KR100544644B1 - Method for manufacturing high carbon wire rod having superior strength - Google Patents

Abstract

The present invention relates to a method for manufacturing high strength carbon steel wire used as a drawing material, and its object is to provide a method for producing high strength wire by using latent heat and latent heat by rapid cooling in manufacturing medium carbon steel wire. will be.

The present invention for achieving the above object, after heating the billet composed of C: 0.6-1.0%, Mn: 0.1-1.0%, Si: 0.1-0.5% by weight Fe and unavoidable impurities after heating the hot wire re-rolling High strength high carbon steel comprising quenching and winding to a temperature of 850 ~ 750 ℃, then cooling to a range of 530 ~ 560 ℃ at a cooling rate of 30 ~ 80 ℃ / sec and then cooling at a rate of 10 ℃ / sec or less The technical summary of the method for producing wire rods shall be taken.

Wire rod, martensite, latent sapwood, ferrite, pearlite

Description

Method for manufacturing high carbon wire rod having superior strength

1 is a graph showing the temperature change of the material during continuous cooling

Figure 2 is a continuous transformation curve of 2. 0.82 wt% C-0.4 wt% Mn-0.3 wt% Si steel

3 is a microstructure photograph according to the change in the cooling rate change temperature when the cooling rate is changed to 80 ℃ / sec 10 ℃ / sec after cooling 0.82 wt% C-0.7 wt% Mn steel

The present invention relates to a high-strength carbon steel wire manufacturing method used as a drawing material, more specifically, bainite martensite-free ferrite + pearlite or pearlite structure by using the latent transformation heat during transformation at a high cooling rate during cooling after rolling It is a method of manufacturing a high strength wire rod.

The method of increasing the strength of steel wire used in PC wire bead wire, wire rope, etc. can be divided into the method by increasing the amount of drawn wire and the method by improving the strength of the material. The method by increasing the amount of drawn wire is relatively easy to approach, but as the amount of drawn wire increases, the ductility of the steel wire decreases, making it difficult to commercialize. Therefore, the strength improvement method by the strength improvement of a material is the most widely used method.

As a method for improving the freshness and strength of the carbon steel of 0.6% by weight or more, there is a method of increasing the amount of carbon or increasing the cooling rate to obtain fine pearlite interlayer spacing. In order to secure a fine perlite interlayer spacing, it is usually required to cool at a rapid cooling rate of 20-30 ° C./sec or more immediately after rolling to increase the supercooling when transforming pearlite from austenite. However, if it is cooled above a certain cooling rate, the austenite is cooled to low temperature before all of it is transformed into pearlite, thereby forming bainite or martensite in some unmodified austenite. The martensite thus formed is a very hard phase and hardly deformed during drawing, thus acting as a location of cracking, causing disconnection during drawing. Therefore, in order to maintain excellent freshness, such martensite formation must be actively suppressed. In addition, when pearlite and bainite are mixed, strength and ductility decrease. Therefore, it is necessary to actively suppress such bainite or martensite formation.

In order to manufacture high strength wire rods, a method of adding an alloying element or increasing the amount of carbon may be used while limiting the cooling rate to a predetermined cooling rate or less. Such a technique is widely used. However, this method may reduce the freshness during the drawing process by increasing the amount of carbon and the increase of the alloying elements. On the other hand, in order to suppress the transformation of untransformed austenite into martensite after quenching in the early stage of cooling, a method of lowering or maintaining the cooling rate around 500 ° C is also proposed. For example, Japanese Patent Application Laid-open No. Hei 7-90495 discloses a method of holding for 5 seconds to 5 minutes after quenching to 650 to 500 ° C. In addition, Japanese Patent Application Laid-open No. Hei 5-156370 suggests a method of cooling again in a high carbon steel at 450 to 550 ° C. or more at 50 ° C./sec or more, and then maintaining the mixture at a temperature of 450 ° C. or more for 2 minutes and then again. However, these methods are effective in suppressing martensite formation, but there is a possibility that low temperature tissue may be generated due to the temperature variation that is inevitably necessary in mass production, or equipment for precise temperature control in actual production. In addition, Japanese Patent Application Laid-open No. Hei 10-17943 proposes a method of manufacturing a high-strength material at a cooling rate close to water cooling at 20 ° C / sec or more using a mist. According to these, after quenching to suppress martensite formation It suggests a method of raising the temperature by using a slow cooling or thermal cover at -2 ~ 3 ℃ / sec. However, such a method requires additional equipment and also has a problem in that bainite is formed in such a temperature range by controlling the quenching completion temperature to 450 to 500 ° C.

