JP6191263B2 - Lap welded member and manufacturing method thereof - Google Patents

Description

  The present invention relates to a lap weld member in which two or more steel plates or molded bodies thereof are overlapped and welded, 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.

  The structural member for automobiles is constructed by welding a molded product of these steel plates with other molded products, generally 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”) is recognized even in a 980 MPa steel plate, a high-tensile steel plate having a martensite structure of 1180 MPa or higher, which has a martensitic structure particularly in a continuous annealing facility (WQ-CAL) having a water cooling function, It is remarkable in the HAZ of spot welds of stamped steel sheets. 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 locally concentrates on the HAZ softened portion and may 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 cylindrical 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. It is required to protect passengers in the cabin by plastic deformation without breaking. However, in severe collision mode, for example, in the case of the American Road Safety Insurance Association (IIHS) SUV side collision test, the B pillar reinforcement using high-strength steel plate is the HAZ softened part of the spot weld of the flange of the B pillar. As a result, the B pillar material 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, strain concentrates on the HAZ softened part of the spot welded part of the flange of the roof rail, which becomes the starting point of breakage and the roof rail breaks, and the amount of penetration of the B pillar into the cabin increases. , Target crash performance may not be achieved.

  For this reason, in the application of these high-strength steel plates of 1180 MPa or more to automobile bodies, it is required that the HAZ most softened portion of the spot welded portion formed on the flange of the automotive structural member does not become the starting point of fracture due to 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 that the high strength of the hot stamped part 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 possibility that the collision performance of the automotive structural member may vary. .

  As disclosed in Non-Patent Document 2, after hot stamping, the B pillar flange is softened by tempering and softening by high-frequency heating over a wide range, and the B pillar is deformed by thermal strain generated in a wide range of tempering. The dimensional accuracy of the pillar may be reduced. High dimensional accuracy is required for structural members for automobiles arranged around door openings such as A pillars and roof rails, as well as B pillars. 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. Moreover, there is a problem that the strength of the structural member for automobile is lowered by tempering the flange over a wide range, and the high strength characteristics of the hot stamp material cannot be enjoyed.

  Also, at the vehicle body design stage, design the structure of automotive structural members such as B pillars so that the HAZ softened part of the spot welded part of the flange of the B pillar does not reach the strain that breaks at the time of collision. It can be changed. However, in this method, it is necessary to add reinforcement to the automobile structural member, increase the thickness of the automotive structural member, etc., and increase the cost of the automobile body due to the increase in the number of parts and the thickness of the automobile body. The increase in weight is inevitable.

  In the above description, spot welding frequently used for assembling an automobile body is taken as an example. However, the situation is the same in welding other 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. Even when welding is performed on a molded product of a high strength steel plate having a tensile strength of 1180 MPa or more, the molded product is welded at the time of collision. An object of the present invention is to provide a lap weld structure of a high-strength steel sheet, a lap weld structure, and a structural member for an automobile that can prevent fracture at a HAZ softened portion of the part at low strain.

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 component members,
This lap weld is
One or more weld metals formed by welding along the longitudinal direction of the member;
In a part or all of the one or more weld metals, a range including the range in which the HAZ softened portion is formed among the ranges in which the HAZ softened portion formed around the weld metal is formed, and at least A softened region provided on the first component side,
The softened region is formed in a range of not more than three times the maximum dimension in the direction perpendicular to the longitudinal direction of the weld metal in a direction perpendicular to the longitudinal direction up to a position that is 4.0 mm or more away from the end of the weld metal in the longitudinal direction. And
The lap weld member, wherein the softened region is provided in a range excluding an end surface along a longitudinal direction of the first component member.

  (2) The lap weld member described in the item (1), comprising a third component member that is a steel plate or a molded body thereof overlapped and welded with the first component member or the second component member.

  (3) The Vickers hardness A (Hv) of the softened region is set to B (Hv) as the Vickers hardness of the first constituent member excluding the softened region and the HAZ softened portion, and the Vickers hardness of the HAZ most softened portion is set to C The lap weld member described in the item (1) or (2) that satisfies the following formulas (1) and (2) when (Hv) is set.

