Name of the invention: A method for manufacturing a spot welded joint, an automobile frame component provided with a spot welded joint, and a spot welded joint.
Technical field
[0001]
　The present disclosure relates to spot welded joints, automobile frame parts including spot welded joints, and methods for manufacturing spot welded joints.
　The present application claims priority based on Japanese Patent Application No. 2018-105233 filed in Japan on May 31, 2018, the contents of which are incorporated herein by reference.
Background technology
[0002]
　In a structure in which a plurality of steel plate members are stacked, joining by resistance spot welding is widely performed on the overlapped portion in which the steel plate members are stacked.
[0003]
　For example, Patent Document 1 describes an energy absorbing member in which a hat material and a closing plate are joined to each other by spot welding.
[0004]
　Currently, as a high-strength steel sheet for automobiles, a high-strength steel sheet having a tensile strength of 980 MPa or more is widely used. In recent years, high-strength steel sheets having a tensile strength of 1100 MPa or more have begun to be applied. High-strength steel sheets with a tensile strength of 1100 MPa or more generally include a hardened structure in order to obtain high strength. When resistance spot welding is performed, a nugget for welding a steel plate is formed, and a heat-affected zone (hereinafter referred to as HAZ) is generated around the nugget. HAZ generally includes a hardened structure. However, when resistance spot welding is performed on a high-strength steel plate having a hardened structure, a region (HAZ softened zone) having a hardness lower than that of the base metal having a hardened structure is formed. This is because the hardened structure of the base metal is tempered by the heat of spot resistance welding.
[0005]
　In general, the strength decreases when there is a region with low hardness. However, such a HAZ softened zone does not affect the evaluation results of the tensile shear test and the cross tensile test (JISZ3137) used for the joint evaluation of resistance spot welding. However, when an in-plane tensile stress is applied, strain may be locally concentrated on the HAZ softened zone, causing fracture in the HAZ softened zone.
[0006]
　Passengers in the cabin need to be protected in the event of a car collision. Therefore, the structural members (layer welded members) constituting the automobile body such as the A pillar, the B pillar, the roof rail, and the side sill need to have high strength. Generally, a structural member constituting an automobile body is manufactured by superimposing a plurality of steel plate members and joining flanges (overlapping portions) by resistance spot welding to form a tubular closed cross section. In order to improve the deformation resistance at the time of collision and absorb more collision energy with a small amount of deformation, methods such as increasing the strength of the material (base material) and increasing the number of welding (spot) hit points are taken.
　In-plane tensile stress may be applied to a part of the flange of the member to be spot-welded by resistance spot welding. Even if the strength of the base metal is increased and the spot impact points are increased, if the above-mentioned HAZ softened portion is generated, the collision resistance performance expected from the strength of the base metal and the shape of the component may not be obtained.
　Therefore, when a steel plate member made of a high-strength steel plate is applied to a structural member of an automobile body, it is required to suppress the peripheral region of the nugget from becoming the starting point of fracture.
[0007]
　Conventionally, studies have been made to improve the characteristics of welded members formed by resistance welding spot welding. For example, Patent Document 2 describes a welded joint in which the characteristics of the spot welded portion are improved by heat-treating the spot welded portion at 100 to 400 ° C. to improve the strength of the L-shaped tensile joint. Further, Patent Document 3 describes a method of post-energizing the spot welded portion to improve the strength of the cross tension joint. Patent Document 4 describes the ratio of TSS to material strength and CTS by heating the spot welded portion and the molten portion by high-frequency induction heating immediately after welding with a coil wound around the spot weld electrode. A welding method for improving the joint strength evaluated from the product with the material strength is described.
[0008]
　However, although the techniques disclosed in Patent Documents 2 to 4 have a certain effect on improving TSS and CTS, the fracture at the HAZ softened portion when an in-plane tensile stress is applied to the steel sheet Not considered.
[0009]
　In response to such a problem, Patent Document 5 describes that the energy absorption capacity is enhanced by having a region called a soft zone having a strength of less than 1100 MPa in a part or all of the flange portion used for spot welding. Pillars are listed.
[0010]
　However, in the B-pillar disclosed in Patent Document 5, since the side flange needs to be softened, the bending performance may be deteriorated. Further, in Patent Document 5, since a softening region is provided in the part before welding, there is also a problem that the shape accuracy of the part is lowered. When the shape accuracy of a part is lowered, a gap is generated between the parts during welding, which makes welding difficult.
