Method for producing carburized member

Abstract

To provide a technology capable of suppressing the occurrence of coarsening of grains in carburization.SOLUTION: A method for producing a carburized member includes the steps of performing a softening heat treatment for heating and softening a case hardening steel which is a base material of the carburized member, performing cold forging on the case hardening steel after the softening heat treatment, and performing carburizing on the case hardening steel after the cold forging to obtain the carburized member. The carburized member has a predetermined chemical composition, and satisfies a relation of Tc-RT≤2000×[Nb]+133×[Mo]+862, where Tc [°C] is a carburizing temperature in the carburizing, RT [°C / min] is a rate of temperature increase until the carburizing temperature is reached in the carburizing, [Nb] is a mass content of Nb, and [Mo] is a mass content of Mo.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a carburized member.

Background Art

[0002] A carburized member is a steel material that has been carburized by infiltrating carbon (C) into the surface layer of low-carbon steel and then hardened by quenching. Carburized members are often used in mechanical parts of drive systems that require high strength, such as automotive parts like gears and pulleys. Various techniques for enhancing the performance of carburized members have been proposed so far.

[0003] For example, Patent Document 1 below discloses a technique for suppressing the coarsening of crystal grains and enhancing the impact resistance of a carburized member by preventing denitrification from the surface of a part during vacuum carburizing. Also, Patent Document 2 below discloses a technique for enhancing the effect of refining crystal grains by titanium (Ti)-based precipitates and enhancing the impact resistance of a carburized member by suppressing the formation of aluminum nitride (AlN) during gas carburizing. Patent Document 3 below discloses a technique for defining the surface hardness and the amount of retained austenite of a carburized member and adjusting the contents of chromium (Cr), silicon (Si), and molybdenum (Mo) to enhance the tempering softening resistance.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the manufacturing process of the carburized member, the skin-burned steel, which is the base material of the carburized member, may be formed into the shape of parts such as gears and pulleys by forging before carburization. As forging methods, for example, hot forging and cold forging are known. Hot forging has the advantage of reducing the deformation resistance of the skin-burned steel, but has the disadvantage of a large amount of carbon dioxide (CO2) emissions. On the other hand, according to cold forging, there is an advantage that the amount of CO2 emitted in the heating process can be reduced compared to hot forging.

[0006] However, in the skin-burned steel subjected to cold forging, since strain tends to remain inside, abnormal grain growth and coarsening of austenite grains may occur during carburization. The occurrence of such coarsening of crystal grains causes deterioration of the fatigue strength of the carburized member and an increase in heat treatment strain, etc., which leads to a decrease in the quality of the carburized member.

[0007] An object of the present invention is to provide a technique capable of suppressing the occurrence of coarsening of austenite grains in carburization of skin-burned steel that has undergone cold forging.

Means for Solving the Problems

[0008] The inventor of the present invention, through repeated research on carburized members, found that by adjusting the heat treatment conditions in carburization according to the content of elements that act on suppressing coarsening of crystal grains in carburization, even when carburization is performed after cold forging, coarsening of crystal grains can be suppressed. The present invention can be realized, for example, in the following forms.

[0009] [First Embodiment] The first embodiment is provided as a method for manufacturing a carburized member. The manufacturing method of the first embodiment includes a step of performing a softening heat treatment for heating and softening a skin-burned steel that is a base material of the carburized member, a step of performing cold forging on the skin-burned steel after the softening heat treatment, and a step of performing carburization on the skin-burned steel after the cold forging to obtain the carburized member. The carburized member consists of, by mass%, C: 0.10% or more and 0.30% or less, Si: 0.03% or more and 1.10% or less, Mn: 0.30% or more and 1.00% or less, Cr: 0.50% or more and 1.80% or less, Al: 0.01% or more and 0.08% or less, B: 0.0003% or more and 0.0300% or less, Ti: 0.010% or more and 0.150% or less, Nb: 0.01% or more and 0.10% or less, Mo: 0.03% or more and 1.00% or less, Cu: 0.50% or less, Ni: 0.50% or less, P: 0.03% or less, and S: 0.03% or less, with the balance being Fe and unavoidable impurities. When the carburizing temperature in the carburization is Tc [°C], the heating rate until reaching the carburizing temperature in the carburization is RT [°C / min], the mass content of Nb is [Nb], and the mass content of Mo is [Mo], the relationship of Tc - RT ≤ 2000×[Nb] + 133×[Mo] + 862 is satisfied.

