Steel manufacturing methods

Abstract

To provide a method for manufacturing steel materials that can produce steel materials with high surface fatigue strength and bending fatigue strength. [Solution] A method for producing steel, comprising: a melting step of melting raw materials to obtain carburizing steel of a specific composition; a vacuum carburizing step of subjecting the carburizing steel to vacuum carburizing treatment under reduced pressure conditions of 2 kPa or less, followed by cooling at more than 0.2 °C / s to obtain carburized steel; and a vacuum heating step of placing the carburized steel inside a vacuum furnace, subjecting it to convection heating in an inert gas for 10 minutes or more, or subjecting it to vacuum heating for 1 hour or more, followed by oil cooling or water cooling, wherein the carbon mass% on the surface is 1.1 to 1.5 mass%, the proportion of carbides of 1 μm or less in the total carbides contained in the surface layer to a depth of 0.1 mm from the surface is 90 area % or more, and the grain boundary oxidation depth is 10 μm or less.

Description

Technical Field

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

Background Art

[0002] In recent years, with the development of electrification, the load on steel components such as gears used in power transmission parts has been increasingly increasing.

[0003] For example, in Patent Document 1, the core part contains, in mass%, C: 0.10 to 0.30%, Si: 0.16 to 1.40%, Mn: 1.40 to 3.00%, P: 0.030% or less, S: 0.060% or less, Cr: 0.01 to 0.29%, Al: 0.010 to 0.300%, and N: 0.003 to 0.030%, and the balance consists of Fe and impurities, and has a chemical composition; the surface has a flat part and an edge part, the carbon concentration in the flat part surface layer region from the surface of the flat part to a depth of 0.05 mm is 0.70% or more and 0.89% or less, the carbon concentration in the edge part surface layer region from the surface of the edge part to a depth of 0.05 mm is 1.20% or less, the depth of the grain boundary oxide layer is 1 μm or less, and the Vickers hardness of the core part is 260 or more. An carburized part is described. <00​​​​​​For example, Patent Document 3 describes a compound with the following composition in mass%, C: 0.15-0.25%, Si: 0.90-1.30%, Mn: 0.70-1.10%, P: 0.030% or less, S: 0.100% or less, Cu: 0.01-0.50%, Ni: 0.01-0.50%, Cr: 0.20-0.50%, Mo: 0.50% or less, Al: 0.30% or less, N: 0.05% or less, and satisfying the following condition of formula (1): [Si]+[Ni]+[Cu]-[Cr]>0.5···Formula (1) (where each element symbol in formula (1) represents the content in mass%), with the remainder being Fe and unavoidable impurities. The present invention describes a method for manufacturing a carburized part, characterized by performing vacuum carburizing on a steel having a certain structure under reduced pressure conditions of 2 kPa or less, such that the surface carbon concentration after slow cooling following carburizing is in the range of 0.9 to 1.5%, then performing the slow cooling by air cooling at a cooling rate that causes pearlite transformation to occur to form a pearlite structure on the surface, and then performing high-frequency induction hardening under heating and cooling conditions that finely break up the cementite in the pearlite structure, thereby producing fine carbides in a range of up to 0.1 mm from the surface, in which carbides of 1 μm or less account for 90% or more of the carbides. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2016-191151 [Patent Document 2] Japanese Patent Publication No. 2024-7540 [Patent Document 3] Japanese Patent Publication No. 2010-90437 [Overview of the project] [Problems that the invention aims to solve]

[0007] This invention provides a method for manufacturing steel materials that can be obtained to produce steel materials with high surface fatigue strength and bending fatigue strength. [Means for solving the problem]

