Hot-rolled steel material and method of producing same

Abstract

Provided is hot-rolled steel material for which thickness of a decarburized layer is sufficiently suppressed and that has excellent cold workability. The hot-rolled steel material includes a steel substrate that has a defined chemical composition and a decarburized layer formed on the surface of the steel substrate. In the decarburized layer, there is an enriched region where at least one of Cu and Ni is enriched. Coverage of the enriched region on a surface of the decarburized layer is 50% or more. Maximum depth of the enriched region in the decarburized layer is 1 μm or more and 150 μm or less. Total decarburization depth of the decarburized layer (DM-T), as specified in JIS G 0558, is 0.80 mm or less. Total area fraction of ferrite and pearlite in the steel substrate is 90.0% or more. Average Vickers hardness in the steel substrate is 250 HV or less.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to hot-rolled steel material and a method of producing same.BACKGROUND

[0002] Components used in automobiles and for other applications are made by first applying preliminary processing such as wire drawing to hot-rolled steel material, which is then shaped into the component by hot working or cold working, and then subjected to cutting work, heat treatment, or the like as required to become a final product. In recent years, the production of components by cold working (for example, cold forging) has become widespread, cold working being advantageous in terms of production costs such as dimensional accuracy and yield rate of parts, or in terms of reducing energy consumption for heating material. Components produced by cold working are either cold worked as they are, or after heat treatment (quenching and tempering heat treatment, induction hardening and tempering heat treatment, and the like) for strength adjustment, depending on the required strength, before becoming final products.

[0003] The surface layer of hot-rolled steel material subjected to the cold working has a decarburized layer produced during hot rolling. When the decarburized layer of hot-rolled steel material is excessively thick, the components after cold working or components after cold working and induction hardening and tempering heat treatment have soft portions with low carbon content remaining in the vicinity of the steel material surface layer, resulting in inferior strength or fatigue resistance as components. Further, when the decarburized layer of hot-rolled steel material is excessively thick, in a component that is 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, and therefore the soft portions described above do not remain in the steel material surface layer. However, the amount of carbon differs between the decarburized layer and the core, and therefore the austenite reverse transformation temperatures are different. In this case, the timing of austenite reverse transformation differs between the steel material surface layer and the core, and therefore the reverse transformation austenite grain size is non-uniform between the steel material surface layer and the core. Such grain size non-uniformity induces coarsening of the crystal grains, leading to decreased fatigue resistance and toughness of the component.

[0004] As mentioned above, when the decarburized layer of the hot-rolled steel material surface layer is excessively thick, various properties of the final product degrade, and therefore it is necessary to suppress excessive thickness of the decarburized layer at the hot rolling stage. Further, when cracking occurs during cold working, use as a final product becomes impossible, and therefore hot-rolled steel material is also required to have excellent cold workability.

[0005] In Patent Literature (PTL) 1, it is described 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 both soften the hot-rolled steel material and suppress the decarburization reaction.CITATION LISTPatent Literature

[0006] PTL 1: JP 2004-250768 ASUMMARYTechnical Problem

[0007] However, according to PTL 1, Te and Se are used, which are rare and toxic elements that require special care in handling, and it is difficult to say that this technology can be generally applied. Further, both the technology of PTL 1 and earlier technologies require precise control of the thermal history after hot rolling, which remains a challenge from the viewpoint of productivity. Thus, a technique that combines the suppression of decarburized layer thickness with excellent cold workability had not been reported.

[0008] In view of the above problem, it would be helpful to provide hot-rolled steel material for which thickness of the decarburized layer is sufficiently suppressed and that has excellent cold workability, and a method of producing same.Solution to Problem

[0009] To solve the above problem, the inventors focused on suppression of the decarburization reaction by adding Cu and Ni, which are elements that are less easily oxidized than Fe, and made the following discoveries. When scale is formed on steel material surface by heating during hot rolling, Fe is preferentially oxidized and becomes scale. On the other hand, Cu and Ni are not oxidized and are left in the steel material surface layer, forming an enriched region where at least one of Cu and Ni is enriched. The formation of such an enriched region effectively suppresses the decarburization reaction that occurs in the steel material surface layer.

[0010] However, it is well known that Cu concentrated in the steel material surface layer penetrates into grain boundaries during hot rolling, embrittling the grain boundaries and inducing cracks during cold working. The inventors conducted further studies and discovered that by optimizing a balance between the amounts of Ni and Cu in the hot-rolled steel material, it is possible to suppress to a certain range the depth of the enriched region where at least one of Cu and Ni is enriched in the steel material surface layer, thereby suppressing occurrence of cracking during cold working.

[0011] In addition, the inventors discovered that the maximum heating temperature during hot rolling needs to be appropriately controlled in order to make the enriched region where at least one of Cu and Ni is enriched in hot-rolled steel material suitable for suppressing the decarburization reaction. Further, the inventors discovered that in order to effectively suppress the decarburization reaction, it is necessary to appropriately control the residence time of steel material in the heating furnace during hot rolling.

[0012] Further, the inventors have made the following discoveries as a result of extensive studies. In hot-rolled steel material with Sn added in addition to Cu and Ni, Sn is also enriched in the enriched region where at least one of Cu and Ni is enriched in the steel material surface layer. Sn has an effect of lowering the melting point of the enriched region, and therefore the enriched region is more easily formed on the steel material surface layer than in a case where Sn is not added, and the decarburization reaction suppression effect of the enriched region is also easier to obtain.

[0013] On the other hand, when 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 grain boundaries and embrittles the grain boundaries, causing cracking during cold working. Therefore, in the production of hot-rolled steel material with Sn added in addition to Cu and Ni, it is necessary to limit the Sn concentration in the enriched region to a defined range to avoid excessive grain boundary penetration depth. As specific methods, effective methods include adjusting the amount of Sn added according to the amount of Cu and Ni, shortening the furnace residence time in the heating furnace during hot rolling according to the amount of Sn, and the like. Primary features of the present disclosure are as follows.

[0014] [1] Hot-rolled steel material comprising: a steel substrate that has a chemical composition containing (consisting of), in mass %,

[0015] C: 0.03% to 0.80%,

[0016] Si: 0.01% to 1.00%,

[0017] Mn: 0.01% to 1.50%,

[0018] Cu: 0.010% to 0.500%,

[0019] Ni: 0.010% to 1.000%, and

[0020] N: 0.0020% to 0.0250%,

[0021] with the balance being Fe and inevitable impurity, wherein a ratio of Ni to Cu, [Ni] / [Cu], is 0.10 or more and 3.00 or less; and a decarburized layer formed on a surface of the steel substrate, wherein

[0022] the decarburized layer comprises an enriched region where at least one of Cu and Ni is enriched,

[0023] coverage of the enriched region on the surface of the decarburized layer is 50% or more,

[0024] maximum depth of the enriched region in the decarburized layer is 1 μm or more and 150 μm or less,

[0025] total decarburization depth of the decarburized layer, DM-T, as specified in JIS G 0558, is 0.80 mm or less,

[0026] total area fraction of ferrite and pearlite in the steel substrate is 90.0% or more, and

[0027] average Vickers hardness in the steel substrate is 250 HV or less.

[0028] [2] The hot-rolled steel material according to [1], wherein the chemical composition further contains, in mass %,

[0029] Sn: 0.001% or more and ([Ni]+[Cu]) / 2 or less,

[0030] in addition to at least one of Cu and Ni, Sn is enriched in the enriched region, and

[0031] in the enriched region, an atomic ratio of Sn concentration to the sum of Cu and Ni concentrations, [Sn] / ([Cu]+[Ni]), is 0.50 or less.

[0032] [3] The hot-rolled steel material according to [1] or [2], wherein the chemical composition further contains, in mass %, at least one element selected from the group consisting of:

[0033] Cr: 0.01% to 1.50%,

[0034] Mo: 0.01% to 0.50%,

[0035] Al: 0.001% to 0.100%,

[0036] Ti: 0.001% to 0.100%,

[0037] V: 0.001% to 0.300%,

[0038] Nb: 0.001% to 0.100%, and

[0039] B: 0.0005% to 0.0050%.

[0040] [4] The hot-rolled steel material according to any one of [1] to [3], wherein the chemical composition further contains, in mass %, at least one element selected from the group consisting of:

[0041] P: 0.001% to 0.100%,

[0042] S: 0.001% to 0.100%, and

[0043] Sb: 0.0010% to 0.0300%.

[0044] [5] The hot-rolled steel material according to any one of [1] to [4], wherein the chemical composition further contains, in mass %, at least one element selected from the group consisting of:

[0045] Pb: 0.01% to 0.50%,

[0046] Bi: 0.001% to 0.100%, and

[0047] Ca: 0.0005% to 0.1000%.

[0048] [6] A method of producing hot-rolled steel material, the method comprising a process of hot rolling steel material that has a chemical composition containing (consisting of), in mass %,

[0049] C: 0.03% to 0.80%,

[0050] Si: 0.01% to 1.00%,

[0051] Mn: 0.01% to 1.50%,

[0052] Cu: 0.010% to 0.500%,

[0053] Ni: 0.010% to 1.000%, and

[0054] N: 0.0020% to 0.0250%,

[0055] with the balance being Fe and inevitable impurity, wherein a ratio of Ni to Cu, [Ni] / [Cu], is 0.10 or more and 3.00 or less, under a set of conditions including a maximum heating temperature T in a heating furnace being 1000° C. or more and 1200° C. or less, and a residence time of the steel material in the heating furnace being a time t1 or shorter, in min, determined by Expression (1) below, to obtain the hot-rolled steel material,t1=1150−0.8T−3([Ni] / [Cu])−10 [Sn]  (1)where T is the maximum heating temperature, in ° C., [Ni] is Ni content in the steel material, in mass %, [Cu] is Cu content in the steel material, in mass %, and [Sn] is Sn content in the steel material, in mass %.

[0057] [7] The method of producing hot-rolled steel material according to [6], wherein the chemical composition further contains, in mass %,

[0058] Sn: 0.001% or more and ([Ni]+[Cu]) / 2 or less.

[0059] [8] The method of producing hot-rolled steel material according to [6] or [7], wherein the chemical composition further contains, in mass %, at least one element selected from the group consisting of:

[0060] Cr: 0.01% to 1.50%,

[0061] Mo: 0.01% to 0.50%,

[0062] Al: 0.001% to 0.100%,

[0063] Ti: 0.001% to 0.100%,

[0064] V: 0.001% to 0.300%,

[0065] Nb: 0.001% to 0.100%, and

[0066] B: 0.0005% to 0.0050%.

[0067] [9] The method of producing hot-rolled steel material according to any one of [6] to [8], wherein the chemical composition further contains, in mass %, at least one element selected from the group consisting of:

[0068] P: 0.001% to 0.100%,

[0069] S: 0.001% to 0.100%, and

[0070] Sb: 0.0010% to 0.0300%.

[0071]

[10] The method of producing hot-rolled steel material according to any one of [6] to [9], wherein the chemical composition further contains, in mass %, at least one element selected from the group consisting of:

[0072] Pb: 0.01% to 0.50%,

[0073] Bi: 0.001% to 0.100%, and

[0074] Ca: 0.0005% to 0.1000%.Advantageous Effect

[0075] The present disclosure provides hot-rolled steel material for which thickness of the decarburized layer is sufficiently suppressed and that has excellent cold workability, and a method of producing same.BRIEF DESCRIPTION OF THE DRAWINGS

[0076] In the accompanying drawings:

[0077] FIG. 1 is a schematic diagram illustrating a cross-section of hot-rolled steel material according to an embodiment of the present disclosure.DETAILED DESCRIPTION

[0078] The following is a description of embodiments of the hot-rolled steel material and the method of producing same. The embodiments described below are example embodiments of the present disclosure, and do not limit configuration to the specific examples described.(Hot-Rolled Steel Material)

[0079] FIG. 1 illustrates a cross-section of hot-rolled steel material according to an embodiment of the present disclosure. Hot-rolled steel material 100 includes a steel substrate 10 and a decarburized layer 20 formed on a surface of the steel substrate 10. Further, scale 30 may be on the decarburized layer 20. Further, in the decarburized layer 20, there is an enriched region 22 where at least one of Cu and Ni is enriched. Coverage of the enriched region 22 on a surface 24 of the decarburized layer 20 is 50% or more. Maximum depth A of the enriched region 22 in the decarburized layer 20 is 1 μm or more and 150 μm or less. Total decarburization depth B of the decarburized layer 20 (DM-T), as specified in JIS G 0558, is 0.80 mm or less. Total area fraction of ferrite and pearlite in the steel substrate 10 is 90.0% or more. Average Vickers hardness in the steel substrate 10 is 250 HV or less.<Steel Substrate>

[0080] First, the steel substrate of the hot-rolled steel material according to an embodiment of the present disclosure is described. The steel substrate has a chemical 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%, with the balance being Fe and inevitable impurity, and a ratio of Ni to Cu, [Ni] / [Cu], is 0.10 or more and 3.00 or less. In the following description of chemical composition, “%” indicating content means “mass %” unless otherwise specified. Further, [Ni] and [Cu] represent Ni content and Cu content in the steel substrate, respectively.[C: 0.03% to 0.80%]

[0081] C is an element added to secure strength of the hot-rolled steel material. When C content of the steel substrate is less than 0.03%, the required strength cannot be secured. The C content of the steel substrate is therefore 0.03% or more. The C content of the steel substrate is preferably 0.05% or more. On the other hand, when the C content of the steel substrate exceeds 0.80%, hardenability becomes too high, resulting in a microstructure containing hard bainite or martensite, which increases rolled material hardness and decreases cold workability. The C content of the steel substrate is therefore 0.80% or less. The C content of the steel substrate is preferably 0.65% or less. The C content of the steel substrate is more preferably 0.50% or less.[Si: 0.01% to 1.00%]

[0082] Si is a deoxidation element during refining and also improves the strength and hardenability of the hot-rolled steel material. When Si content of the steel substrate is less than 0.01%, the effects cannot be obtained. The Si content of the steel substrate is therefore 0.01% or more. On the other hand, when the Si content of the steel substrate exceeds 1.00%, hardenability becomes too high, resulting in a microstructure containing hard bainite or martensite, which increases rolled material hardness and decreases cold workability. The Si content of the steel substrate is therefore 1.00% or less. The Si content of the steel substrate is preferably 0.80% or less. The Si content of the steel substrate is more preferably 0.50% or less.[Mn: 0.01% to 1.50%]