[0010] [Second Embodiment] The softening heat treatment is spheroidizing annealing or low-temperature annealing in the manufacturing method according to the first embodiment above.

[0011] [Third Embodiment] The grain size number of austenite grains in the microscopic test method of JIS G 0551:2020 Steel - Determination of Grain Size of the carburized member is larger than 3 in the manufacturing method according to the first embodiment or the second embodiment above.

[0012] [Fourth Embodiment] The carburizing temperature is 950°C or higher in the manufacturing method according to any one of the first embodiment, the second embodiment, and the third embodiment above. [Advantages of the Invention]

[0013] According to the present invention, in the case of carburizing cold-forged skin-burned steel, the occurrence of coarsening of austenite grains can be suppressed, and a decrease in the fatigue strength of the carburized member and an increase in heat treatment strain can be suppressed.

[0014] The present invention can be realized in various forms other than the method for manufacturing a carburized member. For example, it can be realized in the form of a carburized member manufactured by the manufacturing method, mechanical parts such as gears and pulleys constituted by the carburized member, machines, devices, and instruments using the mechanical parts, etc. Further, it can be realized in the form of a carburizing method for steel materials, a temperature control method in carburizing, etc.

Brief Description of Drawings

[0015]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0016] Hereinafter, embodiments of the carburized member and its manufacturing method according to the present invention will be described.

[0017] 1. Chemical Composition of Carburized Member: The carburized member of the present embodiment comprises at least carbon (C), silicon (Si), manganese (Mn), chromium (Cr), aluminum (Al), boron (B), titanium (Ti), niobium (Nb), molybdenum (Mo), copper (Cu), nickel (Ni), phosphorus (P), and sulfur (S), with the balance being iron (Fe) and inevitable impurities.

[0018] The range of the content rate of each element in the carburized member of the present embodiment and the reasons for preferring the range of the content rate will be detailed below. In the following description, unless otherwise specified, the content rate is the mass content rate, and “%” means “mass %”.

[0019] · C: 0.10% or more and 0.30% or less C is added to ensure the strength (hardness) of the core part of the carburized member. The carburized member of the present embodiment contains C at 0.10% or more. However, if the content rate of C becomes excessive, it will cause a decrease in the machinability and a decrease in the internal toughness of the carburized member. Therefore, in the carburized member of the present embodiment, the content rate of C is 0.30% or less.

[0020] · Si: 0.03% or more and 1.10% or less Si is added to enhance the softening resistance. The carburized member of the present embodiment contains Si at 0.03% or more. However, in the carburized member of the present embodiment, in order to suppress the decrease in workability, the upper limit of the content rate of Si is 1.10% or less.

[0021] · Mn: 0.30% or more and 1.00% or less Mn is added to improve the hardenability. In the carburized member of the present embodiment, Mn is contained at 0.30% or more. However, in the carburized member of the present embodiment, in order to suppress the decrease in workability due to the excessive addition of Mn, the upper limit of the content rate of Mn is 1.00% or less.

[0022] · Cr: 0.50% or more and 1.80% or less Cr is an element effective for improving the hardenability and the surface fatigue strength. The carburized member of the present embodiment contains Cr at 0.05% or more. However, if Cr is added excessively, coarse carbides are likely to precipitate. Therefore, in the carburized member of the present embodiment, the upper limit of the content rate of Cr is 1.80% or less.

[0023] · Al: 0.01% or more and 0.08% or less Al is added for deoxidation. In the carburized member of the present embodiment, Al is added so that the content rate is 0.01% or more. However, if Al is added excessively, it will cause a decrease in the manufacturability of the carburized member. Therefore, in the carburized member of the present embodiment, the upper limit of the content rate of Al is 0.08% or less.

