Iron-chromium alloy and method for manufacturing iron-chromium alloy

Abstract

To achieve an iron chromium alloy that is resistant to an increase in contact resistance from fretting corrosion.SOLUTION: An iron chromium alloy contains 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 inevitable impurities. The iron chromium alloy has a martensitic mono-phase structure or a composite structure with at least 50 vol.% of a martensite phase and a ferrite phase. The surface Ra×RSm value is 550 or less. The εCu phase is dispersed and precipitated in the phase. The total area percentage of the εCu phase in the cross section is 1.0-10.0%.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an iron-chromium alloy and a method for producing the same.

Background Art

[0002] In transportation equipment such as automobiles, with the increase in various electrical equipment such as safety equipment, the usage amount of wire harnesses composed of electric wires and connection terminals is increasing. Since a predetermined contact pressure is required at the connection part, the connection terminal is preferably formed of a material having high strength.

[0003] Generally, copper alloys are used as the material for connection terminals. In recent years, the use of iron-chromium alloys such as stainless steel has been proposed, which has the same strength as copper alloys, is cheaper, and enables sorting by magnetic force, which is difficult with copper alloys. However, while iron-chromium alloys form a passive film on the surface and thus have excellent corrosion resistance, they generally tend to have a large contact resistance.

[0004] Regarding such problems, for example, Patent Document 1 discloses an automotive terminal having a Cu plating layer and a Sn plating layer on the surface of a stainless steel plate. Patent Document 2 discloses a terminal provided with a connection part having a layer with a fine uneven surface. Patent Document 3 describes a stainless steel plate in which precipitation particles of a Cu-rich phase with a particle size of 300 nm or less are dispersed in a phase constituting the matrix to improve conductivity.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

[0006] In transportation equipment such as automobiles, vibrations can easily cause micro-sliding wear at the connection points of terminals. The plating layer described in Patent Document 1 may wear down due to this micro-sliding wear. The terminal disclosed in Patent Document 2 has difficulty in controlling the surface irregularities of the connection point during manufacturing, and there is room for improvement in manufacturing costs. When a Cu-rich phase is deposited as disclosed in Patent Document 3, micro-sliding wear may generate fine powder containing the Cu-rich phase. This fine powder oxidizes, causing an increase in contact resistance.

[0007] One aspect of the present invention aims to realize an iron-chromium alloy that is less susceptible to increased contact resistance due to micro-sliding wear. [Means for solving the problem]

[0008] To solve the aforementioned problems, an iron-chromium alloy according to one aspect of the present invention 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 remainder being Fe. The material consists of unavoidable impurities, and the matrix is ​​a single-phase martensite structure or a multi-phase structure of 50 volume% or more of martensite and ferrite phases, the value of Ra (arithmetic mean roughness) × RSm (average length of roughness curve elements) on the surface is 550 or less, the εCu phase is dispersed and precipitated in the phases 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%.

[0009] An iron-chromium alloy according to one aspect of the present invention may further contain at least one element 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% by mass%.

[0010] To solve the aforementioned problems, an iron-chromium alloy according to one aspect of the present invention 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 remainder being Fe and unavoidable impurities, and the matrix is ​​a single-phase martensite structure. A method for producing an iron-chromium alloy having a multiphase structure of 50 volume% or more of martensite and ferrite phases, where the value of Ra (arithmetic mean roughness) × RSm (average length of roughness curve elements) on the surface is 550 or less, where εCu phase is dispersed and precipitated in the phases 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%, comprising a finish annealing step in which heat treatment is performed under conditions in which the P value shown by the following formula (1) is 11000 to 22000; P-value = T(20 + log t) ... (1) In equation (1) above, T represents the heat treatment temperature (K) expressed in absolute temperature, and t represents the heat treatment time (hr). [Effects of the Invention]

[0011] According to one aspect of the present invention, an iron-chromium alloy can be realized in which an increase in contact resistance due to micro-sliding wear is less likely to occur. [Modes for carrying out the invention]

[0012] One embodiment of the present invention will be described below. However, 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 different embodiments and examples are also included within the technical scope of the present invention. Furthermore, in this specification, "A to B" indicates a range of A or greater and B or less.

