KR101054775B1 - Alloy Steel and Carburizing Heat Treatment Method for Vehicle Transmission Gears - Google Patents

Abstract

By weight%, C 0.28 to 0.33%, Si 0.2% or less, Mn 1.20 to 1.45%, P 0.020% or less, S 0.020% or less, Ni 0.75 to 0.85%, Cr 0.35 to 0.45%, B 0.001 to 0.003%, the rest An alloy steel and a carburizing heat treatment method for a vehicle transmission contribution stream having a composition containing iron and unavoidable impurities are introduced. This method uses a vacuum carburizing furnace to carburize the parts at a relatively low temperature in the A ₁ to A ₃ temperature range and then to heat-treat them with high-pressure gas, whereby the thermal deformation of the parts due to vacuum carburizing heat treatment is significantly reduced.

Annular Gear, Heat Deflection, Carburizing, Heat Treatment

Description

Alloy steel and carburizing heat treatment method for vehicle transmission gears {STEEL ALLOY FOR VEHICLE TRANSMISSION GEARS AND CARBURIZATION HEAT TREATMENT METHOD THEREOF}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to alloy steel for vehicle transmission gears that are sensitive to heat deformation, such as annular gears, and to carburizing heat treatment methods for parts made of such alloy steels.

Gears of the vehicle transmission are components that directly transmit engine power to the differential system, and require fatigue strength, wear resistance, and impact toughness, and are generally carburized and heat treated.

When the carburizing heat treatment is performed, deformation occurs in the component. When the amount of deformation is excessive, the assemblability between components is degraded and abnormal noise problems are caused. In particular, the annular gear, which is a main part of the planetary gear device for an automatic transmission, is a cylindrical part having internal teeth 1, as shown in FIG. 1, and has a large diameter and a thin thickness. be sensitive.

In the past, carburizing heat treatment of parts such as annular gears was performed by high frequency heat treatment after gas carburization and furnace cooling in order to minimize thermal deformation. During the high frequency heat treatment, the parts were bitten by a jig to prevent thermal deformation. However, this method has a problem such as reducing the work efficiency and increase the cost, because the parts must be processed one by one, there was a problem that the tooth deformation of the gear due to forced cooling after fixing the jig.

Recently, a vacuum carburizing method has been introduced, which is excellent in productivity and uniformly cooled, which is advantageous for thermal deformation. This vacuum carburizing method uses a vacuum carburizing furnace to quench the gas after carburizing in a vacuum atmosphere. The vacuum carburizing method reduces the heat treatment cost and reduces the noise due to the uniformity of the teeth. However, the gear shape is distorted. Due to the severe shape, such as roundness, cylindricalness, etc., there is a disadvantage in that it is disadvantageous compared with the conventional gas carburizing method using a jig for hardening.

The present invention has been proposed to solve the above problems, and an object of the present invention is to provide an alloy steel suitable for vehicle transmission gears that can reduce shape deformation during carburizing heat treatment.

It is also an object of the present invention to provide a carburizing heat treatment method suitable for vehicle transmission gears sensitive to heat deformation such as an annular gear.

In addition, it is to provide a carburization heat treatment method that can secure a high productivity by replacing the existing gas carburizing method of high-frequency heat treatment after the furnace cooling.

Alloy steel for vehicle transmission gears according to the present invention for achieving the object as described above, by weight%, C 0.28 ~ 0.33%, Si 0.2% or less, Mn 1.20 ~ 1.45%, P 0.020% or less, S 0.020% or less (0 is not included), Ni 0.75 to 0.85%, Cr 0.35 to 0.45%, B 0.001 to 0.003%, the remaining iron and inevitable impurities.

It is preferable that the dissolved oxygen amount in the said alloy steel is 15 ppm or less.

On the other hand, the carburization heat treatment method according to the invention, the process of heating the carburizing temperature in the A ₁ ~ A ₃ temperature range of the parts made of the alloy steel described above using a vacuum carburizing furnace; Carburizing the surface of the part while supplying carburizing gas at the carburizing temperature; And heat-treating the carburized part with a high-pressure gas so that a martensite phase is formed on the surface portion and a first ferrite mixed phase which does not undergo phase change with the martensite is formed on the core portion.

