JP2016032834A - Lap weld member, lap resistance seam-welding method for the same, and lap weld member for automobile comprising lap weld part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lap weld member which can suppress breaking due to a low strain around a weld part at a lap portion when steel plate members including one or more high-strength steel plate members each including a martensitic structure are welded, and also to provide a lap resistance seam-welding method for the lap weld member and an automotive component comprising the lap weld member.SOLUTION: There is provided a lap weld member comprising a lap resistance seam-welding part. The lap weld member is formed by lapping and welding a plurality of steel plate members including one or more high-strength steel plate members each having a martensitic structure and having a tensile strength of 1,180 MPa or more. In the lap resistance seam-welding part of the lap weld member, nuggets formed by lap resistance seam-welding are present intermittently within the lapped steel plate members, and when the plate thickness of the high-strength steel plate members is defined as t (mm), the size N (mm) of the nugget in a direction in which the nuggets are formed intermittently lies in the range of 2√t≤N≤12√t, and a difference between a maximum value and a minimum value of Vickers hardness of adjacent nuggets among the nuggets is Hv70 or less.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a lap weld member in which a plurality of steel plate members are overlapped and joined, a lap resistance seam welding method, and a lap weld member for an automobile including a lap weld.

  Up to now, in a structure composed of a plurality of steel plate members formed from steel plates, depending on the function and usage environment, the nugget is applied by resistance spot welding to the overlapped portion where the steel plate members are overlapped with each other. It has been widely performed to form a lap welded member in which a plurality of steel plate members are joined by forming a spot welded portion having the following. For example, in an automobile body having a monocoque structure, in order to improve collision safety and fuel consumption, it is generally performed that steel plate members including high-strength steel plates are overlapped and a flange (overlap portion) is resistance spot welded. I came.

  On the other hand, as a constituent material of an automobile body, in recent years, application of high-strength steel plates has progressed, and cold-pressed products of high-strength steel plates having a tensile strength of 1180 MPa class or higher and hot-pressed products of hot stamped steel plates of 1500 MPa class are also used. It has become like this.

  Such a high-strength steel sheet has a hardened martensite-based metal structure. For this reason, when a welded portion is formed by spot welding, tempered martensite in which martensite is tempered is generated around the welded portion, and the hardness is reduced. For example, in the case of a 1500 MPa class hot stamp material, the hardness of the base material is about Hv460, but the hardness of the heat-affected zone (hereinafter referred to as “HAZ”) around the weld is locally Hv300. Decrease to a degree.

  Such a HAZ softened portion around the spot welded portion may become a starting point of breakage of the steel plate member constituting the vehicle body when the vehicle collides. For example, in a center pillar equipped with a 1500 MPa class hot stamp material with a spot weld on the flange, a crack enters the HAZ softened part around the flange spot weld in the side impact test. The pillar may break.

  Such a fracture phenomenon starting from the HAZ softened portion by spot welding occurs in a high-strength steel sheet having a tensile strength exceeding 1180 MPa, and in particular, a tensile strength of 1180 MPa or more in which a quenching structure is formed in a continuous annealing facility having a water cooling function. This phenomenon occurs remarkably in spot welds of high strength steel plates and hot stamping materials (high strength steel plate members) formed by hot pressing.

  As a countermeasure, Non-Patent Document 1 discloses resistance spot welding by reducing the strength of a base material by a heat treatment performed at the time of hot stamping on a portion that is likely to break due to collision in an A pillar formed by hot stamping. Even if it is carried out, a method for preventing breakage of a structural member starting from the HAZ softened portion without causing HAZ softening is disclosed.

  Non-Patent Document 2 discloses that the strength of the base material is reduced by tempering the B pillar formed by hot stamping by high-frequency heating, and HAZ softening does not occur even when resistance spot welding is performed. A method for preventing breakage of a structural member starting from the above is disclosed.

  Furthermore, in Patent Document 1, high-strength steel plates are spot-welded, and then irradiated with laser across the spot-welded part, thereby dividing the HAZ softened part around the spot-welded part and reducing the concentration of strain. Thus, a technique for suppressing breakage is disclosed. Non-Patent Document 3 discloses a technique for applying resistance seam welding to an automobile body. Patent Document 2 discloses a technique for preventing an internal defect of a seam weld and improving welding quality.