[0083] Mn is an element that improves the strength and hardenability of the hot-rolled steel material. When Mn content of the steel substrate is less than 0.01%, the effects cannot be obtained. The Mn content of the steel substrate is therefore 0.01% or more. On the other hand, when the Mn content of the steel substrate exceeds 1.50%, hardenability becomes too high, resulting in a microstructure containing hard bainite or martensite, which increases rolled material hardness and decreases cold workability. The Mn content of the steel substrate is therefore 1.50% or less. The Mn content of the steel substrate is preferably 1.20% or less. The Mn content of the steel substrate is more preferably 1.00% or less.[Cu: 0.010% to 0.500%]

[0084] Cu is an element that is less oxidizable than Fe and forms an enriched region in the steel material surface layer during scale formation during hot rolling, thereby suppressing the decarburization reaction. When Cu content of the steel substrate is less than 0.010%, the enriched region is not sufficiently formed in the steel material surface layer, and the effect of decarburization suppression is not sufficient. The Cu content of the steel substrate is therefore 0.010% or more. On the other hand, when the Cu content in the steel substrate exceeds 0.500%, the enriched region in the steel material surface layer becomes excessively deep and occurrence of cracking during cold working becomes more likely. The Cu content in the steel substrate is therefore 0.500% or less. The Cu content in the steel substrate is preferably 0.400% or less. The Cu content in the steel substrate is more preferably 0.350% or less.[Ni: 0.010% to 1.000%]

[0085] Ni, like Cu, is an element that is less oxidizable than Fe and forms the enriched region in the steel material surface layer during scale formation during hot rolling, thereby suppressing the decarburization reaction. When Ni content of the steel substrate is less than 0.010%, the enriched region is not sufficiently formed in the steel material surface layer, and the effect of decarburization suppression is not sufficient. The Ni content of the steel substrate is therefore 0.010% or more. On the other hand, when the Ni content in the steel substrate exceeds 1.000%, the enriched region in the steel material surface layer becomes excessively deep and occurrence of cracking during cold working becomes more likely. The Ni content of the steel substrate is therefore 1.000% or less. The Ni content of the steel substrate is preferably 0.800% or less. The Ni content of the steel substrate is more preferably 0.600% or less.[N: 0.0020% to 0.0250%]

[0086] N is an element that has an effect of helping prevent crystal grain coarsening by combining with nitride forming elements in steel to form nitride, which act as grain boundary pinning particles. When N content of the steel substrate is less than 0.0020%, the effect cannot be obtained. The N content of the steel substrate is therefore 0.0020% or more. On the other hand, when the N content of the steel substrate exceeds 0.0250%, not only do blowholes form in the steel, but solute N in the steel causes dynamic strain aging, and cracking becomes more likely to occur during cold working. The N content of the steel substrate is therefore 0.0250% or less. The N content of the steel substrate is preferably 0.0200% or less. The N content of the steel substrate is more preferably 0.0180% or less.[Ratio of Ni to Cu (Mass Ratio) [Ni] / [Cu] is 0.10 or More and 3.00 or Less]

[0087] According to the present disclosure, the decarburization reaction is suppressed by the enriched region where at least one of Cu and Ni is enriched in the steel material surface layer, and therefore focusing on the Cu content and Ni content independently is not sufficient, and considering the balance of the content of these elements is necessary. When the ratio of Ni to Cu (mass ratio) [Ni] / [Cu] is less than 0.10, that is, when the Cu content is excessive relative to the Ni content, Cu concentrated in the steel material surface layer penetrates to grain boundaries, resulting in an excessively deep enriched region, and cracking becomes more likely to occur during cold working. [Ni] / [Cu] is therefore 0.10 or more. [Ni] / [Cu] is preferably 0.15 or more. [Ni] / [Cu] is more preferably 0.20 or more. On the other hand, when [Ni] / [Cu] exceeds 3.00, that is, when the Ni content is excessive relative to the Cu content, the enriched region in the steel material surface layer also becomes excessively deep and cracking becomes more likely to occur during cold working. [Ni] / [Cu] is therefore 3.00 or less. [Ni] / [Cu] is preferably 2.50 or less. [Ni] / [Cu] is more preferably 2.00 or less.

[0088] The steel substrate of the hot-rolled steel material according to an embodiment of the present disclosure may further contain the following elements as required.[Sn: 0.001% or more and ([Ni]+[Cu]) / 2 or less]

[0089] Sn has an effect of lowering the melting point of the enriched region where at least one of Cu and Ni is enriched, and therefore the addition of Sn facilitates the formation of the enriched region in the steel material surface layer, and the decarburization reaction suppression effect of the enriched region is easier to obtain. To obtain the effect, when Sn is added to the steel substrate, Sn content is 0.001% or more. On the other hand, excessive addition of Sn causes deep grain boundary penetration depth in the enriched region, which leads to grain boundary embrittlement and cold work cracking. Therefore, when Sn is added to the steel substrate, the Sn content is ([Ni]+ [Cu]) / 2 or less. The Sn content is preferably ([Ni]+[Cu]) / 3 or less.[Cr: 0.01% to 1.50%]

[0090] Cr is an element that improves the hardenability of the steel material. When Cr content of the steel substrate is less than 0.01%, the effect cannot be obtained. Therefore, when Cr is added to the steel substrate, the Cr content is 0.01% or more. On the other hand, when the Cr content of the steel substrate exceeds 1.50%, the effect of the addition is saturated and the hardenability of the steel material becomes excessive, increasing the hardness of the steel material and decreasing cold workability. Therefore, when Cr is added to the steel substrate, the Cr content is 1.50% or less. The Cr content is preferably 1.30% or less. The Cr content is more preferably 1.15% or less.[Mo: 0.01% to 0.50%]

[0091] Mo is an element that greatly improves the hardenability of the steel material when added in small amounts. When Mo content in the steel substrate is less than 0.01%, the effect cannot be obtained. Therefore, when Mo is added to the steel substrate, the Mo content is 0.01% or more. On the other hand, when the Mo content of the steel substrate exceeds 0.50%, the effect of the addition is saturated and the hardenability of the steel material becomes excessive, increasing the hardness of the steel material and decreasing cold workability. Therefore, when Mo is added to the steel substrate, the Mo content is 0.50% or less. The Mo content is preferably 0.30% or less.[Al: 0.001% to 0.100%]

[0092] Al is a deoxidation element and also an element that combines with N in the steel to form nitrides, contributing to crystal grain refinement. When Al content in the steel substrate is less than 0.001%, the effects cannot be obtained. Therefore, when Al is added to the steel substrate, the Al content is 0.001% or more. On the other hand, when the Al content in the steel substrate exceeds 0.100%, the amount of Al oxides in the steel increases and cracking becomes more likely to occur during cold working, and fatigue fracture properties as a component also degrade. Therefore, when Al is added to the steel substrate, the Al content is 0.100% or less. The Al content is preferably 0.080% or less. The Al content is more preferably 0.050% or less.[Ti: 0.001% to 0.100%]

[0093] Ti, like Al, is an element that combines with N in the steel to form nitrides, contributing to crystal grain refinement. When Ti content of the steel substrate is less than 0.001%, the effect cannot be obtained. Therefore, when Ti is added to the steel substrate, the Ti content is 0.001% or more. On the other hand, when the Ti content in the steel substrate exceeds 0.100%, the amount of Ti precipitates in the steel becomes excessive, increasing the hardness of the steel material and decreasing cold workability. Therefore, when Ti is added to the steel substrate, the Ti content is 0.100% or less. The Ti content is preferably 0.080% or less. The Ti content is more preferably 0.050% or less.[V: 0.001% to 0.300%]

[0094] V, like Al and Ti, is an element that combines with N in the steel to form nitrides, contributing to crystal grain refinement. Further, Vis an element that contributes to increased steel material strength. When V content in the steel substrate is less than 0.001%, the effects cannot be obtained. Therefore, when V is added to the steel substrate, the V content is 0.001% or more. On the other hand, when the V content in the steel substrate exceeds 0.300%, the amount of V precipitates in the steel becomes excessive, increasing the hardness of the steel material and decreasing cold workability as well as the toughness of the steel material. Therefore, when V is added to the steel substrate, the V content is 0.300% or less. The V content is preferably 0.200% or less. The V content is more preferably 0.150% or less.[Nb: 0.001% to 0.100%]

[0095] Nb is an element that combines with carbon in the steel to form carbides, contributing to crystal grain refinement. When Nb content of the steel substrate is less than 0.001%, the effect cannot be obtained. Therefore, when Nb is added to the steel substrate, the Nb content is 0.001% or more. On the other hand, when the Nb content in the steel substrate exceeds 0.100%, the amount of Nb carbides in the steel becomes excessive, increasing the hardness of the steel material and decreasing cold workability. Therefore, when Nb is added to the steel substrate, the Nb content is 0.100% or less. The Nb content is preferably 0.050% or less. The Nb content is more preferably 0.030% or less.[B: 0.0005% to 0.0050%]

[0096] Mo is an element that greatly improves the hardenability of the steel material when added in small amounts. When B content of the steel substrate is less than 0.0005%, the effect cannot be obtained. Therefore, when B is added to the steel substrate, the B content is 0.0005% or more. On the other hand, when the B content in the steel substrate exceeds 0.0050%, the effect of improving hardenability becomes saturated. Therefore, when B is added to the steel substrate, the B content is 0.0050% or less. The B content is preferably 0.0040% or less. The B content is more preferably 0.0030% or less.[P: 0.001% to 0.100%]

[0097] P is an element for increasing the strength of the steel material. When P content of the steel substrate is less than 0.001%, the effect cannot be obtained. Therefore, when P is added to the steel substrate, the P content is 0.001% or more. On the other hand, when the P content in the steel substrate exceeds 0.100%, P segregates to grain boundaries and decreases the toughness of the steel material. Therefore, when P is added to the steel substrate, the P content is 0.100% or less. The P content is preferably 0.050% or less. The P content is more preferably 0.030% or less.[S: 0.001% to 0.100%]

[0098] S is an element that combines with Mn in the steel to form MnS inclusions, which have an effect of improving the machinability by cutting of the steel material. When S content in the steel substrate is less than 0.001%, the effect cannot be obtained. Therefore, when S is added to the steel substrate, the S content is 0.001% or more. On the other hand, when the S content in the steel substrate exceeds 0.100%, the large amount of MnS inclusions act as initiation points for cracking during cold working. Therefore, when S is added to the steel substrate, the S content is 0.100% or less. The S content is preferably 0.070% or less. The S content is more preferably 0.050% or less.[Sb: 0.0010% to 0.0300%]

[0099] Sb, like Sn, is an element that is likely to segregate in the steel material surface layer and has an effect of suppressing the decarburization reaction in a similar way as the enriched region where at least one of Cu and Ni is enriched. When Sb content in the steel substrate is less than 0.0010%, the effect cannot be obtained. Therefore, when Sb is added to the steel substrate, the Sb content is 0.0010% or more. On the other hand, when the Sb content in the steel substrate exceeds 0.0300%, the amount of Sb that segregates in the surface layer becomes excessive, degrading the surface characteristics of the steel material. Therefore, when Sb is added to the steel substrate, the Sb content is 0.0300% or less. The Sb content is preferably 0.0200% or less. The Sb content is more preferably 0.0150% or less.[Pb: 0.01% to 0.50%]

[0100] Pb is an element that improves the machinability by cutting of the steel material. When Pb content of the steel substrate is less than 0.01%, the effect cannot be obtained. Therefore, when Pb is added to the steel substrate, the Pb content is 0.01% or more. On the other hand, when the Pb content in the steel substrate exceeds 0.50%, the effect of improving machinability by cutting is saturated, and the amount of inclusions in the steel increases, leading to a decrease in toughness. Therefore, when Pb is added to the steel substrate, the Pb content is 0.50% or less. The Pb content is preferably 0.35% or less.[Bi: 0.001% to 0.100%]

[0101] Bi, like Pb, is an element that improves the machinability by cutting of the steel material. When Bi content of the steel substrate is less than 0.001%, the effect cannot be obtained. Therefore, when Bi is added to the steel substrate, the Bi content is 0.001% or more. On the other hand, when the Bi content in the steel substrate exceeds 0.100%, the effect of improving machinability by cutting is saturated and the amount of inclusions in the steel increases, leading to a decrease in toughness. Therefore, when Bi is added to the steel substrate, the Bi content is 0.100% or less. The Bi content is preferably 0.050% or less.[Ca: 0.0005% to 0.1000%]

[0102] Ca is an element that is solute in sulfides in the steel and has an effect of spheroidizing the sulfides, thereby suppressing degradation of the cold workability and toughness of the steel material. When Ca content in the steel substrate is less than 0.0005%, the effect cannot be obtained. Therefore, when Ca is added to the steel substrate, the Ca content is 0.0005% or more. On the other hand, when the Ca content in the steel substrate exceeds 0.1000%, the amount of Ca inclusions in the steel increases, adversely affecting the toughness and fatigue resistance of the steel material. Therefore, when Ca is added to the steel substrate, the Ca content is 0.1000% or less. The Ca content is preferably 0.0500% or less. The Ca content is more preferably 0.0300% or less.[Total Area Fraction of Ferrite and Pearlite: 90.0% or More]

[0103] To secure cold workability, the microstructure of the steel substrate needs to be a relatively soft ferrite and pearlite mixed structure. Therefore, the total area fraction of ferrite and pearlite in the steel substrate is 90.0% or more. The total area fraction is preferably 95.0% or more. An upper limit of the total area fraction of ferrite and pearlite in the steel substrate is not particularly limited, and the total area fraction may be 100.0%.