[0008] The present invention is as follows (1) to (2). (1) The raw materials are melted down, In mass%, C: 0.10~0.30% Si: 0.50~3.00% Mn: 0.30~3.00% Cr: 0.30~1.00% Cu:1.00% or less Ni: 3.00% or less Mo: 2.00% or less P:0.030% or less S: 0.030% or less Al: 0.300% or less N: 0.050% or less And, Formula (1): [Si]+[Ni]+[Cu]-[Cr]>0.5% (However, in formula (1), [Si], [Ni], [Cu], and [Cr] represent the mass percentage of Si, Ni, Cu, and Cr.) A melting process to obtain carburizing steel that satisfies the following conditions, with the remainder being Fe and unavoidable impurities, The vacuum carburizing process involves subjecting the aforementioned carburizing steel to vacuum carburizing treatment under reduced pressure conditions of 2 kPa or less, followed by cooling at a rate greater than 0.2°C / s to obtain carburized steel. The carburized steel is placed inside a vacuum furnace and subjected to convection heating in an inert gas for 10 minutes or more, or vacuum heating for 1 hour or more, followed by oil cooling or water cooling in a vacuum heating process, Equipped with, The C mass% on the surface is 1.10 to 1.50 mass%, The proportion of carbides smaller than 1 μm in the total carbides contained in the surface layer up to a depth of 0.1 mm from the surface is 90% or more by area. A method for producing steel, which yields steel with a grain boundary oxidation depth of 10 μm or less. (2) The carburizing steel is, by mass %, Nb: 0.001~0.080%, V: 0.50% or less, Ti: 0.050% or less, and, B: 0.0005~0.0030% The method for manufacturing the steel material according to (1) above, further containing at least one selected from the group consisting of

Advantages of the Invention

[0009] According to the present invention, it is possible to provide a method for manufacturing a steel material capable of obtaining a steel material having high surface fatigue strength and bending fatigue strength.

Brief Description of the Drawings

[0010] [Figure 1] It is a diagram (schematic diagram) showing the shape of a test piece used in the surface fatigue strength test. [Figure 2] FIG. 2(A) is a diagram (schematic diagram) showing the shape of a test piece 11 used in the surface fatigue strength test, and FIG. 2(b) is a diagram explaining a method for measuring the pitching life using a roller pitching tester.

Modes for Carrying Out the Invention

[0011] The present invention will be described. The present invention relates to a method for producing a carburizing steel, comprising a melting step of melting raw materials to obtain a carburizing steel having the following composition in mass%: C: 0.10 to 0.30%, Si: 0.50 to 3.00%, Mn: 0.30 to 3.00%, Cr: 0.30 to 1.00%, Cu: 1.00% or less, Ni: 3.00% or less, Mo: 2.00% or less, P: 0.030% or less, S: 0.030% or less, Al: 0.300% or less, N: 0.050% or less, and satisfying the formula (1): [Si] + [Ni] + [Cu] - [Cr] > 0.5% (wherein, in formula (1), [Si], [Ni], [Cu] and [Cr] represent the contents (mass%) of Si, Ni, Cu and Cr), and the balance being Fe and inevitable impurities; a vacuum carburizing step of subjecting the carburizing steel to a vacuum carburizing treatment under a reduced pressure condition of 2 kPa or less and then cooling at a rate exceeding 0.2 °C / s to obtain a carburized steel; and a vacuum heating step of placing the carburized steel inside a vacuum furnace, performing convection heating in an inert gas for 10 minutes or more, or performing vacuum heating for 1 hour or more, and then performing oil cooling or water cooling, thereby obtaining a steel material having a C mass% on the surface of 1.1 to 1.5 mass%, a ratio of carbides having a size of 1 μm or less in all carbides contained in the surface layer from the surface to a depth of 0.1 mm of 90 area% or more, and a grain boundary oxidation depth of 10 μm or less. Such a method for producing a steel material is also referred to as "the production method of the present invention" hereinafter.

[0012] Unless otherwise specified, "%" means "mass%" hereinafter.

