KR101287018B1 - A method for manufacutring laser welded boron sheet with hot stamping process - Google Patents

Abstract

The present invention relates to a method for producing hot stamping of boron steel sheet, the material preparation step of providing different blank material to manufacture a component that requires the local heterogeneous strength; Welding the different blank materials to each other; A heating step of heating the joined blank material; And hot forming the heated blank material at 650 ° C. to 700 ° C. to form a hot stamping step.

Description

A method for manufacutring laser welded boron sheet with hot stamping process

The present invention relates to a method for manufacturing a laser welded boron steel sheet through a hot stamping process.

Recently, automobile regulations require strict body in white (BIW) collision performance to enhance passenger safety. In addition, as the awareness of the environment increases, the fuel economy standards for exhaust gas regulations are strengthened, and the necessity for weight reduction is continuously increasing. As part of efforts to simultaneously satisfy the requirements of improving the collision performance and light weight, the amount of application of high-strength steel sheet to the body is continuously increasing. However, in the case of high strength steel sheet, the elongation is low, so the application is limited to complex shaped parts requiring high formability. In order to overcome this, hot forming (boron steel sheet) heated at high temperature is formed in a press and then hot formed (hot stamping, hot press fomring or press hardening). Technology has been developed since the late 1990s, and 1400MPa class body parts using hot stamping technology have been developed and applied in various ways. TWB boron steel sheet hot stamping technology produces blanks by welding materials of different types or thicknesses (mainly laser welding), and forms parts by hot stamping method to maximize the collision of parts and light weight. Although there is an advantage, there is a conventional difficulty in hot stamping a laser welded blank (TWB) boron steel sheet due to fracture behavior due to laser welding of the boron steel sheet.

Conventional Republic of Korea Application No. 10-2009-0113386 "Taylor welded hot stamping manufacturing method and a molded article having a locally heterogeneous strength using the same" molded body having a locally different strength by joining a material of different materials and its manufacturing method The hot stamping molding temperature, which does not disclose the breakage behavior of the laser-welded boron steel sheet, is disclosed, but does not disclose the hot-stamping of the TWB boron steel sheet. The integrity of the welds could not be ensured, resulting in breakage in the welds.

The present invention is to produce a blank by laser welding a material of different type or thickness with the boron steel sheet, and in forming the part by hot stamping method, it is possible to maximize the collision improvement and light weight of the part and to ensure the integrity of the laser welding portion The purpose of the present invention is to propose a hot stamping method for manufacturing a welded boron steel sheet welded with a laser.

Hot-stamping manufacturing method of the boron steel sheet according to the present invention in order to achieve the object as described above, the material preparation step of providing a different blank material for producing a component that requires a different strength; Welding the different blank materials to each other; A heating step of heating the joined blank material; And hot forming the heated blank material at 650 ° C. to 700 ° C. to form a hot stamping step.

According to the invention, one of the different blank material is characterized in that the boron steel sheet.

According to the invention, the welding in the welding step is characterized in that the laser welding.

According to the present invention, the laser source in the laser welding has a YAG laser source, characterized in that the output of the laser beam is 3kW.

According to the invention, the laser source and the blank is characterized in that the vertical irradiation.

According to the present invention, the laser welding speed during the laser welding is characterized in that the 4.0m / min.

According to the present invention, the protective gas to be sprayed around the laser welding in the welding step is characterized in that helium is used, the injection speed is 20 l / min.

According to the present invention, the components of the boron steel sheet are 0.18 wt% C, 0.21 wt% Si, 1.27 wt% Mn, 0.017 wt% P, 0.002 wt% S, 0.22 wt% Cr, 0.0028 wt% B, and the balance Fe is characterized in that it is included.

According to the present invention, the boron steel sheet is characterized in that the plated Al-Si alloy.

According to the invention, the coating weight of the Al-Si alloy containing 9 wt% Si of the boron steel sheet is 80g / m ² , characterized in that the thickness is 1.4mm thick.

According to the present invention, in the heating step, the blank material is characterized in that the heating for 5 minutes at 950 ℃ in an external furnace.

