U.S. patent application number 16/090306 was filed with the patent office on 2019-04-18 for austenitic stainless steel material.
The applicant listed for this patent is Nippon Steel & Sumitomo Metal Corporation. Invention is credited to Jun Nakamura, Takamitsu Takagi, Masaaki Terunuma, Masaki Ueyama.
Application Number | 20190112694 16/090306 |
Document ID | / |
Family ID | 60001254 |
Filed Date | 2019-04-18 |
United States Patent
Application |
20190112694 |
Kind Code |
A1 |
Takagi; Takamitsu ; et
al. |
April 18, 2019 |
Austenitic Stainless Steel Material
Abstract
There is provided an austenitic stainless steel material having
a consistent high-strength across the overall length of the steel
material, which has a chemical composition consisting of, in mass
percent: C: 0.10% or less, Si: 1.0% or less, Mn: 3 to 8%, P: 0.05%
or less, S: 0.03% or less, Ni: 10 to 20%, Cr: 15 to 30%, N: 0.20 to
0.70%, with the balance being Fe and impurities, the austenitic
stainless steel material having a grain size number of 6.0 or
greater, the grain size number conforming to ASTM E 112 tensile
strength of the austenitic stainless steel material is 800 MPa or
more, and the difference between the maximum value and the minimum
value of the tensile strength is 50 MPa or smaller. The number of
alloy carbo-nitrides having a circle equivalent diameter of larger
than 1000 nm in the steel is 10/mm.sup.2 or more.
Inventors: |
Takagi; Takamitsu;
(Chiyoda-ku, Tokyo, JP) ; Nakamura; Jun;
(Chiyoda-ku, Tokyo, JP) ; Ueyama; Masaki;
(Chiyoda-ku, Tokyo, JP) ; Terunuma; Masaaki;
(Chiyoda-ku, Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nippon Steel & Sumitomo Metal Corporation |
Tokyo |
|
JP |
|
|
Family ID: |
60001254 |
Appl. No.: |
16/090306 |
Filed: |
April 4, 2017 |
PCT Filed: |
April 4, 2017 |
PCT NO: |
PCT/JP2017/014008 |
371 Date: |
October 1, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 6/004 20130101;
C22C 38/46 20130101; C21D 9/525 20130101; C21D 9/08 20130101; C22C
38/02 20130101; C21D 8/00 20130101; C21D 2211/001 20130101; C21D
6/02 20130101; C21D 8/105 20130101; C22C 38/44 20130101; C22C
38/002 20130101; C22C 38/04 20130101; C22C 38/00 20130101; C22C
38/001 20130101; C22C 38/48 20130101; C22C 38/58 20130101 |
International
Class: |
C22C 38/48 20060101
C22C038/48; C22C 38/46 20060101 C22C038/46; C22C 38/44 20060101
C22C038/44; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C21D 9/08 20060101
C21D009/08; C21D 8/10 20060101 C21D008/10; C21D 6/00 20060101
C21D006/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 7, 2016 |
JP |
2016-077628 |
Claims
1. An austenitic stainless steel material comprising: a chemical
composition consisting of, in mass percent: C: 0.10% or less; Si:
1.0% or less; Mn: 3 to 8%; P: 0.05% or less; S: 0.03% or less; Ni:
10 to 20%; Cr: 15 to 30%; N: 0.20 to 0.70%; Mo: 0 to 5.0%; V: 0 to
0.5%; and Nb: 0 to 0.5%, with the balance being Fe and impurities,
wherein a grain size number conforming to ASTM E 112 is 6.0 or
greater, a tensile strength is 800 MPa or more, a difference
between a maximum value and a minimum value of the tensile strength
is 50 MPa or less, and a number of alloy carbo-nitrides with circle
equivalent diameters of larger than 1000 nm in steel is 10/mm.sup.2
or more.
2. The austenitic stainless steel material according to claim 1,
wherein the chemical composition contains one, or two or more
elements selected from the group consisting of: Mo: 1.5 to 5.0%; V:
0.1 to 0.5%; and Nb: 0.1 to 0.5%.
3. The austenitic stainless steel material according to claim 1,
wherein a difference between a maximum value and a minimum value of
the grain size number is 1.5 or smaller.
4. The austenitic stainless steel material according to claim 1,
wherein the austenitic stainless steel material is one of a steel
pipe, a steel bar, and a wire rod.
5. The austenitic stainless steel material according to claim 2,
wherein a difference between a maximum value and a minimum value of
the grain size number is 1.5 or smaller.
6. The austenitic stainless steel material according to claim 2,
wherein the austenitic stainless steel material is one of a steel
pipe, a steel bar, and a wire rod.
7. The austenitic stainless steel material according to claim 3,
wherein the austenitic stainless steel material is one of a steel
pipe, a steel bar, and a wire rod.
8. The austenitic stainless steel material according to claim 4,
wherein the austenitic stainless steel material is one of a steel
pipe, a steel bar, and a wire rod.
Description
TECHNICAL FIELD
[0001] The present invention relates to a stainless steel material
more specifically to an austenitic stainless steel material.
BACKGROUND ART
[0002] Recent years have seen the progress of lively studies for
the practical application of transportation equipment that utilizes
hydrogen in place of fossil fuels as energy. For example, fuel cell
powered vehicles that run on hydrogen as fuel, and hydrogen
stations where fuel cell powered vehicles are supplied with
hydrogen, have been under development.
[0003] When stainless steel is used for hydrogen stations, the
stainless steel is placed in a high-pressure hydrogen gas
environment. For this reason, an excellent strength is demanded of
a stainless steel used for hydrogen stations.
[0004] International Application Publication No. WO 2012/132992
(Patent Literature 1), International Application Publication No. WO
2004/083476 (Patent Literature 2), International Application
Publication No. WO 2004/083477 (Patent Literature 3), and
International Application Publication No. WO 2004/111285 (Patent
Literature 4) propose stainless steels that are used in
high-pressure hydrogen environments and have high strengths.
