KR20210150563A - Spot weld joint, and method of manufacturing spot weld joint - Google Patents

Abstract

This spot weld 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, and the two spot weld metals are included. a first region formed between the two spot weld metals and between the two spot weld metals and in the thickness direction of the first steel sheet from the surface on the side of the second steel sheet in a range of 0.1 mm; a second region formed between two spot weld metals in a range of 0.1 mm from a surface opposite to the surface on the second steel plate side in the plate thickness direction, the average Vickers hardness HV1 of the first region and the second region 1 The average Vickers hardness HVbase of the 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 are HVbase-30 ≤HV2≤HVbase+30 is satisfied.

Description

Spot weld joint, and method of manufacturing spot weld joint

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

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

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

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, high-strength steel sheets having a tensile strength of 1100 MPa or more are also beginning to be applied. A high-strength steel sheet having a tensile strength of 1100 MPa or more generally includes a quenching structure in order to obtain high strength. When resistance spot welding is performed, a nugget (spot weld metal) for welding a steel sheet is formed, and a heat affected zone (hereinafter referred to as HAZ) is formed around the nugget. In general, HAZ includes a quenching organization. However, when resistance spot welding is performed on a high-strength steel sheet having a quenching structure, a region (HAZ softened portion) having a lower hardness than the base material having a quenching 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 (lap welding members) constituting an 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 a collision and to absorb more collision energy with a small deformation amount, methods such as strengthening the material (base material) and increasing the welding (spot) points are taken.

An in-plane tensile stress may be applied to a portion of the flange of the member to be resistance spot welded at the time of an automobile collision. In general, if there is a region with low hardness, such as a HAZ softening zone, the impact resistance performance of the member is lowered. Such a HAZ softened portion has a small influence on the evaluation results of the tensile shear test and cross tensile test (JIS Z3137) used for joint evaluation of resistance spot welding. However, when an in-plane tensile stress is applied, deformation may locally concentrate in the HAZ softened portion to cause fracture in the HAZ softened portion. Therefore, even if the base material is strengthened and the spot spot is increased, if the above-described HAZ softened area is generated, the collision resistance performance assumed from the strength of the base material and the shape of the part may not be obtained.

Accordingly, 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 a fracture origin.

BACKGROUND ART In the past, studies have been made to improve the properties of a weld member formed by resistance spot welding. For example, Patent Document 2 describes a weld joint in which the properties of the spot welded part are improved, and the spot welded part is heat treated at 100 to 400°C to improve the strength of the L-shaped tensile joint. In addition, Patent Document 3 describes a method of improving the strength of a cross-tensile joint by performing post-energization to the spot welded portion. In Patent Document 4, by winding the periphery of the spot welding electrode with a coil, high-frequency induction heating is rapidly performed after welding to temper the spot weld and the molten portion. An improved welding method is described.

However, according to the techniques disclosed in these Patent Documents 2 to 4, although a certain effect is obtained for the improvement of TSS and CTS, in these Patent Documents 2 to 4, in the HAZ softening zone when an in-plane tensile stress is applied to the steel sheet, Fracture is not taken into account.

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

However, in the B-pillar disclosed in Patent Document 5, since it is necessary to soften the side flange, there is a fear that the bending performance, which is the collision resistance performance of the member, may decrease. Moreover, in patent document 5, since a softening area|region is provided in a component before welding, there also exists a subject that the shape precision of a component falls. When the shape precision of a part falls, a gap will arise between parts at the time of welding, and welding becomes difficult.

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

The present disclosure has been made in view of the above problems, and provides a spot weld 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 a spot weld joint The task is to provide

The present inventors analyzed the strain distribution when an in-plane tensile stress was generated in the part in which the HAZ softened part was formed. As a result, by quenching the vicinity of the surface (on the opposite side of the overlapping surface) between the weld metals of the high-strength steel sheet subjected to overlap welding, and at the same time, by tempering the overlapping surface vicinity, the decrease in component strength is minimized while in-plane It has been found that even when a tensile stress is applied, it is possible to avoid the occurrence of fractures in the HAZ softened portion by locally concentrating the deformation in the HAZ softened portion.

This indication was made based on the said knowledge. The gist of the present disclosure is as follows.

