Carburized parts

Abstract

To provide a carburized component in which a stable carburized layer is obtained despite a high Cr material.SOLUTION: A carburized component is produced by carburizing a steel material in which a chemical composition 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%, and N: 0.005 to 0.020%, and the remainder consists of Fe and inevitable impurities, and Cr+9.0Mn<8.6 is satisfied. In the carburized component, the hardness of a surface after carburization is 700Hv or more, and carbon content from the surface after carburization to the 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, and particularly to carburized parts that can suppress the occurrence of carburization inhibition even in 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 incompletely hardened layer that 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 with the remaining state of the carburized abnormal layer, has been proposed (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 incompletely hardened layer softer than martensite and wearing away the grain boundary oxidation together with the incompletely hardened layer.

[0004] Also, in 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 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 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, which causes embrittlement, and improve the anti-pitting characteristics of gears.

[0005] And, in 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 steel composed of Fe and unavoidable impurities. The total of Si, Mn, Cr, Ni, and Mo dissolved in the matrix component 100 - 300 μm from the outermost surface of the steel after the carburizing and quenching pattern shown in Figure 2 and tempering 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 technique 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 inclusions with a diameter of 10 μm or more 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. Thus, as a measure for extending the life and increasing the strength of parts, it is contemplated 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 carburizing, Cr oxide is formed on the surface of the steel material, so there is a problem that the risk of carbon intrusion 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 processed altered layer in which Cr enrichment, which causes Cr oxide formation, occurs before carburizing (see Patent Document 6).

[0011] However, in these proposals, since they are all affected 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 affected 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 further, in order to enhance the carburizing characteristics, suppressing the formation of Mn oxide together with Cr oxide, 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 problems 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%, and the balance is 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 hardness of the surface 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 hardness of the surface 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 contrivance in the 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

Embodiments 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%. 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 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 0.030% or less.

[0023] S: ≤ 0.030% S is an inevitable impurity. If it exceeds 0.030%, the toughness will be reduced due to the formation of MnS, and the fatigue strength will also be reduced. Therefore, S is 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 form on the surface of the steel material, and regardless of the carburizing conditions, carburization will be inhibited. Therefore, Cr is set at 1.60 - 5.00%, and more preferably, Cr is set at 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 is set at 0.003 - 0.050%. More preferably, Al is set at 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, 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 at 0.005 - 0.200%, and more preferably, N is set at 0.050 - 0.150%.

[0027] Ni: 5.00% or less Ni is one of the selected elements. Ni is an element effective in increasing the hardenability of steel, but it is expensive, so minimizing its content is required industrially. Therefore, the content of Ni is set at 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 content of Mo is set at 1.00% or less.

[0029] Nb: 0.02 - 0.10% Nb is one of the selective additive components. It is an element that forms carbonitrides with C and N, and the fatigue strength is improved by refining the crystal grains due to 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 selective additive components. Ti forms carbonitrides with C and N, and the fatigue strength is improved by refining the crystal grains due to the pinning effect. However, if the Ti content is too high, the toughness of the steel will decrease. Therefore, the Ti content is set to 0.20% or less.

[0031] B: 0.0010~0.0050% B is one of the selective additive components. B has the function of improving hardenability and improving toughness by inhibiting the grain boundary segregation of P. To obtain this effect, it is desirable to add B above. However, when B exceeds 0.0010% its effect saturates. Therefore, the content of B added is 0.0050% set to 0.0010~0.0050% and.

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

[0033] Next, the reason for defining the properties of the surface after carburization of the carburized parts made of 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 part is less than 700 Hv, predetermined strength characteristics cannot be obtained in carburized parts such as gears and bearings, and the life of the parts will be 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 should 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 properly and the fatigue strength will be low. Also, if the carbon content from the surface to a depth of 500 μm exceeds 1.00%, too much carbide will precipitate and the effect will saturate. Therefore, the carbon content from the surface to a depth of 500 μm after carburizing should be 0.50% to 1.00%

[0036] (Example) 100 kg steel ingots of steel types A to P with 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 test pieces were obtained by gas carburizing them under three conditions with different carburizing conditions shown in Fig. 2. 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.

[0039] [Evaluation Items] Regarding the characteristics of the test pieces after carburizing, (1) the surface hardness after carburizing (measured using a Hv hardness tester) and (2) the surface carbon concentration after carburizing (the test piece 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 test pieces, any part of the carburized surface was measured 5 times with a load of 300 kgf using an Hv hardness tester, and the average value of the measurement results was taken as the surface hardness (Vickers hardness) after carburization of the test piece.

[0041] (2) 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 determined whether this value satisfied the range of 0.50 to 1.00% in the claims related to the carbon amount.

[0042] Test pieces made of steels with chemical compositions of steel types A to P were carburized under carburizing conditions 1 to 3, and the above evaluations (1) and (2) were performed on each test piece after carburization. The evaluation results of each test piece 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 test piece under carburizing conditions 1 to 3, no carburization inhibition occurred, the surface hardness after carburization 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 carbon amount from the surface to a depth of 500 μm was also insufficient, etc., and carburization was inhibited. For steel type K, the Mn content exceeds 0.60% and the Cr content also exceeds 5.00%. It is considered that this is because the oxide scales of Mn and Cr are generated and carburization inhibition is likely to occur. Also, for steel type L, the Cr content exceeds 5%, and when carburized, Cr oxides are formed on the surface of the steel material, so the intrusion of carbon is inhibited and no hardness is obtained. Steel grade M has an Mn content of 0.95% which exceeds 0.60%. An Mn-based oxide film is formed on the surface of the steel material, inhibiting carburization regardless of the carburizing conditions. As a result, hardness could not be obtained and the surface carbon concentration also seems to have been low.

Explanation of symbols

[0046] 1 Specimen

Claims

【Claim 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 4.92%, Al: 0.003 to 0.050%, N: 0.005 to 0.020%, Furthermore, as a selectively added component, 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: 0.0010 to 0.0050%, the balance being composed of Fe and unavoidable impurities, a carburized component obtained by carburizing a steel material satisfying the formula Cr + 9.0Mn < 8.6, characterized in that the hardness of the surface after carburization is 700 Hv or more and the carbon amount from the surface to a depth of 500 μm is 0.50 to 1.00%.

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