Warm-forged part for carburizing and method for producing the same

Abstract

To provide a warm-forged part for carburizing and a method for producing the same that avoid positive addition of Nb and Ti which significantly deteriorates carburizability, machinability, and cost, suppress grain coarsening during carburization, and allow omission of normalizing after forging.SOLUTION: A warm-forged part for carburizing contains chemical components within specific ranges and satisfies formula 1, formula 1: Beq=3Mn+2Cr+10Mo≤6.8, wherein (a) a metal structure mainly includes ferrite and pearlite and has a bainite fraction of 5% or less, (b) a precipitation ratio of AlN is 0.020 mass% or more, (c) an average particle diameter of AlN observed by SEM is 0.15 μm or less, (d) an average particle diameter of pearlite is 8.0 μm or more, (e) formula 2: f*R / r≥2 is satisfied, where f represents a precipitation ratio of AlN (mass%), R represents an average particle diameter of pearlite (μm), and r represents an average particle diameter of AlN observed by SEM (μm), and (f) an area ratio of a flattened structure is 3.0% or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a warm forged part for carburizing and a method for manufacturing the same. [Background technology]

[0002] In recent years, for example, when manufacturing parts used in automobile transmissions, warm forging, which is performed at a heating temperature of around 900°C and can produce forged parts with high dimensional precision, has been adopted to reduce manufacturing costs. Compared to hot forging, which requires a heating temperature of around 1200°C, warm forging has a lower heating temperature, which is therefore significantly effective in reducing CO2 emissions and energy costs.

[0003] Because the forging heating temperature is low in warm forging, the structure after forging is finer than in hot forging. If the structure before carburizing is fine, the driving force for grain growth during carburizing increases, making coarsening (abnormal grain growth) more likely to occur. Therefore, when conventional materials are warm forged and then carburized, grain coarsening is likely to occur. To address this, heat treatments such as normalizing are performed after forging to make the structure somewhat coarse before carburizing, but this can result in scale formation and other issues that reduce the precision of the forged parts. Furthermore, because the presence or absence of this heat treatment has a significant impact on part cost, it is desirable to be able to omit the heat treatment after forging.

[0004] Also, methods have been proposed that utilize fine precipitates of AlN, Nb, Ti, etc. to prevent coarsening of crystal grains. In conventional manufacturing using hot forging, it has been proposed that AlN, Nb, and Ti are first dissolved during heating such as rolling or hot forging at high temperatures, and then finely precipitated during rolling or heat treatment at lower temperatures, thereby preventing coarsening of crystal grains during carburizing (see, for example, Patent Document 1).

[0005] However, when Nb and Ti are used in warm forging, which uses lower forging temperatures than hot forging, recrystallization after forging becomes a problem. In other words, when Nb or Ti is added to prevent the above-mentioned coarsening, the recrystallization temperature during warm forging increases, making it more likely that a flat structure, which is an unrecrystallized structure, will remain as it is after processing. This flat structure recrystallizes during the carburizing temperature increase, generating very fine grains, which significantly increases the driving force for grain coarsening. In such cases, in order to prevent grain coarsening during the carburizing process, it is necessary to add large amounts of Nb or Ti to obtain a pinning effect that can withstand the significantly increased driving force for grain coarsening.

[0006] On the other hand, adding large amounts of Nb can have a negative effect on the quality of carburizing after carburizing (variation in surface carbon concentration and effective hardening depth). Adding large amounts of Ti also reduces the amount of MnS due to the formation of Ti sulfides, which has a negative effect on machinability. Furthermore, adding large amounts of Nb or Ti increases material costs.

[0007] As mentioned above, in order to omit the heat treatment after warm forging and to suppress the coarsening of crystal grains during carburizing, it is necessary to add large amounts of Nb and Ti. However, there are conflicting demands for the amount of Nb and Ti added to be kept to a minimum from the standpoint of carburizing ability, machinability, and cost. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-321211 Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention has been made in view of the above background, and aims to provide a warm forged product for carburizing, which can avoid the active addition of Nb and Ti, which have a significant adverse effect on carburization ability, machinability, and cost, can suppress the coarsening of crystal grains during carburizing, and can omit normalizing after forging, and a method for manufacturing the same. [Means for solving the problem]

[0010] A first aspect of the present invention is a warm forged product for carburizing that has undergone final hot working and is scheduled for carburizing treatment, The alloy has a chemical composition that satisfies formula 1, wherein the composition contains, in mass%, C: 0.12 to 0.28%, Si: 0.90% or less, Mn: 0.30 to 1.00%, P: 0.035% or less, S: 0.010 to 0.040%, Cr: 0.40 to 2.00%, Al: 0.020 to 0.100%, N: 0.0100 to 0.0250%, and contains Mo: 0.15% or less as an optional element, with the balance being Fe and unavoidable impurities; Formula 1: Beq=3Mn+2Cr+10Mo≦6.8, (Note that the element symbols in Formula 1 represent the content (%) of each element.) In addition, it has the following properties: (a) A metal structure consisting mainly of ferrite and pearlite, with a bainite fraction of 5% or less; (b) The precipitation rate of AlN is 0.020 mass% or more, (c) The average grain size of AlN observed by SEM is 0.15 μm or less, (d) the average particle size of pearlite is 8.0 μm or more; (e) Equation 2: f·R / r≧2, (where f is the precipitation rate of AlN (mass%), R is the average grain size of pearlite (μm), and r is the average grain size of AlN observed by SEM (μm)) (f) Within a 0.6 mm × 0.8 mm field of view, the area ratio of flat tissue with an aspect ratio of 3 or more is 3.0% or less. Warm forged products for carburizing.

[0011] A second aspect of the present invention provides a method for producing a steel slab having a chemical composition, in mass%, containing C: 0.12 to 0.28%, Si: 0.90% or less, Mn: 0.30 to 1.00%, P: 0.035% or less, S: 0.010 to 0.040%, Cr: 0.40 to 2.00%, Al: 0.020 to 0.100%, N: 0.0100 to 0.0250%, Mo: 0.15% or less as an optional element, and the balance being Fe and unavoidable impurities, and satisfying formula 1; Formula 1: Beq=3Mn+2Cr+10Mo≦6.8, (Note that the element symbols in Formula 1 represent the content (%) of each element.) The steel slab is subjected to a product rolling process in which the slab is heated to a product rolling heating temperature of 1150°C to 1350°C and rolled to produce a rolled material; The rolled material is heated to a forging heating temperature of 900 ° C to 1060 ° C, and a warm forging process is carried out so that the temperature after warm forging is 800 ° C to 960 ° C to produce a warm forged product. The method for producing a warm forged product for carburizing comprises cooling the warm forged product to 700°C after the completion of the warm forging step under conditions where the cooling rate is 3.0°C / second or less.

