Method for producing hot-rolled steel material

Abstract

To provide a method for producing a hot-rolled steel material in which the thickness of a decarburized layer is sufficiently suppressed, and which has excellent cold workability.SOLUTION: A method for producing a hot-rolled steel material includes a step of hot-rolling a steel raw material having a predetermined component composition such that a maximum heating temperature T in a heating furnace is 1000°C or higher and 1200°C or lower and a residence time of the steel raw material in a heating furnace satisfies a predetermined condition, thereby obtaining a predetermined hot-rolled steel material.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Parts used in automobiles and other applications are made by first performing preliminary processing such as wire drawing on hot-rolled steel material, then hot or cold working to form the part shape, followed by cutting or heat treatment as needed to create the final product. In recent years, the use of cold working (e.g., cold forging) to manufacture parts has become widespread, as it offers advantages in terms of manufacturing costs, such as dimensional accuracy and yield, as well as in reducing the energy used to heat the material. Parts manufactured using cold working are either left as they are, 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 adding a specific amount of Te, Se, or S as a trace element and appropriately controlling the thermal history after hot rolling makes it possible to achieve both softening of the hot-rolled steel material and suppression of the decarburization reaction. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-250768 Summary of the Invention [Problem to be solved by the invention]

[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 in terms 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. [Means for solving the problem]

[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 oxidize than Fe, and obtained the following findings. When scale forms 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 decarburization reactions that occur 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 and Cu contents of a hot-rolled steel material, it is possible to keep the depth of the 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 present 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, the present inventors have made the following findings as a result of extensive investigations: 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 the steel material surface layer where at least one of Cu and Ni is enriched. Since Sn has the effect of lowering the melting point of the enriched region, the enriched region is more likely to be formed 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 the 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 deeply into the grain boundaries, embrittling the grain boundaries and causing cracks during cold working. Therefore, when manufacturing 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 amounts 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] In mass%, C: 0.03 to 0.80%, Si: 0.01 to 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 base steel having 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 Fe and unavoidable impurities; and a decarburized layer formed on the surface of the base steel, The decarburized layer has a concentrated region in which at least one of Cu and Ni is concentrated, the coverage of the enriched 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.

[0016] [2] The component composition is further expressed 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 [1] above, 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.

[0017] [3] The component composition is further expressed in mass%: 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: 0.001~0.300%, Nb: 0.001 to 0.100%, and B: 0.0005 to 0.0050% The hot-rolled steel material according to the above [1] or [2], containing at least one element selected from the group consisting of:

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

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

[0020] [6] by mass%, C: 0.03 to 0.80%, Si: 0.01 to 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% and 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, under conditions where 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 equal to or less than a time t1 (minutes) determined by the following formula (1), to obtain a hot-rolled steel material. t1=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 component composition is further expressed in mass%: Sn: 0.001% or more ([Ni]+[Cu]) / 2 or less The method for producing a hot-rolled steel material according to [6] above, comprising:

[0022] [8] The component composition is further expressed in mass%: 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: 0.001~0.300%, Nb: 0.001 to 0.100%, and B: 0.0005 to 0.0050% The method for producing a hot-rolled steel material according to the above [6] or [7], wherein the hot-rolled steel material contains at least one element selected from the group consisting of:

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

[0024]

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

[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. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a schematic diagram showing a cross section of a hot-rolled steel material according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE 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) FIG. 1 shows a cross section of a hot-rolled steel material according to one embodiment of the present invention. The hot-rolled steel material 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 in that enriched regions 22 containing at least one of Cu and Ni are present, the coverage of the enriched regions 22 on the surface 24 of the decarburized layer 20 is 50% or more, the maximum depth A of the enriched regions 22 in the decarburized layer 20 is 1 μm or more and 150 μm or less, the total decarburization depth B (DM-T) of the decarburized layer 20 as defined by JIS G 0558 is 0.80 mm or less, the total area fraction of ferrite and pearlite in the base steel 10 is 90.0% or more, and the average Vickers hardness of the base steel 10 is 250 HV or less.

