JP2022184163A - Carburized component - Google Patents

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

The present invention relates to carburized parts. In particular, the present invention relates to a carburized part capable of suppressing the occurrence of carburization inhibition while using a high Cr steel material.

Carburizing and quenching is one of the typical surface hardening treatments for steel materials, and is used for carburized parts such as gears and bearings that require high fatigue strength and wear resistance. Steels such as SCM420 and SCR420 defined by Japanese Industrial Standards (JIS) are generally used as steel materials for such parts. However, since the environment in which parts are used these days is becoming more severe, there is a demand for further extension of the service life and higher strength of carburized parts.

Therefore, in mass%, C: 0.10 to 0.35%, Si: 0.40 to 0.80%, Mn: 0.15 to 1.50%, P: 0.030% Steel containing S: 0.030% or less, Cr: 1.20 to 2.50%, Ni: 0.20% or less, Mo: 0.10% or less, and the balance being Fe and inevitable impurities When the steel is gas carburized, the maximum grain boundary oxidation depth D ₁ is 10 μm or less, the maximum depth D ₂ of the incompletely hardened layer, which is an alloy deficient layer, is 8 to 20 μm, and D ₂ −D ₁ is There has been proposed a case-hardening steel for machine structures having excellent pitting resistance, characterized by having a carburized surface with a thickness of 2 to 15 μm and an abnormal carburized layer remaining (see Patent Document 1).
In this proposal, the incompletely quenched layer, which is softer than martensite, is covered to the depth of the grain boundary oxidation, which can be the starting point of pitting. It is a policy.

Also, in mass%, 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: 2.00-3.50%, Mo: 0.04-0.50%, Al: 0.003-0.100%, N: 0.002-0.050% and Si + 0.5 Cr ≥ 1.5 and the total amount of Si, Cr and Mo is 3.0% or more, the balance being Fe and inevitable impurities, carburizing treatment or carbonitriding treatment and quenching and tempering treatment The C concentration at 20 μm from the surface is 0.7 to 1.0%, Ms ≤ 215 ° C. or less, and the amount of residual γ is 20 ≤ γ ≤ 50% by volume%. A case hardening steel for gears has been proposed that has excellent pitting resistance when used in a hydrogen infiltration environment (see Patent Document 2).
This is a measure for stabilizing the residual γ that suppresses the diffusion rate of hydrogen, which causes embrittlement, and improving the pitting resistance of the gear.

Also, in mass%, 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 one or two selected from Ni: 0.10 to 0.50% and Mo: 0.03 to 0.50% and the balance being Fe and unavoidable impurities, the carburizing and quenching pattern shown in FIG. The total content of Cr, Ni, and Mo is 3.0% or more, the amount of residual γ is 20 to 50 vol%, and the remainder is a martensitic structure. It has been proposed (see Patent Document 3).
This is a technique for improving the spalling life of bearings by suppressing the change in white structure caused by hydrogen in a hydrogen environment.

Also, in terms of 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, and the balance is iron ( Fe) and an alloy steel that is an inevitable impurity, and has 10 or less oxide-based inclusions with a diameter of 10 μm or more per 320 mm 2 of an arbitrary cut surface, is processed into a predetermined shape, and then carburized or A rolling bearing for wind power generation equipment obtained by carbonitriding and quenching and tempering has been proposed (see Patent Document 4).
This is a technique for suppressing structural change from martensite to ferrite due to hydrogen by increasing Cr to achieve high strength.

Both proposals require Cr to be added in an amount greater than 1.20%. As a measure for extending the service life and increasing the strength of parts in this way, it is intended to make the Cr content more than the steel specified in JIS.

JP 2016-98426 A Japanese Patent No. 6347926 Japanese Patent No. 6639839 Japanese Patent No. 5982782 Japanese Patent No. 6308382 Japanese Patent No. 4327812

However, since both of these proposed technologies use high Cr, Cr oxides are likely to be formed on the surface of the steel material during carburization, and the risk of impeding the penetration of carbon during carburization becomes apparent. be.

Therefore, a measure has been proposed to make the Cr oxide layer harmless by making it less than a predetermined thickness (see Patent Document 5). Further, there is also proposed a method of removing a work-affected layer in which Cr is concentrated, which is the cause of Cr oxide formation before carburizing (see Patent Document 6).

