JP2024080918A - Iron-chromium alloy and method for producing the same - Google Patents

Abstract

The present invention provides an iron-chromium alloy that is resistant to an increase in contact resistance caused by fretting wear. [Solution] An iron-chromium alloy containing C: 0.01-0.5%, Si: 0.01-2.0%, Mn: 0.01-2.0%, P: 0.045% or less, S: 0.03% or less, Ni: 0.01-5.0%, Cr: 4.0-25.0%, Cu: 0.5-10%, N: 0.10% or less, Nb: 0.6% or less, Ti: 0.6% or less, and Al: 0.001-3.5%, with the balance being Fe and unavoidable impurities, having a martensite single phase structure or a multi-phase structure of 50% or more by volume of martensite phase and ferrite phase, having a Ra×RSm value of 550 or less on the surface, εCu phase being dispersed and precipitated in the phase, and a total area ratio of εCu phase in a cross section being 1.0-10.0%. [Selected Figure] None

Description

The present invention relates to an iron-chromium alloy and a method for producing the iron-chromium alloy.

In transportation equipment such as automobiles, the use of wire harnesses consisting of electric wires and connection terminals is increasing due to an increase in various electrical equipment such as safety equipment. Since a certain contact pressure is required at the connection part, it is preferable that the connection terminals are made of a material with high strength.

Copper alloys are generally used as materials for connection terminals. In recent years, the use of iron-chromium alloys such as stainless steel has been proposed, which have the same strength as copper alloys but are less expensive and allow for magnetic sorting, which is difficult with copper alloys. However, while iron-chromium alloys have excellent corrosion resistance because they form a passive film on their surface, they generally tend to have high contact resistance.

To address these problems, for example, Patent Document 1 discloses an automobile terminal having a Cu-plated layer and a Sn-plated layer on the surface of a stainless steel sheet. Patent Document 2 discloses a terminal equipped with a connection part having a layer with a finely uneven surface. Patent Document 3 describes a stainless steel sheet in which Cu-rich phase precipitate particles with a particle size of 300 nm or less are dispersed in the phase that constitutes the matrix to improve electrical conductivity.

JP 2017-183144 A JP 2015-220145 A JP 2005-154792 A

Transportation equipment such as automobiles is prone to micro-friction wear at the connection of the connection terminal due to vibration. The plating layer described in Patent Document 1 may be worn away by micro-friction wear. The terminal disclosed in Patent Document 2 is difficult to control the unevenness of the connection surface during manufacturing, leaving room for improvement in manufacturing costs. When a Cu-rich phase is precipitated as disclosed in Patent Document 3, micro-friction wear may generate fine powder containing the Cu-rich phase. The fine powder oxidizes, causing an increase in contact resistance.

One aspect of the present invention aims to realize an iron-chromium alloy that is less susceptible to an increase in contact resistance due to fretting wear.

In order to solve the above problems, the iron-chromium alloy according to one embodiment of the present invention contains, by mass%, C: 0.01 to 0.5%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.0%, P: 0.045% or less, S: 0.03% or less, Ni: 0.01 to 5.0%, Cr: 4.0 to 25.0%, Cu: 0.5 to 10%, N: 0.10% or less, Nb: 0.6% or less, Ti: 0.6% or less, and Al: 0.001 to 3.5%, with the balance being Fe and and unavoidable impurities, the matrix is a martensite single phase structure or a multi-phase structure of 50 volume % or more of martensite phase and ferrite phase, the value of Ra (arithmetic mean roughness) x RSm (average length of roughness curve element) on the surface is 550 or less, εCu phase is dispersed and precipitated in the phases that make up the matrix, and the total area ratio of the εCu phase in the cross section perpendicular to the rolling direction is 1.0 to 10.0%.

The iron-chromium alloy according to one embodiment of the present invention may further contain, by mass%, at least one selected from the group consisting of Mo: 0.01-2.0%, V: 0.6% or less, B: 0.01% or less, Ca: 0.0002-0.015%, Hf: 0.001-0.60%, Zr: 0.01-0.60%, Sb: 0.005-0.60%, Co: 0.6% or less, W: 0.6% or less, Ta: 0.001-1.0%, Sn: 0.002-1.0%, Ga: 0.0002-0.50%, Mg: 0.0003-0.0050%, and REM (rare earth elements): 0.001-0.20%.