[0013] An 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 its surface derived from Cr in the alloy. Examples of iron-chromium alloys include stainless steel with a Cr content of 10.5% by mass or more, but are not limited thereto, and may also be an alloy steel with a Cr content of less than 10.5% by mass. Furthermore, the shape of the iron-chromium alloy is not particularly limited and may be, for example, a steel plate, steel strip, steel pipe, or steel bar.

[0014] [εCu phase] In iron-chromium alloys, the εCu phase is dispersed and precipitated within the matrix (hereinafter sometimes simply referred to as "the matrix"). The εCu phase is also called the Cu-rich phase and consists of particles of Cu that have precipitated in the matrix through aging treatment during processes such as annealing. 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.

[0015] Furthermore, 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 higher electrical conductivity than the passivation film formed on the surface of the iron-chromium alloy. It is also known that a passivation film is less likely to form at the locations where the εCu phase is precipitated on the surface of the iron-chromium alloy, and the εCu phase is more easily 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.

[0016] 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 aforementioned cross section of the iron-chromium alloy. The cross section perpendicular to the rolling direction may be a cross section perpendicular and parallel to the rolling direction (L-section), a cross section perpendicular and right-angled to the rolling direction (T-section), or any other cross section perpendicular to the rolling direction.

[0017] 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 using εCu phase with a particle size of 1 nm to 1000 nm. The particle size of the εCu phase may be expressed by the maximum diameter of the particle.

[0018] The average particle size of the εCu phase precipitated in the iron-chromium alloy matrix may be 1 nm or more, or 5 nm or more. Alternatively, the average particle size of the εCu phase may be 1000 nm or less, or 500 nm or less. The particle size of the εCu phase can be measured, for example, by identifying the εCu phase particles using an EDX device within the field of view observed by a transmission electron microscope. Furthermore, the average particle size of the εCu phase can be calculated as the average value of the particle sizes of particles contained within a predetermined range within the field of view of the transmission electron microscope.

[0019] On the surface of an iron-chromium alloy containing such an εCu phase, a predetermined amount of the εCu phase, which has good electrical conductivity, is present. Because the εCu phase has higher electrical conductivity than the passivation film formed on the surface of the iron-chromium alloy, it functions as a pathway for electricity to flow between the surface of the iron-chromium alloy and a conductor in contact with that surface.

[0020] In addition, the iron-chromium alloy as described above can also be suitably used for connection terminals of transportation equipment such as automobiles, where fretting wear is likely to occur. In the matrix of the iron-chromium alloy, a predetermined amount of εCu phase is dispersed and precipitated in the same manner as on the surface. Therefore, even when fretting wear occurs on the surface of the iron-chromium alloy, the εCu phase in the matrix is successively exposed at the site where fretting wear has occurred. Therefore, if the connection terminal is made of an iron-chromium alloy, the εCu phase can function as an electrical conduction path even in the connection part where fretting wear has occurred, so an increase in contact resistance is unlikely to occur.

[0021] When the total area ratio of the εCu phase is less than 1.0%, the εCu phase existing on the surface and near the surface of the iron-chromium alloy is insufficient. Therefore, the εCu phase with good conductivity does not function sufficiently for improving the contact resistance in the iron-chromium alloy. On the other hand, when the total area ratio of the εCu phase exceeds 10.0%, it is necessary to add a large amount of Cu. The excessive addition of Cu not only causes a decrease in the hot workability of the iron-chromium alloy due to the formation of the CuMn phase, but also may cause an increase in the surface unevenness of the iron-chromium alloy depending on the type of acid when an acidic process is carried out. In addition, the addition of a large amount of Cu also leads to an increase in the manufacturing cost of the iron-chromium alloy.

