EP4663797A1

EP4663797A1 - Austenitic stainless steel material

Info

Publication number: EP4663797A1
Application number: EP24753390.4A
Authority: EP
Inventors: Yoshitomo Fujimura; Jun-Ichi Hamada
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-02-07
Filing date: 2024-02-07
Publication date: 2025-12-17
Also published as: KR20250140582A; JPWO2024166947A1; WO2024166947A1; CN120584210A

Abstract

The present invention has as its topic suppressing intergranular cracking (nitriding resistance) and also suppressing Red iron oxide (oxidation resistance) even in a gas atmosphere containing nitrogen and steam such as ammonia combustion gas in a 500 to 700°C medium/high temperature region and has as its object the provision of such an austenitic stainless steel. The present invention has a predetermined chemical composition giving a nitriding tendency index of 15 or less, polishes the steel product surface to introduce dislocations (strain) and harden the steel sheet surface, and makes the difference of surface hardness and steel product center part hardness 20 Hv or more to thereby form a strong Cr oxide layer and Si oxide layer at a 500°C or more atmosphere at the surface layer and obtain excellent oxidation resistance and nitriding resistance:
Nitriding tendency index: 0.5Cr+10Al+2Mo+3Ti+0.5Cu-1.5Si

Description

FIELD

The present invention relates to an austenitic stainless steel product and a method of production of the same.

BACKGROUND

Global warming is becoming an international environmental issue. Technology is actively being developed for realizing carbon zero, carbon neutral, and other goals of a decarbonized society. Amid such developments, ammonia has come into focus as a fuel to take the place of carbon fuels. The formula for combustion of ammonia is 4NH₃ +3O₂ →2N₂ +6H₂ O. If ammonia burns, water and nitrogen are produced. It is therefore promising as a recyclable fuel with a small environmental load. The combustion temperature of ammonia is, by adiabatic flame temperature, 1750°C. This is lower compared to hydrogen's 2120°C, methane's 1970°C, and gasoline's about 2000°C. The combustion temperature in actual engines and gas turbines also becomes lower than these existing fuels. For this reason, when using ammonia as fuel, the exhaust gas temperature also becomes a lower 500 to 700°C or so than the case of existing fuels. This 500 to 700°C temperature range is a temperature where the steel product used for exhaust pipes etc. easily oxidizes and a temperature region where so-called Red iron oxide easily forms.
PTL 1 proposes austenitic stainless steel having excellent corrosion resistance even in boiler superheater tubes, garbage incinerators, ammonia synthesis apparatuses, and other high sulfur (S), high chlorine (Cl) containing environments.
PTL 2 proposes austenitic stainless steel having excellent corrosion resistance in an ammonia-water absorption heat exchanger etc. without adding chromic acid even if rendered an ammonia atmosphere.

[CITATION LIST]

[PATENT LITERATURE]

[PTL 1] Japanese Unexamined Patent Publication No. 3-126842
[PTL 2] Japanese Unexamined Patent Publication No. 10-280100

SUMMARY

[TECHNICAL PROBLEM]

The use of ammonia as a fuel is being worked on not only for combustion by itself, but also for mixed combustion with other fuels (heavy oil, light oil, hydrogen, etc.) However, while referring to "mixed combustion", in order to burn fuel by adding ammonia with its lower combustion temperature than existing fuels, the combustion exhaust gas temperature becomes 500 to 700°C or so - which is lower than existing fuels. Furthermore, the combustion gas of ammonia includes large amounts of nitrogen and steam.
Due to the inclusion of steam, steam oxidation occurs and Red iron oxide easily forms. Furthermore, the combustion gas temperature is 500 to 700°C or so or also the temperature region in which Red iron oxide easily forms. For this reason, steel products used for ammonia combustion gas systems etc. are required to have oxidation resistance (Red iron oxide resistance).
Furthermore, due to the large amount of nitrogen contained in ammonia combustion exhaust gas, nitrogen penetrates the steel product surface layer (nitrides) leading to embrittlement caused by intergranular cracking. For this reason, steel products for ammonia combustion gas systems are required to have nitriding resistance (intergranular cracking resistance).
The stainless steel of PTL 1 is described as being able to be used for ammonia synthesis apparatuses, but this does not cover the combustion gas of ammonia (combustion exhaust gas). Countermeasures against intergranular cracking due to nitriding and formation of Red iron oxide at 500 to 700°C (oxidizability) are not considered at all.
The stainless steel of PTL 2 is predicated on application to ammonia-water absorption heat exchangers, that is, contact with ammonia gas or ammonia solutions. Measures against intergranular cracking due to nitriding and formation of Red iron oxide at 500 to 700°C (oxidizability) are not considered.
The present invention has as its topic austenitic stainless steel having Red iron oxide resistance (oxidation resistance) and intergranular cracking resistance (nitriding resistance) with respect to gas of 500 to 700°C or so containing large amounts of nitrogen and water (steam) such as ammonia combustion exhaust gas and has as its object the provision of such a steel product (austenitic stainless steel).

[SOLUTION TO PROBLEM]

The inventors engaged in intensive studies to achieve the above object and obtained the following discoveries.

(a) The inventors proceeded with development with the idea of forming a coating having strong oxidation resistance and nitriding resistance on a steel product surface layer. As a result, they discovered that by introducing strain to the steel product surface and hardening the surface, it is possible to form a strong Cr oxide and Si oxide coating (below, together referred to as a "Cr-Si oxide coating"). As to the depth of surface hardening, it was learned that it is possible to form a sufficiently strong Cr-Si oxide coating even 10 to 50 µm or so from the steel product surface.
(b) From the viewpoint of suppressing nitriding, the inventors thought that it was best to optimize the contents of the elements of Cr, Mo, Ti, Al, and Cu promoting nitriding. Furthermore, they discovered that Si not only has the effect of suppressing Red iron oxide due to steam oxidation, but also the effect of suppressing nitriding, though the reason has not been elucidated. They considered the effect of Si together with the contents of the elements of Cr, Mo, Ti, Al, and Cu promoting nitriding so as to derive a nitriding tendency index showing the nitriding tendency of steel products and discovered that it was sufficient to make this nitriding tendency index 15.0 or less: $Nitriding tendency index = 0.5 Cr + 10 Al + 2 Mo + 3 Ti + 0.5 Cu - 1.5 Si \leq 15.0$
(c) The method of hardening the steel product surface layer is not particularly limited, but the inventors discovered that it is sufficient to introduce strain by polishing, shot blasting, shot peening, or otherwise treating the steel sheet surface. From the viewpoint of forming a strong coating of Cr oxide or Si oxide, they discovered that it was possible to produce stainless steel by the ordinary production process, then remove once the oxide coating formed on the surface and introduce strain at the activated surface then heat it to oxidize to thereby form a strong oxide coating. Further, the temperature at the time of this heating and oxidation is 500 to 700°C or so or overlaps the combustion gas temperature of ammonia. For this reason, they discovered that by using the surface treated stainless steel for ammonia combustion apparatuses or exhaust parts and bringing it into contact with the combustion gas, a strong oxide coating is formed at the steel product surface.
(d) Furthermore, the inventors discovered that due to the presence of precipitates at the surface layer of stainless steel products, transport of nitrogen is limited and nitriding of the stainless steel product surface layer is suppressed. Specifically, they discovered that nitriding is suppressed if there are 7/1000 µm² or more particle size 0.1 µm or more precipitates (Nb, Ti, W, B, and V carbonitrides and ε-Cu precipitates) present at the surface layer of stainless steel products (base material) (range from steel product surface down to 20 µm in depth direction).
(e) In particular, they discovered that due to the presence of the above-mentioned precipitates at the grain boundaries which easily become paths for transport of nitrogen (grain boundaries between crystals, below, simply called "grain boundaries"), transport of nitrogen is limited and nitriding is suppressed. Specifically, they discovered that if there are 0.10/µm or more particle size 0.1 µm or more precipitates present at the grain boundaries of the surface layer, nitriding is effectively suppressed.

