JP2011031249A - Laser beam welding method of high tensile steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser beam welding method of a high tensile steel sheet, in which no weld zone is hardened during laser beam welding and in which no blow hole is formed or no brittle fracture takes place by hydrogen melted into a weld bead even if cooling water or a lubricant is used in laser beam welding of the abutted part at both ends of a roll-formed high tensile steel sheet. <P>SOLUTION: A high tensile steel sheet having a tensile strength (TS) in excess of 780 MPa is roll-formed and a filler wire is continuously supplied in laser beam welding of the abutted part at both ends of the sheet to perform laser beam welding. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a laser welding method for a high-tensile steel plate when roll-forming a high-tensile steel plate having a tensile strength (TS) of more than 780 MPa, so-called super high-tensile steel, and laser-welding the butt portions at both ends of the plate. .

  In recent years, steel sheets used for automobile bodies and the like have been increasingly used for high-strength steel sheets in order to achieve both improved safety in the event of a collision and weight reduction for the purpose of reducing fuel consumption. .

  In addition, an example in which laser welding is applied as means for welding the high-tensile steel plate has been frequently seen. Laser welding is a method in which a metal is joined by irradiating with a laser beam focused on the metal as a heat source and locally melting and solidifying the metal. Since laser is light having a single wavelength and no phase difference, it can be focused on an extremely small point by a lens of an optical system and given high density energy to join metals.

  Since laser welding is a welding method for joining metals in this way, it has the features of high penetration at high speed, little influence of welding heat, and little welding deformation. However, on the other hand, there is a drawback that the welded portion is hardened, and particularly when laser welding a high-tensile steel plate, the welded portion (welded deposit) is easy to harden, and there is a problem that low temperature cracks are likely to occur. This is an obstacle to adopting laser welding for welding high-strength steel sheets.

  When roll-forming a high-tensile steel sheet having a tensile strength exceeding 780 MPa, higher frictional heat is generated than in the case of roll-forming a normal steel sheet, and thus it is necessary to cool with cooling water or the like. A lubricating liquid is used to reduce roll friction during roll forming. Therefore, laser welding is performed in a state where moisture is attached to the surface of the high-tensile steel plate or in an atmosphere containing a lot of moisture. Moisture adhering to the surface of a high-strength steel sheet can be reduced by wiping with a waste cloth or blowing away with air blow, but it is extremely difficult to keep it dry in continuous operations during operation. Difficult and impossible. Therefore, blow holes may be formed by the hydrogen dissolved in the weld bead or embrittlement failure may occur, which is not a problem when laser welding the butt portions at both ends of a roll-formed high-tensile steel plate. It was.

  Further, Patent Document 1 proposes a heat treatment method for a high-tensile steel plate laser welded part as a method for solving the problem that the welded part is hardened in laser welding of the high-tensile steel sheet. However, this method is not particularly intended for laser welding of the butt portions at both ends of a roll-formed high-tensile steel plate, and in this method, it is necessary to perform heating by locally heating to a high temperature of 400 ° C. or higher. Therefore, there has been a situation that a special apparatus for irradiating a laser beam having a high power density is required.

  Furthermore, although the method of laser welding the butt portion of both ends of the roll-formed high-tensile steel plate is not particularly targeted, the technique of supplying filler wire when performing laser welding has already been proposed by Patent Documents 2 to 5. Has been.

JP 2004-209497 A Japanese Patent Laid-Open No. 2003-211274 JP 2004-209536 A JP 2005-21912 A JP 2006-159234 A

  The present invention has been made in order to solve the above-mentioned conventional problems. The welded portion is not hardened during laser welding, and cooling is performed when the butt portions at both ends of a roll-formed high-tensile steel plate are laser welded. It is an object of the present invention to provide a laser welding method for high-strength steel sheets in which blow holes are not formed or brittle fracture does not occur due to hydrogen dissolved in the weld bead even when water or lubricating liquid is used. It is.

