Carburized parts

Abstract

To provide a carburized component in which a stable carburized layer is obtained even with high Cr.SOLUTION: Provided is a carburized component that contains, in mass%, C: 0.10 to 0.40%, Si: 0.20 to 0.80%, Mn: 0.20 to 0.60%, P: ≤0.030%, S: ≤0.030%, Cr: 1.60 to 5.00%, Al: 0.003 to 0.050%, N: 0.005 to 0.020%, and the balance being Fe and unavoidable impurities, and is in a carburized state, and in which, when assuming the region near the surface in which O amount is contained by 5% or more to be the scale thickness from the surface, the Mn amount contained in the scale is less than 10% at any depth of the scale thickness, the surface hardness is 700Hv or more, and the carbon amount from the surface to a depth of 500 μm is 0.50 to 1.00%.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to carburized parts. In particular, it relates to carburized parts capable of suppressing the occurrence of carburization inhibition while using high Cr steel materials.

Background Art

[0002] Carburizing and quenching is one of the typical surface hardening treatments for steel materials and is used for carburized parts that require high fatigue strength and wear resistance, such as gears and bearings. As the steel materials for such parts, steels such as SCM420 and SCR420 defined in Japanese Industrial Standards (JIS) are generally used. However, since the usage environments of parts are becoming more severe these days, further extension of the service life and increase in strength of carburized parts are required.

[0003] Therefore, heretofore, a steel containing, by mass%, C: 0.10 to 0.35%, Si: 0.40 to 0.80%, Mn: 0.15 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 1.20 to 2.50%, Ni: 0.20% or less, Mo: 0.10% or less, with the balance being Fe and unavoidable impurities, and having a maximum grain boundary oxidation depth D1 of 10 μm or less, a maximum depth D2 of an incomplete quenched layer which is an alloy-deficient layer of 8 to 20 μm, and D2 - D1 of 2 to 15 μm, and having a carburized surface in a state where a carburized abnormal layer remains, has been proposed as a skin-rolled steel for mechanical structures having excellent anti-pitting characteristics (see Patent Document 1). This proposal is a measure for extending the service life by covering the depth of grain boundary oxidation that can be a starting point of pitting with an incomplete quenched layer that is softer than martensite and wearing away the grain boundary oxidation together with the incomplete quenched layer.

[0004] Also, in terms of mass%, it contains C: 0.10 to 0.35%, Si: 0.25 to 0.80%, Mn: 0.30 to 1.80%, P: 0.030% or less, S: 0.035% or less, Cr: 2.00 to 3.50%, Mo: 0.04 to 0.50%, Al: 0.003 to 0.100%, N: 0.002 to 0.050%, satisfies Si + 0.5Cr ≥ 1.5 and the total amount of Si, Cr, and Mo is 3.0% or more, with the balance being composed of Fe and inevitable impurities. When carburizing treatment, carbonitriding treatment, and quenching and tempering treatment are carried out, the C concentration at 20 μm from the surface is 0.7 to 1.0%, and also Ms ≤ 215°C or less, and the residual γ amount is 20 ≤ γ ≤ 50% by volume. In particular, a carburized steel for gears with excellent anti-pitting characteristics when used in a hydrogen intrusion environment for gears has been proposed (see Patent Document 2). This is a measure to stabilize the residual γ that suppresses the diffusion rate of hydrogen that causes embrittlement and improve the anti-pitting characteristics of gears.

[0005] Also, in terms of mass%, it contains C: 0.13 to 0.35%, Si: 0.20 to 0.65%, Mn: 0.50 to 1.80%, P: 0.030% or less, S: 0.030% or less, Cr: 2.30 to 3.50%, and further contains one or two selected from Ni: 0.10 to 0.50%, Mo: 0.03 to 0.50%, with the balance being steel composed of Fe and inevitable impurities. The total of Si, Mn, Cr, Ni, and Mo dissolved in the matrix component 100 to 300 μm from the outermost surface of the steel after carburizing quenching pattern and tempering shown in Figure 2 is 3.0% or more, and further the residual γ amount is 20 to 50 vol%, and the rest is a martensite structure. A bearing steel with excellent resistance to white structure change and peeling life has been proposed (see Patent Document 3). This is a technology to improve the peeling life of bearings by suppressing white structure change caused by hydrogen in a hydrogen environment.

