JP2022000540A - Carburization method of steel part and manufacturing method of steel part - Google Patents

Abstract

To provide a carburization method of a steel part capable of obtaining a deep carburization depth in a short time, and a manufacturing method of the steel part.SOLUTION: A carburization agent in which an Fe-C alloy in a power state and a binder composed of a sodium silicate aqueous solution for binding the Fe-C alloy in a powder state together are mixed is caused to contact with at least a part of a surface of a steel part, and the steel part is kept for certain period of time at a temperature range of an austenite range that is higher than an eutectic point of a raw material and less than a peritectic point to form a carburization layer with a carburization depth of 1.9 mm or more.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for carburizing steel parts and a method for manufacturing steel parts.

Generally, in mechanical parts such as gears and bearings, in order to increase the hardness near the surface (for example, at a depth of about 1 mm from the surface) to improve various characteristics such as wear resistance, slidability, and fatigue strength. ,
May be charcoal-burned. Such carburizing methods include solid carburizing, liquid carburizing, gas carburizing,
It is classified as vacuum carburizing, and all of them are treatments in which carbon (C) is invaded from the surface of a heated component in the austenite region. Industrially, most of them are gas hydrocarbons, and a heat-absorbing modified gas is flowed as a carrier gas in a furnace heated to a temperature of point A1 or higher in the Fe-C equilibrium diagram, and propane and butane are further enriched. Etc. to supply hydrocarbon gas. The required carburizing depth is determined according to the size of the part, wear amount and fatigue characteristics, chemical composition of the steel material used, usage conditions of the product, etc., so the carburizing time is the desired depth for each part. It is processed.

On the other hand, in the case of rolling bearings, when they rotate under a load, the raceway surfaces of the inner and outer rings and the rolling surface of the rolling element are constantly repeatedly loaded, and the material fatigue causes scaly peeling damage called flaking. Occurs. In order to secure rolling fatigue strength under a large load, the area to which the load is applied may be wide, for example, a carburizing depth of more than 10 mm is required from the surface of each member (see FIG. 4). .. However, when this part is carburized by gas at 950 ° C., a processing time of 200 to 300 hours is required. Therefore, in the carburizing treatment, an excessive lead time is the biggest production issue, and various studies have been made in order to shorten the carburizing treatment time (see Patent Documents 1 to 3).

Patent Document 1 describes a method of carburizing a work heated to a temperature above the eutectic point and below the peritectic point by supplying a hydrocarbon gas as a carburizing source by using high frequency heating. However, in the temperature range above the eutectic point, when carbon invades beyond the solid solution limit of austenite, a liquid phase is formed on the work surface. Since the liquid phase moves easily due to gravity, there is a limit to maintaining the shape of the work. Further, it is difficult to uniformly adsorb carbon on the work surface using a hydrocarbon gas as a carburizing source, and it is difficult to control the amount of carbon inflow below the solid solution limit.

Reference 2 describes a method in which the surface of a work is coated with carbon, heated to a predetermined temperature, and the carbon is diffused inside. However, the hardened layer obtained by this method is only the surface, and supplies a sufficient amount of carbon source to obtain the wide range and sufficient carburizing depth required for rolling bearings in a short carburizing time. There is a need. Further, Cited Document 2 assumes carburization below the eutectic point, and when applied to a temperature range above the eutectic point, a liquid phase is generated on the work surface in contact with carbon, so that the amount of carbon inflow is limited to a solid solution. It is difficult to control below.

Cited Document 3 describes a carburizing method using iron carbide, which is a compound of Fe and C, as a carburizing source. Carbon is supplied from iron carbides that are in equilibrium with austenite. Further, it is described that iron carbide is produced by mechanical milling of iron powder, Gr (graphite) powder and iron alloy powder to produce a carburizing source agent. However, Cited Document 3 also assumes carburization below the eutectic point, and as described above, when applied to a temperature range above the eutectic point, a liquid phase is generated on the work surface in contact with carbon, so the amount of carbon inflow. Is difficult to control below the solid solution limit.

