Method and apparatus for manufacturing steel plate component

Abstract

To provide a method for manufacturing a steel plate component capable of maintaining the overall strength of the component while avoiding delayed fracture.SOLUTION: A method for manufacturing a steel plate component includes locally heating the component by means of a heating electrode disposed on one side and facing the punched portion of the steel plate with a separation therefrom. The steel plate component is a high-strength steel plate having a tensile strength of 780 MPa or higher. The heating temperature is 500 to 830°C. The steel plate is heated in a short time of 10 seconds or less by means of high-frequency induction. The heating electrode is a coil larger than the punched portion and has two or more turns with respect to the punched portion.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a method and an apparatus for manufacturing a steel sheet component. [Background technology]

[0002] Reducing the weight of automobiles is an important issue for reducing CO2 emissions by extending the driving range of BEVs (Battery Electric Vehicles) and improving fuel efficiency of conventional vehicles. Therefore, the use of high-strength steel sheets to integrate parts and reduce their thickness is being considered. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 7207283 Summary of the Invention [Problem to be solved by the invention]

[0004] One of the issues when using high-strength steel sheets is the problem of delayed fracture, which is a phenomenon in which a metal material subjected to a constant tensile load suddenly breaks after a certain period of time has passed.

[0005] It is known that delayed fracture occurs due to the synergistic action of the combination of corrosive environment, material, and tensile stress. In sheet metal parts, high residual stress remains on the punched end surface, making the punched end surface prone to delayed fracture, which is a major barrier to the expansion of the use of high-strength steel plates. For this reason, in conventional sheet metal parts, the only way to deal with areas where delayed fracture is a high concern is to avoid using high-strength steel plates, avoid corrosive environments, or create a structure that does not generate residual stress.

[0006] The method described in Patent Document 1 discloses a technique for improving stretch flange formability by sandwiching the edge end face of a steel sheet between electrodes and passing an electric current through the edge to heat the vicinity of the edge.

[0007] However, regular maintenance of the electrodes is required to stabilize the heating range, resulting in high electrode costs, maintenance, quality control, and other labor costs. In addition, because the non-heated area is sandwiched between two electrodes, it may not be possible to heat the non-heated surface if it is vertical or has a complex shape. Furthermore, induction heating is also possible, but this requires placing heating coils above and below the workpiece plane (placing electrodes facing each other). This method is inefficient because the heating range is wide, and it is not possible to perform localized heating near the edge with high precision.

[0008] The present disclosure has been made to solve such problems, and aims to provide a manufacturing method for a steel plate part that can avoid delayed fracture while maintaining the strength of the entire part. [Means for solving the problem]

[0009] A method for manufacturing a steel plate part according to one embodiment of the present disclosure includes: This method is characterized by localized heating using a heating electrode placed on one side directly opposite the punched portion of the steel plate.

[0010] An apparatus for manufacturing a steel plate part according to one embodiment of the present disclosure includes: A high frequency AC power source; A coil connected to the high frequency AC power source; A mounting base for mounting the steel plate; When the steel plate is attached to the mounting base, a processed portion of the steel plate is configured to directly face the coil, The method is configured to carry out the above-mentioned manufacturing method. Effect of the Invention

[0011] According to the present disclosure, it is possible to provide a manufacturing method for a steel plate part that can avoid delayed fracture while maintaining the strength of the entire part. [Brief description of the drawings]

