JP3385722B2 - Carburizing and quenching method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for carburizing steel parts such as gears and bearings which require surface hardening. 2. Description of the Related Art There is a strong demand for weight reduction and long life of mechanical structural parts such as gears and bearings. Since these parts are characterized by high surface pressures, they are often subjected to surface hardening, especially carburizing. [0003] In recent years, in order to meet the above-mentioned demand, a new treatment method for dispersing cementite by improving the conventional carburizing treatment method for converting the surface into high carbon martensite for the purpose of obtaining higher surface hardness has been proposed. . In the conventional carburizing method, carbon is infiltrated from the atmosphere in the austenite region of the steel below the solid solubility limit (carbon concentration at which cementite precipitates) from the atmosphere. Let it penetrate more. When a new carburizing method is applied, the surface hardness is further increased due to the dispersion of cementite, and improvement in wear resistance and rolling fatigue life can be achieved. For example, JP-A-61-
Japanese Patent Application Laid-Open No. 104065 and Japanese Patent Application Laid-Open No. 2-107755 are inventions based on this concept. However, the carburizing method for precipitating cementite has the following two problems. (1) In cementite precipitation carburization, carbon diffuses toward the center of the base material while precipitating cementite. However, since the diffused carbon is consumed for the precipitation of cementite, a predetermined carburization depth is required. It takes a long time to obtain this. (2) Cementite tends to aggregate and coarsen, and in addition, tends to precipitate at grain boundaries during carburization in the austenite temperature range. Such cementite coarsened or precipitated at the grain boundary contributes to an increase in hardness, but has a bad influence on the toughness of the hardened layer. As one method of shortening the time required for carburizing, there is a method of treating at a high temperature as disclosed in, for example, Japanese Patent Application Laid-Open No. 57-5861. The present invention is a method of carburizing carbon at a temperature of 950 to 1150 ° C. above the austenite solid solubility limit and then cooling, and is characterized by the precipitation of cementite. However, when cementite is precipitated in such a high temperature region (austenite + cementite two phase region),
As described above, coarse rod-like carbides are formed, and the particle size is 1 μm.
It is difficult to obtain cementite below m. [0009] The present invention has been made to solve such a problem. An object of the present invention is to provide a method of carburizing steel capable of shortening the carburizing time in cementite precipitation carburization and uniformly dispersing fine cementite in austenite matrix. The gist of the present invention lies in the following carburizing method. C: 0.05-1.2% by weight, Si: 0.05-%
2%, Mn: 0.3-3%, Cr: 0.1-5%, Ni: 0-3%
And Mo: a steel containing 0 to 2%, in a temperature range of 1000 ° C. or higher and lower than the eutectic point, the lower limit of the maximum value of the carbon concentration distribution on the surface of the carburized layer is set to 0.8% or more, and Increase the amount, also the upper limit, to less than the amount of carbon on which cementite precipitates (Acm line), then cool the carburized layer to at least 500 ° C or less at a cooling rate of 10 ° C / sec or more, and then reheat to _one point or more of Ac A method of carburizing steel, wherein cementite grains having a diameter of 1 μm or less are precipitated on the surface by 10% or more by volume ratio by quenching. In the above steel, Ni and Mo may not be added, and one or two of them may be added as necessary. The lower limit when these are added positively is 0.5% for Ni.
% And 0.1% with Mo. As a basic fact, the diffusion rate increases not only with the diffusion of carbon in steel but also with the increase in temperature. In the austenitic region of steel, the higher the temperature, the higher the solid solubility limit of carbon, and the maximum at the eutectic temperature. The present inventor has further studied in view of these basic facts, and has obtained the following new findings. (1) A high-carbon austenite matrix is generated by utilizing the increase in the amount of carbon solid solution at a high temperature. Thereafter, cooling is performed while preventing precipitation of pro-eutectoid cementite during cooling to generate a supersaturated structure (martensite or bainite, or a mixed structure thereof). By reheating this structure and then quenching, it is possible to generate an extremely fine and uniform dispersed cementite, which has never been seen before. In order to finely disperse cementite, it is important to prevent precipitation of proeutectoid cementite. In addition, it is also important to increase the amount of solute carbon in the austenite matrix. The amount of solute carbon after carburizing is at least 0.8% by weight.