Japanese Patent No. 3899081 Japanese Unexamined Patent Publication No. 2015-108164 Japanese Unexamined Patent Publication No. 2010-1508

The present invention has been made in view of the above matters, and an object of the present invention is to provide a method for carburizing steel parts and a method for manufacturing steel parts, which can obtain a wide range and sufficient carburizing depth in a short carburizing time. It is in.

The present invention has the following configuration.
(1) A carburizing agent obtained by kneading a powdery Fe-C alloy and a binder composed of an aqueous sodium silicate that binds the powdered Fe-C alloy powder to each other is used as a carburizing agent for at least a part of steel parts. The steel part is kept in the temperature range of the austenite region above the eutectic point and below the peritectic point for a certain period of time to form a carburized layer with a carburizing depth of at least 1.9 mm or more. Carburizing method for steel parts.
(2) A method for manufacturing steel parts having a carburized layer.
A carburizing agent obtained by kneading a powdery Fe-C alloy and a binder composed of an aqueous solution of sodium silicate is brought into contact with the surface of at least a part of the steel parts, and the steel parts are packaged at or above the eutectic point of the material. A method for manufacturing a steel part, which is maintained in a temperature range in the austenite region below the crystal point for a certain period of time to form the carburized layer having a carburized depth of at least 1.9 mm or more.

According to the present invention, it is possible to obtain a steel part having a wide range and a sufficient carburizing depth in a short carburizing time.

It is a Fe-C equilibrium state diagram. FIG. 5 is a conceptual diagram showing a state in which a carburizing agent in a mixed state of a solid-phase Fe—C alloy powder and an aqueous sodium silicate solution as a binder is in contact with the work surface. It is a conceptual diagram which shows the state which carburized is started by further heating from the state shown in FIG. It is a conceptual diagram which shows the load load area and carburizing depth (carburizing layer) of a rolling bearing.

Hereinafter, embodiments of the present invention will be described in detail.
Generally, in the carburizing process in which carbon is infiltrated from the surface of a steel material and diffused into the material, the temporal change in carbon concentration at an arbitrary depth position is expressed by Eq. (1) using Fick's second law. .. According to the equation (1), the diffusion of carbon in the carburizing step can be theoretically explained.

C: Carbon concentration at position x C ₀ : Surface carbon concentration D: Diffusion coefficient of carbon in γ-Fe t: Time

According to the above equation (1), in order to improve the carburizing rate, (1) the surface carbon concentration C ₀ , that is, the carbon potential of the heat treatment atmosphere is improved, or (2) the treatment is performed at a high temperature having a large diffusion coefficient D. Is required.

Practically, carburizing in the temperature range of 930 ° C to 1050 ° C (temperature range of A1 point or more and eutectic point or less in hardened steel and alloy steel for machine structure) is the mainstream, but further processing temperature If high temperature is possible, the carburizing time can be significantly shortened in theory. However, even with this technology, there is a limit to improving the carburizing efficiency (mainly heat treatment in a short time) of parts that require a wide range and sufficient carburizing depth.

Therefore, in order to obtain a carburizing depth of more than 10 mm, preferably more than 15 mm in a short carburizing time, as a result of diligent studies to stably establish a carburizing reaction on the component surface in a temperature range above the austenite. It has been found that a method of heating the work in the austenite region of the solid phase and supplying the carbon source with the Fe—C alloy consisting of the solid phase and the liquid phase is suitable. According to this method, the problem that the carbon concentration on the work surface is carburized above the JE line of the Fe-C equilibrium diagram shown in FIG. 1 to form a liquid layer, that is, excessive carburizing exceeding the solid solution limit is avoided. Then, the dissolution phenomenon can be controlled.

If the work surface is carburized above the eutectic temperature and above the austenite solid solution limit, a liquid phase will be generated on the work surface. Therefore, by controlling the carbon concentration contained in the carburizing agent and the heating temperature, the reaction rate at the interface with the work is controlled.

Here, as the Fe—C alloy used as the carburizing agent, cast iron powder, carbon steel powder, tool steel powder, cast iron grid and the like can be used. The shape of the powder (end) may be granular or needle-shaped. Further, by mixing pure iron powder and Gr (graphite) powder with the Fe—C alloy, it is possible to change the carbon concentration in the Fe—C alloy and control the melting temperature with higher accuracy. The amount of C in the Fe—C alloy powder as the carbon supply source is preferably calculated from the range of the liquid phase appearance region (γ + L) in FIG.

