Steel material and method for manufacturing the same

Abstract

This technology improves the manufacturing efficiency of steel materials by enhancing cold forgeability and increasing the efficiency of high-frequency induction hardening while ensuring the necessary strength. [Solution] The steel material has a chemical composition comprising C: 0.30% to 0.45%, Si: 0.01% to 0.30%, Mn: 0.01% to 1.10%, Ni: 0.01% to 0.60%, Cr: 0.01% to 0.60%, Ti: 0.060% or less, and B: 0.0030% or less, with the remainder being Fe and unavoidable impurities, and has a cylindrical shape with radius r, and the hardness at a depth of r / 2 radially from the surface of the cylindrical shape is 185 HV or less, and the tensile reduction value obtained in a tensile test is 50% or more, and the ratio of the depth t from the side surface where the hardness is 400 HV to the radius r is 0.30 or less.

Description

Technical Field

[0001] The present invention relates to steel materials and a method for manufacturing the same.

Background Art

[0002] For example, steel materials constituting mechanical parts such as shafts in automotive parts are usually manufactured by being formed into a cylindrical shape or the like by hot forging or cold forging. In recent years, in the manufacturing process of steel materials, from the viewpoints of improving processing accuracy (dimension accuracy), reducing manufacturing costs, and reducing the amount of carbon dioxide emissions in the manufacturing process, cold forging is often carried out instead of hot forging. Further, in the manufacturing process of steel materials, for example, as also described in Patent Document 1 below, high-frequency quenching may be performed on a material formed through cold forging in order to ensure the strength of the steel material.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Generally, it is preferable that the material before cold forging has a small deformation resistance so that the forging load is reduced, and it is preferable that the material has high ductility so that the occurrence of cracks is suppressed in cold forging. Conventionally, in order to reduce the forging load of the material, spheroidizing annealing (SA) may be performed on the material before cold forging. [[ID=3,6]]

[0005] However, since spheroidizing annealing leads to increased carbon dioxide emissions, there has been a growing demand in recent years for the development of materials that can achieve high cold forgeability without spheroidizing annealing. According to conventional technical knowledge, in order to achieve high cold forgeability without performing spheroidizing annealing on the material, it is preferable to reduce the hardness of the material so that its deformation resistance is reduced.

[0006] The hardness of the material can be reduced by using F+P steel, which has a metallic structure in which ferrite and pearlite coexist. However, reducing the hardness by using F+P steel increases the ferrite area ratio of the material, which increases the likelihood that sufficient strength cannot be ensured after high-frequency induction hardening. This is because an increase in the ferrite area ratio increases the likelihood that soft ferrite structures will be mixed into the martensitic structure of the hardened layer formed by high-frequency induction hardening performed after cold forging.

[0007] Steel materials that have undergone high-frequency induction hardening can have their strength increased by improving the hardened portion's hardness through methods such as extending the heating time during high-frequency induction hardening or adding a hardening and tempering heat treatment process before high-frequency induction hardening. However, extending the heating time during high-frequency induction hardening or adding a heat treatment process can lead to a decrease in the efficiency of steel manufacturing, potentially resulting in increased steel manufacturing costs and increased carbon dioxide emissions.

[0008] Thus, in the manufacturing technology of steel materials produced by cold forging followed by high-frequency induction hardening, there was still room for improvement in enhancing the cold forgeability of the material before high-frequency induction hardening, thereby reducing the forging load and suppressing the occurrence of cracks during cold forging. Furthermore, in steel manufacturing technology, there was a demand to improve the efficiency of high-frequency induction hardening by shortening the heating time while ensuring the minimum necessary strength of the steel material required for the parts to which the steel material is applied, in order to increase the manufacturing efficiency of steel materials.

[0009] The present invention aims to improve the manufacturing efficiency of steel materials by reducing the forging load during cold forging, improving cold forgeability to suppress crack formation, and increasing the efficiency of high-frequency induction hardening while ensuring the necessary strength required for steel materials. [Means for solving the problem]

[0010] The present invention can be realized, for example, in the following forms.

[0011] [First Form] The first form is provided as a steel material. The steel material of the first form has a chemical composition in mass%, comprising C: 0.30% to 0.45%, Si: 0.01% to 0.30%, Mn: 0.01% to 1.10%, Ni: 0.01% to 0.60%, Cr: 0.01% to 0.60%, Ti: 0.060% or less, and B: 0.0030% or less, with the remainder being Fe and unavoidable impurities, and has a cylindrical shape with radius r, a hardness of 185 HV or less at a depth of r / 2 radially from the side surface of the cylindrical shape, and a tensile reduction value of 50% or more obtained in a tensile test, and when high-frequency induction hardening is performed, the ratio of the depth t from the side surface of the cylindrical shape to the radius r at which the hardness is 400 HV, t / r, is 0.30 or less.

[0012] [Second Embodiment] In the steel material described in the first embodiment above, the average value of the minor axis when the ferrite crystal grains are approximated to an elliptical shape may be 20.0 μm or less.

[0013] [Third form] In the steel material described in the first or second form above, t / r may be 0.27 or less.

[0014] [Fourth Embodiment] The steel material described in any one of the first, second, and third embodiments above may be configured in a cylindrical shape by forming a through hole that penetrates the center along the central axis.

