KR100347575B1 - Step cooling method of high carbon wire rod for inhibiting generation of martensite - Google Patents

Abstract

PURPOSE: Provided is a step cooling method of high carbon wire rod for inhibiting the generation of martensite irrespective of the amount of segregation in central part of wire rod by controlling cooling rate according to temperature. CONSTITUTION: In a manufacture method of high carbon steel comprising C 0.4 to 0.9 wt.%, Si 0.01 to 1.0 wt.%, Mn 0.1 to 1.0 wt.%, 0.03 wt.% or less of P, 0.03 wt.% or less of S, Al 0.01 to 1.0 wt.%, a balance of Fe and incidental impurities, the method is characterized in that the high carbon wire rod steel undergoes hot rolling after heating at higher than 1000 deg.C; cold rolling at a cooling rate of 15 to 25 deg.C/sec over the temperature range of 900 to 550°C and at a cooling rate of 5 to 10 deg.C/sec over the temperature range of 550 to 300°C for improving elongation and reducing martensite.

Description

Step cooling method of high carbon wire rod to suppress martensite generation

The present invention relates to a step cooling method of a high carbon wire rod for suppressing the generation of martensite, and more particularly, by controlling the cooling rate according to the cooling temperature effectively prevents the generation of martensite regardless of the amount of segregation of the center of the wire rod for rolling. It relates to a step cooling method that can be done.

In general, high carbon wire is a wire rod that is processed into a wire rope, hard wire, etc., the most important item in the quality of the wire is to ensure the structure of the wire uniformly.

1 is a process chart showing a hot rolling process of a high carbon wire, and FIG. 2 is a process chart showing a drawing and heat treatment process of a high carbon wire.

In the hot rolling process of the high carbon wire rod, the wire rod is heated in the heating furnace of 1000-1250 ° C. before hot rolling to increase the temperature of the wire rod, and the wire rod is hot rolled and wound up in a winding machine to cool the wire rod. Cooling then begins immediately while passing through the winder.

After hot rolling, the cooled high carbon wire goes through a drawing and heat treatment process. The drawing and drawing process of the wire is the most important process in the manufacturing process.

In the drawing and heat treatment process of high carbon wire, hot rolled and cooled wire is drawn and heat treated and fresh again.

Conventionally, step cooling is also used to cool the wire rod after hot rolling.

In the conventional step cooling method, eight coolers are installed at a lower portion of the wire rod passing through the winding machine to cool the wire rod passing through the upper stage by blowing cold air.

Conventionally, when cooling a wire in a coil state when manufacturing a high carbon wire, it is required to secure a fine pearlite structure in order to secure a maximum tensile strength of the product state after drawing, so that cold cooling is applied as much as possible.

In general, when the winding temperature, that is, the cooling start temperature is 800-900 ℃ to 900-550 ℃ to cool at a cooling rate of 18 ℃ / sec, and at 550-300 ℃ to cool at a cooling rate of 15 ℃ / sec.

As in the conventional method, when the cooling is applied, the impurity element content of the central segregation zone is higher than that of the matrix, so segregation and transformation delay occur, and the residual austenite is transformed at low temperature, thereby causing martensite, which is an inferior renal tissue. )

When martensite, which is a hard tissue, is generated in the cooling process, disconnection occurs frequently in the drawing process.

The present invention for solving the above problems is to provide a cooling method that can prevent the generation of martensite regardless of the amount of segregation of the center of the wire rod for wire uniformity by controlling the cooling conditions performed after hot rolling. It aims to do it.

1 is a process chart showing a hot rolling process of a high carbon wire rod,

2 is a process chart showing the drawing and heat treatment process of high carbon wire;

3 is a graph showing the constant temperature transformation curve for each cross-sectional position of the wire rod,

Figure 4a is a wire rod texture picture of the invention steel hot rolled,

Figure 4b is a photograph of the wire structure of the hot rolled comparative steel.

The present invention for achieving the above object, by weight% C: 0.4-0.9%, Si: 0.01-1.0%, Mn: 0.1-1.0%, P: 0.03% or less, S: 0.03% or less, Al: 0.01 In processing high carbon steels containing -1.0% and consisting of residual Fe and other inevitable impurities, the high carbon steels are heated to 1000 ° C. or higher, followed by hot wire re-rolling, and cooling after rolling to 900-550 By cooling at a rate of 15-25 ℃ / sec at ℃, and at a rate of 5-10 ℃ / sec at 550-300 ℃, it is characterized in that it improves the fresh workability and minimize the generation of martensite in the center.

Hereinafter, the present invention will be described in detail.

The present invention contains C: 0.4-0.9% by weight, Si: 0.01-1.0%, Mn: 0.1-1.0%, P: 0.03% or less, S: 0.03% or less, Al: 0.01-1.0% and the balance Fe And a known high carbon steel composed of other inevitably contained impurities.

That is, after hot-rolling high carbon steel composed of the above components, the winding temperature, which is the cooling start temperature when the wire is cooled, is maintained in the range of 800-900 ° C., and the temperature of the pearlite transformation temperature zone is 900-550 ° C. when the wire is cooled. Cooling is performed at a rate of 15-25 ° C./sec, and a cooling rate of 5-10 ° C./sec is applied to 550-300 ° C. after the pearlite transformation temperature zone, for the following reasons.

