KR101481168B1 - Method for manufacturing shaft of automobile - Google Patents

Abstract

The present invention provides a method for manufacturing an automobile shaft which optimizes the component content of an alloy steel for an automobile shaft to omit the spheroidizing annealing process and significantly shortens the normalizing process after cold working, thereby reducing manufacturing costs and improving work productivity. There is a purpose.

In order to achieve the above object, the present invention provides a method of manufacturing a semiconductor device, comprising the steps of: 0.17 to 0.21 wt% of C; 0.10 wt% or less of Si; 0.40 to 0.50 wt% of Mn; 0.02 wt% or less of P; 0.30 wt% or less, Ni: 0.25 wt% or less, 0.95-1.35 wt% of Cr, 0.20-0.4 wt% of Mo, 0.01-0.04 wt% of Ti, 0.01-0.02 wt% of N, And the alloy steel material is subjected to cold working in the form of a shaft without performing a spheroidizing annealing treatment and then subjected to a normalizing treatment at 900 to 950 DEG C for 100 to 150 minutes , And carburizing to increase the surface hardness.

Automobile Shafts, Spheroidal Annealing, Normalizing, Carburizing, Tempering

Description

[0001] METHOD FOR MANUFACTURING SHAFT OF AUTOMOBILE [0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method for manufacturing a shaft for an automobile, and more particularly, to a method for manufacturing a shaft for an automobile which is capable of exhibiting the same level of physical properties as a conventional automobile shaft through a simpler heat treatment process .

Generally, shafts used in automobile transmission and the like are made of alloy steel which is carburized treatment in order to increase the surface hardness. Figs. 1 and 2 briefly disclose a manufacturing process of an automobile shaft using such an alloy steel.

First, an alloy steel material for a shaft is prepared and cut to a size necessary for producing a shaft (S10). (SCR420H1 or the like), which is conventionally used as a shaft alloy steel, contains 0.17 to 0.21 wt% of C, 0.15 wt% or less of Si, 0.60 to 0.85 wt% of Mn, 0.02 wt% or less of P, 0.03 wt% or less of S, (Fe) and unavoidable impurities, in the following order: Cu: not more than 0.30 wt%, Ni: not more than 0.25 wt%, Cr: 1.25 to 1.45 wt%, Mo: 0.55 to 0.65 wt%

Sintered annealing (S11) is performed to increase the cold rolling property of the alloyed steel material for the cut shaft. This spheroidizing annealing is a heat treatment method which makes it possible to form in the cold state by reducing the strength of the material by spheroidizing the carbide in the alloy steel. Typically, spheroidizing annealing is performed for a long time of more than 20 hours at the A1 transformation point boundary (710 ~ 725 ℃) of the material to make the base structure into a composite structure of microcrystalline carbide and ferrite.

The spheroidizing annealing treatment is advantageous in that the hardness and strength of the material is lowered and the cold formability is increased. However, since the heat treatment must be performed for 20 hours or more, it is costly and takes a long time.

The spheroidized annealed alloy steel is processed into a required shape for the shaft through cold working such as forging and turning (S12, S13). Forging is a work of kneading or pressing a material to make a preliminary shape necessary for a shaft. Turning is a work of cutting a surface of a forged material by a tool while rotating the material on a lathe, thereby forming a final shape of the shaft.

The work-hardened material is subjected to a normalizing process by cold working (S14). Since the internal stress remaining in the work-hardened material causes the crystal grains to coarsen during the carburizing process in the subsequent process, it is disadvantageous in terms of strength and hardness, and therefore must undergo a normalizing step.

Thereafter, carburizing to a predetermined depth from the surface is carried out in order to increase the surface hardness of the shaft (S15). The carburizing process consists of preheating, surface carburizing (900 to 910 ℃ for 90 minutes), diffusion (900 to 910 ℃ for 90 minutes) and quenching (800 ℃ for 20 minutes). Since the shaft for an automobile is often thread-formed on the outer surface and gear-coupled, the surface hardness must be high in order to use the shaft for a long time, and the carburizing treatment is usually performed as described above.