International Publication No. 2014/024997 Japanese Patent Application Laid-Open No. 11-073326

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 http://publications.lib.chalmers.se/records/fulltext/144308.pdf http://www.mitsubishi-motors.com/jp/corporate/pressrelease/products/detail1761.html

  However, the method disclosed in Non-Patent Document 1 for adjusting the strength for each portion of the A pillar inevitably forms a low strength portion in a relatively wide range of the A pillar. For this reason, the effect of the hot stamp that high strength can be obtained cannot be fully enjoyed, and the effect of weight reduction is also limited. In addition, in this method, the strength characteristics are likely to vary in a relatively wide transition region inevitably formed between the quenched region and the unquenched region, and there is a possibility that the collision performance of the A pillar varies.

  Further, the method disclosed in Non-Patent Document 2 for tempering the flange of the B pillar by high-frequency heating after hot stamping may cause the B pillar to be deformed by thermal strain generated by the high-frequency heating, resulting in a decrease in dimensional accuracy. is there. Not only the B-pillar but also the structural members arranged around the door opening, such as the A-pillar and roof rail, for example, it is necessary to ensure the installation accuracy so that the gap with the door panel is uniform over the entire circumference of the door panel. is there. For this reason, when the technique disclosed in Non-Patent Document 2 is applied to a structural member around the door opening, the dimensional accuracy of the structural member arranged around the door opening is lowered, and the appearance quality of the vehicle cannot be ensured.

  On the other hand, in the vehicle body design stage, it is conceivable to design a structural member such as a B pillar so that the HAZ softened portion of the spot welded portion of the flange at the time of collision does not reach fracture strain. However, it is difficult to perform such a design because an increase in the thickness of the steel plate constituting the structural member and the addition of reinforcement occur, thereby increasing the cost and weight of the automobile body.

  Since the technique disclosed in Patent Document 1 needs to sequentially perform a spot welding process and a laser welding process, there is a problem of an increase in cost due to an increase in the number of processes.

  Non-Patent Document 3 discloses a technique for applying resistance seam welding to an automobile body, but the aim of seam welding is to improve the rigidity of the vehicle. Not used for seam welds. And the said technique is not aimed at the effect which prevents the fracture | rupture by the HAZ softening of the welding part of a high strength steel plate. Moreover, in the figure of nonpatent literature 3, the nugget which is the melt solidification part of a seam weld part is connected, and in such a seam weld part, it becomes easy to generate a welding defect and it will become easy to fracture within a nugget.

  Patent Document 2 discloses a seam welding technique in which a high current and a low current are energized repeatedly in order to prevent internal defects (blow holes) in seam welds in an aluminum alloy plated steel sheet and improve welding quality. ing. The technique disclosed in Patent Document 2 is intended for a fuel tank that requires confidentiality, and is premised on seam welding so that nuggets are continuously connected. In this method, large nuggets and small nuggets are alternately generated to prevent both blowholes and electrode contamination. However, in the case of a high-strength steel plate, as in the technique disclosed in Patent Document 2, in a welded portion where nuggets are continuously connected, defects such as cracks are likely to occur inside the nugget, and breakage is likely to occur within the nugget.

  The present invention has been made in view of such a problem. When a steel plate member including one or more high-strength steel plate members including a martensite structure is joined by welding, around the welded portion of the overlapped portion. An object of the present invention is to provide a lap weld member capable of suppressing breakage with low strain, a lap resistance seam welding method thereof, and a lap weld member for an automobile including a lap weld portion.

  The inventors of the present invention diligently studied a method for welding overlapping portions in a lap weld member to which steel plate members formed from a plurality of steel plates are joined. As a result, instead of resistance spot welding, resistance seam welding is used to form intermittent melted and solidified portions (hereinafter referred to as “nuggets”), and between the intermittent nuggets to form a quenched structure. As a result, it was found that a weld with high strength reliability can be obtained.