[0012] [Figure 1] FIG. 2 is a schematic perspective view illustrating a method for manufacturing a steel sheet part according to the present disclosure. [Diagram 2] 2A to 2C are diagrams illustrating steps in a method for manufacturing a steel sheet part according to an embodiment. [Diagram 3] FIG. 2 is a diagram illustrating an evaluation method for a hydrochloric acid immersion test according to an embodiment. [Figure 4] FIG. 13 is a diagram illustrating the evaluation results of a hydrochloric acid immersion test in the case of a notched portion without local heating according to a comparative example. [Diagram 5] FIG. 13 is a diagram illustrating the evaluation results of a hydrochloric acid immersion test in the case of a notched portion with local heating according to an embodiment. [Figure 6] FIG. 3 is a diagram illustrating the temperature distribution in the circumferential direction of the punched end after the local heating shown in FIG. 2. [Figure 7] 3 is a diagram illustrating a temperature distribution in a direction perpendicular to the end surface of the punched end after the local heating shown in FIG. 2. [Figure 8] FIG. 2 is a diagram illustrating an example of a target area for local heating according to the embodiment. [Figure 9] FIG. 11 is a diagram illustrating another example of a target portion for local heating according to the embodiment. [Figure 10] FIG. 13 is a diagram illustrating an example of an apparatus for local heating using a laser according to another embodiment. [Figure 11] FIG. 13 is a top view illustrating the shape of the punched end and the coil. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] With reference to FIG. 1, an overview of the manufacturing method of the steel sheet part according to the present disclosure will be described. A method is provided for reducing residual stress and avoiding the occurrence of delayed fracture by applying localized heat to the punched end 20 of a high-strength steel plate, where residual stress is high.

[0014] Therefore, the manufacturing method of the steel plate part according to the present disclosure is characterized in that heat is applied locally by a heating electrode (e.g., a coil) arranged facing and spaced apart from the processed portion of the steel plate (e.g., the punched end 20). In this specification, facing means that the processed portion of the steel plate (e.g., the punched end 20) and the heating electrode (e.g., the coil) can be arranged substantially parallel to each other without contacting each other, within 10 mm. As shown in FIG. 1, the processed portion of the steel plate (e.g., the punched end 20) protrudes toward the heating electrode (e.g., the coil) and extends in a cylindrical shape. In other words, the protruding tip portion (punched end surface) and the heating electrode (e.g., the coil) can be arranged substantially parallel to each other without contacting each other, within 10 mm.

[0015] More specifically, the punched end 20 of a high-strength steel plate having a tensile strength of 780 MPa or more may be heated at 500 to 830°C for a short time of 10 seconds or less. This reduces residual stress and prevents delayed fracture. At this time, it is preferable to use high-frequency induction heating in order to apply heat locally for a short time. The number of turns of the coil can be set arbitrarily, but preferably, as shown in FIG. 1, at least two turns of the coil overlap with the punched end 20. In addition, by arranging the coil 10 so as to directly face the punched end 20, it is possible to heat the punched end 20 in a concentrated manner and to minimize deformation of the panel 2.

[0016] Although it is possible to reduce residual stress and avoid delayed fracture even with heating for 10 seconds or more, the heat-affected zone may spread over a wide area (of panel 2) due to heat transfer, which may reduce the strength of the part. Therefore, it is desirable to finish heating as quickly as possible.

[0017] First embodiment Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 2 is a diagram for explaining each step (loading, heating, removal) in the manufacturing method of the steel plate part. Four L-shaped mounting bases 11 for placing the elongated steel plate parts 2 on are provided near the center of the upper surface of the rectangular base part 12. Furthermore, two coils 10 are provided on both diagonal ends of the upper surface of the rectangular base part 12. An AC power source 15 is connected to the coils 10 for generating high-frequency induced electromotive force by passing an AC current through the coils.

[0018] A punched end 20 is provided at each end of the elongated steel plate part 2. The punched end 20 is processed by drilling a hole in the material and stretching the edge of the hole into a cylindrical shape. This process is also called burring.

