U.S. patent application number 16/310613 was filed with the patent office on 2019-05-02 for austenitic stainless steel.
The applicant listed for this patent is Nippon Steel & Sumitomo Metal Corporation. Invention is credited to Norifumi Kochi, Yoshitaka Nishiyama.
Application Number | 20190127832 16/310613 |
Document ID | / |
Family ID | 60786709 |
Filed Date | 2019-05-02 |
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United States Patent
Application |
20190127832 |
Kind Code |
A1 |
Kochi; Norifumi ; et
al. |
May 2, 2019 |
Austenitic Stainless Steel
Abstract
Provided is an austenitic stainless steel having excellent
anti-carburizing properties even in a high temperature carburizing
environment, and an excellent hot workability in its production.
The austenitic stainless steel according to the present embodiment
includes a chemical composition consisting of, in mass percent, C:
0.03 to less than 0.25%, Si: 0.01 to 2.0%, Mn: 2.0% or less, Cr: 10
to less than 22%, Ni: more than 30.0% to 40.0%, Al: more than 2.5%
to less than 4.5%, Nb: 0.01 to 3.5%, Ca: 0.0005 to 0.05%, Mg:
0.0005 to 0.05%, and N: 0.03% or less, with the balance being Fe
and impurities. In the austenitic stainless steel, a Cr
concentration C.sub.Crz,23 n its outer layer and an Al
concentration C.sub.Aln the outer layer satisfy Formula (1) for a
Cr concentration C.sub.Cr in an other-than-outer-layer region and
an Al concentration C.sub.Al in the other-than-outer-layer region.
0.40.ltoreq.(C.sub.CrC.sub.Cr/C.sub.Al).ltoreq.0.80 (1)
Inventors: |
Kochi; Norifumi;
(Chiyoda-ku, Tokyo, JP) ; Nishiyama; Yoshitaka;
(Chiyoda-ku, Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nippon Steel & Sumitomo Metal Corporation |
Tokyo |
|
JP |
|
|
Family ID: |
60786709 |
Appl. No.: |
16/310613 |
Filed: |
June 28, 2017 |
PCT Filed: |
June 28, 2017 |
PCT NO: |
PCT/JP2017/023657 |
371 Date: |
December 17, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/42 20130101;
C22C 38/46 20130101; C21D 2211/001 20130101; C22C 38/00 20130101;
C22C 38/002 20130101; C22C 30/02 20130101; C21D 9/46 20130101; C22C
38/06 20130101; C22C 38/54 20130101; C22C 38/02 20130101; C21D 8/00
20130101; C21D 8/0268 20130101; C21D 6/004 20130101; C22C 38/04
20130101; C22C 38/50 20130101; C21D 8/105 20130101; C22C 38/34
20130101; C21D 8/0236 20130101; C22C 38/48 20130101; C21D 6/00
20130101; C22C 38/44 20130101; C21D 9/08 20130101; C21D 8/10
20130101; C22C 38/001 20130101; C22C 38/58 20130101 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/34 20060101 C22C038/34; C22C 38/02 20060101
C22C038/02; C22C 38/04 20060101 C22C038/04; C22C 38/06 20060101
C22C038/06; C22C 38/54 20060101 C22C038/54; C22C 38/50 20060101
C22C038/50; C22C 38/48 20060101 C22C038/48; C22C 38/46 20060101
C22C038/46; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/00 20060101 C22C038/00; C22C 30/02 20060101
C22C030/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 29, 2016 |
JP |
2016-128321 |
Claims
1. An austenitic stainless steel comprising a chemical composition
consisting of, in mass percent: C: 0.03 to less than 0.25%; Si:
0.01 to 2.0%; Mn: 2.0% or less; P: 0.04% or less; S: 0.01% or less;
Cr: 10 to less than 22%; Ni: more than 30.0% to 40.0%; Al: more
than 2.5% to less than 4.5%; Nb: 0.01 to 3.5%; N: 0.03% or less;
Ca: 0.0005 to 0.05%; Mg: 0.0005 to 0.05%; Ti: 0 to less than 0.2%;
Mo: 0 to 0.5%; W: 0 to 0.5%; Cu: 0 to 0.5%; V: 0 to 0.2%; and B: 0
to 0.01%, with the balance being Fe and impurities, and satisfying
Formula (1): 0.40.ltoreq.(C.sub.CrC.sub.Cr/C.sub.Al).ltoreq.0.80
(1) where, a Cr concentration (mass percent) in an outer layer of
the austenitic stainless steel is substituted for C.sub.Crn Formula
(1), an Al concentration (mass percent) in the outer layer of the
austenitic stainless steel is substituted for C.sub.Al a Cr
concentration (mass percent) in an other-than-outer-layer region of
the austenitic stainless steel is substituted for C.sub.Cr, and an
Al concentration (mass percent) in the other-than-outer-layer
region of the austenitic stainless steel is substituted for
C.sub.Al.
2. The austenitic stainless steel according to claim 1, wherein the
chemical composition contains one or two or more elements selected
from the group consisting of: Ti: 0.005 to less than 0.2%; Mo: 0.01
to 0.5%; W: 0.01 to 0.5%; Cu: 0.005 to 0.5%; V: 0.005 to 0.2%; and
B: 0.0001 to 0.01%.
Description
TECHNICAL FIELD
[0001] The present invention relates to a stainless steel, more
particularly to an austenitic stainless steel.
BACKGROUND ART
[0002] In facilities used under high temperature carburizing
environment, such as thermal power generation boilers and chemical
plants, austenitic stainless steels containing increased contents
of Cr and increased contents of Ni, or Ni-based alloys containing
increased contents of Cr have been used as heat resistant steels.
These heat resistant steels are austenitic stainless steels or
Ni-based alloys each containing about 20 to 30% by mass of Cr and
about 20 to 70% by mass of Ni.
[0003] Pipes of the facilities such as thermal power generation
boilers and chemical plants are produced from steel material pipes.
The steel material pipe is produced by melting and thereafter
performing hot working on the above austenitic stainless steel or
Ni-based alloy. Therefore, heat resistant steels are requested to
have high hot workabilities. However, austenitic stainless steels
typically have high deformation resistances and low ductilities at
high temperature. For that reason, there is a demand for austenitic
stainless steels having excellent hot workabilities.
[0004] In what is called the shale gas revolution, inexpensive
shale gas has been produced in recent years. As compared with
conventional raw materials such as naphtha, use of shale gas as
source gas in facilities such as chemical plants is likely to cause
carburization, which is a corrosion phenomenon of a metallic tube
(e.g., reaction tube) used in the facilities such as chemical
plants due to carbon (C) derived from the source gas. Therefore,
steels used in facilities such as chemical plants are requested to
have excellent anti-carburizing properties.
[0005] Stainless steels having increased anti-carburizing
properties and anti-coking properties are proposed in, for example,
Japanese Patent Application Publication No. 2005-48284 (Patent
Literature 1).
[0006] A stainless steel disclosed in Patent Literature 1 is made
of a base material including a chemical composition consisting of,
in mass percent, C: 0.01 to 0.6%, Si: 0.1 to 5%, Mn: 0.1 to 10%, P:
0.08% or less, S: 0.05% or less, Cr: 20 to 55%, Ni: 10 to 70%, N:
0.001 to 0.25%, O (oxygen): 0.02% or less, with the balance being
Fe and unavoidable impurities. This stainless steel includes a Cr
depleted zone in its near-surface portion, a Cr concentration in
the Cr depleted zone is 10% or more and less than a Cr
concentration in the base material, and a thickness of the Cr
depleted zone is within 20 .mu.m. Patent Literature 1 states that
the anti-carburizing properties and the anti-coking properties are
increased by forming a protection film mainly made of
Cr.sub.2O.sub.3 coating film.