The gist of the present invention is as follows.
(1) A lap weld member in which a plurality of steel plate members including one or more high strength steel plate members having a martensite structure and having a tensile strength of 1180 MPa or more are overlapped and joined,
The nugget formed by lap resistance seam welding is intermittently present inside the superposed steel plate member, and the nugget size N (mm) in the direction in which the nugget is intermittently formed is the high strength. When the thickness of the steel plate member is t (mm), 2√t ≦ N ≦ 12√t, and the difference between the maximum value and the minimum value of Vickers hardness between adjacent nuggets among the nuggets is Hv70 or less. A lap weld member comprising a lap resistance seam weld.
(2) The lap weld member according to (1), wherein the high-strength steel plate member is a member subjected to hot stamping.
(3) A lap weld member for automobiles, comprising the lap weld portion according to (1) or (2).
(4) A lap resistance seam welding method in which a plurality of steel plate members including one or more high-strength steel plate members having a martensite structure and a tensile strength of 1180 MPa or more are overlapped and joined,
A high current and a low current (including 0) are alternately and repeatedly energized to form the nugget intermittently, and the size N (mm) of the nugget in the direction in which the nugget is intermittently formed is Assuming that the thickness of the high-strength steel plate member is t (mm), 2√t ≦ N ≦ 12√t, and the highest temperature reached between the nuggets is equal to or higher than the Ac3 point of the high-strength steel plate. And the difference between the maximum value and the minimum value of Vickers hardness between adjacent nuggets among the nuggets is set to Hv 70 or less.
(5) The lap resistance seam welding method according to (4), wherein the high-strength steel plate member is a member subjected to hot stamping.
(6) The lap resistance seam welding method, wherein the steel sheet according to (4) or (5) is an automobile member.

  According to the present invention, in a structure in which a plurality of steel plate members including one or more high-strength steel plate members having a martensite structure are overlapped and welded, the welded portion of the overlapped portion is prevented from breaking at low strain. it can. Therefore, it is possible to provide a high-strength lap-welded member for automobiles excellent in passenger protection performance at the time of collision.

It is a figure which shows the example which distortion concentrated on the HAZ softening part around the spot welding formed in the flange, and fractured. It is a figure which shows a tensile test piece. (A) is a tensile test piece A in which three steel plates are joined by spot welding, (b) is a tensile test piece B in which nuggets are intermittently joined by seam welding to intermittently energize and three steel plates are joined, ( c) is a diagram showing a tensile test piece C in which three steel plates are joined by intermittently nugget welding by seam welding in which a high current and a low current are alternately repeated. It is a figure which shows a hardness investigation position and hardness distribution. (A) is the hardness investigation position and hardness distribution of the tensile specimen A, (b) is the hardness investigation position and hardness distribution of the tensile specimen B, and (c) is the hardness investigation position of the tensile specimen C. It is a figure which shows hardness distribution. It is a figure which shows the fracture | ruptured tensile test piece. It is a load-elongation line figure in a tension test. It is a figure which shows the condition which applied this invention to the center pillar. It is a figure which shows the condition which applied this invention to the side sill. It is a figure which shows the condition which applied this invention to the roof rail from A pillar. It is a figure which shows the test piece of the comparative example which joined the steel plate by the seam welding with which the nugget is continuing.

The present invention will be described with reference to the drawings.
The present invention uses resistance seam welding instead of resistance spot welding that has been used so far to join a plurality of steel plate members including one or more high-strength steel plate members having a martensite structure, and a tensile load at the time of collision. A predetermined weld is obtained in which nuggets are intermittently arranged in a direction substantially parallel to the direction assumed to be applied. As a result, a good weld without defects such as cracks can be obtained, and it is possible to prevent the weld from being broken at low strain and to fully exhibit the high strength of a high-strength steel plate member or hot stamp material. be able to.

First, the lap weld member of the present invention will be described in detail.
The present invention includes, for example, a plurality of steel plate members including one or more high-strength steel plate members having a martensite structure having a tensile strength of 1180 MPa class or more, and one or more high-strength steel plate members formed by hot stamping and having a martensite structure. Is a lap welded member in which intermittent nuggets are formed inside the steel plate member (overlapping part) by lap resistance seam welding, and the welded part breaks at low strain. In order to suppress, the size of each nugget and the difference between the maximum value and the minimum value of Vickers hardness between the nuggets are set as follows. The high-strength steel plate member is a member cut into a predetermined contour using a steel plate as a raw material, a member processed into a predetermined shape (for example, a press-formed product), or the like.

