EP4663793A1

EP4663793A1 - Steel component and method for producing same

Info

Publication number: EP4663793A1
Application number: EP23921395.2A
Authority: EP
Inventors: Kentaro Fukumoto
Original assignee: Proterial Ltd
Current assignee: Proterial Ltd
Priority date: 2023-02-10
Filing date: 2023-12-27
Publication date: 2025-12-17
Also published as: JPWO2024166577A1; CN120641585A; WO2024166577A1; EP4663793A4

Abstract

The present invention provides a steel component capable of making the fatigue limit higher than in the prior art. In the present invention, a steel component: has a 10- to 100- µ m thick nitrogen-enriched layer having a hardness of 600 HV or more on the surface of a steel having a component composition of, in mass%, C: 0.3 to 0.5%, Si: 1.0% or less, Mn: 1.5% or less, Cr: 9.0 to 15.0%, Mo and W alone or in combination (Mo+W/2): 0.5 to 3.0%, N: 0.1% or less, the remainder being Fe and unavoidable impurities; and has, in the center of the component, a nitrogen non-absorption region where N is, in mass%, 0.1% or less, and the hardness is at least 10 HV less than the hardness of the nitrogen-enriched layer.

Description

Technical Field

The present invention relates to a steel component and a method for producing the same.

Related Art

Conventionally, martensitic stainless steel containing 12 to 13% Cr has been used as steel for cutting tools, spring materials, valve materials, sliding components etc. This martensitic stainless steel not only achieves high hardness through quenching and tempering, but also excels in corrosion resistance and is difficult to rust, so it is widely used in general. Particularly for spring materials and valve materials, martensitic stainless steel strips having sufficiently high fatigue limit are required to suppress fatigue failure due to repeated stress.
For example, the applicant has proposed in Patent Document 1, as a martensitic stainless steel strip that can achieve further fatigue limit enhancement, a martensitic stainless steel in which the compressive residual stress at the surface of the steel strip is 50 MPa or more, and the area ratio of carbides present in the metallic structure of the steel strip is 0.5% or more and 8.0% or less.
Moreover, Patent Document 2 describes a flapper valve body made of martensitic stainless steel having compressive residual stress on the plate surface and having a solid solution nitrogen enrichment layer in the plate surface layer part, in order to improve the corrosion resistance and fatigue characteristics of the flapper valve body. Here, Patent Document 2 also describes that the residual stress on the surface of martensitic stainless steel may be adjusted to compressive stress by rapid cooling after heating to a temperature equal to or higher than the temperature at which transformation to austenite single phase occurs in an atmosphere containing 20% or more nitrogen and 10% or less (including the case of 0%) oxygen (percentages are volume%).

Citation List

Patent Literature

Patent Document 1: International Publication No. WO 2020/013223
Patent Document 2: Japanese Patent Application Laid-Open Publication No. 10-274161

SUMMARY OF INVENTION

Technical Problem

Since increasing the fatigue limit may make components lighter or extend their service life, and may further increase design freedom, further fatigue limit enhancement of steel is expected. Patent Document 1 is an excellent invention that can achieve high fatigue limit characteristics, but there is still room for further study regarding the point of controlling the surface structure to further enhance the fatigue limit. Moreover, although the martensitic stainless steel described in Patent Document 2 describes forming a solid solution nitrogen enrichment layer, no consideration has been given to the steel composition suitable for forming the solid solution nitrogen enrichment layer or the size of the solid solution nitrogen enrichment layer, leaving room for further improvement.
Therefore, the object of the present invention is to provide a steel component that can achieve even higher fatigue limit than conventional ones.