[0022] Note that the iron-chromium alloy is not limited to fretting wear. For example, even for connection terminals where insertion and extraction are frequently performed, an increase in the contact resistance of the connection part due to wear is unlikely to occur. Therefore, the iron-chromium alloy can be suitably used for various connection terminals.

[0023] According to such an iron-chromium alloy, for example, there is no need to perform plating treatment for reducing contact resistance, and it is possible to save raw materials and energy in the manufacturing process. In addition, separation of scrap is possible by magnetic separation, which is impossible with copper alloys and the like, and metal recycling becomes easy. This can contribute to the achievement of the Sustainable Development Goals (SDGs), such as Goal 12, "Responsibility for Production and Consumption".

[0024] 〔Surface roughness〕 Iron-chromium alloys have a surface surface value of Ra (arithmetic mean roughness) × RSm (average length of roughness curve elements) of 550 or less. Ra (arithmetic mean roughness) and RSm (average length of roughness curve elements) conform to the definitions in JIS B0601:2013.

[0025] When the surface of an 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 contact pressure concentrates on the few contact points. As a result, when micro-sliding wear occurs on the iron-chromium alloy, wear particles (powder generated from surface abrasion) are more easily generated. Oxidized wear particles cause an increase in the contact resistance of the iron-chromium alloy.

[0026] Here, the inventors focused not only on Ra (arithmetic mean roughness) but also on RSm (average length of roughness curve elements). Ra (arithmetic mean roughness) indicates the magnitude of surface irregularities in the direction perpendicular to the surface, while RSm (average length of roughness curve elements) indicates the magnitude in the direction parallel to the surface. Since wear particles are easily generated regardless of the direction of surface irregularities, the amount of wear particles generated can be effectively reduced by adjusting both of these parameters. Thus, the inventors have found that, from the viewpoint of wear particle generation, 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 elements).

[0027] If the surface of the iron-chromium alloy is within the range of Ra × RSm ≤ 550, the excessive generation of wear particles can be prevented. Therefore, even in iron-chromium alloys in which the εCu phase has precipitated in the matrix, an increase in contact resistance due to wear particles can be prevented.

[0028] [Component composition] Iron-chromium alloys contain, 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 remainder being Fe and unavoidable impurities. The following describes each element contained in iron-chromium alloys.

[0029] (C) Carbon (C) 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 iron-chromium alloys. Therefore, iron-chromium alloys contain 0.01% to 0.5% by mass of C. Preferably, the C content is 0.01% to 0.15% by mass.

[0030] (Si) Silicon (Si) is an effective deoxidizing agent and also possesses solid solution strengthening properties. However, excessive Si addition can lead to a decrease in workability and toughness. Therefore, iron-chromium alloys contain Si in an amount between 0.01% and 2.0% by mass. Preferably, the Si content is between 0.2% and 1.5% by mass.

[0031] (Mn) Manganese (Mn) is an effective element for increasing the strength of iron-chromium alloys. However, excessive addition of Mn leads to a decrease in the hot workability of iron-chromium alloys. Therefore, iron-chromium alloys contain between 0.01% and 2.0% by mass of Mn. Preferably, the Mn content is between 0.2% and 1.5% by mass.

[0032] (P) Phosphorus (P) is an unavoidable impurity, and a lower P content is preferable. From a manufacturability standpoint, iron-chromium alloys contain 0.045% by mass or less of P. A P content of 0.045% by mass or less reduces the adverse effects on material properties such as ductility in iron-chromium alloys. Preferably, the P content is between 0.005% by mass and 0.040% by mass.

[0033] (S) Sulfur (S) is an unavoidable impurity, and a lower S content is preferable. From a manufacturability standpoint, iron-chromium alloys contain 0.03% by mass or less of S. A S content of 0.03% by mass or less reduces the adverse effects on material properties such as ductility in iron-chromium alloys. Preferably, the S content is between 0.0001% by mass and 0.003% by mass.

