[0001]The present disclosure relates to a spot weld joint and a method of manufacturing a spot weld joint.
The present application claims priority based on Japanese Patent Application No. 2019-097703 filed in Japan on May 24, 2019, the content of which is incorporated herein by reference. [Related Art]
[0002]
In structures configured by stacking a plurality of steel sheet members, joining by resistance spot welding is widely performed on an overlapping portion where the steel sheet members are overlapped together.
[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, high strength steel sheets having a tensile strength of 980 MPa or more are widely used as high strength steel sheets for vehicles. In recent years, high strength steel sheets having a tensile strength of 1100 MPa or more has begun to be applied. The high strength steel sheets having a tensile strength of 1100 MPa or more generally include a quenched structure in order to obtain a high strength. When resistance spot welding is performed, a nugget (spot weld metal) that welds steel sheets
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is formed, and a heat affected zone (hereinafter, referred to as HAZ) is generated in a periphery of the nugget. In general, the HAZ includes the quenched structure. However, in a case where resistance spot welding is performed on a high strength steel sheet having the quenched structure, a region (HAZ softened portion) having lower hardness than a base metal having a quenched structure is formed. This is because the quenched structure of the base metal is tempered by heat from resistance spot welding.
[0005]
In the event of a vehicle collision, there is a need for protecting passengers in a cabin. Therefore, structural members (overlap welded members) that configure vehicle bodies of vehicles, such as an A pillar, a B pillar, a roof rail, and a side sill need to have a high strength. Ordinarily, the structural member that configures the vehicle body of a vehicle is manufactured by overlapping a plurality of steel sheet members and joining flanges (overlapping portions) by resistance spot welding to form a cylindrical closed cross section. In order to improve deformation resistance in the event of collision and absorb larger collision energy with a small amount of deformation, a method such as high-strengthening of a material (base metal) or an increase in the number of welding points (spots) is employed.
To a portion of the flange of the above-described member that is to be resistance-spot-welded, there is a case where an in-plane tensile stress is applied in the event of collision of a vehicle. In general, when there is a region with low hardness such as the HAZ softened portion, collision-resistant performance of the member decreases. The above-described HAZ softened portion have little influence on evaluation results of a tensile shear test and a cross tension test (JIS Z 3137) that are used for a joint evaluation of resistance spot welding. However, in a case where the in-plane tensile stress is applied, there is a case where a strain locally concentrates in the
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HAZ softened portion and causes breakage in the HAZ softened portion. Even when the high-strengthening of the base metal is performed, and the number of spot points increases, in a case where the above-described HAZ softened portion is generated, there is a case where collision-resistant performance that is assumed from strength of the base metal and shapes of components cannot be obtained.
Therefore, in a case where a steel sheet member made of a high strength steel sheet is applied to a structural member for a vehicle body of a vehicle, there is a demand for suppressing a peripheral region of a nugget acting as a starting point of breakage.
[0006]
In the related art, studies have been made to improve characteristics of welded members formed by resistance spot welding. For example, Patent Document 2 describes, as a weld joint having improved characteristics in a spot-welding portion, a weld joint for which a heat treatment is performed on the spot-welding portion at 100°C to 400°C and an L-tensile joint strength is improved. In addition, Patent Document 3 describes a method in which post energization is performed on a spot-welding portion and a cross tensile joint strength is improved. Patent Document 4 describes a welding method of performing high-frequency induction heating on a periphery of a spot welding electrode immediately after welding with winding coil to temper a spot-welding portion and welded portion, and thus, improve a joint strength that is evaluated from a ratio between TSS and a material strength and a product of CTS and a material strength.
[0007]
In the techniques disclosed in Patent Documents 2 to 4, it is possible to obtain a certain degree of effect in improving the TSS or CTS. However, breakage in the HAZ softened portions when the in-plane tensile stress is applied to the steel sheet is not
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considered in Patent Documents 2 to 4.
[0008]
Regarding the problem, Patent Document 5 describes a B pillar having an energy absorption capability enhanced by providing a region having a strength of lower than 1100 MPa, which is referred to as a soft zone, in a portion or the entirety of a flange portion that is subjected to spot welding.
[0009]
However, in the B pillar disclosed in Patent Document 5, since it is necessary to soften a side flange, there is a concern that bending performance which is collision-resistant performance of a member may degrade. In addition, in Patent Document 5, since a softened region is provided in a component before welding, there is another problem in that shape accuracy of a component decreases. When the shape accuracy of the component decreases, a gap is generated between components during welding, which makes welding difficult. [Prior Art Document] [Patent Document]
[0010]
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2006-142905
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2010-059451
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2015-093282
[Patent Document 4] Japanese Patent No. 5459750
[Patent Document 5] Japanese Patent No. 5894081
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[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0011]
The present disclosure is made in view of the above-described problems, and an object of the present disclosure is to provide a spot weld joint and a method of manufacturing a spot weld joint capable of suppressing breakage from a region interposed between spot weld metals even when an in-plane tensile stress is applied. [Means for Solving the Problem]
[0012]
The present inventors have analyzed a strain distribution when an in-plane tensile stress is generated in a component in which a HAZ softened portion is formed. As a result, the present inventors have found that, by quenching the vicinity of a surface (side opposite to overlapping surface) between the weld metals of a high strength steel sheet subjected to overlap welding, and at the same time, tempering the vicinity of the overlapping surface, it is possible to prevent the strain from being locally concentrated on an HAZ softened portion and prevent breakage from being generated in the HAZ softened portion even when an in-plane tensile stress is applied while minimizing a decrease in a component strength.
