EP4653565A1

EP4653565A1 - Austenitic stainless steel material

Info

Publication number: EP4653565A1
Application number: EP24744646.1A
Authority: EP
Inventors: Kana JOTOKU; Takahiro Osuki; Shinnosuke KURIHARA; Takahiro Izawa; Kiyoko Takeda
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-01-16
Filing date: 2024-01-16
Publication date: 2025-11-26
Also published as: KR20250132512A; EP4653565A4; WO2024154725A1; JPWO2024154725A1; CN120530219A

Abstract

An austenitic stainless steel material having a chemical composition including, in mass%, C: 0.010 to 0.15%, Si: 0.10 to 2.00%, Mn: 0.10 to 3.00%, P: 0.060% or less, S: 0.010% or less, Ni: 16.0 to 25.0%, Cr: 20.0 to 29.0%, Mo: 0.08 to 1.0%, Sn: 0.001 to 0.080%, N: 0.010 to 0.15%, Co: 0.001 to 1.50%, Nb: 0 to 0.20%, and the balance: Fe and impurities, and in which F1 (= 16.0 + 8.0C + 2.4Si - 0.5Mn + 0.3Ni - 0.9Cr - 3.1Mo + 119N + 2.0Co) is 7.5 to 20.0, and F2 (= 286.2 + 337.9C + 23.9Si - 2.7Mn + 0.1Cr - 15.5Mo + 339.4Sn - 286.9Nb) is 320.0 or less.

Description

TECHNICAL FIELD

The present invention relates to an austenitic stainless steel material.

BACKGROUND ART

Austenitic stainless steel materials are excellent in strength and corrosion resistance, and are used for various applications. For this reason, for example, as disclosed in Patent Document 1, austenitic stainless steel materials with enhanced wear resistance have been developed.

LIST OF PRIOR ART DOCUMENT

PATENT DOCUMENT

Patent Document 1: JP5-222512A

SUMMARY OF INVENTION

TECHNICAL PROBLEM

Patent Document 1 discloses an austenitic stainless steel material which is subjected to a heat treatment in a nitrogen atmosphere (hereinafter, also referred to as "nitridation treatment") to harden the surface and thereby improve its properties. Such nitridation treatment is sometimes performed, for example, after a certain amount of processing has been performed, in order to make it difficult for scratches to occur during transportation and the like.
However, when it is necessary to perform further processing such as drawing thereafter, in some cases small cracks occur from the hardened surface (hereinafter, referred to simply as "microcracks") in the austenitic stainless steel material. Such microcracks are undesirable in terms of the appearance and performance of the product.
Furthermore, depending on the application, in some cases an austenitic stainless steel material is required to have weldability. However, when it is attempted to suppress the microcracks, in some cases the weldability may decrease. In other words, it is generally difficult to both suppress microcracks and also improve weldability.
In view of the above situation, an objective of the present invention is to provide an austenitic stainless steel material in which microcracks are suppressed and which has good weldability even when subjected to processing before and after nitridation treatment.

SOLUTION TO PROBLEM

The present invention has been made to solve the problem described above, and the gist of the present invention is an austenitic stainless steel material described hereunder.

