U.S. patent application number 15/760895 was filed with the patent office on 2018-09-13 for austenitic stainless steel and method of manufacturing austenitic stainless steel.
The applicant listed for this patent is Nippon Steel & Sumitomo Metal Corporation. Invention is credited to Hiroyuki Hirata, Kana Jotoku, Jun Nakamura, Tomohiko Omura, Takahiro Osuki, Masaaki Terunuma, Masaki Ueyama.
Application Number | 20180258505 15/760895 |
Document ID | / |
Family ID | 58424115 |
Filed Date | 2018-09-13 |
United States Patent
Application |
20180258505 |
Kind Code |
A1 |
Hirata; Hiroyuki ; et
al. |
September 13, 2018 |
Austenitic Stainless Steel and Method of Manufacturing Austenitic
Stainless Steel
Abstract
An austenitic stainless steel with improved strength, ductility
and weldability is provided. An austenitic stainless steel has a
chemical composition of, in mass %: 0.005 to 0.07% C; 0.1 to 1.2%
Si; 3.2 to 6.5% Mn; 9 to 14% Ni; a total of not less than 0.005%
and less than 3% of at least one of Cu and Co; 19 to 24% Cr; 1 to
4% Mo; 0.05 to 0.4% Nb; 0.15 to 0.50% N; up to 0.05% Al; up to
0.03% P; up to 0.002% S; up to 0.02% O; 0 to 0.5% V; 0 to 0.5% Ti;
0 to 0.01% B; 0 to 0.05% Ca; 0 to 0.05% Mg; 0 to 0.5% REM; and the
balance being Fe and impurities, where the amount of Nb analyzed as
residues after electrolytic extraction is 0.01 to 0.3 mass %.
Inventors: |
Hirata; Hiroyuki;
(Chiyoda-ku, Tokyo, JP) ; Jotoku; Kana;
(Chiyoda-ku, Tokyo, JP) ; Omura; Tomohiko;
(Chiyoda-ku, Tokyo, JP) ; Nakamura; Jun;
(Chiyoda-ku, Tokyo, JP) ; Terunuma; Masaaki;
(Chiyoda-ku, Tokyo, JP) ; Osuki; Takahiro;
(Chiyoda-ku, Tokyo, JP) ; Ueyama; Masaki;
(Chiyoda-ku, Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nippon Steel & Sumitomo Metal Corporation |
Tokyo |
|
JP |
|
|
Family ID: |
58424115 |
Appl. No.: |
15/760895 |
Filed: |
July 6, 2016 |
PCT Filed: |
July 6, 2016 |
PCT NO: |
PCT/JP2016/070042 |
371 Date: |
March 16, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/001 20130101;
Y02P 10/212 20151101; C22C 38/06 20130101; C22C 38/44 20130101;
C21D 8/0263 20130101; C21D 2211/001 20130101; C22C 38/14 20130101;
C22C 38/48 20130101; C21D 8/005 20130101; C22C 38/02 20130101; C22C
38/22 20130101; C22C 38/46 20130101; C22C 38/12 20130101; C21D
8/0226 20130101; C22C 38/04 20130101; C22C 38/32 20130101; C22C
38/26 20130101; C22C 38/38 20130101; C22C 38/58 20130101; C22C
38/54 20130101; C22C 38/20 20130101; C22C 38/24 20130101; Y02P
10/20 20151101; C22C 38/00 20130101; C22C 38/42 20130101; C22C
38/50 20130101; C22C 38/52 20130101; C21D 6/004 20130101 |
International
Class: |
C21D 8/00 20060101
C21D008/00; C22C 38/58 20060101 C22C038/58; C22C 38/00 20060101
C22C038/00; C22C 38/06 20060101 C22C038/06; C22C 38/02 20060101
C22C038/02; C22C 38/20 20060101 C22C038/20; C22C 38/50 20060101
C22C038/50; C22C 38/48 20060101 C22C038/48; C22C 38/44 20060101
C22C038/44 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 30, 2015 |
JP |
2015-192676 |
Claims
1. An austenitic stainless steel having a chemical composition of,
in mass %: 0.005 to 0.07% C; 0.1 to 1.2% Si; 3.2 to 6.5% Mn; 9 to
14% Ni; a total of not less than 0.005% and less than 3% of at
least one of Cu and Co; 19 to 24% Cr; 1 to 4% Mo; 0.05 to 0.4% Nb;
0.15 to 0.50% N; up to 0.05% Al; up to 0.03% P; up to 0.002% S; up
to 0.02% O; 0 to 0.5% V; 0 to 0.5% Ti; 0 to 0.01% B; 0 to 0.05% Ca;
0 to 0.05% Mg; 0 to 0.5% REM; and the balance being Fe and
impurities, where an amount of Nb analyzed as a residue after
electrolytic extraction is 0.01 to 0.3 mass %.
2. The austenitic stainless steel according to claim 1, wherein the
chemical composition includes one or more elements selected from
the group consisting of, in mass %: 0.001 to 0.5% V; 0.001 to 0.5%
Ti; 0.0001 to 0.01% B; 0.0001 to 0.05% Ca; 0.0001 to 0.05% Mg; and
0.001 to 0.5% REM.
3. The austenitic stainless steel according to claim 1, wherein the
austenitic stainless steel has a tensile strength not lower than
690 MPa and a breaking elongation not less than 35% at room
temperature.
4. The austenitic stainless steel according to claim 1, wherein the
austenitic stainless steel is used in high-pressure hydrogen-gas
equipment or in liquid-hydrogen equipment.
