KR20190094941A - Heat treatment method of high strength bolt - Google Patents

Abstract

The present invention relates to a heat treatment method to manufacture a high-tension bolt by applying heat to a bolt. The heat treatment method comprises: a step of inserting the bolt into an atmospheric furnace (10) with a salt bath quenching tank (20) to increase a temperature; a step of maintaining a constant temperature in the atmospheric furnace to transform the bolt into an austenite structure after the temperature increasing step; a step of discharging the bolt in the austenite structure from the atmospheric furnace (10) to insert the bolt into the salt bath quenching tank (20) filled with molten salt with water added thereto to rapidly cool the bolt after the structure transforming step; a step of maintaining a constant-temperature state for a prescribed period of time after the rapid cooling step to transform the bolt into a bainite structure; and a step of discharging the bolt from the salt bath quenching tank (20) to feed the bolt to a washing device to wash the bolt after the bainite structure transforming step. Accordingly, the present invention performs quenching in a salt bath with water added thereto, maintains a constant temperature without material improvement in manufacture to manufacture a high-tension bolt up to 9-16 T used in automobile parts, industrial machine parts, bridges, civil engineering, and buildings simply by a heat treatment of transformation into a bainite structure by austempering, pursues energy saving by shortening processes and eliminates a need for tempering equipment, and provides a product with improved hardness, tension, yield, impact strength and excellent delayed fracture resistance by hydrogen brittleness.

Description

Heat treatment method of high strength bolt

The present invention relates to a heat treatment method of a high-tensile bolt, and more specifically, in manufacturing, only by heat treatment transformed into bainite tissue by maintaining the constant temperature while quenching in a salt bath with added moisture without improving the material The present invention relates to a heat treatment method of high tension bolts capable of manufacturing high tension bolts of 9T to 16T grades used in automobile parts, industrial machine parts, bridges, civil engineering and buildings.

Normally, high-strength bolts are used for friction welding of steel structural members. Friction welding is a method of joining steel structural members and tightening them securely using a torque wrench or an impact wrench. Since 1938, research has been carried out in the United States. It is used in many steel structures in Korea, and compared to riveting, torque wrench clamping has less noise, and there is no risk of fire by welding, and it is easy to correct defective parts. Types are from the first to the third type, the tensile strength is 80 ~ 130kg / ㎠.

As prior art documents that can be referred to in this regard, Korean Unexamined Patent Publication No. 10-1991-0012293 (prior document 1), Korean Unexamined Patent Publication No. 10-1993-0013188 (prior document 2), and Korean Unexamined Patent Publication 10-1994-0002365 (prior document 3) etc. can be referred.

Prior Art 1 omits the spheroidization heat treatment and hardening heat treatment performed during bolting to manufacture wire rods for high-strength bolts by weight% C: 0.13-0.15%, Si: 0.16-0.20%, Mn: 1.10-1.40 Steels containing%, P: 0.030% or less, S: 0.030% or less, Nb: 0.030-0.040%, and V: 0.030-0.040% are 1100-1200 ° C, and the finish rolling temperature is 1000 ° C based on the material surface temperature. After hot rolling in the temperature range directly above -Ar3, air cooling is performed after forced cooling at a cooling rate of 18-22 ° C / sec to 500-550 ° C. As a result, carbon content is significantly reduced in steel components used for high tension bolts, and hot rolling and cooling are appropriately controlled to greatly increase impact toughness and ductility, thereby eliminating the spheroidizing heat treatment and quenching treatment performed during bolt forming. In addition, the present invention proposes a method for producing a heat treatment omitted high tension bolt wire rod which is obtained by adding niobium (Nb) and vanadium (V), which are trace alloy elements.

Prior art document 2, in the manufacturing method of the high-strength bolt steel wire, in weight%, C: 0.09-0.11%, Si: 0.61-0.65%, Mn: 1.30-1.70%, B: 0.0015-0.0025%, Ti: 0.18 Steel composed of -0.20%, Nb: 0.015-0.030%, P: 0.03% or less, S: 0.03% or less, balance Fe and other unavoidable impurities is heated to a temperature of 1100-1250 ° C, and 1000 ° C to Ar3 After the finish hot rolled wire in the, it is characterized in that the air cooled to room temperature after forced cooling to 500 ~ 550 ℃ at a cooling rate of 5-10 ℃ / sec. Accordingly, the present invention proposes a method for manufacturing a heat-treated omitted high-strength bolt steel wire which can manufacture bolts by eliminating spheroidizing heat treatment, hardening, and soaking treatment performed during bolt forming.