In order to solve the above problems, an iron-chromium alloy according to one embodiment of the present invention contains, by mass%, C: 0.01 to 0.5%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.0%, P: 0.045% or less, S: 0.03% or less, Ni: 0.01 to 5.0%, Cr: 4.0 to 25.0%, Cu: 0.5 to 10%, N: 0.10% or less, Nb: 0.6% or less, Ti: 0.6% or less, and Al: 0.001 to 3.5%, with the balance being Fe and unavoidable impurities, and a matrix having a martensite single phase structure. or a multi-phase structure of 50 volume % or more of martensite phase and ferrite phase, the value of Ra (arithmetic mean roughness) × RSm (average length of roughness curve element) on the surface is 550 or less, εCu phase is dispersed and precipitated in the phase constituting the matrix, and the total area ratio of the εCu phase in a cross section perpendicular to the rolling direction is 1.0 to 10.0%, the method for producing an iron-chromium alloy including a finish annealing step of performing a heat treatment under conditions in which the P value represented by the following formula (1) is 11,000 to 22,000;
P value = T (20 + log t) ... (1)
In the formula (1), T represents the heat treatment temperature (K) expressed in absolute temperature, and t represents the heat treatment time (hr).

According to one aspect of the present invention, it is possible to realize an iron-chromium alloy that is less susceptible to an increase in contact resistance due to fretting wear.

One embodiment of the present invention will be described below. Note that the present invention is not limited to the following embodiments or examples, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in the different embodiments and examples are also included in the technical scope of the present invention. In addition, in this specification, "A to B" indicates greater than or equal to A and less than or equal to B.

The iron-chromium alloy according to one embodiment of the present invention (hereinafter, sometimes simply referred to as "iron-chromium alloy") is intended to be an alloy steel that forms a passive film on the surface derived from the Cr in the alloy. The iron-chromium alloy is, for example, a stainless steel with a Cr content of 10.5 mass% or more, but is not limited thereto, and may be an alloy steel with a Cr content of less than 10.5 mass%. In addition, the shape of the iron-chromium alloy is not particularly limited, and may be, for example, a steel plate, a steel strip, a steel pipe, or a bar.

[ε Cu phase]
In the iron-chromium alloy, an εCu phase is dispersed and precipitated in the phase constituting the matrix of the iron-chromium alloy (hereinafter, sometimes simply referred to as "in the matrix"). The εCu phase is also called a Cu-rich phase, and is a particle in which Cu is precipitated in the matrix by aging treatment in an annealing process or the like. Since the εCu phase is mainly composed of Cu, which has excellent electrical conductivity, the dispersion and precipitation of the εCu phase in the matrix can improve the electrical conductivity of the iron-chromium alloy.

In addition, if the εCu phase is dispersed and precipitated in the matrix, the εCu phase will also be present on or near the surface of the iron-chromium alloy. The εCu phase has a higher electrical conductivity than the passive film formed on the surface of the iron-chromium alloy. It is also known that a passive film is less likely to form at positions on the surface of the iron-chromium alloy where the εCu phase is precipitated, and the εCu phase is more likely to be exposed. Therefore, if the εCu phase is precipitated on the surface of the iron-chromium alloy, the contact resistance of the iron-chromium alloy can be reduced.

The iron-chromium alloy has a total area ratio of εCu phase in a cross section perpendicular to the rolling direction of 1.0% to 10.0%. The total area ratio refers to the sum of the area ratios of each precipitate in the cross section of the iron-chromium alloy. The cross section perpendicular to the rolling direction may be a cross section perpendicular to and parallel to the rolling direction (L cross section), a cross section perpendicular to and perpendicular to the rolling direction (T cross section), or another cross section perpendicular to the rolling direction.

The particle size of the εCu phase precipitated in the iron-chromium alloy matrix is not particularly limited. For example, the total area ratio may be calculated for εCu phase having a particle size of 1 nm or more and 1000 nm or less. The particle size of the εCu phase may be expressed by the maximum particle size.

The average particle size of the εCu phase precipitated in the iron-chromium alloy matrix may be 1 nm or more, or may be 5 nm or more. The average particle size of the εCu phase may be 1000 nm or less, or may be 500 nm or less. The particle size of the εCu phase can be measured, for example, by identifying the particles of the εCu phase using an EDX device in the field of view observed by a transmission electron microscope. The average particle size of the εCu phase can be calculated as the average particle size of the particles contained in a specified range in the field of view of the transmission electron microscope.

On the surface of such iron-chromium alloys containing εCu phase, a certain amount of εCu phase with good electrical conductivity is present. The εCu phase has a higher electrical conductivity than the passive film formed on the surface of the iron-chromium alloy, so it functions as a path for electricity to flow between the surface of the iron-chromium alloy and a conductor in contact with the surface.

