KR102336758B1 - Alloy steel for cold-forging and method of manufacturing the same - Google Patents

Abstract

The method of manufacturing an alloy steel for cold forging according to an embodiment of the present invention is (a) carbon (C): 0.17 to 0.23 wt%, silicon (Si): more than 0 and 0.7 wt% or less, manganese (Mn): 0.45 to 0.9 wt%, chromium (Cr): 0.85 to 2.25 wt%, niobium (Nb): 0.015 to 0.035 wt%, nickel (Ni): more than 0 and 0.25 wt% or less, nitrogen (N): 100 to 160 ppm, oxygen (O) ): Normalizing (Normalizing) step of maintaining the steel containing more than 0 15 ppm and the remainder iron (Fe) and other unavoidable impurities at 870 ~ 930 ℃ 2-3 hours; (b) furnace cooling the steel to 680 to 790 °C at a cooling rate of 0.02 to 0.06 °C/s; and (c) maintaining the steel material at 680 to 790° C. for 2 to 3 hours for stress relief (Stress Relief); includes

Description

Alloy steel for cold forging and manufacturing method thereof

The present invention relates to an alloy steel and a method for manufacturing the same, and more particularly, to an alloy steel for cold forging capable of reducing the required time for hardness reduction heat treatment and a method for manufacturing the same.

During the manufacturing process of automobile parts, the cold forging method has high dimensional precision and is easy to mass-produce due to high productivity and automation, so the application of the method is gradually increasing. However, the cold forging load is directly related to the mold life and dimensional accuracy of parts, so the material before cold forging must have a hardness property of 85 HRB or less.

As a related technology, there is Korean Patent Registration No. 10-0285258.

An object of the present invention is to provide an alloy steel for cold forging and a method for manufacturing the same, which reduces the time required for the hardness reduction heat treatment and can be implemented in a continuous heat treatment furnace.

Alloy steel for cold forging according to an aspect of the present invention is carbon (C): 0.17 to 0.23 wt%, silicon (Si): more than 0 and 0.7 wt% or less, manganese (Mn): 0.45 to 0.9 wt%, chromium (Cr) : 0.85 to 2.25 wt%, niobium (Nb): 0.015 to 0.035 wt%, nickel (Ni): more than 0 and 0.25 wt% or less, nitrogen (N): 100 to 160 ppm, oxygen (O): more than 0 15 ppm and It contains iron (Fe) and other unavoidable impurities in the balance, has a microstructure made of ferrite and pearlite, and has a hardness of 85 HRB or less.

The alloy steel for cold forging may further include molybdenum (Mo): 0.33 to 0.65 wt%.

The alloy steel for cold forging may further include boron (B): 10 to 30 ppm.

A method of manufacturing an alloy steel for cold forging according to another aspect of the present invention is (a) carbon (C): 0.17 to 0.23 wt%, silicon (Si): 0 to 0.7 wt% or less, manganese (Mn): 0.45 to 0.9 wt% %, chromium (Cr): 0.85 to 2.25 wt%, niobium (Nb): 0.015 to 0.035 wt%, nickel (Ni): more than 0 0.25 wt% or less, nitrogen (N): 100 to 160 ppm, oxygen (O) : Normalizing step of maintaining the steel containing more than 0 15 ppm and the remainder iron (Fe) and other unavoidable impurities at 870 ~ 930 ℃ for 2-3 hours; (b) furnace cooling the steel to 680 to 790 °C at a cooling rate of 0.02 to 0.06 °C/s; and (c) maintaining the steel material at 680 to 790° C. for 2 to 3 hours for stress relief (Stress Relief); includes

In the manufacturing method of the alloy steel for cold forging, the heat treatment time required in steps (a) to (c) may be 8 hours or less.

In the manufacturing method of the alloy steel for cold forging, the steps (a) to (c) may be performed in a continuous heat treatment furnace.

In the method for manufacturing the alloy steel for cold forging, the steel material may further include molybdenum (Mo): 0.33 to 0.65 wt%.