(1) An austenitic stainless steel material having a chemical composition including, in mass%,
- C: 0.010 to 0.15%,
- Si: 0.10 to 2.00%,
- Mn: 0.10 to 3.00%,
- P: 0.060% or less,
- S: 0.010% or less,
- Ni: 16.0 to 25.0%,
- Cr: 20.0 to 29.0%,
- Mo: 0.08 to 1.0%,
- Sn: 0.001 to 0.080%,
- N: 0.010 to 0.15%,
- Co: 0.001 to 1.50%,
- Nb: 0 to 0.20%, and
- the balance: Fe and impurities,
- wherein:
  - F1 calculated by Formula (i) below is 7.5 to 20.0; and
  - F2 calculated by Formula (ii) below is 320.0 or less; $F 1 = 16.0 + 8.0 C + 2.4 Si - 0.5 Mn + 0.3 Ni - 0.9 Cr - 3.1 Mo + 119 N + 2.0 Co$ $F 2 = 286.2 + 337.9 C + 23.9 Si - 2.7 Mn + 0.1 Cr - 15.5 Mo + 339.4 Sn - 286.9 Nb$
  - where, each symbol of an element in the above formulas represents a content (mass%) of a corresponding element contained in the austenitic stainless steel material, and is assigned a value of 0 if the corresponding element is not contained.
(2) The austenitic stainless steel material according to (1) above, wherein the chemical composition contains, in lieu of a part of the Fe, one or more kinds of element selected from, in mass%:
- Cu: 1.0% or less,
- V: 0.30% or less,
- Ti: 0.020% or less,
- W: 0.10% or less,
- Al: 0.050% or less, and
- B: 0.050% or less.
(3) An austenitic stainless steel material having a chemical composition including, in mass%,
- C: 0.010 to 0.15%,
- Si: 0.10 to 2.0%,
- Mn: 0.10 to 3.0%,
- P: 0.06% or less,
- S: 0.010% or less,
- Ni: 16.0 to 25.0%,
- Cr: 20.0 to 29.0%,
- Mo: 0.08 to 1.0%,
- Sn: 0.001 to 0.080%,
- N: 0.010 to 0.15%,
- Co: 0.001 to 1.5%,
- Nb: 0 to 0.20%, and
- the balance: Fe and impurities,
- wherein:
  - F1 calculated by Formula (i) below is 7.5 to 20.0; and
  - F2 calculated by Formula (ii) below is 320.0 or less; $F 1 = 16.0 + 8.0 C + 2.4 Si - 0.5 Mn + 0.3 Ni - 0.9 Cr - 3.1 Mo + 119 N + 2.0 Co$ $F 2 = 286.2 + 337.9 C + 23.9 Si - 2.7 Mn + 0.1 Cr - 15.5 Mo + 339.4 Sn - 286.9 Nb$
  - where, each symbol of an element in the above formulas represents a content (mass%) of a corresponding element contained in the austenitic stainless steel material, and is assigned a value of 0 if the corresponding element is not contained.
(4) The austenitic stainless steel material according to (3) above, wherein the chemical composition contains, in lieu of a part of the Fe, one or more kinds of element selected from a group consisting of, in mass%:
- Cu: 1.0% or less,
- V: 0.30% or less,
- Ti: 0.020% or less,
- W: 0.10% or less,
- Al: 0.050% or less, and
- B: 0.050% or less.

ADVANTAGEOUS EFFECT OF INVENTION

According to the present invention, an austenitic stainless steel material in which microcracks are suppressed and which has good weldability even when subjected to processing before and after nitridation treatment is obtained.

DESCRIPTION OF EMBODIMENTS

The present inventors have conducted various studies directed at improving weldability while suppressing the microcracks described above, and obtained the findings described in (a) to (c) below.

(a) Microcracks form when a nitridation layer in which nitrogen is concentrated grows too thick due to nitridation treatment. Therefore, the present inventors have investigated the possibility of reducing the thickness of a nitridation layer in order to suppress the occurrence of microcracks. In order to reduce the thickness of a nitridation layer, it is desirable to cause nitrides, such as CrN, to form within the nitridation layer, to thereby consume N that constitutes the nitridation layer.
(b) On the other hand, if nitrides are excessively formed, the formed nitrides may become starting points for microcracks. Therefore, it is important to adjust the balance between the amount of the nitridation layer and the amount of nitrides that are formed. Specifically, it is effective to set the balance with respect to the contents of elements such as C, Si, Mn, Ni, Cr, Mo, N, and Co which affect the formation of the nitridation layer, within an appropriate range.
(c) It is necessary to ensure weldability while adjusting the balance between the amount of the nitridation layer and the amount of nitrides that are formed. Therefore, it is necessary to control the range of the contents of elements such as C, Si, Mn, Cr, Mo, Sn, and Nb.