[0015] [Fifth Form] The fifth form is provided as a method for manufacturing steel. The manufacturing method of the fifth form has a chemical composition in mass%, comprising C: 0.30% to 0.45%, Si: 0.01% to 0.30%, Mn: 0.01% to 1.10%, Ni: 0.01% to 0.60%, Cr: 0.01% to 0.60%, Ti: 0.060% or less, and B: 0.0030% or less, with the remainder being Fe and unavoidable impurities, and has a cylindrical shape with radius r, with a radial direction r / 2 from the side surface of the cylindrical shape. The process includes the steps of preparing a material having a hardness of 185 HV or less at a certain depth and a tensile reduction value of 50% or more obtained in a tensile test, and producing a steel material having a hardened layer with a hardness greater than 400 HV formed on its side surface by performing high-frequency induction hardening on the material, wherein the high-frequency induction hardening is performed under the condition that the ratio of the depth t from the side surface of the cylindrical shape at which the hardness is 400 HV relative to the radius r is 0.30 or less.

[0016] [Sixth Embodiment] In the manufacturing method described in the fifth embodiment, when the heating time for high-frequency induction hardening is a, and the average value of the minor axis when the ferrite crystal grains in the material are approximated to an elliptical shape is b, the high-frequency induction hardening may be performed under conditions that satisfy the relationship 1.4ln(a) + (49.1-b) / 38.8 ≤ 2.0.

[0017] [Seventh Embodiment] The manufacturing method described in the fifth or sixth embodiment may further include a step of forming a through hole that penetrates the center of the steel material along the central axis, thereby making the steel material cylindrical. [Effects of the Invention]

[0018] According to the steel material of the present invention, the hardness at the position of the depth of r / 2 in the radial direction of the material is 185 HV or less, the deformation resistance of the material is small, and the forging load is reduced. Further, according to the steel material of the present invention, since the tensile drawing value of the material is 50% or more, the occurrence of cracks can be suppressed in cold forging. Therefore, the cold forging property of the steel material is improved, the deformation processing by cold forging can be facilitated, and the forging load can be reduced. Further, according to the steel material of the present invention, when a hardened layer is formed on the surface layer by high-frequency quenching, the value t / r, which is an index of the depth of the hardened layer that can obtain the hardness effectively acting on the strength of the steel material, becomes 0.30 or less, and it is suppressed that the hardened layer becomes too thick for ensuring the strength of the steel material. Therefore, for example, it is possible to reduce the consumption energy for high-frequency quenching, such as shortening the heating time of high-frequency quenching for forming the hardened layer. Therefore, while ensuring the minimum required strength of the steel material required for the parts to which the steel material is applied, it is possible to reduce the consumption energy of high-frequency quenching, enable high efficiency of high-frequency quenching, and increase the manufacturing efficiency of the steel material.

[0019] The present invention can be realized in various forms other than steel materials and manufacturing methods of steel materials. The present invention can be realized, for example, in the form of high-frequency quenched high-frequency quenched steel materials, mechanical parts such as automotive parts constituted by steel materials, manufacturing apparatuses of steel materials, methods of high-frequency quenching for steel materials, apparatuses for executing high-frequency quenching, and control methods thereof.

Brief Description of Drawings

[0020] [Figure 1] Schematic perspective view of the steel material. [Figure 2] Explanatory drawing showing an example of the relationship between the depth and hardness from the side surface in the steel material after high-frequency quenching. [Figure 3] Explanatory drawing for explaining the ferrite thickness. [Figure 4] Schematic perspective view showing another configuration example of the steel material.

Embodiments for Carrying Out the Invention

[0021] Hereinafter, embodiments of the steel material and its manufacturing method according to the present invention will be described.

[0022] 1. Embodiment: 1-1. Outline of the steel material: Referring to FIG. 1, the outline of the configuration of the steel material 10 of the present embodiment will be described. FIG. 1 shows a schematic perspective view of the steel material 10 after high-frequency quenching.

[0023] The steel material 10 is formed into a solid cylindrical shape with a radius r by cold forging. In this specification, "cylinder" includes not only a solid configuration as shown in FIG. 1 but also a hollow "cylindrical tube". After the steel material 10 is formed by cold forging, high-frequency quenching is performed so that the strength required for the applied parts is ensured. The steel material 10 is applied to, for example, cylindrical mechanical parts such as automobile shafts and steel pipes. A hardened layer 12 for increasing the strength of the steel material 10 is formed on the surface layer of the side surface of the steel material 10 after high-frequency quenching. Details of high-frequency quenching and the hardened layer 12 will be described later.

[0024] The steel material 10 is composed of a material having the chemical composition described below. This material has hardness and ductility that provide high cold forging properties. The hardness and ductility of the material will be described after the chemical composition.

[0025] 1-2. Chemical composition of the steel material: The steel material 10 is composed of a material having a chemical composition containing at least carbon (C), silicon (Si), manganese (Mn), nickel (Ni), chromium (Cr), titanium (Ti), and boron (B), with the balance being iron (Fe) and inevitable impurities. Hereinafter, the preferred ranges of the contents of the respective elements C, Si, Mn, Ni, Cr, Ti, and B in the steel material 10 and the reasons for their limitations will be described in detail. In the following description, unless otherwise specified, the content is the mass content, and "%" means "mass %".

[0026] · C: 0.30% or more and 0.45% or less Carbon (C) is an effective element for ensuring the hardness and strength of steel. In steel material 10, if the carbon content in the material is less than 0.30%, it becomes difficult to ensure sufficient strength. To further increase the strength of steel material 10, the carbon content in the material is more preferably 0.32% or higher, and even more preferably 0.35% or higher. On the other hand, if the carbon content in the material is greater than 0.45%, the material becomes too hard, increasing the forging load and potentially reducing its cold forgeability. To further improve the cold forgeability of steel material 10, the carbon content in the material is more preferably 0.43% or lower, and even more preferably 0.40% or lower.