Prior art 3 is 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 contained steels heated to a temperature of 1100-1300 ° C., hot rolled to a 70% or higher loading rate, cooled, and then reheated to 900-1050 ° C. and extracted to a temperature below 950 ° C. After hot-rolled wire rolling with the filament rolling blowing temperature below 950 ℃, forced cooling to a temperature range of 450-550 ℃ at a cooling rate of 5-15 ℃ / sec at a temperature range of 750-850 ℃, and then air cooled to room temperature. It is characterized by. Accordingly, the present invention proposes a method for producing a high-strength bolt steel wire which can manufacture high-tensile bolts, omitting softening heat treatment, quenching, and soaking process usually involved in the bolt processing process.

However, according to the above-mentioned prior art, in order to manufacture high tension bolts as high tension bolts, hydrogen embrittlement or delayed destruction due to hydrogen intrusion is not recommended. Currently, it is not recommended to use more than 1200 MPa class. Most of the constituent tissues are tempered martensite tissues, and carbide-based precipitates are distributed at grain boundaries, and the base material is lattice martensite precipitates.

However, the main inhibitory factor in achieving the high strength of the material is the delayed breakdown phenomenon due to the intrusion of hydrogen, which causes the precipitate distribution in the grain boundary to act as a trapped site of hydrogen, thus degrading the strength of the grain boundary. It is known because it is made. Therefore, due to the characteristics of the tempered martensite microstructure, it was limited to use it as a high-strength bolt steel.

In addition, in the salt bath heat treatment method, after the quenching, the post-incineration process is necessary for washing. Usually, the washing process is performed in hot water, and as the number of washings accumulates, salt dissolves and the washing is not performed well. There is a disadvantage that wastewater treatment and replacement with new hot water.

Korean Laid-Open Patent Publication No. 10-1991-0012293 "Method of manufacturing wire rod for high tension bolt" (published date: Aug. 7, 1991) Korean Laid-Open Patent Publication No. 10-1993-0013188 "Manufacturing Method of Steel Wire for High-Tension Bolted High-Strength Bolt" (Publication Date: July 21, 1993) Korean Unexamined Patent Publication No. 10-1994-0002365 "Method of manufacturing steel wire for high tension bolt" (published date: February 17, 1994)

An object of the present invention for fundamentally improving the conventional problems as described above, in the manufacturing process by transforming into bainite (Bainite) structure by maintaining a constant temperature while quenching in a salt bath with added moisture without improving the material It is possible to manufacture high-strength bolts of 9T ~ 16T grades used in automobile parts, industrial machine parts, bridges, civil engineering, and buildings, and it saves energy by shortening the process and eliminating the need for tempering equipment. The present invention aims to provide a method for heat-treatment of high-strength bolts having excellent yield and impact strength and excellent resistance to delayed fracture due to hydrogen embrittlement.

In order to achieve the above object, the present invention provides a heat treatment method for producing a high tension bolt by applying heat to the bolt: charging the bolt in an atmosphere having a salt bath sintering tank to increase the temperature; Maintaining the temperature in the atmosphere furnace at a constant temperature after the step of raising the temperature to transform it into austenite tissue; After the tissue transformation step, austenite-structured bolts are taken out of the atmosphere furnace and charged into a salt bath inlet tank filled with molten salt to which water is added and quenched; Maintaining a constant temperature for a certain time after the quenching step to transform the bainite tissue into bainite tissue; And after the bainite tissue transformation step, removing the bolts from the salt bath inlet bath and putting the bolts in the washing apparatus to wash them.

According to the present invention, the atmosphere is characterized by treating any one selected from carburizing, carburizing and nitriding, and QT (Quenching & Tempering).

According to the present invention, the salt bath inlet tank is characterized in that the vortex to form a vortex in the molten salt (Molten Salt) and a stirrer for stirring the molten salt.

According to the present invention, the salt bath sintering tank is characterized by improving the salt bath hardenability of the high tension bolt by forming a vortex in molten salt.

According to the present invention, the salt bath sintering tank is characterized in that the salt salt quenching property of the bolt is enhanced by stirring molten salt.

According to the present invention, the water after washing in the washing apparatus is not discarded and is supplied to a salt bath sintering tank, and the moisture of molten salt is maintained within 0.05 to 1.5% of the total weight and there is no generation of waste water.

