Hot-rolled steel material and production method therefor

Abstract

Provided is a hot-rolled steel material in which the thickness of a decarburized layer is sufficiently inhibited and that has excellent cold workability. A hot-rolled steel material according to the present invention is characterized by having a base iron having a predetermined component composition, and a decarburized layer formed on the surface of the base iron, and is characterized in that: in the decarburized layer, an enriched region in which at least one of Cu and Ni is enriched exists; the coverage of the enriched region with respect to the surface of the decarburized layer is at least 50%; the greatest depth of the enriched region in the decarburized layer is 1-150 μm; the total decarburization depth (DM-T) of the decarburized layer as determined in accordance with JIS G 0558 is at most 0.80 mm; the total area ratio of ferrite and pearlite in the base iron is at least 90.0%; and the average Vickers hardness of the base iron is at most 250 HV.

Description

Hot-rolled steel and its manufacturing method

[0001] The present invention relates to a hot-rolled steel material and a method for producing the same.

[0002] Parts used in automobiles and other applications are made by subjecting hot-rolled steel material, which is then subjected to preliminary processing such as wire drawing, to form the part shape by hot working or cold working, and then, as necessary, to cutting or heat treatment to form the final product. In recent years, the manufacture of parts by cold working (e.g., cold forging) has become widespread, as it is advantageous in terms of manufacturing costs, such as dimensional accuracy and yield of the parts, or in terms of reducing the energy used to heat the material. Parts manufactured by cold working are either left as cold worked or subjected to heat treatment for strength adjustment (quenching and tempering heat treatment, induction hardening and tempering heat treatment, etc.) depending on the required strength, before becoming the final product.

[0003] The surface layer of the hot-rolled steel material subjected to the above-mentioned cold working has a decarburized layer formed during hot rolling. If the decarburized layer of the hot-rolled steel material is excessively thick, a soft part with a low carbon content remains near the surface layer of the steel material in a part after cold working or a part that has been subjected to cold working and induction hardening and tempering heat treatment, resulting in poor strength or fatigue properties as a part. Furthermore, if the decarburized layer of the hot-rolled steel material is excessively thick, in a part that has been subjected to cold working and quenching and tempering heat treatment, carbon diffuses from the core of the steel material to the decarburized layer during heating and holding during quenching, so the soft part described above does not remain in the surface layer of the steel material. However, since the carbon content differs between the decarburized layer and the core, the austenite reverse transformation temperatures differ. In this case, the timing of austenite reverse transformation differs between the surface layer and the core of the steel material, resulting in non-uniform reverse-transformed austenite grain size between the surface layer and the core of the steel material. Such uneven grain size induces coarsening of the crystal grains, resulting in a deterioration of the fatigue properties and toughness of the parts.

[0004] As described above, if the decarburized layer on the surface of a hot-rolled steel material is excessively thick, various properties of the final product will deteriorate, so it is necessary to prevent the decarburized layer from becoming excessively thick at the hot-rolling stage. In addition, if cracks occur during cold working, the steel material cannot be used as a final product, so hot-rolled steel material is also required to have excellent cold workability.

[0005] Patent Document 1 describes that by adding a specific amount of Te, Se, or S as a trace element and appropriately controlling the thermal history after hot rolling, it is possible to achieve both softening of the hot-rolled steel material and suppression of the decarburization reaction.

[0006] Japanese Patent Application Laid-Open No. 2004-250768

[0007] However, Patent Document 1 uses Te and Se, which are rare and toxic elements that require special care in handling, and therefore it is difficult to say that this technology is generally applicable. Furthermore, both Patent Document 1 and the previous technologies require precise control of the temperature history after hot rolling, which also poses a problem from the viewpoint of productivity. As such, no technology has been reported that achieves both suppression of the thickness of the decarburized layer and excellent cold workability.

[0008] In view of the above problems, an object of the present invention is to provide a hot-rolled steel material in which the thickness of a decarburized layer is sufficiently suppressed and which has excellent cold workability, and a method for manufacturing the same.

[0009] In order to solve the above-mentioned problems, the present inventors focused on suppressing decarburization reactions by adding Cu and Ni, which are elements that are less likely to be oxidized than Fe, and obtained the following findings. When scale is formed on the surface of a steel material due to heating during hot rolling, Fe is preferentially oxidized to form the scale. On the other hand, Cu and Ni are not oxidized and are left behind in the surface layer of the steel material, forming an enriched region in which at least one of Cu and Ni is enriched. By forming this enriched region, it is possible to effectively suppress the decarburization reaction that occurs in the surface layer of the steel material.

[0010] However, it is well known that Cu concentrated in the surface layer of a steel material penetrates into grain boundaries during hot rolling, embrittling the grain boundaries and inducing cracking during cold working. As a result of further intensive research, the present inventors have found that by optimizing the balance between the Ni content and the Cu content of a hot-rolled steel material, it is possible to suppress the depth of an enriched region in which at least one of Cu and Ni is enriched in the surface layer of the steel material within a certain range, thereby suppressing the occurrence of cracking during cold working.

[0011] In addition, the inventors have found that in order to make the enriched region of the hot-rolled steel material, in which at least one of Cu and Ni is enriched, suitable for suppressing the decarburization reaction, it is necessary to appropriately control the maximum heating temperature in the hot rolling. Furthermore, they have found that in order to effectively suppress the decarburization reaction, it is necessary to appropriately control the residence time of the steel material in the heating furnace in the hot rolling.

[0012] Furthermore, as a result of intensive investigations, the present inventors have obtained the following findings: In a hot-rolled steel material to which Sn is added in addition to Cu and Ni, Sn is simultaneously enriched in an enriched region in which at least one of Cu and Ni is enriched in the steel material surface layer. Since Sn has the effect of lowering the melting point of the enriched region, the enriched region is more likely to be generated in the steel material surface layer than in a case where Sn is not added, and the enriched region also more easily achieves the effect of suppressing a decarburization reaction.

[0013] On the other hand, if the Sn concentration in the enriched region where at least one of Cu and Ni is enriched is excessively high, the enriched region penetrates deep into the grain boundaries, embrittling the grain boundaries and causing cracks during cold working. Therefore, when manufacturing a hot-rolled steel material to which Sn is added in addition to Cu and Ni, it is necessary to limit the Sn concentration in the enriched region to a predetermined range so that the grain boundary penetration depth does not become excessively deep. Specific effective methods include adjusting the amount of Sn added depending on the amount of Cu and Ni, and shortening the residence time in the heating furnace during hot rolling depending on the amount of Sn.

