KR100257648B1 - The manufacturing method of high strength impact bar - Google Patents

Abstract

PURPOSE: A heat treating method is provided to manufacture a high strength impact bar by heat treating a steel pipe made of hot coil in the constantly set heating conditions, and a method for manufacturing a high strength impact bar is provided, which improves mechanical properties and prevents bending of the bar due to the homogeneous structure. CONSTITUTION: In a method for manufacturing an impact bar using a steel pipe material comprising the process of adding one or two metals selected from 0.4 wt.% or less of Cr and 0.05 wt.% or less of Nb to the basic constituents comprising 0.17 to 0.20 wt.% of C, 0.2 to 0.35 wt.% of Si, 0.9 to 1.5 wt.% of Mn, 0.02 to 0.05 wt.% of Al, 0.001 to 0.005 wt.% of B and 0.01 to 0.04 wt.% of Ti so as to manufacture an impact bar using a welded steel pipe material, the method comprises a first heating step of protecting heat for a certain period of time after rapidly heating the steel pipe material to an ordinary temperature of 750 to 850 deg.C; a second heating step of protecting heat for a certain period of time after rapidly heating the first heat treated steel pipe material to an ordinary temperature of 900 to 970 deg.C; and a third heating step of reheating the water cooled material in an ordinary temperature of 150 to 190 deg.C after water cooling the steel pipe material, and protecting heat at the same temperature for 30 to 50 minutes.

Description

Manufacturing method of high strength impact bar

1 is a schematic side view of a tension bar pressing device installed for the purpose of preventing deformation near heating and cooling coils for producing the impact bar of the present invention.

2 is a schematic structural cross-sectional view of the heating coil and cooling coil of the present invention when installed to efficiently heat or cool a steel pipe.

Here, (A ₁ ) and (A ₂ ) each represent a heating coil, and (A ₃ ) represents a cooling coil.

3 is a heat treatment cycle of the impact bar steel pipe of the present invention.

4 is a schematic explanatory diagram in which the distance between the primary heating coil and the secondary heating coil and the distance between the secondary heating coil and the cooling coil in the present invention are set.

5 is a schematic explanatory diagram of the heating coil of the present invention.

* Explanation of symbols for main parts of the drawings

1: steel pipe 2: tension bar

3: heating or cooling coil 4: rotating roller

5: Cooling water passage (water in heating coil is heating coil cooling function and water in cooling coil is sprayed out to cool steel pipe)

6: cold water (cooling water) 7: cooling water supply pipe

d1: localized heating d2: small local heating

The present invention relates to a heat treatment method of a high strength impact bar used for reinforcement of automobile doors, the heat treatment for the production of high strength impact bar by heat-treating the steel pipes made of hot rolled coil having the chemical composition described below to a predetermined set heating conditions It is about a method. In particular, the present invention relates to a method for producing a high-impact impact bar for preventing the bending and the improvement of the mechanical properties, especially by a uniform structure.

Advanced automakers are actively conducting research to reduce the weight of the car body by developing high-strength materials and changing the shape of existing materials to improve fuel efficiency. However, countries around the world are enacting and enacting various safety laws to improve the safety of automobiles, and No.214 of the US Federal Safety Standard (FMVSS) provides more than a certain strength to the body including the door in the event of a collision on the side of the vehicle to protect passengers. I'm asking.

Since the early 90's, the use of “c” type beams on doors has been increasing in order to satisfy the above regulations. In recent years, the impact bars, which are high strength steel pipes, have been actively applied instead of the existing “c” type beams. ought.

The production method of the impact bar can be divided into two types: a method using a non-wood steel pipe and a method using a welded steel pipe. In the production of welded steel pipes, there are a method of using a high strength coil in the production of steel pipes to satisfy the high strength required by the impact bar, and a method of improving the strength of the impact bar as a heat treatment after the production of the steel pipe.

However, in the conventional case, due to the limitation of steel pipe manufacturing equipment, if a high-strength coil is used, the pipe can not be piped. Therefore, the pipe is heat-treated after pipe-joining. Problems such as warpage).

In the case of the conventional impact beam, for example, Korean Patent Publication No. 94-3248 and Korean Patent Publication No. 95-3455, in the former case, the tempering is not sufficient because the heating time is too short and the heat holding time is too short at a time. In particular, the tempering results are poor at low temperatures, and problems such as warpage and product cracking are not sufficiently solved. In the latter case, even though there are primary and secondary heating steps, there is no tempering step. The effect of heat retention was difficult to be expected, resulting in problems such as unstable mechanical properties of the product and poor warpage.