[0006] Also proposed is a rolling bearing for a wind power generation facility obtained by processing a material made of alloy steel containing, by mass%, C: 0.10 to 0.30%, Si: 0.20 to 0.50%, Mn: 0.20 to 1.20%, P: 0.020% or less, S: 0.020% or less, Cr: 2.60 to 4.50%, Mo: 0.10 to 0.40%, Ni: 0.20% or less, Cu: 0.20% or less, with the balance being iron (Fe) and inevitable impurities, into a predetermined shape and then performing carburizing or carbonitriding and quenching and tempering. (See Patent Document 4.) This is a technique for suppressing the transformation of martensite to ferrite due to hydrogen by increasing the Cr content and achieving high strength.

[0007] In any of the proposals, Cr needs to be added in an amount exceeding 1.20%. Thus, as a measure to extend the service life and increase the strength of parts, it is intended to contain Cr in an amount greater than that of the steel specified in JIS.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Summary of the Invention

Problems to be Solved by the Invention

[0009] However, since all of these proposed technologies have a high Cr content, Cr oxides are likely to form on the surface of the steel material during carburization, and there is a problem that the risk of carbon intrusion being inhibited during carburization becomes apparent.

[0010] Therefore, a measure has been proposed to render it harmless by making the Cr oxide layer less than a predetermined thickness (see Patent Document 5). In addition, a measure has also been proposed to remove the work-affected layer in which Cr enrichment, which causes Cr oxide formation, occurs before carburization (see Patent Document 6).

[0011] However, in these proposals, since they are all affected by carburization conditions, it is difficult to say that inhibition can be completely avoided, and at present, a sufficient improvement method has not yet been established.

[0012] Therefore, the problem to be solved by the present invention is to provide a carburized part that is made of a high Cr material and suppresses carburization inhibition without being affected by carburization conditions.

Means for Solving the Problem

[0013] As a result of intensive studies to solve the above problems, the inventors of the present invention have found that oxides of Mn, like those of Cr, also have an adverse effect on carburization characteristics, and furthermore, the effect on carburization characteristics is greater than that of Cr oxides. And when the components of the steel are within the range of the components of the present invention and the Mn content is low, Mn oxides, which are particularly harmful to carburization characteristics, are less likely to form during carburization. Thus, it has been found that it is possible to provide a carburized part in which the surface hardness and surface carbon amount after carburization for extending the life of the part stably meet the requirements without being greatly affected by carburization conditions.

[0014] Therefore, the first means for solving the problem of the present invention is By mass, C: 0.10 to 0.40%, Si: 0.20 to 0.80%, Mn: 0.20 to 0.60%, P: ≤0.030%, S: ≤0.030%, Cr: 1.60 to 5.00%, Al: 0.003 to 0.050%, N: 0.005 to 0.020%, the balance being Fe and inevitable impurities, in a carburized state, when the region near the surface containing 5% or more of O amount is defined as the scale thickness from the surface, the Mn amount contained in the scale at any depth of the scale thickness is less than 10%, the hardness of its surface is 700 Hv or more, it is a carburized part with a carbon amount of 0.50 to 1.00% from the surface to a depth of 500 μm.

[0015] Its second means is, by mass, C: 0.10 to 0.40%, Si: 0.20 to 0.80%, Mn: 0.20 to 0.60%, P: ≤0.030%, S: ≤0.030%, Cr: 1.60 to 5.00%, Al: 0.003 to 0.050%, N: 0.005 to 0.020% as the main components, furthermore, as selectively added components, it contains at least one of Nb: 0.02 to 0.10%, Ni: 5.00% or less, Mo: 1.00% or less, Ti: 0.20% or less, B: 0.010 to 0.050%, the balance being Fe and inevitable impurities, in a carburized state, when the region near the surface containing 5% or more of O amount is defined as the scale thickness from the surface, the Mn amount contained in the scale at any depth of the scale thickness is less than 10%, the hardness of its surface is 700 Hv or more, it is a carburized part with a carbon amount of 0.50 to 1.00% from the surface to a depth of 500 μm. It is a carburized part characterized in that the carbon amount is 0.50 to 1.00%.