An embodiment of the present invention has been made based on the above findings. The respective requirements of the austenitic stainless steel material of the present embodiment are described in detail hereunder.

1. Chemical composition

The reasons for limiting each element are as follows. Note that, the symbol "%" in relation to content in the following description means "mass percent".

C: 0.010 to 0.15%

C (carbon) has an effect of improving strength. In addition, C has an effect of suppressing formation of a nitridation layer. Therefore, the content of C is 0.010% or more. The content of C is preferably 0.020% or more, and more preferably is 0.030% or more. However, if C is excessively contained, weldability will decrease. Further, mechanical properties such as toughness will decrease. Therefore, the content of C is 0.15% or less. The content of C is preferably 0.12% or less, and more preferably is 0.10% or less.

Si: 0.10 to 2.00%

Si (silicon) is an element that has a deoxidizing effect. In addition, Si has an effect of suppressing formation of a nitridation layer. Therefore, the content of Si is 0.10% or more. The content of Si is preferably 0.12% or more, more preferably is 0.15% or more, and further preferably is 0.20% or more. However, if Si is excessively contained, weldability will decrease. Therefore, the content of Si is 2.00% or less. The content of Si is preferably 1.70% or less, more preferably is 1.40% or less, and further preferably is 1.00% or less.

Mn: 0.10 to 3.00%

Mn (manganese) has an effect of improving strength. Therefore, the content of Mn is 0.10% or more. The content of Mn is preferably 0.15% or more, more preferably is 0.20% or more, and further preferably is 0.25% or more. However, if Mn is excessively contained, microcracks will easily occur. Further, weldability will also decrease. Therefore, the content of Mn is 3.00% or less. The content of Mn is preferably 2.50% or less, more preferably is 2.30% or less, and further preferably is 2.00% or less.

P: 0.060% or less

P (phosphorus) is an impurity element contained in the steel. P reduces mechanical properties such as strength and toughness. Therefore, the content of P is 0.060% or less. The content of P is preferably 0.050% or less. Although it is preferable to reduce the content of P as much as possible, excessive reduction of P will increase the refining cost. Therefore, the content of P is preferably 0.005% or more.

S: 0.010% or less

S (sulfur) is an impurity element contained in the steel. S reduces mechanical properties such as strength and toughness. Therefore, the content of S is 0.010% or less. The content of S is preferably 0.0050% or less. Although it is preferable to reduce the content of S as much as possible, excessive reduction of S will increase the refining cost. Therefore, the content of S is preferably 0.0001% or more.

Ni: 16.0 to 25.0%

N (nickel) is an element that stabilizes austenite. Further, N has an effect of improving strength. In addition, N has an effect of suppressing formation of a nitridation layer. Therefore, the content of Ni is 16.0% or more. The content of Ni is preferably 16.5% or more, more preferably is 17.0% or more, and further preferably is 18.0% or more. However, if Ni is excessively contained, weldability and hot workability will decrease. Therefore, the content of Ni is 25.0% or less. The content of Ni is preferably 24.8% or less, more preferably is 24.5% or less, and further preferably is 24.0% or less.

Cr: 20.0 to 29.0%

Cr (chromium) has an effect of increasing corrosion resistance. Therefore, the content of Cr is 20.0% or more. The content of Cr is preferably 21.0% or more, more preferably is 21.5% or more, and further preferably is 22.0% or more. However, if Cr is excessively contained, weldability will decrease. Therefore, the content of Cr is 29.0% or less. The content of Cr is preferably 28.0% or less, more preferably is 27.5% or less, and further preferably is 27.0% or less.

Mo: 0.08 to 1.0%

Mo (molybdenum) has an effect of increasing strength and corrosion resistance. Therefore, the content of Mo is 0.08% or more. The content of Mo is preferably 0.10% or more, more preferably is 0.12% or more, and further preferably is 0.15% or more. However, if Mo is excessively contained, mechanical properties such as ductility and toughness will tend to decrease. Therefore, the content of Mo is 1.0% or less. The content of Mo is preferably 0.95% or less, more preferably is 0.90% or less, and further preferably is 0.85% or less.