According to the present invention, in the heating step, the blank material is characterized in that the heating for 5 minutes at 950 ℃ in an external furnace.

According to the present invention, the protective gas to be sprayed around the laser welding in the welding step is characterized in that helium is used, the injection speed is 20 l / min.

It is characterized in that to provide a molded article having a heterogeneous strength produced by the hot stamping manufacturing method of the boron steel sheet according to the present invention.

According to the present invention, a hot stamping method for manufacturing a welded boron steel sheet welded by a laser according to the present invention is a high-temperature tensile test of a laser-welded TWB boron steel sheet with a specimen in which the weld line is horizontal and vertical in the tensile hysteresis condition of the hot stamping process. (1) Type I high temperature tensile test at 0.01 / s showed that the tensile strength of the weld at 650 and 700 ° C was higher than that of the base material, similar at 800 ° C, and (2) Type II high temperature at 0.01 / s. The test results show that the fracture occurred in the base material at 650 and 700 ℃ of the weld, and that the fracture occurred in the weld at 800 ℃. There is no effect of breaking.

1 is a flowchart of a method for producing hot stamping boron steel sheet according to the present invention.
Figure 2 relates to a laser welding machine used in the present invention, Figure 2 (a) is a laser oscillator, Figure 2 (b) shows a six-axis robot system, Figure 2 (c) shows a nozzle for the protective gas injection.
Figure 3 shows a welded cross-sectional shape of the welded specimen according to the present invention.
Figure 4 is a tensile tester used in the present invention, Figure 4 (a) shows a tensile tester with a heating furnace, Figure 4 (b) shows a state in which the demonstration is fixed to the tensile tester grip (grip).
5 shows the shape of the test piece used in the present invention.
FIG. 6 illustrates a Type I specimen in which the tensile direction and the weld line are horizontal, and a Type II specimen in which the tensile direction and the weld line are vertical.
Figure 7 shows the experimental temperature for a temperature hysteresis condition similar to the hot stamping process according to the present invention.
FIG. 8 shows stress-strain plots measured at different temperatures and FIG. 8 (b) shows strain-strain plots measured at different strain rates for Type I welded specimens and substrate specimens. .
FIG. 9 (a) shows tensile strength and elongation according to temperature change, and FIG. 9 (b) shows tensile strength and elongation according to strain rate.
10 shows the shape of a Type I welded specimen that was tensile tested to failure.
FIG. 11 (a) shows the stress-strain plots measured at different temperatures for type II welded specimens and the base specimen, and FIG. 11 (b) shows the stress-strain plots measured at different strain rates.
12 (a) shows the tensile strength and elongation with the change of temperature, Figure 12 (b) shows the tensile strength and elongation with the change of strain rate.
FIG. 13 shows the fracture shape of Type II specimens with temperature. FIG.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. First, it should be noted that the same components or parts among the drawings denote the same reference numerals whenever possible. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted so as not to obscure the subject matter of the present invention.

TWB boron steel sheet hot stamping technology produces blanks by welding materials of different types or thicknesses (mainly laser welding), and forms parts by hot stamping method to maximize the collision of parts and light weight. There are advantages to it. In order to secure the hot stamping technology of the TWB boron steel sheet, it is very important to understand the characteristics of the high temperature deformation of the TWB boron steel sheet. Accordingly, in the present invention, the high temperature tensile properties of the TWB boron steel sheet were evaluated by performing tensile tests on the laser welded boron steel sheet in a process history similar to that of the hot stamping process. It was confirmed.

1 is a flow chart of a method for manufacturing a hot stamping of boron steel sheet according to the present invention, the material preparation step (S100) for preparing a different blank material for producing a component that requires a local heterogeneous strength (S100), the different blank provided Welding step (S200) for joining the materials to each other, heating step (S300) for heating the bonded blank material; And hot forming the heated blank material at 650 ° C. to 700 ° C. (S400).

In the present invention, by performing a hot stamping step (S400) by hot forming the welded blank material at 650 ℃ to 700 ℃ after heating, the following experiment was performed to show that the integrity of the welded portion can be secured.