However, in any case, these proposals depend on the carburizing conditions, so it is difficult to say that the inhibition can be completely avoided, and the current situation is that a sufficient improvement method has not yet been established.

Therefore, the problem to be solved by the present invention is to provide a carburized part that is not influenced by carburizing conditions and that is made of a high-Cr material and that suppresses carburization inhibition.

In order to solve the above problems, the inventors of the present invention conducted intensive studies and found that, together with Cr, oxides of Mn have an adverse effect on the carburization characteristics, and that the effect on the carburization characteristics is greater than that of Cr oxides. There was found.
When the composition of the steel is within the range of the composition of the present invention and low Мn, Mn oxide, which is particularly harmful to the carburizing characteristics, is less likely to form during carburizing. It has been found that it is possible to provide a carburized part stably meeting the requirements for surface hardness and surface carbon content after carburizing for hardening.

Therefore, the first means for solving the problems 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 unavoidable impurities, in a carburized state There is
When a region near the surface containing 5% or more of O 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 hardness of the surface is 700Hv or more,
It is a carburized part with a carbon content of 0.50 to 1.00% from the surface to a depth of 500 μm.

The 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 main components,
Furthermore, as selective additional components, 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%, containing at least one
The remainder consists of Fe and unavoidable impurities,
in a carburized state,
When a region near the surface containing 5% or more of O 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 hardness of the surface is 700Hv or more,
It is a carburized part with a carbon content of 0.50 to 1.00% from the surface to a depth of 500 μm.
50 to 1.00%.

According to the present invention, even though it is a high Cr steel material, by suppressing the formation of Mn oxide, it is not easily affected by carburizing conditions, and even if the carburizing conditions are not specially devised, under general carburizing conditions, , a carburized part in which inhibition of carburization is suppressed can be obtained.
Therefore, according to the present invention, it is possible to obtain a carburized part having an excellent surface hardness of 700 Hv or more after carburization and an appropriate carbon concentration distribution in which the carbon content is 0.50 to 1.00% from the surface to a depth of 500 μm. be able to.

It is explanatory drawing of the test piece used for carburizing evaluation. A front view is shown in FIG. 1(a), and a side view is shown in FIG. 1(b). FIG. 2 is a temperature-time process chart showing an example of carburizing conditions. The heat treatment procedures under the carburizing conditions 1, 2 and 3 used in the examples are shown in order from the top.

Prior to describing the embodiments of the present invention, the reasons for specifying the chemical composition of the steel material used for the carburized parts of the present invention will be described below. The following percentages are percentages by mass.

C: 0.10-0.40%
C is an element necessary for securing the strength of the carburized layer and the core after the carburizing treatment as a mechanical structural part. If it is less than 0.10%, the effect is not sufficiently obtained, and on the contrary, if it exceeds 0.40%, the toughness of the core is lowered. Therefore, the C content is set to 0.10 to 0.40%. More preferably, C is 0.15-0.30%.

Si: 0.20-0.80%
Si is an element necessary for deoxidation during steel smelting, and is an element that has the effect of improving hardenability. If it is less than 0.20%, the deoxidizing effect is not sufficient, and if it exceeds 0.80%, workability is lowered. Therefore, the Si content is set to 0.20 to 0.80%. More preferably, Si is 0.35 to 0.65%.

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 smelting and an element that improves hardenability. If the Mn content is less than 0.20%, the deoxidizing effect is not sufficient, and if it exceeds 0.60%, scale that may inhibit carburization is generated. Therefore, the content of Mn is set to 0.20 to 0.60%. More preferably, Mn is 0.20 to 0.40%.

P: ≤ 0.030%
P is an unavoidable impurity, and if it exceeds 0.030%, grain boundary segregation will lower the toughness. Therefore, P is set to 0.030% or less.

S: ≤ 0.030%
S is an unavoidable impurity, and if it exceeds 0.030%, the formation of MnS reduces toughness and fatigue strength. Therefore, S is set to 0.030% or less.

