CN114007796A

CN114007796A - Spot welded joint and method for manufacturing spot welded joint

Info

Publication number: CN114007796A
Application number: CN202080038325.2A
Authority: CN
Inventors: 钱谷佑; 广濑智史; 古贺敦雄; 滨田幸一; 泰山正则
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2019-05-24
Filing date: 2020-05-22
Publication date: 2022-02-01
Anticipated expiration: 2040-05-22
Also published as: JP6885523B2; CN114007796B; KR20210150563A; WO2020241500A1; JPWO2020241500A1; KR102556695B1

Abstract

A spot welded joint comprising a 1 st steel plate, a 2 nd steel plate overlapped with the 1 st steel plate, and two spot welding metals joining the 1 st steel plate and the 2 nd steel plate, in a cross section including the two spot-welded metals, the 1 st steel plate has a 1 st region and a 2 nd region, the 1 st region is formed between the two spot-welded metals and is in the range of 0.1mm in the thickness direction of the 1 st steel plate from the 2 nd steel plate side surface, the 2 nd region is formed between the two spot welding metals and is in a range of 0.1mm in the plate thickness direction from a surface opposite to the 2 nd steel plate side surface, the average Vickers hardness HV1 of the 1 st region and the average Vickers hardness HVbase of the 1 st steel sheet satisfy HVbase x 0.33+150 HV1 HVbase x 0.33+230, the average Vickers hardness HV2 of the 2 nd region and the average Vickers hardness HVbase of the 1 st steel sheet satisfy HVbase-30 ≦ HV2 ≦ HVbase + 30.

Description

Spot welded joint and method for manufacturing spot welded joint

Technical Field

The present disclosure relates to a spot weld joint and a method of manufacturing the spot weld joint.

This application is based on the priority claim from patent application No. 2019-097703, which was filed 24.05.2019, the contents of which are hereby incorporated by reference.

Background

In a structure formed by overlapping a plurality of steel plate members, joining by resistance spot welding is widely performed on an overlapped portion formed by overlapping the steel plate members.

For example, patent document 1 describes an energy absorbing member in which a cap and a closing plate are joined to each other by spot welding.

Currently, high-strength steel sheets having a tensile strength of 980MPa or more are widely used as high-strength steel sheets for automobiles. In recent years, high-strength steel sheets having a tensile strength of 1100MPa or more have also come to be used. High-strength steel sheets having a tensile strength of 1100MPa or more generally include a quenched structure in order to obtain high strength. In resistance spot welding, a nugget (spot-welded metal) to be welded to a steel sheet is formed, and a heat affected zone (hereinafter referred to as HAZ) is generated around the nugget. Typically the HAZ comprises quenched tissue. However, when resistance spot welding is performed on a high-strength steel sheet having a quenched structure, a region (HAZ softened portion) having a hardness lower than that of a base material serving as the quenched structure is formed. This is because the quenched structure of the base material is tempered by the heat of resistance spot welding.

When a collision occurs in an automobile, it is necessary to protect passengers in a vehicle compartment. Therefore, structural members (lap-welded members) constituting the automobile body, such as the a-pillar, the B-pillar, the roof side rail, and the side sill, need to have high strength. In general, a structural member constituting an automobile body is manufactured by overlapping a plurality of steel plate members and joining flanges (overlapped portions) by resistance spot welding to form a tubular closed cross section. In order to increase the deformation resistance at the time of collision and absorb more collision energy with a small amount of deformation, methods of increasing the strength of the raw material (base material) and increasing the number of welding (spot welding) points can be employed.

A part of the flange of the member subjected to resistance spot welding may be subjected to tensile stress in the load surface at the time of collision of the automobile. Generally, if there is a region with low hardness such as a HAZ softened portion, the impact resistance of the member is reduced. Such a HAZ softened portion has little influence on the evaluation results of a tensile shear test and a cross tensile test (JIS Z3137) used for joint evaluation in resistance spot welding. However, when in-plane tensile stress is applied, strain is locally concentrated on the HAZ softened portion, and breakage may occur in the HAZ softened portion. Therefore, even if the base material is strengthened to increase the spot welding point, if the HAZ softened portion is generated, the collision resistance assumed by the strength of the base material and the shape of the member may not be obtained.

Therefore, when a steel sheet member including a high-strength steel sheet is applied to a structural member of an automobile body, it is required to suppress the peripheral region of the nugget from becoming a starting point of fracture.

Conventionally, studies have been made to improve the characteristics of a welded member formed by resistance spot welding. For example, patent document 2 describes a welded joint in which a spot-welded portion is heat-treated at 100 to 400 ℃ to improve the strength of an L-shaped tensile joint, as a welded joint in which the characteristics of the spot-welded portion are improved. Patent document 3 describes: a method for improving the strength of a cruciform tensile joint by post-energizing the spot-welded portions. Patent document 4 describes a welding method in which the joint strength evaluated by the ratio of TSS to material strength and the product of CTS to material strength is improved by winding the periphery of a spot welding electrode with a coil, rapidly performing high-frequency induction heating after welding, and tempering a spot welded portion and a melted portion.