5. A method of manufacturing the austenitic stainless steel
according to claim 1, comprising the steps of: preparing a raw
material having a chemical composition of, in mass %: 0.005 to
0.07% C; 0.1 to 1.2% Si; 3.2 to 6.5% Mn; 9 to 14% Ni; a total of
not less than 0.005% and less than 3% of at least one of Cu and Co;
19 to 24% Cr; 1 to 4% Mo; 0.05 to 0.4% Nb; 0.15 to 0.50% N; up to
0.05% Al; up to 0.03% P; up to 0.002% S; up to 0.02% O; 0 to 0.5%
V; 0 to 0.5% Ti; 0 to 0.01% B; 0 to 0.05% Ca; 0 to 0.05% Mg; 0 to
0.5% REM; and the balance being Fe and impurities; performing hot
working on the raw material; and performing solution heat treatment
on the raw material after the hot working at a solution heat
treatment temperature of 950 to 1300.degree. C. under a condition
satisfying the formula provided below, (1), wherein cold working is
not performed between the hot working and the solution heat
treatment, 40.times.[%
Nb]+100.ltoreq.T.times.log(1.2+t/60).ltoreq.-200.times.[% Nb]+700
(1), In formula (1), the Nb content in the raw material in mass %
is substituted for [% Nb], the solution heat treatment temperature
in .degree. C. is substituted for T, and the solution heat
treatment time in minutes is substituted for t.
Description
TECHNICAL FIELD
[0001] The present invention relates to an austenitic stainless
steel and a method of manufacturing an austenitic stainless
steel.
BACKGROUND ART
[0002] In recent years, research has been under progress for
putting to practical use transportation equipment that use
hydrogen, instead of fossil fuel, as driving energy. Such practical
use requires the provision of a use environment in which hydrogen
under high pressure can be stored and transported (hereinafter also
referred to as hydrogen equipment). Hydrogen equipment may be, for
example, high-pressure hydrogen-gas equipment or liquid-hydrogen
equipment. Materials used in hydrogen equipment are required to
have hydrogen embrittlement resistance.
[0003] WO 2004/083476 A1, WO 2004/083477 A1, WO 2004/110695 A1 and
WO 2012/132992 A1 each disclose a high-strength austenitic
stainless steel. According to these documents, Mn is increased to
increase the solubility of N, and V and Nb are added to provide
solute strengthening due to N and precipitation strengthening due
to nitrides and cause grains to be finer due to their pinning
effect, thereby increasing strength.
[0004] When an austenitic stainless steel is used as a structure,
the steel is required to allow assembly by welding for cost
reasons. JP Hei5 (1993)-192785 A, JP 2010-227949 A and, again, WO
2004/110695 A1 each disclose a welded joint where Al, Ti and Nb
serve as useful elements and post weld heat treatment is done to
achieve a tensile strength above 800 MPa.
[0005] WO 2013/005570 A1 discloses a welded joint where the N
content in the weld material, the shield gas used during the
welding and the area of the molten pool are controlled to increase
the N content in the weld metal, thereby providing high strength
even without post weld heat treatment.
DISCLOSURE OF THE INVENTION
[0006] Materials used for structures are required to have various
properties in addition to hydrogen embrittlement resistance and
strength. For example, when such materials are used in piping, some
structures may be cold bent or welded under various conditions. As
such, to provide healthy structures, both sufficient ductility and
good weldability are needed.
[0007] Employing techniques as described in the above-listed patent
documents provides a high-strength base material or welded joint
with improved hydrogen embrittlement resistance. Particularly, WO
2004/083476 A1 and WO 2004/083477 A1 each disclose a high-strength
austenitic stainless steel with a ductility corresponding to a
braking elongation above 30%. However, if a high concentration of
Nb is contained as an alloy element for the purpose of using its
effects, weldability may decrease and, during the welding, cracks
may develop in weld heat-affected zones.
[0008] WO 2012/132992 A1 discloses performing cold working after
solution heat treatment and, then, performing heat treatment again
to provide an austenitic stainless steel with a strength of 800 MPa
or higher. However, when a material is to be used as a structure,
it is difficult to perform cold working on all its parts. Thus, a
material is preferred that can provide the required strength and
other properties after the solution heat treatment after the hot
working without any further treatment.
[0009] An object of the present invention is to provide an
austenitic stainless steel with improved strength, ductility and
weldability.
[0010] An austenitic stainless steel according to an embodiment of
the present invention has a chemical composition of, in mass %:
0.005 to 0.07% C; 0.1 to 1.2% Si; 3.2 to 6.5% Mn; 9 to 14% Ni; a
total of not less than 0.005% and less than 3% of at least one of
Cu and Co; 19 to 24% Cr; 1 to 4% Mo; 0.05 to 0.4% Nb; 0.15 to 0.50%
N; up to 0.05% Al; up to 0.03% P; up to 0.002% S; up to 0.02% O; 0
to 0.5% V; 0 to 0.5% Ti; 0 to 0.01% B; 0 to 0.05% Ca; 0 to 0.05%
Mg; 0 to 0.5% REM; and the balance being Fe and impurities, where
an amount of Nb analyzed as a residue after electrolytic extraction
is 0.01 to 0.3 mass %.
[0011] The present invention provides an austenitic stainless steel
with improved strength, ductility and weldability.
EMBODIMENTS FOR CARRYING OUT THE INVENTION
[0012] The present inventors investigated the strength and
ductility of austenitic stainless steels produced by performing hot
working and solution heat treatment on a raw material containing,
in mass %: 0.005 to 0.07% C; 0.1 to 1.2% Si; 3.2 to 6.5% Mn; 9 to
14% Ni; a total of not less than 0.005% and less than 3% of at
least one of Cu and Co; 19 to 24% Cr; 1 to 4% Mo; 0.05 to 0.4% Nb;
0.15 to 0.50% N; up to 0.05% Al; and other elements, and obtained
the following findings.
[0013] The strength and ductility of an austenitic stainless steel
are related to the amount of Nb analyzed as residues after
electrolytic extraction, that is, they are related to the amount of
precipitates containing Nb. The precipitates containing Nb are Nb
carbonitrides and Nb nitrides produced during the manufacture of
the austenitic stainless steel.
[0014] To provide the required strength after the solution heat
treatment after the hot working without any further treatment, the
amount of Nb analyzed as residues after electrolytic extraction
needs to be 0.005 mass % or more. On the other hand, if the amount
of Nb analyzed as residues after electrolytic extraction exceeds
0.3 mass %, the ductility decreases.
[0015] Even in the case of a steel where the amount of Nb analyzed
as residues after electrolytic extraction is 0.005 mass % or more
and the required strength is provided, welding such a steel before
using it may cause another problem: liquation cracks may develop in
weld heat-affected zones adjacent to the fusion line. This is
presumably because, when the amount of Nb analyzed as residues
after electrolytic extraction is small, the effects of the pinning
due to carbonitrides or nitrides of Nb are insufficient and thus,
during welding, grains become coarse. To prevent liquation cracking
during welding, the amount of Nb analyzed as residues after
electrolytic extraction needs to be 0.01 mass % or more.
