KR100328039B1 - A Method Manufacturing Wire Rods for cold Heading - Google Patents

Abstract

PURPOSE: A method of manufacturing wire rods for cold heading is provided, which can reduce the time required to spheroidization even in the case that wire rod is hot rolled at high temperatures. CONSTITUTION: The method includes the steps of reheating a steel billet comprising C 0.2 to 0.6 wt.%, Si 0.1 to 1.0 wt.%, Mn 0.3 to 1.5 wt.%, 0.03 wt.% or less of P, 0.05 wt.% or less of S, Al 0.01 to 0.2 wt.%, Ti 0.008 to 0.030 wt.%, N 0.002 to 0.01 wt.%, a balance of Fe and incidental impurities to the temperature range of 950 to 1200°C; finish hot rolling the steel billet into wire rod at 900 to 1000°C; drawing the wire rod at an elongation ratio of greater than 5 %; heating the wire rod at 740 to 780°C and holding the temperature for more than 1 hour; cooling the wire rod at a cooling rate of 10 to 30°C/hr to the temperature range of 690 to 710°C; holding the temperature for more than 1 hour; and cooling the wire rod to ambient temperature.

Description

A method manufacturing wire rods for cold heading

The present invention relates to a method for manufacturing a cold-rolled wire rod used as a material for bolts and nuts, and more specifically, even when the wire is rolled at a high temperature, the structure of the steel wire state can be made finer, thereby significantly reducing the spheroidization heat treatment time. It relates to a method for producing a cold-rolled wire rod.

In general, bolts and nuts used for fastening parts of automobiles, industrial machines, etc. are used as materials of medium carbon steel wires as shown in Table 1 below, and the manufacturing process is rolled steel wires, drawing, spheroidizing heat treatment, cold It is commercialized through a series of processes for rolling.

Unit: weight% C Si Mn P S 0.42-0.48 0.15-0.35 0.60-0.90 Less than 0.030 Less than 0.030

In the spheroidizing heat treatment, the wire strength in the hot rolled state is about 85 kg / mm 2, and since it is difficult to directly press the high strength steel in cold operation, the softening heat treatment is performed before the rolling process. The spheroidizing heat treatment is usually maintained at a constant temperature above AC ₁ transformation temperature (about 750 ° C.) for a predetermined time, and slowly cooled to a constant temperature below AC ₁ (about 700 ° C.), and then cooled at a constant temperature at this temperature. The time required for such heat treatment is about 15-20 hours, the tensile strength of the heat treated material is reduced to about 55㎏ / ㎜. The reason for the softening of the material is that the ferrite + pearlite structure before heat treatment is changed to ferrite + spherical cementite. The softened material is transferred to the cold rolling process and cold pressed in the form of bolts and nuts. Cold pressing is a process of inserting a material into a specific type of die and then applying a blow to make a bolt shape. In the cold rolling process, there should be no crack in the deformed part and the deformation resistance of the material should not be large. In the case of the conventional material that is commercialized through such cold rolling process, the time required for the spheroidizing annealing heat treatment for softening the steel is too long and the annealing temperature is high, so that the productivity is low and the manufacturing cost is high.

Looking at the known technology for the manufacture of spheroidized steel rods developed to solve this problem is as follows. First, when the wire finish rolling temperature is lowered to 700 ° C by using a new rolling equipment jointly developed by Morgan and US Daedong Special Steel Co., the structure becomes finer and the spheroidization heat treatment time is reduced to 1/2 of conventional materials. Could.

Another example is spheroidization heat treatment by refined ferrite and pearlite structures of rolled material by finishing rolling austenite of medium-carbon steel recrystallized finely recrystallized to 20 μm or less at 850 ° C. in KOBE steel mill in Japan. Reduced time

In both cases, the structure is refined by lowering the rolling temperature below 850 ° C. in order to manufacture spheroidized steel. However, in order to lower the rolling temperature, the capability of the rolling mill should be greatly increased, and in the case of low-temperature rolling materials, the cross-sectional shape of the wire is poor, and the winding shape of the coil is poor when the wire is wound.

Accordingly, the present inventors have been made to solve the above-described conventional problem, to provide a manufacturing method that can significantly shorten the spheroidizing heat treatment time by miniaturizing the structure even when hot rolling.

1 is a photograph showing the microstructure of the rolled material according to the finish rolling temperature

The present invention for achieving the above object, in the manufacturing method of the cold-rolled wire rod, by weight, C: 0.2-0.6%, Si: 0.1-1.0%, Mn: 0.3-1.5%, P: 0.03% or less , S: 0.05% or less, Al: 0.01-0.2%, Ti: 0.008-0.030%, N: 0.002-0.01%, and the steel slab consisting of the balance Fe and other unavoidable impurities after reheating to a temperature of 950-1200 ℃ 900- After hot-rolled at 1000 ℃ finish rolling condition, it is fresh with a processing amount of more than 5% and then heated to a temperature of 740-780 ℃ and maintained for more than 1 hour, followed by 690-710 at a cooling rate of 10-30 ℃ / hr. It is configured to include cooling to room temperature according to the usual method after cooling to ℃ and maintained for 1 hour or more.

