KR102556695B1 - Spot welded joint and manufacturing method of the spot welded joint - Google Patents

Abstract

This spot welded joint includes a first steel plate, a second steel plate overlapping the first steel plate, and two spot weld metals joining the first steel plate and the second steel plate, and includes the two spot weld metals. In the cross section of the first steel sheet, the first region formed in the range of 0.1 mm in the sheet thickness direction of the first steel sheet from the surface on the side of the second steel sheet and between the two spot weld metals; A second region formed in a range of 0.1 mm in the plate thickness direction from a surface opposite to the surface on the side of the second steel plate between two spot weld metals, wherein the average Vickers hardness HV1 of the first region and the second region The average Vickers hardness HVbase of one steel sheet satisfies HVbase×0.33+150≤HV1≤HVbase×0.33+230, and the average Vickers hardness HV2 of the second region and the average Vickers hardness HVbase of the first steel sheet satisfy HVbase-30 ≤HV2≤HVbase+30 is satisfied.

Description

Spot welded joint and manufacturing method of the spot welded joint

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

This application claims priority based on Japanese Patent Application No. 2019-097703 for which it applied to Japan on May 24, 2019, and uses the content here.

In a structure formed by overlapping a plurality of steel plate members, joining by resistance spot welding is widely performed with respect to overlapping portions in which steel plate members are overlapped.

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 sheet for automobiles, a high-strength steel sheet having a tensile strength of 980 MPa or more is widely used. In recent years, a high-strength steel sheet having a tensile strength of 1100 MPa or more has also begun to be applied. A high-strength steel sheet having a tensile strength of 1100 MPa or more generally includes a quenched structure 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 formed around the nugget. In general, HAZs include quenching organizations. However, when resistance spot welding is performed on a high-strength steel sheet having a quenched structure, a region (HAZ softening zone) having a lower hardness than the base material having a quenched structure is formed. This is because the quenching structure of the base metal is tempered by the heat of resistance spot welding.

When a car collides, it is necessary to protect the passengers in the cabin. For this reason, structural members (overlap welding members) constituting the automobile body such as A-pillar, B-pillar, roof rail, and 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 (overlapping portions) by resistance spot welding to form a cylindrical 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) or increasing the number of welding (spot) spots are taken.

A portion of the flange of the member subjected to resistance spot welding may be loaded with in-plane tensile stress at the time of an automobile collision. In general, if there is a region of low hardness such as the HAZ softening zone, the crash resistance performance of the member is lowered. Such a HAZ softening zone has little influence on the evaluation results of a tensile shear test and a cross tensile test (JIS Z3137) used for joint evaluation of resistance spot welding. However, when an in-plane tensile stress is applied, strain locally concentrates on the HAZ softened portion, and fracture may occur in the HAZ softened portion. Therefore, even if the base material is strengthened and the spot spot is increased, if the above-mentioned HAZ softening zone occurs, the crash resistance performance expected from the strength of the base material and the shape of the part 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 area surrounding the nugget from becoming a 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 spot welded portion is heat-treated at 100 to 400° C. to improve the L-shaped tensile joint strength as a welded joint with improved characteristics of the spot welded portion. Further, Patent Literature 3 describes a method of improving the strength of a cross tension joint by applying post-energization to a spot welded portion. In Patent Document 4, the periphery of the spot welding electrode is wound with a coil, and after welding, high-frequency induction heating is performed to temper the spot welded portion and the molten portion. The ratio of TSS and material strength, and the joint strength evaluated from the product of CTS and material strength An improved welding method is described.

However, according to the techniques disclosed in these Patent Literatures 2 to 4, certain effects are obtained in improving TSS and CTS, but in these Patent Literatures 2 to 4, in the HAZ softening zone when the steel sheet is loaded with in-plane tensile stress. Breakage is not considered.

In response to this problem, Patent Document 5 describes a B-pillar in which the energy absorption capability is improved by having a region having a strength of less than 1100 MPa called a soft zone in part or all of the flange portion provided for spot welding.

