KR20100036138A - Method for fabricating steel pipe using electronic resistance welding and sts201 steel pipe using the same - Google Patents

Abstract

PURPOSE: A method for fabricating a steel pipe using electronic resistance welding and a STS201 steel pipe using the same are provided to improve workability and moldability by increasing solid solution heat treatment temperature, higher than general heat treatment temperature of a STS201 steel sheet. CONSTITUTION: A method for fabricating a steel pipe comprises following steps. The austenite steel sheet is slit along the external diameter width of the steel pipe in order to form skelp(S-11). The skelp of a roll form is released, CRT curvature is controlled, and edge is ground(S-12). The skelp is molded in the shape of a pipe(S-13). The both edges of the skelp are welded using electric resistance(S-14). A bead formed on the weld surface is removed. A welding part is high melting heat-processed(S-15).

Description

METHOD FOR FABRICATING STEEL PIPE USING ELECTRONIC RESISTANCE WELDING AND STS201 STEEL PIPE USING THE SAME}

The present invention relates to a method for manufacturing a steel pipe using electric resistance welding, and to an STS201 steel pipe manufactured using the same. By manufacturing a steel pipe using the electric resistance welding method using STS201 material, a steel pipe having excellent moldability while improving corrosion resistance is provided. It relates to a technology that can be manufactured.

Stainless steel pipes are used for piping and vehicle parts, and their range of use and usage are gradually increasing.

Such a stainless steel requires excellent workability, formability and corrosion resistance.

Steel pipes used for automotive parts are for hydroforming. Hydroforming began to develop rapidly in the 1990s as it was used in the overseas auto parts industry.

In addition to the basic functions of transporting passengers and cargo, consumer demands for automobiles are expanding in various aspects such as appearance, driving performance, ride comfort, fuel economy and environmental friendliness.

In particular, demand for weight reduction has continued due to soaring oil prices. In response, advanced automobile industries such as the US, Europe, and Japan have been working on a new technology construction method called hydroforming using steel pipes to secure competitiveness in terms of parts reduction, process simplification, material yield, welding score reduction, and weight reduction. A lot of research is being done.

This trend is applied not only to research institutes but also to the domestic automobile industry now, and the demand for lightweight components is expanding in the future.

One of the important characteristics required for the hydroforming steel pipe of the present invention is that the steel pipe material must be a high strength material capable of satisfying a certain level of elongation that can withstand the deformation of the pipe and durability of the automobile parts and crash safety performance.

The purpose of the development of steel pipe for hydroforming is to advance research and development products in order to meet the demands of parts weight reduction and product strength. An important property required for hydroforming steel pipes is the proper level of elongation to withstand the deformation of pipes when forming parts of the desired shape by using the hydraulic pressure acting inside the pipe during hydroforming, and the durability and collision safety of automobile parts. It should be a high strength material that can satisfy the performance.

Stainless steel pipes currently used at home and abroad are mainly manufactured using STS304 steel.

STS304 steel has chemical composition (wt%) C: 0.08 or less, Si: 1.00 or less, Mn: 2.00 or less, P: 0.040 or less, S: 0.030 or less, Cr: 18.0 ~ 20.0, Ni: 8.00 ~ 11.00 and other inevitable additions It is an austenitic STS product containing impurity, which has high strength and high stretching characteristics.

However, since STS304 is a material containing a large amount of expensive Ni, the price is expensive, and thus there is a problem that the steel pipe manufacturing cost increases.

The present invention is a roll forming process using an austenitic STS201 steel sheet to reduce the Ni content, increase the Mn and Cu content, in place of STS304 steel sheet containing a large amount of expensive Ni in the manufacture of piping materials or hydroforming steel pipe The first brake down roll is equipped with a W bending type, and the second brake down roll is equipped with a center bending type to adjust the process to minimize the occurrence of work hardening. It is an object of the present invention to provide a method for manufacturing a steel pipe using electric resistance welding and an STS201 steel pipe manufactured by using the same, by increasing the heat treatment temperature of the steel sheet to improve workability and formability.

