EP4779046A1

EP4779046A1 - Ferrite-based stainless steel

Info

Publication number: EP4779046A1
Application number: EP24864958.4A
Authority: EP
Inventors: Yoshitomo Fujimura; Atsutaka Hayashi; Junichi Hamada
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-09-13
Filing date: 2024-02-07
Publication date: 2026-07-22
Also published as: CN121712916A; WO2025057448A1; KR20260042291A; JPWO2025057448A1

Abstract

The present invention has as its technical concern the suppression of intergranular cracking (nitriding resistance) and suppression of formation of red iron oxide (oxidation resistance) in the 500 to 700°C medium and high temperature regions even in a gas atmosphere containing nitrogen and water vapor such as ammonia combustion gas and has as its object the provision of such a ferritic stainless steel.

The present invention has a predetermined chemical composition giving a nitriding tendency index of 5 or less while polishes the steel product surface to introduce dislocations (strain), cause the steel product surface to harden, and make the difference in surface hardness and steel product center part hardness 25 Hv or more, whereby a strong Cr-Si-based oxide film strong in a 500°C or more atmosphere is formed at the surface layer and excellent oxidation resistance and nitriding resistance are obtained.
Nitriding tendency index: 10Al+2Mo+3Ti+0.5Cu-1.5Si

Description

FIELD

The present invention relates to ferritic stainless steel and a method of production of the same.

BACKGROUND

Global warming is becoming an international environmental problem. Carbon zero, carbon neutral, and other technologies are being actively developed for realization of a carbon-free society. In the midst of such a trend, ammonia is being focused on as a fuel to take the place of carbon fuel. The reaction formula for combustion of ammonia is 4NH₃ +3O₂ →2N₂ +6H₂ O. The environmental load created by water and nitrogen is small. Much can be expected from this as a sustainable fuel. The combustion temperature of ammonia is an adiabatic flame temperature of 1750°C. This is lower compared with the 2120°C of hydrogen, the 1970°C of methane, and the approximately 2000°C of hydrogen. The actual combustion temperature in an engine or gas turbine also becomes lower compared with existing fuels. For this reason, if using ammonia as a fuel, the exhaust gas temperature also becomes a temperature of 500 to 700°C or so lower than the case of existing fuels. This 500 to 700°C temperature region is a temperature in which the steel product used for exhaust pipes etc. easily oxidize and a temperature region in which so-called red iron oxide easily occurs.
PTL 1 proposes a ferritic steel product, for use for a reformer for a fuel cell, which has an oxidation resistance and red iron oxide resistance even at a 600°C or so high temperature.
PTL 2 proposes a ferritic steel product for an exhaust pipe of an internal combustion engine provided with a pollution control system having urea or ammonia for reducing nitrogen oxides.

[CITATION LIST]

[PATENT LITERATURE]

[PTL 1] Japanese Unexamined Patent Publication No. 2003-160844
[PTL 2] Japanese Unexamined Patent Publication No. 2015-532681

SUMMARY

[TECHNICAL PROBLEM]

Ammonia is not just being used as a fuel for combustion alone. Mixed combustion with other fuels (heavy oil, light oil, hydrogen, etc.) is also being developed. However, while referring to "mixed combustion", ammonia, which has a lower combustion temperature than existing fuels, is added for combustion, therefore the combustion exhaust gas temperature becomes 500 to 700°C or so or lower than existing fuels. Furthermore, the combustion gas of ammonia contains large amounts of nitrogen, water vapor, and hydrogen.
Since water vapor and hydrogen are contained, steam oxidation causes red iron oxide to easily form. Further, the combustion gas temperature is 500 to 700°C or so, therefore is also a temperature region where red iron oxide easily forms. For this reason, oxidation resistance (red iron oxide resistance) is demanded from a steel product used for an ammonia combustion gas system etc.
Furthermore, due to the large amount of nitrogen contained in ammonia combustion exhaust gas, nitrogen penetrates the steel product surface layer (nitrides it) resulting in embrittlement due to intergranular cracking. For this reason, nitriding resistance (intergranular cracking resistance) is also demanded from a steel product used for an ammonia combustion gas system etc.
The stainless steel of PTL 1 is one securing high temperature oxidation resistance and red iron oxide resistance as a steel product for exhaust gas system parts, but countermeasures against intergranular cracking due to surface layer nitriding of the gas containing large amounts of nitrogen such as ammonia combustion gas are not considered.
The stainless steel of PTL 2 to an extent has the effect of suppression of surface layer intergranular cracking due to urea and ammonia, but the nitriding resistance cannot be said to be sufficient for a large amount of nitrogen-containing gas such as the combustion exhaust gas of ammonia.
The present invention has as its technical concern the fact that ferritic stainless steel has a red iron oxide resistance (oxidation resistance) and intergranular cracking resistance (nitriding resistance) even with respect to gas containing large amounts of nitrogen and water (water vapor) and hydrogen at 500 to 700°C or so such as ammonia combustion exhaust gas and has as its object the provision of such a steel product (ferritic stainless steel product).

[SOLUTION TO PROBLEM]

To solve this problem, the inventors engaged in intensive studies and obtained the following discoveries.

(a) The inventors proceeded with development thinking of forming a film having a strong oxidation resistance and nitriding resistance at a steel product surface. As a result, they discovered that if introducing dislocations (strain) to the steel product surface to form a surface hardened layer, diffusion of Cr and Si is promoted and a strong film of Cr oxides and Si oxides (below, called a "Cr-Si-based oxide film) can be formed at the surface. It was learned that if there is a surface hardened layer present from the steel product surface and its thickness is 0.5 µm or more, it is possible to form a strong Cr-Si-based oxide film. The "surface hardened layer" is a region with a difference of hardness from the steel product center part (region of 3/8 to 5/8 of the steel product thickness in the thickness direction from the steel product surface) of a Vickers hardness of 25 Hv or more. The inventors discovered that if the surface hardness of the steel product is different from the steel product center part by 25 Hv or more, there is a surface hardened layer of at least a thickness of 0.5 µm.
(b) The inventors thought that from the viewpoint of suppression of nitriding, the contents of the elements promoting nitriding, that is, Cr, Mo, Ti, Al, and Cu, should be optimized. Further, they discovered that Si not only has an effect of suppression of red iron oxide by steam oxidation, but, though the reason has not been elucidated, also has the effect of suppression of nitriding. It is possible to consider the contents of the elements Cr, Mo, Ti, Al, and Cu promoting nitriding and the effect of suppression of nitriding by Si so as to derive a nitriding tendency index showing the tendency in nitriding of a steel product and make this nitriding tendency index 5.0 or less: $Nitriding tendency index = 10 Al + 2 Mo + 3 Ti + 0.5 Cu - 1.5 Si \leq 5.0$
(c) The method of introduction of strain to the steel product surface layer is not particularly limited, but the inventors discovered that the steel product surface may be polished, shot blasted, shot peened, etc. From the viewpoint of forming a strong oxide film, the inventors discovered that by producing stainless steel by the normal production process, then removing once the oxide film formed on the surface and introducing strain to the activated surface and oxidizing it by heating, it is possible to form a strong oxide film. Further, the temperature at the time of heating and oxidation is 500 to 700°C or so. This overlaps with the combustion gas temperature of ammonia. Therefore, the inventors discovered that by using surface hardened stainless steel for an ammonia combustion apparatus and exhaust part and bringing it into contact with a combustion gas, it is possible to form a strong oxide film on the steel product surface.
(d) Furthermore, the inventors discovered that the presence of precipitates at the surface layer of the stainless steel product limits movement of nitrogen and suppresses nitriding at the surface layer of the stainless steel product. Specifically, the inventors discovered that the presence of particle size 0.1 µm or more precipitates (Nb, Ti, W, B, and V carbonitrides and ε-Cu precipitates) at the surface layer of a stainless steel product (base material) (range down to 20 µm in depth direction from steel product surface) in 10/1000 µm² or more suppresses nitriding.
(e) In particular, the inventors discovered that the presence of the above precipitates at the grain boundaries particularly easily becoming paths for movement of nitrogen (grain boundaries between grains, below simply referred to as "grain boundaries") limits movement of nitrogen and suppresses nitriding. Specifically, they discovered that if precipitates with a particle size of 0.1 µm or more (preferably 2.0 µm or less) are present in 0.17/µm or more at the surface layer, nitriding is effectively suppressed.