However, according to the techniques disclosed in these patent documents 2 to 4, although a certain effect can be obtained in terms of improvement of TSS and CTS, these patent documents 2 to 4 do not consider fracture at the HAZ softened portion when an in-plane tensile stress is applied to the steel sheet.

In order to solve such a problem, patent document 5 describes a B-pillar in which energy absorption capacity is improved by providing a region having a strength of less than 1100MPa, called a soft region, in a part or all of a flange portion to be subjected to spot welding.

However, in the B-pillar disclosed in patent document 5, the side flange (side flange) needs to be softened, and thus there is a possibility that the bending performance as the collision resistance of the member is lowered. In addition, in patent document 5, since a softened region is provided in the member before welding, there is also a problem that the shape accuracy of the member is lowered. If the shape accuracy of the members is lowered, a gap is generated between the members at the time of welding, and it is difficult to perform welding.

Prior art documents

Patent document

Patent document 1: japanese unexamined patent publication No. 2006-142905

Patent document 2: japanese laid-open patent publication No. 2010-059451

Patent document 3: japanese laid-open patent publication No. 2015-093282

Patent document 4: japanese patent No. 5459750 publication

Patent document 5: japanese patent No. 5894081 publication

Disclosure of Invention

The present disclosure has been made in view of the above problems, and an object of the present disclosure is to provide a spot welded joint and a method for manufacturing the spot welded joint, which can suppress fracture from a region sandwiched between spot welding metals even when an in-plane tensile stress is applied.

The present inventors analyzed the strain distribution when in-plane tensile stress is generated in a member in which the HAZ softened portion is formed. As a result, they found that: by quenching the vicinity of the surface (the side opposite to the overlapping surface) of the high-strength steel sheets subjected to overlap welding between the weld metals and tempering the vicinity of the overlapping surface, it is possible to minimize the reduction in the strength of the component, and it is possible to avoid the occurrence of fracture in the HAZ softened portion due to local concentration of strain in the HAZ softened portion even when in-plane tensile stress is applied.

The present disclosure has been completed based on the above-described findings. The gist of the present disclosure is as follows.

[1] A spot welded joint according to one aspect of the present disclosure includes a 1 st steel plate, a 2 nd steel plate, and two spot welding metals joining the 1 st steel plate and the 2 nd steel plate, wherein the 1 st steel plate is a hard martensite-containing steel plate having an average vickers hardness HVbase of 350HV or more, the 2 nd steel plate and the 1 st steel plate overlap, the 1 st steel plate has a 1 st region and a 2 nd region in a whole cross section in a plate thickness direction of the 1 st steel plate including the two spot welding metals, the 1 st region is formed between the two spot welding metals and is in a range of 0.1mm in the plate thickness direction of the 1 st steel plate from a surface on the 2 nd steel plate side, the 2 nd region is formed between the two spot welding metals and is in a range of 0.1mm in the plate thickness direction from a surface opposite to the surface on the 2 nd steel plate side, a metallic structure of the 1 st region includes 50% by area or more of tempered martensite, the average Vickers hardness HV1 of the 1 st region and the average Vickers hardness HVbase of the 1 st steel sheet satisfy the following formula (1), the metal structure of the 2 nd region contains 50 area% or more of hard martensite, the average Vickers hardness HV2 of the 2 nd region and the average Vickers hardness HVbase of the 1 st steel sheet satisfy the following formula (2),

HVbase × 0.33+150 ≦ HV1 ≦ HVbase × 0.33+230 formula (1)

HVbase-30 ≤ HV2 ≤ HVbase +30 formula (2)

[2] In the spot welded joint according to [1], a difference between a maximum value and a minimum value of the vickers hardness in the 1 st region may be 80HV or less.

[3] In the spot welded joint according to [1] or [2], a thickness of the 1 st region in the plate thickness direction may be 30 to 70% of a plate thickness of the 1 st steel plate.

[4] Another aspect of the present disclosure relates to a method for manufacturing a spot welded joint,

overlapping a 1 st steel sheet and a 2 nd steel sheet, wherein the 1 st steel sheet is a steel sheet containing hard martensite and has an average Vickers hardness HVbase of 350HV or more,

forming two spot welding metals joining the overlapped 1 st and 2 nd steel plates,

hardening the 1 st steel plate in a range of 0.1mm from the surface opposite to the 2 nd steel plate side between the two spot-welded metals while tempering the 1 st steel plate in a range of 0.1mm from the 2 nd steel plate side surface between the two spot-welded metals by laser irradiation.

According to the above aspect of the present disclosure, it is possible to obtain a spot welded joint and a method of manufacturing a spot welded joint capable of suppressing fracture from a region sandwiched between spot-welded metals even when an in-plane tensile stress is applied.

Drawings

Fig. 1 is a sectional view in the plate thickness direction of the spot welded joint according to the present embodiment.

Fig. 2 is a view of the spot welded joint according to the present embodiment as viewed from the 1 st steel plate side.