[0014] That is, the gist and configuration of the present invention are as follows.

[0015] [1] A steel substrate having a composition containing, in mass%, C: 0.03 to 0.80%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.50%, Cu: 0.010 to 0.500%, Ni: 0.010 to 1.000%, and N: 0.0020 to 0.0250%, in which the ratio of the Ni amount to the Cu amount [Ni] / [Cu] is 0.10 to 3.00, with the balance being Fe and unavoidable impurities; and a decarburized layer formed on the surface of the steel substrate, wherein the decarburized layer has enriched regions in which at least one of Cu and Ni is enriched, the coverage of the enriched regions on the surface of the decarburized layer is 50% or more, the maximum depth of the enriched regions in the decarburized layer is 1 μm to 150 μm, and the decarburized layer meets JIS G a total decarburization depth (DM-T) as defined in JIS K 0558 of 0.80 mm or less, a total area ratio of ferrite and pearlite in the base steel of 90.0% or more, and an average Vickers hardness of the base steel of 250 HV or less.

[0016] [2] The hot-rolled steel material according to the above [1], wherein the composition further contains, in mass%, Sn: 0.001% or more and ([Ni] + [Cu]) / 2 or less, wherein Sn is enriched in the enriched region in addition to at least one of Cu and Ni, and the ratio of the Sn concentration to the sum of the Cu concentration and the Ni concentration in the enriched region, [Sn] / ([Cu] + [Ni]), is 0.50 or less in atomic ratio.

[0017] [3] The hot-rolled steel material according to the above [1] or [2], wherein the composition further contains, in mass%, at least one element selected from the group consisting of Cr: 0.01 to 1.50%, Mo: 0.01 to 0.50%, Al: 0.001 to 0.100%, Ti: 0.001 to 0.100%, V: 0.001 to 0.300%, Nb: 0.001 to 0.100%, and B: 0.0005 to 0.0050%.

[0018] [4] The hot-rolled steel material according to any one of the above [1] to [3], wherein the component composition further contains, in mass%, at least one element selected from the group consisting of P: 0.001 to 0.100%, S: 0.001 to 0.100%, and Sb: 0.0010 to 0.0300%.

[0019] [5] The hot-rolled steel material according to any one of the above [1] to [4], wherein the composition further contains, in mass%, at least one element selected from the group consisting of Pb: 0.01 to 0.50%, Bi: 0.001 to 0.100%, and Ca: 0.0005 to 0.1000%.

[0020] [6] A steel material having a composition containing, in mass%, C: 0.03 to 0.80%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.50%, Cu: 0.010 to 0.500%, Ni: 0.010 to 1.000%, and N: 0.0020 to 0.0250%, wherein the ratio of the amount of Ni to the amount of Cu, [Ni] / [Cu], is 0.10 to 3.00, with the balance consisting of Fe and unavoidable impurities, is heated in a heating furnace at a maximum heating temperature T of 1000°C to 1200°C, and the residence time of the steel material in the heating furnace is a time t determined by the following formula (1): １ A method for producing a hot-rolled steel material, comprising a step of hot-rolling the steel under conditions of t (min) or less to obtain a hot-rolled steel material. １ = 1150 - 0.8T - 3 ([Ni] / [Cu]) - 10 [Sn] (1) where T is the maximum heating temperature (°C), [Ni] is the amount of Ni in the steel material (mass %), [Cu] is the amount of Cu in the steel material (mass %), and [Sn] is the amount of Sn in the steel material (mass %).

[0021] [7] The method for producing a hot-rolled steel material according to the above [6], wherein the component composition further contains, in mass%, Sn: 0.001% or more ([Ni] + [Cu]) / 2 or less.

[0022] [8] The method for producing a hot-rolled steel material according to the above [6] or [7], wherein the composition further contains, in mass%, at least one element selected from the group consisting of Cr: 0.01 to 1.50%, Mo: 0.01 to 0.50%, Al: 0.001 to 0.100%, Ti: 0.001 to 0.100%, V: 0.001 to 0.300%, Nb: 0.001 to 0.100%, and B: 0.0005 to 0.0050%.

[0023] [9] The method for producing a hot-rolled steel material according to any one of the above [6] to [8], wherein the component composition further contains, in mass%, at least one element selected from the group consisting of P: 0.001 to 0.100%, S: 0.001 to 0.100%, and Sb: 0.0010 to 0.0300%.

[0024]

[10] The method for producing a hot-rolled steel material according to any one of the above [6] to [9], wherein the composition further contains, in mass%, at least one element selected from the group consisting of Pb: 0.01 to 0.50%, Bi: 0.001 to 0.100%, and Ca: 0.0005 to 0.1000%.

[0025] According to the present invention, it is possible to provide a hot-rolled steel material in which the thickness of a decarburized layer is sufficiently suppressed and which has excellent cold workability, and a method for producing the same.

[0026] 1 is a schematic diagram showing a cross section of a hot-rolled steel material according to an embodiment of the present invention.

[0027] Hereinafter, embodiments of the hot-rolled steel material and the manufacturing method thereof according to the present invention will be described. Note that the embodiment described below is an example of the present invention, and the configuration of the present invention is not limited to this specific example.

[0028] (Hot-rolled steel) Figure 1 shows a cross section of a hot-rolled steel according to one embodiment of the present invention. The hot-rolled steel 100 has a base steel 10 and a decarburized layer 20 formed on the surface of the base steel 10. Scale 30 may also be formed on the decarburized layer 20. The decarburized layer 20 is characterized by having enriched regions 22 in which at least one of Cu and Ni is enriched, a coverage of the enriched regions 22 on the surface 24 of the decarburized layer 20 of 50% or more, a maximum depth A of the enriched regions 22 in the decarburized layer 20 of 1 μm or more and 150 μm or less, a total decarburization depth B (DM-T) of the decarburized layer 20 as defined in JIS G 0558 of 0.80 mm or less, a total area fraction of ferrite and pearlite in the base steel 10 of 90.0% or more, and an average Vickers hardness of the base steel 10 of 250 HV or less.

[0029] <Base Steel> First, the base steel of a hot-rolled steel material according to one embodiment of the present invention will be described. The base steel contains, by mass%, 0.03 to 0.80% C, 0.01 to 1.00% Si, 0.01 to 1.50% Mn, 0.010 to 0.500% Cu, 0.010 to 1.000% Ni, and 0.0020 to 0.0250% N, with the ratio of the amount of Ni to the amount of Cu, [Ni] / [Cu], being 0.10 to 3.00, and the balance being Fe and unavoidable impurities. In the following description of the composition, "%" representing the content means "% by mass" unless otherwise specified. Furthermore, [Ni] and [Cu] represent the amount of Ni and the amount of Cu contained in the base steel, respectively.

