JPWO2020241500A1 - Manufacturing method of spot welded joints and spot welded joints - Google Patents

Abstract

The spot welded joint includes a first steel plate, a second steel plate superimposed on the first steel plate, and two spot weld metals joining the first steel plate and the second steel plate. In the cross section including the two spot weld metals, the first steel plate is 0.1 mm in the plate thickness direction of the first steel plate between the two spot weld metals and from the surface on the second steel plate side. It was formed in a range of 0.1 mm in the plate thickness direction between the first region formed in the above range and the two spot weld metals and from a surface opposite to the surface on the second steel plate side. It has a second region, and the average Vickers hardness HV1 of the first region and the average Vickers hardness HVbase of the first steel plate satisfy HVbase × 0.33 + 150 ≦ HV1 ≦ HVbase × 0.33 + 230. The average Vickers hardness HV2 of the second region and the average Vickers hardness HVbase of the first steel plate satisfy HVbase-30 ≦ HV2 ≦ HVbase +30.

Description

The present disclosure relates to a spot welded joint and a method for manufacturing the spot welded joint.
The present application claims priority based on Japanese Patent Application No. 2019-09703 filed in Japan on May 24, 2019, the contents of which are incorporated herein by reference.

In a structure formed by stacking a plurality of steel plate members, joining by resistance spot welding is widely performed on the superposed portion in which the steel plate members are superposed.

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.

Currently, as a high-strength steel plate for automobiles, a high-strength steel plate having a tensile strength of 980 MPa or more is widely used. In recent years, high-strength steel sheets having a tensile strength of 1100 MPa or more have begun to be applied. High-strength steel sheets having a tensile strength of 1100 MPa or more generally include a hardened structure in order to obtain high strength. When resistance spot welding is performed, a nugget (spot weld metal) for welding a steel plate is formed, and a heat-affected zone (hereinafter referred to as HAZ) is generated around the nugget. HAZ generally includes a hardened structure. However, when resistance spot welding is performed on a high-strength steel plate having a hardened structure, a region (HAZ softened zone) having a hardness lower than that of the base metal having a hardened structure is formed. This is because the quenching structure of the base metal is tempered by the heat of resistance spot welding.

Passengers in the cabin need to be protected in the event of a car collision. Therefore, the structural members (layer welded members) constituting the automobile body such as the A pillar, the B pillar, the roof rail, and the side sill need to have high strength. Generally, a structural member constituting an automobile body is manufactured by superimposing a plurality of steel plate members and joining flanges (superposition portions) by resistance spot welding to form a tubular closed cross section. In order to improve the deformation resistance at the time of collision and absorb more collision energy with a small amount of deformation, methods such as increasing the strength of the material (base material) and increasing the number of welding (spot) hit points are taken.
In-plane tensile stress may be applied to a part of the flange of the member to be spot-welded by resistance spot welding at the time of a collision of an automobile. In general, if there is a region with low hardness such as a HAZ softened zone, the collision resistance performance of the member deteriorates. Such a HAZ softened zone has little influence on the evaluation results of the tensile shear test and the cross tensile test (JIS Z3137) used for the joint evaluation of resistance spot welding. However, when an in-plane tensile stress is applied, strain may be locally concentrated on the HAZ softened zone, causing fracture in the HAZ softened zone. Therefore, even if the strength of the base metal is increased and the spot hitting points are increased, if the above-mentioned HAZ softened portion is generated, the collision resistance performance expected from the strength of the base metal and the shape of the component may not be obtained.
Therefore, when a steel plate member made of a high-strength steel plate is applied to a structural member of an automobile body, it is required to suppress the peripheral region of the nugget from becoming the starting point of fracture.

Conventionally, studies have been made to improve the characteristics of welded members formed by resistance spot welding. For example, Patent Document 2 describes a welded joint in which the characteristics of the spot welded portion are improved by heat-treating the spot welded portion at 100 to 400 ° C. to improve the strength of the L-shaped tensile joint. Further, Patent Document 3 describes a method of post-energizing the spot welded portion to improve the strength of the cross tension joint. Patent Document 4 describes the ratio of TSS to material strength and CTS by heating the spot welded portion and the molten portion by high-frequency induction heating immediately after welding with a coil wound around the spot weld electrode. A welding method for improving the joint strength evaluated from the product with the material strength is described.