Fig. 3 is a schematic view showing a relationship between an arrival temperature and a vickers hardness measurement when the spot welded joint according to the present embodiment is irradiated with a laser beam.

FIG. 4 is a schematic view showing a test piece used in the examples.

Detailed Description

A spot-welded joint according to an embodiment of the present disclosure (spot-welded joint according to the present embodiment) and a method for manufacturing the spot-welded joint according to the present embodiment will be described with reference to the drawings.

As shown in fig. 1, the spot welded joint 1 according to the present embodiment includes a 1 st steel plate 11, a 2 nd steel plate 12 overlapping the 1 st steel plate, and two spot welding metals 2 joining the 1 st steel plate 11 and the 2 nd steel plate 12. In fig. 1 and 2, the spot welding metal 2 is a nugget formed by resistance spot welding. Such a spot welded joint is obtained by resistance spot welding the 1 st steel plate 11 and the 2 nd steel plate 12 in a stacked state.

The 1 st steel sheet 11 to be subjected to resistance spot welding is a steel sheet having an average vickers hardness (HVbase) of 350HV or more in consideration of application to automobile frame members such as a B pillar. The 1 st steel sheet is composed of a structure including a quenched structure such as hard martensite. On the other hand, the 2 nd steel plate 12 is not limited.

The average vickers hardness (sometimes simply referred to as hardness) of the 1 st steel sheet 11 means the average vickers hardness of the 1 st steel sheet 11 before welding. The term "average vickers hardness" as measured after spot welding refers to the average vickers hardness measured at a position not affected by the welding heat.

In the spot welded joint 1 according to the present embodiment, in the entire cross section in the plate thickness direction of the 1 st steel plate 11 including the two spot-welded metals 2, the 1 st steel plate 11 has the 1 st region 51 and the 2 nd region 52, the 1 st region 51 being formed between the two spot-welded

metals

2, 2 and ranging from the surface on the 2 nd steel plate 12 side (i.e., the overlapping surface) to 0.1mm, and the 2 nd region 52 being formed between the two spot-welded

metals

2, 2 and ranging from the surface opposite to the surface on the 2 nd steel plate 12 side (i.e., the surface of the joint) to 0.1 mm.

In the spot welded joint 1 according to the present embodiment, the microstructure of the 1 st region 51 includes tempered martensite in an amount of 50 area% or more, and the average vickers hardness HV1 of the 1 st region 51 and the average vickers hardness HVbase of the 1 st steel plate satisfy the following formula (1).

HVbase × 0.33+150 ≦ HV1 ≦ HVbase × 0.33+230 formula (1)

In the spot welded joint 1 according to the present embodiment, the microstructure of the 2 nd region 52 includes hard martensite in an amount of 50 area% or more, and the average vickers hardness HV2 of the 2 nd region 52 and the average vickers hardness HVbase of the 1 st steel plate satisfy the following expression (2).

HVbase-30 ≤ HV2 ≤ HVbase +30 formula (2)

The microstructure satisfying the same conditions of the area% of tempered martensite and the average vickers hardness as those of the 1 st region 51 may extend to the outside of the 1 st region 51. Further, the microstructure satisfying the same conditions of the area% of hard martensite and the average vickers hardness as those of the 2 nd region 52 may be expanded to the outside of the 2 nd region 52.

The reasons for limitations of the respective configurations will be described below.

As described above, high-strength steel sheets having an average vickers hardness of 350HV or more (about 1100MPa or more in terms of tensile strength) often have a structure including a quenched structure (e.g., 50 area% or more) such as hard martensite. Such a structure can be obtained by a production method including a quenching step.

In the case of welding steel sheets including a quenched structure, in the HAZ formed around the weld metal by the heat of welding, hard martensite is changed to a soft structure such as tempered martensite. That is, a region (HAZ softened portion) having a lower hardness than the base material is formed. When tensile stress is generated in the surface of the plate having the welded portion, the HAZ softened portion may become a starting point of fracture.

The results of the studies conducted by the present inventors revealed that: a shape in which (i) a local strength-reduced portion is not generated between two weld metals on which in-plane tensile stress acts, and (ii) a portion where the strength is deteriorated and a portion where the weld metal is easily stretched are provided in a portion other than the HAZ softened portion; (iii) the structure of a region having a heat-affected zone including a HAZ softening portion is modified so that the elongation until fracture of the region increases, thereby making it possible to alleviate and strain concentration at a portion serving as the HAZ softening portion.

In the case where the local strength-reduced portion is not generated, for example, it is conceivable to temper the region including the HAZ softened portion between the spot-welded metals 2 and reduce the hardness of the periphery of the HAZ softened portion to the same degree as the hardness of the HAZ softened portion. However, in this case, although the breakage from the HAZ softened portion can be suppressed, the softened portion becomes large as the whole member, and therefore there is a concern that the collision resistance (bending performance) of the member is lowered.

