JP7568497B2 - Martensitic stainless steel material and its manufacturing method - Google Patents

Description

The present invention relates to martensitic stainless steel materials and their manufacturing methods.

Traditionally, various techniques have been proposed to improve the electrical conductivity of stainless steel materials in order to apply the stainless steel materials to conductive parts (e.g. electrical contact parts).

For example, there are known techniques for reducing the surface contact resistance of stainless steel materials by forming a layer of Ni or a Ni alloy on the surface of the stainless steel material (Patent Document 1) or by modifying the surface of the stainless steel material (Patent Document 2).

Patent Document 2 describes how to modify the surface of a stainless steel material, specifically, (i) concentrating Cu in the passive film or the outermost surface layer of the stainless steel material, and (ii) precipitating a second phase mainly composed of Cu on the surface of the stainless steel material to partially inhibit the formation of the passive film.

For example, Patent Document 3 describes a stainless steel material in which the electrical resistance of the base material is reduced by precipitating a Cu-rich phase in a ferrite phase matrix.

JP 2013-087329 A JP 2001-089865 A JP 2004-277807 A

However, the technology described in Patent Document 1 requires that the base material of the stainless steel material be an austenitic stainless steel and that a Ni-containing layer be formed on the surface of the base material, making it difficult to reduce manufacturing costs. In addition, in general, the electrical resistance of the base material of stainless steel tends to increase as the content of alloy components increases (high alloy content).

While the technology described in Patent Document 2 can reduce the surface contact resistance of the stainless steel material, there is room for improvement in reducing the electrical resistance of the base material in the stainless steel material.

While the technology described in Patent Document 3 can reduce the electrical resistance of the base material in stainless steel materials, there is room for improvement in reducing the surface contact resistance of the stainless steel materials.

One aspect of the present invention has been made in consideration of the above-mentioned problems in the conventional art, and its purpose is to provide a martensitic stainless steel material and a manufacturing method thereof that reduce both electrical resistance and surface contact resistance.

In order to solve the above problems, the martensitic stainless steel material according to one embodiment of the present invention is a martensitic stainless steel material containing 1.0 mass% or more and 15.0 mass% or less of Cu, and is provided with a base material in which a first Cu-rich phase having a grain size of less than 500 nm is formed in a matrix, and a Cu-enriched surface layer portion formed on the surface of the martensitic stainless steel material, in which Cu is more concentrated than in the base material.

A method for producing a martensitic stainless steel material according to one embodiment of the present invention includes an annealing step of heating a rolled material having a martensitic stainless steel composition containing 1.0 mass% or more and 15.0 mass% or less of Cu at a temperature of 780°C or more and 830°C or less for 6 hours or more, an aging treatment step of forming a first Cu-rich phase having a particle size of less than 500 nm in a matrix by performing an aging treatment on a first intermediate material obtained by performing an intermediate step including at least a first pickling step after the annealing step under conditions where the A value defined by the following formula (1) is 15.0 to 20.0, and a second pickling step of performing a pickling treatment on a second intermediate material obtained by the aging treatment step using a mixed solution containing (i) 50 g/L to 150 g/L of nitric acid and (ii) 5 g/L to 15 g/L of hydrofluoric acid, with the mixed solution temperature being 30°C to 60°C;
A=T(20+logt)× ^10-3 ...(1)
(where:
T: Aging temperature (K)
t: aging time (h)
(It is.)

According to one aspect of the present invention, it is possible to provide a martensitic stainless steel material and a manufacturing method thereof that reduce both electrical resistance and surface contact resistance.

FIG. 1 is a cross-sectional view showing a martensitic stainless steel material according to an embodiment of the present invention. 1 is a graph showing an example of a change in Cu intensity in the depth direction near the surface of a martensitic stainless steel material.

One embodiment of the present invention will be described below. Note that the following description is provided to provide a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified. In this specification, "A to B" means greater than or equal to A and less than or equal to B.

The martensitic stainless steel material in one embodiment of the present invention is a martensitic stainless steel material containing 1.0 mass% or more and 15.0 mass% or less of Cu. The martensitic stainless steel material comprises a base material in which a first Cu-rich phase having a grain size of less than 500 nm is formed in a matrix, and a Cu-enriched surface layer portion formed on the surface of the martensitic stainless steel material, in which Cu is more concentrated than in the base material.

The matrix contained in the base material may also include a multi-phase structure of a ferrite phase and 50% by volume or more of a martensite phase. The multi-phase structure means that the part (base material) other than the Cu-rich phase in the metal structure of the base material is substantially composed of a multi-phase structure of a ferrite phase and a martensite phase. The martensite phase contained in the part other than the Cu-rich phase accounts for 50% by volume or more of the entire metal structure of the base material. "Substantially" means that other phases (e.g., precipitates and inclusions) are allowed to be mixed in a range of approximately 3% by volume or less. The matrix of the base material may also be substantially a martensite single-phase structure. The martensite single-phase structure means that the part (base material) other than the Cu-rich phase in the metal structure of the base material is substantially composed of a martensite phase. "Substantially" has the same meaning as above.

(Definition of terms)
The "martensitic stainless steel material" is not limited to a specific shape such as a steel strip or a steel plate. In this embodiment, a martensitic stainless steel strip will be described as an example of a martensitic stainless steel material. In addition, since a "steel plate" can be considered to be a part of a "steel strip", the term "martensitic stainless steel plate" is used to include a "martensitic stainless steel strip".