[0012] A third aspect of the present invention provides a method for producing a slab having a chemical composition, in mass%, of C: 0.12 to 0.28%, Si: 0.90% or less, Mn: 0.30 to 1.00%, P: 0.035% or less, S: 0.010 to 0.040%, Cr: 0.40 to 2.00%, Al: 0.020 to 0.100%, N: 0.0100 to 0.0250%, Mo: 0.15% or less as an optional element, and the balance being Fe and unavoidable impurities, and satisfying formula 1; Formula 1: Beq=3Mn+2Cr+10Mo≦6.8, (Note that the element symbols in Formula 1 represent the content (%) of each element.) The slab is heated to a blooming heating temperature of 1150°C to 1350°C and subjected to blooming rolling to produce a steel slab; The steel slab is subjected to a product rolling process in which the slab is heated to a product rolling heating temperature of 800°C to 1000°C and rolled to produce a rolled material; The rolled material is heated to a forging heating temperature of 900 ° C to 1060 ° C, and a warm forging process is carried out so that the temperature after warm forging is 800 ° C to 960 ° C to produce a warm forged product. The method for producing a warm forged product for carburizing comprises cooling the warm forged product to 700°C after the completion of the warm forging step under conditions where the cooling rate is 3.0°C / second or less. [Effects of the Invention]

[0013] The warm-forged product for carburizing of the first embodiment has a structure suitable for suppressing grain coarsening (pearlite grain size within a certain range and a ferrite-pearlite structure), and also has AlN precipitated in a size and amount suitable for suppressing grain coarsening, so even if normalizing after forging is omitted, it is possible to easily suppress grain coarsening during the subsequent carburizing treatment. Furthermore, a warm-forged product for carburizing with such an appropriate structure and AlN precipitated can be easily manufactured by optimizing the temperature conditions of each process, without relying on the active addition of Nb and Ti, as in the manufacturing methods of warm-forged parts for carburizing of the second and third embodiments, for example. [Brief explanation of the drawings]

[0014] [Figure 1] A metallographic photograph showing an example of pearlite structure. [Figure 2] Metallographic photograph showing examples of AlN particles. [Figure 3] Metallographic photograph showing an example of a flat structure. [Figure 4] FIG. 2 is a perspective view of a forged product produced in Experimental Example 1. [Figure 5] 5 is an explanatory diagram showing evaluation positions and the like in a cross section including the axial center line of the forged product shown in FIG. 4. DETAILED DESCRIPTION OF THE INVENTION

[0015] In the present application, we have discovered a warm forged product for carburization (first embodiment) that avoids the active addition of Nb and Ti, which have a significant adverse effect on carburization ability, machinability, and cost, and that ensures an AlN precipitation state with an optimal size and precipitation rate that can suppress coarsening of crystal grains during carburization even if normalizing before carburizing is omitted, and we have also discovered optimal temperature conditions for the rolling and forging processes as a method for manufacturing this product (second and third embodiments).

[0016] In order to suppress grain coarsening during carburizing without actively adding Nb or Ti, it is important to reduce the driving force for grain coarsening and to optimize the precipitation state of pinning particles and pearlite grain size. As a result of numerous experiments conducted by the inventors, it was found that a specific measure to reduce the driving force for grain coarsening is to make the state of the post-forging structure a ferrite-pearlite structure that minimizes the formation of bainite, and then to further optimize it (suppression of flattened structures and control of pearlite grain size).

[0017] Previously, to prevent grain coarsening during carburizing using AlN, it was necessary to first dissolve AlN in the hot forging process and then finely precipitate it during subsequent heat treatment. However, because the heating temperature in warm forging is lower than that in hot forging, AlN cannot be fully dissolved. Furthermore, if AlN is coarsely precipitated before warm forging, it will aggregate and coarsen during warm forging, preventing it from being finely precipitated, reducing the effect of preventing grain coarsening.

[0018] Therefore, in both the second and third manufacturing methods, most of the AlN is dissolved once during heating in the rolling process (product rolling process (second embodiment) or blooming process (third embodiment)) which is a process before forging, and then finely precipitates during the subsequent warm forging (or product rolling process (third embodiment)), thereby obtaining the effect of preventing coarsening of crystal grains. The rolling heating temperature required to dissolve most of the AlN is 1150°C to 1350°C (preferably 1200 to 1290°C). The rolling heating temperature may be selected using the generally proposed solubility product of AlN to determine the rolling heating temperature at which the precipitates can be dissolved.

[0019] AlN solubility product formula: log(%Al)(%N)=-7400 / T+1.92, (The above (%Al) means the mass % of solute Al in the steel, (%N) means the mass % of solute N in the steel, and T means the heating temperature (°C) during rolling.)

[0020] Although AlN dissolves in the steel due to heating during rolling, if the subsequent heating temperature during rolling or forging is high, the AlN precipitates, grows, and becomes coarse. Coarse AlN has little effect in preventing grain coarsening, which leads to further grain coarsening during carburizing. Therefore, after dissolving, it is necessary to roll and forge at an appropriate temperature, and not only the amount of precipitation but also the morphology (particle size) is important.

[0021] Adding Al increases the recrystallization temperature of austenite. When the recrystallization temperature increases, the forging temperature is lower than in hot forging, so there is a possibility that the crystal grains from warm forging will remain without recrystallization. In this way, if the shape remains flattened and unrecrystallized austenite grains remain during warm forging, this flat structure will recrystallize during carburizing temperature increase, turning into fine crystal grains and significantly increasing the driving force for grain coarsening.

[0022] Even if flattened structures remain after warm forging, if heat treatment such as normalizing is performed after forging, the flattened structures will recrystallize during the heat treatment and will not exist during carburizing. However, if heat treatment after forging is omitted to reduce CO2 emissions and costs, and the forging conditions are not appropriate, flattened structures will remain and significantly increase the driving force for grain coarsening during carburizing.

[0023] The flat structure that did not recrystallize during warm forging and remained unrecrystallized recrystallizes during cooling after forging, and the remaining amount decreases. Therefore, it was found that controlling the cooling rate after forging is effective in promoting recrystallization.

[0024] Furthermore, even when the crystal grains after forging are fine, the driving force for grain coarsening increases because finer crystal grains are generated during the carburizing temperature rise.It was also found that the fineness of the crystal grains after forging (pearlite grains) changes depending on the Al content and the forging heating temperature.

[0025] From the above, in order to suppress the coarsening of crystal grains during carburizing, it is very effective to precipitate AlN at a precipitation rate above a certain level with an appropriate grain size, suppress the remaining flat structure, control the crystal grain (pearlite grain) size after forging to a certain size or more, and reduce the driving force for the coarsening of crystal grains during carburizing, and for this purpose, it is effective to optimize at least the rolling heating temperature, forging heating temperature, and cooling rate after forging. The first to third aspects will be further explained below.