Furthermore, the above-mentioned iron-chromium alloy can be suitably used for connection terminals equipped in transportation equipment such as automobiles, which are prone to micro-friction wear. A certain amount of εCu phase is dispersed and precipitated in the matrix of the iron-chromium alloy, just like on the surface. Therefore, even if micro-friction wear occurs on the surface of the iron-chromium alloy, the εCu phase in the matrix is sequentially exposed at the site where micro-friction wear has occurred. Therefore, if the connection terminal is made of an iron-chromium alloy, the εCu phase can function as a path for electricity even in the connection part where micro-friction wear has occurred, so that the contact resistance is unlikely to increase.

When the total area ratio of the εCu phase is less than 1.0%, the εCu phase present on and near the surface of the iron-chromium alloy is insufficient. Therefore, the εCu phase, which has good electrical conductivity, does not function sufficiently to improve the contact resistance of the iron-chromium alloy. On the other hand, when the total area ratio of the εCu phase exceeds 10.0%, it becomes necessary to add a large amount of Cu. Excessive addition of Cu not only causes a decrease in the hot workability of the iron-chromium alloy due to the formation of CuMn phase, but also can cause the surface of the iron-chromium alloy to become more uneven when an acidic process is performed, depending on the type of acid. In addition, adding a large amount of Cu also leads to an increase in the manufacturing cost of the iron-chromium alloy.

Furthermore, iron-chromium alloys are not limited to micro-sliding wear, and are less likely to cause an increase in contact resistance at the connection due to wear, even in connection terminals that are frequently inserted and removed. Therefore, iron-chromium alloys can be suitably used for a variety of connection terminals.

With such iron-chromium alloys, for example, there is no need to carry out plating processes to reduce contact resistance, making it possible to reduce raw materials and energy consumption in the manufacturing process. In addition, scrap can be separated by magnetic separation, which is not possible with copper alloys, making metal recycling easier. This will contribute to the achievement of Goal 12 of the Sustainable Development Goals (SDGs), "Responsible Consumption and Production."

〔Surface roughness〕
The iron-chromium alloy has a surface having a value of Ra (arithmetic mean roughness) x RSm (average length of roughness curve element) of 550 or less. Ra (arithmetic mean roughness) and RSm (average length of roughness curve element) comply with the definitions in JIS B0601:2013.

When the surface of the iron-chromium alloy becomes excessively rough and the surface irregularities increase, the number of contact points between the iron-chromium alloy and the conductor becomes extremely small, and the load due to the contact pressure becomes concentrated at the few contact points. This makes it easier for wear powder (powder produced by scraping the surface) to be generated when micro-sliding wear occurs in the iron-chromium alloy. Oxidized wear powder causes the contact resistance of the iron-chromium alloy to increase.

Here, the inventors focused on not only Ra (arithmetic mean roughness) but also RSm (average length of roughness curve element). Ra (arithmetic mean roughness) indicates the magnitude of the surface irregularities in the direction perpendicular to the surface, and RSm (average length of roughness curve element) indicates the magnitude in the parallel direction. It can be said that wear debris is likely to be generated regardless of the direction in which the surface irregularities are large, so by adjusting both of these parameters, the amount of wear debris generated can be effectively reduced. Thus, the inventors found that it is preferable to evaluate the surface roughness of iron-chromium alloys using both Ra (arithmetic mean roughness) and RSm (average length of roughness curve element) from the perspective of wear debris generation.

If the surface of the iron-chromium alloy is within the range of Ra × RSm ≦ 550, excessive generation of wear particles can be prevented. Therefore, even in an iron-chromium alloy in which the εCu phase is precipitated in the matrix, an increase in contact resistance due to wear particles can be prevented.

[Component Composition]
The iron-chromium alloy contains, by mass%, C: 0.01-0.5%, Si: 0.01-2.0%, Mn: 0.01-2.0%, P: 0.045% or less, S: 0.03% or less, Ni: 0.01-5.0%, Cr: 4.0-25.0%, Cu: 0.5-10%, N: 0.10% or less, Nb: 0.6% or less, Ti: 0.6% or less, and Al: 0.001-3.5%, with the balance being Fe and unavoidable impurities. Each element contained in the iron-chromium alloy will be described below.

(C)
C (carbon) has a high solid solution strengthening effect and is also effective in increasing the strength of iron-chromium alloys. On the other hand, excessive addition of C leads to a decrease in the workability and corrosion resistance of the iron-chromium alloy. Therefore, the iron-chromium alloy contains 0.01 mass% or more and 0.5 mass% or less of C. The C content is preferably 0.01 mass% or more and 0.15 mass% or less.