The present invention is based on these discoveries and has as its gist the following:

[1] An austenitic stainless steel product comprising, by mass%,
- C: 0 to 0.150%,
- Si: 0.05 to 4.50%,
- Mn: 0.05 to 3.00%,
- P: 0.050% or less,
- S: 0.0050% or less,
- Ni: 8.00 to 21.00%,
- Cr: 15.00 to 30.00%,
- N: 0 to 0.350%,
- Nb: 0 to 1.00%
- Mo: 0 to 3.00%
- Cu: 0 to 3.50%
- Al: 0.002 to 0.800%,
- Ti: 0 to 0.600%
- V: 0 to 1.00%,
- B: 0 to 0.0100%,
- Ca: 0 to 0.0150%
- Sn: 0 to 1.00%,
- Hf: 0 to 0.60%,
- Zr: 0 to 0.60%,
- Sb: 0 to 0.60%,
- Co: 0 to 1.50%,
- W: 0 to 2.00%,
- Ta: 0 to 1.00%,
- Ga: 0 to 0.50%,
- Mg: 0 to 0.0050%, and
- REM: 0 to 0.200% and
- having a balance of Fe and impurities,
- in which austenitic stainless steel product,
  - the following formula 1 is satisfied, and
  - a Vickers hardness of the steel product surface (Hvs) is 20 Hv or more higher than the Vickers hardness of the steel product center part (Hvc) (that is, Hvs-Hvc≥29 Hv):: $0.5 Cr + 10 Al + 2 Mo + 3 Ti + 0.5 Cu - 1.5 Si \leq 15.0$
  - where, in formula 1, the element symbols indicate the contents (mass%) of the elements, 0 being entered when not contained.
[2] The austenitic stainless steel product according to [1] having a surface hardened layer having a Vickers hardness 20 Hv or more higher than a Vickers hardness of the steel product center part in a cross-section vertical to the surface of the steel product and has a thickness of the surface hardened layer of 0.5 µm or more from the surface of the steel product in the depth direction.
[3] The austenitic stainless steel product according to [1]or [2], wherein, in the cross-section vertical to the surface of the steel product, there are particle size 0.1 µm or more precipitates present in 7/1000 µm² or more at the surface layer at a depth of 20 µm from the steel product surface.
[4] The austenitic stainless steel product according to any one of [1] to [3], wherein, in the cross-section vertical to the surface of the steel product, there are particle size 0.1 µm or more precipitates present in 0.10/µm or more at the grain boundaries in the surface layer at a depth of 20 µm from the steel product surface.
[5] The austenitic stainless steel product according to any one of [1] to [4], wherein, in the cross-section vertical to the surface of the steel product, a 50 µm square range is defined as one field and a total of the intergranular crack lengths in any three fields is 15 µm or less.
[6] An austenitic stainless steel product for an ammonia combustion apparatus according to any one of [1] to [5].
[7] A method of production of an austenitic stainless steel product according to any one of [1] to [5], which method of production of an austenitic stainless steel product having the steps of heating and holding a steel product having the constituents according to [1] at 900 to 1100°C after a final cold rolling, then cooling it down to 50°C or less in temperature, dipping it in a pickling solution containing fluoric acid in 2.0% or less and nitric acid in 6 to 15% and having a temperature of 50 to 70°C for 60 to 90 seconds to pickle it, then polishing, shot blasting or shot peening the steel product surface.
[8] The method of production of an austenitic stainless steel product according to [7], wherein a holding time at the heating and holding at 900 to 1100°C is 30 seconds or more and 5 minutes or less and a dwell time at 800°C to 300°C in the temperature range at the cooling is 30 seconds or more and 120 seconds or less.
[9] The method of production of an austenitic stainless steel product according to [7] or [8], wherein the polishing is dry polishing and polishes by #220, then #400 and/or #600 in that order.
[10] A part having an austenitic stainless steel product according to any one of [1] to [5] at least in part.
[11] The part according to [10], which is a part for an ammonia combustion apparatus.

[ADVANTAGEOUS EFFECTS OF INVENTION]

According to the austenitic stainless steel according to the present invention, it is possible to obtain stainless steel excellent in oxidation resistance and nitriding resistance even if contacting gas containing large amounts of nitrogen or water (steam) at a temperature of 500 to 700°C or so such as ammonia combustion exhaust gas. Furthermore, austenitic stainless steel is also excellent in high temperature strength and high temperature corrosion resistance compared with ferritic stainless steel. Therefore, the austenitic stainless steel according to the present invention is extremely effective even in an environment where high temperature corrosion resistance and strength are required yet, like in ammonia combustion exhaust gas, the temperature is a not that high 500 to 700°C and nitrogen and steam are present in large amounts.

DESCRIPTION OF EMBODIMENTS

Below, an embodiment of the present invention (below, simply referred to as the "present invention") will be explained. Unless otherwise indicated, "%" relating to the constituents will indicate the mass% in the steel. If no lower limit is particularly prescribed or if the lower limit is 0%, the case of non-inclusion (0%) is included. Further, unless otherwise indicated, "hardness" indicates the Vickers hardness.

C: 0 to 0.150%

C is an element causing the shapeability (r-value) to fall, therefore the less, the more preferable. The upper limit is 0.150%. From the viewpoint of the shapeability, 0.140% or less, 0.120% or less, or 0.100% or less is preferable. The lower limit is not particularly prescribed, but excessive reduction invites a rise in refining costs, therefore 0.001% or more is preferable. It is furthermore preferably 0.002% or more.

Si: 0.05 to 4.50%

Si is an element effective for suppressing oxidation, in particular steam oxidation, and effective for suppressing nitriding as well. Furthermore, from the viewpoint of forming an SiO₂ internal oxidation layer right under the steel product surface, 0.05% or more is contained. The lower limit of Si may preferably be 0.10%, 0.20%, 0.30%, 0.50%, 0.80%, 1.00%, 1.25%, 1.50%, 1.70%, 1.90%, 2.00%, 2.20%, 2.40%, 2.50%, or 2.60%. On the other hand, if increasing the Si content, the area ratio of the Si oxide layer (or the SiO₂ internal oxidation layer) increases and the workability and weldability are lowered, therefore 4.50% is made the upper limit. The upper limit of Si may preferably be 4.30%, 4.10%, or 4.00%.

Mn: 0.05 to 3.00%

Mn, like Si, is an element effective for oxidation resistance, therefore 0.05% or more may be contained. The lower limit of Mn may preferably be 0.07%, 0.10%, 0.13%, or 0.15%. On the other hand, if including a large amount of Mn, the workability is lowered, so 3.00% or less may be contained. The upper limit of Mn is preferably 2.80%, 2.60%, 2.50%, or 2.40%.