[0026] <Warm forged product for carburizing according to the first embodiment>

[0027] First, the reasons for limiting the chemical composition of the warm forged part for carburizing will be explained.

[0028] C: 0.12~0.28%; Carbon (C) is an element necessary for improving hardness after quenching and obtaining the internal hardness necessary to ensure strength. To achieve this effect, the C content must be 0.12% or more. However, excessive addition of C increases hardness after forging, which can lead to reduced machinability. To prevent this, the upper limit of the C content is set at 0.28%.

[0029] Si: 0.90% or less; Silicon (Si) is added because it contributes to improving strength through solid solution strengthening. However, excessive addition of Si leads to a decrease in machinability, so to prevent this, the upper limit of the Si content is set at 0.90%.

[0030] Mn: 0.30~1.00%; Mn (manganese) is added at a content of 0.30% or more to improve hardenability. However, excessive addition of Mn leads to a decrease in machinability due to an increase in hardness after forging and a decrease in hardness of the carburized layer due to an increase in retained austenite, so to prevent this, the upper limit of Mn content is set at 1.00%.

[0031] P: 0.035% or less; P (phosphorus) is an element contained as an impurity. It is an element that is prone to segregation at austenite grain boundaries, and segregation can cause a decrease in fatigue strength, so the upper limit of the allowable P content is set at 0.035%.

[0032] S: 0.010~0.040%; S (sulfur) is contained in an amount of 0.010% or more, as it improves machinability by forming MnS together with Mn. However, if too much S is contained, the amount of sulfide-based non-metallic inclusions increases, which causes a decrease in strength, so the upper limit of the allowable S content is set at 0.040%.

[0033] Cr: 0.40~2.00%; Cr (chromium) is contained in an amount of 0.40% or more to obtain the effect of improving hardenability. On the other hand, excessive addition of Cr increases hardness after forging, which leads to a decrease in machinability, so to prevent this, the upper limit of the Cr content is set to 2.00%.

[0034] Al: 0.020~0.100%; Al (aluminum) is an element used as a deoxidizer during steelmaking, and when it combines with N to form fine AlN, it has the effect of suppressing abnormal grain growth during carburizing. To obtain these effects, the Al content must be 0.020% or more. However, excessive addition of Al not only reduces workability but also causes the AlN, which acts as pinning particles, to coarsen, reducing the effect of suppressing abnormal grain growth, so the upper limit of the Al content is set at 0.100%.

[0035] N: 0.0100~0.0250%; N (nitrogen) becomes AlN and has the effect of suppressing grain coarsening through its pinning effect, so it is contained at 0.0100% or more. On the other hand, if the N content is too high, it will cause coarsening of AlN, which becomes pinning particles, and the effect of suppressing abnormal grain growth will decrease, so the upper limit of the N content is set to 0.0250%.

[0036] Mo as an optional element: not more than 0.15%; Mo (molybdenum) is an optional element and does not necessarily need to be included; a content of 0% is acceptable. However, when manufacturing by melting scrap in an electric furnace, it may be included as an unavoidable impurity. Furthermore, Mo is an element that is effective in improving hardenability when contained in an amount of 0.01% or more, so it can be added as needed. On the other hand, if the Mo content is too high, it not only leads to increased costs and reduced machinability, but also makes it easier for bainite to form after warm forging, which makes it easier for grains to coarsen during carburizing, so it is limited to 0.15% or less.

[0037] Formula 1: Beq = 3Mn + 2Cr + 10Mo ≤ 6.8; The above chemical composition must satisfy formula 1. By satisfying formula 1, the bainite structure can be suppressed to a predetermined ratio or less, whereas if formula 1 is not satisfied, the bainite structure will be generated in excess of the predetermined ratio, resulting in a decrease in workability and a tendency for crystal grains to become coarse.

[0038] In the above chemical composition, Ti and Nb are not actively added (although the inclusion of 0.01% or less of each as unavoidable impurities is permitted).

[0039] Next, a warm forged product for carburizing must satisfy at least the following requirements (a) to (f).

[0040] (a) The metal structure is composed mainly of ferrite and pearlite, with a bainite fraction of 5% or less; This metallographic structure ensures sufficient workability (machinability). This metallographic structure contains a higher proportion of ferrite and pearlite than other structures, and although a small amount of bainite is permitted, its fraction is limited to 5% or less. Specifically, the bainite fraction is measured using the procedure described in the Examples section below. Having a metallographic structure with the above configuration prevents a significant increase in hardness even when post-forging heat treatment is omitted, does not reduce workability, and can suppress grain coarsening after carburization. On the other hand, if the above metallographic structure conditions are not met, i.e., if the bainite fraction exceeds 5%, workability after warm forging may deteriorate and grain coarsening may occur.

[0041] (b) The precipitation rate of AlN is 0.020 mass% or more; By ensuring that the precipitation rate of AlN is within the above range, it is possible to obtain the effect of suppressing grain coarsening during carburizing. On the other hand, if the precipitation rate of AlN is outside the above range, there is a risk that the effect of suppressing grain coarsening will not be sufficiently obtained.

[0042] (c) The average grain size of AlN observed by SEM is 0.15 μm or less; Precipitated AlN is effective in suppressing crystal grain growth, but this effect is reduced if the grain size is not fine. Consequently, after conducting numerous tests, it was found that the expected effect can be achieved when the average grain size observed by SEM is within the above range. On the other hand, if the average grain size observed by SEM of AlN is outside the above range, the opposite may occur, and the effect of suppressing crystal grain coarsening may not be effectively achieved.

[0043] (d) the average particle size of pearlite is 8.0 μm or more; By having the average particle size of pearlite within the above range, it is possible to effectively suppress the coarsening of crystal grains. On the other hand, if the average particle size of pearlite is outside the above range, the expected effect of coarsening crystal grains may not be obtained. Therefore, it is more preferable that the average particle size of pearlite is 10.0 μm or more.

[0044] (e) Equation 2: f·R / r≧2; f is the precipitation rate (mass%) of AlN described above, R is the average particle size (μm) of pearlite described above, and r is the average particle size (μm) of AlN observed by SEM described above.

[0045] Compared to the use of Nb and Ti, which are effective in preventing grain coarsening, as in the prior application, the pinning effect of AlN is not as great. Therefore, it is not easy to reliably prevent grain coarsening without relying on Nb and Ti, and the effects of each of the above requirements (a) to (d) alone are insufficient. However, even without relying on Nb and Ti, the expected effect can be achieved by combining requirement (e) with requirements (a) to (d). The value of Equation 2 shown in (e) is a numerical expression of the overall effect of combining requirements (b) to (d). Therefore, by satisfying Equation 2, the effect of effectively suppressing grain coarsening during carburizing can be achieved without relying on the addition of Nb and Ti. On the other hand, if Equation 2 is not satisfied, the above effect may not be fully achieved.