In order to minimize the reduction in joint strength and prevent cracking at the HAZ softened portion, the spot welded joint 1 according to the present embodiment forms the 1 st and 2 nd regions having desired microstructures and average vickers hardness by tempering the 1 st steel plate 11 subjected to overlap welding in the vicinity of the overlapping surface 3 between the spot welding metals 2 and quenching the 1 st steel plate between the

spot welding metals

2 and 2 in the vicinity of the surface (opposite to the overlapping surface 3).

< region 1 >

[ the microstructure contains 50 area% or more of tempered martensite, the average Vickers hardness HV1 of the 1 st region and the average Vickers hardness HVbase of the 1 st steel sheet satisfy HVbase x 0.33+ 150. ltoreq.HV 1. ltoreq.HVbase x 0.33+230]

As a result of studies by the present inventors, when the 1 st steel sheet 11 including hard martensite having an average vickers hardness HVbase of 350HV or more is spot welded, the hardness (vickers hardness) of the HAZ softened portion due to the thermal influence of welding is about (the hardness of the 1 st steel sheet 11 before welding × 0.33+150) to (the hardness of the 1 st steel sheet 11 before welding × 0.33+ 230). Therefore, in the spot welded joint according to the present embodiment, the 1 st region between the

spot welding metals

2 and 2 including the HAZ softened portion is tempered to have a structure in which tempered martensite is 50 area% or more, and the vickers hardness is controlled so as to satisfy the following formula (1).

When the average vickers hardness (HV1) of the 1 st region 51 satisfies formula (1), the difference in hardness between the HAZ softened portion and the surroundings thereof is 80HV or less. In this case, the concentration to the HAZ softened portion can be relaxed.

HVbase × 0.33+150 ≦ HV1 ≦ HVbase × 0.33+230 formula (1)

The 1 st region 51 preferably has a difference between the maximum value and the minimum value of the vickers hardness of 80HV or less, except that the average vickers hardness satisfies formula (1). By reducing the difference between the maximum value and the minimum value of the hardness in the 1 st region 51, the concentration of strain can be further relaxed. In other words, when the hardness distribution in the 1 st region 51 is uniform, the local concentration of strain can be avoided. More preferably, the difference between the maximum value and the minimum value of the Vickers hardness is 50HV or less.

[ range of 0.1mm from the 2 nd-plate-side surface formed between two spot-welded metals ]

The 1 st region 51 is formed between the two spot-welded

metals

2 and 2 in the thickness cross section of the 1 st steel plate 11 and has a thickness (thickness) in the range of 0.1mm from the surface on the 2 nd steel plate 12 side. If the thickness of the region having the above hardness in the plate thickness direction is less than 0.1mm, the HAZ softened portion, which is a local strength-reduced portion, remains, and a sufficient effect may not be obtained. The region satisfying the conditions of the hardness and the texture of the 1 st region 51 described above may be extended to the outside of the 1 st region 51. In this case, it is preferable that: the region satisfying the hardness and the structure of the 1 st region 51 extends from the 2 nd steel sheet-side surface to a range of 30% or more of the sheet thickness of the 1 st steel sheet. However, if the region satisfying the hardness and the structure of the 1 st region 51 described above is expanded from the 2 nd steel plate side surface to a range exceeding 90% of the plate thickness of the 1 st steel plate, the average hardness of the entire joint portion may be lowered, and the bending resistance may be lowered, which is not preferable.

< region 2 >

[ the microstructure contains hard martensite in an amount of 50 area% or more, the average Vickers hardness HV2 of the 2 nd region and the average Vickers hardness HVbase of the 1 st steel sheet satisfy HVbase-30. ltoreq. HV 2. ltoreq. HVbase +30]

When in-plane tensile stress is applied to the spot welded joint 1 according to embodiment 1 (the present invention), strain concentrates when a local strength-reduced portion such as a HAZ softened portion is formed between the two spot-welded metals 2, but as a result of studies made by the present inventors, particularly when a strength-reduced portion is present in the vicinity of the surface (the surface opposite to the overlapping surface) of the 1 st steel plate 11, strain tends to concentrate.

Therefore, in the spot welded joint 1 according to the present embodiment, the vicinity of the surface of the 1 st steel plate 11 between the two spot-welded

metals

2 and 2 including the HAZ softened portion is quenched so that the hardness of the quenched region is equal to the average vickers hardness of the 1 st steel plate 11 which is not affected by the welding heat.

Specifically, the average vickers hardness HV2 of the 2 nd region 52 and the average vickers hardness HVbase of the 1 st steel sheet 11 satisfy the following formula (2).

HVbase-30 ≤ HV2 ≤ HVbase +30 formula (2)

When the difference between the average vickers hardness of the 2 nd region 52 and the average vickers hardness of the 1 st steel plate 11 exceeds 30, the concentration of strain at the time of tensile stress in the load surface cannot be sufficiently suppressed.

Since the 1 st region 51 is a tempered structure containing 50 area% or more of tempered martensite and the 2 nd region is a quenched structure containing 50 area% or more of hard martensite, the average vickers hardness (HV2) of the 2 nd region 52 is higher than the average vickers hardness (HV1) of the 1 st region 51.

