JP7278476B2 - Ferritic stainless steel material and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a ferritic stainless steel material and a method for manufacturing the same.

2. Description of the Related Art Conventionally, various techniques for improving the conductivity of stainless steel materials have been proposed in order to apply stainless steel materials to conductive parts (for example, electrical contact parts).

For example, the surface contact resistance of a stainless steel material is reduced 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). technology is known.

Patent Literature 2 describes a technique related to modifying the surface of a stainless steel material. Specifically, (i) Cu is concentrated in the passivation film or the outermost layer of the stainless steel material, and (ii) a second phase mainly composed of Cu is deposited on the surface of the stainless steel material to form a passivation film. is described to partially inhibit the formation of

Further, 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 during aging.

Japanese Patent Application Laid-Open No. 2013-087329 Japanese Patent Application Laid-Open No. 2001-089865 Japanese Patent Application Laid-Open No. 2004-277807

However, the technique described in Patent Document 1 requires that the base material of the stainless steel material be 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 general, in stainless steel, the electrical resistance of the base metal tends to increase as the content of alloying components increases (higher alloy).

Although the technique 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 of the stainless steel material.

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

One aspect of the present invention has been made in view of the conventional problems described above, and an object thereof is to provide a ferritic stainless steel material with reduced electrical resistance and surface contact resistance, and a method for producing the same.

In order to solve the above problems, a ferritic stainless steel material according to an aspect of the present invention is a ferritic stainless steel material containing 1.0% by mass or more and 15.0% by mass or less of Cu, and A base material in which a first Cu-rich phase having a grain size of less than 500 nm is formed, a Cu-enriched surface layer portion formed on the surface of the ferritic stainless steel material, the Cu-enriched surface layer having a higher concentration of Cu than the base material, It has

Further, in a method for manufacturing a ferritic stainless steel material according to an aspect of the present invention, a rolled material having a chemical composition of a ferritic stainless steel containing 1.0% by mass or more and 15.0% by mass or less of Cu is An annealing step of heating at a temperature of 780° C. or higher and 830° C. or lower for 6 hours or more, and after the annealing step, an intermediate step including at least a first pickling step for the first intermediate material obtained, Aging to form a first Cu-rich phase having a grain size of less than 500 nm in the matrix by performing aging treatment under conditions where the A value defined by the following formula (1) is 15.0 or more and 20.0 or less. (i) nitric acid of 50 g/L or more and 150 g/L or less and (ii) hydrogen fluoride of 5 g/L or more and 15 g/L or less for the second intermediate material obtained by the treatment step and the aging treatment step a second pickling step of performing a pickling treatment using a mixed solution containing an acid at a liquid temperature of 30° C. or higher and 60° C. or lower;
A=T(20+logt)×10 ⁻³ (1)
(here,
T: aging temperature (K)
t: aging time (h)
is).

According to one aspect of the present invention, it is possible to provide a ferritic stainless steel material with reduced electrical resistance and surface contact resistance, and a method for producing the same.

1 is a schematic cross-sectional view of a ferritic stainless steel material according to an embodiment of the present invention; FIG. 4 is a graph showing an example of changes in Cu intensity in the depth direction near the surface of a ferritic stainless steel material.

An embodiment of the present invention will be described below. The following description is for better understanding of the gist of the invention, and does not limit the invention unless otherwise specified. Further, in this specification, "A to B" indicates that A or more and B or less.

(Definition of terms)
A "ferritic stainless steel material" may be a steel strip or a steel plate, and the specific shape of the steel material is not limited. In this embodiment, a ferritic stainless steel strip will be described as an example of a ferritic stainless steel material. Further, since the "steel sheet" can be considered to be a part of the "steel strip", the term "ferritic stainless steel sheet" is used to include the "ferritic stainless steel strip".

The "surface contact resistance value" is an index representing the contact electrical resistance on the surface of the stainless steel material, and generally, the surface contact resistance value of the stainless steel material exhibits a high value. This is because a passivation film exists on the surface of the stainless steel material.

"Electrical resistivity" is an index representing the difficulty of current flow in the entire steel (that is, the base material) of a stainless steel material. In general, stainless steel materials exhibit relatively high electrical resistivity due to their high alloy content.

“Scratch resistance” is a property related to the resistance to scratching on the surface of a stainless steel material. It can be said that the harder the surface of the stainless steel material, the better the scratch resistance.

The “Cu-rich phase” refers to a second phase composed mainly of Cu and generated in the material structure of stainless steel containing Cu, and is a phase containing 80 atomic % or more of Cu.

(Regarding the general manufacturing method)
First, an example of a general manufacturing process for a stainless steel strip will be briefly described. A typical stainless steel strip manufacturing process, for example, includes a steelmaking process, a hot rolling process, an annealing process, a pickling process, a cold rolling process, an annealing/pickling process, and a finish rolling process in this order. Since each of these steps in the conventional manufacturing process is a well-known content, detailed explanation will be omitted except for the following.

Each step after the hot rolling step is usually performed using a coil formed of a wound stainless steel strip. That is, the stainless steel strip pulled out from the coil is continuously treated, and the treated stainless steel strip is wound again as a coil.

In recent years, in the annealing process, the stainless steel strip is often continuously annealed from the viewpoint of cost reduction. applied. On the other hand, there is also a method of annealing the coil as it is in a heating furnace (for example, a bell-type annealing furnace) for a relatively long time, and such a method is batch annealing (also called box annealing or bell annealing). called.

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, as a pickling solution used for pickling, a mixed solution of nitric acid and hydrofluoric acid, an acid solution containing sulfuric acid, or the like is used. Also, the annealed stainless steel strip may be subjected to electrolytic pickling using, for example, nitric acid.