[0029] <Subway> First, we will explain the base steel of a hot-rolled steel material according to one embodiment of the present invention. The base steel has a chemical composition containing, 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], satisfying 0.10 to 3.00, and the balance being Fe and unavoidable impurities. In the following explanation of the chemical 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: 0.03~0.80%] 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 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: 0.010~0.500%] Cu is an element that is less susceptible to oxidation than Fe, and is an element that suppresses decarburization reactions by concentrating in the steel surface layer as scale is formed during hot rolling, forming a concentrated region. If the Cu content of the base steel is less than 0.010%, a concentrated region is not sufficiently formed in the steel surface layer, 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 concentrated region in the steel surface layer 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: 0.010~1.000%] Like Cu, Ni is an element that is less susceptible to oxidation than Fe. Ni is an element that suppresses decarburization reactions by concentrating in the steel surface layer as scale forms during hot rolling, forming a concentrated region. If the Ni content of the base steel is less than 0.010%, the concentrated region is not sufficiently formed in the steel surface layer, and the decarburization suppression effect is not fully 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 concentrated 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:0.0020~0.0250%] N is an element that combines 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%, this effect cannot be achieved. 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 should be set to 0.0250% or less, preferably 0.0200% or less, and more preferably 0.0180% or less.

[0036] [The ratio (mass ratio) of Ni to Cu [Ni] / [Cu] is 0.10 or more and 3.00 or less] In the present invention, the decarburization reaction is suppressed by the enriched region in which at least one of Cu and Ni is enriched in the steel surface layer. Therefore, it is not sufficient to focus solely on the Cu content and Ni content 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 content is excessive relative to the Ni content, Cu enriched in the steel surface layer penetrates into grain boundaries, making the enriched region excessively deep and prone to cracking 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, i.e., when the Ni content is excessive relative to the Cu content, the enriched region in the steel surface layer also becomes excessively deep and prone to cracking 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] Since Sn has the effect of lowering the melting point of enriched regions where at least one of Cu and Ni is enriched, adding Sn makes it easier for enriched regions to form in the surface layer of the steel material, making it easier to achieve 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 should be set to ([Ni] + [Cu]) / 2 or less, and preferably ([Ni] + [Cu]) / 3 or less.

[0039] [Cr: 0.01~1.50%] 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 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 saturates, and the hardenability of the steel becomes 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 combines 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 effects cannot be achieved. 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 it more susceptible to cracking during cold working and deteriorating the fatigue fracture properties of the part. 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: 0.001~0.100%] Like Al, Ti is an element that combines 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 achieved. 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:0.001~0.300%] Like Al and Ti, V is an element that combines with N in steel to form nitrides and contributes to grain refinement. Furthermore, V contributes to increasing the strength of steel. If the V content in the base steel is less than 0.001%, the above-mentioned effects cannot be achieved. 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 in 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 combines 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:0.0005~0.0050%] B is an element that greatly improves the hardenability of steel with a small amount added. 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:0.001~0.100%] 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:0.001~0.100%] S is an element that combines with Mn in steel to form MnS inclusions, improving the machinability of steel. If the S content of the base steel is less than 0.001%, this effect cannot be achieved. 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 present 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: 0.0010~0.0300%] Like Sn, Sb is an element that easily segregates in the surface layer of a steel material and has the effect of suppressing decarburization reactions, similar to the enriched regions where at least one of Cu and Ni is enriched. If the Sb content of the base steel is less than 0.0010%, the above-mentioned 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, deteriorating 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: 0.01~0.50%] 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 effect cannot be obtained. Therefore, if 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, if Pb is added to the base steel, the Pb content should be 0.50% or less, and preferably 0.35% or less.