[ range of 0.1mm from the surface opposite to the 2 nd steel plate-side surface formed between two spot-welded metals ]

The 2 nd region 52 is formed between the two spot-welded

metals

2 and 2 in the thickness cross section of the 1 st steel plate 11 and has a thickness (thickness) in the range of 0.1mm from the surface opposite to the 2 nd steel plate side surface. If the thickness of the region having the above hardness in the plate thickness direction is less than 0.1mm, the HAZ softened portion, which is a local strength-reduced portion, remains, and a sufficient effect may not be obtained. The region satisfying the conditions of the hardness and the texture of the 2 nd region 52 described above may also be extended to the outside of the 2 nd region 52. In this case, it is preferable that: the region satisfying the hardness and the structure of the 2 nd region 52 is formed in a range of 10% or more of the sheet thickness of the 1 st steel sheet from the surface opposite to the surface on the 2 nd steel sheet 12 side. However, if the region satisfying the hardness and the structure of the 2 nd region 52 is expanded from the surface opposite to the surface on the 2 nd steel plate 12 side to a range exceeding 70% of the plate thickness of the 1 st steel plate 11, the elongation at break may decrease when the tensile bending load other than bending is applied to the 2 nd region side, which is not preferable.

In the spot welded joint 1 according to the present embodiment, the 1 st region 51 and the 2 nd region 52 are formed in all cross sections in the plate thickness direction of the 1 st steel plate including two spot-welded metals. That is, in the case of the 1 st steel sheet, when the cross section in the sheet thickness direction is observed so as to include two weld metals, the 1 st region 51 and the 2 nd region 52 are observed in all the cross sections.

In other words, when the diameter of the spot weld metal 2 on the overlapping surface 3 is D, the width of the 1 st region 51 and the 2 nd region 52 in the direction perpendicular to the cross section of the 1 st steel plate 11 in the plate thickness direction (the direction perpendicular to the paper surface of fig. 1, and the vertical direction on the paper surface of fig. 2) is 1.0 × D. The microstructure satisfying the same conditions of the area% of tempered martensite and the average vickers hardness as those of the 1 st region 51 may extend to the outside of the 1 st region 51 in the width direction. The microstructure satisfying the same conditions of the area% of hard martensite and the average vickers hardness as those of the 2 nd region 52 may extend to the outside of the 2 nd region 52 in the width direction.

When the automobile is collided, the direction of the in-plane tensile stress is not necessarily parallel to the direction of connecting the spot welding metals 2, 2 (the direction of connecting the centers of the

spot welding metals

2, 2 to each other). That is, there are cases where the angle is constant (stress acts in an oblique direction). If the width of the 1 st region 51 and the 2 nd region 52 is 1.0 × D, even when the direction of the in-plane tensile stress is at a certain angle (stress acts in an oblique direction) with respect to the direction of connecting the spot weld metals 2, it is possible to suppress the concentration of the strain in the HAZ softened portion where the strain can be concentrated. As a result, breakage at the HAZ softened portion can be further suppressed.

On the other hand, if the widths of the 1 st region 51 and the 2 nd region 52 are smaller than 1.0 × D, there is a possibility that a sufficient effect cannot be obtained when the direction of the in-plane tensile stress is at a certain angle (stress acts in an oblique direction) with respect to the direction of connecting the spot-welded

metals

2 and 2.

The average hardness of the 1 st steel plate 11 was measured using a vickers hardness tester with a load of 1.0 kgf.

In a steel sheet including a hard martensite structure, the hardness of a portion affected by the welding heat becomes lower than the hardness before welding. Therefore, the hardness of the 1 st steel plate 11 is measured at the position of the 1 st steel plate 11 not affected by the welding heat and the average value thereof is used. As the position not affected by the welding heat, for example, the hardness of a position separated by 15mm or more from the spot-welded metal 2 in a direction free from other welded metals may be measured.

Specifically, the hardness was measured at 10 locations not affected by the welding heat from the surface of the 1 st steel plate 11 at the position of 1/8, the position of 3/8, the position of 5/8, and the position of 7/8 of the plate thickness using a vickers hardness tester with a load of 1.0kgf, and the average value thereof was used.

The thickness of the 1 st region 51 and the 2 nd region 52 from the surface of the 1 st steel plate 11 and the average vickers hardness thereof were measured by repeating grinding and vickers hardness measurement on the cross section of the 1 st steel plate in the plate thickness direction using a vickers hardness tester with a load of 100gf, and a distribution of vickers hardness in a range sandwiched between the

spot welding metals

2 and 2 was obtained. Based on the distribution, the thicknesses of the 1 st region 51 and the 2 nd region 52 and the average vickers hardnesses thereof are calculated.

The vickers hardness distribution is specifically measured by the following method.

First, a sample was prepared so that a cross section in the thickness direction of the 1 st steel plate 11 passing through the centers of two spot-welded metals (a cross section a-a shown in fig. 2) became a measurement surface.

The Vickers hardness of the measured surface was measured at a position 0.1mm away from the surface and the overlapping surface of the 1 st steel plate 11 in the thickness direction of the 1 st steel plate 11, and at a position 5 times equal therebetween. This measurement was repeated at intervals of 0.5mm in the width direction (the direction in which the spot-welded metal 2 and the other spot-welded metal 2 were joined).

