US20190040506A1

US20190040506A1 - Martensitic stainless steel member and method for manufacturing same, and martensitic stainless steel component and method for manufacturing same

Info

Publication number: US20190040506A1
Application number: US16/074,434
Authority: US
Inventors: Junichi Nishida
Original assignee: Hitachi Metals Ltd
Current assignee: Proterial Ltd
Priority date: 2016-03-04
Filing date: 2017-03-06
Publication date: 2019-02-07
Also published as: JPWO2017150738A1; WO2017150738A1; JP6784960B2; JP2019167630A; JP6631860B2

Abstract

Provided are a stainless steel component having excellent corrosion resistance and abrasion resistance and a method for manufacturing the same, a stainless steel member suitable for fabricating the component, and a method for manufacturing the stainless steel member. The present invention is a stainless steel member having a thickness of 0.3 mm or less and having a component composition comprising, in terms of mass %, 0.10-0.40% C, 1.00% or less of Si, 0.10-1.50% Mn, 10.0-18.0% Cr, and 2.00% or less of N, the remainder being Fe and impurities, wherein the amount of N in a depth range of at least 0.05 mm from the surface of the stainless steel member is 0.80-2.00 mass %. The present invention is also a method for manufacturing the stainless steel member, a stainless steel component which uses the stainless steel member, and a method for manufacturing the stainless steel component.

Description

TECHNICAL FIELD

The present invention relates to a stainless steel member that may be used for stainless steel components such as sliding components such as piston rings and cams of a combustion engine and tool components such as molds and blades, and a method of manufacturing the same. Further, the present invention relates to a stainless steel component and a method of manufacturing the same.

BACKGROUND ART

Conventionally, it is known that various properties can be improved by adding nitrogen to stainless steel. By adding nitrogen, it is possible to enhance the properties of stainless steel without adding expensive alloying elements.
For example, in the case of stainless steel having an austenitic structure, it is advantageous for improving corrosion resistance. In general, the corrosion resistance of the stainless steel may be evaluated by a pitting resistance equivalent (PRE) defined by the component composition of the stainless steel. The larger this value, the higher the corrosion resistance. When evaluated by “Cr+3.3Mo+30N-Mn” proposed as an example of the definition formula of the pitting resistance equivalent, the effect of improving the corrosion resistance when 1 mass % of nitrogen is added is almost equivalent to that when about 9 mass % of molybdenum is added.
Further, for example, in the case of stainless steel having a martensitic structure, it is advantageous for improving the quenching hardness. That is, when quenching is performed on stainless steel, nitrogen in the steel solid-dissolves in the structure like carbon, which is the same invasive element. The hardness of the stainless steel improves according to the amount of nitrogen solid-dissolved in the structure.
As a method of manufacturing stainless steel to which nitrogen is added, for example, there is a method of manufacturing an ingot having a high nitrogen content in which, in a melting step, nitrides are added to molten steel or melted in a pressurized nitrogen atmosphere. However, when the amount of nitrogen added at the time of molten steel is large, nitrogen gas is generated during solidification, and blowholes are generated in the ingot. In addition, when electroslag remelting is used for the dissolution in a nitrogen atmosphere as described above, special remodeling relating to maintenance of a pressurized nitrogen atmosphere is required in a usual electroslag remelting apparatus.
Therefore, as another method of manufacturing stainless steel to which nitrogen is added, there is a method of heating the “solidified” stainless steel in a pressurized nitrogen atmosphere, ammonia, nitrogen plasma, or salt bath. According to this method, nitrogen is solid-dissolved or nitrides are precipitated on the surface of the stainless steel under heating to form a “nitrogen-enriched layer” in which nitrogen is added on the surface of the stainless steel. As a result, corrosion resistance and abrasion resistance of stainless steel may be improved without adding nitrogen to the entire stainless steel.
As a specific method of forming the above-mentioned “nitrogen-enriched layer”, a “nitride precipitation method (so-called nitriding treatment)” in which stainless steel is treated in an environment of about 500° C. using ammonia or nitrogen plasma has been widely put to practical use. By this nitride precipitation method, a nitrogen-enriched layer in which fine nitrides are precipitated is formed on the surface of the stainless steel, and the surface of the stainless steel after the nitrogen-enriched layer is formed is cured. However, in this case, a fragile nitrogen compound such as ε nitride is likely to be generated in the nitrogen-enriched layer.
On the other hand, as another method of forming the “nitrogen-enriched layer”, there is “nitrogen absorption treatment” in which stainless steel is heated and maintained at a temperature of, for example, about 1000° C. in a nitrogen atmosphere. In the case of the nitrogen absorption treatment, nitrogen is added in a solid solution state exclusively to the surface of the stainless steel, so that a large amount of fragile nitrogen compounds is not generated as in the above-described nitride precipitation method. Further, since treatment is at a high temperature as compared with the nitride precipitation method, it is advantageous for forming a thick nitrogen-enriched layer.
As a method of adding nitrogen to the surface of stainless steel by nitrogen absorption treatment, the following has been proposed.
First, for a stainless steel having an austenitic structure, for example, a “method in which a ferritic stainless steel containing 18 to 24% of Cr and 0 to 4% of Mo in terms of mass % comes into contact with an inert gas containing nitrogen gas at 800° C. or more to perform the nitrogen absorption treatment, so that the entire product is austenitized or a part thereof is austenitized to produce a product not containing Ni” has been proposed (Patent Literature 1).
Further, a “method of forming an austenite surface layer containing dissolved nitrogen at 0.30 wt % or more on the stainless steel by nitriding stainless steel components close to the final shape in a nitrogen-containing gas atmosphere at a temperature of 1000 to 1200° C., followed by cooling at a rate such that precipitation of nitrides is avoided” has been proposed (Patent Literature 2).
On the other hand, for stainless steel having a martensitic structure, the following has been proposed. For example, a “chromium-based stainless steel sheet having a chemical composition of, in terms of weight proportion, 13.0 to 20.0% of Cr, 0.1% or less, of C, 0.1% or less of N, the balance being Fe and inevitable impurities, and having a nitride layer and a structure in which a single phase of a ferrite phase appears in an inner layer portion and a martensitic phase appears in a nitrided surface layer portion” has been proposed (Patent Literature 3). For this martensitic phase structure portion, the thickness is in the range of about 10 to 30 μm and the hardness is about 250 HV.
Further, a “martensitic stainless steel obtained by heating a steel material having components including, in terms of wt %, C in the range of 0.26 to 0.40%, Si in the range of 1% or less, Mn in the range of 1% or less, P in the range of 0.04% or less, S in the range of 0.03% or less, Cr in the range of 12 to 14%, N in the range of 0.02% or less, and B in the range of 0.0005 to 0.002%, the balance being Fe and inevitable impurities in a nitrogen atmosphere so that a nitrogen concentration is in the range of 0.25 to 0.3% in the surface layer, and then water quenching the steel material” has been proposed (Patent Literature 4). In heating in the nitrogen atmosphere, a solid-phase nitrogen absorption method of maintaining in a high-temperature nitrogen atmosphere of 1200° C. and 0.1 MPa for 1 to 3 hours is used, and accordingly, nitrogen is absorbed so that the concentration of nitrogen in a steel material surface layer is in the range of 0.25 to 0.3%, and thereby a surface hardness of 700 HV or more is obtained.
Further, a method of heating stainless steel containing 0.4 mass % or less of carbon to an Ac1 point or more and diffusing 0.2 to 0.8 mass % of nitrogen into the surface, and directly quenching and tempering the stainless steel to cure the surface has been proposed (Patent Literature 5).