[0027] ·Si: 0.01% or more, 0.30% or less Si is an effective element for deoxidizing steel, as well as for increasing the strength and hardenability of steel, and for improving the resistance to tempering softening. In steel material 10, if the Si content in the material is less than 0.01%, it becomes difficult to obtain these effects to the fullest extent. On the other hand, if the Si content is greater than 0.30%, the material becomes too hard, increasing the forging load and potentially reducing its cold forgeability. To improve the cold forgeability of steel material 10, the Si content is more preferably 0.20% or less, and even more preferably 0.10% or less.

[0028] ·Mn: 0.01% or more, 1.10% or less Mn is an effective element for improving the high-frequency induction hardenability of steel materials. In steel material 10, if the Mn content in the material is less than 0.01%, it becomes difficult to obtain a sufficient effect in improving the high-frequency induction hardenability. To further improve the high-frequency induction hardenability of steel material 10, the Mn content is more preferably 0.20% or more, and even more preferably 0.30% or more. On the other hand, if the Mn content in the material is greater than 1.10%, the material becomes too hard, increasing the forging load and potentially reducing the cold forgeability. To further improve the cold forgeability of steel material 10, the Mn content in the material is more preferably 0.80% or less, and even more preferably 0.40% or less.

[0029] ·Ni: 0.01% or more, 0.60% or less Ni is an effective element for increasing the strength and hardenability of steel. In steel material 10, if the Ni content in the material is less than 0.01%, the effect cannot be fully obtained. On the other hand, if the Ni content in the material exceeds 0.60%, the material becomes too hard, increasing the forging load and potentially reducing cold forgeability. To further improve the cold forgeability of steel material 10, the Ni content in the material is more preferably 0.30% or less, and even more preferably 0.10% or less.

[0030] ·Cr: 0.01% or more, 0.60% or less Cr is an effective element for improving the high-frequency hardenability of steel materials. In steel material 10, if the Cr content in the material is less than 0.01%, it becomes difficult to obtain a sufficient effect in improving the high-frequency hardenability. To further improve the high-frequency hardenability of steel material 10, the Cr content is more preferably 0.05% or more, and even more preferably 0.08% or more. On the other hand, if the Cr content in the material is greater than 0.60%, the material becomes too hard, increasing the forging load and potentially reducing the cold forgeability. To further improve the cold forgeability of the steel material 10, the Cr content is more preferably 0.40% or less, and even more preferably 0.20% or less.

[0031] ·Ti: 0.060% or less The addition of Ti promotes the formation of carbonitrides during the steel manufacturing process, enabling grain refinement. However, in steel material 10, if the Ti content in the material exceeds 0.060%, TiC precipitation becomes significant, reducing its cold forgeability and potentially increasing the forging load of steel material 10.

[0032] ·B:0.0030% or less The addition of B can result in the softening of the steel material due to the formation of boron-iron carbides, improved high-frequency hardenability, and improved fatigue strength and impact strength of the steel material due to improved grain boundary strength after high-frequency hardening. However, in the case of steel material 10, it is undesirable if the B content in the material exceeds 0.0030% because these effects become saturated.

[0033] The material of steel 10 may contain other chemical components in addition to the C, Si, Mn, Ni, Cr, Ti, and B mentioned above. For example, the material of steel 10 may contain phosphorus (P), sulfur (S), copper (Cu), molybdenum (Mo), aluminum (Al), nitrogen (N), oxygen (O), or equivalent elements. When the material of steel 10 contains the aforementioned P, S, Cu, Mo, Al, N, and O, the preferred range of content for each element is, for example, as follows.

[0034] ·P:0.020% or less If the P content in the steel material 10 is kept below 0.020%, the increase in forging load during cold forging can be suppressed, and the deterioration of cold forgeability can be suppressed.

[0035] ·S: 0.001% or more, 0.020% or less If the steel material 10 contains S together with Mn at a content of 0.001% or more, MnS can be formed in its microstructure, improving the machinability of the steel material 10. If the S content is 0.020% or less, the occurrence of grain boundary embrittlement due to grain boundary segregation in the steel material 10 can be suppressed.

[0036] ·Cu: 0.01% or more, 0.20% or less Cu is an effective element for stabilizing austenite. In steel material 10, it is preferable that the material contains 0.01% or more Cu. If the Cu content is 0.20% or less, the decrease in the hot workability of steel material 10 can be suppressed.

[0037] ·Mo: 0.01% or more, 0.10% or less In steel material 10, if the Mo content in the material is 0.01% or more, the grain boundary strength of steel material 10 after high-frequency induction hardening can be improved, thereby improving the strength characteristics of steel material 10. Furthermore, if the Mo content in steel material 10 is 0.10% or less, it is possible to suppress the material from becoming too hard and reducing its cold workability.

[0038] ·Al: 0.010% or more, 0.050% or less If the material of steel 10 contains 0.010% or more Al, the quality of steel 10 can be improved by the deoxidation effect. If the Al content in the material of steel 10 is 0.050% or less, the formation and retention of AlN in the metal structure can be suppressed, thereby suppressing the decrease in cold workability.

[0039] ·N: 0.020% or less If the steel material contains an excess of nitrogen, the formation of BN may reduce the effects obtained by adding boron as described above. In addition, the formation of TiN may increase the likelihood of cracking during cold forging. For this reason, it is preferable that the nitrogen content in the steel material 10 is 0.020% or less.

[0040] ·O:0.0030% or less In steel materials, an excess of oxygen can lead to the formation of oxide inclusions, potentially reducing cold workability. Therefore, in steel material 10, it is preferable that the oxygen content in the material is 0.0030% or less.

[0041] 1-3. Hardness and tensile reduction of area of ​​steel materials: The steel material 10 is formed into a solid cylindrical shape with radius r, as shown in Figure 1, by cold forging. The hardness of the steel material 10 is 185 HV or less. As a result, after the material is formed into a cylindrical shape, the hardness at a depth of r / 2 radially from the side of the cylinder is 185 HV or less. With this material, the deformation resistance is low and the forging load is reduced, so high cold forgeability can be obtained.