Table 1 below shows the welding conditions in the embodiment according to the present invention.

Laser source Seam Protective gas Angle ( ^o ) Speed (m / min) Gap (mm) Milling Materials Flow rate ( l / min) 0.0 4.0 0.1 Yes He 20.0

As an embodiment of the present invention, the boron steel sheet for hot stamping is used by plating an Al-Si alloy containing 7-11 wt% of Si to prevent oxidation and corrosion during high temperature forming. A 1.4 Mm thick 22 MnB5 boron steel sheet with an Al-Si alloy (9 wt% Si) plating amount of about 80 g / m ² was used. The chemical composition of the boron steel sheet used is summarized in Table 1. TWB boron steel sheets were fabricated by laser butt welding of the same thickness. Figure 1 relates to a laser welding machine used in the present invention, Figure 2 (a) is a laser oscillator, Figure 2 (b) is a six-axis robot system, Figure 1 (c) shows a nozzle for the protective gas injection. Trumpf's HASS HL3006D welder with CW-ND: YAG laser source shown in Figure 2 (a) was used. The output of the laser beam was fixed at 3 kW. The welding used the Yaskawa 6-axis robot shown in FIG. 2 (b). The protective gas was injected through a 4 mm diameter copper nozzle (FIG. 2C), and the distance between the nozzle and the specimen was maintained at 5 mm during welding. The nozzle moved with the laser beam while keeping 60 ^o from the beam to be irradiated. The diameter of the laser beam is 0.6 mm and the focal length of the lens is 200 mm. It is possible to check the profile of the measured laser beam through FIG. 2. Welding was performed on the welding parameters listed in Table 1 in order to fabricate a healthy welded specimen. Appearance of the key holes, undercuts, internal pores, etc. were selected as the specimen manufacturing conditions for high temperature tensile test. The laser beam was irradiated on the vertical direction of the plate, and the welding speed was 4.0 m / min. The butt portion of the material was milled to smooth the surface and the gap was 0.1 mm. Helium was used as the protective gas and was injected at a rate of 20 l / min. 3 is a cross-sectional view of the welded section of the manufactured weld specimen.

In order to evaluate the high temperature tensile characteristics of the laser-welded TWB boron steel sheet according to the present invention, a tensile test was performed at different temperatures and strain rates, and the results were compared with the base material (BM). The high temperature tensile test was carried out in a tensile tester (model name: Shimadzu AG-100kNx) equipped with a heating furnace shown in Fig. 4 (a).

4 is a tensile tester used in the present invention, Figure 4 (a) shows a tensile tester with a heating furnace, Figure 4 (b) shows a state in which the demonstration is fixed to the tensile tester grip (grip). .

5 shows the shape of the test piece used in the present invention.

As shown in FIG. 5, the specimen was modified to conform to ASTME 8M small size specimen shape for high temperature tensile test, and a grip to which the modified specimen could be fastened was used as manufactured in FIG. 4 (b).

FIG. 6 illustrates a Type I specimen in which the tensile direction and the weld line are horizontal, and a Type II specimen in which the tensile direction and the weld line are vertical.

As shown in FIG. 6, two types of specimens having different tensile directions and weld lines directions, that is, specimens having a horizontal tension direction and a weld line (type I), and specimens having a tensile direction and a weld line perpendicular to each other (type II) were fabricated and subjected to a high temperature tensile test. Was performed. For specimens in which the weld line is parallel to the tensile direction, the tensile properties of the weld can be evaluated. On the other hand, specimens in which the weld line is perpendicular to the tensile direction can evaluate the integrity of the welded portion, and the breaking strength of the welded portion can be measured when the welded fracture occurs.

Figure 7 shows the experimental temperature for a temperature hysteresis condition similar to the hot stamping process according to the present invention.

