KR20000027040A - Method for heat treatment of surface of steel to reduce heating transformation - Google Patents

Abstract

PURPOSE: A method for heat treatment of surface of steel is provided which is to reduce heating transformation by lowering quenching temperature, by which good quality of carburized steel can be obtained. CONSTITUTION: A method for heat treatment of surface of steel is accomplished by: carburizing the steel in an atmosphere of hydrocarbon gas and RX gas at 830-930°C for 2-5 hours(step 1); carburizing the steel in an atmosphere of hydrocarbon gas and RX gas at 650-740°C with blowing NH3 in at a rate of 1.0-3.0 liter/min from the temperature of about 850°C of step 1(step 2); oil quenching the steel at 130°C(step 3); tempering the steel at 160°C(step 4).

Description

Surface heat treatment method for steel to reduce heat deformation

The present invention relates to a method for surface heat treatment of steel, specifically, carburizing the steel at 830 to 930 ° C. for 2 to 5 hours (first step); Carburizing the carburized steel in the first step at 650 to 740 ° C. (second step); Oil quenching the carburized steel in the second step at 130 ° C. (third step); And a step (fourth step) of considering the steel quenched in the third step at 160 ° C. (fourth step).

Steel is an alloy containing 0.02 to 2.0% of carbon (C) in iron (Fe). If the amount of carbon is small, it is soft but tough. If the amount of carbon is high, the steel is hard and strong but weak. Therefore, according to the purpose of use, as a component material of the machine, carbon is distributed to the surface portion and less at the center portion, the surface is lighter, the wear resistance is good, and the center portion is softer and tougher. In order to make such a product, a low carbon material is cut and then carbon is diffused on the surface to make a high carbon alloy layer. This process is called carburizing. Carburizing includes solid carburizing, gas carburizing and liquid carburizing.

The solid carburizing method is to fill the product and carburizing agent in the carburized bed and seal it, and to carburize it by heating to 850 ~ 950 ℃. The carburizing agent is charcoal or bone char, and barium carbonate (BaCO ₃ ) or sodium carbonate (Na ₂ CO ₃₎ ) Is used.

Gas carburizing method uses gas such as natural gas, city gas, propane and butane, and in the furnace containing the product, air is properly mixed, heated to 850 ~ 900 ℃, incomplete combustion and carburizing, which is more uniform than solid carburizing. There is an advantage that one carburized layer is obtained.

The solid carburizing method and the gas carburizing method are methods in which carburization is performed by decomposing carbon monoxide (CO) gas generated from a carburizing agent on the surface of steel to generate activated carbon.

The liquid carburizing method is a method in which a product is carburized by putting a product in a salt bath containing sodium cyanide (NaCN) as a main component, and the heating temperature is 750 to 900 ° C. In this method, nitriding occurs along with carburization, so it is also called carbo-nitriding. The decomposition of sodium cyanide results in carbon monoxide and nitrogen gas (N ₂ ), which decomposes on the surface of the steel to produce carbon and nitrogen, which leads to carburization and nitriding.

On the other hand, nitriding is a process of increasing the hardness of the steel surface by diffusing nitrogen (N) into steel containing Al, Cr, Ti, Mo, V and the like to form a solid nitride in a state where nitrogen is bonded to the element. . However, pure iron, carbon steel and steel containing nickel (Ni) and cobalt (Co) are not hardened by nitriding. Therefore, the nitriding steel is limited to a special composition containing the above element, which is called nitride steel. Call. Nitriding is a process in which a product is heated to about 500 ° C. in ammonia (NH ₃ ) gas, and active nitrogen, which is generated by decomposition of ammonia at a high temperature, penetrates into the steel to form steel having excellent wear resistance and corrosion resistance. On the other hand, nitriding is advantageous in that steel of higher hardness is obtained compared to carburizing and the heating temperature is low, so that deformation due to heat is small, but there is a drawback in that the treatment time is long.

In the conventional carburizing heat treatment method, a carburized steel having good mechanical properties such as fatigue strength, tensile strength, and impact strength could be obtained, but after the carburizing process, the hardening process was performed at a relatively high temperature of about 850 ° C. This was a serious problem. Therefore, in order to reduce the heat treatment deformation, plug high frequency quenching treatment may be performed after carburizing, but this is not only a factor of cost increase but also a deformation problem.

