JP2023083966A - Resistance spot-weld joint and production method for the same - Google Patents

Abstract

To provide a resistance spot-weld joint including a high strength steel sheet and capable of inhibiting LME crack at spot welding, and a production method for the above joint.SOLUTION: A resistance spot-weld joint comprises a plurality of overlapped steel sheets, nuggets joining the steel sheets, and a weld zone having a corona bond formed in the surround of nuggets and a heat-affected zone. At least one among the plurality of steel sheets is a high strength steel sheet having the tensile strength of 980 MPa or more, and that high strength steel sheet or the sheet adjacent to the high strength one is galvanized. The joint is characterized in that in a base material surface on which the base material of the sheet contacts a galvanized plating, the hardness in a depth region of 10-60 μm from the base material surface is lower than the hardness in the depth region of 1-10 μm from the above surface and that in the depth region of 60 μm or more from the above surface. The production method for the joint is also disclosed.SELECTED DRAWING: None

Description

The present invention relates to resistance spot welded joints and methods of making same.

In recent years, in the automobile industry, there has been a demand for lighter vehicle bodies from the viewpoint of improving fuel efficiency. Increasing the strength of the steel plates used in frame parts is an effective way to achieve both vehicle weight reduction and collision safety. there is

High-strength steel sheets used in automobiles and the like are required to have excellent weldability. Resistance spot welding is mainly used in processes such as assembly of automobile bodies and attachment of parts. It is necessary to suppress embrittlement (Liquid Metal Embrittle: LME) cracking. This phenomenon is caused by the tensile stress generated by welding acting on zinc that has been liquefied due to the heat input from welding and penetrates into the interior of the steel sheet along the grain boundaries. In spot welding using a high-strength steel sheet, if such an LME crack occurs, the strength of the welded joint cannot be ensured, which may hinder the use of the high-strength steel sheet.

In this regard, Patent Document 1 discloses a joined structure formed by resistance welding a plurality of steel plates that are superimposed, wherein at least one of the plurality of steel plates has a carbon equivalent Ceq of 0. A high-strength steel sheet having a chemical composition of 53% or more and a tensile strength of 590 MPa or more, wherein the high-strength steel sheet includes a zinc-based steel sheet formed on at least one surface of the overlapping surface side and the welding electrode side. A decarburized layer is provided between the coating layer and the base material or on the overlapping surface adjacent to the zinc-based coating layer of the zinc-based plated steel sheet, and the decarburized layer has a thickness of 5 μm or more and 200 μm or less. A bonded structure is described which is characterized by having In addition, in Patent Document 1, the A3 point is higher than that of the base metal, and a decarburized layer that is less likely to undergo austenite transformation (reverse transformation) exists between the zinc-based coating layer and the base metal, so that HAZ (heat Affected part) The surface layer is unlikely to form a coarse austenitic structure, and as a result, the molten zinc in the zinc-based coating layer is dispersed during welding, suppressing embrittlement due to zinc entering the grain boundaries of the HAZ. It is described that cracks can be prevented even if tensile stress acts.

In Patent Document 2, a spot-welded member in which a plurality of steel plates are spot-welded, wherein at least one of the plurality of steel plates is a high-strength cold-rolled steel plate having a tensile strength of 780 MPa or more without a plating layer on the surface. At least one of the plurality of steel sheets is a zinc-based plated steel sheet having a zinc-based coating layer on the surface, and the surface layer Zn concentration inside the corona bond of the spot welded portion is 1% by mass or more and less than 25% by mass. A spot welded member is described. In addition, in Patent Document 2, by controlling the surface layer Zn concentration inside the corona bond of the spot welded part, even though the spot welded member contains a high-strength cold-rolled steel sheet, the coke LME crack (high-strength It is described that it is possible to suppress LME cracking that occurs when the steel plate and the mating material to be welded have a zinc-based coating layer.

In Patent Document 3, it has a predetermined composition, a ferrite area ratio of 15% to 55%, a hard phase area ratio of 40% to 85%, and a retained austenite volume ratio of 4% to 20%. , the carbon concentration in the retained austenite is 0.55% or more and 1.10% or less, the amount of diffusible hydrogen in the steel sheet is 0.80 mass ppm or less, the surface layer softening thickness is 5 μm or more and 150 μm or less, and the steel sheet after the high temperature tensile test A high-strength steel sheet having a steel structure with a surface layer corresponding grain boundary frequency of 0.45 or less and a tensile strength of 980 MPa or more is described. Further, in Patent Document 3, by controlling the corresponding grain boundary frequency of the steel plate surface layer after the high temperature tensile test to 0.45 or less and the surface layer softening thickness to 5 μm or more and 150 μm or less, high-strength steel plate excellent in LME resistance can be realized.

Japanese Patent Application Laid-Open No. 2020-082102 JP 2020-179413 A WO2020/184154

Generally, it is known that LME cracking becomes conspicuous when steel sheets having relatively high strength are spot-welded, and that the higher the strength of the steel sheet, the higher the susceptibility to LME cracking. On the other hand, in the automobile industry and the like, there is a demand for further weight reduction of steel sheets, and in order to achieve such weight reduction, it becomes necessary to increase the strength of steel sheets more than ever before. Therefore, there is a continuing need to solve the problem of LME cracking even when spot welding is performed using a steel plate having a strength equal to or higher than that of conventional welding members. High-strength steel sheets still have room for improvement from this point of view.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a resistance spot welded joint including high-strength steel sheets, which can suppress the occurrence of LME cracking during spot welding, and a method for manufacturing the same. do.

The inventors of the present invention have studied a technique for suppressing or reducing the occurrence of LME cracking, focusing particularly on high-strength steel sheets used in resistance spot welded joints. As a result, the present inventors defined the depth region from the base material surface of the high-strength steel sheet in contact with the zinc-based plating as a first depth region of 1 to 10 μm, a second depth region of 10 to 60 μm, and When divided into a third depth region of 60 μm or more, spot welding is performed by forming a three-layer structure in which the hardness of the second depth region is lower than the hardness of the first and third depth regions. The relatively hard and soft layer (second depth region) can bear the strain introduced into the high-strength steel plate due to the deformation of time, thereby preventing excessive strain in the outermost layer (first depth region). The present inventors have found that the intrusion of molten zinc into the inside of the steel sheet can be suppressed by suppressing the increase in the steel sheet, and have completed the present invention.

The present invention that has achieved the above object is as follows.
(1) a plurality of superimposed steel plates;
A resistance spot welded joint comprising a nugget joining the steel plates and a weld having a corona bond and a heat affected zone formed around the nugget,
At least one of the plurality of steel plates is a high-strength steel plate having a tensile strength of 980 MPa or more,
The high-strength steel sheet has zinc-based plating, or a steel sheet adjacent to the high-strength steel sheet has zinc-based plating,
On the base material surface where the base material of the high-strength steel sheet is in contact with the zinc-based plating,
The hardness in a depth region of 10 to 60 μm from the base material surface is lower than the hardness in a depth region of 1 to 10 μm from the base material surface and the hardness of a depth region of 60 μm or more from the base material surface. A resistance spot weld joint characterized by:
(2) the surface of the base material of the high-strength steel sheet in contact with the zinc-based plating is located on the overlapping surface of the plurality of steel sheets,
Precipitates with a diameter of less than 0.1 μm are present in a depth region of 1 to 10 μm from the base material surface at a number density of 10 to 200/μm ² ,
The resistance spot welded joint according to (1) above, characterized in that the amount of solid solution C in a depth region of 10 to 60 μm from the base material surface is less than 0.20% by mass.
(3) The amount of η phase of the zinc-based plating in the corona bond of the overlapping surfaces of the plurality of steel sheets where the base material surface of the high-strength steel sheet is in contact with the zinc-based plating is 20 area% or less. The resistance spot welded joint according to (2) above, characterized in that:
(4) the surface of the base material of the high-strength steel sheet in contact with the zinc-based plating is located on the outermost surface of the plurality of superimposed steel sheets;
Precipitates with a diameter of less than 0.1 μm are present in a depth region of 1 to 10 μm from the base material surface at a number density of 10 to 200/μm ² ,
The resistance spot welded joint according to (1) above, characterized in that the amount of solid solution C in a depth region of 10 to 60 μm from the base material surface is less than 0.20% by mass.
(5) The amount of η phase of the zinc-based plating at the weld shoulder of the outermost surface of the plurality of steel plates where the base material surface of the high-strength steel sheet is in contact with the zinc-based plating is 20 areas. % or less, the resistance spot welded joint according to (4) above.
(6) a step of pressing a plurality of superimposed steel plates using a pair of opposing electrodes;
forming a nugget and a corona bond by energizing between the electrodes while pressing the plurality of steel plates;
Any one of the above (1) to (5), characterized by comprising a step of continuously reducing the current value between the electrodes to 0 while maintaining the pressure on the plurality of steel plates. A method for manufacturing a resistance spot welded joint according to 1.
(7) a step of pressing a plurality of superimposed steel plates using a pair of electrodes facing each other;
forming a nugget and a corona bond by energizing between the electrodes while pressing the plurality of steel plates;
reducing the current value between the electrodes to 0 while maintaining the pressure on the plurality of steel plates, wherein the current value between the electrodes at the time when the nugget formation is completed is defined as I; When 1/2 of the total plate thickness in the unit mm of the steel plate is defined as tm,
When the current value between the electrodes is reduced to 0 or less than 0.04 seconds after the current value is reduced to 0, the current value between the electrodes is reduced from 0.9 × I to 0.3 × I The method for manufacturing a resistance spot welded joint according to any one of the above (1) to (5), wherein a constant value of 0.12 × tm or more in units of seconds is maintained within the range of .
(8) a step of pressing a plurality of superimposed steel plates using a pair of opposing electrodes;
A step of preheating the steel plates by pre-energizing between the electrodes while pressurizing the plurality of steel plates;
forming a nugget and a corona bond by energizing between the electrodes while pressing the plurality of steel plates;
A method for manufacturing a resistance spot welded joint according to any one of (1) to (5) above, characterized by comprising:
(9) a step of pressing a plurality of superimposed steel plates using a pair of electrodes facing each other;
forming a nugget and a corona bond by energizing between the electrodes while pressing the plurality of steel plates;
reducing the current value between the electrodes to 0 while maintaining pressure on the plurality of steel plates;
and holding the pressure at 0.8×P or more for 0.2 sec or more and 0.4 sec or less with the current value between the electrodes set to 0, where P is the completion of the formation of the nugget. The method for manufacturing a resistance spot welded joint according to any one of (1) to (5) above, wherein the pressure is applied at the time of welding.

