Steel material

Abstract

To provide a steel material having excellent corrosion resistance.SOLUTION: A steel material comprising a Cr-enriched layer having a Cr content of 3.0 mass% or more on a surface layer of the steel material, wherein a maximum value of the Cr content in the Cr-enriched layer is 10.0 to 20.0 mass%, and when a portion excluding the Cr-enriched layer from the steel material is defined as a base metal portion, a chemical composition of the base metal portion contains, by mass%, C: 0.050 to 0.800% and Cr: less than 3.00%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel material. [Background technology]

[0002] Steel materials such as steel sheets for automobiles have low corrosion resistance and are prone to rust. Therefore, to ensure corrosion resistance, the surface of steel materials is often plated.

[0003] For example, Patent Document 1 discloses a hot-dip galvanized hot-rolled steel sheet for use as an automobile part, in which a predetermined blasting treatment is carried out after a pickling process to control the surface shape of the steel sheet and the amount of residual stress in the surface layer of the steel sheet, thereby improving the surface appearance, coating adhesion, and corrosion resistance after processing, and a method for manufacturing the same. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-20026 [Non-patent literature]

[0005] [Non-Patent Document 1] Minoru Hino, Minoru Hiramatsu, Keitaro Horikawa: Surface Treatment, Vol.71, (2020) No.5, 323-329 Summary of the Invention [Problem to be solved by the invention]

[0006] Incidentally, hydrogen is generated during the plating process on the surface of steel material and the pickling processes performed before and after. As shown in Non-Patent Document 1, it is known that hydrogen embrittlement occurs when hydrogen generated during the plating process or the like penetrates into the steel material. Furthermore, the higher the strength of the steel material, the more likely this hydrogen embrittlement occurs. Therefore, this problem has become apparent in high-strength steel, which is in increasing demand from the perspective of reducing the weight of vehicle bodies.

[0007] Furthermore, when plating is applied, in addition to the problem of hydrogen embrittlement described above, there is also the problem that the steel material will rust when the plating peels off.

[0008] The present invention aims to solve the above problems and to provide a steel material having improved corrosion resistance without plating. [Means for solving the problem]

[0009] As a result of extensive investigations to solve the above-mentioned problems, the present inventors have come to the following findings.

[0010] After melting steel and heat treating it under various conditions, they discovered that when a chemical agent containing metallic Cr powder and SiO2 was applied to the surface of the steel, Cr was concentrated in the surface layer of the steel. Cr, which has the effect of improving corrosion resistance, is concentrated in the surface layer of the steel, making it possible to improve the corrosion resistance of the steel.

[0011] The present invention has been made based on the above findings, and the gist of the present invention is the following steel material.

[0012] (1) The surface layer of the steel material has a Cr-enriched layer having a Cr content of 3.0 mass% or more, the maximum Cr content in the Cr-enriched layer is 10.0 to 20.0 mass%, When the portion of the steel material excluding the Cr-enriched layer is used as the base material, The chemical composition of the base material is, in mass%, C: 0.050 to 0.800%, and Cr: less than 3.00%; Steel material.

[0013] (2) When the thickness of the Cr-enriched layer is d, the average value of the Cr content at a position d / 2 in the depth direction from the surface of the steel material is more than 3.0% by mass, the average value of the Cr content at a position d / 10 in the depth direction from the surface of the steel material is higher than the average value of the Cr content at a position d / 2 in the depth direction from the surface of the steel material; The steel material described in (1) above.

[0014] (3) The chemical composition of the base material is, in mass%, C: 0.050~0.800%, Si: 0.01 to 3.00%, Mn: 0.01 to 10.00%, Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.0500% or less, N: 0.0100% or less, O: 0.0200% or less, Cr: less than 3.00%, and The balance is Fe and impurities. The steel material according to (1) or (2) above.

[0015] (4) The chemical composition of the base material is, in mass%, C: 0.050~0.800%, Si: 0.01 to 3.00%, Mn: 0.01 to 10.00%, Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.0500% or less, N: 0.0100% or less, O: 0.0200% or less, and Cr: less than 3.00% and further containing one or more selected from the group consisting of the following groups A, B, and C: The balance is Fe and impurities. The steel material according to (1) or (2) above. [Group A] One or more selected from the group consisting of Mo: 1.0% or less, B: 0.0100% or less, Ti: 0.500% or less, Nb: 0.500% or less, V: 0.500% or less, Cu: 0.500% or less, Ni: 0.500% or less, W: 0.100% or less, Ta: 0.100% or less, and Co: 0.500% or less [Group B] One or more selected from the group consisting of Sn: 0.050% or less, Sb: 0.050% or less, and As: 0.050% or less [Group C] One or more selected from the group consisting of Mg: 0.0500% or less, Ca: 0.050% or less, Zr: 0.050% or less, and REM: 0.150% or less

[0016] (5) The steel material according to (4) above, wherein the base metal portion has a chemical composition containing one or more elements selected from Group A.