Preferably, the carburizing temperature is set within a temperature range such that the phase fraction of ferrite in the part is within 30-50%. In addition, the carburization temperature is preferably 740 to 760 ° C.

As described above, the alloy steel for vehicle transmission gears according to the present invention has a lower A ₁ to A ₃ temperature than the SCr420HB, which has been used as a conventional transmission gear alloy steel, so that a vehicle part made of such alloy steel can be used in the A ₁ to A ₃ temperature range. The change in temperature during the quenching process after carburizing in the interior is small, which greatly reduces the shape deformation of the part.

In addition, the carburizing heat treatment method according to the present invention is carburized in a considerably lower A ₁ to A ₃ temperature range, compared with the conventional gas or vacuum carburizing heat treatment method which has been carried out at the austenitization temperature A ₃ or higher, and therefore, particularly during quenching. In addition, the shape distortion of the part is reduced, and is particularly suitable for carburizing heat treatment of vehicle transmission gears that are sensitive to heat deformation such as an annular gear.

In addition, the carburizing heat treatment method according to the present invention can be continuously carburized after a large amount of components are put into a vacuum carburizing furnace, and thus, productivity is superior to that of a conventional gas carburizing method using a jig during quenching.

Hereinafter, with reference to the accompanying drawings looks at in more detail with respect to the present invention.

The main cause of thermal deformation during carburizing heat treatment is the volume change due to the transformation of microstructure and rapid temperature decrease (ΔT). In the quenching process, the austenite phase (γ) is transformed into a martensite phase (α '), and the crystal structure is changed from FCC (Face Centered Cubic) to BCT (Body Centered Tetragonal). In this case, while the BCT structure has a wider lattice spacing, the volume expands, and as the temperature decreases during quenching, the volume of the component itself rapidly decreases. These various factors combine with each other to cause deformation.

In order to minimize thermal deformation during carburization heat treatment caused by the above reasons, it is necessary to minimize the temperature change (ΔT) during quenching. As a solution for this, the present applicant has carburized in an abnormal region in which an austenite phase and a ferrite phase coexist (dual phase: see FIG. 2) through a Korean Patent Application No. 2008-0087666, and a certain amount in a temperature raising and carburizing diffusion process of a part. The ferrite phase of α is maintained without phase change, and through this, a method of reducing thermal deformation of carburized heat-treated parts by minimizing volume change by lowering the transformation fraction from austenite to martensite is shown. have.

The present invention is an extension of the contents of the invention introduced in the above application, by lowering the carburizing heat treatment temperature than in the case of the present invention, to further reduce the temperature change at the time of quenching, through which the thermal deformation of the parts To further reduce the More specifically, the alloy steel according to the present invention is designed such that the temperature of A1 and A3 is lower than the temperature of the alloy steel used in the present invention. In Table 1 below, Examples are examples of alloy steel according to the present invention, Comparative Example is SCr420HB.

division Ar3 Ac3 (0.8% C) Ac1 Carburizing Temperature Example 776 ℃ 733 ℃ 705 ℃ 740 ~ 760 ℃ Comparative example 796 ℃ 752 ℃ 732 ℃ 770 ~ 790 ℃

In Table 1, Ac3 (0.8% C) is the austenitization temperature at elevated temperature when the carburized part has a carbon concentration of 0.8%. Since the surface carbon concentration of the carburized part is increased to about 0.8% during the carburizing process, the Ac3 (0.8% C) temperature corresponds to one condition necessary to fully austenite the surface of the part. Ar3 is a temperature at which the cornerstone ferrite phase starts to precipitate upon cooling in the austenite single phase region, as is well known.

Hereinafter, the composition of the alloy steel according to the present invention will be described.

It contains 0.28 to 0.33% by weight of carbon (C). If the carbon content is less than 0.28% by weight, the austenite stabilization effect that can lower the phase transformation temperature is limited, making it difficult to achieve the low temperature carburizing purpose. On the other hand, when the content exceeds 0.33% by weight, the hardness of the material increases due to an increase in the base metal carbon content, thereby decreasing various workability, and also increasing the brittleness of parts due to the increase in core hardness after carburizing heat treatment.