Solution to Problem

The present invention has been made in view of the above problems.
That is, one aspect of the present invention is a steel component that has a nitrogen-enriched layer having a hardness of 600HV or more and a thickness of 10µm to 100µm on a surface of a steel, the steel having a component composition containing, in mass%, C: 0.3% to 0.5%; Si: 1.0% or less; Mn: 1.5% or less; Cr: 9.0% to 15.0%; Mo and W: either alone or in combination of (Mo+W/2): 0.5% to 3.0%; and N: 0.1% or less, with a remainder being Fe and unavoidable impurities, and has, in a center part of the component, a nitrogen non-absorption region with N of 0.1% or less in mass% and having a hardness of 10HV or more lower than the hardness of the nitrogen-enriched layer.
Preferably, elongation is 1.0% or more and 7.0% or less.
Another aspect of the present invention is a method for producing steel component, including a nitrogen absorption treatment step of having a steel material absorb nitrogen while heating and holding it under conditions where a heating temperature is 900°C to 1100°C, a heating time is 2min to 20min, and an integrated value of the heating temperature [°C] and the heating time [min] is 2000 to 11000, and rapidly cooling it, the steel material having a component composition containing, in mass%, C: 0.3% to 0.5%; Si: 1.0% or less; Mn: 1.5% or less; Cr: 9.0% to 15.0%; Mo and W: either alone or in combination of (Mo+W/2): 0.5% to 3.0%; and N: 0.1% or less, with a remainder being Fe and unavoidable impurities.

Effects of Invention

According to the present invention, a steel component having a higher fatigue limit than conventional ones can be obtained.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, one embodiment of the present invention will be described. However, the present invention is not limited to the embodiment taken up here, and appropriate combinations and improvements are possible within the scope that does not depart from the technical concept of the present invention. First, the reasons for limiting the component composition of the steel component according to the present invention will be described.

C: 0.3 to 0.5%

C is an important element that dissolves from carbides into a matrix in solid solution during austenitization temperature in quenching and determines the hardness of martensite generated by quenching. Here, C in steel is divided into that which dissolves in the matrix in solid solution and that which precipitates as carbides, but since the ratio is determined by interaction with Cr, it is also important that Cr also falls within the composition range to be described later. In order to obtain a steel component having surface hardness suitable for the present invention, the lower limit of C is set to 0.3%. The preferable lower limit value of C is 0.35%, the more preferable lower limit value is 0.38%, and the further preferable lower limit value is 0.41%. On the other hand, in the case of excessive C amount, nitrogen may excessively dissolve in solid solution in the quenching step in nitrogen atmosphere to be described later, which may cause much austenite to remain on the very surface and reduce surface hardness. Moreover, in the case of excessive C amount, the carbides generated become excessively numerous, so the upper limit of C is set to 0.5%. The preferable upper limit value of C is 0.45%, the more preferable upper limit value is 0.43%, and the further preferable upper limit value is 0.42%.

Si: 1.0% or less

Si is an element that is used as a deoxidizer during refining of steel, and also dissolves in steel in solid solution and suppresses softening in low-temperature tempering, but on the other hand, excessive content reduces the toughness of steel and may reduce cold workability during cold rolling, for example. Therefore, the upper limit of Si amount is set to 1.0%. The preferable upper limit is 0.9%, the more preferable upper limit is 0.8%, the further preferable upper limit is 0.7%, and the particularly preferable upper limit is 0.5%. The lower limit is not particularly limited, but may be set to 0.1%, for example.

Mn: 1.5% or less

Mn is also an element that has a role as a deoxidizer during refining, similar to Si, and it dissolves in the matrix in solid solution and enhances quenching properties. In the case of insufficient Mn amount, the quenching properties of steel decreases, and particularly at the thickness center part of steel, there is a possibility that quenching may not occur. On the other hand, excessive content of Mn reduces hot workability, so the upper limit is set to 1.5%. The preferable upper limit is 1.2%, and the more preferable upper limit is 1.0%. The lower limit is not particularly limited, but may be set to 0.1%, for example, and preferably may be set to 0.2%.