[0017] (6) The steel material according to (4) above, wherein the base metal portion has a chemical composition containing one or more elements selected from the B group.

[0018] (7) The steel material according to (4) above, wherein the base metal portion has a chemical composition containing one or more elements selected from the C group. [Effects of the Invention]

[0019] According to the present invention, steel materials having excellent corrosion resistance can be obtained industrially and stably. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a schematic diagram illustrating the measurement area for the Cr content in the Cr-enriched layer. DETAILED DESCRIPTION OF THE INVENTION

[0021] Each of the requirements of the present invention will be described in detail below.

[0022] 1.Cr enriched layer The steel material according to the present invention has a Cr-enriched layer on the surface thereof. In the present invention, the "Cr-enriched layer" refers to a region where the Cr content is 3.0% by mass or more. In the Cr-enriched layer, the maximum Cr content is 10.0 to 20.0% by mass. In the present invention, the "steel material" includes hot-rolled steel sheets and cold-rolled steel sheets.

[0023] If the maximum Cr content in the Cr-enriched layer is less than 10.0 mass%, the effect of improving corrosion resistance is not sufficiently obtained. On the other hand, if Cr is excessively concentrated in the Cr-enriched layer, the Cr diffusion rate becomes faster than the rate of the decarburization reaction, and the chromium carbide (Cr 23 C6) is promoted. As a result, areas where the Cr content drops to less than 3.0 mass% around chromium carbides are scattered around the steel surface, reducing corrosion resistance. Therefore, the maximum Cr content in the Cr-enriched layer is set to 20.0 mass% or less.

[0024] From the viewpoint of obtaining better corrosion resistance, it is preferable that the Cr content be higher closer to the surface of the steel material. Specifically, when the thickness of the Cr-enriched layer is d, the average Cr content at a position d / 2 in the depth direction from the surface (hereinafter also simply referred to as the "d / 2 position") is more than 3.0 mass%, and the average Cr content at a position d / 10 in the depth direction from the surface (hereinafter also simply referred to as the "d / 10 position") is preferably higher than the average Cr content at the d / 2 position.

[0025] Furthermore, the thickness d of the Cr-enriched layer in the steel material may be more than 0 μm, but when the heat treatment described below is performed, the thickness becomes 15 to 100 μm.

[0026] The Cr content in the Cr-enriched layer is measured using a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). Specifically, a cross section perpendicular to the rolling direction is used as the measurement surface, and the observation sample is rotated so that the right and left edges of the steel surface to be observed are at the same height in the observation field at 50x magnification. Hereinafter, the straight line passing through the right and left edges of the steel surface to be observed within the field of view is referred to as the reference line.

[0027] FIG. 1 is a schematic diagram illustrating the measurement area for the Cr content in the Cr-enriched layer. With the reference line positioned so that the right and left edges of the field of view are at the same height, i.e., the reference line is parallel to the horizontal plane of the field of view, the observation magnification is changed to 500x, and a line parallel to the reference line and passing through the most recessed position a in the depth direction of the steel surface (hereinafter referred to as the "observation start line") is defined as the top end of the observation area, as shown in FIG. 1. The observation start line is defined as one side, and the observation area is a 100 μm × 100 μm region in the depth direction of the steel. The Cr content in this region is measured by EDS at 5 μm intervals (pitch) so that each point is parallel to the observation start line, with each point held for 1 second.

[0028] The average value of the Cr content at 21 measurement points that are located at the same depth from the observation start line is taken as the average Cr content at that depth. The maximum depth at which the average Cr content at each depth is 3.0% or more is defined as the bottom end of the Cr-enriched layer. The distance from the observation start line to the bottom end of the Cr-enriched layer is taken as the thickness d of the Cr-enriched layer. The maximum value of the average Cr content at each depth is taken as the maximum Cr content in the Cr-enriched layer. The d / 2 and d / 10 positions in the depth direction from the surface refer to positions d / 2 and d / 10 away from the observation start line, respectively, in the depth direction.

[0029] Next, the average Cr content at the d / 2 and d / 10 positions was determined by the average of the quantitative EDS analysis values, as described above. Point analysis was then performed again at 21 points at the d / 2 and d / 10 positions from the observation start line, and the average value was defined as the average Cr content at each position in the Cr-enriched layer.