In addition, the "surface contact resistance value" is an index that represents the contact electrical resistance on the surface of a stainless steel material, and generally, the surface contact resistance value of stainless steel material is high. This is because a passive film exists on the surface of stainless steel material.

"Electrical resistivity" is an index that expresses the difficulty of electric current flowing through the entire steel of stainless steel. In general, stainless steel exhibits a relatively high electrical resistivity because it is a high alloy material.

"Scratch resistance" refers to the property of how difficult it is for scratches to form on the surface of a stainless steel material. The harder the surface of the stainless steel material, the better the scratch resistance.

"Cu-rich phase" refers to a second phase that is mainly composed of Cu and is formed in the material structure of stainless steel containing Cu, and is a phase that contains 80 atomic % or more of Cu.

(General manufacturing method)
First, an example of a typical manufacturing process for stainless steel strip will be described in outline. In one example, the typical manufacturing process for stainless steel strip includes a steelmaking process, a hot rolling process, an annealing process, a pickling process, a cold rolling process, an annealing and pickling process, and a finish rolling process, in this order. Each of these steps in the conventional manufacturing process is publicly known, and therefore will not be described in detail except as described below.

The processes after the hot rolling process are usually carried out using a coil formed from a wound stainless steel strip. In other words, the stainless steel strip is continuously processed as it is pulled out of the coil, and the processed stainless steel strip is then wound up again as a coil.

In recent years, in order to reduce costs, the stainless steel strip is often annealed continuously in the annealing process. In this case, the stainless steel strip drawn from the coil is continuously annealed in an annealing line. In contrast, there is also a method in which the coil is annealed directly in a heating furnace (e.g., a bell-type annealing furnace) for a relatively long period of time, and this method is called batch annealing (also called box annealing or bell annealing).

When pickling the annealed stainless steel strip, various methods are used to pickle the scale on the surface of the stainless steel strip. For example, the pickling solution used for pickling is a mixture of nitric acid and hydrofluoric acid, an acid solution containing sulfuric acid, etc. In addition, the annealed stainless steel strip may be subjected to electrolytic pickling using, for example, nitric acid.

(Summary of findings of the invention)
When stainless steel materials are applied to conductive parts, etc., the stainless steel materials are required to have low surface contact resistance and low electrical resistance of the base material.

The use of stainless steel materials for electrical contact parts, a type of conductive part, is also being considered. For example, parts such as terminals can become scratched on their surfaces when they are inserted and removed when connecting to other circuits. Such surface scratches reduce the stability of the parts.

Therefore, stainless steel materials for electrical contact parts are required to have both reduced electrical resistance and surface contact resistance, as well as improved scratch resistance. In addition, stainless steel materials for electrical contact parts are also required to keep manufacturing costs low.

The inventors of the present invention have conducted extensive research into relatively inexpensive martensitic stainless steel materials that can be suitably used for electrical contact parts and the like. As a result, they have obtained the following findings and arrived at the present invention.

That is, by subjecting a steel material having a composition of martensitic stainless steel containing Cu to a long-term batch annealing, a large amount of Cu-rich phase with a relatively large grain size can be precipitated in the base material. Then, by subjecting an intermediate material (first intermediate material) obtained by intermediate processing after batch annealing to an aging treatment, the Cu-rich phase is further precipitated in the base material. By specifying the conditions for this aging treatment, a fine Cu-rich phase (first Cu-rich phase) with a smaller grain size and finer grain size than the Cu-rich phase (second Cu-rich phase) generated by the above-mentioned batch annealing can be precipitated in the base material.

Among the multiple pickling treatments that may be included in the manufacturing process of the martensitic stainless steel material, at least the conditions of the pickling treatment after the aging treatment are specified. As a result, the following phenomenon occurs in the pickling treatment after the aging treatment. That is, Cu ions dissolved from the surface of the intermediate material (second intermediate material) after the aging treatment and contained in the pickling solution are reattached to the surface of the intermediate material during the pickling treatment. Here, by performing the batch annealing and aging treatment, the base material of the intermediate material contains a Cu-rich phase and a fine Cu-rich phase. Therefore, the Cu-rich phase and the fine Cu-rich phase present in the vicinity of the surface of the intermediate material are dissolved, thereby increasing the concentration of Cu ions dissolved in the pickling solution. As a result, a Cu-enriched portion can be appropriately formed on the surface of the martensitic stainless steel material and in its vicinity. In this specification, the surface of the martensitic stainless steel material on which the Cu-enriched portion is formed and its vicinity are referred to as a Cu-enriched surface layer portion. This Cu-enriched surface layer contains a Cu adhesion layer formed by the precipitation of Cu ions in the pickling solution, a Cu-rich phase, and a fine Cu-rich phase. This makes it possible to reduce the surface contact resistance.

The fine Cu-rich phase, like the Cu-rich phase, is a phase that is mainly composed of Cu and is formed in the material structure of stainless steel containing Cu, and contains 80 atomic % or more of Cu.

These effects have resulted in a martensitic stainless steel material that has improved surface hardness while reducing both electrical resistance and surface contact resistance. In addition, the matrix of the base material of the martensitic stainless steel material in one embodiment of the present invention is a multiphase structure of a ferrite phase and a martensite phase that accounts for 50 volume % or more of the base material, or a martensite single-phase structure. Furthermore, the martensitic stainless steel material contains a fine Cu-rich phase in the Cu-enriched surface layer. Due to these configurations, the martensitic stainless steel material has improved surface hardness and scratch resistance. By using such a martensitic stainless steel material, electrical contact parts with stable conductivity can be manufactured.

