Hot forging steel and method for manufacturing the same

Abstract

This invention provides a hot forging steel that possesses high strength and excellent toughness, and also exhibits excellent high-frequency hardenability when subjected to high-frequency induction hardening. [Solution] The material has a component composition containing C: 0.28% to 0.40%, Si: 0.05% to 0.35%, Mn: 0.80% to 2.00%, P: 0.005% to 0.025%, S: 0.030% to 0.080%, Al: 0.015% to 0.050%, Cr: 0.80% to 2.00%, Mo: 0.05% to 0.35%, V: 0.040%, B: 0.0005%, and N: 0.0030% to 0.0200% in a predetermined relationship, with a bainite area ratio of 90% or more in the microstructure and an island martensite area ratio of 3.0% or less in the bainite structure.

Description

Technical Field

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

Background Art

[0002] Conventionally, for automotive undercarriage parts such as steering knuckles and upper arms, and hydraulic parts of construction machinery such as rod ends, high strength and high toughness have been required. For this reason, for these parts, S43C, S45C, S48C, etc. defined in JIS G4051, which are carbon steels for machine structures, are used as materials. After being formed by hot forging, heat treatment such as quenching and tempering (hereinafter also referred to as "quenching and tempering") is performed, and furthermore, depending on the type of part, induction hardening is performed on the surface to ensure the required properties.

[0003] However, these quenching and tempering processes require a huge amount of energy. Therefore, in order to meet the current social demand for energy conservation, the development of steel materials that do not require such quenching and tempering processes has been actively carried out.

[0004] So far, hot forging non-quenched and tempered steels (hot forging non-QT steels) that satisfy the properties as they are in hot forging have been mainly designed to increase the strength by precipitation strengthening of V carbonitrides in the ferrite-perlite structure. However, if the carbon content and V addition amount are simply increased aiming for higher strength, there is a concern of significantly impairing the toughness. Therefore, hot forging non-quenched and tempered steels having a bainite structure have attracted attention and have been developed.

[0005] For example, Patent Document 1 proposes a non-quenched and tempered steel in which 0.05 to 0.50% by mass of V is added to a low-carbon steel containing about 0.10 to 0.30% by mass of C. This non-quenched and tempered steel does not require heat treatment after hot forging and obtains excellent strength and toughness by natural air cooling.

[0006] Patent Document 2 proposes a non-quenched and tempered steel containing more than 0.30% to 0.60% of C and more than 1.60% to 3.00% of Mn, and having a bainite structure as it is in hot forging by adding V or Nb.

[0007] Patent Document 3 proposes a non-heat-treated steel in which C is 0.10-0.35% and V is 0.30-0.70%, and a bainite structure can be obtained without heat treatment after hot forging.

[0008] Patent Document 4 proposes a non-heat-treated steel in which C is 0.25-0.38%, Mn is 1.51-2.2%, and a small amount of Mo is added while V is added, resulting in a bainite-dominant structure after hot forging. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Patent No. 2743116 [Patent Document 2] Patent No. 3196006 [Patent Document 3] Patent No. 3241897 [Patent Document 4] Patent No. 6390685 [Overview of the project] [Problems that the invention aims to solve]

[0010] However, the non-heat-treated steel described in Patent Document 1 had the problem that it could not achieve high strength due to its low carbon content.

[0011] Furthermore, the component design using the precipitate described in Patent Document 2 has the problem of large variations in performance due to differences in cooling rates depending on forging conditions (heating temperature, processing temperature, etc.) and part thickness. In addition, there are concerns about material degradation (reduction in toughness and fatigue limit) due to Mn segregation caused by increasing the amount of Mn.

[0012] Furthermore, the technology described in Patent Document 3 requires the addition of a large amount of V when achieving strength with a relatively low amount of C, but this has the problem of impairing toughness.

[0013] In addition, the technology described in Patent Document 4 raises concerns that the precipitation of V may degrade toughness.

[0014] The present invention was developed to solve the problems of the conventional low-CO2 type hot forging steel that does not require tempering, as described above, and aims to provide a hot forging steel that has high strength and excellent toughness, and also excellent high-frequency hardenability when high-frequency hardening is performed. Furthermore, the present invention aims to provide a method for producing hot forging steel that eliminates the need for quenching and tempering treatment after hot forging, while achieving both high strength and excellent toughness, and also having excellent high-frequency hardenability, by performing appropriate heat treatment after hot forging. [Means for solving the problem]

[0015] Under the aforementioned objectives, the inventors diligently conducted research on steel for hot forging and obtained the following findings.