Prior art literature
Patent documents
[0011]
Patent Document 1: Japanese Patent Application Laid-Open No. 2006-142905
Patent Document 2: Japanese Patent Application Laid-Open No. 2010-059451
Patent Document 3: Japanese Patent Application Laid-Open No. 2015-093282
Patent Document 4: Japanese Patent Application Laid-Open No. 5459750
Patent Document 5: Japanese Patent No. 5894081
Outline of the invention
Problems to be solved by the invention
[0012]
　The present disclosure has been made in view of the above problems, and is a spot welded joint capable of suppressing breakage from a region around the nugget even when an in-plane tensile stress is applied, and an automobile frame component including the spot welded joint. , And a method for manufacturing a spot welded joint thereof.
Means to solve problems
[0013]
　The present inventors have investigated and studied spot welded joints of high-strength steel sheets. As a result, it was found that by controlling the hardness in a predetermined region around the nugget including the HAZ softened zone, fracture at the HAZ softened zone under in-plane stress loading can be suppressed.
[0014]
　The HAZ softened zone, which has a lower hardness than the base metal, is hardly formed on a steel sheet having a tensile strength of less than 980 MPa. The HAZ softened zone can occur in a steel sheet having a tensile strength of 980 MPa or more. In particular, a HAZ softened zone is remarkably generated in a spot welded portion of a high-strength steel plate having a tensile strength of 1100 MPa or more or a hot stamping material (high-strength steel plate member) formed by hot stamping, which has been increased in strength by quenching.
[0015]
　The present disclosure has been made based on the above findings, and the gist thereof is as follows.
(1) The spot-welded joint according to one aspect of the present disclosure includes a first steel sheet having a tensile strength of 1100 MPa or more and mainly having a hard martensite structure, and a second steel sheet laminated on the first steel sheet. A nugget having a diameter D at the interface between the first steel plate and the second steel plate formed between the first steel plate and the second steel plate, and the first steel plate. A region of 0.5 × D to 1.0 × D in the plate surface direction from the center of the nugget and all in the plate thickness direction of the first steel plate in the cross section in the plate thickness direction passing through the center of the nugget. A hardness control region in which the difference between the maximum value of the hardness of the region and the minimum value of the hardness is 80 HV or less, and the maximum value of the hardness in the region is lower than the maximum value of the hardness of the first steel sheet. , Equipped with.
(2) In the spot welded joint according to (1) above, another nugget may be provided in the cross section.
(3) In the spot welded joint according to the above (1) or (2), the hardness control region may be provided in all the cross sections.
(4) In the spot welded joint according to any one of (1) to (3) above, the maximum value of the hardness in the region may be 250 HV to 450 HV.
(5) In the spot welded joint according to any one of (1) to (4) above, the first steel plate and the second steel plate may be plated steel plates.
(6) The automobile frame component according to another aspect of the present disclosure includes the spot welded joint according to any one of (1) to (5) above.
(7) In the method for manufacturing a spot welded joint according to another aspect of the present disclosure, a first steel sheet and a second steel sheet having a tensile strength of 1100 MPa or more and mainly having a hard maltensite structure are superposed to form a resistance spot. By performing welding, a nugget having a diameter D at the interface between the first steel plate and the second steel plate is formed between the first steel plate and the second steel plate, and the first steel plate is formed. A region of 0.5 × D to 1.0 × D in the plate thickness direction of the steel plate passing through the center of the nugget, all in the plate thickness direction of the first steel plate and in the plate surface direction from the center of the nugget. Burn all of.
(8) In the method for manufacturing a spot welded joint according to (7) above, the difference between the maximum value of hardness and the minimum value of hardness in the region is 80 HV or less, and the maximum value of hardness in the region is the first. Tempering may be performed so that the hardness of the steel sheet is lower than the maximum value.
(9) In the method for manufacturing a spot welded joint according to (7) or (8) above, tempering may be performed on the region of all the cross sections.
(10) In the method for manufacturing a spot welded joint according to (8) or (9) above, the maximum value of the hardness in the region may be 250 HV to 450 HV.
The invention's effect
[0016]
　According to the spot welded joint of the present disclosure, even when an in-plane tensile stress is applied, fracture from the HAZ softened portion around the nugget is less likely to occur. Therefore, when used as a structural member constituting an automobile body, high collision resistance can be obtained.
　Further, in the automobile frame component of the present disclosure, breakage from the HAZ softened portion is less likely to occur. Therefore, high collision resistance can be obtained.
　Further, in the method for manufacturing a spot welded joint of the present disclosure, it is possible to manufacture a spot welded joint capable of obtaining high collision resistance when used for a structural member constituting an automobile body.