CITATION LIST

Patent Literature

[Patent Literature 1]

Japanese Unexamined Patent Application Publication No. 2006-316338
[Patent Literature 2] Japanese Unexamined Patent Application Publication No. H7-188733

[Patent Literature 3]

Japanese Unexamined Patent Application Publication No. H5-311336

[Patent Literature 4]

Japanese Unexamined Patent Application Publication No. 2010-138425

[Patent Literature 5]

German Patent Application Publication No. 4033706

SUMMARY OF INVENTION

Technical Problem

The method proposed in each of the above-mentioned items of patent literature is an effective method for obtaining stainless steel to which nitrogen is added. However, in the case of stainless steel obtained by these conventional methods, since the amount of absorbed nitrogen is small, corrosion resistance and abrasion resistance are insufficient under a more severe corrosive environment, and there is room for improvement in improvement of corrosion resistance and abrasion resistance in the state of products (components).
An object of the present invention is to provide a stainless steel member with a nitrogen-enriched layer formed on the surface thereof, which is capable of achieving excellent corrosion resistance and abrasion resistance on the surface of a stainless steel component after quenching and tempering, and a method of manufacturing the same. Further, the present invention provides a stainless steel component having excellent corrosion resistance and abrasion resistance, and a method of manufacturing the same.

Solution to Problem

That is, the present invention is a stainless steel member having a thickness of 0.3 mm or less and having a component composition including, in terms of mass %, 0.10 to 0.40% of C, 1.00% or less of Si, 0.10 to 1.50% of Mn, 10.0 to 18.0% of Cr, 2.00% or less of N, and the remainder being Fe and impurities, wherein the amount of N in a depth range of at least 0.05 mm from the surface of the stainless steel member is in a range of 0.80 to 2.00 mass %.
Further, the present invention is a stainless steel component, including a martensitic structure having an average crystal grain diameter of 20 μm or less, having a thickness of 0.3 mm or less and having a component composition including, in terms of mass %, 0.10 to 0.40% of C, 1.00% or less of Si, 0.10 to 1.50% of Mn, 10.0 to 18.0% of Cr, 2.00% or less of N, and the remainder being Fe and impurities, wherein the amount of N in a depth range of at least 0.05 mm from the surface of the stainless steel component is in a range of 0.80 to 2.00 mass %, and hardness in the depth range is 650 HV or more.
Further, the present invention is a method of manufacturing a stainless steel member, including heating a stainless steel, which has a thickness of 0.3 mm or less and has a component composition including, in terms of mass %, 0.10 to 0.40% of C, 1.00% or less of Si, 0.10 to 1.50% of Mn, 10.0 to 18.0% of Cr, 2.00% or less of N, and the remainder being Fe and impurities, to 860° C. or higher in a nitrogen atmosphere and maintaining it as is, and then cooling the stainless steel.
Further, the present invention is a method of manufacturing a stainless steel component, including quenching and tempering the stainless steel member manufactured by the above-described method of manufacturing a stainless steel member.
Here, in each of the above-described present inventions, the component composition in the stainless steel, stainless steel member, or stainless steel component may further include, in terms of mass %, at least one of 4.00% or less of Mo, 8.00% or less of W, 1.00% or less of Ni and 0.10% or less of Nb.

Advantageous Effects of Invention

According to the present invention, corrosion resistance and abrasion resistance of the surface of the stainless steel component can be improved. As a result, it is possible to improve the characteristics of various sliding parts, molds, knives and the like used under a corrosive environment.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a drawing-substituting photograph showing occurrence of rust after a salt spray test of Sample No. 6 (inventive example) evaluated in examples.

FIG. 2 is a drawing-substituting photograph showing occurrence of rust after a salt spray test of Sample No. 1 (comparative example) evaluated in examples.

FIG. 3 is a microphotograph showing an example of a cross-sectional structure in the thickness direction of Sample No. 1 (comparative example) evaluated in examples.

FIG. 4 is a microphotograph showing an example of a cross-sectional structure in the thickness direction of Sample No. 3 (inventive example) evaluated in examples.

FIG. 5 is a microphotograph showing an example of a cross-sectional structure in the thickness direction of Sample No. 4 (comparative example) evaluated in examples.

FIG. 6 is a microphotograph showing an example of a cross-sectional structure in the thickness direction of Sample No. 5 (inventive example) evaluated in examples.

FIG. 7 is a calibration curve chart showing the relationship between the amount of N and the hardness when the amount of N in the component composition was changed for the stainless steel having the component composition of the base material of Sample No. 5 (inventive example) evaluated in examples.

FIG. 8 is a view showing an example of the hardness distribution in the thickness direction of Sample Nos. 21 and 22 (inventive examples) evaluated in examples and Sample Nos. 23 and 24 (comparative examples) evaluated in examples.

FIG. 9 is a view showing an example of the distribution of the amount of N in the thickness direction of Sample Nos. 21 and 22 (inventive examples) evaluated in examples and Sample Nos. 23 and 24 (comparative examples) evaluated in examples.