(Summary of Findings of the Invention)
When a stainless steel material is applied to a conductive part or the like, the stainless steel material is required to reduce the electrical resistance of the base material as well as reduce the surface contact resistance.

In addition, the application of stainless steel to electrical contact parts, which are a kind of conductive parts, is under study. For example, in a part such as a terminal, an operation of inserting/removing the part into/from a connecting device is performed when connecting to another circuit. This can cause scratches on the part surface. Such surface blemishes reduce the stability of the part.

Therefore, stainless steel materials for electrical contact parts are required to have both reduced electrical resistance and surface contact resistance and improved scratch resistance. In addition, the stainless steel material for electrical contact parts is required not to increase the manufacturing cost.

The present inventors have extensively studied ferritic stainless steel materials that are relatively inexpensive and can be suitably applied to electrical contact parts and the like. As a result, the following findings were obtained and the present invention was conceived.

That is, a large amount of a Cu-rich phase having a relatively large grain size can be precipitated in the base material by subjecting a steel material having the chemical composition of a ferritic stainless steel containing Cu to long-term batch annealing. . Then, an intermediate material (first intermediate material) obtained by intermediate treatment after batch annealing is subjected to aging treatment to further precipitate a Cu-rich phase in the base material. By specifying the conditions in this aging treatment, a fine Cu-rich phase (first Cu-rich phase) having a grain size smaller than that of the Cu-rich phase (second Cu-rich phase) generated by the batch annealing is formed as the base. It can be deposited in the material.

Then, among the plurality of pickling treatments that can be included in the manufacturing process of the ferritic stainless steel material, at least the conditions for the pickling treatment after the aging treatment are specified. This causes the following phenomenon in the pickling treatment after the aging treatment. That is, the Cu ions eluted from the surface of the intermediate material (second intermediate material) after the aging treatment and contained in the pickling solution reattach to the surface of the intermediate material (ferritic stainless steel material) during the pickling treatment. do. Here, the base material of the intermediate material contains a Cu-rich phase and a fine Cu-rich phase due to the batch annealing and aging treatment. Therefore, the Cu-rich phase and the fine Cu-rich phase existing in the vicinity of the surface of the intermediate material are dissolved, so that the concentration of Cu ions dissolved in the pickling solution can be increased. As a result, Cu-enriched portions can be appropriately formed on the surface of the ferritic stainless steel material and in the vicinity thereof. In this specification, the surface of the ferritic stainless steel material and the vicinity thereof on which the Cu-enriched portion is formed are referred to as the Cu-enriched surface layer portion. This Cu-enriched surface layer portion includes a Cu-adhered layer formed by precipitation of Cu ions in the pickling solution, a Cu-rich phase, and a fine Cu-rich phase. Thereby, the surface contact resistance can be reduced.

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

Due to these effects, a ferritic stainless steel material with improved surface hardness and reduced electrical resistance and surface contact resistance was realized. In addition, the ferritic stainless steel material according to one aspect of the present invention has improved surface hardness and scratch resistance due to the fine Cu-rich phase contained in the Cu-enriched surface layer portion. By using such a ferritic stainless steel material, an electrical contact part having stable conductivity can be manufactured.

<Ferritic stainless steel material>
A ferritic stainless steel material according to one embodiment of the present invention will be described below with reference to FIG. FIG. 1 is a cross-sectional view schematically showing a ferritic stainless steel material according to one embodiment of the present invention.

As shown in FIG. 1, the ferritic stainless steel material 1 of this embodiment includes a fine Cu-rich phase (first Cu-rich phase) 21 having a grain size of less than 500 nm and a grain size of 500 nm or more in a matrix 25. A Cu-rich phase (second Cu-rich phase) 22 having Further, in the ferritic stainless steel material 1 of the present embodiment, a Cu-enriched surface layer portion 10 in which Cu is more concentrated than the matrix 25 is formed on the surface of the base material 20 and in the vicinity thereof. The matrix 25 is the main phase (mother phase) in the base material 20 and substantially has a ferrite single-phase structure. The fact that the matrix of the base material 20 has a ferrite single-phase structure means that the metal structure of the base material 20 has a structure in which the portion (base) other than the Cu-rich phase is substantially composed of the ferrite phase. "Substantially" means that other phases (for example, precipitates and inclusions) are allowed to coexist in a range of approximately 3% by volume or less.

Also, the Cu-enriched surface layer portion 10 includes a passive film 11 , a Cu adhesion layer 13 , and a matrix surface layer 25</b>A that is a surface layer portion of the matrix 25 . In the ferritic stainless steel material 1, fine Cu-rich phases 21 and Cu-rich phases 22 are formed in the matrix surface layer 25A. Each of these units will be described later in detail.

Note that FIG. 1 schematically shows the organizational structure of the Cu-enriched surface layer 10 and the base material 20, but the shape, size, and position of each part included in the Cu-enriched surface layer 10 and the base material 20 are provisionally set for illustration and are not intended to limit the invention.

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

Cu is added to improve the electrical conductivity of the ferritic stainless steel material 1 . If the Cu content is less than 1.0% by mass, the conductivity cannot be sufficiently improved by the treatment described later. On the other hand, if the Cu content is too high, the hot workability and corrosion resistance may deteriorate, so the Cu content is limited to 15.0% by mass or less. In addition, when the cost increase due to deterioration of hot workability becomes remarkable, such as when manufacturing a steel plate, it is desirable to contain Cu in the range of 1.0% by mass or more and 8.0% by mass or less. Moreover, the ferritic stainless steel material 1 can further facilitate the reduction of both the surface contact resistance value and the electrical resistivity by containing a relatively large amount of Cu (for example, containing more than 5% by mass of Cu). By containing a relatively large amount of Cu, the density of the fine Cu-rich phase 21 can be made relatively high, and as a result, the surface hardness can be easily improved.