In the manufacturing method of the alloy steel for cold forging, the steel material may further include boron (B): 10 ~ 30 ppm.

Advantageous Effects of Invention According to the present invention, it is possible to provide an alloy steel for cold forging and a method for manufacturing the same that can be realized in a continuous heat treatment furnace with a reduced hardness reduction heat treatment time required.

1 is a graph illustrating a low-temperature annealing process in a method of manufacturing an alloy steel for cold forging according to a comparative example of the present invention.
2 is a graph illustrating a spheroidizing heat treatment process in a method for manufacturing an alloy steel for cold forging according to a comparative example of the present invention.
3 is a graph illustrating a heat treatment process of a method for manufacturing an alloy steel for cold forging according to an embodiment of the present invention.
4 is a process flow diagram schematically illustrating a method of manufacturing an alloy steel for cold forging according to an embodiment of the present invention.
5 is a graph illustrating a normalizing heat treatment and a stress relaxation heat treatment process in the method for manufacturing an alloy steel for cold forging according to the first experimental example of the present invention.
6 is a photograph of the structure of the alloy steel for cold forging according to the first experimental example of the present invention.
7 is a graph illustrating a normalizing heat treatment and a stress relaxation heat treatment process in a method for manufacturing an alloy steel for cold forging according to a second experimental example of the present invention.
8 is a photograph of the structure of the alloy steel for cold forging according to the second experimental example of the present invention.
9 is a graph illustrating a normalizing heat treatment and a stress relaxation heat treatment process in a method for manufacturing an alloy steel for cold forging according to a third experimental example of the present invention.
10 is a photograph of the structure of the alloy steel for cold forging according to the third experimental example of the present invention.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings so that those of ordinary skill in the art can easily practice it. The present invention may be embodied in several different forms, and is not limited to the embodiments described herein. The same reference numerals are used throughout this specification to refer to the same or similar components. In addition, detailed descriptions of well-known functions and configurations that may unnecessarily obscure the gist of the present invention will be omitted.

During the manufacturing process of automobile parts, the cold forging method has high dimensional precision and is easy to mass-produce due to high productivity and automation, so the application of the method is gradually increasing. However, the cold forging load is directly related to the mold life and dimensional accuracy of parts, so the material before cold forging must have a hardness property of 85 HRB or less.

For the existing cold forging material, the steel mill supplied the material with reduced hardness by using low-temperature annealing heat treatment to secure shear cutability. After cutting, additional hardness reduction heat treatment was performed before cold forging. For additional heat treatment, companies mainly used spheroidizing heat treatment, but the spheroidizing heat treatment requires at least 24 hours of heat treatment, so the problem of reduced productivity is always emerging.

In addition, alloy steel used for automobile transmission has been developed in the direction of adding elements such as chromium (Cr), silicon (Si), and molybdenum (Mo) in order to improve the fatigue resistance of the final carburizing heat treatment material. The alloy steel to which the above elements are added should be accompanied by heat treatment for a long time in order to decrease hardness by inhibiting solid solution strengthening and cementite diffusion.

Therefore, the present invention applies normalizing and stress relief heat treatment conditions instead of the conventional low annealing heat treatment process using transmission alloy steel, omitting the spheroidizing heat treatment process, and reducing the heat treatment time than the spheroidizing heat treatment We propose an effective reduction method. In addition, it was confirmed that the required physical properties for cold forging were satisfied by applying the normalizing and stress relief heat treatment conditions proposed in the present invention to the continuous heat treatment furnace, and the productivity was dramatically improved compared to the batch used in existing companies. confirmed to be.

Hereinafter, an alloy steel for cold forging according to an embodiment of the present invention will be described in more detail.