[0016] Accordingly, to provide good strength, ductility and
weldability in an austenitic stainless steel having the
above-indicated chemical composition, it is required that the
amount of Nb analyzed as residues after electrolytic extraction be
0.01 to 0.3 mass %.
[0017] Furthermore, the present inventors found that even better
ductility and weldability may be provided by adjusting the
conditions of the solution heat treatment performed after hot
working depending on the Nb content in the raw material. More
specifically, they found that good ductility and weldability may be
provided by performing solution heat treatment at a solution heat
treatment temperature in the range of 950 to 1300.degree. C. under
a condition that satisfies the following formula, (1):
40.times.[%
Nb]+100.ltoreq.T.times.log(1.2+t/60).ltoreq.-200.times.[% Nb]+700
(1)
[0018] In formula (1), the Nb content in the raw material in mass %
is substituted for [% Nb], the solution heat treatment temperature
in .degree. C. is substituted for T, and the solution heat
treatment time in minutes is substituted for t.
[0019] The present invention was made based on the above-discussed
findings. An austenitic stainless steel according to an embodiment
of the present invention and a method of manufacturing it will be
described in detail below.
[0020] [Chemical Composition]
[0021] The austenitic stainless steel according to the present
embodiment has the chemical composition described below. In the
following description, "%" in the content of an element means mass
percent.
[0022] C: 0.005 to 0.07%
[0023] Carbon (C) is an element effective in stabilizing austenite.
Further, C produces carbides of Nb and contributes to providing
sufficient strength. The C content needs to be 0.005% or higher in
order that these effects are sufficiently present. However, if the
C content is too high, excessive amounts of Nb carbides are
produced, which decreases the ductility of the steel. In view of
this, the C content should be in the range of 0.005 to 0.07%. The
lower limit of C content is preferably 0.01%, and more preferably
0.02%. The upper limit of C content is preferably 0.06%, and more
preferably 0.05%.
[0024] Si: From 0.1 and Up to 1.2%
[0025] Silicon (Si) is an element effective as a deoxidizer and
also effective in improving corrosion resistance. The Si content
needs to be 0.1% or higher in order that these effects are
sufficiently present. However, if the Si content is too high, this
reduces the stability of the austenite microstructure and also
reduces the ductility of the steel. In view of this, the Si content
should be in the range of 0.1 to 1.2%. The lower limit of Si
content is preferably 0.15%, and more preferably 0.2%. The upper
limit of Si content is preferably 1.1%, and more preferably
1.0%.
[0026] Mn: 3.2 to 6.5%
[0027] Manganese (Mn) contributes to deoxidization during
manufacture and is also effective in stabilizing austenite. Mn
further increases the solubility of N to indirectly contribute to
increasing strength. The Mn content needs to be 3.2% or higher in
order that these effects are sufficiently present. On the other
hand, if the Mn content is too high, not only is the steel
saturated in terms of these effects, but also Mn becomes fumes
during welding which stick to the weld, which decreases corrosion
resistance. In view of this, the Mn content should be in the range
of 3.2 to 6.5%. The lower limit of the Mn content is preferably
3.4%, and more preferably 3.5%. The upper limit of Mn content is
preferably 6.3%, and more preferably 6.0%.
[0028] Ni: 9 to 14%
[0029] Nickel (Ni) is indispensable for providing stable austenite,
and increases stacking fault energy and reduces embrittlement
susceptibility in a hydrogen environment. The Ni content needs to
be 9% or higher in order that these effects are sufficiently
present. However, Ni is an expensive element, and high Ni contents
mean increased costs. In view of this, the Ni content should be in
the range of 9 to 14%. The lower limit of Ni content is preferably
9.5%, and more preferably 10%. The upper limit of Ni content is
preferably 13.5%, and more preferably 13%.
[0030] Total of at Least One of Cu and Co: Not Less than 0.005% and
Less than 3%
[0031] Similar to Ni, Cu (copper) and Co (cobalt) are effective in
providing stable austenite microstructure. The total content of Cu
and Co needs to be 0.005% or more in order that their effects are
sufficiently present. Only one of Cu and Co may be contained, or
both may be contained. However, Cu and Co are expensive elements,
and higher contents mean increased costs. Further, excess contents
of Cu and Co lead to decreased ductility of the steel. In view of
this, the total content of Cu and Co should be not lower than
0.005% and lower than 3%. The lower limit of the total content of
Cu and Co is preferably 0.01%, and more preferably 0.02%. The upper
limit of the total content of Cu and Co is preferably 2%, and more
preferably 1%.
[0032] Cr: 19 to 24%
[0033] Chromium (Cr) is indispensable for providing sufficient
corrosion resistance in a use environment. Cr further increases the
solubility of N during the manufacture of the base material to
indirectly contribute to increasing strength. The Cr content needs
to be 19% or higher in order that these effects are sufficiently
present. However, if the Cr content is too high, the austenite
microstructure becomes instable. In view of this, the Cr content
should be in the range of 19 to 24%. The lower limit of Cr content
is preferably 19.5%, and more preferably 20%. The upper limit of Cr
content is preferably 23.5%, and more preferably 23%.
[0034] Mo: 1 to 4%
[0035] Molybdenum (Mo) is an element effective in improving
corrosion resistance in a use environment and increasing strength.
The Mo content needs to be 1% or higher in order that these effects
are sufficiently present. However, Mo is an expensive element, and
high Mo contents mean increased costs. Further, if the Mo content
is too high, the austenite microstructure becomes instable. In view
of this, the Mo content should be in the range of 1 to 4%. The
lower limit of Mo content is preferably 1.2%, and more preferably
1.5%. The lower limit of Mo content is preferably 3.8%, and more
preferably 3.5%.
[0036] Nb: 0.05 to 0.4%
[0037] Niobium (Nb) precipitates in the form of fine carbonitrides
and nitrides in the matrix and is effective in improving strength.
