Lap welding method, lap joint, production method of lap joint, and an automobile part

Abstract

The present invention has as its object to inhibit fracture due to a HAZ softened part in substantially circular laser lap welding taking the place of resistance spot welding for joining a superposed plurality of steel sheets including a high strength steel sheet. To solve this problem, the inventors discovered lap welding comprising superposing a plurality of steel sheets and firing a laser to form a substantially circular laser weld (1), which lap welding firing the laser in a straight line through an outer edge of the substantially circular laser weld (1) so as to form a hardened part (8) at the steel sheets and thereby suppressing fracture arising from the HAZ softened part.

Claims

1. Lap welding method comprising superposing a plurality of steel sheets and firing a laser to form a substantially circular laser weld, which lap welding method comprises scanning the laser in a straight line through an outer edge of said substantially circular laser weld so as to form a hardened part at the steel sheets, wherein the hardened part is a reheated and hardened HAZ-softened part formed by the substantially circular laser weld, wherein a laser scanning width is at least 10% and 50% or less of a diameter of the substantially circular laser weld, wherein said straight line laser scanning operation scans the laser over at least 3 mm from the outer edge of said substantially circular laser weld, and wherein at least one steel sheet among said plurality of steel sheets is a steel sheet having a martensite structure.

2. The lap welding method according to claim 1 wherein said straight line laser scanning operation scans the laser in a principal stress direction found in advance.

3. The lap welding method according to claim 2 wherein said substantially circular is a circular shape, oval shape, circular ring shape, oval ring shape, C-shape, long C-shape, or multiple circular ring shape.

4. The lap welding method according to claim 2 wherein a fired width of said straight line laser fired part is smaller than a curvature diameter of the part of the outer edge of said substantially circular laser weld through which said laser is fired in a straight line.

5. The lap welding method according to claim 1 wherein said substantially circular is a circular shape, oval shape, circular ring shape, oval ring shape, C-shape, long C-shape, or multiple circular ring shape.

6. The lap welding method according to claim 1 wherein a fired width of said straight line laser fired part is smaller than a curvature diameter of the part of the outer edge of said substantially circular laser weld through which said laser is fired in a straight line.

7. The lap welding method according to claim 1 wherein said steel sheet having a martensite structure is a steel sheet having a tensile strength of 1180 MPa or more.

8. A lap joint obtained by superposing a plurality of steel sheets and firing a laser to form substantially circular laser weld so as to join said plurality of steel sheets, which lap joint is provided with a hardened part formed in a straight line by firing the laser through an outer edge of said substantially circular laser weld, wherein the hardened part is a reheated and hardened HAZ-softened part formed by the substantially circular laser weld, wherein a laser scanning width is at least 10% and 50% or less of a diameter of the substantially circular laser weld, wherein said straight line laser scanning operation scans the laser over at least 3 mm from the outer edge of said substantially circular laser weld, and wherein at least one steel sheet among said plurality of steel sheets is a steel sheet having a martensite structure.

9. The lap joint according to claim 8, wherein said straight line hardened part is formed in a principal stress direction found in advance.

10. The lap joint according to claim 8 wherein said substantially circular is a circular shape, oval shape, circular ring shape, oval ring shape, C-shape, long C-shape, or multiple circular ring shape.

11. The lap joint according to claim 8 wherein a width of said straight line hardened part is smaller than a curvature diameter of the part of the outer edge of said substantially circular laser weld through which said straight line hardened part crosses or contacts.

12. The lap joint according to claim 8 wherein said steel sheet having a martensite structure is a steel sheet having a tensile strength of 1180 MPa or more.

13. A method of production of a lap joint produced by superposing a plurality of steel sheets and firing a laser to form a substantially circular laser weld, which method of production of a lap joint comprises scanning the laser in a straight line through the outer edge of said substantially circular laser weld so as to form a hardened part at the steel sheets, wherein the hardened part is a reheated and hardened HAZ-softened part formed by the substantially circular laser weld, wherein a laser scanning width is at least 10% and 50% or less of a diameter of the substantially circular laser weld, wherein said straight line laser firing operation fires the laser over at least 3 mm from the outer edge of said substantially circular laser weld, and wherein at least one steel sheet among said plurality of steel sheets is a steel sheet having a martensite structure.