[0104] The total area fraction of ferrite and pearlite may be determined by the following procedure. Cut the steel material and prepare three samples that include the steel material surface layer, and observe the microstructure of each sample for a cross-section perpendicular to the rolling direction. Observe the microstructure in the surface layer of the sample, excluding the decarburized layer described below, using an optical microscope at a magnification of 100×. Calculate the total area fraction of ferrite and pearlite in the observed area of each field of view in five randomly selected fields of view, where the area of each field of view is 600 μm×800 μm. Calculate the average of the total area fraction obtained in each field of view for each sample, as the total area fraction of ferrite and pearlite of the steel material. For the calculation of the area fraction, the image interpretation software ImageJ may be used.[Average Vickers Hardness is 250 HV or Less]

[0105] When the average Vickers hardness of the steel substrate exceeds 250 HV, cold workability decreases. The average Vickers hardness of the steel substrate is therefore 250 HV or less. The average Vickers hardness is preferably 230 HV or less. The average Vickers hardness is more preferably 220 HV or less. On the other hand, from a practical viewpoint, the average Vickers hardness of the steel substrate is preferably 80 HV or more.

[0106] The average Vickers hardness may be determined by the following procedure. Cut the steel material and prepare three samples, and measure the hardness in a cross-section perpendicular to the rolling direction of the samples using a Vickers hardness tester. When the sample is a round steel material, there are five measurement points: one position corresponding to ½ the diameter of the round steel material in the center direction from the steel material surface (hereinafter referred to as a “D / 2 position”) and four positions corresponding to ¼ the diameter of the round steel material in the center direction from the steel material surface (hereinafter referred to as “D / 4 positions”). When the sample is a sheet material, there are five measurement points: one position corresponding to ½ of the sheet width in the center direction from one end of the sheet material and ½ of the sheet thickness in the depth direction from the steel material surface (hereinafter referred to as a “W / 2−t / 2 position”), and four positions corresponding to ¼ of the sheet width in the center direction from one end of the sheet material and ¼ of the sheet thickness in the depth direction from the steel material surface (hereinafter referred to as “W / 4−t / 4 positions”). Calculate the average value of hardness obtained at each measurement point for each sample as the average Vickers hardness of the steel material. The hardness measurement may be performed with a load of 10 kgf.<Decarburized Layer>

[0107] The following describes the decarburized layer of the hot-rolled steel material.[Total Decarburization Depth of Decarburized Layer (DM-T), as Specified in JIS G 0558, is 0.80 mm or Less]

[0108] Excessive thickness of the decarburized layer leads to a decrease in fatigue resistance and toughness as a component, and therefore excessive thickness of the decarburized layer needs to be suppressed. According to the present disclosure, the thickness of the decarburized layer is evaluated by the total decarburization depth (DM-T) as specified in JIS G 0558 (hereinafter referred to simply as “total decarburization depth”). When the total decarburization depth of the decarburized layer exceeds 0.80 mm, the degradation of properties becomes more pronounced. The total decarburization depth of the decarburized layer is therefore 0.80 mm or less. The total decarburization depth is preferably 0.50 mm or less. The total decarburization depth is more preferably 0.30 mm or less. On the other hand, a 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.

[0109] The total decarburization depth of the decarburized layer may be determined by the following procedure. Cut the steel material and prepare three samples that include the steel material surface layer, and observe the microstructure of each sample for a cross-section perpendicular to the rolling direction of the samples. Observe the surface layer microstructure of the sample using an optical microscope at a magnification of 100× with an area of 600 μm×800 μm for each field of view. Measure the total decarburization depth of the surface layer of the sample in three randomly selected fields of view, by the method described in JIS G 0558, and take the total decarburization depth at the position where the decarburized layer is deepest as the total decarburization depth in each field of view. Calculate the average of the total decarburization depth obtained in each field of view of each sample as the total decarburization depth of the decarburized layer of the steel material.[Decarburized Layer Includes Enriched Region where at Least One of Cu and Ni is Enriched]

[0110] In the decarburized layer of the hot-rolled steel material according to an embodiment of the present disclosure, there is the enriched region where at least one of Cu and Ni is enriched. Further, Sn is preferably enriched in the enriched region in addition to at least one of Cu and Ni. Hereinafter, the enriched region is defined as a region in the surface layer of the hot-rolled steel material in which the concentration of at least one of Cu and Ni is three times or more than a 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 round steel material, and at the W / 4−t / 4 position of the hot-rolled steel material when the hot-rolled steel material is sheet material.[Coverage of Enriched Region on Surface of Decarburized Layer is 50% or More]

[0111] The formation of the enriched region where at least one of Cu and Ni is enriched at the surface of the hot-rolled steel material contributes to suppression of the decarburization reaction. When the coverage of the enriched region on the surface of the decarburized layer is less than 50%, the decarburization suppression effect is insufficient. The coverage of the enriched region on the surface of the decarburized layer is therefore 50% or more. The coverage is preferably 60% or more. The coverage is more preferably 70% or more. On the other hand, an upper limit of the coverage is not particularly restricted, and the coverage of the enriched region may be 100%.

[0112] The coverage of the enriched region on the surface of the decarburized layer may be determined by the following procedure. First, cut the steel material and prepare three samples containing the steel material surface layer, and measure the reference concentrations of Cu and Ni in each sample. In order to eliminate the influence of the enriched region in the steel material surface layer and central segregation on the reference concentration, three randomly selected points in the above-mentioned positions (D / 4 position of the hot-rolled steel material for round steel material and W / 4−t / 4 position of the hot-rolled steel material for sheet material) are analyzed by an electron probe micro-analyzer (EPMA), and the average values of Cu and Ni concentrations at these three points are used as the reference concentrations of Cu and Ni, respectively.

[0113] The measurement conditions of the EPMA may be as follows: magnification of 100×, and area per field of view of 600 μm×800 μm.

[0114] Next, for each sample, perform quantitative analysis (mapping) on three randomly selected fields of view of the steel material surface by EPMA to determine the concentration of Cu and Ni. The measurement conditions of the EPMA may be as follows: magnification of 100×, and area per field of view of 600 μm×800 μm. Determine the coverage of the enriched region in each field of view by defining the area in which at least three times the concentration of at least one of Cu and Ni is present relative to the reference concentrations of Cu and Ni determined above as the enriched region. Calculate the average value of the coverage obtained at each view of each sample as the coverage of the enriched region of the steel material.[Maximum Depth of Enriched Region in Decarburized Layer is 1 μm or More and 150 μm or Less]

[0115] When 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. The maximum depth of the enriched region in the decarburized layer is therefore 1 μm or more. On the other hand, when the maximum depth of the enriched region in the decarburized layer exceeds 150 μm, this is preferred from the viewpoint of suppressing the decarburization reaction, but the depth of the enriched region is too deep and therefore cracking is more likely to occur during cold working. The maximum depth of the enriched region in the decarburized layer is therefore 150 μm or less. The maximum depth is preferably 120 μm or less. The maximum depth is more preferably 100 μm or less.

[0116] The maximum depth of the enriched region in the decarburized layer may be determined by the following method. Cut the steel material to prepare three samples including the steel material surface layer, observe three fields of view of the enriched region (an area of about 600 μm×800 μm per field of view) in a cross-section perpendicular to the rolling direction of each sample by the method described above, and measure the maximum depth of the enriched region. Determine the average of the maximum depth obtained in each field of view of each sample as the maximum depth of the enriched region in the decarburized layer of the steel material. Further, the minimum depth of the enriched region may also be determined by observations in the same manner.

[0117] Further, when the hot-rolled steel material contains Sn, Sn may also be enriched in the enriched region where at least one of Cu and Ni is enriched. Sn is also considered to be enriched when the concentration of Sn is at least three times the reference concentration.[Ratio of Sn Concentration to Sum of Cu and Ni Concentrations in Enriched Region [Sn] / ([Cu]+[Ni]) is 0.50 or Less (Preferred Range)]

[0118] 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, when the Sn concentration in the enriched region is excessively high, the grain boundary penetration depth of the enriched region becomes deeper, causing cold working cracking. The ratio of the Sn concentration to the sum of the Cu and Ni concentrations, [Sn] / ([Cu]+[Ni]), in the enriched region is therefore 0.50 or less in atomic ratio. The ratio is preferably 0.40 or less. The ratio is more preferably 0.30 or less. On the other hand, a lower limit of the atomic ratio [Sn] / ([Cu]+[Ni]) is not particularly limited, and [Sn] / ([Cu]+[Ni]) may be 0.00.

[0119] The ratio of the Sn concentration to the sum of the Cu and Ni concentrations in the enriched region [Sn] / ([Cu]+[Ni]) may be determined by the following procedure. Cut the steel material to prepare three samples including the steel material surface layer, and perform quantitative analysis (mapping) on three fields of view of a cross-section perpendicular to the rolling direction of each sample by EPMA to determine the concentrations of Cu, Ni, and Sn. EPMA measurement conditions may be set to a magnification of 100× and an area per field of view of 600 μm×800 μm. Determine the enriched region by the method described above, and calculate the atomic ratio of [Sn] / ([Cu]+[Ni]) from the measured concentration at the point in the enriched region where the Sn concentration is highest. Calculate the average value of the atomic ratio obtained in each field of view of each sample as [Sn] / ([Cu]+ [Ni]) in the enriched region of the steel material.(Method of Producing Hot-Rolled Steel Material)

[0120] Next, the method of producing hot-rolled steel material is described below. The method of producing hot-rolled steel material according to an embodiment of the present disclosure includes a process of hot rolling steel material that has the chemical composition described above, under a set of conditions including a maximum heating temperature T in a heating furnace being 1000° C. or more and 1200° C. or less, and a residence time of the steel material in the heating furnace being in a defined range.[Steel Material]

[0121] The steel material used in the method of producing hot-rolled steel material has a chemical composition consisting of C, Si, Mn, Cu, Ni, and N, with the balance being Fe and inevitable impurity. In addition to these elements, the steel material may contain the elements mentioned above as required. The content of each element is as described above.[Hot Rolling]

[0122] Hot rolling is applied to the steel material to obtain the hot-rolled steel material. During the hot rolling, when the maximum heating temperature T in the heating furnace is less than 1000° C., scale formation and growth on the steel material surface layer is difficult, and it is difficult for formation of the enriched region to progress. As a result, the coverage of the enriched region on the surface of the decarburized layer becomes small and the decarburization reaction cannot be suppressed. The maximum heating temperature T in the heating furnace is therefore 1000° C. or more. The maximum heating temperature T is preferably 1030° C. or more. On the other hand, when the maximum heating temperature T exceeds 1200° C., the scale growth rate of the steel material surface layer is too fast, and therefore the enriched region generated on the steel material surface layer is shed to the scale side (scale-off). As a result, the coverage of the enriched region on the surface of the decarburized layer becomes small and the decarburization reaction cannot be sufficiently suppressed. The maximum heating temperature T in the heating furnace is therefore 1200° C. or less. The maximum heating temperature T is preferably 1150° C. or less.

[0123] During the hot rolling, the longer the steel material is resident in the heating furnace, the longer the decarburization reaction in the heating furnace occurs. The inventors discovered that by setting the upper limit of the residence time of the steel material in the heating furnace to the time t1 (in min) determined by the following Expression (1), the decarburized layer and the enriched region can be made suitable for suppressing the decarburization reaction. On the other hand, a shorter furnace residence time is advantageous for suppressing the decarburization reaction, and therefore there is no need to define a lower limit for the furnace residence time. However, an excessively short furnace residence time may have adverse effects such as temperature variation of the material at different positions. The furnace residence time is therefore preferably 30 min or longer. Hereinafter, the furnace residence time is the time spent in the heating furnace from the time the material is charged into the heating furnace until the heated material leaves the heating furnace.t1=1⁢1⁢5⁢0-0.8⁢T-3⁢([Ni]⁢ / [Cu])-10[Sn](1)

[0124] Here, T is the maximum heating temperature, in ° C., [Ni] is Ni content in the steel material, in mass %, [Cu] is Cu content in the steel material, in mass %, and [Sn] is Sn content in the steel material, in mass %. When the steel material does not contain Sn, [Sn]=0.

[0125] For processes and conditions not described in the present disclosure, regular methods can be used.EXAMPLES

[0126] Examples are illustrated below to specifically describe the structure and effects according to the present disclosure. Note that the present disclosure is not restricted by any means to these examples and appropriate modifications may be made within the scope of the spirit of the present disclosure, all such modifications being included within the technical scope of the present disclosure.

[0127] 160 mm square billet material having the chemical compositions listed in Tables 1 and 2 was heated in a heating furnace and hot rolled under the hot rolling conditions listed in Tables 3 and 4 to obtain wire rods each having a diameter of 15 mm. Samples for microstructure observation and hardness measurement were taken from the resulting hot-rolled wire rods. Using the method described above, for each sample, the coverage of the enriched region where at least one of Cu and Ni is enriched, the maximum and minimum depths of the enriched region, the total decarburization depth of the decarburized layer (DM-T), the total area fraction of ferrite and pearlite, and the average Vickers hardness were measured. The results are listed in Tables 3 and 4. Further, for the steel samples listed in Table 2, the atomic ratio [Sn] / ([Cu]+[Ni]) was measured using the method described above, and the results are listed in Table 4.