In order to examine the deformation behavior of the TWB boron steel sheet in the temperature history state similar to the hot stamping process conditions, the experiment is performed in the temperature history condition as shown in FIG. The test piece was kept at 950 ° C. for 5 minutes in an external furnace, moved to a tensile tester equipped with a heating furnace, and after about 20 seconds, a tensile test was performed when the temperature of the specimen reached a desired temperature. The high temperature tensile test is the same for both Type I and Type II specimens at temperatures of 650, 700 and 800 ° C at strain rates of 0.01 / s and at strain rates of 0.01 / s, 0.1 / s and 1.0 / s at temperatures of 700 ° C. Do otherwise.

FIG. 8 shows stress-strain plots measured at different temperatures and FIG. 8 (b) shows strain-strain plots measured at different strain rates for Type I welded specimens and substrate specimens. .

FIG. 9 (a) shows tensile strength and elongation according to temperature change, and FIG. 9 (b) shows tensile strength and elongation according to strain rate.

Elongation is shown by comparing tensile strength and elongation at break, respectively.

10 shows the shape of a Type I welded specimen that was tensile tested to failure.

As a result of the high temperature tensile test at different strain rates at 0.01 / s, the tensile strength and tensile strength elongation of both the type I welded specimen and the substrate specimen increased as the temperature was increased in FIGS. 8 (a) and 9 (a). . The elongation at break of the welded specimen increased with temperature, but the elongation at break of the base specimen increased from 650 ° C to 700 ° C, but lowered at 800 ° C.

At 0.01 / s for all 650, 700 and 800 ° C, the tensile strength of the welded specimen is higher than that of the base specimen and the elongation of the welded specimen is lower than that of the base specimen. In addition, the difference between the tensile strength and the elongation of the welded specimen and the parent specimen decreases as the temperature increases, and the difference between the tensile strength and the elongation of the welded specimen and the parent specimen is almost disappeared at 800 ° C. It can be seen that the stress at initial strain at 800 ° C. is higher than that of the welded specimen and the stress at strain of about 40% or more is lower than that of the welded specimen. Considering the rule of mixture, at 650 ° C and 700 ° C strain rate is 0.01 / s, the weld strength of weld is higher than that of the base material and the elongation of weld is lower, while at 800 ° C the property of weld is not large. It can be seen that. In general, at high temperatures, there is almost no difference in tensile strength according to the amount of carbon contained in the steel sheet, but as the temperature is lowered, the tensile strength increases with the amount of carbon. Due to the increased carbon content of the welded portion during the laser welding, the boron steel sheet is melted and quenched, resulting in higher strength of the welded portion than the base metal portion at temperatures of 650 ° C and 700 ° C.

8 (b) and 9 (b) show the results of the high temperature tensile test at different strain rates at a temperature of 700 ° C. It can be seen that as the strain rate increases, the tensile strength of the Type I welded specimen and the substrate specimen increases and the elongation decreases. Tensile strength and tensile strength elongation of welded specimens are higher than that of the parent specimen at strain rates of 0.01, 0.1, and 1 / s. On the other hand, in the case of elongation at break, the base specimen is higher than the weld specimen at 0.01 / s, but the weld specimen is higher than the base specimen at 0.1 / s and 1 / s.

It can be seen that the tensile strength of the welded portion was higher than that of the base metal part at 650 and 700 ° C., and similar at 800 ° C. in the case of Type I.

Therefore, as in type I, the blank material welded with the laser in a state where the tensile direction and the welding line are horizontal is blank material of two different strengths because hot forming at 650 ° C. to 700 ° C. after heating is higher than the base material part. In the molded body requiring the strength of the base material by joining, it is possible to secure the desired tensile strength of the molded body due to higher than the tensile strength of the welded portion than the base material portion.

FIG. 11 (a) shows the stress-strain plots measured at different temperatures for type II welded specimens and the base specimen, and FIG. 11 (b) shows the stress-strain plots measured at different strain rates.

12 (a) shows the tensile strength and elongation with the change of temperature, Figure 12 (b) shows the tensile strength and elongation with the change of strain rate.

 Tensile and tensile strength of Type II welded specimens increased with increasing temperature, but the elongation at break increased from 650 ℃ to 700 ℃ at 800 ℃. It can be seen in Figure 11 (a) and Figure 12 (a) that the sharp drop. This is because the fracture occurs at the base material portion at 650 ℃ and 700 ℃ while the fracture occurs at the weld portion at 800 ℃. This can be clearly seen from the fracture shape of the Type II specimen with temperature in FIG. 13.