14. An automobile part provided with a lap joint according to claim 8.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 show a tensile test piece, wherein FIG. 1A shows the piece as a whole and FIG. 1B shows a cross-section along a centerline of the test piece.

(2) FIG. 2A is a view showing a position for investigation of hardness in a circular shape laser weld at a 1310 MPa class steel sheet, while FIG. 2B is a view showing a hardness distribution of a circular shape laser weld.

(3) FIG. 3 is a view showing a break at a HAZ softened part of a circular shape laser weld.

(4) FIG. 4A is a view showing a laser weld joint of a comparative example in a tensile test, FIG. 4B is a view showing a laser weld joint of Invention Example 1, FIG. 4C is a view showing a laser weld joint of Invention Example 2, and FIG. 4D is a graph of load-elongation of the comparative example, Invention Example 1, and Invention Example 2 in a tensile test.

(5) FIG. 5A is a view showing a break position in a tensile test of Invention Example 1, while FIG. 5B is a view showing a break position in a tensile test of Invention Example 2.

(6) FIG. 6A to FIG. 6G are views showing examples of an embodiment of the present invention.

(7) FIG. 7A to FIG. 7J are also views showing examples of an embodiment of the present invention.

(8) FIG. 8 is a view showing an example of application of the present invention to a center pillar.

(9) FIG. 9 is a view showing an example of application of the present invention to a side sill.

(10) FIG. 10 is a view showing an example of application of the present invention to an A-pillar and roof rail.

(11) FIG. 11A is a view showing a tensile test piece obtained by C-shape laser lap welding, FIG. 11B is a view showing a laser lap weld of a comparative example, FIG. 11C is a view showing a laser lap weld according to an invention example, and FIG. 11D is a graph showing results of a tensile test.

(12) FIG. 12A is a view showing a break position of a comparative example, while FIG. 12B is a view showing a break position of an invention example.

(13) FIG. 13 are views showing the hardness distribution of a C-shape laser lap joint in an invention example, wherein FIG. 13A is a view showing a position of measurement of the hardness distribution of the lap joint according to the present invention, FIG. 13B is a view showing the hardness distribution of a terminal end of a straight line laser scanned part, FIG. 13C is a view showing a hardness distribution of an intermediate part of the straight line laser scanned part, and FIG. 13D is a view showing a hardness distribution of a substantially circular laser lap weld.

DESCRIPTION OF EMBODIMENTS

(14) First, a welding method using a laser beam according to the present invention will be explained.

(15) The laser welder is not particularly limited. As examples, a disk laser, fiber laser, YAG laser, and CO.sub.2 gas laser can be used. A beam diameter of 0.15 to 0.9 mm in range, an output of 1 to 10 kW in range, and a welding speed of 1 to 25 m/min in range can be illustrated. The conditions for laser welding are suitably determined by the types of the steel sheets, the thicknesses of the steel sheets, etc. and are not limited to the above illustrations.

(16) The welding may be general welding by a torch carried by a robot, but is preferably remote laser welding using a galvanomirror. Remote laser welding is the art of combining a long focal point focus lens with a specialized scanning mirror and firing the laser beam through space to make the laser spot scan the surface at a high speed to perform welding. Compared with the robot or NC device used for conventional laser welding, the beam travel time becomes substantially zero, so high efficiency welding becomes possible. This system has the merit of not requiring almost any travel time of the laser spot from a weld location being worked to the next weld location.