Therefore, the present inventors can reduce the heat-treatment deformation such as thermal deformation and transformation deformation caused by the high temperature by lowering the quenching temperature after carburizing to 650 to 740 ° C. near the A ₁ line, as well as the carburizing previously used for heat treatment The present invention has been completed by developing a carburized surface heat treatment method for obtaining a carburized steel having excellent mechanical properties even by using a general carbon steel material instead of a molten alloy steel.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for surface annealing of steel, in which the hardening temperature during carburization is reduced to reduce the heat treatment deformation caused by the high temperature, and the carburized steel having excellent mechanical properties can be obtained even using a general carbon steel material.

1 is a schematic view showing a surface heat treatment method of steel (steel) for reducing the heat deformation of the present invention,

Figure 2 is an optical microscope photograph of the microstructure of the surface of the carburized steel heat-treated by the conventional method (× 400),

Figure 3 is an optical microscope photograph of the microstructure of the inside of the carburized steel heat-treated by the conventional method (× 400),

Figure 4 is an optical microscope photograph of the microstructure of the surface of the carburized steel heat-treated by the surface heat treatment method of the steel for reducing the heat deformation of the present invention (× 400),

FIG. 5 is an optical microscope photograph of microstructure inside a carburized steel made of a dual phase of martensite and ferrite by heat-treatment using a surface heat treatment method of steel for reducing thermal deformation of the present invention ( × 400).

In order to achieve the above object, in the present invention, a step of carburizing at 830 to 930 ° C. for 2 to 5 hours (first step), a step of carburizing at 650 to 740 ° C. (second step), and quenching oil at 130 ° C. It provides a method for surface heat treatment of steel, comprising a (third step) and a step (fourth step) at 160 ° C.

Hereinafter, the present invention will be described in detail.

First, in the present invention, the step of carburizing steel (steel) at 830 ~ 930 ℃ for 2 to 5 hours (first step); Carburizing the carburized steel in the first step at 650 to 740 ° C. (second step); Oil quenching the carburized steel in the second step at 130 ° C. (third step); And a step (fourth step) of considering the steel quenched in the third step at 160 ° C. (fourth step) (FIG. 1).

At this time, the nitriding is carried out by introducing ammonia at 1.0 to 3.0 L / min since the temperature reaches about 850 ° C. during the process from the first step to the second step of the method. Is the process of hardening while nitriding.

Further, the first and second steps can be carried out in a hydrocarbon gas and RX gas atmosphere, preferably in the range of 1.0 to 2.5 L / min propane (C ₃ H ₈ ) gas and in the range of 100 to 400 CFH RX gas. It can be performed in.

In addition, the first step and the second step may be performed by setting the carbon potential to 0.75 to 0.9 before 850 ° C. and 0.75 to 0.8 after 850 ° C.

In the present invention, it is hardened at a considerably lower temperature compared to the conventional carburizing heat treatment method. That is, in the conventional method, while quenching at about 850 ℃, in the present invention, the quenching with nitriding at 650 ~ 740 ℃. Generally, the temperature at which the steel structure is transformed from austenite to martensite is about the temperature at which the quenching of the present invention occurs. In the conventional method, both the inside and the outside of the steel are transformed into martensite by heat treatment at a temperature significantly higher than this temperature. The degree of heat deformation was also severe (see FIGS. 2 and 3). However, in the method of the present invention, the hardening is performed at the temperature of the transformation from austenite to martensite, so that the inside of the steel is not completely transformed, and the outside is completely transformed to martensite. Is soft and tough, so that an ideal carburized steel can be obtained that can be avoided by fragility as a whole (see FIGS. 4 and 5).

In addition, in the method of the present invention, it is possible to obtain a carburized steel having mechanical properties equivalent to that of the carburized alloy steel even when general carbon steel is used instead of carburized alloy steel, thereby reducing the cost.

In the embodiment of the present invention using S45C which is a general carbon steel material, after carburizing at 900 ° C. for 4 hours under conditions of propane gas 2.5 L / min, ammonia 2.5 L / min and RX gas 350 CFH, the conditions of 1.0 L / min ammonia Quenched while nitriding at 720 ° C. for 20 minutes. After carburizing steel was obtained through an oil quenching process at 130 ° C. and an annealing process at 160 ° C., mechanical properties such as heat treatment deformation degree, bending fatigue strength and impact strength were measured. As a result, the carburized steel heat-treated by the method of the present invention has a significantly reduced deformation due to heat treatment compared to the carburized steel obtained by the conventional method, and also by carburizing general carbon steel rather than the carburized alloy steel by the method of the present invention It can be seen that the carburized steel obtained also secures the mechanical properties of the alloy steel level.