[0030] [C: 0.03 to 0.80%] C is an element added to ensure the strength of hot-rolled steel. If the C content of the base steel is less than 0.03%, the necessary strength cannot be ensured. Therefore, the C content of the base steel is set to 0.03% or more, and preferably 0.05% or more. On the other hand, if the C content of the base steel exceeds 0.80%, the hardenability becomes too high, resulting in a microstructure containing hard bainite or martensite, which increases the hardness of the rolled material and reduces cold workability. Therefore, the C content of the base steel is set to 0.80% or less, and preferably 0.65% or less, and more preferably 0.50% or less.

[0031] [Si: 0.01 to 1.00%] Si is a deoxidizing element during refining and also an element that improves the strength and hardenability of hot-rolled steel. If the Si content of the base steel is less than 0.01%, the above effects cannot be obtained. Therefore, the Si content of the base steel is set to 0.01% or more. On the other hand, if the Si content of the base steel exceeds 1.00%, the hardenability becomes too high, resulting in a structure containing hard bainite or martensite, which increases the hardness of the rolled material and reduces cold workability. Therefore, the Si content of the base steel is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.

[0032] [Mn: 0.01 to 1.50%] Mn is an element that improves the strength and hardenability of hot-rolled steel. If the Mn content of the base steel is less than 0.01%, the above effects cannot be obtained. Therefore, the Mn content of the base steel is set to 0.01% or more. On the other hand, if the Mn content of the base steel exceeds 1.50%, the hardenability becomes too high, resulting in a structure containing hard bainite or martensite, which increases the hardness of the rolled material and reduces cold workability. Therefore, the Mn content of the base steel is set to 1.50% or less, preferably 1.20% or less, and more preferably 1.00% or less.

[0033] [Cu: 0.010 to 0.500%] Cu is an element that is less oxidized than Fe and that forms an enriched region by enriching in the surface layer of the steel material as scale is generated during hot rolling, thereby suppressing the decarburization reaction. If the Cu content of the base steel is less than 0.010%, an enriched region is not sufficiently formed in the surface layer of the steel material, and the decarburization suppression effect is not sufficiently achieved. Therefore, the Cu content of the base steel is set to 0.010% or more. On the other hand, if the Cu content of the base steel exceeds 0.500%, the enriched region in the surface layer of the steel material becomes excessively deep, making cracks more likely to occur during cold working. Therefore, the Cu content of the base steel is set to 0.500% or less, preferably 0.400% or less, and more preferably 0.350% or less.

[0034] [Ni: 0.010 to 1.000%] Like Cu, Ni is an element that is less oxidized than Fe. Ni is an element that concentrates in the steel surface layer as scale is generated during hot rolling, forming an enriched region and suppressing decarburization reactions. If the Ni content of the base steel is less than 0.010%, an enriched region is not sufficiently formed in the steel surface layer, and the decarburization suppression effect is not sufficiently achieved. Therefore, the Ni content of the base steel is set to 0.010% or more. On the other hand, if the Ni content of the base steel exceeds 1.000%, the enriched region in the steel surface layer becomes excessively deep, making cracks more likely to occur during cold working. Therefore, the Ni content of the base steel is set to 1.000% or less, preferably 0.800% or less, and more preferably 0.600% or less.

[0035] [N: 0.0020 to 0.0250%] N is an element that bonds with nitride-forming elements in the steel to form nitrides and acts as grain boundary pinning particles, thereby preventing grain coarsening. If the N content of the base steel is less than 0.0020%, the above effect cannot be obtained. Therefore, the N content of the base steel is set to 0.0020% or more. On the other hand, if the N content of the base steel exceeds 0.0250%, not only will blowholes form in the steel, but the solute N in the steel will undergo dynamic strain aging, making it more susceptible to cracking during cold working. Therefore, the N content of the base steel is set to 0.0250% or less, preferably 0.0200% or less, and more preferably 0.0180% or less.

[0036] [Ratio (mass ratio) of Ni to Cu, [Ni] / [Cu], is 0.10 or more and 3.00 or less] In the present invention, since the decarburization reaction is suppressed by the enriched region in which at least one of Cu and Ni is enriched, which occurs in the steel surface layer, it is not sufficient to focus on the Cu and Ni amounts independently; it is necessary to consider the balance of the contents of these elements. When the ratio (mass ratio) of Ni to Cu, [Ni] / [Cu], is less than 0.10, i.e., when the Cu amount is excessive relative to the Ni amount, the Cu enriched in the steel surface layer penetrates into the grain boundaries, making the enriched region excessively deep and making cracks more likely to occur during cold working. Therefore, [Ni] / [Cu] is set to 0.10 or more, preferably 0.15 or more, and more preferably 0.20 or more. On the other hand, when [Ni] / [Cu] exceeds 3.00, that is, when the amount of Ni is excessive relative to the amount of Cu, the enriched region in the surface layer of the steel material becomes excessively deep, and cracks tend to occur during cold working. Therefore, [Ni] / [Cu] is set to 3.00 or less, preferably 2.50 or less, and more preferably 2.00 or less.

[0037] The base steel of the hot-rolled steel material according to one embodiment of the present invention may further contain the following elements as necessary.

[0038] [Sn: 0.001% or more ([Ni] + [Cu]) / 2 or less] Sn has the effect of lowering the melting point of enriched regions where at least one of Cu and Ni is enriched. Therefore, adding Sn makes it easier for enriched regions to form in the surface layer of the steel material, making it easier to obtain the decarburization suppression effect. When Sn is added to the base steel to achieve this effect, the Sn content is set to 0.001% or more. On the other hand, if excessive Sn is added, the grain boundary penetration depth of the enriched regions increases, embrittling the grain boundaries and causing cold work cracks. Therefore, when Sn is added to the base steel, the Sn content is set to ([Ni] + [Cu]) / 2 or less, and preferably ([Ni] + [Cu]) / 3 or less.

[0039] [Cr: 0.01 to 1.50%] Cr is an element that improves the hardenability of steel. If the Cr content of the base steel is less than 0.01%, the above-mentioned effect cannot be obtained. Therefore, when Cr is added to the base steel, the Cr content should be 0.01% or more. On the other hand, if the Cr content of the base steel exceeds 1.50%, the effect of the addition will saturate, and the hardenability of the steel will become excessive, increasing its hardness and reducing its cold workability. Therefore, when Cr is added to the base steel, the Cr content should be 1.50% or less, preferably 1.30% or less, and more preferably 1.15% or less.