P: 0.050% or less

P lowers the toughness, hot workability, and corrosion resistance and otherwise is harmful to stainless steel, therefore the less, the better. It may be made 0.050% or less. Preferably it is made 0.040% or less. However, excessive reduction increases the burden at the time of refining or necessitates use of high cost materials, so practically 0.001% or more may be contained.

S: 0.0050% or less

S lowers the toughness, hot workability, and corrosion resistance and otherwise is harmful to stainless steel, therefore the less, the better. It may be made 0.0050% or less. Preferably it is made 0.0030% or less. However, excessive reduction increases the burden at the time of refining or necessitates use of high cost materials, so practically 0.0001% or more may be contained.

Ni: 8.00 to 21.00%

Ni is an element stabilizing the austenite phase. It has the action of improving corrosion resistance to various types of acids and furthermore improving the low temperature toughness, therefore 8.00% or more may be contained. Preferably 9.00% or more, 10.00% or more, or 11.00% or more may be contained. On the other hand, it is an expensive element, therefore if contained in a large amount, an effect commensurate with the increase in alloy costs cannot be obtained, therefore the content may be 21.00% or less, preferably may be 20.00% or less or 18.00% or less.

Cr: 15.00 to 30.00%

Cr is an important element giving rise to corrosion resistance in stainless steel. 15.00% or more may be contained. Preferably it may be 15.50% or more, 16.00% or more, 17.00% or more, 18.00% or more, 19.00% or more, or 20.00% or more. On the other hand, inclusion in a large amount would invite a drop in workability, therefore the content may be 30.00% or less. Preferably it may be 29.00% or less, 28.00% or less, 27.00% or less, or 26.00% or less.

N: 0 to 0.350%

From the viewpoint of suppressing intergranular crack of the surface, the less the N, which is originally contained in a steel product, the more preferable. Furthermore, N causes a drop in workability and bonds with Cr to cause a drop in corrosion resistance, therefore the less the more preferable. The content may be 0.350% or less. Preferably, it may be 0.300% or less, 0.280% or less, 0.260% or less, 0.240% or less, 0.220% or less, or 0.200% or less. On the other hand, excessive reduction would place a large burden on the refining process, therefore preferably 0.001% or more, 0.005% or more, or 0.010% or more may be contained.

Nb: 0 to 1.00%

Nb has the action of raising the shapeability and corrosion resistance. On the other hand, if the Nb content is too great, recrystallization becomes difficult and the structure becomes coarser, therefore the content may be made 1.00% or less. Preferably, it may be 0.90% or less, 0.80% or less, or 0.70% or less. The lower limit of the Nb content is not particularly prescribed, but to reliably obtain this effect, preferably 0.01% or more may be contained.

Mo: 0 to 3.00%

Mo, by addition, has the action of further raising the high corrosion resistance of stainless steel. On the other hand, not only is it an element promoting nitriding, it also forms a high Cr brittle sigma phase and causes embrittlement and a drop in corrosion resistance, therefore the content may be made 3.00% or less. Preferably it may be 2.50% or less or 2.20% or less. The lower limit of the Mo content is not particularly prescribed, but to reliably obtain the effect of corrosion resistance, preferably 0.01% or more may be contained.

Cu: 0 to 3.50%

Cu, by addition, has the action of further raising the high corrosion resistance of stainless steel. On the other hand, excessive addition does not improve the performance commensurate with the cost in production, therefore the content may be made 3.50% or less. Preferably, it may be 3.20% or less, 3.00% or less, or 2.80% or less. The lower limit of the Cu content is not particularly prescribed, but to reliably obtain the effect, preferably 0.01% or more may be contained.

Al: 0.002 to 0.800%

Al is an element which bonds with N to form AlN and promotes nitriding. Furthermore, excessive addition causes the workability to fall, therefore the Al content may be 0.800% or less. Preferably it may be 0.750% or less, 0.700% or less, 0.600% or less, 0.500% or less, 0.400% or less, 0.300% or less, or 0.200% or less. On the other hand, it has the effect of desulfurization to improve the corrosion resistance, therefore the Al content may be 0.002% or more. Preferably it may be 0.004% or more, 0.006% or more, or 0.008% or more.

Ti: 0 to 0.600%

Ti secures corrosion resistance by the stabilizing action of C or N. On the other hand, Ti is an element promoting nitriding. If excessively added, TiN is remarkably formed and clogging of the nozzles at the time of production or surface defects in the products are invited, therefore the content may be 0.600% or less. Preferably it may be 0.500% or less, 0.400% or less, or 0.300% or less. The lower limit of the Ti content is not particularly prescribed, but to secure its effect, preferably 0.001% or more may be contained.

V: 0 to 1.00%

V, by its addition, has the action of further raising the high corrosion resistance of stainless steel. On the other hand, if contained in a high concentration, it invites a drop in toughness, therefore the upper limit may be 1.00%. Preferably it may be 0.90% or less, 0.70% or less, or 0.50% or less. The lower limit of the V content is not particularly prescribed, but to reliably obtain its effect, preferably 0.01% or more or 0.05% or more may be contained.

B: 0 to 0.0100%

B is an element raising the strength of the grain boundaries and contributes to improvement of the workability. On the other hand, excessive addition conversely invites a drop in elongation and consequent drop in workability, therefore the content may be 0.0100% or less. Preferably it may be 0.0090% or less, 0.0070% or less, or 0.0050% or less. The lower limit of the B content is not particularly prescribed, but to reliably obtain its effect, preferably 0.0001% or more, or 0.0005% or more may be contained.

Ca: 0 to 0.0150%

Ca, if contained in a large amount, rises in concentration in the oxides and promotes the formation of TiN. To eliminate this ability, 0.0150% or less may be contained. Preferably it may be 0.0120% or less, 0.0090% or less, 0.0070% or less, or 0.0050% or less. The lower limit is not particularly prescribed, but Ca is a main constituent of slag and some entrainment is unavoidable. Further, complete removal is difficult. Excessive reduction results in a higher burden at the time of refining. In practical operations, 0.0001% or more or 0.0002% or more may be contained.

Sn: 0 to 1.00%

Sn, by its addition, has the action of further raising the high corrosion resistance of stainless steel. On the other hand, excessive addition leads to a drop in workability, therefore the content may be 1.00% or less. Preferably it may be 0.70% or less, 0.50% or less, or 0.30% or less. The lower limit of the Sn content is not particularly prescribed, but to reliably obtain its effect, preferably 0.01% or more or 0.02% or more may be contained.
In addition, furthermore, by mass%, Hf: 0 to 0.60%, Zr: 0 to 0.60%, Sb: 0 to 0.60%, Co: 0 to 1.50%, W: 0 to 2.00%, Ta: 0 to 1.00%, Ga: 0 to 0.50%, Mg: 0 to 0.0050%, and REM: 0 to 0.200% may also be contained. These elements, by addition, have the actions of raising the corrosion resistance of stainless steel. On the other hand, they are expensive elements, therefore even if excessively contained, effects commensurate with the increase in costs cannot be obtained, therefore upper limits were set. The lower limits of the contents of these elements are not particularly prescribed, but to reliably obtain the effects of inclusion, preferably Mg 0.0001% or more and elements besides Mg respectively 0.001% or more may be contained.
The balance of the above steel constituents is comprised of Fe and impurities. Here, the "impurities" mean, first and foremost, raw materials such as ore and scrap when industrially producing steel, which enter due to various factors in the production process and which are allowed to an extent not detrimentally affecting the present invention.