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

The present invention effectively suppresses the growth of austenite recrystallized with TiN precipitates, thereby making the structure finer even at high temperature rolling, thereby shortening the spheroidization treatment time.

To this end, among the components of the steel of the present invention, the carbon (C) is a steel that does not need to be subjected to spheroidizing heat treatment because the strength of the steel wire is low when the addition amount is less than 0.2%, the target of the present invention for less than 0.2% carbon content I never do that. If the added amount is more than 0.6%, the strength of the final product is too high. Therefore, it is desirable to limit the addition range to 0.2-0.6%.

The silicon (Si) is added to increase the deoxidation effect and strength, but when the addition amount is too small, it is difficult to obtain such an effect, and when the addition amount is large, it promotes decarburization to reduce the surface hardness value and durability of the steel, so that the addition range is 0.1-. It is preferable to limit to 1.0%.

Manganese (Mn) is an element necessary for deoxidation of steel and is added for the purpose of improving hardenability. If the addition amount is too small, it is difficult to obtain such an effect. If the addition amount is too large, the strength is excessively increased, and a hard martensite structure is generated at the center of the steel wire, resulting in deterioration of the material. It is desirable to limit.

When phosphorus (P) is added in an amount of 0.03% or more, cracking is promoted during quenching, and the toughness of steel is lowered, so it should not be added more than 0.03%.

Sulfur (S) is to be added not less than 0.05% because it reduces the impact toughness of the steel when added more than 0.05%.

The aluminum (Al) acts as a deoxidation and also reacts with nitrogen (N) in the steel to form AlN to inhibit austenite grain growth. However, when excessively added, aluminum oxide (Al ₂ O ₃ ) is formed to lower the cold pressure composition and impact toughness. Therefore, aluminum is preferably limited to the addition range of 0.01-0.2%.

The titanium (Ti) is combined with nitrogen in the steel to form titanium nitride (TiN). Since such titanium nitride is stable at high temperatures, it effectively suppresses the growth of recrystallized austenite. However, if the addition amount is too large, the amount of titanium dissolved in the steel is increased to excessively strengthen the steel, and because it is expensive, it is preferable to limit the addition amount to 0.008-0.03% because it increases the manufacturing cost of the steel.

Nitrogen (N), as mentioned above, combines with aluminum and titanium to form nitride, which inhibits the growth of recrystallized austenite, thereby effectively miniaturizing tissue. However, if the amount added is too large, it is preferable to limit the amount added to 0.002-0.01% because the amount of solid solution nitrogen increases to excessively strengthen the steel.

The steel having the composition described above is manufactured into steel pieces and then heated to perform wire rod rolling. In this case, the heating temperature is preferably in the range of 950-1200 ° C. The reason is that when the heating temperature is below 950 ℃, the deformation resistance of the steel is so great that it is difficult to roll it into the wire rod in the hot zone, and when the heating temperature is above 1200 ℃, decarburization occurs badly, which severely degrades the material properties of the final product. This is because, especially, austenite tissue grows significantly, making it difficult to obtain a target fine tissue.

After heating as described above, it is preferable to perform hot rolling under the finishing rolling temperature condition of 900-1000 ° C. The reason is that when the finish rolling temperature is less than 900 ° C, the ferrite structure is hardened because the rolling is performed in the ideal region, and the stiffness is lowered. .

Steels hot-rolled under the above conditions are subjected to cold working by cold rolling and drawing, but the processing amount during cold working is 5% or more. If cold working is carried out with a smaller workability, internal energy accumulation is not sufficient, and cementite fragments do not sufficiently occur and spheroidization does not occur quickly.

The cold finished steel as described above is heated to 740-780 ℃ again and maintained for 1 hour or more, and then cooled to 690-710 ℃ at a rate of 10-30 ℃ / hr, and maintained at this temperature for 1 hour usually Cool to room temperature as instructed.

In such spheroidizing heat treatment conditions, the temperature conditions are ordinary conditions, and the characteristics of the present invention are shortened in time.

When the heating temperature is 740 ° C. or less during the spheroidizing heat treatment, the cementite is not decomposed because the temperature is less than or equal to Ar ₁ . Therefore, at the temperature below 740 ° C, the time required for spheroidization is too long and cannot be used industrially. If the heating temperature is above 780 ° C, regenerated pelletite is produced rather than spheroidized when cementite is cooled because all of the cementite is re-used into austenite without leaving a seed that can nucleate.

The reason for limiting the heating time to 1 hour or more is that at least one hour is required for the cementation and re-use of cementite.