A ≦ C + 50 (1)
B-300 ≦ A (2)

( 4 ) The lap weld member according to any one of items (1) to ( 3 ), wherein the softened region is formed by a tempered structure.

( 5 ) Manufacture of a lap-welded member that welds a steel sheet having a tensile strength of 1180 MPa or more or a first structural member that is a molded body thereof and a second structural member that is another steel sheet or a molded body thereof. A method,
In the range where the HAZ softened portion is formed in the range in which the HAZ softened portion is formed when the weld metal is formed by welding the portion to be welded to the first component member, A first step of providing at least one softened region along the longitudinal direction in a range excluding an end surface along the longitudinal direction;
A method of manufacturing a lap weld member, comprising: a second step of superimposing a first component member and a second component member on which a softened region is formed and welding the softened region inside the softened region.

( 6 ) In the second step, the first component member, the second component member, and the third component member, which is another steel plate or a molded body thereof, are overlapped and welded ( 5 ). The manufacturing method of the lap weld member described in the item.

(7) The Vickers hardness A (Hv) of the softened region is the Vickers hardness B (Hv) of the first constituent member excluding the softened region and the HAZ softened portion, and the Vickers hardness of the HAZ most softened portion is C ( Hv), the method for producing a lap weld member according to the item (5) or (6), which satisfies the following formulas (1) and (2):

A ≦ C + 50 (1)
B-300 ≦ A (2)

( 8 ) The softened region is a range not more than three times the maximum dimension in the direction perpendicular to the longitudinal direction of the weld metal in the direction perpendicular to the longitudinal direction up to a position that is 4.0 mm or more away from the end of the weld metal in the longitudinal direction. The method for producing a lap weld member according to any one of items ( 5 ) to ( 7 ), which is formed in item (1).

( 9 ) The method for manufacturing a lap weld member according to any one of items ( 5 ) to ( 8 ), wherein the softened region is formed by tempering.

  According to the present invention, even when welding is used for assembling a molded product of a high-strength steel sheet having a tensile strength of 1180 MPa or more, the HAZ softened portion of the welded portion can be prevented from breaking at a low strain during a collision. Thereby, the structural member for motor vehicles which is excellent in the passenger | crew protection performance at the time of a collision, for example, and has high intensity | strength and high shape precision can be provided.

1 (a) and 1 (b) are explanatory views showing a situation in which the lap welding member according to the present invention is applied to a B pillar, and FIG. 1 (a) is a perspective view of the B pillar from the side panel outer side. FIG. 1B is a perspective view of the B pillar reinforcement, and FIG. 1B is a cross-sectional view 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) is explanatory drawing which shows typically an example of the vicinity of the flange of the 1st structural member joined to the 2nd structural member which is a molded object of a low strength steel plate, FIG.3 (b) And FIG.3 (c) is a graph which shows an example of the hardness distribution of the cross section in the vicinity of a spot weld part and a softening area | region. FIG. 4A is a graph showing the effect of the present invention. When a tensile load is applied to a tensile test piece 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. 4 (b) is an explanatory view showing a tensile test piece of a comparative example, and FIG. 4 (c) is an explanatory view showing a tensile test piece of an example of the present invention. Fig.5 (a)-FIG.5 (d) 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 C in the first structural member that is a molded product of a high-strength steel plate (1500 MPa class hot stamped steel plate) that constitutes a lap weld member together with a second structural member that is a molded product of a low-strength steel plate. FIG. 6B is an explanatory view conceptually showing the relationship between the letter-shaped laser welded portion and the softened region, and FIG. It is a graph which shows the measurement result of hardness. FIG. 7 (a) shows a ring in a first component member that is a molded product of a high strength steel plate (1500 MPa class hot stamped steel plate) that constitutes a lap weld member together with a second component member that is a molded product of a low strength steel plate. FIG. 7B is a diagram conceptually showing the relationship between the laser beam welded portion and the softened region, and FIG. It is a graph which shows the measurement result. FIG. 8A shows a circle in the first structural member that is a molded product of a high-strength steel plate (1500 MPa class hot stamped steel plate) that constitutes a lap weld member together with a second structural member that is a molded product of a low-strength steel plate. FIG. 8B is a diagram conceptually showing the relationship between the laser beam welded portion and the softened region, and FIG. It is a graph which shows the measurement result. FIG. 9A shows a circle in the first structural member that is a molded product of a high-strength steel plate (1500 MPa class hot stamped steel plate) that constitutes a lap weld member together with a second structural member that is a molded product of a low-strength steel plate. FIG. 9B is a diagram conceptually showing the relationship between the arc-shaped arc welded portion and the softened region, and FIG. It is a graph which shows the measurement result. 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. 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. In the following description, the case where welding is spot welding is taken as an example.