However, in the B-pillar disclosed in Patent Literature 5, since it is necessary to soften the side flange, there is a possibility that the bending performance, which is the crash resistance performance of the member, may decrease. Further, in Patent Literature 5, since a softening region is provided within the component before welding, there is also a problem that the shape accuracy of the component is lowered. If the shape accuracy of the parts is lowered, gaps are created between the parts during welding, which makes welding difficult.

Japanese Unexamined Patent Publication No. 2006-142905 Japanese Unexamined Patent Publication No. 2010-059451 Japanese Unexamined Patent Publication No. 2015-093282 Japanese Patent No. 5459750 Japanese Patent No. 5894081

The present disclosure has been made in view of the above problems, and provides a spot welded joint capable of suppressing fracture 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. It is a task to provide

The inventors of the present invention analyzed strain distribution when in-plane tensile stress occurred in the parts in which the HAZ softening zone was formed. As a result, while quenching the vicinity of the surface (opposite to the overlapping surface) between the weld metals of the high-strength steel sheets subjected to overlap welding, and tempering the vicinity of the overlapping surface, while minimizing the decrease in component strength, in-plane It has been found that even when tensile stress is loaded, it is possible to avoid locally concentrating strain on the HAZ softening section to cause breakage in the HAZ softening section.

This disclosure was made based on the above findings. The gist of this disclosure is as follows.

[1] A 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, a second steel plate overlapping the first steel plate, and the first steel plate In all cross sections in the sheet thickness direction of the first steel sheet including the two spot weld metals joining the second steel sheet, the first steel sheet includes the two spot weld metals. Between the weld metal and between the first region formed in the range of 0.1 mm from the surface on the second steel plate side in the sheet thickness direction of the first steel plate and between the two spot weld metals, and further on the second steel plate side A second region formed in a range of 0.1 mm in the plate thickness direction from a surface opposite to the surface, wherein the metal structure of the first region contains 50 area% or more of tempered martensite, and the average Vickers of the first region Hardness HV1 and average Vickers hardness HVbase of the first steel sheet satisfy the following formula (1), the metal structure of the second region contains 50 area% or more of hard martensite, and the average Vickers hardness of the second region HV2 and the average Vickers hardness HVbase of the first steel sheet satisfy the following formula (2).

[2] In the spot welded joint described in [1], the difference between the maximum value and minimum value of the Vickers hardness in the first region may be 80 HV or less.

[3] In the spot weld joint described in [1] or [2], the thickness of the first region in the sheet thickness direction may be 30 to 70% of the sheet thickness of the first steel sheet.

[4] In a method for manufacturing a spot welded joint according to another aspect of the present disclosure, a first steel sheet having an average Vickers hardness HVbase of 350 HV or more, including hard martensite, and a second steel sheet are overlapped, and the first steel sheet is overlapped. and two spot weld metals joining the second steel plate are formed, and by laser irradiation, between the two spot weld metals of the first steel plate and in a range of 0.1 mm from the surface on the second steel plate side. At the same time as tempering, quenching is performed in a range of 0.1 mm from the surface opposite to the surface on the side of the second steel plate between the two spot weld metals.

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

1 is a sectional view in the plate thickness direction of a spot welded joint according to the present embodiment.
Fig. 2 is a view of the spot welded joint according to the present embodiment when viewed from the first steel plate side to the upper surface.
Fig. 3 is a schematic diagram showing the relationship between the reached temperature and the measurement of the Vickers hardness when laser irradiation is applied to the spot welded joint according to the present embodiment.
4 is a schematic diagram showing test pieces used in Examples.

A spot welded joint according to an embodiment of the present disclosure (the spot welded joint according to the present embodiment) and a manufacturing method of 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 this embodiment comprises a first steel plate 11, a second steel plate 12 overlapping the first steel plate, and a first steel plate 11. It is provided with two spot welding metals (2) joining two steel plates (12). 1 and 2, the spot weld metal 2 is a nugget formed by resistance spot welding. Such a spot welded joint is obtained by overlapping the first steel plate 11 and the second steel plate 12 and performing resistance spot welding.