Therefore, the present inventors focus on this point, and put the first heating and the second heating step, but the heating step to keep the heat for a sufficient time after heating, so as to obtain the uniformity of martensite structure by water cooling, In order to prevent changes in the structure, instability of mechanical properties, and warpage due to temperature changes, the uniformity and stability of the structure can be obtained by limiting the range of the feed rate of the heating coil, the heating coil, the heating coil, the cooling coil, and the steel pipe. It is intended to drastically reduce.

In order to manufacture the impact bar using the welded steel pipe material, C: 0.17 ~ 0.20%, Si: 0.2 ~ 0.35%, Mn: 0.9 ~ 1.5%, Al: 0.02 ~ 0.05%, B: 0.001 ~ 0.005%, Ti: 0.01 to 0.04% as a basic component, and relates to a method for producing an impact bar using a steel pipe material by adding one or two component metals selected from Nb of Cr or less and 0.05% or less of 0.4% or less. Here, the steel pipe material is heated by arranging the primary and secondary tunnel heating coils at 20 to 40 mm intervals in the heating step to prevent rapid air cooling, and again by placing the cooling coils 50 to 100 mm behind the secondary tunnel heating coils. And a high strength impact bar adapted to carry out the cooling process.

According to the present invention, the steel pipe material having the chemical composition range described above is first heated to 750 to 850 ° C. as shown in FIG. 3, and then maintained at this temperature for a predetermined time, but the distance between the heating coil and the heating coil is 20 to 40 mm. This prevents rapid steel pipe cooling. Next, the secondary heating is rapidly heated to 900 ~ 970 ℃. When this temperature is reached, it is kept for a certain time and then water-cooled with water in a copper (Cu) cooling coil. At this time, the cooling coil is placed at a distance of about 50 to 100 mm behind the heating coil to prevent the steel pipe heated to 900 to 970 ° C. from being excessively cooled in the air. This is also to secure mechanical properties such as predetermined tensile strength by securing sufficient martensite structure. Heat retention and cooling cycles at this time are shown in FIG.

Here, the heating rate and heat retention time depend on the type and thickness of the steel pipe. The characteristics of the present invention is to uniformize the tissue by rapidly heating the first heating temperature range to a typical 750 ~ 850 ℃ constant heat after rapid heating, and then rapidly heating to 900 ~ 970 ℃, a heat retention region of 30 to 50 ℃ at the next AC ₃ transformation point To be. If it is less than 900 ° C, sufficient martensite transformation of residual austenite is not achieved. If the temperature is higher than 970 ° C, there is a risk of burning due to energy waste. In the next step of the cooling process, the heating coil as shown in Figure 2 (A ₁₎ (A ₂₎ and a cooling coil (A ₃₎ of the coil (3) The steel pipe material (1) is passed through and heated in the center A cooling water passage 5 is provided in the coil 3 so as to cool the coil facility with the cooling water. In addition, air cooling (6) by air cooling or water is performed at the time of cooling. This is to obtain sufficient martensite structure by water cooling, and by tempering the resulting martensite structure to 150 ~ 190 ℃ to eliminate the deterioration of brittleness and impact resistance by residual austenite, and to prevent the decrease in tensile strength by entangling the inclusion It is in a heat treatment method aimed at preventing the clear heat brittleness. In addition, the steel pipe feed rate was set to 200 ~ 400mm / 10 seconds to prevent the warpage while having impact resistance. If the steel pipe speed is less than 200mm / 10 seconds, the productivity is lowered, and if it exceeds 400mm / 10 seconds, the fear of bending increases.

One feature of the present invention lies in the distance adjustment between the heating and cooling coils in which copper (Cu) pipes are installed, and the method of conveying steel pipes. In addition, two heating coils are installed at regular intervals. In other words, in the second heating, two heating coils with a gap of about 20 to 40 mm are rapidly heated at a temperature of 900 to 970 ° C. The reason for installing two coils at a interval of 20 to 40 mm is 1 If the coil is processed, the heating rate is low, which lowers the productivity. When three or more installations are used, the capacity of the high frequency oscillation facility is increased, which is a limitation on the equipment.

In addition, when the two coil spacing according to the primary and secondary heating is 40mm or more, heat is transferred to the second coil after being heated in the first coil, which may cause air cooling. Occurs. Also, if this distance is within 20mm, there is a risk of ignition.