Then, the sample was polished to 0.5mm, and the Vickers hardness measurement was performed on the developed cross section (the B-B cross section shown in FIG. 2) in the same manner as described above.

Then, grinding and vickers hardness measurement were performed until the spot-welded metal 2 was not included in the cross section, and a vickers hardness distribution of the 1 st steel sheet 11 between the spot-welded

metals

2, 2 was obtained. As for the hardness, the hardness of the opposite cross section is also considered to be equivalent, and therefore, the half cross section may be measured as described above.

Whether or not the 1 st steel plate 11 to be welded contains hard martensite may be determined by performing an etching treatment using a Lepera etching solution on samples prepared from 5 positions each of which is a position 1/8, a position 3/8, a position 5/8 and a position 7/8 of the plate thickness from the surface of the 1 st steel plate 11 which is not affected by welding heat, and observing the field of view of 100 μm square at 1000 × magnification by an optical microscope. In the observation field, the white to reddish brown structure is martensite, and of the martensite, martensite containing carbides is determined to be hard martensite, and martensite containing no carbides is determined to be tempered martensite.

The area ratio of tempered martensite in the 1 st region 51 and the area ratio of hard martensite in the 2 nd region 52 were measured by etching a sample prepared from 5 points in a region to be measured in the same plane as the measurement plane of vickers hardness (the region satisfying the formula (1) in the case of the 1 st region and the formula (2) in the case of the 2 nd region) with a Lepera etchant, observing a field of view of 100 μm square at 1000-fold magnification with an optical microscope, and regarding a structure appearing white to reddish brown as martensite in the observation field of view. The martensite area ratios of the 1 st region 51 and the 2 nd region 52 are obtained by averaging the martensite area ratios in the observed field of view. Then, the same sample was subjected to etching treatment using pical etching solution (picral), and a field of view of 100 μm square was observed at a magnification of 1000 times with an optical microscope, and in the observation field, martensite containing carbides was regarded as hard martensite, and martensite containing no carbides was regarded as tempered martensite, and the proportions of tempered martensite and hard martensite in martensite were determined.

In the present embodiment, a spot welded joint in which a weld metal is formed by spot welding is targeted. Spot welding is also called spot welding and is welding in which two steel plates that are overlapped are joined at one spot. Examples of the spot welding method include arc spot welding, resistance spot welding, and laser spot welding. In contrast, welding performed linearly is called continuous welding. Examples of the continuous welding include arc welding, laser welding, and seam welding. Spot welding requires a shorter time for construction and saves electricity because of a smaller welding area than continuous welding. Namely, the spot welding is excellent in productivity.

In the above description, the case where the spot-welded metal 2 is a nugget of resistance spot welding has been described, but the spot-welded joint 1 according to the present embodiment is not limited to a resistance spot-welded joint in which the spot-welded metal 2 is a nugget of resistance spot welding. For example, the spot welding metal 2 may be formed by laser spot welding, or the spot welding metal 2 may be formed by arc spot welding.

In the spot welded joint 1 according to the present embodiment, for example, it is preferable that: the 1 st steel plate 11 is a hat-shaped member, the 2 nd steel plate 12 is a closing plate, and two spot-welded metals 2 are formed at the overlapping portion of the flange portion of the hat-shaped member and the closing plate. Such a structure is particularly effective for improving the strength and impact resistance of the structural member.

In addition, the spot welded joint 1 according to the present embodiment can obtain its effect if it has two spot welding metals 2 and the area between the two

spot welding metals

2, 2 satisfies the above-described relationship. In the case of application to automobile frame members and the like, a plurality of (two or more) spot-welded metals are formed. Even in the case where more than 2 spot welding metals are formed, as long as the region between the two spot welding metals as the object has the above-described relationship, the effect can be obtained with respect to the region. In the case where there are more than 2 spot-welded metals, it is preferable to control so that the two spot-welded metals 2 satisfy the above-described relationship with each other, particularly in a region where an in-plane tensile stress is supposed to act. If the above-described relationship of hardness is satisfied between all the spot-welded metals 2, even if tensile stress is applied in any direction or location in the surface of the high-strength steel sheet, breakage at the HAZ softened portion can be suppressed, which is more preferable.

The spot welded joint 1 according to the present embodiment can be applied to an a-pillar, a side sill, a B-pillar, and the like. For example, the B-pillar is joined to the closing plate by spot welding metal at the flange portion of the hat-shaped member. If the above-described relationship is satisfied between the spot-welded metal of the B pillar, even when an in-plane tensile stress is applied to the flange portion at the time of an automobile collision, it is possible to suppress fracture at the portion serving as the HAZ softened portion.

The 1 st steel plate 11 and/or the 2 nd steel plate 12 may be a plated steel plate. In this case, the corrosion resistance is improved. Examples of the plated steel sheet include hot-dip galvanized steel sheets, alloyed hot-dip galvanized steel sheets, electrogalvanized steel sheets, aluminum-plated steel sheets, and the like.