[0005] The austenitic stainless steel for high-pressure hydrogen
gas disclosed in Patent Literature 1 contains, in mass percent. C:
0.10% or less, Si: 1.0% or less, Mn: 3% or more and less than 7%,
Cr: 15 to 30%, Ni: 10% or more and less than 17%, Al: 0.10% or
less, N: 0.10 to 0.50%, and at least one of V: 0.01 to 1.0% and Nb:
0.01 to 0.50%, with the balance being Fe and impurities, of which
impurities P accounts for 0.0050% or less, and S accounts for
0.050% or less, wherein the austenitic stainless steel has a
tensile strength of 800 MPa or more, a grain size number (ASTM E
112) is 8 or greater, and the austenitic stainless steel contains
an alloy carbo-nitride with a maximum diameter of 50 to 1000 nm at
0.4/.mu.m.sup.2 or more in cross section observation.
[0006] The stainless steel for hydrogen gas disclosed in Patent
Literature 2 contains, in mass percent, C: 0.02% or less, Si: 1.0%
or less, Mn: 3 to 30.degree. %, Cr: larger than 22% to 30%, Ni: 17
to 30%, V: 0.001 to 1.0%, N: 0.10 to 0.50%/6, and Al: 0.10% or
less, with the balance being Fe and impurities, of which impurities
P accounts for 0.030% or less, S accounts for 0.005% or less, and
Ti, Zr, and Hf each account for 0.01% or less, wherein the contents
of Cr, Mn, and N satisfies 5Cr+3.4Mn.ltoreq.500N.
[0007] The stainless steel for high-pressure hydrogen gas disclosed
in Patent Literature 3 contains, in mass percent, C: 0.04% or less.
Si: 1.0% or less, Mn: 7 to 30%, Cr: 15 to 22%, Ni: 5 to 20%, V:
0.001 to 1.0%, N: 0.20 to 0.50%, and Al: 0.10% or less, with the
balance being Fe and impurities, of which impurities P accounts for
0.030% or less, S accounts for 0.005% or less, and Ti, Zr, and Hf
each account for 0.01% or less, wherein the contents of Cr, Mn, and
N satisfies 2.5Cr+3.4Mn.ltoreq.300N.
[0008] The austenitic stainless steel for hydrogen gas disclosed in
Patent Literature 4 has a chemical composition containing, in mass
percent, C: 0.10% or less, Si: 1.0% or less, Mn: 0.01 to 30%, P:
0.040% or less, S: 0.01% or less, Cr: 15 to 30%, Ni: 5.0 to 30%,
sol. Al: 0.10% or less, and N: 0.001 to 0.30%, with the balance
being Fe and impurities, wherein the austenitic stainless steel
includes a micro-structure in which an X-ray integrated intensity I
(111) on a cross section along a direction perpendicular to a
processing direction is five times or less that in a random
orientation, and an X-ray integrated intensity I (220) on a cross
section along the processing direction satisfies
I(220)/I(111).ltoreq.10.
CITATION LIST
Patent Literature
[0009] Patent Literature 1: International Application Publication
No. WO 2012/132992 [0010] Patent Literature 2: International
Application Publication No. WO 2004/083476 [0011] Patent Literature
3: International Application Publication No. WO 2004/083477 [0012]
Patent Literature 4: International Application Publication No. WO
2004/111285
SUMMARY OF INVENTION
Technical Problem
[0013] Now, a stainless steel to be used for hydrogen stations is
required to have not only an excellent strength but also suppressed
variation in strength. Stainless steels disclosed in Patent
Literature 1 to Patent Literature 4 mentioned above have strengths
of 700 MPa or more even after solution treatment is performed, and
the stainless steel of Patent Literature 4 has a high strength by
being subjected to solution treatment and cold working. However,
these Patent Literatures give no consideration to variation in
strength. Even the stainless steels described in Patent Literature
1 to Patent Literature 4 mentioned above may show large variations
in strength, failing to provide consistent high-strengths.
[0014] An objective of the present invention is to provide an
austenitic stainless steel material having a consistent
high-strength across the overall length of the steel material.
Solution to Problem
[0015] An austenitic stainless steel material according to the
present embodiment has a chemical composition consisting of, in
mass percent, C: 0.10% or less, Si: 1.0% or less, Mn: 3 to 8%, P:
0.05% or less, S: 0.03% or less, Ni: 10 to 20%, Cr: 15 to 30%, N:
0.20 to 0.70%, Mo: 0 to 5.0%, V: 0 to 0.5%, and Nb: 0 to 0.5%, with
the balance being Fe and impurities, the austenitic stainless steel
material having a grain size number of 6.0 or greater, the grain
size number conforming to ASTM E 112. The tensile strength of the
austenitic stainless steel material is 800 MPa or more, and the
difference between the maximum value and the minimum value of the
tensile strength is 50 MPa or less. The number of alloy
carbo-nitrides having a circle equivalent diameter of larger than
1000 nm in the steel is 10/mm.sup.2 or more.
Advantageous Effects of Invention
[0016] The austenitic stainless steel material according to the
present embodiment has a consistent high-strength across the
overall length of the steel material.
DESCRIPTION OF EMBODIMENTS
[0017] The present inventors conducted investigations and studies
on the high strengthening of an austenitic stainless steel material
and variation in strength across the overall length of the steel
material, and obtained the following findings.
[0018] (A) Methods for increasing the strength include
solid-solution strengthening using N and grain refinement. The
austenitic stainless steel of the present embodiment contains 0.20
to 0.70% of N, and the strength thereof is increased through the
solid-solution strengthening. The strength is further increased by
grain refining on grains.
[0019] (B) The variation in strength across the overall length of
the steel material is attributable to the grain size of the steel
material. With a smaller variation in grain size in the steel
material, the variation in strength can be reduced. Specifically,
when the grain size number based on the ASTM E 112 is 6.0 or
greater, and the difference between the maximum value and the
minimum value of the grain size number (hereafter, referred to as a
grain size difference .DELTA.GS) across the overall length of the
steel material is 1.5 or smaller, the difference between the
maximum value and the minimum value of the tensile strength
(hereafter, referred to as a strength difference .DELTA.TS) across
the overall length of the steel material becomes 50 MPa or less,
which enables the variation in strength to be suppressed
sufficiently.