Cr: 9.0 to 15.0%

Cr is an important element for forming a strong passive film on steel and obtaining excellent corrosion resistance. In order to exhibit this corrosion resistance, it is necessary that at least 9.0% of Cr is contained in the steel. On the other hand, excessive Cr amount causes a decrease in martensitic transformation start temperature (Ms point) and becomes a factor of hardness reduction due to increase of retained austenite, so the upper limit of Cr is set to 15%. In the case of wanting to further improve high hardness characteristics, it is preferable to set Cr to 9.0% or more and less than 12.0% to further reduce the retained austenite amount. The preferable upper limit of Cr amount for further improving high hardness characteristics is 11.5%, and the preferable lower limit of Cr amount is 9.5%. In the case of wanting to further improve toughness, it is preferable to set Cr to 12.0% or more and less than 15.0% to further increase the retained austenite amount. The preferable upper limit of Cr amount for further improving toughness is 14.5%, and the preferable lower limit of Cr amount is 12.5%.

Mo and W either alone or in combination (Mo+W/2): 0.5 to 3.0%

Mo and W have similar effects and are specified as (Mo+W/2) based on the relationship of atomic weight. Mo and W may be contained either alone or in combination. Mo and W are effective elements that have a high effect of stabilizing the passive state and for improving corrosion resistance by making the pitting potential noble in chloride solutions. They are also elements that suppress softening in low-temperature tempering, and at least 0.5% is necessary to obtain these effects. On the other hand, excessive addition of Mo and W significantly reduces workability during hot working, so the upper limit is set to 3.0%. The preferable lower limit of (Mo+W/2) amount is 0.8%, and the preferable upper limit of (Mo+W/2) amount is 2.0%.