[0040] [Mo: 0.01 to 0.50%] Mo is an element that significantly improves the hardenability of steel with a small amount added. If the Mo content of the base steel is less than 0.01%, the above effect cannot be obtained. Therefore, when Mo is added to the base steel, the Mo content should be 0.01% or more. On the other hand, if the Mo content of the base steel exceeds 0.50%, the effect of the addition saturates, and the hardenability of the steel becomes excessive, increasing the hardness of the steel and reducing its cold workability. Therefore, when Mo is added to the base steel, the Mo content should be 0.50% or less, and preferably 0.30% or less.

[0041] [Al: 0.001 to 0.100%] Al is a deoxidizing element and also bonds with N in the steel to form nitrides, contributing to grain refinement. If the Al content of the base steel is less than 0.001%, the above-mentioned effect cannot be obtained. Therefore, when Al is added to the base steel, the Al content should be 0.001% or more. On the other hand, if the Al content of the base steel exceeds 0.100%, the amount of Al oxide in the steel increases, making the steel more susceptible to cracking during cold working and deteriorating the fatigue fracture properties of the component. Therefore, when Al is added to the base steel, the Al content should be 0.100% or less, preferably 0.080% or less, and more preferably 0.050% or less.

[0042] [Ti: 0.001 to 0.100%] Like Al, Ti is an element that bonds with N in steel to form nitrides and contributes to grain refinement. If the Ti content of the base steel is less than 0.001%, the above-mentioned effect cannot be obtained. Therefore, when Ti is added to the base steel, the Ti content should be 0.001% or more. On the other hand, if the Ti content of the base steel exceeds 0.100%, the amount of Ti-based precipitates in the steel becomes excessive, increasing the hardness of the steel and reducing its cold workability. Therefore, when Ti is added to the base steel, the Ti content should be 0.100% or less, preferably 0.080% or less, and more preferably 0.050% or less.

[0043] [V: 0.001 to 0.300%] Like Al and Ti, V is an element that bonds with N in steel to form nitrides and contributes to grain refinement. Furthermore, V contributes to increasing the strength of the steel. If the V content of the base steel is less than 0.001%, the above-mentioned effect cannot be obtained. Therefore, when V is added to the base steel, the V content should be 0.001% or more. On the other hand, if the V content of the base steel exceeds 0.300%, the amount of V-based precipitates in the steel becomes excessive, increasing the hardness of the steel, reducing cold workability, and also reducing the toughness of the steel. Therefore, when V is added to the base steel, the V content should be 0.300% or less, preferably 0.200% or less, and more preferably 0.150% or less.

[0044] [Nb: 0.001 to 0.100%] Nb is an element that bonds with carbon in steel to form carbides and contributes to grain refinement. If the Nb content in the base steel is less than 0.001%, the above-mentioned effect cannot be obtained. Therefore, when Nb is added to the base steel, the Nb content should be 0.001% or more. On the other hand, if the Nb content in the base steel exceeds 0.100%, the amount of Nb-based carbides in the steel becomes excessive, increasing the hardness of the steel and reducing its cold workability. Therefore, when Nb is added to the base steel, the Nb content should be 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less.

[0045] [B: 0.0005 to 0.0050%] B is an element that greatly improves the hardenability of steel with a small amount of addition. If the B content of the base steel is less than 0.0005%, the above effect cannot be obtained. Therefore, when B is added to the base steel, the B content should be 0.0005% or more. On the other hand, if the B content of the base steel exceeds 0.0050%, the effect of improving hardenability saturates. Therefore, when B is added to the base steel, the B content should be 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less.

[0046] [P: 0.001 to 0.100%] P is an element that increases the strength of steel. If the P content of the base steel is less than 0.001%, the above effect cannot be obtained. Therefore, when P is added to the base steel, the P content should be 0.001% or more. On the other hand, if the P content of the base steel exceeds 0.100%, P segregates at grain boundaries and reduces the toughness of the steel. Therefore, when P is added to the base steel, the P content should be 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less.

[0047] [S: 0.001 to 0.100%] S combines with Mn in steel to form MnS inclusions, which have the effect of improving the machinability of the steel. If the S content of the base steel is less than 0.001%, the above effect cannot be obtained. Therefore, when S is added to the base steel, the S content should be 0.001% or more. On the other hand, if the S content of the base steel exceeds 0.100%, the large amount of MnS inclusions acts as crack initiation sites during cold working, making cracks more likely to occur during cold working. Therefore, when S is added to the base steel, the S content should be 0.100% or less, preferably 0.070% or less, and more preferably 0.050% or less.

[0048] [Sb: 0.0010 to 0.0300%] Like Sn, Sb is an element that easily segregates in the surface layer of a steel material. Similar to concentrated regions where at least one of Cu and Ni is concentrated, Sb has the effect of suppressing decarburization reactions. If the Sb content of the base steel is less than 0.0010%, the above-described effect cannot be obtained. Therefore, when Sb is added to the base steel, the Sb content should be 0.0010% or more. On the other hand, if the Sb content of the base steel exceeds 0.0300%, the amount of Sb segregating in the surface layer becomes excessive, degrading the surface properties of the steel material. Therefore, when Sb is added to the base steel, the Sb content should be 0.0300% or less, preferably 0.0200% or less, and more preferably 0.0150% or less.

[0049] [Pb: 0.01 to 0.50%] Pb is an element that improves the machinability of steel. If the Pb content of the base steel is less than 0.01%, the above-mentioned effect cannot be obtained. Therefore, when Pb is added to the base steel, the Pb content should be 0.01% or more. On the other hand, if the Pb content of the base steel exceeds 0.50%, the effect of improving machinability saturates, and the amount of inclusions in the steel increases, resulting in a decrease in toughness. Therefore, when Pb is added to the base steel, the Pb content should be 0.50% or less, and preferably 0.35% or less.

[0050] [Bi: 0.001 to 0.100%] Like Pb, Bi is an element that improves the machinability of steel. If the Bi content in the base steel is less than 0.001%, the above-mentioned effect cannot be obtained. Therefore, when Bi is added to the base steel, the Bi content should be 0.001% or more. On the other hand, if the Bi content in the base steel exceeds 0.100%, the effect of improving machinability saturates, and the amount of inclusions in the steel increases, resulting in a decrease in toughness. Therefore, when Bi is added to the base steel, the Bi content should be 0.100% or less, and preferably 0.050% or less.