[0013]
The present disclosure has been made on the basis of the above findings. A gist of the present disclosure is as follows.
[1] According to an aspect of the present disclosure, there is provided a spot weld joint including: a first steel sheet containing full hard martensite and having an average Vickers hardness HVbase of 350 HV or more; a second steel sheet which is stacked on the first steel sheet; and two spot weld metals which join the first steel sheet
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and the second steel sheet, in which in every cross section of the first steel sheet including the two spot weld metals in a sheet thickness direction, the first steel sheet includes a first region which is formed between the two spot weld metals and in a range of 0.1 mm in the sheet thickness direction of the first steel sheet from a surface on a side of the second steel sheet, and a second region which is formed between the two spot weld metals and in a range of 0.1 mm in the sheet thickness direction from a surface opposite to the surface on the side of the second steel sheet, a metallographic structure of the first region contains 50 area% or more of tempered martensite, and an average Vickers hardness HV1 of the first region and the average Vickers hardness HVbase of the first steel sheet satisfy the following Equation (1), and a metallographic structure of the second region contains 50 area% or more of the full hard martensite, and an average Vickers hardness HV2 of the second region and the average Vickers hardness HVbase of the first steel sheet satisfy the following Equation (2).
HVbase x 0.33 + 150 < HV1 < HVbase x 0.33 + 230 ... (1)
HVbase - 30 < HV2 < HVbase + 30 ... (2)
[2] In the spot weld joint according to [1], a difference between a maximum value and a minimum value of a Vickers hardness in the first region may be 80 HV or less.
[3] In the spot weld joint to [1] or [2], a thickness of the first region in the sheet thickness direction may be 30 to 70% of a thickness of the first steel sheet.
[4] According to another aspect of the present disclosure, there is provided a method of manufacturing a spot weld joint, including: overlapping a first steel sheet containing full hard martensite and having an average Vickers hardness HVbase of 350 HV or more and a second steel sheet; forming two spot weld metals which join the first steel sheet and the second steel sheet which are overlapped; and tempering a range of
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the first steel sheet between the two spot weld metals and in a range of 0.1 mm from a surface on a side of the second steel sheet, and at the same time, quenching a range of the first steel sheet between the two spot weld metals in a range of 0.1 mm from a surface opposite to the surface on the side of the second steel sheet, by laser irradiation. [Effects of the Invention]
[0014]
According to the aspects of the present disclosure, it is possible to obtain a spot weld joint capable of suppressing breakage from a region interposed between spot weld metals even when an in-plane tensile stress is applied and a method of manufacturing a spot weld joint. [Brief Description of the Drawings]
[0015]
FIG. 1 is a cross-sectional view of a spot weld joint in a sheet thickness direction according to the present embodiment.
FIG. 2 is a top view when the spot weld joint according to the present embodiment is viewed from a first steel sheet side.
FIG. 3 is a schematic view showing a relationship between measurements of an achieving temperature and a Vickers hardness when the spot weld joint according to the present embodiment is irradiated with a laser.
FIG. 4 is a schematic view showing a test piece used in Examples. [Embodiments of the Invention]
[0016]
A spot weld joint (a spot weld joint according to the present embodiment) according to an embodiment of the present disclosure, and a method of manufacturing a spot weld joint according to the present embodiment will be described with reference to
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drawings.
As shown in FIG. 1, a spot weld joint 1 according to the present embodiment includes a first steel sheet 11, a second steel sheet 12 stacked on the first steel sheet, and two spot weld metals 2 which join the first steel sheet 11 and the second steel sheet 12 to each other. In FIGS. 1 and 2, the spot weld metal 2 is a nugget formed by resistance spot welding. Such a spot weld joint is obtained by overlapping the first steel sheet 11 and the second steel sheet 12 and performing the resistance spot welding.
The first steel sheet 11 to be used for the resistance spot welding is a steel sheet having an average Vickers hardness (HVbase) of 350 HV or more in consideration of application to vehicle frame components such as B pillars. Further, the first steel sheet has a structure including a quenched structure such as full hard martensite. Meanwhile, regarding the second steel sheet 12, there is no limitation.
The average Vickers hardness (sometimes simply referred to as hardness) of the first steel sheet 11 means an average Vickers hardness of the first steel sheet 11 to be welded before welding. When measured after spot welding, the average Vickers hardness means an average Vickers hardness measured at a position not affected by welding heat.