Cr is an essential element for improving the corrosion resistance of steel, and the ferritic stainless steel material 1 preferably contains Cr: 9.0% by mass or more and 20.0% by mass or less. The Cr content is limited to 20.0% by mass or less because excessive addition of Cr may cause a decrease in electrical conductivity and a deterioration in manufacturability.

Considering the balance between both hot workability and electrical resistance, the ferritic stainless steel material 1 contains Cu: 1.5% by mass or more and 5.0% by mass or less, Cr: 11.0% by mass or more and 13.5% by mass. % by mass or less is preferably contained.

For alloying elements other than Cr and Cu, in mass %, C+N: 0.10% or less, Mn: 2.0% or less, Si: 2.0% or less, Ti: 0.5% or less, Nb: One or two of 0.5% or less can be contained, and the balance can be Fe and unavoidable impurities. In addition, in mass %, Mo: 3.0% or less, Ni: 3.0% or less, Al: 5.0% or less, V: 2.0% or less, W: 2.0% or less, Zr: 1.0%. If necessary, one or more of these elements may be contained in the range of 0% or less and REM: 0.1% or less.

When one or two 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 and precipitated in the matrix 25 by the aging precipitation treatment described later. The grain size of the fine Cu-rich phase 21 is greater than 0 and less than 500 nm. The lower limit of the particle diameter is not particularly limited, and the particle diameter may be such that it 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 diameter 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 individual ultrafine particles, observation using a transmission electron microscope reveals that the particle size of the heterogeneous phase dispersed in the matrix 25 is within the range of less than 500 nm. It is sufficiently possible to determine whether or not there is Also, it is possible to determine whether or not the grain size of the fine Cu-rich phase 21 is in the range of 5 nm or more and 20 nm or less.

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

A ferritic stainless steel material in one aspect of the present invention contains at least fine Cu-rich phases 21 in a matrix 25 . Thereby, the electrical resistivity of the base material 20 can be reduced. The ferritic stainless steel material 1 of the present embodiment has a structure structure in which the fine Cu-rich phase 21 and the Cu-rich phase 22 coexist in the matrix 25, and the electrical resistivity of the base material 20 can be further effectively increased. can be reduced. In particular, by co-dispersing the fine Cu-rich phase 21 with a grain size of 5 nm or more and 20 nm or less and the Cu-rich phase 22 with a grain size of 1500 nm or more in the matrix 25, which is a ferrite phase, the effect of improving conductivity is increased.

Although the reason for this is not clear, for example, it is conceivable that an electrical conduction path is formed by shortening the mutual distance between the fine Cu-rich phase 21 and the Cu-rich phase 22 in the base material 20 .

Here, the fine Cu-rich phase 21 and the Cu-rich phase 22 are formed in the matrix 25 so as to have a particle size distribution. Regarding the fine Cu-rich phase 21 and the Cu-rich phase 22, it is difficult to measure the particle size (maximum particle size) of each fine particle (precipitation phase) and quantitatively express the particle size distribution by some index. On the other hand, it is possible to express in what range the average grain size and grain size distribution of the fine Cu-rich phase 21 and the Cu-rich phase 22 are. The particle size of each fine particle can be measured, for example, by transmission electron microscopy.

Therefore, in this specification, the grain size ranges of the fine Cu-rich phase 21 and the Cu-rich phase 22 are defined as in the following rule R1 and rule R2.
Rule R1: A range that includes the average particle size of the measured fine particles.
Rule R2: The range to which the particle size of the majority (80%) of the measured particles belongs.

In the ferritic stainless steel material 1 of the present embodiment, the grain size of the Cu-rich phase 22 may be 500 nm or more and 2500 nm or less, 500 nm or more and 2000 nm or less, or 500 nm or more and 1500 nm or less.

Based on the rules R1 and R2 above, the following can be said. That is, when the ferritic stainless steel material 1 includes, for example, the Cu-rich phase 22 having a grain size defined as 500 nm or more and 2500 nm or less, the matrix 25 contains fine particles (Cu-rich phase 22) having a grain size of 500 nm or more and less than 2000 nm. is also formed.

The ferritic stainless steel material 1 having the Cu-rich phase 22 will be described below in comparison with, for example, the high Cr steel material for current-carrying parts described in Patent Document 3.

The high Cr steel material for current-carrying parts described in Patent Document 3 has a Cu-rich phase that has a particle size of 2000 nm or more and remains undissolved before aging treatment (hereinafter, for convenience of explanation, residual Cu-rich phase RCP ). The residual Cu-rich phase RCP is a different phase from the "aging precipitates" formed in the matrix 25 by the aging treatment.

The Cu-rich phase 22 contained in the ferritic stainless steel material 1 in this embodiment is a precipitate dispersed and precipitated in the matrix 25 by the batch annealing treatment described later, and is different from the residual Cu-rich phase RCP. In the ferritic stainless steel material 1 of the present embodiment, the Cu-rich phase 22 having a small grain size and a large number can exist dispersedly in the matrix 25 compared to the high Cr steel material for current-carrying parts described in Patent Document 3. .

The ferritic stainless steel material 1 according to the present embodiment can lower the electrical resistivity of the base material compared to the high Cr steel material for current-carrying parts described in Patent Document 3. This is because, in the ferritic stainless steel material 1, the distance between the Cu-rich phases (the fine Cu-rich phase 21 and the Cu-rich phase 22) can be relatively short, and as a result, an electric conduction path effective for improving the electric resistivity is formed. It is thought that this is because it is formed effectively.