Further, fine carbonitrides and nitrides that have precipitated
prevent coarsening of grains in heat-affected zones during welding,
thereby reducing liquation cracking susceptibility. The Nb content
needs to be 0.05% or higher in order that these effects are
sufficiently present. However, if the Nb content is too high,
cracking susceptibility in weld heat-affected zones becomes high
and, also, large amounts of carbonitrides and nitrides precipitate,
which decreases the ductility of the material. In view of this, the
Nb content should be in the range of 0.05 to 0.4%. The lower limit
of Nb content is preferably 0.12%, and more preferably 0.15%. The
upper limit of Nb content is preferably 0.38%, and more preferably
0.35%.
[0038] Nb content as used herein means the total amount of Nb
contained in the austenitic stainless steel. That is, it means the
sum of the amount of Nb dissolved in the matrix and the amount of
Nb that is present in the form of precipitates. In the present
embodiment, in addition to Nb content, the amount of Nb that is
present in the form of precipitates, i.e. the amount of Nb analyzed
as residues after electrolytic extraction needs to be in the
specified range.
[0039] N: 0.15 to 0.50%
[0040] Nitrogen (N) dissolves in the matrix, and, together with Nb
and other elements, forms fine carbonitrides and nitrides to
contribute to increasing strength. Further, N is an element
effective in stabilizing the austenite microstructure. The N
content needs to be 0.15% or higher in order that these effects are
sufficiently present. However, if the N content is too high, hot
workability during manufacturing decreases, and excess amounts of
Nb precipitates are produced, which decreases the ductility of the
steel. In view of this, the N content should be in the range of
0.15 to 0.50%. The lower limit of N content is preferably 0.22%,
and more preferably 0.25%. The upper limit of N content is
preferably 0.48%, and more preferably 0.45%.
[0041] Al: Up to 0.05%
[0042] Similar to Si, Al (aluminum) is contained as a deoxidizer.
However, if the Al content is too high, the cleanliness of the
steel deteriorates and hot workability decreases. In view of this,
the Al content should be not higher than 0.05%. The Al content is
preferably not higher than 0.04%, and more preferably not higher
than 0.03%. Although no lower limit of Al content needs to be
provided, excessive reduction leads to increased steel-making
costs. In view of this, the lower limit of Al content is preferably
0.0005%, and more preferably 0.001%.
[0043] The balance of the chemical composition of the austenitic
stainless steel according to the present embodiment is Fe and
impurities. Impurity as used herein means an element originating
from ore or scrap used as raw material for stainless steel or an
element that has entered from the environment or the like during
the manufacturing process.
[0044] The contents of P, S and O, which are impurities, are
limited to the ranges provided below.
[0045] P: Not Higher than 0.03%
[0046] Phosphorus (P) is contained as an impurity in steel. If the
P content is too high, hot workability during manufacturing
decreases, and the liquation cracking susceptibility in weld
heat-affected zones during welding increases. The lower the P
content, the better; however, excessive reduction leads to
increased manufacture costs. In view of this, the P content should
be not higher than 0.03%. The P content is preferably not higher
than 0.025%, and more preferably not higher than 0.02%.
[0047] S: Up to 0.002%
[0048] Sulfur (S) is contained as an impurity in steel. If the S
content is too high, hot workability during manufacturing
decreases, and the ductility of the steel decreases. Further, if
the S content is too high, the liquation cracking susceptibility in
weld heat-affected zones during welding increases. The lower the S
content, the better; however, excessive reduction leads to
increased manufacture costs. In view of this, the S content should
be not higher than 0.002%. The S content is preferably not higher
than 0.0018%, and more preferably not higher than 0.0015%.
[0049] O: Up to 0.02%
[0050] Oxygen (O) is contained as an impurity in steel. If the O
content is too high, hot workability during manufacturing
decreases, and the cleanliness of the steel deteriorates and
ductility decreases. In view of this, the O content should be not
higher than 0.02%. The O content is preferably not higher than
0.015%, and more preferably not higher than 0.01%. Although no
lower limit of O content needs to be provided, excessive reduction
leads to increased steel-making costs. In view of this, the lower
limit of O content is preferably 0.001%, and more preferably
0.002%.
[0051] In the chemical composition of the austenitic stainless
steel according to the present embodiment, some of the Fe may be
replaced by one or more elements selected from V, Ti, B, Ca, Mg and
REM. V, Ti, B, Ca, Mg and REM are optional elements. That is, the
chemical composition of the austenitic stainless steel according to
the present embodiment may contain only some or none of V, Ti, B,
Ca, Mg and REM.
[0052] V: 0 to 0.5%
[0053] Similar to Nb, vanadium (V) precipitates in the form of
carbonitrides and increases the strength of the steel. This effect
is present if a small amount of V is contained. On the other hand,
if the V content is too high, excessive amounts of carbonitrides
precipitate, which decreases the ductility of the steel. In view of
this, the V content should be in the range of 0 to 0.5%. The lower
limit of V content is preferably 0.001%, and more preferably
0.005%, and still more preferably 0.01%. The upper limit of V
content is preferably 0.45%, and more preferably 0.40%.
[0054] Ti: 0 to 0.5%
[0055] Similar to V and Nb, titanium (Ti) precipitates in the form
of carbonitrides and increases the strength of the steel. This
effect is present if a small amount of Ti is contained. On the
other hand, if the Ti content is too high, excessive amounts of
carbonitrides precipitate, which decreases the ductility of the
steel. In view of this, the Ti content should be in the range of 0
to 0.5%. The lower limit of Ti content is preferably 0.001%, and
more preferably 0.003%, and still more preferably 0.005%. The upper
limit of Ti content is preferably 0.45%, and more preferably
0.40%.
[0056] B: 0 to 0.01%
[0057] Boron (B) segregates along grain boundaries and increases
the fixing force at grain boundaries to contribute to increasing
strength, and also improves ductility. B also reduces embrittlement
in a hydrogen environment. These effects are present if a small
amount of B is contained. On the other hand, if the B content is
too high, the liquation cracking susceptibility in weld
heat-affected zones increases. In view of this, the B content
should be in the range of 0 to 0.01%. The lower limit of B content
is preferably 0.0001%, and more preferably 0.0002%, and still more
preferably 0.0005%. The upper limit of B content is preferably
0.008%, and more preferably 0.005%.