[0042] In this specification, "hardness" is expressed in terms of Vickers hardness. Vickers hardness may also be expressed as a Rockwell hardness (HRB) equivalent. Furthermore, the "position at a depth of r / 2 radially from the side surface" refers to a position r / 2 radially away from the side surface of the cylindrical shape toward its central axis CX. "Radial direction" corresponds to the direction perpendicular to the central axis CX.

[0043] Furthermore, the tensile reduction of the steel material 10 is 50% or more. The tensile reduction is a value obtained by a tensile test on a solid cylindrical test piece. The tensile test can be performed, for example, under conditions conforming to JIS Z2241. The tensile reduction is determined by pulling the test piece in the longitudinal direction until it breaks, and then calculating the ratio of the cross-sectional area after fracture to the cross-sectional area before fracture at the fracture site.

[0044] If the tensile reduction of area is 50% or more, high ductility can be obtained. Therefore, cracking can be suppressed when cold forging is performed during the manufacturing process of the steel material 10. It is more preferable that the tensile reduction of area of ​​the steel material 10 is 60% or more. This further enhances the cold forgeability of the base material.

[0045] As described above, the cold forgeability of the steel material 10 is enhanced, reducing the forging load during cold forging and suppressing the occurrence of cracks during cold forging.

[0046] 1-4. Hardness of steel after high-frequency induction hardening: Figure 2 shows a graph illustrating an example of the change in hardness of steel material 10 with respect to depth from the side after high-frequency induction hardening. "Depth from the side of steel material 10" refers to the radial distance from the side of steel material 10 toward the central axis CX. "Radial direction" corresponds to the direction perpendicular to the central axis CX of steel material 10.

[0047] As described above, the induction hardening performed after cold forging forms a hardened layer 12 on the surface of the side of the steel material 10 to increase its strength. Due to the formation of the hardened layer 12 in the steel material 10, as shown in the graph in Figure 2, regions are formed on the side surface where the hardness increases in a stepwise manner as the radial direction approaches from the central axis CX side toward the side.

[0048] The hardened layer 12 has a hardness greater than 400 HV throughout its entire thickness. The steel material 10 has increased strength compared to the material after cold forging because the hardened layer 12 is formed on its sides by high-frequency induction hardening. In order to ensure the strength of the steel material 10, it is preferable that the hardened layer 12 has a hardness greater than 500 HV throughout its entire thickness, and more preferably greater than 550 HV.

[0049] In the steel material 10 after high-frequency induction hardening, the hardness at a depth of r / 2 from the surface of the hardened layer 12 is 185 HV or less. The hardness at a depth of r / 2 from the surface of the hardened layer 12 corresponds to the hardness of the interior of the steel material 10 below the hardened layer 12, and corresponds to the hardness of the material before high-frequency induction hardening as described above.

[0050] Let r be the radius of the steel material 10, and let t be the depth from the side where the hardness is 400 HV, as shown in Figure 2. In this embodiment, when the steel material 10 is high-frequency hardened, the ratio t / r, which is the ratio of the depth t from the cylindrical side where the hardness is 400 HV to the radius r, is 0.30 or less. t / r is an index value that indicates the depth of the hardened layer 12 that provides the hardness that effectively contributes to the strength of the steel material 10. A small value of t / r indicates that the thickness of the hardened layer 12 that is effective in improving the strength of the steel material 10 is small, and that high-frequency hardening to form the hardened layer 12 can be performed efficiently with low energy consumption.

[0051] If the steel material 10 has a t / r value of 0.30 or less, the thickness of the hardened layer 12, which effectively contributes to the strength of the steel material 10, can be reduced as described above. Therefore, the heating time for high-frequency induction hardening can be shortened, reducing the energy consumed by high-frequency induction hardening, thereby increasing the efficiency of high-frequency induction hardening, and improving the manufacturing efficiency of the steel material 10 while ensuring its strength.

[0052] The t / r ratio is more preferably 0.27 or less, and even more preferably 0.25 or less. A t / r ratio of 0.21 or less is even more preferable. This makes it possible to reduce the energy consumption of high-frequency induction hardening while suppressing a decrease in the strength of the steel material 10, and to further improve the manufacturing efficiency of the steel material 10.

[0053] However, if t / r is too small, the thickness of the hardened layer 12 becomes too small, and sufficient residual stress cannot be obtained, which may reduce the strength of the steel material 10. For this reason, t / r is preferably 0.15 or higher, and more preferably 0.16 or higher.

[0054] 1-5. Crystal grains of ferrite in steel materials: Figure 3 schematically shows the metallographic structure MS of the steel material 10 at an arbitrary cross-section. The cross-section in Figure 3 may be assumed to be any cross-section perpendicular to the central axis CX of the material formed into a cylindrical shape by cold forging. In Figure 3, for convenience, one of the ferrite crystal grains FG is hatched.

[0055] The material of steel material 10 is F+P steel in which ferrite and pearlite coexist in the internal region. In the material of steel material 10, it is preferable that the average value of the minor axis FD when the ferrite crystal grain FG is approximated as an ellipse is 20.0 μm or less. In this specification, the value of the minor axis FD is also referred to as the "ferrite thickness". "Approximateing the ferrite crystal grain FG as an ellipse" means forming an ellipse representing the ferrite crystal grain FG such that, as shown in Figure 3, as many corners and ends of the outer contour of the ferrite crystal grain FG as possible are located on or near the outer contour of the ellipse.