<Martensitic stainless steel material>
A martensitic stainless steel material according to one embodiment of the present invention will be described below with reference to Fig. 1. Fig. 1 is a cross-sectional view that shows a schematic diagram of a martensitic stainless steel material 1 according to one embodiment of the present invention.

As shown in FIG. 1, the martensitic stainless steel material 1 of this embodiment has a fine Cu-rich phase (first Cu-rich phase) 21 having a grain size of less than 500 nm and a Cu-rich phase (second Cu-rich phase) 22 having a grain size of 500 nm or more formed in a matrix 25. In addition, the martensitic stainless steel material 1 of this embodiment has a Cu-enriched surface layer 10 formed on the surface of the base material 20 and in its vicinity, in which Cu is more concentrated than in the matrix 25. The matrix 25 is the main phase (parent phase) in the base material 20, and is a multiphase structure of a ferrite phase and a martensite phase that accounts for 50 volume % or more of the base material 20, or a martensite single-phase structure.

The Cu-enriched surface portion 10 includes a passive film 11, a Cu adhesion layer 13, and a matrix surface layer 25A, which is the surface portion of the matrix 25. The matrix surface layer 25A includes a fine Cu-rich phase 21 and a Cu-rich phase 22. Each of these portions will be described in detail later.

Note that in FIG. 1, the structure of the Cu-enriched surface layer 10 and the base material 20 are shown diagrammatically, but the shape, size, and position of each part contained in the Cu-enriched surface layer 10 and the base material 20 are provisionally set for illustration purposes and do not limit the invention.

(Component Composition)
The martensitic stainless steel material 1 contains Cu: 1.0 mass% or more and 15.0 mass% or less. The martensitic stainless steel material 1 has a composition based on the composition of martensitic stainless steel and contains Cu. That is, the martensitic stainless steel material 1 has a Ni content of 1.0 mass% or less.

Cu is added to improve the electrical conductivity of the martensitic stainless steel material 1. If the Cu content is less than 1.0 mass%, the electrical conductivity cannot be sufficiently improved by the treatment described below. On the other hand, if the Cu content is too high, hot workability and corrosion resistance may decrease, so the Cu content is limited to 15.0 mass% or less. Note that, when manufacturing steel plate, etc., where the cost increase due to the deterioration of hot workability becomes significant, it is desirable to contain Cu in the range of 8.0 mass% or less.

Cr is an essential element for improving the corrosion resistance of steel, and it is preferable that martensitic stainless steel material 1 contains Cr: 9.0 mass% or more and 13.0 mass% or less. Adding excessive Cr can cause a decrease in electrical conductivity and deterioration of manufacturability, so the Cr content is limited to 13.0 mass% or less.

In consideration of the balance between hot workability and electrical resistance, it is preferable that martensitic stainless steel material 1 contains Cu: 1.5% by mass to 5.0% by mass, and Cr: 11.0% by mass to 13.0% by mass.

As for alloying elements other than Cr and Cu, the mass percentages are C+N: 0.10% or less, Mn: 2.0% or less, Si: 2.0% or less, and if necessary, one or two of Ti: 0.5% or less, Nb: 0.5% or less may be included, with the balance being Fe and unavoidable impurities. In addition, if necessary, one or more of these elements may be included within the mass percentages of Mo: 3.0% or less, Al: 5.0% or less, V: 2.0% or less, W: 2.0% or less, Zr: 1.0% or less, and REM: 0.1% or less.

When one or both of Ti: 0.5% or less and Nb: 0.5% or less are contained, it is desirable to satisfy the following formula (2):
7(C+N)≦Ti+Nb≦7(C+N)+0.3 (2).

(Cu-rich phase/fine Cu-rich phase)
The fine Cu-rich phase 21 is an aging precipitate dispersed in the matrix 25 by the aging precipitation treatment described later. The particle size of the fine Cu-rich phase 21 is greater than 0 and less than 500 nm. The lower limit of the particle size is not particularly limited, and it is sufficient that the particle size can be observed using a transmission electron microscope. The particle size of the fine Cu-rich phase 21 is preferably 5 nm or more and 20 nm or less. The particle size of each particle of the fine Cu-rich phase 21 is represented by the maximum diameter of the particle. Although it is difficult to quantitatively measure the particle size of each extremely fine particle, it is sufficiently possible to determine whether the particle size of the heterogeneous phase dispersed in the matrix 25 is within a range of less than 500 nm by observing using a transmission electron microscope. It is also possible to determine whether the particle size of the fine Cu-rich phase 21 is within a range of 5 nm or more and 20 nm or less.

The Cu-rich phase 22 is a precipitate dispersed in the matrix 25 by the batch annealing process described below. The grain size of the Cu-rich phase 22 is 500 nm or more, and preferably 1500 nm or more.

The martensitic stainless steel material in one aspect of the present invention contains at least a fine Cu-rich phase 21 in the matrix 25. This allows the electrical resistivity of the base material 20 to be reduced. The martensitic stainless steel material 1 in this embodiment has a structure in which the fine Cu-rich phase 21 and the Cu-rich phase 22 coexist in the matrix 25, allowing the electrical resistivity of the base material 20 to be reduced even more effectively. In particular, the fine Cu-rich phase 21 with a grain size of 5 nm to 20 nm and the Cu-rich phase 22 with a grain size of 1500 nm or more are dispersed together in the matrix 25, which is a martensitic phase, thereby increasing the effect of improving electrical conductivity.