Sn: 0.001 to 0.080%

Sn (tin) has an effect of increasing corrosion resistance. Further, Sn also has an effect of increasing weldability. Therefore, the content of Sn is 0.001% or more. The content of Sn is preferably 0.0015% or more, more preferably is 0.002% or more, and further preferably is 0.003% or more. However, if Sn is excessively contained, hot workability will decrease. Therefore, the content of Sn is 0.080% or less. The content of Sn is preferably 0.075% or less, more preferably is 0.070% or less, and further preferably is 0.060% or less.

N: 0.010 to 0.15%

N (nitrogen) dissolves in the matrix and has an effect of stabilizing austenite. In addition, N has an effect of suppressing formation of a nitridation layer. Therefore, the content of N is 0.010% or more. The content of N is preferably 0.015% or more, more preferably is 0.020% or more, and further preferably is 0.030% or more. However, if N is excessively contained, sensitization will occur in a heat affected zone during welding, and the corrosion resistance will decrease. Therefore, the content of N is 0.15% or less. The content of N is preferably 0.14% or less, more preferably is 0.13% or less, and further preferably is 0.12% or less.

Co: 0.001 to 1.50%

Co (cobalt) is an important element in a steel plate of the present embodiment, and has an effect of suppressing formation of a nitridation layer. Therefore, the content of Co is 0.001% or more. The content of Co is preferably 0.005% or more, more preferably is 0.01% or more, and further preferably is 0.02% or more. However, if Co is excessively contained, the production cost will increase. Therefore, the content of Co is 1.50% or less. The content of Co is preferably 1.30% or less, more preferably is 1.00% or less, and further preferably is 0.80% or less.
In addition to the elements described above, the chemical composition may also contain Nb within the range described below in lieu of a part of Fe. The reason for limiting the element is described below.

Nb: 0 to 0.20%

Nb (niobium) forms carbo-nitrides and has an effect of increasing corrosion resistance. Therefore, Nb may be contained as necessary. However, if Nb is excessively contained, mechanical properties such as toughness will decrease. Therefore, the content of Nb is 0.20% or less. The content of Nb is preferably 0.15% or less, and more preferably is 0.10% or less. On the other hand, in order to obtain the aforementioned advantageous effect, the content of Nb is preferably 0.001% or more, more preferably is 0.005% or more, further preferably is 0.010% or more, and further preferably is 0.015% or more.
In addition to the elements described above, the chemical composition may also contain one or more kinds of element selected from Cu, V, Ti, Al, and B within the ranges described below in lieu of a part of Fe. The reasons for limiting each element are described below.

Cu: 1.0% or less

Cu (copper) stabilizes austenite and has an effect of improving strength. Therefore, Cu may be contained as necessary. However, if the content of Cu is excessive, hot workability will decrease. Therefore, the content of Cu is 1.0% or less. The content of Cu is preferably 0.80% or less, and more preferably is 0.60% or less. On the other hand, in order to obtain the aforementioned advantageous effect, the content of Cu is preferably 0.001% or more.

V: 0.30% or less

V (vanadium) has an effect of improving the strength of the steel by forming carbides. Therefore, V may be contained as necessary. However, if the content of V is excessive, toughness will decrease. Therefore, the content of V is 0.30% or less. The content of V is preferably 0.25% or less, and more preferably is 0.20% or less. On the other hand, in order to obtain the aforementioned advantageous effect, the content of V is preferably 0.001% or more.

Ti: 0.020% or less

Ti (titanium) has an effect of improving the strength of the steel by forming carbo-nitrides. Therefore, Ti may be contained as necessary. However, if the content of Ti is excessive, toughness will decrease. Therefore, the content of Ti is 0.020% or less. The content of Ti is preferably 0.015% or less, and more preferably is 0.010% or less. On the other hand, in order to obtain the aforementioned advantageous effect, the content of Ti is preferably 0.001% or more.