Finally, in order to solve the increased internal stress in the carburizing process, the automotive shaft is manufactured by tempering at 180 DEG C for 120 minutes (S16).

According to the conventional automobile shaft manufacturing process thus configured, the heat treatment process (spheroidizing annealing, normalizing, carburizing, and tempering) is very complicated and takes a long time, resulting in a very low work productivity.

SUMMARY OF THE INVENTION The present invention has been developed in order to solve such problems, and it is an object of the present invention to optimize the content of alloy components such as greatly reducing the content of manganese (Mn) in alloy steels for automobile shafts, thereby omitting spheroidizing annealing treatment and drastically shortening the normalizing treatment after cold working Thereby reducing manufacturing costs and improving work productivity. SUMMARY OF THE INVENTION

P: 0.02 wt% or less, S: 0.03 wt% or less, Cu: 0.30 wt% or less, Ni: 0.25 wt% or less 0.95 to 1.35% by weight of Cr, 0.20 to 0.40% by weight of Mo, 0.01 to 0.04% by weight of Ti, 0.01 to 0.02% by weight of N and 10 to 35 ppm of B and balance of Fe and unavoidable impurities Preparing an alloy steel material; A step of immediately cold-working the alloy steel material in a shaft shape without performing spheroidizing annealing; Normalizing the cold-worked shaft at 900 to 950 DEG C for 100 to 150 minutes; And carburizing the normalized shaft to increase the surface hardness.
In this case, the cold working is forging and turning, and the carburizing treatment is a preheating step, a surface carburizing and diffusing step, which is performed at 900 to 910 ° C in order, and a quenching step. After the carburizing treatment, It is preferable to perform the tempering treatment at 170 to 190 占 폚.

According to the method for manufacturing a shaft for an automobile according to the present invention configured as described above, the cold working can be performed immediately without performing the spheroidizing annealing process, which has conventionally been performed for a long time.

Further, the normalizing process time for reducing the internal stress after the cold working can be greatly shortened.

As described above, according to the present invention, owing to the omission of the spheroidizing annealing process and the shortening of the normalizing time, the shaft having the same physical properties as the conventional one in terms of strength and hardness can be manufactured through a much simpler heat treatment process.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings. FIG. 3 is a flowchart showing a manufacturing process of a vehicle shaft according to the present invention, and FIG. 4 is a view showing a heat treatment method in the manufacturing process of FIG.

The method for manufacturing a motor shaft according to the present invention is mainly manufactured through a process of material preparation-forging-turning-normalizing-carburizing-tempering.

Comparing this with the conventional method for manufacturing a motor shaft shown in FIG. 1, it can be seen that the spheroidizing annealing treatment is not performed before cold working such as forging and turning, and the time for normalizing treatment is greatly shortened. This difference in configuration is due to the optimization of the alloy composition of the material, which is a technical feature of the present invention.

Hereinafter, a method for manufacturing an automobile shaft will be described in detail with reference to the technical features of the present invention.

First, an alloy steel material for a shaft is prepared and cut to a size necessary for producing a shaft (S20). The components of the alloy steel for a shaft according to the present invention are shown in Table 1 below.

division
Alloy component (% by weight) C Si Mn P S Cu Ni Cr Mo Ti N B (ppm) Fe Conventional example 0.17
-
0.21 0.15
Below 0.60
-
0.85 0.02
Below 0.03
Below 0.30
Below 0.25
Below 1.25
-
1.45 0.55
-
0.65 Remainder
Example 0.17
-
0.21 0.1
Below 0.40
-
0.50 0.02
Below 0.03
Below 0.30
Below 0.25
Below 0.95
-
1.35 0.20
-
0.40 0.01
-
0.04 0.01
-
0.02 10
-
35 Remainder

C (carbon) is the most effective element to improve the strength of the material, and it is incorporated in austenite to form a martensite structure during quenching heat treatment. In addition, it combines with Fe, Cr, Mo, etc. to form a carbide, thereby improving strength and hardness. However, during quenching heat treatment, deformation is induced, elongation rate, impact value are decreased, and weldability is lowered.