[0058] Ca: 0 to 0.05%
[0059] Calcium (Ca) improves the hot workability of steel. This
effect is present if a small amount of Ca is contained. On the
other hand, if the Ca content is too high, Ca combines with O such
that the cleanliness of the steel deteriorates and the hot
workability decreases. In view of this, the Ca content should be in
the range of 0 to 0.05%. The lower limit of Ca content is
preferably 0.0001%, and more preferably 0.0005%, and still more
preferably 0.001%. The upper limit of Ca content is preferably
0.03%, and more preferably 0.01%.
[0060] Mg: 0 to 0.05%
[0061] Similar to Ca, magnesium (Mg) improves the hot workability
of steel. This effect is present if a small amount of Mg is
contained. On the other hand, if the Mg content is too high, Mg
combines with O such that the cleanliness of the steel deteriorates
and the hot workability decreases. In view of this, the Mg content
should be in the range of 0 to 0.05%. The lower limit of Mg content
is preferably 0.0001%, and more preferably 0.0005%, and still more
preferably 0.001%. The upper limit of Mg content is preferably
0.03%, and more preferably 0.01%.
[0062] REM: 0 to 0.5%
[0063] Rare-earth metals (REMs) have a strong affinity with S and
improves the hot workability of steel. This effect is present if a
small amount of REM is contained. On the other hand, if the REM
content is too high, REM combines with O such that the cleanliness
of the steel deteriorates and the hot workability decreases. In
view of this, the REM content should be in the range of 0 to 0.5%.
The lower limit of REM content is preferably 0.001%, and more
preferably 0.002%, and still more preferably 0.005%. The upper
limit of REM content is preferably 0.3%, and more preferably
0.1%.
[0064] "REM" is a collective term for the total of 17 elements: Sc,
Y and the lanthanoids, and REM content refers to the total content
of one or more REM elements. REM is typically contained in misch
metal. Thus, for example, misch metal may be added to an alloy to
adjust the REM content to be in the above-provided range.
[0065] [Amount of Nb Analyzed as Residues after Electrolytic
Extraction]
[0066] In the austenitic stainless steel according to the present
embodiment, the amount of Nb analyzed as residues after
electrolytic extraction is in the range of 0.01 to 0.3 mass %.
[0067] Nb contained in the raw material precipitates in the form of
fine carbonitrides and nitrides during the process of solution heat
treatment. Fine carbonitrides and/or nitrides of Nb that have
precipitated improve the strength of the steel, and, during
welding, contributes to preventing coarsening of grains in weld
heat-affected zones to reduce liquation cracking susceptibility. To
produce these effects, the amount of Nb that has precipitated in
the form of carbonitrides and/or nitrides, i.e. the amount of Nb
analyzed as residues after electrolytic extraction needs to be 0.01
mass % or higher. However, if the amount of Nb analyzed as residues
after electrolytic extraction is in excess, the ductility of the
steel decreases. In view of this, the amount of Nb analyzed as
residues after electrolytic extraction should be in the range of
0.01 to 0.3 mass %. The lower limit of the amount of Nb analyzed as
residues after electrolytic extraction is preferably 0.02 mass %,
and more preferably 0.03 mass %. The upper limit of the amount of
Nb analyzed as residues after electrolytic extraction is preferably
0.28 mass %, and more preferably 0.25 mass %.
[0068] The amount of Nb analyzed as residues after electrolytic
extraction may be adjusted by adjusting the Nb content and N
content in the raw material as well as the conditions of the
solution heat treatment. More specifically, the higher the Nb and N
contents in the raw material, the higher the amount of Nb analyzed
as residues after electrolytic extraction. The lower the
temperature for the solution heat treatment and/or the longer the
hold time, the higher the amount of Nb analyzed as residues after
electrolytic extraction. However, if the temperature for solution
heat treatment is low and/or the hold time is short, the amounts of
carbonitrides and/or nitrides of Nb that have been produced in the
steps preceding the solution heat treatment, such as hot working,
and the solution heat treatment itself, and that dissolve during
the solution heat treatment are not sufficient, in which case, too,
the amount of Nb analyzed as residues after electrolytic extraction
is high. Further, during the cooling in the solution heat
treatment, the lower the cooling rate in the temperature range of
1100 to 600.degree. C., where carbonitrides and/or nitrides of Nb
precipitate, the higher the amount of Nb analyzed as residues after
electrolytic extraction.
[0069] The amount of Nb analyzed as residues after electrolytic
extraction is measured in the following manner.
[0070] From an austenitic stainless steel, a test material with a
predetermined size is obtained. With constant-current electrolysis
using, as the electrolyte, 10 volume % acetylacetone-1 mass %
tetramethyl ammonium chloride methanol solution, the test material
is subjected to anodic dissolution at a current density of 20 to 25
mA/cm.sup.2, and carbonitrides and nitrides in the residue are
extracted. The extracted residue is subjected to acid decomposition
and then ICP (high-frequency inductively coupled plasma) emission
analysis is performed to measure the mass of Nb in the residue. The
mass of Nb in the residue is divided by the amount of dissolution
of the test material to determine the amount of Nb present in the
form of carbonitrides and/or nitrides, that is, the amount of Nb
analyzed as residues after electrolytic extraction.
[0071] [Manufacturing Method]
[0072] A method of manufacturing an austenitic stainless steel
according to an embodiment of the present invention will be
described below. The method for the austenitic stainless steel
according to the present embodiment includes the steps of:
preparing a raw material; hot working the raw material; and
performing solution heat treatment on the hot-worked raw
material.
[0073] First, a raw material with the above-listed chemical
composition is prepared. More specifically, for example, a steel
with the above-listed chemical composition is smelted and
refined.
[0074] The raw material is hot worked. The hot working may be, for
example, hot rolling or hot forging.
[0075] The hot-worked raw material is subjected to solution heat
treatment. More specifically, the raw material is held at a
predetermined solution heat treatment temperature for a
predetermined solution heat treatment time before being cooled.