[0017]
Further, in the spot weld joint 1 according to the present embodiment, in the every cross section in a sheet thickness direction of the first steel sheet 11 including the two spot weld metals 2 and 2, the first steel sheet 11 includes a first region 51 which is formed between the two spot weld metals 2 and 2 and in a range of 0.1 mm from a surface (that is, overlapping surface) on the second steel sheet 12 side, and a second region 52 which is formed between the two spot weld metals 2 and 2 and in a range of 0.1 mm from a surface (that is, joint surface) opposite to the surface on the second steel
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sheet 12 side.
Further, in the spot weld joint 1 according to the present embodiment, a metallographic structure of the first region 51 contains 50 area% or more of tempered martensite, and an average Vickers hardness HVl of the first region 51 and the average Vickers hardness HVbase of the first steel sheet satisfy the following Equation (1).
HVbase X 0.33 + 150 < HVl < HVbase x 0.33 + 230 ... (1)
Further, in the spot weld joint 1 according to the present embodiment, a metallographic structure of the second region 52 contains 50 area% or more of full hard martensite, and an average Vickers hardness HV2 of the second region 52 and the average Vickers hardness HVbase of the first steel sheet satisfy the following Equation (2).
HVbase - 30 < HV2 < HVbase + 30 ... (2)
A metallographic structure satisfying the same conditions of the area% of tempered martensite and the average Vickers hardness as in the first region 51 may extend to the outside of the first region 51. Moreover, a metallographic structure satisfying the same conditions of the area% of full hard martensite and the average Vickers hardness as in the second region 52 may extend to the outside of the second region 52.
Hereinafter, the reasons for limiting each configuration will be described.
[0018]
As described above, in most cases, a high strength steel sheet having the average Vickers hardness of 350 HV or more (about 1100 MPa or more in terms of tensile strength) has a structure including a quenched structure (for example, 50 area% or more) such as full hard martensite. Such a structure is obtained by a manufacturing method including a quenching step.
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When welding is performed on a steel sheet containing a quenched structure, the full hard martensite is changed to a soft structure such as tempered martensite in HAZ formed around a weld metal due to the heat of welding. That is, a region (HAZ softened portion) having lower hardness than the base metal is formed. When a tensile stress is generated in a plane of a sheet having a welding portion, this HAZ softened portion may become a starting point of breakage.
[0019]
As a result of studies by the present inventors, it has been found that concentration of a strain at a location which was the HAZ softened portion can be alleviated by (i) preventing a local strength reduction portion from occurring between the two weld metals to which an in-plane tensile stress is applied, (ii) providing a shape in which a portion with inferior strength or a portion which easily extends is provided at a location except for the HAZ softened portion, and (iii) modifying a structure of a region having a heat affected zone including the HAZ softened portion so that an amount of elongation of the region until breakage is large.
When preventing the occurrence of the local strength reduction portion, for example, it is conceivable that a region including the HAZ softened portion between the spot weld metals 2 is tempered to reduce hardness around the HAZ softened portion to the same degree as hardness of the HAZ softened portion. However, in this case, although breakage from the HAZ softened portion can be suppressed, since the softened portion of a component as a whole becomes large, there is a concern that collision-resistant performance (bending performance) of the component may deteriorate.
In the spot weld joint 1 according to the present embodiment, in order to prevent cracking in the HAZ softened portion while minimizing a decrease in joint strength, the vicinity of the overlapping surface 3 between the spot weld metals 2 and 2
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of the first steel sheet 11 subjected to the overlap welding is tempered, the vicinity of the surface (surface on a side opposite to overlapping surface 3) of the first steel sheet between the spot weld metals 2 and 2 is quenched, and thus, the first region and second region having the desired metallographic structure and average Vickers hardness are formed.
[0020]

[The metallographic structure contains 50 area% or more of tempered martensite, and the average Vickers hardness HV1 of the first region and the average Vickers hardness HVbase of the first steel sheet satisfy HVbase x 0.33 + 150 < HV1 < HVbase x 0.33 + 230]
As a result of studies by the present inventors, when the spot welding is performed on the first steel sheet 11 containing full hard martensite and having the average Vickers hardness HVbase of 350 HV or more, the hardness (Vickers hardness) of the HAZ softened portion caused by heat effects of welding is (hardness of the first steel sheet 11 before welding x 0.33 + 150) to (hardness of the first steel sheet 11 before welding x 0.33 + 230). Therefore, in the spot weld joint to the present embodiment, the first region between the spot weld metals 2 and 2 including the HAZ softened portion is tempered to have the tempered martensite of 50 area% or more, and the Vickers hardness is controlled so as to satisfy the following Equation (1).
When the average Vickers hardness (HV1) of the first region 51 satisfies Equation (1), a hardness difference between the HAZ softened portion and the surroundings thereof becomes 80 HV or less. In this case, the strain concentration on the HAZ softened portion can be alleviated.
HVbase X 0.33 + 150 < HV1 < HVbase x 0.33 + 230 ... (1)
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[0021]
In the first region 51, in addition to the average Vickers hardness satisfying Equation (1), preferably, a difference between a maximum value and a minimum value of the Vickers hardness in the first region is 80 HV or less. By reducing the difference between the maximum value and the minimum value of the hardness in the first region 51, the strain concentration can be further alleviated. In other words, when a hardness distribution within the first region 51 is homogeneous, the local strain concentration can be avoided. More preferably, the difference between the maximum value and the minimum value of Vickers hardness is 50 HV or less.