In consideration of this point, it is preferable that C is contained in an amount of 0.17 to 0.21% by weight. If the content of C is less than 0.17% by weight, the strength and hardness required for the shaft can not be obtained. If the content of C is more than 0.21% by weight, the hardness of the material becomes HRB 80 or more, and the spheroidizing annealing process can not be omitted.

Si (silicon) is incorporated into the ferrite to strengthen it. Therefore, in order to lower the strength of the ferrite before the heat treatment, it is preferably contained in an amount of 0.1 wt% or less. The hardness of the material becomes HRB 80 or more so that the spheroidizing annealing treatment can not be omitted. Si is always contained in the steelmaking and steelmaking process, but since the lower limit can not be industrially controlled, usually only the upper limit is specified.

Mn (manganese) is incorporated in the matrix to increase the bending fatigue strength and greatly improve the hardenability without decreasing the elongation. Further, Mn greatly improves the yield strength of the alloy steel by refining pearlite and solidifying ferrite. Accordingly, in the present invention, the content of Mn is greatly lowered compared with the conventional method, so that the cold forming property of the alloy steel material is improved.

In this respect, Mn is preferably contained in an amount of 0.40 to 0.50% by weight. When the content of Mn is less than 0.40% by weight, the bending fatigue strength is reduced by 20% or more compared to the conventional one. When the content of Mn is more than 0.50% by weight, the hardness of the material becomes HRB 80 or more.

P (phosphorus) and S (sulfur) improve the cold workability, but if too much is contained, compounds such as MnS are segregated in the grain boundaries to lower the strength, so that they are preferably contained in an amount of 0.02 wt% or less and 0.03 wt% or less, respectively. Ni (nickel) and Cu (copper) improve the ingotability. However, if too much Ni (Ni) and Cu (copper) decrease the strength due to coarsening of the precipitate, they are preferably contained in an amount of not more than 0.30 wt% and not more than 0.25 wt%, respectively.

The above P, S, Ni and Cu are included in the steelmaking process as Si as described above. However, since the lower limit can not be controlled industrially, the upper limit is usually specified.

Cr (chromium) has the effect of expanding the austenite region to improve the entrapment properties and to prevent low-temperature embrittlement and hydrogen embrittlement, but if too much, it creates a non-magnetic weak phase called σ phase and promotes tempering brittleness. Considering this point, Cr is preferably contained in an amount of 0.95 to 1.35% by weight.

Mo (molybdenum) improves the incombustibility up to 10 times of Ni and prevents the tempering brittleness, but if it is contained too much, it lowers the strength. Considering this point, it is preferable that Mo is contained in an amount of 0.20 to 0.40% by weight.

B (boron) is excellent in improving the incombustibility and has a great influence on the incombustibility even when added in a small amount. Therefore, in the present invention, in order to secure the cold workability, B is added instead of greatly reducing the content of Mn so as to offset the reduction in incineration caused by the decrease of Mn. If the content of B is less than 10 ppm, it is impossible to attain the effect of improving the incombustibility due to reduction of Mn due to the decrease of Mn. If the content exceeds 30 ppm, addition of B during the mass production is difficult and the cost increases greatly.

N (nitrogen) is an interstitial element such as C, which greatly affects the tensile strength, yield strength and elongation of the material. Furthermore, it has been found that by having a large solubility change in the ferrite from 0.1% by weight (580 ° C) to 0.003% by weight (room temperature), various age hardenability and brittleness such as quench aging, strain aging, .