1. Lap welding member 1
First, the lap weld member 1 according to the present invention will be sequentially described with reference to the example of the B pillar shown in FIGS. 1 (a) and 1 (b).

  1 (a) and 1 (b) show a situation in which the lap welding member 1 according to the present invention is applied to the B pillar 1, and FIG. 1 (a) is a perspective view of the B pillar 1 from the side panel outer 3 side. FIG. 1B is a perspective view of the B pillar reinforcement 2, and FIG. 1B is a cross-sectional view of the B pillar 1.

  The lap weld member 1 of the present invention includes at least a first component member 2 and a second component member 3. The number of steel plates constituting the lap weld member 1 is two or three. In the case of stacking three sheets, in addition to the first component member 2 and the second component member 3, a third component member 4 is further provided which is overlapped and welded.

  The first constituent member 2 is a steel plate having a tensile strength of 1180 MPa or more or a molded body thereof (hereinafter referred to as “steel plate” including the case of the molded body) and is a 1500 MPa class hot stamped steel plate. Is done. The steel plate constituting the first component 2 is a high-strength steel plate having a quenching structure including 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.

  When the lap welding member 1 according to the present invention is applied to the B pillar 1, as shown in FIG. 1B, the second component member 3 is a B pillar inner panel and is a 440 MPa class steel plate. And the third component member 4 is a side panel outer, and is exemplified as a 270 MPa grade steel plate.

  In the case of the two-layer structure, the second component member 3 and the first component member 2 are overlapped and spot-welded. As shown in FIG. 1B, in the case of a three-layer structure, the second component member 3 and the third component member 4 are superimposed on the first component member 2 and spot welded.

  In the B pillar 1 shown in FIG. 1B, a hinge reinforcement as a fourth constituent member 5 is partially overlapped inside the B pillar reinforcement 2 which is the first constituent member, and is mutually attached. Two sheets are laminated and welded at the bottom of the groove. The fourth component member 5 is exemplified by a 980 MPa grade steel plate.

  In this way, the lap welding member 1 includes the first component member 2 and the second component member 3 partially overlapped (in the case of the three-layer structure, the third structure member 4 is partially overlapped), and the overlap portion Are assembled by spot welding.

  In the lap weld member 1, the first component 2 includes one or more nuggets 6 formed by spot welding and a softened region 7 formed so as to surround all or part of the nuggets 6. . The softened region 7 surrounds the nugget 6 and the HAZ softened portion formed around the nugget 6 and is provided in a range excluding the end surface 2 b along the longitudinal direction of the first component 2.

  The fracture of the nugget 6 that is the spot welded portion tends to occur more easily as the first structural member 2 that mainly bears the strength of the B pillar that is the lap weld member 1 is a steel plate having a high tensile strength and a high proportion of martensite structure. It is in. Therefore, the present invention exhibits a particularly great effect when the first component member 2 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 2, It is desirable to set it as 2000 MPa.

  The thickness range of the steel plates constituting the first component member 2, the second component member 3 and the third component member 4 is also about 0.6 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 the B pillar 1, as described above, the 270 MPa class galvannealed steel sheet having a thickness of 0.7 mm as the side panel outer (third constituent member), and the B pillar reinforcement (1st component member) 1500 MPa class hot stamped steel plate with a plate thickness of 1.8 mm or 1800 MPa class hot stamped steel plate with a plate thickness of 1.6 mm, and B pillar inner (second component member 3) with a plate thickness of 1.2 mm A three-ply structure with a 440 MPa class cold-rolled steel sheet is exemplified.