(Si)
Silicon (Si) is an element that is effective as a deoxidizer and has a solid solution strengthening effect. On the other hand, excessive addition of Si causes a decrease in workability and toughness. Therefore, the iron-chromium alloy contains 0.01 mass% or more and 2.0 mass% or less of Si. The Si content is preferably 0.2 mass% or more and 1.5 mass% or less.

(Mn)
Mn (manganese) is an element effective in increasing the strength of iron-chromium alloys. On the other hand, excessive addition of Mn leads to a decrease in the hot workability of the iron-chromium alloy. Therefore, the iron-chromium alloy contains 0.01 mass% or more and 2.0 mass% or less of Mn. The content of Mn is preferably 0.2 mass% or more and 1.5 mass% or less.

(P)
P (phosphorus) is an element that is mixed in as an unavoidable impurity, and the lower the P content, the better. From the viewpoint of manufacturability, the iron-chromium alloy contains 0.045 mass% or less of P. If the P content is 0.045 mass% or less, the adverse effect on material properties such as ductility in the iron-chromium alloy can be reduced. The P content is preferably 0.005 mass% or more and 0.040 mass% or less.

(S)
S (sulfur) is an element that is mixed in as an inevitable impurity, and the smaller the S content, the better. From the viewpoint of manufacturability, the iron-chromium alloy contains 0.03 mass% or less of S. If the S content is 0.03 mass% or less, the adverse effect on material properties such as ductility in the iron-chromium alloy can be reduced. The S content is preferably 0.0001 mass% or more and 0.003 mass% or less.

(Ni)
Ni (nickel) is an element effective in improving the corrosion resistance and toughness of iron-chromium alloys. On the other hand, Ni is an expensive element, and excessive addition of Ni leads to an increase in manufacturing costs. Therefore, the iron-chromium alloy contains Ni in an amount of 0.01 mass% or more and 5.0 mass% or less. The Ni content is preferably 0.1 mass% or more and 3.0 mass% or less.

(Cr)
Cr (chromium) is an element effective for ensuring the corrosion resistance of the iron-chromium alloy. On the other hand, if Cr is added excessively, the workability and toughness of the iron-chromium alloy are reduced. Therefore, the iron-chromium alloy contains 4.0 mass% or more and 25.0 mass% or less of Cr. The Cr content is preferably 7.0 mass% or more and 18.0 mass% or less.

(Cu)
Cu (copper) is an element effective in increasing the strength and improving the electrical conductivity of the iron-chromium alloy. Cu is also an element effective in precipitating the εCu phase. On the other hand, if excessive Cu is added, a CuMn phase is generated in the center of the slab during the solidification process of the slab, and the hot workability of the slab decreases. Therefore, the iron-chromium alloy contains 0.5 mass% to 10 mass% of Cu. The Cu content is preferably 1.0 mass% to 7.0 mass%.

(N)
N (nitrogen) is an element that has a solid solution strengthening effect and a corrosion resistance improving effect. On the other hand, if N is added excessively, the workability of the iron-chromium alloy decreases. Therefore, the N content in the iron-chromium alloy is 0.10 mass% or less, and preferably 0.001 mass% or more and 0.08 mass% or less.

(Nb)
Nb (niobium) is an element effective in refining and homogenizing the structure. Nb is also effective in precipitating metal carbides, metal borides, and metal nitrides having electrical conductivity. On the other hand, Ni is an expensive element, and excessive addition of Ni leads to an increase in manufacturing costs. Therefore, the iron-chromium alloy contains 0.6 mass% or less of Nb. The content of Nb is preferably 0.01 mass% or more and 0.5 mass% or less.

(Ti)
Ti (titanium) is an element having a deoxidizing effect. Therefore, the iron-chromium alloy contains 0.6 mass% or less of Ti. The Ti content is preferably 0.01 mass% or more and 0.5 mass% or less.

(Al)
Al (aluminum) is an element that has a deoxidizing effect. On the other hand, excessive addition of Al may deteriorate the surface quality. Therefore, the iron-chromium alloy contains 0.001% by mass or more and 3.5% by mass or less of Al. The content of Al is preferably 0.001% by mass or more and 1.8% by mass or less.