[0013] <Melting step> The melting step in the production method of the present invention will be described. In the melting step, raw materials are melted to obtain a carburizing steel. For example, an ingot is obtained using molten steel obtained by melting raw materials, and then the ingot is hot forged to obtain a bar-shaped carburizing steel. Alternatively, after hot forging the ingot to obtain a bar steel, cold forging and machining may be further performed to obtain a square bar-shaped carburizing steel. Further, the ingot may be hot forged and machined to obtain a gear-shaped carburizing steel. It is preferable to apply annealing after forging. Specifically, after forging, the workpiece is placed in a heating furnace with a furnace temperature of 850 to 1000°C, heated for 1 to 6 hours, and then cooled in the furnace. Alternatively, normalizing (a process of removing the workpiece from the furnace after heating and air-cooling) may be performed after forging.

[0014] The composition of carburized steel is as follows (by mass%): C: 0.10-0.30%, Si: 0.50-3.00%, Mn: 0.30-3.00%, Cr: 0.30-1.00%, Cu: 1.00% or less, Ni: 3.00% or less, Mo: 2.00% or less, P: 0.030% or less, S: 0.030% or less, Al: 0.300% or less, N: 0.050% or less, satisfying formula (1): [Si] + [Ni] + [Cu] - [Cr] > 0.5% (wherein formula (1), [Si], [Ni], [Cu] and [Cr] represent the mass%) content of Si, Ni, Cu and Cr, with the remainder being Fe and unavoidable impurities.

[0015] Furthermore, the carburizing steel may further contain at least one selected from the group consisting of Nb: 0.001 to 0.080%, V: 0.50% or less, Ti: 0.050% or less, and B: 0.0005 to 0.0030% by mass.

[0016] The composition of such carburized steel is the same as the composition of the core (uncarburized portion) of the steel material obtained by the manufacturing method of the present invention. The regulations regarding the content of each component will be described later.

[0017] <Vacuum carburizing process> The vacuum carburizing process in the manufacturing method of the present invention will be described. In the vacuum carburizing process, the carburizing steel obtained in the aforementioned melting process is subjected to vacuum carburizing treatment under reduced pressure conditions of 2 kPa or less.

[0018] In the manufacturing method of the present invention, carbon is introduced to the surface of the carburizing steel by vacuum carburizing treatment so that the mass% of carbon on the surface after cooling is 1.10 to 1.50%. Then, by subsequent cooling, the surface structure is made into a pearlite and / or martensite structure.

[0019] In conventional gas carburizing, the surface carbon mass percentage is limited to about 0.8%, but the manufacturing method of the present invention, by using vacuum carburizing, can achieve a higher surface carbon concentration. Vacuum carburizing, in general, is a method in which the atmosphere inside the furnace is reduced in pressure, and hydrocarbon gases (such as methane, propane, ethylene, acetylene, etc.) are directly introduced into the furnace as a carburizing gas. When the gas comes into contact with the surface of the steel to be carburized, the active carbon that decomposes supplies carbon to the surface of the steel. During the carburizing phase, carbides are formed and carbon is stored, and in the subsequent diffusion phase, the carbides decompose, and the stored carbon dissolves into the matrix, causing the carbon to diffuse inward and carburize. The supply routes for carbon are not limited to those via carbides; there are also routes through direct dissolution. The manufacturing method of the present invention is carried out under a reduced pressure of 2 kPa or less. In this case, a large amount of fine carbides can be generated on the surface of the carburizing steel. This greatly contributes to increasing the strength of the surface layer of the steel material obtained by the manufacturing method of the present invention.

[0020] After this vacuum carburizing process, the steel is cooled to over 0.2°C / s to obtain carburized steel. With such a cooling rate, the surface structure of the resulting carburized steel will be pearlite and / or martensite. Even if the structure is martensite, the inventors believe that because the vacuum heating time described later is relatively long, carbides can be generated, and a hardness equivalent to that of a carburized steel surface composed of pearlite can be obtained. Cooling can be done using oil cooling.