Next, a method for producing a spot welded joint according to the present embodiment will be described.

The spot welded joint according to the present embodiment can be manufactured by a manufacturing method including the following steps. That is, the method for manufacturing a spot welded joint according to the present embodiment includes the steps of:

(I) overlapping a 1 st steel sheet and a 2 nd steel sheet, wherein the 1 st steel sheet is a steel sheet containing hard martensite and having an average Vickers hardness HVbase of 350HV or more,

(II) forming a plurality of spot welding metals joining the overlapped 1 st steel plate and the overlapped 2 nd steel plate,

(III) quenching a range of 0.1mm from the surface opposite to the 2 nd steel plate side of the 1 st steel plate that is between the two spot-welded metals while tempering the range of 0.1mm from the 2 nd steel plate side surface of the 1 st steel plate that is between the two spot-welded metals.

Known steel sheets can be used for the 1 st steel sheet 11 and the 2 nd steel sheet 12 having an average vickers hardness HVbase of 350HV or more and containing hard martensite.

These steel plates are overlapped and spot-welded to form a spot-welded metal, thereby forming a welded joint. The spot welding conditions are not limited, and may be normal conditions.

After spot welding, a 1 st region and a 2 nd region are formed by quenching a part of the 1 st steel sheet and tempering a part of the quenched steel sheet by laser irradiation.

When quenching is performed, the temperature of the target region needs to be increased to a temperature exceeding Ac1 ℃. Preferably Ac1+30 ℃ or higher. However, if the temperature of the quenched area is excessively increased, the area that needs to be tempered also becomes the quenched area due to heat conduction. Therefore, heat input control corresponding to the thickness of the plate is required.

On the other hand, in the case of tempering, the temperature of the target region needs to be heated to a temperature lower than Ac1 ℃. As shown in fig. 3, the heated region becomes tempered martensite until Ac1 ℃, and the hardness decreases with an increase in temperature. On the other hand, when the heating temperature exceeds Ac1 ℃, the structure phase changes to austenite. This austenite is transformed into hard martensite again upon cooling, and therefore exhibits high hardness at a portion heated to a temperature exceeding Ac1 ℃.

In this case, the first and second regions 1 and 2 can be formed by performing laser irradiation from the surface side of the first steel sheet (the surface opposite to the first steel sheet 2) to heat the vicinity of the surface of the first steel sheet to a temperature exceeding Ac1 ℃, and heating the vicinity of the surface opposite thereto to a temperature of Ac1 ℃ or lower.

In the case of performing the heating as described above, in order to provide a hardness distribution in the plate thickness direction, it is necessary to input heat only to the extreme surface layer and to provide the heat in the depth direction by heat conduction. If the heating is performed outside the target region, heat dissipation in the target region may become insufficient, and a tempered structure may not be obtained.

For example, in the case of high-frequency induction heating, since heat is input to a certain depth, a satisfactory hardness distribution cannot be obtained. In addition, it is difficult to heat only a specific region by gas heating or arc heating.

Therefore, in the method of manufacturing a spot welded joint according to the present embodiment, heating is performed by irradiation of a laser beam. In order to heat the entire portion between the weld metals, it is preferable to heat the weld metals while moving a laser beam having a beam width equal to or larger than the diameter of the weld metals at a constant speed.

The conditions for laser irradiation are not particularly limited, and may be determined according to the thickness of the 1 st steel sheet, the thickness of the 1 st region or the 2 nd region to be obtained, and the like, but for example, the following conditions may be exemplified.

Exemplary conditions

Kind of oscillator: semiconductor laser device

Output: 500 to 3000W

Beam shape: in the irradiation surface, width direction: 4-10 mm, direction of travel: 0.5-3 mm rectangle

Laser moving speed: 50 to 500 cm/min

By such laser irradiation, while tempering the range of 0.1mm from the 2 nd steel plate-side surface of the 1 st steel plate between the two spot-welded metals, quenching the range of 0.1mm from the opposite surface to the 2 nd steel plate-side surface of the 1 st steel plate between the two spot-welded metals can be performed.

Examples

The present disclosure will be specifically described below with reference to the following examples and tables. These examples are examples for confirming the effects of the present disclosure, and do not limit the present disclosure.

First, a steel sheet having a thickness of 2.0mm was held at 950 ℃ for 5 minutes in a furnace, and then hot-pressed with a water-cooled mold, thereby being subjected to quenching treatment. After quenching, shot blasting is used to remove oxide scale on the surface of the steel sheet. The vickers hardness of the steel sheets used after quenching is shown in table 1. The steel sheet has a structure containing hard martensite.

Then, a tensile test piece having an inter-gauge distance of 50mm and a parallel portion width of 25mm as shown in FIG. 4 was prepared from the steel sheet. Further, a 25mm square lead plate was produced from the steel plate.

As shown in fig. 4, the prepared tensile test piece was resistance spot-welded under the following conditions using a single-phase ac spot welder with the lead plates placed in parallel with each other and the central portion of each lead plate.