[0056] If the average value of the ferrite thickness FD is 20.0 μm or less, it is possible to suppress the retention of ferrite in the hardened layer 12 without it becoming martensitic through high-frequency induction hardening. Therefore, the effect of the hardened layer 12 on improving the strength of the steel material 10 can be enhanced. It is more preferable that the average value of the ferrite thickness FD be 18.0 μm or less, and even more preferable that it be 17.5 μm.

[0057] Furthermore, it is preferable that, in the steel material 10 after high-frequency induction hardening, there are almost no ferrite grains FG in the region of the hardened layer 12 at a depth of at least 0.2 mm from the side surface. This is because the presence of ferrite grains FG in that region may make it impossible to ensure sufficient strength required for mechanical parts such as automobile parts.

[0058] 1-6. Other examples of steel material configurations: Figure 4 is a schematic perspective view of steel material 10a, which is another example of the configuration of steel material 10 in this embodiment. Steel material 10a is cylindrical in shape. Steel material 10a corresponds to a configuration in which a through hole 15 is formed through the center along the central axis CX of the solid steel material 10 shown in Figure 1, and has substantially the same configuration as steel material 10 in Figure 1 except for having the through hole 15. With steel material 10a, the same effects as those obtained with steel material 10 shown in Figure 1 can be obtained, and it is possible to reduce the weight by the amount of the through hole 15 formed, making it suitable for reducing the weight of machine parts.

[0059] 1-7. Method for manufacturing steel materials: The steel materials 10 and 10a of this embodiment are manufactured by the following manufacturing process.

[0060] (1) Material preparation process: In the first step, a material for steel 10 suitable for high-frequency induction hardening is prepared. This material has a chemical composition in which at least C, Si, Mn, Ni, Cr, Ti, and B are contained in the above-mentioned preferred ranges, with the remainder being Fe and unavoidable impurities. In the first step, cold forging is performed on the material to form a solid cylindrical shape with radius r.

[0061] The material prepared in the first step has a hardness of 185 HV or less before cold forging. Furthermore, after cold forging, as described above, the material has a hardness of 185 HV or less at a depth of r / 2 radially from the side of the cylindrical shape. Having such hardness reduces the deformation resistance of the material during cold forging, thereby reducing the forging load.

[0062] Furthermore, as mentioned above, the material prepared in the first step has a tensile reduction value of 50% or more obtained by tensile testing. This ensures high ductility of the material, thereby suppressing cracking during cold forging.

[0063] In the material prepared in the first step, it is preferable that the average value of the ferrite thickness FD described above is 20.0 μm or less. This suppresses the retention of ferrite in the hardened layer 12 formed by high-frequency induction hardening. Therefore, the strength of the steel material 10 can be increased more effectively.

[0064] (2) High-frequency induction hardening process: In the second step, high-frequency induction hardening is performed on the steel material 10, which is made up of the material prepared in the first step after cold forging. The high-frequency induction hardening in the second step is performed under the condition that the ratio of the depth t from the side of the cylindrical shape that achieves a hardness of 400 HV to the radius r of the steel material 10, t / r, is 0.30 or less. Conditions for high-frequency induction hardening include, for example, the heating temperature and heating time, the power supply frequency, the current and voltage values, and the distance between the material and the coil.

[0065] The heating temperature for induction hardening is, for example, around 700 to 1100°C. The power supply frequency for induction hardening can be around 1 kHz to 500 kHz. For example, the power supply frequency for induction hardening may be 150 kHz.

[0066] The heating time for high-frequency induction hardening is preferably 2.5 seconds or less. By setting the heating time for high-frequency induction hardening to 2.5 seconds or less, it is possible to form a hardened layer 12 that can ensure the strength of the steel material 10 while reducing the energy consumption caused by the high-frequency induction hardening process. In addition, the time required to manufacture the steel material 10 can be shortened, and the manufacturing efficiency of the steel material 10 can be increased. To further increase the manufacturing efficiency of the steel material 10, the heating time for high-frequency induction hardening is more preferably 2.2 seconds or less, and even more preferably 2.0 seconds or less.

[0067] The second step, high-frequency induction hardening, is preferably performed under conditions that satisfy the following inequality (1). Examples of high-frequency induction hardening conditions controlled to satisfy inequality (1) include the heating time for high-frequency induction hardening, the average value of the ferrite thickness FD, the power supply frequency for high-frequency induction hardening, the current and voltage values ​​for high-frequency induction hardening, and the distance between the material and the coil.

[0068] [Mathematics 1] 1.4ln(a)+(49.1-b) / 38.8≦2.0···(1) a: Heating time for high-frequency induction hardening [seconds] b: Average value of ferrite thickness FD [μm]

[0069] The above inequality (1) was derived by the inventor of the present invention based on his own experimental results, as an equation dependent on the heating time of high-frequency induction hardening and the ferrite thickness FD, so that the conditions for obtaining an effective hardened layer 12 by high-frequency induction hardening can be quantitatively determined. The average value of the ferrite thickness FD is the value obtained in the material before high-frequency induction hardening. An example of how to determine the average value of the ferrite thickness FD will be explained in the examples described later.

[0070] In the following, the value obtained by the formula on the left side of inequality (1) will also be called the "ECD (Effective Case Depth) value". As shown in the examples described later, by performing high-frequency induction hardening under conditions where the ECD value is 2.0 or less, it is easy to form a hardened layer 12 with a t / r of 0.30 or less in a heating time of 2.5 seconds or less.

[0071] High-frequency induction hardening is preferably performed under conditions that result in an ECD value of 1.2 or higher, and more preferably under conditions that result in an ECD value of 1.4 or higher. This allows for obtaining a suitable hardened layer depth and appropriately generating residual stress. Therefore, it is easy to ensure the strength of the steel material 10 after high-frequency induction hardening.

