KR20020078830A - A method for manufacturing steel wire rod for cold forging with low deviation in mechanical properties - Google Patents

Abstract

PURPOSE: A method for manufacturing a steel wire rod for cold forging is provided to obtain a steel wire rod with low deviation in mechanical properties by changing microstructures of steel wire rod to ultra low carbon bainite. CONSTITUTION: The method for manufacturing a steel wire rod includes the steps of reheating a steel billet comprising C 0.10 wt.% or less, Si 0.1 to 0.7 wt.%, Mn 1.0 to 3.0 wt.%, P 0.03 wt.% or less, S 0.03 wt.% or less, Cr 0.20 to 1.0 wt.%, Mo 0.10 to 0.50 wt.%, Ni 0.2 to 1.0 wt.%, Cu 0.3 to 1.5 wt.% V 0.05 to 0.20 wt.%, Ti 0.01 to 0.03 wt.%, B 0.0010 to 0.0050 wt.%, a balance of Fe and other unavoidable impurities; hot rolling the steel billet at a finish rolling temperature of higher than 800°C; and cooling the hot rolled steel wire rod from the finish rolling temperature to 300 to 400°C at a cooling rate of 0.1 to 10°C/sec.

Description

A method for manufacturing cold-rolled steel wire with less variation in mechanical properties {A METHOD FOR MANUFACTURING STEEL WIRE ROD FOR COLD FORGING WITH LOW DEVIATION IN MECHANICAL PROPERTIES}

The present invention relates to a method for manufacturing a cold-rolled steel wire used as a material for automobile parts, and more particularly, to a cold-rolled steel for greatly improving the material deviation by changing the microstructure of the steel wire into an ultra low carbon bainite structure. It relates to a method for producing a steel wire.

In general, the medium carbon steel wire rods and cold-rolled non-coarse steel wire rods, which are commonly used as materials for automobile parts, are mainly composed of ferrite and pearlite in the hot-rolled state. The cooling rate is different for each coil, so a difference occurs in the microstructure of each coil position. For example, a relatively fast cooling rate is small in the size of the ferrite, the fraction of pearlite is increased, and the low temperature transformation tissue, such as bainite (Bainite) is partly mixed to increase the strength. On the contrary, in the region where the cooling rate is relatively slow, the size of the ferrite is increased and the fraction of the ferrite is increased, thereby decreasing the strength. Therefore, in the case of the medium carbon steel wire or the non-coarse steel wire mainly composed of ferrite and pearlite, the occurrence of microstructure variation due to the cooling rate variation and the material unevenness due to this cannot be avoided.

In general, the weight of one coil of wire is 2 tons, and when the 80 kg / mm 2 grade steel wire is wound and cooled by a coil of 2 ton weight, the variation in tensile strength in the coil is about 10 kg / mm 2. This material deviation in the wire condition leads to a material deviation of the final product, causing a problem of increasing the product failure rate.

In order to solve the problems as described above, techniques for a manufacturing method that can improve the material deviation of the wire rod has been proposed. For example, in Japanese Patent Laid-Open No. 9-67622, a steel containing by weight% C: 0.15 to 0.35%, Si: 0.05% or less, Mn: 0.70 to 1.50%, N: 0.005% or less, and Cr: 0.20% or less. The steel wire with low material deviation in the coil, characterized in that the steel wire with a tensile strength of 700 ~ 930 N / ㎜ is produced by drawing 20 ~ 30% of the wire obtained by cooling at a rate of 2 ℃ / sec or more after hot rolling using The preparation method is presented. However, this technique has the advantage that the material deviation can be reduced by controlling only the cooling rate of the medium carbon steel of the plain component, but the coil end or the outside of the coil as well as the middle of the coil and the coil having a slow cooling rate Since even the cooling rate should be 2 ℃ / sec or more, there is a problem that a cooling facility for forcibly cooling the wire coil. That is, when the wire rod coil is cooled in the air, such a cooling rate cannot be obtained inside the coil that is not in contact with the atmosphere. Therefore, a facility and a medium capable of quenching the wound wire coil are needed.