A brief description of the drawing
[0017]
FIG. 1 is a cross-sectional view in the plate thickness direction passing through the nugget center of the spot welded joint according to the present embodiment.
FIG. 2 is a cross-sectional view in the plate thickness direction passing through the nugget center of the spot welded joint according to the present embodiment.
FIG. 3 is a cross-sectional view in the plate thickness direction passing through the nugget center of the spot welded joint according to the present embodiment, and is an example in the case where another nugget is included in the cross section.
FIG. 4 is a schematic view of a B-pillar which is an example of an automobile frame component according to the present embodiment.
FIG. 5 is a cross-sectional view taken along the line BB of a B-pillar, which is an example of an automobile frame component according to the present embodiment.
FIG. 6 is a schematic diagram showing a test piece used in an example.
Mode for carrying out the invention
[0018]
　A method for manufacturing a spot welded joint according to an embodiment of the present disclosure (spot welded joint according to the present embodiment), an automobile frame component according to the present embodiment, and a spot welded joint according to the present embodiment will be described with reference to the drawings. To do.
[0019]
　First, the spot welded joint according to the present embodiment will be described.
　As shown in FIGS. 1 and 2, the spot welded joint 1 according to the present embodiment includes a first steel plate 11, a second steel plate 12 stacked on the first steel plate 11, and a first steel plate 11. A nugget 2 having a diameter D at the interface between the first steel plate 11 and the second steel plate 12 formed between the second steel plates 12 is provided. That is, the first steel plate 11 and the second steel plate 12 are joined by the nugget 2.
[0020]
　Further, the spot welded joint 1 according to the present embodiment includes a hardness control region 33 that occupies the first region 32. The first region 32 is a cross section in the plate thickness direction of the first steel plate 11 passing through the center O of the nugget 2 in the plate surface direction (at least one direction perpendicular to the plate thickness direction of the first steel plate 11). For example, between a position 0.5 × D away from the center O of the nugget 2 and a position 1.0 × D away from the center O of the nugget 2 (hereinafter 0.5 × D) in the direction of the arrow 102 in FIG. (Sometimes referred to as a range of ~ 1.0 × D) and the entire thickness of the first steel plate 11.
　In the present embodiment, the hardness control region 33 occupying the first region 32 means that the hardness control region 33 includes the entire first region 32.
[0021]
　Usually, the HAZ softened zone is formed in a region separated from the nugget by a predetermined distance. In the spot welded joint 1 according to the present embodiment, it occurs in the range of 0.5 × D to 1.0 × D in the plate surface direction from the center O of the nugget. Therefore, in the spot welded joint 1 according to the present embodiment, in order to suppress breakage from the periphery of the nugget 2, the hardness control region 33 is tempered so that the hardness of the first region 32 will be described later. Control.
　For example, from the viewpoint of cross tensile strength and the like, there is a possibility that the effect of improving the joint strength can be obtained by tempering only the vicinity of the interface between the first steel plate and the second steel plate. However, in order to prevent fracture from the HAZ softened zone when an in-plane tensile stress is applied, it is necessary to perform tempering treatment on the entire thickness of the first steel sheet 11 to control the hardness.
　Further, as described above, in the present disclosure, as shown in FIG. 1, the spot welded joint in which the HAZ softened zone is generated in the range of 0.5 × D to 1.0 × D is targeted. This cannot sufficiently prevent breakage from the HAZ softened zone even if only the hardness in the range sandwiched between the nugget 2 and the HAZ softened zone is controlled, and a region including at least the HAZ softened zone from the nugget 2 is preferable. This is because it is not possible to prevent breakage from the HAZ softened zone when an in-plane tensile stress is applied unless the hardness up to the region beyond the HAZ softened zone is controlled.
[0022]
　Further, in order to more reliably suppress the breakage from the periphery of the nugget 2, as shown in FIG. 2, in addition to the first region 32, the hardness of the second region 34 is also the hardness of the first region 32. It is desirable to control in the same way as. The second region 34 is between a position 1.0 × D away from the center O of the nugget 2 in the plate surface direction and a position 2.5 × D away (1.0 × D to 2.5 × D). ) And the entire thickness of the first steel plate 11. The hardness control region shown in FIG. 2 includes a first region and a second region. That is, as shown in FIG. 2, the hardness of the first region 32 and the second region 34 is adjusted so that the hardness control region 33 for tempering occupies the first region 32 and the second region 34. It is preferable to control.