[0128] Further, the cold workability of the hot-rolled wire rods was evaluated as follows. For each sample, after the scale on the surface layer of the hot-rolled wire rod was completely removed by pickling, the wire rod was drawn to a diameter of 14 mm and then cut to a height of 21 mm to obtain a cylindrical test piece. The test piece shape was based on the No. 1 test piece described in “Method of Testing for Cold Upsettability” (Journal of the Japan Society for Technology of Plasticity, 22 (1981), 139). The obtained test pieces were subjected to a cold compression test (under end face restraint conditions) in which the test pieces were compressed 60% in the height direction at a strain rate of 10 / s. Cold compression tests were carried out with N=3 for each case, and cracking caused by cold compression was checked by observing the side of the sample after the compression test. In Tables 3 and 4, the steel samples that did not crack in all of the N=3 cases are listed as “No” as passing products, and steel samples that had a crack in at least one of the N=3 cases are listed as “Yes” as failed products.TABLE 1Chemical composition (mass %)No.CSiMnCuNiNOthers[Ni] / [Cu]Classification10.820.880.020.2200.2100.0044—0.95Comparative steel20.641.060.550.0800.1200.0211—1.50Comparative steel30.710.031.600.1100.2400.0144—2.18Comparative steel40.410.070.040.0080.0200.0052—2.50Comparative steel50.430.410.220.5500.5100.0066—0.93Comparative steel60.550.410.550.0500.0070.0088—0.14Comparative steel70.220.760.730.4501.1200.0028—2.49Comparative steel80.040.150.440.3200.1400.0264—0.44Comparative steel90.110.221.120.2200.0200.0023—0.09Comparative steel100.380.210.790.1100.3400.0088—3.09Comparative steel110.550.440.370.0600.1700.0065—2.83Conforming steel120.440.080.220.2800.1300.0048—0.46Conforming steel130.600.120.080.0300.0200.0109—0.67Conforming steel140.780.021.450.4900.9600.0240—1.96Conforming steel150.490.780.080.0900.0300.0052—0.33Conforming steel160.630.200.750.2200.5800.0177—2.64Conforming steel170.110.440.970.2800.0300.0110—0.11Conforming steel180.040.970.550.0100.0100.0022—1.00Conforming steel190.440.711.170.3800.7700.0197—2.03Conforming steel200.200.050.790.1600.3900.0065—2.44Conforming steel210.330.110.020.1500.4400.0077—2.93Conforming steel220.060.490.790.1400.2200.0050Cr: 1.12 1.57Conforming steel230.310.200.660.2200.0700.0069Mo: 0.29   0.32Conforming steel240.180.210.770.2800.4400.0110Al: 0.048 1.57Conforming steel250.330.430.990.1700.1100.0044Ti: 0.048 0.65Conforming steel260.220.020.300.2700.2000.0057V: 0.140 0.74Conforming steel270.280.220.210.2600.5100.0072Nb: 0.029  1.96Conforming steel280.050.410.730.1100.1000.0148 B: 0.00220.91Conforming steel290.100.320.280.0200.0300.0101P: 0.0221.50Conforming steel300.270.070.860.1200.2100.0177S: 0.0501.75Conforming steel310.470.330.330.1600.0500.0045Sb: 0.01250.31Conforming steel320.080.170.110.0200.0200.0025Pb: 0.33 1.00Conforming steel330.330.030.020.2300.1000.0098Bi: 0.041 0.43Conforming steel340.170.220.050.2700.3300.0087Ca: 0.00211.22Conforming steel* Underlining indicates value outside scope of disclosure.* The balance other than the above chemical composition is Fe and inevitable impurity.TABLE 2Chemical composition (mass %)No.CSiMnCuNiSnNOthers[Ni] / [Cu]Classification350.130.060.660.0200.0200.0230.0041—1.00Comparative steel360.210.330.710.1400.0800.0140.0066—0.57Conforming steel370.740.031.480.4800.9500.0110.0233—1.98Conforming steel380.410.760.040.1100.0220.0150.0055—0.20Conforming steel390.640.130.660.2800.5600.0110.0147—2.00Conforming steel400.130.440.230.3200.0600.0120.0087—0.19Conforming steel410.090.020.310.0100.0100.0020.0025—1.00Conforming steel420.350.801.050.3700.8800.0310.0187—2.38Conforming steel430.180.020.820.1300.3700.0500.0071—2.85Conforming steel440.300.100.020.1300.3300.0200.0055—2.54Conforming steel450.080.330.810.1300.2000.0050.0051Cr: 0.88 1.54Conforming steel460.210.030.330.1800.0600.0210.0069Mo: 0.05   0.33Conforming steel470.310.180.650.2200.3100.0300.0102Al: 0.041 1.41Conforming steel480.440.100.980.1500.1000.0210.0047Ti: 0.031 0.67Conforming steel490.170.020.280.2800.1500.0110.0041V: 0.051 0.54Conforming steel500.210.350.100.2200.3300.0170.0103Nb: 0.015  1.50Conforming steel510.070.100.730.0800.0500.0100.0120 B: 0.00180.63Conforming steel520.130.250.550.0200.0100.0040.0025P: 0.0110.50Conforming steel530.220.140.790.1500.0900.0120.0155S: 0.0440.60Conforming steel540.410.310.440.1200.0700.0210.0051Sb: 0.00500.58Conforming steel550.100.200.150.0200.0200.0030.0023Pb: 0.08 1.00Conforming steel560.060.050.020.2300.1100.0140.0071Bi: 0.015 0.48Conforming steel570.210.200.880.2400.1000.0150.0065Ca: 0.00100.42Conforming steel* Underlining indicates value outside scope of disclosure.* The balance other than the above chemical composition is Fe and inevitable impurity.TABLE 3TotalAverageHot rolling conditionsdecarburizationTotal areaVickersMaximumEnriched regiondepth offraction ofhardnessheatingResidenceMinimumMaximumdecarburizedferrite andof steelCrackingtemp. Ttimet1Coveragedepthdepthlayer (DM-T,pearlitesubstrateduring coldClassi-No.(° C.)(min)(min)(%)(μm)(μm)mm)(%)(HV)compressionfication11138110237734710.6687.5261YesComparativeExample21070206290866500.5188.6252YesComparativeExample31102148262698480.7085.1270YesComparativeExample41166194210132130.8594.1210NoComparativeExample51077 97286603155 0.3195.9227YesComparativeExample61135150242246330.8395.3219NoComparativeExample71140 41231883160 0.4797.1192YesComparativeExample81005177345942580.17100.0 105YesComparativeExample91181133205779170 0.28100.0 130YesComparativeExample101055195297845155 0.2292.7166YesComparativeExample111211170173464130 0.8391.8152NoComparativeExample12 985211361 92160.8193.0160NoComparativeExample131110271260533260.8891.1181NoComparativeExample141003231342973148 0.6690.6241NoExample151122177251615410.4294.0227NoExample161075185282754330.4791.7245NoExample171144204234888290.21100.0 115NoExample181022293329522110.22100.0  87NoExample191035250316904115 0.1896.1193NoExample201192 48189797730.7798.4180NoExample211164 89210803410.4194.5201NoExample221108122259865380.10100.0  98NoExample231088235279724270.2795.3179NoExample241085204277889780.2197.1171NoExample251071241291687740.2796.5177NoExample261110179260712590.2297.3168NoExample271101101263825820.2996.2170NoExample281079133284747400.08100.0  90NoExample291066164293772170.11100.0 101NoExample301094 77270723950.2896.4163NoExample311105144265767280.2695.1192NoExample321138 53237742150.10100.0 104NoExample331079168285906440.2896.6187NoExample341077280285794280.1997.1160NoExample* Underlining indicates value outside scope of disclosure.TABLE 4Totaldecar-Enriched regionburizationAverageHot rolling conditions[Sn] / depth ofTotal areaVickersMaximum([Cu] +decar-fraction ofhardnessheatingResidenceMinimumMaximum[Ni])burizedferrite andof steelCrackingtemp. Ttimet1Coveragedepthdepthatomiclayer (DM-pearlitesubstrateduring coldClassi-No.(° C.)(min)(min)(%)(μm)(μm)ratioT, mm)(%)(HV)compressionfication351086200278743161 0.600.15100.0119YesComparativeExample361165220216914155 0.530.2198.8133YesComparativeExample371011200335984141 0.110.4490.4247NoExample381118144255564350.210.3396.3227NoExample391055107300773250.410.1392.2240NoExample401122204252805350.180.10100.0122NoExample411007245341522150.030.21100.0102NoExample421033211316863118 0.460.1093.6211NoExample431188 64191735770.410.3896.1177NoExample441170104206612540.360.2292.5208NoExample451080133281838310.100.12100.0103NoExample461073218290874260.170.2198.7160NoExample471104188262917550.210.1697.5180NoExample481042211314885320.130.1694.1189NoExample491070189292864280.090.1997.3135NoExample501097104268913440.080.1198.8171NoExample511082133282884380.110.06100.088NoExample521044177313752130.030.22100.0112NoExample531091 88275883400.210.2098.3150NoExample541113180258913510.270.1395.1204NoExample551101102266722180.050.18100.0111NoExample561131170244964510.110.21100.088NoExample571061205300883470.100.1899.3131NoExample* Underlining indicates value outside scope of disclosure.The following is a description of each Example and Comparative Example.No. 1, 2, and 3 are Comparative Examples in which the C, Si, or Mn content exceeded the range specified in the present disclosure. These steel samples had excessive C, Si, or Mn content and hardenability was too high, and therefore the microstructure contained bainite or martensite, and the total area fraction of ferrite and pearlite in the steel substrate was less than 90.0%. Further, the average Vickers hardness of the steel substrate also exceeded 250 HV, and therefore cold workability was low and cracking occurred after cold compression tests.No. 4 and 6 are Comparative Examples in which the Cu or Ni content was less than the range specified in the present disclosure. These steel samples had low Cu or Ni content, and therefore the coverage of the enriched region was less than the range specified in the present disclosure and the total decarburization depth exceeded the range specified in the present disclosure.

[0132] No. 5 and 7 are Comparative Examples in which the Cu or Ni content exceeded the range specified in the present disclosure. These steel samples had excessive Cu or Ni content, and the maximum depth of the enriched region exceeded the range specified in the present disclosure, and therefore cracking occurred in the cold compression test.

[0133] No. 8 is a Comparative Example where the N content exceeded the range specified in the present disclosure. This steel sample had high solute N content in the steel, and low cold workability due to the effect of dynamic strain aging, and therefore cracking occurred during the cold compression test.

[0134] No. 9 and 10 are Comparative Examples in which the ratio of Ni to Cu, [Ni] / [Cu], was outside the range specified in the present disclosure. In these steel samples, [Ni] / [Cu] was not appropriate, and therefore the maximum depth of the enriched region exceeded the range specified in the present disclosure, and cracking occurred during the cold compression test.

[0135] No. 11 is a Comparative Example in which the maximum heating temperature T during hot rolling exceeded the range specified in the present disclosure. In this example, the heating temperature of the steel material was too high, and therefore scale growth rate was fast, coverage of the enriched region was low, and the total decarburization depth exceeded the range specified in the present disclosure.

[0136] No. 12 is a Comparative Example in which the maximum heating temperature T during hot rolling was less than the range specified in the present disclosure. In this example, the heating temperature was too low, and therefore formation of the enriched region on the steel material surface layer was insufficient, and the coverage of the enriched region was well below the range specified in the present disclosure, resulting in the total decarburization depth exceeding the range specified in the present disclosure.

[0137] No. 13 is a Comparative Example in which the furnace residence time in the heating furnace during hot rolling exceeded the range specified in the present disclosure. In No. 13, the coverage and maximum depth of the enriched region were in the range specified in the present disclosure, but due to the long time for the decarburization reaction to occur, the total decarburization depth exceeded the range specified in the present disclosure.

[0138] No. 35 is a Comparative Example of steel containing Sn, in which the Sn content exceeded the upper limit ([Cu]+[Ni]) / 2. The atomic ratio of [Sn] / ([Cu]+[Ni]) in the enriched region exceeded 0.5 due to the excessive amount of Sn in this steel sample, and the maximum depth of the enriched region also exceeded the range specified in the present disclosure, and therefore cracking occurred during the cold compression test.

[0139] No. 36 is a Comparative Example of steel containing Sn, in which the furnace residence time in the heating furnace during hot rolling exceeded the range specified in the present disclosure. 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 specified in the present disclosure, and therefore cracking occurred during the cold compression test.

[0140] In contrast to the Comparative Examples described above, No. 14 to 34 and No. 37 to 57 had steel material chemical compositions and hot rolling conditions that were in the ranges according to the present disclosure, as listed in Tables 1 to 4. In these examples of the hot-rolled steel material, the coverage and maximum depth of the enriched region, the total decarburization depth, and the total area fraction of ferrite and pearlite were in the respective ranges specified in the present disclosure. Further, in the examples listed in Tables 2 and 4, [Sn] / ([Cu]+[Ni]) in the enriched region was in the range specified in the present disclosure. Further, these examples had excellent average Vickers hardness and cracking properties during cold compression. That is, No. 14 to 34 and No. 37 to 57 had excellent decarburization reaction suppression and cold workability.INDUSTRIAL APPLICABILITY

[0141] According to the present disclosure, hot-rolled steel material is provided that has sufficient suppression of thickness of the decarburized layer and excellent cold workability, and a method of producing same is provided.REFERENCE SIGNS LIST100 hot-rolled steel material

[0143] 10 steel substrate

[0144] 20 decarburized layer

[0145] 22 enriched region

[0146] 24 decarburized layer surface

[0147] 30 scale

[0148] A maximum depth of enriched region

[0149] B total decarburization depth of decarburized layer

Examples

examples

[0126]Examples are illustrated below to specifically describe the structure and effects according to the present disclosure. Note that the present disclosure is not restricted by any means to these examples and appropriate modifications may be made within the scope of the spirit of the present disclosure, all such modifications being included within the technical scope of the present disclosure.

[0127]160 mm square billet material having the chemical compositions listed in Tables 1 and 2 was heated in a heating furnace and hot rolled under the hot rolling conditions listed in Tables 3 and 4 to obtain wire rods each having a diameter of 15 mm. Samples for microstructure observation and hardness measurement were taken from the resulting hot-rolled wire rods. Using the method described above, for each sample, the coverage of the enriched region where at least one of Cu and Ni is enriched, the maximum and minimum depths of the enriched region, the total decarburization depth of the decar...

TECHNICAL FIELD

[0001] The present disclosure relates to hot-rolled steel material and a method of producing same.BACKGROUND

[0002] Components used in automobiles and for other applications are made by first applying preliminary processing such as wire drawing to hot-rolled steel material, which is then shaped into the component by hot working or cold working, and then subjected to cutting work, heat treatment, or the like as required to become a final product. In recent years, the production of components by cold working (for example, cold forging) has become widespread, cold working being advantageous in terms of production costs such as dimensional accuracy and yield rate of parts, or in terms of reducing energy consumption for heating material. Components produced by cold working are either cold worked as they are, or after heat treatment (quenching and tempering heat treatment, induction hardening and tempering heat treatment, and the like) for strength adjustment, depending on the required strength, before becoming final products.