W: 0.10% or less

W (tungsten) dissolves in the parent phase and has an effect of improving the strength of the steel. Therefore, W may be contained as necessary. However, if the content of W is excessive, hot workability will decrease. Therefore, the content of W is 0.10% or less. The content of W is preferably 0.08% or less, and more preferably is 0.05% or less. On the other hand, in order to obtain the aforementioned advantageous effect, the content of W is preferably 0.001% or more.

Al: 0.050% or less

Al (aluminum) is an element that has a deoxidizing effect. Therefore, Al may be contained as necessary. However, if the content of Al is excessive, inclusions will excessively form and productivity will decrease. Therefore, the content of Al is 0.050% or less. The content of Al is preferably 0.040% or less, and more preferably is 0.030% or less. On the other hand, in order to obtain the aforementioned advantageous effect, the content of Al is preferably 0.001% or more.

B: 0.050% or less

B (boron) forms intermetallic compounds at grain boundaries, and has an effect of increasing grain boundary strength. Therefore, B may be contained as necessary. However, if the content of B is excessive, hot workability will decrease. Therefore, the content of B is to be 0.050% or less. The content of B is preferably 0.040% or less, and more preferably is 0.030% or less. On the other hand, in order to obtain the aforementioned advantageous effect, the content of B is preferably 0.0001% or more.
In the chemical composition of the austenitic stainless steel of the present embodiment, the balance is Fe and impurities. Here, the term "impurities" refers to components which, during industrial production of the steel, are mixed in from raw material such as ore or scrap or due to various causes during the production process, and which are allowed within a range that does not adversely affect the present embodiment.

Formula (i)

In the chemical composition of the austenitic stainless steel of the present embodiment, Fl calculated by Formula (i) below is 7.5 to 20.0. $F 1 = 16.0 + 8.0 C + 2.4 Si - 0.5 Mn + 0.3 Ni - 0.9 Cr - 3.1 Mo + 119 N + 2.0 Co$
In the formula, each symbol of an element in the above formula represents the content (mass%) of a corresponding element contained in the austenitic stainless steel material, and is assigned a value of 0 if the corresponding element is not contained.
If the nitridation layer in which nitrogen is concentrated at the surface of the steel material grows too thick due to nitridation treatment, microcracks will form. Therefore, it is effective to cause nitrides, for example, CrN and the like, to form in order to reduce the thickness of the nitridation layer. Here, the above Formula (i) is an experimentally obtained relational expression with respect to C, Si, Mn, Ni, Cr, Mo, N, and Co, which are elements which affect the formation of nitrides, and F1 is an index of the likelihood of microcracks occurring in a case where processing is performed before and after nitridation treatment. If F1 is less than 7.5, nitrides cannot be formed sufficiently and the thickness of the nitridation layer cannot be reduced, and consequently microcracks will easily occur. Therefore, F1 is 7.5 or more. Fl is preferably 7.6 or more, more preferably is 7.7 or more, and further preferably is 7.8 or more.
On the other hand, if F1 is more than 20.0, nitrides will excessively form and, on the contrary, microcracks will easily occur. Therefore, F1 is 20.0 or less. F1 is preferably 19.0 or less, more preferably is 18.0 or less, and further preferably is 17.0 or less.

Formula (ii)

In the chemical composition of the austenitic stainless steel of the present embodiment, F2 calculated by Formula (ii) below is 320.0 or less: $F 2 = 286.2 + 337.9 C + 23.9 Si - 2.7 Mn + 0.1 Cr - 15.5 Mo + 339.4 Sn - 286.9 Nb$ where, each symbol of an element in the above formula represents the content (mass%) of a corresponding element contained in the austenitic stainless steel material, and is assigned a value of 0 if the corresponding element is not contained.
Here, the aforementioned F2 is an index indicating weldability, and Formula (ii) is an experimentally obtained formula. That is, Formula (ii) is a relational expression with respect to C, Si, Mn, Cr, Mo, Sn, and Nb which mutually affect weldability. If F2 is more than 320.0, weldability will decrease and cracks will easily occur after welding. Therefore, F2 is 320.0 or less. F2 is preferably 319.0 or less, more preferably is 318.0 or less, and further preferably is 317.0 or less. Note that, although a lower limit of F2 is not particularly limited, F2 will be 213.0 or more based on the ranges of the contents of the elements constituting Formula (ii).