Steel pipe manufacturing method using the medium frequency electrical resistance welding according to the present invention is chemical composition (wt%) C: 0.06 ~ 0.10, Si: 0.05 ~ 0.10, Mn: 5.50 ~ 7.50, P: 0.045 or less, S: 0.015 or less, Cr: 17.0 to 18.0, Ni: 3.50 to 5.50, Cu: 1.50 to 3.50, and slits forming a skelp by slitting an austenitic steel sheet containing an unavoidably added impurity to the outer diameter of the steel pipe Rolling step, leveling and edge milling step of adjusting flatness and grinding edge while releasing the roll-shaped skelp, and roll forming to form a pipe shape by forming the skelp through the leveling and edge milling step into a multi-stage roll. And an electric resistance welding step of welding both edges of the skelp through the roll forming step by using an electric resistance, removing beads formed on the surface of the welded part, and the welding part. Characterized in that it comprises a weld heat treatment step of heat-treating pupation.

In addition, the STS201 steel pipe according to the present invention is formed by the above-described steel pipe manufacturing method is characterized by having a tensile strength of 60kg or more and an elongation of 50% or more.

Steel pipe manufacturing method using the electric resistance welding according to the present invention and STS201 steel pipe manufactured using the same by manufacturing a steel pipe using STS201 steel sheet with reduced Ni content, so as to lower the production cost, but have high strength, excellent corrosion resistance and formability To provide the effect.

In addition, the present invention provides the effect of improving the workability and formability by using the initial brake down roll as the W bending type, and increasing the solid solution heat treatment temperature after the medium frequency electric resistance welding than the heat treatment temperature of the general STS201 steel sheet. .

In order to solve the above-mentioned problems, the piping material or the hydroforming steel pipe according to the present invention is a chemical component (wt%) C: 0.06 ~ 0.10, Si: 0.05 ~ in place of STS304 steel sheet containing a large amount of expensive Ni. Austenitic STS201 steel sheet containing 0.10, Mn: 5.50 to 7.50, P: 0.045 or less, S: 0.015 or less, Cr: 17.0 to 18.0, Ni: 3.50 to 5.50, Cu: 1.50 to 3.50 and other unavoidable impurities To form.

Herein, C is an important element that allows martensite transformation to occur after heat treatment in a subsequent process, and may help to improve the martensite fraction as the C content increases. If the C content is less than 0.06 wt%, the desired strength cannot be obtained, and if the C content is excessively increased beyond 0.1 wt%, it adversely affects the workability, so it is important to maintain an appropriate amount of the content according to the present invention.

Next, Si is an austenite stabilizing element, when the Si content is less than 0.05 wt%, or exceeds 0.10 wt%, heat treatment and tissue transformation may be insufficient.

Next, Mn serves to suppress FeS formation and lower martensite transformation start temperature (MS temperature). Therefore, the cooling rate when manufacturing the STS201 steel pipe is dependent on the Mn equivalent, when the Mn content is 5.50 ~ 7.50 wt%, the cooling rate is slowed to be able to manufacture the STS201 steel. In addition, Mn may improve the tensile strength by making the ferrite fine. Mn promotes the solid solution of C to Fe element, and the solidified C is changed to martensite around the ferrite grain boundary after heat treatment to have super high strength.

Next, Cu improves the corrosion resistance of the steel pipe and acts as an austenite stabilizing element. However, the addition of less than 1.50 wt% may not exhibit the effect, and if an excessive amount of more than 3.5 wt% is added, red brittleness may occur. Therefore, it is preferable to maintain an appropriate amount according to the present invention. Do.

As described above, the STS201 steel sheet according to the present invention is an STS product having lowered Ni content and increased Mn / Cu content to lower cost and improve corrosion resistance. However, STS201 steel sheet is less workability and formability than STS304, for the hydroforming process, the steel pipe forming process should be controlled as follows.

First, the roll forming process of forming a steel pipe while passing through the forming roll is improved. Equipped with an initial break down roll of type W bending, this minimizes the formation of both edges of the initial skelp.

Next, from the second break down roll, the precision bending can be performed using the center bending type, and the process is adjusted to minimize the occurrence of work hardening.

Next, the solid solution heat treatment temperature performed after the steel pipe formation is increased than the heat treatment temperature (1010 ~ 1120 ℃) of the general STS201 steel sheet. By increasing the solid solution heat treatment temperature, the grain size (Grain Size) of the STS201 steel sheet can be coarsened, thereby improving workability and formability. At this time, by performing the heat treatment process by the offline heat treatment, it is preferable to improve the stability of the heat treatment conditions, thereby to be able to further improve the workability and formability.