[0024] · B: 0.0003% or more and 0.0300% or less B is added to improve hardenability. In the carburized member of the present embodiment, B is added such that the content is 0.0003% or more. However, if B is added in excess, boron nitride (BN) is generated during carburization, which causes a decrease in the hot workability and castability of the carburized member. Therefore, in the carburized member of the present embodiment, the upper limit of the content of B is set to 0.0300% or less.

[0025] ·Ti: 0.01% or more and 0.15% or less Ti is added to suppress the reaction of B with nitrogen (N). In the carburized member of the present embodiment, Ti is added such that the content is 0.01% or more. If Ti is added in excess, the hardness becomes too high, which causes a decrease in workability. Therefore, in the carburized member of the present embodiment, the upper limit of the content of Ti is set to 0.15% or less.

[0026] ·Nb: 0.01% or more and 0.10% or less Nb is added to generate niobium carbide (NbC) that exhibits a pinning effect and suppress the coarsening of crystal grains. In the carburized member of the present embodiment, Nb is added such that the content is 0.01% or more. However, excessive addition of Nb causes a decrease in the workability of the carburized member. Therefore, in the carburized member of the present embodiment, the upper limit of the content of Nb is set to 0.10% or less.

[0027] ·Mo: 0.03% or more and 1.00% or less Mo is added to suppress the coarsening of crystal grains. Also, Mo is added to improve the hardenability and surface fatigue strength of the carburized member. In the carburized member of the present embodiment, the content of Mo is 0.03% or more. However, excessive addition of Mo causes a decrease in the workability of the carburized member. Therefore, in the carburized member of the present embodiment, the upper limit of the content of Mo is set to 1.00% or less.

[0028] ·Cu: 0.50% or less Cu is an element mixed from the raw material of skin-burned steel. In the carburized member of the present embodiment, in order to suppress a decrease in hot workability, the upper limit of the content of Cu is set to 0.50% or less.

[0029] ·Ni: Below 0.50% Ni is an element mixed in from the raw materials of skin-burning steel. If Ni is present in excess, it will increase the amount of retained austenite and cause a decrease in surface hardness. Therefore, in the carburized member of this embodiment, the upper limit of the Ni content is set to 0.50% or less.

[0030] ·P: Below 0.03% P corresponds to impurities mixed in during the manufacturing process. If the carburized member contains an excessive amount of P, it will cause a decrease in toughness and a decrease in fatigue strength. Therefore, in the carburized member of this embodiment, the upper limit of the P content is set to 0.03% or less.

[0031] ·S: Below 0.03% S corresponds to impurities mixed in during the manufacturing process. If the carburized member contains an excessive amount of S, it will cause a decrease in hot workability. Therefore, in the carburized member of this embodiment, the upper limit of the S content is set to 0.03% or less.

[0032] In addition to the above elements, other elements and inevitable impurities may exist in the carburized member of this embodiment. In this specification, "inevitable impurities" means elements mixed in from ores, scraps used as raw materials for steel, or the manufacturing process environment, etc. The carburized member of this embodiment may contain, for example, nitrogen (N) at a content rate in the range of 0.001% or more and 0.050% or less.

[0033] 2. Manufacturing method of carburized member: FIG. 1 shows a flowchart showing the manufacturing process of the carburized member of this embodiment. Inside the blocks of each process P2 to P4 in FIG. 1, a graph showing an example of the time change of the temperature of the steel material to be processed in that process is illustrated.

[0034] In step P1, the skin-burned steel, which is the base material of the carburized member, is prepared. In step P1, the chemical composition of the skin-burned steel is adjusted so that the carburized member obtained in this manufacturing method has the above-described chemical composition. The Nb content and the Mo content of the skin-burned steel are used to determine the heat treatment conditions for carburization described later.

[0035] In step P2, a softening heat treatment is performed on the skin-burned steel prepared in step P1 to soften it by heating. Due to the softening heat treatment in step P2, the skin-burned steel is softened and its workability is improved, so the formability in the cold forging in the subsequent step P3 is enhanced.

