KR20040032285A - A method for manufacturing medium carbon steel bar-in-coil with low deviation in mechanical properties - Google Patents

Abstract

PURPOSE: A method for manufacturing medium-carbon steel wire rod is provided which is capable of reducing material deviation inside coil due to nonuniformity of cooling rate by adding Ti to steel without addition of cooling facility, thereby forming titanium nitride (TiN) and properly controlling hot rolling conditions. CONSTITUTION: The method for manufacturing medium-carbon steel wire rod having low material deviation comprises the steps of reheating a steel slab comprising 0.42 to 0.48 wt.% of C, 0.15 to 0.35 wt.% of Si, 0.40 to 0.70 wt.% of Mn, 0.03 wt.% or less of P, 0.035 wt.% or less of S, 0.01 to 0.03 wt.% of Ti, 0.004 to 0.01 wt.% of N and a balance of Fe and other inevitable impurities to a temperature of 950 to 1,150 deg.C; finish rolling the reheated steel slab to a temperature of 700 to 850 deg.C; coiling the finish rolled steel; and air cooling the coiled steel.

Description

Method for manufacturing medium carbon steel wire rod with low material deviation {A METHOD FOR MANUFACTURING MEDIUM CARBON STEEL BAR-IN-COIL WITH LOW DEVIATION IN MECHANICAL PROPERTIES}

The present invention relates to a medium-carbon steel wire used as a material for industrial machinery parts and automobile parts, and more particularly, by miniaturizing the austenite structure before transformation through proper control of the components of the steel wire and hot rolling conditions. In addition, the present invention relates to a method for manufacturing a medium carbon steel wire rod which greatly improves the material deviation due to the cooling rate nonuniformity inevitably generated during the cooling process after hot rolling.

The microstructure of hot rolled medium carbon steel wire is mainly composed of ferrite and pearlite. Typically, the weight of a coil (wire) is 2 tons, and the cooling rate is different for each coil position during the cooling process. Therefore, the microstructure of each coil position according to the non-uniformity of the cooling rate occurs.

For example, the region where the cooling rate is relatively high increases the strength due to the smaller ferrite size and the higher fraction of pearlite. On the other hand, the relatively slow cooling rate increases the ferrite size and lower the pearlite fraction, resulting in lower strength. Therefore, in the case of the medium-carbon steel wire mainly composed of ferrite and pearlite, the occurrence of microstructure variation due to the cooling rate variation and the material non-uniformity due to this cannot be avoided.

Typically, when coiling and cooling a 75 kg / mm class ² steel wire having a carbon content of 0.45% in the form of a coil, the variation in tensile strength in the coil of 2 ton weight is about 10 kg / mm ² . This material deviation in the wire state leads to a material deviation of the final product, causing a problem of increasing the product failure rate.

The prior art for reducing the material deviation of the wire rod is Japanese Patent Laid-Open No. 9-67622. In the prior art, the steel is formed by weight%, C: 0.15 to 0.35%, Si: 0.05% or less, Mn: 0.70 to 1.50%, N: 0.005% or less, Cr: 0.20% or less after hot rolling, and then 2 ° C. It proposes a method for producing a steel wire with a low material deviation in the coil, characterized in that the wire obtained by cooling at a rate of more than / seconds 20 to 30% to produce a steel wire with a tensile strength of 700 ~ 930N / mm ² . In the case of the prior art, there is an advantage that the material deviation can be reduced by controlling only the cooling rate of the general medium carbon steel. However, in order to increase the cooling rate to 2 ° C / sec or more, a cooling facility for forcibly cooling the wire coil is required, and thus there is a problem of increasing production cost.

The present invention is to solve the above problems, by adding Ti to the steel without the addition of a cooling facility to form titanium nitride (TiN) and by appropriately adjusting the hot rolling conditions, by minimizing the size of austenite before transformation to cool It is an object of the present invention to provide a method for manufacturing a medium carbon steel wire rod which can reduce material variation in a coil due to speed unevenness.

1 is a graph showing the hardness change of the invention steel and the comparative steel according to the cooling rate change

2 is a microstructure comparison picture of the invention material 2 and the comparative material 2

3 is a graph showing the correlation between austenite size and cure sensitivity

The present invention for achieving the above object by weight, C: 0.42 ~ 0.48%, Si: 0.15 ~ 0.35%, Mn: 0.40 ~ 0.70%, P: 0.03% or less, S: 0.035% or less, Ti: 0.01 ~ 0.03%, N: 0.004% to 0.01%, the steel sheet composed of the remaining Fe and other unavoidable impurities, and reheated to 950 ~ 1150 ℃, finish rolling to 700 ~ 850 ℃, then wound and air-cooled .