Advantageous Effects of Invention According to the present invention, it is possible to provide a resistance spot welded joint including high-strength steel sheets and capable of suppressing the occurrence of LME cracking during spot welding, and a method for manufacturing the same.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows typically manufacture of the resistance spot-welded joint which concerns on embodiment of this invention, (a) shows the state before manufacture of a resistance spot-welded joint, (b) is when resistance spot-welding is performed. state. 1 is a schematic diagram of a resistance spot welded joint according to a preferred embodiment of the present invention when two steel sheets are superimposed; FIG. 1 is a schematic diagram of a resistance spot welded joint according to a preferred embodiment of the present invention when three steel plates are superimposed; FIG. FIG. 4 is a diagram schematically showing changes over time in current in manufacturing method 1; 4 is a diagram schematically showing temporal changes in current and applied pressure in a preferred embodiment of Manufacturing Method 1. FIG. FIG. 10 is a diagram schematically showing changes over time in current in manufacturing method 2;

<Resistance spot welding joint>
A resistance spot welded joint according to an embodiment of the present invention comprises:
a plurality of superimposed steel plates;
A nugget for joining the steel plates, and a weld having a corona bond and a heat affected zone formed around the nugget,
At least one of the plurality of steel plates is a high-strength steel plate having a tensile strength of 980 MPa or more,
The high-strength steel sheet has zinc-based plating, or a steel sheet adjacent to the high-strength steel sheet has zinc-based plating,
On the base material surface where the base material of the high-strength steel sheet is in contact with the zinc-based plating,
The hardness in a depth region of 10 to 60 μm from the base material surface is lower than the hardness in a depth region of 1 to 10 μm from the base material surface and the hardness of a depth region of 60 μm or more from the base material surface. is characterized by

As described above, LME cracking becomes conspicuous when steel sheets having relatively high strength are spot-welded, and it is known that the higher the strength of the steel sheet, the higher the susceptibility to LME cracking. there is When two or more steel plates containing at least one high-strength steel plate are piled up and spot-welded to produce a joint, the striking angle (the angle between the axial direction of the electrode and the direction perpendicular to the surface of the steel plate) and the plate If the degree of disturbance such as a gap between weld metals (nuggets) increases, LME cracks may occur inside or just outside the press contact (corona bond) formed on the outside of the weld metal (nugget) or on the surface on the electrode side. LME cracking occurs when tensile stress is applied while zinc is molten or at a temperature close to its melting point. The magnitude of the tensile stress is affected by many factors such as the pressure applied by the electrode, the expansion and contraction of the welded portion, and the springback when the electrode is released. Since the occurrence of LME cracking is thus affected by temperature and the magnitude of tensile stress, it is important to suppress increases in temperature and tensile stress in order to suppress LME cracking. However, in order to obtain a sufficient nugget diameter for joining, it is necessary to increase the welding heat input to raise the temperature. Therefore, it may be difficult to suppress the temperature rise. Therefore, the present inventors focused on a high-strength steel plate used in resistance spot welded joints in order to cope with LME cracking by suppressing or reducing the stress applied during spot welding, and the structure of the high-strength steel plate was examined from the viewpoint of making it more appropriate.

More specifically, the present inventors first found that the occurrence of LME cracks is greatly affected by the "strain" introduced into the surface layer of the high-strength steel sheet due to the stress applied during spot welding. I found out. For example, it has been found that even with the same energization cycle (thermal history), when spot welding is performed so as to increase the amount of plastic deformation of the steel sheet, the occurrence of LME cracking becomes significant. The reason why LME cracking is more likely to occur with an increase in "strain" is thought to be that as the strain in the surface layer of the steel sheet increases, molten zinc is more likely to penetrate from the surface layer of the steel sheet into the interior. Therefore, by preventing an increase in strain in the surface layer of the steel sheet, it is possible to suppress the occurrence of LME cracking even when a welded joint is manufactured by spot-welding high-strength steel sheets. The present inventors have found that it is effective to provide a difference in hardness in the thickness direction of the surface layer of the steel sheet as a means for preventing an increase in strain in the surface layer of the steel sheet. Specifically, the depth from the base material surface of the high-strength steel sheet in contact with the zinc-based plating is a first depth region of 1 to 10 μm, a second depth region of 10 to 60 μm, and a third depth of 60 μm or more. depth regions, the hardness of the second depth region is controlled to be lower than the hardness of the first and third depth regions. A three-layer structure in which the depth region from the base material surface in the plate thickness direction of the high-strength steel plate is controlled so that the hardness of the second depth region is lower than the hardness of the first and third depth regions By doing so, the strain introduced into the surface layer of the high-strength steel plate due to deformation during spot welding can be borne by a relatively hard and soft layer (second depth region), thereby resulting in the outermost layer (first It is possible to suppress an excessive increase in stress and thus an excessive increase in strain in the depth region). Therefore, by using a high-strength steel sheet having a depth region composed of the above three-layer structure, the penetration of molten zinc into the steel sheet during spot welding is suppressed, and the occurrence of LME cracking is significantly suppressed or prevented. As a result, it is possible to obtain resistance spot welded joints having sound joint strength.

Hereinafter, resistance spot welded joints according to embodiments of the present invention will be described in more detail with reference to the drawings. is not intended to be limited to such specific embodiments.

FIG. 1 is a diagram schematically showing the production of a resistance spot welded joint according to an embodiment of the present invention, FIG. 1(a) shows the state before production of the resistance spot welded joint, and FIG. The state when resistance spot welding is performed is shown. Referring to FIG. 1(a), a high-strength steel plate 11 having a tensile strength of 980 MPa or more has zinc-based plating 12 on both surfaces, and the high-strength steel plate 11 has zinc-based plating. The steel plate 11 ′ and the overlapping surface 15 are overlapped in a state adjacent to each other. Next, as shown in FIG. 1(b), the two superimposed steel plates are pressed using a pair of electrodes A facing each other. Then, a nugget 13 and a corona bond 14 are formed by applying an electric current between the electrodes A while pressing the two steel plates, and finally, the two steel plates 11 and 11 ′ superimposed on each other and these steel plates are connected to each other. A resistance spot welded joint 1 is produced comprising a mating nugget 13 and a weld 17 having a corona bond 14 and a heat affected zone 16 formed around the nugget 13 . Although the shape, structure, etc. of the nugget 13 are not particularly limited, for example, the nugget 13 has a single-layer structure or a double-layer structure, preferably a single-layer structure. After passing through a short cool time (non-energization time) enough for the nugget to solidify during resistance spot welding, there is a case where the current is re-energized. In such a case, the nugget has a double structure because cooling is performed in two steps, cooling during cooling and cooling after re-energization. Since the resistance spot welded joint 1 according to the embodiments of the present invention can be manufactured with or without such a cool time, the nugget 13 can be both single and double structures, but according to the preferred embodiment, For example, the nugget 13 of the resistance spot welded joint 1 has a single-layer construction.

In FIG. 1B, since the high-strength steel plate 11 has the zinc-based plating 12 on both surfaces, the surface of the base material of the high-strength steel plate 11 in contact with the zinc-based plating 12 is the overlapping surface 15, It is located on the opposite side, that is, on both of the outermost (electrode A side) surfaces of the two superimposed steel sheets. In this case, due to strain introduced into the high-strength steel plate 11 due to deformation during spot welding, there is a possibility that LME cracks will occur on both sides of the high-strength steel plate 11, that is, both the overlapping surface 15 side and the electrode A side. be. More specifically, for example, the inside or just outside the corona bond 14 of the overlapping surface 15, or the weld shoulder 18 corresponding to the outer edge of the electrode A-side surface (e.g., the outer edge of the electrode indentation) or its surroundings. ) and the like may cause LME cracking. However, according to the embodiment of the present invention, the depth region from the base material surface of the high-strength steel plate 11 is such that the hardness of the second depth region is equal to the hardness of the first and third depth regions as described above. By having a three-layer structure controlled to be lower than the depth, the second depth region can absorb more strain introduced into the steel plate due to deformation during spot welding than the first depth region. As a result, it is possible to suppress the occurrence of LME cracks on both the lap surface 15 side and the electrode A side of the high-strength steel sheet 11 .

For ease of understanding, FIGS. 1(a) and 1(b) describe the case of a welded joint in which only two steel sheets are superimposed and only one of the high-strength steel sheets 11 is coated with zinc-based plating 12. However, the resistance spot welded joint 1 according to the embodiment of the present invention is not necessarily limited to such a welded joint, and may be a welded joint having various configurations in which the base material of the high-strength steel plate 11 is in contact with the zinc-based plating 12. can include For example, when the high-strength steel sheet 11 having a tensile strength of 980 MPa or more has the zinc-based plating 12, at least one of the plurality of steel sheets, for example, three steel sheets, one of the high-strength steel sheets 11 Alternatively, both surfaces may have the zinc-based plating 12 , and the other steel sheet may be the high-strength steel sheet 11 or may not be the high-strength steel sheet 11 . Also, these other steel sheets may have the zinc-based plating 12 or may not have the zinc-based plating 12 . FIGS. 1A and 1B show the case where the high-strength steel plate 11 has the zinc-based plating 12 on both surfaces. It may have a system plating 12 . In this case, LME cracking may occur inside or just outside the corona bond 14 of the lap surface 15, but according to the embodiment of the present invention, the high-strength steel sheet 11 provided with the zinc-based plating 12 Since the depth region from the surface of the base material has the above-mentioned three-layer structure, the second depth region reliably absorbs the strain introduced into the steel plate due to deformation during spot welding, so that the high-strength steel plate 11 It is possible to reliably suppress the occurrence of LME cracking inside or just outside the corona bond 14 on the overlapping surface 15 side. Similarly, when the high-strength steel sheet 11 has the zinc-based plating 12 only on the electrode A side, usually, LME cracking may occur in the heat-affected zone of the surface on the electrode A side. According to the embodiment, since the depth region from the base material surface of the high-strength steel plate 11 provided with the zinc-based plating 12 has the above-described three-layer structure, the second depth region is deformed during spot welding. By reliably absorbing the strain introduced into the high-strength steel plate 11, it is possible to reliably suppress the occurrence of LME cracking in the heat-affected zone or the like of the surface of the high-strength steel plate 11 on the electrode A side.