[0016] (1) While a certain amount of carbon is necessary to ensure high-frequency induction hardening hardness, if it is too high, the amount of retained austenite in the steel structure increases, causing a rapid deterioration in toughness. (2) Simply increasing the carbon content of low-carbon bainite steel leads to increased strength and decreased toughness. However, by balancing the content of Mn, Mo, and Cr in accordance with the increase in carbon content, it is possible to increase strength while maintaining the bainite structure. Furthermore, the presence of this bainite structure helps to suppress the decrease in toughness. This effect can be obtained even more efficiently by suppressing the amount of V added. (3) When using the precipitation of V carbonite, the material is susceptible to the effects of forging conditions (heating temperature, processing conditions, cooling rate), so reducing V as much as possible leads to material stability. (4) By not incorporating ferrite into the structure of the steel material, the high-frequency induction hardening depth becomes uniform within the cross-section of the steel material, and variations in the hardened layer harden are also suppressed. (5) In order to achieve both high strength and excellent toughness, it is necessary to limit the island-shaped martensite formed in the bainite structure, and appropriate heat treatment is effective for this. (6) In a component system where the reheat quenching after hot forging is omitted and a predetermined strength is satisfied, performing a quenching treatment is not suitable because defects such as cracking may occur.

[0017] The present invention is based on the above findings. That is, the gist of the present invention is as follows. 1. In mass%, C: 0.28% or more and 0.40% or less, Si: 0.05% or more and 0.35% or less, Mn: 0.80% or more and 2.00% or less, P: 0.005% or more and 0.025% or less, S: 0.030% or more and 0.080% or less, Al: 0.015% or more and 0.050% or less, Cr: 0.80% or more and 2.00% or less, Mo: 0.05% or more and 0.35% or less, V: 0.040% or less, B: 0.0005% or less, and N: 0.0030% or more and 0.0200% or less, with Ceq shown in the following formula (1) being 0.80 or more and 0.97% or less, and C, Mn, Cr, and Mo satisfying the following formula (2) within a range, having a component composition consisting of the balance Fe and inevitable impurities, A hot forging steel in which the area ratio of the bainite structure in the microstructure is 90% or more and the area ratio of the island-shaped martensite in the bainite structure is 3.0% or less. Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ··· (1) 0.08 ≤ C - Mn / 18 - Cr / 24 - Mo / 3 ≤ 0.28 ···· (2) The element symbols in the above formulas (1) and (2) mean the content of the corresponding element in the steel. For elements not contained in the steel, the content in the steel is taken as zero.

[0018] 2. The hot forging steel according to 1 above, wherein the component composition further contains, in mass%, one or more selected from Cu: 0.30% or less, Ni: 0.30% or less, Nb: 0.050% or less, Ti: 0.050% or less, Pb: 0.30% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Bi: 0.30% or less, and Sb: 0.0100% or less.

[0019] 3. The hot forging steel according to 1 or 2 above, wherein the component composition further contains, in mass%, Sn: 0.030% or less.

[0020] <{ 4. In mass%, C: 0.28% or more and 0.40% or less, Si: 0.05% or more and 0.35% or less, Mn: 0.80% or more and 2.00% or less, P: 0.005% or more and 0.025% or less, S: {0.030% or more and 0.080% or less}, Al: 0.015% or more and 0.050% or less, Cr: 0.80% or more and 2.00% or less, Mo: 0.05% or more and 0.35% or less, V: 0.040% or less, B: 0.0005% or less, and N: 0.0030% or more and 0.020% or less, and having a component composition consisting of the balance Fe and inevitable impurities, in a range where Ceq shown in the following formula (1) is 0.80 or more and 0.97% or less, and C, Mn, Cr, and Mo satisfy the following formula (2), a steel material is heated to 1100 - 1300 °C and then hot forged, and then cooled to 350 °C or less with an average cooling rate of 0.10 - 3.00 °C / s from 950 °C to 350 °C, and then heated in a temperature range of 450 - 600 °C. 1100 - 1300 °C and then hot forged, and then cooled to 350 °C or less with an average cooling rate of 0.10 - 3.00 °C / s from 950 °C to 350 °C, and then heated in a temperature range of 450 - 600 °C. Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ··· (1) 0.08 ≤ C - Mn / 18 - Cr / 24 - Mo / 3 ≤ 0.28 ··· (2) The element symbols in the above formulas (1) and (2) mean the content of the corresponding element in the steel. For elements not contained in the steel, the content in the steel is taken as 0.