As shown in FIGS. 2 and 4, the steel pipe heated as described above should be wind-cooled or water-cooled in a cooling coil at a distance of about 50 to 100 mm behind the heating coil. To obtain the desired martensitic structure and mechanical properties. However, when the distance between the heating coil and the cooling coil is 100 mm or more, there is a possibility that the heated material is air-cooled in the meantime, and thus the structure and the stone cannot be secured. On the contrary, if it is 50 mm or less, the holding time required for transformation is shortened, and therefore, rapid cooling at least 50 mm or more can obtain a certain martensite and mechanical properties. In order to compensate for this, lowering the heat treatment rate lowers productivity. In addition, the heating coils should be spaced at 20 ~ 40mm intervals to cool the cooling coils after heating and quenching process at a distance of about 50 ~ 100mm behind them. Since the mechanical properties can be obtained and the material must be rotated at high speed to reduce deformation, the feeding and rotating is carried out using a rotating roller device as shown in FIG. 1, and the feeding speed is about 200 to 400 mm / 10 seconds. .

Due to quenching, the steel pipe is heated to 150 ~ 190 ℃ in the electric furnace for 30 ~ 50 minutes in order to remove residual stress and surface stress of the material and transform residual austenite into martensite and stabilize the martensite structure. Then air or furnace cooled.

Since the surface stress and residual stress start to disappear from about 145 ° C, the minimum temperature is set to 150 ° C. Tensile strength was lowered above 200 ℃, and it is not suitable for tempering temperature because the temperature of 250 ~ 280 ℃ is a cold-brittle brittle area of carbon steel.

In addition, the tempering time is 30 to 50 minutes was the most suitable temperature because the thickness of the material is usually about 2.0mm ~ 3.5mm. Uniform heat and cooling are essential factors to secure the uniform structure and tensile strength of steel pipes and prevent bending after heat treatment.

That is, uniform feed speed and high speed rotation are required. As shown in FIG. 2, it is important to have a uniform ambient pressure in all directions to correct this when the steel pipe is bent or the flatness is uneven (FIG. 2A2). This can be prevented as much as possible by installing a tension bar (2) for pressing the steel pipe (1) to be transported while rotating in the heat treatment facilities by hydraulic or pneumatic (see Fig. 2).

EXAMPLE

To prepare the impact bar, C: 0.18%, Si: 0.30%, Mn: 1.1%, Al: 00.02%, B: 0.004%, Ti: 0.03%, Cr: 0.25% Nb: 0.03% Hot rolled coil sample Rapid heating to 835% using a conventional heat treatment cycle and held at this temperature for 17 minutes. The material was then heated with two tunnel heating coils installed at approximately 33mm intervals. Next, the heated steel pipe was cooled with a copper water cooling coil placed at a distance of 80 mm behind the heating coil, and then transferred to the rotary roller feeder at a speed of 30 mm / sec. Next, the resultant was passed through an electric furnace, reheated to 172 ° C., maintained for 35 minutes, and then cooled to prepare an impact bar of the present invention. The mechanical properties thereof were as follows. Here, in the degree of curvature, a very good degree of curvature was obtained because the conventional 1.5 mm thickness was about 0.4 mm.

TABLE 1

Claims (2)

In order to manufacture the impact bar using the welded steel pipe material, C: 0.17 ~ 0.20%, Si: 0.2 ~ 0.35%, Mn: 0.9 ~ 1.5%, Al: 0.02 ~ 0.05%, B: 0.001 ~ 0.005%, In the method for producing an impact bar using a steel pipe material, wherein Ti is 0.01 to 0.04% as a basic component and one or two component metals selected from Nb of 0.05% or less of Cr and 0.4% or less of Cr are added thereto. Steel pipe materials usually have a first heating step of rapidly heating at 750 ° C. to 850 ° C. for a certain time, and a second heating step of rapid heating at 900 ° C. to 970 ° C. for a predetermined time and after cooling the steel pipe material. A method of producing a high-impact impact bar, which is generally reheated to 150-190 ° C. and divided into a third heating step, which is heated at this temperature for 30 to 50 minutes. The method of claim 1, wherein the steel pipe material is heated by placing the primary and secondary tunnel heating coils at intervals of 20 to 40 mm from each other in the heating step, and further, by placing a cooling coil 50 to 100 mm behind the secondary tunnel heating coil. High strength impact bar manufacturing method to perform a heating and cooling process.

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