According to the present invention, the austenite tissue transformation step is characterized in that the bolt is heated to a temperature of 800 ~ 950 ℃ to maintain the steel of the bolt until the austenite to the core.

According to the present invention, the step of transforming into bainite tissue may include a bay at a temperature of 215 to 540 ° C. in a salt bath immersion tank filled with molten salt in which water is mixed within 0.05 to 1.5% of the total weight. It is characterized by transformation into knightite (Bainite) tissue.

According to the present invention, the high-strength bolt manufactured by the heat treatment method has a tensile strength of 800 to 1800 N / mm 2, a yield strength of 600 to 1500 N / mm 2, and an elongation of 12 to 20%.

On the other hand, the terms or words used in the present specification and claims are not to be construed as limiting the ordinary or dictionary meanings, the inventors should use the concept of the term in order to explain the invention in the best way. It should be interpreted as meanings and concepts corresponding to the technical idea of the present invention based on the principle that it can be properly defined. Therefore, the embodiments described in the present specification and the configuration shown in the drawings are only the most preferred embodiments of the present invention, and do not represent all of the technical ideas of the present invention, and various alternatives may be substituted at the time of the present application. It should be understood that there may be equivalents and variations.

As described in the above configuration and operation, the heat treatment method of the high-strength bolt according to the present invention was transformed into bainite tissue by maintaining the constant temperature while quenching in a salt bath containing water without improving the material in manufacturing It is possible to manufacture high-tension bolts of 9T ~ 16T class used in automobile parts, industrial machine parts, bridges, civil engineering, and buildings only by heat treatment, and it is necessary to shorten the process and no tempering equipment, and to save energy. In addition to the improvement of yield, impact strength, and excellent resistance to delayed fracture by hydrogen embrittlement.

1 is a graph showing a heat treatment process applied to the heat treatment method of the high tension bolt according to the present invention.
2 is a flowchart illustrating a heat treatment method of a high tension bolt according to the present invention.
Figure 3 is a photograph of the organization of the high tension bolt according to the present invention observed with an optical microscope (Optical Observation).
4 is a configuration diagram showing an atmosphere furnace applied to the heat treatment method of the high tension bolt according to the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The present invention proposes a heat treatment method for manufacturing a high tension bolt by applying heat to the bolt, which is an atmosphere furnace (10) and a washing apparatus having a salt bath sintering tank 20 having a vortex (23) and a stirrer (25) By using a step of temperature rising, austenite (Austenite) tissue transformation step, quenching step, transformation to bainite (Bainite) tissue, washing step to produce high-tensile bolts up to 9T ~ 16T grade.

According to the present invention, the bolt is charged into the atmosphere furnace 10 having the salt bath sintering tank 20 to increase the temperature. In the temperature raising step, as shown in FIGS. 1 and 2, the bolt is charged into an atmosphere 10 having a salt bath sintering tank 20 to raise the temperature. The atmosphere 10 includes carburizing, carburizing and nitriding, and QT (Quenching & Tempering). ) Process any one selected. Carburization is a kind of surface hardening method that hardens only the surface layer by injecting carbon into the surface layer of a product made of low carbon steel and hardening it. Carburization is a process of making carbon on the surface of low carbon steel to make only the surface of high carbon steel. As a result, the surface is hardened and the inside softened to obtain mechanical properties (strong and non-broken).

And carburizing nitriding is performed for a long time at low temperature, so before the carburizing nitriding treatment, the structure is sufficiently stabilized by Quenching and Tempering and carburizing and nitriding treatment is not possible. In general steel, the effect of nitriding treatment cannot be expected, and for nitriding steels (Al, Ti, Nb, Cr, etc.) is effective only when used, and there is a very strong increase in hardness and increase in fatigue strength. Quenching & Tempering (QT) is also referred to as Quenching and Tempering. This is because the hardening of the hardening material causes brittleness as the material hardens. As it works, it is called Quenching & Tempering, or QT. These tasks are mostly carried out in the atmosphere (10). This heat treatment is a heat treatment operation to change the material properties of the metal by using a temperature change.

Into the atmosphere furnace 10 having the salt bath sintering tank 20 performing this function, the temperature is raised to transform the bolt into an austenite structure, which will be described later.