[0023]
　The hardness control region 33 shown in FIGS. 1 and 2 may have a first region 32 or a peripheral region adjacent to the first region 32 and the second region 34. The peripheral region is a region that is affected by the heat of tempering when the first region 32 or the first region 32 and the second region 34 are tempered to control the hardness. That is, the peripheral region is a hardness transition portion between the high hardness portion (base material portion) that is not affected by the heat of tempering and the first region 32 or the second region 34.
[0024]
　Further, it is desirable that the thickness of the first region 32 and the second region 34 in the direction perpendicular to the cross section (the direction perpendicular to the paper surface of FIG. 1 or 2) is 0.75 × D or more. More preferably, the thickness of the first region 32 and the second region 34 in the direction perpendicular to the cross section is D (1.0 × D) or more. By doing so, since all the positions of the HAZ softening portion 31 where the strain can be concentrated are softened, the breakage at the HAZ softening portion 31 is further suppressed, and the amount of elongation until the breakage can be increased.
[0025]
　In the present embodiment, the diameter D of the nugget 2 is the nugget 2 at the interface between the first steel plate 11 and the second steel plate 12 in the cross section in the plate thickness direction of the first steel plate 11 passing through the center O of the nugget 2. It is the length in the plate surface direction of.
　The nugget 2 is obtained by superimposing the first steel plate 11 and the second steel plate 12 and performing resistance spot welding.
[0026]
　The first steel sheet 11 is a steel sheet mainly composed of a hard martensite structure and having a tensile strength of 1100 MPa or more. This is because the HAZ softened portion 31 does not clearly appear on a steel sheet having a tensile strength of less than 1100 MPa, so that the problem of the invention does not occur. On the other hand, the second steel plate 12 is not limited.
[0027]
　As described above, a high-strength steel sheet having a tensile strength of 1100 MPa or more generally has a structure mainly composed of a hardened structure such as hard martensite (at least 50 area% or more, for example 80 area% or more). ing. Such a structure is obtained by transformation of the structure by quenching. When a spot weld is formed on a steel plate mainly composed of a hardened structure, hard martensite is tempered in HAZ around the nugget by the heat of resistance spot welding. That is, hard martensite changes to a soft structure such as tempered martensite, bainite, and ferrite. As a result, a region (HAZ softened portion 31) having a hardness lower than that of the base material is generated. When in-plane stress is applied, this HAZ softened portion 31 becomes the starting point of fracture.
[0028]
　The microstructure fraction of the hard martensite of the first steel sheet 11 can be confirmed by observing the microstructure at the position not affected by the welding heat under a microscope. Specifically, each of the 5/8 position, 3/8 position, 5/8 position, and 7/8 position from the surface of the first steel plate 11 in the plate thickness direction cross section of the steel plate. Samples collected from various locations are etched with a repera corrosive solution, and a 100 μm square field of view is observed with an optical microscope at a magnification of 1000 times. Measure the area ratio of martensite. By averaging the area ratios of the observed 20 fields of martensite, the martensite area ratio of the first steel sheet 11 can be obtained.
　Then, using the same sample, etching treatment is performed with picral, a field of view of 100 μm square is observed with an optical microscope at a magnification of 1000 times, and the ratio of hard martensite to martensite is determined in the observation field of view. Martensite containing carbide is judged to be hard martensite, and martensite not containing carbide is judged to be tempered martensite.
　The ratio of hard martensite in the martensite of 20 fields is calculated and averaged, and the average value is multiplied by the martensite area ratio of the first steel sheet 11 obtained above to obtain the hard martensite of the first steel sheet 11. The tissue division ratio (area ratio) can be obtained.
　If the average area ratio of hard martensite is 50% or more, it is judged that the hard martensite structure is the main component.
[0029]
　In the spot welded joint 1 according to the present embodiment, the hardness of the first region 32 is controlled as described above. Specifically, the difference between the maximum value of hardness and the minimum value of hardness in the first region 32 is 80 HV or less. Further, the maximum value of hardness in the first region 32 is lower than the maximum value of hardness of the first steel plate 11.
　More preferably, the difference between the maximum hardness and the minimum hardness of the first region 32 and the second region 34 is 80 HV or less. In this case, it is preferable that the maximum hardness in the first region 32 and the second region 34 is lower than the maximum hardness of the first steel plate 11.