Cr: 1.60-5.00%
Cr is an element that improves hardenability. 1.60% or more of Cr must be added to ensure the hardenability of the steel. However, when Cr is added in excess of 5.00%, a Cr-based oxide film is formed on the surface of the steel material, impeding carburization regardless of the carburization conditions. Therefore, Cr should be 1.60 to 5.00%, more preferably 1.70 to 3.00%.

Al: 0.003-0.050%
Al is an element necessary for deoxidation. However, if the amount of Al is less than 0.003%, the effect is not sufficiently obtained, and if the amount of Al added is increased, the amount of alumina-based inclusions formed in the steel increases, thereby lowering the fatigue strength. Therefore, the Al content is set to 0.003 to 0.050%. More preferably, Al is 0.010 to 0.030%.

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. In order to obtain these effects, N must be added in an amount of 0.005% or more. However, if N is added in excess of 0.200%, nitrides are precipitated excessively and the fatigue strength is lowered. Therefore, the content of N is set to 0.005 to 0.200%. More preferably, N is 0.050 to 0.150%.

Optional additional ingredients are described below.

Nb: 0.02-0.10%
Nb is one of the optional additional components. It is an element that forms carbonitrides with C and N and improves fatigue strength by refining crystal grains due to the pinning effect. However, if the Nb content is too high, the toughness of the steel will decrease. Therefore, when Nb is added, it should be 0.02 to 0.10%.

Ni: 5.00% or less Ni is one of the optional additional components. Although Ni is an effective element for enhancing the hardenability of steel, it is expensive, so minimization of its content is required industrially. Therefore, the addition of Ni is made 5.00% or less.

Mo: 1.00% or less Mo is one of the optional additional components. Mo is an element effective in improving hardenability and toughness. However, an excessive amount of Mo leads to a decrease in workability and an increase in material cost. Therefore, the amount of Mo added is 1.00% or less.

Ti: 0.20% or less Ti is one of optional additional components. Ti forms carbonitrides with C and N, and the pinning effect refines crystal grains, thereby improving the fatigue strength. However, if the Ti content is too high, the toughness of the steel will decrease. Therefore, the addition of Ti is made 0.20% or less.

B: 0.010-0.050%
B is one of the optional additional components. B is an element that has the function of improving hardenability and improving toughness by inhibiting grain boundary precipitation of P. In order to obtain this effect, it is desirable to add 0.010% or more of B. However, when B exceeds 0.050%, the effect saturates. Therefore, the amount of B to be added is 0.010 to 0.050%.

Next, the reasons for specifying the properties of the surface after carburizing of the carburized parts obtained by carburizing the steel material specified in the present invention will be described.

The amount of Mn contained in the scale is less than 10% at any depth of the scale thickness, where the area near the surface containing 5% or more of O is defined as the scale thickness from the surface:
The amount of Mn contained in the scale is an index for inhibiting inhibition of carburization during carburization. If the Mn content of this index exceeds 10%, the penetration of carbon is inhibited by the Mn oxide when the part is carburized, making it difficult to obtain hardness after carburizing. If it is satisfied that the Mn content is 10% or less at any depth inside the scale, carburization inhibition can be achieved without being affected by the carburizing conditions and without using specific carburizing conditions. The occurrence can be suppressed.

The hardness of the outermost surface after carburizing is 700Hv or more:
If the surface hardness of the carburized parts is less than 700 Hv, the carburized parts, such as gears and bearings, cannot obtain the desired strength characteristics, resulting in a shortened service life of the parts. Therefore, the hardness of the carburized outermost surface is defined as 700 Hv or more.

After carburizing, the carbon content from the surface to a depth of 500 μm should be 0.50% to 1.00%:
If the carbon content from the surface to a depth of 500 μm after carburization is less than 0.50%, carburization cannot be performed normally, resulting in low fatigue strength. Also, if the carbon content exceeds 1.00% from the surface to a depth of 500 μm, too much carbide precipitates and the effect is saturated. Therefore, the carbon content from the surface to a depth of 500 μm after carburizing is 0.50% to 1.00%.

(Example)
100 kg steel ingots of steel type names A to M having chemical compositions shown in Table 1 were each melted in a vacuum melting furnace. After that, each of these steels A to M is soaked at 1250°C for 12 hours or more, hot forged to produce a steel bar with a diameter of 32 mm, held at 900°C for 4 hours, and then air-cooled to perform normalizing treatment. Thus, the test material was obtained.

