Carburized component

Abstract

To provide a carburized component in which a stable carburized layer can be obtained even when a high-Cr material is used.SOLUTION: A carburized component is produced by carburizing a steel material having a chemical composition comprising, 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%, with the balance consisting of Fe and inevitable impurities, and satisfying the condition Cr+9.0Mn<8.6. The carburized component has a surface hardness of 700 Hv or more after carburizing, and a carbon content of 0.50 to 1.00% from the surface to a depth of 500 μm after carburizing.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to carburized parts, and particularly to carburized parts that can suppress the occurrence of carburization inhibition even for high-Cr materials.

Background Art

[0002] Carburizing and quenching is a typical surface hardening treatment for steel materials and is used for carburized parts that require high fatigue strength and wear resistance such as gears and bearings. For such parts, SCM420 and SCR420 defined in the Japanese Industrial Classification (JIS) have usually been used. However, the usage environment of parts has become more severe in recent years, and longer life and higher strength of parts are required.

[0003] In order to meet such needs, 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 inevitable impurities, the steel having a maximum depth of grain boundary oxidation 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 when gas carburized, and having a carburized surface in the state of the remaining carburized abnormal layer, has been proposed as a skin-rolled steel for mechanical structures with excellent anti-pitting characteristics (see Patent Document 1). This proposal is a measure for extending the life by covering the depth of grain boundary oxidation that can be a starting point of pitting with an incomplete quenched layer 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 - 0.35%, Si: 0.25 - 0.80%, Mn: 0.30 - 1.80%, P: 0.030% or less, S: 0.035% or less, Cr: 2.00 - 3.50%, Mo: 0.04 - 0.50%, Al: 0.003 - 0.100%, N: 0.002 - 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 unavoidable 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 - 1.0%, the Ms point is 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 ingress 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] And, in terms of mass%, it contains C: 0.13 - 0.35%, Si: 0.20 - 0.65%, Mn: 0.50 - 1.80%, P: 0.030% or less, S: 0.030% or less, Cr: 2.30 - 3.50%, and further contains one or two selected from Ni: 0.10 - 0.50%, Mo: 0.03 - 0.50%, with the balance being composed of Fe and unavoidable impurities. In the parent phase component 100 - 300 μm from the outermost surface of the steel after carburizing quenching pattern shown in Figure 2 and tempering, the total of Si, Mn, Cr, Ni, and Mo dissolved is 3.0% or more, and the residual γ amount is 20 - 50 vol%, and the rest is a martensite structure. A steel for shafts 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] In addition, in terms of mass%, it contains 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, and the balance is made of alloy steel of iron (Fe) and inevitable impurities. The material having 10 or less oxide-based inclusions with a diameter of 10 μm or more present per 320 mm 2 in area at any cut surface is processed into a predetermined shape and then carburized or carbonitrided and quenched and tempered to obtain a rolling bearing for a wind power generation facility (see Patent Document 4). This is a technique for suppressing the phase change from martensite to ferrite due to hydrogen by increasing the Cr content and achieving high strength.

[0007] In any of the proposals, it is necessary that the Cr content is 1.50% or more. As a measure for achieving long life and high strength of parts in this way, it is considered to contain Cr in an amount equal to or more than that of JIS steel.

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 techniques involve high Cr, when carburized, Cr oxides are formed on the surface of the steel material, which poses a problem that the risk of carbon penetration being inhibited becomes apparent.

[0010] In response to such problems, measures have been provided to render harmless by making the Cr oxide layer less than a predetermined thickness (see Patent Document 5). In addition, measures have also been provided to remove the work-affected layer where Cr enrichment, which causes Cr oxide formation, occurs before carburizing (see Patent Document 6).

[0011] However, in these proposals, in any case, since they are influenced by carburizing 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 not influenced by carburizing conditions, is a high Cr material, and suppresses carburizing inhibition.

Means for Solving the Problem

[0013] As a result of intensive studies to solve the above problems, the inventors of the present invention found that in order to extend the life of the part, the surface hardness and surface carbon content after carburizing satisfy the regulations, and in order to further improve the carburizing characteristics, suppressing the formation of Mn oxides together with Cr oxides, that is, providing a carburized part using a steel material in which the amounts of Cr and Mn satisfy Cr + 9.0Mn < 8.6 (the following formula (A)) is useful for suppressing carburizing inhibition.