The present invention was made based on these findings and has as its gist the following:

[1] A ferritic stainless steel product having a composition containing, by mass%,
- C: 0 to 0.030%,
- Si: 0.03 to 3.00%,
- Mn: 0.03 to 1.20%,
- P: 0.050% or less,
- S: 0.005% or less,
- Ni: 0 to 1.00%,
- Cr: 12.0 to 31.0%,
- N: 0 to 0.030%,
- Nb: 0 to 1.00%
- Mo: 0 to 2.50%
- Cu: 0 to 3.00%
- Al: 0.002 to 0.500%,
- 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 0.20%,
- satisfying the following formula 1, and
- having a balance of Fe and impurities,
- in which steel product,
- a Vickers hardness (Hvs) of a surface of the steel product and a Vickers hardness (Hvc) of a center part of the steel product satisfy the following formula 2: $10 Al + 2 Mo + 3 Ti + 0.5 Cu - 1.5 Si \leq 5.0$ $Hvs - Hvc \geq 25 Hv$
- where, notations of elements in formula 1 indicate contents of the elements (mass%), 0 being entered when not contained.
Further, the "center part of steel product" indicates a region in the steel product cross-section in a range of 3/8 to 5/8 of the thickness in the thickness direction of the steel product.
[2] The ferritic stainless steel product according to [1], which, in a cross-section vertical to the surface of the steel product, has a surface hardened layer having Vickers hardness higher by 25 Hv or more than a Vickers hardness of a center part of the steel product and has a thickness of the surface hardened layer of 0.5 µm or more in a thickness direction from the surface of the steel product.
[3] The ferritic stainless steel product according to [1] or [2], wherein, in a cross-section vertical to the surface of the steel product, particle size 0.1 µm or more (preferably 2.0 µm or less) precipitates are present in 10/1000 µm² or more at a surface layer of a depth of 20 µm from the steel product surface.
[4] The ferritic stainless steel product according to any one of [1] to [3], wherein, in a cross-section vertical to the surface of the steel product, particle size 0.1 µm or more (preferably 2.0 µm or less) precipitates are present in 0.17/µm or more at grain boundaries in a surface layer of a depth of 20 µm from the steel product surface.
[5] The ferritic stainless steel product according to any one of 1] to [4], wherein, in a cross-section vertical to the surface of the steel product, when a 100 µm square range is 1 field, a total of intergranular crack lengths of any three fields is 5 µm or less.
[6] The ferritic stainless steel product according to any one of [1] to [5], wherein when holding the steel product at 600°C for 10 minutes, then analyzing a region of a distance of 500 nm (0.5 µm) or less in the thickness direction from the surface of the steel product by GD-OES, the obtained peak intensity I_{C r} of Cr, peak intensity I_{S i} of Si, and peak intensity I_N of N (nitrogen) satisfy the following formula 3: $I_{Cr} + 2 \times I_{Si} - 0.5 \times I_{N} \geq 0.30$
[7] A ferritic stainless steel product for use for an ammonia combustion apparatus according to any one of [1] to [6].
[8] A method of production of a ferritic stainless steel product according to any one of [1] to [6], which method of production of a ferritic stainless steel product includes a step of holding a steel product having a composition according to [1] at a 900 to 1100°C temperature region, then cooling it down to a 50°C or less temperature, dipping it in a pickling solution containing fluoric acid in 2.0% or less and nitric acid 6 to 15% and having a temperature of 50 to 60°C for 40 to 60 seconds, then polishing the surface of the steel product and shot blasting or shot peening it.
[9] The method of production of a ferritic stainless steel product according to [8], wherein a holding time in the 900 to 1100°C temperature region is 30 seconds or more and 5 minutes or less and, in the cooling, a dwell time in a temperature range from 800°C to 300°C is 30 seconds or more and 120 seconds or less.
[10] The method of production of a ferritic stainless steel product according to [8] or [9], wherein the polishing is dry polishing and polishes in the order of #220, followed by #400 or/and #600.
[11] A part having the ferritic stainless steel product according to any one of [1] to [6] at least at a portion of it.
[12] A part according to [11], which is a part for an ammonia combustion apparatus.

[ADVANTAGEOUS EFFECTS OF INVENTION]

According to the ferritic stainless steel according to the present invention, it is possible to obtain a ferritic stainless steel excellent in oxidation resistance and nitriding resistance even if contacted by a gas containing a large amount of nitrogen or water (water vapor) of a temperature of 500 to 700°C or so such as ammonia combustion exhaust gas.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing a relationship of a nitriding resistance index and intergranular crack length.

DESCRIPTION OF EMBODIMENTS

Below, an embodiment of the present invention (below, simply referred to as "the present invention") will be explained. Unless otherwise indicated, the "%" relating to the constituents indicate mass% in the steel. In particular, the case when the lower limit is not prescribed or when the lower limit is 0% includes cases where not contained (0%). Further, unless otherwise indicated, the hardness shows the Vickers hardness. Further, in this Description, unless otherwise indicated, the ferritic stainless steel product will simply be called the "steel product". The form is not particularly limited and includes steel sheets, steel bars, wire rods, steel shapes, steel tubes, etc.

C: 0 to 0.030%

C is an element lowering the shapeability (r-value), therefore the less, the more preferable. The upper limit is 0.030%. From the viewpoint of shapeability, 0.020% or less or 0.010% or less is preferable. The lower limit is not particularly prescribed, but excessive reduction would invite a rise in refining costs, therefore 0.001% or more is preferable. The content is more preferably 0.002% or more.

Si: 0.03 to 3.00%

Si is an element effective for suppressing oxidation, in particular steam oxidation, and effective for suppression of nitriding as well. Further, from the viewpoint of forming an SiO₂ internal oxide layer right below the surface of a steel product, 0.03% or more is contained. The lower limit of Si preferably may be 0.05%, 0.10%, 0.15%, 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 film (or SiO₂ internal oxide layer) increases and the workability and weldability are made to worsen, therefore 3.00% is the upper limit. The upper limit of Si is preferably 2.95% or 2.90%.

Mn: 0.03 to 1.20%

Mn, like Si, is an element effective for oxidation resistance, therefore 0.03% or more may be contained. The lower limit of Mn preferably may be 0.05%, 0.07%, 0.09%, 0.11%, 0.13%, or 0.15%. On the other hand, excessive inclusion of Mn would have a detrimental effect on the oxidation resistance and cause the workability to worsen, therefore 1.20% or less may be contained. The upper limit of Mn preferably may be 1.10%, 1.00%, 0.90, or 0.80%.