　With such a welded joint, breakage from the HAZ softened portion 31 is suppressed, and even when an in-plane tensile stress is applied, the base metal breaks. On the other hand, when the difference (hardness difference) between the maximum value of hardness and the minimum value of hardness in the first region 32 exceeds 80 HV, when tensile stress (in-plane tensile stress) is applied from both sides of the first steel plate. In addition, since the strain is concentrated on the HAZ softening portion 31, breakage occurs from the HAZ softening portion 31. Preferably, the difference (hardness difference) between the maximum value and the minimum value of the hardness in the first region 32 described above is 50 HV or less. Further, if the hardness of the second region is controlled in the same manner, the concentration of stress on the HAZ softened zone is further relaxed.
[0030]
　In a steel sheet mainly composed of a hard martensite structure, the hardness of the portion affected by welding heat is lower than the hardness before welding. Therefore, for the maximum value of the hardness of the first steel plate 11, the hardness at a position not affected by heat due to resistance spot welding of the first steel plate 11 is measured, and the maximum value is used. As a position not affected by heat due to resistance spot welding, for example, the hardness at a position 15 mm or more away from the nugget 2 in the direction without other nuggets may be measured.
　Specifically, using a Vickers hardness tester, with a load of 1.0 kgf, 10 positions not affected by heat due to resistance spot welding, 1/8 of the thickness from the surface of the first steel sheet 11. Measure the hardness at the 3/8 position, 5/8 position, and 7/8 position, and use the maximum value.
[0031]
　The maximum and minimum values ​​of hardness in the first region 32 are measured using a Vickers hardness tester with a load of 100 gf. Specifically, in the cross section in the plate thickness direction including the center O of the nugget of the first steel plate 11, the position of 1/8 of the plate thickness from the surface of the first steel plate 11 and the position of 3/8 and 5/8. The hardness of the position, 7/8, is measured at intervals of 0.1 mm from one end in the direction perpendicular to the plate thickness direction (that is, the plate surface direction) of the first region 32 to the other end. Then, the maximum value and the minimum value of the measured hardness are set as the maximum value and the minimum value of the hardness of the first region 32.
[0032]
　In the case of automobile frame parts such as B-pillars, spot welds are generally formed at a plurality of overlapping steel plates.
[0033]
　In the event of a collision, in-plane tensile stress is generated in the direction connecting the multiple nuggets. Therefore, when the spot welded joint 1 according to the present embodiment includes the nugget 2 and another nugget, as shown in FIG. 3, the cross section in the plate thickness direction passing through the center O of the nugget 2 of the first steel plate 11 is observed. It is preferable that the other nugget 2 is contained in the same cross section. In other words, it is preferable to provide the hardness control region 33 at least in the direction from the nugget 2 toward the other nugget 2'(the direction of the arrow 102 in FIG. 3) in order to suppress breakage in the HAZ softening portion 31.
[0034]
　The spot welded joint 1 according to the present embodiment can be applied to other than the B pillar. For example, A pillars and side sills. As described above, the direction connecting the nuggets is the predominant direction in which tensile stress is generated. However, tensile stress can occur in either direction, depending on the parts applied and the conditions of the collision.
　It is preferable to provide the hardness control region 33 in the entire circumferential direction of the nugget 2 in the plate surface direction because it is possible to suppress the breakage at the HAZ softening portion 31 regardless of the direction in which the tensile stress is applied. In other words, it is preferable that the hardness control region 33 exists in all the cross sections in the plate thickness direction including the center O of the nugget 2.
[0035]
　The maximum value of the hardness of the first region 32 included in the hardness control region 33 is preferably 250 HV to 450 HV. Further, it is more preferable that the maximum value of the hardness of the second region 34 is 250 HV to 450 HV. If the maximum value of hardness is less than 250 HV, there is a concern that sufficient strength cannot be obtained as a structural member. Further, if it exceeds 450 HV, there is a concern that the strength of the base material may be exceeded.
[0036]
　The first steel plate 11 and / or the second steel plate 12 may be a plated steel plate. Corrosion resistance can be improved by using a plated steel sheet. Examples of the plated steel sheet include hot-dip galvanized steel sheets, alloyed hot-dip galvanized steel sheets, electrogalvanized steel sheets, and aluminum-plated steel sheets.
[0037]
　Next, the automobile skeleton parts according to the present embodiment will be described.
　The automobile skeleton part according to the present embodiment is an automobile skeleton part including the spot welded joint 1 according to the above-described embodiment in at least one part. For example, the B-pillar 201 as shown in FIGS. 4 and 5.
[0038]
　The automobile skeleton component according to the present embodiment includes the spot welded joint 1 according to the present embodiment in which breakage at the HAZ softened portion is suppressed. Therefore, when a high-strength steel sheet is used, high collision resistance can be obtained.