[0072] (3) Process of forming into a cylindrical shape: Furthermore, by forming a through-hole that penetrates the center along the central axis in the steel material 10 shown in Figure 1, a cylindrical steel material 10a shown in Figure 4 can be obtained. As described above, since the hardness of the interior of the layer below the hardened layer 12 in the steel material 10 is 185 HV, the formation of the through-hole 15 is easy. Therefore, cylindrical steel material 10a can be manufactured efficiently. Note that the process of making the steel material 10 cylindrical may be performed on the steel material 10 before high-frequency induction hardening or on the steel material 10 after high-frequency induction hardening.

[0073] 1-8. Summary of Embodiments: As described above, the steel materials 10 and 10a of this embodiment, and their manufacturing method, can improve the cold forgeability of the steel materials 10 and 10a, thereby reducing the forging load during cold forging and suppressing the occurrence of cracks during cold forging. Furthermore, it is possible to improve the efficiency of high-frequency induction hardening while ensuring the required strength of the steel materials 10 and 10a after high-frequency induction hardening, thereby increasing the manufacturing efficiency of the steel materials 10 and 10a. [Examples]

[0074] 2. Examples of steel materials: Next, examples and comparative examples of the steel material according to the present invention will be described with reference to Tables 1 to 3.

[0075] First, refer to Tables 1 and 2. Table 1 shows the chemical composition of the steel materials of Examples 1 to 26, and Table 2 shows the chemical composition of the steel materials of Comparative Examples 1 to 10. The percentages in Tables 1 and 2 are all mass percentages. Also, "s-Al" in Tables 1 and 2 represents acid-soluble aluminum.

[0076] The steel materials of Examples 1 to 26 and Comparative Examples 1 to 10 were all manufactured by cold forging a material having the chemical composition shown in Tables 1 and 2 into a solid cylindrical shape, followed by high-frequency induction hardening.

[0077] As shown in Table 1, in Examples 1 to 26, the respective content of C, Si, Mn, Ni, Cr, Ti, and B was within the preferred range described in the above embodiments. On the other hand, as shown in Table 2, the content of C in Comparative Example 2, the content of C and Mn in Comparative Example 3, the content of Mn in Comparative Example 4, the content of Ni in Comparative Example 5, and the content of Cr in Comparative Example 6 were all outside the preferred range.

[0078] Furthermore, as shown in Tables 1 and 2, in all of Examples 1 to 26 and Comparative Examples 1 to 10, the respective content of P, S, Cu, Mo, s-Al, N, and O was within the preferred range described in the above embodiments.

[0079] [Table 1]

[0080] [Table 2]

[0081] Table 3 shows the "tensile reduction value," "hardness at a depth of r / 2," and "t / r" for each of Examples 1 to 26 and Comparative Examples 1 to 10.

[0082] The "tensile reduction of area" is a value obtained in a tensile test conducted under conditions conforming to JIS Z2241, and corresponds to the tensile reduction of a solid cylindrical steel material. The tensile reduction of area was calculated as the percentage of the cross-sectional area after fracture to the cross-sectional area before fracture at the fracture site, after the JIS 14A test specimen was pulled in the longitudinal direction and fractured. The dimensions of the JIS 14A test specimen are as follows. • Handles (gripping parts) at both ends: M8 • Central parallel section: φ5 ·Total length: 78L

[0083] "Hardness at a depth of r / 2" is the Rockwell hardness measurement in HRB taken at a depth of r / 2 from the side of the steel material, converted to Vickers hardness. r is the radius of the steel material. Rockwell hardness is the average of measurements taken at four points equally spaced at 90° intervals around the central axis CX of the steel material. This hardness corresponds to the hardness of the material before induction hardening.

[0084] "t / r" was calculated by dividing the value of t, obtained as follows, by the radius r of the steel material. In Examples 1 to 26 and Comparative Examples 1 to 10, Vickers hardness was measured with a load of 500 gf at depth positions at 0.2 mm intervals from the side, and the relationship between the depth position from the side and the Vickers hardness was determined as illustrated in Figure 2. Then, the depth position at which the hardness was 400 HV in that relationship was defined as t.

[0085] Table 3 further shows the "ECD value," "heating time," "ferrite thickness," and "presence or absence of ferrite at a depth of 0.2 mm" for each of Examples 1 to 26 and Comparative Examples 1 to 10.

[0086] The "ECD value" is the value calculated by substituting the following induction hardening heating time for a and the ferrite thickness for b into the formula on the left side of inequality (1) above.

[0087] "Heating time" refers to the heating time during high-frequency induction hardening. During high-frequency induction hardening, the heating time was adjusted to 2.5 seconds or less so that the t / r ratio (as shown below) was 3.0 or less. The power supply frequency for high-frequency induction hardening was 150 Hz.

[0088] "Ferrite thickness" is a value obtained by observing and measuring an arbitrary cross-section perpendicular to the central axis of the steel material using an optical microscope. The optical microscope's magnification was 100x, and the field of view was 0.55m. 2 The dimensions were (660 μm × 840 μm). The cross-section within the field of view of the optical microscope was divided into a 3 × 3 grid using equally spaced dividing lines. The minor axis of the ferrite crystal grains located near the nine grid points was measured when they were approximated as elliptical shapes, and the average value was calculated to determine the ferrite thickness.

[0089] The "presence or absence of ferrite at a depth of 0.2 mm" is determined by observing the microstructure at a depth of 0.2 mm from the side of the steel material using an optical microscope, within the same field of view as when determining the ferrite thickness described above, and confirming the presence or absence of ferrite crystal grains. The observation position is any position along the central axis CX direction of the steel material. If the "presence or absence of ferrite at a depth of 0.2 mm" is "absent," it indicates that the strength required for the machine part is ensured, and if it is "present," it indicates that the strength of the steel material is insufficient.

