steel

Abstract

To provide a steel product that has sufficiently low deformation resistance during a cold forging process and can sufficiently suppress crystal grain coarsening during a carburizing process, even when the compression ratio during the cold forging process is 70%.SOLUTION: A steel product has a composition consisting of C: 0.10 to 0.22 mass%, Si: more than 0 mass% and 0.14 mass% or less, Mn: 0.01 mass% to 0.70 mass%, P: more than 0 mass% and 0.100 mass% or less, S: more than 0 mass% and 0.100 mass% or less, Cr: 1.30 mass% to 2.00 mass%, Al: 0.050 mass% to 0.100 mass%, Ti 0.040 mass% to 0.100 mass%, Nb: 0.001 mass% to 0.100 mass%, N: 0.0020 mass% to 0.0080 mass%, B: 0.0010 mass% to 0.0050 mass%, and the balance of iron and unavoidable impurities, wherein a total area ratio of ferrite and pearlite is 85 to 100%. 0.0030<[N]-10-4. 11007 / [Al]<0.0105...(1). In Formula (1), [N] and [Al] indicate N and Al contents in mass%, respectively.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to steel products. [Background technology]

[0002] Steel parts used in automobiles, construction machinery, and other industrial machinery are manufactured by, for example, sequentially performing the following steps: (1) forming steel material into the desired part shape, (2) carburizing, and (3) finishing such as polishing.

[0003] (1) Conventionally, the forming process has been carried out by hot forging followed by cutting, but in recent years, cold forging has been adopted to reduce manufacturing costs and improve yields. Cold forging places a higher load on the die than hot forging. To reduce the load on the die, steel is required to have low deformation resistance.

[0004] In addition, the strain introduced into the steel by cold forging causes grain coarsening during the (2) carburizing process. This grain coarsening increases the amount of strain in the steel part. Therefore, it is necessary to suppress grain coarsening in the (2) carburizing process.

[0005] Patent Document 1 discloses a steel material (carburizing steel) that has a hardness of HV125 or less before cold forging, and that has no crystal grains with a grain size number of 4 or less after cold forging (upset compression) at a compression ratio of 50% and carburizing treatment. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. WO2012 / 108460 Summary of the Invention [Problem to be solved by the invention]

[0007] However, in Patent Document 1, the deformation resistance during cold forging is indirectly evaluated by measuring hardness, but the actual value of the deformation resistance is unknown. Furthermore, as parts have become larger in recent years, steel materials have also become larger, resulting in steel materials with portions in which the compression ratio during cold forging significantly exceeds 50%. Therefore, steel materials are required to suppress grain coarsening even under high strain load conditions with a compression ratio exceeding 50% (e.g., a compression ratio of 70%). However, in the steel material of Patent Document 1, the grain size after carburizing is evaluated assuming a compression ratio during cold forging of 50%, which may result in insufficient grain coarsening suppression.

[0008] The present invention has been made in view of the above circumstances, and one of its objects is to provide a steel material that has sufficiently low deformation resistance during cold forging and can sufficiently suppress grain coarsening during the carburizing process even when the compression ratio during cold forging is 70%. [Means for solving the problem]

[0009] Aspect 1 of the present invention is The component composition is C: 0.10~0.22% by mass, Si: more than 0 mass% and 0.14 mass% or less, Mn: 0.01~0.70% by mass, P: more than 0% by mass and 0.100% by mass or less, S: more than 0% by mass and 0.100% by mass or less, Cr: 1.30~2.00% by mass, Al: 0.050~0.100% by mass, Ti:0.040~0.100% by mass, Nb:0.001~0.100% by mass, N: 0.0020~0.0080% by mass, B: 0.0010 to 0.0050 mass%; and The balance is composed of iron and inevitable impurities and satisfies the following formula (1): This steel has a total area ratio of ferrite and pearlite of 85 to 100%. 0.0030<[N]-10 -4.1007 / [Al]<0.0105 (1) In formula (1), [N] and [Al] represent the contents of N and Al, respectively, expressed in mass %.