(17) At the step of forming the hardened part, a laser beam is fired at the lap part to form a substantially circular laser weld, then is run across the HAZ softened part formed around the substantially circular laser weld (outer circumference) to form a straight line hardened part and thereby make the HAZ softened part around the substantially circular laser weld harden. Therefore, by firing a laser through the outer edge of the laser lap weld in a straight line, it is possible to form a hardened part so as to split the HAZ softened part. Here, through the outer edge of the laser lap weld indicates the state of crossing the outer edge of the laser lap weld or contacting it.

(18) A laser lap weld is a melted and solidified part shown as shown in FIG. 2B. Therefore, the outer edge of the laser lap weld indicates the boundary between the melted and solidified part and the matrix metal. FIG. 2 shows an example where the center part of the circular shape is also made to melt and solidify. FIG. 13 shows a C-shape, but in this case as well, the laser lap weld is a melted and solidified part. The thinking is the same.

(19) FIG. 13A shows an example of laser hardening so as to cut across a C-shape laser lap weld. This shows the hardness distribution of the surface of a steel sheet in the direction shown by the dotted line in FIG. 13A. FIGS. 13B, 13C, and 13D show the hardness distributions of the surface of a steel sheet at the laser hardened part terminal end (ending end), laser hardened part, and laser lap weld.

(20) The hardened part, as shown in FIG. 13C, indicates the region sandwiched between the local maximum points of hardness appearing sandwiching a laser scanned part when measuring the hardness distribution across the laser hardened part. Around a hardened part, a softened part can be formed due to the heat input at the time of hardening. In FIG. 13C, there is a part showing the local minimum value of hardness at the outside of the peak of hardness. This is the part softened by laser hardening.

(21) The state of hardness of the terminal end (ending end) of the laser hardened part is shown in FIG. 13B. FIG. 13B is a view showing the hardness distribution in the laser firing direction. The local maximum point of the hardness in FIG. 13B is the terminal end (ending end) of the hardened part. The right side becomes the hardened part. As will be understood from FIG. 13B there is a part showing the local minimum point immediately to the outside of the local maximum point of hardness. This is the part softened by laser hardening.

(22) Further, the hardened part is preferably formed at 50% or more of the thickness of the high strength steel sheet (in particular, steel sheet including martensite structure).

(23) Further, the straight line hardened part may be positioned in the principal stress direction envisioned at the time of impact. Alternatively, when laser welding flange shaped steel sheet members, the direction of extension of this flange may be made the principal stress direction. Even if off from the principal stress direction due to the precision of the laser welding etc., the angle of that deviation is preferably as small as possible. Therefore, the straight line hardened part is preferably formed in a range of within 30 with respect to the principal stress direction. If the hardened part is formed in a range of within 30 with respect to the principal stress direction, it can be deemed to have been formed in the principal stress direction. More preferably, the range may be within 15 with respect to the principal stress direction. Still more preferably, the range may be within 10 with respect to the principal stress direction.

(24) Note that, the principal stress direction can be found in advance at the design stage. The method of finding the principal stress direction in advance is not limited. For example, the finite element method (FEM) or other simulation can be used to find it. Alternatively, a model or actual shape test member can be used for testing to find it. There may also be a plurality of principal stress directions. In this case, it is sufficient to fire the laser in a straight line matching each principal stress direction so as to form hardened parts.

(25) By firing a laser for reheating so as to form a straight line hardened part, the HAZ softened part around the substantially circular laser lap weld is split and a drop in strength in the principal stress direction due to the HAZ softened part is suppressed. Note that, the straight line hardened part need only split the HAZ softened part of the laser lap weld, so need not necessarily be continuously formed in a straight line inside the laser lap weld.

(26) The steel sheet to which the present invention can be applied is not limited. However, HAZ softening remarkably occurs at steel having a martensite structure. If applying the present invention to such a steel sheet, the effect is large. As steel sheet having a martensite structure, there is high strength steel having a 1180 MPa or more tensile stress. Such high strength steel is often used for hot stamping (hot press forming) and is sometimes called hot stamp-use steel. Further, the presence or absence of plating is not an issue.

(27) The lap joint according to the present invention may be applied to any steel member. In particular, the obtained effect is large by application to an automobile member in which impact resistance is sought.