[0020] (C) To suppress the variation in strength, controlling
temperature variation in a starting material in the hot working is
effective. Variation in grain size is brought most remarkably in
the hot working. In the starting material, the introduction amount
of strain differs between a portion at a lower temperature and a
portion at a higher temperature. The difference in the introduction
amount of strain causes the difference in how grains are refined in
recrystallization. As a result, the variation in grain size is
increased. Therefore, it is preferable that the temperature
variation in a starting material in hot working is small.
[0021] Specifically, in the starting material, if the difference
between the temperature of a portion for which the hot working is
first completed, at the completion of the working (hereafter,
referred to as an initial temperature) and the temperature of a
portion for which the hot working is last completed, at the
completion of the working (hereafter, referred to as an end
temperature) (temperature difference .DELTA.T) is 100.degree. C. or
less, the grain size difference .DELTA.GS can be controlled to 1.5
or smaller. Consequently, it is possible to control a strength
difference .DELTA.TS to 50 MPa or less.
[0022] (D) By performing heat treatment on a steel material to
cause coarse alloy carbo-nitrides to precipitate, precipitation
strengthening occurs, further increasing the strength of the steel
material. When the grain size number of a steel material is 6.0 or
greater, and the number of alloy carbo-nitrides having circle
equivalent diameters of larger than 1000 nm (hereafter, referred to
as a coarse alloy carbo-nitrides) is 10/mm.sup.2 or more in the
steel, a tensile strength of 800 MPa or more is obtained. By
performing the heat treatment at a heat treatment temperature set
at 930.degree. C. to less than 1000.degree. C., it is possible to
obtain 10/mm.sup.2 or more of the coarse alloy carbo-nitrides.
[0023] Here, the alloy carbo-nitrides contain Cr, V, Nb, Mo, W, Ta,
or the like as a main component, and mean Cr.sub.2N, Z phase,
namely Cr(Nb, V)(C, N) and MX (M: Cr, V, Nb, Mo, W, Ta, etc., X: C,
N). The "main component" means that the component accounts for 40%
or more in mass percent. In addition, the alloy carbo-nitrides in
the present invention include those of which contents of C (carbon)
are extremely low, namely nitrides. The alloy carbo-nitrides
according to the present invention include carbides.
[0024] An austenitic stainless steel material according to the
present embodiment that is completed based on the above findings
has a chemical composition consisting of, in mass percent, C: 0.10%
or less, Si: 1.0% or less, Mn: 3 to 8%, P: 0.05% or less, S: 0.03%
or less, Ni: 10 to 20%, Cr: 15 to 30%, N: 0.20 to 0.70%, Mo: 0 to
5.0%, V: 0 to 0.5%, and Nb: 0 to 0.5%, with the balance being Fe
and impurities, the austenitic stainless steel material having a
grain size number of 6.0 or greater, the grain size number
conforming to ASTM E 112. The tensile strength of the austenitic
stainless steel material is 800 MPa or more, and the difference
between the maximum value and the minimum value of the tensile
strength is 50 MPa or less. The number of alloy carbo-nitrides
having a circle equivalent diameter of larger than 1000 nm in the
steel is 10/mm.sup.2 or more.
[0025] The above chemical composition may contain one, or two or
more elements selected from the group consisting of, in mass
percent, Mo: 1.5 to 5.0%, V: 0.1 to 0.5%, and Nb 0.1 to 0.5%.
[0026] In the above austenitic stainless steel material, the
difference between the maximum value and the minimum value of the
above grain size number is 1.5 or smaller.
[0027] The above austenitic stainless steel material is, for
example, a steel pipe, a steel bar, or a wire rod.
[0028] Hereafter, the austenitic stainless steel material of the
present embodiment will be described in detail. The sign "%"
following each element means mass percent unless otherwise
noted.
[0029] [Chemical Composition]
[0030] The austenitic stainless steel material of the present
embodiment has a chemical composition that consists of the
following elements.
[0031] C: 0.10% or Less
[0032] Carbon (C) is unavoidably contained. C stabilizes austenite
in an fcc structure, which hardly causes hydrogen brittleness. In
addition, C combines with Cr or the like, causing precipitation
strengthening to increase the strength of steel. However, an
excessively high content of C results in the precipitation of
carbides in grain boundaries, decreasing the toughness of steel.
Consequently, the content of C is 0.10% or less. An upper limit of
the content of C is preferably 0.08%, more preferably 0.06%. In
addition, a preferable lower limit of the content of C to stabilize
austenite is 0.005%.
[0033] Si: 1.0% or Less
[0034] Silicon (Si) combines with Ni and Cr to form intermetallic
compounds. In addition, Si contributes to the growth of
intermetallic compounds such as a sigma phase (.sigma. phase).
These intermetallic compounds decrease the hot workability of
steel. Therefore, the content of Si is 1.0% or less. An upper limit
of the content of Si is preferably 0.8%. From the viewpoint of
deoxidizing steel, a preferable lower limit of the content of Si is
0.2%.
[0035] Mn: 3 to 8%
[0036] Manganese (Mn) stabilizes austenite and suppresses the
generation of martensite, which has a high susceptibility to
hydrogen brittleness. In addition, Mn combines with S to form MnS,
increasing the machinability of steel. An excessively low content
of Mn results in failure to provide the effects described above. In
contrast, an excessively high content of Mn results in a decrease
in the ductility and the hot workability of steel. Consequently,
the content of Mn is 3 to 8%. A lower limit of the content of Mn is
preferably 4.0%, more preferably 5.0%. An upper limit of the
content of Mn is preferably 6.0%, more preferably 5.9%.