An electrode: DR type electrode (tip phi 6mm R40)

Pressurizing pressure: 400kgf

Energization time: 24 cycles (cyc)

Two nugget portions having a nugget diameter of 4 × t were formed between the tensile test piece and the lead plate by resistance spot welding^1/2(t: plate thickness of tensile test piece(mm)) of weld metal.

The test pieces (joint nos. 1 to 5, 11, and 12) after spot welding were irradiated with laser light from the tensile test piece side, and heat treatment was performed on the entire parallel portions in the longitudinal direction so that the widthwise central portions of the parallel portions were aligned with the center of the beam. No.6 to 10 of the joints were not irradiated with laser light.

The laser irradiation conditions for the joints No.1 to No. 5 were as follows.

Kind of oscillator: semiconductor laser device

Output: 1200W

Beam shape: in the irradiation surface, width direction: 8mm, direction of travel: 1mm rectangle

Laser moving speed: 250 cm/min

The laser irradiation conditions for the joint No.11 were as follows.

Kind of oscillator: semiconductor laser device

Output: 700W

Laser moving speed: 130 cm/min

The laser irradiation conditions for the joint No.12 were as follows.

Kind of oscillator: semiconductor laser device

Output: 750W

Laser moving speed: 80 cm/min

Then, whether or not the 1 st region and the 2 nd region having a predetermined size are formed is examined by the above-mentioned method, and when formed, the average vickers hardness and the area ratio of tempered martensite or hard martensite of the 1 st region and the 2 nd region are examined by the above-mentioned method. The measurement surface is a cross section in the thickness direction at the center in the width direction of the test piece.

Further, a tensile test (tensile stress in the load surface) was performed on each test piece to investigate the fracture position. The tensile rate in the tensile test was set to 10 mm/min.

The results are shown in Table 1.

In the joints nos. 1 to 5 (examples of the present invention), the range (1 st region) of 0.1mm from the overlapping surface was tempered and satisfied the formula (1), and the range (2 nd region) of 0.1mm from the surface was quenched and satisfied the formula (2). As a result, no cracking was observed in the HAZ softened region.

On the other hand, in the joints nos. 6 to 10 (comparative examples), since laser irradiation was not performed, the formulas (1) and (2) were not satisfied in the range corresponding to the 1 st region or the 2 nd region of the joints nos. 1 to 5. As a result, the HAZ softened portion was cracked in the tensile test.

Joints No.11 to 12 (comparative examples) were irradiated with laser light, but the laser irradiation conditions were not satisfactory. As a result, in comparative example 11, the HAZ softened portion clearly remained in the 1 st region due to insufficient heat input, and was broken at the HAZ softened portion. In comparative example 12, although the heat input was insufficient and the 2 nd zone was quenched so as to satisfy the formula (2), the tempering was insufficient in the 1 st zone, the formula (1) was not satisfied in the 1 st zone, and the difference between the maximum value and the minimum value of the vickers hardness in the 1 st zone was 80HV or more. As a result, the HAZ softened portion was cracked in the tensile test.

Description of the reference numerals

1 Spot welded joint

2 spot welding of metals

3 overlapping surface

11 st Steel plate

12 nd 2 nd steel plate

51 region 1

52 region 2

Claims

1. A spot welded joint comprising a 1 st steel plate, a 2 nd steel plate, and two spot welding metals joining the 1 st steel plate and the 2 nd steel plate, wherein the 1 st steel plate is a steel plate containing hard martensite and having an average Vickers hardness HVbase of 350HV or more, the 2 nd steel plate is overlapped with the 1 st steel plate,

in the entire cross section in the plate thickness direction of the 1 st steel plate including the two spot welding metals, the 1 st steel plate has a 1 st region and a 2 nd region, the 1 st region is formed between the two spot welding metals and ranges from the 2 nd plate-side surface to 0.1mm in the plate thickness direction of the 1 st steel plate, the 2 nd region is formed between the two spot welding metals and ranges from the surface opposite to the 2 nd plate-side surface to 0.1mm in the plate thickness direction,

the microstructure of the 1 st region contains 50 area% or more of tempered martensite, the average Vickers hardness HV1 of the 1 st region and the average Vickers hardness HVbase of the 1 st steel sheet satisfy the following formula (1),

the microstructure of the 2 nd region contains hard martensite in an amount of 50 area% or more, and the average Vickers hardness HV2 of the 2 nd region and the average Vickers hardness HVbase of the 1 st steel sheet satisfy the following formula (2),

HVbase × 0.33+150 ≤ HV1 ≤ HVbase × 0.33+230 formula (1);

HVbase-30 ≤ HV2 ≤ HVbase +30 of formula (2).

2. The spot weld joint according to claim 1,

the difference between the maximum value and the minimum value of the Vickers hardness in the 1 st region is 80HV or less.

3. Spot welded joint according to claim 1 or 2,

the thickness of the 1 st region in the plate thickness direction is 30 to 70% of the plate thickness of the 1 st steel plate.

4. A method of manufacturing a spot welded joint, characterized in that,