On the other hand, the atmosphere furnace 10 of the present invention is provided with a salt bath sintering tank 20, which uses the "salt bath heat treatment apparatus" of Patent Publication No. 10-2010-0074914 filed by the present applicant. The salt bath sintering tank 20 includes a vortex 23 for forming a vortex in molten salt and an agitator 25 for stirring the molten salt. The salt bath sintering tank 20 is composed of a vortex 23 and a stirrer 25 as shown in FIG. The vortex 23 forms a vortex in the molten salt, and the stirrer 25 serves to stir the molten salt. The salt bath sintering tank 20 forms or stirs vortices in molten salt by the vortex 23 and the stirrer 25 to enhance the salt bath quenchability of the high tension bolt.

Meanwhile, in the salt bath sintering tank 20, an agitator is provided for uniform temperature and distribution of molten salt and rapid circulation of molten salt upon quenching. As it is circulated, the uniform distribution of temperature and water cannot be expected and the uniformity of the product cannot be guaranteed. Therefore, if the vortex is provided in the salt bath sintering tank 20 and the stirrer is provided and the impeller is provided at the end of the rotating shaft to rotate in a constant direction, the melt can be rotated at a high speed if it can distribute a constant direction and even temperature and water. A swirling vortex can occur on the top surface of the salt to enhance salt bath quenching of the high tension bolts.

According to the present invention, after the step of raising the temperature, the temperature in the atmosphere 10 is maintained at a constant temperature to transform into austenite tissue. In the tissue transformation step, as shown in FIGS. 1 and 2, the bolt is charged into the atmosphere 10 and maintained at a constant temperature to transform into austenite tissue. This converts the steel structure of the bolt into an austenite structure while charging the bolt into the atmosphere 10 to raise the temperature and then maintaining a constant temperature.

At this time, the austenite tissue transformation step according to the detailed configuration of the present invention is characterized in that the bolt is heated to a temperature of 800 ~ 950 ℃ to maintain the steel of the bolt until the austenite to the core. In the step of transforming the bolt to austenite by transforming the tissue, which is charged into the atmosphere 10 to the bolt as shown in Figure 1 and 2 to raise the temperature to transform the austenite (Austenite) tissue on 800 ~ 950 ℃. This keeps the bolts sufficiently warm for 30 to 180 minutes until the austenite is organized to the core of the steel at a temperature of 800 to 950 ° C. directly above the austenizing (Ac3) to austenite to the core of the steel.

Maintaining a temperature of 800 ~ 950 ℃ to transform into austenite tissue, such as incomplete tissue may be generated due to incomplete austenitization of metal tissue when the temperature below 800 ℃, to obtain a stable structure by making a single phase austenitization The reason for limiting to 950 ° C is to obtain a stable particle size and obtain mechanical properties due to homogeneous particle size and coarsening of particles. The heating time must also be maintained until reaching single phase austenitization. (However, in the case of a special high temperature stabilized steel grade, even if the particle size does not increase even above 950 ℃, it will be meaningless.)

According to the present invention, after the tissue transformation step, the austenite-structured bolts are taken out from the atmosphere furnace 10 and charged into a salt bath inlet 20 filled with molten salt to which moisture is added. The quenching step is carried out and quenched after the tissue transformation step as shown in FIGS. This is carried out through the tissue transformation step in the atmosphere furnace (10) to take out the austenitic bolts organized in the salt bath intake tank 20 is filled with water and molten salt is quenched.

According to the present invention is a step of transforming to bainite (Bainite) tissue by maintaining a constant temperature for a certain time after the quenching step. The tissue transformation step is transformed into bainite tissue by maintaining a constant temperature for a certain time after the quenching step similarly to the austenite tissue step as shown in FIGS. 1 and 2. It charges the bolts into the salt bath inlet 20 filled with water and molten salt, and then quenches them, and then transforms the bolt's steel structure from the austenite tissue to the bainite tissue while maintaining a constant temperature. .

At this time, the step of transforming to the bainite (Bainite) tissue according to the detailed configuration of the present invention in the salt bath sintered tank 20 filled with molten salt (Molten Salt) mixed with moisture within 0.05 ~ 1.5% of the total weight It characterized in that the transformation to bainite (Bainite) tissue at a temperature of 215 ~ 540 ℃. Transforming the steel structure of the bolt from the austenite tissue to the bainite tissue, which maintains a constant temperature for a certain time in the salt bath sintering tank 20 as shown in FIGS. The condition of the bath immersion tank 20 is to hold the molten salt mixed with moisture within 0.05-1.5% of the total weight, and hold it at a temperature of 215-540 ° C. for 20 to 120 minutes to bainite to the core of the steel. Organize

When molten salt is added to water, it should be maintained within 0.05 to 1.5% of the total weight. When the water content is managed to 0.05% or less, the heat capacity of the water is insufficient and cooling ability is lowered. (Quenching) The cooling of the product is delayed, the pearlite is generated in the metallographically, it is impossible to obtain the bainite structure pursued by the present invention.