[0022]
[The first region is formed between two spot weld metals and within a range of 0.1 mm from the surface on the second steel sheet side]
The first region 51 is formed in the thickness cross section of the first steel sheet 11 between the two spot weld metals 2 and 2 and in a range (thickness) of 0.1 mm from the surface on the second steel sheet 12 side. When the thickness of the region having the above hardness in the sheet thickness direction is less than 0.1 mm, the HAZ softened portion, which is a local strength reduction portion, remains, and there is a possibility that a sufficient effect cannot be obtained. The region satisfying the conditions of the hardness and structure of the first region 51 may extend to the outside of the first region 51. In that case, preferably, the region satisfying the hardness and the structure of the first region 51 extends from the surface on the second steel sheet side to a range of 30% or more of the sheet thickness of the first steel sheet. However, when the region satisfying the hardness and structure of the first region 51 extends from the surface on the second steel sheet side to a range of more than 90% of the sheet thickness of the first steel sheet, it is not preferable because there is a concern that an
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average hardness of the entire joint decreases and a bending proof stress decreases.
[0023]

[The metallographic structure contains 50 area% or more of full hard martensite, and the average Vickers hardness HV2 of the second region and the average Vickers hardness HVbase of the first steel sheet satisfy HVbase - 30 < HV2 < HVbase +30]
In a case where an in-plane tensile stress is applied to the spot weld joint 1 according to the first embodiment, when the local strength reduction portion such as the HAZ softened portion is formed between the two spot weld metals 2 and 2, a strain is concentrated. However, as a result of studies by the present inventors, in particular when there is a strength reduction portion in the vicinity of the surface (surface opposite to the overlapping surface) of the first steel sheet 11, it is found that the strain tends to concentrate.
Therefore, in the spot weld joint 1 according to the present embodiment, the vicinity of the surface of the first steel sheet 11 between the two spot weld metals 2 and 2 including the HAZ softened portion is quenched, and the hardness of the quenched region is made equal to the average Vickers hardness of the first steel sheet 11 which is not affected by welding heat.
Specifically, the average Vickers hardness HV2 of the second region 52 and the average Vickers hardness HVbase of the first steel sheet 11 satisfy the following Equation (2).
HVbase - 30 < HV2 < HVbase + 30 ... (2)
When a difference between the average Vickers hardness of the second region 52 and the average Vickers hardness of the first steel sheet 11 is more than 30, it is not
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possible to sufficiently suppress the strain concentration when the in-plane tensile stress is applied.
Since the first region 51 is the tempered structure containing the tempered marten site of 50 area% or more, the second region is the quenched structure containing full hard marten site of 50 area% or more, the average Vickers hardness (HV2) of the second region 52 is larger than the average Vickers hardness (HV1) of the first region 51.
[0024]
[The second region is formed between two spot weld metals and within the range of 0.1 mm from the surface opposite to the surface on the second steel sheet side]
The second region 52 is formed in the thickness cross section of the first steel sheet 11 between the two spot weld metals 2 and 2 and in the range (thickness) of 0.1 mm from the surface opposite to the surface of the second steel sheet 12 side. When the thickness of the region having the above hardness in the sheet thickness direction is less than 0.1 mm, the HAZ softened portion, which is a local strength reduction portion, remains, and there is a possibility that a sufficient effect cannot be obtained. The region satisfying the conditions of the hardness and structure of the second region 52 may extend to the outside of the second region 52. In that case, preferably, the region satisfying the hardness and structure of the second region 52 is formed from the surface opposite to the surface of the second steel sheet side to a range of 10% or more of the sheet thickness of the first steel sheet. However, in a case where the region satisfying the hardness and the structure of the second region 52 extends from the surface on the second steel sheet 12 side to a range of more than 70% of the first steel sheet 11, when a tensile bending load is applied so that the second region side is outside the bending, there is a concern that breakage elongation may decrease, which is not preferable.
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[0025]
In the spot weld joint 1 according to the present embodiment, the first region 51 and the second region 52 are formed in the cross section of the first steel sheet including the two spot weld metals in the sheet thickness direction. That is, when the cross section in the sheet thickness direction of the first steel sheet is observed so as to include two weld metals, the above-mentioned first region 51 and second region 52 are observed in the every cross section.
In other words, when the diameter of the spot weld metal 2 in the overlapping surface 3 is represented by D, widths of the first region 51 and the second region 52 in a direction (direction perpendicular to the paper surface of FIG. 1 and vertical direction on the paper surface of FIG. 2) perpendicular to the cross section in the sheet thickness direction of the first steel sheet 11 are 1.0 x D. A metallographic structure satisfying the same conditions of the area% of tempered martensite and the average Vickers hardness as in the first region 51 may extend to the outside in the width direction of the first region 51. A metallographic structure satisfying the same conditions of the area% of full hard martensite and the average Vickers hardness as in the second region 52 may extend to the outside in the width direction of the second region 52.