The first steel sheet 11 used for resistance spot welding is a steel sheet having an average Vickers hardness (HVbase) of 350 HV or more in consideration of application to automobile frame parts such as B-pillars. In addition, the first steel sheet is made of a structure including a quenched 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 sheet 11 (sometimes simply referred to as hardness) means the average Vickers hardness of the first steel sheet 11 subjected to welding before welding. In the case of measurement 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 cross sections in the plate thickness direction of the first steel plate 11 including the two spot weld metals 2 and 2, the first steel plate 11 The first region 51 formed between the two spot weld metals 2, 2 and in the range of 0.1 mm from the surface on the second steel plate 12 side (that is, the overlapping surface), and the two spot weld metals ( 2, 2), and has a second region 52 formed in a range of 0.1 mm from the surface opposite to the surface on the side of the second steel plate 12 (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 second The average Vickers hardness HVbase of one steel sheet satisfies the following formula (1).

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 of the second region 52 is HV2. and the average Vickers hardness HVbase of the first steel sheet satisfy the following formula (2).

A metal structure satisfying the conditions of the area % of tempered martensite and average Vickers hardness similar to that of the first region 51 may extend to the outside of the first region 51 . In addition, a metal structure satisfying the conditions of the area % of hard martensite and the average Vickers hardness similar to that of the second region 52 may extend to the outside of the second region 52 .

The reasons for limiting each configuration will be described below.

As described above, a high-strength steel sheet having an average Vickers hardness of 350 HV or more (about 1100 MPa or more in terms of tensile strength) has a structure including (for example, 50 area% or more) a quenched structure such as hard martensite. There are many cases. Such a structure is obtained by a manufacturing method including a quenching step.

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

As a result of examination by the present inventors, (i) preventing the local strength decrease part from occurring between two weld metals to which in-plane tensile stress is applied, (ii) a part with poor strength or a part that is easy to elongate other than the HAZ softening zone (iii) by modifying the structure of the region with the heat-affected zone including the HAZ softened zone so that the amount of elongation to fracture increases knew what could be done.

In the case of preventing the occurrence of a local strength decrease part, for example, tempering a region including a HAZ softened part between the spot weld metal 2, and reducing the hardness around the HAZ softened part to the same degree as the hardness of the HAZ softened part can think of However, in this case, although breakage in the HAZ softening zone can be suppressed, there is a concern that the crash resistance performance (bending performance) of the parts is lowered because the softening zone becomes large as a whole.

In the spot welded joint 1 according to the present embodiment, in order to prevent cracking in the HAZ softening zone while minimizing the decrease in joint strength, the spot weld metal 2 of the first steel plate 11 subjected to overlap welding, 2) While tempering the vicinity of the overlapping surface 3 between them, quenching the vicinity of the surface of the first steel plate between the spot weld metals 2 and 2 (on the opposite side to the overlapping surface 3) By doing so, a first region and a second region having a desired metal structure and average Vickers hardness are formed.

<First area>

[The metal structure contains 50 area% or more of tempered martensite, and the average Vickers hardness HV1 of the first region and the average Vickers hardness HVbase of the first steel sheet satisfy HVbase×0.33+150≤HV1≤HVbase×0.33+230 ]

As a result of examination by the present inventors, when a spot is applied to the first steel sheet 11 containing hard martensite having an average Vickers hardness HVbase of 350 HV or more, the hardness (Vickers hardness) of the HAZ softened zone caused by the heat effect of welding is ( Hardness of the first steel plate 11 before welding × 0.33 + 150) to (hardness × 0.33 + 230 of the first steel plate 11 before welding). 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 zone is tempered so that the tempered martensite has a structure of 50 area% or more, and the Vickers hardness is controlled to satisfy the following formula (1).