[0007] However, in the stainless steel of Patent Literature 1, the
protection film mainly includes the Cr.sub.2O.sub.3 coating film.
Therefore, the stainless steel suffers from an insufficient
function of preventing oxygen and carbon from entering from an
external atmosphere, in particular, under a high temperature
carburizing environment. As a result, internal oxidation and
carburizing may occur in the material.
[0008] Hence, International Application Publication No.
WO2010/113830 (Patent Literature 2), International Application
Publication No. WO2004/067788 (Patent Literature 3), and Japanese
Patent Application Publication No. 10-140296 (Patent Literature 4)
disclose techniques relating to protection films that are
alternatives to Cr.sub.2O.sub.3 coating films. Specifically,
according to these literatures, a protection film mainly containing
Al.sub.2O.sub.3, which is thermodynamically stable, is formed on a
surface of heat resistant steel, as a protection film that is an
alternative to the Cr.sub.2O.sub.3 coating films.
[0009] A cast product disclosed in Patent Literature 2 includes a
casting made of a heat resistant alloy that consists of, in mass
percent, C: 0.05 to 0.7%, Si: more than 0% to 2.5% or less, Mn:
more than 0% to 3.0% or less, Cr: 15 to 50%, Ni: 18 to 70%, Al: 2
to 4%, and rare earth metals: 0.005 to 0.4%, as well as W: 0.5 to
10% and/or Mo: 0.1 to 5%, with the balance being Fe and unavoidable
impurities. The casting includes a barrier layer formed on its
surface that is to be brought into contact with a high-temperature
atmosphere, the barrier layer is an Al.sub.2O.sub.3 layer having a
thickness of 0.5 .mu.m or more, 80% by area or more of an outermost
surface of the barrier layer is Al.sub.2O.sub.3, and Cr-based
particles disperse in an interface between the Al.sub.2O.sub.3
layer and the casting, the Cr-based particles having a Cr
concentration higher than that of a base of the alloy. Patent
Literature 2 states that with added Al, a protection film mainly
including an Al.sub.2O.sub.3 protection film is formed, and
anti-carburizing properties are increased.
[0010] A nickel-chromium casting alloy disclosed in Patent
Literature 3 consists of, up to 0.8% of Carbon, up to 1% of
silicon, up to 0.2% of manganese, 15% to 40% of chromium, 0.5% to
13% of iron, 1.5% to 7% of aluminum, up to 2.5% of niobium, up to
1.5% of titanium, 0.01% to 0.4% of zirconium, up to 0.06% of
nitrogen, up to 12% of cobalt, up to 5% of molybdenum, up to 6% of
tungsten, and 0.019% to 0.089% of yttrium, with the rest being
nickel. Patent Literature 3 states that with added REM as well as
Al, the nickel-chromium casting alloy including Al.sub.2O.sub.3,
which serves as a protection film, with enhanced anti-peeling
properties can be provided.
[0011] An austenitic stainless steel disclosed in Patent Literature
4 consists of, in mass percent, C: 0.15% or less, Si: 0.9% or less,
Mn: 0.2 to 2%, P: 0.04% or less, S: 0.005% or less, S (%) and O (%)
at 0.015% or less in total, Cr: 12 to 30%, Ni: 10 to 35%, Al: 1.5
to 5.5%, B: 0.001 to 0.01%, N: 0.025% or less, Ca: 0 to 0.008%, Cu:
0 to 2%, one or more elements of Ti, Nb, Zr, V, and Hf at 0 to 2%
in total, one or more elements of W, Mo, Co, and Re at 0 to 3% in
total, and one or more elements of rare earth metals at 0 to 0.05%
in total, with the balance being Fe and unavoidable impurities.
Patent Literature 4 states that with added Al, a protection film
mainly including an Al.sub.2O.sub.3 protection film is formed, and
an oxidation resistance is increased.
CITATION LIST
Patent Literature
[0012] Patent Literature 1: Japanese Patent Application Publication
No. 2005-48284
[0013] Patent Literature 2: International Application Publication
No. WO2010/113830
[0014] Patent Literature 3: International Application Publication
No. WO2004/067788
[0015] Patent Literature 4: Japanese Patent Application Publication
No. 10-140296
SUMMARY OF INVENTION
Technical Problem
[0016] However, in Patent Literature 2, the heat resistant alloy
contains Cr at 50% at the maximum. Therefore, in a high temperature
carburizing environment such as a hydrocarbon gas atmosphere, Cr
may form its carbide on a steel surface. In this case,
Al.sub.2O.sub.3, which serves as a protection film, is not formed
uniformly. As a result, carburizing may occur.
[0017] In addition, the casting item and the nickel-chromium
casting alloy disclosed in Patent Literatures 2 and 3 each have a
high content of C, which significantly decreases their hot
workabilities.
[0018] Furthermore, in Patent Literature 3, a content of Ni is
high, which significantly increases a raw-material cost.
[0019] In Patent Literature 4, anti-carburizing properties are not
considered. As a result, its anti-carburizing properties may be
low.
[0020] An objective of the present invention is to provide an
austenitic stainless steel that has excellent anti-carburizing
properties even in a high temperature carburizing environment such
as a hydrocarbon gas atmosphere, and provides an excellent hot
workability in its production.
Solution to Problem
[0021] An austenitic stainless steel according to the present
embodiment includes a chemical composition consisting of, in mass
percent, C: 0.03 to less than 0.25%, Si: 0.01 to 2.0%, Mn: 2.0% or
less, P: 0.04% or less, S: 0.01% or less, Cr: 10 to less than 22%,
Ni: more than 30.0% to 40.0%, Al: more than 2.5% to less than 4.5%,
Nb: 0.01 to 3.5%, N: 0.03% or less, Ca: 0.0005 to 0.05%, Mg: 0.0005
to 0.05%, Ti: 0 to less than 0.2%, Mo: 0 to 0.5%, W: 0 to 0.5%, Cu:
0 to 0.5%, V: 0 to 0.2%, and B: 0 to 0.01%, with the balance being
Fe and impurities, and satisfying Formula (1).
0.40.ltoreq.(C.sub.CrC.sub.Cr/C.sub.Al).ltoreq.0.80 (1)
[0022] Here, a Cr concentration (mass percent) in an outer layer of
the austenitic stainless steel is substituted for C.sub.Crn Formula
(1). An Al concentration (mass percent) in the outer layer of the
austenitic stainless steel is substituted for C.sub.AlA Cr
concentration (mass percent) in an other-than-outer-layer region of
the austenitic stainless steel is substituted for C.sub.Cr. An Al
concentration (mass percent) in the other-than-outer-layer region
of the austenitic stainless steel is substituted for C.sub.Al.
Advantageous Effects of Invention
[0023] The austenitic stainless steel according to the present
embodiment has excellent anti-carburizing properties even in a high
temperature carburizing environment such as a hydrocarbon gas
atmosphere, and provides an excellent hot workability in its
production.
DESCRIPTION OF EMBODIMENTS
[0024] The present inventors conducted investigations and studies
about anti-carburizing properties of the austenitic stainless steel
in a high temperature carburizing environment and a hot workability
in its production, and obtained the following findings. The high
temperature carburizing environment refers to an environment in a
hydrocarbon gas atmosphere at 1000.degree. C. or more.
[0025] (A) When an austenitic stainless steel or a Ni-based alloy
is made to contain Cr, Cr.sub.2O.sub.3 that is a protection film is
formed on its steel surface, increasing its anti-carburizing
properties. However, as described above, Cr.sub.2O.sub.3 is
thermodynamically unstable. Hence, in the present invention, an
Al.sub.2O.sub.3 coating film is formed on a surface of the steel.