  The invention according to claim 1 is a laser welding method for a high-tensile steel sheet, in which a high-tensile steel sheet having a tensile strength (TS) of more than 780 MPa is roll-formed, and the butted portions at both ends of the sheet are laser-welded. A laser welding method for a high-strength steel sheet, which is continuously supplied and laser-welded.

  The invention according to claim 2 is characterized in that the continuous supply of the filler wire is performed by setting the supply speed of the filler wire to the same speed as the welding speed of laser welding. Is the method.

  A third aspect of the present invention is a laser welding method for a high-strength steel sheet, wherein the welded portion is heated to 300 to 400 ° C. immediately after the laser welding according to the first or second aspect.

  A fourth aspect of the present invention is the laser welding method for a high-tensile steel sheet according to the third aspect, wherein the welding portion is heated by burner heating.

  According to the laser welding method of the high-strength steel sheet according to claim 1 of the present invention, while taking advantage of the characteristics of laser welding, such as high penetration and deep penetration, less influence of welding heat, and less welding deformation, The weld does not harden, and blow holes are formed by hydrogen dissolved in the weld bead even when cooling water or lubricating liquid is used when laser welding the butt of both ends of the roll-formed high-tensile steel plate. It is possible to achieve an effect that neither the occurrence of brittleness nor the occurrence of brittle fracture occurs.

  According to the high-strength steel plate laser welding method of the second aspect of the present invention, the filler wire can be stably supplied without excess or deficiency.

  According to the laser welding method for a high-strength steel sheet according to claim 3 of the present invention, dehydrogenation can be performed by post-heating at a relatively low temperature compared to the conventional method, and embrittlement of the welded portion can be suppressed.

  According to the laser welding method of the high-strength steel sheet according to claim 4 of the present invention, dehydrogenation can be performed by a simple means such as burner heating, and embrittlement of the welded portion can be suppressed.

It is a perspective view which shows the process of roll-forming a high strength steel plate in one Embodiment of this invention. In the embodiment, it is the longitudinal cross-sectional view which showed the state in which a high-tensile steel plate deform | transforms sequentially in a roll forming process, and the state which supplies a filler wire. It is a perspective view which shows the relationship between the laser head used for the embodiment, and a wire feeding apparatus. It is an external appearance photograph which shows the welding part of the test piece of the comparative example which performed laser welding in the Example. It is an external appearance photograph which shows the welding part of the test piece of the invention example 1 which performed laser welding in the Example. It is an external appearance photograph which shows the welding part of the test piece of the invention example 2 which performed laser welding in the Example. It is a cross-sectional photograph which shows the cross-sectional property in the weld part of the test piece of the comparative example which performed laser welding in the Example. It is a cross-sectional photograph which shows the cross-sectional property in the welding part of the test piece of the invention example 1 which performed laser welding in the Example. It is a cross-sectional photograph which shows the cross-sectional property in the welding part of the test piece of the invention example 2 which performed the laser welding in the Example. It is a graph which shows the Vickers hardness distribution in the welding part of the test piece of the comparative example which performed laser welding in the Example. It is a graph which shows the Vickers hardness distribution in the welding part of the test piece of the invention example 1 which performed laser welding in the Example. It is a graph which shows the Vickers hardness distribution in the welding part of the test piece of the invention example 2 which performed laser welding in the Example.

  When the present inventors roll-form a high-tensile steel plate having a tensile strength (TS) of more than 780 MPa, so-called ultra-high-tensile steel, and laser weld the butt portion at both ends of the plate, hardening of the weld deposit portion occurs. In order to solve the problem of easy cracking and low temperature cracking due to embrittlement caused by diffusible hydrogen caused by cooling water and lubricating liquid during roll forming, we conducted diligent efforts, experiments and studies.