[0051] [Ca: 0.0005 to 0.1000%] Ca is an element that dissolves in sulfides in steel to spheroidize the sulfides, thereby suppressing deterioration of the cold workability and toughness of the steel. If the Ca content in the base steel is less than 0.0005%, the above effect cannot be obtained. Therefore, when Ca is added to the base steel, the Ca content should be 0.0005% or more. On the other hand, if the Ca content in the base steel exceeds 0.1000%, the amount of Ca-based inclusions in the steel increases, adversely affecting the toughness and fatigue properties of the steel. Therefore, when Ca is added to the base steel, the Ca content should be 0.1000% or less, preferably 0.0500% or less, and more preferably 0.0300% or less.

[0052] [Total area ratio of ferrite and pearlite is 90.0% or more] To ensure cold workability, the structure of the base steel needs to be a mixed structure of relatively soft ferrite and pearlite. Therefore, the total area ratio of ferrite and pearlite in the base steel is 90.0% or more, and preferably 95.0% or more. On the other hand, there is no particular upper limit for the total area ratio of ferrite and pearlite in the base steel, and the total area ratio may be 100.0%.

[0053] The total area ratio of ferrite and pearlite in the present invention can be determined by the following procedure. A steel material is cut to prepare three samples including a steel material surface layer, and a structural observation is performed on each cross section perpendicular to the rolling direction. The microstructure of a region in the surface layer structure of the sample, excluding the decarburized layer described below, is observed using an optical microscope at an observation magnification of 100 times. The area of ​​one visual field is set to 600 μm × 800 μm, and the total area ratio of ferrite and pearlite in the observed area of ​​each visual field is calculated for five randomly selected visual fields. The average value of the total area ratios obtained in each visual field of each sample is calculated, and this is used as the total area ratio of ferrite and pearlite for that steel material. ImageJ, an image analysis software, can be used to calculate the area ratio.

[0054] [Average Vickers hardness of 250 HV or less] If the average Vickers hardness of the base steel exceeds 250 HV, cold workability decreases. Therefore, the average Vickers hardness of the base steel is set to 250 HV or less, preferably 230 HV or less, and more preferably 220 HV or less. On the other hand, from a practical standpoint, the average Vickers hardness of the base steel is preferably 80 HV or more.

[0055] The average Vickers hardness in the present invention can be determined by the following procedure. Three samples are prepared by cutting a steel material, and hardness is measured on a cross section perpendicular to the rolling direction of the sample using a Vickers hardness tester. When the sample is a round steel material, the measurement points are five in total: one point at a position corresponding to 1 / 2 of the diameter of the round steel material from the surface toward the center of the steel material (hereinafter referred to as the "D / 2 position"), and four points at positions corresponding to 1 / 4 of the diameter of the round steel material from the surface toward the center of the steel material (hereinafter referred to as the "D / 4 positions"). Furthermore, when the sample is a plate, the measurement points are a total of five points: one point at a position corresponding to 1 / 2 of the plate width from one end toward the center and corresponding to 1 / 2 of the plate thickness in the depth direction from the steel surface (hereinafter referred to as the "W / 2-t / 2 position"), and four points at positions corresponding to 1 / 4 of the plate width from one end toward the center and corresponding to 1 / 4 of the plate thickness in the depth direction from the steel surface (hereinafter referred to as the "W / 4-t / 4 positions"). The average value of the hardness obtained at each measurement point on each sample is calculated and used as the average Vickers hardness of the steel. Note that hardness measurements can be performed under a load of 10 kgf.

[0056] <Decarburized Layer> Next, the decarburized layer of the hot-rolled steel material will be described.

[0057] [Decarburized Layer Total Decarburization Depth (DM-T) Defined in JIS G 0558 is 0.80 mm or Less] An excessively thick decarburized layer leads to a decrease in the fatigue properties and toughness of the component, so it is necessary to prevent the decarburized layer from becoming excessively thick. In the present invention, the thickness of the decarburized layer is evaluated using the total decarburization depth (DM-T) defined in JIS G 0558 (hereinafter simply referred to as "total decarburization depth"). If the total decarburization depth of the decarburized layer exceeds 0.80 mm, the deterioration of properties becomes significant. Therefore, the total decarburization depth of the decarburized layer is set to 0.80 mm or less, preferably 0.50 mm or less, and more preferably 0.30 mm or less. Meanwhile, the lower limit of the total decarburization depth of the decarburized layer is not particularly limited, and the total decarburization depth of the decarburized layer may be 0.00 mm.

[0058] The total decarburization depth of the decarburized layer in the present invention can be determined by the following procedure. A steel material is cut to prepare three samples including the steel material surface layer, and a microstructure observation is performed on the cross section perpendicular to the rolling direction of the sample. The surface layer structure of the sample is observed using an optical microscope at a magnification of 100x, with the area of ​​one field of view being 600 μm × 800 μm. In three randomly selected fields of view, the total decarburization depth of the surface layer of the sample is measured using the method described in JIS G 0558, and the total decarburization depth at the deepest position of the decarburized layer is defined as the total decarburization depth in that field of view. The average value of the total decarburization depths obtained in each field of view of each sample is calculated, and this is defined as the total decarburization depth of the decarburized layer of that steel material.

[0059] [The decarburized layer has an enriched region where at least one of Cu and Ni is enriched] The decarburized layer of the hot-rolled steel according to one embodiment of the present invention has an enriched region where at least one of Cu and Ni is enriched. Furthermore, the enriched region preferably has Sn enriched in addition to at least one of Cu and Ni. In the present invention, the enriched region is defined as a region in the surface layer of the hot-rolled steel where the concentration of at least one of Cu and Ni is three times or more the reference concentration. The reference concentration refers to the concentrations of Cu and Ni at the D / 4 position of the hot-rolled steel when the hot-rolled steel is a round steel, or at the W / 4-t / 4 position of the hot-rolled steel when the hot-rolled steel is a plate steel.

[0060] [Coverage of enriched regions on the surface of the decarburized layer is 50% or more] When enriched regions in which at least one of Cu and Ni is enriched are formed on the surface of the hot-rolled steel material, this contributes to suppressing the decarburization reaction. If the coverage of the enriched regions on the surface of the decarburized layer is less than 50%, the decarburization suppression effect becomes insufficient. Therefore, the coverage of the enriched regions on the surface of the decarburized layer is set to 50% or more, preferably 60% or more, and more preferably 70% or more. On the other hand, the upper limit of the coverage is not particularly limited, and the coverage of the enriched regions may be 100%.