From the viewpoint of suppressing nitriding of steel, in optimizing the steel constituents, the relationship among the contents of the elements affecting nitriding was considered. As elements aggravating nitriding, Cr, Mo, Ti, Al, and Cu are known. In securing the corrosion resistance and other functions as stainless steel, certain amounts may be contained. Furthermore, it was discovered by the present inventors that Si, an important element contained in the steel of the present invention, not only has the effect of suppressing Red iron oxide caused by steam oxidation, but also has the effect of suppressing nitriding, though the reason why is not certain. Further, while explained later, forming an Si oxide coating (SiO₂ coating) on the steel surface is also effective. Therefore, the inventors thought to combine with a good balance the elements Cr, Mo, Ti, and Al aggravating nitriding the Si with the effect of suppressing nitriding and discovered that in the case of austenitic stainless steel, it is possible to evaluate this by 0.5Cr+10Al+2Mo+3Ti+0.5Cu-1.5Si as an index showing the nitriding tendency. From the viewpoint of suppression of nitriding, this nitriding tendency index may be 15.0 or less.

Nitriding tendency $index = 0.5 Cr + 10 Al + 2 Mo + 3 Ti + 0.5 Cu - 1.5 Si \leq 15.0$
where, in formula 1, the element symbols indicate the contents (mass%) of the elements, 0 being entered when not contained.

The "nitriding tendency index", in a word, is an indicator of ease of nitriding. The smaller the value, the more preferable. For this reason, the upper limit value of the nitriding tendency index preferably may be 14.5, 14.0, 13.5, 13.0, 12.5, 12.0, 11.5, 11.0, or 10.0.

If forming a strong coating of Cr oxides and Si oxides (Cr-Si oxide coating) at the surface layer of a steel product, this is effective for nitriding resistance and oxidation resistance, therefore the inventors proceeded with development from the viewpoint of forming a strong Cr-Si oxide coating. As a result, the inventors discovered that if hardening the steel product surface layer, it is possible to form a strong Cr-Si oxide coating even if heating to 500 to 700°C or so in temperature. This is believed to be due to the following:
That is, to harden the steel product surface layer, dislocations (strain) are introduced to the surface layer. The dislocations probably form paths for diffusion of Cr and Si inside the steel product. Furthermore, if heating the steel product to 500°C or more, heat energy and high density strain become driving forces for accelerated diffusion of Cr and Si from the inside of the steel product toward the surface layer. Probably these bond with the oxygen in the outside environment and can form a strong Cr-Si oxide coating at the steel product surface layer.
Furthermore, by forming the strong Cr-Si oxide coating at the surface layer, it is possible to keep oxygen and nitrogen in the outside environment from penetrating and diffusing in the steel product and possible to keep the Cr and Si in the steel product from being unneedlessly consumed, therefore probably no Cr or Fe based oxides are formed in the steel product, a drop in the Cr concentration is suppressed, and corrosion resistance can be maintained. Further, similarly, due to the strong Cr-Si oxide coating of the surface layer, penetration and diffusion of nitrogen are also suppressed, therefore probably nitriding of the steel product surface layer is suppressed and intergranular cracking due to nitriding can be prevented.
It was confirmed that the hardness of the steel product surface should be 20 Hv or more higher than the hardness of an inside part of the steel product (indicating inside of steel product other than surface hardened layer). Here, the hardness at the inside part of the steel product may be represented by the hardness of the steel product center part. The "steel product center part" indicates the vicinity of the center in the sheet thickness direction if the steel product is steel plate or sheet, indicates the vicinity of the center axis of the steel product if the steel product is a bar or rod, and indicates the center part in the pipe and tube thickness direction if the steel product is a pipe or tube. For example, in the cross-section vertical to the surface of the steel product (below, simply referred to as the "steel product cross-section"), it indicates a region of a distance of 3/8 to 5/8 of the thickness (the "thickness" indicates thickness in the case of a plate or sheet shape and indicates the diameter in the case of a bar or rod shape) from the steel sheet surface in the thickness direction (the "thickness direction" is the direction vertical to the steel sheet surface and toward the center of the steel product (center direction)). That is, when designating the Vickers hardness of the steel sheet surface as Hvs and the Vickers hardness of the steel product center part as Hvc, the following formula 2 is satisfied: $Hvs - Hvc \geq 20 Hv$
The hardness difference between the hardness of the steel product surface (Hvs) and the hardness of the steel product center part (Hvc) may preferably be 22 Hv or more, 24 Hv or more, 26 Hv or more, 28 Hv or more, 30 Hv or more, 32 Hv or more, 34 Hv or more, or 35 Hv or more.

Due to the introduction of dislocations (strain) at the steel product surface, there is a surface hardened layer having a thickness in the depth direction at the steel product surface layer (part right under surface). That is, the surface hardened layer is a region having a Vickers hardness 20 Hv or more higher than the Vickers hardness of the steel product center part. Here, the "depth direction" indicates the direction of the steel product center part vertical to the steel product surface in the cross-section vertical to the surface of the steel product (below, simply referred to as the "steel product cross-section").
The thickness of the surface hardened layer is not particularly limited, but at least 0.5 µm is sufficient. Preferably, it may be 0.7 µm or more, 0.9 µm or more, or 1.0 µm or more.
The upper limit of the thickness of the surface hardened layer is not particularly prescribed. The thickness of the surface hardened layer may be determined in accordance with the thickness of the Cr-Si oxide coating formed at the steel product surface. On the other hand, it is difficult to form the surface hardened layer thickly, therefore practically it is sufficient that the region from the steel product surface down to 10.0 µm or less in the depth direction be the surface hardened layer. Preferably, it may be 15.0 µm or less, 20.0 µm or less, 25.0 µm or less, 30.0 µm or less, 35.0 µm or less, 40.0 µm or less, 45.0 µm or less, or 50.0 µm or less.
For example, the "region from the steel product surface down to 10 µm or less in the depth direction" indicates the region from the steel product surface down to 10 µm in the depth direction.

The Vickers hardness is measured by a pressing load of 200 g. The Vickers hardnesses of five points are measured and the average value of these is made the Vickers hardness of that portion.
The hardness of the steel product surface can be measured at five points of any portion of the surface of the steel product (for example, any selected 5 mm square portion) and the arithmetic average value of these made the surface hardness.
The Vickers hardness at the steel product center part and the surface hardened layer are measured at the cross-section of the steel product. For the Vickers hardness of the steel product center part, five points are measured at a portion near the center part at the cross-section of the steel product (for example, if a plate or sheet or a pipe or tube, a 5 mm square portion including the center of the material thickness and 5 mm square portion including center axis if bar or rod) are measured. The arithmetic average value may be made the hardness of the steel product center part.
The Vickers hardness of the surface hardened layer can be found by measuring the Vickers hardness from the steel product surface down to 0.5 µm, 1 µm, 2 µm, and subsequently every 1 µm pitch in the steel product depth direction at the cross-section of the steel product and comparing this with the hardness of the steel product center part to identify the region having a 20 Hv or more hardness (surface hardened layer).