[0037] P: 0.05% or Less,
[0038] Phosphorus (P) is an impurity. P decreases the hot
workability and the toughness of steel. Therefore, the content of P
is 0.05% or less. An upper limit of the content of P is preferably
0.045%, more preferably 0.035%, still more preferably 0.020%. The
content of P is preferably as low as possible.
[0039] S: 0.03% or Less
[0040] Sulfur (S) combines with Mn to form MnS, increasing the
machinability of steel. However, an excessively high content of S
results in a decrease in the toughness of steel. Therefore, the
content of S is 0.03% or less. An upper limit of the content of S
is preferably 0.02%, more preferably 0.01%. The content of S is
preferably as low as possible.
[0041] Ni: 10 to 20%
[0042] Nickel (Ni) stabilizes austenite. In addition, Ni increases
the ductility and the toughness of steel. An excessively low
content of Ni results in failure to provide the effects described
above. In contrast, an excessively high content of Ni results in
saturation of the effects described above, increasing production
costs. Consequently, the content of Ni is 10 to 20%. A lower limit
of the content of Ni is preferably 11.5%, more preferably 12.0%. An
upper limit of the content of Ni is preferably 13.5%, more
preferably 13.4%.
[0043] Cr: 15 to 30%
[0044] Chromium (Cr) increases the corrosion resistance of steel.
In addition, Cr combines with N through heat treatment to form
alloy carbo-nitrides such as Cr.sub.2N, causing precipitation
strengthening to increase the strength of steel. An excessively low
content of Cr results in failure to provide the effects described
above. In contrast, an excessively high content of Cr causes the
generation of M.sub.23C.sub.6 carbides, resulting in decreases in
the ductility and the toughness of steel. Consequently, the content
of Cr is 15 to 30%. A lower limit of the content of Cr is
preferably 20.5%, more preferably 21.0%. An upper limit of the
content of Cr is preferably 23.5%, more preferably 23.4%.
[0045] N: 0.20 to 0.70%
[0046] Nitrogen (N) stabilizes austenite. In addition, N increases
the strength of steel through solid-solution strengthening.
Furthermore, N combines with Cr through heat treatment to form
alloy carbo-nitrides such as Cr.sub.2N, causing precipitation
strengthening to increase the strength of steel. An excessively low
content of N results in failure to provide the effects described
above. In contrast, an excessively high content of N results in a
decrease in the toughness of steel. Consequently, the content of N
is 0.20 to 0.70%. A lower limit of the content of N is preferably
0.21%, more preferably 0.22%. An upper limit of the content of N is
preferably 0.40%, more preferably 0.35%.
[0047] The balance of the chemical composition of the austenitic
stainless steel material according to the present embodiment is Fe
and impurities. Here, the impurities mean elements that are mixed
from ores and scraps used as raw material, a producing environment,
or the like, when the austenitic stainless steel material is
produced in an industrial manner, and are allowed to be mixed
within ranges in which the impurities have no adverse effects on
the austenitic stainless steel material of the present
embodiment.
[0048] [Optional Elements]
[0049] The austenitic stainless steel material according to the
present embodiment may further contain, instead of a part of Fe,
one, or two or more elements selected from the group consisting of
Mo, V, and Nb. All of these elements increase the strength of
steel.
[0050] Mo: 0 to 5.0%
[0051] Molybdenum (Mo) is an optional element and need not be
contained. When contained, Mo subjects austenite to solid-solution
strengthening. In addition, Mo increases the corrosion resistance
of steel. However, an excessively high content of Mo is liable to
cause intermetallic compounds to precipitate, results in a decrease
in the ductility and the toughness of steel. Consequently, the
content of Mo is 0 to 5.0%. A lower limit of the content of Mo is
preferably 1.5%, more preferably 1.9%. An upper limit of the
content of Mo is preferably 3.0%, more preferably 2.9%.
[0052] V: 0 to 0.5%
[0053] Vanadium (V) is an optional element and need not be
contained. When contained, V forms its carbide, increasing the
strength of steel. However, an excessively high content of V
saturates the effect, resulting in an increase in production costs.
Consequently, the content of V is 0 to 0.5%. A lower limit of the
content of V is preferably 0.1%, more preferably 0.12%. An upper
limit of the content of V is preferably 0.3%, more preferably
0.28%.
[0054] Nb: 0 to 0.5%
[0055] Niobium (Nb) is an optional element and need not be
contained. When contained, Nb forms its carbide, increasing the
strength of steel. However, an excessively high content of Nb
saturates the effect, resulting in an increase in production costs.
Consequently, the content of Nb is 0 to 0.5%. A lower limit of the
content of Nb is preferably 0.1%, more preferably 0.12%. An upper
limit of the content of Nb is preferably 0.3%, more preferably
0.28%.
[0056] [Strength and Strength Difference .DELTA.TS]
[0057] In the austenitic stainless steel material of the present
embodiment, a tensile strength is 800 MPa or more, and a difference
between the maximum value and the minimum value of the tensile
strength (hereafter, referred to as a strength difference
.DELTA.TS) is 50 MPa or less. This makes the austenitic stainless
steel material of the present embodiment have a consistent
high-strength across the overall length of the steel material. The
above strength and strength difference .DELTA.TS can be achieved
with, for example, the following structure.
[0058] [Grain Size]
[0059] The austenitic stainless steel material of the present
embodiment has a grain size number specified in ASTM E 112 of 6.0
or greater. The grain size number is measured in conformity with
ASTM E 112. A grain size number less than 6.0 decreases the
strength. With a grain size number of 6.0 or greater, it is
possible to obtain a high strength in the austenitic stainless
steel material having the above chemical composition. Specifically,
a tensile strength of 800 MPa or more required for the austenitic
stainless steel material of the present embodiment is obtained.
[0060] The grain size number is determined by the following method.