[0021] <Vacuum heating process> The vacuum heating step in the manufacturing method of the present invention will now be described. In the vacuum heating process, the carburized steel obtained in the vacuum carburizing process is placed inside a vacuum furnace. Here, the vacuum carburizing furnace used in the aforementioned vacuum carburizing process may be used to perform the vacuum heating process. In other words, after performing the aforementioned vacuum carburizing process using the vacuum carburizing furnace, the carburized steel may be subjected to the vacuum heating process without removing it from the vacuum carburizing furnace. In this case, the aforementioned vacuum carburizing furnace will be used as the vacuum furnace.

[0022] The carburized steel placed inside the vacuum furnace is subjected to convection heating in an inert gas for 10 minutes or more, or vacuum heating for 1 hour or more. In the former case, the aforementioned carburized steel is first placed inside a vacuum furnace, and the inside of the vacuum furnace is filled with an inert gas. Examples of inert gases include nitrogen and argon. The carburized steel is then heated while the inert gas is circulated. The heating temperature is preferably 750 to 900°C. The heating time is 10 minutes or more, but preferably within 2 hours, and more preferably within 1 hour. In the latter case, the aforementioned carburized steel is first placed inside the vacuum furnace, and the inside of the vacuum furnace is evacuated. Then the carburized steel is heated. The heating temperature is preferably 750 to 900°C. The heating time is 1 hour or more, but preferably 2 hours or less.

[0023] The inventors believe that by applying vacuum heating for such a short time, the carbides can be dispersed while remaining fine, without becoming coarse. Furthermore, the inventors believe that by maintaining a vacuum, grain boundary oxidation is suppressed as much as possible, and by quenching at a stable temperature through vacuum heating, variations in hardness are suppressed. When variations in hardness are suppressed, bending fatigue strength and surface fatigue strength tend to increase. Furthermore, the inventors believe that because the alloying elements diffuse into the matrix phase without coarsening the carbides in a vacuum, the hardenability of the matrix phase increases, resulting in a relatively high hardness even when oil-cooled.

[0024] After vacuum heating, the steel material can be obtained by oil cooling or water cooling.

[0025] The steel material obtained by the manufacturing method of the present invention as described above will be explained.

[0026] The steel material obtained by the manufacturing method of the present invention has a carbon mass% on its surface of 1.1 to 1.5% by mass. The steel material obtained by the manufacturing method of the present invention has a high carbon content on its surface, which contributes to its high strength.

[0027] The steel material obtained by the manufacturing method of the present invention has a composition in which carbides of 1 μm or less account for 90 area % or more of the total carbides contained in the surface layer up to a depth of 0.1 mm from the surface. In this case, the surface fatigue strength and bending fatigue strength of the steel material obtained by the manufacturing method of the present invention are increased.

[0028] The steel obtained by the manufacturing method of the present invention has a grain boundary oxidation depth of 10 μm or less. In this case, the bending fatigue strength is improved. The steel obtained by the manufacturing method of the present invention is subjected to vacuum carburizing treatment, and therefore has a small grain boundary oxide layer formed by the carburizing treatment. A smaller grain boundary oxide layer is desirable in order to reduce the incomplete quenching structure. The incomplete quenching structure leads to a decrease in the fatigue strength of the steel, and the degree of decrease in fatigue strength increases as the amount of the incomplete quenching structure increases.

[0029] The composition of the core portion (uncarburized portion) of the steel material obtained by the manufacturing method of the present invention will be described. As described above, the core composition of the steel material obtained by the manufacturing method of the present invention is the same as that of carburized steel.

[0030] C: 0.10~0.30% If the carbon content is below the lower limit, excessive ferrite will form in the core, reducing its strength. If the carbon content is too high, the machinability, especially the workability, will deteriorate.

[0031] Si: 0.50~3.00% If the Si content is too low, carbides are more likely to form during air cooling after vacuum carburizing. Furthermore, hardenability decreases, leading to a reduction in strength. If the Si content is too high, machinability, especially workability, tends to deteriorate.