  Further, 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 roof rail outer reinforcement, The roof rail inner is exemplified by a three-ply structure with a 590 MPa grade cold-rolled steel plate having a thickness of 1.4 mm.

  Further, 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 as a reinforcement inner (second component). A two-ply structure with a 590 MPa grade cold-rolled steel sheet having a thickness of 1.4 mm is exemplified.

  In these examples, only the first constituent member 2 was a high-strength steel plate of 1180 MPa class or higher, but the second constituent member 3 or the third constituent member 4 is a high-strength steel plate of 1180 MPa class or higher. Good. In the case where the second constituent member 3 to be superimposed is a high-strength steel plate of 1180 MPa class or higher, in the case where a fracture at the HAZ softened portion of the spot welded portion of the second constituent member 3 to be superimposed at the time of collision is expected, The softened region 7 may be partially formed on the second component 3 as well. On the other hand, in the case where it is predicted that no fracture will occur at the HAZ most softened portion of the spot welding of the second component member 3 at the time of collision, it is not necessary to form the softened region 7 for the second component member 3.

  The surface of the steel plate of the first component 2 may be non-plated or plated. The galvanized steel sheets used are electrogalvanized steel sheets, electrogalvanized alloy-plated steel sheets (for example, Zn-Ni alloy electroplated steel sheets), hot-dip galvanized steel sheets, alloyed hot-dip galvanized steel sheets, hot-dip aluminum-plated steel sheets, hot-dip zinc-aluminum. Examples include alloy-plated steel sheets. Further, these may be hot stamped steel plates.

  The nugget diameter of the nugget 6 formed at the interface between the first component 2 and the second component 3 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)). The three-layer structure is the same at the interface between the first component member 2 and the third component member 4.

  The Vickers hardness A (Hv) of the softened region 7 in the lap weld member 1 is set to B (Hv) as the Vickers hardness of the base material of the first component member 2 and to the Vickers hardness of the HAZ most softened portion C ( Hv), it is desirable to satisfy (1) Formula: A ≦ C + 50 and (2) Formula: B−300 ≦ A. That is, it is desirable that A ≦ C + 50 in order to reliably suppress the strain concentration on the HAZ softest part of the spot weld 2. Further, in order to prevent a decrease in strength of the lap weld member due to a decrease in the hardness of the base material of the first component member 2, it is desirable that B−300 ≦ A.

  The softened region 7 in the lap weld member 1 is desirably up to a position away from the end of the nugget 6 by 4.0 mm or more in the longitudinal direction of the first component 2 with respect to the target nugget 6 and orthogonal to the longitudinal direction. It is provided in a range not more than three times the diameter of the nugget 6 with respect to the direction to be performed. By sufficiently taking the length in the longitudinal direction of the softened region 7, it is possible to prevent concentration of strain on the HAZ most softened portion. The upper limit of the length of the softened region 7 is not particularly desirable. However, in order to reduce the influence of thermal strain during softening and the decrease in strength as a structural member for automobiles, for example, 50 mm or less from the end of the spot weld 6. It is desirable to be. The same applies to the upper limit of the width of the softened region 7, and it is desirable that it is not more than three times the diameter of the nugget 6.

  The diameter of the nugget 6 is the value of the diameter of the melt-solidified portion at the interface where the two steel plates are stacked. In addition, in the case of three or more overlapping, the value of the melt-solidified portion on the larger diameter side of the melt-solidified portion at the overlap interface with the steel plate of the first component member 2 is defined as the nugget diameter.

  Further, as described above, the welding is not necessarily spot welding. Including such a case, the softened region desirably has a weld metal (melt-solidified) in a direction perpendicular to the longitudinal direction up to a position that is 4.0 mm or more away from the end of the weld metal (melt-solidified portion) in the longitudinal direction. Part) is not more than three times the maximum dimension.

  The softened region 7 is desirably formed by tempering with a laser beam or high frequency heating.

