KR100415664B1 - A method for manufacturing wire rod for bolt with superior spheroidizing heat treatment property - Google Patents

Abstract

The present invention relates to a method for manufacturing a wire rod for bolts, by adding Ni and W in steel and appropriately controlling the hot rolling and cold drawing processes, thereby reducing the spheroidization heat treatment time and improving productivity, and thus the bolt wire rod having excellent spheroidization characteristics. To provide a method of manufacturing, the purpose is.

The present invention provides a method for producing a wire rod for bolts,

By weight% C: 0.2-0.7%, Si: 0.5-1.5%, Mn: 0.3% or less, P: 0.01% or less, S: 0.03% or less, Ni: 0.5-1.5%, W: 0.2-1.0%, balance After reheating the steel pieces composed of Fe and other unavoidable impurities to 950 ~ 1150 ℃, hot rolled them under finish rolling conditions of 800 ~ 950 ℃ to form wires or round rods, and then cold working them at 5% or more. After the reheating in the temperature range of 630 ~ 730 ° C and maintained for 2 hours or more, the method for producing a wire rod for excellent spheroidizing heat treatment characteristics, characterized in that the air cooled to room temperature, the technical gist.

Description

Manufacturing method of bolt wire with excellent spheroidizing heat treatment characteristics {A METHOD FOR MANUFACTURING WIRE ROD FOR BOLT WITH SUPERIOR SPHEROIDIZING HEAT TREATMENT PROPERTY}

The present invention relates to a method for manufacturing a cold-rolled wire rod used as a material for bolts and nuts used for fastening parts of automobiles, industrial machines, and the like, and more specifically, to adjust steel components and to hot-roll and cold drawn wires. By controlling suitably, it is related with the manufacturing method of the wire rod for bolts which can drastically shorten spheroidization heat processing time.

In general, bolts and nuts used for fastening parts of automobiles, industrial machines, etc. have been commercialized through a series of processes of wire-rolling medium carbon steel of the components shown in Table 1 below, drawing, spheroidizing heat treatment, and cold rolling.

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

The medium carbon steel having the above components is a high-strength steel having a tensile strength of about 85 kg / mm 2 in a hot rolled state, and is usually softened and heat-treated before rolling in order to make it difficult to directly press in cold.

In general, the softening heat treatment is performed by maintaining a predetermined time at an Ac ₁ transformation temperature or more and slowly cooling to a predetermined temperature (about 700 ° C.) of Ac ₁ or less, and then maintaining it at this temperature for a certain time and allowing it to cool in the air. The total time required for such heat treatment is about 15 to 20 hours, and 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 a cold pressing process and cold pressed with bolts. Cold press is a process of making a bolt shape by inserting a material into a die of a certain type and then applying a blow. In this process, cracks should not occur in the deformed area. In order to improve, the deformation resistance of the material should not be large. In other words, the material must be sufficiently softened before entering the cold rolling process.

In the case of the conventional material which is commercialized through such a cold rolling process, the time required for the spheroidizing annealing heat treatment for softening of the steel is too long and the annealing temperature is high, resulting in low productivity and high manufacturing cost. In addition, there is a problem that the heat treatment is not easy because the heat treatment pattern is complicated.

Conventional technology for the manufacture of spheroidized steel wire rods developed to solve these problems. Kobe Steel Works and Daedong Special Steel Co., Ltd. in Japan use the intermediate water cooling system for wire rolling material. By cooling to 750 ° C. and rolling the steel using a powerful finishing mill at this temperature, a technique has been disclosed in which the rolled structure of the wire can be refined to reduce the spheroidization heat treatment time to a conventional level of 1/2.

However, in the case of the above technique, in order to manufacture the spheroidized steel, the rolling temperature is reduced to 750 ° C. to refine the structure, but in order to lower the rolling temperature, an additional facility for rapidly cooling the material being rolled is needed, and the performance of the finishing mill is reduced. There is a problem that must be increased significantly. In addition, in the case of low-temperature rolling material, there is a problem in that the cross-sectional shape of the wire rod is poor and the winding shape of the coil becomes poor when the wire rod is wound.

