KR890000665B1 - Process of the wire rod for high tension bolt - Google Patents

Abstract

Steel comprises 0.16-0.20% C, 0.16-0.20% Si, 1.10-1.30% MN, 0.01- 0.02% Nb, under 0.03% P and S and the balance impurities. Steel is heated to 1,150-1,230≰C at heating furnace and before hot rolling, the wire rods are precooled to 950 -1,140≰C and therefore hot-rolled to rolling reduction of 65% and finish-rolled and at 895≰C-910≰C wound up in ring and quenched at 6.5-7.5≰C/sec.

Description

Manufacturing method of wire rod for high tension bolt

Figure showing the moldability of the present invention.

The present invention relates to a method for producing a wire rod for high tension bolts having excellent workability.

In general, high-strength bolts such as automobiles, electricity, construction, and mechanical parts, which require a tensile strength of 70kg / mm2 (7T) or more, should be freshly loaded with rods (hereinafter referred to as "wires"), and then cut into cold working or cold forging. It is difficult to form due to the work hardening of the material, or the material is weak, so that the material is softened by spheroidizing heat treatment before processing, and hardened and hardened after the molding of the bolt to give strength and toughness.

However, the method described above is required to reduce the manufacturing cost by simplifying the process and eliminating the heat treatment because the manufacturing cost is increased by the heat treatment such as spheroidization, quenching, thinning, etc., the manufacturing process is complicated and the productivity is low.

In this regard, the Japanese Patent Application Laid-Open No. 55-31170 discloses a process improvement method that omits the complicated heat treatment process by making the wire component C: 0.3-0.6% and Mn: 1.9-2.5%. Is a method of bolt forming in a warm zone and is not applicable to bolt forming in cooling zones such as cold forgings.

On the other hand, the invention of Japanese Patent Application Laid-Open No. 58-11492 provides a method for producing a wire rod which can form a bolt by adding Ti, B, Cr, Cu, Ni, etc. to low carbon steel and omitting heat treatment by cold working.

However, in the invention of this method, it is necessary to complexly add alloy elements as described above, so that not only the adjustment of complex chemical components is required in the steelmaking process, but also the cost increase due to the addition of alloy elements is inevitable.

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a method capable of manufacturing a high-strength bolt wire having low manufacturing cost and excellent workability, as well as eliminating heat treatment during cold bolt forming.

In other words, the present invention is C: 0.16-0.20, Si: 0.16-0.20%, Mn: 1.10-1.30%, Nb: 0.01-0.02%, p and S: 0.03% or less, and other unavoidable impurities in the heating furnace After heating 1150 ℃-1230 ℃ and hot rolling, the temperature before final rolling is set to 950 ℃ -1040 ℃, and the final rolling is carried out with 65% or more of rolling reduction, followed by winding at 895 ℃ -910 ℃. It is characterized by cooling to ℃ -7.5 ℃ / Sec.

In the present invention, the range of additive element components, rolling conditions, and cooling temperature ranges are as follows.

1. Carbon (C)

Since carbon is an element that increases strength but lowers ductility and toughness, the upper and lower limits are set at 0.16% -0.20%.

2. Silicon (Si)

Silicon is an element that improves deoxidation and strength, but in many cases, ductility is lowered, so the upper and lower limits are set at 0.16% -0.20%.

3. Manganese (Mn)

Manganese is an element necessary for deoxidation, like silicon, and is effective in improving hot workability. In addition, manganese has a density of 1.10-1.30% because densification of metamorphic tissue improves the balance of strength ductility and, in many cases, lowers the transformation point during cooling.

4. Nb

Nb is dissolved in grains when heated to a high temperature to inhibit grain growth of austenite and to prevent grain coarsening.In cooling and rolling, Nb precipitates as NbC or NBN, delaying recrystallization, and resulting from carbonitride precipitation. To increase. For this purpose, the additive component range was set at 0.01% -0.02%.

5. Rolling Condition and Cooling Speed

Prior to rolling the steel with the component of the present invention, the furnace temperature was set to 1150 ° C-1230 ° C. This temperature range is somewhat higher than that of ordinary steel, but it is the temperature range where the added element Nb can be completely dissolved. In addition, in the present invention, if the processing temperature is not controlled during hot rolling, recrystallization occurs, and thus, in the present invention, the final finishing rolling temperature is controlled to 950-1040 ° C. using a precooling system.

On the other hand, in order to produce a good ductile wire by promoting ferrite nucleation in crystal grains and generating a large amount of fine ferrite, the rolling reduction during final finishing rolling is 65% or more and wound at 895-910 ° C. The fine grains of the hot austenite are refined and cooled at a cooling rate of 6.5-7.5 ° C / Sec.

According to the present invention as described above, if the material strength of the wire rod is 60Kg / mm2 or more, the hardness of the bolt is 70Kg / mm2 or more due to the work hardening of the drawing work, and the result of good bolt formability is obtained. Quenching) will be described in more detail through an embodiment in which omission is made.

EXAMPLE

When the steel of the component shown in Table 1 is used as a sample, it is heated to 1200 ℃, and after the final finishing rolling and the coiling temperature are shown in Table 2, the cooling diameter averages 7 ℃ / Sec. Tensile strength, coiling temperature and cross-sectional reduction rate for the same are shown in Table 2.

This is shown in Table 3 the workability and the changed strength of cold processing to increase the strength, which is the bolt processing process, as shown in Table 3. The hardening heat treatment which is a conventional bolt manufacturing process is omitted.

In addition, the drawing shows the result of the forging test, the hatched portion is generally required cold bolt formability limit and in the case of the test NO 1, 2, 3 according to the present invention all satisfy the moldability limit shown in the prior art It can be seen that the spheroidization heat treatment, which is a pretreatment step in the bolt process, can be omitted.

As described above, the steel produced by the manufacturing method of the present invention is manufactured by the manufacturing process that eliminates the heat treatment and processing steps such as spherical heat treatment, quenching, warm drawing after warm drawing, and the like, which are carried out in the conventional method, thereby reducing manufacturing costs. The manufacturing process is a method of manufacturing a wire rod for high tension bolts is simplified.

TABLE 1

TABLE 2

TABLE 3

Claims (1)

C: 0.016-0.20%, Si: 0.16-0.20%, Mn: 1.10-1.30%, p and S: 0.03% & below, Nb: 0.01-0.02% and steels containing unavoidable impurities in the furnace at 1150 ° C After heating to -1230 ℃, during hot rolling, the final pre-rolling temperature is 950 ℃ -1040 ℃ and the final reduction rolling with 65% or more reduction, followed by winding at 895 ℃ -910 ℃ to 6.5 ℃ Method for producing a high-strength bolt wire rod, characterized in that cooled to -7.5 ℃ / Sec.

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