N: 0.1% or less

In the present invention, if there is too much nitrogen contained in the steel material before the nitrogen absorption treatment described later is performed, gas defects may occur during casting, so it is limited to 0.1% or less because. The preferable upper limit is 0.07%, and the more preferable upper limit is 0.05%. The lower limit may be set to, for example, 0.001%. The steel component of the present invention has a nitrogen non-absorption region in the center part of the component where the effect of the nitrogen absorption treatment does not reach, as described later. Therefore, in order to measure nitrogen contained in the steel material, N amount may be measured in the above-mentioned nitrogen non-absorption region.
In the present embodiment, components other than the above are Fe and unavoidable impurities. Unavoidable impurity elements include P, S, Al, Ti and O, but they may be contained as long as they are within the ranges shown below that do not inhibit the effects of the present invention.
P ≦ 0.04%, S ≦ 0.03%, Al ≦ 0.1%, Ti ≦ 0.1% and O ≦0.05%.
Next, the steel component of the present invention having the above-mentioned component composition will be described. The steel component of the present embodiment has a nitrogen-enriched layer with a hardness of 600HV or more and a thickness of 10 to 100µm on the steel surface, and has a nitrogen non-absorption region in the center part of the component with N of 0.1% or less in mass% and a hardness that is 10HV or more lower than the hardness of the nitrogen-enriched layer. Thereby, the steel component of the present embodiment has high fatigue limit characteristics. In the present embodiment, by forming the nitrogen-enriched layer only on the surface part, the surface hardness of the component is improved, and further, by dissolving the nitrogen in solid solution, the martensite start temperature is lowered, and the retained austenite amount in the surface layer part can be appropriately increased. The retained austenite in the surface layer part tends to be reduced because it reduces the hardness and strength of the steel component, but by having an appropriate amount of retained austenite present in the martensite, crack propagation can be suppressed without reducing hardness and strength, and the fatigue limit can be further improved. In the case where the thickness of the nitrogen-enriched layer is less than 10µm, the surface hardness of the component is low and the fatigue limit tends to decrease. In the case where the thickness of the nitrogen-enriched layer exceeds 100µm, the hardness and ductility of the component tend to decrease due to nitrogen being excessively dissolved in solid solution. The lower limit of the thickness of the nitrogen-enriched layer is preferably 20µm, and 40µm is preferable. Moreover, the upper limit of the nitrogen-enriched layer is preferably 80µm, and 60µm is preferable.
The hardness of the nitrogen-enriched layer in the present embodiment is a value measured at room temperature (normal temperature) and is 600HV or more. It is preferably 610HV or more, more preferably 620HV or more, further preferably 640HV or more, and particularly preferably 700HV or more. The upper limit is not particularly limited, but it may be about 800HV due to manufacturing constraints.
The steel component of the present embodiment has a nitrogen non-absorption region of 0.1% or less in mass% in the center part of the component. The nitrogen non-absorption region has no solid solution strengthening by nitrogen and has high ductility, making it possible to maintain the ductility of the entire component. Since the thickness of this nitrogen non-absorption region is determined by the thickness of the component and the thickness of the nitrogen-enriched layer, the lower limit is not particularly limited, but in order to stably obtain the ductility improvement effect, it is preferable to secure a thickness of the nitrogen non-absorption region of at least 100µm or more. A more preferable thickness is 150µm or more, and a further preferable thickness is 200µm or more. The thickness of this nitrogen non-absorption region may also be determined by measuring the nitrogen amount at the material center part (geometric center) and setting the location where the nitrogen amount increases as the boundary of the nitrogen non-absorption region. Moreover, the nitrogen amount in the present embodiment may be measured by mechanically polishing and removing the surface of the sample to be measured for fatigue limit etc., and performing chemical analysis of the remainder of the sample. Moreover, since the nitrogen non-absorption region has a hardness that is 10HV or more lower compared to the nitrogen-enriched layer, it is also possible to identify the presence or absence of the nitrogen non-absorption region by focusing on hardness.
The thickness of the steel component of the present embodiment is preferably 0.20mm or more. The steel component of the present invention has high fatigue strength even in steel components with a thickness of 0.20mm or more. More preferably, it is 0.30mm or more, 0.40mm or more, and 0.50mm or more. The upper limit is not particularly limited, but 3.0mm or less is realistic. The shape of the steel component of the present embodiment is not particularly limited; shapes such as plate shape, block shape, rod shape, etc. may also apply.
The steel component of the present embodiment preferably includes a retained austenite amount of 10.5 to 35.0%. As described above, the retained austenite amount generally reduces the hardness and strength of steel components, so it is common to reduce it. However, by having an appropriate amount of retained austenite present in the martensite, crack propagation can be suppressed without reducing hardness and strength, and the fatigue limit can be further improved. The preferable lower limit of retained austenite is 11.0%, and the preferable upper limit of retained austenite amount is 32.0%, more preferably 31.0%. The retained austenite amount in the present embodiment is measured at the component surface layer part, in other words, at the nitrogen-enriched layer.
The steel component of the present embodiment preferably has an elongation of 1.0% or more and 7.0% or less. In the case of elongation being less than 1.0%, according to use as cutting tools, spring materials, valve materials, or sliding components etc., the ductility is low and it is difficult to use as a component. The preferable lower limit of elongation is 1.2% or more, more preferably 1.5% or more, and even more preferably 2.0% or more. On the other hand, in the case of elongation being 7.0% or more, it is difficult to maintain the hardness of the nitrogen-enriched layer. The preferable upper limit of elongation is 5.0% or less, more preferably 3.0% or less. The Young's modulus is not particularly limited and may be 150GPa or more. It is preferably 180GPa or more, and more preferably 200GPa or more.
The elongation and Young's modulus in the present embodiment are evaluated by tensile test using a unique proportional test specimen with a gauge length of 25mm, because it is difficult to prepare a JIS-13B standard test specimen specified in JIS-Z2241.
Next, the method for producing the steel component of the present invention will be described. In the present invention, steel having the above-mentioned component range and thickness is subjected to quenching after absorbing nitrogen while heating and holding under conditions where the heating temperature is 900 to 1100°C, the heating time is 2 to 20min, and the integrated value of heating temperature [°C] and heating time [min] is 2000 to 11000. In the present component system, in the case of heating temperature being less than 900°C, nitrogen in the atmosphere and carbides in the material do not sufficiently dissolve into austenite in solid solution, so surface hardness becomes low. Moreover, in the case of heating temperature exceeding 1100°C, most of the carbides dissolve in solid solution, causing crystal grains to coarsen and toughness to decrease, so the heating temperature was set to 900 to 1100°C. The preferable lower limit of heating temperature is 950°C, and the more preferable lower limit is 1000°C. The preferable upper limit of temperature is 1070°C, and the more preferable upper limit is 1050°C. Moreover, in the case of heating time exceeding 20min, austenite remains on the very surface due to nitrogen being excessively dissolved in solid solution, and surface hardness becomes low, resulting in decreased fatigue limit. Therefore, the preferable upper limit of heating time is 15.0min, the more preferable upper limit is 10.0min, and the even more preferable upper limit is 5.0min. Moreover, in the case of heating time being shorter than 2min, the nitrogen-enriched layer is not sufficiently formed on the surface layer, and surface hardness and fatigue limit decrease, so the preferable lower limit of heating time is 2.5min, and the more preferable lower limit is 3.0min.
In the producing method of the present embodiment, tempering is performed after the nitrogen absorption treatment step. By setting the tempering temperature to 250 to 400°C, a steel component with surface hardness of 600HV or more may be obtained. In the present component system, in the case of tempering temperature being less than 250°C, carbides do not sufficiently precipitate from the martensitic structure, so toughness becomes low. On the other hand, in the case of tempering temperature exceeding 400°C, a large amount of carbides precipitate from the martensitic structure, causing hardness reduction.
A hot-rolled material with a thickness of 2.0mm having component composition shown in Table 1 (remainder Fe and unavoidable impurities) was annealed in a batch annealing furnace. Subsequently, cold rolling and strain relief annealing were performed to finish the material to the predetermined thickness shown in Table 2. Nitrogen absorption treatment and tempering were performed after holding the material at specified temperature and time in the atmosphere shown in Table 2, followed by rapid cooling, and then holding at 270°C or 350°C for 100 seconds. Oxides and fine scratches generated on the surface were polished and removed by barrel polishing for 30 minutes to prepare components A to T. For the thickness of the nitrogen-enriched layer, chemical analysis was performed on a sample in which the surface of component was ground to a depth of 10 to 150µm, and the depth in which the amount of the nitrogen was 0.01% or more higher than that of the component before heat treatment was defined as the nitrogen-enriched layer. In the present example, nitrogen-enriched layer thickness of 10 to 100µm was judged as "○" (nitrogen-enriched layer present), 0 to less than 10µm as "×" (no nitrogen-enriched layer), and 100µm or more as "Δ" (excessive nitrogen-enriched layer).