Al.sub.2O.sub.3 acts as a protection film. Al.sub.2O.sub.3 is
thermodynamically more stable than Cr.sub.2O.sub.3 in the high
temperature carburizing environment. That is, the Al.sub.2O.sub.3
coating film can increase the anti-carburizing properties of
austenitic stainless steel even in an environment at 1000.degree.
C. or more.
[0026] (B) When Cr is excessively contained in Al-containing
austenitic stainless steel or Ni-based alloy, Cr binds with C
derived from atmospheric gas in the high temperature carburizing
environment. Cr binding with C forms a Cr carbide on the steel
surface. The Cr carbide physically inhibits uniform formation of
the Al.sub.2O.sub.3 coating film on the steel surface. As a result,
the anti-carburizing properties of steel are decreased. Therefore,
the content of Cr needs to be limited to a certain content.
[0027] Meanwhile, Cr promotes uniform formation of the
Al.sub.2O.sub.3 coating film. Hereafter, this effect is called a
Third Element Effect of Cr (referred to as a TEE effect below). A
mechanism of the TEE effect is as follows. At the very beginning of
a heat treatment process to be described later, Cr is
preferentially oxidized first in the steel surface, and
Cr.sub.2O.sub.3 is formed. Therefore, an oxygen partial pressure in
the steel surface locally decreases. As a result, Al does not
undergo the inside oxidation but forms a uniform Al.sub.2O.sub.3
coating film in proximity to the surface. Afterward, oxygen used in
a form of Cr.sub.2O.sub.3 is incorporated into Al.sub.2O.sub.3.
Then, at the end of the heat treatment process, a protection film
made only of Al.sub.2O.sub.3 is formed. Likewise, Cr has the TEE
effect even under the high temperature carburizing environment.
That is, Cr promotes the uniform formation of the Al.sub.2O.sub.3
coating film even under the high temperature carburizing
environment. Therefore, to form a uniform Al.sub.2O.sub.3 coating
film, Cr needs to be contained at a certain content or more.
[0028] Accordingly, in order to promote inhibition of the
production of a Cr carbide and promote the formation of the
Al.sub.2O.sub.3 coating film under the high temperature carburizing
environment, a content of Cr is set at 10 to less than 22% in the
present invention.
[0029] (C) For austenitic stainless steel, it is effective to make
a ratio of a Cr concentration in an outer layer to an Al
concentration in the outer layer moderately lower than a ratio of a
Cr concentration in an other-than-outer-layer region to an Al
concentration in the other-than-outer-layer region. That is, when
an austenitic stainless steel satisfies Formula (1), the
anti-carburizing properties in the high temperature carburizing
environment is increased.
0.40.ltoreq.(C.sub.CrC.sub.Cr/C.sub.Al).ltoreq.0.80 (1)
Here, a Cr concentration (mass percent) in an outer layer of the
austenitic stainless steel is substituted for C.sub.Crn Formula
(1). An Al concentration (mass percent) in the outer layer of the
austenitic stainless steel is substituted for C.sub.AlA Cr
concentration (mass percent) in an other-than-outer-layer region of
the austenitic stainless steel is substituted for C.sub.Cr. An Al
concentration (mass percent) in the other-than-outer-layer region
of the austenitic stainless steel is substituted for C.sub.Al.
[0030] Define F1 as F1=(C.sub.CrC.sub.Cr/C.sub.Al). When F1 is 0.40
or more, the TEE effect by Cr is sufficiently provided on the steel
surface in the high temperature carburizing environment. In this
case, the formation of the Al.sub.2O.sub.3 coating film is
promoted. When F1 is 0.80 or less, the formation of the Cr carbide
on the steel surface is inhibited in the high temperature
carburizing environment. Therefore, the uniform Al.sub.2O.sub.3
coating film is formed. As a result, the anti-carburizing
properties are increased.
[0031] (D) When a chemical composition of an austenitic stainless
steel contains 0.0005% or more of calcium (Ca) and 0.0005% or more
of magnesium (Mg), the hot workability is increased. In contrast,
when contents of these elements are excessively high, a toughness
and a ductility of an austenitic stainless steel at high
temperature are decreased, resulting in a decrease in hot
workability. For this reason, Ca: 0.0005 to 0.05%, and Mg: 0.0005
to 0.05% are contained.
[0032] An austenitic stainless steel according to the present
embodiment that is made based on the above findings includes a
chemical composition consisting of, in mass percent, C: 0.03 to
less than 0.25%, Si: 0.01 to 2.0%, Mn: 2.0% or less, P: 0.04% or
less, S: 0.01% or less, Cr: 10 to less than 22%, Ni: more than
30.0% to 40.0%, Al: more than 2.5% to less than 4.5%, Nb: 0.01 to
3.5%, N: 0.03% or less, Ca: 0.0005 to 0.05%, Mg: 0.0005 to 0.05%,
Ti: 0 to less than 0.2%, Mo: 0 to 0.5%, W: 0 to 0.5%, Cu: 0 to
0.5%, V: 0 to 0.2%, and B: 0 to 0.01%, with the balance being Fe
and impurities, and satisfying Formula (1).
0.40.ltoreq.(C.sub.CrC.sub.Cr/C.sub.Al).ltoreq.0.80 (1)
Here, a Cr concentration (mass percent) in an outer layer of the
austenitic stainless steel is substituted for C.sub.Crn Formula
(1). An Al concentration (mass percent) in the outer layer of the
austenitic stainless steel is substituted for C.sub.AlA Cr
concentration (mass percent) in an other-than-outer-layer region of
the austenitic stainless steel is substituted for C.sub.Cr. An Al
concentration (mass percent) in the other-than-outer-layer region
of the austenitic stainless steel is substituted for C.sub.Al.
[0033] The above chemical composition may contain one or two or
more types selected from the group consisting of Ti: 0.005 to less
than 0.2%, Mo: 0.01 to 0.5%, W: 0.01 to 0.5%, Cu: 0.005 to 0.5%, V:
0.005 to 0.2%, and B: 0.0001 to 0.01.
[0034] Hereafter, the austenitic stainless steel according to the
present embodiment will be described in detail. The sign "%"
following each element means mass percent unless otherwise
noted.
[Chemical Composition]
[0035] A chemical composition of the austenitic stainless steel
according to the present embodiment contains the following
elements.
[0036] C: 0.03 to Less than 0.25%
[0037] Carbon (C) binds mainly with Cr to form a Cr carbide in the
steel, increasing a creep strength in use in the high temperature
carburizing environment. An excessively low content of C results in
failure to provide this effect. In contrast, an excessively high
content of C causes a large number of coarse eutectic carbides to
be formed in a solidification micro structure after the steel is
cast, resulting in a decrease in a toughness of the steel.
Consequently, a content of C is 0.03 to less than 0.25%. A lower
limit of the content of C is preferably 0.05%, more preferably
0.08%. An upper limit of the content of C is preferably 0.23%, more
preferably 0.20%.
[0038] Si: 0.01 to 2.0%
[0039] Silicon (Si) deoxidizes steel. If the deoxidation can be
sufficiently performed using another element, a content of Si may
be reduced as much as possible. In contrast, an excessively high
content of Si results in a decrease in the hot workability.
Consequently, the content of Si is 0.01 to 2.0%. A lower limit of
the content of Si is preferably 0.02%, more preferably 0.03%. An
upper limit of the content of Si is preferably 1.0%.
[0040] Mn: 2.0% or Less
[0041] Manganese (Mn) is unavoidably contained. Mn binds with S
contained in the steel to form MnS, increasing the hot workability
of the steel. However, an excessively high content of Mn makes the
steel too hard, resulting in decreases in the hot workability and
weldability. Consequently, a content of Mn is 2.0% or less. A lower
limit of the content of Mn is preferably 0.1%, more preferably
0.2%. An upper limit of the content of Mn is preferably 1.2%.