[1] A spot welded joint according to an aspect of the present disclosure includes a first steel sheet containing hard martensite having an average Vickers hardness HVbase of 350 HV or more, a second steel sheet superimposed on the first steel sheet, the first steel sheet, In all cross-sections in the plate thickness direction of the first steel sheet including two spot weld metals joining the second steel plates, and including the two spot weld metals, the first steel plate has the two spots A first region formed in a range of 0.1 mm between the weld metal and the surface on the side of the second steel sheet in the sheet thickness direction of the first steel sheet, between the two spot weld metals, and on the side of the second steel sheet It has a second region formed in a range of 0.1 mm in the thickness direction from the surface opposite to the surface, the metal structure of the first region contains 50 area% or more of tempered martensite, and the average Vickers of the first region The hardness HV1 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 of the second region The average Vickers hardness HVbase of HV2 and the said 1st steel plate satisfy|fills following formula (2).

[2] In the spot weld joint according to [1], the difference between the maximum and minimum values of Vickers hardness in the first region may be 80 HV or less.

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

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

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

1 is a cross-sectional view in the plate thickness direction of a spot weld joint according to the present embodiment.
2 is a view when the spot welded joint according to the present embodiment is viewed from the first steel plate side in the upper surface.
It is a schematic diagram which shows the relationship between the reached temperature and the measurement of Vickers hardness at the time of laser-irradiating the spot weld joint which concerns on this embodiment.
It is a schematic diagram which shows the test piece used in the Example.

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

The first steel sheet 11 provided 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 skeleton parts such as B-pillars. Moreover, the 1st steel plate consists of a structure|tissue containing a quenching structure, such as hard martensite. In addition, about the 2nd steel plate 12, it is not limited.

The average Vickers hardness of the first steel sheet 11 (it may simply be referred to as hardness) means the average Vickers hardness before welding of the first steel sheet 11 used for welding. When measured after spot welding, it means the average Vickers hardness measured at a position not affected by welding heat.

In addition, in the spot weld 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 A first region 51 formed between the two spot weld metals 2 and 2 and in a range of 0.1 mm from the face (ie, overlapping face) on the side of the second steel plate 12, and the two spot weld metals ( Between 2 and 2), there is provided 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).

Moreover, in the spot weld joint 1 which concerns on this embodiment, the metal structure of the 1st area|region 51 contains 50 area% or more of tempered martensite, and the average Vickers hardness of the 1st area|region 51 HV1 and The average Vickers hardness HVbase of 1 steel plate satisfy|fills following formula (1).

Moreover, in the spot weld joint 1 which concerns on this embodiment, the metal structure of the said 2nd area|region 52 contains 50 area% or more of hard martensite, and the average Vickers hardness of the said 2nd area|region 52 HV2. and the average Vickers hardness HVbase of the first steel sheet satisfies the following formula (2).

The metal structure which satisfy|fills the conditions of the area % of tempered martensite similar to the 1st area|region 51 and average Vickers hardness may be extended to the outer side of the 1st area|region 51. Moreover, the metal structure which satisfy|fills the conditions of the area % of hard martensite similar to the 2nd area|region 52 and average Vickers hardness may be extended to the outer side of the 2nd area|region 52.

Hereinafter, the reason for limitation of each structure is demonstrated.

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

When welding is performed on a steel sheet having a quenching structure, hard martensite is changed into 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 softened portion) having a lower hardness than the base material is formed. When a tensile stress arises in the surface of the plate which has a welding part, this HAZ softened part may become a fracture|rupture origin.

As a result of the investigation by the present inventors, (i) between two weld metals to which in-plane tensile stress is applied, a local strength reduction area does not occur, (ii) a site having inferior strength or a site prone to elongation other than the HAZ softened area By setting the shape to be provided, and (iii) modifying the structure of the heat affected zone including the HAZ softened zone to increase the elongation to breakage of the zone, the concentration of strain in the area that was the HAZ softened zone is alleviated. knew what could be done.

In the case of preventing the occurrence of a local strength reduction zone, for example, by tempering the area including the HAZ softened zone between the spot weld metal 2, the hardness around the HAZ softened zone is reduced to the same degree as the hardness of the HAZ softened zone. can think of However, in this case, although the fracture|rupture in a HAZ softened part can be suppressed, since a softened part becomes large as a part as a whole, there is a concern that the collision-resistance performance (bending performance) of a part falls.