[0021] 5. The method for producing hot forging steel according to item 4, wherein the component composition further contains one or more selected from, by mass%, Cu: 0.30% or less, Ni: 0.30% or less, Nb: 0.050% or less, Ti: 0.050% or less, Pb: 0.30% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Bi: 0.30% or less, and Sb: 0.0100% or less.

[0022] 6. The method for producing hot forging steel according to 4 or 5, wherein the component composition further contains Sn: 0.030% or less by mass%. [Effects of the Invention]

[0023] According to the present invention, it is possible to provide a hot forging steel that has high strength and excellent toughness, and also excellent high-frequency hardenability. Furthermore, by performing a tempering treatment after hot forging according to the method of the present invention, a hot forging steel with high strength, high toughness, and excellent high-frequency hardenability can be obtained. [Modes for carrying out the invention]

[0024] The present invention will be described in detail below. [Steel for hot forging] First, the reason for limiting the component composition of the steel of the present invention to the range described above will be explained. In the following explanation, the content (%) of each element refers to mass % unless otherwise specified.

[0025] C: 0.28~0.40% Carbon (C) is an element necessary to ensure strength and fatigue strength, and also enhances surface hardness during high-frequency induction hardening. To achieve these effects, a content of 0.28% or more is required. Preferably, it is 0.29% or more, and more preferably 0.30% or more. On the other hand, if the C content exceeds 0.40%, the amount of island martensite increases too much, reducing toughness, so the upper limit for the C content is set at 0.40%. Preferably, it is 0.39% or less, and more preferably 0.38% or less.

[0026] Si: 0.05~0.35% Si is useful as a deoxidizing agent during the steelmaking process, i.e., the steelmaking process, and must be contained in an amount of 0.05% or more. Preferably, it is 0.06% or more, and more preferably 0.09% or more. On the other hand, if the Si content exceeds 0.35%, the toughness decreases and the hardening depth during high-frequency induction hardening also varies, so the upper limit for the Si content is set at 0.35%. Preferably, it is 0.30% or less, and more preferably 0.28% or less.

[0027] Mn: 0.80~2.00% Mn is a useful element for improving the hardenability of steel and for bainitizing the microstructure. Specifically, if the Mn content is less than 0.80%, the hardenability will be insufficient, the amount of bainite structure formed will be small, and sufficient strength will not be obtained. Therefore, the Mn content should be 0.80% or more. Preferably, it should be 0.82% or more, and more preferably 0.85% or more. On the other hand, if the Mn content exceeds 2.00%, the hardenability becomes too high and the formation of retained austenite is promoted, resulting in a decrease in toughness and a decrease in the fatigue limit. Furthermore, the hardening depth during high-frequency induction hardening will also vary, so the upper limit for the Mn content was set at 2.00%. Preferably, it should be 1.95% or less, and more preferably 1.90% or less.

[0028] P: 0.005~0.025% P is an element that segregates at prior austenite grain boundaries and reduces toughness. Since the adverse effects on toughness and fatigue strength become significant when the P content exceeds 0.025%, the upper limit was set at 0.025%. Preferably, it is 0.024% or less, and more preferably 0.023% or less. On the other hand, while toughness improves as the P content decreases, the refining cost increases, so the lower limit was set at 0.005%.

[0029] S: 0.030~0.080% S is a useful element for improving machinability in processes such as outer turning and drilling, and a content of 0.030% or more is necessary to obtain its effect. In addition, MnS particles are effective in refining and aligning crystal grains, and are also effective in improving toughness and reducing variations in hardening depth during high-frequency induction hardening. For the above reasons, 0.030% was set as the lower limit. Preferably, it is 0.035% or more. On the other hand, excessive addition exceeding 0.080% causes the generated MnS to act as a fracture initiation point, reducing toughness and fatigue strength. It also exacerbates variations in hardening depth during high-frequency induction hardening. Therefore, the upper limit for S is set at 0.080%. Preferably, it is 0.078% or less. More preferably, it is 0.075% or less.

[0030] Al: 0.015~0.050% Al is an element with a strong deoxidizing effect, but a sufficient deoxidizing effect cannot be obtained if the content is less than 0.015%, so the lower limit of the Al content was set to 0.015%. Preferably, it is 0.019% or more. On the other hand, if the Al content exceeds 0.050%, not only will the effect of its addition saturate, but the excessive inclusions will also reduce toughness and fatigue limit, so the upper limit of the Al content was set to 0.050%. Preferably, it is 0.045% or less, and more preferably 0.040% or less.