However, according to the techniques disclosed in these Patent Documents 2 to 4, although a certain effect can be obtained in improving TSS and CTS, in these Patent Documents 2 to 4, in-plane tensile stress is applied to the steel sheet. No consideration is given to breakage at the HAZ softened portion when it is made.

In response to such a problem, Patent Document 5 describes that the energy absorption capacity is enhanced by having a region called a soft zone having a strength of less than 1100 MPa in a part or all of the flange portion used for spot welding. Pillars are listed.

However, in the B-pillar disclosed in Patent Document 5, since the side flange needs to be softened, the bending performance, which is the collision resistance performance of the member, may be deteriorated. Further, in Patent Document 5, since a softening region is provided in the part before welding, there is also a problem that the shape accuracy of the part is lowered. When the shape accuracy of a part is lowered, a gap is generated between the parts during welding, which makes welding difficult.

Japanese Patent Application Laid-Open No. 2006-142905 Japanese Patent Application Laid-Open No. 2010-509451 Japanese Patent Application Laid-Open No. 2015-093282 Japanese Patent No. 5459750 Japanese Patent No. 5894081

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

The present inventors analyzed the strain distribution when an in-plane tensile stress was generated in a part in which a HAZ softened zone was formed. As a result, the strength of the parts is increased by quenching the vicinity of the surface (opposite the overlapping surface) between the weld metals of the high-strength steel plate that has been overlap-welded, and at the same time, tempering the vicinity of the overlapping surface. It has been found that it is possible to prevent strain from being locally concentrated on the HAZ softened portion and causing breakage in the HAZ softened portion even when an in-plane tensile stress is applied while suppressing the decrease to the minimum.

The present disclosure has been made on the basis of the above findings. The gist of this disclosure is as follows.
[1] The spot welded joint according to one aspect of the present disclosure includes a first steel plate containing hard martensite having an average Vickers hardness HVbase of 350 HV or more, and a second steel plate laminated on the first steel plate. In all cross sections of the first steel plate in the plate thickness direction, including the two spot-welded metals joining the first steel plate and the second steel plate, and including the two spot-welded metals. The first region formed in the range of 0.1 mm between the two spot-welded metals and from the surface on the second steel plate side in the plate thickness direction of the first steel plate. And a second region formed in a range of 0.1 mm in the plate thickness direction from the surface opposite to the surface on the second steel plate side between the two spot weld metals. The metal structure of the first region 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 are expressed by the following formula (1). The metal structure of the second region contains 50 area% or more of hard martensite, and the average Vickers hardness HV2 of the second region and the average Vickers hardness HVbase of the first steel plate are as follows. (2) is satisfied.
HVbase × 0.33 + 150 ≦ HV1 ≦ HVbase × 0.33 + 230 Equation (1)
HVbase-30 ≤ HV2 ≤ HVbase + 30 Equation (2)
[2] In the spot welded joint according to [1], the difference between the maximum value and the minimum value of Vickers hardness in the first region may be 80 HV or less.
[3] In the spot welded joint according to [1] or [2], even if the thickness of the first region in the plate thickness direction is 30 to 70% of the plate thickness of the first steel plate. good.
[4] In the method for manufacturing a spot welded joint according to another aspect of the present disclosure, a first steel plate containing hard maltensite having an average Vickers hardness HVbase of 350 HV or more and a second steel plate are superposed. Two spot weld metals that join the first steel plate and the second steel plate that are overlapped are formed, and by laser irradiation, the first steel plate is between the two spot weld metals and the second. At the same time, tempering is performed in a range of 0.1 mm from the surface on the steel plate side of the above, and at the same time, tempering is performed in a range of 0.1 mm between the two spot weld metals and the surface opposite to the surface on the second steel plate side. ..

According to the above aspect of the present disclosure, a spot welded joint capable of suppressing breakage from a region sandwiched between spot weld metals even when an in-plane tensile stress is applied, and a method for manufacturing the spot welded joint can be obtained.

It is a cross-sectional view in the plate thickness direction of the spot welded joint which concerns on this embodiment. It is a figure when the spot welded joint which concerns on this embodiment is seen from the top view from the 1st steel plate side. It is a schematic diagram which shows the relationship between the ultimate temperature and the measurement of Vickers hardness when the spot welded joint which concerns on this embodiment is irradiated with a laser. It is a schematic diagram which shows the test piece used in an Example.