(Nugget size N (mm): 2√t ≦ N ≦ 12√t)
In the lap resistance seam weld of the present invention, the nugget is intermittently formed, and the size N (mm) of the nugget in the direction in which the nugget is intermittently formed is the thickness of the high-strength steel plate member t (mm). Then, 2√t ≦ N ≦ 12√t. This is because when the nugget size is less than 2√t, the nugget is small and the strength of the joint is insufficient. In addition, when the nugget size is larger than 12√t, in the case of a high-strength steel sheet having a carbon content generally exceeding 0.14% by mass, the solid-liquid coexistence temperature range increases during welding, and the hardness increases in the high temperature range. Since the region where the thickness is lowered is widened, the influence of increasing the deformation of the welded portion due to the pressurization of the electrode is superimposed, so that internal defects such as solidification cracks and shrinkage nests are likely to occur in the nugget. Preferably, 5√t ≦ N ≦ 10√t.

  When a plurality of high-strength steel plate members are overlapped and resistance seam welding is performed, the size of the nugget needs to satisfy 2√t ≦ N ≦ 12√t with respect to the thickness of each high-strength steel plate member. . In general, in a fuel tank (a raw material is a low-strength steel plate) in which seam welding is often used, a continuous melt-solidified portion is formed in order to ensure airtightness.

(Difference between maximum and minimum values of Vickers hardness between adjacent nuggets: Hv 70 or less)
In the present invention, in order to suppress breakage at the time of collision in the high-strength steel plate member, an intermittent nugget is formed, and further, between the nugget intermittently formed in the high-strength steel plate member and the nugget adjacent thereto, The difference between the maximum value and the minimum value of Vickers hardness is set to Hv 70 or less.

  This is an intermittent nugget formation, which suppresses defects such as cracks and shrinkage nests, and prevents the formation of a structure that is significantly softer than other parts in the lap resistance seam weld, thereby breaking due to HAZ softening This is to prevent the formation of the starting point. When a region softened locally by Hv70 or more is formed in the heat-affected zone between the nugget and the nugget, the decrease in the elongation at break of the part becomes remarkable. It is more desirable to suppress the hardness reduction to Hv 40 or less. A region softened by Hv 70 or more is formed in a direction perpendicular to the direction in which the nuggets are intermittently arranged, but does not coincide with the assumed load direction and thus does not break with a small strain.

Next, the lap resistance seam welding method of the present invention will be described in detail.
The present invention is a lap resistance seam welding method in which a plurality of steel plate members including one or more high-strength steel plate members having a martensite structure are overlapped and joined. Therefore, the energization is performed on the steel sheet as follows.

  Resistance seam welding is a method in which a material is welded by applying pressure using a roller electrode made of a copper alloy and rotating the electrode while energizing. Resistance seam welders for this purpose include a DC inverter type, an AC inverter type, and a single-phase AC type. Any of these types of resistance seam welders can be used in the present invention. However, it is desirable to use a DC inverter type that can reduce the size of the transformer and can be easily mounted on a robot.

  The roller electrode used in the present invention is not particularly limited, but is made of a copper alloy and has a diameter of 30 to 300 mm, an electrode tip width of 5 to 15 mm, an electrode tip R (curvature radius) of about 8 to 50 mm, or a flat (curvature radius). Is infinite). For example, in the case where a thin steel plate, a thick steel plate, and a thick steel plate are overlapped, in order to promote nugget formation on the thin steel plate side where it is difficult to form a nugget, The electrode tip R, the electrode diameter, and the electrode tip width are reduced. The welding pressure is 200 to 1000 kgf, the current value is 3 to 18 kA, and the welding speed is 1 to 10 m / min.