% Or more is required. (2) By utilizing the high temperature treatment and the fact that cementite is not precipitated during carburization, it is possible to carry out the treatment not only by conventional carburization of cementite but also at higher speed than ordinary carburization. (3) By uniformly dispersing the fine cementite in the surface layer, it is possible to impart not only surface hardness but also toughness. First, the reasons for limiting the chemical composition of the steel (base material) and the structure of the product steel as the object of the method of the present invention as described above will be described. % Means% by weight. (1) C: 0.05 to 1.2% C has an effect of improving the strength of the base material and an effect of reducing the toughness of the base material. If it is less than 0.05%, the strength as a mechanical component is insufficient, so the lower limit was made 0.05%. On the other hand, 1.2
%, The toughness of the base material is greatly reduced, and it becomes difficult to melt the steel due to the problem of center segregation. Therefore, the upper limit is set to 1.2%. Desirable is 0.10-0.30%. (2) Si: 0.05 to 2% Since Si is used as a deoxidizing agent when smelting steel, at least 0.05% is contained. On the other hand, if it exceeds 2%, the machinability and workability of the base material deteriorate, so the upper limit was made 2%. Desirable is 1% or less. (3) Mn: 0.3-3% Mn has the effect of improving the hardenability of steel. In the present invention, since it is indispensable to harden the carburized layer and the base material by reheating and quenching, it is also necessary to ensure the hardenability of steel. To secure this, Mn of at least 0.3% is required.
On the other hand, if it exceeds 3%, the machinability and workability of the base material deteriorate, so the upper limit was made 3%. Desirable is 2% or less. (4) Cr: 0.1 to 5% Cr has an effect of improving the hardenability of steel. In addition, there is an effect of refining cementite by reheating.
To achieve these effects, a minimum Cr content of 0.1% is required. On the other hand, even if it exceeds 5%, the effect of refining cementite does not improve remarkably, so the upper limit was made 5%. Desirable is 4% or less. (5) Ni, Mo: The upper limit of Ni is 3%, and the upper limit of Mo is 2%. In the steel to be subjected to the method of the present invention, one or two of Ni and Mo are contained in addition to the above components. You may. These have an effect of improving the toughness and hardenability of the matrix of the base material and the hardened layer, and are added according to the purpose. The lower limit for positive addition is 0.5% for Ni and 0.5% for Mo.
It is desirable to set it to 0.1%. However, Ni is 3%, Mo is 2
%, The machinability of the base material deteriorates. Therefore, the upper limits are set to 3% and 2%, respectively. Desirable upper limits are 2% for Ni and 1% for Mo. (6) Other elements: When using aluminum-killed steel, Al as a deoxidizing agent, S, Pb, etc. for improving machinability may be added as necessary. In addition, P, N, O (oxygen) and the like are included as inevitable impurities. (7) Particle size and amount of cementite grains on the surface of the product steel The particle size of cementite formed in the hardened layer on the surface is 1 μm.