P: 0.050% or less

P is an element causing the toughness, hot workability, and corrosion resistance to drop etc. and is otherwise harmful to stainless steel, therefore the less, the better. The content may be 0.050% or less, preferably may be 0.040% or less. However, excessive reduction would raise the load at the time of refining or require the use of high priced starting materials, therefore practically 0.001% or more may be contained.

S: 0.0050% or less

S is an element causing the toughness, hot workability, and corrosion resistance to drop etc. and is otherwise harmful to stainless steel, therefore the less, the better. The upper limit may be 0.0050% or less, preferably may be 0.0030% or less. However, excessive reduction would raise the load at the time of refining or require the use of high priced starting materials, therefore practically 0.0001% or more may be contained.

Ni: 0 to 1.00% or less

Ni, by addition, has the action of further raising the high corrosion resistance of stainless steel. On the other hand, it is an expensive element, therefore even if contained in a large amount, an effect commensurate with the increase in alloying costs cannot be obtained, therefore the content may be made 1.00% or less. Preferably, it may be 0.80% or less, 0.60% or less, or 0.50% or less. The lower limit of Ni content is not particularly prescribed, but, to reliably obtain that effect, 0.01% or more may be contained.

Cr: 12.00 to 31.00%

Cr is an important element giving rise to corrosion resistance in stainless steel. It may be contained in 12.00% or more, preferably 12.50% or more, 13.00% or more, 14.00% or more, 15.00% or more, 18.00% or more, or 20.00% or more. On the other hand, inclusion of a large amount invites a drop in workability, therefore the content may be 31.00% or less, preferably 30.00% or less, 29.00% or less, 28.00% or less, 26.00% or less, or 24.00% or less

N: 0 to 0.030%

From the viewpoint of suppressing intergranular cracking of the surface, the less the N inherently contained in the steel product, the more preferable. Furthermore, N causes the workability to drop and bonds with Cr to cause the corrosion resistance to drop, therefore the less, the more preferable. The content may be 0.030% or less, preferably may be 0.025% or less, 0.020% or less, 0.015% or less, or 0.010% or less. On the other hand, excessive reduction would cause a large load in the refining step, therefore preferably 0.001% or more, 0.003% or more, or 0.005% or more may be contained.

Nb: 0 to 1.00%

Nb has the action of raising the shapeability or corrosion resistance. On the other hand, if more than 1.00% is added, recrystallization becomes difficult and the structures become coarser, therefore the content may be 1.00% or less, preferably may be 0.90% or less, 0.80% or less, or 0.70% or less. The lower limit of Nb content is not particularly prescribed, but to reliably obtain that effect, preferably 0.01% or more may be contained.

Mo: 0 to 2.50%

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, but also it forms brittle sigma phases with high Cr and invites embrittlement and a drop in corrosion resistance, therefore the content may be 2.50% or less, preferably may be 2.20% or less or 2.00% or less. The lower limit of Mo content is not particularly prescribed, but to reliably obtain that effect of corrosion resistance, preferably 0.01% or more may be contained.

Cu: 0 to 3.00%

Cu, by addition, has the action of further raising the high corrosion resistance of stainless steel. On the other hand, addition in a further larger amount would not improve the performance to a level commensurate with the cost in production therefore the content may be 3.00% or less, preferably may be 2.50% or less, 2.20% or less, or 1.90% or less. The lower limit of Cu content is not particularly prescribed, but, to reliably obtain that effect, preferably 0.01% or more may be contained.

Al: 0.002 to 0.500%

Al is an element bonding with N to form AlN and promoting nitriding. Furthermore, excessive addition would cause the workability to fall. Therefore, the Al content may be 0.500% or less, preferably may be 0.450% or less, 0.400% or less, 0.350% or less, 0.300% or less, 0.250% 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 may be 0.004% or more, 0.007% or more, or 0.010% or more.

Ti: 0 to 0.600%

Ti ensures the corrosion resistance by the stabilization action of C and N. On the other hand, Ti is an element assisting nitriding. If excessively added, TiN would be remarkably formed and nozzle clogging at the time of production or surface defects of the product would be invited, therefore the content may be 0.600% or less, preferably may be 0.500% or less, 0.400% or less, or 0.300% or less. The lower limit of Ti content is not particularly prescribed, but, to reliably obtain that effect, preferably 0.001% or more may be contained.

V: 0 to 1.00%

V, by addition, has the action of further raising the high corrosion resistance of stainless steel. On the other hand, if contained in a high concentration, a drop in the toughness is invited, therefore the upper limit may be 1.00%, preferably may be 0.90% or less, 0.70% or less, or 0.50% or less. The lower limit of V content is not particularly prescribed, but to reliably obtain that 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 grain boundaries and contributing to improvement of the workability. On the other hand, excessive addition would conversely invite a drop in workability due to the drop in elongation, therefore the content may be 0.0100% or less, preferably may be 0.0090% or less, 0.0070% or less, or 0.0050% or less. The lower limit of B content is not particularly prescribed, but to reliably obtain that 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 oxides promoting the formation of TiN and causes a loss of that ability, therefore may be contained in 0.0150% or less, preferably 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 major component of slag and some level of entrainment is unavoidable. Further, complete removal is difficult. Excessive reduction would raise the load at the time of refining, therefore in actual operations, 0.0001% or more or 0.0002% or more may be contained.

Sn: 0 to 1.00%

Sn, by addition, has the effect of further raising the high corrosion resistance of stainless steel. On the other hand, excessive addition would lead to a drop in workability, therefore the content may be 1.00% or less, preferably may be 0.70% or less, 0.50% or less, or 0.30% or less. The lower limit of Sn content is not particularly prescribed, but to reliably obtain that effect, preferably 0.001% or more or 0.002% or more may be contained.
In addition, furthermore, by mass%, Hf: 0 to 0.600%, Zr: 0 to 0.600%, Sb: 0 to 0.600%, Co: 0 to 1.500%, W: 0 to 2.000%, Ta: 0 to 1.000%, Ga: 0 to 0.500%, Mg: 0 to 0.0050%, and REM: 0 to 0.200% may be contained. These elements, by addition, have the action of raising the corrosion resistance of stainless steel. On the other hand, it is an expensive element, therefore even if contained in a further larger amount, an effect commensurate with the increase in cost is not obtained, therefore an upper limit was set. The lower limits of contents of these elements are not particularly prescribed, but to reliably obtain the effect of inclusion, preferably Mg is 0.0001% or more and elements other than Mg may be contained in respectively 0.001% or more.
The balance of the steel constituents is Fe and impurities. Here, "impurities" are constituents entering due to various factors in the production process such as the ores, scraps, and other starting materials when industrially producing steel and allowed in a range not having a detrimental effect on the present invention.

In optimizing the steel constituents from the viewpoint of suppressing the nitriding of the steel, the relationship of the contents of the elements affecting the nitriding was considered. As elements promoting nitriding, Cr, Mo, Ti, and Al are known, but certain amounts may be contained in securing the corrosion resistance and other functions as stainless steel. Furthermore, the inventors discovered that the important element Si contained in the steel according to the present invention not only has the effect of suppression of red iron oxide due to steam oxidation, but also, although the reason is not certain, has the effect of suppression of nitriding. Further, while explained later, forming an Si oxide film (SiO₂ film) on the steel surface is also effective. Therefore, the inventors thought to combine with a good balance the elements Mo, Ti, Al, and Cu promoting nitriding and the Si with the effect of nitriding suppression and discovered that in the case of ferritic stainless steel, it is possible to use 10A1+2Mo+3Ti+0.5Cu-1.5Si for evaluation as an index showing the nitriding tendency. They discovered that from the viewpoint of suppression of nitriding, this nitriding tendency index may be made 5.0 or less.
$Nitriding tendency index = 10 Al + 2 Mo + 3 Ti + 0.5 Cu - 1.5 Si \leq 5.0$ where, notations of elements in formula 1 indicate contents of the elements (mass%), 0 being entered when not contained.
The nitriding tendency index, in a word, is an index of the 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 4.8, 4.6, 4.4, 4.2, 4.0, 3.9, 3.8, 3.7, 3.6, or 3.5.