[0046] (f) The area ratio of squamous tissue is 3.0% or less; A flat structure is defined as a pearlite grain structure with an aspect ratio of 3 or greater. Since the magnitude of strain during forging varies depending on the part, the strain (= aspect ratio) of the unrecrystallized grains also varies. However, the presence of unrecrystallized grains with larger strain has a greater adverse effect on grain coarsening during carburizing. Therefore, whether a forged part has a flat structure is determined by the aspect ratio, regardless of the magnitude of strain. By ensuring that the area ratio of the flat structure is within the above range, the adverse effect on grain coarsening due to the presence of unrecrystallized grains can be prevented. On the other hand, if the area ratio of the flat structure is outside the above range, the adverse effect on grain coarsening due to the presence of unrecrystallized grains may not be prevented. Therefore, it is more preferable that the area ratio of the flat structure be 1.5% or less.

[0047] Here, the above requirements (a) to (d) are obtained by observing a cross section or by taking a sample from the cross section. The cross section from which the observation or sample is taken may be any representative portion that can be called a steady or general portion of the forged product. For example, in the case of a forged product having a roughly rod-shaped form, the cross section may be anywhere on the cross section between a position L / 4 from the front end and a position L / 4 from the rear end, relative to the total length L. Furthermore, the direction of the cross section may be either along the axial direction or perpendicular to the axial direction.

[0048] In addition, for forged products with unique shapes where the axial direction cannot be easily identified, imagine an imaginary axis passing through the position where the outer diameter dimension is largest, and with respect to the total length L on that axis, the area on a cross section along the axial direction between a position L / 4 from the front end and a position L / 4 from the rear end can be considered a representative portion that can be called a steady portion or general portion.

[0049] The identification of the metal structure and the bainite fraction (area fraction) in (a) can be determined from the entire observation cross-section because the observed cross-section of the representative portion is generally uniform at any position. However, for example, a representative position that can be considered steady or general, such as an axial position, can be selected from between L / 4 from the front end and L / 4 from the rear end, relative to the total length L, as described above. For a radial position, if the cross-section perpendicular to the axial direction (transverse cross-section) is substantially circular, a position between D / 4 and 3D / 4 from the center, relative to its diameter D, can be selected. Then, a cross-section along the axial direction at that position can be observed, and the observation results can be used as a representative value indicating the overall condition. Furthermore, if the transverse cross-section is not substantially circular, a position considered to be steady or general, between the outer surface and the center, can be used as the representative value.

[0050] In addition, in the present application, the bainite fraction (area fraction) of (a) and the average grain size of pearlite of (d) remain almost unchanged even when the observation magnification or field of view is changed. Therefore, for example, the bainite fraction and the average grain size of pearlite can be measured by image analysis using images taken at 100x magnification (field of view of 0.6 mm × 0.8 mm) with an optical microscope, and these values ​​can be used as representative values ​​indicating the overall state.

[0051] The AlN precipitation rate in (b) is analyzed using a sample taken from the cross section. As in (a), this sample can be taken from the representative position described above, and the state at this point can be used as a representative value. That is, as described above, the axial position can be selected from a position between L / 4 from the front end and L / 4 from the rear end, with respect to the total length L. The radial position can be selected from a position between D / 4 and 3D / 4 from the center, with respect to the diameter D, if the cross section (transverse cross section) perpendicular to the axial direction is approximately circular. The AlN precipitation rate can be measured by analyzing the residue extracted from the sample by the bromine-methanol method using optical emission spectroscopy with an ICP (inductively coupled plasma) as a light source.

[0052] (c) SEM observation of AlNThe average grain size can also be determined by observing a representative position in the cross section of the representative portion described above using a scanning electron microscope (SEM), as in the case of (a), and the state obtained here can be used as a representative value. Specifically, as shown in the experimental example described later, for example, 10 fields of view are observed, and the average can be determined.

[0053] Regarding requirement (f), the area ratio of flattened structures (unrecrystallized grains) must be 3% or less in any cross section, as viewed in a 0.6 mm × 0.8 mm field of view (100x magnification) of an optical microscope. Considering that regions with aspect ratios of 3 or greater tend to occur more frequently in regions with high strain, it is preferable to identify regions with relatively high strain and measure them preferentially over other regions to facilitate identification. Specifically, the location of the greatest strain (maximum strain location) in a cross section along the axial direction of the forged product can be determined in advance using FEM analysis, and the maximum strain location can be observed and used to make a judgment. In other words, if the unrecrystallized grains in a 0.6 mm × 0.8 mm field of view in a region with high strain are 3% or less, it can be determined that the unrecrystallized grains in the same field of view in other regions with lower strain are 3% or less. Therefore, it is not necessary to measure the entire region. When determining the area ratio of unrecrystallized grains, it is necessary to measure the area ratio of unrecrystallized grains while including other structures that exist around the unrecrystallized grains, and the range must be selected appropriately. However, because a field of view of 0.6 mm × 0.8 mm corresponds to an area containing several thousand crystal grains, there is little chance that the field of view will be too narrow and the unrecrystallized grains will be preferentially included in the measured field of view by chance, or conversely, even though unrecrystallized grain structures are present, only the surrounding structures will be selected, resulting in an inappropriate area ratio (0% in this case). Therefore, this field of view range was set as being appropriate for calculating the area ratio of unrecrystallized grains.

[0054] Next, it is preferable that the surface hardness of the warm forged product for carburizing, measured at the surface layer position of the maximum diameter portion (large diameter portion 52), is 220 HV or less in Vickers hardness. This ensures sufficient workability (machinability) of the warm forged product for carburizing even if heat treatment after forging is omitted. On the other hand, even if the surface hardness exceeds 220 HV, this may not be a major obstacle as long as the processing equipment can tolerate it. Therefore, this hardness requirement is not essential.

[0055] The warm forged product for carburizing has the above-mentioned structure and AlN precipitation state to be suitable for suppressing grain coarsening, and is planned to be carburized in its original state without subsequent hot working or heat treatment. The carburizing conditions include, for example, holding at 900 to 960°C for 4 hours.

[0056] Next, the method for manufacturing the warm forged product for carburizing includes, for example, the second and third modes.

[0057] <Manufacturing method of the second embodiment>

[0058] In the second mode, a steel billet having the above chemical composition is prepared, and a product rolling step is carried out in which the steel billet is heated to a product rolling heating temperature of 1150°C to 1350°C and rolled to produce a rolled material.

[0059] The steel slab used as the raw material in this product rolling process can be a continuous cast slab of product rolling size such as billet CC, a slab that has already been bloomed, etc., regardless of the manufacturing history up to that point. The heating temperature in the product rolling process (product rolling heating temperature) means the heating temperature in the heating device before rolling, and as mentioned above, the temperature range is 1150°C to 1350°C.