[0019] When the elongated steel plate part 2 is placed on the four L-shaped mounting bases 11, the two punched ends 20 at both ends of the steel plate part 2 are positioned so as to face the coil 10 in a substantially parallel manner. A positioning mechanism (e.g., the pin hole 23 of the steel plate part 2 in FIG. 2 and the pin 13 on the base part 12, etc.) may be provided so that the steel plate part 2 faces the coil 10 in a substantially parallel manner. The axes of the two coils 10 may be perpendicular to the upper surface of the base part 12. The outermost periphery of the coil 10 is aligned so as to partially overlap the punched ends 20. This suppresses heating of the panel 2 and realizes local heating of the punched ends 20. The diameter of the coil is slightly larger than the diameter of the punched ends 20. The coil may be a coil copper tube (copper pipe coil). As shown in FIG. 11, the coil diameter CD may be at most equal to or less than the diameter PED of the punched ends plus two pipe diameters PD of the pile P forming the coil. This prevents the punched end from being heated widely beyond the punched end, and allows the punched end to be heated more appropriately and locally. In addition, the input end S and output end E of the coil are arranged on one side of the center O of the coil (on the radius of the coil from the center O of the coil) along the diameter L of the coil. This allows the number of turns of the coil to be arranged approximately evenly over the entire punched end, thereby achieving even more uniform heating. As a result, delayed fracture can be prevented. The coil 10 can be, for example, a coil with two or more turns for the punched end 20 in order to ensure a sufficient magnetic field for rapid heating. If it is a one-turn coil, the coil is not connected at the folded portion (i.e., the input end S and the output end E). The magnetic field spreads only in this discontinuous portion, and the current density does not increase. As a result, the temperature rise in that portion is low, and temperature variations occur overall. Therefore, by using a coil with two or more turns, uniform heating is possible. As a result, delayed fracture in the discontinuous portion can be prevented. 1 and 2, at least two turns of the coil are overlapped with respect to the punched end 20. Then, the AC power source 15 is turned on to pass an AC current through the coil 10, thereby generating a high-frequency induced electromotive force, and the punched ends 20 at both ends of the steel sheet part 2 are locally heated.

[0020] After heating for a short time (e.g., 10 seconds or less) using the manufacturing device (local heating device) for a steel plate part described above, the AC power source 15 is turned off and the steel plate part 2 is removed. The punched end 20 of the steel plate part 2 is in an evenly heated state, as will be described in detail later.

[0021] According to the first embodiment, delayed fracture can be avoided by removing processing strain of the high-strength steel plate by high-frequency induction heating. Localized heating at 500 to 830°C is performed for a short time of 10 seconds or less. By locally heating the punched end with a coil facing the punched end face, softening of the entire part can be prevented and the strength of the part can be maintained. By arranging the coil facing the punched end face, thermal deformation of the panel can be suppressed. In any step after the press punching step or before the painting step, the punched end face can be heated as described with reference to FIG. 2. In other embodiments, this localized heating step may be performed within the press step.

[0022] A hydrochloric acid immersion test of the manufactured steel sheet parts will be described with reference to Figs. 3 to 5. A high-strength steel plate was cut into a rectangular shape with bolt holes 35 drilled at both ends. A notch of R5 was punched into the center of the high-strength steel plate, and the plate was bent at a tip R6 and a bending angle of 90 degrees. Stress was then applied to the apex of the bend by tightening a bolt, and a hydrochloric acid immersion test (PH3, 100H) was performed.

[0023] Figure 3 shows the procedure for evaluating delayed fracture during bending. 1) A rectangular blank 30 (C110 mm × L30 mm) was prepared. A notch 31 of R5 was formed in the longitudinal center of the blank by punching. 2) A V-bend 32 (tip R6, bending angle 90°) was formed near the notch 31 of the blank 30. 3) The stress generated at the bend apex 34 was adjusted by tightening the bolt 33 inserted into the bolt hole 35. 4) To evaluate delayed fracture during bending, a hydrochloric acid immersion test (PH3, 100H) was conducted.

[0024] When the end surface heating according to this embodiment is applied, local heating is performed on the portion that will become the notch 31 when the blank is produced.

