KR940007365B1 - Method of manufacturing steel rod - Google Patents

Abstract

The steel wire material for a high tension bolt is mfd. by (a) heating a steel composed of 0.15-0.25 wt.% C, 0.1-0.5 wt.% Si, 1.0-2.0 wt.% Mn, 0.01-0.1 wt.% Al, at most 0.03 wt.% P, at most 0.03 wt.% S, residual Fe and impurity at 1100-1300 deg.C, (b) hot rolling, and cooling it, (c) reheating it at 900-1050 deg.C, and extracting it at 950 deg.C or less, (d) hot rolling it at 950 deg.C or less, and (e) cooling it at 5-15 deg.C/sec. cooling velocity from 750-850 deg.C to 450-550 deg.C, and air cooling it at room temperature.

Description

Manufacturing method of steel wire for high tension bolt

1 is a manufacturing process diagram of a conventional high tension bolt.

2 is a microstructure photograph of the present invention steel and comparative steel in the wire rolled state.

The present invention relates to a method for manufacturing a steel wire used for high-tensile bolts, and more particularly, for high-strength bolts that can produce high-tensile bolts without omission of softening heat treatment, hardening, and soaking process usually involved in the bolt processing process. It relates to a method for producing a steel wire.

Structural bolts currently used have a tensile strength of 30-160kg / mm ² , and high tensile bolts with a double tensile strength of 70kg / mm ² or higher after cooling 15-20% of wire with a diameter of 5.5-20mm as shown in FIG. The material is softened by spheroidizing heat treatment, and then bolted by cold pressing. After that, quenching and tempering heat treatment is performed to secure the required strength and toughness. The spheroidizing heat treatment during the process is heat-treated for about 9-13 hours at the temperature near the A ₁₁ transformation point (about 650-750 ° C.). The cementite of is uniformly distributed and changes into existing spheroidized tissue, and the material is softened greatly. Quenching and tampering heat treatments performed after bolt forming are carried out by heating to 830-880 ° C. and oil-cooling to harden the material, and then heating to 550-650 ° C. and cooling to impart toughness to the hardened steel.

To present the specifications of the final bolt (7T class) processed as described above is shown in Table 1.

TABLE 1

Currently, as a wire rod for 70-80kg / mm class ² high-tension bolts manufactured through the above heat treatment process, S45C having a carbon content of about 0.45% and a manganese (Mn) content of about 0.7% has been mainly used, but the bolt is being processed. Recently, new steel grades have been proposed that can reduce cost by eliminating softening heat treatment and hardening treatment. Known methods for producing a heat treatment process omission steel can be roughly divided into two, which will be described in detail as follows.

The first method is to make carbon (C) in the steel component 0.1-0.2%, which is significantly reduced compared to the conventional heat treatment material. Instead, niobium (Nb) or vanadium (V) is added in small amounts, and then the wire rod is controlled and controlled by cooling. As a method for producing a method, impact toughness is secured through reduction of carbon content and microstructure, and strength is secured through addition of trace alloy element, increase of manganese content, and microstructure.

However, this method has a problem in that it is necessary to add a trace element such as expensive niobium and to raise the temperature of the wire heating furnace at least 1100 ° C. in order to fully re-use these alloy elements.

The second method improves the problems of the first method described above, in which the carbon content is significantly reduced compared to conventional heat treatment materials at 0.2-0.3%, and the low-temperature low temperature steel, which has greatly increased the manganese content to around 1.5%. Control cooling after rolling or special cooling after high speed and high temperature rolling.

However, in this method, when cold rolling is required to refine the final structure, the rolling speed must be reduced to lower the rolling temperature, resulting in a decrease in productivity. There is a problem that it is necessary to control the tissue by performing cooling to refine the tissue, or additionally conduct a constant temperature heat treatment in the 450-580 ℃ region after wire production.

In addition, since the low speed, low temperature rolling, and the high speed-high temperature rolling method all add about 1.5% of manganese, manganese may be segregated in the center of the steel sheet.