  In order to obtain a martensite-based structure in which the difference between the maximum value and the minimum value of Vickers hardness between the nugget and the nugget is Hv 70 or less, the steel plate (member) between the nugget and the nugget is not welded. It is necessary that the highest temperature is not less than Ac3 point (for example, about 820 ° C. in the case of 1500 MPa class hot stamp material) and not more than the melting point (about 1500 ° C.) of the high-strength steel plate. In addition, when a plurality of high-strength steel plate members having different component compositions are overlapped and resistance seam welding is performed, the maximum ultimate temperature between the nugget and the nugget is set to the Ac3 point or more and the melting point or less of all the steel plate members during welding. It is necessary to.

  The object of the present invention is a member in which a plurality of relatively thin steel plate members are overlapped and joined. For example, the total thickness of the plurality of sheets is about 7 mm or less. In such a thin laminated steel plate member, the cooling rate after the steel plate is heated to a temperature of the Ac3 point or higher by energization is faster than the critical cooling rate of the steel plate. That is, after cooling, the heated part becomes a martensite structure. By setting the maximum temperature to be not less than Ac3 point and not more than the melting point of the steel plate, the high strength steel plate member having high hardenability is quenched in the cooling process after welding. Further, when the hot stamp material (steel plate member) is heated to a temperature of Ac3 or higher by cooling and cooled, the hot stamp material (steel plate member) has a martensitic structure because it is cooled faster than the critical cooling rate of the steel plate member.

  The Ac3 points are edited by the Japan Institute of Metals, “Steel Materials” 2006, published by the Japan Institute of Metals, p. It is calculated | required using the formula described in 43. It can also be obtained by measuring the temperature change of thermal expansion. The critical cooling rate can be evaluated by obtaining a continuous cooling transformation diagram (CCT curve). As an example, in the case of a 1500 MPa class hot stamp material (steel plate member), the critical cooling rate is 30 ° C./second.

  Repeated high-current and low-current energization to form a nugget when energized with a high current, and subsequent low-current energization does not form a nugget, but the highest temperature of the high-strength steel plate member is higher than the Ac3 point of the high-strength steel plate member A martensite structure is obtained by supplying a current having a temperature of In the case of intermittent energization with high current and current off (zero), that is, intermittent energization with current on / off, by reducing the distance between the nuggets, even when the energization is off, the maximum temperature reached between the nuggets is Ac3. It is possible to ensure a temperature above the point.

  By doing in this way, the target seam welded part in which HAZ is not tempered and softened can be obtained. If the maximum temperature reached is less than the Ac3 point, the part that was originally a martensite structure is tempered and the HAZ softens. The metal structure of the steel plate member can be confirmed by cutting the steel plate member, polishing the cross section, and observing with an optical microscope after nital etching. The size of the nugget can be measured by cutting the steel plate member after welding, polishing the cross section, and observing with a magnifier.

  In the lap resistance seam welding method of the present invention, the current value, energization time, and the like are set so that the maximum temperature reached between the nuggets is a temperature not lower than the Ac3 point and not higher than the melting point of the high-strength steel plate member. Whether the temperature has reached the Ac3 point or higher can be confirmed by observing the metal structure of the high-strength steel sheet member after welding and confirming that the structure is not a tempered martensite but a martensite-based structure. Whether it is below the melting point can be determined by the fact that no nugget is formed. It can also be confirmed by measuring the hardness distribution between the nugget with a Vickers hardness meter. In this case, if the measurement pitch (interval) of the Vickers hardness measurement is wide, the HAZ softened portion cannot be detected. Further, if the load in the hardness measurement is too low, the hardness varies due to non-uniformity of the metal structure, so the measurement load needs to be 4.9 to 49N. As a measurement condition of the Vickers hardness, a load of 9.8 N and a measurement pitch of 0.2 mm are exemplified.

  The length for performing seam welding of the present invention is not particularly limited, and may be performed at a site where breakage is likely to occur at the time of collision. In the case of an automobile part, about 100 to 1500 mm is exemplified.

Next, the present invention will be described based on specific examples.
When a car collides, for example, when it hits the center pillar, a tensile load is applied to the flange part for welding the center pillar inner panel and the center pillar reinforcement (high strength steel plate), As shown in FIG. 1, the strain concentrates on the HAZ softened portion around the spot weld formed on the flange and breaks.