On the other hand, when the high-strength steel sheet 11 does not have the zinc-based plating 12, the steel sheet that is overlapped with the high-strength steel sheet 11 among the one or more other steel sheets has at least the zinc-based plating on the overlapping surface 15 side. 12 is sufficient. When an additional steel plate is included in addition to these two steel plates, the additional steel plate may or may not be the high-strength steel plate 11 and has the zinc-based plating 12. or may not have zinc-based plating 12 . For example, a set of three steel sheets in which high-strength steel sheets 11 having no zinc-based coating 12 are superimposed on both sides of a relatively low tensile strength steel sheet 11 ′ having zinc-based coating 12 on both surfaces is spot-welded. In this case, LME cracking may occur inside the corona bonds 14 on the lap surfaces 15 on both sides of the steel plate 11'. However, by using the high-strength steel plates having the three-layer structure depth region as the other two high-strength steel plates 11, the second depth region in these high-strength steel plates is reduced during spot welding. By reliably absorbing the strain introduced into the steel sheet due to deformation, it is possible to reliably suppress the occurrence of LME cracking inside the corona bond 14 on the overlapping surface 15 side of the high-strength steel sheet 11 or the like.

FIG. 2 is a schematic diagram of a resistance spot welded joint according to a preferred embodiment of the present invention when two steel sheets are superimposed. In both FIGS. 2(a) and (b), the high-strength steel sheet 11 has zinc-based plating 12 on both surfaces, while FIG. 2(a) has a configuration similar to that of FIG. In FIG. 2(b), two high-strength steel sheets 11 having zinc-based plating 12 are overlapped in a state of being adjacent to each other on the overlap surface 15. In FIG. In FIG. 2( a ), as described with reference to FIG. 1 , two regions on the overlapping surface 15 side and the electrode A side of one high-strength steel plate 11 are exposed to the risk of LME cracking. On the other hand, in FIG. 2(b), a total of three regions on the overlapping surface 15 side and the electrode side of each high-strength steel plate 11 are exposed to the risk of LME cracking. However, as described above, the depth region from the base material surface of the high-strength steel plate 11 is the first depth region with a hardness of 1 to 10 μm in the second depth region of 10 to 60 μm, and the third depth region of 60 μm or more. By using a three-layer structure with hardness lower than that of the depth region, the risk of LME cracking can be avoided or reliably suppressed in all of the above regions. Also, in FIG. 2(b), only one of the two high-strength steel plates 11 may be a steel plate having the three-layer structure, and such an embodiment is also included in the present invention. In this case, both high-strength steel plates 11 use the high-strength steel plates having the three-layer structure, although the joint strength may be somewhat reduced compared to the resistance spot weld joint having the three-layer structure. The number and positions of the joints may be appropriately determined in consideration of the desired joint strength and the like.

FIG. 3 is a schematic diagram of a resistance spot welded joint according to a preferred embodiment of the present invention when three steel sheets are superimposed. In all the embodiments of FIGS. 3(a)-(e), the high strength steel plate 11 has zinc-based plating 12 on both surfaces. In this case, all base material surfaces of the high-strength steel sheet 11 are in contact with the zinc-based plating 12 . Therefore, in FIGS. 3(a) to 3(e), all regions on the side of the overlapping surface 15 where the high-strength steel plate 11 is arranged and on the side of the electrodes are exposed to the risk of LME cracking. However, as described above, the depth region from the base material surface of the high-strength steel plate 11 is the first depth region with a hardness of 1 to 10 μm in the second depth region of 10 to 60 μm, and the third depth region of 60 μm or more. By using a three-layer structure with hardness lower than that of the depth region, the risk of LME cracking can be avoided or reliably suppressed in all of the above regions. In addition, in FIGS. 3(c) to 3(e), only one of the plurality of high-strength steel plates 11 may be a steel plate having the three-layer structure, and such an embodiment is also included in the present invention. In this case, the high-strength steel plates 11 having the three-layer structure are used, although the joint strength may be somewhat reduced compared to the resistance spot welded joint having the three-layer structure. The number and positions of the joints may be appropriately determined in consideration of the desired joint strength and the like.

In FIGS. 3A to 3E, for ease of understanding, the case where all high-strength steel sheets 11 have zinc-based plating 12 on both surfaces was described. The resistance spot welded joint 1 is not necessarily limited to such a welded joint, and may include welded joints having various configurations in which the base material of the high-strength steel plate 11 is in contact with the zinc-based plating 12 . For example, when the high-strength steel sheets 11 have the zinc-based plating 12, at least one high-strength steel sheet 11 may have the zinc-based plating 12 on one or both surfaces, and the other steel sheets have high strength. It may be the steel plate 11 or it may not be the high-strength steel plate 11 . Also, these other steel sheets may have the zinc-based plating 12 or may not have the zinc-based plating 12 . On the other hand, when the high-strength steel sheet 11 does not have the zinc-based plating 12, the steel sheet that is overlapped with the high-strength steel sheet 11 among the other two steel sheets has the zinc-based plating 12 at least on the overlap surface 15 side. should have The remaining one steel sheet may or may not be the high-strength steel sheet 11, and may or may not have the zinc-based plating 12. may The high-strength steel sheets used in the resistance spot-welded joints according to the embodiments of the present invention will be described below in more detail.

[High-strength steel plate]
The high-strength steel sheet according to the embodiment of the present invention has a tensile strength of 980 MPa or more, and the depth region from the base material surface in contact with the zinc-based plating is the second depth region (10 to 60 μm from the base material surface A three-layer structure that is controlled so that the hardness of the first and third depth regions (depth regions of 1 to 10 μm and 60 μm or more from the surface of the base material) is lower than the hardness of the depth region) Optional material. In general, a steel sheet tends to be more susceptible to LME cracking as its strength increases, and this tendency is particularly strong when the tensile strength is 980 MPa or more. Therefore, by applying the above three-layer structure to a high-strength steel sheet having a tensile strength of 980 MPa or more, the effect of suppressing LME cracking becomes particularly remarkable. For example, the tensile strength may be 1080 MPa or higher, 1180 MPa or higher, or 1200 MPa or higher. Although the upper limit is not particularly limited, for example, the tensile strength may be 2300 MPa or less, 2000 MPa or less, 1800 MPa or less, or 1500 MPa or less. Tensile strength is measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece is parallel to the rolling direction of the steel plate, and performing a tensile test in accordance with JIS Z 2241:2011.

The hardness of the second depth region is not particularly limited as long as it is lower than the hardness of the first and third depth regions. Here, in the present invention, the hardness of the second depth region (the depth region of 10 to 60 μm from the base material surface) refers to the average Vickers hardness of the second depth region. Therefore, in the present invention, "the hardness of the second depth region (a depth region of 10 to 60 μm from the base material surface) is the same as the hardness of the first and third depth regions (1 to 10 μm and 60 μm or more from the base material surface. "lower than the hardness of the depth region)" means that the average Vickers hardness of the second depth region is lower than the average Vickers hardness of the first and third depth regions. For example, the average Vickers hardness of the second depth region may be less than or equal to 0.98 times the average Vickers hardness of the first and/or third depth regions. From the viewpoint of suppressing an increase in strain occurring in the first depth region, it is preferable that the average Vickers hardness of the second depth region is relatively lower. Therefore, the average Vickers hardness of the second depth region is preferably 0.95 times or less than the average Vickers hardness of the first and/or third depth regions, and is 0.92 times or less. is more preferable, and 0.90 times or less is most preferable. Although the lower limit is not particularly limited, for example, the average Vickers hardness of the second depth region is 0.60 times or more or 0.70 times or more the average Vickers hardness of the first and / or third depth regions may be

In the present invention, the "hardness of the first depth region", "hardness of the second depth region" and "hardness of the third depth region" are based on JIS Z 2244-1: 2020. It is determined as follows by performing a Vickers hardness test. First, the Vickers hardness was measured at three points at intervals of 3 μm in the plate thickness direction from a depth of 2 μm from the base material surface in the outer region of the heat-affected zone of the high-strength steel plate, which was not affected by heat, with an indentation load of 20 g. , their average value is determined as the hardness of the first depth region. Next, the Vickers hardness was measured at 6 points at intervals of 10 μm in the plate thickness direction from a depth of 10 μm from the base material surface with an indentation load of 20 g, and their average value was taken as the hardness of the second depth region. It is determined. Finally, the Vickers hardness was measured at 5 points at intervals of 100 μm in the plate thickness direction from a depth of 60 μm from the base material surface with an indentation load of 20 g, and their average value was taken as the hardness of the third depth region. It is determined.

[Preferred embodiment of high-strength steel plate]
Hereinafter, a preferred method for realizing a high-strength steel sheet having a depth region having a three-layer structure in which the hardness of the second depth region is lower than the hardness of the first and third depth regions Although configurations will be described, these descriptions are intended to be merely exemplary of preferred embodiments of the invention and are not intended to limit the invention to such specific embodiments.