[0030] <About the decarburized layer> Furthermore, the steel material according to the present invention preferably has a decarburized layer at its surface. In the present invention, the term "decarburized layer" refers to a region in which the C content is 10% or more lower than the C content in the chemical composition of the steel material excluding the Cr-enriched layer (hereinafter referred to as the "base material"). Forming a decarburized layer by heat treatment, which will be described later, can suppress the formation of chromium carbides when Cr is enriched. As a result, the reduction in the Cr content around chromium carbides can be suppressed, and the effect of improving the corrosion resistance of the steel material due to Cr enrichment can be maintained. There are no particular restrictions on the thickness of the decarburized layer, but from the viewpoint of suppressing the precipitation of chromium carbides, it is preferably equal to or greater than the thickness d of the Cr-enriched layer. In this case, it is more preferable that the average C content at the position d in the depth direction from the surface (hereinafter simply referred to as the "position d") be 0.15 mass% or less.

[0031] The thickness of the decarburized layer and the carbon content at position d are measured using an electron probe microanalyzer (EPMA). Specifically, in the same manner as in the measurement of the Cr content in the Cr-enriched layer described above, a linear analysis of 21 points is performed at 5 μm intervals (pitch) parallel to the observation start line, with each point held for 1 second. The average carbon content at the 21 measurement points that coincide in the depth direction from the observation start line is used as the carbon content at that depth, and the maximum depth where the carbon content is 10% or more lower than the carbon content of the steel is taken as the decarburized layer thickness. The carbon content at position d is also determined.

[0032] The Cr-enriched layer and decarburized layer are formed by subjecting the steel material to the heat treatment described below, and are maintained at or at a ratio similar to that formed by the heat treatment during hot working, cold working after cooling to room temperature, and forming into, for example, a product or a component constituting the product. Furthermore, the formation and thickness of the layer can be increased by further subjecting the above-described heat treatment to intermediate materials such as ingots, billets, and hot-rolled and cold-rolled materials after processing. Steel billets include not only slabs but also billets, blooms, and the like.

[0033] As described above, the steel material according to the present invention preferably has a Cr-enriched layer and a decarburized layer. However, since the Cr-enriched layer and the decarburized layer are formed in the surface layer of the steel material, they are not thought to affect the chemical composition and metal structure of the base material. Therefore, it is possible to improve the corrosion resistance while maintaining the mechanical properties of the steel material, such as strength and workability.

[0034] 2. Dimensions There is no particular limitation on the dimensions of the steel material according to the present invention. When the steel material according to the present invention is used as a steel plate, the plate thickness is preferably 1.0 to 20.0 mm.

[0035] 3.Chemical composition The chemical composition of the base metal of the steel material according to the present invention contains, in mass%, C: 0.050 to 0.800%, and Cr: less than 3.00%. The reasons for limiting the content of each element are as follows. In the following description, "%" for the content means "mass%".

[0036] C: 0.050 to 0.800% C is an element effective in increasing the strength of steel. C is also an element effective in ensuring hardenability. Therefore, the C content is set to 0.050% or more. On the other hand, if C is added excessively, toughness may decrease, so the C content is set to 0.800% or less. The C content is preferably set to 0.100% or more, 0.200% or more, or 0.300% or more. Furthermore, the C content is preferably set to 0.700% or less, 0.600% or less, or 0.500% or less.

[0037] Cr: Less than 3.00% Cr is an element that contributes to improving corrosion resistance and strength. Cr also improves hardenability. However, excessive Cr content can deteriorate weldability and / or hot workability. For this reason, the Cr content is set to less than 3.00%. Furthermore, the Cr content is preferably set to 2.50% or less, 2.00% or less, or 1.50% or less. While the Cr content may be 0%, to fully obtain the above-mentioned effects, the Cr content is preferably set to 0.001% or more. The Cr content is preferably set to 0.005% or more, 0.010% or more, or 0.100% or more.

[0038] In the present invention, even if the Cr content in the base metal part of the steel material is less than 3.00%, excellent corrosion resistance can be obtained by having a Cr-enriched layer in the surface layer of the steel material with a Cr content of 3.0 mass% or more and setting the maximum Cr content in the Cr-enriched layer to 10.0 to 20.0 mass%.

[0039] Furthermore, it is preferable that the base metal portion of the above steel material contains the elements shown below in addition to C and Cr. The reasons for limiting each element are as follows. In the following description, "%" for the content means "% by mass."

[0040] Si: 0.01 to 3.00% Si is an element effective in ensuring hardenability and ductility. Therefore, the Si content is preferably 0.01% or more. On the other hand, excessive Si content may cause embrittlement in the center portion of the steel sheet and deteriorate ductility. Therefore, the Si content is preferably 3.00% or less. From the viewpoint of ensuring ductility, the Si content is more preferably 0.10% or more, 0.30% or more, or 0.50% or more. Si is also an element effective in suppressing coarsening of iron-based carbides in the center portion of the steel sheet and improving strength and formability. Furthermore, Si is an element that contributes to increasing the strength of steel materials through solid solution strengthening. From these viewpoints, the Si content is more preferably 0.70% or more, or 0.90% or more. On the other hand, the Si content is more preferably 2.50% or less, or 2.00% or less.