[0010] Aspect 2 of the present invention is The steel material according to aspect 1 further contains at least one selected from the group consisting of more than 0% by mass and not more than 0.05% by mass of Mo, more than 0% by mass and not more than 0.2% by mass of Cu, and more than 0% by mass and not more than 0.2% by mass of Ni.

[0011] Aspect 3 of the present invention is The steel material according to aspect 1 or 2, further comprising at least one selected from the group consisting of V: ​​more than 0% by mass and not more than 0.1% by mass and Hf: more than 0% by mass and not more than 0.1% by mass.

[0012] A fourth aspect of the present invention is The steel material according to any one of Aspects 1 to 3, further comprising at least one selected from the group consisting of: more than 0% by mass and not more than 0.005% by mass of Ca; more than 0% by mass and not more than 0.005% by mass of Mg; more than 0% by mass and not more than 0.005% by mass of Zr; more than 0% by mass and not more than 0.1% by mass of Te; and more than 0% by mass and not more than 0.02% by mass of REM.

[0013] A fifth aspect of the present invention is The steel material according to any one of aspects 1 to 4, further containing at least one selected from the group consisting of Pb: more than 0 mass% and not more than 0.1 mass%, Bi: more than 0 mass% and not more than 0.1 mass%, and Sb: more than 0 mass% and not more than 0.1 mass%. [Effects of the Invention]

[0014] According to an embodiment of the present invention, it is possible to provide a steel material that has sufficiently low deformation resistance during cold forging and can sufficiently suppress grain coarsening during the carburizing process even when the compression ratio during cold forging is 70%. DETAILED DESCRIPTION OF THE INVENTION

[0015] The inventors have conducted research from various angles in order to realize a steel material that has sufficiently low deformation resistance during cold forging and can sufficiently suppress grain coarsening during the carburizing process, even when the compression ratio during cold forging is 70%.

[0016] Patent Document 1 discloses that the Ti content of steel is controlled to form TiC, which has a pinning effect that prevents grain coarsening. However, as a result of studies by the present inventors, it was found that simply controlling the Ti content is not enough to sufficiently suppress grain coarsening, at least during the carburizing treatment process.

[0017] The present inventors considered controlling the Ti content and utilizing AlN, which has a pinning effect similar to TiC. However, as disclosed in Patent Document 1, it was previously believed that AlN and solute N, which increases deformation resistance, are almost absent in Ti-added steel because Ti easily bonds with N. In contrast, the present inventors discovered that if the Al content is 0.050 mass% or more, not only can a significant amount of AlN be present in Ti-added steel, but also a significant amount of solute N. They then considered minimizing the amount of solute N to reduce deformation resistance during cold forging while suppressing grain coarsening through the pinning effect of AlN.

[0018] The present inventors estimated the amount of solute N as follows, referring to the equation by Darken et al. described in the literature "Properties of Untempered AlN-Containing High-Tensile Steel, Iron and Steel, 58th Year (1972), No. 13, pp. 1791-1805." Amount of dissolved N = [N] - (amount of N required to form AlN) =[N]-(10 1.95-(7400 / T) / [Al])···(2) In formula (2), [N] and [Al] are the contents of N and Al expressed in mass%, respectively, and T is the solubility temperature (K) of AlN. The inventors have adopted 1223 K as the solubility temperature T suitable for the embodiment of the present invention.

[0019] To reduce the amount of solute N in formula (2), it is necessary to reduce [N] and / or [Al]. However, since there is a manufacturing limit to how much [N] can be reduced, it is necessary to reduce [Al]. However, if [Al] is reduced too much, it will limit the generation of AlN, and it will not be possible to sufficiently suppress the coarsening of crystal grains. As a result of extensive research, the inventors have found that by adjusting the component composition and metal structure and controlling the amount of solute N estimated by formula (2) to a specific range of more than 0.0030 and less than 0.0105, it is possible to realize a steel material that has sufficiently low deformation resistance during cold forging and can sufficiently suppress grain coarsening during the carburizing process, even when the compression ratio during cold forging is 70%.