[0042] P: 0.04% or Less
[0043] Phosphorus (P) is an impurity. P decreases the weldability
and the hot workability of the steel. Consequently, a content of P
is 0.04% or less. An upper limit of the content of P is preferably
0.03%. The content of P is preferably as low as possible. A lower
limit of the content of P is, for example, 0.0005%.
[0044] S: 0.01% or Less
[0045] Sulfur (S) is an impurity. S decreases the weldability and
the hot workability of the steel. Consequently, a content of S is
0.01% or less. An upper limit of the content of S is preferably
0.008%. The content of S is preferably as low as possible. A lower
limit of the content of S is, for example, 0.001%.
[0046] Cr: 10 to Less than 22%
[0047] Chromium (Cr) exhibits the above TEE effect to promote the
formation of the Al.sub.2O.sub.3 coating film in the heat treatment
process and under the high temperature carburizing environment. In
addition, Cr binds with C in the steel to form a Cr carbide,
increasing the creep strength. An excessively low content of Cr
results in failure to provide these effects. In contrast, an
excessively high content of Cr causes Cr to bind with C derived
from atmospheric gas (hydrocarbon gas) under the high temperature
carburizing environment and form a Cr carbide on the steel surface.
The formation of the Cr carbide on the steel surface causes local
depletion of Cr in the steel surface. This lessens the TEE effect,
resulting in failure to form the uniform Al.sub.2O.sub.3 coating
film. An excessively high content of Cr further causes the Cr
carbide on the steel surface to physically inhibit the formation of
the uniform Al.sub.2O.sub.3 coating film. Consequently, a content
of Cr is 10 to less than 22%. A lower limit of the content of Cr is
preferably 11%, more preferably 12%. An upper limit of the content
of Cr is preferably 21%, more preferably 20%. In the present
specification, the Cr carbide is divided into a Cr carbide formed
in the steel and a Cr carbide formed on the steel surface. For the
austenitic stainless steel according to the present embodiment, the
Cr carbide in the steel is allowed to form, and the Cr carbide on
the steel surface is inhibited.
[0048] Ni: More than 30.0% to 40.0%
[0049] Nickel (Ni) stabilizes an austenite, increasing the creep
strength. In addition, Ni increases the anti-carburizing properties
of the steel. An excessively low content of Ni results in failure
to provide these effects. In contrast, an excessively high content
of Ni results not only in saturation of these effects but also in
an increase in raw-material costs. Consequently, a content of Ni is
more than 30.0% to 40.0%. A lower limit of the content of Ni is
preferably 31.0%, more preferably 32.0%. An upper limit of the
content of Ni is preferably 39.0%, more preferably 38.0%.
[0050] Al: More than 2.5% to Less than 4.5%
[0051] Aluminum (Al) forms the Al.sub.2O.sub.3 coating film on the
steel surface in the heat treatment process and under the high
temperature carburizing environment, increasing the
anti-carburizing properties of the steel. In particular, in the
high temperature carburizing environment assumed in the present
invention, the Al.sub.2O.sub.3 coating film is thermodynamically
stable as compared with Cr.sub.2O.sub.3 coating films
conventionally used. An excessively low content of Al results in
failure to provide these effects. In contrast, an excessively high
content of Al leads to a decrease in structural stability,
resulting in a significant decrease in the creep strength.
Consequently, a content of Al is more than 2.5% to less than 4.5%.
A lower limit of the content of Al is preferably 2.55%, more
preferably 2.6%. An upper limit of the content of Al is preferably
4.2%, more preferably 4.0%. In the austenitic stainless steel
according to the present invention, the content of Al means a total
amount of Al contained in the steel material.
[0052] Nb: 0.01 to 3.5%
[0053] Niobium (Nb) forms intermetallic compounds to be
precipitation strengthening phases (Laves phase and Ni.sub.3Nb
phase) to cause precipitation strengthening in crystal grain
boundaries and in grains, increasing the creep strength of the
steel. In contrast, an excessively high content of Nb causes the
intermetallic compounds to be produced excessively, resulting in a
decrease in the toughness of the steel. In addition, an excessively
high content of Nb also results in a decrease in the toughness
after long-time aging. Consequently, a content of Nb is 0.01 to
3.5%. A lower limit of the content of Nb is preferably 0.05%, more
preferably 0.1%. An upper limit of the content of Nb is preferably
less than 3.2%, more preferably 3.0%.
[0054] N: 0.03% or Less,
[0055] Nitrogen (N) stabilizes austenite and is unavoidably
contained. In contrast, an excessively high content of N causes
coarse nitride and/or carbo-nitride, which remains undissolved even
after heat treatment, to be produced. The coarse nitride and/or
carbo-nitride decreases the toughness of the steel. Consequently, a
content of N is 0.03% or less. An upper limit of the content of N
is preferably 0.01%. A lower limit of the content of N is, for
example, 0.0005%.
[0056] Ca: 0.0005 to 0.05%
[0057] Calcium (Ca) immobilizes S in a form of its sulfide,
increasing the hot workability. In contrast, an excessively high
content of Ca results in a decrease in the toughness and the
ductility. As a result, the hot workability decreases. In addition,
an excessively high content of Ca results in a decrease in
cleanliness. Consequently, a content of Ca is 0.0005 to 0.05%. A
lower limit of the content of Ca is preferably 0.0006%, more
preferably 0.0008%. An upper limit of the content of Ca is
preferably 0.01%, more preferably 0.008%.
[0058] Mg: 0.0005 to 0.05%
[0059] Magnesium (Mg) immobilizes S in a form of its sulfide,
increasing the hot workability of the steel. In contrast, an
excessively high content of Mg results in a decrease in the
toughness and the ductility. As a result, the hot workability
decreases. In addition, an excessively high content of Mg results
in a decrease in cleanliness. Consequently, a content of Mg is
0.0005 to 0.05%. A lower limit of the content of Mg is preferably
0.0006%, more preferably 0.0008%. An upper limit of the content of
Mg is preferably 0.01%, more preferably 0.008%.
[0060] The balance of the chemical composition of the austenitic
stainless steel 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 is produced in an
industrial manner, and are allowed to be mixed within ranges in
which the impurities have no adverse effect on the present
invention.
[Optional Elements]
[0061] The above chemical composition of the austenitic stainless
steel may further contain Ti in lieu of a part of Fe.
[0062] Ti: 0 to Less than 0.2%
[0063] Titanium (Ti) is an optional element and need not be
contained. If contained, Ti forms intermetallic compounds to be
precipitation strengthening phases (Laves phase and Ni.sub.3Ti
phase) to cause precipitation strengthening, increasing the creep
strength. In contrast, an excessively high content of Ti causes the
intermetallic compounds to be produced excessively, resulting in a
decrease in high-temperature ductility and the hot workability. In
addition, an excessively high content of Ti results in a decrease
in the toughness after long-time aging. Consequently, a content of
Ti is 0 to less than 0.2%. A lower limit of the content of Ti is
preferably 0.005%, more preferably 0.01%. An upper limit of the
content of Ti is preferably 0.15%, more preferably 0.1%.
[0064] The above chemical composition of the austenitic stainless
steel may further contain, in lieu of a part of Fe, one or two
elements selected from the group consisting of Mo and W. All of
these elements are optional elements and increase the creep
strength of the steel.
[0065] Mo: 0 to 0.5%
[0066] Molybdenum (Mo) is an optional element and need not be
contained. If contained, Mo is dissolved in the austenite, a parent
phase. The dissolved Mo causes solid-solution strengthening,
increasing the creep strength. In contrast, an excessively high
content of Mo results in a decrease in the hot workability.
Consequently, a content of Mo is 0 to 0.5%. A lower limit of the
content of Mo is preferably 0.01%, more preferably 0.05%. An upper
limit of the content of Mo is preferably 0.4%, more preferably
0.3%.