(Other elements)
In addition to the above elements, the iron-chromium alloy may further contain, by mass%, at least one selected from the group consisting of Mo: 0.01 to 2.0%, V: 0.6% or less, B: 0.01% or less, Ca: 0.0002 to 0.015%, Hf: 0.001 to 0.60%, Zr: 0.01 to 0.60%, Sb: 0.005 to 0.60%, Co: 0.6% or less, W: 0.6% or less, Ta: 0.001 to 1.0%, Sn: 0.002 to 1.0%, Ga: 0.0002 to 0.50%, Mg: 0.0003 to 0.0050%, and REM (rare earth element): 0.001 to 0.20%.

(Mo)
Mo (molybdenum) is an element effective in improving the corrosion resistance of iron-chromium alloys. On the other hand, since Mo is an expensive element, excessive addition is not preferable. Therefore, the iron-chromium alloy may contain Mo in an amount of 0.01 mass% or more and 2.0 mass% or less. The Mo content is more preferably 0.1 mass% or more and 1.5 mass% or less.

(V, W)
Both V (vanadium) and W (tungsten) are elements effective in improving the corrosion resistance of iron-chromium alloys. On the other hand, since V and W are expensive elements, excessive addition is not preferable. Therefore, the iron-chromium alloy may contain at least one of 0.6 mass% or less of V and 0.6 mass% or less of W. The content of V is preferably 0.05 mass% or more and 0.5 mass% or less. The content of W is preferably 0.05 mass% or more and 0.5 mass% or less.

(B)
B (boron) is an element that improves the hot workability of the iron-chromium alloy and is an element that is effective in reducing the occurrence of edge breaks and splitting during hot rolling. Therefore, the iron-chromium alloy may contain 0.01 mass% or less of B. The content of B is preferably 0.0005 mass% or more and 0.005 mass% or less.

(Ca)
Ca (calcium) is effective in preventing edge breakage during hot rolling. On the other hand, excessive addition of Ca leads to a decrease in corrosion resistance. Therefore, the iron-chromium alloy may contain 0.0002 mass% or more and 0.015 mass% or less of Ca. The Ca content is preferably 0.0002 mass% or more and 0.005 mass% or less.

(Hf)
Hf (hafnium) is an element that improves corrosion resistance and high-temperature strength. On the other hand, excessive addition of Hf may cause deterioration of workability and manufacturability. Therefore, the iron-chromium alloy may contain 0.001 mass% or more and 0.60 mass% or less of Hf. The content of Hf is preferably 0.005 mass% or more and 0.50 mass% or less.

(Zr)
Zr (zirconium) is an element that improves the hot workability of the iron-chromium alloy and is also effective in improving oxidation resistance. Therefore, the iron-chromium alloy may contain 0.01 mass% or more and 0.60 mass% or less of Zr. The content of Zr is preferably 0.05 mass% or more and 0.50 mass% or less.

(Sb)
Sb (antimony) is an element that improves high-temperature strength. On the other hand, excessive addition of Sb reduces weldability and toughness. Therefore, the iron-chromium alloy may contain 0.005% by mass or more and 0.60% by mass or less of Sb. The content of Sb is preferably 0.01% by mass or more and 0.40% by mass or less.

(Co)
Co (cobalt) is an element that improves high-temperature strength. On the other hand, the addition of excessive Co reduces toughness, leading to reduced manufacturability. Therefore, the iron-chromium alloy may contain 0.6 mass% or less of Co. The content of Co is preferably 0.05 mass% or more and 0.5 mass% or less.

(Ta)
Ta (tantalum) is an element that improves high-temperature strength. On the other hand, excessive addition of Ta reduces weldability and toughness. Therefore, the iron-chromium alloy may contain 0.001 mass% or more and 1.0 mass% or less of Ta. The Ta content is preferably 0.005 mass% or more and 0.5 mass% or less.

(Sn)
Sn (tin) is an element that improves corrosion resistance and high-temperature strength. On the other hand, excessive addition of Sn may cause a decrease in toughness and manufacturability. Therefore, the iron-chromium alloy may contain 0.002 mass% or more and 1.0 mass% or less of Sn. The content of Sn is preferably 0.002 mass% or more and 0.5 mass% or less.

(Ga)
Ga (gallium) is an element that improves corrosion resistance and hydrogen embrittlement resistance. On the other hand, excessive addition of Ga reduces weldability and toughness. Therefore, the iron-chromium alloy may contain 0.0002 mass% or more and 0.50 mass% or less of Ga. The Ga content is preferably 0.0002 mass% or more and 0.30 mass% or less.

(Mg)
Magnesium (Mg) is a deoxidizing element, and also refines the structure of the slab and improves formability. On the other hand, excessive addition of Mg leads to deterioration of corrosion resistance, weldability and surface quality. Therefore, the iron-chromium alloy may contain 0.0003% by mass or more and 0.0050% by mass or less of Mg. The content of Mg is preferably 0.0003% by mass or more and 0.0030% by mass or less.