[0020] The details of each requirement stipulated in the embodiments of the present invention are described below. Regarding the difference between "steel material" and "steel part" in this specification, "steel material" refers to a material that has not been subjected to part shaping, while "steel part" refers to a material that has been subjected to part shaping. For example, steel material may have a simple shape that extends linearly in one direction, such as a cylindrical shape or a rectangular parallelepiped shape, as a result of rolling and / or wiredrawing. On the other hand, steel parts may have a complex shape that does not extend linearly in one direction and has a cross-section of the same shape and dimensions, such as having ground portions, bent portions, and / or openings, as a result of further part shaping, such as forging and cutting, in addition to rolling and / or wiredrawing.

[0021] <1. Ingredient composition> A steel material according to an embodiment of the present invention preferably has a chemical composition including 0.10 to 0.22 mass% C, more than 0 to 0.14 mass% Si, 0.01 to 0.70 mass% Mn, more than 0 to 0.100 mass% P, more than 0 to 0.100 mass% S, 1.30 to 2.00 mass% Cr, 0.050 to 0.100 mass% Al, 0.040 to 0.100 mass% Ti, 0.001 to 0.100 mass% Nb, 0.0020 to 0.0080 mass% N, and 0.0010 to 0.0050 mass% B, with the balance being iron and inevitable impurities. Furthermore, the steel material according to an embodiment of the present invention satisfies the following formula (1): 0.0030<[N]-10 -4.1007 / [Al]<0.0105 (1) In formula (1), [N] and [Al] represent the contents of N and Al, respectively, expressed in mass %. Each requirement is explained in detail below.

[0022] (C:0.10~0.22% by mass) C is added to ensure the core hardness of steel parts. Therefore, the C content is set to 0.10% by mass or more, preferably 0.12% by mass or more, and more preferably 0.13% by mass or more. On the other hand, if excessive C is added, the core hardness becomes too high, which reduces toughness and, conversely, reduces low-cycle strength. Therefore, the C content is set to 0.22% by mass or less, preferably 0.21% by mass or less, and more preferably 0.20% by mass or less.

[0023] (Si: more than 0 mass% and 0.14 mass% or less) Si dissolves in ferrite and reduces workability. Therefore, the Si content is set to 0.14 mass% or less, preferably 0.12 mass% or less, and more preferably 0.10 mass% or less. On the other hand, Si may be contained in an amount exceeding 0 mass% due to manufacturing reasons. Furthermore, since increasing the purity increases manufacturing costs, the Si content is preferably set to 0.02 mass% or more, and more preferably 0.04 mass% or more.

[0024] (Mn:0.01~0.70% by mass) Mn is an element that improves the hardenability of steel materials to ensure core hardness of steel parts. Furthermore, since it lowers the Ms point of steel materials, it is an important element for the formation of retained austenite after carburizing. To achieve these effects, the Mn content is set to 0.01% by mass or more, preferably 0.02% by mass or more, and more preferably 0.03% by mass or more. On the other hand, excessive addition of Mn reduces forgeability. Therefore, the Mn content is set to 0.70% by mass or less, preferably 0.65% by mass or less, and more preferably 0.60% by mass or less.

[0025] (P: more than 0% by mass and 0.100% by mass or less) P segregates at grain boundaries and reduces the impact properties of steel parts, so it is desirable to reduce it as much as possible. Therefore, the P content is set to 0.100 mass% or less, preferably 0.080 mass% or less, and more preferably 0.060 mass% or less. On the other hand, P is an element that is inevitably contained in steel, that is, it can be contained in amounts greater than 0 mass%. Furthermore, since increasing purity increases production costs, the P content is preferably 0.003 mass% or more, more preferably 0.005 mass% or more.

[0026] (S: more than 0% by mass and 0.100% by mass or less) S combines with Mn to form MnS inclusions, which contribute to improved machinability. However, excessive S segregates at grain boundaries, reducing the impact properties of parts and increasing costs. Therefore, the S content is set to 0.100% by mass or less, preferably 0.080% by mass or less, and more preferably 0.060% by mass or less. On the other hand, S is an element that is inevitably contained in steel, meaning that it can be contained in amounts greater than 0% by mass. Furthermore, increasing purity increases manufacturing costs and also reduces machinability, so the S content is preferably 0.003% by mass or more, and more preferably 0.005% by mass or more.