[0014] And the first means for solving the problem of the present invention is, By mass, C: 0.10 to 0.35%, 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 unavoidable impurities, A carburized part obtained by carburizing a steel material that satisfies the formula Cr + 9.0Mn < 8.6, The carburized part is characterized in that the surface hardness after carburization is 700 Hv or more, and the carbon content from the surface to a depth of 500 μm is 0.50 to 1.00%.

[0015] The second means is By mass, C: 0.10 to 0.35%, 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 composed of Fe and unavoidable impurities, a carburized part obtained by carburizing a steel material that satisfies the formula Cr + 9.0Mn < 8.6, The carburized part is characterized in that the surface hardness after carburization is 700 Hv or more, and the carbon content from the surface to a depth of 500 μm is 0.50 to 1.00%.

Advantages of the Invention

[0016] According to the present invention, although it is a high-Cr material, it is not easily affected by carburizing conditions, and a carburized part with suppressed carburization inhibition can be obtained without any special device for carburizing conditions. Therefore, a carburized part with an excellent surface hardness of 700 Hv or more after carburization and an appropriate carbon concentration distribution with a carbon content 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] The reasons for defining the chemical composition of the steel material used for the carburized parts of the present invention are described below. The following % is by mass.

[0019] C: 0.10 to 0.35% C is an element necessary to ensure the carburized layer and the core strength after carburizing treatment as parts for mechanical structures. If it is less than 0.10%, the effect cannot be obtained sufficiently. On the contrary, if it exceeds 0.35%, the toughness of the core part will be reduced. Therefore, the content of C is set to 0.10 to 0.35%. Preferably, C is 0.10 to 0.32%, and 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, and if it exceeds 0.80%, the workability will be reduced. Therefore, the content of Si is set to 0.25 to 0.80%. More preferably, Si is 0.35 to 0.65%.

[0021] Mn: 0.20 to 0.60% Mn is the most important element component in the present invention. It is an element necessary for deoxidation during steel melting and is also an element that improves hardenability. If Mn is less than 0.20%, the deoxidation effect is not sufficient, and if it exceeds 0.60%, oxidation scale that may cause carburizing inhibition will be generated. Therefore, the content of Mn is set to 0.20 to 0.60%. More preferably, Mn is 0.20 to 0.40%.

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

[0023] S: ≤ 0.030% S is an inevitable impurity. When it exceeds 0.030%, toughness decreases due to the formation of MnS, and fatigue strength also decreases. Therefore, S should be 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, when Cr is added in excess of 5.00%, a Cr-based oxide film is formed on the surface of the steel material, and regardless of the carburizing conditions, carburization will be inhibited. Therefore, Cr should be 1.60 - 5.00%, and more desirably, Cr should be 1.70 - 3.00%.

[0025] Al: 0.003 - 0.050% Al is an element necessary for deoxidation. However, if the amount of Al is less than 0.003%, its effect cannot be fully obtained. 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 should be 0.003 - 0.050%. More desirably, Al should be 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 this effect, it is necessary to add 0.005% or more. However, when N is added in excess of 0.200%, too much nitride precipitates and fatigue strength decreases. Therefore, the content of N should be 0.005 - 0.200%, and more desirably, N should be 0.050 - 0.150%.

[0027] Ni: 5.00% or less Ni is one of the selectable elements. Ni is an element effective in increasing the hardenability of steel, but it is expensive, so its content minimization is required industrially. Therefore, the content of Ni should be 5.00% or less.

[0028] Mo: 1.00% or less Mo is one of the selected elements. 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 Mo content should be 1.00% or less.

[0029] Nb: 0.02 - 0.10% Nb is one of the selectively added components. It is an element that forms carbonitrides with C and N, and 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, the Nb content is set to 0.02 - 0.10%.

[0030] Ti: 0.20% or less Ti is one of the selectively added 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 Ti content should be 0.20% or less.

[0031] B: 0.010 - 0.050% B is one of the selectively added components. B has the function of improving hardenability and improving toughness by inhibiting the grain boundary precipitation of P. To obtain this effect, it is desirable to add B at 0.010% or more. However, if B exceeds 0.050%, the effect will saturate. Therefore, the content of B to be added is 0.010 - 0.050%.

[0032] Cr + 9.0Mn < 8.6 This formula index is for suppressing carburization inhibition during carburizing. If the value of this index exceeds 8.6, when carburized, Mn oxide will be formed together with Cr oxide on the surface of the steel material, so the intrusion of carbon will be inhibited, making it difficult to obtain hardness. When the components of the steel material satisfy Cr + 9.0Mn < 8.6, during carburizing, the occurrence of carburization inhibition can be suppressed regardless of the carburizing conditions.