[0003] The surface layer of hot-rolled steel material subjected to the cold working has a decarburized layer produced during hot rolling. When the decarburized layer of hot-rolled steel material is excessively thick, the components after cold working or components after cold working and induction hardening and tempering heat treatment have soft portions with low carbon content remaining in the vicinity of the steel material surface layer, resulting in inferior strength or fatigue resistance as components. Further, when the decarburized layer of hot-rolled steel material is excessively thick, in a component that is 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, and therefore the soft portions described above do not remain in the steel material surface layer. However, the amount of carbon differs between the decarburized layer and the core, and therefore the austenite reverse transformation temperatures are different. In this case, the timing of austenite reverse transformation differs between the steel material surface layer and the core, and therefore the reverse transformation austenite grain size is non-uniform between the steel material surface layer and the core. Such grain size non-uniformity induces coarsening of the crystal grains, leading to decreased fatigue resistance and toughness of the component.

[0004] As mentioned above, when the decarburized layer of the hot-rolled steel material surface layer is excessively thick, various properties of the final product degrade, and therefore it is necessary to suppress excessive thickness of the decarburized layer at the hot rolling stage. Further, when cracking occurs during cold working, use as a final product becomes impossible, and therefore hot-rolled steel material is also required to have excellent cold workability.

[0005] In Patent Literature (PTL) 1, it is described 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 both soften the hot-rolled steel material and suppress the decarburization reaction.CITATION LISTPatent Literature

[0006] PTL 1: JP 2004-250768 ASUMMARYTechnical Problem

[0007] However, according to PTL 1, Te and Se are used, which are rare and toxic elements that require special care in handling, and it is difficult to say that this technology can be generally applied. Further, both the technology of PTL 1 and earlier technologies require precise control of the thermal history after hot rolling, which remains a challenge from the viewpoint of productivity. Thus, a technique that combines the suppression of decarburized layer thickness with excellent cold workability had not been reported.

[0008] In view of the above problem, it would be helpful to provide hot-rolled steel material for which thickness of the decarburized layer is sufficiently suppressed and that has excellent cold workability, and a method of producing same.Solution to Problem

[0009] To solve the above problem, the inventors focused on suppression of the decarburization reaction by adding Cu and Ni, which are elements that are less easily oxidized than Fe, and made the following discoveries. When scale is formed on steel material surface by heating during hot rolling, Fe is preferentially oxidized and becomes scale. On the other hand, Cu and Ni are not oxidized and are left in the steel material surface layer, forming an enriched region where at least one of Cu and Ni is enriched. The formation of such an enriched region effectively suppresses the decarburization reaction that occurs in the steel material surface layer.

[0010] However, it is well known that Cu concentrated in the steel material surface layer penetrates into grain boundaries during hot rolling, embrittling the grain boundaries and inducing cracks during cold working. The inventors conducted further studies and discovered that by optimizing a balance between the amounts of Ni and Cu in the hot-rolled steel material, it is possible to suppress to a certain range the depth of the enriched region where at least one of Cu and Ni is enriched in the steel material surface layer, thereby suppressing occurrence of cracking during cold working.

[0011] In addition, the inventors discovered that the maximum heating temperature during hot rolling needs to be appropriately controlled in order to make the enriched region where at least one of Cu and Ni is enriched in hot-rolled steel material suitable for suppressing the decarburization reaction. Further, the inventors discovered that in order to effectively suppress the decarburization reaction, it is necessary to appropriately control the residence time of steel material in the heating furnace during hot rolling.

[0012] Further, the inventors have made the following discoveries as a result of extensive studies. In hot-rolled steel material with Sn added in addition to Cu and Ni, Sn is also enriched in the enriched region where at least one of Cu and Ni is enriched in the steel material surface layer. Sn has an effect of lowering the melting point of the enriched region, and therefore the enriched region is more easily formed on the steel material surface layer than in a case where Sn is not added, and the decarburization reaction suppression effect of the enriched region is also easier to obtain.

[0013] On the other hand, when 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 grain boundaries and embrittles the grain boundaries, causing cracking during cold working. Therefore, in the production of hot-rolled steel material with Sn added in addition to Cu and Ni, it is necessary to limit the Sn concentration in the enriched region to a defined range to avoid excessive grain boundary penetration depth. As specific methods, effective methods include adjusting the amount of Sn added according to the amount of Cu and Ni, shortening the furnace residence time in the heating furnace during hot rolling according to the amount of Sn, and the like. Primary features of the present disclosure are as follows.

[0014] [1] Hot-rolled steel material comprising: a steel substrate that has a chemical composition containing (consisting of), in mass %,

[0015] C: 0.03% to 0.80%,

[0016] Si: 0.01% to 1.00%,

[0017] Mn: 0.01% to 1.50%,

[0018] Cu: 0.010% to 0.500%,

[0019] Ni: 0.010% to 1.000%, and

[0020] N: 0.0020% to 0.0250%,

[0021] with the balance being Fe and inevitable impurity, wherein a ratio of Ni to Cu, [Ni] / [Cu], is 0.10 or more and 3.00 or less; and a decarburized layer formed on a surface of the steel substrate, wherein

[0022] the decarburized layer comprises an enriched region where at least one of Cu and Ni is enriched,

[0023] coverage of the enriched region on the surface of the decarburized layer is 50% or more,

[0024] maximum depth of the enriched region in the decarburized layer is 1 μm or more and 150 μm or less,

[0025] total decarburization depth of the decarburized layer, DM-T, as specified in JIS G 0558, is 0.80 mm or less,

[0026] total area fraction of ferrite and pearlite in the steel substrate is 90.0% or more, and

[0027] average Vickers hardness in the steel substrate is 250 HV or less.

[0028] [2] The hot-rolled steel material according to [1], wherein the chemical composition further contains, in mass %,

[0029] Sn: 0.001% or more and ([Ni]+[Cu]) / 2 or less,

[0030] in addition to at least one of Cu and Ni, Sn is enriched in the enriched region, and

[0031] in the enriched region, an atomic ratio of Sn concentration to the sum of Cu and Ni concentrations, [Sn] / ([Cu]+[Ni]), is 0.50 or less.

[0032] [3] The hot-rolled steel material according to [1] or [2], wherein the chemical composition further contains, in mass %, at least one element selected from the group consisting of:

[0033] Cr: 0.01% to 1.50%,

[0034] Mo: 0.01% to 0.50%,

[0035] Al: 0.001% to 0.100%,

[0036] Ti: 0.001% to 0.100%,

[0037] V: 0.001% to 0.300%,

[0038] Nb: 0.001% to 0.100%, and

[0039] B: 0.0005% to 0.0050%.

[0040] [4] The hot-rolled steel material according to any one of [1] to [3], wherein the chemical composition further contains, in mass %, at least one element selected from the group consisting of:

[0041] P: 0.001% to 0.100%,

[0042] S: 0.001% to 0.100%, and

[0043] Sb: 0.0010% to 0.0300%.

[0044] [5] The hot-rolled steel material according to any one of [1] to [4], wherein the chemical composition further contains, in mass %, at least one element selected from the group consisting of:

[0045] Pb: 0.01% to 0.50%,

[0046] Bi: 0.001% to 0.100%, and

[0047] Ca: 0.0005% to 0.1000%.

[0048] [6] A method of producing hot-rolled steel material, the method comprising a process of hot rolling steel material that has a chemical composition containing (consisting of), in mass %,

[0049] C: 0.03% to 0.80%,

[0050] Si: 0.01% to 1.00%,

[0051] Mn: 0.01% to 1.50%,

[0052] Cu: 0.010% to 0.500%,

[0053] Ni: 0.010% to 1.000%, and

[0054] N: 0.0020% to 0.0250%,

[0055] with the balance being Fe and inevitable impurity, wherein a ratio of Ni to Cu, [Ni] / [Cu], is 0.10 or more and 3.00 or less, under a set of conditions including a maximum heating temperature T in a heating furnace being 1000° C. or more and 1200° C. or less, and a residence time of the steel material in the heating furnace being a time t1 or shorter, in min, determined by Expression (1) below, to obtain the hot-rolled steel material,t1=1150−0.8T−3([Ni] / [Cu])−10 [Sn]  (1)where T is the maximum heating temperature, in ° C., [Ni] is Ni content in the steel material, in mass %, [Cu] is Cu content in the steel material, in mass %, and [Sn] is Sn content in the steel material, in mass %.

[0057] [7] The method of producing hot-rolled steel material according to [6], wherein the chemical composition further contains, in mass %,

[0058] Sn: 0.001% or more and ([Ni]+[Cu]) / 2 or less.

[0059] [8] The method of producing hot-rolled steel material according to [6] or [7], wherein the chemical composition further contains, in mass %, at least one element selected from the group consisting of:

[0060] Cr: 0.01% to 1.50%,

[0061] Mo: 0.01% to 0.50%,

[0062] Al: 0.001% to 0.100%,

[0063] Ti: 0.001% to 0.100%,

[0064] V: 0.001% to 0.300%,

[0065] Nb: 0.001% to 0.100%, and

[0066] B: 0.0005% to 0.0050%.

[0067] [9] The method of producing hot-rolled steel material according to any one of [6] to [8], wherein the chemical composition further contains, in mass %, at least one element selected from the group consisting of:

[0068] P: 0.001% to 0.100%,

[0069] S: 0.001% to 0.100%, and

[0070] Sb: 0.0010% to 0.0300%.

[0071]

[10] The method of producing hot-rolled steel material according to any one of [6] to [9], wherein the chemical composition further contains, in mass %, at least one element selected from the group consisting of:

[0072] Pb: 0.01% to 0.50%,

[0073] Bi: 0.001% to 0.100%, and

[0074] Ca: 0.0005% to 0.1000%.Advantageous Effect

[0075] The present disclosure provides hot-rolled steel material for which thickness of the decarburized layer is sufficiently suppressed and that has excellent cold workability, and a method of producing same.BRIEF DESCRIPTION OF THE DRAWINGS

[0076] In the accompanying drawings:

[0077] FIG. 1 is a schematic diagram illustrating a cross-section of hot-rolled steel material according to an embodiment of the present disclosure.DETAILED DESCRIPTION

[0078] The following is a description of embodiments of the hot-rolled steel material and the method of producing same. The embodiments described below are example embodiments of the present disclosure, and do not limit configuration to the specific examples described.(Hot-Rolled Steel Material)

[0079] FIG. 1 illustrates a cross-section of hot-rolled steel material according to an embodiment of the present disclosure. Hot-rolled steel material 100 includes a steel substrate 10 and a decarburized layer 20 formed on a surface of the steel substrate 10. Further, scale 30 may be on the decarburized layer 20. Further, in the decarburized layer 20, there is an enriched region 22 where at least one of Cu and Ni is enriched. Coverage of the enriched region 22 on a surface 24 of the decarburized layer 20 is 50% or more. Maximum depth A of the enriched region 22 in the decarburized layer 20 is 1 μm or more and 150 μm or less. Total decarburization depth B of the decarburized layer 20 (DM-T), as specified in JIS G 0558, is 0.80 mm or less. Total area fraction of ferrite and pearlite in the steel substrate 10 is 90.0% or more. Average Vickers hardness in the steel substrate 10 is 250 HV or less.<Steel Substrate>

[0080] First, the steel substrate of the hot-rolled steel material according to an embodiment of the present disclosure is described. The steel substrate has a chemical 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%, with the balance being Fe and inevitable impurity, and a ratio of Ni to Cu, [Ni] / [Cu], is 0.10 or more and 3.00 or less. In the following description of chemical composition, “%” indicating content means “mass %” unless otherwise specified. Further, [Ni] and [Cu] represent Ni content and Cu content in the steel substrate, respectively.[C: 0.03% to 0.80%]

[0081] C is an element added to secure strength of the hot-rolled steel material. When C content of the steel substrate is less than 0.03%, the required strength cannot be secured. The C content of the steel substrate is therefore 0.03% or more. The C content of the steel substrate is preferably 0.05% or more. On the other hand, when the C content of the steel substrate exceeds 0.80%, hardenability becomes too high, resulting in a microstructure containing hard bainite or martensite, which increases rolled material hardness and decreases cold workability. The C content of the steel substrate is therefore 0.80% or less. The C content of the steel substrate is preferably 0.65% or less. The C content of the steel substrate is more preferably 0.50% or less.[Si: 0.01% to 1.00%]

[0082] Si is a deoxidation element during refining and also improves the strength and hardenability of the hot-rolled steel material. When Si content of the steel substrate is less than 0.01%, the effects cannot be obtained. The Si content of the steel substrate is therefore 0.01% or more. On the other hand, when the Si content of the steel substrate exceeds 1.00%, hardenability becomes too high, resulting in a microstructure containing hard bainite or martensite, which increases rolled material hardness and decreases cold workability. The Si content of the steel substrate is therefore 1.00% or less. The Si content of the steel substrate is preferably 0.80% or less. The Si content of the steel substrate is more preferably 0.50% or less.[Mn: 0.01% to 1.50%]

[0083] Mn is an element that improves the strength and hardenability of the hot-rolled steel material. When Mn content of the steel substrate is less than 0.01%, the effects cannot be obtained. The Mn content of the steel substrate is therefore 0.01% or more. On the other hand, when the Mn content of the steel substrate exceeds 1.50%, hardenability becomes too high, resulting in a microstructure containing hard bainite or martensite, which increases rolled material hardness and decreases cold workability. The Mn content of the steel substrate is therefore 1.50% or less. The Mn content of the steel substrate is preferably 1.20% or less. The Mn content of the steel substrate is more preferably 1.00% or less.[Cu: 0.010% to 0.500%]

[0084] Cu is an element that is less oxidizable than Fe and forms an enriched region in the steel material surface layer during scale formation during hot rolling, thereby suppressing the decarburization reaction. When Cu content of the steel substrate is less than 0.010%, the enriched region is not sufficiently formed in the steel material surface layer, and the effect of decarburization suppression is not sufficient. The Cu content of the steel substrate is therefore 0.010% or more. On the other hand, when the Cu content in the steel substrate exceeds 0.500%, the enriched region in the steel material surface layer becomes excessively deep and occurrence of cracking during cold working becomes more likely. The Cu content in the steel substrate is therefore 0.500% or less. The Cu content in the steel substrate is preferably 0.400% or less. The Cu content in the steel substrate is more preferably 0.350% or less.[Ni: 0.010% to 1.000%]