Furthermore, development efforts have been underway from the viewpoint of suppression of entry of nitrogen from outside the steel product. As a result, it was discovered that if there are precipitates present at the surface layer of a stainless steel product, the transport of nitrogen is limited and nitriding of the stainless steel surface layer is suppressed. Here, the "surface layer of a stainless steel product (base material)" indicates a range from the surface of the steel product down to 20 µm in the depth direction at the cross-section of the steel product.
It was found that, at the cross-section of the steel product if the number density of particle size 0.1 µm or more precipitates at the steel product surface layer (surface layer precipitate density) is 7/1000 µm² or more, nitriding is suppressed. The surface layer precipitate density may preferably be 8/1000 µm² or more, 10/1000 µm² or more, 12/1000 µm² or more, 15/1000 µm² or more, 18/1000 µm² or more, or 20/1000 µm² or more. The upper limit of the surface layer precipitate density is not particularly prescribed, but if the surface layer precipitate density is too high, the properties besides nitriding (for example, toughness) will be affected, so preferably it may be 200/1000 µm² or less, 150/1000 µm² or less, 100/1000 µm² or less, 70/1000 µm² or less, or 50/1000 µm² or less .
The upper limit of the particle size of the precipitates covered by measurement is not particularly prescribed, but if the particle size of the precipitates is too large, the strength or corrosion resistance of the steel product is also affected, therefore it is preferable that large precipitates be made to not form. From this viewpoint, the particle size of the precipitates is preferably kept to 2.0 µm or less, therefore the particle size of the precipitates covered by measurement may also preferably be 2.0 µm or less.
The type of the precipitates is not particularly limited, but for example may be one or more of Nb(C, N), Ti(C, N), W(C, N), B(C, N), V(C, N), and ε-Cu. Here, M(C, N) indicates a carbonitride of an element M (one of carbide or nitride or composite compound of both).

The inventors proceeded with studies on limiting transport of nitrogen due to precipitates, whereupon they learned that if there are precipitates present at the grain boundaries, which easily form paths for transport of nitrogen, it is possible to block transport of nitrogen and efficiently limit the transport of nitrogen. The inventors used experiments to investigate the relationship between nitriding and the precipitates formed at the grain boundaries in the surface layer and as a result learned that, at the grain boundaries in the surface layer, if the number of particle size 0.1 to 2.0 µm precipitates per length of grain boundaries (grain boundary precipitate density) is 0.10/ µm or more, nitriding is effectively suppressed. The grain boundary precipitate density is preferably 5/µm or more, 7/µm or more, 10/µm or more, 15/µm or more, or 20/µm or more. The upper limit of the grain boundary precipitate density is not particularly prescribed, but if the grain boundary precipitate density is too high, the properties other than the nitriding (for example the toughness or corrosion resistance) are affected, therefore preferably may be 100/ µm or less, 70/µm or less, 50/µm or less, 40/µm or less, or 30/µm or less.
The upper limit of the particle size of the precipitates covered by measurement is not particularly prescribed in the same way as the precipitates of the surface layer, but may preferably be 2.0 µm or less.
The type of the precipitates, in the same way as the precipitates of the surface layer, is not particularly limited, but for example may be one or more of Nb(C, N), Ti(C, N), W(C, N), B(C, N), V(C, N), and ε-Cu.

The method of examination of the precipitates at the surface layer will be explained next. At the cross-section of the steel product of the steel product sample examined, the range of depth from the surface down to 20 µm is examined. For example, it is sufficient to examine 50 µm in a direction parallel to the surface at 20 µm from the surface in the depth direction. The area of the examined field at this time becomes 1000µ². The examined field of the sample is observed using an EF-EPMA (for example, JXA-8100, JXA-8350F) and the obtained examined image is analyzed. For example, the examined image obtained by conditions of an accelerated voltage: 15 kV and current: 1x10^-7 A can be analyzed and the precipitates examined. The examined precipitates can be mapped by software attached to the EPMA. The mapped precipitates are measured for particle size and the particle size 0.1 to 2 µm or more precipitates are extracted. By dividing the number of precipitates extracted by the area of the examined field, it is possible to calculate the surface layer precipitate density.
Note that, the particle size of the precipitates was made the average value of the long axis and short axis ((long axis+short axis)/2) when defining the maximum width of the precipitates examined as the "long axis" and defining the maximum width in the direction vertical to the long axis as the "short axis".
Similarly, it is possible to identify grain boundaries from the obtained examined image, extract precipitates present on the grain boundaries and use image processing to calculate the grain boundary precipitate density (/µm) of the line density of precipitates from grain boundary length and the number of precipitates extracted on the grain boundary.
At least three fields are measured and the obtained surface layer precipitate densities and grain boundary precipitate densities are arithmetically averaged to obtain the surface layer precipitate density and grain boundary precipitate density of the steel product.

By making the steel product surface layer the one explained above, nitriding of the steel product surface is suppressed. As a result, intergranular cracking due to penetration of nitrogen (N) is suppressed. Intergranular cracking can be measured for length by examining the crystal grain boundaries. Three locations of any 50 µm square ranges of the surface layer of the steel product cross-section (part including at least nitrided part) may be selected. The total of the intergranular cracking lengths there should be 15 µm or less. If the total of the intergranular cracking lengths at three examined surfaces is 15 µm or less, it is possible to suppress embrittlement of the steel product surface and possible to secure the steel product strength at the 500 to 700°C temperature region. The shorter the total of the intergranular cracking lengths, the more preferable. It is more preferably 14 µm or less, 13 µm or less, 12 µm or less, 11 µm or less, or 10 µm or less.
The steel product can measured for intergranular cracking length of the surface layer in the following way. A cross-section of the steel product used as a sample is examined under an optical microscope for an examined field of a 50 µm square so as to measure the intergranular cracking length. At that time, the closer to the steel product surface, the more susceptible to the effects of nitrogen, therefore it is sufficient to make the part corresponding to right under the steel product surface the examined field. At the time of measurement, it is preferable to use image processing for measurement. For example, it is possible to mark the intergranular cracking on the measurement image and use image processing to measure its length.

From the viewpoint of suppressing nitriding cracking of the surface, the shallower the nitrided depth, the better. The steel product according to the present invention is adjusted in constituents so that nitriding is suppressed and has an Si oxide coating on its surface, therefore the nitrided depth becomes shallower if averaged. In particular, clearly, the smaller the value of the nitriding tendency index (including negative value), the shallower the nitrided depth tends to become. The nitrided depth differs somewhat even in the nitrogen (N) content of the gas contacted, but it was confirmed that if generally 100 µm or less, surface embrittlement was suppressed. The nitrided depth is preferably 90 µm or less, 80 µm or less, 70 µm or less, 60 µm or less, 50 µm or less, 40 µm or less, 30 µm or less, 20 µm or less, 10 µm or less, 5 µm or less, 4 µm or less, 3 µm or less, or 2 µm or less. Note that the range with a nitrogen concentration of 2 mass% or more is defined as the "nitrided layer".