The reason for this is unclear, but it is thought that, for example, an electrical conduction path is formed by shortening the distance between the fine Cu-rich phase 21 and the Cu-rich phase 22 in the base material 20.

(Cu-enriched surface layer)
As shown in FIG. 1, a Cu-enriched surface layer 10 in which Cu is more concentrated than in a matrix 25 is formed on the surface of the martensitic stainless steel material 1. The Cu-enriched surface layer 10 is formed by performing a pickling treatment (pickling treatment using a mixed acid) described later after the base material 20 is made to contain at least a Cu-rich phase 22 by a batch annealing treatment and an aging treatment described later. Hereinafter, in this specification, the pickling treatment that forms the Cu-enriched surface layer 10 is referred to as a Cu adhesion pickling treatment. This Cu adhesion pickling treatment is performed as at least the final pickling treatment in the manufacturing process of the martensitic stainless steel material 1. It is preferable that the Cu adhesion pickling treatment is also performed in an intermediate process between the batch annealing and the final pickling treatment. The reason for this will be described later.

The Cu-enriched surface layer 10 will be described with reference to FIG. 2. FIG. 2 is a graph showing an example of the change in Cu intensity in the depth direction detected by performing glow discharge optical emission surface analysis on the surface of the martensitic stainless steel material 1. Specifically, the distribution of Cu in the depth direction from the surface of the martensitic stainless steel material 1 was measured by glow discharge optical emission surface analysis (GDS) using a test piece 50 mm wide x 50 mm long. The Cu concentration in the matrix 25 was determined to be the average value in the portion at a depth of, for example, 40 nm to 50 nm.

2, in the martensitic stainless steel material 1 of this embodiment, when the composition analysis is performed, the Cu concentration (surface layer strength I) at the surface and the vicinity thereof is clearly greater than the intensity I0 of the Cu concentration in the matrix 25. Such a surface layer portion in the martensitic stainless steel material 1 is referred to as the Cu-enriched surface layer portion 10. The Cu-enriched surface layer portion 10 is formed in a region from the surface of the martensitic stainless steel material 1 to a depth of about 3 nm. The surface layer strength I means the intensity of the peak in the graph showing the Cu concentration in the Cu-enriched surface layer portion 10. In the example shown in FIG. 2, the surface layer strength I is about 0.84. In the martensitic stainless steel material 1 of this embodiment, the ratio of the surface layer strength I in the Cu-enriched surface layer portion 10 to the intensity I0 of the Cu concentration in the matrix 25 (surface layer Cu intensity ratio) is 1.5 or more, for example, 1.6 to 2.5. From the viewpoint of the stability of the surface contact resistance value, the martensitic stainless steel material 1 preferably has a surface layer Cu intensity ratio of 1.7 or more. In addition, the martensitic stainless steel material 1 preferably has a surface layer Cu intensity ratio of 2.0 or less. This is because if the surface layer Cu intensity ratio exceeds 2.0, the corrosion resistance of the martensitic stainless steel material 1 may decrease. The martensitic stainless steel material 1 preferably has a surface layer Cu intensity ratio of 1.7 or more and 2.0 or less.

The detailed structure of the Cu-enriched surface layer 10 is not clear, but since the surface layer Cu intensity ratio is 1.5 or more as described above, it is believed to have a structure including a Cu adhesion layer 13, a passive film 11, and a matrix surface layer 25A. The matrix surface layer 25A is a portion near the outermost surface of the matrix 25 (e.g., a portion from the outermost surface to a depth of several nm), and includes a fine Cu-rich phase 21 and a Cu-rich phase 22 (see Figure 1). The matrix surface layer 25A is a part of the matrix 25.

The Cu-enriched surface portion 10 includes a Cu adhesion layer 13 formed by Cu ions dissolved in the pickling solution in the Cu adhesion pickling process adhering (re-adhering) to the surface of the base material 20. The Cu adhesion layer 13 is a phase containing 80 atomic % or more of Cu, and may contain oxidized or hydroxided Cu. The Cu adhesion layer 13 has a thickness of, for example, 2 nm to 20 nm. The Cu adhesion layer 13 is formed in a spatially sparse state (for example, a porous state) by the Cu ions adhering to the surface of the base material 20. Therefore, the Cu adhesion layer 13 is formed to have a communication hole that communicates between the atmosphere and the matrix surface layer 25A. The communication hole has a shape that allows at least oxygen to move from the atmosphere to the matrix surface layer 25A.

In this embodiment, the Cu-enriched surface layer 10 of the martensitic stainless steel material 1 has a passive film 11 formed on the underside of the Cu adhesion layer 13 in the atmosphere.

The passive film 11 is formed on at least a portion of the interface between the Cu adhesion layer 13 and the matrix surface layer 25A by a reaction between (i) oxygen that comes into contact with the matrix surface layer 25A through the porous Cu adhesion layer 13 (specifically, the above-mentioned communicating holes) and (ii) Cr and the like contained in the matrix surface layer 25A.

More specifically, in the above-mentioned Cu adhesion pickling treatment, the passivation film that existed before the treatment is destroyed, and a Cu adhesion layer 13 is formed on the surface of the matrix 25 (or the matrix surface layer 25A). After the above-mentioned Cu adhesion pickling treatment, oxygen is supplied through the porous Cu adhesion layer 13 in the atmosphere, and a passivation film 11 is formed at the interface between the matrix surface layer 25A and the Cu adhesion layer 13.

In addition, the matrix surface layer 25A in the Cu-enriched surface portion 10 includes a fine Cu-rich phase 21 and a Cu-rich phase 22.