[0060] If the product rolling heating temperature is lower than the above-mentioned lower limit temperature, it becomes difficult to sufficiently dissolve AlN before warm forging, while if it is higher than the upper limit temperature, there is a problem that the cost of the energy required for heating increases. Therefore, the product rolling heating temperature is preferably in the range of 1200°C to 1290°C.

[0061] It is preferable that the temperature (finishing temperature) of the rolled material after the product rolling process is 900°C or less at the end of final rolling. Although this requirement is not essential, if the finishing temperature of the rolled material after the product rolling process is high, bainite is more likely to form and the hardness after rolling tends to increase. Therefore, by setting the temperature to 900°C or less, the formation of bainite structure is further suppressed, and it becomes possible to further suppress the deterioration of workability. As a result, it becomes easier to cut the rolled material that will become the base material during forging.

[0062] Furthermore, the rolled material that has completed the product rolling process is preferably cooled under conditions where the cooling rate after the end of final rolling is 3.0°C / sec or less to 700°C. Although this requirement is not essential, by satisfying the above cooling rate conditions, it is possible to further suppress the formation of bainite structures and further suppress the deterioration of workability due to an increase in rolling hardness.

[0063] Next, the rolled material obtained by the product rolling process is heated to a forging temperature of 900°C to 1060°C, and a warm forging process is carried out in which one or more warm forgings are performed to produce a warm forged product. This warm forging process is carried out so that the temperature after the final warm forging is 800°C to 960°C. By satisfying both the forging temperature range and the temperature after the final warm forging range, it becomes possible to finely precipitate AlN in the warm forging process and maintain the precipitation state.

[0064] If the forging temperature is higher than the upper limit, the energy-saving effect is reduced, and there are problems such as the generation of scale, which increases the manufacturing cost for removing it, and further, there is a risk of bainite being generated, which deteriorates machinability and causes grain coarsening during carburizing. On the other hand, if the forging temperature is lower than the lower limit, it becomes difficult to satisfy the temperature range after the final warm forging, and there is a concern that the unrecrystallized flat structure will increase. Therefore, the forging temperature is more preferably 960°C to 1060°C.

[0065] By setting the temperature after the final warm forging in the range of 800°C to 960°C, it becomes easier to produce a steel with a pearlite grain size within the target range after forging, and the temperature can be set to be suitable for suppressing the remaining of unrecrystallized grains. If the temperature after the final warm forging is higher than the above upper limit, there are problems such as a decrease in the energy-saving effect, the generation of scale, and an increase in the production cost for removing it. Furthermore, there is a risk of bainite being generated, which may lead to a deterioration in machinability and coarsening of grains during carburizing. On the other hand, if the temperature after the final warm forging is lower than the above lower limit, there is a problem that a flat structure is more likely to remain, which increases the risk of coarsening of grains during carburizing.

[0066] Next, the warm-forged product is cooled to 700°C after the completion of the warm forging process at a cooling rate of 3.0°C / sec or less. This ensures sufficient time for the forged product to recrystallize during cooling, suppresses the formation of bainite, and reduces adverse effects on workability. If the cooling rate is too fast, there is insufficient time for the unrecrystallized grains to recrystallize, leaving a flattened structure. This structure recrystallizes during carburizing, increasing the risk of grain coarsening during carburizing.

[0067] <Production method of the third embodiment>

[0068] In the third mode, a slab having the above chemical composition is prepared, and the slab is heated to a blooming heating temperature of 1150°C to 1350°C and rolled to produce a steel slab.

[0069] The heating temperature in this blooming process (blooming heating temperature) refers to the heating temperature in the heating device before rolling, and as mentioned above, its temperature range is 1150°C to 1350°C. If the blooming heating temperature is lower than the above-mentioned lower limit, it becomes difficult to sufficiently dissolve AlN, while if it is higher than the upper limit, there is a problem that the cost of the energy required for heating increases. Therefore, the blooming heating temperature is preferably in the range of 1200°C to 1290°C.

[0070] Next, the slab obtained in the blooming process is heated to a product rolling heating temperature of 800°C to 1000°C and rolled to produce a rolled material. By setting the product rolling heating temperature, which is the heating temperature in the heating device before rolling, within this temperature range, it becomes possible to finely precipitate the AlN solid-solved in the blooming process. In other words, the key to rolling is to perform the rolling at a relatively low temperature so that the precipitated AlN remains finely precipitated without growing coarsely.

[0071] If the product rolling heating temperature is lower than the lower limit temperature, the deformation resistance of the material increases, which causes a problem of an increased rolling load and a higher load on the rolling equipment, while if the temperature is higher than the upper limit temperature, the precipitated AlN becomes coarse, which may reduce the effect of suppressing coarsening during subsequent carburization. Therefore, the product rolling heating temperature is more preferably 800°C to 950°C.

[0072] It is preferable that the temperature (finishing temperature) of the rolled material after the product rolling process is 900° C. or less at the end of final rolling. Although this requirement is not essential, if the finishing temperature of the rolled material after the product rolling process is 900° C. or less, it becomes possible to further suppress the formation of bainite structure in the rolled material after rolling, thereby making it possible to keep the rolling hardness low and further suppress the deterioration of workability.

[0073] Furthermore, the rolled material that has completed the product rolling process is preferably cooled at a cooling rate of 0.1°C / sec to 3.0°C / sec after the end of final rolling to 700°C. Although this requirement is not essential, by satisfying the above cooling rate conditions, it becomes possible to further suppress the formation of bainite structure, to keep the rolling hardness low, and to further suppress the deterioration of workability.

[0074] That is, if the cooling rate exceeds the upper limit, the effect of suppressing bainite formation decreases, and rolling hardness tends to increase. Meanwhile, unlike the second embodiment, AlN is in a state of fine precipitation in the product rolling process, so if the cooling rate is slowed below the lower limit, the finely precipitated AlN may grow and coarsen, potentially reducing the effect of suppressing grain coarsening during carburizing.

[0075] Next, the rolled material obtained by the product rolling process is heated to a forging temperature of 900°C to 1060°C, and a warm forging process is carried out in which one or more warm forgings are performed to produce a warm forged product. This warm forging process is carried out so that the temperature immediately after the final warm forging is 800°C to 960°C. By providing both the forging temperature range and the temperature immediately after the final warm forging range, it is possible to maintain the fine precipitation state of AlN precipitated in the previous product rolling process.

[0076] If the forging temperature is higher than the upper limit, the energy-saving effect is reduced, and there are problems such as the generation of scale, which increases the manufacturing cost for removing it, and further, there is a risk of bainite being generated, which deteriorates machinability and causes grain coarsening during carburizing. On the other hand, if the forging temperature is lower than the lower limit, it becomes difficult to satisfy the temperature range after the final warm forging. Therefore, the forging temperature is more preferably 960°C to 1060°C.