(Cu-enriched surface layer)
As shown in FIG. 1 , the surface of the ferritic stainless steel material 1 is formed with a Cu-enriched surface layer portion 10 in which Cu is more concentrated than the matrix 25 . The Cu-enriched surface layer portion 10 is made to include at least a Cu-rich phase 22 in the base material 20 by batch annealing treatment and aging treatment described later, and then undergoes a pickling treatment (pickling treatment using a mixed acid) described below. It is formed by doing. In the present specification, the pickling treatment for forming the Cu-enriched surface layer portion 10 is hereinafter referred to as Cu-adhering pickling treatment. This Cu adhesion pickling treatment is performed as at least the final pickling treatment in the manufacturing process of the ferritic stainless steel material 1 . Cu adhesion pickling treatment is preferably performed also in an intermediate step between batch annealing and final pickling treatment. The reason for this will be described later.

The Cu-enriched surface layer portion 10 will be described with reference to FIG. FIG. 2 is a graph showing an example of changes in Cu intensity in the depth direction detected by performing glow discharge luminescence surface analysis on the surface of the ferritic stainless steel material 1 . Specifically, the distribution state of Cu in the depth direction from the surface of the ferritic stainless steel material 1 was measured by glow discharge luminescence surface spectroscopy (GDS) using a test piece having a width of 50 mm and a length of 50 mm. Here, it can be said that the Cu concentration of the matrix surface layer 25</b>A in which the fine Cu-rich phases 21 and the Cu-rich phases 22 are formed is the same as the Cu concentration of the base material 20 . Therefore, as the Cu concentration of the base material 20 (shown as base material strength in FIG. 2), for example, an average value at a depth of 40 nm to 50 nm from the surface of the ferritic stainless steel material 1 was adopted.

As shown in FIG. 2, when the ferritic stainless steel material 1 of the present embodiment is subjected to composition analysis, the Cu concentration peak intensity (surface layer intensity) I at the surface and its vicinity is the intensity of the Cu concentration in the base material 20 ( Base material strength) becomes clearly larger than I0. Such a surface layer portion is called a Cu-enriched surface layer portion 10 . The Cu-concentrated surface layer portion 10 has a Cu adhesion layer 13 which is a portion where the Cu concentration is concentrated, that is, a portion where the surface layer strength I of the Cu concentration is obtained. The Cu adhesion layer 13 is formed in a region from the surface of the ferritic stainless steel material 1 to a depth of about 10 nm. The surface layer strength I means the peak strength in a graph showing the Cu concentration of the Cu-enriched surface layer portion 10 . In the example shown in FIG. 2, the surface intensity I is approximately 0.6.

In the ferritic stainless steel material 1 of the present embodiment, the ratio of the surface layer strength I in the Cu-enriched surface layer portion 10 (more specifically, the Cu adhesion layer 13) to the Cu concentration strength I0 in the base material 20 (surface layer Cu strength ratio) is 1.5 or more, for example, 1.6 or more and 2.5 or less. From the viewpoint of the stability of the surface contact resistance value, the ferritic stainless steel material 1 preferably has a surface layer Cu strength ratio of 1.7 or more. Further, the ferritic stainless steel material 1 preferably has a surface layer Cu strength ratio of 2.0 or less. This is because if the surface layer Cu strength ratio exceeds 2.0, the corrosion resistance of the ferritic stainless steel material 1 may decrease. The ferritic stainless steel material 1 may have a surface layer Cu intensity ratio of 1.5 or more and 2.0 or less, preferably 1.7 or more and 2.0 or less.

Although the detailed structure of the Cu-enriched surface layer portion 10 is not clear, since the surface layer Cu strength ratio is 1.5 or more as described above, the Cu adhesion layer 13, the passive film 11, and the matrix surface layer 25A and is considered to have a structure including The matrix surface layer 25A is a portion near the outermost surface of the matrix 25 (for example, a portion from the outermost surface to a depth of several μm) and contains fine Cu-rich phases 21 and Cu-rich phases 22 (see FIG. 1). Matrix surface layer 25A is part of matrix 25 .

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

The Cu-enriched surface layer portion 10 of the ferritic stainless steel material 1 in this embodiment has a passive film 11 formed in the lower layer (inner side) than the Cu adhesion layer 13 in the atmosphere.

The passive film 11 is formed by a reaction between (i) oxygen in contact with the matrix surface layer 25A through the porous Cu adhesion layer 13 (specifically, the above-mentioned communicating holes) and (ii) Cr or the like contained in the matrix surface layer 25A. , are formed on at least part of the interface between the Cu adhesion layer 13 and the matrix surface layer 25A. The porous Cu adhesion layer 13 includes irregularly shaped cells (pores) and may have an open cell shape in which the spaces within the cells are partially communicated.

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

Further, the matrix surface layer 25A in the Cu-enriched surface layer portion 10 includes fine Cu-rich phases 21 and Cu-rich phases 22. As shown in FIG.

As shown in FIG. 1, the Cu-enriched surface layer portion 10 is formed in a region where the fine Cu-rich phase 21 or the Cu-rich phase 22 exists on the surface of the matrix 25 (that is, the interface between the Cu adhesion layer 13 and the matrix surface layer 25A). , the passive film 11 may not be formed. This is because the amount of Cr present in the region is insufficient, making it difficult to form the passive film 11 . In this case, the Cu adhesion layer 13 may be in contact with the fine Cu-rich phase 21 or the Cu-rich phase 22, thereby forming a good conductive region (electrically conductive path) with relatively high conductivity. .