In the spot welded joint 1 according to the present embodiment, in order to prevent cracking in the HAZ softened zone while minimizing the decrease in joint strength, the spot welded metal 2 of the first steel sheet 11 subjected to overlap welding; 2) Tempering the vicinity of the overlapping surface 3 between each other, and quenching the vicinity of the surface (opposite side to the overlapping surface 3) of the first steel sheet between the spot weld metals 2 and 2 . Thereby, the 1st area|region and 2nd area|region provided with the desired metal structure and average Vickers hardness are formed.

<First Area>

[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 the investigation by the present inventors, when a spot is performed on 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 ( The hardness of the first steel sheet 11 before welding x 0.33 + 150) to (hardness of the first steel sheet 11 before welding x 0.33 + 230). Therefore, in the spot weld joint 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 structure in which tempered martensite is 50 area% or more, and the Vickers hardness is controlled so that the following formula (1) is satisfied.

When the average Vickers hardness HV1 of the first region 51 satisfies Formula (1), the hardness difference between the HAZ softened portion and its surroundings becomes 80 HV or less. In this case, it is possible to relieve the concentration of strain on the HAZ softening part.

As for the 1st area|region 51, it is preferable that the difference between the maximum value and minimum value of Vickers hardness in an area|region other than that an average Vickers hardness satisfy|fills Formula (1) is 80 HV or less. By reducing the difference between the maximum and minimum hardness values in the first region 51 , 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 minimum value of Vickers hardness is 50 HV or less.

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

The first region 51 is in the range (thickness) of the plate thickness cross section of the first steel plate 11 between the two spot weld metals 2 and 2 and 0.1 mm from the surface on the second steel plate 12 side. ) is formed. If the thickness in the plate thickness direction of the region having the hardness is less than 0.1 mm, there is a possibility that the HAZ softened portion, which is a local strength lowering portion, remains, and a sufficient effect may not be obtained. A region satisfying the conditions of hardness and texture of the first region 51 may be extended to the outside of the first region 51 . In that case, it is preferable that the region satisfying the hardness and the structure of the first region 51 extends from the surface on the second steel plate side to 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 exceeding 90% of the plate thickness of the first steel plate, the average hardness of the entire joint portion decreases. This is not preferable because there is a concern that the bending strength may decrease.

<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 in-plane tensile stress is applied to the spot welded joint 1 according to the first embodiment, if a local strength-reduced portion such as a HAZ softened portion is formed between the two spot weld metals 2 and 2, the deformation is concentrated However, as a result of studies by the present inventors, it can be seen that, in particular, when the strength-reduced portion exists near the surface (surface opposite to the overlapping surface) of the first steel sheet 11, the strain tends to be concentrated.

Therefore, in the spot weld joint 1 according to the present embodiment, quenching is performed in the vicinity of the surface of the first steel sheet 11 between the two spot weld metals 2 and 2 including the HAZ softening part, , The hardness of the quenched region is made equal to the average Vickers hardness of the first steel sheet 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 sheet 11 satisfy the following formula (2).

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

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

The second region 52 is in the range of 0.1 mm from the surface of the plate thickness cross section of the first steel plate 11 between the two spot weld metals 2 and 2 and opposite to the surface on the second steel plate side. (thickness) is formed. If the thickness in the plate thickness direction of the region having the above hardness is less than 0.1 mm, the HAZ softened portion, which is a local strength lowering portion, remains, and there is a possibility that a sufficient effect may not be obtained. A region satisfying the conditions of hardness and texture of the second region 52 may be extended to the outside of the second region 52 . In that case, the region satisfying the hardness and texture of the second region 52 is formed in a range of 10% or more of the 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 more than 70% of the first steel plate 11, the second region side When a tensile-bending load other than bending is applied, since there is a concern that the elongation at break will fall, it is not preferable.

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 sheet including two spot weld metals, is formed That is, when a cross section in the plate thickness direction is observed with respect to the first steel sheet so that two weld metals are included, the first region 51 and the second region 52 are observed in all cross sections.