  As a result, high strength steel sheet is roll-formed, and filler wire is continuously supplied when laser welding the butt portion at both ends of the plate, and the filler wire is melted and solidified together with both ends of the plate to soften the weld deposit. Thus, the inventors have found that the toughness can be increased and the occurrence of brittle fracture can be suppressed, and the present invention has been completed.

  Moreover, since diffusible hydrogen has temperature dependence, dehydrogenation can be achieved by performing post-heating at 300 to 400 ° C., and the brittleness of the welded portion can be more reliably suppressed. It was also found that the occurrence of brittle fracture can be more reliably suppressed.

  Hereinafter, the present invention will be described in more detail based on embodiments.

  The high-tensile steel plate laser welding method of the present invention is a high-strength steel plate when roll-forming a high-tensile steel plate having a tensile strength (TS) exceeding 780 MPa, so-called super high-tensile steel, and laser-welding the butt portions at both ends of the plate. The laser welding method is targeted.

  The reason for targeting high-tensile steel sheets having a tensile strength (TS) of over 780 MPa is that, when roll-forming high-tensile steel sheets having a tensile strength (TS) of over 780 MPa, than when rolling a normal steel sheet. Since high frictional heat is generated, it is necessary to cool with cooling water or the like. The cooling water adheres to the surface of the high-tensile steel plate or is contained in the atmosphere as water vapor. Sometimes it becomes hydrogen and dissolves in the weld bead, forming blowholes and causing embrittlement failure, and there is a higher need for improvement than when roll forming ordinary steel sheets. is there.

  In roll forming, as shown in FIG. 1, a plurality of forming rolls 3 arranged in tandem are passed through high-tensile steel plates 1 such as coil materials, hoop materials, and cut plates, and sequentially subjected to bending in the width direction, As shown in FIG. 2, it is the process which shape | molds from a flat-shaped board to the target shape. In the present invention, as shown in FIG. 2, a high-tensile steel plate can be formed into a substantially square cross section by roll forming, and the butted portions at both ends of the plate are joined by laser welding.

  Moreover, in this invention, as shown in FIG. 2, the filler wire 2 is melted simultaneously with the both ends of the high-tensile steel plate 1 canned by roll forming by continuously supplying the filler wire 2 during the laser welding. Solidify. By continuously supplying the filler wire, the weld deposit can be softened and the toughness can be increased. As a result, the occurrence of brittle fracture can be suppressed.

  In general, the high-tensile steel plate 1 to be laser welded is a thin plate having a thickness of 1 to 2 mm. When the butt portions at both ends of the high-tensile steel plate 1 are laser welded without supplying the filler wire 2, the butt portion is completely dissolved, but the weld deposit is hardened and the toughness is lowered. There is. In order to prevent the lowering of the toughness, the weld deposit can be softened and the toughness can be increased by continuously supplying the filler wire 2 which is inferior in strength to the base material (high-strength steel plate 1) by laser welding.

  As shown in FIG. 3, the filler wire is continuously supplied to the welded portion by a wire feeding device 5 fixed to the laser head 4. The filler wire 2 can be supplied without shortage.

  In the present invention, it is desirable to heat the weld to 300 to 400 ° C. immediately after the laser welding. By subsequent heating, dehydrogenation can be achieved, and embrittlement of the welded portion can be suppressed more reliably. Further, the post-heating can temper the weld metal whose hardness has been increased, thereby improving the residual stress of the welded portion.

  The reason for setting the post-heating temperature to 300 to 400 ° C. is that if the post-heating temperature is less than 300 ° C., the dehydrogenation and the improvement of the residual stress in the welded portion cannot be sufficiently achieved, while the 400 ° C. If it exceeds, dehydrogenation effect cannot be obtained.

  In addition, it is sufficient to perform this post-heating by heating with a burner, dehydrogenation can be performed by simple means, and embrittlement of the welded portion can be reliably suppressed. In addition to the heating by the burner, heating by a thermal coil can be employed.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and the present invention is implemented with appropriate modifications within a range that can meet the gist of the present invention. These are all included in the technical scope of the present invention.