A test specimen for microscopic observation is fabricated from a
center portion of a cross section perpendicular to a lengthwise
direction of the austenitic stainless steel material. In the
surface of the test specimen, a surface corresponding to the above
cross section (referred to as an observation surface) is used, and
a microscopic test method for grain size specified in ASTM E 112 is
performed, and the grain size number is evaluated. Specifically,
the observation surface is subjected to mechanical polishing, and
thereafter etched using a well-known etching reagent (e.g.,
Glyceregia, Kalling's reagent, or Marble's reagent), and crystal
grain boundaries on the observation surface are caused to appear.
For each of ten visual fields on the etched surface, a grain size
number is determined. The area of each visual field is about 10.2
mm.sup.2. By performing a comparison with a grain size standard
chart specified in ASTM E 112, the grain size number of each visual
field is evaluated. The average of the grain size numbers of the
respective visual fields is defined as the grain size number of the
austenitic stainless steel material of the present embodiment.
[0061] [Grain Size Difference .DELTA.GS]
[0062] In addition, in the austenitic stainless steel material of
the present embodiment, the difference between the maximum value
and the minimum value of grain size numbers that are measured in
any plurality of portions across overall length of the austenitic
stainless steel material (referred to as the grain size difference
.DELTA.GS) is 1.5 or smaller. When the grain size difference
.DELTA.GS is more than 1.5, the difference between the maximum
value and the minimum value of tensile strengths that are measured
in a plurality of portions of the steel material (strength
difference .DELTA.TS) becomes larger than 50 MPa, and variation in
strength across the overall length of the steel material becomes
large. When the grain size difference .DELTA.GS is 1.5 or smaller,
the strength difference .DELTA.TS becomes 50 MPa or less, and the
variation in strength across the overall length of the steel
material is suppressed. As a result, the austenitic stainless steel
material of the present embodiment has a consistent
high-strength.
[0063] The grain size difference .DELTA.GS is measured by the
following method. From any plurality of portions across the overall
length of the austenitic stainless steel material in the lengthwise
direction, the same test specimens for microscopic observation as
those described above are taken. Using each of the test specimens,
the microscopic test method for grain size specified in ASTM E 112
is performed in the same manner as described above, and the grain
size number is determined. Of the obtained grain size numbers, a
maximum value and a minimum value are selected, and the difference
between the maximum value and the minimum value is defined as the
grain size difference .DELTA.GS. In a case where the austenitic
stainless steel material is a steel pipe, a steel bar, a wire rod,
or the like, the test specimens are taken from both end portions of
the steel material in a hot working direction (rolling direction,
extruding direction, or the like) (a top portion and a bottom
portion), and the grain size difference .DELTA.GS is determined.
Here, the top portion is defined as a portion extending from a
front end of the steel material toward the center portion of the
steel material within a range of 200 mm, and the bottom portion is
defined as a portion extending from a rear end of the steel
material toward the center portion of the steel material within a
range of 200 mm.
[0064] The smaller the grain size difference .DELTA.GS, the more
preferable it is. An upper limit of the grain size difference
.DELTA.GS is preferably 1.3, more preferably 1.0.
[0065] [Alloy Carbo-Nitrides]
[0066] By performing heat treatment on a steel material to cause
coarse alloy carbo-nitrides to precipitate, precipitation
strengthening occurs, increasing the strength of the steel
material.
[0067] The alloy carbo-nitrides contain Cr, V, Nb, Mo, W, Ta, or
the like as a main component and include Cr.sub.2N, Z phase, namely
Cr(Nb, V)(C, N) and MX (M: Cr, V, Nb, Mo, W, Ta, etc., X: C, N). In
addition, the carbo-nitrides in the present invention include those
of which contents of C (carbon) are extremely low, namely nitrides.
The carbo-nitrides in the present invention also include
carbides.
[0068] In the present embodiment, the number of alloy
carbo-nitrides having a circle equivalent diameter of larger than
1000 nm (coarse alloy carbo-nitrides) in the steel is 10/mm.sup.2
or more. In this case, it is possible to obtain a high tensile
strength through the precipitation strengthening. If the number of
the coarse alloy carbo-nitrides is excessively large, the toughness
of the steel may decrease, and thus an upper limit of the number of
the coarse alloy carbo-nitrides in the steel is preferably
1.5.times.10.sup.5/mm.sup.2. By performing the heat treatment with
a heat treatment temperature set at 930.degree. C. to less than
1000.degree. C., it is possible to obtain 10/mm.sup.2 or more of
the coarse alloy carbo-nitrides.
[0069] [Method for Measuring Number of Coarse Alloy
Carbo-Nitrides]
[0070] The number of the coarse alloy carbo-nitrides is defined as
follows. A sample is taken that includes a center portion of a
cross section of the austenitic stainless steel material, the cross
section being perpendicular to the lengthwise direction of the
austenitic stainless steel material (an observation region having a
radius of 10 mm around the central axis of the steel material). The
above observation region of the sample is subjected to mirror
polish. Thereafter, in each of any ten visual fields (200
.mu.m.times.200 .mu.m) in the observation region, alloy
carbo-nitrides are identified from precipitates and inclusions
using a scanning electron microscope (SEM) equipped with an
energy-dispersive X-ray spectroscope (EDS). In each visual field, a
circle equivalent diameter of each of the identified alloy carbides
is determined by image analysis. The circle equivalent diameter
means a diameter (nm) of a circle into which the area of an alloy
carbide in the visual field is converted. The number of alloy
carbo-nitrides having circle equivalent diameters of larger than
1000 nm (coarse alloy carbo-nitrides) is counted. The average value
of the coarse alloy carbo-nitrides obtained in each of the ten
visual fields is defined as the number of coarse alloy
carbo-nitrides (/mm.sup.2) in the present specification.
[0071] [Producing Method]
[0072] Description will be made about an example of a production
method for an austenitic stainless steel material according to the
present embodiment. The present producing method includes a
preparation step of preparing a starting material, a hot working
step of performing hot working on the starting material to produce
an intermediate material, a cooling step of cooling the
intermediate material subjected to the hot working, and as
necessary, a heat treatment step of performing heat treatment on
the cooled intermediate material. The producing method will be
described below.
