 19
0.09/0.08 20 A B 9 S 60 14 0.09/0.07 45 B A 10 L 60 6 0.06/0.04 45
A A 11 H L 50 16 0.08/0.08 20 B A 12 I* S 60 24 0.17/0.15* 20 X X
Comparatives 13 J* S 60 26 0.18/0.16* 20 X X 14 K* S 60 27
0.17/0.16* 20 X X 15 L* S 60 23 0.18/0.17* 20 B X 16 M* S 60 22
0.19/0.18* 20 B X 17 N* S 60 32*(24) 0.20/0.19* 20 X X 18 O* S 60
23 0.18/0.16* 20 X X 19 P* S 60 32*(21) 0.14/0.14 20 B X 20 Q* S 60
33*(26) 0.17/0.16* 20 X X (Note 1) Roll: small diameter roll =>
S, large diameter roll => L (Note 2) {211} .+-.
10-degree-oriented grain: notations of size are in a rolling
direction/rolling vertical direction. (Note 3) Hydrogen
embrittlement resistance A: relative tensile strength of 0.98 or
more and relative elongation of 0.85 or more B: relative tensile
strength of 0.98 or more and relative elongation of 0.75 or more X:
one or both of relative tensile strength of less than 0.98 and
relative elongation of less than 0.75 (Note 4) Low-temperature
embrittlement resistance A: energy transition temperature of -40
degrees C. or less B: energy transition temperature of -10 degrees
or less X: energy transition temperature of more than -10 degrees
C. (Note 5) *indicates that values are outside the respective
ranges defined in the invention. (Note 6) Nos. 17, 19, 20:
numerical values in parentheses for an area ratio of {211} .+-.
10-degree-oriented grain are measured values at a thickness center
t/2.
[0114] The obtained Cr-based stainless steel sheet was subjected to
an evaluation for hydrogen embrittlement and low-temperature
embrittlement. As comparative materials, an SUS316L steel sheet
(17.5%Cr-12%Ni-2%Mo) and an SUS316 steel sheet
(17.5%Cr-10%Ni-2%Mo), each of which had a thickness of 2 mm and was
commercially available, were used to evaluate the hydrogen
embrittlement resistance.
[0115] The hydrogen embrittlement was evaluated according to the
following steps.
[0116] A tensile test piece (width: 4 mm, length: 20mm) was
prepared from a parallel portion. A surface thereof was polished
with a dry #600 Emery paper and was subsequently degreased with an
organic solvent immediately prior to a tensile test in a high
pressure hydrogen gas. As shown in Table 1, the tensile test in the
high pressure hydrogen gas was performed at a hydrogen gaseous
pressure of 20 MPa or 45 MPa, a test temperature of -40 degrees C.
and a strain rate of 10.sup.-51s. A comparative tensile test was
performed in nitrogen (-40 degrees C. and 0.1 MPa). A tensile
strength in the high pressure hydrogen gas divided by a tensile
strength in nitrogen (0.1 MPa) was defined as a relative tensile
strength. A fracture elongation in the high pressure hydrogen gas
divided by a fracture elongation in nitrogen (0.1 MPa) was defined
as a relative elongation. The hydrogen embrittlement resistance was
evaluated using the relative tensile strength and the relative
elongation as evaluation indices. Evaluation criteria were as
follows. A and B were evaluated to pass.
[0117] A: The relative tensile strength of 0.98 or more and the
relative elongation of 0.85 or more were satisfied.
[0118] B: Except for steel sheets satisfying the above A, the
relative tensile strength of 0.98 or more and the relative
elongation of 0.75 or more were satisfied.
[0119] X: One or both of the relative tensile strength of less than
0.98 and the relative elongation of less than 0.75 were
obtained.
[0120] Here, when the hydrogen gaseous pressure was 45 MPa and the
test temperature was -40 degrees C., the SUS316L steel sheet had
the relative elongation of less than 0.75 and thus was evaluated as
X. When the hydrogen gaseous pressure was 20 MPa and the test
temperature was -40 degrees C., the SUS316 steel sheet had the
relative elongation of less than 0.75 and thus was evaluated as
X.
[0121] The low-temperature embrittlement was evaluated by a Charpy
impact test according to JIS Z 2242. A test piece was set to have a
V-notch shape (1.5 to 2.5 mm thickness.times.10 mm width.times.55
mm length). A test temperature was set to be in a range from -100
degrees C. to a room temperature (20 degrees C.). An energy
transition temperature was calculated based on an absorption energy
measured in the Charpy test and set to be an evaluation index of
low-temperature embrittlement resistance. Evaluation criteria were
as follows. A and B were evaluated to pass.
[0122] A: The energy transition temperature of -40 degrees C. or
less was satisfied.
[0123] B: The energy transition temperature of more than -40
degrees C. and -10 degrees or less was satisfied.
[0124] X: The energy transition temperature was more than -10
degrees C.
[0125] Test results are all shown in Table 2.
[0126] Nos. 1 to 11 were each a Cr-based stainless steel sheet
having chemical composition and a texture that were within
respective ranges of the invention, thereby showing favorable
hydrogen embrittlement resistance and low-temperature embrittlement
resistance. In particular, Nos. 5, 6, 9 and 10, which were within
preferable ranges of the components and the texture, were evaluated
as "B" or "A" for the index of the hydrogen embrittlement
resistance at a hydrogen gaseous pressure of 45 MPa. The hydrogen
embrittlement resistance thereof was superior to that of SUS316L.
In Nos. 6, 8 and 10, the {211}.+-.10-degree-oriented grains were
reduced using a large-diameter roll. Although having the same
chemical composition, the hydrogen embrittlement resistance thereof
further improved as compared with that of Nos. 5, 7 and 9.
[0127] Nos. 12 to 20 were each a Cr-based stainless steel sheet
that did not have chemical composition within the respective ranges
of the invention and thus unable to form a texture within the range
of the invention, so that one or both of the hydrogen embrittlement
resistance and low-temperature embrittlement resistance thereof
were deteriorated. In Nos. 17, 19 and 20, the area ratio of the
{211}.+-.10-degree-oriented grains at a thickness center was less
than 30% but the area ratio thereof in the sheet surface was more
than 30%. Accordingly, it has been found that controlling of the
area ratio in the sheet surface is important in order to obtain
both the hydrogen embrittlement resistance and the low-temperature
embrittlement resistance.
[0128] In view of the above evaluation results, by having the
components and the texture that were within the respective ranges
of the invention, each of the Cr-based stainless steel sheets
exhibited superior hydrogen embrittlement resistance to that of the
commonly available SUS316. It also has been found that each of the
Cr-based stainless steel sheets is controlled using the
large-diameter roll to have a preferable texture with preferable
components, thereby achieving the hydrogen embrittlement resistance
superior to that of SUS316L.
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