Effects of the Invention

[0016] According to the present invention, even though it is a high-Cr steel material, by suppressing the formation of Mn oxide, it is less affected by carburizing conditions. Without any special device for carburizing conditions, as long as it is a general carburizing condition, a carburized part with suppressed carburizing inhibition can be obtained. Therefore, according to the present invention, a carburized part with excellent surface hardness of 700 Hv or more after carburizing and an appropriate carbon concentration distribution with a carbon amount of 0.50 to 1.00% from the surface to a depth of 500 μm can be obtained.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0018] Prior to the description of the embodiments of the present invention, first, the reason for defining the chemical composition of the steel material used for the carburized parts of the present invention will be explained below. The following % is by mass.

[0019] C: 0.10 to 0.40% C is an element necessary to ensure the carburized layer and the core strength after carburizing as parts for mechanical structures. If it is less than 0.10%, the effect cannot be obtained sufficiently. On the contrary, if it exceeds 0.40%, the toughness of the core part will be reduced. Therefore, the content of C is set to 0.10 to 0.40%. More preferably, C is 0.15 to 0.30%.

[0020] Si: 0.20 to 0.80% Si is an element necessary for deoxidation during steel melting and has an effect of improving hardenability. If it is less than 0.20%, the deoxidation effect is not sufficient. If it exceeds 0.80%, the workability will be reduced. Therefore, the content of Si is set to 0.20 to 0.80%. More preferably, Si is 0.35 to 0.65%.

[0021] Mn: 0.20 - 0.60% Mn is the most important elemental component in the present invention. It is an element necessary for deoxidation during steel melting and an element that improves hardenability. If Mn is less than 0.20%, the deoxidation effect is insufficient, and if it exceeds 0.60%, a scale that may cause carburization inhibition will be generated. Therefore, the content of Mn is set to 0.20 - 0.60%. More preferably, Mn is 0.20 - 0.40%.

[0022] P: ≤0.030% P is an inevitable impurity. If it exceeds 0.030%, toughness will decrease due to grain boundary segregation. Therefore, P is set to 0.030% or less.

[0023] S: ≤0.030% S is an inevitable impurity. If it exceeds 0.030%, toughness will decrease due to the formation of MnS, and fatigue strength will also decrease. Therefore, S is set to 0.030% or less.

[0024] Cr: 1.60 - 5.00% Cr is an element that improves hardenability. To ensure the hardenability of steel, it is necessary to add Cr at 1.60% or more. However, if Cr is added in excess of 5.00%, a Cr-based oxide film will be formed on the surface of the steel material, and regardless of the carburizing conditions, carburization will be inhibited. Therefore, Cr is 1.60 - 5.00%, and more desirably, Cr is 1.70 - 3.00%.

[0025] Al: 0.003 - 0.050% Al is an element necessary for deoxidation. However, if Al is less than 0.003%, its effect cannot be fully obtained, and when the addition amount of Al is increased, the amount of alumina-based inclusions generated in the steel increases, resulting in a decrease in fatigue strength. Therefore, the content of Al is set to 0.003 - 0.050%. More desirably, Al is 0.010 - 0.030%.

[0026] N: 0.005 - 0.200% N easily combines with Al, Nb, Ti, etc. to form nitrides, is effective in refining crystal grains, and has the effect of increasing fatigue strength. To obtain these effects, N needs to be added at 0.005% or more. However, if N is added in excess of 0.200%, too much nitride will precipitate and the fatigue strength will decrease. Therefore, the content of N is set to 0.005 - 0.200%. More preferably, N is 0.050 - 0.150%.

[0027] The following describes the selective additive components.

[0028] Nb: 0.02 - 0.10% Nb is one of the selective additive components. It forms carbonitrides with C and N, and is an element that improves fatigue strength by refining crystal grains through the pinning effect. However, if the Nb content is too high, the toughness of the steel will decrease. Therefore, when adding Nb, it is set to 0.02 - 0.10%.

[0029] Ni: 5.00% or less Ni is one of the selective additive components. Ni is an element effective in increasing the hardenability of steel, but it is expensive, so minimizing its content is required industrially. Therefore, the addition of Ni is set to 5.00% or less.