[0073] [Preparation Step]
[0074] A molten steel having the chemical composition described
above is produced. As necessary, a well-known degassing treatment
is performed on the produced molten steel. From the molten steel
subjected to the degassing treatment, a starting material is
produced. Examples of the producing method for the starting
material include a continuous casting process. By the continuous
casting process, a continuous casting material (the starting
material) is produced. The continuous casting material is, for
example, a slab, a bloom, a billet, and the like. The molten steel
may be subjected to an ingot-making process into an ingot.
[0075] [Hot Working Step]
[0076] The starting material (continuous casting material or ingot)
is subjected to hot working by a well-known method to be produced
into the intermediate material of the austenitic stainless steel
material. Examples of the intermediate material include a steel
pipe, a steel bar, a wire rod, and the like. The intermediate
material is produced by, for example, hot extrusion working
according to the Ugine-Sejoumet process.
[0077] In the hot working step, a heating temperature and a
reduction of area are as follows.
[0078] Heating Temperature: 1160.degree. C. or Less
[0079] An excessively high heating temperature causes grains to
coarsen, with the result that the grain size number of structures
of the steel becomes less than 6.0. Therefore, the heating
temperature is 1160.degree. C. or less. An upper limit of the
heating temperature is preferably 1100.degree. C.
[0080] A lower limit of the heating temperature may be a well-known
temperature. An excessively low heating temperature makes the
coarse alloy carbo-nitrides hard to be generated even with the
heating treatment to be described later that is performed after the
hot working. Consequently, a lower limit of the heating temperature
is preferably 1060.degree. C.
[0081] Reduction of Area: Larger than 70%
[0082] When the cross-sectional area of the starting material
before the hot working is denoted by A0 (mm.sup.2), and the
cross-sectional area of the starting material after the final
operation of the hot working is denoted by A1 (mm.sup.2), a
reduction of area RA (%) is defined by Formula (1).
RA=(A0-A1)/A0.times.100 (1)
[0083] When the above reduction of area is 70% or less, an amount
of strains to be introduced into the steel material becomes
insufficient, and grains are hard to be refined. When the reduction
of area is 70% or more, an adequate amount of strains is introduced
into the steel material through the hot working, which refines the
grains, making the grain size number 6.0 or greater. A lower limit
of the reduction of area is preferably 75%.
[0084] Temperature Difference .DELTA.T in the Starting Material in
the Hot Working: 100.degree. C. or Less
[0085] In the hot working step, in the starting material, the
difference between the temperature of a portion for which the hot
working is first completed, at the completion of the hot working,
(referred to as the initial temperature) and the temperature of a
portion for which the hot working is last completed, at the
completion of the hot working, (referred to as the end temperature)
(temperature difference .DELTA.T) is 100.degree. C. or less.
[0086] For example, in a case where the intermediate product is
produced by performing piercing-rolling, hot extrusion, and hot
rolling, the portion for which the hot working is first completed
of the starting material is a top portion, and the portion for
which the hot working is last completed of the starting material is
a bottom portion. Therefore, in this case, the initial temperature
is a temperature of the top portion at the completion of the hot
working, and the end temperature is a temperature of the bottom
portion at the completion of the hot working.
[0087] When the temperature difference .DELTA.T of the starting
material is larger than 100.degree. C., variation in temperature
across the overall length of the steel material becomes excessively
large. In this case, the grain size of the top portion and the
grain size of the bottom portion differ from each other
significantly, and the grain size difference .DELTA.GS becomes
larger than 1.5. As a result, the strength difference .DELTA.TS
becomes larger than 50 MPa.
[0088] When the temperature difference .DELTA.T of the starting
material is 100.degree. C. or less, variation in grain size between
the top portion and the bottom portion is suppressed, and the grain
size difference .DELTA.GS becomes 1.5 or less. As a result, the
strength difference .DELTA.TS becomes 50 MPa or less. An upper
limit of the temperature difference .DELTA.T is preferably
90.degree. C., more preferably 80.degree. C.
[0089] [Cooling Step]
[0090] In the cooling step, the intermediate product subjected to
the hot working is cooled at 0.10.degree. C./sec or more. When the
cooling rate is less than 0.10.degree. C./sec, a phases
precipitate. The .sigma. phases decrease corrosion resistance. To
increase the corrosion resistance, the generation of .sigma. phases
needs to be suppressed. Furthermore, when the cooling rate is less
than 0.10.degree. C./sec, grains coarsen, which decreases the
strength of the steel. Consequently, the cooling rate is
0.10.degree. C./sec or more.
[0091] On the intermediate product after the cooling, straightening
may be performed to straighten the bends of the intermediate
product. In a case of performing the straightening, for example, a
straightener is disposed in line or off line on a downstream side
of a cooling device and/or an upstream side of a heating
device.
[0092] On the intermediate product subjected to the cooling or the
straightening, descaling treatment may be performed. The descaling
treatment is performed in the form of, for example, pickling or
shotblasting. The descaling treatment is performed to remove oxide
scale that is unavoidably formed on the surface of the intermediate
product due to being heated in the step prior thereto. Through the
above steps, the austenitic stainless steel material of the present
embodiment is produced.
[0093] [Heat Treatment Step]
[0094] In the heat treatment step, 10/mm.sup.2 or more of coarse
alloy carbo-nitrides are caused to precipitate. Through this step,
the tensile strength of the austenitic stainless steel material is
further increased. A heat treatment temperature is as follows.
[0095] Heat Treatment Temperature: 930.degree. C. to Less than
1000.degree. C.
[0096] A heat treatment temperature less than 930.degree. C. leads
to a failure to obtaining structures of an austenite single phase,
resulting in a decrease in strength. The heat treatment temperature
less than 930.degree. C. further leads to the generation of .sigma.
phases, resulting in a decrease in the corrosion resistance of
steel. In contrast, a heat treatment temperature of 1000.degree. C.
or more causes coarse alloy carbo-nitrides in steel to become
smaller or totally dissolved, and the number of coarse alloy
carbo-nitrides becomes less than 10/mm.sup.2. As a result,
precipitation strengthening cannot be obtained.