When the average Vickers hardness HV1 of the first region 51 satisfies Expression (1), the hardness difference between the HAZ softened zone and its surroundings becomes 80 HV or less. In this case, the concentration of strain on the HAZ softening zone can be alleviated.

It is preferable that the difference between the maximum value and the minimum value of the Vickers hardness in the region is 80 HV or less, in addition to that the average Vickers hardness of the first region 51 satisfies Expression (1). By reducing the difference between the maximum and minimum values of hardness in the first region 51, the concentration of strain can be further alleviated. In other words, if the hardness distribution in the first region 51 is homogeneous, local concentration of deformation can be avoided. More preferably, the difference between the maximum value and the minimum value of the Vickers hardness is 50 HV or less.

[It is formed in the range of 0.1 mm from the surface on the side of the second steel plate between the two spot weld metals]

The first region 51 is in the range (thickness) of 0.1 mm from the surface between the two spot weld metals 2, 2 of the plate thickness cross section of the first steel plate 11 and on the side of the second steel plate 12. ) is formed. If the thickness of the above hardness region in the plate thickness direction is less than 0.1 mm, there is a possibility that a sufficient effect may not be obtained because the HAZ softening zone, which is a local strength lowering zone, remains. A region that satisfies the conditions of 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 side of the second steel plate to a range of 30% or more of the sheet 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 side of the second steel plate to a range exceeding 90% of the plate thickness of the first steel plate, the average hardness of the entire joint portion decreases. This is undesirable because there is a concern that the bending yield strength may be lowered.

<Second Area>

[The metal structure 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 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, deformation is concentrated when a local strength lowering portion such as a HAZ softening zone is formed between the two spot weld metals 2, 2. However, as a result of examination by the present inventors, it has been found that deformation tends to be concentrated particularly when a strength-decreasing portion exists in the vicinity of the surface of the first steel plate 11 (surface opposite to the overlapping surface).

Therefore, in the spot welded joint 1 according to the present embodiment, quenching is performed with respect to the vicinity of the surface of the first steel plate 11 between the two spot welded metals 2 and 2 including the HAZ softening zone. , the hardness of the quenched region is made equal to the average Vickers hardness of the first steel plate 11 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 are made to satisfy the following formula (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 exceeds 30, the concentration of strain upon in-plane tensile stress loading cannot be sufficiently suppressed.

The average of the first regions 51 in that the first region 51 is a tempered structure containing 50 area% or more of tempered martensite, and the second region is a quenched structure containing 50 area% or more of hard martensite. The average Vickers hardness HV2 of the second region 52 is greater than the Vickers hardness HV1.

[It is formed in the range of 0.1 mm from the surface opposite to the surface on the side of the second steel plate between the two spot weld metals]

The second region 52 is in the range of 0.1 mm from the surface opposite to the surface between the two spot weld metals 2, 2 of the plate thickness cross section of the first steel plate 11 and on the second steel plate side. (thickness). If the thickness in the thickness direction of the region having the above hardness is less than 0.1 mm, there is a possibility that a sufficient effect may not be obtained because the HAZ softening zone, which is a local strength lowering zone, remains. 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 sheet thickness of the first steel plate from the surface opposite to the surface on the second steel plate 12 side. desirable. 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 exceeding 70% of the first steel plate 11, the second region side It is undesirable because there is a concern that the elongation at break will decrease when a tensile bending load outside the bending is applied.

In the spot welded joint 1 according to the present embodiment, the first region 51 and the second region 52 are all cross sections in the plate thickness direction of the first steel plate containing two spot weld metals, is formed That is, when cross sections in the thickness direction of the first steel sheet are observed so that two weld metals are included, the above-described first region 51 and second region 52 are observed in all cross sections.