[0041] Mn: 0.01 to 10.00% Mn is an element effective in increasing the strength of steel. Mn is also an element effective in ensuring hardenability. Therefore, the Mn content is preferably 0.01% or more. On the other hand, excessive Mn content may cause large fluctuations in the hardness distribution in the steel surface layer due to Mn segregation. Therefore, the Mn content is preferably 10.00% or less. The Mn content is more preferably 0.10% or more, 1.00% or more, or 1.50% or more. Furthermore, the Mn content is more preferably 8.00% or less, 6.00% or less, or 5.00% or less.

[0042] Al: 0.001 to 0.500% Al is an element that acts as a deoxidizer. Therefore, the Al content is preferably 0.001% or more. On the other hand, excessive Al content may cause the formation of coarse oxides, which may deteriorate properties such as workability. Therefore, the Al content is preferably 0.500% or less. The Al content is more preferably 0.005% or more, 0.010% or more, or 0.050% or more. Furthermore, the Al content is more preferably 0.400% or less, 0.300% or less, or 0.200% or less.

[0043] P:0.100% or less P tends to segregate in the center of the steel thickness, and excessive P content can embrittle welds. Therefore, the P content is preferably 0.100% or less. The P content is more preferably 0.080% or less, 0.060% or less, or 0.050% or less. The lower limit of the P content is not particularly limited and may be 0%, but from the perspective of manufacturing costs, the P content is more preferably more than 0%, 0.001% or more, or 0.005% or more.

[0044] S: 0.0500% or less S is an element that gets mixed in during the manufacturing process and forms inclusions. Excessive S content reduces toughness. There is a risk of deterioration of properties such as toughness. Therefore, the S content is preferably 0.0500% or less. The S content is more preferably 0.0300% or less, 0.0100% or less, or 0.0050% or less. Note that the lower limit of the S content is not particularly limited and may be 0%, but from the viewpoint of production costs, the S content is more preferably more than 0%, 0.0001% or more, or 0.0005% or more.

[0045] N: 0.0100% or less If N is contained in excess, it may form coarse nitrides and deteriorate bendability. Therefore, the N content is preferably 0.0100% or less. Furthermore, since N may cause blowholes during welding, a lower N content is preferable. From this perspective, the N content is more preferably 0.0080% or less, 0.0060% or less, or 0.0030% or less. Note that the lower limit of the N content is not particularly limited and may be 0%, but from the perspective of manufacturing costs, the N content is more preferably more than 0%, 0.0005% or more, or 0.0010% or more.

[0046] O: 0.0200% or less O is an element that promotes the formation of oxides in steel and may deteriorate bendability. Therefore, the O content is preferably 0.0200% or less. The O content is more preferably 0.0170% or less. Note that the lower limit of the O content is not particularly limited and may be 0%, but from the viewpoint of production costs, the O content is more preferably more than 0%, 0.0001% or more, or 0.0005% or more.

[0047] In the above chemical composition, the balance is Fe and impurities. Here, "impurities" refer to components that are mixed in during industrial steel production due to various factors in raw materials such as ore and scrap, and in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention.

[0048] In addition to the above elements, the chemical composition of the base metal portion of the steel material according to the present invention may further contain one or more elements selected from the following Group A (Mo: 1.0% or less, B: 0.0100% or less, Ti: 0.500% or less, Nb: 0.500% or less, V: 0.500% or less, Cu: 0.500% or less, Ni: 0.500% or less, W: 0.100% or less, Ta: 0.100% or less, and Co: 0.500% or less) in the ranges shown below in order to improve the mechanical properties of the steel material. Note that these elements are not necessarily essential in the base metal portion of the steel material, and therefore the lower limit of their content is 0%. The reasons for limiting each element will be explained below.

[0049] Mo: 1.0% or less Mo is an element that contributes to improving strength. Mo also improves hardenability, so it may be added as needed. However, excessive Mo content can deteriorate pickling properties, weldability, and / or hot workability. Therefore, the Mo content is preferably 1.0% or less. The Mo content is more preferably 0.80% or less, 0.60% or less, or 0.40% or less. To achieve the above effects, the Mo content is more preferably 0.001% or more, 0.005% or more, 0.010% or more, or 0.10% or more.

[0050] B: 0.0100% or less B is an element that contributes to improving strength. Furthermore, B also improves hardenability, so it may be added as needed. However, excessive B content can deteriorate pickling properties, weldability, and / or hot workability. Therefore, the B content is preferably 0.0100% or less. It is more preferable that the B content be 0.0080% or less, 0.0060% or less, or 0.0050% or less. To achieve the above effects, the B content is more preferably 0.0001% or more, 0.0010% or more, 0.0020% or more, or 0.0030% or more.