Therefore, the present inventors conducted research and experiments to solve the above problems, and based on the results, the present invention was proposed, and in the method of manufacturing a wire rod for bolts, Ni and W in steel were added and hot It is an object of the present invention to provide a method for producing a wire rod for bolts having excellent spheroidization characteristics that can shorten spheroidization heat treatment time and improve productivity by appropriately controlling rolling and cold drawing.

1 is a microstructure photograph after spheroidization heat treatment of invention steel and comparative steel

Figure 2 is a graph comparing the hardness of the invention and the comparative material after the spheroidizing heat treatment

The present invention provides a method for producing a wire rod for bolts,

By weight% C: 0.2-0.7%, Si: 0.5-1.5%, Mn: 0.3% or less, P: 0.01% or less, S: 0.03% or less, Ni: 0.5-1.5%, W: 0.2-1.0%, balance After reheating the steel pieces composed of Fe and other unavoidable impurities to 950 ~ 1150 ℃, hot rolled them under finish rolling conditions of 800 ~ 950 ℃ to form wires or round rods, and then cold working them at 5% or more. After the reheating in the temperature range of 630 ~ 730 ℃ and maintained for 2 hours or more, and relates to a method for producing a wire rod for excellent spheroidizing heat treatment characteristics, characterized in that the air-cooled to room temperature.

Hereinafter, the reason for limiting the component range, cold processing conditions, and spheroidizing heat treatment conditions of the added element in the present invention will be described in detail.

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

In the present invention, the carbon (C) is less than 0.2% of the steel wire is low because the strength of the steel wire does not need to be subjected to spheroidizing heat treatment, so the carbon amount less than 0.2% was excluded from the object of the present invention, the addition amount In the case of exceeding 0.7%, the strength is still high even after spheroidizing heat treatment, making it difficult to cold work, and the product strength in the final state after the hardening treatment is too high, so the amount of C added is 0.2 to 0.7%. It is preferable to limit to.

The Si is a component that is usually added in order to deoxidize during steelmaking and to secure the required strength of the product. In addition, in the present invention, the Siantite phase is unstable, thereby promoting the decomposition of cementite, thereby promoting spheroidization. In the present invention, when the addition amount of Si is less than 0.5%, such an effect is difficult to obtain, and when it exceeds 1.5%, it is excessively promoted decarburization to greatly reduce the surface hardness value and durability of steel, The range of addition is preferably limited to 0.5 to 1.5%.

When Mn is a large amount of elements or additions necessary for deoxidation of steel, it stabilizes cementite and delays spheroidization, so the content thereof is preferably limited to 0.3% or less.

P promotes cracking upon quenching, reduces toughness of steel, and inhibits cementite decomposition, so the content is preferably limited to 0.01% or less.

Since said S reduces impact toughness of steel, it is preferable to set the addition amount to 0.03% or less.

Ni is an element that increases the strength of steel and promotes cementite decomposition, and when the added amount is less than 0.5%, it is difficult to obtain such an effect, whereas, when Ni exceeds 1.5%, the strength due to solid solution strengthening and hardening increase Since the increase is excessively large, the amount of addition is preferably limited to 0.5 to 1.5%.

W is a component that plays a role of increasing strength without inhibiting the decomposition of cementite, and when the addition amount is less than 0.2%, it is difficult to obtain such an effect, and when the addition amount is more than 1.0%, gravity segregation occurs and steel materials Since the characteristic of deteriorates, it is preferable to limit the addition amount to 0.2 to 1.0%.

Steel of the composition as described above is produced in steel strips and reheated, and then hot rolled to produce wire rods and round rods. At this time, the reheating temperature is a temperature range of 950 ~ 1150 ℃, the mari rolling temperature is preferably set to 800 ~ 950 ℃. The reason is that when the reheating temperature is higher than 1150 ° C., surface decarburization occurs badly, and the surface hardness and durability of the final product are severely reduced. It's hard. In addition, when the finish rolling temperature is higher than 950 ℃, the austenite tissues grow excessively before transformation, and the pearlite transformation tissue becomes coarse at room temperature, so that the time required for globularization is long, thereby achieving the desired goal. It is difficult to achieve. On the other hand, if the finish rolling temperature is less than 800 ℃ there is a problem that the winding is difficult and the surface state of the product worsens.