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 a 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 first steel plate 11, a second steel plate 12 stacked on the first steel plate, and a first steel plate 11 and a second steel plate. It is provided with two spot welded metals 2 for joining the twelve. In FIGS. 1 and 2, the spot weld metal 2 is a nugget formed by resistance spot welding. Such a spot welded joint is obtained by superimposing the first steel plate 11 and the second steel plate 12 and performing resistance spot welding.
The first steel plate 11 to be used for resistance spot welding is a steel plate having an average Vickers hardness (HVbase) of 350 HV or more in consideration of application to automobile frame parts such as B pillars. Further, the first steel sheet has a structure including a hardened structure such as hard martensite. On the other hand, the second steel plate 12 is not limited.
The average Vickers hardness of the first steel plate 11 (sometimes simply referred to as hardness) means the average Vickers hardness of the first steel plate 11 to be welded before welding. When measured after spot welding, it means the average Vickers hardness measured at a position not affected by welding heat.

Further, in the spot welded joint 1 according to the present embodiment, in all the cross sections of the first steel plate 11 including the two spot welded metals 2 and 2 in the plate thickness direction, the first steel plate 11 is the two spot welded metals. Between the first region 51 formed between 2 and 2 and within a range of 0.1 mm from the surface (that is, the overlapped surface) on the side of the second steel plate 12 and between the two spot welded metals 2 and 2. It has a second region 52 formed in a range of 0.1 mm from a surface opposite to the surface on the second steel plate 12 side (that is, the surface of the joint).
Further, in the spot welded joint 1 according to the present embodiment, the metal structure of the first region 51 contains 50 area% or more of tempered martensite, and the average Vickers hardness HV1 of the first region 51 and the first steel plate. The average Vickers hardness HVbase satisfies the following equation (1).
HVbase × 0.33 + 150 ≦ HV1 ≦ HVbase × 0.33 + 230 Equation (1)
Further, in the spot welded joint 1 according to the present embodiment, the metal structure of the second region 52 contains 50 area% or more of hard martensite, and the average Vickers hardness HV2 of the second region 52 and the first region 52. The average Vickers hardness HVbase of the steel plate of the above satisfies the following equation (2).
HVbase-30 ≤ HV2 ≤ HVbase + 30 Equation (2)
A metal 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. Further, a metal structure satisfying the same conditions of the area% of 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.

As described above, a high-strength steel plate having an average Vickers hardness of 350 HV or more (about 1100 MPa or more in terms of tensile strength) has a structure containing a hardened structure such as hard martensite (for example, 50 area% or more). Often have. Such a structure is obtained by a manufacturing method including a quenching step.
When welding is performed on a steel sheet containing a hardened structure, hard martensite changes to a soft structure such as tempered martensite in HAZ formed around the weld metal due to the heat of welding. That is, a region (HAZ softened zone) having a hardness lower than that of the base material is formed. When tensile stress is generated in the plane of the plate having the welded portion, this HAZ softened portion may become the starting point of fracture.

As a result of the studies by the present inventors, (i) to prevent a local strength decrease from occurring between the two weld metals to which in-plane tensile stress is applied, and (ii) to prevent a portion having inferior strength and elongation. A shape that is likely to be provided in addition to the HAZ softened zone, and (iii) modifying the structure of the region having a heat-affected zone including the HAZ softened zone so that the amount of elongation until breakage is large. , It was found that the concentration of strain on the part that was the HAZ softened zone can be relaxed.
When preventing the occurrence of a local strength reduction portion, for example, the region including the HAZ softened zone between the spot weld metals 2 is tempered so that the hardness around the HAZ softened zone is about the same as the hardness of the HAZ softened zone. It is possible to reduce it. However, in this case, although the breakage from the HAZ softened portion can be suppressed, the softened portion as a whole becomes large, so that there is a concern that the collision resistance performance (bending performance) of the part may deteriorate.
The spot welded metal 2 according to the present embodiment is the spot weld metal 2 of the first steel plate 11 which has been lap welded in order to prevent cracking at the HAZ softened portion while minimizing a decrease in joint strength. Desirable by tempering the vicinity of the overlapped surface 3 between the two and quenching the vicinity of the surface of the first steel plate (the side opposite to the overlapped surface 3) between the spot weld metals 2 and 2. A first region and a second region having the metallographic structure of the above and the average Vickers hardness are formed.