In other words, when D is the diameter of the overlapping surface 3 of the spot weld metal 2, cross sections of the first steel sheet 11 in the first region 51 and the second region 52 in the thickness direction The width in the direction perpendicular to (the direction perpendicular to the paper in FIG. 1 and the vertical direction on the paper in FIG. 2) is 1.0×D. A metal structure satisfying the conditions of the area % of tempered martensite and average Vickers hardness similar to that of the first region 51 may extend to the outside of the first region 51 in the width direction. A metal structure satisfying the conditions of the area % of hard martensite and the average Vickers hardness similar to that of the second region 52 may extend to the outside of the second region 52 in the width direction.

When an automobile collision occurs, the direction of the in-plane tensile stress is not necessarily parallel to the direction connecting the spot weld metals 2, 2 (the direction connecting the centers of the spot weld metals 2, 2). . That is, there is a case where it has a certain angle (stress is applied in an oblique direction). If the width of the first region 51 and the second region 52 is 1.0 × D, when the direction of application of the in-plane tensile stress becomes a constant angle with respect to the direction connecting the spot weld metals 2 and 2 (inclination direction), strain concentration to the HAZ softening zone where strain can be concentrated is suppressed. As a result, breakage in the HAZ softening 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 has a constant angle with respect to the direction connecting the spot weld metals 2 and 2. In the case where (stress is applied in an oblique 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 with a load of 1.0 kgf.

In a steel sheet containing a hard martensitic structure, the hardness of the portion affected by welding heat is lower than the hardness before welding. For this reason, the hardness of the 1st steel plate 11 measures the hardness of the position which does not receive the heat effect by welding of the 1st steel plate 11, and uses the average value. As a position that is not affected by heat due to welding, for example, the hardness may be measured at a position separated from the spot weld metal 2 by 15 mm or more in a direction where there is no other weld metal.

Specifically, using a Vickers hardness tester, with a load of 1.0 kgf, 10 locations that are not affected by heat by welding, 1/8 of the sheet thickness from the surface of the first steel sheet 11, 3/8 The longitude of the position, the 5/8 position, and the 7/8 position are measured, and the average value is used.

For the thickness of the first region 51 and the second region 52 from the surface of the first steel plate 11 and their average Vickers hardness, a Vickers hardness tester with a load of 100 gf was used, and the plate thickness of the first steel plate With respect to the cross section in the direction, polishing and Vickers hardness measurement are repeated, and a Vickers hardness distribution in a range spanning the spot weld metal 2, 2 is obtained. Based on this distribution, the thickness of the first region 51 and the second region 52 and their average Vickers hardness are calculated.

Distribution of the Vickers hardness is specifically measured by the following method.

First, a sample is taken so that the cross section (A-A cross section shown in Fig. 2) of the first steel plate 11 in the sheet thickness direction passing through the centers of two spot weld metals becomes the measurement plane.

With respect to this measurement plane, in the plate thickness direction of the first steel plate 11, Vickers hardness was measured at positions 0.1 mm from the surface and overlapping surface of the first steel plate 11, respectively, and at positions divided into 5 equal parts therebetween. do 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).

Thereafter, the sample is polished to a thickness of 0.5 mm, and the same Vickers hardness measurement as described above is performed on the raised cross section (B-B cross section shown in Fig. 2).

Further, polishing and Vickers hardness measurement are performed until the cross section does not contain the spot weld metal 2, and the Vickers hardness distribution of the first steel sheet 11 between the spot weld metals 2 and 2 is obtained. . Since the hardness is considered to be equivalent to the cross section on the opposite side, it is sufficient to measure the hardness on half the cross section as described above.

Whether or not the first steel sheet 11 subjected to welding contains hard martensite is determined by a position of 1/8 of the sheet thickness from the surface of the first steel sheet 11 at a position not affected by heat by welding, 3/ Samples taken from each of 5 locations, 8 locations, 5/8 locations, and 7/8 locations, were etched using Repera etchant, and an optical microscope was used to obtain a 100 μm square field of view at a magnification of 1000 times. Observe and judge. Martensite that appears white or reddish brown within the observation field is martensite, and among martensite, martensite containing carbides is judged to be hard martensite, and martensite without carbides is judged to be tempered martensite.