When the vehicle collision occurs, the direction of the in-plane tensile stress is not necessarily parallel to a direction connecting the spot weld metals 2 and 2 (direction connecting the centers of the spot weld metals 2 and 2). That is, the direction of the in-plane tensile stress may have a certain angle (stress is applied in an oblique direction). In a case where the widths of the first region 51 and the second region 52 are 1.0 x D, even when a direction in which the in-plane tensile stress is applied is a certain angle with respect to the direction connecting the spot weld metals 2 and 2 (even when stress is applied in the oblique direction), the strain concentration on the HAZ
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softened portion where the strain is concentrated is suppressed. As a result, breakage at the HAZ softened portion is further suppressed.
Meanwhile, when the widths of the first region 51 and the second region 52 are less than 1.0 x D, in a case where the direction of the in-plane tensile stress is a certain angle with respect to the direction connecting the spot weld metals 2 and 2 (stress is applied in the diagonal direction), there is a concern that a sufficient effect cannot be obtained.
[0026]
The average hardness of the first steel sheet 11 is measured using a Vickers hardness tester having a load of 1.0 kgf.
In a steel sheet having a full hard martensite structure, the hardness of a portion affected by welding heat becomes lower than the hardness before welding. Therefore, for the hardness of the first steel sheet 11, the hardness at a position not affected by heat due to welding of the first steel sheet 11 is measured, and the average value thereof is used. As the position that is not affected by heat due to welding, for example, the hardness at a position 15 mm or more away from the spot weld metal 2 in a direction free of other weld metals may be measured.
Specifically, the hardness is measured using a Vickers hardness tester under a load set to 1.0 kgf at a 1/8 thickness position, a 3/8 thickness position, a 5/8 thickness position, and a 7/8 thickness position from the surface of the first steel sheet 11 in 10 places that are not affected by heat due to welding, and the average value thereof is used.
[0027]
Regarding the thicknesses and the average Vickers hardnesses in the first region 51 and the second region 52 from the surface in the first steel sheet 11, using the
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Vickers hardness tester with a load of 100 gf, polishing and Vickers hardness measurements are repeated for the cross section of the first steel sheet in the sheet thickness direction, and a distribution of Vickers hardness in the range interposed between the spot weld metals 2 and 2 is obtained. Based on this distribution, the thicknesses and the average Vickers hardnesses of the first region 51 and the second region 52 are calculated.
Specifically, the distribution of Vickers hardness is measured by the following method.
First, a sample is collected so that the cross section (A-A cross section shown in FIG. 2) in the sheet thickness direction of the first steel sheet 11 passing through the centers of the two spot weld metals is a measurement surface.
With respect to this measurement surface, in the sheet thickness direction of the first steel sheet 11, the Vickers hardness is measured at positions of 0.1 mm from each of the surface and the overlapping surface of the first steel sheet 11, and positions which divide a portion therebetween into five equal portions. This measurement is repeated at intervals of 0.5 mm in the width direction (the direction connecting one spot weld metal 2 and the other spot weld metal 2).
Then, the sample is polished by 0.5 mm, and the Vickers hardness measurement in the same manner as described above is performed on the exposed cross section (B-B cross section shown in FIG. 2).
Further, the polishing and the measurement of the Vickers hardness are performed until the cross section does not contain the spot weld metal 2, and the Vickers hardness distribution of the first steel sheet 11 between the spot weld metals 2 and 2 is obtained. Since the hardness is considered to be the same for the cross section on the opposite side, the measurement may be performed for half the cross section as described
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above.
[0028]
Whether or not the first steel sheet 11 to be welded includes full hard martensite may be determined by obtaining samples from five places of a 1/8 thickness position, a 3/8 thickness position, a 5/8 thickness position, and a 7/8 thickness position from the surface of the position of the first steel sheet 11 that is not affected by heat due to welding and etching the samples using a Lepera etachnt to observe a 100 um square field of view with an optical microscope at a magnification of 1000 times. In the observed visual field, what appears to be white to reddish brown is martensite, among the martensite, martensite containing carbide is determined as the full hard martensite, and martensite that does not contain carbide is determined as tempered martensite.
[0029]
Further, an area ratio of the tempered martensite in the first region 51 and an area ratio of the full hard martensite in the second region 52 are measured as follows. That is, on the same surface as the Vickers hardness measurement surface described above, samples are obtained from five places of a target region (region satisfying Equation (1) in the case of the first region and region satisfying Equation (2) in the case of the second region), the samples are etched using a Lepera etachnt to observe a 100 urn square field of view with an optical microscope at a magnification of 1000 times, and in the observed visual field, the area ratio of martensite is measured assuming that what appears white to reddish brown is martensite. By averaging the area ratios of martensite in the observed fields of view, the area ratios of the martensite in the first region 51 and the second region 52 can be obtained. Then, the same sample is etched with picral to observe a 100 urn square field of view with an optical microscope at a magnification of 1000 times, and in the observed visual field, the martensite containing
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carbide is determined as the full hard martensite, the martensite that does not contain carbide is determined as the tempered martensite, and thus, proportions of the tempered martensite and full hard martensite in the martensite are obtained.