Alloy steel for cold forging

Alloy steel for cold forging according to an aspect of the present invention is carbon (C): 0.17 to 0.23 wt%, silicon (Si): more than 0 and 0.7 wt% or less, manganese (Mn): 0.45 to 0.9 wt%, chromium (Cr) : 0.85 to 2.25 wt%, niobium (Nb): 0.015 to 0.035 wt%, nickel (Ni): more than 0 and 0.25 wt% or less, nitrogen (N): 100 to 160 ppm, oxygen (O): more than 0 15 ppm and The remainder contains iron (Fe) and other unavoidable impurities. In addition, the alloy steel for cold forging may further include molybdenum (Mo): 0.33 to 0.65 wt%. The alloy steel for cold forging may further include boron (B): 10 to 30 ppm.

Hereinafter, the role and content of each component included in the alloy steel for cold forging according to an embodiment of the present invention will be described in detail.

Carbon (C): 0.17 to 0.23 wt%

Carbon (C) is an element that increases strength and hardness, and when it is excessively added, cold forgeability is reduced. If the carbon content is less than 0.17 wt%, the strength and hardness required for a transmission or the like cannot be obtained, and if it exceeds 0.23 wt%, toughness is reduced, so the upper limit is limited to 0.23 wt%.

Silicon (Si): greater than 0 0.7 wt% or less

Silicon (Si) is a ferrite generation inducing element, dissolved in ferrite to strengthen it and increase tempering softening resistance. When the silicon content exceeds 0.7% by weight, forging resistance increases and the upper limit is limited to 0.7% by weight.

Manganese (Mn): 0.45 to 0.9 wt%

Mn (manganese) is dissolved in the matrix structure to increase the rotational bending fatigue strength without reducing the elongation and has a great influence on the improvement of hardenability. In addition, manganese refines pearlite and strengthens ferrite in solid solution, thereby greatly improving the yield strength of alloy steel. When the manganese content is less than 0.45 wt%, it is difficult to implement the above-described effects, and when it exceeds 0.9 wt%, it is difficult to control the hardness of the alloy steel to 85 HRB or less, thereby limiting the upper limit to 0.9 wt%.

Chromium (Cr): 0.85 to 2.25 wt%

Chromium (Cr) is a cementite stabilizing element and an element that increases hardenability and improves strength. If the content of chromium is less than 0.85% by weight, it is difficult to expect the above-described effects, and if it exceeds 2.25% by weight, the strength increases but cold forgeability deteriorates, so the content of chromium is adjusted to 0.85 to 2.25% by weight.

Niobium (Nb): 0.015 to 0.035 wt%

Niobium (Nb) produces fine carbides, nitrides, and carbonitrides to refine austenite grains, and increases cold forgeability and fatigue strength of steel. When the content of niobium is less than 0.015% by weight, it is difficult to expect the above-described effect, and when it exceeds 0.035% by weight, the effect is saturated and the machinability of the steel is deteriorated. Therefore, the content of niobium is adjusted to 0.015 to 0.035 wt%.

Nickel (Ni): greater than 0 0.25 wt% or less

Nickel (Ni) is an element that refines the structure of steel and increases hardenability. However, if the content of nickel is more than 0.25 wt%, the toughness of the alloy steel is improved, but the machinability is lowered and the manufacturing cost of the part is increased, which is not economical, so it is limited to 0.25 wt% or less.

Nitrogen (N): 100 to 160 ppm

Nitrogen (N) reacts with niobium to form nitrides or carbonitrides, thereby refining austenite grains and increasing cold forgeability and fatigue strength of steel. When the nitrogen content is less than 100 ppm, it is difficult to expect such an effect. On the other hand, when the nitrogen content exceeds 160 ppm, the nitrogen dissolved in the alloy steel increases the deformation resistance during cold forging, and when the nitrogen content is high when the alloy steel contains boron, BN is generated and the effect of adding boron ( hardenability improvement), it is preferable to adjust the nitrogen content to 100 ~ 160 ppm.

Oxygen (O): >0 15 ppm

Oxygen (O) exists as oxide-based inclusions in steel and is an element that lowers fatigue strength, so the lower it is, the more preferable it is, but up to 15 ppm is acceptable. Usually, it is difficult to set the oxygen content to 0% by weight, but it is desirable to minimize it if possible.