In addition, the area ratio of tempered martensite in the first region 51 and the area ratio of hard martensite in the second region 52 are on the same plane as the above-described Vickers hardness measurement plane, and the target region (th For the area that satisfies formula (1) for area 1 and the area that satisfies formula (2) for area 2), samples taken from 5 locations were etched using Repera etching solution and examined under an optical microscope. Observe a 100 μm square field of view at a magnification of 1000 times, and measure the area ratio of martensite assuming that what appears white to reddish brown in the observation field is martensite. By averaging the martensite area rates of the observed fields of view, the martensite area rates of the first region 51 and the second region 52 are obtained. Thereafter, the same sample was subjected to an etching process using a picral, and a 100 μm square field of view was observed at a magnification of 1000 times with an optical microscope, and martensite containing carbides was hardened within the observation field of view. Martensite and martensite containing no carbide are referred to as tempered martensite, and the respective ratios of tempered martensite and hard martensite among martensite are obtained.

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 that connects two overlapping steel sheets at points. As means of spot welding, arc spot welding, resistance spot welding, and laser spot welding are exemplified. On the other hand, welding performed in a linear manner is referred to as continuous welding. As a means of continuous welding, arc welding, laser welding, seam welding, etc. are mentioned. Compared to continuous welding, spot welding has a smaller welding area, so the construction time is shorter and it is power saving. That is, spot welding is excellent in productivity.

In the above, the case where the spot weld metal 2 is a resistance spot welding nugget has been described, but in the spot welded joint 1 according to the present embodiment, the spot weld metal 2 is a resistance spot welding nugget. Not limited to joints. 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 weld metals 2 are a hat member. It is preferable to form the overlapping part of the flange part of and the closing plate. Such a configuration acts particularly effectively to improve strength and crash resistance as a structural member.

Further, the spot welded joint 1 according to the present embodiment has two spot weld metals 2, and if the area between the two spot weld metals 2 and 2 satisfies the above relationship, the effect is obtained. When applied to automobile frame parts and the like, a plurality of (two or more) spot weld metals are formed. Even when spot weld metals exceeding 2 are formed, as long as the area between the target two spot weld metals has the above relationship, the effect is obtained for that area. In the case of having more than 2 spot weld metals, it is preferable to control the distance between the two spot weld metals 2 so as to satisfy the above relationship, especially in a region where in-plane tensile stress is expected to be applied. Among all the spot weld metals 2, if the hardness relationship as described above is satisfied, fracture at the HAZ softening zone can be suppressed even when tensile stress is applied to any direction and location in the surface of the high-strength steel sheet, so it is more preferable.

The spot welded joint 1 according to this embodiment can be applied to an A-pillar, a side sill, or a B-pillar. For example, in the B-pillar, the flange portion of the hat member is joined to the closing plate by a spot weld metal. Between the spot weld metals of the B-pillar, if the above relationship is satisfied, even when an in-plane tensile stress is applied to the flange portion at the time of a vehicle collision, breakage at the portion that was the HAZ softening portion can be suppressed.

The first steel plate 11 and/or the second steel plate 12 may be plated steel plates. In this case, corrosion resistance is improved. As a plated steel sheet, a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, an electrogalvanized steel sheet, an aluminum-plated steel sheet etc. are illustrated, for example.

Next, the manufacturing method of the spot welded joint according to this 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 manufacturing method of the spot welded joint according to the present embodiment,

(I) overlapping a first steel sheet containing hard martensite having an average Vickers hardness HVbase of 350 HV or more and a second steel sheet;

(II) forming a plurality of spot weld metals joining the superimposed first steel plate and the second steel plate;

(III) Tempering is performed in a range of 0.1 mm from the surface on the side of the second steel plate between the two spot weld metals of the first steel plate, and between the two spot weld metals and the second steel plate Quenching is performed in the range of 0.1 mm from the surface opposite to the side surface.

have a fair

For the first steel sheet 11 containing hard martensite having an average Vickers hardness HVbase of 350 HV or more and the second steel sheet 12, known steel sheets can be used, respectively.