The form of the steel product is not particularly limited. For example, it may be plate and sheet, bars and rods, pipes and tubes, structures, etc. Further, the steel product may be parts obtained by working steel plate and sheet. The working method is not particularly limited. For example, it may be press-forming, drawing, cutting, etc.

Next, the method of production will be explained. The method of production explained below is one embodiment for obtaining a steel product according to the present invention. The present invention is not limited to this method of production. The method of production is not limited if the steel product according to the present invention is obtained.
One embodiment of the method of production of a steel product according to the present invention comprises using an ordinary method to produce a steel product, then acid washing it, then polishing, shot blasting, shot peening, or otherwise treating the surface to introduce strain. Due to this, it is possible to obtain a steel product according to the present invention with a surface hardness higher than the center part. Below, the case where the steel product is a steel sheet and polishing is used as the method of introducing strain to the surface will be explained as an example.
Before the final annealing, the steel sheet may be produced by the usual method of production. For example, it can be produced by the steps of steelmaking-hot rolling, steelmaking-hot rolling-annealing, or steelmaking-hot rolling-pickling-cold rolling.
However, in steelmaking, the method of smelting steel containing constituents adjusted to give the constituents explained above in a converter or electric furnace, then performing secondary refining is suitable. Molten steel adjusted to predetermined constituents in this way is made into a slab according to a known casting method (for example, the continuous casting method). The slab is heated to a predetermined temperature and then hot rolled to a predetermined sheet thickness. After hot rolling, this may be rolled cold (cold rolled) in accordance with need. The cold rolling may also be performed by an ordinary method.
The various conditions in the production process may be suitably selected. For example, the slab thickness, hot rolled sheet thickness, etc. may be suitably selected. The hot rolled sheet may also be dipped in a water cooling pool after coiling. The pickling step after hot rolling or after annealing the hot rolled sheet is not particularly limited. Shot blasting, bending, brushing or other mechanical descaling may also be suitably selected. The pickling solution after hot rolling is also not particularly limited, therefore for example sulfuric acid, nitrofluoric acid, or other existing conditions may be used. Furthermore, after that, the surface of the coil may also be ground down.
The thus obtained hot rolled steel sheet, hot rolled annealed steel sheet, or cold rolled steel sheet is made to recrystallize by final annealing. The annealing atmosphere is not particularly limited and may be the air atmosphere as well. The annealing may be performed at a temperature of 900 to 1100°C in temperature region.
From the viewpoint of formation of precipitates at the surface layer, even if the annealing temperature is too high, precipitation does not proceed while in the solid solution state, therefore the temperature may preferably be 1100°C or less or 1050°C or less. On the other hand, if the annealing temperature is too low, the number of precipitates becomes smaller, therefore it may preferably be 900°C or more, 930°C or more, or 950°C or more. The mechanism has not been elucidated, but if the annealing temperature is low, it is believed that nuclei for formation of precipitates are not generated. The holding time is not particularly limited, but if too short, the precipitation time is liable to become insufficient, therefore it is preferably 30 seconds or more or 60 seconds or more. On the other hand, if the holding time is too long, the precipitates coarsen and the toughness and corrosion resistance are liable to be affected, therefore the holding time preferably may be 5 minutes or less, 3 minutes or less, or 2 minutes or less.
After annealing, the steel sheet is cooled. The cooling conditions are not particularly limited, but to make the precipitates grow to a predetermined particle size, the dwell time of the steel sheet in the temperature range from 800°C to 300°C preferably may be 30 seconds or more and 120 seconds or less. The dwell time more preferably may be 40 seconds or more, 50 seconds or more, or 60 seconds or more and may be 110 seconds or less, 100 seconds or less, or 90 seconds or less.
After the final annealing, the steel sheet is cooled down to 50°C or less, then pickled to etch off the top layer Cr oxide layer and Fe oxide layer. For this reason, the pickling solution is a pickling solution containing fluoric acid (HF) 3.0% or less and nitric acid 6 to 15% and is adjusted to give a dipping time of 60 to 90 seconds at a temperature of 50 to 70°C. Due to this, it is possible to obtain a steel sheet surface from which the top layer Fe oxides, Cr oxides, and Si oxides formed by the production process are removed.
After pickling, the steel sheet is polished on its surface to impart dislocations (strain) to the steel sheet and make the surface harden. It is possible to use the polishing conditions at this time (polishing time, pressing strength, etc.) to adjust the thickness of the surface hardened layer. The polishing conditions are not particularly limited, but dry polishing, wet polishing, etc. can be applied. For example, in the case of dry polishing, a #220 polishing agent may be used to polish the steel sheet surface, then a #400 to #600 polishing agent used to polis the steel sheet surface. For example, after the #220 polishing agent, a #400 and/or #600 polishing agent may be used to polish the surface of the steel sheet. This is because by polishing in an order from a coarse polishing agent to a fine polishing agent, it is possible to reduce the steel sheet surface roughness and eliminate defects. The combination of the polishing agents is not particularly limited. The polishing amount is also not particularly limited, but 10 µm to 50 µm or so is preferable in terms of actual production. In practice, the conditions differ depending on the steel product, therefore it is sufficient to polish the surface in advance and suitably determine them while confirming the surface hardness and surface defects.
In the case of wet polishing, oil in which a polishing agent is mixed in advance is coated on the steel sheet surface. A polishing cloth can be rotated while polishing the steel sheet surface. At this time as well, it is sufficient to polish in order from a coarse polishing agent to a fine one. After polishing, the surface may be rinsed or washed by alkali to remove the polishing agent remaining on the steel sheet surface.
Even when employing shot blasting or shot peening as the method for imparting dislocations (strain) to the surface, the conditions of the shot blasting or shot peening are not particularly limited. Experiments may be conducted in advance to suitably determine them while checking for surface hardness or surface defects.

The steel product according to the present invention has good nitriding resistance, therefore the steel product surface layer is only slightly penetrated by nitrogen and therefore is suppressed in intergranular cracking even if used in a gas atmosphere with a large nitrogen (N) content. Furthermore, it is also provided with oxidation resistance and is effective also against the formation of Red iron oxide, which becomes a problem in conventional stainless steel, in particular in the 500 to 700°C or so medium/high temperature region. From this, for example, the invention can be used for combustion apparatuses for ammonia in which the nitrogen content is high and the gas temperature is a 500 to 700°C medium/high temperature region. In particular, it can be used for exhaust parts etc. of ammonia combustion apparatuses.
Of course, from the nature of nitriding resistance and oxidation resistance, for example, even if used for a container or piping part directly coming into contact with ammonia, urea, etc., the nitrogen ions in the solution or the nitrogen in the gas formed by evaporation of the ammonia or urea in are kept from penetrating the steel product surface layer and intergranular cracking is suppressed.
In addition, this effect can be obtained if applying the steel product according to the present invention to parts in which nitriding resistance and oxidation resistance are demanded.