Molybdenum (Mo): 0.33 to 0.65 wt%

Molybdenum (Mo) is an element that enhances the hardenability of steel for cold forging. When the content of molybdenum is less than 0.33% by weight, it is difficult to expect the above-described effect, and when the content of molybdenum exceeds 0.65% by weight, there may be a problem in that ductility is lowered.

Boron (B): 10 to 30 ppm

Boron (B) has an excellent effect of improving hardenability and has a great effect on hardenability even when added in a small amount. Therefore, instead of reducing the content of manganese to ensure cold workability, boron may be added to offset the reduction in hardenability caused by the reduction of manganese. In addition, boron is an element that improves hardenability. When the content of boron is less than 10 ppm, it is difficult to expect the above-described effect, and when it exceeds 30 ppm, there is a problem in that toughness is deteriorated and production cost is increased, so it is preferable to adjust it to 10 to 30 ppm.

The alloy steel for cold forging having the above composition has a microstructure made of ferrite and pearlite. In particular, cementite constituting the pearlite may have a segmented form. The hardness of the alloy steel for cold forging may be 85 HRB or less.

The alloy steel for cold forging according to an embodiment of the present invention described above may be manufactured by the method of an embodiment as follows.

Manufacturing method of alloy steel for cold forging

1 is a graph illustrating a low-temperature annealing process in a method of manufacturing an alloy steel for cold forging according to a comparative example of the present invention, and FIG. 2 is a spheroidization of the alloy steel for cold forging according to a comparative example of the present invention It is a graph illustrating the heat treatment process.

Referring to FIG. 1 , in the method of manufacturing an alloy steel for cold forging according to a comparative example of the present invention, a low-temperature annealing heat treatment may be performed to secure cutability of the alloy steel for cold forging. The low-temperature annealing heat treatment process includes a step of maintaining the temperature directly below A1 for a predetermined time, and then performing a step of cooling.

In addition, in order to additionally satisfy the required hardness of the cold forging material, the spheroidizing heat treatment performed by the company has been performed in many ways to reduce the heat treatment time required. Among the methods, as shown in FIG. 2 to reduce hardness and promote spheroidization of cementite, a method of heating at a temperature directly above A1 for a long time and then cooling after maintaining at a temperature below A1 for a certain period of time is widely used in heat treatment companies. However, in this method, heat treatment time of 24 hours or more is usually required in order to satisfy the physical properties required by cold forging companies. In addition, as heat treatment companies mainly perform spheroidizing heat treatment in batch-type furnaces, problems of increasing production speed and cost are emerging.

3 is a graph illustrating a heat treatment process of a method for manufacturing an alloy steel for cold forging according to an embodiment of the present invention, and FIG. 4 is a process schematically illustrating a method for manufacturing an alloy steel for cold forging according to an embodiment of the present invention It is a flow chart.

3 and 4, the method of manufacturing an alloy steel for cold forging according to an embodiment of the present invention is (a) carbon (C): 0.17 to 0.23 wt%, silicon (Si): more than 0 0.7 wt% or less , manganese (Mn): 0.45 to 0.9 wt%, chromium (Cr): 0.85 to 2.25 wt%, niobium (Nb): 0.015 to 0.035 wt%, nickel (Ni): more than 0 and 0.25 wt% or less, nitrogen (N) : 100 ~ 160 ppm, Oxygen (O): Normalizing step (S100) of maintaining the steel containing more than 0 15 ppm and the remainder iron (Fe) and other unavoidable impurities at 870 ~ 930 ℃ for 2-3 hours (S100) ; (B) furnace cooling the steel to 680 ~ 790 ℃ at a cooling rate of 0.02 ~ 0.06 ℃ / s (S200); and (c) maintaining the steel material at 680 to 790° C. for 2 to 3 hours for stress relief (Stress Relief) step (S300); includes

In the method for manufacturing the alloy steel for cold forging, the heat treatment time required in steps (a) to (c) may be 8 hours or less, and steps (a) to (c) are performed in a continuous heat treatment furnace can be

In the manufacturing method of the alloy steel for cold forging, the steel material may further include molybdenum (Mo): 0.33 to 0.65 wt%, and boron (B): 10 to 30 ppm.