Accordingly, the inventors of the present invention have repeatedly studied and experimented to solve the above problems and proposed the present invention based on the results, and the present invention controls the steel components and cooling conditions after rolling to control the structure of the wire rod. By using the ultra-low carbon bainite, to provide a method for producing a cold-rolled steel wire rod with a very small material variation in the wire coil, an object thereof.

1 is a graph showing the change in hardness according to the cooling rate

(A), (b), (c) are micrographs showing the microstructure of the inventive steel, comparative steel 1, and comparative steel 2;

The present invention for achieving the above object,

By weight%, C: 0.10% or less, Si: 0.1-0.7%, Mn: 1.0-3.0%, P: 0.03% or less, S: 0.03% or less, Cr: 0.20-1.0%, Mo: 0.10-0.50%, Ni: 0.2 ~ 1.0%, Cu: 0.3 ~ 1.5% V: 0.05 ~ 0.20%, Ti: 0.01 ~ 0.03%, B: 0.0010 ~ 0.0050%, Re-heated steel sheet composed of residual Fe and other unavoidable impurities Hot rolling is carried out in the form of a wire rod at a temperature of 800 ° C. or higher, and cooling the temperature section of 300 to 400 ° C. at a rate of 0.1 to 10 ° C./second from the finish rolling temperature is characterized in that the variation in mechanical properties is achieved. The present invention relates to a method for producing a cold rolled steel wire.

Hereinafter, the steel component and manufacturing conditions of this invention are demonstrated.

First, in the present invention, it is preferable to manage the content of carbon (C) to 0.10% or less in order to reduce the material deviation caused by the difference in cooling rate while having excellent mechanical properties. The reason is that carbon (C) has a great influence on the microstructure and mechanical properties of the steel. If the content is 0.10% or more, the target ultra low carbon bainite structure cannot be obtained, and the material deviation of the steel becomes large and the toughness is high. Deterioration, even if the bainite structure is obtained through cooling control, the material deviation of the steel wire is increased, the brittleness is very bad. Therefore, it is preferable to set the carbon content to 0.10% or less because very low carbon bainite structure having a low carbon content must be obtained in order to reduce the material deviation caused by the difference in cooling rate with excellent mechanical properties.

Silicon (Si) is usually added for deoxidation during steelmaking, and also plays a role of securing strength required for products. However, when the content of Si is less than 0.1%, deoxidation is not performed smoothly, and it is difficult to secure the required strength. When the content of Si is more than 0.7%, the deformation resistance during cold working is greatly increased, thereby rapidly decreasing the cold pressure composition. Not.

Manganese (Mn) is an element that increases the hardenability of steel and has a structure refining effect, and increases the strength of steel without deteriorating impact toughness. In order to increase the hardenability of the steel by adding manganese and to stably obtain bainite structure even in a slow cooling rate section, it is preferable to set the content to 1.0% or more. Since martensite tissue is formed in the toughness deterioration, the addition range of the manganese is preferably limited to 1.0 ~ 3.0%.

Phosphorus (P) is segregated at the grain boundary, and the toughness of the steel is reduced, it is preferable to limit the content to 0.03% or less.

Since sulfur (S) reduces the impact toughness of the steel, it is preferable to limit the content to 0.03% or less.

On the other hand, chromium (Cr) increases the hardenability of the steel and acts to stably obtain the ultra-low carbon bainite structure. If the content is small, it is difficult to obtain such an effect, and if the amount is large, brittle martensite structure is generated. Therefore, the content of chromium is preferably set to 0.2 to 1.0%.

In addition, boron (B) is a characteristic element of the present invention is an element that serves to increase the hardenability of steel to stably obtain the ultra-low carbon bainite structure. In particular, in order to stably secure the pure bainite structure containing no ferrite to be obtained in the present invention, it is necessary to suppress the transformation into the ferrite structure that appears at a higher temperature than the bainite structure in general during the cooling process. When boron (B) is added in an appropriate amount, boron (B) is segregated in the austenite grain boundary in the steel at a high temperature, thereby lowering the grain boundary energy, thereby effectively inhibiting transformation into ferrite tissue during cooling. Done. Thus, it is possible to obtain pure bainite structure in which ferrite is not incorporated. Here, when the content of the boron (B) is less than 0.0010%, it is difficult to obtain the above-described functional effect, and when the content is added more than 0.0050%, the effect is saturated, so it is preferable to add it in 0.0010 ~ 0.0050%.