[0077] By setting the temperature after the final warm forging in the range of 800°C to 960°C, it becomes easier to adjust the pearlite grain size after forging to a predetermined range, and the temperature can be set appropriately to prevent the remaining flat structure (= unrecrystallized structure). If the temperature after the final warm forging is higher than the above upper limit, there are problems such as a decrease in the energy-saving effect, the generation of scale, and an increase in manufacturing costs for its removal. Furthermore, there is a risk of bainite being generated, which may lead to a deterioration in machinability and coarsening of grains during carburizing. On the other hand, if the temperature after the final warm forging is lower than the above lower limit, there is a risk of the remaining flat structure leading to coarsening of grains during carburizing.

[0078] Next, the warm-forged product is cooled to 700°C after the warm forging process is completed at a cooling rate of 3.0°C / sec or less. This ensures sufficient time for the unrecrystallized grains to recrystallize during cooling, and also suppresses the formation of bainite. If the cooling rate is too fast, grains affected by processing will remain unrecrystallized, leaving a flat structure that will recrystallize during carburizing, potentially resulting in coarsening of the grains during carburizing. [Example]

[0079] (Experimental Example 1) The present warm forged product for carburizing and its manufacturing method (second embodiment) will be described using examples. In these examples, an experiment was conducted to manufacture a warm forged product for carburizing using the manufacturing method of the second embodiment described above. In these examples, 13 types of steel (Examples 1 to 10, Comparative Examples 21 to 23) were prepared as shown in Table 1, and test pieces corresponding to the warm forged products were made and evaluated for various properties. Here, Mo, an optional element, was not actively added because it was mixed in from scrap as an impurity, and the analytical values ​​shown were those contained as an impurity, except for Example 4.

[0080] [Table 1]

[0081] <Production of forged products> Each steel material was melted and cast in an electric furnace to obtain a slab, which was then subjected to blooming, product rolling, and warm forging to produce the forged product 5 shown in Figures 4 and 5. The conditions for each process are shown in Table 2.

[0082] [Table 2]

[0083] In the second embodiment, the precipitation state of AlN is controlled by the conditions in product rolling, so the heating temperature for the dividing rolling in the blooming process is not specified as a particular condition. In this example, however, it was set to 1120°C to 1300°C, and blooming was performed to obtain a steel slab. The product rolling process was then carried out on this steel slab. The heating temperature for the product rolling in the product rolling process was set to a heating temperature of 1150°C to 1350°C. After blooming, the rolled material obtained by completing the product rolling process was measured for its temperature at the end of final rolling, and the results are shown in Table 2. Specifically, the temperature at the end of final rolling was measured using a radiation thermometer at the center of the rolled material in the longitudinal direction.

[0084] Furthermore, the cooling rate of this rolled material after the end of final rolling was measured down to 700° C. Specifically, the steel material temperature was measured with a radiation thermometer at regular time intervals from the end of rolling and calculated.

[0085] Next, the rolled material was subjected to a continuous warm forging process consisting of three steps to obtain a forged product 5. Specifically, after heating, a φ50 mm × 120 mm round bar-shaped rolled material was subjected to a continuous forging process consisting of a crushing process (first step) in which it was compressed to shorten its overall length by 5 mm, a second step in which it was formed into a rough shape close to the final shape using a die that reduced the outer diameter of the front and rear ends, and a finishing process (third step) in which it was formed into the final forged product shape, ultimately obtaining a forged product 5 with a total length of approximately 150 mm, having a large diameter portion 52 of φ50 mm between a front axle portion 51 and a rear axle portion 53 of φ25 mm, as shown in Figure 4.

[0086] The position in forged product 5 where the highest strain occurs during the warm forging process can be identified in advance by performing FEM analysis. In forged product 5, the front and rear axle sections 51 and 53 are subjected to diameter reduction processing by extrusion, so the part with the highest strain from the three processes combined is position P3, which is slightly inward from the outer periphery of rear axle section 53, as shown in Figure 5.

[0087] The forging heating temperature in the warm forging step was set to 900°C to 1060°C in Examples 1 to 10 and Comparative Examples 21 and 22, with the exception of Comparative Example 23, which was set to 885°C. After forging, the forged products obtained by completing the warm forging step were measured for their temperatures after final warm forging, and the results are shown in Table 2. Specifically, the temperature after final warm forging (final temperature) was measured using a radiation thermometer at position P1 on the outer periphery of large diameter portion 52 of forged product 5 after forming was completed.

[0088] Furthermore, the cooling rate of this forged product up to 700°C after the warm forging process was measured. Specifically, the temperature of the maximum diameter part (large diameter part 52) ​​of the forged product flowing on the conveyor was measured with a radiation thermometer at regular intervals and calculated.

[0089] <Evaluation of rolled materials> Hardness: For rolled materials, the material was cut perpendicular to the axial direction at a position L / 4 from the tip of the material in the longitudinal direction of the rolled material with respect to the total length L, and after polishing the cut surface, Vickers hardness was measured at a position D / 4 from the outer surface with respect to the diameter D.

[0090] Presence or absence of bainite structure: The cross section of the rolled material was polished at the same position as the Vickers hardness measurement position described above, and then the presence or absence of bainite structure was determined by image analysis using images taken at the D / 4 position with an optical microscope at 100x magnification (0.6mm x 0.8mm field of view). Three fields of view were observed, and if bainite was observed in even one location, it was determined that bainite was present.

[0091] <Evaluation of forged products> Microstructure: At the position P2 shown in Figure 5, i.e., at a position 3 mm from the surface in the longitudinal cross section (cross section including the axial center line) of the large diameter portion 52 of the forged product 5, an image (microstructure image) was taken at 100x magnification (field of view of 0.6 mm x 0.8 mm) with an optical microscope.

[0092] Bainite fraction: Ten lines were drawn evenly vertically and ten lines horizontally on the microstructure image, and the structure at the 100 intersections was determined to be bainite, pearlite, or ferrite. The bainite fraction was then calculated from the bainite ratio. Whether the structure was a ferrite-pearlite structure (α+P) or a ferrite-pearlite-bainite structure (α+P+B) was determined by whether each structure accounted for 5% or more. If the bainite fraction was 5% or less, it was determined to be essentially a ferrite-pearlite structure (α+P).

[0093] · R: Average grain size of pearlite (μm): The above microstructure image was binarized using image processing software, and only pearlite was extracted. The grain size of each pearlite was calculated using the equivalent circle diameter, and the average value of all the extracted pearlite grain sizes was taken as the average to calculate the average grain size of pearlite. An average grain size of pearlite of 8 μm or more is considered acceptable.

[0094] Hardness: The hardness of the forged product 5 was measured by Vickers hardness at the position P2 (large diameter portion 52), which was the same as the observation position of the microstructure described above.

[0095] An example of a pearlite structure is shown in Figure 1. This figure is a photograph of the position P2 in Figure 5 in forged product 5 of Example No. 1, which will be described later, where the white parts are ferrite and the black parts are pearlite.