Thus, coarse carbonitrides and/or nitrides of Nb that have
precipitated during the hot working and other steps are dissolved,
and, during the process of cooling, are precipitated again in the
form of fine carbonitrides and/or nitrides. The fine carbonitrides
and/or nitrides of Nb that have precipitated contribute to
improving the strength and ductility of the steel.
[0076] The solution heat treatment temperature is preferably in the
range of 950 to 1300.degree. C. If the solution heat treatment
temperature is lower than 950.degree. C., the amounts of
carbonitrides and/or nitrides of Nb that have precipitated during
the hot working and that dissolve during the solution heat
treatment are not sufficient and the amount of Nb analyzed as
residues after electrolytic extraction may not be 0.3 mass % or
lower. On the other hand, if the solution heat treatment
temperature exceeds 1300.degree. C., grains become coarse and some
grain boundaries may begin to melt.
[0077] The cooling for the solution heat treatment is preferably
water cooling. In the cooling after the solution heat treatment,
the lower the cooling rate in the temperature range of 1100 to
600.degree. C. in which carbonitrides and/or nitrides of Nb
precipitate, the larger the amount of Nb analyzed as residues after
electrolytic extraction becomes. The cooling rate in this
temperature range is preferably not lower than 0.5.degree. C./sec.,
and more preferably not lower than 1.degree. C./sec.
[0078] The solution heat treatment is preferably performed under a
condition that satisfies the following formula, (1).
40.times.[%
Nb]+100.ltoreq.T.times.log(1.2+t/60).ltoreq.-200.times.[% Nb]+700
(1)
[0079] In formula (1), the Nb content in the raw material in mass %
is substituted for [% Nb], the solution heat treatment temperature
in .degree. C. is substituted for T, and the solution heat
treatment time in minutes is substituted for t. log (x) is the
common logarithm of x.
[0080] If T.times.log(1.2+t/60) is smaller than 40.times.[%
Nb]+100, the amounts of coarse carbonitrides and/or nitrides of Nb
that dissolve are not sufficient such that the amounts of fine
carbonitrides and/or nitrides that precipitate in the process
including the cooling are not sufficient. This decreases the
improvement in strength and ductility. This is because the higher
the Nb content in the raw material, the higher the temperature
and/or the longer the time required to dissolve coarse
carbonitrides and/or nitrides of Nb. Thus, it is preferable that
the higher the Nb content in the raw material, the larger the value
of T.times.log(1.2+t/60).
[0081] On the other hand, if T.times.log(1.2+t/60) exceeds
-200.times.[% Nb]+700, grains significantly coarsen, which
increases the liquation cracking susceptibility during welding.
Since Nb is an element that increases liquation cracking
susceptibility, it is preferable that the higher the Nb content in
the raw material, the smaller the value of
T.times.log(1.2+t/60).
[0082] In the method of manufacturing an austenitic stainless steel
according to the present embodiment, it is preferable that cold
working is not performed between the hot working and the solution
heat treatment. This is because performing cold working would cause
distortion-induced precipitates to be produced during the
temperature increase of the solution heat treatment, which would
require a higher temperature or a longer time for the solution heat
treatment.
[0083] An embodiment of the present invention has been described.
The present embodiment provides an austenitic stainless steel with
improved strength, ductility and weldability.
[0084] The above-described embodiments are merely examples for
carrying out the present invention. Thus, the present invention is
not limited to the above-described embodiments, and the
above-described embodiments may be modified as appropriate without
departing from the spirit of the present invention.
Examples
[0085] The present invention will now be described more
specifically with the help of examples. The present invention is
not limited to these examples.
[0086] Raw materials for Steel Types A to F having the chemical
compositions shown in Table 1 were melted in a laboratory and cast
into ingots, which were subjected to hot forging and hot rolling to
produce plates with a plate thickness of 14 mm. Thereafter,
solution heat treatment was performed with varied temperatures and
times. The cooling after the solution heat treatment was water
cooling. The plates that have been subjected to the solution heat
treatment were machined into a plate thickness of 12 mm to provide
samples. "-" in Table 1 means that the content of the relevant
element was at an impurity level.
TABLE-US-00001 TABLE 1 Steel Chemical composition (in mass %,
balance Fe and impurities) Cu + type C Si Mn P S Ni Cu Co Cr Mo Nb
Al N O other Co A 0.03 0.30 4.55 0.017 0.0010 12.43 0.06 -- 21.65
2.11 0.20 0.009 0.33 0.002 0.06 B 0.02 0.38 4.68 0.015 0.0015 11.89
-- 0.02 22.00 2.06 0.39 0.014 0.27 0.002 V: 0.22, 0.02 B: 0.002,
Ca: 0.001 C 0.03 0.42 5.05 0.015 0.0008 11.95 0.51 0.49 21.10 2.14
0.07 0.008 0.42 0.005 Ti: 0.004, 1.00 Mg: 0.001, REM: 0.002 D 0.03
0.32 5.65 0.025 0.0018 13.26 0.05 0.06 20.42 1.85 0.42* 0.010 0.28
0.006 Ca: 0.003 0.11 E 0.04 0.48 6.20 0.002 0.0019 11.95 1.48 1.96
22.24 1.98 0.22 0.018 0.44 0.006 Ti: 0.34 3.44* F 0.02 0.28 3.85
0.015 0.0012 12.01 0.03 -- 20.58 1.96 --* 0.010 0.19 0.003 0.03
*indicates that the relevant value is outside the range specified
by the present invention.
[0087] [Residue Analysis]
[0088] From the samples were obtained test materials with a width
and height of 10 mm and a length of 50 mm, and the amount of Nb
analyzed as residues after electrolytic extraction was measured by
the method described in connection with the above-described
embodiment.
[0089] [Tensile Test]
[0090] From the samples were obtained No. 14A round-bar test pieces
indicated in JIS Z2201 (2013) with a parallel-portion diameter of 8
mm and a parallel-portion length of 55 mm, and tensile testing was
conducted at room temperature. A test piece with a tensile strength
of 690 MPa or higher, which is required from hydrogen equipment,
was determined to have passed the test. A test piece with a tensile
strength of 800 MPa or higher was determined to have a particularly
good tensile strength. Regarding ductility, a test piece with a
breaking elongation of 35% or higher during the tensile testing was
determined to have passed the test. A test piece with a breaking
elongation of 40% or higher was determined to have a particularly
good ductility.