[0027] (Cr:1.30~2.00% by mass) Cr improves the hardenability of steel materials to ensure the core hardness of steel parts, and is effective in ensuring the static strength and / or fatigue strength of steel parts. Therefore, the Cr content is set to 1.30% by mass or more, preferably 1.40% by mass or more, and more preferably 1.45% by mass or more. On the other hand, excessive addition of Cr increases the hardness of the steel material, thereby reducing machinability. Therefore, the Cr content is set to 2.00% by mass or less, preferably 1.90% by mass or less, and more preferably 1.80% by mass or less.

[0028] (Al:0.050~0.100% by mass) Even if Ti, which easily reacts with N, is added, Al can generate AlN with a pinning effect when added at 0.050% by mass or more. Furthermore, within the N content range described below, the solid solution temperature of AlN decreases, and the coarse AlN crystallized during casting dissolves during rolling. Therefore, the Al content is set to 0.050% by mass or more, preferably 0.055% by mass or more, and more preferably 0.060% by mass or more. On the other hand, excessive addition of Al reduces manufacturability and processability. Therefore, the Al content is set to 0.100% by mass or less, preferably 0.090% by mass or less, and more preferably 0.085% by mass or less.

[0029] (Ti:0.040~0.100% by mass) Ti bonds with C in the steel to form TiC as pinning particles, suppressing grain coarsening during carburization. Furthermore, it preferentially bonds with N to form TiN, thereby ensuring solute B. Therefore, the Ti content is set to 0.040% by mass or more, preferably 0.045% by mass or more, and more preferably 0.050% by mass or more. On the other hand, excessive addition of Ti generates coarse TiN, making it difficult to suppress grain coarsening. Therefore, the Ti content is set to 0.100% by mass or less, preferably 0.090% by mass or less, and more preferably 0.080% by mass or less.

[0030] (Nb:0.001~0.100% by mass) Nb bonds with C and N in the steel to form Nb(C,N), which, like TiC, acts as a pinning particle to effectively suppress grain coarsening during carburization. Therefore, the Nb content is set to 0.001% by mass or more, preferably 0.002% by mass or more, and more preferably 0.003% by mass or more. On the other hand, adding excessive Nb forms crystallized products that do not contribute to suppressing grain coarsening, making it more difficult to suppress grain coarsening. Therefore, the Nb content is set to 0.100% by mass or less, preferably 0.090% by mass or less, and more preferably 0.080% by mass or less.

[0031] (N:0.0020~0.0080% by mass) N combines with Al and Nb in the steel to form fine nitrides, carbonitrides, etc., and suppresses grain coarsening during carburizing. Therefore, the N content is set to 0.0020% by mass or more, preferably 0.0025% by mass or more, and more preferably 0.0030% by mass or more. On the other hand, excessive N content forms TiN, which does not contribute to suppressing grain coarsening, and causes grain coarsening during carburizing. Therefore, the N content is set to 0.0080% by mass or less, preferably 0.0075% by mass or less, and more preferably 0.0070% by mass or less.

[0032] (B:0.0010~0.0050% by mass) Addition of trace amounts of B significantly improves the hardenability of steel. Therefore, the B content is set to 0.0010% by mass or more, preferably 0.0016% by mass or more, and more preferably 0.0020% by mass or more. On the other hand, even if excessive B is added, these effects become saturated and workability is further reduced. Therefore, the B content is set to 0.0050% by mass or less, preferably 0.0040% by mass or less, and more preferably 0.0030% by mass or less.

[0033] The steel material according to the embodiment of the present invention preferably contains the above-described chemical composition, with the balance being iron and inevitable impurities in one embodiment of the present invention. Elements introduced due to the conditions of raw materials, materials, manufacturing facilities, etc. are permitted as inevitable impurities. For example, elements such as P and S are generally preferable when their content is low, and therefore are considered inevitable impurities, but their composition ranges are separately specified as described above. Therefore, in this specification, the term "unavoidable impurities" is used to refer to elements excluding elements whose composition ranges are separately specified.