[0085] Ni, like Cu, is an element that is less oxidizable than Fe and forms the enriched region in the steel material surface layer during scale formation during hot rolling, thereby suppressing the decarburization reaction. When Ni content of the steel substrate is less than 0.010%, the enriched region is not sufficiently formed in the steel material surface layer, and the effect of decarburization suppression is not sufficient. The Ni content of the steel substrate is therefore 0.010% or more. On the other hand, when the Ni content in the steel substrate exceeds 1.000%, the enriched region in the steel material surface layer becomes excessively deep and occurrence of cracking during cold working becomes more likely. The Ni content of the steel substrate is therefore 1.000% or less. The Ni content of the steel substrate is preferably 0.800% or less. The Ni content of the steel substrate is more preferably 0.600% or less.[N: 0.0020% to 0.0250%]

[0086] N is an element that has an effect of helping prevent crystal grain coarsening by combining with nitride forming elements in steel to form nitride, which act as grain boundary pinning particles. When N content of the steel substrate is less than 0.0020%, the effect cannot be obtained. The N content of the steel substrate is therefore 0.0020% or more. On the other hand, when the N content of the steel substrate exceeds 0.0250%, not only do blowholes form in the steel, but solute N in the steel causes dynamic strain aging, and cracking becomes more likely to occur during cold working. The N content of the steel substrate is therefore 0.0250% or less. The N content of the steel substrate is preferably 0.0200% or less. The N content of the steel substrate is more preferably 0.0180% or less.[Ratio of Ni to Cu (Mass Ratio) [Ni] / [Cu] is 0.10 or More and 3.00 or Less]

[0087] According to the present disclosure, the decarburization reaction is suppressed by the enriched region where at least one of Cu and Ni is enriched in the steel material surface layer, and therefore focusing on the Cu content and Ni content independently is not sufficient, and considering the balance of the content of these elements is necessary. When the ratio of Ni to Cu (mass ratio) [Ni] / [Cu] is less than 0.10, that is, when the Cu content is excessive relative to the Ni content, Cu concentrated in the steel material surface layer penetrates to grain boundaries, resulting in an excessively deep enriched region, and cracking becomes more likely to occur during cold working. [Ni] / [Cu] is therefore 0.10 or more. [Ni] / [Cu] is preferably 0.15 or more. [Ni] / [Cu] is more preferably 0.20 or more. On the other hand, when [Ni] / [Cu] exceeds 3.00, that is, when the Ni content is excessive relative to the Cu content, the enriched region in the steel material surface layer also becomes excessively deep and cracking becomes more likely to occur during cold working. [Ni] / [Cu] is therefore 3.00 or less. [Ni] / [Cu] is preferably 2.50 or less. [Ni] / [Cu] is more preferably 2.00 or less.

[0088] The steel substrate of the hot-rolled steel material according to an embodiment of the present disclosure may further contain the following elements as required.[Sn: 0.001% or more and ([Ni]+[Cu]) / 2 or less]

[0089] Sn has an effect of lowering the melting point of the enriched region where at least one of Cu and Ni is enriched, and therefore the addition of Sn facilitates the formation of the enriched region in the steel material surface layer, and the decarburization reaction suppression effect of the enriched region is easier to obtain. To obtain the effect, when Sn is added to the steel substrate, Sn content is 0.001% or more. On the other hand, excessive addition of Sn causes deep grain boundary penetration depth in the enriched region, which leads to grain boundary embrittlement and cold work cracking. Therefore, when Sn is added to the steel substrate, the Sn content is ([Ni]+ [Cu]) / 2 or less. The Sn content is preferably ([Ni]+[Cu]) / 3 or less.[Cr: 0.01% to 1.50%]

[0090] Cr is an element that improves the hardenability of the steel material. When Cr content of the steel substrate is less than 0.01%, the effect cannot be obtained. Therefore, when Cr is added to the steel substrate, the Cr content is 0.01% or more. On the other hand, when the Cr content of the steel substrate exceeds 1.50%, the effect of the addition is saturated and the hardenability of the steel material becomes excessive, increasing the hardness of the steel material and decreasing cold workability. Therefore, when Cr is added to the steel substrate, the Cr content is 1.50% or less. The Cr content is preferably 1.30% or less. The Cr content is more preferably 1.15% or less.[Mo: 0.01% to 0.50%]

[0091] Mo is an element that greatly improves the hardenability of the steel material when added in small amounts. When Mo content in the steel substrate is less than 0.01%, the effect cannot be obtained. Therefore, when Mo is added to the steel substrate, the Mo content is 0.01% or more. On the other hand, when the Mo content of the steel substrate exceeds 0.50%, the effect of the addition is saturated and the hardenability of the steel material becomes excessive, increasing the hardness of the steel material and decreasing cold workability. Therefore, when Mo is added to the steel substrate, the Mo content is 0.50% or less. The Mo content is preferably 0.30% or less.[Al: 0.001% to 0.100%]

[0092] Al is a deoxidation element and also an element that combines with N in the steel to form nitrides, contributing to crystal grain refinement. When Al content in the steel substrate is less than 0.001%, the effects cannot be obtained. Therefore, when Al is added to the steel substrate, the Al content is 0.001% or more. On the other hand, when the Al content in the steel substrate exceeds 0.100%, the amount of Al oxides in the steel increases and cracking becomes more likely to occur during cold working, and fatigue fracture properties as a component also degrade. Therefore, when Al is added to the steel substrate, the Al content is 0.100% or less. The Al content is preferably 0.080% or less. The Al content is more preferably 0.050% or less.[Ti: 0.001% to 0.100%]

[0093] Ti, like Al, is an element that combines with N in the steel to form nitrides, contributing to crystal grain refinement. When Ti content of the steel substrate is less than 0.001%, the effect cannot be obtained. Therefore, when Ti is added to the steel substrate, the Ti content is 0.001% or more. On the other hand, when the Ti content in the steel substrate exceeds 0.100%, the amount of Ti precipitates in the steel becomes excessive, increasing the hardness of the steel material and decreasing cold workability. Therefore, when Ti is added to the steel substrate, the Ti content is 0.100% or less. The Ti content is preferably 0.080% or less. The Ti content is more preferably 0.050% or less.[V: 0.001% to 0.300%]

[0094] V, like Al and Ti, is an element that combines with N in the steel to form nitrides, contributing to crystal grain refinement. Further, Vis an element that contributes to increased steel material strength. When V content in the steel substrate is less than 0.001%, the effects cannot be obtained. Therefore, when V is added to the steel substrate, the V content is 0.001% or more. On the other hand, when the V content in the steel substrate exceeds 0.300%, the amount of V precipitates in the steel becomes excessive, increasing the hardness of the steel material and decreasing cold workability as well as the toughness of the steel material. Therefore, when V is added to the steel substrate, the V content is 0.300% or less. The V content is preferably 0.200% or less. The V content is more preferably 0.150% or less.[Nb: 0.001% to 0.100%]

[0095] Nb is an element that combines with carbon in the steel to form carbides, contributing to crystal grain refinement. When Nb content of the steel substrate is less than 0.001%, the effect cannot be obtained. Therefore, when Nb is added to the steel substrate, the Nb content is 0.001% or more. On the other hand, when the Nb content in the steel substrate exceeds 0.100%, the amount of Nb carbides in the steel becomes excessive, increasing the hardness of the steel material and decreasing cold workability. Therefore, when Nb is added to the steel substrate, the Nb content is 0.100% or less. The Nb content is preferably 0.050% or less. The Nb content is more preferably 0.030% or less.[B: 0.0005% to 0.0050%]

[0096] Mo is an element that greatly improves the hardenability of the steel material when added in small amounts. When B content of the steel substrate is less than 0.0005%, the effect cannot be obtained. Therefore, when B is added to the steel substrate, the B content is 0.0005% or more. On the other hand, when the B content in the steel substrate exceeds 0.0050%, the effect of improving hardenability becomes saturated. Therefore, when B is added to the steel substrate, the B content is 0.0050% or less. The B content is preferably 0.0040% or less. The B content is more preferably 0.0030% or less.[P: 0.001% to 0.100%]

[0097] P is an element for increasing the strength of the steel material. When P content of the steel substrate is less than 0.001%, the effect cannot be obtained. Therefore, when P is added to the steel substrate, the P content is 0.001% or more. On the other hand, when the P content in the steel substrate exceeds 0.100%, P segregates to grain boundaries and decreases the toughness of the steel material. Therefore, when P is added to the steel substrate, the P content is 0.100% or less. The P content is preferably 0.050% or less. The P content is more preferably 0.030% or less.[S: 0.001% to 0.100%]

[0098] S is an element that combines with Mn in the steel to form MnS inclusions, which have an effect of improving the machinability by cutting of the steel material. When S content in the steel substrate is less than 0.001%, the effect cannot be obtained. Therefore, when S is added to the steel substrate, the S content is 0.001% or more. On the other hand, when the S content in the steel substrate exceeds 0.100%, the large amount of MnS inclusions act as initiation points for cracking during cold working. Therefore, when S is added to the steel substrate, the S content is 0.100% or less. The S content is preferably 0.070% or less. The S content is more preferably 0.050% or less.[Sb: 0.0010% to 0.0300%]

[0099] Sb, like Sn, is an element that is likely to segregate in the steel material surface layer and has an effect of suppressing the decarburization reaction in a similar way as the enriched region where at least one of Cu and Ni is enriched. When Sb content in the steel substrate is less than 0.0010%, the effect cannot be obtained. Therefore, when Sb is added to the steel substrate, the Sb content is 0.0010% or more. On the other hand, when the Sb content in the steel substrate exceeds 0.0300%, the amount of Sb that segregates in the surface layer becomes excessive, degrading the surface characteristics of the steel material. Therefore, when Sb is added to the steel substrate, the Sb content is 0.0300% or less. The Sb content is preferably 0.0200% or less. The Sb content is more preferably 0.0150% or less.[Pb: 0.01% to 0.50%]

[0100] Pb is an element that improves the machinability by cutting of the steel material. When Pb content of the steel substrate is less than 0.01%, the effect cannot be obtained. Therefore, when Pb is added to the steel substrate, the Pb content is 0.01% or more. On the other hand, when the Pb content in the steel substrate exceeds 0.50%, the effect of improving machinability by cutting is saturated, and the amount of inclusions in the steel increases, leading to a decrease in toughness. Therefore, when Pb is added to the steel substrate, the Pb content is 0.50% or less. The Pb content is preferably 0.35% or less.[Bi: 0.001% to 0.100%]

[0101] Bi, like Pb, is an element that improves the machinability by cutting of the steel material. When Bi content of the steel substrate is less than 0.001%, the effect cannot be obtained. Therefore, when Bi is added to the steel substrate, the Bi content is 0.001% or more. On the other hand, when the Bi content in the steel substrate exceeds 0.100%, the effect of improving machinability by cutting is saturated and the amount of inclusions in the steel increases, leading to a decrease in toughness. Therefore, when Bi is added to the steel substrate, the Bi content is 0.100% or less. The Bi content is preferably 0.050% or less.[Ca: 0.0005% to 0.1000%]

[0102] Ca is an element that is solute in sulfides in the steel and has an effect of spheroidizing the sulfides, thereby suppressing degradation of the cold workability and toughness of the steel material. When Ca content in the steel substrate is less than 0.0005%, the effect cannot be obtained. Therefore, when Ca is added to the steel substrate, the Ca content is 0.0005% or more. On the other hand, when the Ca content in the steel substrate exceeds 0.1000%, the amount of Ca inclusions in the steel increases, adversely affecting the toughness and fatigue resistance of the steel material. Therefore, when Ca is added to the steel substrate, the Ca content is 0.1000% or less. The Ca content is preferably 0.0500% or less. The Ca content is more preferably 0.0300% or less.[Total Area Fraction of Ferrite and Pearlite: 90.0% or More]

[0103] To secure cold workability, the microstructure of the steel substrate needs to be a relatively soft ferrite and pearlite mixed structure. Therefore, the total area fraction of ferrite and pearlite in the steel substrate is 90.0% or more. The total area fraction is preferably 95.0% or more. An upper limit of the total area fraction of ferrite and pearlite in the steel substrate is not particularly limited, and the total area fraction may be 100.0%.

[0104] The total area fraction of ferrite and pearlite may be determined by the following procedure. Cut the steel material and prepare three samples that include the steel material surface layer, and observe the microstructure of each sample for a cross-section perpendicular to the rolling direction. Observe the microstructure in the surface layer of the sample, excluding the decarburized layer described below, using an optical microscope at a magnification of 100×. Calculate the total area fraction of ferrite and pearlite in the observed area of each field of view in five randomly selected fields of view, where the area of each field of view is 600 μm×800 μm. Calculate the average of the total area fraction obtained in each field of view for each sample, as the total area fraction of ferrite and pearlite of the steel material. For the calculation of the area fraction, the image interpretation software ImageJ may be used.[Average Vickers Hardness is 250 HV or Less]

[0105] When the average Vickers hardness of the steel substrate exceeds 250 HV, cold workability decreases. The average Vickers hardness of the steel substrate is therefore 250 HV or less. The average Vickers hardness is preferably 230 HV or less. The average Vickers hardness is more preferably 220 HV or less. On the other hand, from a practical viewpoint, the average Vickers hardness of the steel substrate is preferably 80 HV or more.