[0030] Mo: 1.00% or less Mo is one of the selective additive components. Mo is an element effective in improving hardenability and toughness. However, if there is too much Mo, it will lead to a decrease in workability and an increase in material cost. Therefore, the addition of Mo is set to 1.00% or less.

[0031] Ti: 0.20% or less Ti is one of the selective additive components. Ti forms carbonitrides with C and N, and improves fatigue strength by refining crystal grains through the pinning effect. However, if the Ti content is too high, the toughness of the steel will decrease. Therefore, the addition of Ti is set to 0.20% or less.

[0032] B: 0.010 - 0.050% B is one of the selective additive components. B is an element that has the function of improving hardenability and improving toughness by inhibiting the grain boundary precipitation of P. In order to obtain this effect, it is desirable to add B in an amount of 0.010% or more. However, when B exceeds 0.050%, the effect saturates. Therefore, the addition of B is set to 0.010 to 0.050%.

[0033] Next, the reason for defining the properties of the surface after carburizing of the carburized parts made of the steel material defined in the present invention will be described.

[0034] When the region near the surface containing 5% or more of the O amount is defined as the scale thickness from the surface, the amount of Mn contained in the scale is less than 10% at any depth of the scale thickness: The amount of Mn contained in the scale is an index for suppressing carburizing inhibition during carburizing. When the amount of Mn of this index exceeds 10%, when carburizing the parts, the intrusion of carbon is inhibited by Mn oxide, and it becomes difficult to obtain hardness after carburizing. If the amount of Mn is 10% or less at any depth inside the scale, when carburizing, the occurrence of carburizing inhibition can be suppressed without using specific carburizing conditions regardless of the carburizing conditions.

[0035] The hardness of the outermost surface after carburizing is 700 Hv or more: If the surface hardness of the carburized parts is less than 700 Hv, in carburized parts such as gears and bearings, predetermined strength characteristics cannot be obtained, and the life of the parts is shortened. Therefore, the hardness of the outermost surface after carburizing is defined to be 700 Hv or more.

[0036] The carbon amount from the surface after carburizing to a depth of 500 μm shall be 0.50% to 1.00%: If the carbon amount from the surface after carburizing to a depth of 500 μm is less than 0.50%, carburizing cannot be carried out normally, and the fatigue strength is lowered. Also, if the carbon amount from the surface to a depth of 500 μm exceeds 1.00%, carbides precipitate too much and the effect saturates. Therefore, the carbon amount from the surface after carburizing to a depth of 500 μm shall be 0.50% to 1.00%.

[0037] (Example) Steel ingots of 100 kg each of steel types A to M having the chemical compositions shown in Table 1 were melted in a vacuum melting furnace. Then, each of these steels A to M was soaked at 1250 °C for 12 hours or more, hot forged into a bar steel with a diameter of 32 mm, held at 900 °C for 4 hours, and then air-cooled to perform a normalizing treatment to obtain test specimens.

[0038]

Table 1

[0039] These test specimens were processed into the dimensions shown in Figure 1 and gas carburized under three conditions with different carburizing conditions shown in Figure 2 to obtain test pieces. Here, carburizing condition 1 is the commonly used carburizing condition, carburizing condition 2 has a lower carburizing temperature, and carburizing condition 3 has a shorter carburizing time set.

[0040] <Evaluation items> Regarding the characteristics of the test pieces after carburizing, (1) when the scale thickness was defined as the range where the surface to O amount contains 5% or more, the Mn amount inside the scale (measured using a glow discharge optical emission spectrometer), (2) the surface hardness after carburizing (measured using a Vickers hardness tester), and (3) the surface carbon concentration after carburizing (the test piece was cut out after carburizing and measured using an electron probe microanalyzer (EPMA)) were evaluated respectively. The detailed measurement methods will be described below.