(28) The present invention will be explained based on specific examples. For example, if the center pillar of an automobile is impacted, a tensile load will be applied to the flange part where the inner member panel and outer member panel are lap welded. For this reason, strain will concentrate at the HAZ softened part of the circular shape laser lap welded formed at the flange causing it to break. The inventors etc. ran tests envisioning such a case.

(29) FIG. 1 is an explanatory view showing a tensile test piece. A 440 MPa class steel sheet (thickness 1.2 mm), a 1310 MPa class steel sheet (thickness 1.4 mm), and a 270 MPa class hot dipped galvannealed steel sheet (thickness 0.7 mm) were superposed and fastened by resistance spot welding at the gripping parts of the test piece. Circular shape laser lap welding was performed at the center part of the test piece. A fiber laser was used for laser welding under conditions of a beam diameter of 0.6 mm, an output of 2.0 kW, and a welding speed of 2.0 m/min so as to form a test piece having a laser lap weld shown in FIG. 4A of a circular shape of a diameter of about 7 mm welded inside the circle as well.

(30) FIG. 2A is an explanatory view showing the position for investigation of hardness of a circular shape laser weld in a 1310 MPa class steel sheet, while FIG. 2B is an explanatory view showing the hardness distribution of a circular shape laser weld.

(31) As shown in FIG. 2A and FIG. 2B, in circular shape laser welding of 1310 MPa class steel sheet, the HAZ around the hardened part was tempered and softened.

(32) FIG. 3 is an explanatory view showing a break position of a circular shape laser lap welded joint. When performing a tensile test, as shown in FIG. 3, it was learned that strain concentrated at the HAZ softened part around the laser weld of the 1310 MPa class steel sheet causing it to break.

(33) FIG. 4 show a comparative example in the tensile test (FIG. 4A), Invention Example 1 (FIG. 4B), Invention Example 2 (FIG. 4C), and a load-elongation graph of the same (FIG. 4D). The comparative example (FIG. 4A) shows the case of only laser lap welding. Invention Example 1 (FIG. 4B) shows the case of firing a laser in a straight line from an outer edge of the laser lap welding along the axial direction of the test piece (principal stress direction) to form a hardened part. The laser was fired from a point 12 mm separated from the lap weld edge and cut across the laser lap weld so as to extend in a straight line in a length from the point 12 mm separated from the lap weld edge of 31 mm. Invention Example 2 (FIG. 4C) is the case of forming the laser hardened part up to the gripping parts of the test piece. It is a test piece designed so that the terminal end of the laser hardened part does not form a starting point of fracture. A tensile test was run with a distance between prepared welds evaluated of 50 mm and a tensile speed of 3 mm/min. FIG. 5 are explanatory views showing break positions of Invention Example 1 (FIG. 5A) and Invention Example 2 (FIG. 5B).

(34) The results of measurement of the elongation at break are shown in FIG. 4D. It will be understood that the comparative example had an elongation at break of a small one of 2.3% or so and that low strain breakage occurred. Invention Example 1 had an elongation at break of 4.3%. Even compared with the comparative example, the elongation at break was improved over the comparative example by about 87%. Further, it could be confirmed that the location of break was the terminal end of the laser hardened part (FIG. 5A). As a result, it could be confirmed that Invention Example 1 solved the problem of low strain breakage compared with the comparative example. Note that, the melted metal part of the lap weld softens due to being mixed with elements of the steel sheets, but this part is comprised of three steel sheets superposed and is large in thickness, so does not break at the melted metal part.

(35) For Invention Example 2, the elongation at break was 7.3% or so. Compared with the comparative example, it was improved about 317%. It could be confirmed that the location of break was the middle of the laser hardened part (FIG. 5B). That is, it could be confirmed that the example did not break at the HAZ softened part of the lap welding. As a result, it could be confirmed that Invention Example 2 solved the problem of low strain breakage compared with the comparative example and Invention Example 1. Note that, for observation of the broken surface, the broken surface of the high strength steel sheet in the test piece (in the above test piece, the 1310 MPa class steel sheet) was observed.