If hardening the steel product surface layer, it is possible to form a strong Cr-Si-based oxide film in the 500 to 700°C or so temperature region. This is believed due to the following such reason.
That is, hardening the steel product surface layer means introducing dislocations (strain) to the surface layer. These dislocations are believed to become paths for diffusion of Cr and Si inside of the steel product. Furthermore, it is believed that if heating the steel product to 500°C or more, the heat energy and high density strain become drive forces causing the diffusion of Cr or Si from inside the steel product toward the surface layer to accelerate and causing these to bond with the oxygen in the external environment whereby a strong oxide film is formed at the steel product surface.
Furthermore, due to the formation of a strong Cr-Si-based oxide film at the surface, the oxygen or nitrogen of the external environment can be kept from penetrating and diffusing into the steel product and the Cr and Si in the steel product can be kept from being needlessly consumed, therefore it is believed possible to keep the Cr concentration from dropping and maintain the corrosion resistance. Further, similarly, due to the strong Cr-Si-based oxide film at the surface, penetration and diffusion of nitrogen are also suppressed, therefore nitriding of the steel product surface layer (part right under surface of steel product) is suppressed and intergranular cracking due to nitriding can be prevented.
To introduce dislocations (strain) at the steel product surface, there should be a surface hardened layer at the steel product surface layer (right under surface of steel product). The surface hardened layer is a region having a Vickers hardness 25 Hv or more higher than the steel product center part. The hardness of the surface hardened layer was confirmed to be 25 Hv or more higher than the hardness of the inside of the steel product (indicating inside of steel product other than surface hardened layer). Here, the hardness of the inside of the steel product can be represented by the hardness of the steel product center part. The "center part of the steel product" indicates a region at the cross-section vertical to the surface of the steel product (below, simply called the "steel product cross-section") of a distance of from 3/8 to 5/8 of the thickness ("thickness" indicating thickness in the case of a sheet and diameter in the case of a bar) in the thickness direction from the steel product surface ("thickness direction" being direction vertical to steel product surface heading toward steel product center (center direction)). If the steel product is a steel sheet, it indicates the vicinity of the center in the sheet thickness direction, if the steel product is a steel bar or wire rod, it indicates the vicinity of the center axis of the steel product, and if the steel product is a steel tube, it indicates a center part in the wall thickness direction of the steel tube.
The surface hardened layer is present right below the steel product surface and has a thickness in the thickness direction from the steel product surface. The lower limit of the thickness of the surface hardened layer is not particularly prescribed, but may be at least 0.5 µm. It preferably may be 0.6 µm or more, 0.7 µm or more, 0.8 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. However, it is difficult to form the surface hardened layer thick, therefore the thickness of the surface hardened layer may be 50.0 µm or less. Preferably, it may be 45.0 µm or less, 40.0 µm or less, 35.0 µm or less, 30.0 µm or less, 25.0 µm or less, or 20.0 µm or less.
For example, the thickness of the surface layer hardened layer being T µm means the region from steel product surface down to T µm in the thickness direction. That is, it indicates the region from the steel product surface down to 0 to T µm in range in the thickness direction. Therefore, T may be 0.5 µm or more, 0.6 µm or more, 0.7 µm or more, 0.8 µm or more, 0.9 µm or more, or 1.0 µm or more and may be 50.0 µm or less, 45.0 µm or less, 40.0 µm or less, 35.0 µm or less, 30.0 µm or less, 25.0 µm or less, or 20.0 µm or less.

It was confirmed that if the hardness of the steel product surface is 25 Hv or more higher than the hardness of the inside of the steel product (indicating inside of steel product other than surface hardened layer), the thickness of the surface hardened layer is at least 0.5 µm. That is, when the Vickers hardness of the steel product surface is Hvs and the Vickers hardness of the steel product center part is Hvc, if the following formula 2 is satisfied, there is a thickness 0.5 µm or more surface layer hardened layer.
$Hvs - Hvc \geq 25 Hv$
The difference of hardness of the hardness (Hvs) of the steel product surface and the hardness (Hvc) of the steel product center part is preferably 26 Hv or more, 28 Hv or more, 30 Hv or more, 32 Hv or more, 34 Hv or more, 36 Hv or more, 38 Hv or more, or 40 Hv or more.
Note that, for the hardness (Hvc) of the steel product center part, the hardnesses are measured at any five positions at the steel product cross-section and the arithmetic average value of the same is deemed the hardness (Hvc) of the steel product center part. Further, for the hardness of the steel product surface, the hardnesses are measured at the steel product surface at any five points and the arithmetic average value of the same is deemed the hardness (Hvs) of the steel product surface.

For measurement of the Vickers hardness, the Vickers hardnesses are measured by a pressing load of 200 g at five points. The arithmetic average value of these is defined as the Vickers hardness at that portion.
For measurement of hardness of the steel product surface, five points are measured at any portion of the surface of the steel product (for example, any selected 5 mm square portion) and the arithmetic average value of these is defined as the surface hardness.
For measurement of the Vickers hardnesses at the inside of the steel product and the surface hardened layer, the hardnesses are measured at the steel product cross-section. For the Vickers hardness of the steel product center part, five points are measured at the steel product cross-section in a rectangular (square) region of 1/4 (25%) of the sheet thickness centered about a center line of sheet thickness if a steel sheet or steel tube or in a circular region (concentric circle of cross-section of the steel bar or wire) having a diameter of 1/4 (25%) of the diameter of the steel product centered about a center axis if a steel bar or wire rod and the arithmetic average value of these is defined as the hardness of the steel product center part.
The surface hardened layer is measured for the Vickers hardness in the steel product thickness direction from the steel product surface at the steel product cross-section and can be identified as the region having a hardness of a predetermined hardness difference (25 Hv) or more compared with the hardness of the steel product center part (surface hardened layer). For example, the Vickers hardness may be measured by a 0.1 µm pitch down to a thickness 1.0 µm in the thickness direction from the surface and at a 1.0 µm pitch from a thickness 1.0 µm or more.

Furthermore, development has proceeded from the viewpoint of suppressing penetration of nitrogen from outside of the steel product. As a result, it was discovered that due to the presence of precipitates at the surface layer of the stainless steel product, movement of nitrogen is restricted and the nitriding of the stainless steel product surface layer is suppressed. Here, the "surface layer of the stainless steel product (base material)" indicates the range at the steel product cross-section from the surface of the steel product down to 20 µm in the depth direction.
It was learned that in the steel product cross-section, if the number density of the particle size 0.1 µm or more precipitates at the steel product surface layer (surface layer precipitate density) is 10/1000 µm² or more, nitriding is suppressed. The surface layer precipitates density may preferably be 11/1000 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 becomes too high, the properties other than the nitriding (for example, the toughness) are adversely affected, so the density is preferably 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 to be measured is not particularly prescribed, but if the precipitate particle size is too large, the strength and corrosion resistance of the steel product are also affected, therefore not forming large precipitates is preferable. From this viewpoint, the particle size of the precipitates is preferably kept down to 2.0 µm or less. The particle size of the precipitates to be measured preferably may also be 2.0 µm or less.
The type of the precipitates is not particularly limited, but, for example, one or more of Nb(C, N), Ti(C, N), W(C, N), B(C, N), V(C, N), and ε-Cu are possible. Here, M(C, N) indicates a carbonitride (one or a composite compound of both of a carbide or nitride) of an element M.