Hereinafter, the present invention will be described in detail by Examples and Comparative Examples.

However, the following examples are illustrative of the present invention, and the content of the present invention is not limited by the examples.

Example 1 Surface Heat Treatment of General Carbon Steel S45C (1)

After carburizing for 4 hours at 900 ° C using propane gas 2.5 L / min, ammonia 2.5 L / min and RX gas 350 CFH using general carbon steel material S45C, 20 min at 720 ° C. under 1.0 L / min ammonia Quenched while nitriding. After carburizing at 130 ℃ for oil quenching and soaking at 160 ℃ to obtain carburized steel, the obtained carburized steel was measured by using low and reverse annulus gear for passenger car. 1 is shown. In addition, the mechanical properties such as bending fatigue strength and impact strength were measured and shown in Table 2.

Thermal Deformation Measurement Result (1) division Profile Lead Runout over ball diameter (O.B.D) Bias Ff C _α Bias C _β 105.387 105.28 One 6.9 20.5 2.3 17.1 -1.5 59.1 105.222 105.159 2 2.0 17.7 4.6 9.2 -1.1 83.4 105.225 105.162 3 7.7 14.9 3.4 12.1 -2.6 63.2 105.212 105.178 4 6.4 19.4 4.2 8.3 -2.0 99.9 105.222 105.167 5 2.1 18.1 4.6 12.2 0.6 86.9 105.210 105.173 6 4.2 16.6 4.0 30.2 -2.4 80.3 105.216 105.167 7 6.0 16.7 3.2 26.1 -2.1 77.5 105.221 105.147 8 4.2 17.6 4.5 10.1 -2.0 56.2 105.217 105.181 9 2.6 16.8 1.8 23.0 -1.0 52.0 105.200 105.171 Average 4.7 17.6 3.6 16.5 -1.6 73.2 105.216 105.167

Mechanical property measurement result division Surface Heat Treatment with S45C Bending fatigue strength 70 Kgf / ㎡ Impact strength 0.5 Kgm / ㎠

Example 2 Surface Heat Treatment of General Carbon Steel S45C (2)

After carburizing for 3 hours at 900 ° C. with propane gas 2.5 L / min, ammonia 2.5 L / min, RX gas 350 CFH using S45C, which is a general carbon steel material, 20 minutes at 740 ° C. under 1.0 L / min ammonia Quenched while nitriding. After oil quenching at 130 ° C. and a carburizing process at 160 ° C. to obtain carburized steel, the degree of heat treatment deformation was measured in the same manner as in Example 1 for the obtained carburized steel, and the results are shown in Table 3.

Thermal Deformation Measurement Result (2) division profile lead Runout O.B.D. bias Ff C _α bias C _β 105.387 105.28 One 7.3 11.1 2.6 12.2 -1.8 75.7 105.165 105.122 2 5.1 11.7 2.5 26.5 0.1 29.6 105.149 105.117 3 6.4 10.7 2.9 15.0 -2.2 45.9 105.161 105.092 4 4.6 10.4 3.4 28.0 -1.7 61.0 105.154 105.071 5 4.5 13.0 2.6 26.5 -0.9 55.1 105.167 105.078 6 5.1 14.4 3.9 24.8 -0.5 49.6 105.138 105.075 7 2.8 13.4 2.2 25.6 -1.3 26.0 105.115 105.089 8 7.6 11.1 2.2 11.2 0.4 37.6 105.130 105.084 9 4.9 9.9 1.8 8.6 -0.1 64.3 105.139 105.081 Average 5.4 11.7 2.7 19.8 -0.89 49.4 105.146 105.089

Example 3 Surface Heat Treatment of General Carbon Steel S45C (3)

After carburizing at a temperature of 930 ° C. for 3 hours using propane gas 2.5 L / min, ammonia 2.5 L / min and RX gas 350 CFH using general carbon steel material S45C, 30 minutes at 700 ° C. under 2.0 L / min ammonia Quenched while nitriding. After oil quenching at 130 ° C. and a carburizing process at 350 ° C. to obtain carburized steel, the degree of heat treatment deformation was measured in the same manner as in Example 1, and the results are shown in Table 4.