These steel plates are overlapped and spot welding is performed 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, part of the first steel sheet is quenched by laser irradiation, and part of the first steel sheet is tempered to form a first region and a second region.

When quenching is performed, it is necessary to raise the temperature of the target region to more than Ac1°C. Preferably it is Ac1+30 degreeC or more. However, if the temperature of the quenching region is too high, the region that needs to be tempered will also become a quenching region due to heat conduction. Therefore, heat input control according to the plate thickness is required.

On the other hand, when tempering is performed, it is necessary to heat the temperature of the target region to a temperature lower than Ac1°C. As shown in Fig. 3, the heated region becomes tempered martensite, and the hardness decreases with the increase in temperature up to Ac1 °C. On the other hand, when the heating temperature exceeds Ac1°C, the structure transforms into austenite. Since this austenite transforms into hard martensite again during cooling, high hardness is exhibited at a site heated above Ac1°C.

Using this, laser irradiation is performed from the surface side of the first steel sheet (surface opposite to the second steel sheet), the temperature near the surface of the first steel sheet is heated to exceed Ac1°C, and the temperature near the opposite surface When is heated to Ac1° C. or less, a predetermined first region and a second region can be formed.

In the case of performing the above heating, in order to impart a hardness distribution in the plate thickness direction, it is necessary to input heat only to the extreme surface layer and apply heat in the depth direction by heat conduction. In addition, when heating to outside the target region, heat dissipation in the target region becomes insufficient, and a tempered structure may not be obtained.

For example, in high-frequency induction heating, since heat is input to a certain depth, a desirable hardness distribution cannot be obtained. Further, in gas heating or arc heating, it is difficult to target and heat only a specific region.

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

The laser irradiation conditions are not particularly limited, and may be determined by the sheet thickness of the first steel sheet, the thickness of the first region or the second region to be obtained, and the like. For example, the following conditions are exemplified.

conditions exemplified

ㆍType of oscillator: semiconductor laser

ㆍOutput: 500 to 3000W

ㆍBeam shape: Rectangular width direction: 4 to 10 mm, travel direction: 0.5 to 3 mm in the irradiation surface

ㆍLaser movement speed: 50 to 500 cm/min

By such laser irradiation, tempering is performed in the range of 0.1 mm from the surface on the side of the second steel plate between the two spot weld metals of the first steel plate, and also between the two spot weld metals and the second Quenching can be performed in the range of 0.1 mm from the surface opposite to the surface on the steel plate side.

Example

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

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

Next, a tensile test piece having a gauge-to-gauge distance of 50 mm and a parallel section width of 25 mm as shown in FIG. 4 was taken from the steel sheet. Further, a square tap plate with one side of 25 mm was taken from the steel plate.

As shown in Fig. 4, a tab plate was placed on the parallel portion of the sampled tensile test piece, and resistance spot welding was performed on the central portion of each tab plate using a single-phase alternating current spot welding machine under the conditions shown below.

Electrode: DR type electrode (tip φ6mm R40)

Pressing force: 400kgf

Energization time: 24cyc

By resistance spot welding, two weld metals having a nugget diameter of 4 x √t (t: sheet thickness (mm) of the tensile test piece) were formed between the tensile test piece and the tab plate.

For the test specimens (joints No. 1 to 5, 11, and 12) after spot welding, a laser was irradiated from one side of the tensile test, and the central part in the width direction of the parallel part coincided with the center of the beam over the entire parallel part in the long side direction. Heat treatment was performed. Laser irradiation was not performed on joints Nos. 6 to 10.

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

ㆍType of oscillator: semiconductor laser

ㆍOutput: 1200W

ㆍBeam shape: Rectangular in the irradiation surface, width direction: 8 mm, travel direction: 1 mm

ㆍLaser movement speed: 250 cm/min

In addition, the laser irradiation conditions for joint No. 11 were as follows.