DESCRIPTION OF EMBODIMENTS

A characteristic feature of the present invention is that, in a “stainless steel component” produced by quenching and tempering a “stainless steel member” on which a nitrogen-enriched layer is formed by nitrogen absorption treatment, corrosion resistance and abrasion resistance of the surface (i.e., the working surface of various stainless steel components) on which the nitrogen-enriched layer is formed are improved.
That is, as for the abrasion resistance, first, the stainless steel itself as the base material is adjusted to have a component composition “exhibiting a martensitic structure” by quenching and tempering. Furthermore, by forming a nitrogen-enriched layer in which “0.80 to 2.00 mass %” of a large amount of nitrogen is added to the above-described component composition on the surface of the above-described base material, a high hardness of “650 HV or more” is achieved for the surface (that is, the nitrogen-enriched layer) of the stainless steel component after quenching and tempering.
Further, as for the corrosion resistance, in addition to the formation of the nitrogen-enriched layer, by further adjusting the carbon content to a lower level in the component composition “exhibiting a martensitic structure” of the stainless steel, the formation of coarse carbides in the martensitic structure of the stainless steel component and the occurrence of corrosion starting from this carbide are curbed. In addition, by setting the average crystal grain diameter confirmed in the above-mentioned martensitic structure to “20 μm or less”, the grain boundary which is the starting point of destruction and corrosion is dispersed to enhance fatigue characteristics and corrosion resistance.
Hereinafter, the stainless steel member of the present invention, a stainless steel component formed using the stainless steel member, and a preferable manufacturing method for attaining the stainless steel member and component will be described together.
(1) The stainless steel member of the present invention has a component composition including, in terms of mass %, 0.10 to 0.40% of C, 1.00% or less of Si, 0.10 to 1.50% of Mn, 10.0 to 18.0% of Cr, 2.00% or less of N, and the remainder being Fe and impurities.
As described above, the stainless steel member of the present invention has a quenched and tempered structure with a component composition “exhibiting a martensitic structure” in a stainless steel component produced by performing quenching and tempering on the stainless steel member. The component composition is as follows.
*C: 0.10 to 0.40 mass % (hereinafter simply referred to as “%”)
C is an element that curbs the stabilization of ferrite and increases the hardness of the martensitic structure. In the above-described martensitic structure, it is an element that curbs coarsening of crystal grains.
However, when there is too much C, coarse Cr-based carbides crystallize in the solidified structure at the time of solidification in the melting process. Further, the coarse Cr-based carbide does not disappear even in the martensitic structure after quenching and tempering, which becomes the starting point of corrosion and deteriorates the corrosion resistance of the surface of the stainless steel component. In addition, the cold workability deteriorates, and the yield at finishing to a stainless steel member or stainless steel component of a predetermined shape decreases. Here, in the present invention, since a high hardness of the surface of the stainless steel component is achieved to a large extent by forming the nitrogen-enriched layer which will be described below, the component composition of the stainless steel itself may be designed to have “low carbon” content.
Therefore, the content of C is set to 0.10 to 0.40%. Further, the lower limit thereof is preferably 0.11%, more preferably 0.12%, and still more preferably 0.13%. The upper limit thereof is preferably 0.38%, more preferably 0.36%, and still more preferably 0.34%.
*Si: 1.00% or less
Si is an element which is used as a deoxidizing agent and the like at the time of steelmaking and is inevitably contained. When too much Si is contained, the hardenability deteriorates. Therefore, the content of Si is set to 1.00% or less, and preferably 0.80% or less, more preferably 0.65% or less, and still more preferably 0.50% or less. The lower limit is not particularly limited. Further, it is practical for 0.01% or more of Si to be contained.
*Mn: 0.10 to 1.50%
Mn is an element which is used as a deoxidizer and the like during steel making and is inevitably contained. In the present invention, it is an element having an effect of promoting the dissolution of nitrogen in the structure.
However, when the content of Mn is too large, austenite becomes stable, and it becomes difficult to obtain the martensitic structure.
Therefore, the content of Mn is set to 0.10 to 1.50%. The lower limit thereof is preferably 0.20%, more preferably 0.30%, and still more preferably 0.40%. The upper limit thereof is preferably 1.30%, more preferably 1.10%, and still more preferably 1.00%.
*Cr: 10.0 to 18.0%
Cr is an element which forms an amorphous passivation film on the surface of stainless steel and imparts corrosion resistance to the stainless steel. It also has an effect of increasing the amount of nitrogen which may be dissolved in the stainless steel, and is an element that effectively works to form a nitrogen-enriched layer which will be described below.
On the other hand, when the amount of Cr is too large, ferrite is stabilized, so that martensitization does not proceed sufficiently at the center portion of the stainless steel component of the present invention, and the strength of the whole component is lowered. Further, at the time of forming the nitrogen-enriched layer, which will be described below, it takes time for the surface structure of the stainless steel being heated under nitrogen absorption treatment to solid-dissolve nitrogen to austenite, and the production efficiency is lowered.
Therefore, the content of Cr is set to 10.0 to 18.0%, preferably less than 15.0%, more preferably 14.0% or less. Also, the content of Cr is preferably 12.0% or more.
*N (nitrogen): 2.00% or less
In the present invention, it is assumed that nitrogen that may be contained in “stainless steel” before nitrogen absorption treatment is merely an “impurity”. For example, the amount of nitrogen may be 0.02% or less. However, in the case in which the shape of the stainless steel is a material having a small thickness (thin material) such as a plate material, a band material, a toil material or the like, when nitrogen absorption treatment is performed, nitrogen may be absorbed up to the center portion of the stainless steel (that is, throughout the stainless steel) beyond the surface of the stainless steel which is the target part of absorbing nitrogen.
Therefore, the content of nitrogen in the state of the stainless steel member is 2.00% or less, taking into account the amount that may be added by the nitrogen absorption treatment from the level of the impurity before the nitrogen absorption treatment.
In the stainless steel member of the present invention, a component composition including the above-described types of elements and the remainder being Fe and impurities may be used as a basic component composition. Further, it is also possible for elements of the following types in addition to this basic component composition to be contained.
*Mo: 4.00% or less as necessary
Mo is an element which is effective for increasing corrosion resistance of stainless steel. Further, in the solid solution state, it has the effect of enhancing the function of the passive film formed by Cr. The passive film formed by Cr has its own self-restoring function as well. Further, Mo has a function of enhancing the restoring force of the passive film by increasing the amount of Cr in a scratched place when the passive film formed by Cr is scratched. Furthermore, Mo has a great effect of promoting nitrogen absorption of stainless steel.
On the other hand, when too much Mo is contained, as in Cr, the ferrite is stabilized and the strength of the whole stainless steel component decreases. In addition, it takes time for the nitrogen absorption treatment to form the nitrogen-enriched layer which will be described later, and the production efficiency decreases.
Accordingly, Mo may be contained at 4.00% or less as necessary. The content of Mo is preferably 3.00% or less, more preferably 2.50% or less, and still more preferably 2.00% or less. Further, when Mo is contained, the content thereof is preferably 0.10% or more, more preferably 0.50% or more, and still more preferably 1.00% or more.
*W: 8.00% or less as necessary
W has the same effect as Mo. Since the atomic weight of W is about twice that of Mo, the content of W for obtaining an effect equivalent to that of Mo may be regarded as twice the content of Mo.
Therefore, W may be contained at 8.00% or less as necessary. The content thereof is preferably 6.00% or less, more preferably 5.00% or less, and still more preferably 4.00% or less. In consideration of cost and the like, it is particularly preferably 2.00% or less. Further, when W is contained, the content thereof is preferably 0.10% or more, more preferably 0.30% or more, and still more preferably 0.50% or more, in consideration of cost and the like.
*Ni: 1.00% or less as necessary Ni has an effect of curbing further progress of corrosion in the early stage of corrosion. Ni also has the effect of increasing the toughness of the base in the structure.
On the other hand, when the amount of Ni is too large, the austenite becomes stable, and it becomes difficult to obtain a martensitic structure.
Therefore, Ni may be contained at 1.00% or less as necessary. In the present invention where the stainless steel component has a martensitic structure, it is important to set the content of Ni to 1.00% or less. The content of Ni is preferably 0.90% or less, and more preferably 0.80% or less. When Ni is contained, the content thereof is preferably 0.10% or more, more preferably 0.20% or more, and still more preferably 0.40% or more.