[0097] A heat treatment temperature of 930.degree. C. to less than
1000.degree. C. causes coarse alloy carbo-nitrides to precipitate,
and the number of the coarse alloy carbo-nitrides becomes
10/mm.sup.2 or more. As a result, precipitation strengthening
occurs, increasing the strength of the steel material. In addition,
a heat treatment temperature less than 1000.degree. C. causes
coarse alloy carbo-nitrides to precipitate sufficiently, with the
result that a strength of 800 MPa or more is obtained with stable
also when the grain size number is within a range of 6.0 to less
than 8.0.
[0098] Note that, even if the heat treatment temperature falls out
of the above range, it is possible to obtain a high strength as
long as the grain size number is 6.0 or greater, and the number of
the coarse alloy carbo-nitrides in steel is 10/mm.sup.2 or more,
and it is possible to control the strength difference .DELTA.TS to
50 MPa or less as long as the grain size difference .DELTA.GS is
1.5 or smaller.
[0099] A retention time in the heat treatment at the above heat
treatment temperature is, for example but not specially limited to,
one minute or longer.
[0100] The production method according to the present embodiment
may include a cold working step of performing cold working, after
the heat treatment step. However, solid solution heat treatment is
not performed after the cold working step because there is the
possibility of failing to obtain coarse alloy carbo-nitrides.
Examples
[0101] Molten steels having chemical compositions shown in Table 1
were produced.
TABLE-US-00001 TABLE 1 Chemical Composition (In Mass %, Balance
Being Fe and Impurities) Steel C Si Mn P S Ni Cr N Mo V Nb A 0.03
0.3 4.5 0.02 0.0003 12.0 22.3 0.32 2.1 0.2 0.2 B 0.03 0.4 4.4 0.01
0.0008 12.1 21.9 0.32 2.1 0.2 0.2 C 0.01 0.3 5.2 0.02 0.0002 13.1
22.0 0.32 2.2 0.2 0.2 D 0.01 0.4 5.3 0.01 0.0004 12.9 21.8 0.31 --
-- -- E 0.01 0.3 5.2 0.02 0.0020 13.2 22.0 0.32 -- -- 0.1 F 0.03
0.4 4.5 0.02 0.0020 15.6 22.1 0.19 2.1 0.1 0.1
[0102] Using the molten steels, ingots each weighing 3400 kg were
produced. The ingots were subjected to the hot working to be
produced into austenitic stainless steel bars (intermediate
products) (45 to 75 mm in diameter.times.3000 mm in length). In the
hot working, initial temperatures (temperatures of top portions at
the completion of hot extrusion), end temperatures (temperatures of
bottom portions at the completion of the hot extrusion), and
reductions of area RA (%) were those shown in Table 2.
TABLE-US-00002 TABLE 2 Heat Heating Initial End Temperature
Reduction Cooling Treatment Test Temperature Temperature
Temperature Difference of Area Rate Temperature Number Steel
(.degree. C.) (.degree. C.) (.degree. C.) .DELTA.T (.degree. C.) RA
(%) (.degree. C./Sec) (.degree. C.) 1 B 1102 1102 1003 99 80.2 1
993 2 B 1038 1038 1038 0 88.9 0.7 973 3 D 1150 1150 1101 49 76 1.1
994 4 E 1150 1150 1101 49 76 0.2 979 5 A 1210 1210 1161 49 91.4 2.1
960 6 A 1180 1180 1147 33 89.2 0.2 1002 7 B 1162 1162 1113 49 93.8
0.5 984 8 C 1150 1150 709 441 12 1 1029 9 C 1150 1150 1134 16 10
0.7 985 10 C 1150 1150 1117 33 90 0.07 980 11 C 1130 1130 1048 82
85 0.3 600 12 C 1150 1150 1117 33 97 0.9 1200 13 F 1150 1150 1150 0
76 0.4 1031 14 A 1140 1140 1124 16 71 0.1 1100 15 B 1093 1093 1060
33 76 0.5 1097 16 C 1157 1157 1013 144 72 0.4 960 17 C 1143 1143
1021 122 71 1.2 1046 18 A 1150 1150 1101 49 76 1 -- Number of Grain
Coarse Size Grain Size Alloy Top Bottom Strength Number Number of
Grain size Carbo- Portion Portion Difference Average Test of Top
Bottom Difference Nitrides TS TS .DELTA.TS TS Number Portion
Portion .DELTA.GS (/mm.sup.2) (MPa) (MPa) (MPa) (MPa) 1 7.6 8.3 0.7
141 821 840.8 19.8 830.9 2 8.9 8.9 0 183 860.7 857.4 3.3 859.1 3 7
7.3 0.3 139 804.5 811.3 6.8 807.9 4 7.1 7.3 0.2 170 808.4 817.7 9.3
813.1 5 4.3 4.6 0.3 210 751.8 762.3 10.5 757.1 6 4.6 4.8 0.2 122
757.8 778.5 20.7 768.2 7 5.8 6.1 0.3 160 773.8 800.1 26.3 787 8 3.6
5.3 1.7 65 738.2 790.3 52.1 764.3 9 5.8 5.9 0.1 158 770.3 777.1 6.8
773.7 10 4.8 5.2 0.4 156 777.5 793.2 15.7 785.4 11 7.2 7.7 0.5
156400 792.7 799 6.3 795.9 12 4 4.2 0.2 0 741.6 743 1.4 742.3 13
6.1 6.1 0 61 730.4 739.3 8.9 734.9 14 6.2 6.3 0.1 0 762.7 768.9 6.2
765.8 15 6.5 6.7 0.2 0 768.3 787.1 18.8 777.7 16 6 8.8 2.8 210 802
860.7 58.7 831.4 17 6.1 8.9 2.8 29 800.5 862.1 61.6 831.3 18 6.3
6.8 0.5 0 782.4 795.2 12.8 788.8
[0103] The produced material pipes were cooled at cooling rates
shown in Table 2. In addition, on the cooled material pipes, the
straightening and the descaling treatment were performed.