When the inventors proceeded to study the limitation of movement of nitrogen by precipitates, they learned that if precipitates are present at grain boundaries easily becoming paths for movement of nitrogen, they can block movement of nitrogen and efficiently limit movement of nitrogen. The inventors performed experiments to investigate the relationship between nitriding and the precipitates forming at the grain boundaries in the surface layer and as a result leaned that if the number of particle size 0.1 µm or more precipitates per length of grain boundaries present at the grain boundaries of the surface layer (intergranular precipitate density) is 0.17/µm or more, nitriding is effectively suppressed. The intergranular precipitate density preferably may be 0.20/µm or more, 0.25/µm or more, 0.30/µm or more, 0.35/µm or more, or 0.40/µm or more. The upper limit of the intergranular precipitate density is not particularly prescribed, but if the intergranular precipitate density is too high, the properties other than nitriding (for example, toughness and corrosion resistance) are affected, so the limit preferably 10.00/µm or less, 7.00/µm or less, 5.00/µm or less, 4.00/µm or less, or 3.00/µm or less.
The upper limit of particle size of the precipitates to be measured is not particularly prescribed in the same way as surface layer precipitates, but preferably may be 2.0 µm or less.
The type of precipitates is not particularly prescribed in the same way as surface layer precipitates, but, for example, can be one or more types of Nb(C, N), Ti(C, N), W(C, N), B(C, N), V(C, N), and ε-Cu.

The method of examination of precipitates at the surface layer will be explained here. A range of depth from the surface down to 20 µm at a steel product cross-section of the examined steel product sample is examined. For example, a field extending 50 µm in a direction parallel to the surface from the surface down to 20 µm may be examined. The area of the field examined at this time becomes 1000µ² . The examined field of the sample is observed using an FE-EPMA (for example, JXA-8100, JXA-8350F). The obtained observed image is analyzed. For example, the observed image can be analyzed under conditions of an acceleration voltage: 15 kV and current: 1×10^{- 7} A to examine the precipitates. The examined precipitates can be mapped by software attached to the EPMA. The particle size of the mapped precipitates is measured and particle size 0.1 to 2 µm or more precipitates are extracted. The number of extracted precipitates can be divided by the area of the field examined to calculate the number of surface layer precipitates.
Note that, the particle size of a precipitate is determined by measuring the maximum width of the precipitate examined as the long axis and the maximum width in a direction vertical to the long axis as the short axis. The average value of the long axis and the short axis ((long axis+short axis)/2) is defined as the particle size.
Similarly, the grain boundaries can be identified from the obtained examined image, the precipitates present on the grain boundaries extracted, and the number of intergranular precipitates (/ µm) of the line density of precipitates calculated by image processing from the intergranular lengths and the number of extracted precipitates on the grain boundaries.
The measurement is conducted at least at three fields and the obtained numbers of surface layer precipitates and numbers of intergranular precipitates are arithmetically averaged to obtain the surface layer precipitate density and intergranular precipitate density of the steel product.

By providing the steel product surface layer explained above, nitriding of the steel product surface is suppressed and, as a result, intergranular cracking due to penetration of nitrogen (N) is suppressed. For the intergranular cracking, it is possible to examine the crystal grain boundaries to measure the lengths, select any three ranges of 100 µm square at the surface layer of the steel product cross-section (part including at least nitride parts), and make the total of the intergranular crack lengths at these 5.0 µm or less. If the total of the intergranular crack lengths at the examined surfaces of the three locations is 5 µm or less, it is possible to suppress embrittlement of the steel product surface and possible to secure steel product strength at the 500 to 700°C temperature region. The shorter the total of the intergranular crack lengths, the more preferable. 4.5 µm or less, 4.0 µm or less, 3.5 µm or less, 3.0 µm or less, 2.5 µm or less, or 2.0 µm or less is more preferable.
The intergranular crack lengths of the steel product surface layer can be measured in the following way. A cross-section of the steel product used as a sample is examined under an optical microscope. A 100 µm square rectangular field is examined and the intergranular crack lengths are measured. At this time, the closer to the steel product surface, the more susceptible to the effects of nitrogen, therefore the portion corresponding to right under the steel product surface may be made the examined field. Measurement by image processing at the time of measurement is preferable. For example, it is possible to mark the intergranular crack parts on the measured image and measure their lengths by image processing.

The shallower the nitrided depth, the better from the viewpoint of suppressing nitriding cracking of the surface. The steel product according to the present invention is adjusted in constituents so that nitriding is suppressed and has a Cr-Si-based oxide film at the surface, therefore is high in effect of suppression of nitriding and thin in nitrided layer thickness. In particular, clearly the smaller the value of the nitriding tendency index (including negative values), the thinner the nitrided layer thickness tends to become. The nitrided layer thickness differs somewhat even by the content of nitrogen (N) of the gas contacted, but it was confirmed that surface embrittlement is suppressed if generally 45 µm or less. The nitrided layer thickness preferably may be 40 µm or less, 35 µm or less, 30 µm or less, 20 µm or less, or 10 µm or less. Note that, the range with a nitrogen concentration of 2 mass% or more is deemed the nitrided layer.

The form of the steel product is not particularly limited. For example, it may be a steel sheet, steel bar, wire rod, steel shape, steel tube, etc. Further, the steel product may also be a part obtained by working a steel sheet etc. The method of working is not particularly limited. For example, it may be pressing, drawing, cutting, etc.

At the 500 to 700°C temperature region, composite oxides become greater in number, therefore an Cr-Si-based oxide film of oxides mainly comprised of Cr or Si is formed as strong oxides and oxidation and nitriding of the steel product can be suppressed. In particular, the effect of suppression of nitriding is high. An attempt was made to evaluate the nitriding resistance of a Cr-Si-based oxide film. As a result, the inventors discovered the nitriding suppression effect index using the peak intensities of emission intensities of the elements measured by glow discharge optical emission spectrometry (GD-OES). That is, the inventors held the steel product at 600°C for 10 minutes, then analyzed the region with a distance in the thickness direction from the surface of the steel product of 500 nm (0.5 µm) or less by GD-OES to measure the emission intensities. They identified the peak intensities of Cr, Si, and N detected (qualitative analysis). They discovered that when designating the peak intensities of Cr, Si, and N respectively as I_{C r}, I_{S i}, and I_N, the nitriding resistance index shown by the following formula 3 should be 0.30 or more.
$Nitriding resistance index = I_{Cr} + 2 \times I_{Si} - 0.5 \times I_{N} \geq 0.30$
The lower limit value of the nitriding resistance index preferably may be 0.32, 0.34, 0.35, 0.36, 0.38, 0.40, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, or 0.50.