[0034] (Ni) Nickel (Ni) is an effective element for improving the corrosion resistance and toughness of iron-chromium alloys. However, Ni is an expensive element, and excessive addition leads to increased manufacturing costs. Therefore, iron-chromium alloys contain between 0.01% and 5.0% by mass of Ni. Preferably, the Ni content is between 0.1% and 3.0% by mass.

[0035] (Cr) Chromium (Cr) is an effective element for ensuring the corrosion resistance of iron-chromium alloys. However, excessive addition of Cr reduces the workability and toughness of iron-chromium alloys. Therefore, iron-chromium alloys contain 4.0% to 25.0% by mass of Cr. Preferably, the Cr content is between 7.0% and 18.0% by mass.

[0036] (Cu) Copper (Cu) is an effective element for increasing the strength and improving the conductivity of iron-chromium alloys. Furthermore, Cu is also effective in the precipitation of the εCu phase. However, excessive addition of Cu leads to the formation of a CuMn phase in the center of the slab during solidification, reducing its hot workability. Therefore, iron-chromium alloys contain 0.5% to 10% Cu by mass. Preferably, the Cu content is between 1.0% and 7.0% by mass.

[0037] (N) Nitrogen (N) is an element that has solid solution strengthening and corrosion resistance improving effects. On the other hand, excessive addition of N reduces the workability of iron-chromium alloys. Therefore, the N content in iron-chromium alloys is preferably 0.10% by mass or less, and more preferably between 0.001% by mass and 0.08% by mass.

[0038] (Nb) Niobium (Nb) is an effective element for refining and homogenizing the microstructure. Nb is also effective in the precipitation of conductive metal carbides, metal borides, and metal nitrides. On the other hand, nickel (Ni) is an expensive element, and excessive addition leads to increased manufacturing costs. Therefore, iron-chromium alloys contain 0.6% by mass or less of Nb. Preferably, the Nb content is between 0.01% by mass and 0.5% by mass.

[0039] (Ti) Titanium (Ti) is an element with deoxidizing properties. Therefore, iron-chromium alloys contain 0.6% by mass or less of Ti. Preferably, the Ti content is between 0.01% by mass and 0.5% by mass.

[0040] (Al) Aluminum (Al) is an element with deoxidizing properties. However, excessive addition of Al can degrade surface quality. Therefore, iron-chromium alloys contain between 0.001% and 3.5% by mass of Al. Preferably, the Al content is between 0.001% and 1.8% by mass.

[0041] (Other elements) In addition to the elements mentioned above, the iron-chromium alloy may further contain at least one element 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% by mass%.

[0042] (Mo) Mo (molybdenum) is an effective element for improving the corrosion resistance of iron-chromium alloys. However, since Mo is an expensive element, excessive addition is undesirable. Therefore, iron-chromium alloys may contain 0.01% to 2.0% by mass of Mo. A Mo content of 0.1% to 1.5% by mass is more preferable.

[0043] (V, W) V (vanadium) and W (tungsten) are both effective elements for improving the corrosion resistance of iron-chromium alloys. However, since V and W are expensive elements, excessive addition is undesirable. Therefore, iron-chromium alloys may contain at least one of V (0.6% by mass or less) and W (0.6% by mass or less). The V content is preferably 0.05% by mass or more and 0.5% by mass or less. The W content is also preferably 0.05% by mass or more and 0.5% by mass or less.

[0044] (B) Boron (B) is an element that improves the hot workability of iron-chromium alloys and is effective in reducing the occurrence of edge splitting and double splitting during hot rolling. Therefore, iron-chromium alloys may contain 0.01% by mass or less of boron. Preferably, the boron content is between 0.0005% by mass and 0.005% by mass.