<First area>
[The metal structure contains 50 area% or more of tempered martensite, and the average Vickers hardness HV1 in the first region and the average Vickers hardness HVbase of the first steel sheet are HVbase × 0.33 + 150 ≦ HV1 ≦ HVbase × 0. Satisfy 33 + 230]
As a result of the examination by the present inventors, when the first steel plate 11 containing hard maltensite having an average Vickers hardness HVbase of 350 HV or more is spotted, the hardness of the HAZ softened portion (Vickers hardness) caused by the thermal effect of welding. ) Is about (hardness of the first steel plate 11 before welding × 0.33 + 150) to (hardness of the first steel plate 11 before welding × 0.33 + 230). Therefore, in the spot welded joint according to the present embodiment, the first region between the spot weld metals 2 and 2 including the HAZ softened portion is tempered to have a tempered martensite structure of 50 area% or more, and its Vickers hardness. Is controlled so as to satisfy the following equation (1).
When the average Vickers hardness (HV1) of the first region 51 satisfies the formula (1), the hardness difference between the HAZ softened portion and its surroundings becomes 80 HV or less. In this case, the concentration of strain on the HAZ softened zone can be relaxed.
HVbase × 0.33 + 150 ≦ HV1 ≦ HVbase × 0.33 + 230 Equation (1)

In the first region 51, in addition to the average Vickers hardness satisfying the equation (1), the difference between the maximum value and the minimum value of the Vickers hardness in the region is preferably 80 HV or less. By reducing the difference between the maximum value and the minimum value of hardness in the first region 51, the concentration of strain can be further relaxed. In other words, if the hardness distribution within the first region 51 is homogeneous, 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.

[Formed between two spot weld metals and within a range of 0.1 mm from the surface on the second steel plate side]
The first region 51 is formed in the thickness cross section of the first steel plate 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 plate 12 side. Has been done. If the thickness of the region having the above hardness in the plate thickness direction is less than 0.1 mm, the HAZ softened portion, which is a local strength reducing portion, remains, and there is a possibility that a sufficient effect cannot be obtained. A region satisfying the hardness and structure of the first region 51 may extend to the outside of the first region 51. In that case, the region satisfying the hardness and structure of the first region 51 preferably extends from the surface on the second steel plate side to a range of 30% or more of the plate thickness of the first steel plate. However, if the region satisfying the hardness and structure of the first region 51 extends from the surface on the second steel plate side to a range of more than 90% of the plate thickness of the first steel plate, the entire joint portion It is not preferable because there is a concern that the average hardness will decrease and the bending strength will decrease.

<Second area>
[The metal structure contains 50 area% or more of hard martensite, and the average Vickers hardness HV2 in the second region and the average Vickers hardness HVbase of the first steel sheet satisfy HVbase-30 ≤ HV2 ≤ HVbase +30].
When an in-plane tensile stress is applied to the spot welded joint 1 according to the first embodiment, a local strength reduction portion such as a HAZ softened portion is formed between the two spot welded metals 2 and 2. If so, the strain is concentrated, but as a result of the examination by the present inventors, the strain is likely to be concentrated especially when there is a strength reduction portion near the surface of the first steel plate 11 (the surface opposite to the overlapped surface). I found out.
Therefore, in the spot welded joint 1 according to the present embodiment, the vicinity of the surface of the first steel plate 11 between the two spot weld metals 2 and 2 including the HAZ softened zone is hardened to reduce the hardness of the hardened region. , Equivalent to the average Vickers hardness of the first steel plate 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 plate 11 satisfy the following formula (2).
HVbase-30 ≤ HV2 ≤ HVbase + 30 Equation (2)
If the 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, the concentration of strain under in-plane tensile stress load cannot be sufficiently suppressed.
Since the first region 51 is a tempered structure containing 50 area% or more of tempered martensite and the second region is a hardened structure containing 50 area% or more of hard martensite, the first region 51 has a tempered structure. The average Vickers hardness (HV2) of the second region 52 is larger than the average Vickers hardness (HV1).