[0034] Furthermore, the steel material according to the embodiment of the present invention satisfies the following formula (1). 0.0030<[N]-10 -4.1007 / [Al]<0.0105 (1) In formula (1), [N] and [Al] represent the contents of N and Al, respectively, expressed in mass %.

[0035] The lower limit of formula (1) is set to more than 0.0030, preferably 0.00375 or more, from the viewpoint of sufficiently suppressing grain coarsening during the carburizing treatment process. The upper limit of formula (1) is set to less than 0.0105, from the viewpoint of sufficiently reducing the deformation resistance during cold forging. This allows the deformation resistance during cold forging, described below, to be less than 700 MPa. The upper limit of formula (1) is preferably 0.0050 or less. This allows the deformation resistance during cold forging, described below, to be less than 685 MPa.

[0036] Furthermore, the steel material according to the embodiment of the present invention may selectively contain one or more of the following optional elements as necessary, and the properties of the steel material are further improved depending on the components contained.

[0037] (at least one of the group consisting of Mo: more than 0 mass% and not more than 0.05 mass%, Cu: more than 0 mass% and not more than 0.2 mass%, and Ni: more than 0 mass% and not more than 0.2 mass%) Mo, Cu, and Ni improve the hardenability of steel and the toughness of parts, so they may each be added in amounts exceeding 0% by mass. However, excessive addition increases manufacturing costs and causes the formation of supercooled structures during rolling or annealing, which coarsen grains during carburizing. Therefore, the Mo content is preferably 0.05% by mass or less, more preferably 0.04% by mass or less, and even more preferably 0.03% by mass or less. Similarly, the Cu content and Ni content are each preferably 0.2% by mass or less, more preferably 0.1% by mass or less, and even more preferably 0.05% by mass or less.

[0038] (at least one of the group consisting of V: ​​more than 0 mass% and not more than 0.1 mass% and Hf: more than 0 mass% and not more than 0 mass%) V and Hf are elements effective in improving hardenability, forming carbides and / or nitrides in steel to increase the hardness of steel parts. Therefore, V and Hf may each be added in amounts exceeding 0% by mass. However, excessive addition saturates the effect and increases manufacturing costs. Therefore, the V content and Hf content are each preferably 0.1% by mass or less, more preferably 0.075% by mass or less, and even more preferably 0.05% by mass or less.

[0039] (at least one of the group consisting of Ca: more than 0 mass% and not more than 0.005 mass%, Mg: more than 0 mass% and not more than 0.005 mass%, Zr: more than 0 mass% and not more than 0.005 mass%, Te: more than 0 mass% and not more than 0.1 mass%, and REM: more than 0 mass% and not more than 0.02 mass%) Ca, Mg, Zr, Te, and REM may each be added in amounts exceeding 0% by mass to suppress the elongation of MnS and improve the toughness of parts. However, excessive addition of these elements generates coarse oxides and reduces machinability. Therefore, the amounts of Ca and Mg are preferably 0.005% by mass or less, more preferably 0.004% by mass or less, and even more preferably 0.003% by mass or less. The amount of Zr is preferably 0.005% by mass or less, more preferably 0.004% by mass or less, and even more preferably 0.003% by mass or less. The amount of Te is preferably 0.1% by mass or less, more preferably 0.05% by mass or less, and even more preferably 0.03% by mass or less. The amount of REM is preferably 0.02% by mass or less, more preferably 0.01% by mass or less, and even more preferably 0.005% by mass or less.

[0040] (at least one of the group consisting of Pb: more than 0 mass% and not more than 0.1 mass%, Bi: more than 0 mass% and not more than 0.1 mass%, and Sb: more than 0 mass% and not more than 0 mass%) Pb, Bi, and Sb are elements that improve machinability, so they may each be added in amounts exceeding 0% by mass. However, excessive addition of these elements reduces toughness, so the Pb, Bi, and Sb contents are each preferably 0.1% by mass or less, more preferably 0.05% by mass or less, and even more preferably 0.03% by mass or less.