  Next, as a specific example of the application, with reference to FIG. 2 (a) and FIG. 2 (b) as well as FIG. 1 (a) and FIG. Further explanation will be given.

  As shown in FIGS. 1 (a) and 1 (b), in the B pillar 1, a B pillar reinforcement 2 as a first constituent member, a B pillar inner 3 as a second constituent member, The side panel outer 4 that is the constituent member 3 is overlapped, and a plurality of spot welds 6 are formed in the longitudinal direction in the flange portion (the flange 2a in the B pillar reinforcement 2). In this example, a softened region 7 is formed at the upper part of the B pillar reinforcement 2 so as to surround the spot welded portion 6, and the softened region 7 is similarly formed at the lower part of the B pillar reinforcement 2 so as to surround the spot welded portion 6. In contrast, the softened region 7 corresponding to the spot welded portion 6 is not formed in the intermediate portion in the height direction of the B-pillar force 2.

  The formation of the upper softened region 7 prevents breakage due to breakage at the HAZ most softened portion of the spot welded portion 6 at the upper portion of the B pillar 1 at the time of collision, and the lower softened region 7 is formed by large deformation due to the collision. This prevents the spot welded portion 6 in the region from being broken at the HAZ most softened portion. The softened region 7 is not formed in the central portion in the height direction of the flange 2a of the B pillar reinforcement 2 where the HAZ softest portion does not reach the breaking strain. As described above, the formation of the softened region 7 may be the minimum necessary to prevent breakage starting from the HAZ most softened portion of the spot welded portion 6 at the time of collision.

  2 (a) and 2 (b) are explanatory views showing a situation where the lap welding member according to the present invention is applied to the roof rail 9, and FIG. 2 (a) shows the roof rail 9 on the roof rail inner 9-1 side. FIG. 2 is a perspective view of the roof rail outer force 8 seen through, and FIG. 2 (b) is a cross-sectional view of the roof rail 9.

  In the roof rail 9, the side panel outer 10 that is the third component member, the roof rail outer force 8 that is the first component member, and the roof rail inner 9-1 that is the second component member are overlapped to each other. A plurality of spot welds 6 are formed in the longitudinal direction in the flange portion (8a in the roof rail outer ring force 8). A local softened region 7 is formed in a part of the spot welded portion 6 of the roof rail outer ring force 8. Thereby, the fracture | rupture in the HAZ softening part of spot welding can be suppressed, without reducing the intensity | strength of the roof rail outer ring force 8 as much as possible. In addition, the code | symbol 11 in Fig.2 (a) shows A pillar.

Next, the technical significance of the softened region 7 in the present invention will be described.
FIG. 3A is an explanatory view schematically showing an example of the vicinity of the flange 2a of the first component member 2 joined to the second component member 3 which is a formed body of a low-strength steel plate. FIG. 3B and FIG. 3C are graphs showing an example of the hardness distribution of the cross section in the vicinity of the spot weld 6 and the softened region 7. In FIG. 3A, reference numeral 2b indicates an end face of the flange 2a, reference numeral 2c indicates a bent portion of the first component 2, and reference numeral L indicates the length of the softened region 7 in the longitudinal direction of the flange 2a. The symbol W is the length (width) of the softened region 7 in the direction orthogonal to the longitudinal direction of the flange 2a. Moreover, the hardness in FIG.3 (b) and FIG.3 (c) is the measurement in the hardness measurement position (The board | substrate thickness direction center position of the 1st structural member 2) C shown with a broken line in Fig.3 (a). Value. In addition, the measurement result shown as "independent" in the graph of FIG. 3B and FIG. The measurement result of the hardness of the 1st structural member 2 is shown.