[The number density of precipitates with a diameter of less than 0.1 μm in the first depth region of 1 to 10 μm from the base material surface is 10 to 200/μm ² ]
In a high-strength steel sheet, it is preferable that precipitates having a diameter of less than 0.1 μm are present at a number density of 10 to 200/μm ² in a first depth region of 1 to 10 μm from the base material surface. Due to the presence of a large number of such fine precipitates, the steel sheet structure in the first depth region is refined, and as a result, the strength and hardness in the first depth region of the steel plate are reduced to those in the second depth region. can be higher than the strength and hardness of Therefore, it is possible to increase the deformation resistance of the steel plate in the first depth region when it is heated to a high temperature during spot welding. Therefore, when the electrode is pressed against the steel plate and energized during spot welding, and a load is applied while the steel plate is maintained at a high temperature, an increase in plastic strain in the first depth region of the steel plate can be suppressed. From the viewpoint of increasing the deformation resistance during welding and suppressing LME cracking, the lower limit of the number density of precipitates with a diameter of less than 0.1 μm in the first depth region is preferably 10 / μm ² or more, and 15 / It may be μm ² or more, 30/μm ² or more, 60/μm ² or more, or 90/μm ² or more. On the other hand, if the number density of precipitates is too high, the presence of high-density oxides increases the electrical resistance of the steel sheet surface, increasing the amount of heat generated in the steel sheet surface layer, and as a result, weldability may deteriorate. . Therefore, the number density of precipitates having a diameter of less than 0.1 μm in the first depth region is preferably 200/μm ² or less, and may be 150/μm ² or less or 120/μm ² or less. . The above precipitates may be arbitrary precipitates and are not particularly limited, but include, for example, Ti precipitates and W precipitates, more specifically Ti oxides and Ti carbides. Precipitates in the present invention are, for example, particles of oxides and carbides, such as TiO, TiO ₂ , Ti ₂ O ₃ , Ti ₃ O ₅ and TiC.

[Method for measuring the number density of precipitates with a diameter of less than 0.1 μm in the first depth region of 1 to 10 μm from the base material surface]
Measurement of the diameter and number density of precipitates in the first depth region of 1-10 μm from the base metal surface is determined by microstructural observation of the cross-section of the steel. Since the dispersed state of precipitates does not change depending on the direction of observation, RD (rolling direction of steel sheet) or TD (width direction of steel sheet), the structure is observed in a plane perpendicular to the ND plane (base material surface). should be done. Samples are taken from the base metal surface in the outer region of the heat-affected zone of the high-strength steel plate. A focused ion beam (FIB) processing device cuts out a sample for observation from the surface layer of the material that has undergone preliminary processing to mirror-finish the polished surface by mechanical polishing, and a field emission transmission electron microscope (FE) is used. - TEM: Field Emission Transmission Electron Microscopy) observation at a magnification of 50,000 times and composition analysis by an energy dispersive X-ray detector (EDX: Energy Dispersive X-ray Spectrometry) are used together to identify precipitates and individual Determine the diameter of the precipitate particles. The field of view for observation is 10 μm in the plate thickness direction, and when the plate thickness direction is the height direction in the observation image, which is a two-dimensional view, the length of the horizontal direction perpendicular to the height direction is 5 μm. The number of precipitates per unit area (number density) is determined by dividing the total number of precipitates with a diameter of less than 0.1 µm obtained by observation and compositional analysis by this area, ie, 50 µm ² . In addition, although there is no limit to the area of the sample to be observed as long as the size includes this region, it is desirable that the height of the sample exceeds 60 μm in order to measure the amount of solid solution C in the surface layer portion, which will be described later. Furthermore, since the total number of carbides to be measured may change when the film thickness of the sample changes, it is preferable to set the film thickness of the sample to 10 to 30 nm and prepare the sample with a film thickness of 15 to 25 nm.

[Amount of dissolved C in the second depth region of 10 to 60 μm from the base material surface is less than 0.20% by mass]
In general, the amount of dissolved C affects the strength of steel, and the greater the amount of dissolved C, the higher the deformation resistance. On the other hand, the smaller the amount of dissolved C, the lower the strength of the steel, that is, the steel becomes relatively soft. As described above, in order to suppress the occurrence of LME cracking during spot welding, it is important to prevent an increase in strain in the surface layer of the steel sheet. Therefore, in the high-strength steel sheet according to the embodiment of the present invention, the amount of dissolved C in the second depth region of 10 to 60 μm from the base material surface is reduced, preferably compared to the C content of the entire steel sheet. By doing so, the strength and hardness of the second depth region can be made lower than that of the first depth region. As a result, the second depth region can absorb more strain introduced into the steel sheet due to hot deformation during spot welding than the first depth region, making it possible to suppress LME cracking. becomes. If the amount of dissolved C in the second depth region is too high, the strength and hardness of the steel in the second depth region increase, and the strain generated in the first depth region increases. may not be sufficiently suppressed. For this reason, the amount of dissolved C in the second depth region is preferably less than 0.20% by mass, and is 0.19% by mass or less, 0.18% by mass or less, or 0.17% by mass or less. good too. The lower limit is not particularly limited and may be 0% by mass. For example, the amount of dissolved C may be 0.01% by mass or more, 0.05% by mass or more, or 0.10% by mass or more.

[Method for measuring the amount of dissolved C in the second depth region of 10 to 60 μm from the base material surface]
The amount of solid solution C in the second depth region is obtained by cutting out a sample for evaluation in the same manner as the procedure described in relation to the method for measuring the number density of precipitates in the first depth region, observing with FE-TEM and Obtained from EDX analysis. In order to determine the composition of the second depth region from 10 to 60 μm from the base material surface, the height of the sample should be at least greater than 60 μm. When C exists as precipitates instead of dissolved C, its existence forms are limited to two types: nonmetallic inclusions containing oxides and carbides. The concentration of C in nonmetallic inclusions and carbides containing oxides has a value exceeding twice the average component value of the steel sheet. For this reason, in the map analysis values by FE-TEM and EDX in the second depth region of 10 to 60 μm from the base material surface, the region of twice or less the average composition of the steel plate is regarded as the steel matrix, and the average of that region Let the amount of C be the amount of dissolved C.

The C content in the chemical composition of the entire high-strength steel sheet is preferably 0.20-0.40% by mass. In the high-strength steel sheet according to the embodiment of the present invention, the average carbon concentration is the carbon concentration of the base material inside (the center of the steel sheet in the depth direction) of the second depth region of 10 to 60 μm from the base material surface. almost the same or completely the same. Therefore, by setting the C content of the high-strength steel sheet to 0.20 to 0.40% by mass, a harder layer is formed in the base material than in the second depth region of 10 to 60 μm from the base material surface. It can be ensured that it exists in a region deeper than 60 μm from the surface. The C content of the high-strength steel sheet may be 0.21% by mass or more, 0.22% by mass or more, 0.23% by mass or more, or 0.24% by mass or more. Similarly, the C content of the high-strength steel sheet may be 0.38% by mass or less, 0.36% by mass or less, or 0.34% by mass or less.

[Thickness]
The plate thickness of the high-strength steel plate is not particularly limited, but is preferably 0.2 mm or more from the viewpoint of increasing rigidity, and may be 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, or 2.0 mm or more. may On the other hand, if the plate thickness is too thick, the forming load during stretch forming will increase, which may lead to mold wear and a decrease in productivity. Therefore, the thickness of the high-strength steel sheet is preferably 6.0 mm or less, and may be 5.0 mm or less or 4.0 mm or less.

As a more specific embodiment when using the above preferred high-strength steel sheet, the base material surface of the high-strength steel sheet in contact with the zinc-based plating is positioned on the overlapping surface of the plurality of steel sheets. , Embodiment 2 located on the outermost (ie, electrode side) surface of the stacked steel sheets, and both (Embodiment 3), as follows.
[Embodiment 1]
The surface of the base material of the high-strength steel sheet in contact with the zinc-based plating is located on the overlapping surface of the plurality of steel sheets, and the diameter of the base material is 0.1 μm in the first depth region of 1 to 10 μm from the surface of the base material. Precipitates are present at a number density of 10 to 200/μm ² , and the solid solution C amount in the second depth region of 10 to 60 μm from the base material surface is less than 0.20% by mass. .
[Embodiment 2]
The base material surface of the high-strength steel sheet in contact with the zinc-based plating is located on the outermost surface of the plurality of superimposed steel sheets, and the diameter is 0 in a first depth region of 1 to 10 μm from the base material surface. Precipitates of less than 1 μm are present at a number density of 10 to 200/μm ² , and the solid solution C amount in the second depth region of 10 to 60 μm from the base material surface is less than 0.20% by mass. is.
[Embodiment 3]
The base material surface where the base material of the high-strength steel sheet is in contact with the zinc-based plating is located on the overlapping surface and the outermost surface of the plurality of stacked steel sheets, and the first depth region of 1 to 10 μm from the base material surface. Precipitates with a diameter of less than 0.1 μm are present at a number density of 10 to 200/μm ² , and the solid solution C amount in the second depth region of 10 to 60 μm from the base material surface is 0.20. It is less than % by mass.

[The amount of η phase of zinc-based plating in the corona bond of the overlapping surfaces of a plurality of steel sheets is 20 area% or less]
According to a further preferred embodiment of the present invention, which relates to Embodiments 1 and 3, the zinc-based coating in the corona bond of the overlapping surfaces of a plurality of steel sheets where the base material surface of the high-strength steel sheet is in contact with the zinc-based coating is located. The amount of η phase in the plating is 20 area % or less. The η phase of zinc-based plating means a phase containing Zn as the main component (for example, the Zn content is 95 to 100% by mass) and containing other elements such as Fe in a solid solution state. As the alloying progresses between the zinc-based coating and the steel sheet due to the heat input of welding, the proportion of the η phase, which is mainly composed of Zn, decreases from the initial zinc-based coating in the corona bond formed on the lap surface of the steel sheet. It will be done. By implementing a welding method with a higher heat input than the normal welding method, it is possible to further promote the alloying between the zinc-based coating and the steel sheet. It becomes possible to reduce the amount of η phase to 20 area % or less. The melting point of the zinc-based coating increases due to the progress of alloying of the zinc-based coating on the lap surface side due to the heat input during spot welding. Therefore, it is possible to suppress or reduce the penetration of molten zinc into the steel sheet as compared with the case where the proportion of the η phase is relatively high, and it is possible to further enhance the effect of suppressing LME cracking at the lap surface. . From the viewpoint of enhancing the effect of suppressing LME cracking, the amount of the η phase of the zinc-based plating in the corona bond is preferably smaller, and may be, for example, 18 area % or less or 15 area % or less.