[Formed between two spot weld metals and within a range of 0.1 mm from the surface opposite to the surface on the second steel plate side]
The second region 52 is a range (thickness) of 0.1 mm (thickness) between the two spot weld metals 2 and 2 in the thickness cross section of the first steel plate 11 and from the surface opposite to the surface on the second steel plate side. It is formed in. If the thickness of the region having the above hardness in the plate thickness direction is less than 0.1 mm, the HAZ softened portion, which is a local strength reducing portion, remains, and there is a possibility that a sufficient effect cannot be obtained. A region that satisfies the conditions of hardness and structure of the second region 52 may extend to the outside of the second region 52. In that case, the region satisfying the hardness and structure of the second region 52 is formed in a range of 10% or more of the plate thickness of the first steel plate from the surface opposite to the surface on the second steel plate 12 side. It is preferable to have. However, if the region satisfying the hardness and structure of the second region 52 extends from the surface on the second steel plate 12 side to a range of more than 70% of the first steel plate 11, the second region side bends. It is not preferable because there is a concern that the breaking elongation may decrease when an external tensile bending load is applied.

In the spot welded joint 1 according to the present embodiment, the first region 51 and the second region 52 are formed in all cross sections of the first steel plate containing the two spot weld metals in the plate thickness direction. .. That is, when the cross-sections in the thickness direction of the first steel plate are observed so as to include two weld metals, the above-mentioned first region 51 and second region 52 are observed in all the cross-sections.
In other words, when the diameter of the superposed surface 3 of the spot weld metal 2 is D, the direction perpendicular to the thickness direction cross section of the first steel plate 11 in the first region 51 and the second region 52 (FIG. 1). The width in the direction perpendicular to the paper surface and the vertical direction on the paper surface in FIG. 2) is 1.0 × D. A metal 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 metal structure satisfying the same conditions of the area% of 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 a vehicle collision occurs, the direction of in-plane tensile stress is not necessarily parallel to the direction connecting the spot weld metals 2 and 2 (the direction connecting the centers of the spot weld metals 2 and 2). That is, it may have a certain angle (stress is applied in an oblique direction). If the widths of the first region 51 and the second region 52 are 1.0 × D, the in-plane tensile stress application direction is at a constant angle with respect to the direction connecting the spot weld metals 2 and 2 ( Even when stress is applied in the oblique direction), strain concentration on the HAZ softened portion where strain can be concentrated is suppressed. As a result, breakage at the HAZ softened zone is further suppressed.
On the other hand, when the width of the first region 51 and the second region 52 is less than 1.0 × D, the direction of the in-plane tensile stress is a constant 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.

The average hardness of the first steel plate 11 is measured using a Vickers hardness tester having a load of 1.0 kgf.
In a steel sheet containing a hard martensite structure, the hardness of the portion affected by welding heat is lower than the hardness before welding. Therefore, for the hardness of the first steel plate 11, the hardness at a position not affected by heat due to welding of the first steel plate 11 is measured, and the average value thereof is used. As a 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, using a Vickers hardness tester, with a load of 1.0 kgf, 10 positions that are not affected by heat due to welding, 1/8 of the plate thickness from the surface of the first steel plate 11, 3 Measure the hardness at the / 8 position, 5/8 position, and 7/8 position, and use the average value.

Regarding the thickness of the first steel plate 11 in the first region 51 and the second region 52 from the surface and their average Vickers hardness, a Vickers hardness tester with a load of 100 gf was used to determine the thickness of the first steel plate. The polishing and the Vickers hardness measurement are repeated with respect to the cross section in the plate thickness direction to obtain the Vickers hardness distribution in the range sandwiched between the spot weld metals 2 and 2. Based on this distribution, the thickness of the first region 51 and the second region 52 and their average Vickers hardness are calculated.
Specifically, the distribution of Vickers hardness is measured by the following method.
First, a sample is taken so that the cross section in the thickness direction (cross section AA shown in FIG. 2) of the first steel plate 11 passing through the centers of the two spot weld metals is the measurement surface.
With respect to this measurement surface, in the plate thickness direction of the first steel plate 11, Vickers hardness is set at a position of 0.1 mm from the surface and the superposition surface of the first steel plate 11, respectively, and a position obtained by dividing the space into five equal parts. Measure the hardness. This measurement is repeated at intervals of 0.5 mm in the width direction (the direction connecting the 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 (BB cross section shown in FIG. 2).
Further, polishing and Vickers hardness are measured until the cross section does not contain the spot weld metal 2, and the Vickers hardness distribution of the first steel plate 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 above.