  As shown in FIG. 3 (a), the first constituent member 2 which is a molded product of a 1500 MPa class cold-rolled steel sheet is overlapped with the second constituent member 3 which is a molded product of a 440 MPa class steel plate via a flange 2a. Then, spot welds 6 are intermittently formed in the extending direction of the flange 2a (FIG. 3A shows one spot weld 6), and the spot welds 6 of the flange 2a of the first component 2 are formed. A softened region 7 is provided so as to surround

  As shown in the graph of FIG. 3B, the hardness distribution in the vicinity of the welded portion 6 may have a peak of the HAZ most softened portion around the nugget 6, or the graph of FIG. As shown, the peak of the HAZ most softened portion 12 may not be recognized. If the hardness of the softened region 7 is sufficiently small, for example, if it is smaller than the hardness of the HAZ most softened portion 12 when spot welding is performed without providing the softened region 7, the HAZ maximum is obtained as shown in FIG. It is considered that the peak of the softened portion 12 is not clearly recognized.

  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-elongation diagram when a tensile load is applied to the test piece, FIG. 4B is an explanatory view showing a tensile test piece 13 of a comparative example, and FIG. It is explanatory drawing which shows the test piece.

  4 (c), that is, the first constituent member 2 which is a 1500 MPa class steel plate is overlapped with the second constituent member 3 which is a 440 MPa class steel plate, and the spot weld 6 (nugget diameter: 5). -6 mm (approximately 4√t)), and a tensile test piece 14 in which a softened region 7 (width: 10 mm, length: 15 mm) is provided in the first component 2 so as to surround the spot weld 6. Then, as shown in the graph of FIG. 4A, as compared with the tensile test piece 13 having no softening region as shown in FIG. The absorbed energy increased.

  This is because, when a tensile load is applied to the tensile test pieces (that is, the laminated structural members) 13 and 14, in the tensile test piece 13 in which the softening region 7 is not provided, the strain is locally concentrated in the HAZ most softened portion. On the other hand, since the tensile test piece 14 has the softened region 7 having a spread, local strain concentration is suppressed, and the deformation amount until the tensile test piece 14 breaks is improved.

  In addition, according to the present invention, the deformation of the component due to thermal strain is extremely small as compared with the conventional method of tempering the flange in a wide range and the method of forming the softened region in the wide range of the lap weld member. Therefore, it is particularly 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 7 is not formed in a range along the longitudinal end face of the first component member 2, the strength deterioration of the lap weld member is small, and the material characteristics of a high strength material such as a hot stamp material are sufficient. You can make use of it.

  FIGS. 5A to 5D are explanatory views conceptually showing the relationship between the spot welded portion 6 and the softened region 7 in the first component member 2.

  FIG. 5A shows a rectangular softened region 7 partially formed in a range including the spot weld 6 of the flange 2a of the high-strength steel plate 2 and excluding the end face 2b.

  FIG. 5 (b) shows a structure in which the softened region 7 is partially formed on the bent R portion 2c side in the range including the spot welded portion 6 of the flange 2a of the high-strength steel plate 2 and excluding the end face 2b. .

FIG. 5 (c) shows an elliptical softened region 7 partially formed in the range including the spot weld 6 of the flange 2a of the high-strength steel plate 2 and excluding the end face 2b. (D) includes a spot-welded portion 6 of the flange 2a of the high-strength steel plate 2 and a rectangular softened region 7 formed in a range excluding the end surface 2b, and adjacent softened regions 7 are connected to each other. Is.

  In this way, as illustrated in FIGS. 1A, 1B, 2A, and 2B, the B-pillar is an important member that protects the passenger in the cabin by a side collision. By applying the present invention to structural members for automobiles, such as roof rails and side sills, which are arranged surrounding the cabin, it is possible to suppress breakage at the HAZ softest part of the spot welds 6 of these structural members for automobiles, Safety against side collision can be improved. Further, by applying the present invention to the bumper reinforcement, door beam, floor member, front side member, and rear side member, the strength characteristics of these structural members for automobiles can be enhanced.

  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.

2. Next, a method of manufacturing the lap weld member 1 in the present invention will be described. 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 6 and softened regions 7 are formed in the longitudinal direction of the lap weld member 1 as shown in FIG.

  In the method for producing the lap weld member 1 according to the present invention, in the first step, the softened region 7 is partially formed in a range that does not reach the end face in the longitudinal direction of the first component member 2 having a tensile strength of 1180 MPa or more. In the second step, the nugget 6 is formed by spot welding inside the softened region 7 formed.