In addition, the Cu-enriched surface layer portion 10 has relatively high conductivity because the Cu-adhered layer 13 and the matrix surface layer 25A are in contact with each other in a region where the supply of oxygen through the porous Cu-adhered layer 13 is insufficient. A good conductive region may be formed.

Cu-enriched surface layer portion 10 includes Cu adhesion layer 13 and matrix surface layer 25A on the surface, and includes passive film 11 at least part of the interface between Cu adhesion layer 13 and matrix surface layer 25A. In the Cu-enriched surface layer portion 10, the Cu adhesion layer 13 and the fine Cu-rich phase 21 or the Cu-rich phase 22 are in contact with each other to form an electrical conduction path with relatively high conductivity. Thereby, the ferritic stainless steel material 1 can reduce surface contact resistance more than general ferritic stainless steel materials.

The ferritic stainless steel material 1 according to the present embodiment is not subjected to a polishing treatment during the manufacturing process prior to the Cu adhesion pickling treatment. Therefore, the ferritic stainless steel material 1 does not have polishing marks on the surface of the matrix 25 or the matrix surface layer 25A. As a result, the ferritic stainless steel material 1 does not have polishing marks on the surface. The ferritic stainless steel material 1 may have a surface roughness (arithmetic mean roughness Ra) of, for example, 0.5 μm or less, or 0.01 μm or more and 0.5 μm or less.

(Conductivity)
The ferritic stainless steel material 1 of this embodiment has an electrical resistivity of 60 μΩ·cm or less and a surface contact resistance of 45 mΩ or less. The ferritic stainless steel material 1 preferably has an electrical resistivity of 60 μΩ·cm or less and a surface contact resistance of 30 mΩ or less.

The ferritic stainless steel material 1 may have an electrical resistivity of 50 μΩ·cm or less, or 40 μΩ·cm or less. The ferritic stainless steel material 1 may have a surface contact resistance value of 20 mΩ or less, or 10 mΩ or less. The ferritic stainless steel material 1 may have an electrical resistivity of 50 μΩ·cm or less and a surface contact resistance of 20 mΩ or less, or may have an electrical resistivity of 50 μΩ·cm or less and a surface contact resistance of 20 mΩ or less. The contact resistance value may be 10 mΩ or less.

The electrical resistivity and surface contact resistance value may be measured using the methods described in Examples below.

<Manufacturing method>
An example of the method for manufacturing the ferritic stainless steel sheet of the present embodiment will be described below.

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

(Hot rolling process)
In the hot rolling step, a hot rolled steel strip is produced by hot rolling the steel ingot after the pretreatment step. The temperature in the hot rolling process may be within a general range, for example, about 800°C to 1250°C. The hot rolling process may produce a hot rolled steel strip at a temperature of 1150° C. to 1250° C. and a time of 30 minutes to 120 minutes. This makes it easier to dissolve Cu into the material structure of the hot-rolled steel strip. In the hot rolling step, when the Cr content in the chemical composition is, for example, 9.0% by mass or more and 16.5% by mass or less, the hot rolled steel strip is heated at a temperature of 1100 ° C. to 1180 ° C. for 30 minutes to 120 minutes. may be manufactured.

(First annealing step)
In the method for producing a ferritic stainless steel sheet according to the present embodiment, the hot-rolled steel strip having the chemical composition of ferritic stainless steel is annealed (batch annealing) using, for example, a batch annealing furnace (bell annealing furnace). ) is included in the annealing process. This annealing process is called a first annealing process (batch annealing process). 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 heating time. In the first annealing step, annealing may be performed in either an air atmosphere or a mixed atmosphere of N ₂ and H ₂ .

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

Further, in the first annealing step, the heating temperature is set to 830° C. or lower so as not to cause phase transformation (α phase→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 range in which it transforms into the austenite phase (so-called γ loop in the phase diagram). If the matrix 25 transforms into the austenite phase, the Cu-rich phase 22 will not precipitate in the matrix 25 .

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

Then, in the first annealing step, batch annealing is performed on the hot-rolled steel strip under the conditions defined using the A value defined by the following formula (1) in the same manner as in the aging treatment step described later. may
A=T(20+logt)×10 ⁻³ (1)
Here, T is the heating temperature (K) in the first annealing step, and t is the heating time (h) in the first annealing step.

The first annealing step may be performed under conditions in which the heating temperature ranges from 780° C. to 830° C. and the A value is larger than that of the aging treatment step described later, for example, the A value is 20.0. above and 24.0 or less (heating temperature and heating time). When the first annealing step is performed under the condition that the A value is 20.0 or less, the Cu-rich phase 22 having a sufficient size is not generated, and it is difficult to sufficiently improve the electrical conductivity of the base material. On the other hand, when the first annealing step is performed under the condition that the A value exceeds 24.0, the Cu-rich phase 22 becomes too coarse and the distribution of the Cu-rich phase 22 becomes sparse. In this case, (i) the formation of an electrically conductive path may be insufficient, and (ii) sufficient workability cannot be obtained because the coarse Cu-rich phase 22 serves as a fracture starting point.

(intermediate process)
In the method of manufacturing the ferritic stainless steel material 1 according to the present embodiment, an intermediate process including at least a first pickling process is performed after the first annealing process.

The annealed steel strip (first annealed material) obtained in the first annealing step is pickled in the first pickling step. In this first pickling step, the annealed steel strip is descaled.

In addition, in the first pickling step, it is preferable to perform pickling treatment for Cu adhesion using a mixed acid under the same conditions as the final pickling step described later. Specifically, a pickling solution (mixed acid) containing (i) 50 g/L or more and 150 g/L or less of nitric acid and (ii) 5 g/L or more and 15 g/L or less of hydrofluoric acid is used. It is preferable to pickle the annealed steel strip at a liquid temperature of 30° C. or higher and 60° C. or lower.