[0051] Ti: 0.500% or less Nb: 0.500% or less V:0.500% or less Ti, Nb, and V improve the strength of steel by forming carbides. Furthermore, Ti, Nb, and V contribute to grain refinement through the pinning effect, and their presence in solid solution reduces the diffusion rate of Fe. Therefore, Ti, Nb, and V may be added as needed. On the other hand, excessive Ti, Nb, and V may cause carbide coarsening, deteriorating properties such as formability. Therefore, the Ti, Nb, and V contents are preferably 0.500% or less. More preferably, the Ti, Nb, and V contents are 0.400% or less, 0.300% or less, or 0.200% or less, respectively. To achieve the above effects, the Ti, Nb, and V contents are preferably 0.001% or more, 0.005% or more, 0.010% or more, or 0.100% or more, respectively.

[0052] Cu:0.500% or less Ni: 0.500% or less Cu and Ni are elements that contribute to improving strength, so they may be added as needed. However, excessive Cu and Ni content may deteriorate pickling properties, weldability, and / or hot workability. Therefore, the Cu and Ni contents are preferably 0.500% or less, respectively. The Cu and Ni contents are more preferably 0.400% or less, 0.300% or less, or 0.200% or less, respectively. To achieve the above effects, the Cu and Ni contents are more preferably 0.001% or more, 0.005% or more, 0.010% or more, or 0.100% or more, respectively.

[0053] W:0.100% or less W is an element effective in improving strength, so it may be added as needed. On the other hand, excessive W content causes the precipitation of many fine W carbides, which increases the strength of the steel material and reduces its ductility, thereby reducing its bendability. Therefore, the W content is preferably 0.100% or less. The W content is more preferably 0.080% or less. To achieve the above effects, the W content is more preferably 0.001% or more, or 0.002% or more.

[0054] Ta:0.100% or less Ta is an element effective in improving strength, so it may be added as needed. On the other hand, excessive Ta content causes the precipitation of many fine Ta carbides, which increases the strength of the steel material and reduces its ductility, thereby reducing its bendability. Therefore, the Ta content is preferably 0.100% or less. The Ta content is more preferably 0.080% or less. To achieve the above effects, the Ta content is more preferably 0.001% or 0.002% or more.

[0055] Co:0.500% or less Co is an element that contributes to improving strength, so it may be added as needed. On the other hand, excessive Co content reduces the ductility of the steel. Therefore, the Co content is preferably 0.500% or less. The Co content is more preferably 0.400% or less. If the above effects are to be achieved, the Co content is more preferably 0.001% or more.

[0056] In addition to the above elements, the chemical composition of the base metal of the steel material according to the present invention may further contain one or more elements selected from the following Group B (Sn: 0.050% or less, Sb: 0.050% or less, and As: 0.050% or less) in the ranges shown below in order to improve the surface properties of the steel material. Note that these elements are not necessarily essential in the base metal of the steel material, and therefore the lower limit of their content is 0%. The reasons for limiting each element will be explained below.

[0057] Sn: 0.050% or less Sb: 0.050% or less As: 0.050% or less Sn, Sb, and As are elements that inhibit easily oxidizable elements such as Mn, Si, and / or Al in the steel material from diffusing to the surface and forming oxides, thereby contributing to improving surface properties. Therefore, they may be added as needed. However, even if Sn, Sb, and As are added in excess, the above effects are saturated. Therefore, the Sn, Sb, and As contents are preferably 0.050% or less, respectively. The Sn, Sb, and As contents are more preferably 0.035% or less, respectively. To achieve the above effects, the Sn, Sb, and As contents are more preferably 0.005% or more, or 0.010% or more, respectively.

[0058] In addition to the above elements, the chemical composition of the base metal of the steel material according to the present invention may further contain one or more elements selected from the following C group (Mg: 0.0500% or less, Ca: 0.050% or less, Zr: 0.050% or less, and REM: 0.150% or less) in the ranges shown below, for the purpose of deoxidation and inclusion control. Note that these elements are not necessarily essential in the base metal of the steel material, and therefore the lower limit of their content is 0%. The reasons for limiting each element will be explained below.

[0059] Mg: 0.0500% or less Mg is an element that can control the morphology of sulfides with trace addition, so it may be added as needed. On the other hand, excessive Mg content causes a decrease in bendability due to the formation of coarse inclusions. Therefore, the Mg content is preferably 0.0500% or less. The Mg content is more preferably 0.0400% or less. To achieve the above effects, the Mg content is more preferably 0.0001% or more, or 0.0005% or more.

[0060] Ca:0.050% or less Like Mg, Ca is an element that can control the morphology of sulfides with trace addition, so it may be added as needed. On the other hand, excessive Ca content generates coarse Ca oxides, which leads to a decrease in bendability. Therefore, the Ca content is preferably 0.050% or less. The Ca content is more preferably 0.040% or less. To achieve the above effects, the Ca content is more preferably 0.001% or more.