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

In the present invention, by adding Ti to the steel to form titanium nitride (TiN) and by appropriately adjusting the hot rolling conditions, the production of medium-carbon steel wire rods by miniaturizing the size of austenite before transformation to reduce material deviation in the coil due to uneven cooling rate It is about a method.

First, the steel component of this invention is demonstrated in detail.

C: 0.42-0.48 wt%

C is an element having the greatest influence on the mechanical properties of steel. If the content of C is less than 0.42% by weight, it is impossible to secure the target strength. If the content of C is more than 0.48% by weight, the strength is excessively increased. Therefore, the content is preferably limited to 0.42 to 0.48% by weight.

Si: 0.15 to 0.35 wt%

The Si is a component normally added for deoxidation and strength at steelmaking. If the content is less than 0.15% by weight, the deoxidation is not smoothly performed and it is difficult to secure necessary strength. In addition, since the strength is excessively increased when the content exceeds 0.35% by weight, it is preferable to limit the content to 0.15 to 0.35% by weight.

Mn: 0.40 to 0.70 wt%

Mn is a component that increases the strength of the steel. If the content of Mn is less than 0.40% by weight, the strength is insufficient. If the content of Mn is more than 0.70% by weight, the strength is excessively increased. Therefore, the content is preferably limited to 0.40 to 0.70% by weight.

P: 0.03 wt% or less

P is an element that segregates at grain boundaries and degrades the toughness of the steel, and when the content exceeds 0.03% by weight, the toughness of the steel is remarkably decreased, so the content is preferably limited to 0.03% by weight or less.

S: 0.035 wt% or less

S is an element for reducing the impact toughness of the steel, and when the content exceeds 0.035% by weight, the impact toughness of the steel is significantly reduced, it is preferable to limit the content to 0.035% by weight or less.

Ti: 0.01% to 0.03% by weight

The Ti is a key component in the present invention, in combination with nitrogen at high temperature to form nitride (TiN) to inhibit the growth of austenite grains and to refine the structure, thereby increasing the toughness of the steel and reducing material deviations Play a role. If the content of Ti is less than 0.01% by weight, it is difficult to obtain the effect of reducing the material deviation. If the content is more than 0.03% by weight, the effect of reducing the material deviation is saturated and the production cost increases due to the addition of expensive Ti. It is preferable to limit to 0.03% by weight.

N: 0.004-0.01 wt%

N is a component that combines with Ti at high temperature to form nitride, thereby making the steel structure fine and increasing the toughness of the steel. If the content of N is less than 0.004% by weight, the above effect cannot be obtained, and if the content is more than 0.01% by weight, the amount of free nitrogen which does not form nitride increases, which lowers the toughness of the steel, and thus the content is 0.004 to 0.01% by weight. It is preferable to limit to.

The steel strips prepared as described above are reheated at a temperature of 950-1150 ° C. If the reheating temperature is less than 950 ° C., the high temperature deformation resistance of the steel is excessively increased, and the mill load is excessively generated. If the reheating temperature is higher than 1150 ° C., titanium nitride (TiN) precipitated in the steel is re-dissolved to lose austenite growth inhibiting function. Since it is, the reheating temperature is preferably limited to 950 ~ 1150 ℃.

After the reheating, hot rolling is performed. At this time, finish rolling is performed at 700-850 degreeC. If the finish rolling temperature is less than 700 ℃ rolling is made in an abnormal region, the ferrite is excessively stretched, causing a decrease in the impact value of the steel and an excessive increase in hardness due to the accumulation of a lot of energy therein. In addition, if the finish rolling temperature exceeds 850 ℃ recrystallization and grain growth occurs actively after hot deformation, the ferrite nucleation site is reduced during cooling, the amount of ferrite is reduced and the amount of pearlite is relatively increased, so the material deviation is increased, It is preferable to perform the said finish rolling at 700-850 degreeC.

After the said finish rolling, it winds up and air-cools. At this time, when cooling the wire coil wound as in the present invention, a cooling rate of 0.5 ~ 2 ℃ / second comes out. If the coiled wire coil is forcedly cooled using a medium such as water or compressed air, a low temperature transformation structure is generated locally in the coil to increase the material deviation, so that the bay is air-cooled in the air and adversely affects mechanical properties. It is preferable that low-temperature transformation tissues, such as knight and martensite, do not generate | occur | produce.