[0039]
　Next, a method of manufacturing the spot welded joint 1 according to the present embodiment will be described.
　The spot welded joint 1 according to the present embodiment is formed by performing resistance spot welding by superimposing a first steel plate 11 having a tensile strength of 1100 MPa or more and mainly having a hard maltensite structure and a second steel plate 12. , A nugget 2 having a diameter D at the interface between the first steel plate 11 and the second steel plate 12 is formed between the first steel plate 11 and the second steel plate 12, and then the nugget of the first steel plate 11 is formed. In all the thickness directions of the first steel plate 11 having a cross section in the thickness direction passing through the center O of 2, and in all the regions of 0.5 × D to 1.0 × D in the plate surface direction from the center O of the nugget 2. It can be manufactured by tempering a first region 32.
　In order to temper the first region 32, the hardness control region 33 occupying the first region 32 may be tempered.
　It is desirable to temper not only the first region 32 but also the second region 34 in order to obtain high joint strength. Therefore, it is preferable that the hardness control region 33 occupies both the first region 32 and the second region 34.
[0040]
　In the method for manufacturing a spot welded joint 1 according to the present embodiment, the hardness control region 33 is provided in at least one direction in which in-plane tensile stress is applied to the first steel sheet 11 subjected to resistance spot welding. That is, the HAZ softening portion 31 in at least one direction from the center O of the nugget 2 and the region around it are burnt back. Thereby, the hardness difference in the first region 32 including the HAZ softening portion 31 can be reduced. In this way, the spot welded joint 1 according to the present embodiment in which breakage from the HAZ softened portion 31 is suppressed can be manufactured.
[0041]
　Resistance spot welding may be performed under conditions according to the application, and is not limited.
　The tempering conditions are not particularly limited, but in order to form the tempered structure of hard martensite, which is a quenching structure, it is preferable to perform tempering at a temperature of 500 to 700 ° C. If the tempering temperature is less than 500 ° C., the hardness difference may not be sufficiently reduced. On the other hand, it is also not preferable that the tempering temperature exceeds 700 ° C. This is because a part of the structure is transformed into austenite, and the austenite becomes a hardened structure again in the cooling after tempering. When the quenching structure is obtained again, the hardness of the first region 32 may not be sufficiently reduced. From the viewpoint of joint strength, it is preferable to temper the hardness control region 33 that occupies the first region 32 without reducing the strength of the base metal portion. In that case, for tempering, it is preferable to use a method capable of locally heating, such as heat treatment using a laser or heat treatment by energization.
[0042]
　In order to surely obtain the effect of the invention, the aim of the hardness by tempering is that the hardness of the first region 32 occupied by the hardness control region 33 in at least one direction from the nugget 2 is the base material portion of the first steel plate 11. The difference between the maximum value of hardness and the minimum value of hardness in the first region 32 is 80 HV or less in Vickers hardness, which is lower than the maximum value of hardness. In order to more reliably suppress the breakage from the HAZ softened portion 31, it is desirable that the aim of these hardnesses is applied not only to the first region 32 but also to the second region 34.
[0043]
　Further, it is desirable to provide the hardness control region 33 in the in-plane direction of the first steel plate, in the entire circumferential direction of the nugget 2. That is, it is desirable that tempering is performed so that the first region 32 satisfies the above hardness in all the cross sections in the plate thickness direction passing through the center O of the nugget 2 of the first steel plate. This is because by tempering the nugget 2 in the entire circumferential direction, the effect can be obtained regardless of the direction in which the in-plane tensile stress is generated in the first steel sheet. Further, it is desirable to bake the second region 34 as well in order to more reliably suppress the breakage from the HAZ softened portion 31.
[0044]
　Further, if the conditions are controlled and tempering is performed so that the maximum value of the hardness of the first region 32 in the hardness control region 33 is 250 HV to 450 HV, sufficient strength is secured as a structural member. Therefore, it is preferable because a welded joint in which breakage from the HAZ softened portion 31 is suppressed can be manufactured. In order to more reliably suppress breakage from the HAZ softened portion 31, it is desirable that the maximum value of hardness in the second region 34 is 250 HV to 450 HV.
[0045]
　Specific tempering conditions for obtaining a predetermined hardness are determined by, for example, a method of tempering a spot welded joint manufactured in advance under similar conditions under various conditions and examining the change in hardness. can do.
Example
[0046]
　Hereinafter, the present disclosure will be specifically described by way of examples with reference to FIGS. 6 and 1. These examples are examples for confirming the effect of the present disclosure, and do not limit the present disclosure.