Subsequently, hardness, elongation, Young's modulus, fatigue limit, and retained austenite amount of the steel components after heat treatment were investigated. Regarding hardness, Vickers hardness was measured at the surface and cross-section center part (nitrogen non-absorption region) of components A to N, with a load of 300gf for all measurements, and evaluation was performed using the average value of 3 points. For hardness at the cross-section center part, Vickers hardness was evaluated at the thickness center part of the cross-sectional structure parallel to the rolling direction of rolling processing. Vickers hardness was evaluated according to the method specified in JIS-Z2244. Moreover, elongation was performed according to the method specified in JIS-Z2241 and evaluated by fracture elongation. The test specimen was not a JIS13B standard test specimen, but evaluation was performed using a unique proportional test specimen with a gauge length of 25mm. Moreover, Young's modulus was calculated from the slope of a stress-strain diagram obtained in the above tensile test. Regarding fatigue limit, a complete alternating bending test with a constant stress amplitude (stress ratio R = -1, number of repetitions 1×10⁷ times) was performed, and the maximum value of stress with no fracture at 1×10⁷ times was defined as the fatigue limit. The retained austenite amount was calculated using an X-ray diffraction apparatus and the integrated intensity of peaks of austenite phase and ferrite phase. Table 3 shows the measurement results of each characteristic.