2. Cr depleted zone

In the austenitic stainless steel material of the present embodiment, as mentioned above, the occurrence of microcracks is suppressed by controlling the chemical composition to adjust the balance between the amount of a nitridation layer and the amount of nitrides that are formed. However, in a case where a Cr depleted zone is thickly formed at the surface of the austenitic stainless steel material, when nitridation treatment is performed, microcracks are slightly more likely to occur because the amount of precipitated nitrides increases due to an increase in N activity in the Cr depleted zone. From such a viewpoint, it is preferable that the thickness of the Cr depleted zone formed at the surface of the austenitic stainless steel material is thin, and specifically, the thickness is preferably 10 µm or less, and more preferably 5 µm or less. Note that it is possible to control the thickness of the Cr depleted zone to be thin by performing a heat treatment during production, to be described later, using a hydrogen furnace.
Note that, in the present invention the thickness of the aforementioned Cr depleted zone is to be measured by the following method. After cutting out a cross section perpendicular to the surface of the austenitic stainless steel material, a field emission electron probe microanalyzer (FE-EPMA) is used to perform a line analysis to a depth of 50 µm from the surface toward the central portion of the wall thickness at a pitch of 0.16 µm, and measurement of the Cr concentration in the depth direction is performed. At such time, the measurement is to be performed under conditions of an acceleration voltage of 15 kV and an analysis diameter of 0.1 µm. A region where the Cr concentration is 0.9 times or less for the content of Cr in the aforementioned austenitic stainless steel material is defined as a Cr depleted zone.

3. Shape and applications

The shape of the austenitic stainless steel material of the present embodiment is not particularly limited. For example, the austenitic stainless steel material may be a steel plate, or may be a steel bar or wire rod. Further, the austenitic stainless steel material of the present embodiment is suitable for use in applications in which processing such as drawing is performed after nitridation treatment.

4. Production method

The austenitic stainless steel material of the present embodiment can be stably produced, for example, by the following production method.
Steel having the aforementioned chemical composition is melted, and a raw steel such as a slab or a billet is produced by continuous casting or the like. The obtained raw steel is subjected to hot working to produce an austenitic stainless steel material having a desired shape. The hot working is, for example, hot forging, hot extrusion, hot rolling or the like. The shape may be, for example, a steel plate, a steel bar, a wire rod or the like. Note that, the conditions of the hot working are not particularly limited and may be adjusted as necessary. After the hot working, cold working may be performed as necessary. The cold working is, for example, cold drawing or cold rolling.
After hot working, or after cold working, usually a heat treatment is performed. Well-known conditions can be used as the heat treatment conditions. For example, the steel material may be heated to a temperature range of 1060 to 1350°C, and thereafter rapidly cooled. An atmosphere furnace is generally used when performing the heat treatment. However, as described above, when it is desired to suppress formation of a Cr depleted zone to reduce the thickness of the Cr depleted zone, it is preferable to use a hydrogen furnace.
Further, when performing the heat treatment using an atmosphere furnace, pickling or the like may be performed as necessary. These production conditions are also not particularly limited and may be adjusted as appropriate. The production conditions are controlled so that the final structure is an austenitic stainless steel structure.
In the obtained austenitic stainless steel material, it will be difficult for microcracks to occur even if, thereafter, cold working such as drawing is performed after performing nitridation treatment.
Hereunder, the austenitic stainless steel material according to the present invention is described more specifically by way of examples, although the present embodiment is not limited to these examples.