When a powder mainly composed of Fe—C alloy is used as a carburizing agent, it is important not to oxidize it to a high temperature. The powder has a large surface area, and depending on the atmosphere, there is a problem that the melting temperature rises due to the decarburization of carbon in the Fe—C alloy powder, and the carburizing source cannot be supplied by the liquid layer. Such a problem can be solved by treating in an inert gas atmosphere or by mixing an antioxidant such as sodium silicate or calcium sulfate (gypsum) with the Fe—C alloy powder.

Sodium silicate is particularly preferably supplied as an aqueous solution. The sodium silicate aqueous solution exhibits a viscous glassy state up to a high temperature. By kneading the Fe—C alloy powder and the sodium silicate aqueous solution and supplying them to the work surface, decarburization of the Fe—C powder itself can be suppressed.

The second effect of sodium silicate is the ability to form a powdered deoxidizer. The sodium silicate aqueous solution has a function of reacting with _{CO 2 gas in the air and solidifying.} Therefore, it is possible to improve the adhesion between the Fe-C powders and the interface between the Fe-C powder and the work surface. In addition to this, the effect of blocking oxygen can also be obtained.

Further, calcium sulfate (gypsum), like sodium silicate, can improve the adhesion between Fe—C alloy powders and the surface of the work, and can also obtain oxygen blocking performance.

This carburizing method is a method of supplying carbon as an Fe-C alloy in a temperature range above the eutectic point, and the carbon concentration of the work is set to the solid solution limit concentration (J- in the Fe-C phase diagram shown in FIG. 1). E line) It is characterized by being held in the austenite region below. Further, it is characterized in that sodium silicate or calcium sulfate is mixed with the carburizing agent to make the carburizing agent non-oxidized and carburized.

The carburizing method will be conceptually described with reference to FIGS. 2 and 3.
FIG. 2 is a conceptual diagram showing a state in which a carburizing agent in a mixed state of a solid phase Fe—C alloy powder 11 and a sodium silicate aqueous solution 13 as a binder is in contact with the surface of the work W.
The work W is, for example, a steel part made of hardened steel or alloy steel for machine structure.
The sodium silicate aqueous solution 13 binds the Fe—C alloy powders 11 to each other by its own adhesiveness, and also fixes the Fe—C alloy powders 11 to the work surface. Therefore, the carburizing agent is less likely to be peeled off from the surface of the work, and there is no restriction on the application site to the work.

When heated from the state of FIG. 2, as shown in FIG. 3, the carburizing agent melts a part of the Fe—C alloy powder 11 to form a solid phase Fe—C alloy powder 11 and a liquid phase Fe—C alloy. 15 and the aqueous sodium silicate solution 13 as a binder are in a mixed state and come into contact with the surface of the work W. In this case, the Fe-C alloy 15 that has become a liquid phase spreads and contacts the surface of the work, so that the contact area between the carburizing agent and the work W increases, and the carbon supply rate to the work W is significantly improved. .. Moreover, when heated above the eutectic point, decarburization of the Fe—C alloy can be suppressed by the action of the sodium silicate aqueous solution 13, so that carburization of the work surface is promoted. As a result, the increase in the melting temperature of the Fe—C alloy is prevented, and the carburizing source can be easily supplied in the liquid phase.

It is also conceivable that the solid phase Fe—C alloy powder 11 is further liquid-phased from the state of FIG. 3, and the carburizing agent is in a mixed state of the liquid-phase Fe—C alloy 15 and the binder. Even in that case, the sodium silicate aqueous solution 13 as a binder functions as an antioxidant of the Fe—C alloy and can prevent the melting temperature of the Fe—C alloy from rising.

In order to confirm the effect of the above carburizing method, the following experiment was conducted using a test piece simulating the surface layer of the rolling bearing. As an example, SCr420 steel material (carbon 0.18 to 0.23%, median 0.205%, austenite region 880 to 1470 ° C.) of JIS G 4053 (alloy steel material for machine structure) was selected, and the outer diameter was selected. A test piece having a blind hole having a diameter of 20 mm and an inner diameter of φ15 mm was produced.