[0090] [Table 3]

[0091] In all of Examples 1 to 26, the hardness of the hardened layer was greater than 400 HV, ensuring sufficient strength as a steel material. Furthermore, in all of Examples 1 to 26, the hardness at a depth of r / 2 radially from the surface of the hardened layer was 185 V or less, indicating a reduction in the deformation resistance of the material before induction hardening, resulting in high cold forgeability.

[0092] In all of Examples 1 to 26, the tensile reduction in area was 50% or more, indicating high ductility, improved cold forgeability of the material, and suppression of cracking during cold forging. In Examples 8, 15, 16, and 17, the tensile reduction in area was 60% or more, further improving the cold forgeability of the material and suppressing cracking during cold forging.

[0093] In all of Examples 1 to 26, the t / r ratio was 0.30 or less. Furthermore, the heating time for high-frequency induction hardening was shortened to 2.5 seconds or less, improving the efficiency of high-frequency induction hardening and thus the efficiency of steel production. In addition, in all of Examples 1 to 26, there were almost no ferrite crystal grains at a depth of 0.2 mm from the side surface, ensuring the strength of the steel.

[0094] Thus, in Examples 1 to 26, the cold forgeability of the material before induction hardening was ensured, and the efficiency of induction hardening was improved while suppressing the decrease in strength of the steel after induction hardening, thereby increasing the efficiency of steel manufacturing.

[0095] On the other hand, in Comparative Examples 1, 2, 4, 5, and 6, the hardness at a depth of r / 2 radially from the surface of the hardened layer exceeded 185V, indicating low cold forgeability of the material before high-frequency induction hardening. In Comparative Example 2, the tensile reduction value was less than 50%, indicating low cold forgeability of the material. In Comparative Example 3, ferrite crystal grains were present at a depth of 0.2 mm from the side, indicating insufficient strength as a steel material applicable to machine parts. In Comparative Examples 8, 9, and 10, where t / r was greater than 0.30, the heating time for high-frequency induction hardening exceeded 2.5 seconds in all cases, indicating low efficiency of high-frequency induction hardening. Thus, in Comparative Examples 1 to 10, the cold forgeability of the material was not ensured, the strength of the steel material after high-frequency induction hardening decreased, and the efficiency of high-frequency induction hardening decreased, preventing the achievement of the high effectiveness seen in Examples 1 to 26.

[0096] In these examples, the t / r ratio was 0.27 or less in all of Examples 1 to 26. From this, it can be said that a t / r ratio of 0.27 or less is preferable. In addition, the t / r ratio was 0.25 or less in Examples 1 to 8, 12 to 14, and 16 to 26, and 0.21 or less in all of Examples 1 to 8, 12 to 14, and 16 to 26.

[0097] The ECD values ​​for Examples 1 to 26 were all 2.0 or less. In contrast, in Comparative Examples 8, 9, and 10, where the ECD values ​​were greater than 2.0, the t / r ratio was greater than 3.0, the heating time for high-frequency induction hardening was 2.6 seconds or longer, and the efficiency of high-frequency induction hardening was low. This result indicates that it is preferable to set the high-frequency induction hardening conditions so that the ECD value is 2.0 or less.

[0098] In Examples 1 to 26, in most cases where the ferrite thickness was 20.0 μm or less, ferrite grains were not present at a depth of 0.2 mm from the side. This indicates that the presence of ferrite grains at a depth of 0.2 mm from the side after high-frequency induction hardening is more suppressed when the ferrite thickness is 20.0 μm or less. Furthermore, in Examples 1 to 26, the presence of ferrite grains at a depth of 0.2 mm from the side was suppressed when the ferrite thickness was 18.0 μm or less. Therefore, it can be said that a ferrite thickness of 18.0 μm or less is more preferable, and 17.5 μm or less is even more preferable.

[0099] 3. Other forms: In addition to the configurations described in the above embodiments and examples, the present invention can also be realized in the following forms.

[0100] 3-1.First form: The first form is a steel material suitable for high-frequency induction hardening. The steel material of the first form has a chemical composition in mass%, comprising C: 0.30% to 0.45%, Si: 0.01% to 0.30%, Mn: 0.01% to 1.10%, Ni: 0.01% to 0.60%, Cr: 0.01% to 0.60%, Ti: 0.060% or less, and B: 0.0030% or less, with the remainder being Fe and unavoidable impurities. It has a cylindrical shape with radius r, and the hardness at a depth of r / 2 radially from the side surface of the cylindrical shape is 185 HV or less, and the tensile strength obtained in a tensile test is 185 HV or less. The material is composed of a material with a tension drawing value of 50% or more, and when high-frequency induction hardening is performed under conditions that satisfy the relationship 1.4ln(a) + (49.1-b) / 38.8 ≤ 2.0, which is defined by the heating time a of high-frequency induction hardening and the average value b of the minor axis when the crystal grains of ferrite in the high-frequency induction hardening steel are approximated to an elliptical shape, the hardness at a position radially r / 2 from the surface of the hardened layer is 185 HV or less, and when the depth from the side where the hardness is 400 HV is t is denoted by t, then t / r is 0.30 or less. The first form of steel material allows for high cold forgeability, reducing the forging load during cold forging and suppressing crack formation during cold forging. Furthermore, it is easy to reduce the energy consumption of high-frequency induction hardening and improve its efficiency while ensuring the required strength for the machine parts to which the steel material is applied. Therefore, the manufacturing efficiency of the steel material can be easily increased.