The method of production will be explained here. The method of production explained below is one embodiment for obtaining the steel product according to the present invention and is not limited to this method of production. The method of production is not limited so long as 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 includes producing a steel product by an ordinary method, then washing it by acid, then introducing strain by polishing, shot blasting, shot peening, etc. the surface. 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 example where the steel product is a steel sheet and where polishing is performed as the method for introducing strain to the surface will be explained.
The steel sheet before the final annealing may be produced by an ordinary 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 the steelmaking, the preferred method is melting steel containing constituents adjusted to give the constituents explained above in a converter or electric arc furnace, then performing secondary refining. The molten steel adjusted to predetermined constituents in this way is formed into a slab by a known casting method (for example, continuous casting). The slab is heated to a predetermined temperature and is hot rolled to a predetermined sheet thickness. After hot rolling, cold rolling may be performed in accordance with need. The cold rolling may also be performed by an ordinary method.
The conditions in the production process may be suitably selected. For example, the slab thickness, hot rolled sheet thickness, etc. may be suitably set. The hot rolled sheet may be coiled up, then dipped in a water cooling pool. The pickling step after hot rolling or after hot rolling and annealing is not particularly limited. The shot blasting, bending, brushing, or other mechanical descaling method may be suitably selected. The pickling solution after hot rolling is also not particularly limited, therefore, for example, may be performed by sulfuric acid, nitrofluoric acid, or other existing conditions. Furthermore, after that, the surface may also be ground in the coil state.
The thus obtained hot rolled steel sheet, hot rolled and annealed steel sheet, or cold rolled steel sheet is annealed to make it recrystallized. The annealing atmosphere is not particularly limited and may be an air atmosphere. The sheet may be held at a temperature of 900 to 1100°C in temperature region, then cooled down to a 50°C or less temperature. From the viewpoint of forming precipitates at the surface layer, even if the annealing temperature is too high, the dissolved state continues and precipitation does not proceed, therefore the temperature preferably may 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 the temperature preferably may be 900°C or more, 930°C or more, or 950°C or more. The mechanism is not elucidated, but it is guessed that if the annealing temperature is low, nuclei for formation of precipitates are not formed. The holding time at the temperature 900 to 1100°C temperature region is not particularly limited, but if too short, the precipitation time is liable to be insufficient, therefore preferably may be 30 seconds or more or 60 seconds or more. On the other hand, if the holding time is too long, the precipitates become coarsened and detrimentally affect the toughness or corrosion resistance, therefore the holding temperature preferably may be 5 minutes or less, 3 minutes or less, or 2 minutes or less. The above annealing may be performed after the steel product finishes being worked, for example, after hot rolling, after hot rolling and annealing, or after final cold rolling.
After annealing, the steel sheet is cooled. The cooling conditions are not particularly limited, but to make the precipitates grow to the predetermined particle size, the dwell time of the steel sheet at the 800°C to 300°C temperature range preferably may be 30 seconds or more and 120 seconds or less. The dwell time further 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.
The above annealed steel sheet is cooled down to 60°C or less, then is pickled and etched to remove the upper layer Cr oxide layer and Fe oxide layer. Therefore, the pickling solution is prepared to include fluoric acid (HF) 2.0% or less and nitric acid 6 to 15%. The above annealed steel sheet may be dipped in a temperature 50 to 60°C pickling solution for 40 to 60 seconds to pickle it. Due to this, it is possible to obtain a steel sheet surface from which the upper layers of Fe oxides, Cr oxides, and Si oxides formed in the production process have been removed.
After pickling, the steel sheet is polished at its surface to introduce dislocations (strain) to the surface and harden the surface. The polishing conditions at this time (polishing time, pressing time, etc.) can be used 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, After that, a #400 to #600 polishing agent may be used to polish the steel sheet surface. For example, after the #220 polishing agent, a #400 or/and a #600 polishing agent may be used to polish the surface of the steel sheet. This is because by successively polishing using the coarse polishing agent to the fine polishing agent, it is possible to reduce the steel sheet surface roughness and eliminate the defects. The combination of the polishing agents is not particularly limited. The amount of polishing is also not particularly limited, but in actual production, 10 µm to 50 µm or so is preferable. In actuality, the conditions differ depending on the steel product, therefore these may be suitably determined while polishing the sample in advance and checking for surface hardness and surface defects.
In the case of wet polishing, oil in which the polishing agent is mixed can be coated on the steel sheet surface and a polishing cloth made to rotate while polishing the steel sheet surface. At this time as well, the polishing agent may be successively changed from a coarse one to a fine one for polishing.
After polishing, the surface may be rinsed or washed with alkali to remove the polishing agent remaining on the steel sheet surface.
When employing shot blasting or shot peening as the method for imparting dislocations (strain) to the surface as well, the conditions of the shot blasting or shot peening are not particularly limited. They may be suitably determined while performing experiments in advance to check the surface hardness and check for surface defects.

Even if the steel product according to the present invention is used in a gas atmosphere with a large nitrogen (N) content, there is little penetration of nitrogen to the steel product surface layer due to the excellent nitriding resistance and intergranular cracking is suppressed. Further, oxidation resistance is also provided. In particular, in the 500 to 700°C or so medium to high temperature region, this is effective against the formation of red iron oxide - a problem in conventional stainless steel. Due to this, for example, this can be used for ammonia combustion apparatuss where the nitrogen content is high and the gas temperature is the 500 to 700°C medium to high temperature region. Devices or parts as a whole may be produced by the steel product according to the present invention. Further, the steel product according to the present invention may also be applied to at least part of a device or a part. For example, it can be applied to a part for an ammonia combustion apparatus having the steel product according to the present invention at least in part etc.
Of course, from the nature of nitriding resistance and oxidation resistance, for example, even if used for a container or piping part etc. coming directly into contact with ammonia, urea, etc., penetration the nitrogen ions in the solution or nitrogen in the gas of evaporation of ammonia or urea to the steel product surface layer is suppressed and intergranular cracking is suppressed. In addition, if applying the steel product according to the present invention to a part where nitriding resistance and oxidation resistance are demanded, that effect can be enjoyed.