As shown in FIG. 1, in the Cu-enriched surface layer 10, the passivation film 11 may not be formed in the area where the fine Cu-rich phase 21 or Cu-rich phase 22 is present on the surface of the matrix 25 (i.e., the interface between the Cu adhesion layer 13 and the matrix surface layer 25A). This is because the amount of Cr present in this area is insufficient, making it difficult for the passivation film 11 to form. In this case, the Cu adhesion layer 13 may be in contact with the fine Cu-rich phase 21 or Cu-rich phase 22.

In addition, in the Cu-enriched surface portion 10, in areas where the supply of oxygen through the porous Cu adhesion layer 13 is insufficient, the Cu adhesion layer 13 and the matrix surface layer 25A may be in contact with each other, forming a relatively highly conductive area.

The Cu-enriched surface layer 10 includes a Cu adhesion layer 13 and a matrix surface layer 25A on the surface, and includes a passive film 11 on at least a portion of the interface between them. In the Cu-enriched surface layer 10, a relatively highly conductive electrical conduction path is formed by the Cu adhesion layer 13 and the fine Cu-rich phase 21 or Cu-rich phase 22. As a result, the martensitic stainless steel material 1 can reduce the surface contact resistance more than general martensitic stainless steel materials.

(Conductive)
The martensitic stainless steel material 1 of the present embodiment has a reduced electrical resistivity of 60 μΩ·cm or less and a reduced surface contact resistance of 45 mΩ or less. The martensitic stainless steel material 1 preferably has a reduced electrical resistivity of 60 μΩ·cm or less and a reduced surface contact resistance of 30 mΩ or less.

<Production Method>
An example of a method for manufacturing the martensitic stainless steel sheet of this embodiment will be described below.

(Pretreatment process)
In the pretreatment step, first, a vacuum melting furnace is used to produce steel having a composition adjusted to fall within the range of the present invention. This steel is then cast to produce a steel ingot.

(Hot rolling process)
In the hot rolling process, the steel ingot after the pretreatment process is hot rolled to produce a hot rolled steel strip. The temperature in the rolling process may be within a general range, for example, about 800°C to 1250°C.

(First annealing step)
The manufacturing method of the martensitic stainless steel sheet of this embodiment includes an annealing step of annealing the rolled material having the component composition of martensitic stainless steel (batch annealing) using, for example, a batch annealing furnace (bell annealing furnace) for the hot rolled steel strip. This annealing step is called the first annealing step (batch annealing step). The heating temperature in the first annealing step is 780°C or higher and 830°C or lower, and the heating time is 6 hours or longer. In the first annealing step, the hot rolled steel strip is held at the heating temperature for the treatment time. In the first annealing step, annealing may be performed in either an air atmosphere or a mixed atmosphere of _N2 and _H2 .

By subjecting the hot-rolled steel strip to the first annealing process, a large amount of Cu-rich phase 22 can be precipitated in the matrix 25. In the first annealing process, the heating temperature is set to 780°C or higher to soften the hot-rolled steel strip.

In addition, in the first annealing process, the heating temperature is set to 830°C or lower so as to prevent phase transformation (α phase to γ phase) in the matrix 25 of the hot-rolled steel strip. Depending on the steel composition, the matrix 25 of the hot-rolled steel strip may have a temperature region in which it transforms into the austenite phase (the so-called γ loop in a phase diagram). The austenite phase transforms into the martensite phase after cooling, which may reduce the toughness of the steel strip and cause manufacturing defects.

Therefore, the heating temperature range in the first annealing process is specified as a relatively narrow range of 780°C or higher and 830°C or lower.

(Intermediate process)
In the method for producing the martensitic stainless steel material 1 in this embodiment, after the first annealing step, an intermediate step including at least a first pickling step is performed.

The annealed steel strip (first annealed material) obtained by the first annealing process is subjected to pickling treatment by the first pickling process. In this first pickling process, the annealed steel strip is descaled.

In the first pickling step, it is preferable to perform Cu adhesion pickling treatment using a mixed acid under the same conditions as the final pickling step described below. Specifically, it is preferable to use a pickling solution (mixed acid) containing (i) nitric acid at 50 g/L or more and 150 g/L or less and (ii) hydrofluoric acid at 5 g/L or more and 15 g/L or less, and to perform the pickling treatment on the annealed steel strip with the temperature of the pickling solution set to 30°C or more and 60°C or less.

In the manufacturing method of the martensitic stainless steel material 1 in this embodiment, a Cu adhesion pickling process is performed in the first pickling process under the same conditions as the final pickling process described below.

In the first pickling process, the Cu adhesion pickling process is performed, whereby descaling occurs on the surface of the annealed steel strip, and part of the matrix 25 (including the Cu-rich phase 22) dissolves, and Cu ions are eluted into the pickling solution. In the first pickling process, the Cu ions contained in the pickling solution are redeposited on the surface of the annealed steel strip. As a result, a Cu adhesion layer is formed on the surface of the annealed steel strip. Hereinafter, the Cu adhesion layer formed on the surface of the treated material during the manufacturing process of the martensitic stainless steel material 1 is referred to as an intermediate Cu adhesion layer to distinguish it from the Cu adhesion layer 13 in the martensitic stainless steel material 1. This intermediate Cu adhesion layer may have the same composition as the Cu adhesion layer 13. The total amount of Cu contained in the intermediate Cu adhesion layer is relatively smaller than the total amount of Cu contained in the Cu adhesion layer 13.