[Table 1]

Material No.	Chemical composition (mass%)
Material No.	C	Si	Mn	Cr	Mo	N	Remainder
1	0.30	0.25	0.70	10.1	2.0	0.035	Fe and unavoidable impurities
2	0.38	0.35	0.36	13.4	1.2	0.017
3	0.38	0.35	0.27	13.9	1.2	0.010
4	0.39	0.36	0.39	13.4	1.2	0.014
5	0.40	0.25	0.71	10.1	2.0	0.040
6	0.51	0.25	0.70	10.6	2.0	0.035
7	0.50	0.47	0.79	13.4	1.3	0.034

[Table 2]

Sample No.	Material		Nitrogen absorption treatment			Tempering	Remark
Sample No.	Material No.	Thickness (µm)	Heating conditions	Integrated value of temperature and time	Nitrogen-enriched layer	Heating temperature	Remark
A	1	0.50	1050°C-2.7min-nitrogen	2835	○	350°C	Example of present invention
B	2	0.50	1050°C-2.7min-nitrogen	2835	○	350°C	Example of present invention
C	3	0.31	1000°C-2.7min-nitrogen	2700	○	350°C	Example of present invention
D	3	0.31	1050°C-2.7min-nitrogen	2835	○	350°C	Example of present invention
E	3	0.31	1100°C-2.7min-nitrogen	2970	○	350°C	Example of present invention
F	5	0.50	1050°C-2.7min-nitrogen	2835	○	350°C	Example of present invention
G	6	0.50	1070°C-2.7min-Ar	2889	×	350°C	Comparative example
H	7	0.50	1070°C-2.7min-Ar	2889	×	350°C	Comparative example
I	2	0.50	1070°C-2.7min-Ar	2889	×	350°C	Comparative example
J	2	0.50	1070°C-2.7min-Ar	2835	×	270°C	Comparative example
K	3	0.31	1050°C-2.7min-Ar	2835	×	350°C	Comparative example
L	3	0.31	1150°C-2.7min-nitrogen	3105	○	350°C	Comparative example
M	2	0.50	1050°C-60min-nitrogen	63000	Δ	350°C	Comparative example
N	1	0.50	1050°C-60min-nitrogen	63000	Δ	350°C	Comparative example
O	4	0.50	1050°C-60min-nitrogen	63000	Δ	350°C	Comparative example
P	4	0.50	900°C-1.7min-nitrogen	1530	×	350°C	Comparative example
Q	4	0.50	1050°C-4.7min-nitrogen	4935	○	350°C	Example of present invention
R	4	0.50	1050°C-10.0min-nitrogen	10500	○	350°C	Example of present invention
S	2	0.50	1050°C-2.0min-nitrogen	2100	○	350°C	Example of present invention
T	2	0.50	1050°C-4.0min-nitrogen	4200	○	350°C	Example of present invention

[Table 3]