[0031] Cr: 0.80~2.00% Cr, like Mn, is an element necessary for bainitizing the microstructure. However, this effect is insufficient when the Cr content is less than 0.80%. On the other hand, when the Cr content exceeds 2.00%, it promotes the formation of retained austenite, which reduces toughness and fatigue limit, and also exacerbates variations in hardening depth during induction hardening. Therefore, the Cr content was set to a range of 0.80 to 2.00%. Preferably, it is 0.82% or more. Preferably, it is 1.98% or less.

[0032] Mo: 0.05~0.35% Mo is an essential element for suppressing ferrite and pearlite transformations, bainitizing the microstructure, refining the bainite lath to improve toughness, and suppressing the amount of retained austenite. However, the above effects are not sufficiently achieved with a Mo content of less than 0.05%, so the lower limit of the Mo content was set at 0.05%. Preferably, it is 0.07% or more, and more preferably 0.10% or more. On the other hand, if the Mo content exceeds 0.35%, the cost increases, the formation of retained austenite is suppressed, and the impact value and fatigue limit decrease, so the upper limit of the Mo content was set at 0.35%. Preferably, it is 0.33% or less, and more preferably 0.31% or less.

[0033] V:0.040% or less V has a strong affinity for C and N and precipitates as carbonitrides in steel. However, this effect is small in bainite structures, and conversely, it reduces toughness and exacerbates variations in hardening depth during high-frequency induction hardening. Therefore, the V content needs to be kept below 0.040%. From the viewpoint of improving low-temperature toughness, it is preferably below 0.038%, and more preferably below 0.035%. On the other hand, there is no particular lower limit to the V content, but it is preferably around 0.001%, which is the amount of impurities mixed into the raw material.

[0034] B: 0.0005% or less B is an element that enhances hardenability and increases strength, but its effect on hardenability is highly dependent on the cooling rate, leading to variations in strength within the material after hot forging and reducing toughness. Therefore, the upper limit is set at 0.0005%. Preferably, it is 0.0003% or less. On the other hand, there is no particular lower limit to the B content, but it is preferably around 0.00005%, which is the amount of impurities mixed into the raw material.

[0035] N: 0.0030% or more and 0.0200% or less N forms nitrides in steel, which has the effect of suppressing the coarsening of crystal grain size during heating. To produce this effect, at least 0.0030% of N is required. Preferably, it is 0.0050% or more. More preferably, it is 0.0060% or more. On the other hand, excessive addition can lead to increased cracking defects in the material, a decrease in toughness, and variations in hardening depth during high-frequency induction hardening, so the upper limit is set at 0.0200%. Preferably, it is 0.0190% or less. More preferably, it is 0.0180% or less.

[0036] Furthermore, in the present invention, the component composition must satisfy the following formulas (1) and (2). Note 0.80≦Ceq(=C+Si / 24+Mn / 6+Ni / 40+Cr / 5+Mo / 4+V / 14)≦0.97...(1) 0.08≦C-Mn / 18―Cr / 24―Mo / 3≦0.28 (2) In formulas (1) and (2) above, the element symbols represent the content of that element in the steel. Elements not present in the steel are represented as 0 (zero).

[0037] Here, both equations (1) and (2) above serve as indicators for controlling the bainite structure, and by keeping these elements within the range of the indicators according to the above equations, it is possible to obtain a predetermined strength and excellent toughness in the bainite structure.

[0038] In other words, if equation (1) falls below the lower limit of 0.80%, the strength and fatigue strength decrease, and the required hardening depth cannot be obtained. On the other hand, if equation (1) exceeds the upper limit of 0.97%, the island martensite increases, and the toughness decreases.

[0039] Furthermore, if equation (2) falls below the lower limit of 0.08%, island-like martensite increases, reducing toughness. On the other hand, if it exceeds the upper limit of 0.28%, strength and toughness decrease, and the variation in hardening depth during high-frequency induction hardening also increases.

[0040] The basic components of the present invention have been described above, but the remainder of the hot forging steel of the present invention, other than the above components, consists of Fe and unavoidable impurities. However, the present invention may optionally include the following additional components as needed.

[0041] In other words, it is one or more elements selected from Cu: 0.30% or less, Ni: 0.30% or less, Nb: 0.050% or less, Ti: 0.050% or less, Pb: 0.30% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Bi: 0.30% or less, Sb: 0.0100% or less, and Sn: 0.030% or less.