[0036] In this embodiment, as the softening heat treatment, spheroidizing annealing or low-temperature annealing is performed. In spheroidizing annealing, the skin-burned steel is heated to the two-phase region between the A1 transformation point and the A3 transformation point in a heating furnace and then slowly cooled in the heating furnace. According to spheroidizing annealing, the cold formability of the skin-burned steel can be enhanced. In low-temperature annealing, the skin-burned steel is heated to a temperature of 600 °C or higher and lower than the A1 transformation point and then slowly cooled. According to low-temperature annealing, the skin-burned steel can be easily softened by a low-temperature heat treatment.

[0037] Note that the softening heat treatment in step P2 is not limited to spheroidizing annealing or low-temperature annealing. The type of softening heat treatment in step P2 may be appropriately selected according to the performance required for the skin-burned steel in step P3. In other embodiments, for example, full annealing or normalizing may be performed as the softening heat treatment.

[0038] In step P3, cold forging is performed on the skin-burned steel that has been subjected to the softening heat treatment. In cold forging, the skin-burned steel is formed into the shape of a desired part by repeatedly applying pressure at room temperature. If the skin-burned steel is formed by cold forging, it is possible to reduce the CO2 emission amount compared to the case of forming by hot forging. In addition, since the occurrence of dimensional errors due to the heat influence is suppressed, it is possible to obtain higher dimensional accuracy than in the case of hot forging.

[0039] In Process P4, carburization is performed on the skin-burned steel after cold forging to obtain a carburized member. In Process P4 of the present embodiment, for example, vacuum carburization is carried out as the carburization. However, the carburization in Process P4 is not limited to vacuum carburization. In other embodiments, for example, gas carburizing treatment may be carried out instead of vacuum carburization. As the carburization in Process P4, gas carburizing nitriding treatment, vacuum carburizing nitriding treatment, etc. may also be carried out.

[0040] In carburization, the skin-burned steel is heated to a predetermined carburization temperature Tc, held for a predetermined carburization time tc, and then slowly cooled. In the present embodiment, the carburization temperature Tc may be, for example, about 900 to 1050°C. Also, the carburization time tc may be about 50 to 180 minutes. The carburization time tc can be set shorter as the carburization temperature Tc is higher. From the viewpoint of shortening the carburization time tc, it is preferable that the carburization temperature Tc be 950°C or higher.

[0041] The inventor of the present invention, while conducting repeated research on the carburized member, found that by adjusting the heat treatment conditions in carburization according to the content ratio of elements that act to suppress the coarsening of crystal grains in carburization, even when carburization is carried out after cold forging, the coarsening of crystal grains can be suppressed. In the present embodiment, the heat treatment conditions of the carburization in Process P4, namely, the carburization temperature Tc [°C] and the heating rate RT [°C / min] until reaching the carburization temperature Tc, are determined based on the content ratios of Nb and Mo so that the relationship of the following inequality (1) is satisfied.

[0042] Tc - RT ≦ 2000×[Nb] + 133×[Mo] + 862 ····(1)

[0043] In the above inequality (1), [Nb] represents the content ratio of Nb, and [Mo] represents the content ratio of Mo. The content ratios of Nb and Mo here are the content ratios in the skin-burned steel before carburization.

[0044] With reference to FIGS. 2 and 3, the reason for deriving the above inequality (1) will be explained.

[0045] Figure 2 is a schematic diagram showing the change in the state of crystal grains when carburizing is applied to skin-burned steel after cold forging. At the left end of Figure 2, the state S1 of the austenite crystal grains PT of the skin-burned steel after cold forging is shown. In the upper part on the right side of state S1, the growth states S2 and S3 of the crystal grains PT when the heating rate during carburizing is low are shown. In the lower part on the right side of state S1, the growth states S4 and S5 of the crystal grains PT when the heating rate during carburizing is high are shown.

[0046] As shown in state S1, in the metal structure of the skin-burned steel after cold forging, the crystal grains PT are flattened under the pressure during cold forging. In state S1 after cold forging, when carburizing is started at a low heating rate, recovery is likely to occur during heating and recrystallization is less likely to occur. Therefore, as shown in state S2, fine crystal grains PT are likely to occur. On the other hand, in state S1 after cold forging, when carburizing is started at a high heating rate, recrystallization is likely to occur before recovery during heating. Therefore, as shown in state S4, fine crystal grains PT are less likely to occur.