[0045] (Ca) Calcium (Ca) is effective in preventing edge breakage during hot rolling. However, excessive addition of Ca leads to a decrease in corrosion resistance. Therefore, iron-chromium alloys may contain Ca in an amount of 0.0002% to 0.015% by mass. Preferably, the Ca content is between 0.0002% and 0.005% by mass.

[0046] (Hf) Hf (hafnium) is an element that improves corrosion resistance and high-temperature strength. On the other hand, excessive addition of Hf may lead to a decrease in processability and manufacturability. Therefore, iron-chromium alloys may contain 0.001% to 0.60% by mass of Hf. Preferably, the Hf content is 0.005% to 0.50% by mass.

[0047] (Zr) Zr (zirconium) is an element that improves the hot workability of iron-chromium alloys and is also effective in improving oxidation resistance. Therefore, iron-chromium alloys may contain 0.01% to 0.60% by mass of Zr. Preferably, the Zr content is 0.05% to 0.50% by mass.

[0048] (Sb) Antimony (Sb) is an element that improves high-temperature strength. On the other hand, excessive addition of Sb reduces weldability and toughness. Therefore, iron-chromium alloys may contain 0.005% to 0.60% by mass of Sb. Preferably, the Sb content is 0.01% to 0.40% by mass.

[0049] (Co) Co (cobalt) is an element that improves high-temperature strength. On the other hand, the addition of excessive cobalt reduces toughness, leading to decreased manufacturability. Therefore, iron-chromium alloys may contain 0.6% by mass or less of cobalt. Preferably, the cobalt content is between 0.05% by mass and 0.5% by mass.

[0050] (Ta) Tantalum (Ta) is an element that improves high-temperature strength. On the other hand, excessive addition of Ta reduces weldability and toughness. Therefore, iron-chromium alloys may contain 0.001% to 1.0% by mass of Ta. Preferably, the Ta content is 0.005% to 0.5% by mass.

[0051] (Sn) Tin (Sn) is an element that improves corrosion resistance and high-temperature strength. On the other hand, excessive addition of Sn may lead to a decrease in toughness and manufacturability. Therefore, iron-chromium alloys may contain 0.002% to 1.0% by mass of Sn. Preferably, the Sn content is 0.002% to 0.5% by mass.

[0052] (Ga) Gallium (Ga) is an element that improves corrosion resistance and hydrogen embrittlement resistance. On the other hand, excessive addition of Ga reduces weldability and toughness. Therefore, iron-chromium alloys may contain Ga in an amount of 0.0002% to 0.50% by mass. Preferably, the Ga content is between 0.0002% and 0.30% by mass.

[0053] (Mg) Magnesium (Mg) is a deoxidizing element that also refines the slab structure and improves moldability. On the other hand, excessive addition of Mg leads to a decrease in corrosion resistance, weldability, and surface quality. Therefore, iron-chromium alloys may contain Mg in an amount of 0.0003% to 0.0050% by mass. Preferably, the Mg content is between 0.0003% and 0.0030% by mass.

[0054] (REM) REM (Rare Earth Elements) refers to the collective term for Sc (Scandium) and 15 elements (lanthanides) from La (Lanthanum) to Lu (Lutetium). Iron-chromium alloys may contain REM as a single element or as a mixture of multiple elements. REM improves the cleanliness of iron-chromium alloys and effectively prevents edge cracking during hot rolling. On the other hand, excessive addition of REM increases alloy costs and reduces manufacturability. Therefore, iron-chromium alloys may contain 0.001% by mass or more and 0.20% by mass or less of REM. Preferably, the REM content is 0.005% by mass or more and 0.10% by mass or less.

[0055] The iron-chromium alloy may contain, as REM, at least one of La (0.1% by mass or less) and Ce (cerium) (0.05% by mass or less).