Molybdenum (Mo) is an element that increases the hardenability of the steel to obtain an ultra low carbon bainite structure stably. If the content is small, it is difficult to obtain this effect. Preference is given to adding in%.

Copper (Cu) serves to increase the strength by combining with iron (Fe) in the steel during the cooling process to form a precipitate. Particularly, in the present invention steel, due to a large amount of precipitation in the region of the slow cooling rate, that is, 0.1 ~ 1.0 ℃ / second, to compensate for the decrease in strength due to the decrease in the cooling rate to reduce the material deviation. When the content of copper (Cu) is less than 0.3%, it is difficult to obtain the above-described effects, and when it is added more than 1.5%, the strength is excessively increased to promote material deviations, so the content is preferably set to 0.3 to 1.5%. Do

Nickel (Ni) increases the hardenability of the steel to obtain a very low carbon bainite structure stably, and serves to increase the strength of the steel without reducing the toughness. When the content of nickel is less than 0.2%, it is difficult to obtain the above-mentioned effects, and when more than 1.0% is added, the strength of the steel is excessively increased. Therefore, the content is preferably set to 0.2 to 1.0%.

Vanadium (V) is an element that combines with carbon in steel to form carbides to increase the strength of steel.If the content is less than 0.05%, it is difficult to achieve the above-mentioned effects, and when 0.20% or more is added, the steel becomes brittle. Preference is given to adding in%.

Titanium (Ti) is an element that combines with carbon and nitrogen to form carbonitrides to increase the toughness and strength of steel. In order to obtain such an effect, the content is preferably 0.01% or more, but is excessively added to 0.03% or more. In this case, since the steel is embrittled, the upper limit of the titanium content is preferably set to 0.03%.

On the other hand, the steel strips prepared as described above are reheated and then wire rod hot rolled, and finish-rolled at a temperature of 800 ℃ or more. The reason why the finish rolling is carried out at a temperature of 800 ° C. or higher is that if the finish rolling temperature is less than 800 ° C., ferrite may be formed during rolling deformation, resulting in a decrease in strength and unevenness of materials.

Subsequently, when the hot wire rolling is finished, it is preferable to cool the temperature section between 300 to 400 ° C. at a rate of 0.1 to 10 ° C./second from the temperature at which the hot rolling is finished, because the cooling rate is 0.1 If it is less than ℃ / seconds ferritic structure occurs during metamorphosis, the strength of the steel is severely reduced, if faster than 10 ℃ / seconds hard martensite structure of the brittle strong during transformation, the strength of the steel is excessively increased and toughness is reduced. In other words, without cold artificial steel air cooling or artificial thermal cooling to the hot-rolled wire rod without applying artificial slow cooling in the air at a cooling rate of 0.1 ~ 10 ℃ / second air cooling rate, ultra-low-carbon bainite structure of the desired structure By securing, it is possible to reduce the deviation of the mechanical properties. Here, the temperature section, that is, the temperature section between 300 ~ 400 ℃ hot rolling finish means a section in which the tissue is transformed into ultra-low carbon bainite structure, cooling to 300 ℃ ~ room temperature is a process convenience Natural cooling may be continued as it is, or the cooling rate may be artificially changed.

The steel wire produced as described above contains a small amount of carbon (C) and boron (B) in the steel component, so that the structure becomes an extremely low carbon bainite structure regardless of the coil position, so that the cooling rate is slow. Without cooling the inside separately, even if the cooling from the hot wire rod rolling to the temperature range where transformation to ultra-low carbon bainite is completed is 0.1 to 10 ° C / sec, the mechanical deviation is significantly reduced. This low material deviation extends to the product, which is advantageous in terms of the product.

On the other hand, the wire rod is cold forged after the cold wire processing through a conventional method is processed into a product.