[0061] Zr: 0.050% or less Zr, like Mg and Ca, is an element that can control the morphology of sulfides with trace addition, so it may be added as needed. On the other hand, excessive Zr content generates coarse Zr oxides, reducing bendability. Therefore, the Zr content is preferably 0.050% or less. The Zr content is more preferably 0.040% or less. To achieve the above effects, the Zr content is more preferably 0.001% or more.

[0062] REM: 0.150% or less REM (rare earth element) is an element that can control the morphology of sulfides with a small amount, so it may be added as needed. On the other hand, if REM is added in excess, coarse oxides containing REM are generated, resulting in reduced bendability. Therefore, the REM content is preferably 0.150% or less. The REM content is more preferably 0.120% or less. If the above effects are to be achieved, the REM content is more preferably 0.001% or more.

[0063] Here, REM is a general term for 17 elements in total: Sc, Y, and lanthanides, and the REM content refers to the total amount of these elements. Note that lanthanides are industrially added in the form of misch metals.

[0064] The chemical composition of the base metal of the steel material in the present invention is the average chemical composition of the portion excluding the Cr-enriched layer from the surface of the steel material. The chemical composition of the base metal of the steel material can be measured by a well-known chemical analysis method after removing the steel material from the surface to a depth of 100 μm. For example, the chemical composition of the steel material may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry), a carbon-sulfur analyzer, or an oxygen-nitrogen analyzer. In the case of a steel plate, the chemical composition of the steel plate is measured after removing the steel plate from the surface to a depth of 100 μm on both the front and back sides. Furthermore, unless otherwise specified, the chemical composition of the steel material in the present invention refers to the chemical composition of the base metal of the steel material.

[0065] 4. Manufacturing method A manufacturing method for forming a Cr-enriched layer on a steel material according to the present invention will be described. A steel material having the above-described chemical composition is coated with a chemical agent containing metallic Cr powder and SiO2 and then heat-treated to concentrate Cr in the surface layer of the steel material. Here, the steel material can be an ingot, a steel billet, or any intermediate material such as a hot-rolled material or a cold-rolled material after processing. The Cr-enriched layer can remain in the final product even after subsequent hot rolling, cold rolling after cooling to room temperature, forming into a product or a component constituting the product, and annealing, as necessary.

[0066] The processing conditions will be explained in detail.

[0067] It is preferable to heat the steel material, with the agent containing metal Cr powder and SiO2 applied to the surface, at 1100 to 1350°C for 1 hour or more in an atmosphere with an O2 concentration of 2 to 30% by volume.

[0068] <Chemical agent containing metal Cr powder and SiO2> The agent containing metal Cr powder and SiO2 is an SiO2-based oxidation inhibitor containing metal Cr powder at a volume percentage of 50% or more. The application amount of the agent is 0.1 mg / cm2. 2 It is preferable that the above is set.

[0069] If the steel is heated without applying the agent having the above composition, a Cr-enriched layer will not form. Furthermore, because no film of Cr2O3 or SiO2 is formed and the oxidation of Fe progresses rapidly, decarburization will not occur in the surface layer of the steel. As a result, the corrosion resistance of the steel cannot be improved.

[0070] <Atmosphere> The O2 concentration in the atmosphere during heating is preferably 2 to 30% by volume. By heating the steel material coated with the above-mentioned agent in an atmosphere with an O2 concentration of 2 to 30% by volume, a Cr-enriched layer is formed due to Cr diffusion from the agent to the surface of the steel material, and C present in the surface layer of the steel material is oxidized and released as CO2 into the atmosphere, forming a decarburized layer.

[0071] On the other hand, by applying the above-mentioned agent and heating, a portion of the metallic Cr contained in the agent is oxidized to form Cr2O3. Furthermore, the SiO2 in the agent also forms a coating, which blocks the penetration of O2 into the steel material. This prevents the oxidation of Fe in the steel material, thereby suppressing the formation of scale. As a result, the steel material according to the present invention also has excellent surface properties. The O2 concentration in the atmosphere is preferably 25% by volume or less, and more preferably 20% by volume or less.

[0072] <Heating temperature> The heating temperature is preferably 1100 to 1350°C. By heating in this temperature range, the Cr in the applied agent can be diffused into the surface layer of the steel material, allowing a sufficient Cr-enriched layer to be formed. The heating temperature is more preferably 1130°C or higher. Furthermore, the heating temperature is more preferably 1330°C or lower, and even more preferably 1300°C or lower.

[0073] <Retention time> The holding time during heating is preferably 1 hour or more. By holding for 1 hour or more, a sufficient Cr-enriched layer can be formed. It also improves hot workability and eliminates segregation. On the other hand, if heating is performed for more than 10 hours, the effect saturates and costs only increase, so from the viewpoint of manufacturability, the holding time is preferably 10 hours or less.