[0056] [Organization] The matrix of an iron-chromium alloy is either a single-phase martensite structure or a multi-phase structure consisting of 50% or more by volume of martensite and ferrite phases. The matrix of an iron-chromium alloy can be configured to any desired structure by adjusting its component composition. Note that the εCu phase is a particulate precipitate within the phases constituting the matrix of an iron-chromium alloy and is distinct from the matrix itself. Therefore, a single-phase martensite structure may contain precipitated phases such as the εCu phase within the matrix. Furthermore, the matrix of an iron-chromium alloy may contain unavoidable cambium other than the martensite or ferrite phases.

[0057] Specifically, in iron-chromium alloys, if the M value shown in formula (2) below is 100 or more, it will have a single-phase martensite structure; if it is between 0 and 100, it will have a ferrite-martensite multi-phase structure; and if it is less than 0, it will have a single-phase ferrite structure. Furthermore, if the M value shown in formula (2) below is between 50 and 100, it will have a multi-phase structure of 50% or more by volume of martensite and ferrite.

[0058] M value = 420C-11.5Si+7Mn+23Ni-11.5Cr-12Mo-10V+9Cu-49Ti-52Al+470N+189 ··· (2) In equation (2) above, the elemental symbols are replaced with the content (mass%) of each element contained in the iron-chromium alloy, and 0 is substituted for elements that are not added.

[0059] If the matrix of the iron-chromium alloy has such a microstructure, the generation of wear particles due to micro-sliding wear can be effectively reduced. Furthermore, the εCu phase is thought to precipitate more finely in the martensite phase than in the ferrite phase. This is because the martensite phase contains more strain fields that facilitate the precipitation of the εCu phase compared to the ferrite phase. In order to precipitate a fine εCu phase, it is preferable that the matrix of the iron-chromium alloy has at least 50 volume percent or more of the martensite phase.

[0060] [Manufacturing method] The method for manufacturing an iron-chromium alloy according to one embodiment of the present invention may include general manufacturing processes for iron-chromium alloys such as stainless steel, with respect to steps other than the finish annealing step.

[0061] Iron-chromium alloys can be manufactured by heat treatment in the finish annealing process under conditions where the P value, as shown by the following formula (1), is between 11,000 and 22,000.

[0062] P-value = T(20 + log t) ... (1) In equation (1) above, T represents the heat treatment temperature (K) expressed in absolute temperature, and t represents the heat treatment time (hr).

[0063] When the P value is less than 11,000, the εCu phase does not precipitate in the iron-chromium alloy matrix. Also, when the P value is greater than 22,000, Cu tends to remain in a fine solid solution state in the iron-chromium alloy matrix even during heat treatment, so the εCu phase is less likely to precipitate.

[0064] By performing a finish annealing process with such heat treatment temperature and time, the εCu phase can be dispersed and precipitated in the matrix of the iron-chromium alloy. The finish annealing process may be carried out by batch annealing or by continuous annealing.

[0065] The following is an example of a method for manufacturing iron-chromium alloy, but it is not limited to this method.

[0066] In the method for manufacturing iron-chromium alloy, for example, slabs are produced by continuous casting of molten steel with adjusted composition. Then, the slabs produced by continuous casting are heated to 1100°C to 1300°C, and then hot-rolled to produce hot-rolled steel strips. The hot-rolled steel strips may be pickled. Alternatively, a first intermediate annealing step may be performed before pickling the hot-rolled steel strips, or pickling may be performed without annealing.

[0067] Next, a cold rolling process is carried out in which the hot-rolled steel strip, after pickling, is cold-rolled until it reaches a predetermined thickness to obtain a cold-rolled steel strip. In the cold rolling process, intermediate rolling may be carried out as needed, and a second intermediate annealing process may be carried out in which annealing is performed.

[0068] The cold-rolled steel strip is subjected to the finish annealing process described above. Furthermore, to further increase the strength of the steel strip after finish annealing, temper rolling may be performed as needed. This temper rolling may also be performed for the purpose of surface finishing, as described below.