The intermediate process may include a cold rolling process following the first pickling process. When the intermediate process includes a cold rolling process, the annealed steel strip descaled by the first pickling process is cold rolled, for example, at a reduction rate of 50 to 80%, to produce a cold-rolled steel strip.

The intermediate process may include a second annealing process and a second pickling process in which the cold-rolled steel strip is annealed and pickled following the cold rolling process.

This second annealing process may be continuous annealing, and the processing time may be, for example, several tens of seconds to several minutes. The soaking temperature in the second annealing process is 1000°C or higher. The second annealing process is performed to transform the cold-rolled steel strip from the ferrite phase to the austenite phase (martensite phase after cooling). In the second annealing process, the steel strip after heating is air-cooled.

Next, the annealed steel strip after the second annealing process is subjected to a second pickling process. The annealed steel strip is descaled by the second pickling process. In this second pickling process, it is preferable to perform Cu adhesion pickling under the same conditions as the final pickling process described below. Specifically, it is preferable to use a pickling solution containing (i) nitric acid of 50 g/L to 150 g/L and (ii) hydrofluoric acid of 5 g/L to 15 g/L, and to perform the pickling process on the annealed steel strip with the temperature of the pickling solution being 30°C to 60°C.

In the second pickling process, scale is removed from the surface of the annealed steel strip, and the intermediate Cu adhesion layer, the passive film 11, and part of the matrix 25 (including the Cu-rich phase 22) are dissolved. Here, the intermediate Cu adhesion layer and part of the matrix surface layer 25A are dissolved on the surface of the annealed steel strip, thereby increasing the concentration of Cu ions in the pickling solution.

As a result, an intermediate Cu adhesion layer is effectively formed on the surface of the annealed steel strip after the second pickling process.

(Aging treatment process)
The method for producing the martensitic stainless steel material 1 of this embodiment includes an aging treatment step in which an aging treatment is performed on the steel strip of the intermediate product (first intermediate material) after the above-mentioned intermediate step. The aging treatment step is performed under conditions in which the A value defined by the following formula (1) is 15.0 or more and 20.0 or less.
A=T(20+logt)× ^10-3 ...(1)
where T is the aging temperature (K) and t is the aging time (h).

By carrying out the aging treatment process as described above, fine Cu-rich phase 21 can be dispersed and precipitated in the matrix 25 of the steel strip of the intermediate product.

In the above aging treatment process, the aging time t is preferably 0.016 h or more (approximately 60 sec or more) assuming processing on an operating line. In addition, the aging temperature T is preferably 830°C or less at which a large amount of fine Cu-rich phase 21 can be precipitated.

In addition, in the manufacturing method of the martensitic stainless steel material 1 in this embodiment, ΔHV calculated by the following formula (2) is greater than 35HV:
ΔHV=HV2-HV1...(2)
Here, HV1 is the hardness of the intermediate product (first intermediate material) before the aging treatment process, and HV2 is the hardness of the intermediate product (second intermediate material) after the aging treatment process.

The martensitic stainless steel material 1 contains a fine Cu-rich phase 21 in the matrix 25 and has a Cu-enriched surface layer 10. This allows the material to be manufactured so that ΔHV is greater than 35 HV. As a result, the surface hardness can be increased, and scratch resistance is improved. In addition, the martensitic stainless steel material 1 preferably has ΔHV greater than 60 HV, and more preferably greater than 200 HV.

(Final pickling process)
The manufacturing method of the martensitic stainless steel material 1 in this embodiment includes a final pickling process in which a final pickling process is performed on the intermediate product (second intermediate material) obtained by the aging treatment process. In the final pickling process, a mixed acid is used for pickling (mixed acid treatment). The pickling solution (mixed acid) used in the final pickling process contains 50 g/L of nitric acid and 3 to 15 g/L of hydrofluoric acid, and has a solution temperature of 30 to 60°C.

In mixed acid treatment, Cu ions in the dissolved oxide scale or base material are preferentially deposited (adhered) to the surface compared to other ions. As a result, Cu is concentrated in the surface layer. This effect makes it possible to reduce the surface contact resistance. On the other hand, in nitric acid electrolysis, the surface is constantly dissolved by the electrolytic treatment, so deposition (adhesion) of dissolved ions is not observed.

By the above-mentioned final pickling process, a Cu adhesion layer 13 is formed on the surface of the treated material, similar to the first pickling process and the second pickling process described above. As a result, a martensitic stainless steel material 1 having a Cu-enriched surface layer portion 10 can be obtained.

(Advantageous Effects)
As described above, in the martensitic stainless steel material 1 of this embodiment, the base material 20 contains the fine Cu-rich phase 21 and the Cu-rich phase 22, so that the electrical resistance of the base material 20 is reduced. Furthermore, the surface contact resistance is reduced by providing the Cu-enriched surface layer portion 10. Furthermore, as described above, the surface hardness can be increased. That is, the martensitic stainless steel material 1 has reduced electrical resistance and surface contact resistance, and has improved scratch resistance.

The martensitic stainless steel material 1 has a Cu adhesion layer 13 formed as follows. That is, as described above, an intermediate Cu adhesion layer is formed on the surface of the first intermediate material by performing mixed acid treatment in the intermediate process. Then, in the subsequent aging treatment process, Cu in the intermediate Cu adhesion layer is diffused into the temper color (thin oxide scale) to form a Cu-rich temper color. The Cu-rich temper color is then dissolved in the final pickling process. As a result, more Cu can be adhered to the surface of the matrix surface layer 25A in the final pickling process, forming the Cu adhesion layer 13.