EXAMPLE

Steel types A to S having the chemical compositions shown in Table 1 were melted in a laboratory and subjected to hot forging to produce steel materials with a thickness of 20 mm. Thereafter, hot rolling was performed to a predetermined thickness to produce steel plates (austenitic stainless steel materials) of 1.25 mm and 7 mm in thickness. The obtained steel plates were then subjected to a heat treatment in which the steel plates were heated to a temperature of 1060°C or more in an atmosphere furnace described in Table 2, followed by rapid cooling. For some of the steel plates, the thickness of a Cr depleted zone was measured in the as heat treatment condition. Thereafter, for steel type N which had been subjected to heat treatment using an atmosphere furnace, a descaling treatment was performed by pickling. Further, for all of the steel materials, evaluation of the occurrence of microcracks and evaluation of weldability were carried out by the methods described below.

[Table 2]

Table 2

Steel	Atmosphere furnace	Evaluation of microcrack occurrence	Evaluation of resistance to weld crack
Steel	Atmosphere furnace	Number of cracks observed/Number of observations	Maximum length of crack (mm)
A	Hydrogen furnace	0/5	0.32
B	Hydrogen furnace	0/5	0.31
C	Hydrogen furnace	0/5	0.32
D	Hydrogen furnace	0/5	0.38
E	Hydrogen furnace	0/5	0.41
F	Hydrogen furnace	0/5	0.36
G	Hydrogen furnace	0/5	0.37
H	Hydrogen furnace	0/5	0.31
I	Hydrogen furnace	0/5	0.25
J	Hydrogen furnace	0/5	0.29
K	Hydrogen furnace	0/5	0.39
L	Hydrogen furnace	0/5	0.37
M	Hydrogen furnace	0/5	0.38
N	Atmosphere furnace	1/5	0.37
O	Hydrogen furnace	5/5	0.33
P	Hydrogen furnace	5/5	1.08
Q	Hydrogen furnace	3/5	0.40
R	Hydrogen furnace	3/5	1.22
S	Hydrogen furnace	0/5	1.20
T	Hydrogen furnace	2/5	0.43

(Measurement of thickness of Cr depleted zone)

For the as-heat-treated steel plates of 1.25 mm in thickness obtained from steel types C and N, a cross section perpendicular to the surface was cut out, and thereafter an FE-EPMA was used to perform a line analysis to a depth of 50 µm from the base metal surface toward the central portion of the wall thickness at a pitch of 0.16 µm to measure the Cr concentration in the depth direction. At such time, the measurement was performed under conditions of an acceleration voltage of 15 kV and an analysis diameter of 0.1 µm. A region where the Cr concentration was 0.9 times or less for the content of Cr in the austenitic stainless steel material was defined as a Cr depleted zone, and the thickness of the Cr depleted zone was measured.
As a result, it was found that although in steel type C which was heat-treated using a hydrogen furnace, the thickness of the Cr depleted zone was approximately 3 µm, in steel type N which was heat-treated using an atmosphere furnace, the thickness of the Cr depleted zone was approximately 15 µm.

(Evaluation of occurrence of microcracks)

The occurrence of microcracks was evaluated by a tensile test, and a subsequent structural observation and the like. Specifically, based on JIS Z 2241: 2022, a No. 5 test coupon specified in JIS was taken from each 1.25-mm thick steel plate. Strain was introduced into the test coupon by performing cold working so that the cold reduction ratio was 30% using a tensile testing machine. Thereafter, nitridation treatment was performed in which the test coupon was held at a heat treatment temperature of 1040°C for a heat treatment time of 30 minutes in a 100% nitrogen atmosphere, and then cooled with gas.
After nitridation treatment, a tensile test was performed once more, and strain was introduced into the tensile test coupon by cold working so that the cold reduction ratio was 25%. At such time, strain was introduced in a manner so that the tensile test coupon did not rupture, and thereafter the surface of the test coupon was observed with a scanning microscope to check for the presence or absence of microcracks. At that time, the observation magnification was set to 500×, and the number of visual fields was set to five visual fields. The number of visual fields in which cracks were observed is shown in Table 2. In the present Example, if the number of visual fields in which cracks were observed among the five visual fields was one or less, it was determined that the test coupon was acceptable.