[0030]
In the present embodiment, the spot weld joint in which the weld metal is formed by the spot welding is targeted. The spot welding, also called point welding, is welding in which two overlapped steel sheets are connected by dots. Examples of the means of the spot welding include arc spot welding, resistance spot welding, and laser spot welding. Meanwhile, welding performed linearly is called continuous welding. Examples of the means of the continuous welding include arc welding, laser welding, seam welding, or the like. Compared to the continuous welding, the point welding has a smaller welding area, and thus, a construction time is shorter and power is saved. That is, the point welding is excellent in productivity.
In the above, the case where the spot weld metal 2 is the nugget of the resistance spot welding is described. However, the spot weld joint 1 according to the present embodiment is not limited to the resistance spot weld joint in which the spot weld metal 2 is the nugget of the resistance spot welding. For example, the spot weld metal 2 may be formed by laser spot welding, or the spot weld metal 2 may be formed by arc spot welding.
[0031]
In the spot weld joint 1 according to the present embodiment, for example, the first steel sheet 11 is a hat member, the second steel sheet 12 is a closing plate, and preferably, the two spot weld metals 2 are formed in the overlapping portion of the flange portion of the hat member and the closing plate. With such a configuration, it is particularly effective in improving the strength and collision-resistant performance of
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the structural member.
[0032]
Moreover, the spot weld joint 1 according to the present embodiment has the two spot weld metals 2, and when the region between the two spot weld metals 2 and 2 satisfy the above relationship, the effects can be obtained. When applied to a vehicle frame component or the like, a plurality of (two or more) spot weld metals are formed. Even in a case where more than two spot weld metals are formed, when the region between the two target spot weld metals has the above relationship, the effects can be obtained for that region. When there are more than 2 spot weld metals, preferably, a relationship between the two spot weld metals 2 is controlled to satisfy the relationship, particularly in a portion where the in-plane tensile stress is expected to be applied. In a case where relationships between all the spot weld metals 2 satisfy the above relationship of the hardness, it is more preferable because breakage at the HAZ softened portion can be suppressed even when tensile stress is applied in any direction and location in the plane of the high strength steel sheet.
[0033]
The spot weld joint 1 according to the present embodiment can be applied to an A pillar, a side sill, a B pillar, or the like. For example, in the B pillar, a flange portion of a hat member is joined to a closing plate by a spot weld metal. When the above relationship is satisfied between the spot weld metals of the B pillar, even in a case where an in-plane tensile stress is applied to the flange portion at the time of a vehicle collision, breakage at a portion that is the HAZ softened portion can be suppressed.
[0034]
The first steel sheet 11 and/or the second steel sheet 12 may be plated steel sheets. In this case, corrosion resistance is improved. As the plated steel sheet, for
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example, a hot-dip galvanized steel sheet, a galvannealed steel sheet, an electrolytic zinc-plated steel sheet, an aluminized steel sheet, and the like are exemplified.
[0035]
Next, a method of manufacturing a spot weld joint according to the present embodiment will be described.
The spot weld joint to the present embodiment can be manufactured by a manufacturing method including the following steps. That is, the method of manufacturing a spot weld joint to the present embodiment includes
(I) overlapping a first steel sheet containing full hard martensite and having an
average Vickers hardness HVbase of 350 HV or more and a second steel sheet,
(II) forming a plurality of spot weld metals which join the first steel sheet and
the second steel sheet which are overlapped, and
(III) tempering a range of the first steel sheet between the two spot weld metals
in a range of 0.1 mm from a surface on the second steel sheet side, and at the same time,
quenching a range of the first steel sheet between the two spot weld metals in a range of
0.1 mm from a surface opposite to the surface on the second steel sheet side.
[0036]
As the first steel sheet 11 containing the full hard martensite and having the average Vickers hardness HVbase of 350 HV or more and the second steel sheet 12, known steel sheets can be used.
The steel sheets are overlapped and spot-welded to form a spot weld metal to form a weld joint. The spot welding conditions are not limited and may be normal conditions.
[0037]
After the spot welding, a portion of the first steel sheet is quenching and a
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portion thereof is tempered by laser irradiation to form a first region and a second region.
When performing the quenching, it is necessary to raise a temperature of a target region to more than Acl°C. It is preferably Acl + 30°C or higher. However, when a temperature of a quenching region is too high, a region that needs to be tempered will also become the quenching region due to heat conduction. Therefore, it is necessary to control the heat input according to the sheet thickness.
Meanwhile, when tempering is performed, it is necessary to heat the temperature of the target region to a temperature of less than Acl°C. As shown in FIG. 3, the heated region becomes the tempered martensite, and the hardness decreases as the temperature rises up to Acl°C. Meanwhile, when the heating temperature exceeds Acl°C, the structure is transformed into austenite. Since this austenite is transformed into full hard martensite again by cooling, a portion heated to more than Acl°C has high hardness.
Taking advantage of this, by performing the laser irradiation on the surface (the surface opposite to the second steel sheet) side of the first steel sheet, heating a temperature near the surface of the first steel sheet so as to exceed Acl°C, and heating a temperature near the opposite surface to Acl°C or lower, a predetermined first region and second region can be formed.