If it is not finer than m, the toughness will be significantly reduced. For example, when an impact is applied, cracks may occur. The amount of cementite is a factor that affects the characteristics of cementite-precipitated carburized steel, such as an increase in surface hardness and an improvement in wear resistance and heat resistance. Precipitated so as to have a volume ratio of 10% or more,
Unless uniformly dispersed, the above characteristics cannot be exhibited, so the lower limit of the amount of cementite was set to 10% by volume. A desirable upper limit is also 30%. Next, the carburizing method of the present invention will be described in detail. According to the method of the present invention, the lower limit of the maximum value of the carbon concentration distribution on the surface of the carburized layer is set to 0.8% or more in a temperature range of 1000 ° C. or higher and lower than the eutectic point. And the upper limit is also increased to less than the carbon amount (Acm line) on which cementite precipitates. Next, the carburized layer is cooled to at least 500 ° C. at a cooling rate of 10 ° C./sec or more. Next, it is reheated to _one or more Ac and quenched. The surface is treated with a diameter of 1 μm
The following cementite grains are precipitated in a volume ratio of 10% or more. (1) Carburizing temperature One of the objects of the present invention is to achieve a shortened carburizing time. For this purpose, it is effective to increase the processing temperature. The lower limit of the carburizing temperature was set to 1000 ° C., which can reduce the time by 50% or more as compared with the conventional case (925 to 950 ° C.). On the other hand, the processing temperature must not be higher than the eutectic point of the steel. The reason is as follows. In general, carburizing is divided into a carburizing period and a diffusion period. In the carburizing period, the C concentration on the surface is increased, and in the diffusion period, the C concentration is diffused inside to obtain a predetermined carburizing depth. In the carburizing period, the carbon concentration may be excessive with respect to the carbon solid solubility limit of the austenite matrix for the purpose of increasing the surface carbon concentration. In this case, cementite precipitation occurs at a normal processing temperature, and this can be eliminated by diffusion. However, when the processing temperature is increased, the excess carbon concentration portion is liquefied, and the base material undergoes a shape change. Such liquefaction occurs in the temperature range above the eutectic point of steel. This eutectic point is about 1150 ° C. for the Fe—Fe ₃ C system, and there is a slight change due to the addition of alloying elements. From the viewpoint of shortening the carburizing time, it is ideal that the carburizing treatment is performed at a temperature just below the eutectic point. However, in consideration of variation during mass production, a desirable upper limit temperature is about −30 ° C. The advantages of high-temperature carburizing include shortening of carburizing time,
That is, the amount of carbon on the surface can be increased. The higher the temperature, the higher the solid solubility of C in the austenitic base,
Austenite having a higher carbon concentration can be obtained on the surface, and a large amount of cementite can be generated by quenching after reheating. (2) Surface Carbon Concentration Carbon on the surface has the effect of increasing the surface hardness obtained by reheating and quenching performed after carburization. In addition, it has the effect of finely and uniformly dispersing cementite precipitation after reheating and quenching. In order to obtain sufficient surface hardness and uniform dispersion of fine cementite as a carburized steel, it is necessary to secure a minimum concentration of 0.8% of solute carbon in the austenite matrix on the surface after carburization. The upper limit of the carbon content of the target base material of the method of the present invention is 1.2%. Even when the base material carbon content is 0.8% or more, the method of the present invention allows sufficient surface hardening.