Whether or not the first steel plate 11 to be welded contains hard martensite is determined by the position of 1/8 of the plate thickness and the position of 3/8 from the surface of the position not affected by the welding of the first steel plate 11. Samples collected from 5 locations each at 5/8 and 7/8 are etched with a repera corrosive solution, and a 100 μm square field is observed with an optical microscope at a magnification of 1000 times to make a judgment. Just do it. Martensite that looks white to reddish-brown in the observation field is martensite. Of the martensite, martensite containing carbide is hard martensite, and martensite not containing carbide is tempered martensite. Judge.

Further, the area ratio of the tempered martensite in the first region 51 and the area ratio of the hard martensite in the second region 52 are the target regions (first region) on the same surface as the above-mentioned Vickers hardness measurement surface. If it is a region, the region that satisfies the formula (1), if it is the second region, the region that satisfies the formula (2)), the samples collected from five locations are etched with a repera corrosive liquid, and then subjected to an optical microscope. A 100 μm square field is observed at a magnification of 1000 times, and the area ratio of martensite is measured assuming that martensite appears to be white to reddish brown in the observation field. By averaging the area ratio of martensite in the observed field of view, the area ratio of martensite in the first region 51 and the second region 52 can be obtained. Then, using the same sample, etching treatment with picral, observing a 100 μm square field with an optical microscope at a magnification of 1000 times, and in the observation field, martensite containing carbide was changed to hard martensite. Assuming that martensite containing no carbide is tempered martensite, the proportions of tempered martensite and hard martensite among the martensite are determined.

In this embodiment, a spot welded joint in which a weld metal is formed by spot welding is targeted. Spot welding, also called spot welding, is welding in which two stacked steel plates are connected by dots. Examples of the spot welding means include arc spot welding, resistance spot welding, and laser spot welding. On the other hand, welding performed linearly is called continuous welding. Examples of the means of continuous welding include arc welding, laser welding, seam welding and the like. Compared to continuous welding, spot welding has a smaller welding area, so the construction time is shorter and power is saved. That is, spot welding is excellent in productivity.
In the above, the case where the spot weld metal 2 is a nugget for resistance spot welding has been described, but 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 a nugget for 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.

In the spot welded joint 1 according to the present embodiment, for example, the first steel plate 11 is a hat member, the second steel plate 12 is a closing plate, and the two spot welded metals 2 are the flange portion of the hat member and the closing. It is preferably formed in the overlapped portion with the plate. With such a configuration, it is particularly effective in improving the strength and collision resistance of the structural member.

Further, if the spot welded joint 1 according to the present embodiment has two spot welded metals 2 and the region between the two spot welded metals 2 and 2 satisfies the above relationship, the effect is effective. can get. When applied to automotive skeleton parts and the like, a plurality of (two or more) spot weld metals are formed. Even when more than two spot weld metals are formed, if the region between the two target spot weld metals has the above relationship, the effect can be obtained for that region. When there are more than 2 spot weld metals, it is preferable to control so that the relationship between the two spot weld metals 2 is satisfied, especially in a portion where in-plane tensile stress is expected to be applied. .. If the above-mentioned hardness relationship is satisfied between all the spot weld metals 2, the fracture at the HAZ softened portion is suppressed regardless of the direction and location in the plane of the high-strength steel plate. It is more preferable because it can be used.

The spot welded 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, the flange portion of the hat member is joined to the closing plate by a spot weld metal. If the above relationship is satisfied between the spot-welded metals of the B-pillar, even if an in-plane tensile stress is applied to the flange portion at the time of an automobile collision, fracture at the portion that was the HAZ softened portion is suppressed. can.

The first steel plate 11 and / or the second steel plate 12 may be a plated steel plate. In this case, 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, and aluminum-plated steel sheets.

Next, a method of manufacturing 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 this embodiment is
(I) The first steel plate containing hard martensite and the second steel plate having an average Vickers hardness HVbase of 350 HV or more are superposed.
(II) A plurality of spot-welded metals for joining the first steel plate and the second steel plate that are overlapped with each other are formed.
(III) The first steel plate is tempered between the two spot weld metals and within a range of 0.1 mm from the surface on the second steel plate side, and at the same time, between the two spot weld metals and the above. It has a step of quenching in a range of 0.1 mm from the surface opposite to the surface on the second steel plate side.