*Nb: 0.10% or less as necessary
Nb has an effect of curbing coarsening of crystal grains of the martensitic structure in the stainless steel component after quenching and tempering.
However, when the content of Nb is too large, nitrogen produces Nb nitrides, and the amount of solid solution nitrogen decreases, thereby decreasing the effect of improving the hardness.
Therefore, Nb may be contained at 0.10% or less as necessary. The content of Nb is preferably 0.09% or less, and more preferably 0.08% or less. When Nb is contained, the content is preferably 0.01% or more, and more preferably 0.03% or more.
In addition to being applicable to the “stainless steel member” of the present invention, the above-described component composition may also be applied to the “stainless steel component” of the present invention obtained by quenching and tempering the stainless steel member. Further, the component composition may also be applied to “stainless steel” which is a base material of the stainless steel member and stainless steel component before being subject to the nitrogen absorption treatment which will be described below (before the nitrogen-enriched layer is formed on the surface). When comparing the component composition of the stainless steel member (or steel component) and the stainless steel as the base material, the amount of increase in N in the stainless steel member by the nitrogen absorption treatment, which will be described below, is slight with respect to the total component composition. Therefore, the component composition of the stainless steel member (or the steel component) and the stainless steel which is the base material of the stainless steel member (or the steel component) may be regarded as substantially the same in their entirety except that the amount of N of stainless steel is “less than 2.00% (that is, less than the upper limit of the amount of N contained in the stainless steel member or the stainless steel component)”.
(2) The stainless steel member of the present invention has a nitrogen-enriched layer on the surface of the stainless steel described in the above (1), and the amount of nitrogen in the nitrogen-enriched layer is equivalent to or more than the amount of nitrogen in the center portion of the stainless steel and is 0.80 to 2.00 mass %.
In order to achieve excellent corrosion resistance and abrasion resistance in the state of a stainless steel component produced by performing quenching and tempering on the stainless steel member of the present invention, the stainless steel member of the present invention has a “nitrogen-enriched layer” formed on the surface thereof. The nitrogen-enriched layer is formed by adding “directly” nitrogen in the component composition of the above-mentioned stainless steel by nitrogen absorption treatment or the like which will be described below.
That is, the component composition of the nitrogen-enriched layer according to the present invention is a component composition after “reanalyzing” the component composition of the stainless steel after nitrogen is added thereto by adding a predetermined amount of nitrogen to the component composition of the stainless steel (base material) before the nitrogen-enriched layer is formed thereon. In the nitrogen-enriched layer according to the present invention, the amount of nitrogen is equivalent to or more than the amount of nitrogen contained in the base material of the above-described stainless steel (that is, the amount of nitrogen of the center portion of the stainless steel after the nitrogen enrichment layer is formed thereon), and an effective amount of N (nitrogen) of “0.80 to 2.00 mass %” is contained.
By adding nitrogen to the surface of the stainless steel to be a base material to form a nitrogen-enriched layer, the surface of the stainless steel component (i.e., the nitrogen-enriched layer) can have a high hardness. Regarding the effect of the high hardness, carbon has the same effect as that of nitrogen. However, as for the amount of each element added to stainless steel, in the case of carbon, the hardness improvement effect is saturated when the added amount reaches 0.80 mass %. On the other hand, in the case of nitrogen, the hardness is improved even after the addition amount reaches 0.80 mass %, but when the added amount of nitrogen exceeds about 2.00 mass %, the toughness decreases. Therefore, the amount of nitrogen of the nitrogen-enriched layer according to the present invention is 0.80 to 2.00 mass % in the definition of the above-described component composition. The amount of nitrogen is preferably 0.85 mass % or more, more preferably 0.90 mass % or more, and still more preferably 1.00 mass % or more. Depending on the content of nitrogen, the surface of the stainless steel component may achieve a high hardness of 650 HV or more, preferably 670 HV or more, more preferably 700 HV or more, and still more preferably 720 HV or more. Although it is not necessary to determine the upper limit of this hardness, 800 HV or lower is practical.
Here, solid-dissolving of the nitrogen added to the surface of the stainless steel in the structure of the stainless steel is promoted by adjusting the component composition of the stainless steel to the above-described component composition “exhibiting the martensitic structure”. For the effect of this solid-dissolution, carbon has the same effect as nitrogen. However, at the time of heating in the nitrogen absorption treatment which will be described below, nitrogen has a solid-solubility in an austenitic structure greater than that of carbon. Since nitrogen has a high affinity with Cr contained in a large amount in the stainless steel according to the present invention, the amount of solid dissolution of nitrogen in the nitrogen-enriched layer may be increased. On the other hand, the amount of brittle ε nitride formed in the nitrogen-enriched layer decreases, and the amount of coarse carbide formed due to low carbonization of the stainless steel decreases. Due to these overall effects, in addition to the above-mentioned high hardness, an improvement in corrosion resistance which is effective for austenitic stainless steel is also achieved on the surface of the stainless steel component of the present invention.
Further, the nitrogen-enriched layer having the component composition including such an effective amount of N being formed to have an effective thickness of at least about 0.05 mm on the surface of the stainless steel member (or the stainless steel component) is effective for improving corrosion resistance and abrasion resistance of the stainless steel component. It is effective that the thickness of the nitrogen-enriched layer is 0.1 mm or more, and more preferably 0.15 mm or more. It is possible to improve the corrosion resistance and abrasion resistance of stainless steel component by forming the “effective nitrogen-enriched layer” with the above-described effective amount of N and effective thickness on the surface of the stainless steel member.
In the case in which the shape of the stainless steel member of the present invention is, for example, a plate material, a band material, or a foil material and the thickness thereof is 0.3 mm or less, when a depth range of at least 0.05 mm from the surface of the stainless steel member is set as the nitrogen-enriched layer having the amount of N of 0.80 to 2.00 mass %, a nitrogen-enriched layer having a sufficient thickness (depth) with respect to the thickness of the stainless steel member can be ensured, and corrosion resistance and abrasion resistance of the stainless steel component can be improved. In the stainless steel member of the present invention having a thickness of 0.3 mm or less, it is preferable to have the nitrogen-enriched layer on both surfaces (front and back surfaces) thereof. The component composition of the nitrogen-enriched layer may be denoted as “a component composition including, in terms of mass %, 0.10 to 0.40% of C, 1.00% or less of Si, 0.10 to 1.50% of Mn, 10.0 to 18.0% of Cr, 0.80 to 2.00% of N, and the remainder being Fe and impurities” as a whole.
The setting of the lower limit of the thickness of the stainless steel member of the present invention is not particularly required. However, in terms of manufacturing efficiency and handling properties, for example, 0.02 mm or more is practical,
In the stainless steel member of the present invention having the thickness T of 0.3 mm or less, the amount of N of the center of the thickness of the stainless steel member (that is, the position at the depth of T/2 from the surface of the stainless steel member) and the center portion (that is, the range excluding the range from the surface of the stainless steel member to the depth of at least 0.05 mm) may not be the amount of N of the nitrogen-enriched layer (that is, the amount of N of 0.80 mass % or more). Here, the amount of N at the center and the center portion of the stainless steel member is maintained, for example, as the amount of N in the case of a base material of stainless steel, specifically, it is conceivable that the amount of N is in the range of less than 2.00%, which is lower than the amount of N of the nitrogen-enriched layer, or the amount of N is less than 0.80%. It is conceivable that the hardness of the center and the center portion of the stainless steel component obtained by quenching and tempering such a stainless steel member is less than 650 HV. For example, the hardness may be the hardness when the base material of stainless steel is quenched and tempered “as is”.
On the other hand, the amount of N in the center and center portion of the stainless steel member of the present invention may be equivalent (equivalent value) to the amount of N (0.80 to 2.00 mass %) of the nitrogen-enriched layer. When the thickness of the base material of the stainless steel is thin (small), nitrogen may be absorbed to the center of the base material of the above-mentioned stainless steel when the nitrogen absorption treatment is performed. In other words, it is the case that the “whole” of the stainless steel member is a nitrogen-enriched layer.