[0041] The "glow discharge optical emission spectrometer" used for measuring the Mn amount inside the scale is a device capable of performing quantitative and qualitative analysis of elements in the sample depth direction by detecting the wavelength and intensity of light generated by colliding (sputtering) Ar ions applied with a high voltage onto the sample surface and exciting the ejected atoms into a plasma state. In this example, the test piece as carburized was set in the glow discharge optical emission spectrometer, and regarding the sputtering conditions, a copper electrode with a diameter of φ2 mm, an Ar gas pressure of 600 Pa, and a high-frequency applied voltage of 25 W were used to measure the amount of Mn contained in the scale.

[0042] (2) Also, for each test piece, any location on the carburized surface was measured 5 times with a Hv hardness tester at a load of 300 kgf, and the average value of the measurement results was taken as the surface hardness (Vickers hardness) of the test piece after carburization.

[0043] (3) The test piece after carburization was halved, embedded in a conductive resin so that the cut surface appeared on the surface, and polished. Then, using EPMA, the carbon amount from the carburized surface to a depth of 500 μm in the depth direction was measured at 10-μm intervals, and the average value was taken as the surface carbon concentration. It was evaluated whether this value satisfied 0.50 - 1.00%.

[0044] Test pieces made of steels with chemical compositions of steel grades A to M were carburized under carburizing conditions 1 to 3, and the above evaluations (1), (2), and (3) were performed on each test piece after carburization. The evaluation results of each test piece are shown in Table 2.

[0045]

Table 2

[0046] For steel grades A to J composed of the steel of the chemical composition of the steel of the present invention, for any test piece under carburizing conditions 1 to 3, the Mn amount in the scale did not exceed 10% at any depth, so carburization inhibition did not occur. Also, the surface hardness after carburization was 700 Hv or more, and the surface carbon concentration was also 0.50 - 1.00%. Therefore, sufficient strength can be obtained by general carburization without finding special carburizing conditions, and the fatigue life can be ensured.

[0047] On the other hand, for steel grades K to M composed of the chemical composition of the comparative steel, most of them had the maximum Mn amount in the scale exceeding 10%. For any test piece exceeding 10%, the surface hardness could not be obtained, and the surface carbon concentration was also insufficient, resulting in inhibited carburization.

[0048] Steel grade K has a Mn content of 0.60% and a Cr content exceeding 5.00%. It is considered that sufficient scales of Cr and Mn are generated, making carburization inhibition likely to occur. Also, steel grade L has a Cr content exceeding 5%. When carburized, Cr oxides are formed on the surface of the steel material, inhibiting the intrusion of carbon and resulting in the inability to obtain hardness. Steel grade M has a Mn content exceeding 0.60%. An Mn-based oxide film is formed on the surface of the steel material, inhibiting carburization regardless of the carburization conditions, resulting in the inability to obtain hardness and a low surface carbon concentration.

Explanation of symbols

[0049] 1 Specimen

Claims

1. By mass, C: 0.10 to 0.32%, Si: 0.20 to 0.80%, Mn: 0.20 to 0.60%, P: ≤0.030%, S: ≤0.030%, Cr: 1.60 to 5.00%, Al: 0.003 to 0.050%, N: 0.005 to 0.020%, the balance being Fe and inevitable impurities, in a carburized state, when the region near the surface containing 5% or more of O is taken as the scale thickness from the surface, the amount of Mn contained in the scale is less than 10% at any depth of the scale thickness, the hardness of its surface is 700 Hv or more, and the carbon amount from the surface to a depth of 500 μm is 0.50 to 1.00%. Carburized part.

2. Taking C: 0.10 to 0.32%, Si: 0.20 to 0.80%, Mn: 0.20 to 0.60%, P: ≤0.030%, S: ≤0.030%, Cr: 1.60 to 5.00%, Al: 0.003 to 0.050%, N: 0.005 to 0.020% as the main components by mass, further containing at least one of Nb: 0.02 to 0.10%, Ni: 5.00% or less, Mo: 1.00% or less, Ti: 0.20% or less, B: 0.010 to 0.050% as selectively added components, the balance being Fe and inevitable impurities, in a carburized state, when the region near the surface containing 5% or more of O is taken as the scale thickness from the surface, the amount of Mn contained in the scale is less than 10% at any depth of the scale thickness, the hardness of its surface is 700 Hv or more, and a carburized part in which the carbon amount from the surface to a depth of 500 μm is 0.50 to 1.00%.

Images