(R.E.M.)
REM (rare earth element) refers to a collective term for Sc (scandium) and 15 elements (lanthanoids) from La (lanthanum) to Lu (lutetium). The iron-chromium alloy may contain REM as a single element or as a mixture of multiple elements. REM improves the cleanliness of the iron-chromium alloy and effectively prevents edge breakage during hot rolling. On the other hand, the addition of excessive REM increases the alloy cost and reduces manufacturability. Therefore, the iron-chromium alloy may contain 0.001% by mass or more and 0.20% by mass or less of REM. The content of REM is preferably 0.005% by mass or more and 0.10% by mass or less.

The iron-chromium alloy may contain at least one of the following REMs: 0.1% by mass or less of La and 0.05% by mass or less of Ce (cerium).

[Organization]
The matrix of the iron-chromium alloy is a martensite single phase structure or a multi-phase structure of 50 volume % or more of martensite phase and ferrite phase. The matrix of the iron-chromium alloy can have a desired structure by adjusting the component composition. The εCu phase is a phase that precipitates in the form of particles in the phase that constitutes the matrix of the iron-chromium alloy, and is different from the phase that constitutes the matrix. Therefore, the martensite single phase structure may contain a precipitated phase such as the εCu phase in the matrix. The matrix of the iron-chromium alloy may also contain an unavoidably formed layer other than the martensite phase or the ferrite phase.

Specifically, if the iron-chromium alloy has an M value shown in the following formula (2) of 100 or more, it will have a martensite single-phase structure, if it is 0 or more and less than 100, it will have a ferrite-martensite multi-phase structure, and if it is less than 0, it will have a ferrite single-phase structure. Also, if the M value shown in the following formula (2) is 50 or more and less than 100, it will have a multi-phase structure with 50 volume percent or more of martensite and ferrite phases.

M value = 420C - 11.5Si + 7Mn + 23Ni - 11.5Cr - 12Mo - 10V + 9Cu - 49Ti - 52Al + 470N + 189 ... (2)
In the element symbols of the formula (2), the content (mass%) of each element contained in the iron-chromium alloy is substituted, and 0 is substituted for elements that are not added.

If the matrix of the iron-chromium alloy has such a structure, the generation of wear particles due to fretting wear can be effectively reduced. In addition, it is believed that the εCu phase is more likely to precipitate finely in the martensite phase than in the ferrite phase. This is because the martensite phase contains more strain fields that make it easier for the εCu phase to precipitate than the ferrite phase. In order to precipitate the fine εCu phase, it is preferable that the matrix of the iron-chromium alloy has at least 50% by volume of the martensite phase.

〔Production method〕
The method for producing an iron-chromium alloy according to one embodiment of the present invention may include general production processes for iron-chromium alloys such as stainless steel, except for the final annealing process.

Iron-chromium alloys can be manufactured by performing heat treatment in the final annealing process under conditions that result in a P value of 11,000 or more and 22,000 or less, as shown in the following formula (1):

P value = T (20 + log t) ... (1)
In the formula (1), T represents the heat treatment temperature (K) expressed in absolute temperature, and t represents the heat treatment time (hr).

When the P value is less than 11,000, the εCu phase does not precipitate in the iron-chromium alloy matrix. When the P value is more than 22,000, Cu tends to remain finely dissolved in the iron-chromium alloy matrix even during heat treatment, making it difficult for the εCu phase to precipitate.

The final annealing process, in which heat treatment is performed at such a heat treatment temperature and for such a heat treatment time, allows the εCu phase to be dispersed and precipitated in the matrix of the iron-chromium alloy. The final annealing process may be performed by batch annealing or continuous annealing.

The following is an example of a method for producing an iron-chromium alloy, but it is not limited to this.

In a method for producing an iron-chromium alloy, for example, a slab is produced by continuously casting molten steel with adjusted composition. The slab produced by continuous casting is then heated to 1100°C to 1300°C and then hot-rolled to produce a hot-rolled steel strip. The hot-rolled hot-rolled steel strip may be pickled. A first intermediate annealing step may be performed in which annealing is performed before pickling the hot-rolled steel strip, or pickling may be performed without annealing.

Then, the cold rolling process is carried out in which the hot-rolled steel strip after pickling is cold-rolled until it has a predetermined thickness to obtain a cold-rolled steel strip. In the cold rolling process, intermediate rolling may be carried out as necessary, and a second intermediate annealing process in which annealing is carried out may also be carried out.