The present inventors have conducted research and experiments to solve the above problems of the conventional methods, and based on the results, the present invention proposes the present invention. By minimizing manganese segregation, wire milling is performed at high speed and low temperature by using strong mill power, which can eliminate the heat treatment process without any decrease in productivity or introduction of cooling facilities, and it has excellent strength and toughness. To provide a steel wire for high-strength bolt, the purpose is to.

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

The present invention is by weight% C: 0.15-0.25%, Si: 0.10-0.50%, Mn: 1.0-2.0%, Al: 0.01-0.10%, P0.03% or less, S: 0.03% or less, all Fe and other The steel, which is inevitably contained in the impurity, is heated to a temperature of 1100-1300 ° C., hot rolled to a reduction ratio of 70% or more, cooled, then reheated to 900-1050 ° C., extracted to a temperature of 950 ° C. or lower, and then rolled. Hot-rolled wire with blowing temperature below 950 ℃, forced cooling to temperature range of 450-550 ℃ with cooling rate of 5-15 ℃ / sec at temperature range of 750-850 ℃ and air cooled to room temperature It relates to a method for producing a steel wire.

EMBODIMENT OF THE INVENTION Hereinafter, the component range of the addition element in this invention, and the reason for limitation of rolling conditions are demonstrated in detail.

The carbon (C) can be significantly lower than the conventional heat treatment material (S45C) to increase the volume fraction of ferrite, thereby greatly increasing the impact toughness and ductility of the steel, but when the carbon content is too low, it is impossible to secure the required strength. The content of is preferably limited to 0.15-0.25%.

The silicon (Si) is a necessary component for deoxidation, but when the amount added is too large, the toughness and deterioration of boltability are reduced, so the content of the silicon is preferably limited to 0.10-0.50%.

The manganese (Mn) increases the hardenability and lowers the transformation temperature from austenite to ferrite, thereby miniaturizing the tissue, and the toughness is reduced even when added to about 2.0% in ordinary steel. Since the strength is increased without increasing the strength, the manganese content is preferably limited to 1.0-2.0% in order to obtain the required strength without deterioration of toughness.

The aluminum (Al) contributes to the improvement of toughness due to the refinement of grains and the reduction of solid solution nitrogen, but it is preferable to limit the amount to 0.01-0.10% because the added amount may damage the cleanliness of the steel.

Since phosphorus (P) is segregated at grain boundaries and degrades toughness, the upper limit is preferably limited to 0.030%.

The sulfur (S) is lowered in the toughness to form an emulsion has a bad effect on the material, it is preferable to limit the upper limit to 0.030%.

After casting the steel composition as described above, and then heated to 1100-1300 ℃ to perform a steel sheet rolling with a total pressure drop ratio of more than 70%, when the heating temperature is 1100 ℃ or less, it is difficult to achieve sufficient manganese diffusion and deformation resistance Since it is difficult to roll the steel slab due to its large size, 1100 ° C. or more is preferable, and the reduction ratio is preferably limited to 70% or more in order to sufficiently obtain the effect of reducing manganese segregation due to breakage of the cast structure and recrystallization.

If the heating temperature is higher than necessary when reheating the steel strip for wire rod rolling, the austenite grains grow significantly, which adversely affects microstructure and decarburization becomes severe. If the reheating temperature is too low, the ferrite + austenitic ideal zone The reheating temperature is preferably limited to 900-1050 ° C because the final structure is subjected to a lot of processing and a strong ferrite structure in which internal energy is accumulated is formed to degrade the material.

In order to manufacture wire rods with fine structure, rolling should be performed at the lowest possible temperature to minimize austenite grain growth.In the case of high-speed wire rod rolling (diameter: 100m / sec for 5.5mm wire rod), the material is cooled during rolling. Rather, since the material temperature is increased due to the processing heat, in order to lower the rolling temperature without decreasing the rolling speed, the material extraction temperature must be lowered. In the present invention, the extraction temperature of the material is preferably limited to 950 ° C. or less.