[0067] W: 0 to 0.5%
[0068] Tungsten (W) is an optional element and need not be
contained. If contained, W is dissolved in the austenite, the
parent phase. The dissolved W causes solid-solution strengthening,
increasing the creep strength. In contrast, an excessively high
content of W results in a decrease in the hot workability.
Consequently, a content of W is 0 to 0.5%. A lower limit of the
content of W is preferably 0.01%, more preferably 0.05%. An upper
limit of the content of W is preferably 0.4%, more preferably
0.3%.
[0069] The above chemical composition of the austenitic stainless
steel may further contain Cu in lieu of a part of Fe.
[0070] Cu: 0 to 0.5%
[0071] Copper (Cu) is an optional element and need not be
contained. If contained, Cu stabilizes the austenite. In addition,
Cu causes precipitation strengthening, increasing a strength of the
steel. In contrast, an excessively high content of Cu results in a
decrease in the ductility and the hot workability of the steel.
Consequently, a content of Cu is 0 to 0.5%. A lower limit of the
content of Cu is preferably 0.005%, more preferably 0.01%. An upper
limit of the content of Cu is preferably 0.3%, more preferably
0.1%.
[0072] The above chemical composition of the austenitic stainless
steel may further contain V in lieu of a part of Fe.
[0073] V: 0 to 0.2%
[0074] Vanadium (V) is an optional element and need not be
contained. If contained, V forms intermetallic compounds, as with
Ti, increasing the creep strength of the steel. In contrast, an
excessively high content of V makes a volume ratio of the
intermetallic compounds in the steel excessively high, resulting in
a decrease in the hot workability. Consequently, a content of V is
0 to 0.2%. A lower limit of the content of V is preferably 0.005%,
more preferably 0.01%. An upper limit of the content of V is
preferably 0.15%, more preferably 0.1%.
[0075] The above chemical composition of the austenitic stainless
steel may further contain B in lieu of a part of Fe.
[0076] B: 0 to 0.01%
[0077] Boron (B) is an optional element and need not be contained.
If contained, B segregates in grain boundaries, promoting
precipitation of intermetallic compounds in the grain boundaries.
This increases the creep strength of the steel. In contrast, an
excessively high content of B results in decreases in the
weldability and the hot workability of the steel. Consequently, the
content of B is 0 to 0.01%. A lower limit of the content of B is
preferably 0.0001%, more preferably 0.0005%. An upper limit of the
content of B is preferably 0.008%, more preferably 0.006%.
[Formula (1)]
[0078] The austenitic stainless steel according to the present
embodiment further satisfies Formula (1).
0.40.ltoreq.(C.sub.CrC.sub.Cr/C.sub.Al).ltoreq.0.80 (1)
Here, a Cr concentration (mass percent) in an outer layer of the
austenitic stainless steel is substituted for C.sub.Crn Formula
(1). An Al concentration (mass percent) in the outer layer of the
austenitic stainless steel is substituted for C.sub.AlA Cr
concentration (mass percent) in an other-than-outer-layer region of
the austenitic stainless steel is substituted for C.sub.Cr. An Al
concentration (mass percent) in the other-than-outer-layer region
of the austenitic stainless steel is substituted for C.sub.Al.
[0079] In the present specification, the outer layer of the
austenitic stainless steel means a range of 2 .mu.m depth from the
surface of the austenitic stainless steel. The 2 .mu.m depth from
the surface means 2 .mu.m depth from a surface of the base metal.
When the austenitic stainless steel includes the Al.sub.2O.sub.3
coating film on its surface, the 2 .mu.m depth from the surface of
the base metal means 2 .mu.m depth from the surface of the base
metal after the Al.sub.2O.sub.3 coating film is removed by
descaling treatment. That is, the Cr concentration (mass percent)
in the range of 2 .mu.m depth from the surface of the austenitic
stainless steel (when the austenitic stainless steel includes the
Al.sub.2O.sub.3 coating film on its surface, it is the surface of
the base metal after the Al.sub.2O.sub.3 coating film is removed by
the descaling treatment) is substituted for C.sub.Crn Formula (1).
The Al concentration (mass percent) in the range of 2 .mu.m depth
from the surface of the austenitic stainless steel (when the
austenitic stainless steel includes the Al.sub.2O.sub.3 coating
film on its surface, it is the surface of the base metal after the
Al.sub.2O.sub.3 coating film is removed by the descaling treatment)
is substituted for C.sub.Alin Formula (1). The Cr concentration of
the other-than-outer-layer region (mass percent) means an average
Cr concentration (mass percent) in a region of the base material
other than the outer layer. The Al concentration of the
other-than-outer-layer region (mass percent) means an average Al
concentration (mass percent) in the region of the base material
other than the outer layer.
[0080] As shown in Formula (1), in the austenitic stainless steel
according to the present embodiment, the ratio of the Cr
concentration in the outer layer to the Al concentration in the
outer layer is made moderately lower than the ratio of the Cr
concentration of the base material to the Al concentration of the
base material. In this case, the formation of the Al.sub.2O.sub.3
coating film is promoted as described above. As a result, the
anti-carburizing properties are increased in the high temperature
carburizing environment.
Define F1 as F1=(C.sub.CrC.sub.Cr/C.sub.Al). F1 is an index of Cr
behavior.
[0081] When F1 is more than 0.80, the ratio of the Cr concentration
of the outer layer to the Al concentration of the outer layer is
excessively higher than the ratio of the Cr concentration of the
base material to the Al concentration of the base material. That
is, C.sub.Cr.infin. the Cr concentration of the outer layer, is
excessively high. In this case, in the high temperature carburizing
environment, a Cr carbide is formed on the steel surface,
physically inhibiting the formation of the uniform Al.sub.2O.sub.3
coating film.
[0082] When F1 is less than 0.40, the ratio of the Cr concentration
of the outer layer to the Al concentration of the outer layer is
excessively lower than the ratio of the Cr concentration of the
base material to the Al concentration of the base material. That
is, C.sub.Cr which is the Cr concentration of the outer layer, is
excessively low. In this case, the TEE effect by Cr is not provided
in the high temperature carburizing environment. Therefore, the
uniform Al.sub.2O.sub.3 coating film is not formed on the steel
surface.
[0083] Consequently, F1 is 0.40 to 0.80. A lower limit of F1 is
preferably 0.42, more preferably 0.44. An upper limit of F1 is
preferably 0.79, more preferably 0.78.
[0084] The Cr concentration C.sub.Crn the outer layer and the Al
concentration C.sub.Aln the outer layer described above are
determined by the following method. The austenitic stainless steel
is cut perpendicularly to its surface. In the range of 2 .mu.m
depth from the surface of the cut austenitic stainless steel (when
the austenitic stainless steel includes the Al.sub.2O.sub.3 coating
film on its surface, it is the surface of the base metal after the
Al.sub.2O.sub.3 coating film is removed by the descaling
treatment), any five points (measurement points) are selected. The
Cr concentrations and the Al concentrations at the measurement
points are measured by EDX (Energy Dispersive X-ray Spectroscopy).
Values determined by averaging the measured values are defined as
C.sub.Cr %) and C.sub.Al %).
[0085] When the austenitic stainless steel includes the
Al.sub.2O.sub.3 coating film on its surface, the Cr concentration
C.sub.Crn the outer layer and the Al concentration C.sub.Aln the
outer layer are measured after the descaling treatment is
performed. Conditions for descaling the austenitic stainless steels
conform to JIS Z 2290 (2004).
[0086] Analysis of the Cr concentration C.sub.Cr in the
other-than-outer-layer region and the Al concentration C.sub.Al in
the other-than-outer-layer region described above can be conducted
by a well-known component analysis method. Specifically, they are
determined by the following method. The austenitic stainless steel
is cut perpendicularly to its longitudinal direction (in a case of
a steel pipe, it is its axis direction), and a measurement surface
is prepared. A wall-thickness center portion of the measurement
surface is pierced with a drill. By the piercing, machined chips
are produced, and the machined chips are collected. The machined
chips are collected at four spots of the same measurement surface.