[0096] f: AlN precipitation rate (mass%): Drilling chips (turnings) were collected from position P2 in Figure 5 (3 mm from the surface of the large diameter section 52), dissolved in bromine methanol, and the extracted residue was analyzed by optical emission spectroscopy using ICP (inductively coupled plasma) as a light source to determine the AlN precipitation rate. An AlN precipitation rate of 0.020 mass% or more was considered acceptable.

[0097] r: SEM observation of AlN average particle size (μm): After electrolytic polishing with phosphoric chromic acid, the particle size was observed with a scanning electron microscope (SEM), and the particle size was approximately 450 μm per field of view.2 Ten fields of view were observed, and precipitates thought to be AlN were analyzed for composition using an EDX (energy dispersive X-ray fluorescence analyzer) to confirm that they were AlN. After that, 10 AlN particles were observed and the average was calculated based on the definition of the average of the 10 particles to measure the average particle size of the AlN. An average particle size of AlN of 0.15 μm or less was deemed acceptable.

[0098] An example of AlN particles is shown in Figure 2. This figure is an SEM photograph of Example 1, which will be described later, and the white parts visible at the tips of the white arrows are AlN particles. The particle size of these AlN particles is approximately 0.10 µm.

[0099] · Flat structure area ratio: The microstructure image is processed using image processing software, and the pearlite grain structure with a measured aspect ratio of 3 or more is identified as flat structure. The flat structure area ratio was calculated by binarizing the microstructure image using image processing software, calculating the circle equivalent diameter of the pearlite structure, and extracting the area with an aspect ratio of 3 or more. A flat structure area ratio of 3.0% or less is considered acceptable.

[0100] In this application, as shown in Figure 5, the position P3 where the distortion was highest on the cross section along the axial direction of the forged part was identified in advance by FEM analysis, and one field of view was observed at that position at a magnification of 100 times (field of view size 0.6 mm × 0.8 mm), and the image was processed to determine the area ratio of the flat structure.

[0101] Measurement is performed at the site with the highest strain because sites with high strain are more likely to produce unrecrystallized grains with high aspect ratios. If the site with the highest area ratio of unrecrystallized grains is 3% or less, it can be predicted that the other sites will almost certainly be 3% or less.

[0102] An example of a flat structure is shown in Figure 3. This figure is a photograph of the microstructure of Comparative Example No. 23, which will be described later, and the flat structure is visible at the end of the white arrow.

[0103] Prior austenite (γ) grains: The forged product was observed in the longitudinal direction after carburizing at 100x magnification, and the area of ​​grains that were three or more numbers away from the most frequent grain size was measured. If the area of ​​grains that were three or more numbers away was 25% or more, it was judged as failing (×), and if it was less than 25%, it was judged as passing (○).

[0104] The evaluation results of the above rolled material and forged product are shown in Table 2.

[0105] Examples 1 to 10 showed satisfactory results in all evaluation items, and fine AlN particles were observed dispersed in the forged products, which resulted in the suppression of grain coarsening during carburizing.

[0106] In Comparative Example 21, the heating temperature in the product rolling process was too low, and AlN could not be sufficiently dissolved, which resulted in an insufficient pinning effect of AlN, and the effect of suppressing grain coarsening during the subsequent carburization was not sufficiently obtained, and grain coarsening was confirmed.

[0107] In Comparative Example 22, the heating temperature in the warm forging process was too high, resulting in the generation of bainite in the microstructure at a fraction of 11%, which resulted in the insufficient effect of suppressing grain coarsening during the subsequent carburizing, and coarsening was confirmed.

[0108] In Comparative Example 23, the heating temperature in the warm forging process was too low, resulting in a small average grain size of pearlite in the forged product, insufficient fine precipitation of AlN during warm forging resulting in a large average grain size, and the area ratio of unrecrystallized grains exceeded 3%, which resulted in insufficient suppression of grain coarsening during subsequent carburizing, and coarsening was confirmed.

[0109] (Experimental Example 2) In this example, an experiment was conducted to manufacture a warm forged product for carburizing using the manufacturing method of the third embodiment described above. In this example, 17 types of steel (Examples 11 to 20, Comparative Examples 24 to 30) were prepared as shown in Table 3, and test pieces corresponding to the warm forged products were made and evaluated for various properties. Here, Mo, an optional element, is mixed in as an impurity from scrap, so except for Comparative Example 24, it was not actively added, and the analytical values ​​shown are those contained as an impurity.

[0110] [Table 3]

[0111] <Production of forged products> Each steel material was melted and cast in an electric furnace to obtain a slab, which was then subjected to blooming, product rolling, and warm forging to produce a forged product having the shape shown in Fig. 4, as in the previous example. The conditions for each process are shown in Table 4.

[0112] [Table 4]

[0113] The dividing rolling heating temperature in the blooming process was intentionally set to a range of 1150°C to 1350°C in Examples 11 to 20, and set to a range of 1050°C to 1250°C in Comparative Examples 24 to 30, and rolling was performed to obtain steel slabs. These steel slabs were then subjected to a product rolling process. The product rolling heating temperature in the product rolling process was intentionally set to a low heating temperature of 800°C to 1000°C in Examples 11 to 20, and appropriately set to a range of 900°C to 1050°C in Comparative Examples 24 to 30. After rolling, the rolled material obtained by completing the product rolling process was measured for its temperature at the end of final rolling, and the results are shown in Table 4. Furthermore, the cooling rate of this rolled material from the end of final rolling to 700°C was measured.

[0114] Next, the rolled material was subjected to a warm forging process to obtain forged products. The forging heating temperature in the warm forging process was set to 900°C to 1060°C for Examples 11 to 20 and Comparative Examples 24 to 28 and 30, with Comparative Example 29 being set to 865°C. Then, after three forgings (manufactured in three steps from a base material cut from the rolled material) as in Experimental Example 1, the forged products obtained by completing the warm forging process were measured for their temperatures after the final warm forging (final temperatures), and the results are shown in Table 4.

[0115] The rolled material and forged product were evaluated in the same manner as in Experimental Example 1. The measurement procedures were the same as in Experimental Example 1. Table 4 shows the evaluation results of the rolled material and forged product of this example.

[0116] Examples 11 to 20 showed satisfactory results in all evaluation items, and fine AlN particles were observed dispersed in the forged products, which resulted in the suppression of grain coarsening during carburizing.

[0117] Comparative Example 24 is an example in which conventional steel SCM420 was used as the steel material. However, due to the Mo content being higher than the appropriate range specified in the present invention, Beq was not satisfied, and bainite was generated in the microstructure at a fraction of 38%, which resulted in coarsening of the crystal grains. During the subsequent carburization, the effect of suppressing crystal grain coarsening was not sufficiently obtained, and coarsening was confirmed.