Therefore, in the present invention, the content of N must be precisely controlled in order to compensate for the decrease in the strength of the material by adding B. The N content should be controlled to 20 ppm or less in order to obtain the effect of improving the incombustibility according to the addition of B. However, since it is industrially difficult to control N independently to 20 ppm or less, in the present invention, the content of N is controlled to 0.01 to 0.02% by weight and the rest is controlled by adding Ti.

For this purpose, Ti (titanium) is controlled to be contained in an amount of 0.01 to 0.04% by weight. As described above, B is added in order to offset the decrease in the incombustibility due to the decrease in Mn. When B is present in the form of carbide (BC) or nitride (BN) in the matrix, B does not exhibit the improvement effect on entanglement, ≪ / RTI >

Therefore, in order to maximize the effect of addition of B, the B content in the solute form, which is not carbide or nitride, is important. Therefore, Ti is added to suppress the formation of "BN" type boron nitride. Ti shows strong affinity with O, N, C, S, H, and the like. Thus, the addition of Ti increases the B content of the bridged form by forming a nitride such as "TiN" to inhibit the nitride formation of "BN ". In addition, "TiN" also indicates the effect of suppressing crystal grain growth due to dislocation peening.

In this respect, if the content of Ti is less than 0.01% by weight, the N-binding effect is lost, so that the solubility of B is decreased and the effect of improving the ingotability is not achieved. When the content of Ti exceeds 0.04% by weight, The cost of manufacturing will rise.

The optimization of the alloying element according to the present invention can be achieved by replacing the strength reduction with the decrease of Mn content by the improvement of the ingotability according to the addition of B instead of improving the cold workability by drastically reducing the content of Mn will be. In addition, the contents of Ti and N were precisely controlled in order to enhance the effect of improving the incombustibility according to the addition of B.

As described above, the alloy steel material whose alloy composition is optimized is processed into a shaft shape through cold working such as forging and turning (S21, S22). Unlike the prior art, since the cold workability is improved through optimization of the alloy component, cold working is performed without performing a spheroidizing annealing process. This omission of the spheroidizing annealing step is considered to be the greatest technical feature of the heat treatment process according to the present invention.

The cold-rolled alloy steel is subjected to normalizing treatment at 900 to 950 ° C for 100 to 150 minutes in order to reduce internal stress due to work hardening (S23). The normalizing treatment is intended to prevent the crystal grains from being coarsened during the carburizing treatment which is a post-process due to the internal stress remaining in the work-hardened material.

According to the present invention, since the magnitude of the internal stress remaining after forging and turning is smaller than that of the conventional alloy steel due to the improvement of the cooling processability due to the optimization of the alloy component, the normalizing process time can be drastically shortened. For example, as shown in FIG. 2, according to the present invention, the normalizing process is performed for about 120 minutes at the same temperature, which is conventionally performed at about 930 ° C. for about 300 minutes. This shortening of the normalizing treatment time is another feature of the heat treatment process according to the present invention.

Since the cold rolling alloyed steel structure is subjected to austenitization and air cooling, the normalizing temperature is determined by the austenitizing temperature according to the alloy steel material according to the present invention. Therefore, when the temperature is lower than 900 ° C, the austenitization does not sufficiently proceed and the effect of reducing the internal stress is low, and when the temperature exceeds 950 ° C, the strength is lowered.

As the normalizing time is the same, the normalizing effect can not be obtained when the time is less than 100 minutes, and the normalizing effect is saturated when the time exceeds 150 minutes.

The normalized alloy steel material is carburized to a predetermined depth on the surface in order to increase the surface hardness of the shaft (S24). The carburizing process consists of preheating, surface carburizing (900 to 910 ° C for 90 minutes), diffusion (900 to 910 ° C for 90 minutes) and quenching (800 ° C for 20 minutes) .