The difference in tensile strength of the weld and the base material described for the Type I welded specimens described above can be inferred from the cause of failure of the Type II welded specimen. In case of Type I welded specimens, the fracture is highly related to the elongation at break of the base material and the welded part, depending on the iso-strain condition. Type II welded specimens are closely related to strength and thickness depending on the force equilibrium conditions. Since the tensile strength of the weld is higher than that of the base material at 650 ° C. and 700 ° C., the deformation is concentrated in the base material part with low tensile strength, and fracture occurs at the base material part. On the other hand, at 800 ℃, there is no difference in tensile strength of the welded part and the base material, but since the tensile strength elongation is 32.3% (± 1.63) for the welded part and the base material part is 59.9% (± 8.25), the tensile strength of the base material is increased to the elongation of the base material. Concentration breaks. Elongation at break is 49.3% (± 5.03). Therefore, at 800 ° C. during hot forming with TWB boron steel, fracture of the weld occurs.

Therefore, as in type I, the blank material welded with the laser in a state where the tensile direction and the welding line are horizontal is blank material of two different strengths because hot forming at 650 ° C. to 700 ° C. after heating is higher than the base material part. It is clear that in the molded body requiring the strength of the base material by joining, the desired tensile strength of the molded body can be secured by being higher than the tensile strength of the welded part than the base material part. In the blank material that is laser-welded in the state, hot forming at 650 ° C. to 700 ° C. after heating has a higher tensile strength of the weld than the base material, so that a blank material of two different strengths is bonded to each other to require the strength of the base material. Since the tensile strength of the welded part is higher than that of the base material part, the desired tensile strength of the molded body can be secured. At 800 ℃, the tensile strength of the welded part and the base material is not different, but since the tensile strength elongation is greater than that of the welded part, the welded part is increased to the tensile strength elongation of the base material, and then the deformation is concentrated at the welded part so that the fracture occurs at the welded part. The strength cannot be secured.

From the results of Type I and Type II, it can be seen that the integrity of the weld can be ensured regardless of the welding direction only when the laser-welded blank material is hot formed at 650 ° C to 700 ° C after heating.

While the invention has been shown and described in connection with specific embodiments thereof, it is conventional in the art that various changes, modifications and variations are possible without departing from the spirit and scope of the invention as indicated by the appended claims. Anyone who knows the knowledge of is easy to know.

Claims (14)

A material preparation step of preparing different blank materials, one of which is a boron steel sheet, in order to manufacture parts requiring locally different strengths;
Welding the different blank materials to each other in a horizontal or vertical state in a tensile direction and a direction of a welding line by laser welding;
A heating step of heating the joined blank material for 5 minutes at 950 ° C. in an external heating furnace; And
It comprises a hot stamping step of hot-molding after mounting the heated blank material is moved to a tensile tester equipped with a heating furnace,
In the welding step, helium is used as the protective gas sprayed around the laser welding part, and the spraying speed is 20 l / min, and during laser welding, the laser source has a YAG laser source and is irradiated perpendicularly to the blank material, and output of the laser beam. Is 3kW, laser welding speed is 4.0m / min,
The components of the boron steel sheet are 0.18 wt% C, 0.21 wt% Si, 1.27 wt% Mn, 0.017 wt% P, 0.002 wt% S, 0.22 wt% Cr, 0.0028 wt% B, and the balance Fe is included, and the boron steel sheet is plated with an Al-Si alloy, the coating weight of the Al-Si alloy containing 9 wt% Si of the boron steel sheet is 80 g / m ² , the thickness is 1.4mm thick,
The hot stamping step,
By hot forming the heated blank material at a temperature of 650 ℃ to 700 ℃ to increase the tensile strength of the weld portion than the tensile strength of the base material to ensure the desired tensile strength to the molded body bonded to the blank material of two different strengths Hot stamping manufacturing method of the boron steel.
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