[0047] Here, the following Gladman's formula (2) shows that when the initial austenite grain size R0 is small, the critical precipitate particle radius r crit becomes small and grain coarsening is likely to occur. Conversely, Gladman's formula (2) shows that when the initial austenite grain size R0 is large, the critical precipitate particle radius r crit becomes large and the occurrence of grain coarsening is suppressed.

[0048]

Equation

[0049] That is, according to the above-mentioned Gladman's formula (2), in the case of state S2 in FIG. 2 where the heating rate during carburizing is low, the austenite grains are refined, and the initial austenite grain size R0 is small, as shown in state S3, grain coarsening is likely to occur. Conversely, in the case of state S4 in FIG. 2 where the heating rate during carburizing is high, the austenite grains are not refined, and the initial austenite grain size R0 is large, as shown in state S5, the occurrence of grain coarsening is suppressed.

[0050] Therefore, the inventor of the present invention focused on the heating rate and carburizing temperature, which are heat treatment conditions in carburizing, and the contents of Nb and Mo, which are elements that act to suppress grain coarsening in carburizing. The inventor of the present invention variously changed the heat treatment conditions in carburizing and the chemical composition of the skin-burned steel including the contents of Nb and Mo, and verified the presence or absence of the occurrence of grain coarsening in carburizing. Then, by performing regression analysis using the results, the distribution diagram shown in FIG. 3 was obtained, and the relationship between the heat treatment conditions of carburizing that can suppress grain coarsening and the contents of the elements that act to suppress grain coarsening was successfully quantified.

[0051] <Method for manufacturing experimental carburized member> (i) 150 kg ingots of skin-burned steel having various compositions with different contents of elements including Nb and Mo were prepared. These ingots were melted and forged in a vacuum melting furnace, solution-treated at 1300°C for 1 hour, and then annealed at 1000°C to simulate the thermal history when the steel material was rolled, and spheroidizing annealing was performed as a thermal softening treatment to produce a skin-burned steel material. (ii) The skin-burned steel material was machined into test pieces with a diameter of 15 mm and a length of 22.5 mm, the end faces were restrained cold, and a compression test equivalent to cold forging was performed. (iii) Subsequently, a pseudo-carburizing treatment in which quenching was performed in an air atmosphere simulating carburizing was carried out on the test pieces using any of the combinations of carburizing temperature Tc and carburizing time tc shown in Table 1 below to obtain test pieces of carburized members. According to the pseudo-carburizing treatment, it is possible to confirm the common action effects of various types of carburizing such as gas carburizing and vacuum carburizing by observing the inside of the test pieces.

[0052]

Table 1

[0053] <Method for Judging the Occurrence of Coarsening of Crystal Grains> (i) A test piece of the carburized member obtained by the pseudo-carburizing treatment was cut, and the cut surface was mirror-polished to expose the prior austenite grain boundaries by corrosion. (ii) The cut surface was observed, and the presence or absence of coarsening of the prior austenite grains was judged according to the following judgment criteria. <Judgment Criteria> The grain size number was judged according to JIS G 0551:2020 Steel - Microscopic Test Method for Grain Size. When austenite grains with a grain size number of 3 or less were present, it was judged that the coarsening of crystal grains had occurred, and when austenite grains with a grain size number of 3 or less were not present, it was judged that the coarsening of crystal grains had not occurred.

[0054] Figure 3 is a distribution diagram obtained by performing regression analysis on the above experimental results by the inventor of the present invention.

[0055] The horizontal axis of the distribution diagram in Figure 3 corresponds to the value obtained by the formula on the right side of the above inequality (1). The right side of inequality (1) is expressed by a formula with the contents of No and Mo as parameters. Hereinafter, the value obtained by the formula on the right side of inequality (1) is also referred to as "component-related value CV" (see the following formula (1a)).

[0056] CV = 2000×[Nb] + 133×[Mo] + 862 ···(1a)

[0057] The vertical axis of the distribution diagram in Figure 3 corresponds to the value obtained by the formula on the left side of the above inequality (1). The left side of inequality (1) is expressed by a formula with the carburizing temperature Tc and the heating rate RT, which are the heat treatment conditions in carburizing, as parameters. Hereinafter, the value obtained by the formula on the left side of inequality (1) is also referred to as "temperature-related value TV" (see the following formula (1b)).