In addition, the martensitic stainless steel material 1 has a low Ni content and does not require a special coating on the surface. This helps to keep manufacturing costs low.

Therefore, when the martensitic stainless steel material 1 is applied to, for example, electrical contact parts, it has excellent electrical conductivity because both electrical resistance and surface contact resistance are reduced. Furthermore, since the scratch resistance is improved, the possibility of electrical conductivity deteriorating can be reduced, and as a result, the electrical contact parts can be used stably. Therefore, the martensitic stainless steel material 1 can be suitably used as an electrical contact part.

[Additional Notes]
The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in the above description are also included in the technical scope of the present invention.

Below, examples of martensitic stainless steel materials according to one aspect of the present invention are described, but the present invention is not limited to these examples.

In this embodiment, the martensitic stainless steels shown in Table 1 were melted in a vacuum melting furnace and hot-rolled (1200°C, 2 hours) to produce hot-rolled sheets with a thickness of 3 mm. Next, batch annealing was performed by heating in a heating furnace at 600 to 800°C for 6 hours. After that, the sheets were pickled and cold-rolled to produce cold-rolled sheets with a thickness of 1.0 mm. Each cold-rolled sheet was annealed by soaking at 1050°C for 1 minute and air-cooling, and then pickled. Then, aging treatment was performed at various temperatures from 350°C to 900°C, and pickling treatment was performed using mixed acid (see the above embodiment for the composition) or pickling treatment using nitric acid electrolysis. The aging treatment conditions are shown in Tables 2 and 3.

Although numerical values are omitted in Table 1, the Ni content of steel types C1 to C8 was 1.0 mass% or less. In the "Classification" section of Table 1, steel types containing 1.0 mass% or more and 15.0 mass% or less of Cu are referred to as "steels subject to the present invention," and steel types with a Cu content of less than 1.0 mass% are referred to as "comparison steels."

The steel sheets after batch annealing and aging treatment were observed with a transmission electron microscope to examine the grain size of the Cu-rich phase dispersed in the martensite phase matrix. Regarding the steel sheets after aging treatment, in order to distinguish between the Cu-rich phase precipitated after batch annealing and after aging treatment, a steel sheet that was not batch annealed was separately manufactured, and this steel sheet was subjected to aging treatment to observe the grain size of the Cu-rich phase after aging treatment.

The steel sheets after aging were also tested for surface contact resistance, electrical resistivity, and Cu concentration in the surface layer. Surface contact resistance was measured with an electrical contact simulator using a test piece 50 mm wide x 50 mm long. Electrical resistivity was measured with a four-terminal method (JIS C 2525) using a test piece 3 mm wide x 100 mm long. The distribution of Cu in the depth direction from the surface in the surface layer was measured with GDS using a test piece 50 mm wide x 50 mm long. The surface layer Cu intensity ratio was determined by dividing the surface layer intensity I, which is the peak value of the Cu concentration in the Cu-enriched surface layer 10, by the intensity I0 of the Cu concentration in the base material (matrix 25).

In addition, the surface hardness of the steel sheet before and after aging treatment was measured, and the difference between them was calculated.

The results are shown in Tables 2 and 3. In Table 3, steel sheets having the following properties (A) and (B) are listed as "invention steels". In Table 2, steel sheets not having either property (A) or (B) are listed as "comparison steels".
Characteristic (A): The surface contact resistance is 45 mΩ or less.
Property (B): Electric resistivity is 60 μΩ·cm or less.

As shown in Table 2, in Comparative Examples 2, 4, and 6, in which nitric acid electrolysis was performed in the final pickling treatment, the surface contact resistance was high regardless of whether aging treatment was performed or not. In Comparative Example 4, although a fine Cu-rich phase 21 was formed, the surface contact resistance was high. This is thought to be because the Cu-enriched surface layer 10, especially the Cu adhesion layer 13, was not sufficiently formed. In Comparative Example 4, the surface layer Cu intensity ratio was 1.1, which is smaller than the standard value of 1.5, so it is presumed that the Cu-enriched surface layer 10 was not sufficiently formed.

In addition, as can be seen from the results of Comparative Example No. 1, when using steel type C1 with a low Cu concentration, it was not possible to reduce the surface contact resistance and electrical resistivity even when using the manufacturing method of the above embodiment.

In Comparative Examples 3 and 5, the conditions for the aging treatment were outside the scope of the present invention, and precipitation of the fine Cu-rich phase hardly occurred. In particular, in Comparative Example 5, the temperature and time for the aging treatment were high, so the fine Cu-rich phase dissolved and Cu dissolved in the base material. In Comparative Example 7, aging treatment was not performed, and precipitation of the fine Cu-rich phase hardly occurred. As a result, in Comparative Examples 3, 5, and 7, the electrical resistivity of the base material was higher than 60 μΩ·cm, and the ΔHV indicating the surface hardness was 35 or less.

In contrast, as shown in Table 3, the steel sheets of Examples 11 to 25, which were manufactured under conditions within the scope of the present invention, had reduced surface contact resistance and electrical resistivity, and had large ΔHV and excellent scratch resistance.