[0032] Mn: 0.30~3.00% If the Mn content is too low, it is necessary to ensure hardenability. Ferrite will form in the core, leading to a decrease in strength. If the Mn content is too high, machinability, especially workability, tends to deteriorate.

[0033] Cr: 0.30~1.00% If the chromium content is too low, the hardenability decreases, which tends to lead to a reduction in strength. Furthermore, chromium promotes the formation of chromium carbides, causing chromium to form carbides during post-carburization treatment. This prevents the acquisition of a single-phase pearlite structure. Additionally, excessive addition tends to degrade workability, particularly machinability.

[0034] Cu:1.00% or less Cu is a component that suppresses the formation of carbides. If the Cu content is too high, it tends to reduce hot workability. Cu is not required to be included.

[0035] Ni: 3.00% or less Ni is a component that suppresses the formation of carbides. If the Ni content is too high, it tends to lead to bainite formation in rolled and forged materials, reducing the machinability of the steel. Ni does not need to be included.

[0036] Mo: 2.00% or less If the Mo content is too high, the machinability of the steel, especially its workability, tends to deteriorate. Mo does not need to be included.

[0037] P: 0.030% or less, S: 0.030% or less These are impurities, components undesirable for the mechanical properties of carburized parts, and their content is regulated to remain below the above-mentioned upper limit.

[0038] Al: 0.300% or less Al is added as a deoxidizing agent. It also has the effect of refining the crystal grains and improving strength. However, if the amount exceeds 0.30%, alumina will form in the steel, leading to a decrease in strength. Furthermore, to ensure the effect of refining the crystal grains and improving strength, it is desirable to add 0.010% or more of Al. A more desirable range for Al is 0.010–0.040%.

[0039] N: 0.050% or less N has the effect of preventing grain coarsening. This effect saturates at around 0.050%, so it should be kept below that level. Furthermore, it is desirable to maintain a nitrogen content of 0.0020% or higher. Lowering the content further would increase costs. A more desirable range for N is 0.010 to 0.030%.

[0040] Here, equation (1): [Si] + [Ni] + [Cu] - [Cr] > 0.5 is satisfied. In formula (1), [Si], [Ni], [Cu], and [Cr] represent the mass percentage of Si, Ni, Cu, and Cr. Si, Ni, and Cu suppress the formation of carbides, while Cr increases them. In the manufacturing method of the present invention, by balancing the amounts of Si, Ni, Cu, and Cr added, even when high-concentration carburization is performed by vacuum carburizing, a single-phase pearlite structure can be generated by subsequent cooling.

[0041] The composition of the steel obtained by the manufacturing method of the present invention is as described above, but it may further contain at least one selected from the group consisting of Nb: 0.001 to 0.080%, V: 0.50% or less, Ti: 0.050% or less, and B: 0.0005 to 0.0030%.

[0042] Nb: 0.001~0.080% Nb combines with N and / or C in steel to form fine carbides, nitrides, or carbonitrides, which have the effect of suppressing grain growth during vacuum carburizing (surface hardening heat treatment). Even a small amount of Nb can provide some degree of this effect. If the Nb content is too high, the grain coarsening suppression effect saturates, and machinability tends to decrease.

[0043] V:0.50% or less V forms V precipitates such as V carbides and V carbonitrides. These V precipitates suppress grain coarsening in the steel during vacuum carburizing due to their pinning effect. If the V content is too high, the hardness of the steel becomes excessively high. As a result, the machinability of the steel tends to decrease.

[0044] Ti: 0.050% or less Ti combines with N in the steel to form TiN, thereby suppressing BN. It is preferable to add Ti at the same time as B. If the Ti content is too high, excessive transfer tends to reduce the machinability of the steel.