Furthermore, the heat treatment was performed at heat treatment
temperatures shown in Table 2 to produce austenitic stainless steel
materials (steel pipes). The retention time was 45 minutes. On a
test number 18, the heat treatment was not performed. Note that the
tensile strength (the grain size) is greatly influenced by a work
completion temperature in the hot working, there are a tendency for
the top portion, at a high temperature, to have a high strength
(small grain size) and a tendency for the bottom portion, at a low
temperature, to have a low strength (large grain size). For that
reason, a maximum value and a minimum value of tensile strength
were measured for each of the top portion and the bottom
portion.
[0104] [Measuring Grain Size Number]
[0105] Using test specimens taken from a top portion and a bottom
portion of the produced steel material of each test number
subjected to the hot working, a grain size test was conducted based
on the above ASTM E 112. The samples were taken from positions
corresponding to the top portion and the bottom portion of each
steel material (wall-thickness center portions). The grain size
numbers of the top portion and the bottom portion were determined,
and in addition, the grain size difference .DELTA.GS was
determined. The resultant grain size numbers and grain size
differences .DELTA.GS are shown in Table 2.
[0106] [Counting Number of Coarse Alloy Carbo-Nitrides]
[0107] From a wall-thickness center portion of the steel material
of each test number, a test specimen was taken. Using the taken
test specimen, the number of coarse alloy carbo-nitrides
(/mm.sup.2) was determined by the above method.
[0108] [Tension Test]
[0109] From center portions of the top portion and the bottom
portion of the steel material of each test number, a round-bar
tensile test specimen was taken. The round-bar tensile test
specimen includes a wall-thickness center portion of the steel
material (steel pipe), and a parallel portion of the round bar
specimen was parallel to a lengthwise direction of the steel
material. The diameter of the parallel portion was 5 mm. Using the
round bar specimen, in conformance with JIS Z2241(2011), a tension
test was performed in the atmosphere at a normal temperature
(25.degree. C.), and tensile strengths TS (MPa) of the top portion
and the bottom portion of each test number were determined. In
addition, the strength difference .DELTA.TS (MPa) was determined
for each test number.
[0110] [Test Results]
[0111] Table 2 shows test results.
[0112] Referring to Table 2, as to the steels of test numbers 1 to
4, their chemical compositions and producing conditions were
appropriate. As a result, the grain size numbers were 6.0 or
greater, and the grain size differences .DELTA.GS were 1.5 or
smaller. Furthermore, the numbers of coarse alloy carbo-nitrides
were 10/mm.sup.2 or more. Therefore, the tensile strengths were as
high as 800 MPa or more, in addition, the strength differences
.DELTA.TS were 50 MPa or less, and thus consistent high-strengths
were obtained across the overall lengths of the steel
materials.
[0113] In contrast, as to test numbers 5 to 7, their chemical
compositions were appropriate, but their heating temperatures in
the hot working were excessively high. Therefore, the grain size
numbers of the top portion and/or the bottom portion were less than
6.0. As a result, the strengths of the steels were less than 800
MPa, indicating low strengths.
[0114] As to a test number 8, its chemical composition was
appropriate, but its temperature difference .DELTA.T in the hot
working was larger than 100.degree. C., and its reduction of area
was less than 70%. Therefore, the grain size number thereof was
less than 6.0, and the grain size difference .DELTA.GS was larger
than 1.5. As a result, the strength of the steel was less than 800
MPa, indicating a low strength. In addition, the strength
difference .DELTA.TS was larger than 50 MPa, indicating a large
variation in strength.
[0115] As to a test number 9, its chemical composition was
appropriate, but its reduction of area in the hot working was less
than 70%. Therefore, the grain size number was less than 6.0. As a
result, the tensile strength was less than 800 MPa, indicating a
low strength.
[0116] As to a test number 10, its chemical composition was
appropriate, but its cooling rate after the hot working was less
than 0.10.degree. C./sec. Therefore, the grain size number was less
than 6.0. As a result, the strength of the steel was less than 800
MPa, indicating a low strength.
[0117] As to a test number 11, its chemical composition was
appropriate, but its heat treatment temperature after the cooling
was less than 930.degree. C. As a result, the strength of the steel
was less than 800 MPa, indicating a low strength.
[0118] As to a test number 12, its chemical composition was
appropriate, but its heat treatment temperature after the cooling
was as excessively high as 1200.degree. C. For that reason, the
number of coarse alloy carbo-nitrides was less than 10/mm.sup.2,
and the grain size number was less than 6.0. As a result, the
tensile strength was less than 800 MPa.
[0119] As to a test number 13, the content of N was excessively
low. As a result, the tensile strength was less than 800 MPa.
[0120] As to test numbers 14 and 15, their chemical compositions
were appropriate, but their heat treatment temperatures after the
cooling were 1000.degree. C. or more. Therefore, the numbers of
coarse alloy carbo-nitrides were less than 10/mm.sup.2. As a
result, the tensile strengths were less than 800 MPa.
[0121] As to test numbers 16 and 17, their chemical compositions
were appropriate, but their temperature differences .DELTA.T of the
steel materials in the hot working were larger than 100.degree. C.
Therefore, the grain size differences .DELTA.GS were more than 1.5.
As a result, the strength differences .DELTA.TS were more than 50
MPa, indicating large variations in strength.
[0122] In the test number 18, the heat treatment was not performed.
Therefore, no coarse alloy carbo-nitrides were present. As a
result, the tensile strength was less than 800 MPa.
[0123] As described above, the embodiment according to the present
invention has been described. However, the aforementioned
embodiment is merely an example for practicing the present
invention. Therefore, the present invention is not limited to the
aforementioned embodiment, and the aforementioned embodiment can be
modified and implemented as appropriate without departing from the
scope of the present invention.
* * * * *