If the base material carbon amount exceeds 0.8%, the surface carbon concentration is adjusted to at least the base material carbon amount. On the other hand, after the carburizing treatment, the surface solid solution carbon concentration must be lower than the solid solubility limit of austenite. That is, when the concentration of the solute carbon exceeds the austenitic solid solubility limit and becomes excessive, cementite precipitates. Cementite precipitated from austenite during carburization is coarse. Coarse cementite grains of about 2 to 3 μm are formed even in a low-temperature carburized region where the grain size can be relatively reduced, and may be formed at austenite grain boundaries, thereby deteriorating the toughness of the hardened layer. One of the features of the method of the present invention is that there is no coarse cementite which is easily formed during carburization. For this reason, it is necessary to dissolve the carbide during the diffusion period to make the concentration of the solid solution carbon at the surface portion equal to or lower than the austenite solid solubility limit. As an effect of controlling the concentration of solute carbon as described above, there is also an effect of refining old austenite grains. By dissolving 0.8% or more of carbon in solid solution and reheating and quenching, fine cementite of 1 μm or less can be obtained, but at the same time, prior austenite grains on the surface become extremely fine. Steel alloying elements, carbon content, reheating temperature,
Although it depends on the holding time, the crystal grains can be reduced to 10 μm or less. When the prior austenite grains are refined, the martensite is also refined, and the fracture unit is reduced, thereby improving the toughness. As a carburizing furnace used for the carburizing treatment, a gas carburizing furnace is currently industrially widespread, but the problem of this furnace is that the oxygen potential of the atmosphere is high. If the oxygen potential is high, alloying elements are preferentially oxidized, and the carburizing property of the steel, particularly on the surface, changes. In the method of the present invention, since atmosphere control is required to prevent the precipitation of cementite during carburizing treatment, preferential oxidation of alloying elements during treatment in a gas carburizing furnace and a change in carburizability of the steel surface due to this, It makes it difficult to control the desired carburization. As the carburizing furnace, a type of furnace capable of controlling the oxygen potential of the atmosphere to an extremely low level, such as a plasma carburizing furnace, is suitable. (3) Coarse cementite does not exist at the end of carburizing due to the atmosphere control during the cooling carburizing treatment or the control and the dissolution of C into the central part of the base material during the diffusion period. In order to maintain this state, precipitation of proeutectoid cementite in the austenite region must be prevented even during the cooling process. Cementite precipitated during the cooling process is also coarse if precipitated in the austenite region, and deteriorates the toughness of the hardened layer like cementite precipitated during carburization. Therefore, it is necessary to cool at least the carburized layer at a cooling rate of 10 ° C./sec or more in order to prevent the formation of proeutectoid cementite during cooling. A desirable upper limit is 100 ° C./sec. One of the goals of the present invention is the uniform precipitation of fine cementite. Fine cementite can be obtained by reheating and quenching. For that purpose, it is necessary to form a supersaturated structure during the cooling. The supersaturated structure is specifically martensite or bainite, or a mixed structure thereof. In order to form these structures, it is necessary to cool at the above-mentioned rate conditions at least until the carburized layer becomes 500 ° C. or less. The above cooling condition only needs to be satisfied for at least the carburized layer, and there is no particular limitation on the product steel as a whole. However, it is desirable that the cooling rate of the base material portion be as low as possible in consideration of heat treatment distortion. (4) Since reheating and high-temperature carburizing are performed, the structure after cooling is coarser in both the carburized layer and the base material than in the original base material, and the toughness is reduced. Therefore, it cannot be used as a mechanical part in this state. Reheating has the effect of making the structure of the coarse-grained carburized layer and the base metal finer. It also has the effect of precipitating fine cementite from the supersaturated structure. In order to harden the base sufficiently by quenching in the next step, the reheating temperature is A
c Must have at least _one point. A desirable upper limit is 900 ° C. The heating time is not particularly limited, but the heating time should be such that no ferrite (martensite, bainite) of the prestructure remains. If the pre-structure remains, it causes a variation in hardness and adversely affects fatigue life and the like. (5) Quenching is performed to increase the hardness of the quenched carburized layer and the base material. In order to obtain sufficient strength and toughness after quenching, both the hardened layer and the base metal are mainly composed of martensite, and fine cementite with a diameter of 1 μm or less is uniformly dispersed and precipitated in the hardened layer. This is the organization described above. The quenching method is not particularly limited. EXAMPLES (Test 1) Table 1 shows the chemical composition of the test steel. A-1 to A
For -3 steel, hot forging material is normalized, A-1 steel and A-3 steel remain as-normalized, and A-2 steel is further spheroidized and then machined to have a diameter of φ20 mm and a length of 30 mm. Was manufactured. These test pieces were carburized and reheated and quenched under the conditions shown in Table 2, and the structure was observed, the hardness was measured, the C concentration was analyzed, and the amount of cementite was measured. The results are shown in Table 2. [Table 1] [Table 2] However, in the conventional carburizing treatment of the A-3 steel, the C concentration on the surface is first increased to about 0.8% by carburizing for 120 minutes, then cooled to _three points or less of Ar, and then heated again.