  First, in the first step, for example, a substantially rectangular softened region 7 is formed at a necessary portion of the first component 2 where the nugget 6 is planned to be formed by spot welding. That is, before the spot welding, the softening region 7 is formed at the planned position of the spot welding. At this time, the partial softening region 7 is formed in a range including the range in which the HAZ softened portion is formed when spot welding is performed and excluding the end surface 2b of the high-strength steel plate 2, and the second step. Thus, spot welding is performed inside the softened region 7.

  That is, the softened region 7 is around the portion where the nugget 6 is planned to be formed, and the HAZ softened portion is within the range in which the HAZ softened portion is formed when spot welding is performed at this position of the high-strength steel plate. Is formed in a range including the range in which the first component member 2 is removed except for the end face in the longitudinal direction.

The desirable range of the size, shape or hardness of the softened region 7 is as described above.
There are no particular restrictions on the welding conditions for spot welding, and the nugget diameter is at least 4√t to 7√t at the overlap interface of the target steel plate 2 (t: plate thickness (mm) on the thin side of the overlap surface). What is necessary is just to determine suitably so that the following nugget 6 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 appropriately adjusted according to the steel type, plate thickness, and the like.

  Examples of the method for forming the softened region 7 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 plate 2 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 7 described above.

  In the second step, the first constituent member 2 is overlapped with the second constituent member 3 and joined to the second constituent member 3 by forming a nugget 6 by spot welding.

  In the case where the second constituent member 3 is a high-strength steel plate of 1180 MPa class or higher, or the like, in the case where a fracture at the HAZ most softened portion of the spot welded portion 6 of the second constituent member 3 is expected at the time of collision, For the component member 3 as well, the softened region 7 may be formed in advance in the same manner as described in the first step.

  In the above description, the case where the number of stacked sheets is two has been taken as an example, but when the number of stacked sheets is three, that is, as shown in FIG. 1, for example, the second component member 3 and the third component member 4 In the case of a structure in which the first constituent member 3 is disposed between the two, the third constituent member 4 may be further overlapped and spot-welded in the second step.

  Further, when both the first and second constituent members 2 and 3 are made of a high strength steel plate having a tensile strength of 1180 MPa or more and the third constituent member 4 is made of a steel plate having a tensile strength of, for example, 270 to 980 MPa. The first and second constituent members 2 and 3 are first overlapped and spot welded, and then the softened region 7 is formed on one or both of the first constituent member 2 and the second constituent member 3 ( It is determined whether the softened region 7 is formed in one or both according to necessity), and the third component 4 may be overlapped with the first component 2 and spot-welded.

  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 constituent member 3 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 6-1 to 6-4 in the 1st structural member 2 which is goods, and the softening area | region 7, FIG.6 (b)-FIG.9 (b) It is a graph which shows the measurement result of the hardness of the 1st structural member 2 in the hardness measurement position C in sectional drawing in Fig.6 (a)-FIG.9 (a), respectively. In addition, the measurement result shown as "single" in the graph of FIG.6 (b)-FIG.9 (b) is the case where only each laser welding part 6-1 to 6-4 is formed, and the softening area | region 7 is not formed. The measurement result of the hardness of the 1st structural member 2 in the hardness measurement position C about is shown.

  FIG. 6A shows a structure in which a C-shaped laser welded portion 6-1 is formed on the flange 2a of the first component 2, and a softened region 7 is formed surrounding the C-shaped laser welded portion 6-1. It is.

  In FIG. 7A, a ring-shaped laser welded portion 6-2 is formed on the flange 2a of the second component member 2, and a softened region 7 is formed surrounding the ring-shaped laser welded portion 6-2. .

  FIG. 8A shows a case where a circular laser welded portion 6-3 is formed on the flange 2a of the second component member 2, and a softened region 7 is formed surrounding the circular laser welded portion 6-3. .

  Further, FIG. 9A shows a case where a circular arc welded portion 6-4 is formed on the flange 2a of the second component member 2, and a softened region 7 is formed surrounding the circular arc welded portion 6-4. It is.