In the case in which the thickness T of the stainless steel member of the present invention is 0.1 mm or less, when the surface thereof has the above-mentioned effective nitrogen-enriched layer (thickness≥0.05 mm), the amount of N at the center of the stainless steel member (that is, the position at a depth of T/2 from the surface of the stainless steel member) is also “necessarily” equivalent (equivalent value) to that of the nitrogen-enriched layer. When the stainless steel member of the present invention has the above-mentioned effective nitrogen-enriched layer on both surfaces (front and rear surfaces) of the stainless steel member, the component composition of the stainless steel member (or component) may be denoted as “a component composition including, in terms of mass %, 0.10 to 0.40% of C, 1.00% or less of Si, 0.10 to 1.50% of Mn, 10.0 to 18.0% of Cr, 0.80 to 2.00% of N, and the remainder being Fe and impurities” as a whole. Even such a stainless steel member can achieve excellent corrosion resistance and abrasion resistance when finished into a stainless steel component.
(3) In the stainless steel member of the present invention, the nitrogen-enriched layer having the above-described component composition is preferably formed by heating the stainless steel as the base material to 860° C. or higher in a nitrogen atmosphere and maintaining the stainless steel as is. Further, it is preferable to cool the stainless steel (member) after forming the nitrogen-enriched layer.
The effect of the amount of nitrogen contributing to the improvement of hardness of martensitic stainless steel is somewhat lower than that of the amount of carbon. Therefore, in the case of using the low-carbon stainless steel according to the present invention as a base material, during increasing hardness by forming the nitrogen-enriched layer on the surface thereof, a method through which a large amount of nitrogen may be added into the nitrogen-enriched layer is necessary.
Here, as a technique through which nitrogen may be added to the surface of stainless steel, “nitrogen absorption treatment” of heating the stainless steel in a nitrogen atmosphere and maintaining the stainless steel as is, is effective. As the nitrogen atmosphere, for example, nitrogen gas may be used. Further, as a specific example, the nitrogen atmosphere may be an atmosphere containing 90 vol % or more of nitrogen gas. However, conventionally, when nitrogen of “0.80 mass % or more” is added to the surface of the stainless steel by this nitrogen absorption treatment, it is necessary that the stainless steel is heated to a high temperature exceeding 1000° C. and maintained as is for a long time. Further, by heating and maintaining at the high temperature for a long time, a large amount of nitrogen may be added, but at the same time, the crystal grains of the entire stainless steel are coarsened. When the crystal grains are coarsened, the strength characteristics deteriorate and the fatigue strength deteriorates.
In contrast, in the case of the stainless steel proposed by the present invention, the component composition is adjusted so that the solid solution amount of nitrogen in the austenitic structure is increased so that 0.80 mass % or more of nitrogen may be added to the nitrogen-enriched layer at a maintenance temperature of 860° C. or higher. In the case of conventional stainless steel, the amount of nitrogen added to the nitrogen-enriched layer is limited to about 0.5 mass % at the same maintenance temperature. In addition, when the maintenance time is as short as several minutes assuming a continuous heating furnace or the like, the amount of nitrogen is limited to about 0.3 mass %. Furthermore, since the thickness of the proposed stainless steel of the present invention is 0.3 mm or less, even when N absorbed from the surface of the stainless steel by the nitrogen absorption treatment described above diffuses to the center of the stainless steel, a sufficient amount of N may be secured even on the surface of the stainless steel, and thus a nitrogen-enriched layer having an effective thickness (depth) may be formed.
In the case of the present invention, it is important to form the stainless steel member by “temporarily cooling” the stainless steel on which the nitrogen-enriched layer is formed after forming the nitrogen-enriched layer on the surface of the stainless steel. For example, the stainless steel may be cooled to at most 200° C. (including room temperature). Then, when the cooled stainless steel member is heated again to the quenching temperature and quenched, the crystal grains may be refined by repeatedly passing through the transformation point. By performing quenching and heating, new austenite grains are formed in the structure of stainless steel, so that the crystal grains are subdivided and the average crystal grain diameter (that is, the prior austenite grain size) in the martensitic structure of the stainless steel component may be set to “20 μm or less”. By setting the average crystal grain diameter to 20 μm or less, the fatigue resistance and corrosion resistance of the stainless steel components may be improved. The average crystal grain diameter is preferably 18 μm or less, more preferably 16 μm or less, and still more preferably 14 vim or less.
It is not necessary to designate the lower limit of the above-described average crystal grain diameter. However, 8 μm or more is practical.
In the nitrogen absorption treatment according to the present invention, the upper limit of the maintenance temperature is preferably 1000° C. or less in consideration of coarsening of crystal grains. In this temperature range, it is preferable to set an optimal combination of the maintenance temperature and the maintenance time. For example, the maintenance time may be 1 hour or more or 2 hours or more. Alternatively, the maintenance time may be 6 hours or less or 5 hours or less. Further, by setting the nitrogen atmosphere during the nitrogen absorbing treatment to the “pressurized atmosphere” (including atmospheric pressure), absorption of nitrogen to the surface of the stainless steel is accelerated, which is effective particularly for shortening of the maintenance time.
In the nitrogen absorption treatment according to the present invention, the preferable maintenance temperature is 870° C. or higher, more preferably 880° C. or higher, still more preferably 890° C. or higher, and particularly preferably 900° C. or higher. Further, a more preferable maintenance temperature is 980° C. or less, and more preferably 970° C. or less.
In the case of the present invention, the stainless steel (member) after completion of the above-described nitrogen absorption treatment is temporarily cooled as described above, and the structure of the formed nitrogen-enriched layer is divided into a ferrite structure or a martensitic structure. Then, when quenching is performed on the cooled stainless steel member, the structure of the nitrogen-enriched layer is austenitized again in the heating process, new austenite grains are generated, and fine crystal grains are achieved. The quenching temperature is, for example, in the range of 950 to 1200° C.
The heating atmosphere at the quenching temperature is preferably a “non-oxidizing atmosphere” capable of curbing chemical influences (change in N amount) on the nitrogen-enriched layer. As the non-oxidizing atmosphere, for example, a vacuum environment (including a reduced-pressure atmosphere) or a non-oxidizing gas such as hydrogen gas may be used. As a specific example, the non-oxidizing atmosphere may be a non-oxidizing atmosphere with a purity of 90 vol % or more of non-oxidizing gases. After quenching, subzero treatment is preferably carried out in order to promote transformation into a martensitic structure and stabilize fine crystal grains.
By performing tempering on the quenched stainless steel member, it is possible to obtain a stainless steel component having an average crystal grain diameter in the structure of 20 μm or less and a hardness of the nitrogen-enriched layer on the surface of 650 HV or more. The tempering temperature is, for example, in the range of 150 to 650° C. When corrosion resistance is considered important, tempering is preferably “low temperature tempering”. For example, the tempering temperature may be in the range of 200 to 400° C. By lowering the tempering temperature, precipitation of Cr-based carbides, nitrides and the like in the nitrogen-enriched layer may be curbed, and deficiency of Cr in a portion adjacent to this precipitation site may be curbed, so that high corrosion resistance of the nitrogen-enriched layer may be maintained.
In the case of the present invention, the stainless steel before carrying out the above-described nitrogen absorption treatment may be, for example, subjected to a soaking treatment which is maintaining at a high temperature of about 1200° C. for a long time during the time of being an ingot. In the case of the stainless steel according to the present invention, component design is performed so that coarse Cr-based carbides do not crystallize during solidification. However, in large ingots, coarse Cr-based carbides may crystallize due to segregation. In this case, the coarse Cr-based carbide may be solid-dissolved in the structure by the above-described soaking treatment.
Further, it is preferable that the stainless steel before carrying out the above-described nitrogen absorbing treatment be adjusted to a substantially component (product) shape by machining or the like. In the state of low-carbon steel containing no nitrogen, processing is easy, and a production yield is large. Therefore, it is desirable to process the steel to a shape that is as close as possible to the final shape before curing by addition of nitrogen.