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

EXAMPLE

After the steel composition as shown in Table 1 to produce a small steel piece of 110mm x 110mm x 250 mm, and reheated to 1100 ℃ for 90 minutes and then subjected to hot rolling. At this time, the invention steel is 900 ℃, 850 ℃ and 800 ℃, the comparative steel was finish-rolled to 1000 ℃, 900 ℃ and 800 ℃. In order to measure the austenite size immediately after the finish rolling, the material immediately after the rolling was quenched and the austenite size was measured.

On the other hand, some finish-rolled material is cooled to room temperature through air cooling. The hardness of the inventive materials 1 to 2 and the comparative materials 1 to 4 thus cooled was measured and shown in FIG. 1, and the microstructures of the inventive material 2 and the comparative material 2 were photographed in FIG. 2. Indicated.

Steel component (wt%) C Si Mn P S Ti N Invention steel 0.45 0.22 0.51 0.007 0.010 0.015 0.006 Comparative steel 0.45 0.24 0.70 0.007 0.009 - 0.003

Reheating Temperature (℃) Finish rolling temperature (℃) Austenitic Size (μm) Invention steel Invention 1 1100 800 15.6 Invention 2 850 19.5 Comparative Material 1 900 27.4 Comparative steel Comparative Material 2 800 25.5 Comparative Material 3 900 36.0 Comparative Material 4 1000 93.1

As can be seen from Table 2, the invention materials (1-2) are very small austenite size immediately after the finish rolling compared to the comparative materials (1-4). This is because Ti contained in the inventive steel forms titanium nitride (TiN) to effectively suppress austenite growth upon reheating, and also has a low finish rolling temperature. When the austenite size is 20 µm or less, as in the invention materials 1 to 2, even if there is some cooling speed deviation in the cooling process in the wire coil does not lead to severe material deviation. On the contrary, it can be seen that the comparative steels have relatively larger austenite sizes than the invention steels, and that even when the rolling temperature is lowered to 800 ° C., austenite of 20 μm or less cannot be obtained.

1 is a graph showing the change in hardness according to the cooling rate change, the hardness of both the invention steel and the comparative steel increases as the cooling rate increases. This is because as the cooling rate increases, the soft phase ferrite decreases and the hard phase pearlite increases. However, in the same range of cooling rate changes, the invention steel has a much smaller increase in hardness than the comparative steel. This means that when there is a difference in the cooling rate according to the coil position, the hardness variation of the inventive steel is much reduced compared to the comparative steel. 1, when the cooling rate is low, the lowest hardness value that can be obtained is 195 Hv. The allowable variation of hardness value in the same coil is about 25 Hv, so the maximum hardness value should not exceed 220Hv even if the cooling rate is increased. When the invention steel is rolled to a finish rolling temperature of 850 ° C. or lower (inventive materials 1 to 2), the expected hardness change in a given cooling speed deviation range is 195 to 215 Hv, and it can be seen that the material deviation is 20 Hv or less. However, in the case of the comparative materials (1 to 4), the expected change in hardness is 195 ~ 250Hv, it can be seen through Figure 1 that the material deviation is 30 ~ 55Hv.

2 is a microstructure photograph of the invention material 2 and the comparative material 2 after the end of cooling. In the case of the inventive material (2), the finish rolling temperature is 850 ° C., compared to the comparative material (2) which was finish rolled at 800 ° C., although the finish rolling temperature is high, the structure is finer and has the greatest influence on the hardness increase. It can be seen that the perlite content is less than that of the comparative material (2). Looking at the microstructure in this way it can be seen that the material deviation of the invention materials (1-2) compared to the comparative materials (1-4).

3 shows the sensitivity of the austenite size before transformation to increase in hardness due to the change in cooling rate. The larger the curing sensitivity value, the greater the change in hardness for the same amount of cooling rate variation, which means that the material deviation is increased. As can be seen in Figure 3, as the austenite size increases, the sensitivity value for the material deviation increases. In the case of the invention materials (1 ~ 2), it can be seen that the austenite size is 20um or less and the curing sensitivity value is 100 or less compared to the comparative materials (1-4).

As described above, the present invention makes it possible to manufacture a wire rod having a small material deviation due to the variation in the cooling speed for each position in the wire coil, thereby achieving effects such as productivity improvement and defect rate reduction.

Claims (1)

By weight%, C: 0.42 to 0.48%, Si: 0.15 to 0.35%, Mn: 0.40 to 0.70%, P: 0.03% or less, S: 0.035% or less, Ti: 0.01 to 0.03%, N: 0.004 to 0.01% , The method for producing a medium carbon steel wire with a low material deviation, including re-heating the steel pieces composed of the remaining Fe and other unavoidable impurities to 950 ~ 1150 ℃, finish rolling to 700 ~ 850 ℃, then winding and air-cooled .