[0058] TV = Tc - RT ···(1b)

[0059] The inventor of the present invention performed a regression analysis using the above experimental results, and obtained a regression equation that can express the boundary condition for whether or not grain coarsening occurs as a linear relationship between the content of elements that act to suppress grain coarsening and the heat treatment conditions in carburizing, thereby obtaining the distribution diagram of FIG. 3. This distribution diagram shows that when TV ≤ CV and the relationship of the above inequality (1) is satisfied, the occurrence of grain coarsening due to carburizing is suppressed.

[0060] According to the distribution diagram of FIG. 3, when the carburized member has the above chemical composition and the contents of Nb and Mo, the heating rate RT in carburizing, and the carburizing temperature RT satisfy the relationship of the above inequality (1), it is possible to suppress the occurrence of grain coarsening in carburizing after cold forging. This effect can be obtained regardless of the type of carburizing.

[0061] In addition, in order to improve the formability in cold forging, it is possible to obtain the effect whether spheroidizing annealing or low-temperature annealing is performed as softening heat treatment. By performing spheroidizing annealing or low-temperature annealing as softening heat treatment, it is possible to more effectively improve the formability in cold forging than in the case of performing other heat treatments.

[0062] When the grain size number of austenite grains is determined by the microscopic test method for crystal grain size of steel in JIS G 0551:2020, if the grain size number is larger than 3, it can be said that grain coarsening that affects the performance of the carburized member does not occur in carburizing. Therefore, in a carburized member obtained by carburizing cold-forged skin-burned steel, the deterioration of fatigue strength and the increase of heat treatment strain are further suppressed.

[0063] When manufacturing a carburized member having the above composition, if the carburizing temperature Tc is increased within the range satisfying the relationship of the above inequality (1), even if the carburizing time tc is shortened, coarsening of crystal grains can be suppressed. Therefore, it is possible to set the carburizing temperature Tc to, for example, 950 °C or higher, and it is also possible to set it to 1000 °C or higher. If the carburizing temperature Tc can be set higher, the carburizing time tc can be shortened accordingly. Therefore, the processing efficiency of carburizing can be increased.

[0064] 3. Summary of Embodiment: As described above, according to the manufacturing method of the present embodiment, even when carburizing is performed on skin-burned steel that has undergone cold forging, coarsening of austenite crystal grains can be suppressed. Therefore, in the carburized member, the occurrence of problems caused by cold forging, such as deterioration of fatigue strength and increase in heat treatment strain, can be suppressed. Also, by performing cold forging, it is possible to reduce the CO2 emissions in the manufacturing process of the carburized member compared to the case of performing hot forging. Therefore, it can contribute to the improvement of the global environment.

Examples

[0065] Next, examples of the carburized member and its manufacturing method according to the present invention will be described.

[0066] A. Chemical Composition of Carburized Member: Examples and comparative examples of the carburized member according to the present invention were manufactured using steel grades having the chemical compositions shown in Table 2 below.

[0067]

Table 2

[0068] As shown in Table 2, Steel Grades No. 1 to 18 had each element described in the above embodiment within the suitable range of each content rate. On the other hand, for Steel Grades No. 19 to 22, the content rates of some of the elements described in the above embodiment were outside the suitable range. For Steel Grade No. 19, the content rates of Ti, Nb, and B were 0%. For Steel Grade No. 20, the content rate of C was 0.34%, which was greater than the upper limit of 0.30% described in the above embodiment. For Steel Grade No. 21, the content rate of Nb was 0.007%, which was less than the lower limit of 0.01% described in the above embodiment. For Steel Grade No. 22, the content rate of Nb was 0.002%, which was less than the lower limit of 0.01% described in the above embodiment.

[0069] In Tables 3 and 4 below, for Examples 1 to 44 and Comparative Examples 1 to 35, the steel grade number, the content rates [%] of Nb and Mo, the component-related value CV, the carburizing temperature Tc [°C], the heating rate RT [°C / min], and the temperature-related value TV are summarized.