  FIG. 2 is a diagram illustrating a tensile test piece in a test that simulates a phenomenon in which tensile stress is applied to a joined flange member to deform and break. 2 (a) is a tensile test piece A in which three steel plate members are joined by spot welding, and FIG. 2 (b) is a seam welding in which energization is intermittently conducted, and the maximum attained temperature of HAZ deviates from the scope of the present invention. Tensile test piece B with surrounding HAZ heated to less than Ac3 point, martensite tempered and softened, Fig. 2 (c) shows nugget by seam welding in which high current and low current are alternately turned on. A tensile test piece C (example of the present invention) is shown in which three steel plate members are joined intermittently so that the maximum temperature between the nuggets is Ac3 or higher.

  As shown in FIG. 3, the tensile test pieces were overlapped with low-strength steel 1, high-strength steel 2 and low-strength steel 3, and resistance spot welding was performed at a total of eight places on the left and right grips (four figures). 2) 3 steel plates are fixed. Low strength steel 1 is a 440 MPa class non-plated steel sheet with a thickness of 1.2 mm, high strength steel 2 is a 1500 MPa class aluminized hot stamped steel sheet with a thickness of 1.6 mm (high strength steel sheet having a martensite structure), and low strength. As the steel 3, a 270 MPa class galvannealed steel plate having a thickness of 0.7 mm was used.

  The tensile test piece A is further fixed by spot welding at the center of the parallel part. The tensile test piece B is further fixed by seam welding in which current is intermittently applied to the parallel portion of the tensile test piece. Further, the tensile test piece C is fixed by seam welding in which a high current and a low current are alternately and repeatedly supplied to the parallel portion of the tensile test piece.

  3A is a diagram showing the hardness survey position and hardness distribution of the spot welded portion measured using the tensile test piece A, and FIG. 3B is a measurement using the tensile test piece B. FIG. It is a figure which shows the hardness investigation position and hardness distribution of the seam weld part of a comparative example. FIG. 3C is a diagram showing the hardness survey position and hardness distribution of the seam welded portion of the present invention, measured using the tensile test piece C. FIGS. 3A to 3C are diagrams showing the hardness distribution obtained by measuring the hardness at the hardness investigation position 4. In all cases, the Vickers hardness was measured at a load of 9.8 N and a measurement pitch of 0.2 mm.

  As shown in FIGS. 3 (a) and 3 (b), in spot welding and seam welding with intermittent energization, the martensite of the base material is tempered in the HAZ around the nugget 5 to become tempered martensite. Compared with the base material of the high-strength steel plate member, it was softened by about Hv150. As shown in FIG. 3 (c), in the seam welding in which a high current and a low current are alternately repeated, the maximum temperature reached between the nuggets 5 is higher than the Ac3 point of the high-strength steel plate member, and the HAZ is not tempered. . The space between the nuggets 5 is (all) continuously and is almost martensitic. Therefore, as shown in FIG. 3C, a hardness distribution is obtained in which the difference between the maximum value and the minimum value of Vickers hardness between adjacent nuggets is HV70 or less. In addition, the arrow in a figure shows the magnitude | size N (mm) of the longitudinal direction of a nugget.

  FIG. 4 is a diagram showing a fractured tensile test piece. This was a tensile test of the tensile test piece A, and the strain concentrated on the HAZ softened portion around the spot welded portion and fractured.

  FIG. 5 is a load-elongation diagram in a tensile test. (A) is a tensile test piece A, (b) is a tensile test piece B, and (c) is an elongation and load-elongation diagram of the tensile test piece C in a tensile test. The breaking elongation of the comparative examples (a) and (b) is as small as 3.0% or less. On the other hand, the breaking elongation of (c) which is an invention example is 6.0% or more.

  Although the above is the member and the lap resistance seam welding method provided with the lap resistance seam weld of the present invention, it is preferable to use the lap weld member of the present invention for an automobile member.