In the present invention, normalizing (N.R) heat treatment and stress relaxation (S.R) heat treatment process are applied instead of the conventional low annealing heat treatment process of alloy steel, and heat treatment conditions that satisfy the physical properties required for cutting and cold forging are proposed.

Specifically, after rolling the material, it is maintained at a temperature of 870 to 930° C. for 2 to 3 hours for homogenization and hardness reduction. After that, in order to minimize the stress generated during phase transformation, furnace cooling is performed at a cooling rate of 0.02 ~ 0.06 ℃/s to a temperature of 680 ~ 790 ℃. After furnace cooling, in order to remove the stress generated during phase transformation and to fragment or re-precipitate cementite, it is maintained at 680 ~ 790 ℃ for 2-3 hours and then cooled to room temperature.

As a result of performing normalizing heat treatment and stress relief heat treatment on alloy steel for cold forging using a continuous heat treatment furnace under the above-described conditions, it was confirmed that the hardness 85HRB or less, the physical property required by cold forging companies, was satisfied.

Experimental example

Hereinafter, preferred experimental examples are presented to help the understanding of the present invention. However, the following experimental examples are only for helping understanding of the present invention, and the present invention is not limited by the following experimental examples.

C
(wt%) Si
(wt%) Mn
(wt%) Cr
(wt%) Mo
(wt%) Nb
(wt%) Ni
(wt%) B
(ppm) N
(ppm) O
(ppm) Experimental Example 1 0.17 to 0.23 0.15 to 0.35 0.55 to 0.90 0.85-1.25 - 0.015~0.035 ~0.25 10-30 100-160 ~15 Experimental Example 2 0.17~0.21 ~0.15 0.60 to 0.85 1.25 to 1.45 0.55 to 0.65 0.015~0.035 ~0.25 - 100-160 ~15 Experimental Example 3 0.17 to 0.23 0.50 to 0.70 0.45 to 0.75 1.95~2.25 0.33 to 0.43 0.015~0.035 ~0.25 - 100-160 ~15

Table 1 shows the composition range of the steel according to the experimental examples of the present invention.

The steel material according to Experimental Example 1 of Table 1 is carbon (C): 0.17 to 0.23 wt%, silicon (Si): 0.15 to 0.35 wt%, manganese (Mn): 0.55 to 0.9 wt%, chromium (Cr): 0.85 to 1.25 wt%, niobium (Nb): 0.015 to 0.035 wt%, nickel (Ni): greater than 0 and 0.25 wt% or less, boron (B): 10 to 30 ppm, nitrogen (N): 100 to 160 ppm, oxygen (O) ): satisfies the composition range consisting of more than 0 and not more than 15 ppm and the remainder being iron (Fe). In the steel according to Experimental Example 1 satisfying the above-described composition, it is predicted that the A1 temperature is 710°C, and the A3 temperature is 805°C.

The steel material according to Experimental Example 2 is carbon (C): 0.17 to 0.21 wt%, silicon (Si): more than 0 and 0.15 wt% or less, manganese (Mn): 0.60 to 0.85 wt%, chromium (Cr): 1.25 to 1.45 wt% %, molybdenum (Mo): 0.55 to 0.65 wt%, niobium (Nb): 0.015 to 0.035 wt%, nickel (Ni): greater than 0 0.25 wt% or less, nitrogen (N): 100 to 160 ppm, oxygen (O) : It satisfies the composition range consisting of more than 0 and not more than 15 ppm and the remainder being iron (Fe). The steel material according to Experimental Example 1 satisfying the above-described composition has an A1 temperature of 738°C and a A3 temperature of 822°C.