[0050] [Bi: 0.001~0.100%] 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 effect cannot be achieved. 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 machinability improvement effect 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: 0.0005~0.1000%] Ca is an element that dissolves in sulfides in steel to spheroidize them, 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 base steel must have a structure that is a mixed structure of relatively soft ferrite and pearlite. Therefore, the total area ratio of ferrite and pearlite in the base steel is set to 90.0% or more, and preferably 95.0% or more. However, there is no particular upper limit to 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 the steel material surface layer, and the structure of each sample is observed on a cross section perpendicular to the rolling direction. The microstructure of the surface layer structure of the sample, excluding the decarburized layer described below, is observed using an optical microscope at a magnification of 100x. The area of ​​one field of view is set to 600 μm × 800 μm, and the total area ratio of ferrite and pearlite in the observed area of ​​each field of view is calculated for five randomly selected fields of view. The average value of the total area ratios obtained in each field of view 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 is 250HV or less] If the average Vickers hardness of the base steel exceeds 250 HV, cold workability deteriorates. 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 equivalent 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 equivalent 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"). If the sample is a plate, the measurement points are five in total: one at a position equivalent to 1 / 2 of the plate width from one end toward the center and equivalent to 1 / 2 of the plate thickness from the steel surface in the depth direction (hereinafter referred to as the "W / 2-t / 2 position"), and four at positions equivalent to 1 / 4 of the plate width from one end toward the center and equivalent to 1 / 4 of the plate thickness from the steel surface in the depth direction (hereinafter referred to as the "W / 4-t / 4 positions"). The average value of the hardness values ​​obtained at each measurement point on each sample is calculated and used as the average Vickers hardness of the steel. 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] [The total decarburization depth (DM-T) of the decarburized layer specified in JIS G 0558 is 0.80 mm or less] If the decarburized layer is excessively thick, it will lead to a decrease in the fatigue properties and toughness of the part, 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 by the total decarburization depth (DM-T) specified 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. On the other hand, there is no particular lower limit for the total decarburization depth of the decarburized layer.

[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 an 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 defined as the total decarburization depth of the decarburized layer of that steel material.

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

[0060] [Coverage of enriched areas on the surface of the decarburized layer is 50% or more] When an enriched region in which at least one of Cu and Ni is enriched is formed on the surface of a hot-rolled steel material, it contributes to suppressing the decarburization reaction. If the coverage of the enriched region on the surface of the decarburized layer is less than 50%, the decarburization suppression effect becomes insufficient. Therefore, the coverage of the enriched region 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, there is no particular upper limit to the coverage, and the coverage of the enriched region 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, the steel is cut to prepare three samples containing the steel surface layer, and the reference concentrations of Cu and Ni are measured for each sample. To eliminate the effects of the enriched region and central segregation in the steel surface layer, three randomly selected points at the above-mentioned positions (D / 4 position of hot-rolled steel for round steel, W / 4-t / 4 position of hot-rolled steel 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 an 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 standard 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 coverage obtained in each field of each sample is calculated and used as the coverage of the enriched regions for that steel.

[0063] [Maximum depth of the enriched region in the decarburized layer is 1 μm or more and 150 μm or less] If the maximum depth of the enriched 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 enriched region in the decarburized layer is set to 1 μm or more. On the other hand, if the maximum depth of the enriched region in the decarburized layer exceeds 150 μm, although this is preferable from the viewpoint of suppressing the decarburization reaction, the enriched region is too deep and is therefore prone to cracking during cold working. Therefore, the maximum depth of the enriched 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 the 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 enriched in the enriched region where at least one of Cu and Ni is enriched. Regarding Sn, Sn is considered to be enriched when its concentration is three times or more the reference concentration.

[0066] [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 (preferred range)] As mentioned 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 set to an atomic ratio of 0.50 or less, preferably 0.40 or less, and more preferably 0.30 or less. On the other hand, there is no particular lower limit for the atomic ratio [Sn] / ([Cu] + [Ni]), and [Sn] / ([Cu] + [Ni]) may be 0.00.

[0067] The ratio of the Sn concentration to the sum of the Cu and Ni concentrations in the enriched region, [Sn] / ([Cu] + [Ni]), can be determined by the following procedure. Three samples containing the steel surface are prepared by cutting the steel, and quantitative analysis (mapping) is performed using an EPMA on three fields of view in the 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 100x and an area of ​​600 μm × 800 μm per field. The enriched region is determined using the above 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 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.