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

[0034] The hardness of the outermost surface after carburizing is 700 Hv or more If the surface hardness of the carburized component is less than 700 Hv, in carburized components such as gears and bearings, predetermined strength characteristics cannot be obtained, and the life of the component is shortened. Therefore, the hardness of the outermost surface should be 700 Hv or more.

[0035] The carbon content from the surface to a depth of 500 μm after carburizing shall be 0.50% to 1.00% If the carbon content from the surface to a depth of 500 μm after carburizing is less than 0.50%, carburizing cannot be carried out normally, and the fatigue strength is lowered. Also, if the carbon content from the surface to a depth of 500 μm exceeds 1.00%, too much carbide precipitates and the effect saturates. Therefore, the carbon content from the surface to a depth of 500 μm after carburizing shall be 0.50% to 1.00%.

[0036] (Example) 100 kg steel ingots of steel types A to P having the chemical compositions shown in Table 1 were each melted in a vacuum melting furnace. Then, each of these steels A to P 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.

[0037] [Table 1]

[0038] These test specimens were processed into the dimensions shown in FIG. 1 and gas carburized under three conditions with different carburizing conditions shown in FIG. 2 to obtain test pieces. Here, carburizing condition 1 is a commonly used carburizing condition, carburizing condition 2 has a lower carburizing temperature, and carburizing condition 3 has a shorter carburizing time set.

[0039] <Evaluation item> Regarding the characteristics of the specimens after carburizing, (1) the surface hardness after carburizing (measured using a Hv hardness tester) and (2) the surface carbon concentration after carburizing (the specimen was cut out after carburizing and measured using an electron probe microanalyzer (EPMA)) were evaluated. The detailed measurement methods are described below.

[0040] (1) For the above specimens, any location on the carburized surface was measured 5 times with a load of 300 kgf using a Hv hardness tester, and the average value of the measurement results was taken as the surface hardness (Vickers hardness) after carburizing of the specimen.

[0041] (2) The specimen after carburizing 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 determined whether this value satisfied the range of 0.50 to 1.00% which is the range of the claim regarding the carbon amount.

[0042] Specimens made of steels with chemical compositions of steel types A to P were carburized under carburizing conditions 1 to 3, and for each specimen after carburizing, the evaluations in (1) and (2) above were performed. The evaluation results of each specimen are shown in Table 2.

[0043]

Table 2

[0044] For steel types A to J composed of the steel of the chemical composition of the steel of the present invention, for any specimen under carburizing conditions 1 to 3, no carburizing inhibition occurred, the surface hardness after carburizing was 700 Hv or more, and the carbon amount from the surface to a depth of 500 μm was 0.50 to 1.00%.

[0045] On the other hand, for steel types K to M composed of the chemical composition of the comparative steel, depending on the carburizing conditions, the surface hardness was not obtained, and the carburizing was inhibited such as the carbon amount from the surface to a depth of 500 μm being insufficient. Steel grade K has a Mn content exceeding 0.60% and a Cr content also exceeding 5.00%. It is considered that this is because the oxidation scales of Mn and Cr are generated, making carburization inhibition likely to occur. Also, steel grade L has a Cr content exceeding 5%, and when carburized, Cr oxides are formed on the surface of the steel material, so the intrusion of carbon is inhibited, and it is considered that hardness could not be obtained. Steel grade M has a Mn content of 0.95% which exceeds 0.60%. A Mn-based oxide film is formed on the surface of the steel material, and regardless of the carburization conditions, carburization was inhibited, so it is considered that hardness could not be obtained and the surface carbon concentration was also low.

Explanation of symbols

[0046] 1 Specimen

Claims

1. In 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%, and the balance being Fe and unavoidable impurities; A carburized part made of a steel material satisfying the formula Cr+9.0Mn<8.6, A carburized part characterized in that the surface hardness after carburizing is 700Hv or more, and the carbon content from the surface to a depth of 500μm is 0.50-1.00%.

2. In 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%, Further, as optional additional components, 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, and B: 0.010 to 0.050% is contained; The balance is Fe and unavoidable impurities, A carburized part made by carburizing a steel material satisfying the formula Cr + 9.0Mn < 8.6, A carburized part characterized in that the surface hardness after carburizing is 700Hv or more, and the carbon content from the surface to a depth of 500μm is 0.50-1.00%.

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