[0041] <2.Metal structure> In the metal structure of the steel material according to the embodiment of the present invention, the total area ratio of ferrite and pearlite is 85 to 100%. When the total area ratio of ferrite and pearlite is 85 to 100%, cold forgeability is improved. Preferably, the total area ratio of ferrite and pearlite is 90% or more and 100% or less. Furthermore, from the viewpoint of improving cold forgeability, the remainder other than ferrite and pearlite is preferably bainite.

[0042] <3. Manufacturing method> The method for producing the steel material according to the embodiment of the present invention is not particularly limited as long as it does not deviate from the object of the present disclosure, and known methods can be used. For example, the steel material according to the embodiment of the present invention can be obtained by melting the steel material according to a normal melting method so as to satisfy the above-mentioned component composition, and then appropriately performing casting, hot rolling / hot forging, etc.

[0043] The method for manufacturing a steel part according to an embodiment of the present invention includes a step of cold forging the steel material according to the embodiment of the present invention described above, and a step of carburizing the steel material after the cold forging. The cold forging and carburizing steps can be performed by known methods. The method for manufacturing a steel part according to an embodiment of the present invention may also include other steps within the scope of the present disclosure. Examples of steel parts obtainable by the above-mentioned method include gears, shafts, bearings or CVT pulleys. [Example]

[0044] The following examples are provided to more specifically describe the embodiments of the present invention. The embodiments of the present invention are not limited to the following examples, and may be modified as appropriate within the scope of the above-described and below-described aims, and all such modifications are within the technical scope of the embodiments of the present invention.

[0045] Steel having the chemical composition shown in Table 1 was melted using a vacuum induction furnace (VIF) and cast into billets, which were then bloomed at 1200°C to 1300°C and then hot rolled / hot forged at 800°C to 1100°C to obtain steel materials (steel bars) with a diameter of 24 mm. In Table 1, "-" indicates that no intentional addition was made. In the following tables, values ​​marked with * indicate that the values ​​are outside the range of the embodiments of the present invention.

[0046] [Table 1]

[0047] <Metal structure evaluation> Samples were taken from each steel material so that cross sections perpendicular to the longitudinal direction of steel materials No. 1 to No. 7 could be observed. The cross sections of each sample were subjected to nital etching to reveal the microstructure, and photographs were taken using an optical microscope at 400x magnification (field of view: 220 μm horizontal x 165 μm vertical), centered at a position D / 4 (D: diameter) from the surface. Ten equally spaced vertical lines and ten equally spaced horizontal lines were drawn on the resulting photographs in a grid pattern. The number of points of each microstructure (ferrite, pearlite, bainite, etc.) present at 100 intersections was counted, and the number of points divided by 100 was used as the area fraction (%) of each microstructure. The results are shown in Table 2. In Table 2, "F" indicates ferrite, "P" indicates pearlite, and "B" indicates bainite, and "(F+P) area fraction" indicates the total area fraction of ferrite and pearlite.

[0048] [Table 2]

[0049] <Evaluation of deformation resistance during cold forging> Steel materials Nos. 1 to 7, each measuring 24 mm in diameter, were processed to a diameter of 20 mm and a length of 30 mm. The samples were further cold-worked (compressed) in the longitudinal direction using a 1600-ton press, and the deformation resistance was evaluated when the sample length was 50% of its pre-cold-working length. The evaluation criteria were as follows: less than 685 MPa was considered good (◎), less than 700 MPa was considered sufficient (◯), and 700 MPa or more was considered insufficient (×).

[0050] <Evaluation of grain size during carburizing process> After evaluating the deformation resistance during cold forging, the cold forging was continued until the length of the sample was 30% of that before cold forging (i.e., a compression rate of 70%). The sample was held in a heating furnace at 950°C for 3 hours and then quenched by water cooling. After quenching, the sample was cut across the central axis in the longitudinal direction, and the cut surface was polished and corroded. The area around the central axis was observed with an optical microscope, and a section of approximately 200 mm 2 The largest grain in the area was photographed, and the grain size number was calculated with reference to the specimen diagram in JIS: G0551. However, since it is difficult to measure coarse grains exceeding a negative number, the maximum grain size number was set to 0. According to JIS, grain size numbers less than 5.0 are considered coarse grains, and grain size numbers of 5.0 or more are considered fine grains, so grains of 5.0 or more were considered sufficient (◯), and those less than 5.0 were considered insufficient (×).