(36) FIG. 6A to FIG. 6G are explanatory views showing various substantially circular laser lap welds 1 to 7 assuming flanges. In this case, the principal stress direction is the long direction of the flange (left-right direction in the figure). The present invention, as shown in FIG. 6A to FIG. 6G, can be applied to laser lap welding of a circular shape 1, oval shape 2, C-shape 3, long C-shape 4, circular ring shape 5, oval ring shape 6, and double circular ring shape 7.

(37) FIG. 7A to FIG. 7J are explanatory views showing straight line laser hardened parts 8 formed cutting across HAZ softened parts formed around circular shape laser lap welds 1. FIG. 7 also assume a flange. In the same way as FIG. 6, the principal stress direction is the long direction of the flange (left-right direction in the figure).

(38) As shown in FIG. 7A, the straight line laser hardened part 8 need not necessarily run through the center of the circular shape laser weld 1.

(39) As shown in FIG. 7B, the direction of formation of the straight line laser hardened part 8 may be within 30 of the assumed principal stress direction.

(40) As shown in FIG. 7C, the left and right lengths of the straight line laser hardened part 8 need not necessarily be the same, but the laser hardened part 8 has to be formed up to a position separated by at least 3 mm outward from the end of the substantially circular laser weld 1.

(41) As shown in FIG. 7D, the end of the straight line laser hardened part 8 may be made broader so as to suppress stress concentration. However, the size has to be smaller than the circular shape laser weld 1.

(42) As shown in FIG. 7E, the straight line laser hardened part 8 may be bent if within 30 of the assumed principal stress direction.

(43) As shown in FIG. 7F, a plurality of circular shaped laser hardened parts 1, 1 were continuously connected by a single straight line laser weld 8.

(44) As shown in FIG. 7G and FIG. 7H, the straight line laser hardened part 8 need not run through the inside of the circular shape laser lap weld 1.

(45) As shown in FIG. 7I, there may be a plurality of laser hardened parts 8. If there are a plurality of principal stress directions, the laser hardened parts may be formed in those directions.

(46) As shown in FIG. 7J, the width of the laser hardened part may also be changed. In particular, as shown in FIG. 7I, the width of the laser hardened part should be broader than the outer edge of the laser lap weld and narrower at the terminal end.

(47) The state of application of the present invention to an automobile part will be explained next. FIG. 8 is an explanatory view showing the state of application of the present invention to a center pillar 9.

(48) In the process of production of a side panel, a side panel outer member (not shown) comprised of a 270 MPa class hot dipped galvannealed steel sheet, a center pillar reinforcement member 10 comprised of a hot stamped member, and a center pillar inner member (not shown) comprised of a 590 MPa class steel sheet are superposed at flanges formed at their edge parts and resistance spot welded to tack them by spot welds 11.

(49) The assembled side panel is assembled with the under body at the main body line, is tack welded, then is further welded by remote laser welding.

(50) At this time, as shown in FIG. 8, circular shape laser lap welding is performed to form the laser lap weld 1, then the HAZ softened part around the circular shape laser lap weld 1 is split by firing a laser in a straight line to form the laser hardened part 8.

(51) Note that, as shown in FIG. 8, the operation of firing a laser for splitting the HAZ softened part around the circular shape laser lap weld 1 does not have to be performed for all of the laser lap welds 1. It need only be performed for a laser lap weld 1 with a possibility of break at the HAZ softened part.

(52) FIG. 9 is an explanatory view showing the state of application of the present invention to a side sill 12. The side sill 12 also, in the same way as the center pillar 9, is assembled with the under body at the main body line, tack welded, then increased in welds by remote laser welding. The side sill 12 is comprised of a sill inner member panel comprised of a 590 MPa class hot dipped galvannealed steel sheet, a sill inner member reinforcement member comprised of a 1180 MPa class hot dipped galvannealed steel sheet, and a sill outer member panel comprised of a 270 MPa class hot dipped galvannealed steel sheet superposed by flanges formed at their edge parts. At this time, circular shape laser welding is performed to form the laser lap welds 1, then a laser is fired to split the HAZ softened parts and form the laser hardened parts 8.