0.2 wt% or less of silicon (Si) is included. Silicon is a ferrite stabilizing element, making it difficult to reduce the main phase transformation temperature when added in excess of 0.2% by weight.

Manganese (Mn) is contained in 1.20 ~ 1.45% by weight. Manganese is an austenite stabilizing element, and when it is less than 1.20% by weight, it is difficult to reduce the phase transformation temperature for low temperature carburization, and when it exceeds 1.45% by weight, it becomes difficult to process by giving viscosity to the steel.

Nickel (Ni) is contained 0.75 to 0.85% by weight. Nickel is also an austenite stabilizing element, which is 0.75% by weight or more to enable low temperature carburization, but is limited to 0.85% by weight or less to minimize the use of expensive nickel. In addition, in the present invention, in order to prevent the drop in hardenability due to the reduction of the chromium content, the content of nickel having an effect of improving the hardenability is designed to be 0.75 to 0.85% by weight.

The content of chromium (Cr) is 0.35 to 0.45 wt%. Chromium is the most important ferrite stabilizing element and, when added, increases the transformation temperature of steel. Therefore, the maximum content of chromium is limited to 0.45% by weight or less in order to achieve the low temperature carburizing purpose. On the other hand, in order to secure the hardenability after carburization, the content of chromium needs to be added at least 0.35% by weight.

Phosphorus (P) and sulfur (S) are representative impurities that degrade the quality of alloy steel such as toughness and need to be managed at 0.020% by weight or less (including 0), and dissolved in order to maintain the cleanliness of alloy steel, that is, reduce non-metallic inclusions. The amount of oxygen needs to be managed to be 15 ppm or less (including 0). On the other hand, boron (B) is contained 10 ~ 30ppm to improve the heat treatment hardenability. If the boron content exceeds 30pm brittleness is a problem.

Referring to Figure 3, looks at an example of the carburizing heat treatment process according to the present invention.

According to the present invention, the carburization heat treatment is performed in a vacuum atmosphere using a vacuum carburizing furnace. Vacuum atmosphere refers to a low pressure anoxic atmosphere, that is, a condition where the oxygen partial pressure is low and controlled at a low pressure so as to prevent oxidation of the surface of the part during the carburizing heat treatment process. This carburization heat treatment includes a heating step, a carburization step, and a hardening step using a high pressure gas, as shown in FIG.

Heating process

According to the invention carburization takes place in the A ₁ to A ₃ temperature range, preferably at 740 to 760 ° C. The part is heated to this carburizing temperature using a vacuum carburizing furnace and then held or soaked. Typically, a large amount of parts are introduced into a vacuum carburizing furnace at a time. In this case, since there is a temperature difference depending on the position in the furnace, the parts put into the furnace need to be sufficiently soaked until the temperature becomes uniform. The temperature rise time of the part is determined in consideration of productivity as well as thermal deformation, and the soaking time is considered to be sufficient for approximately 30 minutes to 1 hour.

Typically, carburizing parts have a mixed structure of pearlite and ferrite at room temperature. When these parts are soaked in the A ₁ to A ₃ temperature range, all pearlite at room temperature is austenitized, only part of the ferrite at room temperature is austenitized and the rest (i.e., the first ferrite) remains unchanged. . That is, the part has a mixed structure of austenite and ferrite through a heating process. The important thing here is that the phase fraction of ferrite in the carburizing temperature should be within 30-50% (details will be described later), and the carburizing temperature should be determined in consideration of these matters.

Carburizing process

Existing gas and vacuum carburizing processes are characteristically carried out in the austenite single phase region above the A ₃ temperature. In contrast, according to the present invention, the carburization process is performed in an abnormal region where austenite and ferrite coexist, that is, in the temperature range of A ₁ to A ₃ . As is known, A ₁ is the temperature at which the austenite is transformed into ferrite and cementite, and A ₃ is the stenitization temperature. On the other hand, according to the present invention, the carburizing temperature is preferably 740 to 760 ° C. Under 740 ° C, the diffusion coefficient is low and the carburizing time is long, and the ferrite fraction exceeds 50%. Can be. In addition, carburization is performed at 760 ° C. or less to minimize thermal deformation due to temperature change during quenching.