Moreover, above all, water has a volume change when the phase changes from liquid to gaseous state, and about 1,700 times the volume of the liquid expands when evaporated from 1 atm to 100 ° C, and at 300 ° C at the same pressure, the volume is about When the water is increased by 2,600 times and the molten salt is increased by more than 1.5%, the moisture in the molten salt is transformed into gas almost instantly due to the high temperature product which has a large amount of heat during the quenching process. It may be dangerous to swell together with molten salt at the time of expansion, so it is desirable to manage the moisture of molten salt within 0.05 ~ 1.5%.

And to maintain at a temperature of 215 ~ 540 ℃ in the salt bath sintering tank 20 is the range to precipitate the bainite (Bainite) structure, which is the grade (T) range of the bolt. That is, if it exceeds this temperature range, a lot of bainite may precipitate, but elongation etc. may fall. Therefore, it is important to maintain the temperature of the salt bath sintering tank 20 215 ~ 540 ℃ to obtain the grade (T) of the bolt to be obtained by the present invention.

According to the present invention, after the bainite tissue transformation step, the bolts are removed from the salt bath inlet 20 to be added to the washing apparatus for washing. In the washing step, bainite-structured bolts are removed from the salt bath inlet tank 20 as shown in FIGS. 1 and 2 and put into the washing apparatus for cleaning. This stabilizes the bainite tissue through the washing apparatus after the bainite tissue transformation through the salt bath sintering tank 20.

At this time, the water after washing in the washing apparatus according to the detailed configuration of the present invention is supplied to the salt bath sintering tank 20 without being discarded and the water of the molten salt (Molten Salt) is maintained within 0.05 ~ 1.5% of the total weight of waste water It is characterized by no occurrence. The washing apparatus does not discard the water to be washed in the process of stabilizing the bolt to bainite tissue, and supplies it to the salt bath sintering tank 20 so that the moisture of molten salt is 0.05 to 1.5% of the total weight. Keep within. Accordingly, when the hot water is washed after quenching in the general salt bath incineration tank 20, the salinity accumulates in the washing solution and the washing power is reduced, so that the washing solution is replaced by a predetermined cycle and the washing solution condensed water is replaced. Moisture is added to the tank 20, and the water can maintain the salinity in the washing apparatus at a constant level while supplying a certain amount of hot water of the washing apparatus so that waste water is not generated because it is reused without replacing the washing liquid. to be.

According to the present invention, the high-strength bolt manufactured by the heat treatment method has a tensile strength of 800 to 1800 N / mm 2, a yield strength of 600 to 1500 N / mm 2, and an elongation of 12 to 20%. Through the process of the heat treatment method as shown in FIG. 3 to produce a high-strength bolt through the step of heating, transforming into austenite tissue, quenching, transforming to bainite tissue, washing step do. These high tensile bolts have a tensile strength of 800 ~ 1800N / mm2, yield strength of 600 ~ 1500N / mm2, and an elongation of 12 ~ 20%.

On the other hand, the heat treatment method of the present invention must be washed after quenching and immersing in oil in the existing method, and the tempering process must be carried out, but the present invention is a constant temperature quenching, eliminating the tempering process, shortening the process and no tempering equipment, Energy related to tempering is not necessary.

Moreover, the product, ie high-strength bolts, has increased resistance to hydrogen embrittlement with improved hardness, tensile and impact strength, which is maintained at temperatures between 215 and 540 ° C, heated to temperatures of 800 to 950 ° C, directly above the austenite. By quenching in the salt bath sintering tank 20 maintained as it is possible to obtain a bainite (Bainite) structure without the mixed structure of ferrite or pearlite. To do this, sufficient cooling capacity (curing capacity) must be supported during salt bath quenching. In order to support such cooling ability, the water is maintained in the salt bath incineration tank 20 within 0.05-1.5% of the total weight to provide a stable and bainite structure that is resistant to hydrogen embrittlement without being mixed with ferrite or pearlite when quenched. You can get it. The bainite structure has a very high tensile, yield and impact strength compared to the tempered martensite structure, and has a resistance to hydrogen embrittlement, thereby increasing resistance to delayed fracture due to hydrogen embrittlement, thereby producing high tension bolts.