[0025] FIG. 4 is a diagram illustrating the evaluation results of a hydrochloric acid immersion test on a notched portion without local heating according to a comparative example. Each image in Figure 4 shows an enlarged photograph of the apex of the bend when the bolt tightening amount is 4mm, 8mm, 12mm, 16mm, and 20mm. As shown in Figure 4, when the bolt tightening amount is 4mm, 8mm, and 12mm, no cracks occurred at the apex of the bend, but when the bolt tightening amount is 16mm and 20mm, cracks occurred at the apex of the bend.

[0026] FIG. 5 is a diagram illustrating the evaluation results of a hydrochloric acid immersion test on the notched portion with local heating according to the first embodiment. The notched portion 31 was locally heated to 400° C., 600° C., and 800° C. according to the present embodiment, and then naturally cooled. Hydrochloric acid immersion tests were carried out when the tightening amount was 16 mm and 20 mm (these are the conditions under which cracks occurred without heating, as shown in Figure 4).

[0027] As shown in FIG. 5, when the bolt tightening amount is 16 mm, no cracks occurred at the bend apex at any of 400°C, 600°C, and 800°C. On the other hand, when the bolt tightening amount is 20 mm, a crack penetrating the bend apex occurred at 400°C. When the bolt tightening amount is 20 mm, a crack occurred at 600°C, but not penetrating the bend apex. When the bolt tightening amount is 20 mm, no cracks occurred at the bend apex at 800°C. That is, when the bolt tightening amount is 20 mm, cracks could be reduced by increasing the heating temperature of the local heating according to this embodiment, and cracks could be prevented at 800°C. In this way, delayed fracture could be suppressed by the local heating according to this embodiment. The required temperature may vary depending on the material and residual stress conditions.

[0028] A CAE (Computer Aided Engineering) analysis of the heating temperature will be described with reference to FIG. 6 and FIG. The temperature rise at the punched end was analyzed when an alternating current with a frequency of 240 kHz was passed through it at a constant voltage of 200 V for 1 second.

[0029] Fig. 6 is a diagram for explaining the temperature distribution in the circumferential direction of the punched end 20 after the local heating shown in Fig. 2. The upper diagram of Fig. 6 shows a CAE analysis of the heating temperature when the punched end is viewed from above. After heating for 1 second, the temperature rose to a maximum of 450°C around the end face. As shown in the graph in the lower part of Figure 6, the temperature distribution in the center of the end face is a maximum of 70°C even around the circumference of the end face, and the temperature difference is small and stable. In this way, by arranging the punched end 20 so that it faces the coil 10 almost parallel to it, and performing local heating by high-frequency induction heating with the coil 10, the punched end 20 can be heated almost evenly.

[0030] Fig. 7 is a diagram for explaining the temperature distribution in the direction perpendicular to the end face of the punched end 20 after the local heating shown in Fig. 2. The upper diagram in Fig. 7 shows a CAE analysis of the heating temperature when the punched end is viewed obliquely from above. The temperature was measured by lowering the outer periphery of the end face by 0.5 mm each time from the upper corner. As shown in the graph in the lower diagram of Figure 7, the temperature returned to the general part temperature about 6 mm from the upper corner, and the range of heat influence was limited to the vertical flange part, so it is considered that the influence on the strength of the part is small. Therefore, by heating the burring part, which is the punched end 20, by the local heating according to this embodiment, it is possible to avoid delayed fracture while maintaining the strength of the whole part.

[0031] Target area The method according to the present disclosure is used for parts that use high-strength steel plates with a tensile strength of 780 MPa or more and are subjected to large distortion due to punching and bending. The main target parts are those that are expected to be used in locations where tensile stress is applied during use and where there is a large amount of environmental hydrogen, such as in a corrosive environment. Specific target parts are as follows:

[0032] FIG. 8 shows a bush press-fitting portion of an arm, which is an example of a target portion. In arms such as rear control arms, there are press-in portions 20a (an example of a processed portion of a steel plate) where bushes are pressed in. These are formed by hole expansion forming after punching, so very high residual stress remains. Furthermore, the press-in of the part applies tensile residual stress, which easily induces delayed fracture. Therefore, the local heating method according to the present disclosure can also be applied to the bush press-in portions processed in this way.