These specimens were machined to the dimensions shown in FIG. 1 and gas carburized under three different carburizing conditions shown in FIG. 2 to obtain test pieces. Here, the carburizing condition 1 is the carburizing condition that is normally used, the carburizing condition 2 is set to a lower carburizing temperature, and the carburizing condition 3 is set to a shorter carburizing time.

<Evaluation items>
Regarding the characteristics of the test piece after carburization, (1) the amount of Mn inside the scale (measured using a glow discharge emission spectrometer) when the thickness of the scale is from the surface to the range where the amount of O is 5% or more. , (2) surface hardness after carburization (measured using an Hv hardness tester), (3) surface carbon concentration after carburization (a test piece is cut out after carburization and measured using an electron probe microanalyzer (EPMA) ) were evaluated respectively. A detailed measurement method will be described below.

(1) The "glow discharge optical emission spectrometer" used to measure the amount of Mn inside the scale is to cause Ar ions applied to a high voltage to collide (sputter) the sample surface and excite the sputtered atoms into a plasma state. By detecting the wavelength and intensity of the light generated by this, it is possible to perform quantitative and qualitative analysis of elements in the depth direction of the sample.
In this example, the as-carburized test piece was set in a glow discharge emission spectrometer, and the sputtering conditions were a copper electrode of φ2 mm, an Ar gas pressure of 600 Pa, and a high-frequency applied voltage of 25 W. The amount of Mn was measured.

(2) In addition, for each test piece, an arbitrary location on the carburized surface was measured five times with a load of 300 kgf using an Hv hardness tester, and the average value of the measurement results was calculated after carburizing the test piece. The surface hardness (Vickers hardness) was used.

(3) The test piece after carburization was cut in half, embedded in a conductive resin so that the cut surface appeared on the surface, and polished. After that, using EPMA, the amount of carbon was measured at intervals of 10 μm from the carburized surface to a depth of 500 μm in the depth direction, and the average value was taken as the surface carbon concentration. Evaluate whether this value satisfies 0.50 to 1.00%.

Test pieces made of steel with chemical compositions of steel types A to M were carburized under carburizing conditions 1 to 3, and the test pieces after each carburization were evaluated in the above (1), (2), and (3). did Table 2 shows the evaluation results of each test piece.

Steel types A to J, which have the chemical components of the steel of the present invention, did not exceed 10% in the amount of Mn in the scale at any depth for any of the test pieces under carburizing conditions 1 to 3, so carburization was inhibited. In addition, the surface hardness after carburizing was 700 Hv or more, and the surface carbon concentration was 0.50 to 1.00%. Therefore, even if no special carburizing conditions are found, general carburizing can provide sufficient strength, ensuring the fatigue life.

On the other hand, most of the steel grades K to M, which have the chemical compositions of the comparative steels, have a maximum Mn amount in the scale of more than 10%, and any test specimen exceeding 10% has no surface hardness. In addition, the surface carbon concentration was insufficient, which hindered carburization.

Steel type K has a Mn content of 0.60% and a Cr content of more than 5.00%, so that Cr and Mn scales are sufficiently formed, and carburization is easily inhibited. I think that the. In addition, steel type L has a Cr content of more than 5%, and when carburized, Cr oxides are formed on the surface of the steel material, which hinders the penetration of carbon, and hardness cannot be obtained. I think that the. In steel type M, the amount of Mn exceeded 0.60%, and an oxide film mainly composed of Mn was formed on the surface of the steel material. The concentration was probably too low.

1 test piece

Claims (2)

% 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 unavoidable impurities, in a carburized state There is
When a region near the surface containing 5% or more of O 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 hardness of the surface is 700Hv or more,
A carburized part having a carbon content of 0.50 to 1.00% from its surface to a depth of 500 μm. % 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 main components,
Furthermore, as selective additional components, 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%, containing at least one
The remainder is composed of Fe and unavoidable impurities, and is in a carburized state,
When a region near the surface containing 5% or more of O 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 hardness of the surface is 700Hv or more,
A carburized part with a carbon content of 0.50 to 1.00% from the surface to a depth of 500 μm.

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