(53) FIG. 10 is an explanatory view showing the state of application of the present invention to an A-pillar 13 and roof rail 14.

(54) As shown in FIG. 10, with the A-pillar 13 and roof rail 14 as well, like the center pillar 9, a side panel including the A-pillar 13 and roof rail 14 is assembled with the under body by the main body line and then further welded by remote laser welding. The A-pillar 13 and the roof rail 14 are respectively comprised of shaped panels comprised of two hot stamped members and a 270 MPa class hot dipped galvannealed steel sheet superposed. At this time, circular shape laser welding is performed to form the laser lap welds 1, then a laser is fired to split the HAZ softened parts around the circular shape laser lap welds 1 and form the laser hardened parts 8.

(55) Above, the present invention was explained based on the example of automobile parts having a circular shape laser lap welds. As mentioned above, the embodiments of the present invention are not limited to the above examples. The effects of the present invention are exhibited so long as including the requirements of the present invention. These are included in the scope of the present invention. Below, examples of embodiments different from the above will be explained.

Example 1

(56) Example 1 is an example of application of the present invention to C-shape laser lap welding. FIG. 11A is an explanatory view showing a tensile test piece, FIG. 11B is an explanatory view showing a laser weld of a comparative example, FIG. 11C is an explanatory view showing a laser weld of an invention example, and FIG. 11D is a graph showing the results of a tensile test.

(57) For the tensile test piece, three sheets of a thickness 0.7 mm 270 MPa class hot dipped galvannealed steel sheet, a thickness 1.4 mm 1310 MPa class steel sheet, and a thickness 1.2 mm 440 MPa class steel sheet were stacked in that order and the gripping parts of the test piece were welded by resistance spot welding to prepare the tensile test piece shown in FIG. 11A.

(58) Next, using a fiber laser under conditions of a beam diameter of 0.6 mm, output of 2.7 kW, and welding speed of 2.0 m/min and use of a galvanomirror, a test piece formed with a diameter approximately 7 mm C-shape laser weld shown in FIG. 11B (comparative example) and a test piece formed with a C-shape weld shown in FIG. 11C, then fired upon by laser in a straight line (invention example) were prepared.

(59) The tensile test was conducted at a tensile speed of 3 mm/min with the prepared welds at the evaluation distance 50 mm.

(60) FIG. 12A is an explanatory view showing a break position of the comparative example, while FIG. 12B is an explanatory view showing a break position of an invention example.

(61) As shown in FIG. 11D, in laser welding a C-shape of the comparative example, the elongation at break was 2.7%. Further, as shown in FIG. 12A, the sample broke along the outer circumference of the C-shape.

(62) As opposed to this, in the invention examples, as shown in FIG. 11D, the elongation at break was 4.3%. Further, as shown in FIG. 12B, the break position was at the front end (terminal end) of the laser hardened part.

(63) In this way, the invention examples are improved in elongation at break compared with the comparative example.

INDUSTRIAL APPLICABILITY

(64) The present invention can be utilized for a lap joint joining steel sheets including a high strength steel sheet. In particular, it can be utilized for automobile parts and other machine structure parts.

REFERENCE SIGNS LIST

(65) 1. substantially circular (circular shape) laser weld 2. substantially circular (oval shape) laser weld 3. substantially circular (C-shape) laser weld 4. substantially circular (long C-shape) laser weld 5. substantially circular (circular ring shape) laser weld 6. substantially circular (oval ring shape) laser weld 7. substantially circular (double circular ring shape) laser weld 8. straight line hardened part 9. center pillar 10. center pillar reinforcement member 11. spot weld 12. side sill 13. A-pillar 14. roof rail