If the cooling start temperature of the wire is set to a high temperature of more than 900 ℃, there is a fear of grain boundary cementite, and when starting cooling at a low temperature below 800 ℃, martensite structure occurs due to excessive transformation driving force have.

Therefore, it is preferable to maintain the cooling start temperature of the wire rod in the range of 800-900 ° C. to prevent the generation of grain boundary cementite and to prevent the formation of martensite structure.

When cooling at a cooling rate higher than 25 ℃ / sec in the temperature range of 900-550 ℃ before transformation in the cooling rate there is a problem that the martensite hard tissue, which cannot be drawn in the post-process to the base structure. On the other hand, in the case of cooling at a cooling rate lower than 15 ° C / sec, pearlite having a thick cementite thickness is generated, which is a factor that hinders freshness.

On the other hand, in the case of cooling at a cooling rate higher than 10 ° C / sec in the temperature range of 550 ~ 300 ° C after transformation, there is a problem that the high cooling occurs, such as martensite or bainite. On the other hand, when the wire is cooled at a cooling rate of less than 5 ° C / sec, the thickness of cementite in the pearlite structure is excessively thick, which impairs the freshness and causes disconnection.

Therefore, cooling at 15-25 ° C / sec at 900-550 ° C and 5-10 ° C / sec at 550-300 ° C results in excessive martensite generation and excessive cementite thickness in the pearlite tissue. It is preferable because the problem of thickening can be solved.

In addition, by quenching at the cooling rate of 15-25 ℃ / sec to the pearlite transformation temperature region of steel, the gap between lamellae in the pearlite structure is narrowed to secure the tensile strength suitable for hard steel wire, and 5-10 ℃ / after the pearlite transformation temperature region. The slow cooling of sec is performed to induce untransformed austenite in the center to perlite.

3 shows a comparison of the constant temperature transformation curves of the cross sections of wire rods when they are cooled by the conventional cooling method and when they are cooled by the cooling method of the present invention.

In the case of cooling by the conventional cooling method, the martensite structure is generated by cooling at almost the same cooling rate during the entire cooling process, and in particular, martensite structure is generated by the strong cooling in the central segregation part.

On the other hand, in the cooling method of the present invention, the occurrence of martensite is suppressed by quenching in the early stage, and the martensite structure is suppressed in the central segregation portion by slow cooling in the step of cooling the central segregation portion, so that a large number of pearlite structures are generated in the center portion. It was.

Therefore, it can be seen that the cooling method of the present invention is more effective in suppressing the generation of martensite than the conventional cooling method.

The present invention will be described in more detail with reference to the following Examples.

<Example>

Steel having a composition as shown in Table 1 was prepared as steel sections having a cross-sectional dimension of 160 mm × 160 mm.

After preparing Comparative Steel 1 to Comparative Steel 2 and Invented Steel 1 to Invented Steel 2 having the composition shown in Table 1, the steel was heated under the conditions as shown in Table 2, followed by wire rod rolling and cooling.

After the wire was rolled under the conditions as shown in Table 2, the structure and mechanical properties of the comparative steel and the inventive steel were measured, and the results are shown in Table 3 below.

As can be seen from the results of Table 3, in the case of Comparative Steel 1, the martensite generation area ratio, which is an inferior renal tissue, was 5%, whereas in the Inventive Steel 1 to Inventive Steel 2 and Comparative Steel 2, 0% of martensite did not occur. .

In terms of tensile strength, the tensile strength of the inventive steels 1 to 2 is 115-117 kg / mm ² , whereas the tensile strength of the comparative steels 1 to 2 is 114 kg / mm ² . Higher than tensile strength.

In the case of pearlite generation area ratio, Comparative Steel 1 and Comparative Steel 2 were 10%, while Inventive Steel 1 was 5% and Inventive Steel 2 was 8%.

Figures 4a and 4b shows a picture of the wire structure of the hot rolled comparative steel and the inventive steel.

In the case of the inventive steel, the martensite, which is a hard tissue, does not occur, but the structure is uniform, but in the case of the comparative steel, martensite is generated in 5% of the area, and thus the tissue is not uniform.

The high carbon wire produced by cooling according to the method of the present invention does not generate martensite, which is an insulated wire structure, and prevents thickening of cementite in the pearlite tissue during cooling of the wire. It was able to solve the problem of disconnection when drawing.

In addition, the lamellar spacing in the pearlite structure is narrowed to secure an appropriate tensile strength for hard wire.

Claims (1)

% By weight C: 0.4-0.9%, Si: 0.01-1.0%, Mn: 0.1-1.0%, P: 0.03% or less, S: 0.03% or less, Al: 0.01-1.0%, balance Fe and other unavoidable In processing high carbon steel composed of impurity, the hot carbon steel is heated to 1000 ° C or higher, and hot wire rolling is carried out, and after rolling, cooling is performed at a speed of 15-25 ° C / sec at 900-550 ° C. By cooling and cooling at a rate of 5-10 ° C./sec at 550-300 ° C., stepwise cooling method of high carbon wire rod to suppress martensite generation, which improves fresh workability and minimizes martensite generation at the center. .

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