Example 1

10 kg of molten metal melted in a high frequency induction melting furnace was cast to produce a stainless steel ingot having the chemical components shown in Table 1. Further, the content of N of the ingot was 0.02% or less. Next, these ingots were subjected to hot forging with a forging ratio of about 10, annealed at 780° C. after cooling, and an annealed material was obtained. Then, a plate material having a thickness of 1 mm was cut out from the annealed material, and cold rolling was performed on the plate material to obtain a strip material having a thickness T of 0.15 mm.
“Nitrogen absorption treatment” was performed on the stainless steel base material formed of the above-mentioned strip material by heating the stainless steel in a nitrogen atmosphere with nitrogen gas (purity of 99%) at atmospheric pressure, then the stainless steel was furnace-cooled to 600° C. or lower, and taken out from the furnace (room temperature) to produce a stainless steel member. The heating temperature and the maintenance time in the nitrogen absorption treatment are as shown in Table 1. The nitrogen content of the nitrogen-enriched layer formed by the nitrogen absorption treatment is also shown in Table 1.
In the present example, it was confirmed by an electron beam microanalyzer (EPMA) that, since the shape of the stainless steel subjected to the nitrogen absorption treatment was a thin strip material having a thickness of 0.15 mm, nitrogen of an equivalent concentration was added throughout the entire region in the thickness direction from both surfaces (front and back surfaces) of the stainless steel. Accordingly, the amount of nitrogen contained in the nitrogen-enriched layer was replaced by the amount of nitrogen determined for the entire sample including the surface to the center of the sample. In the analysis of the amount of nitrogen in the entire sample, the amount of nitrogen generated by melting the entire sample was obtained from the thermal conductivity, and this amount was taken as the amount of nitrogen contained in the nitrogen-enriched layer.
Next, the stainless steel member was quenched and tempered to produce a stainless steel component. Quenching was carried out by placing the above-described stainless steel member in a furnace in an atmosphere of hydrogen gas (atmospheric pressure, purity of 99%) heated to 1100° C. for 2 minutes and then rapidly cooling the stainless steel member. After quenching, sub-zero treatment at −75° C. was carried out. The tempering temperature was 350° C.
Then, the average crystal grain diameter of the martensitic structure of the stainless steel component and the hardness of the nitrogen-enriched layer were measured.
The average crystal grain diameter was measured by a line segment method. First, a structure of a cross section in the thickness direction (so-called TD cross section) of a stainless steel component was observed with an optical microscope (×1000) (FIG. 3). Next, a straight line having a length corresponding to the thickness (150 μm) was drawn in the observed field of view, and the number of crystal grain boundaries intersecting the straight line was counted. Then, the above-described 150 μm length was divided by this count number, and the resultant was defined as a provisional average crystal grain diameter. Then, this operation was carried out with a total of 10 different straight lines including 5 lines in the thickness direction of the stainless steel component and 5 lines in the direction orthogonal to the thickness direction (longitudinal direction) to obtain 10 provisional average crystal grain diameters, and the average crystal grain diameter was obtained by averaging the obtained provisional average crystal grain diameters.
The measurement position of the hardness is defined as the center in the thickness direction of a strip material which may be regarded as the position of the “nitrogen-enriched layer” for the above-mentioned reason.
These measurement results are shown in Table 1.
Further, a salt spray test was conducted by spraying 5% salt water at 35° C. for 5 hours on the surface of stainless steel component, and corrosion resistance was evaluated. The corrosion resistance was evaluated by observing the occurrence of rust on the surface. Based on the rust generation conditions shown in FIGS. 1 and 2, in the criteria for evaluation, the case in which the occurrence of rust was slight compared to FIG. 1 was represented by “⊚ (greatly effective)”, the case in which the occurrence of rust was significant compared to FIG. 1, but was slighter than in FIG. 2 was represented by “∘ (effective)” and the case in which the occurrence of rust was more conspicuous than in FIG. 2 was represented by “Δ (ineffective)”. The results are also shown in Table 1.

	TABLE 1

	Nitrogen-enriched layer	Average

		Nitrogen	Amount	crystal
	Component composition	absorption	of	grain	Hard-	Salt
Sample	of stainless steel (base material) (mass %)	treatment	nitrogen	diameter	ness	spray

No,

C

Si

Mn

Ni

Cr

W

Mo

Nb

Fe

conditions

(mass %)

(μm)

(HRC)

test

Note

1	0.65	0.40	0.78	—	13.13	—	—	—	Bal.	—	0.001	10	633	Δ	Comparative
															Example

2	0.14	0.39	0.80	—	13.04	—	—	—	Bal.	850° C. × 3	h	0.55	18	628	⊚	Comparative
																Example
3	0.14	0.39	0.80	—	13.04	—	—	—	Bal.	950° C. × 3	h	0.97	16	695	◯	Inventive
																Example
4	0.14	0.39	0.80	—	13.04	—	—	—	Bal.	1100° C. × 5	min	0.23	45	540	Δ	Comparative
																Example
5	0.15	0.41	0.79	—	12.94	—	1.96	—	Bal.	950° C. × 3	h	1.17	12	707	⊚	Inventive
																Example
6	0.15	0.39	0.82	—	12.97	—	—	0.05	Bal.	950° C. × 3	h	1.13	10	713	⊚	Inventive
																Example
7	0.31	0.41	0.79	—	13.02	—	—	—	Bal.	850° C. × 3	h	0.45	18	603	⊚	Comparative
																Example
8	0.31	0.41	0.79	—	13.02	—	—	—	Bal.	950° C. × 3	h	0.97	16	729	⊚	Inventive
																Example
9	0.30	0.39	0.81	—	7.93	—	—	—	Bal.	950° C. × 3	h	0.42	25	666	Δ	Comparative
																Example
10	0.21	0.34	0.77	0.72	10.90	0.93	0.81	—	Bal.	950° C. × 3	h	0.85	17	709	◯	Inventive
																Example
11	0.14	0.07	0.53	0.51	10.46	1.67	0.37	0.08	Bal.	950° C. × 3	h	0.92	12	711	◯	Inventive
																Example
12	0.20	0.39	0.79	—	17.00	—	1.97	0.03	Bal.	950° C. × 3	h	1.89	10	742	⊚	Inventive
																Example
13	0.23	0.31	0.54	0.65	12.07	—	—	—	Bal.	950° C. × 3	h	0.96	18	685	◯	Inventive
																Example