  In this example, a high-tensile steel plate having a tensile strength (TS) of 1270 MPa was canned by roll forming, and butt portions at both ends of the plate were laser welded. Here, a test piece in which the butted portions at both ends of the plate are joined only by irradiating the laser beam is a comparative example, and a test piece in which a filler wire is simultaneously supplied and the butted portions at both ends of the plate are joined at the time of laser irradiation is invented Inventive Example 2 is a test piece burned to 300 to 400 ° C. immediately after 10 seconds within 10 seconds of supplying a filler wire and laser welding.

  FIG. 4 shows a photograph of the appearance of the welded portion of the comparative example, FIG. 5 shows a photograph of the appearance of the welded portion of Invention Example 1, FIG. 6 shows a photograph of the appearance of the welded portion of Invention Example 2, and FIG. A photograph of the cross-sectional property of the welded portion, FIG. 8 shows a photograph of the cross-sectional property of the welded portion of Invention Example 1, and FIG. 9 shows a photograph of the cross-sectional property of the welded portion of Inventive Example 2. 10 shows the Vickers hardness distribution near the welded portion of the comparative example, FIG. 11 shows the Vickers hardness distribution near the welded portion of Invention Example 1, and FIG. 12 shows the Vickers hardness distribution near the welded portion of Invention Example 2. Respectively.

  In addition, the measurement of Vickers hardness was measured based on the Vickers hardness test-test method prescribed | regulated to JISZ2244.

  Comparing FIG. 10 showing the Vickers hardness distribution in the vicinity of the welded portion of the comparative example and FIG. 11 showing the Vickers hardness distribution in the vicinity of the welded portion of Invention Example 1, the filler wire is supplied at the time of laser welding. It can be seen that the depot is softened. Moreover, in FIG. 12 which shows the Vickers hardness distribution of the welded part vicinity of the example 2 of an invention, it turns out that the Vickers hardness has fallen to the whole by the tempering by post-heating.

  In general, it is considered that the weld bead portion often has a tensile residual stress field, and is easily broken. In addition, assuming that the spring back force at the time of forming works and that the steel pipe is a member accompanied by plastic deformation, in this example, three types of test pieces of Comparative Example, Invention Example 1 and Invention Example 2 are used. Using, the time to failure when a bending load was loaded by a three-point bending apparatus was measured. The test results are shown in Table 1. In Table 1, “Load: large” indicates a state where a bending load near the breaking stress is loaded, and “load: medium” indicates a state where a bending load exceeding the yield stress is loaded. Show. Further, the test was continuously performed for 3 hours, and when the weld bead portion was not cracked, “no crack” was displayed.

  According to the test results, the high strength steel plate having a tensile strength (TS) of 1270 MPa can be processed by roll forming, and the filler wire is continuously supplied when laser welding the butt portion at both ends of the plate, thereby embrittlement. It turns out that destruction does not occur.

DESCRIPTION OF SYMBOLS 1 ... High tension steel plate 2 ... Filler wire 3 ... Forming roll 4 ... Laser head 5 ... Wire feeding apparatus

Claims (4)

  A high-strength steel sheet laser welding method in which a high-tensile steel sheet having a tensile strength (TS) of more than 780 MPa is roll-formed, and butt portions at both ends of the sheet are laser-welded. A laser welding method for high strength steel sheets.   The high-strength steel plate laser welding method according to claim 1, wherein the continuous supply of the filler wire is performed by setting the supply speed of the filler wire to be the same as the welding speed of laser welding.   3. A laser welding method for a high-tensile steel sheet, wherein the welded portion is heated to 300 to 400 [deg.] C. immediately after laser welding according to claim 1 or 2.   The method of laser welding a high-tensile steel sheet according to claim 3, wherein the welding part is heated by burner heating.

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