In the manufacturing method of the ferritic stainless steel material 1 in the present embodiment, the case where the Cu adhesion pickling treatment is performed in the first pickling process under the same conditions as the final pickling process to be described later will be described.

By performing the Cu adhesion pickling treatment in the first pickling step, descaling occurs on the surface of the annealed steel strip, and a part of the matrix 25 (including the Cu-rich phase 22) dissolves, Cu ions are eluted into the pickling solution. In the first pickling step, the Cu ions contained in the pickling solution are reattached to 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 material to be treated during the manufacturing process of the ferritic stainless steel material 1 is referred to as an intermediate Cu adhesion layer in order to distinguish it from the Cu adhesion layer 13 in the ferritic stainless steel material 1. This intermediate Cu adhesion layer may be of similar composition to the Cu adhesion layer 13 . The total amount of Cu contained in the intermediate Cu adhesion layer is relatively less than the total amount of Cu contained in the Cu adhesion layer 13 .

The intermediate step may include a cold rolling step subsequent to the first pickling step. When the intermediate step includes a cold rolling step, the annealed steel strip descaled in the first pickling step is cold rolled at a rolling reduction of 50 to 80%, for example. Steel strip. By thinning the annealed steel strip, the ferritic stainless steel material 1 that is applied to electrical contact parts and the like can be suitably manufactured.

The intermediate step may include, subsequent to the cold rolling step, a second annealing step and a second pickling step of annealing and pickling the cold rolled steel strip.

The second annealing step may be continuous annealing, and the treatment time may be, for example, several tens of seconds to several minutes. The soaking temperature in the second annealing step may be about 800°C. The second annealing step is performed to soften the cold-rolled steel strip. A second annealed steel strip is formed by air-cooling the steel strip after heating in the second annealing step.

In the second annealing step, it is preferable to secure supersaturated Cu in the matrix 25 for the later-described aging treatment step. Therefore, the Cu-rich phase 22 precipitated in the first annealing step may be slightly dissolved by performing the second annealing step under the following temperature conditions. become a little smaller.

The second annealing step may be performed at a temperature of 780° C. or higher. This is to ensure a temperature sufficient to soften the material. On the other hand, the second annealing step is performed at a temperature of 850° C. or lower. This is because when the temperature exceeds 850° C., the Cu-rich phase 22 redissolves vigorously.

Also, the second annealing step is performed at a temperature of 850° C. or less and less than Ac1 point. The Ac1 point can be calculated using the following formula.
Ac1=750.8-26.6C+17.6Si-11.6Mn-22.9Cu-23Ni+24.1Cr+22.5Mo-39.7V-5.7Ti+232.4Nb-169.4Al-894.7B
Here, mass % in the component composition is substituted for the symbol of the element.

In the second annealing step, the cold-rolled steel strip is heated at a desired temperature and for a desired time, and then rapidly cooled. As a result, the possibility of reprecipitation of Cu in the matrix 25 during the cooling process after heating can be reduced. As a result, the second annealed steel strip having the matrix 25 in which supersaturated Cu is ensured can be obtained.

After the second annealing step, the second annealed steel strip is subjected to a second pickling step. The descaling treatment of the second annealed steel strip is performed by the second pickling step. Also in this second pickling process, it is preferable to perform the Cu-adhering pickling treatment under the same conditions as the final pickling process, which will be described later. Specifically, a pickling solution containing (i) 50 g/L or more and 150 g/L or less of nitric acid and (ii) 5 g/L or more and 15 g/L or less of hydrofluoric acid is used, and the pickling solution The second annealed steel strip is preferably subjected to a pickling treatment with the liquid temperature of 30° C. or higher and 60° C. or lower.

In the second pickling step, scale is removed from the surface of the second 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 removed. Dissolve. Here, the concentration of Cu ions in the pickling solution can be increased by partially dissolving the intermediate Cu adhesion layer and the matrix surface layer 25A on the surface of the second annealed steel strip.

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

(Aging treatment process)
The method of manufacturing the ferritic stainless steel material 1 of the present embodiment includes an aging treatment step of aging the steel strip of the intermediate product (first intermediate material) after the intermediate step. The aging treatment process is performed under the condition that 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)
Here, T is the aging temperature (K) and t is the aging time (h).

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

In the aging treatment step, the aging time t is preferably 0.016 hours or longer (approximately about 60 seconds or longer), assuming treatment in an operating line. In addition, the aging temperature T is preferably 830° C. or lower so as not to cause phase transformation in the matrix 25 (see the description of the first annealing step above). The aging temperature T may be 500° C. or higher and 830° C. or lower, or may be 500° C. or higher and 700° C. or lower.

Further, in the method for manufacturing the ferritic stainless steel material 1 in the present embodiment, ΔHV calculated by the following formula (2) is larger 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 ferritic stainless steel material 1 contains fine Cu-rich phases 21 in a matrix 25 and has a Cu-enriched surface layer portion 10 . This makes it possible to produce a ΔHV greater than 35HV. As a result, surface hardness can be increased. Therefore, the scratch resistance is improved. In addition, the ferritic stainless steel material 1 preferably has a ΔHV larger than 60HV, more preferably larger than 200HV.

(Final pickling process)
The method for manufacturing the ferritic stainless steel material 1 in the present embodiment includes a final pickling step of performing a final pickling treatment on the intermediate product (second intermediate material) obtained by the aging treatment step. In the final pickling step, a pickling treatment (mixed acid treatment) is performed using a mixed acid. The pickling solution (mixed acid) used in the final pickling step contains (i) nitric acid of 50 g/L or more and 150 g/L or less and (ii) hydrofluoric acid of 5 g/L or more and 15 g/L or less. The liquid temperature of the pickling liquid is 30° C. or higher and 60° C. or lower.