The cold-rolled steel strip after the cold rolling process is subjected to the above-mentioned finish annealing process. In addition, to further increase the strength of the steel strip after the finish annealing process, temper rolling may be performed as necessary. This temper rolling may also be performed for the purpose of surface finishing as described below.

The iron-chromium alloy may be surface-finished to the extent that the Ra×RSm value on the surface is 550 or less. Examples of surface-finishing methods that can be used on iron-chromium alloys include bright annealing (BA) finishing, pickling finishing, light rolling finishing after pickling, temper rolling finishing, hairline (HL) finishing, and dull finishing. HL finishing is a method of imparting a polished texture to the surface of the iron-chromium alloy. Dull finishing is a method of using a coarse-grained roll during temper rolling to transfer the surface condition of the roll to the surface of the iron-chromium alloy.

For iron-chromium alloys, if the Ra x RSm value on the surface after finish annealing is 550 or less, such surface finishing does not need to be performed.

The results of evaluation of the iron-chromium alloy according to an embodiment of the present invention and a comparative example are described below.

[Evaluation conditions]
<Component Composition>
The component compositions (mass%) and M values of the iron-chromium alloys according to the examples of the present invention (Invention Examples 1 to 16) and the iron-chromium alloys according to the comparative examples (Comparative Examples 1 to 6) are shown in Table 1 below. The M values were calculated using the above formula (2). As described above, an iron-chromium alloy with an M value of 100 or more has a martensite single-phase structure, and an iron-chromium alloy with an M value of 0 or more and less than 100 has a ferrite-martensite dual-phase structure. In addition, all iron-chromium alloys with a ferrite-martensite dual-phase structure contain 50% or more by volume of martensite phase.

In Table 1, underlined values indicate values outside the range specified by this invention.

<Production Method>
The iron-chromium alloys according to the invention examples and comparative examples were produced by the following method. The iron-chromium alloys having the composition shown in Table 1 were melted and subjected to the hot rolling process to the finish annealing process to obtain rolled iron-chromium alloy materials. The heat treatment in the finish annealing process was performed according to the conditions of the heat treatment temperature T and the heat treatment time t shown in Table 1.

The iron-chromium alloy according to Comparative Example 3 was manufactured without performing a final annealing process. The iron-chromium alloy according to Comparative Example 5 was manufactured under the same conditions as those of Invention Example 7, except that in the final annealing process, heat treatment was performed so that the P value was less than 11,000. The iron-chromium alloy according to Comparative Example 6 was manufactured under the same conditions as those of Invention Example 1, except that in the final annealing process, heat treatment was performed so that the P value was more than 22,000.

The iron-chromium alloy according to Comparative Example 1 was produced by applying a dull finish so that the value of Ra x RSm exceeded 550. The iron-chromium alloys according to Comparative Examples 2 and 4 were produced with composition outside the range specified by the present invention.

<Evaluation method>
The methods for evaluating the surface Ra, RSm, Ra×RSm, total area ratio of εCu phase in the cross section, and contact resistance of the iron-chromium alloys according to the invention examples and comparative examples are shown below. The results obtained by these evaluation methods are shown in Table 1.

(Total area ratio of ε Cu phase)
A cross section (L cross section) of the iron-chromium alloy perpendicular to and parallel to the rolling direction was mirror-polished, and the cross section was observed by a transmission electron microscope to obtain image data of the field of view. The precipitated particles of the εCu phase were identified by an EDX device attached to the electron microscope. The area of each precipitated particle of the εCu phase identified in the field of view was calculated, and the area of the entire field of view was calculated. The ratio of the total area of all precipitated particles of the εCu phase to the area of the entire field of view was calculated as the total area ratio (%). The calculation of each area was performed using the image processing software "ImageJ".

(Surface roughness)
The Ra (arithmetic surface roughness) and RSm (average length of roughness curve element) on the surface of the iron-chromium alloy were measured using a stylus surface roughness measuring instrument (SURFCOM2900DX manufactured by Tokyo Seimitsu Co., Ltd.) in accordance with JIS B0601: 2013. The obtained Ra value (μm) and RSm value (μm) were multiplied to obtain the value of Ra × RSm (μm).

(Contact resistance)
The contact resistance of the iron-chromium alloy was evaluated by a fretting wear test. For the fretting wear test, a plate 40 mm long and 40 mm wide was prepared using a rolled material with a thickness of 0.3 mm, and was bent 90 degrees at r1.5 mm to prepare a test piece. The two test pieces were brought into contact with each other at the bent apexes, and slid at a frequency of 1 Hz with a contact pressure of 5 N, a sliding distance of 100 μm (50 μm one way) during reciprocation, and one reciprocating sliding as one cycle. The sliding was performed by reciprocating only one of the test pieces.