EXAMPLES

Below, examples will be used to explain the present invention more specifically, 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 obtained hot rolled steel sheet was annealed at a 900 to 1100°C temperature region, pickled, and cold rolled to obtain a sheet thickness 1.5 mm cold rolled steel sheet. The obtained cold rolled steel sheet was held at a 900 to 1100°C temperature for 90 seconds to anneal it (final annealing), then was cooled so that the dwell time at 800°C to 300°C became 90 seconds and was cooled down to 50°C or less. After that, the steel sheet was dipped in a 40 to 60°C pickling solution (2% fluoric acid+10% nitric acid+water) for 40 to 90 seconds (final pickling) and rinsed to obtain a test material.
Note, in Sample 7, the holding time of the final annealing was 30 seconds and the sample was cooled to give a dwell time at 800°C to 300°C in the subsequent cooling of 90 seconds. In Sample 12, the holding time at the final annealing was 30 seconds and the sample was cooled to give a dwell time at 800°C to 300°C in the subsequent cooling of 30 seconds.
From each obtained test material, five 20 mm×25 mm test pieces were cut out and were polished at the surfaces. The polishing was dry polishing. Polishing paper coated with the polishing agent was wrapped around a plastic holder, the holder was pressed against the test piece surface and made to move back and forth by a fixed speed (10 m/min) over 300 mm. The polishing pressure was set as shown in Table 2 for polishing. For the polishing agent, for the first polishing, a #220 polishing agent was used, while for the second polishing, a #400 polishing agent was used.
One among the obtained test materials was cut so that a cross-section vertical to the surface was obtained and the surface Vickers hardness and the center part Vickers hardness were measured by loads of 50 g. Further, in addition to measurement of the hardness of the surface, the hardness was measured in the thickness direction from the surface at 0.1 µm pitch until the thickness 1.0 µm and at 1 µm pitch from 1.0 µm on to identify the thickness of the surface hardened layer. Furthermore, at the same steel product cross-section, a region of 50 µm in the surface direction and a depth from the surface of 20 µm is made the examined field. FE-EPMA is used to analyze the image of the state of precipitates in the examined field. For the particle size 0.1 to 2.0 µm precipitates, the density of precipitates of the surface layer (surface layer precipitate density) and the number of precipitates per intergranular 1 µm obtained by dividing the number of precipitates present at the grain boundaries between grains by the total of intergranular lengths in the examined field (intergranular precipitate density) were found.
Three test materials of the remaining ones were nitrided and oxidized envisioning ammonia combustion gas. The nitriding and oxidation treatments were performed by introducing into the annealing furnace an atmospheric gas of a gas of ammonia 10 vol%, water vapor 10 vol%, and a balance of nitrogen (N), placing the three test pieces in the furnace, heating them to a temperature of 600°C, then holding them there for 50 hours, then cooling and taking them out and measuring the intergranular crack lengths and nitrided layer thicknesses.
The intergranular crack length was determined by cutting the test piece after nitriding and oxidizing 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 piecer. The examination was conducted on ranges of 100 µm×100 µm right below the steel sheet surface as fields for three randomly selected locations in the sample cross-section and the intergranular crack lengths were measured. If the total of the intergranular crack lengths of the examined surfaces of the three locations is 5 µm or less, it can be judged that the nitriding resistance is excellent.
The nitrided layer thickness was determined by cutting a test piece after nitriding and oxidation, analyzing the cross-section by EPMA to measure the nitriding concentration distribution in the thickness direction from the surface, and defining the range with a nitrogen concentration of 2 mass% or more as a nitrided layer.
The red iron oxide property (oxidation resistance) was visually checked. Samples where no red iron oxide could be found were judged as passing ("good") while samples where red iron oxide was found even slightly were judged as failing ("poor"). The measurement results are shown in Table 2. From the data of Table 2, it is learned that the steel sheet according to the present invention did not have any length of intergranular cracking. A nitrided layer thickness of 45 µm or less was judged as passing. The steel sheet according to the present invention had a nitrided layer thickness of 1.0 µm or less. It was confirmed that the nitrided layer was substantially not formed.
The remaining test piece was heated in a heat treatment furnace up to 600°C and held there as is for 10 minutes to form an oxide film on the surface. The surface layer (region of distance in thickness direction from surface of 0.5 µm or less) was measured for emission intensity by GD-OES analysis and the Cr peak intensity (I_{C r}), peak intensity (I_{S i}), and N (nitrogen) peak intensity (I_N) were obtained. The measurement conditions of the GD-OES were as follows:

· GD-OES apparatus: glow discharge optical emission spectrometer GDA750 made by Rigaku
· Pulse frequency: 650V, 2.8hPa, 200Hz
· sputter speed: 0.5 µm/min
·anode diameter: 4.0 mm
· carrier gas: Ar (argon)

The peak values of Cr, Si, and N were entered into formula 3 and the nitriding resistance of the oxide film was evaluated. The results of evaluation are shown in Table 2. FIG. 1 is a view plotting the values of intergranular crack length and nitriding resistance index (formula 3 left side). From FIG 1, it is learned that if the nitriding resistance index (formula 3 left side) is 0.30 or more, intergranular cracking is kept down to a level where it does not occur. That is, it was confirmed that nitriding is suppressed by the Cr-Si-based oxide film.

[Table 1]

Steel no.	Constituents (mass%) (balance: Fe and impurities)
Steel no.	C	Si	Mn	P	S	Ni	Cr	N	Nb	Mo	Cu	Al	Ti	Others
1	0.009	0.54	0.20	0.015	0.0050	0.11	18.22	0.027	0.41	0.02	0.41	0.021	0.001	Co: 0.95, Ta: 0.21
2	0.010	0.38	0.35	0.041	0.0010	0.15	22.01	0.011	0.21	1.02	0.05	0.081	0.240	Sn: 0.14
3	0.025	0.33	0.95	0.035	0.0010	0.19	18.09	0.012	0.75	1.95	0.51	0.018	-	Zr: 0.24, Sb: 0.18
4	0.006	0.29	0.26	0.033	0.0009	0.29	17.01	0.007	0.51	0.05	1.46	0.331	0.190	B: 0.0012
5	0.004	0.51	0.30	0.022	0.0010	0.19	18.09	0.008	0.39	0.01	2.38	0.210	0.150	REM: 0.061, Mg: 0.0025
6	0.010	0.59	0.31	0.028	0.0010	0.11	14.07	0.008	-	-	1.19	0.078	0.180
7	0.005	0.32	0.08	0.024	0.0020	0.09	17.21	0.007	0.01	0.98	-	0.090	0.180	Ca: 0.0003, Ga: 0.11
8	0.006	0.58	0.89	0.019	0.0007	0.07	17.09	0.007	0.49	2.43	1.29	0.029	0.00	W: 1.52
9	0.010	1.39	1.09	0.028	0.0010	0.35	13.41	0.011	0.41	-	0.11	0.041	0.001	Hf: 0.22
10	0.007	0.25	0.31	0.021	0.0010	0.81	17.02	0.007	0.15	0.53	1.25	0.021	0.150
11	0.007	2.51	0.39	0.029	0.0010	0.11	18.02	0.007	0.38	0.01	0.12	0.038	0.001
12	0.006	0.29	0.23	0.029	0.0010	0.09	29.51	0.012	0.18	1.85	0.11	0.026	0.180	V: 0.43
13	0.006	0.22	0.41	0.030	0.0010	0.51	10.88	0.008	0.21	1.91	0.21	0.192	0.001
14	0.009	0.34	0.31	0.028	0.0020	0.43	17.21	0.009	0.39	4.01	0.11	0.032	0.001
15	0.041	0.58	0.32	0.031	0.0020	0.31	34.01	0.011	0.01	1.95	0.19	0.149	0.180
16	0.005	0.19	1.89	0.029	0.0010	0.33	18.01	0.010	0.21	0.01	0.01	1.510	0.210
17	0.007	0.78	0.51	0.031	0.0010	0.27	19.08	0.010	0.21	1.98	0.02	0.099	0.950
18	0.005	0.32	0.33	0.029	0.0020	0.09	17.98	0.009	0.41	2.55	0.01	0.019	0.160
19	0.012	0.79	0.98	0.019	0.0010	0.09	17.08	0.007	0.00	0.05	0.05	0.021	0.001
20	0.013	0.69	0.55	0.025	0.0010	0.21	18.15	0.010	0.45	2.11	3.21	0.210	0.150

[Table 2]