[0042] Cu, Ni, and Nb are effective elements for increasing strength. However, excessive addition of Cu, Ni, and Nb can lead to a decrease in surface properties, an increase in manufacturing costs, a decrease in toughness, or variations in hardening depth during high-frequency induction hardening. Therefore, it is preferable to set the upper limits for each element's addition to Cu: 0.30%, Ni: 0.30%, and Nb: 0.050%. However, to obtain a strength-enhancing effect, it is desirable to add Cu, Ni, and Nb at concentrations of 0.01% or more, 0.01% or more, and 0.005% or more, respectively.

[0043] Ti forms TiN and other materials, which suppresses grain coarsening during heating and improves toughness. However, excessive addition can lead to a decrease in toughness and fatigue strength due to the formation of coarse precipitates, as well as variations in hardening depth during high-frequency induction hardening. Therefore, it is preferable to limit the addition to 0.050%. However, to obtain the toughness improvement effect, an addition of 0.005% or more is desirable.

[0044] Pb, Ca, Mg, and Bi are all elements that are effective in improving machinability. However, adding them in large quantities will not only saturate their effect but may even decrease toughness and fatigue strength. Therefore, it is preferable to set the upper limits for each element's addition to Pb: 0.30%, Ca: 0.0050%, Mg: 0.0050%, and Bi: 0.30%, respectively. However, to obtain the desired machinability improvement effect, it is desirable to add P, Ca, Mg, and Bi at concentrations of 0.05%, 0.0005%, 0.0005%, and 0.05%, respectively.

[0045] Sb has the effect of suppressing decarburization of the surface layer during high-temperature heating and increasing fatigue strength. On the other hand, excessive addition reduces toughness and fatigue strength, so it is preferable to limit the addition to 0.0100%. However, to obtain the effect of improving fatigue strength, an addition of 0.0015% or more is desirable.

[0046] Sn has the effect of suppressing decarburization of the surface layer during high-temperature heating and increasing fatigue strength. On the other hand, excessive addition reduces toughness and fatigue strength, so it is preferable to limit the addition to 0.030%. However, to obtain the fatigue strength improvement effect, an addition of 0.0010% or more is desirable.

[0047] The remaining unavoidable impurities in the component composition are impurities that are inevitably introduced from raw materials, manufacturing processes, or manufacturing equipment, and are acceptable to be included to the extent that they do not hinder the objectives of the present invention. Examples of raw materials include iron ore, reduced iron, or scrap. Examples of impurities include O, H, Zn, and As.

[0048] In this invention, the steel structure is defined as follows: Area percentage of bainite tissue: 90% or more In order to obtain the required strength and high toughness, the hot forging steel of the present invention must have a bainite structure in its microstructure that accounts for 90% or more of the area, along with the aforementioned steel components. Preferably, it is 92% or more, and may be 100%. Other structures include ferrite, pearlite, and residual γ phase.

[0049] Furthermore, when surface hardening is performed by high-frequency heat treatment, a bainite structure with a uniform carbon distribution has the advantage of less hardness non-uniformity compared to a ferrite-pearlite structure with unevenly distributed carbon.

[0050] Area percentage of island-like martensites in bainite tissue: 3.0% or less While a bainite structure is necessary to obtain the required strength, the hard martensite phase that forms between the laths, so-called island martensite, adversely affects toughness. Therefore, it is necessary to limit the amount of island martensite. Preferably, it is 2.5% or less, more preferably 2.0% or less.

[0051] To achieve the above-described steel structure, it is necessary to adjust the heating temperature before hot forging and the heat treatment conditions after hot forging when forming the steel material into the part shape by hot forging. Specifically, the manufacturing conditions must all be satisfied, including a heating temperature of 1100-1300°C before hot forging, an average cooling rate of 0.10-3.00°C / s after hot forging, and a heating temperature of 450-600°C (after cooling).

[0052] Heating temperature before hot forging: 1100~1300℃ When performing hot forging, if the heating temperature is too low, the aforementioned steel structure cannot be obtained. In other words, if the heating temperature before hot forging is lower than 1100°C, ferrite is more likely to form, and it becomes impossible to achieve a bainite area ratio of 90% or more in the microstructure. On the other hand, while a bainite structure is more easily obtained at higher heating temperatures, it leads to problems such as reduced toughness due to microstructure coarsening, reduced yield due to scale loss, and increased energy costs. Therefore, the upper limit of the heating temperature is set at 1300°C. This heating temperature is specified as the core temperature of the steel material.