Next, a situation in which the present invention is applied to automobile parts will be described.
FIG. 6 is a diagram showing a situation where the present invention is applied to a center pillar. In the side panel manufacturing process, a side panel outer (not shown) made of a 270 MPa class alloyed hot-dip galvanized steel sheet, a center pillar reinforcement 6 made of 1500-1800 MPa hot stamp material, and a 440-780 MPa class non-plated steel sheet A center pillar inner (not shown) is overlapped with flanges formed at respective edges, and resistance spot welding 7 is performed to temporarily fix the center pillar inner (not shown). Thereafter, three steel plate presses are stacked and seam welded 8 in the main body line.

  As shown in FIG. 6, the seam welding 8 does not need to be performed on all the welds of the flange, and may be only the upper and lower parts of the center pillar having a high risk of fracture. That is, it is only necessary to perform the seam welding of the present invention only on the portion where the fracture due to the collision is expected in spot welding, and the other portion may be spot welding. In addition, the flange width of the center pillar reinforcement made of 1500-1800 MPa hot stamp material is shortened, and seam welding is performed with two sheets of center pillar inner made of 440-780 MPa class non-plated steel sheet, and 440-780 MPa class non-plated steel sheet And 270 MPa class galvannealed steel sheet may be spot welded.

  FIG. 7 is a diagram showing a situation where the present invention is applied to a side sill. As with the center pillar, the side sill is assembled with the underbody at the main body line, temporarily fixed with spot welding 7 and then seam welded 8. The side sill includes a side sill inner panel made of a 590 to 1800 MPa grade steel plate (GA plating or aluminum plating hot stamp), a side sill outer reinforcement made of a 1180 to 1800 MPa grade steel plate (GA plating or aluminum plating hot stamp), and a 270 MPa grade. A side sill outer panel made of an alloyed hot-dip galvanized steel sheet is overlapped with a flange formed at each edge. At this time, the seam welded portion 8 of the present invention is formed at a site where the fracture is expected.

  FIG. 8 is a diagram showing a situation where the present invention is applied from the A pillar to the roof rail. As shown in FIG. 8, the side panel including the A rail from the A pillar to the roof rail is assembled with the underbody in the main body line and seam welded 8 in the same manner as the center pillar. Each of the A pillar and the roof rail includes a molded panel made of a 590-1800 MPa class steel plate on the inner side, a molded panel made of a 1500 to 1800 MPa class hot stamp material on the outer side, and a 270 MPa class galvannealed steel plate on the outermost side. Consists of three sheets stacked with the panel outer. At this time, the seam welded portion of the present invention is formed at a site where fracture is expected. In addition, seam welding 8 may be performed so as to be superimposed on a portion where spot welding 7 is temporarily fixed at the time of assembly. The HAZ softened part of spot welding is re-quenched by the seam welding of the present invention, and breakage can be suppressed.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

  Table 1 shows the material of the test piece.

  As shown in FIG. 3, test pieces were prepared by superimposing materials 1 to 3. In Table 1, 270 MPa-0.7 t indicates a 270 MPa class galvannealed steel sheet, and 440 MPa-1.2 t indicates a 440 MPa class non-plated cold-rolled steel sheet. 1500 MPa-1.6 t is a 1500 MPa class aluminized hot stamped steel sheet, 1800 MPa-1.6 t is a 1800 MPa class non-plated hot stamped steel sheet, and 1180 MPa-1.6 t is a 1180 MPa class non-plated cold-rolled steel sheet. These three steel plates are high-strength steel plates containing a martensite structure in the metal structure.

  Table 2 shows the welding method, the size (diameter) of the nugget in the longitudinal direction, and welding energization setting conditions.

Seam Welding Using an inverter DC seam welder, a chrome copper electrode with an electrode diameter of 100 mm, an electrode tip at the thin plate side R15, and a thick plate side R50 mm, a pressurizing force of 400 kgf, current of 3.0 to 9.5 kA, welding speed 3 Linear seam welding was performed by changing the current waveform at 0.0 m / min, and test pieces of comparative examples and inventive examples were obtained.

Spot welding Using an inverter DC spot welder, a DR electrode, a tip of 6 mm, a pressure of 400 kgf, an energization time of 333 msec, and a current value at which a nugget diameter of 5√t was obtained were obtained to obtain spot welding test pieces.