[0118] In Comparative Example 25, the heating temperature in the blooming process was too low, so that sufficient solid solution of AlN was not obtained, and fine AlN could not be precipitated in either the subsequent product rolling process or the warm forging process.As a result, the effect of suppressing grain coarsening during the subsequent carburizing was not sufficiently obtained, and coarsening was confirmed.

[0119] In Comparative Examples 26 and 27, the heating temperature in the product rolling process was too high, and the AlN dissolved in the blooming process precipitated and grew, causing coarsening of the AlN. As a result, the effect of suppressing grain coarsening during the subsequent carburizing was not sufficient, and coarsening was confirmed.

[0120] In Comparative Example 28, the heating temperature in the warm forging process was too high, resulting in the generation of bainite in the microstructure at a fraction of 8%. As a result, the effect of suppressing grain coarsening during the subsequent carburization was not sufficient, and coarsening was confirmed.

[0121] In Comparative Example 29, the heating temperature in the warm forging process was too low, and the temperature after the final warm forging was also too low, resulting in a small average grain size of pearlite in the forged product and a high area ratio of flattened structure. As a result, the effect of suppressing grain coarsening during the subsequent carburizing was not sufficient, and coarsening was confirmed.

[0122] In Comparative Example 30, the cooling rate of the forged product to 700°C after the completion of the warm forging process exceeded 3.0°C / s, and bainite was generated in the microstructure to a fraction of 10%. As a result, the effect of suppressing grain coarsening during the subsequent carburizing was not sufficiently achieved, and coarsening was confirmed.

Claims

1. A warm forged product for carburizing that has undergone final hot working and is scheduled for carburizing treatment, The alloy has a chemical composition that satisfies formula 1, wherein the composition is: C: 0.12 to 0.28%, Si: 0.90% or less, Mn: 0.30 to 1.00%, P: 0.035% or less, S: 0.010 to 0.040%, Cr: 0.40 to 2.00%, Al: 0.020 to 0.100%, N: 0.0100 to 0.0250%, and contains Mo: 0.15% or less as an optional element, with the balance being Fe and unavoidable impurities; Formula 1: Beq=3Mn+2Cr+10Mo≦6.8, (Note that the element symbols in Formula 1 represent the content (%) of each element.) In addition, it has the following properties: (a) A metal structure mainly composed of ferrite and pearlite, with a bainite fraction of 5% or less, (b) the precipitation rate of AlN is 0.020 mass% or more; (c) the average grain size of AlN observed by SEM is 0.15 μm or less; (d) the average particle size of pearlite is 8.0 μm or more; (e) Formula 2: f・R / r≧2, (wherein f is the precipitation rate of AlN (mass%), R is the average particle size of pearlite (μm), and r is the average particle size of AlN observed by SEM (μm)). (f) the area ratio of flat tissues with an aspect ratio of 3 or more within a 0.6 mm x 0.8 mm observation field is 3.0% or less; Warm forged products for carburizing.

2. A steel slab is prepared, which has a chemical composition that satisfies Formula 1, containing, in mass%, C: 0.12 to 0.28%, Si: 0.90% or less, Mn: 0.30 to 1.00%, P: 0.035% or less, S: 0.010 to 0.040%, Cr: 0.40 to 2.00%, Al: 0.020 to 0.100%, N: 0.0100 to 0.0250%, and optionally containing Mo: 0.15% or less, with the balance being Fe and unavoidable impurities; Formula 1: Beq=3Mn+2Cr+10Mo≦6.8, (Note that the element symbols in Formula 1 represent the content (%) of each element.) The steel slab is subjected to a product rolling process in which the slab is heated to a product rolling heating temperature of 1150°C to 1350°C and rolled to produce a rolled material; The rolled material is heated to a forging heating temperature of 900 ° C to 1060 ° C, and a warm forging process is carried out so that the temperature after warm forging is 800 ° C to 960 ° C to produce a warm forged product. After the completion of the warm forging process, the warm forged product is cooled to 700 ° C. at a cooling rate of 3.0 ° C. / second or less. Manufacturing method for warm forged products for carburizing.

3. 3. The method for manufacturing a warm forged product for carburizing according to claim 2, wherein the product rolling heating temperature is 1200°C to 1290°C.

4. 3. The method for manufacturing a warm forged product to be carburized according to claim 2, wherein the temperature of the rolled material after the product rolling step is 900°C or less at the end of rolling.

5. 3. The method for manufacturing a warm forged product for carburizing according to claim 2, wherein the rolled material that has completed the product rolling step is cooled to 700°C at a cooling rate of 3.0°C / second or less after the end of rolling.

6. 3. The method for manufacturing a warm forged product for carburizing according to claim 2, wherein the forging heating temperature is 960°C to 1060°C.

7. A cast slab is prepared, which contains, in mass%, C: 0.12 to 0.28%, Si: 0.90% or less, Mn: 0.30 to 1.00%, P: 0.035% or less, S: 0.010 to 0.040%, Cr: 0.40 to 2.00%, Al: 0.020 to 0.100%, N: 0.0100 to 0.0250%, and contains Mo: 0.15% or less as an optional element, with the balance being Fe and unavoidable impurities, and which has a chemical composition that satisfies Formula 1; Formula 1: Beq=3Mn+2Cr+10Mo≦6.8, (Note that the element symbols in Formula 1 represent the content (%) of each element.) The slab is heated to a blooming heating temperature of 1150°C to 1350°C and subjected to blooming to produce a slab; The steel slab is subjected to a product rolling process in which the slab is heated to a product rolling heating temperature of 800°C to 1000°C and rolled to produce a rolled material; The rolled material is heated to a forging heating temperature of 900 ° C to 1060 ° C, and a warm forging process is carried out so that the temperature after warm forging is 800 ° C to 960 ° C to produce a warm forged product. After the completion of the warm forging process, the warm forged product is cooled to 700 ° C. at a cooling rate of 3.0 ° C. / second or less. Manufacturing method for warm forged products for carburizing.

8. 8. The method for manufacturing a warm forged product for carburizing according to claim 7, wherein the blooming heating temperature is 1200°C to 1290°C.

9. The method for manufacturing a warm forged product for carburizing according to claim 7, wherein the product rolling heating temperature is 800°C to 950°C.

10. 8. The method for manufacturing a warm forged product to be carburized according to claim 7, wherein the rolled material that has completed the product rolling step has a temperature of 900°C or less at the end of rolling.

11. The method for manufacturing a warm forged product for carburizing according to claim 7, wherein the rolled material that has completed the product rolling process is cooled under conditions where the cooling rate to 700 ° C after the end of rolling is 0.1 ° C / sec to 3.0 ° C / sec.

12. 8. The method for manufacturing a warm forged product for carburizing according to claim 7, wherein the forging heating temperature is 960°C to 1060°C.

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