EXAMPLES

Below, examples will be used to explain the present invention in more detail, but the present invention is not limited to these examples.
Steel of each of the chemical compositions shown in Table 1 was smelted and cast into a slab. The slab was hot rolled to obtain a sheet thickness 4 mm hot rolled steel sheet. After that, the hot rolled sheet was annealed at 900 to 1100°C in temperature, then was pickled and cold rolled to obtain sheet thickness 1.5 mm cold rolled steel sheet. The obtained cold rolled steel sheet was held at 900 to 1100°C in temperature for 90 seconds to anneal it (final annealing), then was cooled for a 800°C to 300°C dwell time for 90 seconds and cooled down to 50°C or less. After that, the steel sheet was dipped in a 50 to 70°C pickling solution (2% fluoric acid+10% nitric acid+water) for 60 to 90 seconds (final pickling), then was rinsed to obtain a test material.
Note that, Sample 3 had a holding time of the final annealing of 30 seconds and was cooled in the later cooling to give a dwell time at 800°C to 300°C of 90 seconds. Sample 8 had a holding time of the final annealing of 30 seconds and was cooled in the later cooling to give a dwell time at 800°C to 300°C of 30 seconds.
From each obtained test material, four 20 mm×25 mm test pieces were cut out and were polished at their surfaces. The polishing was dry polishing. Polishing paper coated with a polishing agent was wound around a plastic holder. The holder was pressed against the surface of the test piece and was moved back and forth at a constant speed (10 m/min) over 300 mm for polishing while setting the polishing pressure as shown in Table 2. The polishing agent used was a #220 polishing agent the first time and a #400 one the second time.
One of the obtained test pieces was cut to obtain a cross-section vertical to the surface. This was measured for the surface Vickers hardness and the center part Vickers hardness by a load of 50 g. Further, the hardness was measured at 0.5 µm, 1.0 µm, 2.0 µm, and every subsequent 1.0 µm pitch from the surface in the depth direction to identify the depth of the surface hardened layer. Furthermore, in the same surface material cross-section, the range of 50 µm in the surface direction and a depth of 20 µm from the surface was made the measured field and FE-EPMA was used to analyze the state of formation of precipitates in the examined field by image. The particle size 0.1 to 2.0 µm precipitates were examined to find the precipitate density of the surface layer (surface layer precipitate density) and the number of precipitates per grain boundary 1 µm (grain boundary precipitate density) obtained by dividing the number of precipitates present at the grain boundaries between the crystal grains by the total of the grain boundary lengths in the examined field.
The remaining three test pieces were treated to nitridize and oxidize them assuming ammonia combustion gas. The nitriding and oxidation treatment was performed by introducing into the atmosphere in the annealing furnace a gas containing ammonia 10 vol%, steam 10 vol%, and a balance of nitrogen (N), placing the remaining test pieces in the furnace, heating them to a temperature of 600°C, then holding them there for 50 hours, then cooling and take them out and measuring the intergranular cracking lengths and the nitrided depths.
The intergranular cracking length was determined by cutting a test piece after nitriding and oxidation treatment to enable a cross-section in the sheet thickness direction to be examined and using an optical microscope to examine the cross-section of the test piece. In the examination, the 50 µm×50 µm range right below the steel sheet surface was defined as one field, three randomly selected locations in the sample cross-section were examined, and the intergranular cracking lengths were measured. If the total of the intergranular cracking lengths at the three examined surfaces was 15 µm or less, the nitriding resistance was good.
The nitrided layer thickness was determined by cutting the test piece after the nitriding and oxidizing treatment and using EPMA analysis to measure the nitrogen concentration distribution at the cross-section from the surface in the thickness direction. The range with a nitrogen concentration of 2 mass% or more was deemed the nitrided layer.
The Red iron oxide property (oxidation resistance) was visually confirmed. Cases where no Red iron oxide could be found were deemed as "good" and case where even a little Red iron oxide was found were deemed as "poor".

The measurement results are shown in Table 2. From the data of Table 2, it will be understood that the steel sheets according to the present invention are reduced in intergranular cracking length.

[Table 1]

Steel	Composition (mass%) (bal.: Fe and impurities)
No.	C	Si	Mn	P	s	Ni	Cr	N	Nb	Mo	Cu	Al	Ti	Others
1	0.050	3.28	0.78	0.025	0.0004	13.80	19.03	0.021	0.05	0.15	0.14	0.023	0.005
2	0.056	0.80	1.22	0.018	0.0011	19.16	26.09	0.016	0.01	0.01	-	0.008	0.020	Sn: 0.13
3	0.097	1.92	1.47	0.027	0.0004	12.18	24.37	0.221	0.00	0.60	0.23	0.121	0.01	B: 0.0009
4	0.055	1.81	0.71	0.002	0.0009	11.02	20.01	0.011	0.13	0.03	0.08	0.012	0.020	REM: 0.097,Mg: 0.0029
5	0.060	2.45	0.81	0.029	0.0010	9.01	18.09	0.025	0.01	0.01	0.12	0.630	0.010	V: 0.25
6	0.061	2.11	1.08	0.024	0.0010	10.01	18.02	0.013	-	0.81	2.01	0.041	0.420	Ca: 0.0006,Ga: 0.15
7	0.063	2.08	0.81	0.029	0.0010	13.10	18.12	0.018	0.00	0.91	2.06	0.021	0.010	Hf: 0.18
8	0.071	2.01	1.56	0.019	0.0007	13.09	21.07	0.198	0.28	1.89	0.16	0.029	-	Zr: 0.39,Sb: 0.09
9	0.069	3.01	0.77	0.023	0.0008	14.10	18.81	0.031	0.09	0.89	2.23	0.034	0.010	Co: 0.34,Ta: 0.29
10	0.053	4.11	1.51	0.027	0.0004	13.91	18.01	0.029	0.01	0.02	3.39	0.041	0.020	W: 1.01
11	0.049	4.45	0.98	0.029	0.0010	15.12	19.01	0.021	0.01	-	-	0.019	0.010
12	0.211	0.81	0.41	0.030	0.0010	8.09	14.56	0.031	0.03	3.01	0.21	0.991	0.001
13	0.081	0.80	2.55	0.028	0.0020	14.12	18.09	0.029	0.01	4.21	0.11	0.032	0.002
14	0.090	0.58	2.99	0.031	0.0020	23.03	33.01	0.018	0.01	1.09	1.09	0.081	0.080
15	0.081	0.51	2.09	0.028	0.0010	15.07	18.01	0.022	0.02	0.01	4.09	1.690	0.010
16	0.061	0.29	0.51	0.031	0.0010	10.88	17.99	0.034	0.51	0.98	0.02	0.079	1.450
17	0.095	0.44	4.19	0.039	0.0020	11.09	20.12	0.029	1.31	2.95	0.02	0.090	0.003
18	0.091	5.31	3.21	0.029	0.0020	18.99	25.02	0.029	0.01	3.78	0.03	0.310	0.004
19	0.041	0.81	0.81	0.035	0.0010	9.01	25.11	0.018	0.00	2.01	0.00	0.021	0.010
20	0.061	2.01	1.01	0.039	0.0010	11.98	18.09	0.017	0.00	0.15	0.00	0.008	0.000
21	0.082	1.09	1.95	0.036	0.0010	10.01	20.01	0.021	0.01	3.51	0.00	0.031	0.010

[Table 2]