[0047]
　A tensile test piece having a distance between gauge points of 50 mm as shown in FIG. 6 was collected from a steel sheet having a thickness of 1.6 mm obtained through quenching. Three tensile test pieces were taken from the same steel sheet, and one was subjected to a tensile test as it was according to JISZ2241: 2011 to determine the tensile strength of the steel sheet.
　In addition, repera corrosion was performed on samples collected from 5 locations each of 1/8, 3/8, 5/8, and 7/8 of the thickness of the steel plate in the plate thickness direction. After etching with a liquid, a 100 μm square field of view was observed with an optical microscope at a magnification of 1000 times, and the area ratio of martensite was measured assuming that martensite was white to reddish brown in the field of view. The martensite area ratio was obtained by averaging the area ratios of the observed 20 fields of martensite. Then, using the same sample, etching treatment was performed using picral, and a 100 μm square field of view was observed with an optical microscope at a magnification of 1000 times, and the ratio of hard martensite to martensite was determined within the field of view. .. Then, the ratio of hard martensite in the martensite in 20 fields of view was calculated and averaged, and the average value was multiplied by the martensite area ratio to obtain the area ratio of hard martensite.
[0048]
　For the remaining two tensile test pieces, a tab plate with a thickness of 1.6 mm and a square of 20 mm was placed on one parallel portion, and resistance spot welding was performed using a single-phase AC spot welder under the conditions shown below. .. By resistance spot welding, a nugget having a nugget diameter D of 5 × √t (t: plate thickness (mm) of the tensile test piece) was formed between the tensile test piece and the tab plate.
　Electrode: DR type electrode (tip
　φ6mm R40) Pressurizing pressure: 400kgf Energization
　time 20cyc
[0049]
　After resistance spot welding, one of the two welded test pieces was tempered by irradiating a laser from the first steel plate side. Tempering was performed on the region including the entire range of joint numbers 1 to 5 and the center of the nugget to 0.5 × D to 2.5 × D. For joint numbers 6 to 10, tempering was performed on a region including the entire range of 0.5 × D to 1.0 × D from the center of the nugget.
　For joint numbers 11 to 19, the width is 0.75 × D in the direction parallel to the longitudinal direction of the nugget test piece, and the width is 0.5 × D to 2.5 × D from the center of the nugget. I made a return.
　For joint number 20, resistance spot welding was performed after tempering the position corresponding to the hardness control region after welding with a laser in advance before welding. No tempering was performed after welding.
[0050]
　After cooling to room temperature, all the test pieces were subjected to a tensile test at a tensile speed of 10 mm / min to evaluate the breaking position and the improvement margin of the breaking elongation. The rupture elongation was evaluated based on the percentage of improvement in rupture elongation with respect to the standard, based on the test piece that was not tempered before and after welding.
　With respect to the welded material, if the elongation at break is 100 to 150%, it is Poor, if it is 150 to 250%, it is Good, if it is 250% or more, it is Excellent, and if it is Good or more, a sufficient effect is obtained. I decided.
[0051]
　In addition, the cross section of the steel plate in the thickness direction is in the range of 0.5 × D to 2.5 × D (joint numbers 1 to 5, 11 to 20) from the center of the nugget or 0.5 × D to 1 from the center of the nugget. The minimum value of hardness and the maximum value of hardness in the range of 0.0 × D (joint numbers 6 to 10) were measured.
　Specifically, the position of 1/8 of the plate thickness, the position of 3/8, the position of 5/8, and the position of 7/8 from the surface of the steel plate in the target region are set at one end in the plate surface direction. The hardness was measured at intervals of 0.1 mm from one end to the other end to obtain the maximum and minimum hardness values.
[0052]
　Further, in order to obtain the maximum value of the hardness of the steel sheet, a load of 1.0 kgf was set at 10 locations 15 mm to 20 mm away from one nugget in the direction opposite to the other nugget by using a Vickers hardness tester. The hardness at 1/8, 3/8, 5/8, and 7/8 of the plate thickness from the surface was measured.
　The results are shown in Table 1.