Sample No. 1 had an aim of increasing the hardness of the surface of the stainless steel component by the “carbon addition effect” by increasing the carbon content of the stainless steel used as the base material and not performing the nitrogen absorption treatment. The average crystal grain diameter in the martensitic structure was 20 μm or less. The result of the salt spray test was the rust generation situation shown in FIG. 2, and rust was generated as a whole. For reference, FIG. 3 is a microphotograph obtained by observing a cross-sectional structure in the thickness direction of the Sample No. 1 when the above-described tempering temperature was 550° C. with an optical microscope (×1000) (upper and lower limits in the figure correspond to both surfaces of the strip material). By observing the tempering temperature at 550° C., it is easy to confirm the shape of the carbide. As shown in FIG. 3, somewhat coarse carbides were confirmed in the Sample No. 1.
Sample Nos. 2 to 4 are obtained by subjecting the stainless steels having the same component composition as the base material to nitrogen absorption treatment under various conditions.
Sample No. 2 exhibited excellent corrosion resistance due to an appropriate component composition of the stainless steel as the base material and the like. Further, the average crystal grain diameter of the martensitic structure was 20 μm or less. However, since the heating or maintenance temperature during the nitrogen absorption treatment was low and the like, the amount of nitrogen contained in the nitrogen-enriched layer was low, the hardness was less than 650 HV.
Sample No. 3 also exhibited sufficient corrosion resistance due to an appropriate component composition of the stainless steel base material although some nitrides were formed on the nitrogen-enriched layer. The average crystal grain diameter of the martensitic structure was also 20 μm or less. In addition, since the appropriate heating or maintenance temperature during the nitrogen absorption treatment was 950° C. and the like, the amount of nitrogen contained in the nitrogen-enriched layer was as high as 0.97 mass %, and high hardness reaching 700 HV was achieved. For reference, FIG. 4 is a microphotograph of the cross-sectional structure of Sample No. 3 when the above-described tempering temperature was 550° C. The method of observation is the same as in FIG. 3 (the same also applies to FIGS. 5 and 6). Coarse carbides were not observed in contrast to the Sample No. 1 (FIG. 3), and a uniform martensitic structure was obtained.
Sample No. 4 was obtained by increasing the heating or maintenance temperature during the nitrogen absorption treatment to 1100° C. However, the maintenance time was set to 5 minutes, and after the maintenance, “direct quenching” was carried out as it was without “cooling” before the quenching and heating which is the next step (that is, it did not go through the state of the stainless steel “member” according to the present invention). As a result, the amount of nitrogen contained in the nitrogen-enriched layer was as small as 0.23 mass %. Further, since the number of transit times of the transformation point was small during the series of heat treatment processes, the average crystal grain diameter was also more than 20 μm. There was no improvement in both hardness and corrosion resistance properties. For reference, FIG. 5 shows a microphotograph of the structure of the Sample No. 4 when the above-described tempering temperature was 550° C.
Sample No. 5 is the stainless steel as the base material containing about 2% Mo. Due to the inclusion of Mo, it became easier for the base metal to absorb nitrogen and the amount of nitrogen contained in the nitrogen-enriched layer increased. Further, in the Sample No. 5, in conjunction with the fact that the structure of the entire sample was almost completely martensitized (FIG. 6 is a microphotograph of the cross-sectional structure of Sample No. 5 when the above-described tempering temperature was 550° C.), high hardness exceeding 700 HV was achieved on the nitrogen-enriched layer. Further, Sample No. 5 also had excellent corrosion resistance. The average crystal grain diameter was 20 μm or less.
Sample No. 6 is a stainless steel as a base material to which a trace amount of Nb was added. The amount of nitrogen contained in the nitrogen-enriched layer was high, and the average crystal grain diameter of the martensitic structure was also 20 μm or less, which had both of excellent hardness and corrosion resistance properties.
Sample Nos. 7 and 8 are the stainless steels having the same component composition adjusted to have high carbon content as a base material and subjected to nitrogen absorption treatment under various conditions. In both samples, there was no formation of coarse carbides in the structure, and excellent corrosion resistance was achieved. However, in Sample No. 7, the content of nitrogen contained in the nitrogen-enriched layer was low due to a low heating or maintenance temperature during the nitrogen absorption treatment, and the hardness thereof was less than 650 HV. In both samples, the average crystal grain diameter of the martensitic structure was 20 μm or less.
Sample No. 9 was obtained by adjusting the amount of Cr included in the stainless steel as the base material to about 8%. By reducing the amount of Cr, it became difficult for the base material to absorb nitrogen and the amount of nitrogen contained in the nitrogen-enriched layer was low. Further, sufficient corrosion resistance was not obtained. The average crystal grain diameter of the structure exceeded 20 μm.
Sample Nos. 10 and 11 are those obtained by adding W to the stainless steel as the base material. As in the case where Mo was added, the amount of nitrogen contained in the nitrogen-enriched layer was increased to achieve high hardness exceeding 700 HV and sufficient corrosion resistance. In both samples, the average crystal grain diameter of the martensitic structure was 20 μm or less.
Sample No. 12 is obtained by increasing Cr contained in the base material stainless steel to 17% and adding about 2% of Mo. Accordingly, it becomes easy for the base material to absorb nitrogen, and it was possible to add a large amount of nitrogen to the nitrogen-enriched layer even with nitrogen absorption treatment under almost the same conditions as other samples. Further, the structure was sufficiently martensitized, and the nitrogen-enriched layer reached a high hardness of about 750 HV. Further, Sample No. 12 also had excellent corrosion resistance. The average crystal grain diameter of the structure was 20 μm or less.
Sample No. 13 is a stainless steel as a base material to which Ni was added. The amount of nitrogen contained in the nitrogen-enriched layer was high, and the average crystal grain diameter of the martensitic structure was also 20 μm or less, thereby achieving excellent hardness and sufficient corrosion resistance.