As the first steel plate 11 containing hard martensite and the second steel plate 12 having an average Vickers hardness HVbase of 350 HV or more, known steel plates can be used.
These steel plates are superposed and spot welded to form a spot weld metal to form a welded joint. The spot welding conditions are not limited and may be normal conditions.

After spot welding, a part of the first steel sheet is hardened and a part is tempered by laser irradiation to form a first region and a second region.
When quenching, it is necessary to raise the temperature of the target region to more than Ac 1 ° C. It is preferably Ac1 + 30 ° C. or higher. However, if the temperature of the quenching region is too high, the 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 plate thickness.
On the other hand, when tempering is performed, it is necessary to heat the temperature of the target region to a temperature of less than Ac 1 ° C. As shown in FIG. 3, the heated region becomes tempered martensite, and the hardness decreases as the temperature rises up to Ac1 ° C. On the other hand, when the heating temperature exceeds Ac 1 ° C., the tissue is transformed into austenite. Since this austenite transforms into hard martensite again when cooled, it exhibits high hardness at the site heated to Ac 1 ° C. or higher.
Taking advantage of this, laser irradiation is performed from the surface side of the first steel sheet (the surface opposite to the second steel sheet), and the temperature near the surface of the first steel sheet is heated so as to exceed Ac 1 ° C. By heating the temperature near the opposite surface to Ac 1 ° 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 plate thickness direction, it is necessary to heat only the polar surface layer and heat in the depth direction by heat conduction. Further, if the material is heated to the outside of the target area, the heat removal in the target area may be insufficient and the tempered structure may not be obtained.
For example, in high-frequency induction heating, heat is input to a certain depth, so that 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 for manufacturing a spot welded joint according to the present embodiment, heating is performed by irradiating a laser beam. In order to heat the entire space between the weld metals, it is preferable to heat the laser beam having a beam width equal to or larger than the diameter of the weld metal while moving it at a constant speed.
The laser irradiation conditions are not particularly limited and may be determined depending on the thickness of the first steel plate, the thickness of the first region or the second region to be obtained, and the like. For example, the following conditions are exemplified.
Examples of conditions ・ Oscillator type: Semiconductor laser ・ Output: 500-3000W
-Beam shape: Rectangle with width direction: 4 to 10 mm and traveling direction: 0.5 to 3 mm on the irradiation surface-Laser moving speed: 50 to 500 cm / min

By such laser irradiation, tempering is performed between the two spot weld metals of the first steel sheet and within a range of 0.1 mm from the surface on the second steel plate side, and at the same time, between the two spot weld metals. Moreover, quenching can be performed in a range of 0.1 mm from the surface opposite to the surface on the second steel plate side.

Hereinafter, the present disclosure will be specifically described by way of examples with reference to figures and tables. These examples are examples for confirming the effect of the present disclosure, and do not limit the present disclosure.

First, a steel plate having a thickness of 2.0 mm was held in a furnace at 950 ° C. for 5 minutes, and then quenching was performed by hot stamping with a water-cooled die. After quenching, the oxide scale on the surface of the steel plate was removed by shot blasting. The Vickers hardness of the used steel sheet after quenching was as shown in Table 1. In addition, this steel sheet had a structure containing hard martensite.

Next, a tensile test piece having a distance between reference points of 50 mm and a parallel portion width of 25 mm as shown in FIG. 4 was collected from the steel plate. In addition, a 25 mm square tab plate was collected from the same steel plate.

As shown in FIG. 4, a tab plate was placed on the parallel portion of the collected tensile test piece, and resistance spot welding was performed on the central portion of each tab plate using a single-phase AC spot welder under the conditions shown below. ..
Electrode: DR type electrode (tip φ6 mm R40)
Pressurized pressure: 400 kgf
Energizing time: 24 cyc
By resistance spot welding, two weld metals having a nugget diameter of 4 × √t (t: thickness (mm) of the tensile test piece) were formed between the tensile test piece and the tab plate.