In other words, when the diameter of the overlapping surface 3 of the spot weld metal 2 is D, the first region 51 and the second region 52 are cross-sections in the thickness direction of the first steel sheet 11 . A width in a direction perpendicular to (a direction perpendicular to the paper surface in FIG. 1 , an up-down direction on the paper surface in FIG. 2 ) is 1.0×D. The metal structure which satisfies the conditions of the area % of tempered martensite similar to the 1st area|region 51 and average Vickers hardness may be extended to the width direction outer side of the 1st area|region 51. The metal structure which satisfies the conditions of the area % of hard martensite similar to the 2nd area|region 52 and average Vickers hardness may be extended to the width direction outer side of the 2nd area|region 52.

In the case of an automobile collision, the direction of the 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, there are cases where it has a constant angle (stress is applied in the 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 connecting direction between the spot weld metals 2 and 2 (inclination) direction), strain concentration to the HAZ softened portion where strain can be concentrated is suppressed. As a result, fracture in the HAZ softened portion is further suppressed.

On the other hand, if 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 between the spot weld metals 2 and 2 . In the case (stress is applied in the oblique direction), there is a concern that a sufficient effect cannot be obtained.

The average hardness of the first steel sheet 11 is measured using a Vickers hardness tester with a load of 1.0 kgf.

In the steel sheet containing the hard martensitic structure, the hardness of the portion affected by the welding heat becomes lower than the hardness before welding. For this reason, the hardness of the 1st steel plate 11 measures the hardness of the position which is not affected by the heat by welding of the 1st steel plate 11, and the average value is used. What is necessary is just to measure the hardness of the position spaced apart by 15 mm or more from the spot weld metal 2 in the direction where there is no other weld metal as a position which is not affected by the heat by welding, for example.

Specifically, using a Vickers hardness tester, the load was set to 1.0 kgf, and the position of 1/8 of the sheet thickness from the surface of the first steel sheet 11 at 10 locations not affected by heat by welding, 3/8 Measure the position of , position of 5/8, position of 7/8, and use its average value.

About the thickness from the surface of the 1st steel plate 11 of the 1st area|region 51 and the 2nd area|region 52, and their average Vickers hardness, using the Vickers hardness tester set to the load of 100 gf, the plate|board thickness of the 1st steel plate With respect to the directional cross section, grinding and measurement of Vickers hardness are repeated to obtain a distribution of Vickers hardness in a range sandwiched between spot weld metals 2 and 2 . Based on this distribution, the thickness of the 1st area|region 51 and the 2nd area|region 52 and their average Vickers hardness are computed.

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

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

With respect to this measurement surface, in the plate thickness direction of the first steel plate 11, the Vickers hardness was measured at a position of 0.1 mm from the surface and the overlapping surface of the first steel plate 11, respectively, and a position divided into 5 parts therebetween. do This measurement is repeatedly performed at 0.5 mm intervals in the width direction (the direction connecting the spot weld metal 2 and the other spot weld metal 2).

Then, the sample is grind|polished by 0.5 mm, and Vickers hardness measurement similar to the above is performed with respect to the protruding cross section (B-B cross section shown in FIG. 2).

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

Whether the first steel sheet 11 provided for welding contains hard martensite is determined by a position of 1/8 of the plate thickness from the surface of a position not affected by heat by welding of the first steel sheet 11, 3/ With respect to the samples taken from each of the 5 locations at the position of 8, 5/8, and 7/8, etching treatment was performed using Repera etchant, and an optical microscope at a magnification of 1000 times and a field of view of 100 µm in all directions can be observed and judged. In the observation field, what appears white to reddish-brown is martensite, and among martensite, martensite containing carbide is judged to be hard martensite, and martensite not containing carbide 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 the same as the above-mentioned Vickers hardness measurement surface. Samples taken from 5 locations were etched using Repera etchant for 1 area, an area that satisfies Equation (1), and if it was a 2nd area, an area that satisfies Equation (2)) A field of view of 100 µm square is observed at a magnification of 1000 times, and the area ratio of martensite is measured assuming that it is martensite that appears white to reddish-brown in the observation field. By averaging the martensite area ratios of the observed visual fields, the martensite area ratios of the first region 51 and the second region 52 are obtained. After that, using the same sample, etching treatment using a picral is performed, a 100 µm square field of view is observed with an optical microscope at a magnification of 1000 times, and martensite containing carbide is hardened within the observation field. Martensite and martensite containing no carbide are used as tempered martensite, and the respective ratios of tempered martensite and hard martensite among martensite are obtained.