[0068] (Hot-rolled steel manufacturing method) Next, a method for producing a hot-rolled steel material will be described. The method for producing a 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 more and 1200°C or less, 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] Hot-rolled steel is obtained by hot-rolling a steel material. If the maximum heating temperature T in the heating furnace during hot rolling is less than 1000°C, scale formation and growth in the steel surface layer are difficult, and the formation of enriched regions is difficult to progress. As a result, the coverage of enriched regions on the surface of the decarburized layer is small, 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 1030°C or higher is preferable. On the other hand, if the maximum heating temperature T exceeds 1200°C, the scale growth rate on the steel surface layer is too fast, and the enriched regions formed on the steel surface layer are expelled to the scale side (scale off). As a result, the coverage of enriched regions on the surface of the decarburized layer is small, 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 1150°C or lower is preferable.

[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 discovered that by setting the upper limit of the residence time of the steel material in the heating furnace to time t1 (minutes) determined by the following formula (1), the decarburized layer and concentrated 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 specifically set a lower limit for the residence time. However, an excessively short residence time may have adverse effects such as the temperature of the material varying depending on the position. Therefore, the residence time is preferably 30 minutes or more. In this invention, the residence time refers to the time the material stays in the heating furnace from the time the material is charged into the heating furnace until the heated material leaves the heating furnace. t1=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. [Example]

[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 the scale on the surface of the hot-rolled wire rods was completely removed by pickling, they were drawn to obtain wire rods with a diameter of 14 mm. These 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 Test Method" (Plasticity and Processing, 22 (1981), 139.). The obtained test specimens were subjected to a cold compression test (end-constrained condition) in which they were compressed 60% in the height direction at a strain rate of 10 / s. N=3 for each cold compression test was performed, and the side surfaces of the specimens were inspected after the test to determine whether cracks occurred during cold compression. In Tables 3 and 4, steels that showed no cracks in all three samples were designated as passing products and indicated with "absent," while steels that showed cracks in at least one of the three samples were designated as failing products and indicated with "present."

[0076] [Table 1]

[0077] [Table 2]

[0078] [Table 3]

[0079] [Table 4]

[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, resulting in 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 the occurrence of cracks after cold compression testing.

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

[0083] Nos. 5 and 7 are comparative examples in which the Cu and Ni contents exceeded the ranges set forth in the present invention. These steels contained excessive amounts of Cu or Ni, and the maximum depth of the enriched region exceeded the ranges set forth in 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 high amount of dissolved N in the steel, and the cold workability was poor due to the effects 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 Ni content to the Cu content, [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 enriched regions were not sufficiently formed in the steel surface layer, and the coverage of the enriched regions was significantly below the range of the present invention. As a result, the total decarburization depth exceeded 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 of a Sn-containing steel in which the Sn content exceeded the upper limit of ([Cu] + [Ni]) / 2. Because the Sn content in 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 specified in 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 all 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 crack resistance during cold compression. That is, Nos. 14 to 34 and Nos. 37 to 57 were excellent in suppressing the decarburization reaction and in cold workability. [Industrial Applicability]

[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. [Explanation of symbols]

[0093] 100 Hot rolled steel 10 Subway 20 Decarburized layer 22 Concentrated region 24 Decarburized layer surface 30 scale A Maximum depth of the enriched region B Total decarburization depth of the decarburized layer

Claims

1. In mass%, 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% and further comprising [Group A] by 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% At least one element selected from the group consisting of: [Group B] by mass%, P: 0.001-0.100%, S: 0.001 to 0.100%, and Sb: 0.0010-0.0300% At least one element selected from the group consisting of: [Group C] by mass%, Pb: 0.01 to 0.50%, Bi: 0.001 to 0.100%, and Ca: 0.0005-0.1000% At least one element selected from the group consisting of A steel material having 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 Fe and unavoidable impurities, is heated in a heating furnace at a maximum heating temperature T of 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 determined by the following formula (1): １ and a hot-rolled steel material having a base steel having the above-described chemical composition and a decarburized layer formed on the surface of the base steel, 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 or more and 150 μm or less, the decarburized layer has a total decarburization depth (DM-T) defined in JIS G 0558 of 0.80 mm or less, the total area fraction of ferrite and pearlite in the base steel is 90.0% or more, and the average Vickers hardness of the base steel is 250 HV or less. t 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.

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

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