  The welding location of the test piece was the same as that of the tensile test pieces A to C in FIG. In other words, resistance spot welding is performed at 8 points in total on the left and right grips of the test piece, and three steel plates are fixed. In seam welding, seam welding is further performed on the parallel part of the test piece, and spot welding is performed. In welding, spot welding was further performed at the center of the parallel part of the test piece.

  Further, as a comparative example, a test piece in which nuggets are continuous in seam welding was prepared. FIG. 9 shows a test piece of a comparative example in which steel plates are joined by seam welding in which nuggets are continuous.

  In the 14 test levels shown in Table 2, two test pieces were prepared at the same level. At each level, one specimen was used for tensile testing only. The other test piece was subjected to measurement of the nugget size (diameter) in the direction in which the nugget was intermittently formed (that is, the longitudinal direction of the test piece), hardness distribution measurement, and observation of the metal structure. .

  In the tensile test, the elongation at break was evaluated by carrying out a tensile test at a tensile speed of 3 mm / min with a score distance of 50 mm as a welded part. The test piece not used in the tensile test was cut in the longitudinal direction, the cut surface was polished, and the Vickers hardness was measured between the nugget and the nugget as shown in FIG. The measurement conditions are a load of 9.8 N and a measurement pitch of 0.2 mm. In addition, the size of the nugget was measured using a magnifying glass. The metal structure was confirmed by etching the polished surface with nital and observing with an optical microscope. The results are shown in Table 2.

  As shown in Table 2, the elongation at break of 6.0% or more was obtained in the seam welded portion of the present invention. On the other hand, in the comparative example, the elongation at break was small. In seam welding, when the nugget diameter was small as in the comparative example, the elongation at break did not decrease, but the strength in the tensile shear test was low. In addition, when the nugget diameter was too large or when the nuggets were continuously connected, a fracture with low strain inside the nugget, which was considered to be caused by cracks in the weld, occurred.

  According to the present invention, in a structure in which a plurality of steel plate members including one or more high-strength steel members having a martensite structure and having a strength of 1180 MPa or more are welded by overlapping, the welded portion of the overlapped portion is fractured at a low strain. Can be suppressed. Therefore, the present invention has high industrial applicability.

DESCRIPTION OF SYMBOLS 1 Low strength steel 2 High strength steel 3 Low strength steel 4 Hardness investigation position 5 Nugget 6 Center pillar reinforcement 7 Resistance spot welding 8 Seam welding A Tensile specimen B Tensile specimen C Tensile specimen

Claims (6)

A lap weld member in which a plurality of steel plate members including one or more high strength steel plate members having a martensite structure and having a tensile strength of 1180 MPa or more are overlapped and joined,
The nugget formed by lap resistance seam welding is intermittently present inside the superposed steel plate member, and the nugget size N (mm) in the direction in which the nugget is intermittently formed is the high strength. When the thickness of the steel plate member is t (mm), 2√t ≦ N ≦ 12√t, and the difference between the maximum value and the minimum value of Vickers hardness between adjacent nuggets among the nuggets is Hv70 or less. A lap weld member comprising a lap resistance seam weld.   The lap weld member according to claim 1, wherein the high-strength steel plate member is a member subjected to hot stamping.   A lap weld member for an automobile, comprising the lap weld portion according to claim 1. A lap resistance seam welding method in which a plurality of steel plate members including one or more high strength steel plate members having a martensite structure and a tensile strength of 1180 MPa or more are overlapped and joined,
A high current and a low current (including 0) are alternately and repeatedly energized to form the nugget intermittently, and the size N (mm) of the nugget in the direction in which the nugget is intermittently formed is Assuming that the thickness of the high-strength steel plate member is t (mm), 2√t ≦ N ≦ 12√t, and the highest temperature reached between the nuggets is equal to or higher than the Ac3 point of the high-strength steel plate. And the difference between the maximum value and the minimum value of Vickers hardness between adjacent nuggets among the nuggets is set to Hv 70 or less.   The lap resistance seam welding method according to claim 4, wherein the high-strength steel plate member is a member subjected to hot stamping.   The lap resistance seam welding method, wherein the steel sheet according to claim 4 or 5 is an automobile member.