In this embodiment, the spot welded joint in which the weld metal was formed by spot welding is made into object. Spot welding, also called spot welding, is welding that connects two overlapping steel sheets by a point. As a means of spot welding, arc spot welding, resistance spot welding, and laser spot welding are mentioned. On the other hand, welding performed linearly is called continuous welding. Arc welding, laser welding, seam welding, etc. are mentioned as a means of continuous welding. Compared with continuous welding, spot welding has a smaller welding area, so construction time is short 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. However, in the spot weld joint 1 according to the present embodiment, the spot weld metal 2 is a nugget for resistance spot welding. It is not limited to joints. For example, the spot weld metal 2 may be formed by laser spot welding, and 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, for example, a hat member. It is preferably formed in the overlapping portion of the flange portion and the closing plate of the. With such a structure, it acts especially effectively for the improvement of the intensity|strength as a structural member, and anti-collision performance.

In addition, the spot weld joint 1 according to the present embodiment has two spot weld metals 2, and if the region between the two spot weld metals 2 and 2 satisfies the above relationship, the 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 more than 2 spot weld metals are formed, if the area between the target two spot weld metals has the above relationship, an effect is obtained for that area. In the case of having more than 2 spot weld metals, it is preferable to control so that the relationship between the two spot weld metals 2 is satisfied, particularly in a region where in-plane tensile stress is expected to be applied. Among all the spot weld metals 2, if the relationship of hardness as described above is satisfied, even if a tensile stress is applied in any direction and place in the plane of the high-strength steel sheet, fracture in the HAZ softening zone can be suppressed, so it is more preferable.

The spot welded joint 1 according to the present embodiment can be applied to an A pillar, a side seal, a B pillar, or the like. For example, in the B-pillar, in the flange portion of the hat member, it is joined to the closing plate by a spot weld metal. B-pillar spot weld metal WHEREIN: If the above relationship is satisfied, even when an in-plane tensile stress is applied to the flange part at the time of a collision of a motor vehicle, fracture|rupture at the site|part which used to be a HAZ softened part can be suppressed.

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

Next, the manufacturing method of the spot welded joint which concerns on this embodiment is demonstrated.

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) the average Vickers hardness HVbase is 350HV or more, the first steel sheet containing hard martensite and the second steel sheet are overlapped,

(II) forming a plurality of spot weld metals for joining the overlapping first and second steel plates;

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

have a process

A well-known steel plate can be used for the 1st steel plate 11 and the 2nd steel plate 12 containing hard martensite which average Vickers hardness HVbase is 350 HV or more, respectively.

These steel plates are overlapped and spot welded, a spot weld metal is formed, and it is set as a weld joint. Spot welding conditions are not limited, What is necessary is just to set it as normal conditions.

After spot welding, a part of the first steel sheet is quenched and tempered by laser irradiation to form a first region and a second region.

In the case of quenching, it is necessary to increase 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 due to heat conduction also becomes a quenching region. Therefore, heat input control according to plate|board thickness is required.

On the other hand, when performing tempering, it is necessary to heat the temperature of the area|region made into object to the temperature of less than Ac1 degreeC. 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 Ac1°C, the structure transforms into austenite. Since this austenite transforms back into hard martensite upon cooling, it shows high hardness in the site|part heated to more than Ac1 degreeC.

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

When heating as described above, in order to provide hardness distribution in the plate thickness direction, it is necessary to input heat only to the pole surface layer and to provide heat by heat conduction in the depth direction. Moreover, when heating to the outside of the target area|region, heat radiation in the target area|region becomes inadequate, and a tempering structure|tissue may not be obtained.

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

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

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

Illustrative conditions

ㆍType of oscillator: semiconductor laser

ㆍOutput: 500 to 3000W

ㆍBeam shape: in the irradiation surface, the width direction: 4 to 10 mm, the traveling direction: a rectangle of 0.5 to 3 mm

ㆍLaser movement speed: 50 to 500 cm/min

By such laser irradiation, tempering is performed within a range of 0.1 mm between the two spot weld metals of the first steel sheet and from the surface on the second steel sheet side, and between the two spot weld metals and the second Quenching can be performed in a 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-cooled die. After quenching, the steel sheet using shot blasting The oxide scale on the surface was removed.The Vickers hardness after quenching of the used steel plate was as shown in Table 1. In addition, this steel plate had the structure|tissue containing hard martensite.