Carburizing was performed for 120 minutes. All carburizing furnaces use a plasma carburizing furnace,
Propane and hydrogen were used as atmosphere gases. In the evaluation of the hardened layer, the hardening depth was used, and the depth at which Hv550 was exhibited in the hardness distribution was used as a reference. As is apparent from Table 2, in the example of the present invention, a carburizing time of 40 minutes can obtain a deeper hardening depth than the conventional carburizing of 2 hours. In the conventional example, since the precipitated cementite is generated during carburization in the austenite region, many of the precipitated cementite are present at the austenite grain boundaries or are coarsened. Since the dissolved high-concentration carbon is rapidly precipitated by reheating and quenching, it becomes finer than 1 μm. Examples of these structures are shown in FIG. 1 and FIG. FIG. 1 is a diagram showing an example of the surface microstructure in the case of the present invention. The steel A-1 in Table 2 has a cementite particle size of 0.9 μm, a surface carbon concentration of 1.7%, and an amount of cementite of 12 vol.%. As shown in the figure, it can be seen that the fine cementite is uniformly dispersed in the matrix. FIG. 2 is a diagram showing an example of the surface microstructure in the case of the conventional example. This is that of steel A-3 in the conventional example shown in Table 2. It can be seen that fine cementite uniformly dispersed in the matrix does not exist. (Test 2) Hot forging materials of A-4 to A-12 steels shown in Table 1 were normalized, and A-5 and A-6 steels were further subjected to spheroidizing annealing. As it is,
A test piece having a diameter of φ20 mm and a length of 30 mm was manufactured by machining. These specimens were carburized and reheated and quenched,
The same survey was conducted. Table 3 shows the results. [Table 3] The processing conditions at this time were as follows. Carburizing treatment-cooling conditions: carburizing furnace: plasma carburizing furnace atmosphere: propane, hydrogen carburizing temperature: 1130 ° C., holding time: 40 minutes (carburizing: 20 minutes, diffusion: 20 minutes) cooling rate: 50 ° C./sec Oil-cooled reheating and quenching conditions up to 50 ° C: Heating temperature: 950 ° C, holding time: 30 minutes, then oil-cooling to 50 ° C As shown in Table 3, when all the conditions specified in the present invention are satisfied In this case, a good surface-hardened layer in which fine cementite having a particle size of 1 μm or less is dispersed can be obtained. According to the carburizing method of the present invention, fine cementite having a particle size of 1 μm or less can be precipitated in a uniform dispersion state on the surface in a short time, and the carburized surface layer after carburizing can be deposited. Has good toughness. INDUSTRIAL APPLICABILITY The method of the present invention is suitable as a surface hardening method for steel parts for machine structures such as gears and bearings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing an example of a surface microstructure obtained by the method of the present invention. FIG. 2 is a diagram showing an example of a surface microstructure obtained by a conventional method.

Claims (1)

(57) [Claims] [Claim 1] C: 0.05-1.2%, Si: 0.05-% by weight
2%, Mn: 0.3-3%, Cr: 0.1-5%, Ni: 0-3%
And Mo: 0% to 2% of steel, in a temperature range of 1000 ° C. or higher and lower than the eutectic point, the lower limit of the maximum value of the carbon concentration distribution on the surface of the carburized layer is 0.8% or higher, and the carbon content of the base metal is exceeds the amount, also limit the increase to less than the amount of carbon cementite precipitates (Acm line), then cooled at least carburized layer to 500 ° C. or less at 10 ° C. / sec or more cooling rate, re-heated above the Ac ₁ point A method of carburizing steel, characterized in that cementite grains having a diameter of 1 μm or less are precipitated on the surface by 10% or more by volume ratio by quenching.