Cold working the hot rolled steel under the same conditions as above, the method may be applied to cold rolling and drawing. It is preferable to make the processing amount at the time of cold working at 5% or more. The reason is that when cold working is carried out with a smaller workability, the processing energy is not sufficiently accumulated inside the steel and the segmentation of cementite does not occur sufficiently. Because it does not happen quickly.

After cold working, the steel is heated again to 630 ~ 730 ℃ for 2 hours, and then cooled to room temperature. At this time, if the heating temperature is less than 630 ° C., the diffusion rate of C is low and the time for cementite decomposition is too long, and thus the time for spheroidization is too long, so that it cannot be industrially used. On the other hand, when the heating temperature is higher than 730 ° C, not only the driving force for spheroidization decreases but also heat energy consumption increases.

After heating to the heating temperature described above is maintained for 2 hours or more, because time is required for the decomposition and spheroidization of the plate cementite. In other words, if the holding time is less than 2 hours, the spheroidization is not completed and the desired low hardness value cannot be obtained.

The wire rod after spheroidization heat treatment as described above is fresh to the required wire diameter, and then coated and processed into bolts by cold press processing.

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

(Example)

Inventive steel and conventional steel having the composition as shown in Table 2 were produced as small ingots (160 mm × 160 mm × 250 mm), and then heated at 1050 ° C. for 2 hours and hot rolled at a finish rolling temperature of 900 ° C.

division C Si Mn P S Ni W Invention steel 0.51 1.22 0.29 0.001 0.003 0.55 0.25 Comparative steel 0.48 0.19 0.76 0.007 0.008 － －

After hot rolling as above. After heating to 680 ° C. and holding for 2 hours, air cooled was used as the invention material (1). The comparative material was heated to 740 ° C. and held for 5 hours, and then cooled to 720 ° C. at a cooling rate of 15 ° C./hr, and then again this temperature. The mixture was kept at 4 hours, cooled to 650 ° C. at a cooling rate of 15 ° C./hr, and further cooled to room temperature. Therefore, in the case of the comparative material, the total heat treatment time was about 15 hours.

Then, the microstructure was observed after the spheroidizing heat treatment for the inventive material and the comparative material is shown in FIG.

As shown in Fig. 1, in the microstructure photograph (a) of the invention 1 maintained at 680 ° C. for 2 hours, no plate-like cementite was observed, only spherical cementite was observed. Of course, spherical cementite was also observed in the microstructure photograph (b) of the comparative material, but there was a disadvantage in that the holding time was long when reheating.

Therefore, it could be confirmed that spheroidization was sufficiently performed after the hot rolling for 2 hours or more in the reheating temperature range of the present invention.

On the other hand, Figure 2 shows the hardness value after the heat treatment for the invention material and the comparative material. As can be seen from FIG. 2, in the case of the inventive material 1 heat-treated at 680 ° C. for 2 hours, the hardness value was 135.3 in Hv units, almost 128.5 Hv of the comparative material 2 heat-treated at 740 ° C. for 15 hours. There was no difference. That is, although it is numerically about 7 high, there is no problem in industrial use.

In addition, it was found that the invention material 2 maintained at 680 ° C. for 15 hours, which is the holding temperature of the comparative material, had a low hardness value of 115.2 Hv and was softer than the conventional material.

As described above, the present invention has the effect of significantly reducing the heat treatment cost by shortening the spheroidization heat treatment time by adjusting the steel component and appropriately controlling the hot rolling and cold drawing wire, and shortening the delivery time and productivity. .

Claims (1)

In the manufacturing method of the wire rod for bolts, By weight% C: 0.2-0.7%, Si: 0.5-1.5%, Mn: 0.3% or less, P: 0.01% or less, S: 0.03% or less, Ni: 0.5-1.5%, W: 0.2-1.0%, balance After reheating the steel pieces composed of Fe and other unavoidable impurities to 950 ~ 1150 ℃, hot rolled them under finish rolling conditions of 800 ~ 950 ℃ to form wires or round rods, and then cold working them at 5% or more. After reheating in the temperature range of 630 ~ 730 ℃ and maintained for 2 hours or more, and then air-cooled to room temperature characterized in that the method for producing a wire rod for bolts excellent in spheroidizing heat treatment characteristics