[0053] Average cooling rate after hot forging: 0.10~3.00℃ / s If the average cooling rate within the specified temperature range after hot forging falls below 0.10°C / s, the steel structure becomes a ferrite-pearlite structure, resulting in a decrease in strength. Furthermore, an average cooling rate below 0.10°C / s exacerbates variations in hardening depth during high-frequency induction hardening. On the other hand, if the average cooling rate exceeds 3.00°C / s, the resulting steel becomes too hard, significantly reducing its toughness and exacerbating variations in hardening depth during subsequent high-frequency induction hardening. Note that this average cooling rate is specified at the surface temperature of the hot forged material, and the specified temperature range refers to a surface temperature range of 950 to 350°C.

[0054] The reason for setting the average cooling rate within the temperature range of 950 to 350°C as specified above is as follows: forging is completed at approximately 1000°C, and the transformation completion temperature for the above-mentioned components to obtain the required strength and structure is approximately 400°C.

[0055] The cooling process after hot forging involves cooling to 350°C or below. There is no particular need to limit the cooling rate outside the 950-350°C temperature range. The reason for setting the cooling stop temperature below 350°C is that, for the aforementioned components required to obtain the desired strength and structure, the transformation is generally completed at approximately 400°C, ensuring that the transformation is reliably completed by cooling to 350°C or below.

[0056] Heating temperature after cooling: 450~600℃ To decompose the island-like martensite phase and reduce its fraction to 3.0% or less, the heating temperature after cooling must be 450°C or higher. On the other hand, if the heating temperature is 600°C or higher, the decrease in strength and fatigue strength becomes significant. Therefore, the heating temperature after cooling should be 450 to 600°C. Preferably, it is 470 to 580°C. Furthermore, the holding time in this temperature range is preferably about 15 to 200 minutes.

[0057] Furthermore, other manufacturing conditions for a preferred method of producing hot-forging steel according to the present invention will be described.

[0058] Molten steel having the above-mentioned component composition is melted using conventional melting methods such as converters and electric furnaces, and then produced as steel material by conventional continuous casting or the inflection method. Here, the content of each element in the steel can be determined by ICP emission spectroscopy or the like.

[0059] Next, the steel material is heated as needed and formed into steel bars by hot rolling, such as billet rolling or wire rolling. The above heating and rolling conditions are not particularly limited and can be appropriately determined according to the required material. For example, microstructure control such as MnS control may be performed to improve machinability, which is advantageous for subsequent forging or machining for part formation.

[0060] For manufacturing conditions not specified herein, general steel manufacturing methods should be followed. [Examples]

[0061] Next, embodiments of the present invention will be described. Note that the following embodiments are provided to illustrate the present invention more specifically, and the present invention is not limited to the scope of these embodiments.

[0062] [Example 1] Ingots with the component composition shown in Table 1 (the remainder being Fe and unavoidable impurities) were hot-rolled into 36mm diameter round bars. These were then heated to 1250°C and hot-forged into 25mm diameter round bars. Next, they were cooled to below 350°C with an average cooling rate of 0.8°C / s from 950°C to 350°C, followed by heat treatment at 550°C for 60 minutes, and then air-cooled.

[0063] Cooled round bars were used as test materials, and the microstructure, tensile strength, yield strength, fatigue limit, and impact value were measured for each test material (one per test condition) using the method described below. High-frequency induction hardenability was also evaluated using the method described below.

[0064] First, the methods for measuring microstructure, tensile strength, fatigue limit, and impact value are as follows: (1) The microstructure was determined by polishing the cross-section of the test material, then etching it with Nital, observing the exposed cross-section with an optical microscope, taking photographs, and performing image processing on the obtained images to determine the bainite fraction (bainite area ratio).

[0065] Specifically, three fields of view were captured at 400x magnification from the surface of the test specimen, approximately 1 / 4D of diameter D, with a total measurement area of ​​105,600 μm². 2 (35,200 μm per field of view) 2 ) Furthermore, the bainite phase was identified as a composite phase of ferrite lath and carbides other than ferrite, pearlite, and the residual γ phase, and the bainite fraction was derived by image analysis using the software ImageJ. The island-like martensite phase was quantified using the same method from the region excluding the ferrite phase, pearlite phase, and residual γ phase.

[0066] (2) Tensile strength and yield strength were measured by taking a JIS No. 4 tensile test specimen from the round bar of the test specimen and performing a tensile test at a tensile speed of 1 mm / s in accordance with JIS Z 2241.

[0067] (3) The fatigue limit was defined as the highest stress reached without fracture after 10⁷ rotations of an 8 mmφ smooth test specimen using the Ono-type rotary bending fatigue test in accordance with JIS Z 2274.