[0061] The coverage of the enriched region on the surface of the decarburized layer can be determined by the following procedure. First, a steel material is cut to prepare three samples including the steel surface layer, and the reference concentrations of Cu and Ni are measured for each sample. In order to eliminate the influence of the enriched region and central segregation in the steel surface layer, three randomly selected points at the above-mentioned positions (the D / 4 position of the hot-rolled steel material for round steel material, and the W / 4-t / 4 position of the hot-rolled steel material for plate steel) are analyzed using an EPMA (electron probe microanalyzer), and the average values ​​of the Cu and Ni concentrations at these three points are taken as the reference concentrations of Cu and Ni, respectively. The EPMA measurement conditions can be a magnification of 100x and an area per field of view of 600 μm × 800 μm.

[0062] Next, quantitative analysis (mapping) is performed using EPMA on three randomly selected fields on the steel surface of each sample to determine the concentrations of Cu and Ni. The EPMA measurement conditions can be a magnification of 100x and an area of ​​600 μm × 800 μm per field. Regions where at least one of Cu and Ni is present at a concentration three times or more the reference concentrations of Cu and Ni determined above are defined as enriched regions, and the coverage of the enriched regions in each field is determined. The average of the coverages obtained in each field of each sample is calculated and used as the coverage of the enriched regions of that steel.

[0063] [Maximum depth of concentrated region in decarburized layer is 1 μm or more and 150 μm or less] If the maximum depth of the concentrated region in the decarburized layer is less than 1 μm, the effect of suppressing the decarburization reaction cannot be obtained. Therefore, the maximum depth of the concentrated region in the decarburized layer is set to 1 μm or more. On the other hand, if the maximum depth of the concentrated region in the decarburized layer exceeds 150 μm, this is preferable from the viewpoint of suppressing the decarburization reaction, but the concentrated region is too deep and is therefore prone to cracking during cold working. Therefore, the maximum depth of the concentrated region in the decarburized layer is set to 150 μm or less, preferably 120 μm or less, and more preferably 100 μm or less.

[0064] Here, the maximum depth of the enriched region in the decarburized layer can be determined by the following method. A steel material is cut to prepare three samples including a steel surface layer, and the enriched region is observed in three fields of view (each field having an area of ​​approximately 600 μm × 800 μm) in a cross section perpendicular to the rolling direction of the sample using the method described above, and the maximum depth of each enriched region is measured. The average value of the maximum depths obtained in each field of view of each sample is calculated, and this is defined as the maximum depth of the enriched region in the decarburized layer of that steel material. Furthermore, the minimum depth of the enriched region can also be determined by observing in the same way.

[0065] Furthermore, when the hot-rolled steel material contains Sn, Sn may be further concentrated in the enriched region where at least one of Cu and Ni is enriched. Regarding Sn, it is also considered that Sn is enriched when its concentration is three times or more the reference concentration.

[0066] [The ratio of the Sn concentration to the sum of the Cu concentration and the Ni concentration in the enriched region, [Sn] / ([Cu] + [Ni]), is 0.50 or less in atomic ratio (preferred range)] As described above, Sn has the effect of lowering the melting point of the enriched region where at least one of Cu and Ni is enriched. However, if the Sn concentration in the enriched region is excessively high, the grain boundary penetration depth of the enriched region increases, causing cold work cracks. Therefore, the ratio of the Sn concentration to the sum of the Cu concentration and the Ni concentration in the enriched region, [Sn] / ([Cu] + [Ni]), is 0.50 or less in atomic ratio, preferably 0.40 or less, and more preferably 0.30 or less. On the other hand, the lower limit of the atomic ratio [Sn] / ([Cu] + [Ni]) is not particularly limited, and [Sn] / ([Cu] + [Ni]) may be 0.00.

[0067] The ratio of the Sn concentration to the sum of the Cu concentration and Ni concentration in the enriched region, [Sn] / ([Cu] + [Ni]), can be determined by the following procedure. A steel material is cut to prepare three samples including the steel surface layer, and quantitative analysis (mapping) is performed using EPMA on three fields of view in a cross section perpendicular to the rolling direction of the sample to determine the concentrations of Cu, Ni, and Sn. The EPMA measurement conditions can be a magnification of 100 times and an area per field of view of 600 μm × 800 μm. The enriched region is determined using the above-mentioned method, and the atomic ratio of [Sn] / ([Cu] + [Ni]) is calculated from the measured concentration at the point with the highest Sn concentration in the enriched region. The average value of the atomic ratios obtained in each field of view of each sample is calculated, and this is defined as the [Sn] / ([Cu] + [Ni]) in the enriched region of that steel material.

[0068] (Method for manufacturing hot-rolled steel material) Next, a method for manufacturing hot-rolled steel material will be described. A method for manufacturing hot-rolled steel material according to one embodiment of the present invention includes a step of hot-rolling a steel material having the above-described chemical composition under conditions in which the maximum heating temperature T in a heating furnace is 1000°C or higher and 1200°C or lower, and the residence time of the steel material in the heating furnace is within a predetermined range, to obtain a hot-rolled steel material.

[0069] [Steel material] The steel material used in the method for producing a hot-rolled steel material has a chemical composition containing C, Si, Mn, Cu, Ni, and N, with the balance being Fe and unavoidable impurities. In addition to these, the steel material may contain the above-mentioned elements as necessary. The content of each element is as described above.

[0070] [Hot rolling] A steel material is hot-rolled to obtain a hot-rolled steel material. If the maximum heating temperature T in the heating furnace during hot rolling is less than 1000°C, scale is unlikely to form and grow on the surface of the steel material, and the formation of enriched regions is unlikely to progress. As a result, the coverage of enriched regions on the surface of the decarburized layer is reduced, and the decarburization reaction cannot be suppressed. Therefore, the maximum heating temperature T in the heating furnace is set to 1000°C or higher, and preferably 1030°C or higher. On the other hand, if the maximum heating temperature T exceeds 1200°C, the scale growth rate on the surface of the steel material is too fast, and the enriched regions formed on the surface of the steel material are expelled to the scale side (scale off). As a result, the coverage of enriched regions on the surface of the decarburized layer is reduced, and the decarburization reaction cannot be sufficiently suppressed. Therefore, the maximum heating temperature T in the heating furnace is set to 1200°C or lower, and preferably 1150°C or lower.