[0070] Also, in Tables 3 and 4 below, for Examples 1 to 44 and Comparative Examples 1 to 35, it is shown whether the relationship of the above inequality (1) was satisfied or not, and the evaluation results of the occurrence of grain coarsening. In Tables 3 and 4, when the relationship of inequality (1) is satisfied, "〇" is displayed in the column of "TV≤CV", and when the relationship of inequality (1) is not satisfied, "×" is displayed in the column of "TV≤CV". Also, in Tables 3 and 4, when grain coarsening has not occurred, "〇" is displayed in the column of "Suppression of coarsening", and when grain coarsening has occurred, "×" is displayed in the column of "Coarsening".

[0071]

Table 3

[0072]

Table 4

[0073] Examples 1 to 44 and Comparative Examples 1 to 35 are test pieces of carburized members obtained by the same steps (i) to (iii) as the "method for manufacturing an experimental carburized member" described in the above embodiment, using each skin-burn steel of the steel types shown in Tables 3 and 4. The determination of the presence or absence of grain coarsening in Examples 1 to 44 and Comparative Examples 1 to 35 was carried out in the same manner as the "method for determining the presence or absence of grain coarsening" in the experimental example described in the above embodiment.

[0074] As shown in Table 3, in Examples 1 to 44 carburized under the heat treatment conditions having the composition described in the above embodiment and satisfying the above inequality (1), the occurrence of grain coarsening was suppressed in all cases. In Examples 1 to 44, the occurrence of grain coarsening was suppressed regardless of whether the carburizing temperature tc was 950 °C, 980 °C, or 1020 °C.

[0075] On the other hand, as shown in Table 4, for Comparative Examples 1 to 23 having the composition described in the above embodiment but carburized under heat treatment conditions not satisfying the above inequality (1), grain coarsening occurred in all cases. Also, for Comparative Examples 24 to 35 not having the composition described in the above embodiment and carburized under heat treatment conditions not satisfying the above inequality (1), grain coarsening occurred.

[0076] As described above, the results of the examples and comparative examples show that according to the method for manufacturing a carburized member according to the present invention described in the above embodiment, the occurrence of grain coarsening can be suppressed during carburizing of skin-burn steel that has undergone cold forging.

[0077] The present invention is not limited to the above embodiments and examples, and various modifications are possible without departing from the spirit of the present invention.

Explanation of Reference Signs

[0078] S1 to S5... states of crystal grains, PT... crystal grains

Claims

1. A step of performing a softening heat treatment to heat and soften the skin-burning steel that is the base material of the carburized member; A step of performing cold forging on the skin-burning steel after the softening heat treatment; A step of carburizing the skin-burning steel after the cold forging to obtain the carburized member; comprising The carburized member, in mass%, C: 0.10% or more and 0.30% or less, Si: 0.03% or more and 1.10% or less, Mn: 0.30% or more and 1.00% or less, Cr: 0.50% or more and 1.80% or less, Al: 0.01% or more and 0.08% or less, B: 0.0003% or more and 0.0300% or less, Ti: 0.010% or more and 0.150% or less, Nb: 0.01% or more and 0.10% or less, Mo: 0.03% or more and 1.00% or less, Cu: 0.50% or less, Ni: 0.50% or less, P: 0.03% or less, and S: 0.03% or less, consisting of, with the balance being Fe and unavoidable impurities, When the carburizing temperature in the carburizing is Tc [°C], the heating rate until reaching the carburizing temperature in the carburizing is RT [°C / min], the mass content of Nb is [Nb], and the mass content of Mo is [Mo], Tc - RT ≤ 2000×[Nb] + 133×[Mo] + 862 A manufacturing method of a carburized member in which the relationship is satisfied.

2. The manufacturing method according to claim 1, wherein the softening heat treatment is spheroidizing annealing or low-temperature annealing.

3. The manufacturing method according to claim 1, wherein the grain size number of austenite grains in the carburized member by the microscopic test method of JIS G 0551:2020 Steel - Method for microscopic examination of crystal grain size is greater than 3.

4. The carburizing temperature is 950°C or higher, and the manufacturing method according to any one of claims 1 to 3.

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