[0068] (4) The impact value was measured by taking three Charpy impact test specimens of a 10 mm square with a U-shaped notch that was 2 mm wide and 5 mm deep, cooling and holding them at -50°C, and then performing a Charpy impact test, and calculating the average value.

[0069] Furthermore, high-frequency induction hardening properties were evaluated as follows, using the standard deviation of surface hardness, average hardened layer depth, and effective hardened layer depth after high-frequency induction hardening.

[0070] Using high-frequency heating at a frequency of 200 Hz, quenching conditions were searched and set for each test specimen to obtain an effective hardened layer depth of more than 2.00 mm in a unidirectional measurement within the cross-section. After quenching under these conditions, tempering was performed at 160°C for 1 hour.

[0071] Surface hardness was evaluated as the lowest value obtained from three Rockwell hardness (HRC) measurements. Subsequently, a cross section was cut perpendicular to the height direction (axial direction) of the cylinder (round bar), and the hardened layer depth within this cross section circle was measured using a Vickers hardness tester with a load of 2.94 N (300 gf). Measurements were taken at 90° intervals, with each interval being 0.2 mm, from three directions toward the center of the cross section circle. The length from the steel surface on the cross section circle at the location where Hv400 was obtained was determined in each direction, and the average value from the three directions was taken as the average hardened layer depth.

[0072] Furthermore, the variation in the effective hardened layer depth in the three directions was determined by calculating the standard deviation σ of the average hardened layer depth mentioned above. The test results and evaluation results described above are shown in Table 2.

[0073] [Table 1] TIFF2026076954000002.tif255141

[0074] [Table 2] TIFF2026076954000004.tif255161

[0075] As is clear from Table 2, the inventive steels No. A-L, O, P, R, T, V, Y, AC, AE, AG, and AI all exhibit the following characteristics in their microstructure: bainite area ratio: 90% or more, island martensite area ratio: 3.0% or less, tensile strength: 900 MPa or more, yield strength: 720 MPa or more, and impact value at -50°C: 25 J / cm². 2 In summary, fatigue strength: 600 N / mm 2 In summary, the surface hardness after high-frequency treatment was HRC: 48 or higher, the average hardened layer depth after high-frequency treatment was 2.00 mm or higher, and the standard deviation σ of the variation in effective hardened layer depth was 0.12 mm or less.

[0076] Furthermore, since a tensile strength of 900 MPa or higher and a yield strength of 720 MPa or higher indicates superior strength, this invention uses "superior strength" as its evaluation criterion.

[0077] Impact value at -50℃: 25 J / cm 2 In cases meeting the above criteria, the toughness is excellent, and therefore, in this invention, the evaluation criterion is "excellent toughness."

[0078] Fatigue strength: 600 N / mm 2 In cases meeting the above criteria, the fatigue strength is excellent, and therefore, in this invention, "excellent fatigue strength" is used as the evaluation criterion.

[0079] Furthermore, since a surface hardness of HRC:48 or higher after high-frequency treatment exhibits excellent fatigue strength and wear resistance, the present invention uses the evaluation criteria of excellent fatigue strength and wear resistance, as well as excellent high-frequency hardenability. In this invention, the evaluation criteria for superior fatigue strength are defined as having excellent fatigue strength and excellent high-frequency hardenability, provided that the average hardened layer depth after high-frequency treatment is 2.00 mm or more and the standard deviation variation σ of the effective hardened layer depth is 0.12 mm or less.

[0080] In contrast, the comparative examples of steels No. M, N, Q, S, U, W, X, Z, AA, AB, AD, AF, AH, AJ~BL are inferior to the inventive example in any of the following respects: microstructure, tensile strength, yield strength, impact value at -50°C, fatigue strength, surface hardness after high-frequency treatment, average hardened layer depth, and standard deviation variation σ of the effective hardened layer depth.

[0081] [Example 2] Next, ingots of steel compositions shown in Table 1 (Nos. A, B, and J) were hot-rolled into 36mm diameter round bars, which were then hot-forged into 25mm diameter round bars and cooled to below 350°C. The heating temperature before hot forging, the average cooling rate after hot forging, and the heating temperature after cooling were performed under various conditions as shown in Table 3.

[0082] The round bars obtained in this manner were subjected to the same microstructure, tensile strength, yield strength, fatigue strength, and impact value at -50°C as described in Example 1 above, and their high-frequency hardenability was also evaluated. These results are also shown in Table 3.