[0071] During hot rolling, the longer the residence time of the steel material in the heating furnace, the longer the time for which the decarburization reaction occurs in the heating furnace. The inventors have determined that the upper limit of the residence time of the steel material in the heating furnace is the time t １ It has been discovered that by setting the residence time to (minutes), the decarburized layer and the enriched region can be made into a state suitable for suppressing the decarburization reaction. On the other hand, since a shorter residence time is more advantageous for suppressing the decarburization reaction, there is no need to particularly set a lower limit for the residence time, but if the residence time is too short, adverse effects such as the temperature of the material varying depending on the position can be considered. Therefore, it is preferable that the residence time be 30 minutes or more. In the present invention, the residence time is the time that the material stays in the heating furnace from the time the material is charged into the heating furnace until the heated material is released from the heating furnace. t １ = 1150 - 0.8T - 3 ([Ni] / [Cu]) - 10 [Sn] (1) where T is the maximum heating temperature (°C), [Ni] is the amount of Ni in the steel material (mass %), [Cu] is the amount of Cu in the steel material (mass %), and [Sn] is the amount of Sn in the steel material (mass %). If the steel material does not contain Sn, [Sn] = 0.

[0072] For steps and conditions not described in the present invention, conventional methods can be used.

[0073] The following examples are provided to specifically explain the configuration and effects of the present invention. The present invention is not limited to the following examples, and may be modified as appropriate within the scope of the present invention, and all such modifications are within the technical scope of the present invention.

[0074] A 160 mm square billet material having the chemical composition shown in Tables 1 and 2 was heated in a heating furnace under the hot rolling conditions shown in Tables 3 and 4, and then hot rolled to obtain a wire rod with a diameter of 15 mm. Samples for microstructural observation and hardness measurement were taken from the obtained hot-rolled wire rod. Using the methods described above, the coverage of the enriched region where at least one of Cu and Ni was enriched, the maximum and minimum depth of the enriched region, the total decarburization depth (DM-T) of the decarburized layer, the total area ratio of ferrite and pearlite, and the average Vickers hardness were measured for each example. The results are shown in Tables 3 and 4. Furthermore, for the steel types listed in Table 2, the atomic ratio of [Sn] / ([Cu] + [Ni]) was measured using the method described above, and the results are shown in Table 4.

[0075] The cold workability of the obtained hot-rolled wire rods was evaluated as follows. After completely removing the scale from the surface of the hot-rolled wire rods by pickling, the wire rods were drawn to obtain wire rods with a diameter of 14 mm. These wire rods were then cut to a height of 21 mm to obtain cylindrical test specimens. The test specimen shape conformed to the No. 1 test specimen described in the literature "Cold Upsetting Property Test Method" (Plasticity and Processing, 22 (1981), 139.). The obtained test specimens were subjected to a cold compression test (under end-face restraint conditions) in which the specimens were compressed 60% in the height direction at a strain rate of 10 / s. N = 3 cold compression tests were performed for each example, and the side surfaces of the specimens were observed after the compression test to confirm the presence or absence of cracks due to cold compression. In Tables 3 and 4, steel types that did not exhibit cracks in all N = 3 samples were designated as passing products and indicated with "absent," while steel types that exhibited cracks in at least one of the N = 3 samples were designated as failing products and indicated with "present."

[0076]

[0077]

[0078]

[0079]

[0080] Each of the invention examples and comparative examples will be described below.

[0081] Nos. 1, 2, and 3 are comparative examples in which the amounts of C, Si, and Mn exceeded the ranges set forth in the present invention. These steels contained excessive amounts of either C, Si, or Mn, resulting in excessively high hardenability. Therefore, they exhibited a microstructure containing bainite or martensite, and the total area ratio of ferrite and pearlite in the base steel was below 90.0%. Furthermore, the average Vickers hardness of the base steel exceeded 250 HV, resulting in poor cold workability and cracking after a cold compression test.

[0082] Nos. 4 and 6 are comparative examples in which the Cu and Ni contents were below the ranges defined by the present invention. Because the Cu and Ni contents were low in these steels, the coverage of the enriched region was below the range defined by the present invention, and the total decarburization depth exceeded the range defined by the present invention.

[0083] Nos. 5 and 7 are comparative examples in which the Cu and Ni contents exceeded the ranges of the present invention. In these steels, the Cu or Ni contents were excessive, and the maximum depth of the enriched region exceeded the ranges of the present invention, causing cracks to occur in the cold compression test.

[0084] No. 8 is a comparative example in which the amount of N exceeded the range of the present invention. This steel type had a large amount of dissolved N in the steel, and the cold workability was poor due to the influence of dynamic strain aging, resulting in cracks occurring during the cold compression test.

[0085] Nos. 9 and 10 are comparative examples in which the ratio of the amount of Ni to the amount of Cu, [Ni] / [Cu], was outside the range of the present invention. In these steels, the [Ni] / [Cu] ratio was inappropriate, so the maximum depth of the enriched region exceeded the range of the present invention, and cracks occurred during the cold compression test.

[0086] No. 11 is a comparative example in which the maximum heating temperature T during hot rolling exceeded the range of the present invention. In this example, the heating temperature of the steel material was too high, so the growth rate of scale was fast, the coverage rate of the enriched region was low, and the total decarburization depth exceeded the range of the present invention.

[0087] No. 12 is a comparative example in which the maximum heating temperature T during hot rolling was below the range of the present invention. In this example, the heating temperature was too low, so that enriched regions were not sufficiently formed in the steel material surface layer, and the coverage of the enriched regions was significantly below the range of the present invention, resulting in a total decarburization depth exceeding the range of the present invention.

[0088] No. 13 is a comparative example in which the residence time in the heating furnace during hot rolling exceeded the range of the present invention. In No. 13, the coverage and maximum depth of the enriched region were within the range of the present invention, but the total decarburization depth exceeded the range of the present invention because the time during which the decarburization reaction occurred was long.

[0089] No. 35 is a comparative example in which the Sn content of the Sn-containing steel exceeded the upper limit of ([Cu] + [Ni]) / 2. Because the Sn content of this steel was excessive, the atomic ratio of [Sn] / ([Cu] + [Ni]) in the enriched region exceeded 0.5, and the maximum depth of the enriched region also exceeded the range of the present invention, resulting in cracks during the cold compression test.

[0090] No. 36 is a comparative example in which the residence time in the heating furnace during hot rolling exceeded the range of the present invention for a Sn-containing steel. In this example, the atomic ratio of [Sn] / ([Cu] + [Ni]) in the enriched region exceeded 0.5, and the maximum depth of the enriched region also exceeded the range of the present invention, resulting in cracks during the cold compression test.