Hereinafter, with reference to the accompanying drawings will be described an embodiment of a steel pipe manufacturing method using a medium frequency electrical resistance welding according to the present invention.

In this process, the thickness of the lines or the size of the components shown in the drawings may be exaggerated for clarity and convenience of description.

In addition, terms to be described later are terms defined in consideration of functions in the present invention, which may vary according to the intention or custom of a user or an operator.

Therefore, definitions of these terms should be made based on the contents throughout the specification.

1 is a process chart showing a steel pipe manufacturing method according to the present invention.

1, the slitting step (S-11) of cutting the STS201 hot rolled steel coil to form a skelp, the leveling and edge milling step (S-12), the roll forming step of forming the skelp into a tubular shape ( S-13), an ERW welding step (S-14) for welding the abutted edge surface by using a medium frequency Electronic Resistance Welding (ERW) process, and a heat treatment step (S-15) for heat treatment of the welded part. do.

Hereinafter, each process will be described in detail.

Figure 2 is a schematic diagram schematically showing a method for manufacturing a steel pipe using a medium frequency electric resistance welding according to the present invention.

Referring to FIG. 2, first, a slitting step (S-11) of cutting the coiled hot rolled coil 20 to a width corresponding to the circumference of a steel pipe to be manufactured is performed. Slitting is performed by the slitter and the cut coil is called a skelp (21).

Next, the leveling and edge milling step (S-12) is performed.

The leveling device 22 is provided with planar rolls and flattens the skelp 21 while releasing the skelp 21, and the milling device 23 is provided on a surface perpendicular to the direction of travel of the skelp 21. Both edges of the skelp 21 that will be abutted by welding in the subsequent process are cut by milling.

Here, the operation is performed on-line from the leveling and edge milling step (S-12) to the welding step (S-14) described below, and the tube speed at this time is preferably 25.0 to 30.0 m / min.

It is obvious that the higher the pipe speed, the higher the productivity, but it is determined in consideration of the work hardening or the quality of the weld which occurs in the subsequent roll forming step (S-13).

Subsequently, a roll forming step S-13 is performed in which the flat skelp 21, which has undergone the leveling and edge milling steps S-13, becomes a pipe shape while passing through the multi-stage forming roll 25.

Next, the ERW welding step (S-14) of completing the steel pipe is performed by welding both edge portions of the pipe-shaped skelp 26 using the medium frequency electric resistance welding apparatus 27. At this time, since the bead is generated in the weld by welding, the process of trimming the weld using the bead removal device 28 is performed.

Next, the weld heat treatment step (S-15) of annealing the weld seam (weld seam) deformed by welding is performed. When the heat treatment is performed, the Cr layer is evenly distributed, thereby improving corrosion resistance, making the structure of the welded part in which the heat deformation is made into a uniform austenite structure, and making the grain size uniform.

Welding heat treatment step (S-15) according to the present invention is to maintain a temperature range of 1050 ~ 1200 ℃, the heating furnace 29 is preferably provided in an offline manner. In the case of heat treatment in an offline manner, since the heat treatment effect is more stable, workability and formability may be further improved.

3 is a view showing a forming roll arrangement of the steel pipe manufacturing method according to the present invention.

As shown in Fig. 3, the multi-stage forming roll according to the present invention is a four-stage breakdown roll 31, a three-stage side roll 32, a four-stage pin pass roll 33, and a three-stage squeeze. It consists of a roll 34.

The break down roll 31 is composed of a horizontal roll driven to perform an initial forming, including the edge shaping of the skelp, to the shape of a semicircle.

At this time, the first break down roll 25a is applied with a W banding type that minimizes molding of both edges of the skelp 21 as shown in FIG. 4, and subsequent rolls are used to minimize the occurrence of precision molding and work hardening. Center banding type is applied.

Side roll 32 is composed of a non-driven vertical roll, disposed between the horizontal roll or in a cluster (cluster) serves to hold the shape and position rather than forming.

Pin pass roll 33 is a horizontal roll for the last step of the molding, the pin plate attached to the upper roll serves to control the angle of the edge surface, finish forming, centering.

Squeeze roll 34 is a roll for preparing a welding process, and serves to facilitate the welding by controlling the amount of upset and the edge encounter phenomenon.