When a carburizing gas such as acetylene or ethylene gas is supplied into the chamber of the vacuum carburizing furnace in which the part is placed, carbon is carburized and diffused into the part. The surface of the component carburized and diffused carbon has a high carbon concentration, resulting in austenitization, and the core of the component unable to diffuse carbon has a mixed phase of austenite and ferrite, which is a structure after the heating process. Therefore, it can be said that the ferrite phase percentage in the core portion is about 30 to 50%.

Hardening  fair

The carburized parts are quenched to a martensite transformation start temperature (Ms) using high pressure gas, and the initial cooling rate should be maintained at 12 ° C / sec or more. Nitrogen, helium, hydrogen, and the like may be used for the gas, and quenching using oil or salt bath is avoided because it causes deformation of parts due to uneven cooling. The mixed structure of martensite and residual austenite is formed in the surface portion of the quenched part, and the mixed structure of the first ferrite and martensite is formed therein.

The core ferrite percentage of the hardened parts, in particular of transmission gear parts such as annular gears, needs to be about 30-50%. If the amount of ferrite in the core is less than 30%, there is a large amount of deformation due to phase transformation, so that the effect of reducing heat deformation of the parts may be insignificant. Carburizing heat treatment does not reach the required level for transmission gear parts.

Unlike the conventional case, according to the present invention, it is not necessary to separately cool the carburized parts before hardening. The furnace cooling, which was carried out according to the conventional gear carburizing heat treatment method, aims at lowering the quenching start temperature in order to reduce the thermal deformation of the parts. Since it is much lower, the need for furnace cooling is significantly reduced. Naturally, if some productivity is not taken into account, furnace cooling can be carried out before quenching to improve quality.

On the other hand, even in the case of the conventional gas or vacuum carburizing process, the ferrite and martensite can coexist in the core of the carburized heat-treated component. That is, when the object is carburized in the austenite single phase region and then maintained in the abnormal (γ + α) region through the furnace cooling, it is quenched to generate a mixed structure of martensite and ferrite. However, this ferrite is the first transformation of the austenite in the carburizing process and then the second transformation of the ferrite in the stomach furnace cooling and quenching process, thus causing the shape distortion of the object.

While specific embodiments of the present invention have been illustrated and described, those of ordinary skill in the art may vary the present invention without departing from the spirit of the invention as set forth in the following claims. It is to be understood that modifications and variations are possible.

1 shows a conventional annular gear,

2 is an example of a conventional Fe-C equilibrium diagram,

3 is a view showing a carburization heat treatment process according to an embodiment of the present invention;

Claims (5)

delete delete As a carburization heat treatment method using a vacuum carburizing furnace, Heating the part to a carburizing temperature within the A ₁ to A ₃ temperature range; Carburizing the surface of the part while supplying carburizing gas at the carburizing temperature; And Heat treating the carburized part with a high pressure gas such that a martensite phase is formed at the surface portion and a first ferrite mixed phase which is not subjected to phase change is formed at the core portion. The parts are in weight%, C 0.28 ~ 0.33%, Si 0.2% or less (0 is not included), Mn 1.20 ~ 1.45%, P 0.020% or less (including 0), S 0.020% or less (including 0), Ni 0.75 ~ Carburizing heat treatment method characterized in that it has a composition containing 0.85%, Cr 0.35 ~ 0.45%, B 0.001 ~ 0.003%, dissolved oxygen amount 0.0015% or less (including 0), the remaining iron and inevitable impurities. The method of claim 3, wherein the carburization temperature is set within a temperature range such that the percentage of ferrite in the part is within 30 to 50%. The method of claim 3, wherein the carburizing temperature is 740 ~ 760 ℃ carburizing heat treatment method, characterized in that.

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