The test piece (joints Nos. 1 to 5, 11 and 12) after spot welding is irradiated with a laser from one side of the tensile test piece, and the central part in the width direction of the parallel part coincides with the center of the beam in the longitudinal direction. The entire parallel portion was heat-treated. Joint No. Laser irradiation was not performed on 6 to 10.
Joint No. The laser irradiation conditions for 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 the irradiation surface-Laser moving speed: 250 cm / min
In addition, the joint No. The laser irradiation conditions for 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 the irradiation surface-Laser moving speed: 130 cm / min
In addition, the joint No. The laser irradiation conditions for 12 were as follows.
・ Oscillator type: Semiconductor laser ・ Output: 750W
-Beam shape: Rectangle with width direction: 8 mm and traveling direction: 1 mm on the irradiation surface-Laser moving speed: 80 cm / min

After that, whether or not the first region and the second region of a predetermined size were formed, and if so, the average Vickers hardness of the first region and the second region, and tempered martensite or The area ratio of hard martensite was 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 carried out on each test piece (in-plane tensile stress was applied) to investigate the fracture position. The tensile speed during the tensile test was 10 mm / min.
The results are shown in Table 1.

Joint No. In 1 to 5 (example of the present invention), a range of 0.1 mm (first region) from the overlapping surface is tempered to satisfy the formula (1), and a range of 0.1 mm from the surface (first region) is satisfied. Region 2) was hardened to satisfy equation (2). As a result, no crack was observed in the HAZ softened zone.
On the other hand, the joint No. In 6 to 10 (comparative example), the laser irradiation was not performed, so that the joint No. Equations (1) and (2) were not satisfied in the range corresponding to the first region or the second region of 1 to 5. As a result, in the tensile test, cracks occurred in the HAZ softened zone.
Joint No. Laser irradiation was performed in 11 to 12 (Comparative Examples), but the laser irradiation conditions were not preferable. As a result, in Comparative Example 11, the HAZ softened zone clearly remained in the first region due to insufficient heat input, and the HAZ softened zone was broken. Comparative Example 12 also had insufficient heat input, and although the second region was quenched so as to satisfy the formula (2), the tempering was insufficient in the first region, and the formula (1) was expressed in the first region. It was not satisfied, 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, cracks occurred in the HAZ softened zone.

1 Spot welded joint 2 Spot weld metal 3 Superposition surface 11 First steel plate 12 Second steel plate 51 First area 52 Second area

Claims (4)

A first steel sheet containing hard martensite with an average Vickers hardness HVbase of 350 HV or more, and
A second steel plate stacked on the first steel plate and
Two spot-welded metals that join the first steel plate and the second steel plate,
Including
In all cross sections in the plate thickness direction of the first steel sheet containing the two spot weld metals,
The first steel plate is
A first region formed between the two spot-welded metals and within a range of 0.1 mm in the plate thickness direction of the first steel plate from the surface on the second steel plate side.
It has a second region formed between the two spot-welded metals and in a range of 0.1 mm in the plate thickness direction from a surface opposite to the surface on the second steel plate side.
The metal structure of the first region 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 are expressed by the following formula (1). The filling,
The metal structure of the second region contains 50 area% or more of hard martensite, and the average Vickers hardness HV2 of the second region and the average Vickers hardness HVbase of the first steel sheet are expressed by the following formula (2). Meet,
A spot welded joint characterized by that.
HVbase × 0.33 + 150 ≦ HV1 ≦ HVbase × 0.33 + 230 Equation (1)
HVbase-30 ≤ HV2 ≤ HVbase + 30 Equation (2) The spot welded joint according to claim 1, wherein the difference between the maximum value and the minimum value of the Vickers hardness in the first region is 80 HV or less. The thickness of the first region in the plate thickness direction is 30 to 70% of the plate thickness of the first steel plate.
The spot welded joint according to claim 1 or 2. A first steel plate containing hard martensite and a second steel plate having an average Vickers hardness HVbase of 350 HV or more are superposed.
Two spot-welded metals that join the first steel plate and the second steel plate that are overlapped are formed.
By laser irradiation, the first steel plate is tempered between the two spot weld metals and within a range of 0.1 mm from the surface on the second steel plate side, and at the same time, between the two spot weld metals. Hardening is performed in a range of 0.1 mm from the surface opposite to the surface on the second steel plate side.
A method for manufacturing a spot welded joint.

Images