[0051] The results are shown in Table 3. The chemical compositions shown in Table 3 are a selection of the chemical compositions shown in Table 1 so that the differences between steel materials Nos. 1 to 7 can be clearly seen.

[0052] [Table 3]

[0053] The results in Table 3 can be considered as follows: Steel materials Nos. 1 to 3 in Table 3 all satisfied the requirements specified in the embodiments of the present invention, had sufficiently low deformation resistance during cold forging, and were able to sufficiently suppress grain coarsening during the carburizing process even when the compression ratio during cold forging was 70%. On the other hand, test Nos. 4 to 7 in Table 3 all did not satisfy the requirements specified in the embodiments of the present invention, and had insufficient deformation resistance during cold forging and / or insufficient grain size during the carburizing process.

[0054] In Test No. 4, the Al content, the Ti content, and the value of formula (1) were below the predetermined ranges, and Nb was not added, so that coarsening of crystal grains could not be sufficiently suppressed.

[0055] In Test No. 5, the C content, the Al content, and the value of formula (1) were below the predetermined ranges, and Nb was not added, so that coarsening of crystal grains could not be sufficiently suppressed.

[0056] In Test No. 6, the Si content, Mn content, and N content exceeded the specified range, the Cr content and Al content were below the specified range, and Ti, Nb, and B were not added, so grain coarsening could not be sufficiently suppressed.

[0057] In Test No. 7, the Si content, Mn content, N content, and the value of formula (1) exceeded the specified range, the Cr content, the Al content, and the total area ratio of ferrite and pearlite were below the specified range, and Ti, Nb, and B were not added. As a result, the deformation resistance during cold forging could not be sufficiently reduced, and grain coarsening could not be sufficiently suppressed.

Claims

1. The component composition is C: 0.10 to 0.22% by mass, Si: more than 0% by mass and not more than 0.14% by mass, Mn: 0.01 to 0.70% by mass, P: more than 0% by mass and not more than 0.100% by mass, S: more than 0% by mass and not more than 0.100% by mass, Cr: 1.30 to 2.00% by mass, Al: 0.050 to 0.100% by mass, Ti: 0.040 to 0.100% by mass, Nb: 0.001 to 0.012% by mass, N: 0.0020 to 0.0080% by mass, B: 0.0010 to 0.0050 mass%, and The balance is composed of iron and inevitable impurities and satisfies the following formula (1): A steel material in which the total area ratio of ferrite and pearlite is 85 to 100%. 0.00375≦[N]-10 －４．１００７ / [Al]≦0.0050 ・・・(1) In formula (1), [N] and [Al] represent the contents of N and Al, respectively, expressed in mass %.

2. The steel material according to claim 1, further containing at least one selected from the group consisting of Mo: more than 0 mass% and not more than 0.05 mass%, Cu: more than 0 mass% and not more than 0.2 mass%, and Ni: more than 0 mass% and not more than 0.2 mass%.

3. The steel material according to claim 1 or 2, further containing at least one selected from the group consisting of V: ​​more than 0 mass% and 0.1 mass% or less and Hf: more than 0 mass% and 0.1 mass% or less.

4. The steel material according to any one of claims 1 to 3, further containing at least one or more selected from the group consisting of Ca: more than 0% by mass and not more than 0.005% by mass, Mg: more than 0% by mass and not more than 0.005% by mass, Zr: more than 0% by mass and not more than 0.005% by mass, Te: more than 0% by mass and not more than 0.1% by mass, and REM: more than 0.02% by mass.

5. Pb: more than 0 mass% and not more than 0.1 mass%, Bi: more than 0 mass% and not more than 0.1 mass%, and Sb: more than 0 mass% and not more than 0.1 mass%. The steel material according to any one of claims 1 to 4, further containing at least one or more selected from the group consisting of: more than 0 mass% and not more than 0.1 mass%.

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