Reference Signs List 1 Martensitic stainless steel material 10 Cu-enriched surface layer 11 Passive film 13 Cu adhesion layer 20 Base material 21 Fine Cu-rich phase 22 Cu-rich phase 25 Matrix 25A Matrix surface layer

Claims (11)

In mass%, Cu: 1.0% or more and 15.0 % or less, Cr: 9.0% or more and 13.0% or less, C + N: 0.10% or less, Mn: 2.0% or less, Si: 2.0% or less,
Further, if necessary, one or both of Ti: 0.5% or less and Nb: 0.5% or less are contained;
The Ni content is 1.0 mass% or less,
A martensitic stainless steel material having a chemical composition consisting of the balance being Fe and unavoidable impurities ,
A base material in which a first Cu-rich phase having a grain size of less than 500 nm is formed in a matrix;
A Cu-enriched surface layer portion formed on a surface of the martensitic stainless steel material, in which Cu is enriched more than in the base material ,
A ratio of a peak value of Cu intensity in the Cu-enriched surface layer portion to a value of Cu intensity in the base material, which is detected by a composition analysis of Cu, is 1.5 or more;
The base material has a second Cu-rich phase having a grain size of 500 nm or more and 2000 nm or less formed in the matrix,
The Cu-enriched surface portion includes: (i) a porous Cu adhesion layer containing 80 atomic % or more of Cu; and (ii) a passive film partially located at the interface between the Cu adhesion layer and a surface layer of the matrix,
A martensitic stainless steel material having an electrical resistivity of 60 μΩ·cm or less and a surface contact resistance of 45 mΩ or less . The Cu-enriched surface layer portion has a ratio of peak values of Cu intensity of 1.7 or more,
2. The martensitic stainless steel material according to claim 1, wherein the surface contact resistance is 10 mΩ or less. The Cu adhesion layer is
The thickness is 2 to 20 nm,
having communicating holes communicating with a surface layer of the matrix,
3. The martensitic stainless steel material according to claim 1, wherein the communicating pores have a shape that allows oxygen to move from the atmosphere to a surface layer of the matrix. The martensitic stainless steel material according to any one of claims 1 to 3, wherein the grain size of the first Cu-rich phase is 5 nm or more and 20 nm or less. The martensitic stainless steel material according to any one of claims 1 to 4, wherein the matrix contains a multi-phase structure of a ferrite phase and a martensite phase that accounts for 50% or more by volume of the base material, or is a martensite single-phase structure. The martensitic stainless steel material according to any one of claims 1 to 5, wherein the first Cu-rich phase and the second Cu-rich phase located in the surface layer of the matrix are in contact with the Cu adhesion layer. In mass%, Cu: 1.0% or more and 15.0 % or less, Cr: 9.0% or more and 13.0% or less, C + N: 0.10% or less, Mn: 2.0% or less, Si: 2.0% or less,
Further, if necessary, one or both of Ti: 0.5% or less and Nb: 0.5% or less are contained;
The Ni content is 1.0 mass% or less,
An annealing process in which a rolled material having a component composition of martensitic stainless steel, the balance of which is Fe and unavoidable impurities , is heated at a temperature of 780°C to 830°C for 6 hours or more;
After the annealing step, an aging treatment step is performed on a first intermediate material obtained by performing an intermediate step including at least a first pickling step, under conditions in which the A value defined by the following formula (1) is 15.0 or more and 20.0 or less, thereby forming a first Cu-rich phase having a grain size of less than 500 nm in a matrix;
a final pickling step in which the second intermediate material obtained by the aging treatment step is subjected to a pickling treatment using a mixed solution containing (i) 50 g/L to 150 g/L of nitric acid and (ii) 5 g/L to 15 g/L of hydrofluoric acid, with the mixed solution having a temperature of 30° C. to 60° C .;
a method for producing a martensitic stainless steel material, wherein the martensitic stainless steel material obtained by the final pickling step has an electrical resistivity of 60 μΩ cm or less and a surface contact resistance of 45 mΩ or less ;
A=T(20+logt)× ^10-3 ...(1)
(where:
T: Aging temperature (K)
t: aging time (h)
(It is.) 8. The method for producing a martensitic stainless steel material according to claim 7, wherein in the first pickling step, the first annealed material obtained by the annealing step is subjected to a pickling treatment using a mixed solution containing (i) nitric acid at 50 g/L or more and 150 g/L or less and ( ii ) hydrofluoric acid at 5 g/L or more and 15 g/L or less, with the mixed solution having a liquid temperature of 30°C or more and 60°C or less. The method for producing a martensitic stainless steel material according to claim 7 or 8 , wherein ΔHV calculated by the following formula (2) is greater than 35HV;
ΔHV=HV2-HV1...(2)
(where:
HV1: hardness of the first intermediate material, HV2: hardness of the second intermediate material). The martensitic stainless steel material obtained by the final pickling process is
A base material in which the first Cu-rich phase is formed in a matrix;
A Cu-enriched surface layer portion formed on a surface of the martensitic stainless steel material, in which Cu is enriched more than in the base material,
A ratio of a peak value of Cu intensity in the Cu-enriched surface layer portion to a value of Cu intensity in the base material, which is detected by a composition analysis of Cu, is 1.5 or more;
The base material has a second Cu-rich phase formed in the matrix, the second Cu-rich phase having a grain size of 500 nm or more and 2000 nm or less,
The method for producing a martensitic stainless steel material according to any one of claims 7 to 9, wherein the Cu-enriched surface layer portion includes: (i) a porous Cu adhesion layer containing 80 atomic % or more of Cu; and (ii) a passive film partially located at the interface between the Cu adhesion layer and a surface layer of the matrix. The Cu-enriched surface layer portion has a ratio of peak values of Cu intensity of 1.7 or more,
The method for producing a martensitic stainless steel material according to claim 10, wherein the surface contact resistance is 10 mΩ or less.

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