Sample No.	Tempering						Remark
Sample No.	Surface hardness (HV)	Center part hardness (HV)	Elongation (%)	Young's modulus (GPa)	Retained austenite amount (%) of surface layer part	10⁷ cycle fatigue limit (MPa)	Remark
A	639	543	6.8	216	14.0	1220	Example of present invention
B	628	602	2.2	210	20.6	1270	Example of present invention
C	612	486	5.0	226	11.6	1300	Example of present invention
D	633	546	4.6	226	22.2	1350	Example of present invention
E	624	590	4.8	189	25.9	1300	Example of present invention
F	650	636	2.2	216	19.9	1140	Example of present invention
G	615	656	2.8	198	10.4	1030	Comparative example
H	598	619	4.6	210	9.9	1060	Comparative example
I	540	585	7.8	212	9.7	900	Comparative example
J	609	617	1.6	220	4.3	<950	Comparative example
K	576	542	4.8	206	3.6	1070	Comparative example
L	591	574	3.1	222	28.9	1070	Comparative example
M	537	591	1.9	201	33.7	-	Comparative example
N	587	646	0.2	203	6.1	-	Comparative example
O	514	594	1.2	218	-	800	Comparative example
P	444	407	8.4	222	0	<900	Comparative example
Q	659	597	2.8	205	21.5	1100	Example of present invention
R	660	585	2.3	209	30.2	1100	Example of present invention
S	644	554	2.3	214	24.7	1150	Example of present invention
T	647	581	1.3	216	23.6	1200	Example of present invention

From the results of Tables 2 and 3, components A to F and Q to T of the Example of the present inventions had surface hardness of 600HV or more, had nitrogen-enriched layers with thickness of 10 to 100µm, and had nitrogen non-absorption regions in the center part of the component with hardness 10HV or more lower than the hardness of the nitrogen-enriched layers. Furthermore, in components A to F and Q to T, the retained austenite amount was 11.3% or more and 30.5% or less, and it was confirmed that they had good fatigue limits. On the other hand, in comparative examples G and J, although the surface hardness was good at 600HV or more, because heat treatment was performed in Ar atmosphere, they did not have nitrogen-enriched layers, resulting in low fatigue limits. Moreover, in comparative examples M, N, and O, due to excessive amount of nitrogen being dissolved in solid solution at the surface, austenite remained at the very surface, resulting in low surface hardness and small elongation. Comparative example L is also considered to have reduced surface hardness for the same reason as comparative examples M, N, and O. In comparative examples H, I, and K, because heat treatment was performed in Ar atmosphere, they did not have nitrogen-enriched layers in the first place, resulting in low surface hardness and consequently low fatigue limits. In comparative example P, although the heating temperature was at the lower limit temperature, the heating time was short and the temperature-time integrated value was small, so the thickness of the nitrogen-enriched layer became less than 10µm, resulting in remarkably low fatigue strength. Regarding the nitrogen-enriched layers, it was thereby confirmed that the Example of the present inventions can simultaneously obtain high hardness, good elongation, and fatigue limit compared to conventional examples.

Claims

A steel component, comprising a nitrogen-enriched layer having a hardness of 600HV or more and a thickness of 10µm to 100µm on a surface of a steel, the steel having a component composition containing, in mass%, C: 0.3% to 0.5%; Si: 1.0% or less; Mn: 1.5% or less; Cr: 9.0% to 15.0%; Mo and W: either alone or in combination of (Mo+W/2): 0.5% to 3.0%; and N: 0.1% or less, with a remainder being Fe and unavoidable impurities; and comprising, in a center part of the component, a nitrogen non-absorption region with N of 0.1% or less in mass% and having a hardness of 10HV or more lower than the hardness of the nitrogen-enriched layer.
The steel component according to claim 1, wherein elongation is 1.0% or more and 7.0% or less.
A method for producing steel component, comprising a nitrogen absorption treatment step of having a steel material absorb nitrogen while heating and holding it under conditions where a heating temperature is 900°C to 1100°C, a heating time is 2min to 20min, and an integrated value of the heating temperature [°C] and the heating time [min] is 2000 to 11000, and rapidly cooling it, the steel material having a component composition containing, in mass%, C: 0.3% to 0.5%; Si: 1.0% or less; Mn: 1.5% or less; Cr: 9.0% to 15.0%; Mo and W: either alone or in combination of (Mo+W/2): 0.5% to 3.0%; and N: 0.1% or less, with a remainder being Fe and unavoidable impurities.
The method for producing steel component according to claim 3, wherein tempering is performed after the nitrogen absorption treatment step.