[0106] The average Vickers hardness may be determined by the following procedure. Cut the steel material and prepare three samples, and measure the hardness in a cross-section perpendicular to the rolling direction of the samples using a Vickers hardness tester. When the sample is a round steel material, there are five measurement points: one position corresponding to ½ the diameter of the round steel material in the center direction from the steel material surface (hereinafter referred to as a “D / 2 position”) and four positions corresponding to ¼ the diameter of the round steel material in the center direction from the steel material surface (hereinafter referred to as “D / 4 positions”). When the sample is a sheet material, there are five measurement points: one position corresponding to ½ of the sheet width in the center direction from one end of the sheet material and ½ of the sheet thickness in the depth direction from the steel material surface (hereinafter referred to as a “W / 2−t / 2 position”), and four positions corresponding to ¼ of the sheet width in the center direction from one end of the sheet material and ¼ of the sheet thickness in the depth direction from the steel material surface (hereinafter referred to as “W / 4−t / 4 positions”). Calculate the average value of hardness obtained at each measurement point for each sample as the average Vickers hardness of the steel material. The hardness measurement may be performed with a load of 10 kgf.<Decarburized Layer>

[0107] The following describes the decarburized layer of the hot-rolled steel material.[Total Decarburization Depth of Decarburized Layer (DM-T), as Specified in JIS G 0558, is 0.80 mm or Less]

[0108] Excessive thickness of the decarburized layer leads to a decrease in fatigue resistance and toughness as a component, and therefore excessive thickness of the decarburized layer needs to be suppressed. According to the present disclosure, the thickness of the decarburized layer is evaluated by the total decarburization depth (DM-T) as specified in JIS G 0558 (hereinafter referred to simply as “total decarburization depth”). When the total decarburization depth of the decarburized layer exceeds 0.80 mm, the degradation of properties becomes more pronounced. The total decarburization depth of the decarburized layer is therefore 0.80 mm or less. The total decarburization depth is preferably 0.50 mm or less. The total decarburization depth is more preferably 0.30 mm or less. On the other hand, a 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.

[0109] The total decarburization depth of the decarburized layer may be determined by the following procedure. Cut the steel material and prepare three samples that include the steel material surface layer, and observe the microstructure of each sample for a cross-section perpendicular to the rolling direction of the samples. Observe the surface layer microstructure of the sample using an optical microscope at a magnification of 100× with an area of 600 μm×800 μm for each field of view. Measure the total decarburization depth of the surface layer of the sample in three randomly selected fields of view, by the method described in JIS G 0558, and take the total decarburization depth at the position where the decarburized layer is deepest as the total decarburization depth in each field of view. Calculate the average of the total decarburization depth obtained in each field of view of each sample as the total decarburization depth of the decarburized layer of the steel material.[Decarburized Layer Includes Enriched Region where at Least One of Cu and Ni is Enriched]

[0110] In the decarburized layer of the hot-rolled steel material according to an embodiment of the present disclosure, there is the enriched region where at least one of Cu and Ni is enriched. Further, Sn is preferably enriched in the enriched region in addition to at least one of Cu and Ni. Hereinafter, the enriched region is defined as a region in the surface layer of the hot-rolled steel material in which the concentration of at least one of Cu and Ni is three times or more than a 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 round steel material, and at the W / 4−t / 4 position of the hot-rolled steel material when the hot-rolled steel material is sheet material.[Coverage of Enriched Region on Surface of Decarburized Layer is 50% or More]

[0111] The formation of the enriched region where at least one of Cu and Ni is enriched at the surface of the hot-rolled steel material contributes to suppression of the decarburization reaction. When the coverage of the enriched region on the surface of the decarburized layer is less than 50%, the decarburization suppression effect is insufficient. The coverage of the enriched region on the surface of the decarburized layer is therefore 50% or more. The coverage is preferably 60% or more. The coverage is more preferably 70% or more. On the other hand, an upper limit of the coverage is not particularly restricted, and the coverage of the enriched region may be 100%.

[0112] The coverage of the enriched region on the surface of the decarburized layer may be determined by the following procedure. First, cut the steel material and prepare three samples containing the steel material surface layer, and measure the reference concentrations of Cu and Ni in each sample. In order to eliminate the influence of the enriched region in the steel material surface layer and central segregation on the reference concentration, three randomly selected points in the above-mentioned positions (D / 4 position of the hot-rolled steel material for round steel material and W / 4−t / 4 position of the hot-rolled steel material for sheet material) are analyzed by an electron probe micro-analyzer (EPMA), and the average values of Cu and Ni concentrations at these three points are used as the reference concentrations of Cu and Ni, respectively.

[0113] The measurement conditions of the EPMA may be as follows: magnification of 100×, and area per field of view of 600 μm×800 μm.

[0114] Next, for each sample, perform quantitative analysis (mapping) on three randomly selected fields of view of the steel material surface by EPMA to determine the concentration of Cu and Ni. The measurement conditions of the EPMA may be as follows: magnification of 100×, and area per field of view of 600 μm×800 μm. Determine the coverage of the enriched region in each field of view by defining the area in which at least three times the concentration of at least one of Cu and Ni is present relative to the reference concentrations of Cu and Ni determined above as the enriched region. Calculate the average value of the coverage obtained at each view of each sample as the coverage of the enriched region of the steel material.[Maximum Depth of Enriched Region in Decarburized Layer is 1 μm or More and 150 μm or Less]

[0115] When 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. The maximum depth of the enriched region in the decarburized layer is therefore 1 μm or more. On the other hand, when the maximum depth of the enriched region in the decarburized layer exceeds 150 μm, this is preferred from the viewpoint of suppressing the decarburization reaction, but the depth of the enriched region is too deep and therefore cracking is more likely to occur during cold working. The maximum depth of the enriched region in the decarburized layer is therefore 150 μm or less. The maximum depth is preferably 120 μm or less. The maximum depth is more preferably 100 μm or less.

[0116] The maximum depth of the enriched region in the decarburized layer may be determined by the following method. Cut the steel material to prepare three samples including the steel material surface layer, observe three fields of view of the enriched region (an area of about 600 μm×800 μm per field of view) in a cross-section perpendicular to the rolling direction of each sample by the method described above, and measure the maximum depth of the enriched region. Determine the average of the maximum depth obtained in each field of view of each sample as the maximum depth of the enriched region in the decarburized layer of the steel material. Further, the minimum depth of the enriched region may also be determined by observations in the same manner.

[0117] Further, when the hot-rolled steel material contains Sn, Sn may also be enriched in the enriched region where at least one of Cu and Ni is enriched. Sn is also considered to be enriched when the concentration of Sn is at least three times the reference concentration.[Ratio of Sn Concentration to Sum of Cu and Ni Concentrations in Enriched Region [Sn] / ([Cu]+[Ni]) is 0.50 or Less (Preferred Range)]

[0118] 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, when the Sn concentration in the enriched region is excessively high, the grain boundary penetration depth of the enriched region becomes deeper, causing cold working cracking. The ratio of the Sn concentration to the sum of the Cu and Ni concentrations, [Sn] / ([Cu]+[Ni]), in the enriched region is therefore 0.50 or less in atomic ratio. The ratio is preferably 0.40 or less. The ratio is more preferably 0.30 or less. On the other hand, a lower limit of the atomic ratio [Sn] / ([Cu]+[Ni]) is not particularly limited, and [Sn] / ([Cu]+[Ni]) may be 0.00.

[0119] The ratio of the Sn concentration to the sum of the Cu and Ni concentrations in the enriched region [Sn] / ([Cu]+[Ni]) may be determined by the following procedure. Cut the steel material to prepare three samples including the steel material surface layer, and perform quantitative analysis (mapping) on three fields of view of a cross-section perpendicular to the rolling direction of each sample by EPMA to determine the concentrations of Cu, Ni, and Sn. EPMA measurement conditions may be set to a magnification of 100× and an area per field of view of 600 μm×800 μm. Determine the enriched region by the method described above, and calculate the atomic ratio of [Sn] / ([Cu]+[Ni]) from the measured concentration at the point in the enriched region where the Sn concentration is highest. Calculate the average value of the atomic ratio obtained in each field of view of each sample as [Sn] / ([Cu]+ [Ni]) in the enriched region of the steel material.(Method of Producing Hot-Rolled Steel Material)

[0120] Next, the method of producing hot-rolled steel material is described below. The method of producing hot-rolled steel material according to an embodiment of the present disclosure includes a process of hot rolling steel material that has the chemical composition described above, under a set of conditions including a maximum heating temperature T in a heating furnace being 1000° C. or more and 1200° C. or less, and a residence time of the steel material in the heating furnace being in a defined range.[Steel Material]

[0121] The steel material used in the method of producing hot-rolled steel material has a chemical composition consisting of C, Si, Mn, Cu, Ni, and N, with the balance being Fe and inevitable impurity. In addition to these elements, the steel material may contain the elements mentioned above as required. The content of each element is as described above.[Hot Rolling]

[0122] Hot rolling is applied to the steel material to obtain the hot-rolled steel material. During the hot rolling, when the maximum heating temperature T in the heating furnace is less than 1000° C., scale formation and growth on the steel material surface layer is difficult, and it is difficult for formation of the enriched region to progress. As a result, the coverage of the enriched region on the surface of the decarburized layer becomes small and the decarburization reaction cannot be suppressed. The maximum heating temperature T in the heating furnace is therefore 1000° C. or more. The maximum heating temperature T is preferably 1030° C. or more. On the other hand, when the maximum heating temperature T exceeds 1200° C., the scale growth rate of the steel material surface layer is too fast, and therefore the enriched region generated on the steel material surface layer is shed to the scale side (scale-off). As a result, the coverage of the enriched region on the surface of the decarburized layer becomes small and the decarburization reaction cannot be sufficiently suppressed. The maximum heating temperature T in the heating furnace is therefore 1200° C. or less. The maximum heating temperature T is preferably 1150° C. or less.

[0123] During the hot rolling, the longer the steel material is resident in the heating furnace, the longer the decarburization reaction in the heating furnace occurs. The inventors discovered that by setting the upper limit of the residence time of the steel material in the heating furnace to the time t1 (in min) determined by the following Expression (1), the decarburized layer and the enriched region can be made suitable for suppressing the decarburization reaction. On the other hand, a shorter furnace residence time is advantageous for suppressing the decarburization reaction, and therefore there is no need to define a lower limit for the furnace residence time. However, an excessively short furnace residence time may have adverse effects such as temperature variation of the material at different positions. The furnace residence time is therefore preferably 30 min or longer. Hereinafter, the furnace residence time is the time spent in the heating furnace from the time the material is charged into the heating furnace until the heated material leaves the heating furnace.t1=1⁢1⁢5⁢0-0.8⁢T-3⁢([Ni]⁢ / [Cu])-10[Sn](1)

[0124] Here, T is the maximum heating temperature, in ° C., [Ni] is Ni content in the steel material, in mass %, [Cu] is Cu content in the steel material, in mass %, and [Sn] is Sn content in the steel material, in mass %. When the steel material does not contain Sn, [Sn]=0.

[0125] For processes and conditions not described in the present disclosure, regular methods can be used.EXAMPLES

[0126] Examples are illustrated below to specifically describe the structure and effects according to the present disclosure. Note that the present disclosure is not restricted by any means to these examples and appropriate modifications may be made within the scope of the spirit of the present disclosure, all such modifications being included within the technical scope of the present disclosure.

[0127] 160 mm square billet material having the chemical compositions listed in Tables 1 and 2 was heated in a heating furnace and hot rolled under the hot rolling conditions listed in Tables 3 and 4 to obtain wire rods each having a diameter of 15 mm. Samples for microstructure observation and hardness measurement were taken from the resulting hot-rolled wire rods. Using the method described above, for each sample, the coverage of the enriched region where at least one of Cu and Ni is enriched, the maximum and minimum depths of the enriched region, the total decarburization depth of the decarburized layer (DM-T), the total area fraction of ferrite and pearlite, and the average Vickers hardness were measured. The results are listed in Tables 3 and 4. Further, for the steel samples listed in Table 2, the atomic ratio [Sn] / ([Cu]+[Ni]) was measured using the method described above, and the results are listed in Table 4.