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

(Example 1)

To produce steel as shown in Table 1 as a small steel piece of 110 mm x 110 mm x 250 mm After heating at 1100 ° C. for 2 hours, hot rolling was performed to complete rolling in the form of a plate at 850 ° C. After the rolling is completed, air and water are sprayed on the rolled plate with steel to simulate the difference in cooling rate between the wire coil and the outside in the actual site. After cooling while changing to 0.1-100 ° C./s, the hardness was measured and the results are shown in FIG. 1.

division Steel component (wt%) C Si Mn P S Cr Mo Ni Cu V Ti B Invention steel 0.02 0.39 1.80 0.002 0.004 0.51 0.30 0.50 0.49 0.103 0.017 0.0033 Comparative Steel 1 0.45 0.24 0.71 0.01 0.008 － － － － － － － Comparative Steel 2 0.12 0.05 1.79 0.002 0.004 0.98 0.49 － － 0.052 0.017 0.0026

As shown in Figure 1, in the case of the invention steel it can be seen that the hardness change is very small in the section where the cooling rate is changed to 0.1 ~ 10 ℃ / sec. In particular, in the section where the cooling rate is changed to 0.1 ~ 4 ℃ / sec, the hardness deviation is 17Hv, when converted into the tensile strength is 5.5kg / ㎜.

However, in the case of the comparative steel (1) having a ferrite and pearlite structure, it can be seen that the hardness deviation is very large in the cooling rate change range of 0.1 ~ 10 ℃ / sec. In particular, in the section where the cooling rate is changed to 0.1 ~ 4 ℃ / sec, the hardness deviation is 52Hv and converted to tensile strength was 16.8kg / ㎜.

In addition, the comparative steel (2), which is a low carbon steel with a carbon content of 0.10% or more, has a hardness deviation of 49 Hv in a section in which the cooling rate changes from 0.1 to 4 ° C./sec. kg / mm 2. That is, it can be seen that due to the increase in the carbon content, the microstructure becomes a low carbon bainite structure, resulting in a very large material deviation caused by the difference in cooling rate.

Therefore, when comparing these results with the invention steel, it turns out that the invention steel of this invention is very excellent.

2 (a), 2 (b) and 2 (c) are photographs showing the microstructures of the inventive steel, the comparative steel 1, and the comparative steel 2 when the cooling rate was 1 ° C / sec. .

As shown in FIG. 2 (b), in the case of the comparative steel 1, the carbon (C) has a microstructure composed of pearlite (black portion) and ferrite (white portion), and as shown in FIG. 2 (c). In the case of the comparative steel (2) having a high content of), it can be seen that the microstructure is a general low carbon bainite structure.

On the other hand, the inventive steel of Figure 2 (a) shows a very low carbon carbon bainite structure of a single structure.

As described above, the inventive steel of the present invention has a very low carbon bainite structure, which is a single structure, so that even if the cooling rate is different from the inside of the coil and the outside of the coil, the variation in mechanical properties due to the difference in cooling rate is small.

As described above, according to the present invention, since a wire rod which can minimize the variation in mechanical properties caused by the variation in the cooling rate can be manufactured, when the final product is manufactured by cold pressing using such wire rod, productivity is improved and a defective rate is achieved. A reduction and the like effect can be obtained.

Claims (1)

By weight%, C: 0.10% or less, Si: 0.1-0.7%, Mn: 1.0-3.0%, P: 0.03% or less, S: 0.03% or less, Cr: 0.20-1.0%, Mo: 0.10-0.50%, Ni: 0.2 ~ 1.0%, Cu: 0.3 ~ 1.5% V: 0.05 ~ 0.20%, Ti: 0.01 ~ 0.03%, B: 0.0010 ~ 0.0050%, Re-heated steel sheet composed of residual Fe and other unavoidable impurities Hot rolling is carried out in the form of a wire rod at a temperature of 800 ° C. or higher, and cooling the temperature section of 300 to 400 ° C. at a rate of 0.1 to 10 ° C./second from the finish rolling temperature is characterized in that the variation in mechanical properties is achieved. Process for producing cold rolled steel wire rods.