The area ratio of the η phase of the zinc-based plating in the corona bond is measured as follows. A SEM-EDS is used to photograph the Zn and Fe element distribution images of the corona bond in the cross section of the weld zone. The η phase in the image is defined as the region with Zn concentration above 95% and Fe concentration below 5%. The portion that satisfies this definition and the other portion are binarized using image analysis software, and the area ratio of the η phase in the zinc-based plating within the corona bond is calculated. The measurement area may be, for example, a rectangle with a width of 100 μm and a height of 10 μm. However, it is necessary to increase the height in accordance with the thickness of the plating layer inside the corona bond. When the total plating layer thickness exceeds 10 μm, the height of the measurement area is set to a value exceeding 10 μm. The area ratio of the η phase can be obtained by dividing the area of the η phase within the measurement region by the area of the region in which Zn exists. The eta-phase area fraction is measured at the outer edge of the corona bond.

[The amount of η phase of the zinc-based plating in the weld shoulders on the outermost surfaces of the plurality of steel sheets is 20 area% or less]
According to a further preferred embodiment of the present invention related to Embodiments 2 and 3, welding of the outermost surface, that is, the electrode-side surface of a plurality of steel sheets where the base material surface of the high-strength steel sheet is in contact with the zinc-based plating The amount of η phase of the zinc-based plating on the shoulder is 20 area % or less. When the alloying progresses between the zinc-based coating and the steel sheet due to the heat input of welding, not only in the corona bond described above, but also in the heat-affected zone on the electrode side, the initial zinc-based coating is mainly composed of Zn. The ratio of the η phase to be used is reduced. Therefore, by implementing a welding method with a higher heat input than a normal welding method, the alloying between the zinc-based plating and the steel sheet can be further promoted, so the heat-affected zone on the electrode side It is possible to reduce the amount of η phase in the zinc-based plating to 20 area % or less. In the present embodiment, the amount of η phase of the zinc-based plating on the shoulder portion of the weld is defined as a representative of the heat affected zone on the electrode side. As described above, the melting point of the zinc-based coating increases as the zinc-based coating on the electrode side is alloyed by the heat input during spot welding. For this reason, it is possible to suppress or reduce the penetration of molten zinc into the steel sheet compared to the case where the proportion of the η phase is relatively high, and it is possible to further enhance the effect of suppressing LME cracks on the surface on the electrode side. becomes. From the viewpoint of enhancing the effect of suppressing LME cracking, the amount of the η phase of the zinc-based plating in the shoulder portion of the weld is preferably smaller, and may be, for example, 18 area % or less or 15 area % or less.

The measurement of the area ratio of the η phase of the zinc-based plating on the weld shoulder is performed as follows. By SEM-EDS, the outer boundary of the weld shoulder in the cross section of the weld (the part where the shape suddenly changes from the part with the curvature reflecting the electrode shape due to the electrode pressure during welding to the flat part where the electrode does not contact) ) in a rectangular range of 100 μm in the horizontal direction and 10 μm in the vertical direction. However, it is necessary to increase the height in accordance with the thickness of the plating layer of the shoulder portion. When the total plating layer thickness exceeds 10 μm, the height of the measurement area is set to a value exceeding 10 μm. The η phase in the image is defined as the region with Zn concentration above 95% and Fe concentration below 5%. The part that satisfies this definition and the other part are binarized by image analysis software, and the area ratio of the η phase in the zinc-based plating on the shoulder of the weld is calculated. Specifically, the area ratio of the η phase can be obtained by dividing the area of the η phase within the measurement region by the area of the region in which Zn exists.

[Preferred chemical composition of high-strength steel sheet]
As described above, the high-strength steel sheet according to the embodiment of the present invention has a tensile strength of 980 MPa or more, and the depth region from the base material surface in contact with the zinc-based plating is the hardness of the second depth region. It may be any material in a three-layer structure with a controlled hardness lower than the hardness of the first and third depth regions. Therefore, the chemical composition of the high-strength steel sheet is not particularly limited, and may be appropriately determined within the range that satisfies the above requirements. More specifically, as described above, the object of the present invention is to provide a resistance spot-welded joint that includes a high-strength steel plate and can suppress the occurrence of LME cracking during spot welding, and has a tensile strength of 980 MPa or more. The object is achieved by using a high-strength steel sheet having strength and having a depth region of the three-layer structure described above. Therefore, it is clear that the chemical composition of the high-strength steel sheet itself is not an essential technical feature for achieving the object of the present invention. Preferred chemical compositions of high-strength steel sheets according to embodiments of the present invention will be described in detail below, but these explanations are intended to be mere examples of preferred chemical compositions of high-strength steel sheets for satisfying the above requirements. Therefore, it is not intended that the present invention be limited to high strength steel sheets having such a specific chemical composition. Also, in the following description, the unit of content of each element, "%", means "% by mass" unless otherwise specified. Furthermore, in this specification, the term "to" indicating a numerical range is used to include the numerical values before and after it as lower and upper limits, unless otherwise specified.

[C: 0.20 to 0.40%]
C is an element that increases the tensile strength at low cost and is an important element for controlling the strength of steel. In order to sufficiently obtain such effects, the C content is preferably 0.20% or more. The C content may be 0.21% or more, 0.22% or more, 0.23% or more, or 0.24% or more. On the other hand, an excessive C content may lead to a decrease in elongation. Therefore, the C content is preferably 0.40% or less. The C content may be 0.38% or less, 0.36% or less, or 0.34% or less.

[Si: 0.01 to 2.00%]
Si is an element that acts as a deoxidizing agent and suppresses the precipitation of carbides during the cooling process during cold-rolled sheet annealing. In order to sufficiently obtain such effects, the Si content is preferably 0.01% or more. The Si content may be 0.10% or more, 0.30% or more, or 0.80% or more. On the other hand, an excessive Si content may increase the strength of the steel and decrease the elongation. Therefore, the Si content is preferably 2.00% or less. The Si content may be 1.80% or less, 1.50% or less, or 1.20% or less.

[Mn: 0.10 to 4.00%]
Mn is a factor that affects the ferrite transformation of steel and is an element effective in increasing strength. In order to sufficiently obtain such effects, the Mn content is preferably 0.10% or more. The Mn content may be 0.50% or more, 1.00% or more, or 1.50% or more. On the other hand, an excessive Mn content may increase the strength of the steel and decrease the elongation. Therefore, the Mn content is preferably 4.00% or less. The Mn content may be 3.30% or less, 3.00% or less, or 2.70% or less.

[P: 0.0200% or less]
P is an element that strongly segregates at ferrite grain boundaries and promotes embrittlement of the grain boundaries. Since the P content is preferably as small as possible, it is ideally 0%. However, excessive reduction of the P content causes a significant increase in cost, so the P content may be 0.0001% or more, 0.0010% or more, or 0.0050% or more. On the other hand, an excessive P content may cause embrittlement of the steel due to grain boundary segregation as described above. Therefore, the P content is preferably 0.0200% or less. The P content may be 0.0180% or less, 0.0150% or less, or 0.0100% or less.

[S: 0.0200% or less]
S is an element that forms non-metallic inclusions such as MnS in steel and causes a decrease in ductility of steel parts. Ideally, the S content is 0% because the smaller the S content, the better. However, since an excessive reduction in the S content causes a significant increase in cost, the S content may be 0.0001% or more, 0.0002% or more, 0.0010% or more, or 0.0050% or more. There may be. On the other hand, an excessive S content may cause cracks starting from non-metallic inclusions during cold forming. Therefore, the S content is preferably 0.0200% or less. The S content may be 0.0180% or less, 0.0150% or less, or 0.0100% or less.

[Al: 1.500% or less]
Al is an element that acts as a deoxidizing agent for steel and stabilizes ferrite, and may be contained as necessary. Since Al may not be contained, the lower limit of the Al content is 0%. In order to sufficiently obtain the effect, the Al content is preferably 0.001% or more, and may be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, when Al is contained excessively, ferrite transformation and bainite transformation are excessively accelerated in the cooling process of cold-rolled steel sheet annealing, which may reduce the strength of the steel sheet. Therefore, the Al content is preferably 1.500% or less. The Al content may be 1.400% or less, 1.200% or less, or 1.000% or less.

[N: 0.0200% or less]
N is an element that forms coarse nitrides in the steel sheet and reduces the workability of the steel sheet. Also, N is an element that causes blowholes during welding. Ideally, the N content is 0% because the smaller the N content, the better. However, since an excessive reduction in the N content causes a significant increase in manufacturing costs, the N content may be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0050% or more. may be On the other hand, when N is contained excessively, it combines with Ti to generate a large amount of TiN, so the amount of solid solution Ti in the steel sheet decreases, and the generation of precipitates (for example, Ti oxides) in the steel sheet surface layer cannot be controlled. may disappear. Therefore, the N content is preferably 0.0200% or less. The N content may be 0.0150% or less, 0.0100% or less, or 0.0080% or less.

[Ti: 0.005 to 0.500%]
Ti is an element necessary for forming fine precipitates (e.g., Ti oxides) on the surface layer of the steel sheet by combining with oxygen that enters the surface layer of the steel from the annealing atmosphere in the heating and soaking steps in cold-rolled steel annealing. is. In order to sufficiently form such precipitates, the Ti content is preferably 0.005% or more. The Ti content may be 0.010% or more, 0.050% or more, 0.100% or more, or 0.150% or more. On the other hand, an excessive Ti content may cause the formation of excessive precipitates or promote ferrite transformation in the cooling process during cold-rolled steel annealing, resulting in a decrease in strength. Therefore, the Ti content is preferably 0.500% or less. The Ti content may be 0.450% or less, 0.400% or less, 0.350% or less, or 0.300% or less.

A preferred basic chemical composition of the high-strength steel sheet is as described above. Furthermore, the high-strength steel sheet, if necessary, replaces part of the remaining Fe with Co: 0 to 0.5000%, Ni: 0 to 1.0000%, Mo: 0 to 1.0000%, Cr : 0-2.0000%, O: 0-0.0200%, B: 0-0.0100%, Nb: 0-0.5000%, V: 0-0.5000%, Cu: 0-0. 5000%, W: 0-0.1000%, Ta: 0-0.1000%, Sn: 0-0.0500%, Sb: 0-0.0500%, As: 0-0.0500%, Mg: 0-0.0500%, Ca: 0-0.0500%, Y: 0-0.0500%, Zr: 0-0.0500%, La: 0-0.0500%, and Ce: 0-0. It may contain one or more selected from the group consisting of 0500%. Each element may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

In the high-strength steel sheet according to the embodiment of the present invention, the balance other than the above elements consists of Fe and impurities. Impurities are components and the like that are mixed due to various factors in the manufacturing process, including raw materials such as ores and scraps, when industrially manufacturing high-strength steel sheets.