The steel material according to Experimental Example 3 is carbon (C): 0.17 to 0.23 wt%, silicon (Si): 0.50 to 0.70 wt%, manganese (Mn): 0.45 to 0.75 wt%, chromium (Cr): 1.95 to 2.25 wt% , molybdenum (Mo): 0.33 to 0.43 wt%, niobium (Nb): 0.015 to 0.035 wt%, nickel (Ni): greater than 0 0.25 wt% or less, nitrogen (N): 100 to 160 ppm, oxygen (O): It satisfies the composition range consisting of more than 0 and 15 ppm or less and the remainder being iron (Fe). In the steel according to Experimental Example 1 satisfying the above-described composition, it is predicted that the A1 temperature is 760°C, and the A3 temperature is 830°C.

Low-temperature annealing and spheroidizing heat treatment (comparative example) Normalizing and Stress Relief Heat Treatment (Example) Experimental Examples 1, 2, 3 Hardness: 85 HRB or less
Microstructure: spheroidized tissue
Heat treatment time required: 26 hours Hardness: 85 HRB or less
Microstructure: ferrite and pearlite structure
Heat treatment time required: 8 hours

Table 2 shows the comparison of hardness, microstructure, and heat treatment time required according to the heat treatment conditions of the alloy steel for cold forging according to the experimental examples of the present invention. The low-temperature annealing heat treatment is the heat treatment described with reference to FIG. 1, the spheroidizing heat treatment is the heat treatment described with reference to FIG. 2, and the normalizing and stress relief heat treatment is the heat treatment described with reference to FIG. 3 .

Referring to Table 1 and Table 2 together, when low-temperature annealing and spheroidizing heat treatment were performed on the alloy steels for cold forging of Experimental Example 1, Experimental Example 2, and Experimental Example 3 having the composition of Table 1 (Comparative Example) , it can be seen that the hardness is 85 HRB or less, the microstructure has a spheroidized structure, and the heat treatment time required is 26 hours. On the other hand, it was confirmed that the hardness before the spheroidizing heat treatment after the low annealing heat treatment was 89 to 99 HBR.

In contrast, when normalizing heat treatment and stress relief heat treatment were performed on the alloy steel for cold forging of Experimental Example 1, Experimental Example 2 and Experimental Example 3 having the composition of Table 1 (Example), the hardness was It can be seen that 85 HRB or less, the microstructure is ferrite and pearlite, and the heat treatment time required is 8 hours or less. A detailed description thereof is provided below.

5 is a graph illustrating the normalizing heat treatment and stress relaxation heat treatment process in the method for manufacturing the alloy steel for cold forging according to the first experimental example of the present invention, and FIG. 6 is the alloy steel for cold forging according to the first experimental example of the present invention organization picture.

5 and 6, a normalizing step of maintaining the steel material satisfying the composition range of Experimental Example 1 in Table 1 at 880 ℃; furnace cooling the steel to 690 °C at a cooling rate of 0.05 °C/s; And Stress relief (Stress Relief) step of maintaining the steel at 690 ℃; As a result of cooling to room temperature after performing , it can be confirmed that the final microstructure of the alloy steel for cold forging consists of ferrite and pearlite, and the hardness is 80.3 HRB.

7 is a graph illustrating the normalizing heat treatment and stress relaxation heat treatment process in the manufacturing method of the alloy steel for cold forging according to the second experimental example of the present invention, and FIG. 8 is the alloy steel for cold forging according to the second experimental example of the present invention organization picture.

7 and 8, the normalizing step of maintaining the steel material satisfying the composition range of Experimental Example 2 in Table 1 at 900 ℃; furnace cooling the steel to 740 °C at a cooling rate of 0.04 °C/s; And Stress relief (Stress Relief) step of maintaining the steel at 740 °C; As a result of cooling to room temperature after performing , it can be confirmed that the final microstructure of the alloy steel for cold forging consists of ferrite and pearlite, and the hardness is 79.9 HRB.