On the other hand, as the cooling method for controlling the hot rolled wire, the Stelmoir method is widely used in most industries. The method is a method of winding hot rolled wire at about 850C ~ 900 ℃ and blowing air on a conveyor to cool it. At this time, by changing the moving speed of the coil in the conveyor can be changed the holding time in the cooling table. That is, if the moving speed is changed under the same conditions, the temperature at which cooling is terminated may be changed by changing the cooling time.

In the present invention, in producing a wire rod containing 0.6 wt% or more of C, a high-strength wire rod is used as a result of using latent heat and latent heat by rapid cooling.

Wire rod manufacturing method of the present invention for achieving the above object, by weight% C: 0.6-1.0%, Mn: 0.1- 1.0%, Si: 0.1-0.5%, after heating the billet composed of the remaining Fe and unavoidable impurities After the hot wire rolling, it is quenched and wound at a temperature of 850 ~ 750 ℃, then cooled to a range of 530 ~ 560 ℃ at a cooling rate of 30 ~ 80 ℃ / sec, and then cooled at a rate of 10 ℃ / sec or less. do.

Hereinafter, the present invention will be described.

In the present invention, the cooling stand is installed on the stealmo with a certain length and changing the conveyor moving speed can change the cooling process of the material, using the latent heat of transformation during the pearlite transformation during cooling, without the occurrence of low-temperature tissue The present invention has been completed by focusing on the fact that the strength can be increased significantly. The composition range of the steel component system of the present invention will be described first.

· C: 0.6 ~ 1.0%

The C is an element which is very effective in increasing the strength, and may be added in an amount of 0.6% or more depending on the intended use. However, in steels containing more than 1.0% of carbon, the cornerstone cementite and cementite fractions increase, resulting in a sharp drop in freshness.

· Si: 0.1 ~ 0.5%

Si is an element necessary for deoxidation of steel, and if its content is less than 0.1%, the deoxidation effect is not sufficient, and it is preferable to limit the upper limit to 0.5% because Si encourages decarburization of steel ingots in the heating furnace during wire rod manufacture. .

· Mn: 0.1 ~ 1.0%

Mn is preferably added in an amount of 0.1% or more because it forms a manganese emulsion (MnS) together with sulfur in the material to prevent red brittleness due to sulfur, as well as deoxidation effect in the production of steel. In addition, manganese is an element that is very effective in increasing the strength and minimizing the interlayer spacing, but when excessively added, the amount of manganese in the segregation zone is greatly increased, thereby increasing the possibility of martensite formation.

By rolling and winding the cold wire with the billet formed as described above, the cooling conditions are controlled to use the latent heat of transformation, thereby avoiding low temperature structures such as bainite and martensite, and manufacturing the wire having a ferrite + pearlite structure. The setting of the cooling conditions will be described in detail in terms of metallurgy.

Figure 1 is an example of the temperature change of the material when the steel is cooled from the austenite region to room temperature after rolling, it can be seen that the temperature rises due to latent heat due to the transformation when the transformation starts while cooling at a constant cooling rate. Therefore, in order to use such latent transformation heat, the maximum cooling rate before transformation during continuous cooling should be at least a cooling rate at which pearlite transformation can start. At higher cooling rates, the perlite is not transformed, but the transformation begins with low temperature tissue. In addition, at lower cooling rates, pearlite or ferrite transformation starts at a relatively high temperature, resulting in lower strength. On the other hand, if the cooling rate is continuously increased after the initiation of the perlite transformation, martensite is generated from the untransformed perlite.

That is, Figure 2 shows the transformation curve according to the cooling rate of 0.82% by weight C-0.4% by weight Mn-0.3% by weight Si steel. After rapid cooling with the pearlite transformation nose part around 550 ° C. and maintaining the temperature at this temperature, it can be transformed to pearlite, but subsequent cooling may result in bainite or martensite structure. Able to know. On the other hand, if austenite is cooled at a faster rate than this, bainite is first precipitated from austenite. Therefore, the maximum cooling rate to obtain a pearlite structure is the cooling rate up to the pearlite transformation nose. However, continuous cooling at such a cooling rate generates low temperature tissues from untransformed austenite, so that the cooling rate must be reduced within the range where the strength is not lowered to obtain perlite or pearlite + ferrite structure without low temperature structure. Tensile strength can be obtained.

Therefore, in the present invention, as described above, the billet is hot-rolled and quenched and wound to 850 to 750 ° C., and then cooling is started, and the cooling rate at this time is 30 to 80 ° C./sec. If the cooling rate is faster than 80 ℃ / sec, bainite is first precipitated from austenite, so the cooling rate should be lowered. If the cooling rate is later than 30 ° C / sec it is not possible to increase the strength compared to the currently produced products.