The chemical composition of the high-strength steel sheet can be measured by a general analytical method. For example, the chemical composition of a high-strength steel sheet may be measured using Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). C and S can be measured using a combustion-infrared absorption method, N can be measured using an inert gas fusion-thermal conductivity method, and O can be measured using an inert gas fusion-nondispersive infrared absorption method. When zinc-based plating is provided on the surface of the high-strength steel sheet, the chemical composition may be analyzed after removing the zinc-based plating by mechanical grinding.

[Manufacturing method of high-strength steel plate]
Next, a preferred method for manufacturing the high-strength steel sheet according to the embodiment of the present invention will be described. The following description is intended to illustrate a characteristic method for manufacturing the high-strength steel sheet according to the embodiment of the present invention, and the high-strength steel sheet is manufactured by the manufacturing method described below. It is not intended to be limited to

A method for producing a high-strength steel sheet according to an embodiment of the present invention includes, for example, hot-rolling a billet having the chemical composition described above in relation to the high-strength steel sheet, followed by coiling at 580° C. or less;
a step of pickling the obtained hot-rolled steel sheet to remove oxide scale existing on the surface of the hot-rolled steel sheet and removing at least 5 μm of the surface layer of the hot-rolled steel sheet; and cold-rolling the hot-rolled steel sheet. Then, a step of annealing, wherein the obtained cold-rolled steel sheet is held in an atmosphere with a dew point of -20 to 20 ° C. in a temperature range of 200 to 400 ° C. for 20 to 180 seconds, and then the dew point is -20. It is characterized by including a step of holding in an atmosphere of up to 20°C in a temperature range of 740°C to 900°C for 45 to 300 seconds. Each step will be described in detail below.

[Hot rolling and winding process]
In this process, a billet having the chemical composition described above in relation to high strength steel is subjected to hot rolling. The steel slabs to be used are preferably cast by continuous casting from the viewpoint of productivity, but may be produced by ingot casting or thin slab casting. Further, the cast steel slab may optionally be subjected to rough rolling before finish rolling for plate thickness adjustment and the like. Conditions for such rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured. Hot rolling is not particularly limited, but is generally carried out under such conditions that the completion temperature of finish rolling is 650° C. or higher. This is because if the finish rolling completion temperature is too low, the rolling reaction force increases, making it difficult to stably obtain a desired plate thickness. The upper limit is not particularly limited, but generally the finish rolling completion temperature is 950° C. or less.

[Winding temperature]
After hot rolling, the obtained hot-rolled steel sheet is coiled at a coiling temperature of 580°C or less. The coiling temperature is an important factor controlling the transformation behavior of the steel structure from austenite to ferrite, pearlite, bainite and martensite, and controlling the precipitation behavior of Ti. If the steel is coiled at a relatively high temperature, coarse Ti precipitates may form in the steel structure after coiling. In such a case, after cold-rolled steel sheet annealing, which will be described in detail later, the surface layer structure of the steel sheet has a sufficient gradient of properties (strength, hardness, etc.) (that is, the hardness of the second depth region is the first). It becomes impossible to impart a characteristic gradient that is lower than the hardness of the depth region of . Therefore, in order to suppress the formation of such coarse Ti precipitates, the coiling temperature is preferably as low as possible, specifically 580° C. or less. The winding temperature is preferably 550° C. or less. For example, the coiling temperature may be room temperature or below, but in order to wind the steel sheet at a temperature below room temperature, it is necessary to lower the temperature of the water for cooling the steel sheet below room temperature, which causes an increase in manufacturing costs. In addition, since the residual stress in the steel sheet increases due to rapid cooling, for example, if the steel sheet is coiled at a temperature of less than 10 ° C., the steel sheet will crack when the coil is uncoiled in the subsequent pickling process, resulting in a decrease in productivity. . Therefore, although not particularly limited, the lower limit of the winding temperature is generally 10°C or higher, preferably 50°C or higher.

[Pickling process]
The wound hot-rolled steel sheet is unwound and subjected to pickling. By pickling, the oxide scale existing on the surface of the hot-rolled steel sheet can be removed, and the chemical convertability and the platability of the cold-rolled steel sheet can be improved. Oxide scale refers to an oxide layer (external oxide layer) formed on the _{surface of a steel sheet. Fayalite (Fe 2 SiO 4} ), etc. In addition, the pickling promotes the dissolution of the surface layer of the steel sheet, and completely removes oxides (internal oxides) formed under the oxide scale on the surface layer of the hot-rolled steel sheet, that is, inside the steel sheet. By completely removing the oxides generated inside the steel sheet, that is, by setting the thickness of the internal oxide layer generated inside the steel sheet to 0 μm, the combination of Ti in the steel with oxygen is suppressed and Ti is brought into a solid solution state. It is possible to exist in Here, the thickness of the internal oxide layer is the distance from the surface of the steel sheet to the farthest position where the internal oxide exists when proceeding from the surface of the steel sheet in the thickness direction of the steel sheet (direction perpendicular to the surface of the steel sheet). It means. By leaving solid solution Ti in the inside of the sheet thickness from the new surface that appeared after pickling, a large amount of fine Ti precipitates can be generated in the outermost layer of the steel sheet after cold-rolled sheet annealing. Sufficient inclination can be provided. Pickling may be done once, but in order to more reliably remove the oxides in the steel formed under the oxide scale of the hot-rolled steel sheet, it may be carried out several times. It may be subjected to mechanical polishing such as. Alternatively, instead of measuring the change in plate thickness before and after pickling, the removal amount of the steel sheet surface layer may be obtained from the change in coil weight before and after pickling. If the amount of removal of the steel sheet surface layer is less than 5 μm, the oxide under the oxide scale is not completely removed, that is, the thickness of the internal oxide layer exceeds 0 μm, and remains on the steel sheet surface layer in the heating process during cold-rolled steel annealing. Oxygen is supplied from the internal oxides, Ti oxides precipitate and coarsen in the steel sheet surface layer, and after cold-rolled steel sheet annealing, the surface layer structure of the steel sheet cannot be imparted with a sufficient gradient in properties. Therefore, the removal amount of the surface layer of the steel sheet is 5 μm or more, more specifically, 5 μm or more, preferably 7 μm or more, and more preferably 10 μm or more on one side. Although it is preferable to remove the surface layer of the steel sheet by pickling as much as possible, excessive erosion of the steel causes a decrease in pickling speed and a decrease in productivity due to a decrease in yield. Therefore, the upper limit is generally 150 μm or less, and may be 120 μm or less, 100 μm or less, 70 μm or less, 50 μm or less, or 30 μm or less.

[Cold rolling and annealing process]
Finally, the obtained hot-rolled steel sheet is cold-rolled and then subjected to predetermined annealing (hereinafter also referred to as "cold-rolled steel sheet annealing") to obtain the high-strength steel sheet according to the embodiment of the present invention. The rolling reduction in cold rolling is not particularly limited, and may be any appropriate value. For example, the rolling reduction may be 5% or more, 10% or more, or 30% or more, and/or may be 90% or less, 75% or less, or 50% or less. The cold-rolled sheet annealing will be described in detail below.

[Cold-rolled sheet annealing]
[Dew point in the temperature range of 200 to 400 ° C.]
In the heating process during cold-rolled steel annealing, increasing the dew point of the gas atmosphere in the furnace, specifically controlling the dew point within the range of -20 to 20 ° C. promotes the penetration of oxygen into the steel plate, Fine Ti precipitates can be produced in the outermost layer of the steel sheet. Using these fine Ti precipitates as nuclei, 10 or more precipitates with a diameter of less than 0.1 μm are generated/μm ² in the first depth region of 1 to 10 μm from the base material surface in the soaking treatment following the heat treatment. , the hardness of the outermost layer in the steel sheet after cold-rolled steel annealing can be increased. If the dew point is too low, the amount of oxygen that enters the steel sheet is insufficient, and the number of fine Ti precipitate nuclei decreases. I can't do it. Therefore, the lower limit of the dew point is -20°C or higher, preferably -15°C or higher. On the other hand, if the dew point is high, the amount of oxygen that enters the steel sheet becomes excessive, and coarse Ti precipitates are formed at a low number density. Therefore, the upper limit of the dew point is 20°C or lower, preferably 15°C or lower.

[Holding time in the temperature range of 200 to 400°C]
In the heating process during cold-rolled steel annealing, it is effective to control the dew point and the holding time in the temperature range of 200 to 400° C. in order to produce fine Ti precipitates on the outermost layer of the steel sheet. Here, the holding time means the time during which the temperature stays in the temperature range of 200 to 400°C, and thus includes the time when the temperature is gradually raised between 200 to 400°C. be. If the holding time is short, the amount of oxygen that penetrates into the steel sheet is insufficient, and the number of fine Ti precipitate nuclei decreases. I can't do it. Therefore, the lower limit of the retention time is set to 20 seconds or longer, preferably 30 seconds or longer. On the other hand, if the holding time is long, the amount of oxygen entering the steel sheet becomes excessive, and coarse Ti precipitates are formed with a low number density. Therefore, the upper limit of the retention time is set to 180 seconds or less, preferably 150 seconds or less.