ㆍType of oscillator: semiconductor laser

ㆍOutput: 700W

ㆍBeam shape: Rectangular in the irradiation surface, width direction: 8 mm, travel direction: 1 mm

ㆍLaser moving speed: 130cm/min

In addition, the laser irradiation conditions for joint No. 12 were as follows.

ㆍType of oscillator: semiconductor laser

ㆍOutput: 750W

ㆍBeam shape: Rectangular in the irradiation surface, width direction: 8 mm, travel direction: 1 mm

ㆍLaser moving speed: 80cm/min

After that, whether or not the first region and the second region of a predetermined size have been formed, if formed, the average Vickers hardness of the first region and the second region, and the area ratio of tempered martensite or hard martensite It was investigated by the method described above. The measurement plane was made into a cross section in the thickness direction at the center of the width direction of the test body.

In addition, a tensile test was performed on each test body (in-plane tensile stress was applied) to investigate the fracture location. The tensile speed at the time of the tensile test was 10 mm/min.

The results are shown in Table 1.

In joints No. 1 to 5 (examples of the present invention), a range of 0.1 mm from the overlapping surface (first area) is tempered, satisfying Expression (1), and a range of 0.1 mm from the surface (second area). was called ?? and satisfied Expression (2). As a result, no cracks were observed in the HAZ softening zone.

On the other hand, joints Nos. 6 to 10 (comparative examples) were not subjected to laser irradiation, and in the range corresponding to the first area or the second area of joints Nos. 1 to 5, equations (1) and (2) did not meet As a result, cracks occurred in the HAZ softening zone in the tensile test.

Joints Nos. 11 to 12 (comparative examples) were subjected to laser irradiation, but the laser irradiation conditions were not favorable. As a result, in Comparative Example 11, the HAZ softened portion clearly remained in the first region due to insufficient heat input, and the HAZ softened portion was broken. Comparative Example 12 also lacks heat input, and the second region is quenched to satisfy Expression (2), but the first region becomes insufficient in tempering, and does not satisfy Expression (1) in the first region, and also the above The difference between the maximum value and minimum value of the Vickers hardness in the first region was 80 or more. As a result, cracks occurred in the HAZ softening zone in the tensile test.

1: spot welding joint
2: spot welding metal
3: overlapping plane
11: first steel plate
12: second steel plate
51 first area
52 second area

Claims (4)

A first steel sheet containing hard martensite having an average Vickers hardness HVbase of 350 HV or more;
A second steel plate overlapping the first steel plate;
Two spot weld metals joining the first steel plate and the second steel plate
including,
In all cross sections in the sheet thickness direction of the first steel sheet including the two spot weld metals,
The first steel plate,
A first region formed between the two spot weld metals and in a range of 0.1 mm in the sheet thickness direction of the first steel sheet from the surface on the side of the second steel sheet;
A second region formed in a range of 0.1 mm in the sheet thickness direction from a surface opposite to the surface on the side of the second steel sheet 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 satisfy 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 sheet satisfy the following formula (2),
A spot welded joint, characterized in that.

The spot welded joint according to claim 1, wherein a difference between a maximum value and a minimum value of Vickers hardness in the first region is 80 HV or less. The method according to claim 1 or 2, wherein the thickness of the first region in the sheet thickness direction is 30 to 70% of the sheet thickness of the first steel sheet.
A spot welded joint, characterized in that. A first steel sheet containing hard martensite having an average Vickers hardness HVbase of 350 HV or more and a second steel sheet are overlapped,
Forming two spot weld metals joining the overlapped first steel plate and the second steel plate,
By laser irradiation, tempering is performed in a range of 0.1 mm from the surface of the first steel plate between the two spot weld metals and the second steel plate side, and also between the two spot weld metals. 2 Perform quenching in the range of 0.1 mm from the surface opposite to the surface on the steel plate side,
Method for manufacturing a spot welded joint, characterized in that.

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