[0069] Iron-chromium alloys may be surface-finished to the extent that the Ra × RSm value on the surface is 550 or less. Examples of surface finishing methods applied to iron-chromium alloys include BA (bright annealing) finish, pickling finish, pickling and light rolling finish, temper rolling finish, HL (hairline) finish, and dull finish. HL finish is a method of applying polished marks to the surface of the iron-chromium alloy. Dull finish is a method of transferring the surface condition of the roll to the surface of the iron-chromium alloy by using a coarse roll during temper rolling.

[0070] For iron-chromium alloys after finish annealing, if the Ra × RSm value on the surface is 550 or less, such surface finishing is not required. [Examples]

[0071] The results of the evaluation of an iron-chromium alloy according to one embodiment and a comparative example of the present invention are described below.

[0072] [Evaluation Criteria] <Component composition> Table 1 below shows the component composition (mass%) and M value of the iron-chromium alloy according to one embodiment of the present invention (Examples 1-16) and the iron-chromium alloy according to comparative examples (Comparative Examples 1-6). The M value was calculated using formula (2) above. As described above, if the M value is 100 or more, it is an iron-chromium alloy having a single-phase martensite structure, and if it is 0 or more and less than 100, it is an iron-chromium alloy having a ferrite-martensite multi-phase structure. Furthermore, all iron-chromium alloys having a ferrite-martensite multi-phase structure contain 50 volume% or more of the martensite phase.

[0073] In Table 1, underlined values ​​indicate that they are outside the specified range for this invention.

[0074] [Table 1]

[0075] <Manufacturing method> The iron-chromium alloys for each example of the invention and comparative example were manufactured by the following method. Iron-chromium alloys having the component compositions shown in Table 1 were melted and subjected to a hot rolling process followed by a finish annealing process to obtain rolled iron-chromium alloys. The heat treatment in the finish annealing process was carried out according to the heat treatment temperature T and heat treatment time t conditions shown in Table 1.

[0076] The iron-chromium alloy in Comparative Example 3 was manufactured without a finish annealing process. The iron-chromium alloy in Comparative Example 5 was manufactured under the same conditions as in Invention Example 7, except that the finish annealing process involved heat treatment to reduce the P value to less than 11,000. The iron-chromium alloy in Comparative Example 6 was manufactured under the same conditions as in Invention Example 1, except that the finish annealing process involved heat treatment to increase the P value to more than 22,000.

[0077] The iron-chromium alloy in Comparative Example 1 was manufactured by applying a dull finish so that the Ra × RSm value exceeded 550. The iron-chromium alloys in Comparative Examples 2 and 4 were manufactured with component compositions outside the range specified by the present invention.

[0078] <Evaluation Method> The following describes the evaluation methods for the surface Ra, RSm, and Ra×RSm values, the total area ratio of the εCu phase in the cross-section, and the contact resistance of the iron-chromium alloys according to each example of the invention and comparative example. The results obtained by these evaluation methods are shown in Table 1 above.

[0079] (Total area ratio of the εCu phase) A cross-section (L-section) of an iron-chromium alloy, perpendicular to and parallel to the rolling direction, was mirror-polished, and the cross-section was observed using a transmission electron microscope to obtain image data of the field of view. Deposited particles of the εCu phase were identified using the EDX device attached to the electron microscope. The area of ​​each identified εCu phase precipitate particle within the field of view was calculated, as well as the area of ​​the entire field of view. The ratio of the total area of ​​all εCu phase precipitate particles to the total area of ​​the field of view was calculated as the total area percentage (%). Each area was calculated using the image processing software "ImageJ".

[0080] (Surface roughness) The Ra (arithmetic surface roughness) and RSm (average length of roughness curve elements) on the surface of the iron-chromium alloy were measured in accordance with JIS B0601:2013 using a stylus-type surface roughness measuring instrument (SURFCOM2900DX, manufactured by Tokyo Seimitsu Co., Ltd.). The obtained Ra value (μm) and RSm value (μm) were integrated to obtain the Ra × RSm value (μm).