Example 2

Four types of strip material (or plate materials) having a thickness T shown in Table 2 were produced in the same manner as in Example 1 using an ingot having a component composition of the base metal of Sample No. 5 in Table 1. Then, the “nitrogen absorption treatment” was performed by heating the stainless steel base material formed of the strip materials in a nitrogen atmosphere with nitrogen gas (purity of 99%) at atmospheric pressure, then stainless steel was furnace-cooled to 600° C. or lower, and taken out from the furnace (room temperature) to produce a stainless steel member. The heating temperature and the maintenance time in this nitrogen absorption treatment were respectively 950° C. and 3 hours. The depth from the surface of the nitrogen-enriched layer (N≥0.80 mass %) formed on the surface of the stainless steel member by this nitrogen absorption treatment was measured. Here, the amount of N for determining the nitrogen-enriched layer was measured at the time of being the stainless steel component because it is substantially the same even when measured in the state of a stainless steel component after quenching and tempering which will be described below.
Next, quenching and tempering was performed on the above-described stainless steel member to produce a stainless steel component. Quenching was carried out by placing the above-described stainless steel member in a furnace in an atmosphere of hydrogen gas (atmospheric pressure, purity of 99%) heated to 1100° C. for 2 minutes in common for all thicknesses, and then rapidly cooling the stainless steel member. After quenching, sub-zero treatment at −75° C. was carried out. The tempering temperature was 350° C.
The average crystal grain diameter of the martensitic structure of the stainless steel component, and the amount of N, depth and hardness of the nitrogen-enriched layer formed in the stainless steel component were measured.
The measurement method of the average crystal grain diameter was based on Example 1. Regarding the measurement of the amount of N, the depth and the hardness of the nitrogen-enriched layer, first, for the stainless steel having the component composition of the base metal of the Sample No. 5, a “calibration curve” showing the relationship between the amount of N and the hardness when the amount of N of the component composition was changed was prepared (FIG. 7). Then, in the structure of the cross section in the thickness direction of the stainless steel component (so-called TD cross section), the hardness distribution in the thickness direction was actually measured, and the amount of N corresponding to the hardness obtained by the actual measurement was obtained by comparison using the above-described calibration curve, and thereby the distribution of the converted amount of N corresponding to the hardness distribution of the four types of stainless steel component shown in Table 2 was confirmed. FIG. 8 shows the hardness distribution in the thickness direction of the above-described four types of stainless steel component.
Then, the amount of N from the surface of each of the above-mentioned four types of stainless steel component was evaluated, and a portion where the value was “0.80 to 2.00 mass %” was defined as a nitrogen-enriched layer, and it was possible to determine the depth of the nitrogen-enriched layer. As a result of the measurement using the above-described calibration curve, in the above four types of stainless steel component, the converted amount of N was “2.00 mass % or less” over the entire range in the thickness direction. FIG. 9 shows the N amount distribution in the thickness direction of the above-described four types of stainless steel component.
Then, a salt spray test of spraying 5% salt water at 35° C. for 5 hours was performed on the surface of the stainless steel components, and the corrosion resistance was evaluated. Evaluation of corrosion resistance was performed using “⊚ (greatly effective)”, “∘ (effective)”, “Δ (ineffective)” based on Example 1. The above-described results are summarized in Table 2.

TABLE 2

	Thickness T of	Depth of	Average
Sam-	stainless steel	nitrogen-	crystal grain	Salt
ple	component	enriched	diameter	spray
No.	(mm)	layer (mm)	(μm)	test	Note

21	0.15	0.075	15	⊚	Inventive
22	0.3	0.15	18	⊚	Example
23	0.5	0.03	45	⊚	Comparative
24	1.0	0.02	49	⊚	Example

In Table 2, Sample Nos. 21 and 22 are stainless steel components (members) having a thickness of 0.3 mm or less, and N absorbed from both surfaces (front and back surfaces) by the nitrogen absorption treatment reached the center. Further, the amount of N was “0.80 to 2.00 mass %” over the entire range of the thickness (that is, the entire stainless steel component (member) may be regarded as the nitrogen-enriched layer), and a surface hardness of 650 HV or more was achieved after quenching and tempering. The average crystal grain diameter was also 20 μm or less, and the corrosion resistance was also excellent.
On the other hand, Sample Nos. 23 and 24 are stainless steel components (members) having a thickness of more than 0.3 mm. In Sample Nos. 23 and 24, the amount of N in the center was higher than the amount of nitrogen (0.02% or less) in the base material of the stainless steel, and N absorbed from the surface by the nitrogen absorption treatment reached the center. However, the thickness (the depth from the surface) of the nitrogen-enriched layer which was the site where the amount of N is “0.80 mass % or more” was as small as less than 0.05 mm, and the hardness at a position 0.05 mm deep from the surface after quenching and tempering was less than 650 HV. The average crystal grain diameter exceeded 20 μm. The corrosion resistance was equivalent to that of Sample Nos. 21 and 22.

Claims

1. A martensitic stainless steel member having a thickness of 0.3 mm or less and having a component composition comprising, in terms of mass %, 0.10 to 0.40% of C, 1.00% or less of Si, 0.10 to 1.50% of Mn, 10.0 to 18.0% of Cr, 2.00% or less of N, and the remainder being Fe and impurities, wherein the amount of N in a depth range of at least 0.05 mm from the surface of the martensitic stainless steel member is in a range of 0.80 to 2.00 mass %.

2. The martensitic stainless steel member according to claim 1, wherein the component composition of the martensitic stainless steel member further comprises, in terms of mass %, 4.00% or less of Mo.

3. The martensitic stainless steel member according to claim 1, wherein the component composition of the martensitic stainless steel member further comprises, in terms of mass %, 8.00% or less of W.

4. The martensitic stainless steel member according to claim 1, wherein the component composition of the martensitic stainless steel member further comprises, in terms of mass %, 1.00% or less of Ni.

5. The martensitic stainless steel member according to claim 1, wherein the component composition of the martensitic stainless steel member further comprises, in terms of mass %, 0.10% or less of Nb.

6. A martensitic stainless steel component, comprising a martensitic structure having an average crystal grain diameter of 20 μm or less, having a thickness of 0.3 mm or less and having a component composition comprising, in terms of mass %, 0.10 to 0.40% of C, 1.00% or less of Si, 0.10 to 1.50% of Mn, 10.0 to 18.0% of Cr, 2.00% or less of N, and the remainder being Fe and impurities, wherein the amount of N in a depth range of at least 0.05 mm from the surface of the martensitic stainless steel component is in a range of 0.80 to 2.00 mass %, and hardness in the depth range is 650 HV or more.

7. The martensitic stainless steel component according to claim 6, wherein the component composition of the martensitic stainless steel component further comprises, in terms of mass %, 4.00% or less of Mo.

8. The martensitic stainless steel component according to claim 6, wherein the component composition of the martensitic stainless steel component further comprises, in terms of mass %, 8.00% or less of W.

9. The martensitic stainless steel component according to claim 6, wherein the component composition of the martensitic stainless steel component further comprises, in terms of mass %, 1.00% or less of Ni.

10. The martensitic stainless steel component according to claim 6, wherein the component composition of the martensitic stainless steel component further comprises, in terms of mass %, 0.10% or less of Nb.

11. A method of manufacturing a martensitic stainless steel member, comprising heating a martensitic stainless steel, which has a thickness of 0.3 mm or less and has a component composition comprising, in terms of mass %, 0.10 to 0.40% of C, 1.00% or less of Si, 0.10 to 1.50% of Mn, 10.0 to 18.0% of Cr, 2.00% or less of N, and the remainder being Fe and impurities, to 860° C. or higher in a nitrogen atmosphere and maintaining as is, and then cooling the martensitic stainless steel.

12. The method of manufacturing a martensitic stainless steel member according to claim 11, wherein the component composition of the martensitic stainless steel further comprises, in terms of mass %, 4.00% or less of Mo.

13. The method of manufacturing a martensitic stainless steel member according to claim 11, wherein the component composition of the martensitic stainless steel further comprises, in terms of mass %, 8.00% or less of W.

14. The method of manufacturing a martensitic stainless steel member according to claim 11, wherein the component composition of the martensitic stainless steel further comprises, in terms of mass %, 1.00% or less of Ni.

15. The method of manufacturing a martensitic stainless steel member according to claim 11, wherein the component composition of the martensitic stainless steel further comprises, in terms of mass %, 0.10% or less of Nb.

16. A method of manufacturing a martensitic stainless steel component, comprising quenching and tempering the martensitic stainless steel member manufactured by the method of manufacturing a martensitic stainless steel member according to claim 11.