Here, in order to ensure the welding quality, it is preferable to maintain the interval between the edge portions of the skelp in the pipe shape by the squeeze roll 34 in the range of 5 mm to 8 m.

In the above-described ERW welding step (S-14), the edge portion of the skelp discharged from the squeeze roll 34 is welded using a medium frequency electric resistance. In this case, edge bending and circular bending are performed. Otherwise, the work hardening will be severe during steel pipe forming. In this case, the medium frequency uses a frequency of 1 kHz to 100 kHz. If the frequency is lower than 1 kHz, the welding efficiency may decrease due to over-infiltration and the defect of the welded part. Cold spot phenomenon may occur that is not performed.

In addition, the use of a medium-low frequency welding machine may result in non-uniform heat and heat, and widen the weld quality range. And since the gap between the edges of the skelp is controlled by the squeeze roll, it is impossible to secure the weld quality because the gap of the squeeze roll is not minimized, so that the proper amount of heat can not be secured. For example, when the gap of the squeeze roll falls by more than 10 mm, the calorific value is reduced by 75%. Therefore, it is important to minimize the weld gap spacing within 1-10 mm.

Therefore, in the present invention, the gap adjustment between the squeeze rolls and the welding conditions are optimized within the above ranges, so that problems such as cracking of the weld core and incomplete welding due to excessive stress are not generated.

Thus, while passing through a multi-stage forming roll, the flat skelp has a roll flower as shown in FIG. 5 and is processed into a pipe shape.

The roll flower is a continuous representation of the cross section that deforms with each passing roll.

As shown in Fig. 5, the roll flower of the manufacturing method according to the present invention is uniformly distributed in deformation amount and thickness, so that less work hardening occurs at the edge and a better quality steel pipe can be produced.

As described above, the present invention manufactures the STS201 steel pipe using the medium frequency electric resistance welding method, and in order to find out the physical characteristics of the steel pipe according to the present invention by collecting three specimens from the welded part and the base material, respectively, the following Table 1 and As shown.

The diameter of the steel pipe manufactured by the manufacturing method according to the present invention was 42.7 mm, the thickness was 2.0 mm, and the solid solution heat treatment temperature was 1150 ° C. And, the physical properties were measured for tensile strength (TS), yield strength (YS), elongation (Total EL), respectively.

division Specimen Collection Location TS (MPa) YS (MPa) Total EL (%) Ф42.7 * 2.0t Psalm 1 Weld 640 332 55.1 Psalm 2 642 331 55.0 Psalm 3 646 332 54.8 Psalm 4 Mother and child department 639 326 55.8 Psalm 5 644 326 55.2 Psalm 6 647 325 55.6

As can be seen from the results of Table 1, it can be seen that the tensile strength of the welded portion and the base metal portion is uniformly distributed in the 640Mpa grade, the yield strength is 330Mpa grade, and the elongation is about 55%. Therefore, it can be seen that uniform processing is performed over the entire steel pipe, thereby realizing the uniformity of steel hardening and minimizing the amount of work hardening.

In addition, the STS201 steel pipe according to the present invention has a high strength and high elongation characteristics similar to the conventional STS304 steel pipe, in order to prove this will be compared to the physical properties of the STS201 steel pipe and STS304 steel pipe.

The STS304 steel pipe, which is a reference for comparison, was formed with a diameter of 42.7 mm, two thicknesses of 1.2 mm and 2.0 mm, and a solid solution heat treatment temperature of 1150 ° C.

Next, the STS201 steel pipe diameter according to the present invention was formed in a diameter of 42.7mm, the thickness was formed in two cases of 1.2mm and 2.0mm, the solid solution heat treatment temperature is three cases respectively 1050 ℃, 1100 ℃, 1150 ℃ The physical properties were measured by dividing.

In addition, the property measurement items were made as tensile strength (TS), yield strength (YP), total elongation (%), uniform elongation (%) and work hardening index, the results are shown in Table 2 below.

Table 2 is a data comparing the heat treatment temperature of the STS304 steel pipe of the high-nickel component and the STS201 steel pipe according to the present invention and the properties according to the specifications.