[0081] (contact resistance) The contact resistance of the iron-chromium alloy was evaluated by a micro-sliding abrasion test. For the micro-sliding abrasion test, a plate measuring 40 mm in length and 40 mm in width was prepared from a rolled material with a thickness of 0.3 mm, and bent at a 90-degree angle with a radius of 1.5 mm to form a test specimen. The bend apex of two test specimens were brought into contact with each other, and the sliding motion was performed at a contact pressure of 5 N, a sliding distance of 100 μm during reciprocating motion (50 μm one way), and one reciprocating motion as one cycle, at a speed of 1 Hz. The sliding motion was performed by reciprocating only one test specimen at a time.

[0082] A constant current was passed between the test pieces, and the change in voltage across the test pieces was measured using the four-terminal method to determine the contact resistance value (mΩ) between the test pieces. A test was considered successful if the number of sliding cycles at which the contact resistance value became at least twice the minimum value at the start of measurement or after the start of measurement was 50 cycles or more. In Table 1, "O" indicates a pass in the micro-sliding wear test, and "×" indicates a fail.

[0083] 〔result〕 As shown in Table 1, in all of the iron-chromium alloys according to the present invention, the εCu phase was dispersed and precipitated in the matrix at a predetermined total area ratio, and the Ra×RSm value was also within the range specified by the present invention.

[0084] The iron-chromium alloy in Comparative Example 1 had a rough surface and a Ra×RSm value exceeding 550, resulting in failure in the micro-sliding abrasion test. This is thought to be because wear particles were easily generated by micro-sliding abrasion, and these particles oxidized and accumulated at the abraded areas, causing a rapid increase in contact resistance.

[0085] Comparative Examples 2-6 all had a total area ratio of less than 1.0% for the εCu phase and failed the micro-sliding wear test. This is thought to be because the insufficient amount of εCu phase exposed at the locations where micro-sliding wear occurred made it easier for the contact resistance to increase. Furthermore, a comparison of each inventive example with Comparative Examples 2-6 showed that by keeping the Cu content or the P value in the finish annealing process within the specified range of the present invention, precipitation of the εCu phase with a total area ratio of 1.0-10.0% can be obtained.

[0086] As shown above, all of the iron-chromium alloys according to the present invention passed the micro-sliding abrasion test, demonstrating that the increase in contact resistance due to micro-sliding abrasion was small.

[0087] [Additional Notes] The present invention is not limited to the embodiments and examples described above, 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

1. In mass%, it contains 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 remainder being Fe and unavoidable impurities. The matrix is ​​either a single-phase martensite structure or a multi-phase structure consisting of 50% or more by volume of martensite and ferrite. The value of Ra (arithmetic mean roughness) × RSm (average length of roughness curve elements) on the surface is 550 or less. An iron-chromium alloy in which εCu phases are dispersed and precipitated in the phases constituting the matrix, and the total area ratio of the εCu phases in a cross section perpendicular to the rolling direction is 1.0 to 10.0%.

2. The iron-chromium alloy according to claim 1, further containing, in 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 elements): 0.001 to 0.20%.

3. In mass%, it contains 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 remainder being Fe and unavoidable impurities. The matrix is ​​either a single-phase martensite structure or a multi-phase structure consisting of 50% or more by volume of martensite and ferrite. The value of Ra (arithmetic mean roughness) × RSm (average length of roughness curve elements) on the surface is 550 or less. A method for producing an iron-chromium alloy, wherein 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%, A method for producing an iron-chromium alloy, comprising a finish annealing step in which heat treatment is performed under conditions that the P value shown by the following formula (1) is between 11,000 and 22,000; P-value = T(20 + log t) ... (1) In equation (1) above, T represents the heat treatment temperature (K) expressed in absolute temperature, and t represents the heat treatment time (hr).

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