A constant current was passed between the test pieces, and the change in voltage between the test pieces was measured using the four-terminal method to determine the contact resistance (mΩ) between the test pieces. If the number of sliding cycles at which the contact resistance value reached a value that was at least twice the minimum value at or after the start of measurement was 50 or more, the test piece was deemed to have passed. In Table 1 above, cases in which the fretting wear test was judged to have passed were indicated with an "O" and cases in which the fretting wear test was judged to have failed were indicated with an "X".

〔result〕
As shown in Table 1, in all of the iron-chromium alloys according to the examples of the present invention, the εCu phase was dispersed and precipitated in the matrix at a predetermined total area ratio, and the value of Ra×RSm was also within the range specified by the present invention.

The iron-chromium alloy in Comparative Example 1 had a rough surface, with an Ra x RSm value exceeding 550, and failed the fretting wear test. This is thought to be because fretting wear was prone to generating wear particles, which then oxidized and accumulated at the wear points, causing a rapid increase in contact resistance.

In all of Comparative Examples 2 to 6, the total area ratio of the εCu phase was less than 1.0%, and they all failed the fretting wear test. This is thought to be because there was a shortage of εCu phase exposed at the locations where fretting wear had occurred, making the contact resistance more likely to increase. Furthermore, a comparison between each of the invention examples and Comparative Examples 2 to 6 showed that by setting the Cu content or the P value in the finish annealing process within the range specified by the present invention, precipitation of the εCu phase with a total area ratio of 1.0 to 10.0% can be obtained.

As shown above, all of the iron-chromium alloys in the examples of the present invention were judged to have passed the fretting wear test, indicating that the increase in contact resistance due to fretting wear was small.

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

Claims (3)

The alloy contains, by mass%, C: 0.01 to 0.5%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.0%, P: 0.045% or less, S: 0.03% or less, Ni: 0.01 to 5.0%, Cr: 4.0 to 25.0%, Cu: 0.5 to 10%, N: 0.10% or less, Nb: 0.6% or less, Ti: 0.6% or less, and Al: 0.001 to 3.5%, with the balance being Fe and unavoidable impurities;
The matrix is a martensite single phase structure or a multi-phase structure containing 50 volume % or more of martensite and ferrite phases,
The value of Ra (arithmetic mean roughness) × RSm (average length of roughness curve element) on the surface is 550 or less,
An iron-chromium alloy, wherein an εCu phase is dispersed and precipitated in the phase constituting the matrix, and a total area ratio of the εCu phase in a cross section perpendicular to the rolling direction is 1.0 to 10.0%. The iron-chromium alloy according to claim 1, further comprising, by mass%, at least one selected from the group consisting of Mo: 0.01-2.0%, V: 0.6% or less, B: 0.01% or less, Ca: 0.0002-0.015%, Hf: 0.001-0.60%, Zr: 0.01-0.60%, Sb: 0.005-0.60%, Co: 0.6% or less, W: 0.6% or less, Ta: 0.001-1.0%, Sn: 0.002-1.0%, Ga: 0.0002-0.50%, Mg: 0.0003-0.0050%, and REM (rare earth elements): 0.001-0.20%. The alloy contains, by mass%, C: 0.01 to 0.5%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.0%, P: 0.045% or less, S: 0.03% or less, Ni: 0.01 to 5.0%, Cr: 4.0 to 25.0%, Cu: 0.5 to 10%, N: 0.10% or less, Nb: 0.6% or less, Ti: 0.6% or less, and Al: 0.001 to 3.5%, with the balance being Fe and unavoidable impurities;
The matrix is a martensite single phase structure or a multi-phase structure containing 50 volume % or more of martensite and ferrite phases,
The value of Ra (arithmetic mean roughness) × RSm (average length of roughness curve element) on the surface is 550 or less,
A method for producing an iron-chromium alloy, in which an εCu phase is dispersed and precipitated in the matrix phase, and the total area ratio of the εCu phase in a cross section perpendicular to the rolling direction is 1.0 to 10.0%,
A method for producing an iron-chromium alloy, comprising a finish annealing step of performing heat treatment under conditions in which the P value represented by the following formula (1) is 11,000 to 22,000;
P value = T (20 + log t) ... (1)
In the formula (1), T represents the heat treatment temperature (K) expressed in absolute temperature, and t represents the heat treatment time (hr).