[Dew point in the temperature range of 740 to 900 ° C.]
In cold-rolled sheet annealing, after optimizing the dew point and holding time in the temperature range of 200 to 400 ° C. to generate fine Ti precipitates in the outermost layer of the steel sheet, these fine Ti precipitates are used as nuclei, By controlling the dew point at 740 to 900° C., a sufficient amount of precipitates can be deposited in the outermost layer of the steel sheet. Furthermore, holding at 740 to 900°C promotes the diffusion of alloying elements in the steel compared to holding at 200 to 400°C, so that C dissolved in the steel combines with oxygen, desorption (decarburization reaction), and the amount of dissolved C decreases. Due to this effect, the amount of dissolved C in the second depth region of 10 to 60 μm from the base material surface can be reduced to less than 0.20% by mass, making it possible to form a new soft layer in this region. If the dew point is too low, the amount of oxygen that penetrates into the steel sheet is insufficient, so Ti precipitates and Si and Mn oxides with the Ti precipitates as nuclei are insufficiently coarsened, and the steel sheet after cold-rolled sheet annealing. In addition, the solid solution C amount in the second depth region of 10 to 60 μm from the base material surface cannot be reduced. Therefore, the lower limit of the dew point is -20°C or higher, preferably -15°C or higher. On the other hand, if the dew point is high, the amount of oxygen that enters the steel sheet becomes excessive, and it becomes impossible to suppress the coarsening and coalescence of Ti precipitates and Si and Mn oxides with the Ti precipitates as nuclei. The number density of objects decreases. Therefore, the upper limit of the dew point is 20°C or lower, preferably 15°C or lower.

[Holding time in the temperature range of 740 to 900 ° C.]
In cold-rolled sheet annealing, after optimizing the dew point and holding time in the temperature range of 200 to 400 ° C. to generate fine Ti precipitates in the outermost layer of the steel sheet, these fine Ti precipitates are used as nuclei, By controlling the holding time at 740 to 900° C., a sufficient amount of precipitates can be deposited in the outermost layer of the steel sheet. Furthermore, holding at 740 to 900°C promotes the diffusion of alloying elements in the steel compared to holding at 200 to 400°C, so that C dissolved in the steel combines with oxygen, desorption (decarburization reaction), and the amount of dissolved C decreases. Due to this effect, the amount of dissolved C in the second depth region of 10 to 60 μm from the base material surface can be reduced to less than 0.20% by mass, making it possible to form a new soft layer in this region. Here, the holding time means the time during which the temperature stays in the temperature range of 740 to 900°C, and thus includes the time when the temperature is gradually increased between 740 to 900°C. be. If the holding time is short, the amount of oxygen that enters the steel sheet is insufficient, so that Ti precipitates and Si and Mn oxides with the Ti precipitates as nuclei are insufficiently coarsened, and the steel sheet after cold-rolled sheet annealing. In addition, the solid solution C amount in the second depth region of 10 to 60 μm from the base material surface cannot be reduced. Therefore, the lower limit of the retention time is 45 seconds or longer, preferably 48 seconds or longer, or 60 seconds or longer. On the other hand, if the holding time is long, the amount of oxygen that enters the steel sheet becomes excessive, making it impossible to suppress coarsening and coalescence of Ti precipitates and Si and Mn oxides with the Ti precipitates as nuclei. The number density of precipitates decreases. Therefore, the upper limit of the holding time is 300 seconds or less, preferably 250 seconds or less.

In the method for producing a high-strength steel sheet according to the embodiment of the present invention, as described above, Ti is contained in order to form fine precipitates (for example, Ti oxide) on the surface layer of the steel sheet. More specifically, By coiling the hot-rolled steel sheet containing Ti at a relatively low coiling temperature of 580°C or less in the coiling process, the formation of coarse Ti precipitates in the steel structure is suppressed, and the heat is reduced in the subsequent pickling process. By setting the removal amount of the surface layer of the rolled steel sheet to 5 μm or more, not only the external oxide but also the internal oxide are completely removed, suppressing the combination of Ti in the steel with oxygen and allowing Ti to exist in a solid solution state. enable Next, in the annealing step, controlled annealing at 200 to 400°C (dew point: -20 to 20°C, holding time: 20 to 180 seconds) produces fine Ti precipitates, followed by annealing at 740 to 900°C. By controlled annealing (dew point: -20 to 20°C, holding time: 40 to 300 seconds), a sufficient amount of precipitates can be precipitated on the outermost layer of the steel sheet using the fine Ti precipitates as nuclei. . Precipitation strengthening resulting from such precipitates can increase the hardness of the outermost layer. Furthermore, under such high temperature, the diffusion of alloying elements in the steel is promoted, and C solid solution in the steel is combined with oxygen and desorbed into the atmosphere, resulting in a second The amount of dissolved C in the depth region can be reduced to less than 0.20% by mass. As a result, a softer layer than the outermost layer can be formed in this region. That is, in the method for producing a high-strength steel sheet according to the embodiment of the present invention, on the premise of the presence of Ti, the production conditions for the steel containing Ti are appropriately controlled, particularly in the coiling process, pickling process, and annealing process. Thus, it is possible to impart a sufficient gradient of properties such as hardness to the surface layer structure of the steel sheet.

Operations such as cooling after annealing, tempering, and plating are not particularly limited, and any appropriate conditions may be appropriately selected in consideration of other desired properties.

[Steel sheets other than high-strength steel sheets]
Any appropriate steel plate can be used as the steel plate other than the high-strength steel plate described above among the plurality of steel plates used in the resistance spot welded joint according to the embodiment of the present invention. Such a steel plate may be, for example, a high-strength steel plate that similarly has a tensile strength of 980 MPa or more and does not have the three-layer structure described above in the depth region from the surface of the base material. On the other hand, when such a steel sheet is a low-strength steel sheet, the steel sheet is not particularly limited in chemical composition, metallographic structure, shape, etc., except that the steel sheet has a tensile strength of less than 980 MPa. Therefore, as for steel sheets other than the high-strength steel sheets described above, an appropriate steel sheet may be appropriately selected according to the application and desired properties of the resistance spot welded joint, such as desired joint strength.

[Zinc-based plating]
Zinc-based plating is not particularly limited, and may be any appropriate zinc-based plating. The zinc-based plating may be either hot dip galvanizing or electrogalvanizing. Hot-dip galvanizing includes, for example, hot-dip galvanizing (GI), hot-dip galvanizing (GA), hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, hot-dip Zn-Al-Mg-Si Including alloy plating, etc. Electro-galvanizing includes, for example, electro-galvanizing (EG) as well as electro-Zn—Ni alloy plating. Preferably, the zinc-based plating is hot dip galvanizing (GI), hot dip galvannealing (GA), or electrogalvanizing (EG). The plating amount is not particularly limited, and a general amount may be used. The zinc-based plating may be in contact with the base material of the high-strength steel sheet at the final resistance spot-welded joint, and may be applied to either the high-strength steel sheet or other steel sheets before spot welding.

<Method for manufacturing resistance spot welded joint>
A resistance spot welded joint according to an embodiment of the present invention is a plurality of steel sheets obtained by applying zinc-based plating to the above-described high-strength steel sheet and superimposing it on another steel sheet, or the above-described high-strength steel sheet has zinc-based plating. Multiple steel plates stacked with other steel plates can be manufactured using any suitable spot welding method known to those skilled in the art. Therefore, for example, a plurality of steel plates stacked as described above are pressurized using a pair of electrodes facing each other, and an electric current is passed between the electrodes under normal conditions to form a corona bond on the nugget and its surroundings. It is thus possible to manufacture resistance spot welded joints according to embodiments of the present invention. In the following, a more preferable method for manufacturing a resistance spot-welded joint that can further suppress LME cracking as compared with such a normal spot method will be described in detail.

[Manufacturing method 1: Use of down slope]
Manufacturing method 1 includes a plurality of superimposed steel sheets including a high-strength steel sheet having zinc-based plating or including a steel sheet having zinc-based plating and a high-strength steel sheet adjacent to the steel sheet, using a pair of electrodes facing each other. and pressurizing (S1),
(S2) forming a nugget and a corona bond by energizing between the electrodes while pressing the plurality of steel plates;
(S3) continuously reducing the current value between the electrodes to 0 while maintaining the pressure on the plurality of steel plates;
It is characterized by comprising

FIG. 4 is a diagram schematically showing changes over time in current in manufacturing method 1. FIG. Referring to FIG. 4, in step S1, a plurality of steel plates including a high-strength steel plate are pressed using a pair of opposing electrodes, and then in step S2, a current is applied between the electrodes while pressing the plurality of steel plates to obtain a nugget. And after forming the corona bond, the current value between the electrodes is continuously decreased to 0 while maintaining the pressure on the plurality of steel plates in step S3. By using such a continuous decrease in the current value, that is, the downslope of the current value, compared to the case where the current value I is rapidly decreased in one step, the increase in tensile stress during cooling is suppressed. It becomes possible to LME cracking is caused by the action of tensile stress generated during welding. Therefore, it is possible to suppress the occurrence of LME cracks by suppressing an increase in tensile stress by a manufacturing method using such a downslope of the current value. In addition, by using the downslope of the current value, the welding heat input can be increased compared to the case where the current value is rapidly decreased in one step, and therefore the gap between the zinc-based coating and the steel sheet can be increased. can promote the alloying of As a result, it is possible to reduce the amount of η phase in the zinc-based plating in the corona bond and/or in the weld shoulder, more specifically, to 20 area % or less. Such alloying raises the melting point of the zinc-based coating, so compared to the case where the proportion of the η phase is relatively high, it is possible to suppress or reduce the intrusion of molten zinc into the steel sheet. It is possible to further enhance the effect of suppressing LME cracks on the surface and/or the electrode-side surface.

[Preferred embodiment of production method 1]
In a preferred embodiment of manufacturing method 1, a plurality of superimposed steel sheets including a high-strength steel sheet having zinc-based plating or including a steel sheet having zinc-based plating and a high-strength steel sheet adjacent to the steel sheet are placed in an opposing pair. A step of applying pressure using the electrode of (S1),
(S2) forming a nugget and a corona bond by energizing between the electrodes while pressing the plurality of steel plates;
(S3) continuously reducing the current value between the electrodes to 0 while maintaining the pressure on the plurality of steel plates;
In a first period, which is a current value I between the electrodes when the formation of the nugget is completed, and a period from the time when the formation of the nugget is completed to the time when the current value is set to 0 The average value Iave of the current values between the electrodes satisfies the relationship of 0.30 × I ≤ Iave ≤ 0.90 × I,
The length of a second period, which is a period from when the current value is set to 0.90×I to when the current value is set to 0.30×I, is 420 msec or longer, and
The pressurizing force in the first period is always 1.1 times or more the pressurizing force P at the time when the formation of the nugget is completed.