[0033] FIG. 9 shows the bolt fastening parts of trading arms, which are examples of target parts. Trading arms are often used by fastening them with bolts. The bolt fastening portion 20b (an example of a processed portion of a steel plate) is prone to delayed fracture because of residual stress caused by punching the fastening hole, as well as tensile residual stress that may occur during fastening due to poor surface accuracy of the fastening surface. Therefore, the local heating method according to the present disclosure can also be applied to the bolt fastening portion processed in this way.

[0034] In general, residual stress may be high near the boundary between the shear surface and the fracture surface of a punched end surface. The location of this boundary on the punched end surface depends on the material and plate thickness. The local heating according to this embodiment is preferably performed near this boundary.

[0035] FIG. 10 is a diagram for explaining an example of an apparatus for locally heating using a laser according to another embodiment. It is possible to achieve the same effect as in the above-mentioned embodiment by using laser heating. For example, the laser source 40 is attached to the tip of the arm of a six-axis robot 50. As described above, the laser source 40 may be moved in the circumferential direction along the punched edge to irradiate the laser.

[0036] The present invention is not limited to the above-described embodiment, and can be modified as appropriate without departing from the spirit and scope of the present invention. [Explanation of symbols]

[0037] 2. Steel plate parts 10 Coil 11 Mounting stand 12 Base section 13-pin 15 AC power supply 20 Punched end 20a Press-in part 20b Bolt fastening part 23 Pin Hole 30 Blank 31 Notch 32 V-bend 33 Volts 34 Bend apex 35 Bolt Holes 40 Laser Source 50 Robot

Claims

1. A manufacturing method for steel plate parts, characterized in that localized heating is performed using a heating electrode arranged on one side directly opposite the punched portion of the steel plate.

2. The method for manufacturing a steel plate part according to claim 1, wherein the steel plate is a high-strength steel plate having a tensile strength of 780 MPa or more.

3. The method for producing a steel sheet part according to claim 1, wherein localized heating is performed by high-frequency induction for a short period of time of 10 seconds or less.

4. The method for producing a steel plate part according to claim 1, wherein the heating temperature is 500 to 830°C.

5. The processed portion of the steel plate is formed by punching so as to protrude to one side, The method for manufacturing a steel sheet part according to claim 1 , wherein the heating electrode is disposed so as to directly face the punched end surface, which is a tip end formed to protrude.

6. The processed portion of the steel plate is a bush press-fit portion formed by hole expansion forming after punching, The method for manufacturing a steel plate part according to claim 1 , wherein the heating electrode is disposed so as to directly face the bush press-fit portion.

7. The processed portion of the steel plate is a bolt fastening portion formed by punching, The method for manufacturing a steel plate part according to claim 1 , wherein the heating electrode is disposed so as to directly face the bolt fastening portion.

8. The method for manufacturing a steel plate part according to claim 1 , wherein the heating electrode is a coil larger than the punched portion and has two or more turns with respect to the punched portion.

9. The method for manufacturing a steel plate part according to claim 8 , wherein an outermost periphery of the coil is arranged so as to match a punched end surface formed by punching.

10. 9. The method for manufacturing a steel sheet part according to claim 8, wherein the coil is a pipe coil, and the diameter of the pipe coil is equal to or less than twice the pipe diameter of the pipe coil in addition to the diameter of the punched end.

11. The method of claim 8 , wherein the input and output ends of the coil are located on a radius of the coil from a center of the coil.

12. A high frequency AC power source; A coil connected to the high frequency AC power source; A mounting base for mounting the steel plate part, When the steel plate part is attached to the mounting base, a processed portion of the steel plate is configured to directly face the coil, An apparatus for manufacturing a steel plate part configured to carry out the manufacturing method according to any one of claims 1 to 11.

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