In the mixed acid treatment, Cu ions in the dissolved oxide scale or in the base material preferentially precipitate (attach) to the surface compared to other ions. Therefore, concentration of Cu is recognized in the surface layer. Such an effect of the mixed acid treatment makes it possible to reduce the surface contact resistance value. On the other hand, in nitric acid electrolysis, since the surface is always dissolved by electrolytic treatment, deposition (adhesion) of dissolved ions is not observed.

By the final pickling process, a Cu adhesion layer 13 is formed on the surface of the treated material in the same manner as the first pickling process and the second pickling process. As a result, the ferritic stainless steel material 1 having the Cu-enriched surface layer portion 10 can be obtained.

(advantageous effect)
As described above, the ferritic stainless steel material 1 according to the present embodiment includes the fine Cu-rich phases 21 and the Cu-rich phases 22 in the base material 20, so that the electric resistance of the base material 20 is reduced. The surface contact resistance is reduced by providing the Cu-enriched surface layer portion 10 . Furthermore, as mentioned above, the hardness of the surface can be increased. That is, the ferritic stainless steel material 1 has both reduced electrical resistance and surface contact resistance, and improved scratch resistance.

The ferritic stainless steel material 1 has a Cu adhesion layer 13 formed as follows. That is, as described above, the intermediate Cu adhesion layer is formed on the surface of the first intermediate material by performing the mixed acid treatment in the intermediate step. Then, in the subsequent aging treatment process, the Cu of 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 a final pickling step. Thereby, in the final pickling process, more Cu can be adhered to the surface of the matrix surface layer 25A to form the Cu adhered layer 13 .

Further, since the ferritic stainless steel material 1 is steel with ferritic components, the Ni content is low, for example, the Ni content is 1.0% by mass or less. And no special coating is required on the surface. Therefore, it is possible to prevent the manufacturing cost from increasing.

Therefore, when the ferritic stainless steel material 1 is applied to, for example, an electrical contact component, both the electrical resistance and the surface contact resistance are reduced, and thus the electrical conductivity is excellent. Moreover, since the scratch resistance is improved, the possibility of deterioration of conductivity can be reduced, and as a result, the electrical contact part can be used stably. Therefore, the ferritic stainless steel material 1 can be suitably used as an electrical contact component.

[Additional notes]
The present invention is not limited to the above-described embodiments, and can be modified in various ways within the scope of the claims. It is included in the technical scope of the invention.

Examples of the ferritic stainless steel material according to one aspect of the present invention will be described below, but the present invention is not limited to these examples.

In this example, ferritic stainless steels of the steel grades shown in Table 1 were melted in a vacuum melting furnace and hot-rolled (1200° C., 2 hours) into hot-rolled sheets having a thickness of 3 mm. Batch annealing was then performed by heating in a heating furnace at 700 to 830° C. for 6 hours. Then, it was pickled and cold-rolled into a cold-rolled sheet having a thickness of 1.0 mm. Each cold-rolled sheet was annealed by soaking at 800° C. for 1 minute and air-cooled, and then pickled. After that, aging treatment was performed at various temperatures of 350° C. to 900° C., and pickling treatment using a mixed acid (see the above embodiment for the component composition) or pickling treatment by nitric acid electrolysis was performed. Tables 2 and 3 show the aging treatment conditions.

Although numerical values are omitted in Table 1, the Ni content of each of the steel types C1 to C10 was 1.0% by mass or less. In the "classification" of Table 1, a steel type containing 1.0% by mass or more and 15.0% by mass or less of Cu is referred to as "steel subject to the present invention", and a steel type with a Cu content of less than 1.0% by mass is referred to as " It is called "comparative steel".

The steel sheets after batch annealing and after aging treatment were observed with a transmission electron microscope to investigate the grain size of the Cu-rich phase dispersed in the ferrite phase matrix. Regarding the steel sheet after aging treatment, in order to distinguish the Cu-rich phase that precipitates after batch annealing and after aging treatment, a steel sheet that has not undergone batch annealing is separately manufactured, and the steel sheet is subjected to aging treatment to determine the Cu-rich phase during aging treatment. Particle size was observed.

Moreover, the surface contact resistance value, the electrical resistivity, and the Cu concentration in the surface layer of the steel sheets after the aging treatment were tested. The surface contact resistance value was measured with an electrical contact simulator using a test piece of width 50 mm×length 50 mm. The surface contact resistance value was measured using an "electrical contact simulator" manufactured by Yamazaki Seiki Laboratory Co., Ltd. under measurement conditions of a contact load of 100 gf. The electrical resistivity was measured by the 4-probe method (JIS C 2525) using a test piece of width 3 mm×length 100 mm. Using a test piece having a width of 50 mm and a length of 50 mm, the distribution state of Cu in the depth direction from the surface in the surface layer was measured by GDS. The obtained surface layer strength I, which is the peak value of the Cu concentration in the Cu-enriched surface layer 10, is defined as the strength I0 of the Cu concentration of the base material 20 (average value at a depth of 40 nm to 50 nm from the surface of the ferritic stainless steel material 1). The numerical value obtained by dividing by was taken as the surface layer Cu intensity ratio.

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

Tables 2 and 3 show the results. In Table 3, steel sheets having the following properties (A) and (B) are listed as "invention examples". Table 2 lists steel sheets that do not have either property (A) or (B) as "comparative examples."
Characteristic (A): The surface contact resistance value is 45 mΩ or less.
Characteristic (B): Electrical resistivity is 60 μΩ·cm or less.