9 is a graph illustrating the normalizing heat treatment and stress relaxation heat treatment process in the manufacturing method of the alloy steel for cold forging according to the third experimental example of the present invention, and FIG. 10 is the alloy steel for cold forging according to the third experimental example of the present invention organization picture.

9 and 10, the normalizing step of maintaining the steel material satisfying the composition range of Experimental Example 3 in Table 1 at 900 ℃; furnace cooling the steel to 770 °C at a cooling rate of 0.03 °C/s; And Stress relief (Stress Relief) step of maintaining the steel at 770 ℃; As a result of cooling to room temperature after performing , it can be confirmed that the final microstructure of the alloy steel for cold forging consists of ferrite and pearlite, and the hardness is 81.1 HRB.

So far, an alloy steel for cold forging and a method for manufacturing the same according to the experimental examples of the present invention have been described. The present invention simplifies the process and heat treatment time by omitting the spheroidizing heat treatment using normalizing and stress relief heat treatment in order to satisfy the raw material properties that can be easily cut and cold forged of alloy steel for transmission The condition was developed to shorten the time by 31% compared to the previous case. In addition, it has been confirmed that the normalizing and stress relief heat treatment conditions can be implemented in a continuous heat treatment furnace, which is expected to significantly contribute to productivity improvement compared to the batch type furnace used in the company.

In the above, the embodiments of the present invention have been mainly described, but various changes or modifications can be made at the level of those skilled in the art. As long as such changes and modifications do not depart from the scope of the present invention, it can be said that they belong to the present invention. Accordingly, the scope of the present invention should be judged by the claims described below.

Claims (8)

Carbon (C): 0.17 to 0.23 wt%, Silicon (Si): more than 0 and 0.7 wt% or less, Manganese (Mn): 0.45 to 0.9 wt%, Chromium (Cr): 0.85 to 2.25 wt%, Niobium (Nb): 0.015 to 0.035 wt%, Nickel (Ni): greater than 0 and 0.25 wt% or less, Boron (B): 10 to 30 ppm, Nitrogen (N): 100 to 160 ppm, Oxygen (O): greater than 0 15 ppm and the balance Contains iron (Fe) and other unavoidable impurities,
It has a microstructure made of ferrite and pearlite,
Characterized in that the hardness is 85 HRB or less,
Alloy steel for cold forging. The method of claim 1,
Molybdenum (Mo): 0.33 to 0.65% by weight further comprising,
Alloy steel for cold forging. delete (a) carbon (C): 0.17 to 0.23 wt%, silicon (Si): more than 0 and 0.7 wt% or less, manganese (Mn): 0.45 to 0.9 wt%, chromium (Cr): 0.85 to 2.25 wt%, niobium ( Nb): 0.015 to 0.035 wt%, Nickel (Ni): greater than 0 0.25 wt% or less, Boron (B): 10 to 30 ppm, Nitrogen (N): 100 to 160 ppm, Oxygen (O): greater than 0 15 ppm And a normalizing (Normalizing) step of maintaining the steel material containing the remainder iron (Fe) and other unavoidable impurities at 870 ~ 930 ℃ 2 ~ 3 hours;
(b) furnace cooling the steel to 680 ~ 790 ℃ at a cooling rate of 0.02 ~ 0.06 ℃ / s; and
(c) maintaining the steel material at 680 ~ 790 ℃ for 2 ~ 3 hours stress relief (Stress Relief) step; containing,
Method for manufacturing alloy steel for cold forging. 5. The method of claim 4,
The heat treatment time required in steps (a) to (c) is characterized in that it is 8 hours or less,
Method for manufacturing alloy steel for cold forging. 5. The method of claim 4,
Steps (a) to (c) are characterized in that performed in a continuous heat treatment furnace,
Method for manufacturing alloy steel for cold forging. 5. The method of claim 4,
Molybdenum (Mo): 0.33 to 0.65% by weight further comprising,
Method for manufacturing alloy steel for cold forging. delete

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