In the case of heating as described above, in order to give a hardness distribution in the sheet thickness direction, it is necessary to apply heat only to the extreme surface layer and to apply heat in the depth direction by heat conduction. Further, when heating is performed to the outside of a target region, heat removal in the target region may be insufficient and the tempered structure may not be obtained.
For example, in high-frequency induction heating, heat is input to a certain
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depth, and thus, a preferable hardness distribution cannot be obtained. Further, in gas heating or arc heating, it is difficult to heat only a specific region.
Therefore, in the method of manufacturing a spot weld joint according to the present embodiment, heating is performed by laser beam irradiation. In order to heat the entire space between the weld metals, it is preferable to heat a laser beam having a beam width equal to or larger than a diameter of the weld metal while moving the laser beam at a constant speed.
The laser irradiation conditions are not particularly limited and may be determined depending on the thickness of the first steel sheet, the first region or the second region to be obtained, or the like. For example, the following conditions are exemplified.
Exemplified Conditions
Oscillator type: Semiconductor laser
Output: 500 to 3000W
Beam shape: Rectangle with width direction: 4 to 10 mm and traveling direction: 0.5 to 3 mm on an irradiation surface
Laser moving speed: 50 to 500 cm/min
[0038]
By the laser irradiation, it is possible to temper the first steel sheet between the two spot weld metals and in the range of 0.1 mm from the surface on the second steel sheet side, and at the same time, quench the first steel sheet between the two spot weld metals and in the range of 0.1 mm from the surface opposite to the surface on the second steel sheet side. [Examples]
[0039]
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Hereinafter, the present disclosure will be specifically described using examples with reference to drawings and Table. These examples are merely examples for confirming the effect of the present disclosure and do not limit the present disclosure.
[0040]
First, a steel sheet having a sheet thickness of 2.0 mm was held in a furnace at 950°C for 5 minutes, and then quenching was performed by hot stamping the steel sheet with a water cooling die. After the quenching, oxide scale on the surface of the steel sheet was removed by shot blasting. The Vickers hardness of the used steel sheet after the quenching was as shown in Table 1. In addition, this steel sheet had a structure containing full hard martensite.
[0041]
Next, a tensile test piece having a gauge length of 50 mm and a parallel portion width of 25 mm as shown in FIG. 4 was collected from the steel sheet. In addition, a tab sheet of 25 mm X 25 mm was collected from the same steel sheet.
[0042]
As shown in FIG. 4, the tab sheet was placed on the parallel portion of the collected tensile test piece, and resistance spot welding was performed on a center portion of each tab sheet using a single-phase AC spot welder under the conditions shown below.
Electrode: DR-type electrode (tip (p: 6mm, R40)
Welding pressure: 400 kgf
Energization time: 24 eye
Due to the resistance spot welding, weld metals having a nugget diameter of 4 x Vt (t: sheet thickness (mm) of tensile test piece) were formed at two locations between
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the tensile test piece and the tab sheet.
[0043]
The test piece (joints Nos. 1 to 5, 11 and 12) after the spot welding were irradiated with laser from one side of the tensile test piece, and a heat treatment was performed over the entire parallel portion in a longitudinal direction so that a central portion in a width direction of the parallel portion coincided with a center of the beam. The laser irradiation was not performed on joint Nos. 6 to 10.
Laser irradiation conditions for joint Nos. 1 to 5 were as follows.
Oscillator type: Semiconductor laser
Output: 1200W
Beam shape: Rectangle with width direction: 8 mm and traveling direction: 1 mm on irradiation surface
Laser moving speed: 250 cm/min
In addition, laser irradiation conditions for joint No. 11 were as follows.
Oscillator type: Semiconductor laser
Output: 700W
Beam shape: Rectangle with width direction: 8 mm and traveling direction: 1 mm on irradiation surface
Laser moving speed: 130 cm/min
In addition, laser irradiation conditions for joint No. 12 were as follows.
Oscillator type: Semiconductor laser
Output: 750W
Beam shape: Rectangle with width direction: 8 mm and traveling direction: 1 mm on irradiation surface
Laser moving speed: 80 cm/min
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[0044]
After that, it was determined whether or not the first region and the second region having a predetermined size were formed, and when the regions were formed, the average Vickers hardness and the area ratios of the tempered martensite or the full hard martensite of the first region and the second region were investigated by the above method. The measurement surface was a cross section in the thickness direction at the center of the width direction of the test piece.
In addition, a tensile test was performed on each test piece (in-plane tensile stress was applied) to investigate a breakage position. The tensile speed during the tensile test was 10 mm/min.
The results are shown in Table 1.