As the carburizing agent, the carbon material was made into a powder of Fe-3.1% C and kneaded with a 10% by weight aqueous sodium silicate solution. 10 g of this carburizing agent containing 9 g of Fe-3.1% C powder was brought into close contact with the carburized surface of the test piece. After that, carburizing was carried out by heating and holding at 1200 ° C. (intermediate temperature of eutectic point 1147 ° C. and peritectic point 1494 ° C. of SCr420 steel material from the Fe-C equilibrium phase diagram) for 30 minutes in a nitrogen gas atmosphere. Cooled with.

As a comparative example, a test piece simulating the surface layer of a rolling bearing made of SCr420 steel was prepared in the same manner as in the example, and the carbon potential was set to 1.05 in an atmosphere of Rx gas (heat absorbing type modified gas) + enriched gas. After performing gas carburizing for 5 hours at 950 ° C (the temperature of the eutectic point of SCr429 steel material less than 1147 ° C from the Fe-C phase diagram), which is 10 times that of the example, it is oil-cooled and is generally large. Gas carburizing, which is a carburizing process for bearings, was simulated.

For the carburizing effect by the experiment, EPMA analysis was performed on the cross section of each test piece, and the total carburizing depth was obtained from the C% profile. E. The carburizing rate constant K in Harris's empirical formula (x = K√t) was determined.

The results of the above comparative experiments are summarized in Table 1.

In the example, the carburizing rate constant K = 2.69, and it was confirmed that the carburizing was rapid with respect to the carburizing rate constant K = 0.76 in the gas carburizing of the comparative example (conventional method). That is, in the past, it took 5 hours to obtain a carburizing depth of 1.7 mm, but according to this carburizing method, a carburizing depth of 1.9 mm, which exceeds the carburizing depth of the comparative example, is only 0.5 hours. Depth will be obtained.

In the past, at high temperatures, the amount of carbon that dissolves in austenite is large, so solid carburizing tends to result in excessive carburizing. Therefore, in the case of high temperature carburizing, it was limited to gas carburizing in which the carbon potential can be easily controlled. On the other hand, in this carburizing method, by using sodium silicate, it is possible to control the carbon potential at the time of high temperature carburizing in the austenite region with high accuracy. Sodium silicate has been used as a carburizing inhibitor for some time, but there is no other technique for controlling carbon potential using such an anticarburizing agent. This carburizing method is a low-cost and epoch-making carburizing method that utilizes the high high-temperature viscosity of the carburizing inhibitor to keep the carburizing agent in close contact with the work surface during high-temperature carburizing to form a deep carburized layer. be.

Further, when the carburizing agent is in a mixed state of the solid phase and the liquid phase, the work surface and the carburizing agent come into contact with each other in a wide range, and the carburizing can be performed uniformly as compared with the case where all of them are in the solid phase. Therefore, high-quality carburizing can be realized in a short time.

As described above, according to this carburizing method, a high carbon potential state can be maintained as compared with the carbon potential of gas carburizing at the time of high temperature carburizing, for example, in a large rolling bearing that requires a wide range and sufficient carburizing depth. Carburizing is possible in a significantly shorter time than before.

As the sodium silicate used as an antioxidant and the sodium silicate aqueous solution, as described in JIS K 1408, water glass Nos. 1 to 3 (sodium silicate aqueous solution) and sodium metasilicates Nos. 1 and 2 (crystals) are used. Can be used.
As the calcium sulfate, as described in JIS R 9111, gypsum for special grade to B grade ceramic mold materials can be used.

The Fe—C alloy, which is a carbon material, may contain other elements. In this configuration, the heating method is based on furnace heating, but high-frequency heating can also be used. The inert gas, in industry can be used reducing reformed gas mainly composed of N ₂ gas and CO gas.

As described above, the present invention is not limited to the above-described embodiment, and can be modified or applied by those skilled in the art based on the mutual combination of the configurations of the embodiments, the description of the specification, and the well-known technique. It is also a matter of the present invention to do so, and it is included in the scope of seeking protection.