Next, from the steel sheet, a tensile test piece having a distance between gages of 50 mm and a width of 25 mm in parallel portions as shown in FIG. 4 was taken. Further, a square tab plate having a side of 25 mm was taken from the copper steel plate.

As shown in Fig. 4, a tab plate was placed on a parallel portion of the taken tensile test piece, and resistance spot welding was performed on the central portion of each tab plate using a single-phase AC 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, between the tensile test piece and the tab plate, two weld metals having a nugget diameter of 4x√t (t: plate thickness (mm) of the tensile test piece) were formed.

A laser is irradiated from one side of the tensile test to the specimen (joint Nos. 1 to 5, 11, and 12) after spot welding, and the longitudinal center of the parallel portion coincides with the center of the beam over the entire parallel portion 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 joint Nos. 1 to 5 were as follows.

ㆍType of oscillator: semiconductor laser

ㆍOutput: 1200W

ㆍBeam shape: In the irradiation surface, the width direction: 8 mm, the traveling direction: a rectangle of 1 mm

ㆍLaser movement speed: 250cm/min

In addition, the laser irradiation conditions with respect to joint No. 11 were carried out as follows.

ㆍType of oscillator: semiconductor laser

ㆍOutput: 700W

ㆍBeam shape: In the irradiation surface, the width direction: 8 mm, the traveling direction: a rectangle of 1 mm

ㆍLaser movement speed: 130cm/min

In addition, the laser irradiation conditions with respect to joint No. 12 were carried out as follows.

ㆍType of oscillator: semiconductor laser

ㆍOutput: 750W

ㆍBeam shape: In the irradiation surface, the width direction: 8 mm, the traveling direction: a rectangle of 1 mm

ㆍLaser movement speed: 80cm/min

Then, whether the first region and the second region of a predetermined size have been formed, if they are 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 surface was made into the thickness direction cross section at the center of the width direction of the test body.

Moreover, a tensile test was performed about each test piece (in-plane tensile stress was applied), and the fracture|rupture position was investigated. The tensile speed at the time of the tensile test was 10 mm/min.

A result is shown in Table 1.

In joints Nos. 1 to 5 (invention example), a range of 0.1 mm from the overlapping surface (first area) is tempered, the formula (1) is satisfied, and a range of 0.1 mm from the surface (second area) was named, and Equation (2) was satisfied. As a result, no cracks were observed in the HAZ softened portion.

On the other hand, joints Nos. 6 to 10 (comparative examples) were not irradiated with laser, and in a range corresponding to the first or second areas of joints Nos. 1 to 5, formulas (1) and (2) did not meet As a result, in the tensile test, it became a crack in the HAZ softened part.

The joints Nos. 11 to 12 (comparative examples) were irradiated with laser, but laser irradiation conditions were unfavorable. 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 fractured. Comparative Example 12 also lacks heat input, and the second region is quenched to satisfy the formula (2), but in the first region, the tempering is insufficient, and the first region does not satisfy the formula (1), and the above The difference between the maximum value and the minimum value of Vickers hardness in a 1st area|region became 80 or more. As a result, in the tensile test, it became a crack in the HAZ softened part.

1: spot weld joint
2: spot weld metal
3: Overlapping face
11: first steel plate
12: second steel plate
51: first area
52: second area

Claims (4)

An average Vickers hardness HVbase of 350HV or more, a first steel sheet containing hard martensite,
a second steel plate superimposed on 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 plate 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 from the surface on the side of the second steel plate in the plate thickness direction of the first steel plate;
a second region formed between the two spot weld 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 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),
Spot weld joint, characterized in that.

The spot welded joint according to claim 1, wherein the difference between the maximum value and the minimum value of 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 according to claim 1 or 2,
Spot weld joint, characterized in that. The average Vickers hardness HVbase is 350HV or more, the first steel sheet containing hard martensite and the second steel sheet are overlapped,
Forming two spot weld metals for joining the overlapping first and second steel plates,
By laser irradiation, tempering is performed within a range of 0.1 mm between the two spot weld metals and the surface on the second steel plate side of the first steel sheet, and between the two spot weld metals and the second steel sheet. 2 Quenching is performed in a range of 0.1 mm from the surface opposite to the surface on the steel plate side,
A method for manufacturing a spot welded joint, characterized in that.

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