[0045] B: 0.0005~0.0030% B enhances the hardenability of steel. If the B content is too high, the hardness becomes excessively high, and BN formation tends to reduce machinability. [Examples]

[0046] The present invention will be described using examples. The present invention is not limited to the examples described below.

[0047] A 150 kg steel ingot with the chemical composition shown in Table 1 was melted in a vacuum induction melting furnace. In the subsequent hot forging process, the ingot was heated at 1250°C for 4 hours and forged to a diameter of φ70 mm. After that, it was heated again at 1250°C for 2 hours and forged to a diameter of φ22-30 mm to produce steel bars. Annealing was performed after forging (before machining). Specifically, after forging, the bars were placed in a heating furnace set to a temperature of 930°C and heated for 2 hours before being furnace-cooled. Next, the steel bar was machined to produce a test specimen 10 for use in a bending fatigue strength test, as shown in Figure 1. Similarly, a test piece 11 for surface fatigue strength testing was fabricated by machining, with the shape shown in Figure 2(A). In the same figure, the diameter d1 of the contact portion 11a is φ26 mm, the diameters d2 of the smaller diameter portions 11b on both sides are φ23 mm, the width w1 of the contact portion 11a is 28 mm, and the width w2 of the smaller diameter portions 11b is 51 mm. The test pieces 10 and 11 obtained in this manner correspond to the carburizing steel in the present invention.

[0048] Next, each of the test pieces 10 and 11 (carburizing steel) is placed in a vacuum furnace and subjected to vacuum carburizing treatment under reduced pressure conditions of 2 kPa or less, and then cooled at more than 0.2 °C / s to obtain carburized steel. The vacuum carburizing temperatures are as shown in Table 1.

[0049] Next, the carburized steel was placed inside a vacuum furnace, subjected to vacuum heating for more than one hour, and then cooled in oil. The vacuum heating temperatures are shown in Table 1. The temperature range for vacuum heating was 10°C. In Comparative Example 5, high-frequency heating was applied. The high-frequency heating range (temperature range) was 50°C.

[0050] The following tests were conducted on the steel material obtained after this treatment.

[0051] <Surface carbon concentration> The carbon mass percentage on the surface was measured by spark discharge emission spectrometry (JIS G 1253). The results are shown in Table 1.

[0052] <Surface hardness and variability range> The Vickers hardness (Hv) was measured according to the test method specified in JIS Z2244. Five measurements were taken, and the average value was calculated. The results are shown in Table 1. In addition, the variability of the five measurements was measured. The variability was calculated as the percentage of the variability from the average value of the five measurements. The results are shown in Table 1.

[0053] <Percentage of carbides smaller than 1 μm> The following is a method for measuring the proportion of carbides smaller than 1 μm in the total carbides contained in the surface layer of steel material up to a depth of 0.1 mm from the surface. The surface of the carburized parts was mirror-polished, then etched with a picric acid alcohol solution. The area was measured by image analysis of the SEM images and converted to an equivalent circle diameter. The total area of ​​carbides with an equivalent circle diameter of 1 μm or less was used as the area ratio to the total area of ​​carbides calculated by image analysis.

[0054] <Depth of grain boundary oxidation layer> The obtained steel material (bending fatigue test specimen) was cut approximately perpendicular to an arbitrary surface, the cut surface was mirror-polished, and observed with an optical microscope set to 1000x magnification. Then, the black oxide extending in a continuous streak from the surface inward was observed, and the maximum depth from the surface reached by the streak of black oxide was measured as the grain boundary oxide layer depth. The results are shown in Table 1.