[0074] By carrying out the heat treatment described above, it is possible to suppress the formation of scale. However, descaling, shot blasting, and pickling may be carried out as necessary, and an example of such treatment will be described below.

[0075] <Descaling conditions> First, shot blasting is carried out to crush the scale. The shot particles used to crush the scale are preferably large in size, with high kinetic energy and little loss due to air resistance, and a large number of shot particles is more efficient. It is desirable for the shot particles to be of a material that does not adhere to the steel, but steel balls can be used since pickling will be carried out afterwards. It is also desirable to spray with a high pressure within a range that does not cause adhesion or indentation of the steel surface.

[0076] Next, an aqueous solution containing 1 to 10% hydrofluoric acid and 2 to 20% nitric acid is sprayed from a nozzle to scatter and remove the scale. It is most desirable that the aqueous solution containing hydrofluoric acid and nitric acid corrodes and removes only the scale without corroding the Cr-enriched layer, and a low concentration is preferable. The concentration of hydrofluoric acid is preferably 8% or less, more preferably 6% or less. The concentration of nitric acid is preferably 15% or less, more preferably 12% or less. The lower limit of the concentration of each acid is preferably 1% or more for hydrofluoric acid and 2% or more for nitric acid in order to corrode and remove the scale.

[0077] The manufacturing method of the present invention is not limited to the above-described heat treatment of an ingot, but can also be applied to any steel material having the above-described chemical composition for the purpose of forming a Cr-enriched layer. That is, (1) the descaling is performed after the heat treatment of an ingot or billet, (2) the descaling is performed after the heat treatment of an ingot or billet and hot rolling, (3) descaling is performed by a conventional method on a general hot-rolled material, followed by the heat treatment, and (4) the descaling is performed after the heat treatment of a general cold-rolled material. The presence of a Cr-enriched layer in the product exhibits excellent effects.

[0078] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples. [Example]

[0079] Steel having the chemical composition shown in Table 1 was melted and formed into billets, and then a chemical agent having the composition shown in Table 2 was added at 0.1 mg / cm 2 After coating and heating under various conditions, the test materials were hot-rolled, cold-rolled, and annealed. The Cr in the chemical composition was metallic Cr powder. The heating atmosphere was a mixed gas atmosphere containing O2 at the concentrations shown in Table 2, with the remainder being N2. Hot rolling was performed at a width of 100 mm and a cross-sectional area reduction of 95%.

[0080] After hot rolling, the specimens were shot blasted to remove the scale formed on the surface. They were then cold rolled to a cross-sectional area reduction of 75%. After cold rolling, the specimens were annealed by holding them at a temperature of 600°C or higher for a specified time (1 to 30 minutes), and then the scale was removed by shot blasting and pickling. The pickling solution contained 6% hydrofluoric acid and 12% nitric acid. The test specimens after scale removal were then subjected to structural observation and evaluation tests. Comparison of cross-sectional observations before and after scale removal confirmed that only the scale had been removed from all test specimens.

[0081] [Table 1]

[0082]

Table 2

[0083] <Measurement of Cr Content in Cr Enrichment Layer> First, test pieces for microstructure observation were cut out from the above test materials. Then, a cross-section perpendicular to the rolling direction was used as the observation surface, and compositional distribution observation and quantitative analysis were carried out by EDS. Specifically, a cross-section perpendicular to the rolling direction was used as the measurement surface, and in the observation field at a magnification of 50 times, the reference line was determined by the method described above. And in a state where the reference line passes through the same height at the right end and the left end within the field of view, that is, in a state where the reference line is parallel to the horizontal of the field of view, the observation magnification was changed to 500 times. After that, the observation start line was determined by the method described above, and the Cr content was measured by holding each point for 1 second at an interval (pitch) of 5 μm in a region of 100 μm × 100 μm in the depth direction of the sample with the observation start line as one side.

[0084] The maximum depth at which the average value of the Cr content at 21 measurement points where the positions in the depth direction from the observation start line coincide is 3.0% or more was defined as the lower end of the Cr enrichment layer, and the distance from the observation start line was defined as the thickness d of the Cr enrichment layer. Furthermore, the average value of the Cr content at 21 measurement points where the positions in the depth direction from the observation start line coincide was taken as the average value of the Cr content at each depth position.