After cooling at the cooling rate described above, the cooling rate is lowered to 10 ° C / sec in the range of 530 to 560 ° C. If the cooling rate is continued and cooled at a cooling rate faster than 10 ° C./sec, low-temperature structures of bainite or martensite may come out.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

Billets having a chemical composition of 0.82% by weight C-0.7% by weight Mn-0.3% by weight Si were subjected to continuous cooling at a cooling rate of 800 ° C. to 80 ° C./sec after continuous rolling. Perlite structure could be secured at the cooling rate below 30 ℃ / sec, but the pearlite transformation was preferential in the cooling rate range of 35 ℃ / sec ~ 80 ℃ / sec, and then some bainite or martensite depend on the cooling rate. Precipitated.

The critical cooling rate at which martensite is formed is shown in Table 1 below.

Steel grade Martensite Formation Critical Cooling Rate (℃ / sec) 0.72C + 0.4Mn 45 0.72C + 0.7Mn 40 0.82C 38 0.82C + 0.7Mn 35

In addition, it can be seen that at a cooling rate faster than 80 ° C./sec, all of the martensite was not transformed into pearlite. Therefore, in the case of a wire rod used as a wire rod, bainite or martensite formation cannot be avoided at a cooling rate of 30 to 50 ° C./sec or more regardless of steel type change during continuous cooling.

Therefore, such martensite formation is any of the degree of cooling at 80 ° C./sec, which is the maximum cooling rate at which pearlite transformation can start from 800 ° C. after hot rolling of 0.82% by weight C-0.7% by weight Mn steel which is precipitated at the lowest cooling rate. When the cooling rate is changed from 10 ° C./sec to temperature, the microstructure change according to the cooling rate change temperature is shown in FIG. 3. As shown in FIG. 3, the bainite phase was not formed when the cooling rate was changed above 530 ° C., but the bainite phase was formed when the cooling rate was changed at a temperature below that. Therefore, in continuous cooling, bainite or martensite is formed at 35 ° C / sec or more, but when the cooling rate is reduced to 10 ° C / sec or less from 530 ° C or more during cooling, a pearlite structure without low temperature structure can be obtained.

Table 2 shows the change in tensile strength according to the change in cooling rate when cooling is controlled. At this time, the cooling rate change temperature was controlled by changing the time maintained in the quenching equipment during continuous cooling at 850 ℃. In other words, after installing a device having a cooling efficiency of 35 ~ 80 ℃ / sec on the normal Stelmore facilities, the conveyor movement speed is moved to a speed of about 0.5 ~ 1.0m / sec and quenched for about 4 ~ 10 seconds, then cooled Was controlled to 10 ° C./sec or less using a normal Stelmore equipment.

Austenitic Grain Size Initial cooling rate 2nd cooling speed Low temperature tissue formation Tensile strength (kg / mm2) Remarks 0.82C + 0.7Mn 12 25 - radish 121.7 Comparative material 12 80 10 radish 130 Invention 12 95 10 U 120 Comparative material 12 80 18 U - Comparative material 13 58 8 radish 128 Invention 14 46 5 radish 126 Invention 15 35 5 radish 124 Invention 0.72C + 0.4Mn 12 25 - radish 106 Comparative material 12 80 10 radish 115 Invention 13 58 8 radish 111 Invention 14 46 5 radish 110 Invention 15 35 5 radish 109 Invention

As shown in Table 2, when the secondary cooling rate is increased, some fine martensite is formed from untransformed austenite, and when the primary cooling is 95 ° C / sec, it is almost transformed into martensite. The desired characteristics were not obtained. However, as shown in Table 2, by appropriately controlling the cooling rate before and after the increase and the transformation of the cooling rate, it was possible to increase the strength of the pearlite structure by more than about 10% compared to the material produced by the continuous cooling.

As described above, in the present invention, there is a useful effect that a high-strength wire can be manufactured by using latent heat and latent heat by rapid cooling.

Claims (2)

By weight, C: 0.6-1.0%, Mn: 0.1- 1.0%, Si: 0.1-0.5%, Billet composed of remaining Fe and unavoidable impurities is heated and quenched after hot wire re-rolling and wound to a temperature of 850 ~ 750 ℃ And then cooling to a range of 530-560 ° C. at a cooling rate of 30 ° C./80° C. and then cooling at a rate of 10 ° C./sec or less. The method of claim 1, wherein the wire has a ferrite + pearlite or pearlite structure.

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