Since the finish rolling temperature directly affects the structure after transformation, it should be lowered as much as possible to minimize grain growth, and therefore, in the present invention, the finish rolling temperature is preferably limited to 950 ° C or less.

The hot-rolled material should be quenched as far as the temperature where the low temperature tissue is formed and transformed into a fine ferrite structure. When the cooling rate is 15 ° C / sec or higher, the low temperature structure is likely to occur, and the cooling rate is 5 ° C /. In the case of seconds or less, since the effect of microstructure is not great, the cooling rate range in the present invention is preferably limited to 5-15 ° C / sec.

On the other hand, when the cooling end temperature is 450 ℃ or less may be generated bainite structure that hurts the toughness, when the end temperature is 550 ℃ or more because the particle finer effect is not so large that the cooling end temperature is limited to 450-550 ℃ desirable.

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

EXAMPLE

A specimen prepared as shown in Table 2 and having a size of 250 × 350 × L was heated at 1250 ° C. for 2 hours, and then hot rolled at a total reduction ratio of 70.7% to prepare a steel sheet having 160 × 60 × L.

TABLE 2

The rolled steel strips were heated to the maximum temperature of 1050 ° C. and then lowered to 950 ° C. for extraction.

A wire rod having a final size of 10 mm in diameter was produced, wherein the filament rolling blowing temperature was 950 ° C based on the surface temperature.

The finished material was quenched to 780 ° C. immediately after passing through the water cooling system, and then cooled to about 500 ° C. at 7 ° C./sec by forced air blowing in the transformation section, and then naturally cooled.

The microstructures of the specimens prepared as described above were observed, and the results of the ferrite volume fractions were shown in Table 3 below. The microstructures of the inventive steels (1) and the comparative steels in FIG. Indicated.

As shown in FIG. 2, it can be seen that the inventive steel (1) [second FIG. A] is a finer ferrite + perlite structure than the comparative steel [second FIG. B], as shown in Table 3 below. , It can be seen that the inventive steels (1) and (2) are about 50% larger than the comparative steel (S45C).

TABLE 3

In addition, it was confirmed that the ferrite grain size of the inventive steels (1) and (2) appeared significantly finer than S20C having the same carbon content as compared with the rolling and cooling under the same conditions as the inventive steel.

The wire rod manufactured as described above was cooled and drawn with a reduction rate of 18%, and then the bolt head was cold pressed by a double header process without softening heat treatment that is commonly performed during high tension bolt processing.

Next, the screw part of the bolt was processed from cold to dry processing, the hardening annealing heat treatment was omitted, and galvanized, and then the stress relief annealing was performed at 200 ° C. for 4 hours in the same manner as the heat treatment bolt to make a final product.

As described above, the mechanical properties of the raw material state and the bolted bolt state before bolt processing were measured, and the measurement results are shown in Table 4 below.

TABLE 4

As shown in Table 4, it can be seen that the inventive steels (1) and (2) manufactured according to the present invention satisfactorily satisfy the 7T-class JIS standard without spherical heat treatment, quenching, and soaking.

As described above, the present invention eliminates the softening heat treatment, quenching, and sour processing that is usually involved in the bolting process, and thus has high productivity, and does not require a separate cooling facility. There is an effect that can provide a manufacturing method.

Claims (1)

By weight, C: 0.15-0.25%, Si: 0.10-0.50%, Mn: 1.0-2.0%, Al: 0.01-0.10%, P: 0.03% or less, S: 0.03% or less, balance Fe and other inevitably The steel composed of impurity contained is heated to a temperature of 1100-1300 ° C, hot rolled to 70% or more of loading rate, cooled, then reheated to 900-1050 ° C and extracted to a temperature of 950 ° C or lower, followed by filament rolling. The hot wire is rolled at a temperature of 950 ° C. or lower, and then forcedly cooled to a temperature range of 450-550 ° C. at a cooling rate of 5-15 ° C./sec at a temperature range of 750-850 ° C., followed by air cooling to room temperature. Method for manufacturing steel wire for high tension bolts.