When the austenitic stainless steel is a steel pipe, the machined
chips are collected at four spots provided at 45.degree. pitches.
The collected machined chips are subjected to ICP-OES (Inductively
Coupled Plasma Optical Emission Spectrometry) to conduct an
elemental analysis of its chemical composition. A procedure of the
analysis according to the ICP-OES conforms to JIS G 1258 (2007).
Averages of the measured values for the four spots are defined as
the Cr concentration C.sub.Cr in the other-than-outer-layer region
(%) and the Al concentration C.sub.Al in the other-than-outer-layer
region (%).
[0087] The austenitic stainless steel according to the present
embodiment includes the Al.sub.2O.sub.3 coating film on its surface
after the heat treatment process to be described later. Therefore,
the austenitic stainless steel of the present embodiment may
include the Al.sub.2O.sub.3 coating film on its surface. However,
the Al.sub.2O.sub.3 coating film can be removed by a well-known
method such as pickling treatment and shot peening performed after
the heat treatment process. Therefore, in the austenitic stainless
steel of the present embodiment, the Al.sub.2O.sub.3 coating film
may be removed from its surface.
[Grain Size]
[0088] The austenitic stainless steel according to the present
embodiment preferably has a grain size of 30 to 80 .mu.m. When the
grain size is 30 .mu.m or more, the creep strength of the steel
further increases. When the grain size is 80 .mu.m or less, grain
boundary diffusion of Al is promoted, which further promotes the
formation of the Al.sub.2O.sub.3 coating film. The grain size is
determined by the microscopic test method for a grain size
specified in JIS G 0551 (2013).
[0089] A shape of the austenitic stainless steel according to the
present embodiment is not limited to a particular shape. The
austenitic stainless steel is, for example, a steel pipe. An
austenitic stainless steel pipe is used as a reaction tube for a
chemical plant. The austenitic stainless steel may be a plate
material, a bar material, a wire rod, or the like.
[Producing Method]
[0090] As an example of a method for producing the austenitic
stainless steel of the present embodiment, description will be made
about a method for producing a steel pipe.
[Preparation Process]
[0091] A molten steel having the chemical composition described
above is produced. The molten steel is subjected to a well-known
degassing treatment as necessary. The molten steel is cast to
produce a starting material. The starting material may be an ingot
made by an ingot-making process, or a cast piece such as a slab,
bloom, and billet made by a continuous casting process.
Alternatively, a tube-shaped casting may be produced by a
centrifugal casting process.
[Hot Forging Process]
[0092] Hot forging may be performed on the produced starting
material to produce a cylindrical starting material. By performing
the hot forging, an interior structure of the molten steel produced
in the preparation process can be modified from a solidification
micro structure to a regulated-grain-sized structure, which is
formed by homogeneous grains. A temperature of the hot forging is,
for example, 900 to 1200.degree. C.
[Hot Working Process]
[0093] Hot working is performed on the starting material produced
through the preparation process or the starting material produced
by the hot forging (cylindrical starting material) to produce a
steel material pipe. For example, a through hole is formed at a
center of the cylindrical starting material by machining. The
cylindrical starting material with the through hole formed is
subjected to hot extrusion to produce the steel material pipe. A
machining temperature of the hot extrusion is, for example, 900 to
1200.degree. C. The steel material pipe may be produced by
performing piercing-rolling (the Mannesmann process etc.) on the
cylindrical starting material.
[Cold Working Process]
[0094] Cold working is performed on the steel material pipe
subjected to the hot working to produce an intermediate material.
The cold working is, for example, cold drawing or the like. In the
cold working process, giving strain to the steel surface allows
elements such as Al and Cr to move to the steel surface easily. In
this case, the TEE effect is provided sufficiently. It is thereby
possible to obtain an austenitic stainless steel in which Cr is
moderately depleted in an outer layer of the steel and that
satisfies Formula (1). This effect cannot be provided when a
working ratio of the cold working is excessively low. An upper
limit of the working ratio of the cold working is not particularly
specified, but cold working with an excessively high working ratio
is practically difficult to perform. Consequently, the working
ratio of the cold working is 10 to 90%.
[Heat Treatment Process]
[0095] Heat treatment is performed on the produced intermediate
material in an air atmosphere. By performing the heat treatment in
the air atmosphere, the uniform Al.sub.2O.sub.3 coating film is
formed on the steel surface. At that time, Cr in the outer layer of
the steel is moderately depleted by the TEE effect. As a result, it
is possible to obtain the austenitic stainless steel satisfying
Formula (1).
[0096] A temperature of the heat treatment is 900 to less than
1100.degree. C., and a duration of the heat treatment is 3.0 to
30.0 minutes.
[0097] If the temperature of the heat treatment is less than
900.degree. C., or the duration of the heat treatment is less than
3.0 minutes, the TEE effect cannot be provided sufficiently. In
this case, the Cr concentration C.sub.Crn the outer layer of the
steel becomes excessively high, failing to satisfy Formula (1).
Accordingly, a Cr carbide is formed on the steel surface under the
high temperature carburizing environment, and the uniform
Al.sub.2O.sub.3 coating film is not formed sufficiently. As a
result, the anti-carburizing properties are decreased.
Consequently, the temperature of the heat treatment is 900.degree.
C. or more, and the duration of the heat treatment is 3.0 minutes
or more. In addition, when the temperature of the heat treatment is
900.degree. C. or more, and the duration of the heat treatment is
3.0 minutes or more, a grain size becomes 30 .mu.m or more.
[0098] In contrast, if the temperature of the heat treatment is
1100.degree. C. or more, scales mainly made of Cr.sub.2O.sub.3 are
formed excessively on the steel surface. Therefore, Cr in the outer
layer of the steel is excessively depleted. In this case, the Cr
concentration C.sub.Crn the outer layer of the steel becomes
excessively low, failing to satisfy Formula (1). Accordingly, the
TEE effect by Cr under the high temperature carburizing environment
is lessened, and the uniform Al.sub.2O.sub.3 coating film is not
formed sufficiently. As a result, the anti-carburizing properties
are decreased. If the duration of the heat treatment duration is
more than 30.0 minutes, scales mainly made of Al.sub.2O.sub.3 are
formed excessively on the steel surface. Therefore, Al in the outer
layer of the steel is excessively depleted. In this case, the Al
concentration C.sub.Aln the outer layer of the steel becomes
excessively low, failing to satisfy Formula (1). Accordingly, the
uniform Al.sub.2O.sub.3 coating film is not formed sufficiently
under the high temperature carburizing environment. As a result,
the anti-carburizing properties are decreased. Consequently, the
temperature of the heat treatment is less than 1100.degree. C., and
the duration of the heat treatment is 30.0 minutes or less. In
addition, when the temperature of the heat treatment is less than
1100.degree. C., and the duration of the heat treatment is 30.0
minutes or less, a grain size becomes 80 .mu.m or less.
[0099] When the temperature of the heat treatment is 900 to less
than 1100.degree. C., and the duration of the heat treatment is 3.0
to 30.0 minutes, the TEE effect is provided sufficiently and
appropriately, and the steel having a chemical composition
satisfying Formula (1) is obtained. As a result, the
anti-carburizing properties under the high temperature carburizing
environment are increased.
[0100] For the purpose of removing the scales formed on the
surface, pickling treatment may be performed on the intermediate
material subjected to the heat treatment. For the pickling, for
example, a mixed acid solution of nitric acid and hydrochloric acid
is used. A duration of the pickling is, for example, 30 minutes to
60 minutes.