As described above, the following matters are disclosed in this specification.
(1) A steel part in which a carburizing agent containing a powdery Fe-C alloy and a binder for binding the powdered Fe-C alloy powder to each other is brought into contact with at least a part of the surface. ,
A method for carburizing a steel part, which maintains the temperature in the austenite region above the eutectic point and below the peritectic point for a certain period of time.
According to this carburizing method for steel parts, a deep carburizing depth can be formed in a short time while avoiding excessive carburizing. Further, at the time of carburizing, the powders of Fe—C alloy can be bonded to each other so that the carburizing agent is in close contact with the surface of the steel part. Therefore, the carburizing agent does not peel off from the steel parts due to factors such as gravity and heating, and the steel parts can be reliably carburized.

(2) The method for carburizing steel parts according to (1), wherein the carburizing agent has a eutectic point or higher and contains the Fe—C alloy in a mixed state of a solid phase and a liquid phase.
According to this carburizing method for steel parts, high-quality carburizing can be realized in a short time.

(3) The method for carburizing steel parts according to (2), wherein the binder has a eutectic point or higher and has an antioxidant function of the Fe—C alloy.
According to this method of carburizing steel parts, having an antioxidant function suppresses an increase in the melting point of the Fe—C alloy and does not affect the liquid phase formation of the Fe—C alloy.

(4) The method for carburizing steel parts according to (3), wherein the binder contains either sodium silicate or calcium sulfate.
According to this carburizing method for steel parts, reliable carburizing can be performed at low cost.

(5) The method for carburizing steel parts according to (3), wherein the binder is an aqueous solution of sodium silicate.
According to this method of carburizing steel parts, Fe-C powders can be reliably bonded to each other with viscosity.

(6) The method for carburizing a steel part according to any one of (1) to (5), wherein the steel part is heated in an atmosphere of an inert gas.
According to this carburizing method for steel parts, decarburization of the Fe—C alloy can be suppressed and an increase in the melting temperature can be prevented, so that the carburizing source can be easily supplied in the liquid phase.

(7) A steel part in which a carburizing agent containing a powdery Fe-C alloy and a binder for binding the powdered Fe-C alloy powder to each other is brought into contact with at least a part of the surface thereof. ,
A steel part obtained by holding the steel part in the temperature range of the austenite region having the eutectic point or more and less than the peritectic point for a certain period of time.
According to this steel part, a carburized layer having excellent wear resistance, slidability, and fatigue strength is formed in a deep region in a short time.

(8) A carburizing agent containing a powdery Fe-C alloy and a binder for binding the powder particles to each other at a eutectic point or higher of a steel material.
According to this carburizing agent, a deep carburizing depth can be formed in a short time while avoiding excessive carburizing of the steel material.

11 Fe-C alloy powder (solid phase)
13 Sodium silicate aqueous solution 15 Fe-C alloy (liquid phase)
W work

Claims (5)

A carburizing agent obtained by kneading a powdered Fe-C alloy and a binder composed of an aqueous sodium silicate that binds the powders of the powdered Fe-C alloy to each other is applied to the surface of at least a part of the steel part. Steel parts that are brought into contact with each other to hold the steel parts in the temperature range of the austenite region above the eutectic point and below the peritectic point for a certain period of time to form a carburized layer with a carburizing depth of at least 1.9 mm. Carburizing method. The method for carburizing a steel part according to claim 1, wherein the carburizing agent has a eutectic point or higher and the Fe—C alloy is contained in a mixed state of a solid phase and a liquid phase. The method for carburizing a steel part according to claim 1, wherein the binder has a eutectic point or higher and has an antioxidant function of the Fe—C alloy. The method for carburizing a steel part according to any one of claims 1 to 3, wherein the steel part is heated in an atmosphere of an inert gas. A method for manufacturing steel parts with a carburized layer.
A carburizing agent obtained by kneading a powdery Fe-C alloy and a binder composed of an aqueous solution of sodium silicate is brought into contact with the surface of at least a part of the steel parts, and the steel parts are packaged at or above the eutectic point of the material. A method for manufacturing a steel part, which is maintained in a temperature range in the austenite region below the crystal point for a certain period of time to form the carburized layer having a carburized depth of at least 1.9 mm or more.

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