As shown in Table 2, Comparative Example No. 1 in which nitric acid electrolysis was performed in the finishing pickling treatment. In 2, 4, 7 and 10, the surface contact resistance value was high regardless of the presence or absence of aging treatment. Comparative example no. In No. 4, the fine Cu-rich phase 21 was formed, but the surface contact resistance value was high. The reason for this is thought to be that the Cu-enriched surface layer portion 10, particularly the Cu adhesion layer 13, is not sufficiently formed. Comparative example no. In No. 4, the surface layer Cu intensity ratio is 1.1, which is smaller than the standard 1.5.

Also, Comparative Example No. As can be seen from the result of No. 1, when using steel type C1 with a low Cu concentration, the surface contact resistance value and electrical resistivity could not be reduced even by using the manufacturing method of the above-described embodiment.

Comparative example no. In Nos. 3, 5, 6 and 8, the aging treatment conditions were outside the scope of the present invention, and the electrical resistivity of the base material was high. Specifically, Comparative Example No. In No. 5, since the aging treatment was performed at a high temperature and for a long time, the fine Cu-rich phase dissolved and Cu dissolved in the base material. Also, Comparative Example No. In 3 and 6, the temperature in the aging treatment was low, and precipitation of fine Cu-rich phases hardly occurred. Therefore, Comparative Example No. In Nos. 3, 5 and 6, the electrical resistivity of the base material was high. Also, Comparative Example No. In No. 8, aging treatment was not performed, and precipitation of fine Cu-rich phases hardly occurred. As a result, Comparative Example No. In 3, 5, 6, and 8, the electrical resistivity of the base material was higher than 60 μΩ·cm, and ΔHV, which indicates surface hardness, was 35 or less.

Comparative example no. In No. 9, the grain size of the precipitated Cu-rich phase is small due to the low batch annealing temperature. Therefore, it is considered that the amount of the fine Cu-rich phase precipitated during the aging treatment was relatively small. As a result, the electrical resistivity of the base material was higher than 60 μΩ·cm, and ΔHV, which indicates surface hardness, was 35 or less.

On the other hand, as shown in Table 3, Example Nos. manufactured under the conditions within the scope of the present invention. The steel sheets of Nos. 11 to 29 had both reduced surface contact resistance and electrical resistivity, and had large ΔHV and excellent scratch resistance.

1 ferritic stainless steel material 10 Cu-enriched surface layer 20 base material 21 fine Cu-rich phase 22 Cu-rich phase 25 matrix

Claims (9)

In % by mass, Cu: 1.0% or more and 15.0% or less, Cr: 9.0% or more and 20.0% or less, C+N: 0.10% or less, Mn: 2.0% or less, Si: 2.0% or less. Contains 0% or less ,
Furthermore, if necessary, it contains one or two of Ti: 0.5% or less and Nb: 0.5% or less,
A ferritic stainless steel material having a chemical composition with the balance being Fe and inevitable impurities ,
a base material in which a first Cu-rich phase having a particle size of less than 500 nm is formed in the matrix;
a Cu-enriched surface layer portion formed on the surface of the ferritic stainless steel material in which Cu is more concentrated than the base material ,
The base material is a ferritic stainless steel material in which a second Cu-rich phase having a grain size of 500 nm or more is formed in the matrix. 2. The ferritic stainless steel according to claim 1, wherein the ratio of the peak value of Cu intensity in said Cu-enriched surface layer portion to the value of Cu intensity in said base material, which is detected by compositional analysis of Cu, is 1.5 or more. steel. % by mass, Mo: 3.0% or less, Ni: 3.0% or less, Al: 5.0% or less, V: 2.0% or less, W: 2.0% or less, Zr: 1.0% 3. The ferritic stainless steel material according to claim 1 or 2, having a chemical composition further containing one or more selected from the group consisting of REM: 0.1% or less. The ferritic stainless steel material according to any one of claims 1 to 3, wherein the first Cu-rich phase has a grain size of 5 nm or more and 20 nm or less. The ferritic stainless steel material according to any one of claims 1 to 4 , which has an electrical resistivity of 60 µΩ·cm or less. The ferritic stainless steel material according to any one of claims 1 to 5 , having a surface contact resistance value of 45 mΩ or less. A method for producing a ferritic stainless steel material according to any one of claims 1 to 6,
An annealing step of heating the rolled material of ferritic stainless steel having the chemical composition at a temperature of 780° C. or more and 830° C. or less for 6 hours or more;
After the annealing step, the first intermediate material obtained by performing the intermediate step including at least the first pickling step has an A value defined by the following formula (1) of 15.0 or more and 20.0. an aging treatment step of forming the first Cu-rich phase in the matrix by performing an aging treatment under conditions of 0 or less;
(i) nitric acid of 50 g/L or more and 150 g/L or less and (ii) hydrofluoric acid of 5 g/L or more and 15 g/L or less for the second intermediate material obtained by the aging treatment step a final pickling step of subjecting the mixture to a pickling treatment at a liquid temperature of 30° C. or higher and 60° C. or lower using a mixed solution;
A=T(20+logt)×10−3 (1)
(here,
T: aging temperature (K)
t: aging time (h)
is). In the first pickling step, (i) nitric acid of 50 g/L or more and 150 g/L or less and (ii) 5 g/L or more and 15 g/L or less of the first annealed material obtained in the annealing step 8. The method for producing a ferritic stainless steel material according to claim 7 , wherein a mixed liquid containing hydrofluoric acid is used, and the liquid temperature of the mixed liquid is set to 30° C. or higher and 60° C. or lower, and the pickling treatment is performed. The method for producing a ferritic 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)
(here,
HV1: hardness of the first intermediate material HV2: hardness of the second intermediate material).

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