[0101] In addition, for the purpose of removing the scales on the
steel surface and giving strain to the steel surface of the
intermediate material subjected to the pickling treatment, shot
peening may be performed on the steel surface. In the shot peening,
a starting material and a shape of shot media, and treatment
conditions are not specified, but the starting material and the
shape, and the treatment conditions are set to be sufficient for
peeling the scales on the steel surface and giving the strain to
the steel surface. The scales refer to, for example,
Al.sub.2O.sub.3By well-known methods of the pickling treatment,
shot peening, and the like, the Al.sub.2O.sub.3 coating film can be
removed.
[0102] By the above producing method, the austenitic stainless
steel according to the present embodiment is produced. The above
description is made about the method for producing a steel pipe.
However, a plate material, bar material, wire rod, or the like may
be produced by a similar producing method (preparation process, hot
forging process, hot working process, cold working process, heat
treatment process). It is particularly preferable to apply the
austenitic stainless steel according to the present embodiment to
steel pipes. Hence, the austenitic stainless steel according to the
present embodiment is preferably an austenitic stainless steel
pipe.
Examples
[Producing Method]
[0103] Molten steels having chemical compositions shown in Table 1
were produced using a vacuum furnace.
TABLE-US-00001 TABLE 1 CHEMICAL COMPOSITION (mass %, BALANCE: Fe
AND IMPURITIES) STEEL ESSENTIAL ELEMENT OPTIONAL ELEMENT TYPE C Si
Mn P S Cr Ni Al Nb N Ca Mg Ti Mo W Cu V B A 0.112 1.20 1.85 0.038
0.004 14.6 38.5 2.54 2.29 0.0044 0.0114 0.0263 -- -- -- -- -- -- B
0.089 1.28 1.59 0.024 0.002 11.3 31.8 2.67 0.65 0.0270 0.0448
0.0381 -- -- -- -- -- -- C 0.136 2.00 1.44 0.001 0.001 11.8 34.9
3.44 0.11 0.0070 0.0323 0.0161 -- -- -- -- -- -- D 0.033 0.82 0.44
0.002 0.003 16.1 32.7 2.85 3.25 0.0185 0.0334 0.0098 -- -- -- -- --
-- E 0.205 0.15 1.43 0.002 0.008 20.3 35.3 2.76 0.28 0.0077 0.0481
0.0160 -- -- -- -- -- -- F 0.183 0.24 1.17 0.028 0.006 20.2 33.7
2.89 2.13 0.0002 0.0320 0.0410 -- -- -- -- -- -- G 0.210 1.51 0.84
0.027 0.002 12.1 30.4 4.22 0.36 0.0186 0.0323 0.0015 0.15 -- -- --
-- -- H 0.224 1.13 2.00 0.027 0.009 10.1 34.0 2.81 1.69 0.0118
0.0384 0.0185 -- 0.09 -- -- -- -- I 0.091 1.30 1.61 0.024 0.002
11.4 31.9 2.69 0.68 0.0052 0.0453 0.0023 -- -- 0.50 -- -- -- J
0.030 0.97 0.08 0.025 0.006 12.9 30.3 3.34 0.82 0.0156 0.0032
0.0164 -- -- -- 0.15 -- -- K 0.221 0.83 1.57 0.016 0.009 12.4 36.4
2.90 0.54 0.0098 0.0021 0.0196 -- -- -- -- 0.14 -- L 0.085 0.54
1.36 0.028 0.009 14.1 34.7 3.59 2.75 0.0271 0.0209 0.0269 -- -- --
-- -- 0.0035 M 0.128 0.33 0.05 0.007 0.007 5.9 34.8 3.08 1.27
0.0221 0.0211 0.0131 -- -- -- -- -- -- N 0.166 1.34 1.02 0.040
0.003 32.8 32.5 3.47 1.86 0.0169 0.0463 0.0240 -- -- -- -- -- -- O
0.119 0.58 1.28 0.023 0.001 15.3 33.7 1.55 3.33 0.0157 0.0092
0.0123 -- -- -- -- -- -- P 0.137 0.62 1.15 0.031 0.004 15.2 19.2
4.36 1.59 0.0119 0.0441 0.0066 -- -- -- -- -- -- Q 0.039 0.81 1.57
0.006 0.001 20.1 36.8 3.75 2.23 0.0124 0.0305 0.0001 -- -- -- -- --
-- R 0.185 0.28 0.22 0.026 0.003 17.6 35.4 3.38 3.28 0.0162 0.0224
0.1485 -- -- -- -- -- --
[0104] Using the above molten steels, column-shaped ingots having
an outer diameter of 120 mm (30 kg) were produced. The ingots were
subjected to the hot forging and the hot rolling. After the hot
rolling, the cold rolling was performed in conditions shown in
Table 2 to produce intermediate materials having a thickness of 15
mm. From each of the intermediate materials of respective steel
types, two 8 mm.times.20 mm.times.30 mm plate materials were
produced by machining. The heat treatment was performed on the
plate materials at temperatures and for durations shown in Table 2.
After the heat treatment, the plate materials were water-cooled to
produce test steel plates.
TABLE-US-00002 TABLE 2 Cr Al CONCENTRATION CONCENTRATION COLD HEAT
HEAT C.sub.cr' IN OUTER C.sub.Al' IN OUTER ROLLING TREATMENT
TREATMENT GRAIN LAYER LAYER TEST STEEL WORKING TEMPERATURE DURATION
SIZE AFTER HEAT AFTER HEAT NUMBER TYPE RATIO (%) (.degree. C.)
(min) (.mu.m) TREATMENT (%) TREATMENT (%) 1 A 36 1000 10 54 8.24
2.11 2 B 52 900 5 37 6.99 2.2 3 C 40 900 20 47 5.6 2.91 4 D 41 1050
10 60 10.01 2.28 5 E 62 1000 10 42 11.94 2.23 6 F 39 1000 5 54
13.22 2.54 7 G 62 1050 10 45 7.29 3.42 8 H 55 950 10 71 5.07 2.17 9
I 33 900 20 46 6.35 3.67 10 J 39 950 20 49 8.44 3.11 11 K 48 1000
10 40 7.78 3.65 12 L 49 1000 10 44 10.02 3.46 13 A 7 1050 5 76 4.25
2.11 14 B 32 700 5 21 9.36 2.21 15 C 57 1300 20 131 4.21 3.18 16 D
61 1000 0.5 22 14.29 2.38 17 E 50 1050 90 95 12.68 1.82 18 M 45
1000 10 58 1.92 2.35 19 N 47 900 20 43 15.89 3.06 20 O 64 900 5 35
9.34 1.23 21 P 34 1050 5 58 10.15 3.88 22 Q 30 1050 20 67 12.78
3.31 23 R 31 1000 5 59 11.49 2.97 Cr CONCENTRATION Al CONCENTRATION
C.sub.cr IN OTHER-THAN- C.sub.AI IN OTHER-THAN- ENTERING C TEST
OUTER-LAYER OUTER-LAYER QUANTITY REDUCTION NUMBER
C.sub.cr'/C.sub.AI' REGION (%) REGION (%) C.sub.cr/C.sub.AI F1 (%)
OF AREA 1 3.91 14.60 2.54 5.75 0.68 0.29 .largecircle. 2 3.18 11.3
2.67 4.23 0.75 0.23 .largecircle. 3 1.92 11.8 3.44 3.43 0.56 0.09
.largecircle. 4 4.39 16.1 2.85 5.65 0.78 0.18 .largecircle. 5 5.35
20.3 2.76 7.36 0.73 0.22 .largecircle. 6 5.2 20.2 2.89 6.99 0.74
0.14 .largecircle. 7 2.13 12.1 4.22 2.87 0.74 0.11 .largecircle. 8
2.34 10.1 2.81 3.59 0.65 0.22 .largecircle. 9 1.73 11.4 2.69 4.24
0.41 0.28 .largecircle. 10 2.71 12.9 3.34 3.86 0.7 0.12
.largecircle. 11 2.13 12.4 2.9 4.28 0.5 0.17 .largecircle. 12 2.9
14.1 3.59 3.93 0.74 0.07 .largecircle. 13 2.01 14.6 2.54 5.75 0.35
0.51 .largecircle. 14 4.24 11.3 2.67 4.23 1 0.65 .largecircle. 15
1.32 11.8 3.44 3.43 0.39 0.58 .largecircle. 16 6 16.1 2.85 5.65
1.06 0.69 .largecircle. 17 6.97 20.3 2.76 7.36 0.95 0.54
.largecircle. 18 0.82 5.9 3.08 1.92 0.43 0.75 .largecircle. 19 5.19
32.8 3.47 9.45 0.55 0.6 .largecircle. 20 7.59 15.3 1.55 9.87 0.77
0.83 .largecircle. 21 2.62 15.2 4.36 3.49 0.75 0.52 .largecircle.