[0128] Further, the cold workability of the hot-rolled wire rods was evaluated as follows. For each sample, after the scale on the surface layer of the hot-rolled wire rod was completely removed by pickling, the wire rod was drawn to a diameter of 14 mm and then cut to a height of 21 mm to obtain a cylindrical test piece. The test piece shape was based on the No. 1 test piece described in “Method of Testing for Cold Upsettability” (Journal of the Japan Society for Technology of Plasticity, 22 (1981), 139). The obtained test pieces were subjected to a cold compression test (under end face restraint conditions) in which the test pieces were compressed 60% in the height direction at a strain rate of 10 / s. Cold compression tests were carried out with N=3 for each case, and cracking caused by cold compression was checked by observing the side of the sample after the compression test. In Tables 3 and 4, the steel samples that did not crack in all of the N=3 cases are listed as “No” as passing products, and steel samples that had a crack in at least one of the N=3 cases are listed as “Yes” as failed products.TABLE 1Chemical composition (mass %)No.CSiMnCuNiNOthers[Ni] / [Cu]Classification10.820.880.020.2200.2100.0044—0.95Comparative steel20.641.060.550.0800.1200.0211—1.50Comparative steel30.710.031.600.1100.2400.0144—2.18Comparative steel40.410.070.040.0080.0200.0052—2.50Comparative steel50.430.410.220.5500.5100.0066—0.93Comparative steel60.550.410.550.0500.0070.0088—0.14Comparative steel70.220.760.730.4501.1200.0028—2.49Comparative steel80.040.150.440.3200.1400.0264—0.44Comparative steel90.110.221.120.2200.0200.0023—0.09Comparative steel100.380.210.790.1100.3400.0088—3.09Comparative steel110.550.440.370.0600.1700.0065—2.83Conforming steel120.440.080.220.2800.1300.0048—0.46Conforming steel130.600.120.080.0300.0200.0109—0.67Conforming steel140.780.021.450.4900.9600.0240—1.96Conforming steel150.490.780.080.0900.0300.0052—0.33Conforming steel160.630.200.750.2200.5800.0177—2.64Conforming steel170.110.440.970.2800.0300.0110—0.11Conforming steel180.040.970.550.0100.0100.0022—1.00Conforming steel190.440.711.170.3800.7700.0197—2.03Conforming steel200.200.050.790.1600.3900.0065—2.44Conforming steel210.330.110.020.1500.4400.0077—2.93Conforming steel220.060.490.790.1400.2200.0050Cr: 1.12 1.57Conforming steel230.310.200.660.2200.0700.0069Mo: 0.29   0.32Conforming steel240.180.210.770.2800.4400.0110Al: 0.048 1.57Conforming steel250.330.430.990.1700.1100.0044Ti: 0.048 0.65Conforming steel260.220.020.300.2700.2000.0057V: 0.140 0.74Conforming steel270.280.220.210.2600.5100.0072Nb: 0.029  1.96Conforming steel280.050.410.730.1100.1000.0148 B: 0.00220.91Conforming steel290.100.320.280.0200.0300.0101P: 0.0221.50Conforming steel300.270.070.860.1200.2100.0177S: 0.0501.75Conforming steel310.470.330.330.1600.0500.0045Sb: 0.01250.31Conforming steel320.080.170.110.0200.0200.0025Pb: 0.33 1.00Conforming steel330.330.030.020.2300.1000.0098Bi: 0.041 0.43Conforming steel340.170.220.050.2700.3300.0087Ca: 0.00211.22Conforming steel* Underlining indicates value outside scope of disclosure.* The balance other than the above chemical composition is Fe and inevitable impurity.TABLE 2Chemical composition (mass %)No.CSiMnCuNiSnNOthers[Ni] / [Cu]Classification350.130.060.660.0200.0200.0230.0041—1.00Comparative steel360.210.330.710.1400.0800.0140.0066—0.57Conforming steel370.740.031.480.4800.9500.0110.0233—1.98Conforming steel380.410.760.040.1100.0220.0150.0055—0.20Conforming steel390.640.130.660.2800.5600.0110.0147—2.00Conforming steel400.130.440.230.3200.0600.0120.0087—0.19Conforming steel410.090.020.310.0100.0100.0020.0025—1.00Conforming steel420.350.801.050.3700.8800.0310.0187—2.38Conforming steel430.180.020.820.1300.3700.0500.0071—2.85Conforming steel440.300.100.020.1300.3300.0200.0055—2.54Conforming steel450.080.330.810.1300.2000.0050.0051Cr: 0.88 1.54Conforming steel460.210.030.330.1800.0600.0210.0069Mo: 0.05   0.33Conforming steel470.310.180.650.2200.3100.0300.0102Al: 0.041 1.41Conforming steel480.440.100.980.1500.1000.0210.0047Ti: 0.031 0.67Conforming steel490.170.020.280.2800.1500.0110.0041V: 0.051 0.54Conforming steel500.210.350.100.2200.3300.0170.0103Nb: 0.015  1.50Conforming steel510.070.100.730.0800.0500.0100.0120 B: 0.00180.63Conforming steel520.130.250.550.0200.0100.0040.0025P: 0.0110.50Conforming steel530.220.140.790.1500.0900.0120.0155S: 0.0440.60Conforming steel540.410.310.440.1200.0700.0210.0051Sb: 0.00500.58Conforming steel550.100.200.150.0200.0200.0030.0023Pb: 0.08 1.00Conforming steel560.060.050.020.2300.1100.0140.0071Bi: 0.015 0.48Conforming steel570.210.200.880.2400.1000.0150.0065Ca: 0.00100.42Conforming steel* Underlining indicates value outside scope of disclosure.* The balance other than the above chemical composition is Fe and inevitable impurity.TABLE 3TotalAverageHot rolling conditionsdecarburizationTotal areaVickersMaximumEnriched regiondepth offraction ofhardnessheatingResidenceMinimumMaximumdecarburizedferrite andof steelCrackingtemp. Ttimet1Coveragedepthdepthlayer (DM-T,pearlitesubstrateduring coldClassi-No.(° C.)(min)(min)(%)(μm)(μm)mm)(%)(HV)compressionfication11138110237734710.6687.5261YesComparativeExample21070206290866500.5188.6252YesComparativeExample31102148262698480.7085.1270YesComparativeExample41166194210132130.8594.1210NoComparativeExample51077 97286603155 0.3195.9227YesComparativeExample61135150242246330.8395.3219NoComparativeExample71140 41231883160 0.4797.1192YesComparativeExample81005177345942580.17100.0 105YesComparativeExample91181133205779170 0.28100.0 130YesComparativeExample101055195297845155 0.2292.7166YesComparativeExample111211170173464130 0.8391.8152NoComparativeExample12 985211361 92160.8193.0160NoComparativeExample131110271260533260.8891.1181NoComparativeExample141003231342973148 0.6690.6241NoExample151122177251615410.4294.0227NoExample161075185282754330.4791.7245NoExample171144204234888290.21100.0 115NoExample181022293329522110.22100.0  87NoExample191035250316904115 0.1896.1193NoExample201192 48189797730.7798.4180NoExample211164 89210803410.4194.5201NoExample221108122259865380.10100.0  98NoExample231088235279724270.2795.3179NoExample241085204277889780.2197.1171NoExample251071241291687740.2796.5177NoExample261110179260712590.2297.3168NoExample271101101263825820.2996.2170NoExample281079133284747400.08100.0  90NoExample291066164293772170.11100.0 101NoExample301094 77270723950.2896.4163NoExample311105144265767280.2695.1192NoExample321138 53237742150.10100.0 104NoExample331079168285906440.2896.6187NoExample341077280285794280.1997.1160NoExample* Underlining indicates value outside scope of disclosure.TABLE 4Totaldecar-Enriched regionburizationAverageHot rolling conditions[Sn] / depth ofTotal areaVickersMaximum([Cu] +decar-fraction ofhardnessheatingResidenceMinimumMaximum[Ni])burizedferrite andof steelCrackingtemp. Ttimet1Coveragedepthdepthatomiclayer (DM-pearlitesubstrateduring coldClassi-No.(° C.)(min)(min)(%)(μm)(μm)ratioT, mm)(%)(HV)compressionfication351086200278743161 0.600.15100.0119YesComparativeExample361165220216914155 0.530.2198.8133YesComparativeExample371011200335984141 0.110.4490.4247NoExample381118144255564350.210.3396.3227NoExample391055107300773250.410.1392.2240NoExample401122204252805350.180.10100.0122NoExample411007245341522150.030.21100.0102NoExample421033211316863118 0.460.1093.6211NoExample431188 64191735770.410.3896.1177NoExample441170104206612540.360.2292.5208NoExample451080133281838310.100.12100.0103NoExample461073218290874260.170.2198.7160NoExample471104188262917550.210.1697.5180NoExample481042211314885320.130.1694.1189NoExample491070189292864280.090.1997.3135NoExample501097104268913440.080.1198.8171NoExample511082133282884380.110.06100.088NoExample521044177313752130.030.22100.0112NoExample531091 88275883400.210.2098.3150NoExample541113180258913510.270.1395.1204NoExample551101102266722180.050.18100.0111NoExample561131170244964510.110.21100.088NoExample571061205300883470.100.1899.3131NoExample* Underlining indicates value outside scope of disclosure.The following is a description of each Example and Comparative Example.No. 1, 2, and 3 are Comparative Examples in which the C, Si, or Mn content exceeded the range specified in the present disclosure. These steel samples had excessive C, Si, or Mn content and hardenability was too high, and therefore the microstructure contained bainite or martensite, and the total area fraction of ferrite and pearlite in the steel substrate was less than 90.0%. Further, the average Vickers hardness of the steel substrate also exceeded 250 HV, and therefore cold workability was low and cracking occurred after cold compression tests.No. 4 and 6 are Comparative Examples in which the Cu or Ni content was less than the range specified in the present disclosure. These steel samples had low Cu or Ni content, and therefore the coverage of the enriched region was less than the range specified in the present disclosure and the total decarburization depth exceeded the range specified in the present disclosure.

[0132] No. 5 and 7 are Comparative Examples in which the Cu or Ni content exceeded the range specified in the present disclosure. These steel samples had excessive Cu or Ni content, and the maximum depth of the enriched region exceeded the range specified in the present disclosure, and therefore cracking occurred in the cold compression test.

[0133] No. 8 is a Comparative Example where the N content exceeded the range specified in the present disclosure. This steel sample had high solute N content in the steel, and low cold workability due to the effect of dynamic strain aging, and therefore cracking occurred during the cold compression test.

[0134] No. 9 and 10 are Comparative Examples in which the ratio of Ni to Cu, [Ni] / [Cu], was outside the range specified in the present disclosure. In these steel samples, [Ni] / [Cu] was not appropriate, and therefore the maximum depth of the enriched region exceeded the range specified in the present disclosure, and cracking occurred during the cold compression test.

[0135] No. 11 is a Comparative Example in which the maximum heating temperature T during hot rolling exceeded the range specified in the present disclosure. In this example, the heating temperature of the steel material was too high, and therefore scale growth rate was fast, coverage of the enriched region was low, and the total decarburization depth exceeded the range specified in the present disclosure.

[0136] No. 12 is a Comparative Example in which the maximum heating temperature T during hot rolling was less than the range specified in the present disclosure. In this example, the heating temperature was too low, and therefore formation of the enriched region on the steel material surface layer was insufficient, and the coverage of the enriched region was well below the range specified in the present disclosure, resulting in the total decarburization depth exceeding the range specified in the present disclosure.

[0137] No. 13 is a Comparative Example in which the furnace residence time in the heating furnace during hot rolling exceeded the range specified in the present disclosure. In No. 13, the coverage and maximum depth of the enriched region were in the range specified in the present disclosure, but due to the long time for the decarburization reaction to occur, the total decarburization depth exceeded the range specified in the present disclosure.

[0138] No. 35 is a Comparative Example of steel containing Sn, in which the Sn content exceeded the upper limit ([Cu]+[Ni]) / 2. The atomic ratio of [Sn] / ([Cu]+[Ni]) in the enriched region exceeded 0.5 due to the excessive amount of Sn in this steel sample, and the maximum depth of the enriched region also exceeded the range specified in the present disclosure, and therefore cracking occurred during the cold compression test.

[0139] No. 36 is a Comparative Example of steel containing Sn, in which the furnace residence time in the heating furnace during hot rolling exceeded the range specified in the present disclosure. 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 specified in the present disclosure, and therefore cracking occurred during the cold compression test.

[0140] In contrast to the Comparative Examples described above, No. 14 to 34 and No. 37 to 57 had steel material chemical compositions and hot rolling conditions that were in the ranges according to the present disclosure, as listed in Tables 1 to 4. In these examples of the hot-rolled steel material, the coverage and maximum depth of the enriched region, the total decarburization depth, and the total area fraction of ferrite and pearlite were in the respective ranges specified in the present disclosure. Further, in the examples listed in Tables 2 and 4, [Sn] / ([Cu]+[Ni]) in the enriched region was in the range specified in the present disclosure. Further, these examples had excellent average Vickers hardness and cracking properties during cold compression. That is, No. 14 to 34 and No. 37 to 57 had excellent decarburization reaction suppression and cold workability.INDUSTRIAL APPLICABILITY

[0141] According to the present disclosure, hot-rolled steel material is provided that has sufficient suppression of thickness of the decarburized layer and excellent cold workability, and a method of producing same is provided.REFERENCE SIGNS LIST100 hot-rolled steel material

[0143] 10 steel substrate

[0144] 20 decarburized layer

[0145] 22 enriched region

[0146] 24 decarburized layer surface

[0147] 30 scale

[0148] A maximum depth of enriched region

[0149] B total decarburization depth of decarburized layer

Examples

examples

[0126]Examples are illustrated below to specifically describe the structure and effects according to the present disclosure. Note that the present disclosure is not restricted by any means to these examples and appropriate modifications may be made within the scope of the spirit of the present disclosure, all such modifications being included within the technical scope of the present disclosure.

[0127]160 mm square billet material having the chemical compositions listed in Tables 1 and 2 was heated in a heating furnace and hot rolled under the hot rolling conditions listed in Tables 3 and 4 to obtain wire rods each having a diameter of 15 mm. Samples for microstructure observation and hardness measurement were taken from the resulting hot-rolled wire rods. Using the method described above, for each sample, the coverage of the enriched region where at least one of Cu and Ni is enriched, the maximum and minimum depths of the enriched region, the total decarburization depth of the decar...

Claims

1. Hot-rolled steel material comprising: a steel substrate that has a chemical 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%, andN: 0.0020% to 0.0250%,optionally at least one element selected from the group consisting ofCr: 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%,B: 0.0005% to 0.0050%,P: 0.001% to 0.100%,S: 0.001% to 0.100%,Sb: 0.0010% to 0.0300%,Pb: 0.01% to 0.50%,Bi: 0.001% to 0.100%, andCa: 0.0005% to 0.1000%,with the balance being Fe and inevitable impurity, wherein a ratio of Ni to Cu, [Ni] / [Cu], is 0.10 or more and 3.00 or less; and a decarburized layer formed on a surface of the steel substrate, whereinthe decarburized layer comprises an enriched region where at least one of Cu and Ni is enriched,coverage of the enriched region on the surface of the decarburized layer is 50% or more,maximum depth of the enriched region in the decarburized layer is 1 μm or more and 150 μm or less,total decarburization depth of the decarburized layer, DM-T, as specified in JIS G 0558, is 0.80 mm or less,total area fraction of ferrite and pearlite in the steel substrate is 90.0% or more, andaverage Vickers hardness in the steel substrate is 250 HV or less.

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

3. (canceled)4. (canceled)5. (canceled)6. A method of producing hot-rolled steel material, the method comprising a process of hot rolling steel material that has a chemical 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%, andN: 0.0020% to 0.0250%,optionally at least one element selected from the group consisting ofCr: 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%,B: 0.0005% to 0.0050%,P: 0.001% to 0.100%,S: 0.001% to 0.100%,Sb: 0.0010% to 0.0300%,Pb: 0.01% to 0.50%,Bi: 0.001% to 0.100%, andCa: 0.0005% to 0.1000%,with the balance being Fe and inevitable impurity, wherein a ratio of Ni to Cu, [Ni] / [Cu], is 0.10 or more and 3.00 or less, under a set of conditions including a maximum heating temperature T in a heating furnace being 1000° C. or more and 1200° C. or less, and a residence time of the steel material in the heating furnace being a time t1 or shorter, in min, determined by Expression (1) below, to obtain the hot-rolled steel material,t1=1⁢1⁢5⁢0-0.8⁢T-3⁢([Ni]⁢ / [Cu])-10[Sn](1)where T is the maximum heating temperature, in ° C., [Ni] is Ni content in the steel material, in mass %, [Cu] is Cu content in the steel material, in mass %, and [Sn] is Sn content in the steel material, in mass %.

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

8. (canceled)9. (canceled)10. (canceled)

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