[0083] [Table 3]

[0084] As is clear from the description in Table 3, when the heating temperature before hot forging, the average cooling rate after hot forging, and the tempering temperature are within the range of the present invention, the following are obtained: bainite area ratio in the microstructure: 90% or more, island martensite area ratio: 3.0% or less, tensile strength: 900 MPa or more, yield strength: 720 MPa or more, impact value at -50°C: 25 J / cm² 2 In summary, fatigue strength: 600 N / mm 2 In summary, the surface hardness after high-frequency treatment was HRC: 48 or higher, the average hardened layer depth after high-frequency treatment was 2.00 mm or higher, and the standard deviation variation σ of the effective hardened layer depth was 0.12 mm or less.

[0085] In contrast, if the heating temperature, average cooling rate, and tempering temperature after hot forging are outside the range of the present invention, any of the following will not meet the desired values ​​and will be inferior: microstructure, tensile strength, yield strength, impact value at -50°C, fatigue strength, surface hardness after high-frequency treatment, average hardened layer depth, and standard deviation variation σ of effective hardened layer depth.

Claims

1. In mass percent, C: 0.28% or more and 0.40% or less, Si: 0.05% or more and 0.35% or less, Mn: 0.80% or more and 2.00% or less, P: 0.005% or more and 0.025% or less, S: 0.030% or more and 0.080% or less, Al: 0.015% or more and 0.050% or less, Cr: 0.80% or more and 2.00% or less, Mo: 0.05% or more and 0.35% or less, V: 0.040% or less, B: 0.0005% or less and N: 0.0030% or more and 0.0200% or less The composition has the following characteristics: Ceq shown in formula (1) is 0.80% or more and 0.97% or less, and C, Mn, Cr, and Mo are contained within the range satisfying formula (2), with the remainder being Fe and unavoidable impurities. Hot forging steel having a bainite structure area ratio of 90% or more and an island martensite area ratio of 3.0% or less within the bainite structure. Ceq=C+Si / 24+Mn / 6+Ni / 40+Cr / 5+Mo / 4+V / 14...(1) 0.08≦C-Mn / 18-Cr / 24-Mo / 3≦0.28... (2) In formulas (1) and (2) above, the element symbols represent the content of that element in the steel. Elements not present in the steel are represented as having a content of 0.

2. The aforementioned component composition is further expressed in mass%, Cu: 0.30% or less, Ni: 0.30% or less, Nb: 0.050% or less, Ti: 0.050% or less, Pb: 0.30% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Bi: 0.30% or less and Sb: 0.0100% or less The hot forging steel according to claim 1, comprising one or more selected from among them.

3. The above component composition is further expressed in mass% Sn: 0.030% or less A hot forging steel according to claim 1 or 2, comprising the above.

4. In mass percent, C: 0.28% or more and 0.40% or less, Si: 0.05% or more and 0.35% or less, Mn: 0.80% or more and 2.00% or less, P: 0.005% or more and 0.025% or less, S: 0.030% or more and 0.080% or less, Al: 0.015% or more and 0.050% or less, Cr: 0.80% or more and 2.00% or less, Mo: 0.05% or more and 0.35% or less, V: 0.040% or less, B: 0.0005% or less and N: 0.0030% or more and 0.0200% or less A steel material having a component composition in which the Ceq shown in the following formula (1) is 0.80 to 0.97%, and C, Mn, Cr, and Mo are contained within the range satisfying the following formula (2), with the remainder being Fe and unavoidable impurities, A method for manufacturing steel for hot forging, comprising heating to 1100-1300°C, performing hot forging, then cooling to below 350°C with an average cooling rate of 0.10-3.00°C / s from 950°C to 350°C, and finally heating to a temperature range of 450-600°C. Ceq=C+Si / 24+Mn / 6+Ni / 40+Cr / 5+Mo / 4+V / 14...(1) 0.08≦C-Mn / 18-Cr / 24-Mo / 3≦0.28... (2) In formulas (1) and (2) above, the element symbols represent the content of that element in the steel. Elements not present in the steel are represented as having a content of 0.

5. The aforementioned component composition is further expressed in mass%, Cu: 0.30% or less, Ni: 0.30% or less, Nb: 0.050% or less, Ti: 0.050% or less, Pb: 0.30% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Bi: 0.30% or less and Sb: 0.0100% or less A method for producing hot forging steel according to claim 1, comprising one or more selected from among them.

6. The above component composition is further expressed in mass% Sn: 0.030% or less A method for producing hot forging steel according to claim 4 or 5, comprising the above.

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