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

  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 15 used in this example.
The test piece 15 formed the softened region 7 by irradiating the steel plate as the test material 16 with laser. For the laser irradiation, a direct diode laser was used, the beam size was 1.2 × 4 mm to 15 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, steel plates 17 as test materials were overlapped, and spot welding was performed inside the softened region 7 to form the spot welded portion 6. 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 softest part of the spot welded HAZ is SQ1500: 290, SQZ1500: 290, SQ1800: 390, and SPC1270: 280.

  Next, a tensile test was performed on each test piece 15. 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 fracture strain is 3.5% or more, whereas in the comparative example, the HAZ softened portion of spot welding breaks with a small strain, and the fracture strain is 3.5. %.

1 Lap welded member (B pillar) according to the present invention
2 B pillar reinforcement (first component)
2a Flange 2b Flange end face 2c End face 3 Side panel outer (second component)
4 B pillar inner panel (third component)
5 Hinge reinforcement (fourth component)
6 Nuggets (welded metal, spot welds)
6-1 C-shaped laser welded portion 6-2 Ring-shaped laser welded portion 6-3 Circular laser welded portion 6-4 Circular arc welded portion 7 Softening region 8 Roof rail outer force (first component)
8a Flange 9 Roof rail 9-1 Roof rail inner (second component)
10 Side panel outer (third component)
DESCRIPTION OF SYMBOLS 11 A pillar 12 HAZ softening part 13 Tensile test piece 14 of comparative example 15 Tensile test piece 15 of this invention example Test piece 16 and 17 Test material

Claims (9)

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
One or more weld metals formed along the longitudinal direction of the member by the welding;
In a part or all of the one or more weld metals, the range includes the range in which the HAZ softened portion is formed in the range in which the HAZ softened portion formed around the weld metal is formed. And at least a softened region provided on the first component side,
The softened region is not more than three times the maximum dimension in the direction perpendicular to the longitudinal direction of the weld metal in a direction perpendicular to the longitudinal direction up to a position that is 4.0 mm or more away from the end of the weld metal in the longitudinal direction. Is formed in the range of
The lap weld member, wherein the softened region is provided in a range excluding an end surface along a longitudinal direction of the first component member.   The lap weld member according to claim 1, further comprising a third constituent member that is a steel plate or a molded body thereof that is overlapped and welded to the first constituent member or the second constituent member. The Vickers hardness A (Hv) of the softened region is set to B (Hv) as the Vickers hardness of the first constituent member excluding the softened region and the HAZ softened portion, and the Vickers hardness of the HAZ most softened portion is used. The lap-welded member according to claim 1 or 2, which satisfies the following formulas (1) and (2) when C is H (C).
A ≦ C + 50 (1)
B-300 ≦ A (2)   The lap weld member according to any one of claims 1 to 3, wherein the softened region is formed of a tempered structure. 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
In the range including the range in which the HAZ softened portion is formed in the range in which the HAZ softened portion is formed when the weld metal is formed by performing welding on the portion to be welded on the first component member. A first step of providing at least one softened region along the longitudinal direction in a range excluding an end surface along the longitudinal direction;
A lap weld member, comprising: a second step of superimposing the first component member formed with the softened region and the second component member and welding the softened region inside the softened region. Method.   6. 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. The manufacturing method of the lap-welded member described in 1 .. The Vickers hardness A (Hv) of the softened region is the Vickers hardness B (Hv) of the first component excluding the softened region and the HAZ softened portion, and the Vickers hardness of the HAZ most softened portion is The manufacturing method of the lap-welded member described in Claim 5 or Claim 6 which satisfies following (1) Formula and (2) Formula, when it is set to C (Hv).
A ≦ C + 50 (1)
B-300 ≦ A (2)   The softened region is not more than three times the maximum dimension in the direction perpendicular to the longitudinal direction of the weld metal in a direction perpendicular to the longitudinal direction up to a position that is 4.0 mm or more away from the end of the weld metal in the longitudinal direction. The method for manufacturing a lap weld member according to any one of claims 5 to 7, wherein the lap weld member is formed in a range of.   The method for manufacturing a lap weld member according to any one of claims 5 to 8, wherein the softened region is formed by tempering.