Sample no.	Steel no.	Nitriding tendency index	Polishing pressure (kg/cm²)		Vickers hardness (Hv)			Surface hardened layer thickness	Intergranular cracking length	Surface layer precipitate density	Grain boundary precipitate density	Oxidation resistance (Red iron oxide property)	Nitrided depth	Remarks
Sample no.	Steel no.	Nitriding tendency index	1st time #220	2nd time #400	Center	Surface	Hardness difference	Surface hardened layer thickness	Intergranular cracking length	Surface layer precipitate density	Grain boundary precipitate density	Oxidation resistance (Red iron oxide property)	Nitrided depth	Remarks
			1st time #220	2nd time #400	Center	Surface	Hardness difference	(µm)	(µm)	/1000 µm²	/µm		(µm)
Sample 1-1	1	5.2	1.0	1.0	172	215	43	14	0	8	0.20	Good	0.6	Ex.
Sample 1-2			0.2	0.1	172	184	12	0.4	16	9	0.17	Poor	39.8	Comp. ex
Sample 1-3			No polishing	No polishing	172	175	3	0	21	7	0.17	Poor	48.1	Comp. ex.
Sample 2	2	12.0	2.0	1.0	162	195	33	5	0.1	8	0.13	Good	0.8	Ex.
Sample 3	3	11.9	2.0	1.0	198	239	41	10	0	8	0.07	Good	1.1	Ex.
Sample 4	4	7.6	2.0	1.0	183	223	40	11	0	15	0.23	Good	0.8	Ex.
Sample 5	5	11.8	5.0	1.0	180	231	51	13	0.2	17	0.23	Good	0.9	Ex.
Sample 6	6	10.1	2.0	1.0	172	221	49	12	0	36	0.33	Good	0.6	Ex.
Sample 7	7	9.0	1.0	1.0	175	208	33	7	0	12	0.17	Good	0.7	Ex.
Sample 8	8	11.7	1.0	1.0	195	229	34	9	0	6	0.03	Good	1.8	Ex.
Sample 9	9	8.2	5.0	1.0	172	228	56	10	0	18	0.30	Good	0.8	Ex.
Sample 10	10	5.0	2.0	1.0	186	226	40	12	0	19	0.40	Good	0.7	Ex.
Sample 11	11	3.1	2.0	1.0	191	223	32	9	0.1	7	0.10	Good	0.7	Ex.
Sample 12	12	22.1	1.0	1.0	201	234	33	8	16	8	0.07	Poor	31.9	Comp. ex
Sample 13	13	16.6	0.2	0.1	189	195	6	0.3	22	6	0.10	Poor	79.1	Comp. ex
Sample 14	14	19.4	0.1	0.1	205	210	5	0.2	24	7	0.10	Poor	54.1	Comp. ex.
Sample 15	15	27.2	1.0	1.0	210	245	35	8	21	21	0.40	Poor	73.7	Comp. ex.
Sample 16	16	15.7	No polishing	0.2	189	193	4	0	17	51	0.60	Poor	85.2	Comp. ex.
Sample 17	17	16.2	0.2	No polishing	183	185	2	0	22	48	0.57	Poor	73.5	Comp. ex.
Sample 18	18	15.2	No polishing	0.3	195	205	10	0.4	18	5	0.03	Poor	51.2	Comp. ex.
Sample 19	19	15.6	No polishing		220	223	3	0	20	4	0.07	Poor	51.2	Comp. ex
Sample 20	20	6.4	No polishing	0.2	181	188	7	0.3	16	0	0.00	Poor	49.1	Comp. ex.
Sample 21	21	15.7	No polishing		190	193	3	0	24	3	0.03	Poor	74.1	Comp. ex

[INDUSTRIAL APPLICABILITY]

The present invention can be utilized in the automobile industry, general machinery industry, and all sorts of other industries.

Claims

An austenitic stainless steel product comprising, by mass%,
C: 0 to 0.150%,

Si: 0.05 to 4.50%,

Mn: 0.05 to 3.00%,

P: 0.050% or less,

S: 0.0050% or less,

Ni: 8.00 to 21.00%,

Cr: 15.00 to 30.00%,

N: 0 to 0.350%,

Nb: 0 to 1.00%

Mo: 0 to 3.00%

Cu: 0 to 3.50%

Al: 0.002 to 0.800%,

Ti: 0 to 0.600%

V: 0 to 1.00%,

B: 0 to 0.0100%,

Ca: 0 to 0.0150%

Sn: 0 to 1.00%,

Hf: 0 to 0.60%,

Zr: 0 to 0.60%,

Sb: 0 to 0.60%,

Co: 0 to 1.50%,

W: 0 to 2.00%,

Ta: 0 to 1.00%,

Ga: 0 to 0.50%,

Mg: 0 to 0.0050%, and

REM: 0 to 0.200% and

having a balance of Fe and impurities,

in which austenitic stainless steel product,
the following formula 1 is satisfied, and

a Vickers hardness of the steel product surface (Hvs) is 20 Hv or more higher than the Vickers hardness of the steel product center part (Hvc): $0.5 Cr + 10 Al + 2 Mo + 3 Ti + 0.5 Cu - 1.5 Si \leq 15.0$

where, in formula 1, the element symbols indicate the contents (mass%) of the elements, 0 being entered when not contained.
The austenitic stainless steel product according to claim 1 having a surface hardened layer having a Vickers hardness 20 Hv or more higher than a Vickers hardness of the steel product center part in a cross-section vertical to the surface of the steel product and has a thickness of the surface hardened layer of 0.5 µm or more from the surface of the steel product in the depth direction.
The austenitic stainless steel product according to claim 1 or 2, wherein, in the cross-section vertical to the surface of the steel product, there are particle size 0.1 µm or more precipitates present in 7/1000 µm² or more at a surface layer at a depth of 20 µm from the steel product surface.
The austenitic stainless steel product according to any one of claims 1 to 3, wherein, in the cross-section vertical to the surface of the steel product, there are particle size 0.1 µm or more precipitates present in 0.10/µm or more at the grain boundaries in the surface layer at a depth of 20 µm from the steel product surface.
The austenitic stainless steel product according to any one of claims 1 to 4, wherein, in the cross-section vertical to the surface of the steel product, a 50 µm square range is defined as one field and a total of intergranular crack lengths in any three fields is 15 µm or less.
An austenitic stainless steel product for an ammonia combustion apparatus according to any one of claims 1 to 5.
A method of production of an austenitic stainless steel product according to any one of claims 1 to 6, which method of production of an austenitic stainless steel product having the steps of heating and holding a steel product having the constituents according to claim 1 at 900 to 1100°C after a final cold rolling, then cooling the steel product down to 50°C or less in temperature, pickling the steel product by dipping the steel product in a pickling solution containing fluoric acid in 2.0% or less and nitric acid in 6 to 15% and having a temperature of 50 to 70°C for 60 to 90 seconds, then polishing, shot blasting or shot peening the steel product surface.
The method of production of an austenitic stainless steel product according to claim 7, wherein a holding time at the heating and holding at 900 to 1100°C is 30 seconds or more and 90 seconds and a dwell time at temperature range of 800°C to 300°C at the cooling is 30 seconds or more and 120 seconds or less.
The method of production of an austenitic stainless steel product according to claim 7 or 8, wherein the polishing is dry polishing and polishes by #220, then #400 and/or #600 in that order.
A part having an austenitic stainless steel product according to any of claims 1 to 6 at least in part.
The part according to claim 10, which is a part for an ammonia combustion apparatus.