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[0045]
[Table 1]

Joint No. Base metal First region Second region Difference between average hardness of base metal and average hardness of second region (HV) Difference between maximum
value and
minimum value of
Vickers hardness
in first region
(HV) Cracking in
HAZ
softened
portion Remark

Average
hardness
(HV) Average
hardness
(HV) Tempered
mart en site
area ratio
(%) Thickness in sheet thickness direction
with respect to
thickness of steel
sheet (%) Average
hardness
(HV) Full hard tnartensite area ratio Thickness in sheet thickness direction
with respect to
thickness of steel
sheet (%)

Present Invention Example 1 365 270 51 40 345 51 60 20 40 Monexistence Present Invention Example
Present Invention Example 2 410 306 55 44 385 70 56 25 45 Monexistence Present Invention Example
Present Invention Example 3 455 329 60 45 429 80 55 26 50 Nonexistence Present Invention Example
Present Invention Example 4 500 365 70 51 475 85 49 25 55 Nonexistence Present Invention Example
Present Invention Example 5 600 408 80 55 570 90 45 30 65 Nonexistence Present Invention Example
Comparative Example 6 365 - - 0 - - 100 - - Existence No laser irradiation
Comparative Example 7 410 - - 0 - - 100 - - Existence No laser irradiation
Comparative Example 8 455 - - 0 - - 100 - - Existence No laser irradiation
Comparative Example 9 555 - - 0 - - 100 - - Existence No laser irradiation
Comparative Example 10 600 - - 0 - - 100 - - Existence No laser irradiation
Comparative Example 11 455 445 2 - 330 30 - - - Existence Laser irradiation:
Heat input insufficient:
Condition unsuitable:
Surface is tempered and
sheet-sheet interface is
not tempered
Comparative Example 12 455 400 32 70 460 86 30 5 135 Existence Laser irradiation: Heat input insufficient: Condition unsuitable: Surface is quenched and tempering of sheet-sheet interface is insufficient
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[0046]
In the joint Nos. 1 to 5 (present invention example), the portion within the range of 0.1 mm from the overlapping surface (first region) was tempered to satisfy Equation (1), and the portion within the range of 0.1 mm from the surface (second region) is quenched to satisfy Equation (2). As a result, no cracking was observed in the HAZ softened portion.
Meanwhile, in the joint Nos. 6 to 10 (comparative examples), the laser irradiation was not performed, and thus, Equations (1) and (2) were not satisfied in the range corresponding to the first region or the second region of joint Nos. 1 to 5. As a result, in the tensile test, cracking occurred in the HAZ softened portion.
In joint Nos. 11 and 12 (comparative examples), the laser irradiation was performed, but the laser irradiation conditions were not preferable. As a result, in the comparative example 11, the HAZ softened portion clearly remained in the first region due to insufficient heat input, and the HAZ softened portion was broken. Moreover, in the comparative example 12, heat input was insufficient, and though the second region was quenched to satisfy Equation (2), tempering was insufficient in the first region, Equation (1) was not satisfied in the first region, and the difference between the maximum value and the minimum value of Vickers hardness in the first region was 80 or more. As a result, in the tensile test, cracking occurred in the HAZ softened portion. [Brief Description of the Reference Symbols]
[0047]
1 Spot weld joint
2 Spot weld metal
3 Overlapping surface
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11 First steel sheet
12 Second steel sheet

51 First region
52 Second region

WE CLAIMS

1.A spot weld joint comprising:
a first steel sheet containing full hard martensite and having an average Vickers hardness HVbase of 350 HV or more;
a second steel sheet which is stacked on the first steel sheet; and
two spot weld metals which join the first steel sheet and the second steel sheet,
wherein in every cross section of the first steel sheet including the two spot weld metals in a sheet thickness direction,
the first steel sheet includes
a first region which is formed between the two spot weld metals and in a range of 0.1 mm in the sheet thickness direction of the first steel sheet from a surface on a side of the second steel sheet, and
a second region which is formed between the two spot weld metals and in a range of 0.1 mm in the sheet thickness direction from a surface opposite to the surface on the side of the second steel sheet,
a metallographic structure of the first region contains 50 area% or more of tempered martensite, and an average Vickers hardness HV1 of the first region and the average Vickers hardness HVbase of the first steel sheet satisfy the following Equation (1), and
a metallographic structure of the second region contains 50 area% or more of the full hard martensite, and an average Vickers hardness HV2 of the second region and the average Vickers hardness HVbase of the first steel sheet satisfy the following Equation (2).
HVbase X 0.33 + 150 < HV1 < HVbase x 0.33 + 230 ... (1)
HVbase - 30 < HV2 < HVbase + 30 ... (2)
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2. The spot weld joint according to claim 1,
wherein a difference between a maximum value and a minimum value of a Vickers hardness in the first region is 80 HV or less.
3. The spot weld joint according to claim 1 or 2,
wherein a thickness of the first region in the sheet thickness direction is 30 to 70% of a thickness of the first steel sheet.
4. A method of manufacturing a spot weld joint, comprising:
overlapping a first steel sheet containing full hard martensite and having an
average Vickers hardness HVbase of 350 HV or more and a second steel sheet;
forming two spot weld metals which join the first steel sheet and the second steel sheet which are overlapped; and
tempering a range of the first steel sheet between the two spot weld metals and in a range of 0.1 mm from a surface on a side of the second steel sheet, and at the same time, quenching a range of the first steel sheet between the two spot weld metals and in a range of 0.1 mm from a surface opposite to the surface on the side of the second steel sheet, by laser irradiation.