[0055] <Surface fatigue strength> The surface fatigue strength was measured using the following method, and the ratio to Comparative Example 1 was calculated. The results are shown in Table 1. A test specimen 11 after carburizing treatment (see Figure 2(A)) was used as a small roller, and as shown in Figure 2(B), the contact portion 11a was brought into contact with a larger opposing roller 12 (made of SUJ2) with a diameter D0 = φ130 mm and a width W0 = 18 mm under high surface pressure. Using a roller pitting tester, they were rotated at a slip ratio of 60%, and the pitting life (surface fatigue strength) until pitting occurred was evaluated. The evaluation results are shown in Table 2 below. The other conditions were as follows: Small roller rotation speed: 1500 rpm, Lubricant type: ATF, Oil temperature: 90℃

[0056] <Bending fatigue strength> The bending fatigue strength was measured using the following method, and the ratio to Comparative Example 1 was calculated. The results are shown in Table 1. A rotary bending fatigue test using the Ono method was performed on the carburized specimen 10 in accordance with JIS Z 2274. The test conditions were a rotation speed of 3600 rpm and a test temperature of room temperature. Rotational bending fatigue tests were performed on the specimens under various load conditions until fracture, and the S / N curve was obtained. The number of cycles was 10. 7 The maximum stress at which the material does not break in a single cycle is defined as the life strength (unit: MPa).

[0057] [Table 1]

[0058] Comparative Example 1 had a high Cr content, which led to the formation of coarse carbides during carburizing, making it difficult to disperse fine carbides during secondary heating. Since the coarse carbides acted as crack initiation points, the fatigue strength was reduced.

[0059] In Comparative Example 2, the surface carbon concentration was not high, resulting in either no or only trace amounts of the desired carbide being formed, thus lowering the hardness.

[0060] Comparative Example 3 failed to adequately break down the coarse grain boundary carbides and carbides in the plate-like perlite generated by vacuum carburizing. Although the hardness was high due to the large amount of carbides, the fatigue strength was low because the coarse / plate-like carbides acted as crack initiation points and promoted crack propagation. Furthermore, the hardness was unstable and varied greatly because the fragmentation of the coarse and plate-like carbides was incomplete.

[0061] Comparative Example 4 had low hardness and reduced fatigue strength because the carbides disappeared during vacuum heating.

[0062] In Comparative Example 5, grain boundary oxides were generated during high-frequency heating, which acted as crack initiation points, resulting in reduced fatigue strength. Furthermore, due to the rapid heating process, the heating temperature varied even with short processing times, leading to variations in hardness.

Claims

1. The raw materials are melted down, In mass percent, C: 0.10-0.30% Si: 0.50-3.00% Mn: 0.30-3.00% Cr:0.30~1.00% Cu: 1.00% or less Ni: 3.00% or less Mo: 2.00% or less P: 0.030% or less S: 0.030% or less Al: 0.300% or less N: 0.050% or less And, Formula (1): [Si] + [Ni] + [Cu] - [Cr]>0.5% (However, in formula (1), [Si], [Ni], [Cu], and [Cr] represent the mass percentage of Si, Ni, Cu, and Cr.) A melting process to obtain carburizing steel that satisfies the following conditions, with the remainder being Fe and unavoidable impurities, The vacuum carburizing process involves subjecting the aforementioned carburizing steel to vacuum carburizing treatment under reduced pressure conditions of 2 kPa or less, followed by cooling at a rate greater than 0.2°C / s to obtain carburized steel. The carburized steel is placed inside a vacuum furnace and subjected to convection heating in an inert gas for 10 minutes or more, or vacuum heating for 1 hour or more, followed by oil cooling or water cooling in a vacuum heating process. Equipped with, The mass percentage of C on the surface is 1.10 to 1.50 mass%, The proportion of carbides smaller than 1 μm in the total carbides contained in the surface layer up to a depth of 0.1 mm from the surface is 90% or more by area. A method for producing steel, which yields steel with a grain boundary oxidation depth of 10 μm or less.

2. The carburizing steel is, in mass%, Nb: 0.001-0.080%, V: 0.50% or less, Ti: 0.050% or less, and, B: 0.0005-0.0030% A method for manufacturing steel according to claim 1, further comprising at least one selected from the group consisting of the following.

Images