[0085] After that, when the thickness of the Cr enrichment layer was d, the average values of the Cr content at 21 points measured at a pitch of 5 μm again at positions of d / 2 and d / 10 in the depth direction from the observation start line were taken as the average values of the Cr content at each depth position. <00,00370> <About Decarburized Layer> The thickness of the decarburized layer and the C content at position d were measured using an EPMA. Specifically, similar to the measurement of the Cr content in the Cr-enriched layer described above, line analysis was performed at 21 points, parallel to the observation start line, at intervals (pitch) of 5 μm, with each point held for 1 second. The average value of the C content at the 21 measurement points, which were located at the same depth direction from the observation start line, was adopted as the C content at that depth position, and the maximum depth position where the C content was 10% or more lower than the C content of the steel was defined as the decarburized layer thickness. The C content at position d was also determined.

[0087] <Evaluation of corrosion resistance> Next, a corrosion resistance evaluation test was conducted. The corrosion resistance evaluation test was conducted as follows: First, a test piece measuring 150 mm in length, 70 mm in width, and 1.5 mm in thickness was cut out from the above test material so that the thickness direction coincided with the thickness direction of the test piece, and only the end where there was no Cr-enriched layer and the base material of the steel was exposed was protected with adhesive tape.

[0088] Then, a cyclic corrosion test was conducted in accordance with JASO M 609. The cycle conditions were salt spray (5 mass% sodium chloride aqueous solution, 35°C, 2 hours), dry (60°C, 25% RH, 4 hours), and wet (50°C, 95% RH, 2 hours). The number of cycles was 15. After the corrosion test, the appearance was photographed, and samples with a rust area ratio of less than 20% were rated as having good corrosion resistance (Good), and samples with a rust area ratio of 20% or more were rated as poor.

[0089] The results are shown in Table 2.

[0090] As is clear from the results shown in Table 2, Test Nos. 1 to 11 and 16 to 25, which satisfy the requirements of the present invention, exhibited excellent corrosion resistance. In contrast, Test Nos. 12 to 15 are comparative examples that do not comply with the requirements of the present invention. Specifically, in Test No. 12, no chemical agent was applied, and in Test Nos. 13 and 14, the heating conditions were inappropriate, so a sufficient Cr-enriched layer was not formed, resulting in poor corrosion resistance. In Test No. 15, the C content was high, so the C in the Cr-enriched layer precipitated as chromium carbide, resulting in poor corrosion resistance. [Industrial Applicability]

[0091] According to the present invention, steel materials having excellent corrosion resistance can be obtained industrially and stably.

Claims

1. The steel material has a surface layer having a Cr-enriched layer with a Cr content of 3.0 mass% or more, The maximum value of the Cr content in the Cr-enriched layer is 10.0 to 20.0 mass%, When the portion of the steel material excluding the Cr-enriched layer is used as the base material, The chemical composition of the base material portion is, in mass%, C: 0.050 to 0.800%, and Cr: less than 3.00%; Steel material.

2. When the thickness of the Cr-enriched layer is d, the average value of the Cr content at a position of d / 2 in the depth direction from the surface of the steel material is more than 3.0 mass%, an average value of the Cr content at a position d / 10 in a depth direction from the surface of the steel material is higher than an average value of the Cr content at a position d / 2 in a depth direction from the surface of the steel material; The steel material according to claim 1.

3. The chemical composition of the base material portion is, in mass%, C: 0.050-0.800%, Si: 0.01-3.00%, Mn: 0.01-10.00%, Al: 0.001-0.500%, P: 0.100% or less, S: 0.0500% or less, N: 0.0100% or less, O: 0.0200% or less, Cr: less than 3.00%, and The balance is Fe and impurities. The steel material according to claim 1 or 2.

4. The chemical composition of the base material portion is, in mass%, C: 0.050-0.800%, Si: 0.01-3.00%, Mn: 0.01-10.00%, Al: 0.001-0.500%, P: 0.100% or less, S: 0.0500% or less, N: 0.0100% or less, O: 0.0200% or less, and Cr: less than 3.00% and further containing one or more selected from the group consisting of the following Group A, Group B, and Group C: The balance is Fe and impurities. The steel material according to claim 1 or 2. [Group A] One or more selected from the group consisting of Mo: 1.0% or less, B: 0.0100% or less, Ti: 0.500% or less, Nb: 0.500% or less, V: 0.500% or less, Cu: 0.500% or less, Ni: 0.500% or less, W: 0.100% or less, Ta: 0.100% or less, and Co: 0.500% or less [Group B] One or more elements selected from the group consisting of Sn: 0.050% or less, Sb: 0.050% or less, and As: 0.050% or less [Group C] One or more selected from the group consisting of Mg: 0.0500% or less, Ca: 0.050% or less, Zr: 0.050% or less, and REM: 0.150% or less

5. The steel material according to claim 4 , wherein the base metal portion has a chemical composition containing one or more elements selected from Group A.

6. The steel material according to claim 4 , wherein the base metal portion has a chemical composition containing one or more elements selected from the B group.

7. The steel material according to claim 4, wherein the base metal portion has a chemical composition containing one or more elements selected from the C group.

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