Sample no.	Steel no.	Nitriding tendency index (formula 1)	Polishing pressure (kg/cm²)		Vickers hardness (Hv)			Oxide film nitriding resistance index (formula 3)	Surface layer hardened layer thickness (µm)	Intergranular crack length (µm)	Surface layer precipitate density	Intergranular precipitate density	Oxidation resistance (red iron oxide property)	Nitrided layer thickness (µm)	Remarks Ex.:Example Comp. ex.: Comparative example
			1 st time	2nd time	Center	Surface	Hardness difference (formula 2)				Surface layer precipitate density	Intergranular precipitate density
			#220	#400	Center	Surface	Hardness difference (formula 2)				/1000 µm²	/µm
Sample 1	1	-0.4	1.0	1.0	175	206	31	0.50	10	0	13	0.23	Good	0.1	Ex.
Sample 1-2	1	-0.4	0.1	1.0	175	184	9	0.20	0.2	5.2	12	0.20	Poor	47.0	Comp. ex.
Sample 1-3	1	-0.4	No polishing	No polishing	175	175	0	0.10	0	10.1	11	0.20	Poor	173.0	Comp. ex.
Sample 2	2	3.0	2.0	1.0	181	219	38	0.60	11	0	16	0.27	Good	0.0	Ex.
Sample 3	3	3.8	2.0	1.0	183	225	42	0.51	12	0	20	0.33	Good	0.2	Ex.
Sample 4	4	4.3	2.0	1.0	179	211	32	0.55	10	0	25	0.40	Good	0.0	Ex.
Sample 5	5	3.0	5.0	1.0	180	238	58	0.52	15	0	35	0.60	Good	0.1	Ex.
Sample 6	6	1.0	2.0	2.0	153	195	42	0.42	15	0	14	0.23	Good	0.4	Ex.
Sample 7	7	2.9	1.0	1.0	159	193	34	0.45	9	0	11	0.16	Good	0.4	Ex.
Sample 8	8	4.9	1.0	2.0	185	225	40	0.49	14	0	39	0.67	Good	0.2	Ex.
Sample 9	9	-1.6	5.0	1.0	161	208	47	0.56	14	0	10	0.17	Good	0.0	Ex.
Sample 10	10	2.0	2.0	1.0	162	201	39	0.43	11	0	17	0.20	Good	0.4	Ex.
Sample 11	11	-3.3	2.0	1.0	177	213	36	0.58	11	0	13	0.23	Good	0.0	Ex.
Sample 12	12	4.1	2.0	1.0	192	239	47	0.63	12	0	9	0.16	Good	0.8	Ex.
Sample 13	13	5.5	1.0	1.0	148	179	31	0.15	9	4.9	6	0.07	Poor	111.0	Comp. ex.
Sample 14	14	7.9	0.2	0.1	198	205	7	0.12	0.3	54.0	10	0.10	Poor	194.0	Comp. ex.
Sample 15	15	5.2	0.1	0.1	207	212	5	0.15	0.1	29.0	9	0.17	Good	178.0	Comp. ex.
Sample 15-2	15	5.2	1.0	1.0	207	241	34	0.22	13	8.3	6	0.10	Good	101.0	Comp. ex.
Sample 16	16	15.5	No polishing	0.2	176	181	5	0.08	0.2	49.0	11	0.13	Poor	162.0	Comp. ex.
Sample 17	17	6.6	0.2	No polishing	184	191	7	0.15	0.3	37.0	42	0.57	Good	156.0	Comp. ex.
Sample 18	18	5.3	No polishing	0.1	187	192	5	0.14	0.1	28.0	15	0.23	Good	203.0	Comp. ex.
Sample 18-2	18	5.3	No polishing	No polishing	187	187	0	0.12	0	33.0	15	0.27	Good	221.0	Comp. ex.
Sample 19	19	-0.8	No polishing	0.2	169	174	5	0.18	0.2	11.0	1	0.00	Poor	182.0	Comp. ex.
Sample 20	20	7.34	No polishing	No polishing	172	172	0.0	0.14	0	35.0	29	0.53	Good	201.0	Comp. ex.

[INDUSTRIAL APPLICABILITY]

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

Claims

A ferritic stainless steel product having a composition containing, by mass%,
C: 0 to 0.030%,

Si: 0.03 to 3.00%,

Mn: 0.03 to 1.20%,

P: 0.050% or less,

S: 0.005% or less,

Ni: 0 to 1.00%,

Cr: 12.0 to 31.0%,

N: 0 to 0.030%,

Nb: 0 to 1.00%

Mo: 0 to 2.50%

Cu: 0 to 3.00%

Al: 0.002 to 0.500%,

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 0.20%,

satisfying the following formula 1, and

having a balance of Fe and impurities,

in which steel product,

a Vickers hardness (Hvs) of a surface of the steel product and a Vickers hardness (Hvc) of a center part of the steel product satisfy the following formula 2: $10 Al + 2 Mo + 3 Ti + 0.5 Cu - 1.5 Si \leq 5.0$ $Hvs - Hvc \geq 25 Hv$

where, notations of elements in formula 1 indicate contents of the elements (mass%), 0 being entered when not contained.

Further, the "center part of steel product" indicates a region in the steel product cross-section in a range of 3/8 to 5/8 of the thickness in the thickness direction of the steel product.
The ferritic stainless steel product according to claim 1, wherein, in a cross-section vertical to the surface of the steel product, has a surface hardened layer having Vickers hardness higher by 25 Hv or more than a Vickers hardness of a center part of the steel product and has a thickness of the surface hardened layer of 0.5 µm or more in a thickness direction from the surface of the steel product.
The ferritic stainless steel product according to claim 1 or 2, wherein, in a cross-section vertical to the surface of the steel product, particle size 0.1 µm or more precipitates are present in 10/1000 µm² or more at a surface layer of a depth of 20 µm from the steel product surface.
The ferritic stainless steel product according to any one of claims 1 to 3, wherein, in a cross-section vertical to the surface of the steel product, particle size 0.1 µm or more precipitates are present in 0.17/µm or more at grain boundaries in a surface layer of a depth of 20 µm from the steel product surface.
The ferritic stainless steel product according to any one of claims 1 to 4, wherein, in a cross-section vertical to the surface of the steel product, when a 100 µm square range is 1 field, a total of intergranular crack lengths of any three fields is 5 µm or less.
The ferritic stainless steel product according to claim 1 or 2, wherein when holding the steel product at 600°C for 10 minutes, then analyzing a region of a distance of 0.5 µm or less in the thickness direction from the surface of the steel product by GD-OES, the obtained peak intensity I_{C r} of Cr, peak intensity I_{S i} of Si, and peak intensity I_N of N (nitrogen) satisfy the following formula 3: $I_{Cr} + 2 \times I_{Si} - 0.5 \times I_{N} \geq 0.30$
A ferritic stainless steel product for use for an ammonia combustion apparatus according to any one of claims 1 to 6.
A method of production of a ferritic stainless steel product according to claim 1 or 2, wherein method of production of a ferritic stainless steel product includes a step of holding a steel product having a composition according to claim 1 at a 900 to 1100°C temperature region, then cooling it down to a 50°C or less temperature, dipping it in a pickling solution containing fluoric acid in 2.0% or less and nitric acid 6 to 15% and having a temperature of 50 to 60°C for 40 to 60 seconds, then polishing the surface of the steel product and shot blasting or shot peening it.
The method of production of a ferritic stainless steel product according to claim 8, wherein a holding time in the 900 to 1100°C temperature region is 30 seconds or more and 5 minutes or less and, in the cooling, a dwell time in a temperature range from 800°C to 300°C is 30 seconds or more and 120 seconds or less.
The method of production of a ferritic stainless steel product according to claim 8 or 9, wherein the polishing is dry polishing and polishes in the order of #220, followed by #400 or/and #600.
A part having the ferritic stainless steel product according to any one of claims 1 to 6 at least at a portion of it.
A part according to claim 11, which is a part for an ammonia combustion apparatus.