(Evaluation of weldability)

Weldability was evaluated by examining the resistance to weld crack (solidification cracking) susceptibility in a Trans-Varestraint test. Specifically, a test specimen with dimensions of 4 mm in thickness × 100 mm in width × 100 mm in length was taken from each 7-mm thick steel plate, and the Trans-Varestraint test was performed. Here, the term "Trans-Varestraint test" refers to a test to examine cracking (solidification cracking) that occurs in a weld metal during welding. At a central part in the width direction of the surface of the aforementioned test specimen, welding was performed using GTAW (Gas Tungsten Arc Welding) to form a weld metal extending in the longitudinal direction. Further, at the moment of arc extinction during the welding, a bending strain (2%) was forcibly applied to cause cracks to occur in the weld metal. Note that, the welding condition was set to 6 kJ/cm.
The maximum length of the occurred cracks was measured. The results are shown in Table 2. In the present Example, if the maximum length of the cracks was 1.00 mm or less, it was determined that the resistance to weld crack susceptibility was good. On the other hand, if the maximum length of the cracks was more than 1.00 mm, it was determined that the resistance to weld crack susceptibility was failed.
In steel types A to M which satisfied the requirements of the austenitic stainless steel material of the present embodiment, microcracks did not occur, and weldability was good. Further, in steel type N which was heat-treated using an atmosphere furnace, microcracks were observed in only one visual field, which was within the allowable range, and weldability was also good. On the other hand, in steel types O to T which did not satisfy the requirements of the austenitic stainless steel material of the present embodiment, at least one of microcrack occurrence and a decrease in weldability occurred.
Specifically, in steel type O, the content of N was low and, furthermore, the value of F1 was less than the specified value, and consequently the thickness of the nitridation layer was excessive and microcracks occurred. In steel type P, the content of Cr was excessive, the value of F1 was less than the specified value and, in addition, the value of F2 was more than the specified value. As a result, microcracks occurred, and the weld crack resistance was inferior.
In steel type Q, the value of F1 was less than the specified value, and consequently the thickness of the nitridation layer was excessive and microcracks occurred. In steel type R, the value of F1 was excessive, and consequently nitrides excessively formed and microcracks occurred. In addition, the value of F2 was excessive, and consequently the weld crack resistance was inferior. In steel type S, the value of F2 was excessive, and consequently the weld crack resistance was inferior. In steel type T, Co was not contained, and consequently the thickness of the nitridation layer was excessive and microcracks occurred.

Claims

An austenitic stainless steel material having a chemical composition comprising, in mass%,
C: 0.010 to 0.15%,

Si: 0.10 to 2.00%,

Mn: 0.10 to 3.00%,

P: 0.060% or less,

S: 0.010% or less,

Ni: 16.0 to 25.0%,

Cr: 20.0 to 29.0%,

Mo: 0.08 to 1.0%,

Sn: 0.001 to 0.080%,

N: 0.010 to 0.15%,

Co: 0.001 to 1.50%,

Nb: 0 to 0.20%, and

the balance: Fe and impurities,

wherein:
F1 calculated by Formula (i) below is 7.5 to 20.0; and

F2 calculated by Formula (ii) below is 320.0 or less; $F 1 = 16.0 + 8.0 C + 2.4 Si - 0.5 Mn + 0.3 Ni - 0.9 Cr - 3.1 Mo + 119 N + 2.0 Co$ $F 2 = 286.2 + 337.9 C + 23.9 Si - 2.7 Mn + 0.1 Cr - 15.5 Mo + 339.4 Sn - 286.9 Nb$

where, each symbol of an element in the above formulas represents a content (mass%) of a corresponding element contained in the austenitic stainless steel material, and is assigned a value of 0 if the corresponding element is not contained.
The austenitic stainless steel material according to claim 1, wherein the chemical composition contains, in lieu of a part of the Fe, one or more kinds selected from, in mass%:
Cu: 1.0% or less,

V: 0.30% or less,

Ti: 0.020% or less,

W: 0.10% or less,

Al: 0.050% or less, and

B: 0.050% or less.