The reason for limiting the cooling rate at the time of cooling to 690-710 degreeC after heating is demonstrated. If the cooling rate is less than 10 ℃ / hr, there is no time for the precipitation and growth of cementite, the spheroidization does not occur properly. If the cooling rate is more than 30 ℃ / hr, the particles of spheroidized cementite become too coarse to deteriorate the material. .

The reason why the cooling target temperature is 690-710 ° C. is because the transformation driving force decreases at 710 ° C. or higher, and the spheroidization rate is slowed down.

The reason for maintaining at 690-710 ° C for more than 1 hour is to soften the steel to allow the cold rolling process by sufficiently spheroidizing cementite.

Subsequently, cooling to room temperature is carried out as usual, and the method includes cooling the steel wire to about 660 ° C. to about 20 ° C. per hour and then air-cooling it. The wire rod after spheroidization heat treatment is drawn to the required wire diameter and then coated and processed into bolts through cold press processing.

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

Example 1

Chemical composition (% by weight) division C Si Mn P S Ti N Al Conventional Steel 0.47 0.25 0.73 0.001 0.003 - - - Invention steel 0.47 0.25 0.73 0.001 0.002 0.010 0.0087 0.030

The invention steel and the conventional steel having the composition as shown in Table 2 were made of small ingots (160mm x 160mm) and then heated at 1200 ° C. for 2 hours, and hot-rolled while changing the finish rolling temperature to 900, 950 ° C. and 1000 ° C.

At this time, hot rolling is performed by primary rolling at 30% reduction rate at 1150 ° C, and then finish rolling at 40% reduction rate at finishing rolling temperature, and then cooled to room temperature at a rate of 1 ° C / sec, and then microstructures are applied. 1 is shown.

As shown in Figure 1, it can be seen that the microstructure becomes finer as the finishing rolling temperature is lowered. Further, since the microstructure of the invention steel was much finer than that of the conventional steel, even when the conventional steel was rolled at 900 ° C, it was coarser than the invention steel which was finish-rolled at 1000 ° C. Therefore, it was confirmed that the present invention steel can secure a very fine structure compared to the conventional steel even if the finishing rolling temperature is increased.

Example 2

In Example 1, the hot-rolled steel was heated at 750 ° C. for 1.5 hours and then cooled to 700 ° C. at a cooling rate of 20 ° C./hr, and maintained at this temperature for 1.5 hours, then cooled to 20 ° C./hr up to 650 ° C. After cooling at slow rate and air-cooling to room temperature, spherical heat treatment was performed, and hardness was measured. The results are shown in Table 3 below. In this case, in the case of the conventional material, the spheroidization heat treatment is obtained by measuring the hardness value by obtaining the material heat-treated for 15 hours according to the conventional method.

Finish rolling temperature (℃) Nodular Heat Treatment Time (hr) Hardness (Hv) Remarks Invention One 900 8 134 Invention steel 2 950 8 134 3 1000 8 137 Comparative material One 900 8 140 Conventional Steel 2 950 8 155 3 1000 8 160 Conventional 1000 15 132

As shown in Table 3, the invention material (1-3) was able to secure the same level of hardness even if the spheroidizing heat treatment time is shortened to almost 1/2 compared with the conventional material.

On the contrary, in the case of the comparative material (1-3) in which the conventional steel was spheroidized by heat treatment for a short time (8 hours), it was confirmed that the spherical heat treatment for a long time was necessary because of its high hardness.

In conclusion, the invention material (1-3) containing a small amount of titanium and nitrogen was able to obtain a fine ferrite + pearlite structure which can accelerate the spheroidization rate even at the finishing rolling temperature of 900-1000 ° C. Therefore, in the present invention, it is possible to produce a spheroidization feeling without lowering the finishing rolling temperature below 850 ° C as in the prior art. It was confirmed that a wire-sensitive material capable of spheroidizing promotion could be produced without problems.

As described above, the present invention can significantly reduce the spheroidization time by appropriately adding titanium and nitrogen content in steel to control the steel composition and performing hot rolling appropriately. Therefore, the heat treatment cost can be reduced in size, and the production process is shortened, thereby reducing the delivery time and improving productivity.

Claims (1)

In the manufacturing method of the wire rod for cold pressing, By weight%, C: 0.2-0.6%, Si: 0.1-1.0%, Mn: 0.3-1.5%, P: 0.03% or less, S: 0.05% or less, Al: 0.01-0.2%, Ti: 0.008-0.030% , N: 0.002-0.01% and remainder Fe and other unavoidable impurities are reheated to a temperature of 950-1200 ° C, hot rolled to a finish rolling condition of 900-1000 ° C, and After heating to a temperature of 740-780 ℃ and maintained for more than 1 hour, followed by cooling to 690-710 ℃ at a cooling rate of 10-30 ℃ / hr and maintained for more than 1 hour and then cooled to room temperature in the usual way Method for producing a cold-rolled wire rod characterized in that.

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