[0101] 3-2.Second form: The second form is provided as a high-frequency induction hardened steel material whose strength has been increased by high-frequency induction hardening. The steel material of the second form has a chemical composition in mass%, comprising C: 0.30% to 0.45%, Si: 0.01% to 0.30%, Mn: 0.01% to 1.10%, Ni: 0.01% to 0.60%, Cr: 0.01% to 0.60%, Ti: 0.060% or less, and B: 0.0030% or less, with the remainder being Fe and unavoidable impurities. It has a cylindrical shape with radius r, a hardness of 185 HV or less at a depth of r / 2 radially from the side surface of the cylindrical shape, a tensile reduction value of 50% or more obtained in a tensile test, a hardened layer with a hardness greater than 400 HV formed on the surface of the side surface, and a ratio of t / r, which is the depth t from the side surface where the hardness is 400 HV relative to the radius r, to 0.30 or less. The second embodiment of the steel material has a substantially equivalent structure to the high-frequency induction hardened steel material described in the above embodiment, and therefore can achieve various effects similar to those described in the above embodiment.

[0102] 3-3.Third form: The third embodiment is provided as a method for manufacturing steel. The manufacturing method of the third embodiment has a chemical composition in mass%, comprising C: 0.30% to 0.45%, Si: 0.01% to 0.30%, Mn: 0.01% to 1.10%, Ni: 0.01% to 0.60%, Cr: 0.01% to 0.60%, Ti: 0.060% or less, and B: 0.0030% or less, with the remainder being Fe and unavoidable impurities, and has a cylindrical shape with radius r, and the hardness at a position radially r / 2 depth from the side surface of the cylindrical shape is 185HV or less. The process comprises the steps of preparing a material having a tensile reduction value of 50% or more obtained in a tensile test, and producing a steel material having a hardened layer with a hardness greater than 400 HV formed on its sides by performing high-frequency induction hardening on the material, wherein the high-frequency induction hardening is performed under the condition that the relationship 1.4ln(a) + (49.1 - b) / 38.8 ≤ 2.0 is satisfied, where a is the heating time and b is the average value of the minor axis when the ferrite crystal grains in the material are approximated to an elliptical shape. According to the third method of manufacturing, the cold forgeability of the material is enhanced, which reduces the forging load during cold forging and suppresses the occurrence of cracks during cold forging. Furthermore, by performing high-frequency induction hardening under conditions that satisfy the inequality, the thickness of the hardened layer can be reduced while suppressing a decrease in the strength of the steel material. Therefore, the efficiency of high-frequency induction hardening can be increased, and the manufacturing efficiency of steel materials can be improved.

[0103] The present invention is not limited to the above embodiments and examples, or other forms, and various modifications are possible without departing from the spirit of the invention. For example, it is possible to omit some of the elements that are not essential, add elements other than those mentioned above to the extent that they do not significantly affect the main elements, or change some of the manufacturing conditions that are not essential. [Explanation of Symbols]

[0104] 10,10a...Steel material, 12...Hardened layer, 15...Through hole, CX...Central axis, FG...Crystal grain, MS...Metal structure

Claims

1. In mass percent, C: 0.30% or more and 0.45% or less, Si: 0.01% or more and 0.30% or less, Mn: 0.01% or more and 1.10% or less, Ni: 0.01% or more and 0.60% or less, Cr: 0.01% or more and 0.60% or less, Ti: 0.060% or less, and, B: 0.0030% or less, It has a chemical composition that includes, with the remainder being Fe and unavoidable impurities. It has a cylindrical shape with radius r, The hardness at a depth of r / 2 in the radial direction from the side surface of the cylindrical shape is 185 HV or less. It is made of a material whose tensile reduction value obtained in a tensile test is 50% or more. A steel material in which, when high-frequency induction hardening is performed, the ratio of the depth t from the side surface of the cylindrical shape to the radius r, which is t / r, is 0.30 or less.

2. The steel material according to claim 1, wherein the average value of the minor axis when the ferrite crystal grains are approximated to an elliptical shape is 20.0 μm or less.

3. The steel material according to claim 1, wherein t / r is 0.27 or less.

4. The steel material according to any one of claims 1 to 3, wherein it is configured in a cylindrical shape by forming a through hole that penetrates the center along the central axis.

5. In mass percent, C: 0.30% or more and 0.45% or less, Si: 0.01% or more and 0.30% or less, Mn: 0.01% or more and 1.10% or less, Ni: 0.01% or more and 0.60% or less, Cr: 0.01% or more and 0.60% or less, Ti: 0.060% or less, and, B: 0.0030% or less, A step of preparing a material having a chemical composition comprising, with the remainder being Fe and unavoidable impurities, having a cylindrical shape with radius r, having a hardness of 185 HV or less at a depth of r / 2 radially from the side surface of the cylindrical shape, and having a tensile reduction value of 50% or more obtained in a tensile test, A process for producing a steel material in which a hardened layer with a hardness greater than 400 HV is formed on the side surface by performing high-frequency induction hardening on the aforementioned material, Equipped with, The above-mentioned high-frequency induction hardening is performed under the condition that the ratio of the depth t from the side surface of the cylindrical shape to the radius r, which is t / r, is 0.30 or less.

6. Let a be the heating time for the high-frequency induction hardening, and let b be the average value of the minor axis when the ferrite crystal grains in the material are approximated to an elliptical shape. The manufacturing method according to claim 5, wherein the high-frequency induction hardening is performed under the condition that the relationship 1.4ln(a) + (49.1 - b) / 38.8 ≤ 2.0 is satisfied.

7. The manufacturing method according to claim 5 or claim 6, further comprising the step of forming a through hole that penetrates the center of the steel material along its central axis, thereby making the steel material cylindrical.

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