Thermal Deformation Measurement Results (3) division profile lead Runout O.B.D. bias Ff C _α bias C _β 105.387 105.28 One 8.3 30.5 8.3 27.1 -1.4 117 105.262 105.126 2 7.9 21.6 4.6 9.1 -0.7 72.4 105.252 105.137 3 8.3 20.4 5.4 29.1 0.5 88.2 105.272 105.118 4 24.1 31.6 7.8 28.1 -1.1 219 105.326 105.028 5 4.9 17.3 4.4 11.1 -1.4 74.4 105.228 105.139 Average 10.7 24.3 6.1 20.9 -0.82 114.2 105.268 105.109

Comparative Example Surface Heat Treatment of Carburized Alloy Steel SCM318H

After carburizing at 920 ° C. for 2.5 hours using the existing carburizing alloy steel SCM318H, the degree of heat treatment deformation was measured in the same manner as in Example 1 for the carburized steel obtained by the plug high frequency quenching treatment. Indicated. In addition, the mechanical properties were measured and shown in Table 6.

Result of heat treatment deformation by conventional carburizing division profile lead Runout O.B.D. bias Ff C _α bias C _β 105.387 105.28 One 21.9 28.1 6.4 56.6 0.8 116 105.344 105.249 2 24.6 21.0 6.0 45.7 8.6 98.6 105.323 105.261 3 11.1 23.4 7.3 81.1 3.8 117 105.307 105.259 4 8.1 6.9 2.9 18.2 5.1 130 105.305 105.241 Average 16.4 19.9 5.7 50.4 4.6 115.4 105.319 105.253

Mechanical Properties of Carburized Alloy Steel Heat Treated by Conventional Methods division Surface Heat Treatment with SCM318H Bending fatigue strength 75 Kgf / ㎡ Impact strength 0.9 Kgm / ㎠

As shown in Table 1 to Table 6, the carburized steel heat-treated by the method of the present invention is significantly reduced deformation due to heat treatment compared to the carburized steel obtained by the conventional method, and also the alloy steel for carburization by the method of the present invention It was found that the carburized steel obtained by carburizing the ordinary carbon steel secured the mechanical properties of the alloy steel level.

As described above, in the surface heat treatment method for reducing the heat deformation of the present invention, by reducing the quenching temperature after carburization to near the A ₁ line, heat deformation such as heat deformation and transformation deformation caused by high temperature is reduced. Not only the carburizing alloy steel used for heat treatment but also general carbon steel can be used to obtain carburized steel having excellent mechanical properties, thereby reducing the cost of carburizing steel. In addition, better mechanical properties can be obtained by using carburized alloy steel as well as carbon steel.

Furthermore, in the case of the annular gears having large heat treatment deformation during the conventional carburizing heat treatment, the surface heat treatment method for reducing the heat deformation of the present invention reduces the distortion of the teeth, thereby preventing the occurrence of abnormal stress, thereby increasing the durability and reducing the noise. Can be.

Claims (6)

Carburizing steel at 830-930 ° C. for 2-5 hours (first step); Carburizing the carburized steel in the first step at 650 to 740 ° C. (second step); Oil quenching the carburized steel in the second step at 130 ° C. (third step); And The step of considering the oil-hardened steel in the third step at 160 ° C. (fourth step) Surface heat treatment method of steel consisting of. The method of claim 1, wherein the nitriding is performed by introducing ammonia (NH ₃ ) at 1.0 to 3.0 L / min from the first stage to the second stage when the temperature reaches about 850 ° C. Method of surface heat treatment of steel. The method of claim 1, wherein the carburization and nitriding of the second step is a process in which hardening is performed while nitriding. The method of claim 1, wherein the first and second steps are performed in a hydrocarbon gas and RX gas atmosphere. The method of claim 1, wherein the first and second steps are performed in the range of 1.0 to 2.5 L / min propane (C ₃ H ₈ ) gas and in the range of RX gas 100 to 400 CFH. . The method of claim 1, wherein the first and second steps are performed with a carbon potential of 0.75 to 0.9 before 850 ° C and 0.75 to 0.8 after 850 ° C.