The surface carburization and diffusion at the same temperature is the most important step of the carburizing treatment. The surface carburizing is to make the carburizing at a high carbon concentration in the atmosphere. The diffusion is carried out at a low concentration of the carbon atmosphere in the furnace, To a certain depth.

The quenching step is an essential step for increasing the surface hardness after carburizing by quenching at a high temperature of about 900 ° C through room temperature to about 120 ° C at room temperature. The quenching step consists of two steps of first cooling to about 800 ° C in consideration of thermal deformation and then quenching to 120 ° C.

Lastly, in order to solve the internal stress, which is increased during the carburizing process, the shaft is reheated at 180 ° C. and tempered for 120 minutes (S 25).

In order to examine the physical properties of the automobile shaft of the present invention manufactured according to the above-described process, the following experiment was conducted.

First, the conventional alloy steel material and the alloy steel material according to the present invention listed in Table 2 below were prepared, and the automobile shaft was manufactured through the processes shown in FIGS. 2 and 4, respectively.

division
Alloy component (% by weight) C Si Mn P S Cu Ni Cr Mo Ti N B (ppm) Fe Comparative Example 0.2 0.1 0.75 0.01 0.02 0.15 0.20 1.3 0.57 - - - Remainder Example 0.21 0.05 0.4 0.01 0.01 0.15 0.18 1.2 0.35 0.03 0.01 30 Remainder

In order to compare the physical properties of each automotive shaft, surface hardness and core hardness (carried out at 300 gf using a micro Vickers hardness meter according to KS B 0811 measurement method), emulsion hardening depth (micro-Vickers hardness meter according to KS D 0215 measurement method And the tensile strength was measured in a 20-ton tester using a standard tensile specimen of KS B 0801 and a standard size 8-psi specimen in accordance with KS B 0802 tensile test). As a result, Are shown in Table 3 below.

division Surface hardness Effective hardened layer depth Deep hardness The tensile strength standard Measures standard Measures standard Measures Measures Comparative Example 650
More than 768 0.8 ~
1.0 0.7 350 ~
450 425
1020 MPa Example 770 0.75 440 1015 MPa

As shown in Table 3, the automobile shaft manufactured according to the present invention satisfied all the standard specifications and showed almost the same physical properties as the conventional automobile shaft.

In addition, even in the case of contact fatigue strength (surface pressure: 332 kgf / mm2, slip ratio: 40%), both automobile shafts satisfied the standard specifications of over 10 million cycles, and the rotational bending strength Example (400 kgfm) and Example (395 kgfm) showed equivalent physical properties.

1 is a flowchart showing a conventional manufacturing process of a vehicle shaft;

Fig. 2 is a view showing a heat treatment method in the manufacturing process of Fig. 1; Fig.

3 is a flowchart showing a manufacturing process of a vehicle shaft according to the present invention.

Fig. 4 is a view showing a heat treatment method in the manufacturing process of Fig. 3; Fig.

Claims (4)

P: 0.02 wt% or less, S: 0.03 wt% or less, Cu: 0.30 wt% or less, Ni: 0.25 wt% or less 0.95 to 1.35% by weight of Cr, 0.20 to 0.40% by weight of Mo, 0.01 to 0.04% by weight of Ti, 0.01 to 0.02% by weight of N and 10 to 35 ppm of B and balance of Fe and unavoidable impurities Preparing an alloy steel material; A step of immediately cold-working the alloy steel material in a shaft shape without performing spheroidizing annealing; Normalizing the cold-worked shaft at 900 to 950 DEG C for 100 to 150 minutes; And And carburizing the normalized shaft to increase the surface hardness of the shaft. The method according to claim 1, Wherein the cold working step is forging and turning. The method according to claim 1, Wherein the carburizing treatment comprises a preheating step, a surface carburization and diffusion step, which is performed at 900 to 910 占 폚 successively, and a quenching step. The method according to any one of claims 1 to 3, Wherein the carburizing step is followed by a tempering treatment at 170 to 190 DEG C to reduce internal stress.

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