[0091] [Low-Strain-Rate Tensile Test]
[0092] Low-strain-rate tensile testing was conducted on the samples
that have passed the tensile test, in order to evaluate the
hydrogen embrittlement resistance in a high-pressure hydrogen
environment. More specifically, plate-shaped low-strain-rate
tensile test pieces were obtained from the samples, and
low-strain-rate tensile testing was conducted in the atmosphere and
in a high-pressure hydrogen environment at 45 MPa. The strain rate
was 3.times.10.sup.-5/sec. A test piece in which the value of the
reduction of area due to the break test in the high-pressure
hydrogen environment was 90% or more of the value of the reduction
of area due to the break test in the atmosphere was determined to
have passed the test.
[0093] [Weld Test]
[0094] A test for evaluating weldability was conducted on the
samples that have passed the tensile test and the low-strain-rate
tensile test. More specifically, steel plates with a width of 50 mm
and a length of 100 mm were prepared and, in a cross section along
the longitudinal direction of each plate, a V groove was formed
with an edge angle of 30.degree. and a root thickness of 1 mm. The
four sides of each of these steel plates were restraint-welded on
an SM400B steel plate specified by JIS G 3106 (2008) with a
thickness of 25 mm, a width of 200 mm and a length of 200 mm, using
a covered arc-welding rod Eni6625 specified by JIS Z 3224 (2010).
Thereafter, a filler wire corresponding to SNi 6082 specified in
JIS Z 3334 (2011) was used to perform laminated welding in the
groove at a heat input of 10 to 15 kJ/cm to produce a welded
joint.
[0095] Specimens were obtained from five locations in each produced
welded joint, where the observed surface was represented by a
transverse surface of the joint (i.e. cross section perpendicular
to the weld bead). Each of the obtained specimens were polished and
etched before being observed by optical microscopy to determine
whether cracks were present in the weld heat-affected zones. A
joint in which the five specimens included one or fewer specimens
with cracks found was determined to have passed the test. A joint
in which no cracks were found in any of the specimens was
determined to have particularly good weldability.
[0096] The solution heat treatment conditions and the results of
the tests are shown in Table 2.
TABLE-US-00002 TABLE 2 Nb Residue amount Nb Tensile test Low- Steel
(mass T t amount Tensile strain-rate Weld Mark type %) (.degree.
C.) (min.) fn1 fn2 fn3 (mass %) strength Elongation tensile test
test A1 A 0.20 950 60 108 325.3 660 0.12 excellent excellent passed
excellent A2 A 0.20 1050 3 108 101.8# 660 0.14 excellent good
passed excellent A3 A 0.20 1050 5 108 113.8 660 0.12 excellent
excellent passed excellent A4 A 0.20 1050 10 108 142.4 660 0.11
excellent excellent passed excellent A5 A 0.20 1050 60 108 359.5
660 0.10 excellent excellent passed excellent A6 A 0.20 1050 90 108
452.9 660 0.08 excellent excellent passed excellent A7 A 0.20 1050
120 108 530.4 660 0.11 excellent excellent passed excellent A8 A
0.20 1050 180 108 654.4 660 0.14 excellent excellent passed
excellent A9 A 0.20 1050 210 108 705.7# 660 0.16 excellent
excellent passed good A10 A 0.20 1220 1 108 103.9# 660 0.10
excellent good passed excellent A11 A 0.20 1220 3 108 118.2 660
0.08 excellent excellent passed excellent A12 A 0.20 1220 5 108
132.2 660 0.07 excellent excellent passed excellent A13 A 0.20 1220
10 108 165.5 660 0.07 excellent excellent passed excellent A14 A
0.20 1220 60 108 417.8 660 0.06 excellent excellent passed
excellent A15 A 0.20 1220 90 108 526.3 660 0.06 excellent excellent
passed excellent A16 A 0.20 1220 120 108 616.3 660 0.07 excellent
excellent passed excellent A17 A 0.20 1220 180 108 760.4# 660 0.10
excellent excellent passed good A18 A 0.20 1250 60 108 428.0 660
0.05 excellent excellent passed excellent A19 A 0.20 1300 60 108
445.1 660 0.04 excellent excellent passed excellent B1 B 0.39 950
60 115.6 325.3 622 0.28 excellent excellent passed excellent B2 B
0.39 1050 5 115.6 113.8# 622 0.24 excellent good passed excellent
B3 B 0.39 1050 10 115.6 142.4 622 0.22 excellent excellent passed
excellent B4 B 0.39 1050 60 115.6 359.5 622 0.25 excellent
excellent passed excellent B5 B 0.39 1050 120 115.6 530.4 622 0.29
excellent excellent passed excellent B6 B 0.39 1050 180 115.6
654.4# 622 0.26 excellent excellent passed good B7 B 0.39 920 210
115.6 618.3 622 0.31* excellent unacceptable -- -- B8 B 0.39 1220 5
115.6 132.2 622 0.19 excellent excellent passed excellent B9 B 0.39
1220 10 115.6 165.5 622 0.16 excellent excellent passed excellent
B10 B 0.39 1220 60 115.6 417.8 622 0.14 excellent excellent passed
excellent B11 B 0.39 1220 120 115.6 616.3 622 0.18 excellent
excellent passed excellent B12 B 0.39 1220 180 115.6 760.4# 622
0.20 excellent excellent passed good B13 B 0.39 1300 60 115.6 445.1
622 0.09 excellent excellent passed excellent C1 C 0.07 950 60
102.8 325.3 686 0.05 excellent excellent passed excellent C2 C 0.07
1050 5 102.8 113.8 686 0.07 excellent excellent passed excellent C3
C 0.07 1050 10 102.8 142.4 686 0.06 excellent excellent passed
excellent C4 C 0.07 1050 60 102.8 359.5 686 0.02 excellent
excellent passed excellent C5 C 0.07 1050 120 102.8 530.4 686 0.04
excellent excellent passed excellent C6 C 0.07 1050 180 102.8 654.4
686 0.06 excellent excellent passed excellent C7 C 0.07 1050 210
102.8 705.7# 686 0.06 excellent excellent passed good C8 C 0.07
1220 5 102.8 132.2 686 0.06 excellent excellent passed excellent C9
C 0.07 1220 10 102.8 165.5 686 0.04 excellent excellent passed
excellent C10 C 0.07 1220 60 102.8 417.8 686 0.01 excellent
excellent passed excellent C11 C 0.07 1220 120 102.8 616.3 686 0.04
excellent excellent passed excellent C12 C 0.07 1220 180 102.8
760.4# 686 0.04 excellent excellent passed good C13 C 0.07 1350 60
102.8 462.3 686 0.004* unacceptable excellent -- -- D1 D* 0.42*
1050 10 116.8 142.4 616 0.28 excellent excellent passed
unacceptable D2 D* 0.42* 1220 10 116.8 165.5 616 0.18 excellent
excellent passed unacceptable E1 E* 0.22 1050 10 108.8 142.4 656
0.18 excellent unacceptable -- -- E2 E* 0.22 1220 10 108.8 165.5
656 0.11 excellent unacceptable -- -- F1 F* --* 1050 10 100.0 142.4
700 <0.001 unacceptable excellent -- -- F2 F* --* 1220 10 100.0
165.5 700 <0.001 unacceptable excellent -- -- fn1 = 40 .times.