[0053]
　In joint numbers 1 to 5 and 17 to 19 (example of the present invention), the difference between the maximum value and the minimum value of Vickers hardness in the range of 0.5 × D to 2.5 × D from the center of the tempered portion (in the example of the present invention). Was 80 HV or less, and the maximum value of Vickers hardness in this region was lower than the maximum value of Vickers hardness of the base metal part. As a result, the break position was not the HAZ softened part, and the break elongation was tempered. It was 250% or more with respect to the case where
　the tempering was not performed. In the joint numbers 6 to 10 (example of the present invention), the tempered portion (in the range of 0.5 × D to 1.5 × D from the center of the nugget). The difference between the maximum and minimum Vickers hardness was 80 HV or less, and the maximum Vickers hardness in this region was lower than the maximum Vickers hardness of the base metal. As a result, the breakage was a nugget. Although it was the surrounding tempered part, the breaking elongation was improved to 150% or more compared to the case where the tempering was not performed.
　On the other hand, in the joint numbers 11 to 16 and 20 (comparative example), the Vickers of the tempered part. The difference between the maximum value and the minimum value of hardness was 80 HV or more, or the maximum value of Vickers hardness in this region was higher than the maximum value of Vickers hardness of the base metal part. The reason is that during tempering. It is considered that the heating is insufficient. Further, regarding the joint number 20, it is considered that the HAZ softened portion generated by the welding affected the cracking.
[0054]
[table 1]

Industrial applicability
[0055]
　According to the spot welded joint of the present disclosure, even when an in-plane tensile stress is applied, it is possible to suppress breakage from the HAZ softened zone in the region around the nugget. Collision resistance performance can be obtained. Further, in the automobile frame component of the present disclosure, breakage from the HAZ softened portion can be suppressed, so that a predetermined collision resistance can be obtained. Further, in the method for manufacturing a spot welded joint of the present disclosure, it is possible to manufacture a spot welded joint capable of obtaining a predetermined collision resistance when used for a structural member constituting an automobile body.
　Therefore, it has high industrial applicability.
Description of the sign
[0056]
　1 Spot welded joint
　2 Nugget
　2'Nugget (other nugget)
　O Nugget center
　11 First steel plate
　12 Second steel plate
　31 HAZ softened zone
　32 First region
　33 Hardness control region
　34 Second region
　102 Plate surface direction
　201 B pillar
The scope of the claims
[Claim 1]
　A first steel sheet having a tensile strength of 1100 MPa or more and mainly having a hard maltensite structure,
　a second steel sheet
　laminated on the first steel sheet, the first steel sheet, and the second steel sheet. A nugget having a diameter D at the interface between the first steel plate and the second steel plate formed between the first steel plate and
　a cross section of the first steel plate in the thickness direction passing through the center of the nugget. It occupies a region of 0.5 × D to 1.0 × D in the plate surface direction from the center of the nugget and all in the plate thickness direction of the first steel plate, and the maximum value and the minimum value of the hardness in the region. A
spot-welded joint comprising a hardness control region in which the difference from the above is 80 HV or less and the maximum value of the hardness in the region is lower than the maximum value of the hardness of the first steel sheet .
[Claim 2]
　The spot welded joint according to claim 1, further comprising another nugget in the cross section.
[Claim 3]
　The spot welded joint according to claim 1 or 2, wherein all the cross sections have the hardness control region.
[Claim 4]
　The spot welded joint according to any one of claims 1 to 3, wherein the maximum value of the hardness in the region is 250 HV to 450 HV.
[Claim 5]
　The spot welded joint according to any one of claims 1 to 4, wherein the first steel plate and the second steel plate are plated steel plates.
[Claim 6]
　An automobile frame component comprising the spot welded joint according to any one of claims 1 to 5.
[Claim 7]
　By superimposing the first steel sheet and the second steel sheet having a tensile strength of 1100 MPa or more and mainly having a hard maltensite structure and performing resistance spot welding, the first steel sheet and the second steel sheet are combined. In the meantime, a nugget having a diameter D at the interface between the first steel plate and the second steel plate is formed, and
　the cross section of the first steel plate in the plate thickness direction passing through the center of the nugget, said first. A
method for manufacturing a spot welded joint , in which all of the steel plate in the thickness direction and the entire region of 0.5 × D to 1.0 × D from the center of the nugget in the plate surface direction is tempered .
[Claim 8]
　Tempering is performed so that the difference between the maximum value of the hardness in the region and the minimum value of the hardness is 80 HV or less, and the maximum value of the hardness in the region is lower than the maximum value of the hardness of the first steel plate. The method for manufacturing a spot welded joint according to claim 7.
[Claim 9]
　The method for manufacturing a spot welded joint according to claim 7 or 8, wherein the region of all the cross sections is tempered.
[Claim 10]
　The method for manufacturing a spot welded joint according to claim 8 or 9, wherein the maximum value of the hardness in the region is 250 HV to 450 HV.