[0091] In contrast to the above comparative examples, Nos. 14 to 34 and Nos. 37 to 57 had steel compositions and hot rolling conditions within the ranges of the present invention, as shown in Tables 1 to 4. In these hot-rolled steels, the coverage and maximum depth of the enriched region, the total decarburization depth, and the total area ratio of ferrite and pearlite were within the ranges of the present invention. Furthermore, in the examples shown in Tables 2 and 4, the [Sn] / ([Cu] + [Ni]) ratio in the enriched region was within the range of the present invention. Furthermore, these examples were excellent in average Vickers hardness and cracking properties during cold compression. That is, Nos. 14 to 34 and Nos. 37 to 57 were excellent in suppressing decarburization reactions and in cold workability.

[0092] According to the present invention, it is possible to provide a hot-rolled steel material in which the thickness of the decarburized layer is sufficiently suppressed and which has excellent cold workability, and a method for producing the same.

[0093] 100 Hot-rolled steel material 10 Base steel 20 Decarburized layer 22 Enriched region 24 Decarburized layer surface 30 Scale A Maximum depth of enriched region B Total decarburization depth of decarburized layer

Claims

1. In mass percent, C: 0.03 to 0.80%, Si: 0.01-1.00%, Mn: 0.01 to 1.50%, Cu: 0.010-0.500%, Ni: 0.010 to 1.000%, and N: 0.0020-0.0250% a ratio of the amount of Ni to the amount of Cu, [Ni] / [Cu], of 0.10 or more and 3.00 or less, with the balance being Fe and unavoidable impurities; and a decarburized layer formed on a surface of the base steel, The decarburized layer has a concentrated region in which at least one of Cu and Ni is concentrated, a coverage rate of the concentrated region on the surface of the decarburized layer is 50% or more; The maximum depth of the concentrated region in the decarburized layer is 1 μm or more and 150 μm or less, The total decarburization depth (DM-T) of the decarburized layer as defined in JIS G 0558 is 0.80 mm or less, The total area ratio of ferrite and pearlite in the base steel is 90.0% or more, The base steel has an average Vickers hardness of 250 HV or less.

2. The composition further comprises, in mass%, Sn: 0.001% or more ([Ni] + [Cu]) / 2 or less Contains In the enriched region, Sn is enriched in addition to at least one of Cu and Ni, The hot-rolled steel material according to claim 1, wherein in the enriched region, the ratio of the Sn concentration to the sum of the Cu concentration and the Ni concentration, [Sn] / ([Cu]+[Ni]), is 0.50 or less in atomic ratio.

3. The composition further comprises, in mass%, Cr: 0.01-1.50%, Mo: 0.01-0.50%, Al: 0.001-0.100%, Ti: 0.001 to 0.100%, V: 0.001 to 0.300%, Nb: 0.001 to 0.100%, and B: 0.0005-0.0050% The hot-rolled steel material according to claim 1, containing at least one element selected from the group consisting of:

4. The composition further comprises, in mass %, Cr: 0.01-1.50%, Mo: 0.01-0.50%, Al: 0.001-0.100%, Ti: 0.001 to 0.100%, V: 0.001 to 0.300%, Nb: 0.001 to 0.100%, and B: 0.0005-0.0050% The hot-rolled steel material according to claim 2, containing at least one element selected from the group consisting of:

5. The composition further comprises, in mass%, P: 0.001-0.100%, S: 0.001 to 0.100%, and Sb: 0.0010-0.0300% The hot-rolled steel material according to any one of claims 1 to 4, containing at least one element selected from the group consisting of:

6. The composition further comprises, in mass%, Pb: 0.01 to 0.50%, Bi: 0.001 to 0.100%, and Ca: 0.0005-0.1000% The hot-rolled steel material according to any one of claims 1 to 4, containing at least one element selected from the group consisting of:

7. The composition further comprises, in mass %, Pb: 0.01 to 0.50%, Bi: 0.001 to 0.100%, and Ca: 0.0005-0.1000% The hot-rolled steel material according to claim 5, containing at least one element selected from the group consisting of:

8. In mass percent, C: 0.03 to 0.80%, Si: 0.01-1.00%, Mn: 0.01 to 1.50%, Cu: 0.010-0.500%, Ni: 0.010 to 1.000%, and N: 0.0020-0.0250% The steel material has a composition in which the ratio of the amount of Ni to the amount of Cu, [Ni] / [Cu], is 0.10 or more and 3.00 or less, with the balance being composed of Fe and unavoidable impurities, and the maximum heating temperature T in a heating furnace is 1000° C. or more and 1200° C. or less, and the residence time of the steel material in the heating furnace is a time t １ (min) or less to obtain a hot-rolled steel material. t １ [_]_____________________________・・・・・・・ Here, T is the maximum heating temperature (°C), [Ni] is the amount of Ni in the steel material (mass%), [Cu] is the amount of Cu in the steel material (mass%), and [Sn] is the amount of Sn in the steel material (mass%).

9. The composition further comprises, in mass%, Sn: 0.001% or more ([Ni] + [Cu]) / 2 or less The method for producing a hot-rolled steel material according to claim 8, comprising:

10. The composition further comprises, in mass%, Cr: 0.01-1.50%, Mo: 0.01-0.50%, Al: 0.001-0.100%, Ti: 0.001 to 0.100%, V: 0.001 to 0.300%, Nb: 0.001 to 0.100%, and B: 0.0005-0.0050% The method for producing a hot-rolled steel material according to claim 8, further comprising the step of:

11. The composition further comprises, in mass %, Cr: 0.01-1.50%, Mo: 0.01-0.50%, Al: 0.001-0.100%, Ti: 0.001 to 0.100%, V: 0.001 to 0.300%, Nb: 0.001 to 0.100%, and B: 0.0005-0.0050% The method for producing a hot-rolled steel material according to claim 9, further comprising the step of:

12. The composition further comprises, in mass%, P: 0.001-0.100%, S: 0.001 to 0.100%, and Sb: 0.0010-0.0300% The method for producing a hot-rolled steel material according to any one of claims 8 to 11, comprising at least one element selected from the group consisting of:

13. The composition further comprises, in mass%, Pb: 0.01 to 0.50%, Bi: 0.001 to 0.100%, and Ca: 0.0005-0.1000% The method for producing a hot-rolled steel material according to any one of claims 8 to 11, comprising at least one element selected from the group consisting of:

14. The composition further comprises, in mass %, Pb: 0.01 to 0.50%, Bi: 0.001 to 0.100%, and Ca: 0.0005-0.1000% The method for producing a hot-rolled steel material according to claim 12, further comprising the step of: