JP6149522B2 - Lap welded member of high strength steel plate and method for producing the same - Google Patents

Description

  The present invention relates to a lap weld member of a high-strength steel plate and a method for manufacturing the same.

  In order to achieve both improved collision safety and improved fuel efficiency, the application of high-strength steel sheets to structural members that form the framework of the monocoque body that constitutes the body of an automobile (hereinafter referred to as “structural members for automobiles”) has expanded. Yes. Currently, high-strength steel sheets having a tensile strength of 980 MPa are used for structural members for automobiles, and recently, application of high-tensile steel sheets having a tensile strength of 1180 MPa or more is also being studied. In addition, the production of high-strength automotive structural members having a tensile strength of 1500 MPa or more has been promoted by using a hot stamp method in which quenching is performed simultaneously with press molding. According to the hot stamping method, the steel sheet is pressed in a soft state at a high temperature, so that there is little problem with dimensional accuracy after forming, and press forming can be performed in a high temperature and high ductility state. There is a great merit that it is excellent in performance.

  Generally, the structural member for automobiles is constructed by welding a molded product of these steel plates to other molded products by spot welding. However, the base material of high-strength steel plates such as high-strength steel plates and hot stamped steel plates of 1180 MPa or higher is strengthened by quenching, so that when spot welding is performed, the heat-affected zone (HAZ) of the spot welded portion is tempered. Softer than the base material. Although softening of HAZ (hereinafter also referred to as “HAZ softening”) hardly occurred in, for example, a 590 MPa steel plate, it was recognized in a 980 MPa steel plate, and in particular, a martensite structure in a continuous annealing facility (WQ-CAL) having a water cooling function. It occurs remarkably in the HAZ of spot welds of high-tensile steel plates and hot stamped steel plates of 1180 MPa or higher. For example, in a 1180 MPa grade cold-rolled steel sheet, the hardness of the base material is about Vickers hardness Hv 370 to 420, whereas the hardness of the most softened portion of the HAZ is reduced to about Hv 300.

  FIG. 11 is a graph showing an example of the hardness distribution of the spot welded portion of a member made of a high-strength steel plate (1500 MPa class hot stamped steel plate member).

  As shown in the graph of FIG. 11, in the case of a 1500 MPa class hot stamp material, the base material has a Vickers hardness of about Hv450, but the HAZ softest part is about Hv300, and the HAZ softest part is the base material. Hv is reduced by about 150.

  FIG. 12 is an explanatory view showing a state in which the hot stamped steel plate member having the hardness distribution of the spot welded portion shown in FIG. 11 is broken starting from the HAZ most softened portion, and FIG. 12 (a) is a top view, FIG. 12 (b) is a sectional view.

  Although this HAZ softening does not affect the results of the tensile shear test and cross tension test, which are joint evaluations of spot welding, a tensile load is applied to the entire flange portion of the hot stamping part including the spot weld. Then, as shown in FIGS. 12 (a) and 12 (b), strain may concentrate locally on the HAZ softest part and break. As described above, the HAZ softening of the spot welded portion in the press-formed product of a steel plate of 1180 MPa or more may be a starting point of the fracture of the press-formed product at the time of collision.

  For example, a tubular structural member of an automobile body having a closed cross section, such as an A pillar, a B pillar, a roof rail, and a side sill and having a flange that is spot welded to form the cross section, is torn at the time of a car collision. Therefore, it is required to effectively absorb impact energy and protect passengers in the cabin by plastic deformation. However, in severe collision mode, for example, in the case of the American Road Safety Insurance Association (IIHS) SUV side collision test, B pillar reinforcement using high strength steel plate is the softest HAZ of the spot welds on the flange of the B pillar. In some cases, strain concentrates on the part and becomes a starting point of breakage, and the B-pillar breaks during the collision, and the amount of the B-pillar entering the cabin increases, and the target collision performance may not be obtained. Also, in the case of Euro NCAP pole side collision test, the target collision performance is obtained by the fact that strain concentrates on the HAZ softened part of the spot welded part of the flange of the roof rail and the roof rail breaks as a starting point of breakage. It may not be possible.

  For this reason, in the application of these high-strength steel plates of 1180 MPa or more to the automobile body, the HAZ softest part of the spot weld formed on the flange of the automobile structural member needs not to be the starting point of breakage due to the collision.

  Non-Patent Document 1 describes that HAZ softening is possible even when spot welding is performed by reducing the strength of a base material by heat treatment performed at the time of hot stamping at a portion of a roof rail that is hot stamped at risk of fracture. There is disclosed a method for preventing breakage of a structural member for an automobile starting from a HAZ softened portion.

  Non-Patent Document 2 describes that the strength of the base metal is reduced by tempering the B pillar, which is a hot stamped product, by high-frequency heating, and HAZ softening does not occur even if spot welding is performed. A method for preventing breakage of a structural member for an automobile is disclosed.

Tailored Properties for Press-hardened body parts Dr. Camilla Wastlund, Automotive Circle International, Insight edition 2011 Ultra-high strength steels in car body lightweight design-current challenges and future potential Tempering of hot-formed steel using induction heating (http://publications.lib.chalmers.se/publication/144308)

  As disclosed in Non-Patent Document 1, in the method of adjusting the strength of the roof rail part over a wide range with the parts integrated from the roof rail to the A pillar, a low strength part is inevitably formed in a relatively wide range of the roof rail. As a result, the advantage of the hot stamped part that high strength can be obtained cannot be fully enjoyed, and the effect of weight reduction is also limited. In addition, in this method, strength characteristics are likely to vary in a relatively wide transition region inevitably formed between the quenching region and the unquenched region, and there is a risk of variation in the collision performance of components. Furthermore, since the area including the spot welded portion is softened, the tensile shear strength of the spot welded joint may be reduced.

  As disclosed in Non-Patent Document 2, the method of softening the B pillar flange by high-frequency heating after hot stamping by high-frequency heating is that the B pillar is deformed by the thermal strain generated by the wide range of tempering. There is a risk that the dimensional accuracy of the will decrease. Not only the B pillar but also the structural members for automobiles arranged around the door opening such as the A pillar and the roof rail are required to have a particularly high dimensional accuracy in order to ensure the accuracy of building the vehicle body. The gap (parting) is required to be uniform over the entire periphery of the door panel. For this reason, the reduction in the dimensional accuracy of the structural member for automobiles arranged around the door opening part significantly impairs the appearance quality of the automobile. Further, by tempering the flange over a wide range, the strength of the structural member for automobile is lowered, and the high strength characteristics of the hot stamp material cannot be enjoyed. Furthermore, since the area including the spot welded portion is softened, the tensile shear strength of the spot welded joint may be reduced.

  In addition, in the design stage of the structure of an automotive structural member such as a B-pillar, the structural member for an automotive structural member may be prevented so that the HAZ softened portion of the spot welded portion of the flange of the B-pillar does not reach a strain that breaks at the time of collision. It is also possible to add reinforcement or increase the thickness of structural members for automobiles. However, this method inevitably increases the cost of the automobile body and increases the weight of the automobile body due to an increase in the number of parts and the plate thickness.

  In the above description, the case where the welding is spot welding has been taken as an example, but the situation is the same when the welding is other welding than spot welding such as laser welding or arc welding.

  The present invention has been made in view of such problems of the prior art, and even when welding (typically spot welding) is performed on a high-strength steel sheet having a tensile strength of 1180 MPa or more or a molded product thereof. An object of the present invention is to provide a lap-welded member of a high-strength steel sheet that can be prevented from breaking at low strain starting from the HAZ softened part of the welded part at the time of collision, and a method for producing the same.

The present invention is listed below.
(1) At least a first structural member which is a steel plate having a tensile strength of 1180 MPa or more or a molded body thereof, and another steel plate or a molded body thereof which is overlapped and welded to the first structural member. A lap weld member comprising a component member, the lap weld member being provided at least on the first component member side adjacent to the weld metal in the longitudinal direction of the weld metal formed by welding and the lap weld member. And a softened region, and the softened region is provided so as not to be applied to the HAZ most softened portion formed around the weld metal. In this specification, it is also referred to as a first lap weld member.

  (2) At least a first structural member that is a steel plate having a tensile strength of 1180 MPa or more or a molded body thereof, and another steel plate or a molded body that is overlapped and welded to the first structural member. A lap weld member including a constituent member, wherein the lap weld member includes a plurality of weld metals formed in a longitudinal direction of the lap weld member by welding, and at least a pair of two adjacent weld metals among the plurality of weld metals. A softened region provided on at least the first component side between the two, and the softened region is provided so as not to be applied to the HAZ softest part formed around each of the two weld metals. A lap weld member characterized by the above. In this specification, it is also referred to as a second lap weld member.

  (3) At least a first structural member which is a steel plate having a tensile strength of 1180 MPa or more or a molded body thereof, and another steel plate or a molded body thereof which is overlapped and welded to the first structural member. A lap weld member comprising a constituent member, wherein the lap weld member includes at least one weld metal formed by welding and at least one of the weld metals in the longitudinal direction of the lap weld member. Two softening regions provided on the first component side so as to be adjacent to each other, and the softening region does not reach the HAZ most softened portion formed around the weld metal. A lap weld member characterized by being provided. In this specification, it is also referred to as a third lap weld member.

  (4) Any one of the items (1) to (3), comprising a third component member that is a steel plate or a molded body thereof that is overlapped and welded with the first component member or the second component member. The lap weld member described in 1.

  (5) When the Vickers hardness A (Hv) of the softened region is set to B (Hv) as the Vickers hardness of the steel sheet having a tensile strength of 1180 MPa or more, and the Vickers hardness of the HAZ most softened portion is set to C (Hv) The lap welding described in any one of items (1) to (4) satisfying (1) formula: A ≦ B−50 and (2) formula: C−120 ≦ A ≦ C + 50 Element.

  (6) The softened region is formed 2.0 mm or more away from the end of the weld metal in the longitudinal direction, and the length of the softened region in the longitudinal direction is 10 mm or more and 50 mm or less, and is softened in the direction orthogonal to the longitudinal direction. The length of the region is the lap weld member described in any one of the items (1) to (5), which is 1 to 4 times the maximum length of the weld metal in the direction orthogonal to the longitudinal direction.

  (7) The lap weld member according to any one of items (1) to (6), wherein the softened region is formed by tempering with a laser beam or high-frequency heating.

  (8) An automobile comprising the lap weld member described in any one of items (1) to (7) as a structural member.

  (9) Manufacture of a lap-welded member that overlaps and welds at least a first structural member that is a steel sheet having a tensile strength of 1180 MPa or more or a molded body thereof and a second structural member that is another steel sheet or a molded body thereof. A first step of providing at least one softened region partially on the first component, and the first component formed with the softened region and the second component are overlapped, A second step of welding at least a portion adjacent to the softened region in the longitudinal direction of the lap weld member, and forming the weld metal so that the HAZ softest portion around the weld metal formed by this welding does not reach the softened region; A lap weld member manufacturing method comprising: In this specification, it is also referred to as a first manufacturing method.

  (10) In the second step, the first constituent member, the second constituent member, and a third constituent member that is another steel plate or a molded body thereof are overlapped and welded. A method for manufacturing a lap weld member according to claim 9.

  (11) Manufacture of a lap-welded member that overlaps and welds at least a first structural member that is a steel plate having a tensile strength of 1180 MPa or more or a molded body thereof and another steel plate or a second structural member that is a molded body thereof. A method comprising: a first step of overlapping and welding a first component member and a second component member; and a weld metal formed by welding in the first step in a longitudinal direction of the lap weld member. And a second step of providing a softened region at least on the first component side so as not to cover the HAZ softened portion formed around the weld metal. Method. In this specification, it is also referred to as a second manufacturing method.

  (12) When the Vickers hardness A (Hv) of the softened region is set to B (Hv) and the Vickers hardness of the HAZ most softened portion is set to C (Hv) (1) Formula: A ≦ B−50, and (2) Formula: C−120 ≦ A ≦ C + 50 satisfying the lap weld member described in any one of items (9) to (11) Manufacturing method.

  (13) The softened region is provided 2.0 mm or more away from the end of the weld metal in each of the two welds in the longitudinal direction, and the length of the softened region in the longitudinal direction is 10 mm or more and 50 mm or less, and the longitudinal direction The length of the softened region in the direction orthogonal to the longitudinal direction is 1 to 4 times the maximum length of the weld metal in the direction orthogonal to the longitudinal direction. Any one of items (9) to (12) A method of manufacturing the lap weld member described.

  (14) The method for manufacturing a lap weld member according to any one of (9) to (13), wherein the softened region is provided by tempering with a laser beam or high-frequency heating.

  According to the present invention, even when welding is used for assembling a high-strength steel plate having a tensile strength of 1180 MPa or more or a molded product thereof, it is possible to suppress the HAZ softest portion of the welded portion from breaking at a low strain during a collision. As a result, for example, it is possible to provide an automobile structural member that has excellent occupant protection performance at the time of a collision, high strength, and high shape accuracy.

FIGS. 1A and 1B show a state in which the first to third lap weld members according to the present invention are applied to a B pillar, and FIG. 1A is a perspective view of the B pillar from the side panel outer side. FIG. 1B is a cross-sectional view taken along line AA of the B pillar. 2 (a) and 2 (b) show the situation where the first to third lap weld members according to the present invention are applied to the roof rail, and FIG. 2 (a) shows the roof rail seen through the roof rail inner side. It is a perspective view of an outer ring force, and FIG.2 (b) is BB sectional drawing of a roof rail. FIG. 3 (a) shows a spot welded portion and a softened region in a first structural member that is a molded body of a high-strength steel plate that constitutes a lap weld member together with a second structural member that is a molded body of a low-strength steel plate. It is explanatory drawing which shows a relationship notionally, FIG.3 (b) is a graph which shows the measurement result of the hardness of the 1st structural member in the measurement position C in Fig.3 (a). FIG. 4A is a graph showing the effect of the present invention. FIG. 4A is a stress-elongation diagram when a tensile load is applied to a tensile test piece simulating a spot-welded flange of a 1500 MPa class hot stamp material. FIG. 4 (b) is an explanatory view showing a tensile test piece of a comparative example, and FIG. 4 (c) is an example of the present invention (first). 3 is an explanatory view showing a tensile test piece of 3 lap welded members). Fig.5 (a)-FIG.5 (e) are explanatory drawings which show notionally the relationship between the spot weld part and softening area | region in the 1st structural member which is a high strength steel plate. FIG. 6A shows a C-shaped laser welded portion and softening in the first structural member that is a formed body of the high-strength steel plate, which constitutes the lap weld member together with the second structural member that is a formed body of the low-strength steel plate. FIG. 6B is a diagram conceptually showing the relationship between regions, and FIG. 6B is a graph showing the measurement results of the hardness of the first component member at the hardness measurement position C in the cross-sectional view in FIG. It is. FIG. 7A shows a ring-shaped laser welded portion and a softened region in a first constituent member that is a formed body of a high-strength steel plate that constitutes a lap weld member together with a second constituent member that is a formed body of a low-strength steel plate. FIG. 7B is a graph showing a measurement result of the hardness of the first component at the hardness measurement position C in the cross-sectional view in FIG. 7A. is there. FIG. 8A shows a circular laser welded portion and a softened region in a first constituent member that is a formed body of a high-strength steel plate that constitutes a lap weld member together with a second constituent member that is a formed body of a low-strength steel plate. FIG. 8B is a graph showing the measurement results of the hardness of the first component member at the hardness measurement position C in the cross-sectional view in FIG. 8A. is there. FIG. 9A shows a circular arc welded portion and a softened region in a first constituent member that is a formed body of a high-strength steel plate that constitutes a lap weld member together with a second constituent member that is a formed body of a low-strength steel plate. FIG. 9B is a graph showing the measurement result of the hardness of the first component member at the hardness measurement position C in the cross-sectional view in FIG. 9A. is there. FIG. 10 is an explanatory diagram showing the shape of the test piece used in this example. FIG. 11 is a graph showing an example of the hardness distribution of the spot welded portion of a member made of a high-strength steel plate (1500 MPa class hot stamped steel plate member). FIG. 12 is an explanatory view showing a state in which the hot stamped steel plate member having the hardness distribution of the spot welded portion shown in FIG. 11 is broken starting from the HAZ most softened portion, and FIG. 12 (a) is a top view, FIG. 12 (b) is a sectional view.

  A mode for carrying out the present invention will be described with reference to the accompanying drawings. In the following description, the case where welding is spot welding is taken as an example.

The present invention relates to a lap weld member of a high-strength steel plate that welds an overlap portion of two or more steel plates including at least one high-strength steel plate having a tensile strength of 1180 MPa or more, and a manufacturing method thereof.
1. Lap Welding Member Since the lap welding member 1 according to the present invention is grasped as a first lap welding member, a second lap welding member, or a third lap welding member described below, these are shown in FIG. ) And will be described sequentially with reference to the example of the B pillar shown in FIG.

  FIGS. 1A and 1B show a state in which the first to third lap weld members according to the present invention are applied to a B pillar, and FIG. 1A is a perspective view of the B pillar from the side panel outer side. FIG. 1B is a cross-sectional view taken along line AA of the B pillar.

[First lap welding member]
The first lap weld member includes at least a first component member 1 and a second component member 7. The number of the steel plates constituting the first lap weld member is two or three. In the case of stacking three sheets, in addition to the first component member 1 and the second component member 7, a third component member 11 that is overlapped and welded thereto is provided.

  The first constituent member 1 is a steel plate having a tensile strength of 1180 MPa or more or a molded body thereof (hereinafter referred to as a steel plate including a molded body). The second constituent member 7 and the third constituent member 11 are other steel plates different from the first constituent member 1. In the case of the two-layer structure, the second component member 7 and the first component member 1 are overlapped and spot welded. In the case of a three-layer structure as shown in FIG. 1, the second component member 7 and the third component member 11 are overlapped with the first component member 1 and spot welded.

  In this way, the first lap weld member includes the first component member 1 and the second component member 7 partially overlapped (in the case of the three-layer structure, the third structure member 11). The mating portion is spot welded to have an assembled structure.

  In the first lap weld member, the first component 1 has a nugget 2 and a softened region 3 formed by spot welding. The softened region 3 is provided at least on the first component 1 side adjacent to the nugget 2 in the longitudinal direction of the first lap weld member. The softened region 3 is provided so as not to reach the HAZ most softened portion formed around the nugget 2.

  The steel plate constituting the first component 1 is a high-strength steel plate having a quenching structure containing martensite in a continuous annealing facility or a hot stamping steel plate heated to an austenite temperature or higher and fired while being formed in a water-cooled mold. The hot stamped steel plate which raised the intensity | strength by putting is illustrated.

  The fracture of the spot welded portion 2 tends to occur more easily as the first structural member 1 mainly responsible for the strength of the member is a steel plate having a higher martensite structure and a higher tensile strength. Therefore, the present invention exhibits a particularly great effect when the first component 1 has a tensile strength of 1500 MPa class or more by, for example, hot stamping. Although there is no restriction | limiting in particular in the upper limit of the tensile strength of the steel plate which comprises the 1st structural member 1, It is desirable to set it as 2000 MPa.

  The thickness range of the steel plates constituting the first component member 1, the second component member 7 and the third component member 11 is also about 0.6 mm to 2.6 mm, which is used for ordinary automotive strength member applications. Is sufficiently applicable.

  For example, when the present invention is applied to a B-pillar, a 270 MPa class galvannealed steel sheet having a thickness of 0.7 mm as a side panel outer (third constituent member 11) and a reinforcement (first constituent member 1). ) As a 1500 MPa class hot stamped steel sheet having a thickness of 1.8 mm or a 1800 MPa class hot stamped steel sheet having a thickness of 1.6 mm, and a 440 MPa class cold rolled steel sheet having a thickness of 1.2 mm as the inner (second component 7). The three-layer structure is exemplified.

  When the present invention is applied to a roof rail, a 270 MPa class galvannealed steel plate having a thickness of 0.7 mm as a side panel outer, a 1500 MPa class hot stamped steel plate having a thickness of 1.2 mm as a reinforcement, and an inner A three-ply structure with a 590 MPa grade cold-rolled steel sheet having a thickness of 1.4 mm is exemplified.

  When the present invention is applied to a bumper reinforcement, a 1800 MPa class hot stamped steel plate having a thickness of 1.4 mm as a reinforcement outer (first component), and a plate thickness of 1 as a reinforcement inner (second component). It is exemplified by a two-ply structure with a 4 mm 590 MPa grade cold rolled steel sheet.

  In these examples, only the first component member 1 was a high-strength steel plate of 1180 MPa class or higher, but even if the second component member 7 or the third component member 11 is a high-strength steel plate of 1180 MPa class or higher. Good. In the case where the second constituent member 7 to be superposed is a high-strength steel plate of 1180 MPa class or more, in the case where a fracture at the HAZ softened portion of the spot welded portion of the second constituent member 7 superposed at the time of collision is expected, The softened region 3 may be partially formed also for the second component member 7. On the other hand, in the case where it is expected that no fracture will occur at the HAZ most softened portion of spot welding of the second component member 7 at the time of collision, it is not necessary to form the softened region 3 for the second component member 7.

  The surface of the steel plate of the first component 1 may be non-plated or plated. Examples of the plated steel sheet used include an electrogalvanized steel sheet, an electrogalvanized alloy-plated steel sheet (for example, a Zn-Ni alloy electroplated steel sheet), a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, a hot-dip aluminum-plated steel sheet, and hot-dip zinc-aluminum. Examples include alloy-plated steel sheets. Further, these may be hot stamped steel plates.

  The nugget diameter of the nugget 2 formed at the interface between the first component member 1 and the second component member 7 by spot welding is 4√t or more and 7√t or less (t: plate thickness on the thin side of the overlapping surface (mm )) Is desirable. The three-layer structure is the same at the interface between the first component member 1 and the third component member 11.

[Second lap welding member]
In the subsequent description of the second and third lap weld members, portions that are different from the first lap weld member described above will be described, and overlapping descriptions of the common portions will be omitted as appropriate.

  The second lap weld member has a plurality of nuggets 2 and a softened region 3. A plurality of nuggets 2 are arranged in the longitudinal direction of the lap weld member by spot welding. The pitch of spot welding, that is, the distance between adjacent nuggets 2 is, for example, about 20 to 100 mm, but is not limited to this, and may be set as appropriate according to product specifications.

  The softened region 3 is provided on at least the first component 1 side between at least a pair of two adjacent nuggets 2 among the plurality of nuggets 2. The softened region 3 is provided so as not to reach the HAZ softest part formed around each of the two nuggets 2 and 2.

[Third lap welding member]
The third lap weld member has one or more nuggets 2 and two softened regions 3 and 3 formed by spot welding. The two softened regions 3 are provided on the first component 1 side so as to sandwich the nugget 2 adjacent to both sides in the longitudinal direction of the lap weld member in at least one nugget 2 of the nuggets 2. The two softened regions 3 are provided so as not to reach the HAZ softest part formed around the nugget 2.

  The Vickers hardness A (Hv) of the softened region 3 in the first lap weld member to the third lap weld member is set to B (Hv) as the Vickers hardness of the base material of the first component member 1 and HAZ maximum. When the Vickers hardness of the softened portion is C (Hv), it is desirable to satisfy (1) Formula: A ≦ B-50 and (2) Formula: C-120 ≦ A ≦ C + 50. That is, it is desirable that A ≦ B−50 in order to surely suppress the strain concentration on the HAZ softest part of the spot weld 2. Moreover, in order to prevent the strength reduction of the first lap weld member to the third lap weld member due to the hardness reduction of the base material of the first component member 1, it is desirable that C−120 ≦ A. In order to suppress strain concentration on the HAZ most softened portion of the spot welded portion 2, it is desirable that A ≦ C + 50.

  The softened region 3 in the first lap weld member to the third lap weld member is formed such that the softened region 3 disposed between the hit points of the spot welded portion 2 is separated from the end of the nugget 2 by 2.0 mm or more. Is desirable. This is because if the softened region 3 is too close to the nugget 2, the tensile shear strength decreases.

  As shown in FIG. 2A described later, the length of the softened region 3 in the longitudinal direction of the first lap weld member to the third lap weld member is not less than 10 mm and not more than 50 mm, and is orthogonal to the longitudinal direction. The length (width) of the softened region in the direction is preferably 1 to 4 times the nugget diameter of the nugget 2. This is because if the length of the softened region 3 is less than 10 mm, it cannot be said to be a sufficient strain absorbing region, and if the softened region 3 exceeding 50 mm is formed, the strain increases. Further, if the width of the softened region 3 is smaller than the nugget diameter, sufficient strain absorption cannot be performed. If the width of the softened region 3 exceeds 4 times the nugget diameter, the influence of thermal strain at the time of softening increases and the strength of the structural parts for automobiles decreases. This is because of the increase.

  Here, the nugget diameter is the value of the diameter of the melt-solidified portion at the interface where two steel plates are stacked. In addition, in the case of a plurality of three or more overlaps, the value of the melted and solidified portion on the larger diameter side of the melted and solidified portion at the overlapping interface with the steel plate of the first component 1 is defined as the nugget diameter.

  Further, as described above, the welding is not necessarily spot welding. Including such a case, the length (width) of the softened region in the direction orthogonal to the longitudinal direction is 1 to 4 times the maximum length in the direction orthogonal to the longitudinal direction of the weld metal (molten solidified portion). It is desirable.

  Furthermore, the softened region 3 in the first lap weld member to the third lap weld member is preferably formed by tempering by laser beam or high frequency heating.

  Next, as a specific example, application examples in the B pillar and the roof rail will be further described with reference to FIGS. 1A and 1B, FIG. 2A and FIG. 2B, respectively.

  FIG. 1A and FIG. 1B are explanatory views showing a situation in which the first to third lap weld members according to the present invention are applied to a B pillar, and FIG. FIG. 1B is a perspective view of a B pillar reinforcement seen through from the panel outer side, and FIG. 1B is a cross-sectional view of the B pillar taken along line AA.

  In the B pillar 0, the B pillar reinforcement 1 as the first component, the inner material 7 as the second component, and the side panel outer material 11 as the third component are overlapped, A plurality of spot welds 2 are intermittently formed on the flange (the flange 1a in the B pillar reinforcement 1). In this example, a softened region 3 is formed between the spot welds 2 in the upper part of the B pillar reinforcement 1, and a softened region 3 is similarly formed between the spot welds 2 in the lower part of the B pillar reinforcement 1. In addition, the softened region 3 is not formed between the spot welds 2 in the intermediate portion in the height direction of the B pillar.

  The formation of the upper softened region 3 prevents breakage due to breakage at the HAZ softest portion of the spot welded portion at the upper part of the B pillar 0 at the time of collision, and the formation of the lower softened region 3 is a large deformation region due to the collision. To prevent breakage at the HAZ softest part of spot welding. A softened region is not formed in the central portion in the height direction of the flange 1a of the B pillar reinforcement 1 that does not reach the strain at which the HAZ softest portion breaks. As described above, the formation of the softened region 3 may be a minimum necessary in order to prevent breakage starting from the HAZ most softened portion of the spot welded portion 2 at the time of collision.

  As described above, in the flange portion of the B pillar 0, one softened region 3 is formed adjacent to one spot weld portion 2 in accordance with the height direction position of the B pillar 0 (first lap weld member). In addition, one softened region 3 is formed between two adjacent spot welds 2 (second lap weld member), and two softened regions 3 are formed adjacent to both sides of one spot weld 2. (Third lap welding member).

  In the example of FIG. 1, a hinge reinforcement 15 (for example, a 980 MPa class cold-rolled steel plate) as a fourth constituent member is overlapped inside the B pillar reinforcement 1 and two sheets are welded at the bottom of each groove. Has been.

  2 (a) and 2 (b) are explanatory views showing a situation in which the first to third lap weld members according to the present invention are applied to the roof rail, and FIG. 2 (a) shows the roof rail on the roof rail inner side. FIG. 2 is a perspective view of the roof rail outer force seen through, and FIG. 2B is a cross-sectional view of the roof rail taken along line BB.

  In the roof rail 00, the side panel outer 5 that is the third component, the roof rail outer 4 that is the first component, and the roof rail inner 12 that is the second component are overlapped to form a flange portion (the roof rail outer 4). Then, a plurality of spot welds 2 are formed intermittently in 4a). A local softened region 3 is formed between the spot welds 2 of the roof rail outer 4. Thereby, the fracture | rupture in the HAZ softest part of spot welding can be suppressed, without reducing the intensity | strength of the roof rail outer 4 as much as possible. In addition, the code | symbol 6 in Fig.2 (a) shows A pillar.

  As described above, in the flange portion of the roof rail 00, one softened region 3 is formed adjacent to one spot welded portion 2 (first lap weld member) in accordance with the longitudinal position of the roof rail 00. One softened region 3 is formed between two spot welds 2 (second lap weld member), and two softened regions 3 are formed adjacent to both sides of one spot weld 2 (see FIG. It can be said that this is a third lap weld member.

Next, the technical significance of the softened region 3 in the present invention will be described.
FIG. 3 (a) is an explanatory view schematically showing an example of the vicinity of the flange 1a of the first constituent member 1 joined to the second constituent member 7 which is a formed body of a low-strength steel plate. (B) is a graph which shows an example of the hardness distribution of the cross section in the vicinity of the spot weld part 2 and the softening area | region 3. FIG. In addition, the hardness in FIG.3 (b) is a measured value of the position (plate thickness center position of the 1st structural member 1) of the broken line shown to Fig.3 (a).

  As shown in FIG. 3 (a), a 1500 MPa-class cold-rolled steel sheet molded product (first constituent member) 1 is overlapped with a 440 MPa-class steel plate molded product 7 (second constituent member) via a flange 1a. Then, spot welded portions 2 are formed intermittently in the extending direction of the flange 1a (one spot welded portion 2 is shown in FIG. 3A), and the spot welded portion 2 of the flange 1a of the first component 1 is formed. A softened region 3 is formed adjacent to.

  As shown in the graph of FIG. 3 (b), the HAZ most softened portion where the hardness is reduced to about 300 Hv is formed around the spot welded portion 2, but the softened region where the hardness is reduced to about 300 Hv. 3 is provided so as not to reach the HAZ most softened portion.

  FIG. 4 (a) is a graph showing the effect of the present invention, and is a tensile simulating a flange of a member in which a 440 MPa class cold-rolled steel plate is spot welded to a 1500 MPa class hot stamp material as shown in FIG. 3 (a). FIG. 4B is a stress-strain diagram when a tensile load is applied to a test piece, FIG. 4B is an explanatory view showing a tensile test piece of a comparative example, and FIG. 4C is a tensile test of an example of the present invention. It is explanatory drawing which shows a piece (3rd lap welding member).

  The tensile test piece shown in FIG. 4C, that is, a 1500 MPa class steel plate (first constituent member) 1 is overlapped with a 440 MPa class steel plate (second constituent member) 7 to form a spot welded portion 2, and further In the tensile test piece in which the softened region 3 (width 10 mm, length 15 mm) in a narrow range is provided on the first component 1 with the spot weld 2 interposed therebetween, a softened region as shown in FIG. As shown in FIG. 4 (a), the amount of strain at which breakage occurred with respect to the tensile load was remarkably increased and the absorbed energy was increased as compared with a tensile test piece having no tensile strength. This is because, when a tensile load is applied to the test piece (that is, the laminated structure member), the local concentration of strain is suppressed only in the HAZ softened portion of spot welding. Even if the softened portion exists, as shown in the graph of FIG. 4A, the amount of deformation at which the lap weld member breaks is improved. By such a mechanism, the fracture | rupture with few distortions which a HAZ softest part starts is suppressed.

  Further, in order to form the softened region 3 by softening only a predetermined range between the intermittent spot welds 2 of the high-strength steel plate, a conventional method of tempering the flange 1a over a wide range, and a wide range of lap weld members Compared to the method of forming the softened region, the deformation of the component due to thermal strain is extremely small. Therefore, it is suitable for application to automobile structural members that require high shape accuracy such as B pillars, roof rails, and A pillars. In addition, since the softened region 3 is a local portion between the spot welds 2, the strength reduction of the lap weld member is small, and the material characteristics of a high strength material such as a hot stamp material can be fully utilized.

  In this way, as shown in FIGS. 1 (a) to 1 (d), the B pillar, roof rail, side sill and the like, which are important members for protecting passengers in the cabin by side collision, are arranged so as to surround the cabin. By applying the present invention to structural members for automobiles, it is possible to suppress breakage at the HAZ most softened portion of the spot welded portion of these structural members for automobiles, and it is possible to improve safety against side collision. Furthermore, the strength characteristics of these parts can be enhanced by applying the present invention to the perlin force, door beam, floor member, front side member, and rear side member.

  The softened region can take various forms as shown in FIG. FIG. 5A to FIG. 5E are explanatory views conceptually showing the relationship between the spot welded portion 2 and the softened region 3 in the first structural member 1 that is a high-strength steel plate.

  FIG. 5A shows a structure in which a softened region 3 is formed in a straight line with the spot welded portion 2 between the spot welded portions 2 of the flange 1 a of the first component 1.

  FIG. 5B shows a structure in which a softened region 3 is formed between the spot welded portions 2 of the flange 1a of the first component 1 and closer to the end surface 1b of the first component 1 than the spot welded portion 2. It is.

FIG. 5C shows a part of the softened region 3 formed between the spot welded portions 2 of the flange 1a of the first component 1 up to the bent R portion 1c side. Is formed by the softening region 3 extending to the end face 1b side of the first component 1 between the spot welds 2 of the flange 1a of the first component 1.

  Furthermore, FIG.5 (e) shows the softening area | region 3 formed in the ellipse shape between the spot welding parts 2 of the flange 1a of the 1st structural member 1. FIG.

2. Method for Manufacturing Lap Weld Member There are two methods for manufacturing a lap weld member in the present invention. That is, (1) a first manufacturing method for spot welding after forming a softened region, and (2) a second manufacturing method for forming a softened region after spot welding. Hereafter, the manufacturing method of these lap welding methods is demonstrated.

  The following description is mainly about the two-layer structure, but the same applies to the three-layer structure. Further, description will be made assuming a member in which a plurality of welds and softened regions are formed in the longitudinal direction of the lap weld member as shown in FIG.

(1) First manufacturing method in which spot welding is performed after forming the softened region In the first step, the softened region 3 is partially formed on the base material of the first component member 1 having a tensile strength of 1180 MPa or more. The nugget 2 is formed by performing spot welding on a portion adjacent to the softened region 3 at the base material portion outside the softened region 3 in a second step, overlapping with the steel plate (second constituent member).

  First, in the first step, for example, a substantially rectangular softened region 3 is formed between portions where the nugget 2 is scheduled to be formed intermittently by spot welding. That is, before spot welding, a partial softened region 3 is formed between the spot welding scheduled positions, and spot welding is performed in the second step.

  There are no particular restrictions on the welding conditions for spot welding, and the nugget diameter is at least 4√t and not more than 7√t (t: plate thickness (mm) on the thin side of the overlapping surface) at least at the overlapping interface of the target steel plates. What is necessary is just to determine suitably so that the nugget 2 may be formed. For example, a single-phase AC spot welder or an inverter DC spot welder is used, the tip diameter of the welding electrode is in the range of 6 to 8 mm, the curvature radius R of the tip is, for example, 40 mm, and the applied pressure is 2.5 to 6.0 kN. The current value of the welding current may be in the range of 7 to 11 kA, and the energization time may be in the range of 10/60 to 40/60 seconds. It goes without saying that the spot welding conditions are not limited to the exemplified conditions, and may be adjusted as appropriate according to the steel type, plate thickness, and the like.

  Examples of the method of forming the softened region 3 include tempering by high-frequency heating or heating using a laser beam. Heating with a laser beam is preferable. This is because the energy of the laser beam is stable. As the laser beam, a disk laser, a fiber laser, a direct diode laser, a YAG laser, and a carbon dioxide gas laser are used, the beam diameter is in the range of 5 to 25 mm, the output is in the range of 1 to 10 kW, and the welding speed is in the range of 1 to 20 m / min. Is exemplified. More preferably, it is desirable to apply a direct diode laser capable of rectangular condensing and having a rectangular energy distribution.

  According to tempering with a laser beam, since the laser irradiation is relatively short, the surface of the steel sheet is slightly oxidized, but it is usually not recognized that the spot weldability is lowered. However, an inert shielding gas such as argon, nitrogen, carbon dioxide, helium may be used as necessary. The irradiation conditions of the laser beam are not limited to the exemplified conditions, and may be irradiation conditions that can obtain the predetermined softened region described above.

  In the second step, the first constituent member 1 is overlapped with the second constituent member and joined to the second constituent member by forming a nugget 2 by spot welding.

  In the case where the second constituent member is a high strength steel plate of 1180 MPa class or higher, and the HAZ softened portion of the spot welded portion of the second constituent member is expected to break at the time of collision, the second constituent member In the same manner as described in the first step, the softened region 3 may be partially formed in advance.

  In the above description, the case where the number of stacked sheets is two is taken as an example. However, when the number of stacked sheets is three, that is, as shown in FIG. 1, for example, the second component member 7 and the third component member 11 In the case of the structure in which the first constituent member 1 is disposed between the third constituent members, the third constituent member may be further overlapped and spot-welded in the second step.

  Further, when both the first and second constituent members 1 and 7 are made of a high strength steel plate having a tensile strength of 1180 MPa or more and the third constituent member 11 is made of a steel plate having a tensile strength of, for example, 270 to 980 MPa. The first and second constituent members 1 and 7 are first overlapped and spot welded, and then the softened region 3 is formed on one or both of the first constituent member 1 and the second constituent member 7 ( It is determined whether the softened region 3 is formed in one or both according to necessity), and the third component member 11 may be overlapped with the first component member 1 and spot welded.

(2) Second Method of Forming Softened Region 3 After Spot Welding In the first step, the first constituent member 1 having a tensile strength of 1180 MPa or more is overlapped with the second constituent member 7 and intermittently by spot welding. Thus, the nugget 2 is formed to join the second component member. In the second step, the softened region 3 is formed on the nugget 2 spot-welded of a high strength steel plate having a tensile strength of 1180 MPa or more.

  The size, hardness, formation method, and the like of the softened region 3 are the same as those in the first manufacturing method in which spot welding is performed after forming the softened region (1).

  The lap welded structure according to the present invention described above is suitable for application to an automotive structural member including a high-strength steel plate having a tensile strength of 1180 MPa or more. For example, the present invention can be applied to B pillars, A pillars, roof rails, side sills, bumper reinforcements, door beams, floor members, front side members, rear side members, and the like.

  In the above description, the case where the welding is spot welding has been taken as an example, but the present invention is not limited to spot welding, and is equally applied to other welding other than spot welding such as laser welding and arc welding. The

  6 (a) to 9 (a) are all formed of a high-strength steel plate (1500 MPa class hot stamped steel plate) that constitutes a lap weld member together with the second component member 7 that is a molded product of a low-strength steel plate. It is explanatory drawing which shows notionally the relationship between the various welding parts 2-1 to 2-4 in the 1st structural member 1 which is goods, and the softening area | region 3, FIG.6 (b)-FIG.9 (b) It is a graph which shows the measurement result of the hardness of the 1st structural member 1 in the hardness measurement position C in sectional drawing in Fig.6 (a)-FIG.9 (a), respectively.

  FIG. 6A shows that a C-shaped laser welded portion 2-1 is formed on the flange 1a of the first component 1, and between the adjacent C-shaped laser welded portions 2-1, 2-1 respectively. Two softened regions 3 are formed in a straight line with the C-shaped laser welded part 2-1.

  FIG. 7A shows a ring-shaped laser welded portion 2-2 formed on the flange 1a of the first component 1, and a ring-like shape between the adjacent ring-shaped laser welded portions 2-2 and 2-2. Two softened regions 3 are formed in a straight line with the laser welded portion 2-2.

  FIG. 8A shows a circular laser welded portion 2-3 formed on the flange 1a of the first component 1, and a circular shape is formed between the adjacent circular laser welded portions 2-3 and 2-3. Two softened regions 3 are formed in a straight line with the laser weld 2-3.

  Furthermore, Fig.9 (a) forms the circular arc welding part 2-4 in the flange 1a of the 1st component 1, and each between the adjacent circular arc welding parts 2-4 and 2-4, Two softened regions 3 are formed in a straight line with the circular arc welded portion 2-4.

The present invention will be described more specifically with reference to examples.
Table 1 shows chemical components of the test materials. The units in Table 1 are mass%, and the balance other than those shown in Table 1 is Fe and irregular substances.

  The test material SQ1500 in Table 1 is a hot stamped steel plate with a tensile strength of 1500 MPa obtained by hot stamping a non-plated cold rolled steel plate, and the test material SQZ1500 is obtained by hot stamping an alloyed hot dip galvanized steel plate. The obtained tensile strength is a 1500 MPa class hot stamped steel plate, the test material SQ1800 is a hot stamped steel plate having a tensile strength of 1800 MPa obtained by hot stamping a non-plated cold-rolled steel plate, and the test material JSC1270 is a non-plated steel plate. It is a cold-rolled steel sheet having a tensile strength of 1270 MPa. Moreover, the specimen JSC440 in Table 1 is a non-plated 440 MPa grade steel plate.

  A laser beam was irradiated to form a substantially rectangular softened region, and then spot welded to obtain a test piece.

FIG. 10 is an explanatory diagram showing the shape of the test piece 8 used in this example.
The test piece 8 was tempered by irradiating a steel plate as the specimen 9 with a laser. For the laser irradiation, a direct laser was used, the beam size was 1.2 × 6 mm to 24 mm, the speed was 2.0 to 4.0 m / min, and the shielding gas was argon 20 l / min. In order to control the softening region 3, the output was changed in the range of 1.5 kW to 4 kW.

  Next, as shown in FIG. 10, the steel plate 10 as a test material was overlapped, and spot welding was performed at the overlapped portion to form spot welds 2 and 2. Spot welding uses a single-phase AC spot welder, DR type electrodes (tip diameter 6 mm, curvature radius 40 mm), applied pressure: 400 kgf, energization time 20 cyc, current value: current with nugget diameter 6.0 mm. Value. The Vickers hardness of the base material is SQ1500: 470, SQZ1500: 470, SQ1800: 560, SPC1270: 420. Moreover, the Vickers hardness of the softest part of HAZ of spot welding is SQ1500: 290, SQZ1500: 290, SQ1800: 390, and SPC1270: 280.

  Next, a tensile test was performed on each test piece 8. In the tensile test, the distance between ratings was 50 mm, and the tensile speed was constant at 3 mm / min. 2. When the strain until breakage (grading interval is 50 mm, the steel plate is cracked and the load suddenly drops on the chart) is less than 3%, x is 3% or more and less than 3.5%, and Δ. 5% or more and less than 4% was indicated by □, and 4% or more by ◯. The test results are shown in Table 2.

  As shown in Table 2, in the example of the present invention, the breaking strain is 4.0% or more, whereas in the comparative example, the HAZ softened part of spot welding breaks with a small strain, and the breaking strain is 3. It was less than 5%.

0 B pillar / lap weld member 00 roof rail / lap weld member 1 B pillar reinforcement / first component 1a flange 1b end face 1c bend R portion 2 spot weld / nugget (welded metal)
2-1 C-shaped laser weld (welded metal)
2-2 Ring-shaped laser weld (welded metal)
2-3 Circular laser weld (welded metal)
2-4 Circular arc welds (welded metal)
3 Softening region 4 Roof rail outer / first component member 4a Flange 5, 11 Side panel outer / third component member 6 A pillar 7 B pillar inner / second component member 8 Test pieces 9, 10 Test material 12 Roof rail inner / Second component 15 hinge reinforcement / fourth component

Claims (14)

At least a first structural member that is a steel plate having a tensile strength of 1180 MPa or more or a molded body thereof, and a second structural member that is another steel plate or a molded body thereof and is overlapped and welded to the first structural member. A lap weld member comprising:
The lap weld member has a weld metal formed by the welding, and a softened region provided at least on the first component side adjacent to the weld metal in the longitudinal direction of the lap weld member,
The lap weld member, wherein the softened region is provided so as not to be applied to a HAZ most softened portion formed around the weld metal. At least a first structural member that is a steel plate having a tensile strength of 1180 MPa or more or a molded body thereof, and a second structural member that is another steel plate or a molded body thereof and is overlapped and welded to the first structural member. A lap weld member comprising:
The lap weld member is at least a first constituent member between a weld metal formed in the longitudinal direction of the lap weld member by the welding and at least a pair of two adjacent weld metals among the plurality of weld metals. With a softened region provided on the side,
The lap weld member, wherein the softened region is provided so as not to be applied to the HAZ most softened portion formed around each of the two weld metals. At least a first structural member that is a steel plate having a tensile strength of 1180 MPa or more or a molded body thereof, and a second structural member that is another steel plate or a molded body thereof and is overlapped and welded to the first structural member. A lap weld member comprising:
The lap weld member is sandwiched between one or more weld metals formed by the welding and at least one of the weld metals adjacent to both sides in the longitudinal direction of the lap weld member. Two softening regions provided on the first component side,
The lap weld member, wherein the softened region is provided so as not to be applied to a HAZ most softened portion formed around the weld metal.   It is provided in any one of Claim 1 to 3 provided with the 3rd structural member which is the steel plate which overlapped with the said 1st structural member or the said 2nd structural member, and was welded, or its molded object. Lap welded parts. The Vickers hardness A (Hv) of the softened region is a case where the Vickers hardness of the steel sheet having the tensile strength of 1180 MPa or more is B (Hv) and the Vickers hardness of the HAZ most softened portion is C (Hv) The lap weld member according to any one of claims 1 to 4, which satisfies the following formulas (1) and (2):
A ≦ B-50 (1)
C−120 ≦ A ≦ C + 50 (2)   The softened region is formed 2.0 mm or more away from the end of the weld metal in the longitudinal direction, the length of the softened region in the longitudinal direction is 10 mm or more and 50 mm or less, and a direction orthogonal to the longitudinal direction The lap welding according to any one of claims 1 to 5, wherein a length of the softened region is 1 to 4 times a maximum length of the weld metal in a direction orthogonal to a longitudinal direction. Element.   The lap weld member according to any one of claims 1 to 6, wherein the softened region is formed by tempering with a laser beam or high-frequency heating.   An automobile comprising the lap weld member according to any one of claims 1 to 7 as a structural member. It is a method for manufacturing a lap weld member in which at least a first structural member that is a steel plate having a tensile strength of 1180 MPa or more or a molded body thereof and a second structural member that is another steel plate or a molded body thereof are overlapped and welded. And
A first step of partially providing at least one softened region in the first component;
The first component member formed with the softened region and the second component member are overlapped, welded at least at a site adjacent to the softened region in the longitudinal direction of the lap weld member, and formed by the welding. And a second step of forming the weld metal so that the HAZ softened portion around the weld metal does not cover the softened region.   10. In the second step, the first component member, the second component member, and a third component member that is another steel plate or a molded body thereof are overlapped and welded. The manufacturing method of the lap-welded member described in 1 .. It is a method for manufacturing a lap weld member in which at least a first structural member that is a steel plate having a tensile strength of 1180 MPa or more or a molded body thereof and a second structural member that is another steel plate or a molded body thereof are overlapped and welded. And
A first step of superposing and welding the first component member and the second component member;
The HAZ softest part formed around the weld metal at least on the first component side adjacent to the weld metal formed by the welding in the first step in the longitudinal direction of the lap weld member. And a second step of providing a softened region so as not to be applied to the lap weld member. The Vickers hardness A (Hv) of the softened region is a case where the Vickers hardness of the steel sheet having the tensile strength of 1180 MPa or more is B (Hv) and the Vickers hardness of the HAZ most softened portion is C (Hv) The manufacturing method of the lap weld member described in any one of Claim 9 to 11 which satisfies the following (1) Formula and (2) Formula.
A ≦ B-50 (1)
C−120 ≦ A ≦ C + 50 (2)   The softened region is provided 2.0 mm or more away from the end of the weld metal in each of the two welds in the longitudinal direction, and the length of the softened region in the longitudinal direction is 10 mm or more and 50 mm or less, The length of the softened region in the direction orthogonal to the longitudinal direction is 1 to 4 times the maximum length of the weld metal in the direction orthogonal to the longitudinal direction. The manufacturing method of the lap weld member described in the item.   The method for manufacturing a lap weld member according to any one of claims 9 to 13, wherein the softened region is provided by laser beam or tempering by high-frequency heating.