22 3.86 20.1 3.75 5.36 0.72 0.14 X 23 3.87 17.6 3.38 5.21 0.74 0.21
X
[Measurement of Austenite Grain Size]
[0105] For each of the steel plates of the respective test numbers,
from a center portion of its cross section in a direction
perpendicular to its rolling direction, a test specimen for
microscopic observation was fabricated. Of the surfaces of the test
specimen, a surface corresponding to the above cross section
(referred to as an observation surface) was used, and the
microscopic test method specified in ASTM E 112 was performed, and
the grain size was measured. Specifically, the observation surface
was subjected to mechanical polishing, and thereafter etched using
etching reagent, and crystal grain boundaries in the observation
surface were exposed. An average grain size of ten visual fields on
the etched surface was determined. The area of each visual field
was about 0.75 mm.sup.2.
[Measurement of Cr Concentration C.sub.Crn Outer Layer and Al
Concentration C.sub.Aln Outer Layer]
[0106] The steel plates of the respective test numbers were
subjected to the descaling treatment under conditions conforming to
JIS Z 2290 (2004). Each of the steel plates subjected to the
descaling treatment was cut perpendicularly to its rolling
direction, and a sample including a surface of the steel plate was
taken. Each of the samples was embedded in resin, and its
observation surface including a cross section of the vicinity to
the surface of the steel plate was polished. On the polished
observation surface, the above method was used to determine
C.sub.Cr the Cr concentration and C.sub.Al the Al concentration in
the outer layer (range of 2 .mu.m depth from the surface of the
steel plate).
[Measurement of Cr Concentration C.sub.Cr in Other-than-Outer-Layer
Region and Al Concentration C.sub.Al in Other-than-Outer-Layer
Region]
[0107] The above method was used to determine the Cr concentration
C.sub.Cr in the other-than-outer-layer region and the Al
concentration C.sub.Al in the other-than-outer-layer region.
[Carburizing Test]
[0108] The steel plates of the respective test numbers were
retained in H.sub.2--CH.sub.4--CO.sub.2 atmosphere at 1100.degree.
C..times.96 hours. After the carburizing, scales and the like were
removed from surfaces of the steel plates by performing manual dry
polishing on the surfaces using #600 abrasive paper. From the
surfaces of the steel plates, machined chips for analysis of four
layers were taken at 0.5 mm pitches. On the taken machined chips
for analysis, the C concentrations were measured by a high
frequency combustion infrared absorption method. Values obtained by
subtracting the C concentration originally contained in the steel
from results of the measurement were determined as C concentration
increase quantities. An average of C concentration increase
quantities of the four layers was determined as an entering C
quantity.
[High-Temperature Tensile Test]
[0109] For each of the produced ingots, from its wall-thickness
center portion, a column-shaped tensile test specimen having a
diameter of 10 mm and a length of 130 mm was cut out. Each tensile
test specimen was subjected to a tensile test at a tensile speed
(strain rate) of 10/s, and its hot workability was evaluated. In
the present invention, when a reduction of area of a test specimen
after the tensile test at 900.degree. C. was 60% or more, the test
specimen was determined as good (.largecircle.), and when the
reduction of area was less than 60%, the test specimen was
determined as no good (x).
[Test Results]
[0110] Results of the tests are shown in Table 2.
[0111] Referring to Table 2, as to a test number 1 to a test number
12, their chemical compositions were appropriate, their producing
conditions were also appropriate, and F1 satisfied Formula (1). As
a result, their entering C quantities were 0.4% or less, and they
exhibited excellent anti-carburizing properties. In addition, their
values of reduction of area in the high-temperature tensile test
were 60% or more, and they exhibited excellent hot
workabilities.
[0112] In contrast, as to a test number 13, its working ratio of
the cold rolling was excessively low. Accordingly, F1 was 0.35,
failing to satisfy Formula (1). As a result, its entering C
quantity was 0.51%, exhibiting poor anti-carburizing
properties.
[0113] As to a test number 14, its temperature of the heat
treatment was excessively low. Accordingly, F1 was 1.00, failing to
satisfy Formula (1). As a result, its entering C quantity was
0.65%, exhibiting poor anti-carburizing properties. In addition, as
to the test number 14, its grain size was 21 .mu.m.
[0114] As to a test number 15, its temperature of the heat
treatment was excessively high. Accordingly, F1 was 0.39, failing
to satisfy Formula (1). As a result, its entering C quantity was
0.58%, exhibiting poor anti-carburizing properties. In addition, as
to the test number 15, its grain size was 131 .mu.m.
[0115] As to a test number 16, its duration of the heat treatment
was excessively short. Accordingly, F1 was 1.06, failing to satisfy
Formula (1). As a result, its entering C quantity was 0.69%,
exhibiting poor anti-carburizing properties. In addition, as to the
test number 16, its grain size was 22 .mu.m.
[0116] As to a test number 17, its duration of the heat treatment
was excessively long. Accordingly, F1 was 0.95, failing to satisfy
Formula (1). As a result, its entering C quantity was 0.54%,
exhibiting poor anti-carburizing properties. In addition, as to the
test number 17, its grain size was 95 .mu.m.
[0117] As to Test Number 18, its content of Cr was excessively low.
Accordingly, the TEE effect by Cr was lessened. As a result, its
entering C quantity was 0.75%, exhibiting poor anti-carburizing
properties.
[0118] As to Test Number 19, its content of Cr was excessively
high. Accordingly, its Cr carbide inhibited the formation of the
Al.sub.2O.sub.3 coating film. As a result, its entering C quantity
was 0.60%, exhibiting poor anti-carburizing properties.
[0119] As to Test Number 20, its content of Al was excessively low.
Accordingly, its Al.sub.2O.sub.3 coating film was not formed
sufficiently. As a result, its entering C quantity was 0.83%,
exhibiting poor anti-carburizing properties.
[0120] As to Test Number 21, its content of Ni was excessively low.
As a result, its entering C quantity was 0.52%, exhibiting poor
anti-carburizing properties.
[0121] As to Test Number 22, its content of Mg was excessively low.
As a result, its value of reduction of area was less than 60%,
exhibiting a low hot workability.
[0122] As to Test Number 23, its content of Mg was excessively
high. As a result, its value of reduction of area was less than
60%, exhibiting a low hot workability.
[0123] The embodiment according to the present invention has been
described above. 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.
INDUSTRIAL APPLICABILITY
[0124] The austenitic stainless steel according to the present
invention is available even in the high temperature carburizing
environment such as a hydrocarbon gas atmosphere, in which there
are concerns about carburizing and coking. In particular, the
austenitic stainless steel is suitable for application to steel for
reaction tube in chemical industry plants such as ethylene
producing plants, and the like.
* * * * *