Steel grade standard Heat treatment temperature TS (Mpa) YP (Mpa) T-El (%) U-El (%) n-value STS304 Ф42.7 * 2.0t 1150 ℃ 650 321 55.6 47.5 0.362 Ф42.7 * 1.2t 1150 ℃ 634 310 55.0 48.7 0.373 STS201 Ф42.7 * 2.0t 1150 ℃ 644 327 55.3 44.1 0.365 Ф42.7 * 2.0t 1100 ℃ 652 336 55.4 43.9 0.361 Ф42.7 * 2.0t 1050 ℃ 686 393 50.1 38.2 0.322 STS201 Ф42.7 * 1.2t 1150 ℃ 648 334 53.4 43.9 0.364 Ф42.7 * 1.2t 1100 ℃ 656 330 54.4 44.8 0.371 Ф42.7 * 1.2t 1050 ℃ 676 369 51.2 41.6 0.343

As can be seen from the results of Table 2, all of the STS201 steel pipe according to the present invention exhibits a tensile strength equal to or higher than that of the STS304 steel pipe, it can be seen that the elongation or work hardening index is also represented at the same level.

Here, it can be seen that the physical properties change little by little depending on the heat treatment temperature. The heat treatment range according to the present invention is to increase the temperature range as a whole than 1050 ~ 1200 ℃ 1010 ~ 1120 ℃ conventional.

As the heat treatment temperature increases, the tensile strength decreases little by little, but it can be seen that the elongation and the work hardening index are improved to have excellent moldability. Therefore, the STS201 steel pipe according to the present invention can be an optimized material as a piping material or a steel pipe for hydroforming.

Although the embodiments of the present invention have been described above with reference to the accompanying drawings, the present invention is not limited to the above embodiments and can be modified in various forms, and having ordinary skill in the art to which the present invention pertains. It will be understood by those skilled in the art that the present invention may be embodied in other specific forms without changing the technical spirit or essential features of the present invention. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive.

1 is a process chart showing a steel pipe manufacturing method according to the present invention.

Figure 2 is a schematic diagram schematically showing a method for manufacturing a steel pipe using a medium frequency electric resistance welding according to the present invention.

Figure 3 is a view showing the forming roll arrangement of the steel pipe manufacturing method according to the present invention.

4 is a sectional view showing a first breakdown roll according to the present invention.

5 is a cross-sectional view showing a roll flower in the steel pipe manufacturing method according to the present invention.

Claims (8)

Chemical composition (wt%) C: 0.06 ~ 0.10, Si: 0.05 ~ 0.10, Mn: 5.50 ~ 7.50, P: 0.045 or less, S: 0.015 or less, Cr: 17.0 ~ 18.0, Ni: 3.50 ~ 5.50, Cu: 1.50 ~ Slitting an austenitic steel sheet containing 3.50 and other unavoidable impurities to the outer diameter width of the steel pipe to form a skelp; Leveling and edge milling steps to adjust flatness and polish edges while releasing the roll-shaped skelp; A roll forming step of forming the skelp through the leveling and edge milling steps into a multi-stage roll to form a pipe; A medium frequency electric resistance welding step of welding both edges of the skelp through the roll forming step using electric resistance; Removing the beads formed on the weld surface; And Welded part heat treatment step of heat-treating the welded portion; steel pipe manufacturing method comprising a. The method of claim 1, The steel pipe manufacturing method, characterized in that the leveling and the edge milling step is continuously made on-line from the medium frequency welding step, the tube speed is in the range of 25 ~ 30m / min. The method of claim 1, The roll forming step is performed by passing a four-stage break down roll, a three-stage side roll, a four-stage pin pass roll, and a three-stage squeeze roll. Steel pipe manufacturing method The method of claim 3, wherein The first brake down roll of the brake down rolls is used as a W bending type for minimizing the formation of both edges of the initial skelp, and the second and subsequent brake down rolls have a center bending type (Center Bending Type) steel pipe manufacturing method characterized in that used. The method of claim 1, When the welding of both edges of the skelp, the gap of the both edges is adjusted to be 5mm ~ 8mm steel pipe manufacturing method. The method of claim 1, The medium frequency electrical resistance welding step is a steel pipe manufacturing method, characterized in that using a medium frequency of 1kHz ~ 100kHz. The method of claim 1, The welding unit heat treatment step is a steel pipe manufacturing method characterized in that for heating the weld to 1050 ~ 1200 ℃ using an offline furnace. STS201 steel pipe manufactured by the method of claim 1.

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