[% Nb] + 100, fn2 = T .times. log(1.2 + t/60), fn3 = -200 .times.
[% Nb] + 700 *indicates that the relevant value is outside the
range specified by the present invention. #indicates that the
relevant value is outside the preferred range of the present
invention.
[0097] In Table 2, "Nb amount (mass %)" indicates the Nb content in
the raw material. "T (.degree. C.)" and "t (min.)" indicate
solution heat treatment temperature and solution-temperature time,
respectively. fn1, fn2 and fn3 indicate the left side, middle side
and right side, respectively, of formula (1). "Residue Nb amount
(mass %)" indicates the amount of Nb analyzed as residues after
electrolytic extraction.
[0098] The columns labeled "Tensile test" list the results of
tensile testing. In the column labeled "Tensile strength",
"excellent" means that the tensile strength of the relevant sample
was not lower than 800 MPa, while "unacceptable" means that the
value was lower than 690 MPa. In the column labeled "Elongation",
"excellent" means that the breaking elongation of the relevant
sample was not less than 40%, "good" means that the value was not
less than 35% and less than 40%, and "unacceptable" means that the
value was less than 35%.
[0099] The column labeled "Low-strain-rate tensile test" lists the
results of the low-strain-rate tensile testing. In this column,
"passed" means that, in the sample, the reduction of area due to
the break test in the high-pressure hydrogen environment was 90% or
more of the reduction of area due to the break test in the
atmosphere.
[0100] The column labeled "Weld test" lists the results of the weld
testing. In this column, "excellent" means that no cracks were
found in any of the five specimens, "good" means that cracks were
found in only one of the specimens, and "unacceptable" means that
cracks were found in two or more of the specimens.
[0101] "-" in the columns with "Low-strain-rate tensile test" and
"Weld test" means that the relevant test was not conducted.
[0102] As shown in Table 2, the samples with Marks A1 to A19, B1 to
B6, B8 to B13 and C1 to C12 passed all of the tensile test,
low-strain-rate tensile test and weld test. Particularly, the
samples with Marks A1, A3 to A8, A11 to A16, A18, A19, B1, B3 to
B5, B8 to B11, B13, C1 to C6 and C8 to C11 had excellent results in
both the tensile and weld tests. More specifically, in each sample,
the tensile strength was not lower than 800 MPa, the breaking
elongation was not less than 40%, and no cracks were found in any
of the five specimens.
[0103] The samples with Marks A2, A10 and B2 each had a breaking
elongation not less than 35% and less than 40% and a ductility
slightly inferior to Mark A1, for example. This is presumably
because the value of T.times.log(1.2+t/60) was too low relative to
the Nb content in the raw material and the amounts of coarse
carbonitrides and/or nitrides of Nb that dissolve were not
sufficient.
[0104] In each of the samples with Marks A9, A17, B6, B12, C7 and
C12, cracks were found in only one of the five specimens, which
means a weldability slightly inferior to Mark A, for example. This
is presumably because the value of T.times.log(1.2+t/60) was too
high relative to the Nb content and the increased grain size led to
increased liquation cracking susceptibility.
[0105] Mark B7 had a sufficient tensile strength, but had a
breaking elongation less than 35%. This is presumably because the
amount of Nb analyzed as residues after electrolytic extraction was
too large. The amount of Nb analyzed as residues after electrolytic
extraction was too large presumably because the temperature for the
solution heat treatment was low, which led to a lower temperature
at which the cooling started such that the rate of cooling during
the passage through the temperature range in which precipitates
were produced was too low, resulting in excess amounts of
precipitates.
[0106] Mark C13 had a tensile strength lower than 690 MPa. This is
presumably because the amount of Nb analyzed as residues after
electrolytic extraction was too low. The amount of Nb analyzed as
residues after electrolytic extraction was too low presumably
because the temperature for the solution heat treatment was high,
which means that the temperature at which the cooling started was
also high such that the rate of cooling during the passage through
the temperature range in which precipitates were produced was too
high, preventing the production of precipitates.
[0107] In each of Marks D1 and D2, the tensile strength and
ductility were sufficient but cracks were found in two or more of
the five specimens. This is presumably because the Nb content in
Steel Type D was too high, which increased liquation cracking
susceptibility.
[0108] In each of Marks E1 and E2, the tensile strength was
sufficient but the breaking elongation was less than 35%. This is
presumably because the total content of Cu and Co of Steel Type E
was too high.
[0109] Marks F1 and F2 contained no Nb and thus each had a tensile
strength lower than 690 MPa.
[0110] This shows that the present invention provides an austenitic
stainless steel with improved strength, ductility and
weldability.
INDUSTRIAL APPLICABILITY
[0111] The present invention provides an austenitic stainless steel
with improved strength, ductility and weldability. Thus, the
present invention can be suitably used in various steels in
high-pressure hydrogen-gas equipment or liquid-hydrogen storage
tanks.
* * * * *