Hereinafter, preferred examples and experimental examples are presented to help understand the present invention. However, the following Examples and Experimental Examples are provided only to more easily understand the present invention, and the contents of the present invention are not limited by the Examples.

Experimental Example

Example A 350 is subjected to the heat treatment step of the present invention, Comparative Example B (350) obtained a test sample, heat treatment different from the present invention, each 10 pieces of high-strength bolts obtained by processing five tensile specimens to proceed the tensile test Hardness test and tissue observation are conducted. After the tensile test, the hardness and tissue items were carried out with the same sample, and the measured values were measured without specification classification. The test results are shown in Table 1 below.

[Table 1] After the tensile test proceeds to the processed tensile specimen, the hardness measurement using the unprocessed head.

As described above, Example A and Comparative Example B is the result of hardness measurement using the unprocessed head after the tensile test progress, Example A is tensile strength, yield strength, elongation, core hardness compared to Comparative Example B It can be seen that the back result is much better.

As described above, the present invention is manufactured by autothermally maintaining a constant temperature while quenching in a salt bath to which moisture is added without improving materials, and transforming it into a bainite structure by using only heat treatment for automobile parts, industrial machine parts, bridges, civil works, and the like. It is possible to manufacture high-tension bolts of 9T ~ 16T class used in buildings, and it is possible to shorten the process and do not need tempering equipment, and to save energy, and to improve hardness, tensile, yield, impact strength, and hydrogen embrittlement. Excellent delayed fracture resistance.

It is apparent to those skilled in the art that the present invention is not limited to the described embodiments, and that various modifications and variations can be made without departing from the spirit and scope of the present invention. Therefore, such modifications or variations will have to belong to the claims of the present invention.

Claims (9)

In the heat treatment method for producing a high tension bolt by applying heat to the bolt:
Charging the bolt to an atmosphere furnace (10) having a salt bath sintering tank (20) to raise the temperature;
Maintaining the temperature in the atmosphere 10 at a constant temperature after the step of raising the temperature to transform it into austenite tissue;
After the tissue transformation step, austenite-structured bolts are taken out from the atmosphere (10) and charged into a salt bath inlet tank (20) filled with molten salt to which moisture is added;
Maintaining a constant temperature for a certain time after the quenching step to transform the bainite tissue into bainite tissue; And
The bainite (Bainite) tissue transformation step after the step of removing the bolt from the salt bath inlet tank 20 to the washing apparatus; heat treatment method comprising a high tension bolt. The method of claim 1,
The atmosphere furnace (10) is a heat treatment method of high tension bolt, characterized in that any one selected from carburizing, carburizing nitriding, QT (Quenching & Tempering). The method of claim 1,
The salt bath sintering tank 20 has a vortex (23) for forming a vortex in molten salt and a stirrer (25) for stirring the molten salt, characterized in that the heat treatment method of the high tension bolt. The method of claim 1,
The salt bath sintering tank 20 is a heat treatment method of the high tension bolt, characterized in that to form a vortex in the molten salt (Molten Salt) to improve the salt bath hardenability of the high tension bolt. The method of claim 1,
The salt bath sintering tank 20 is a heat treatment method of the high tension bolt, characterized in that to promote the salt bath hardenability of the bolt by stirring the molten salt (Molten Salt). The method of claim 1,
The water after washing in the washing apparatus is not discarded, is supplied to the salt bath sintering tank 20, the high-strength bolt, characterized in that the moisture of molten salt is maintained within 0.05 ~ 1.5% of the total weight and there is no generation of waste water Method of heat treatment. The method of claim 1,
The austenite tissue transformation step is to heat the bolt to a temperature of 800 ~ 950 ℃ heat treatment method of the high-strength bolt, characterized in that to maintain the steel of the bolt until the austenitization to the core. The method of claim 1,
The transformation into the bainite tissue may include bainite at a temperature of 215-540 ° C. in a salt bath immersion tank 20 filled with molten salt in which water is mixed within 0.05 to 1.5% of the total weight. (Bainite) Heat treatment method of high tension bolt, characterized in that transformed into a tissue. The high-tensile bolts prepared according to claim 1 to 8 has a tensile strength of 800 ~ 1800N / mm2, yield strength of 600 ~ 1500N / mm2, elongation is 12 to 20% heat treatment method.

Images