FIG. 5 is a diagram schematically showing changes over time in current and applied pressure in a preferred embodiment of manufacturing method 1. FIG. Referring to FIG. 5, first, in step S1, a plurality of steel plates including a high-strength steel plate are pressed using a pair of opposing electrodes, and then in step S2, by energizing between the electrodes while pressing the plurality of steel plates, , to form nuggets and corona bonds. Next, in step S3, the pressure is constantly applied to the nugget for the first period corresponding to the period of step S3, that is, the first period corresponding to the period from the time when the formation of the nugget is completed to the time when the current value is set to 0. The current value I between the electrodes is continuously reduced to 0 while maintaining the pressure P at 1.1 times the pressure P at the time of completion of the formation of . In addition, in the present embodiment, in the first period, the average value Iave corresponding to the average of the time integral values of the current values between the electrodes satisfies the relationship of 0.30×I≦Iave≦0.90×I, and Control is performed so that the length of the second period corresponding to the period from when the current value is set to 0.90×I to when the current value is set to 0.30×I is 420 msec or longer. LME cracking occurs when a tensile stress is applied to the high-strength steel sheet while the high-strength steel sheet and liquid zinc are in contact with each other, and such tensile stress tends to increase particularly when the electrodes are released. In addition, for example, the high-strength steel plate and liquid zinc can be in contact with corona bond when the temperature of corona bond is 907 ° C. (the temperature at which zinc vapor may liquefy) and 420 ° C. (the temperature at which liquid zinc solidifies). possible temperature) or higher. Therefore, the pressure applied by the electrodes for a period of time to ensure that the temperature of the corona bond is below about 420° C., preferably for a first period, is 1.1 times the pressure P at which nugget formation is complete. By maintaining the above, it is possible to relax the tensile stress generated in the corona bond while the high-strength steel plate and the liquid zinc are in contact with each other.

In addition, the current value I when the formation of the nugget is completed and the average value Iave of the current values in the first period satisfy the relationship of 0.30×I≦Iave≦0.90×I, and the second If the length of the period is set to 420 msec or more, the cooling rate of the welded portion is lowered. Also, in this case, heat removal by the electrodes is reduced, and heat transfer from the welded portion to the surrounding steel plate is promoted. As a result, when the temperature of the welded portion is lowered, the contraction of the welded portion becomes moderate, while the restraining force of the welded portion by the steel plate around the welded portion becomes smaller. It is believed that such a mechanism further reduces the tensile stress in the weld. Therefore, according to the preferred embodiment of the manufacturing method 1, it is possible to achieve a higher LME crack suppressing effect than the LME crack suppressing effect due to the use of the down slope described in relation to the manufacturing method 1. . Although not particularly limited, I and Iave may be controlled so as to satisfy the relationship 0.45×I≦Iave≦0.85×I.

[Manufacturing method 2: Use of post-energization]
Manufacturing method 2 includes a plurality of superimposed steel sheets including a high-strength steel sheet having zinc-based plating or including a steel sheet having zinc-based plating and a high-strength steel sheet adjacent to the steel sheet, using a pair of electrodes facing each other. and pressurizing (S1),
(S2) forming a nugget and a corona bond by energizing between the electrodes while pressing the plurality of steel plates;
(S3) reducing a current value between the electrodes to 0 while maintaining pressure on the plurality of steel plates;
When the current value between the electrodes at the time when the formation of the nugget is completed is defined as I, and 1/2 of the total plate thickness in mm of the steel plate is defined as tm,
When the current value between the electrodes is reduced to 0 or less than 0.04 seconds after the current value is reduced to 0, the current value between the electrodes is reduced from 0.9 × I to 0.3 × I It is characterized in that it is held at a constant value of 0.12×tm or more in the unit sec within the range of up to .

FIG. 6 is a diagram schematically showing changes over time in current in manufacturing method 2. As shown in FIG. Referring to FIG. 6, in step S1, a plurality of steel plates including a high-strength steel plate are pressed using a pair of opposing electrodes, and then in step S2, a current is applied between the electrodes while pressing the plurality of steel plates to obtain a nugget. And after forming the corona bond, when the current value between the electrodes is reduced to 0 while maintaining the pressure on the plurality of steel plates in step S3, the current value I in step S2 is 0.9 × I to 0.3×I for a predetermined time. In the process of solidifying the weld metal during spot welding, an element such as P contained in the weld metal may solidify and segregate, reducing the joint strength. On the other hand, after the nuggets and corona bonds are formed in step S2, the current value I is not rapidly decreased, and is once set to a predetermined value within the range of 0.9×I to 0.3×I. By holding (post-energizing) for a long period of time, solidification segregation can be dispersed, and such a segregation mitigation effect can suppress a decrease in joint strength. In addition, by using such post-energization, similarly to the case of using the down slope, the welding heat input can be increased compared to the case where the current value I is rapidly decreased in one step. , so that the alloying between the zinc-based coating and the steel sheet can be promoted. As a result, it is possible to reduce the amount of η phase in the zinc-based plating in the corona bond and/or in the weld shoulder, more specifically, to 20 area % or less. Such alloying raises the melting point of the zinc-based coating, so compared to the case where the proportion of the η phase is relatively high, it is possible to suppress or reduce the intrusion of molten zinc into the steel sheet. It is possible to further enhance the effect of suppressing LME cracks on the surface and/or the electrode-side surface.

From the viewpoint of reliably obtaining the segregation relaxation effect and the alloying promotion effect, the current value during the post-energization is preferably high, specifically within the range of 0.9 × I to 0.8 × I. is preferably a constant value. The holding time needs to be longer as the total thickness of the multiple steel sheets becomes thicker. , it must be 0.12×tm or more in units of seconds. Although the upper limit is not particularly limited, the retention time may be 0.6 sec or less, for example. From the viewpoint of enhancing the segregation mitigation effect, in step S3, it is preferable to keep the current value at a constant value within the range of 0.9×I to 0.3×I without passing through a cooling time during which no current is applied. However, for a relatively short period of time, the current value may be maintained at the above constant value after a cool time during which no current is applied in step S3. Such a cool time (non-energization time) is preferably less than 0.04 seconds or 0.03 seconds or less after the current value is decreased to zero. As a result, while maintaining the segregation mitigation effect to some extent, it is possible to further enhance the effect of suppressing LME cracks on the lapped surface and/or the electrode-side surface of the steel sheet based on the alloying promotion effect. The current value I is not particularly limited, but is generally 5-12 kA.

[Manufacturing method 3: preliminary energization]
Manufacturing method 3 includes a plurality of superimposed steel sheets including a high-strength steel sheet having zinc-based plating or including a steel sheet having zinc-based plating and a high-strength steel sheet adjacent to the steel sheet, using a pair of electrodes facing each other. a step of pressurizing with
A step of preheating the steel plates by pre-energizing between the electrodes while pressurizing the plurality of steel plates;
forming a nugget and a corona bond by energizing between the electrodes while pressing the plurality of steel plates;
It is characterized by comprising

In manufacturing method 3, by performing preliminary energization before the main energization that forms the nugget and corona bond, the welding heat input can be increased compared to the case where no preliminary energization is performed. It can promote alloying with the steel plate. As a result, as in the case of manufacturing methods 1 and 2, it is possible to reduce the amount of the η phase of the zinc-based plating in the corona bond and/or in the weld shoulder to 20 area % or less. As a result, penetration of molten zinc into the steel sheet can be suppressed or reduced, and the effect of suppressing LME cracking on the lapped surface and/or the electrode-side surface of the steel sheet can be further enhanced. Specific conditions for preliminary energization are not particularly limited, and any appropriate conditions known to those skilled in the art may be selected. For example, the preliminary energization may be performed by energizing a constant current value lower than that of the main energization for a predetermined period of time, or may be energization in which the current is gradually increased, that is, up-slope energization.

[Manufacturing method 4: retention extension]
Manufacturing method 4 includes a plurality of superimposed steel sheets including a high-strength steel sheet having zinc-based plating or including a steel sheet having zinc-based plating and a high-strength steel sheet adjacent to the steel sheet, using a pair of electrodes facing each other. and pressurizing (S1),
(S2) forming a nugget and a corona bond by energizing between the electrodes while pressing the plurality of steel plates;
a step of reducing the current value between the electrodes to 0 while maintaining pressure on the plurality of steel plates (S3);
a step of maintaining the applied pressure at 0.8×P or more for 0.2 sec or more and 0.4 sec or less with the current value between the electrodes set to 0 (S4);
and P is the pressurization force at the time when the formation of the nugget is completed.

In production method 4, after the current value between the electrodes is reduced to 0, the electrodes are not released immediately, but the electrodes are held for a predetermined period of time and with an applied pressure, and then released to release the tensile stress. We are trying to reduce the occurrence. In more detail, first, in step S1, the steel plate is pressed by the electrodes at room temperature, and the tensile stress applied to the steel plate increases, and then in step S2, the material strength of the steel plate decreases as the temperature rises due to the energization. Tensile stress decreases with Next, when the energization is stopped in step S3, the steel sheet is cooled and the tensile stress is recovered accordingly. In this state, if the electrode is released immediately or after a relatively short holding time, the tensile stress will increase greatly at the moment of release, and since the temperature is relatively high, the molten zinc will not solidify sufficiently. , the risk of LME cracking increases. Therefore, in manufacturing method 4, after step S3, in step S4, the applied pressure is slightly reduced and held for a time of 0.2 sec or more and 0.4 sec or less, so that the electrode is immediately or relatively removed for about 0.1 sec. It is possible to suppress the increase in tensile stress compared to the case of releasing with a short holding time, and furthermore, because the electrode is released after the temperature is sufficiently lowered, the molten zinc can be completely or sufficiently solidified. can. Therefore, according to manufacturing method 4, the risk of LME cracking can be greatly reduced compared to the case where step S4 is not included. If the electrode holding time exceeds 0.4 sec, the quenching speed of the welded portion increases, and the hardness of the welded portion tends to increase. In this case, the toughness of the welded portion, particularly the nugget, deteriorates, and joint strength and resistance to hydrogen embrittlement may be inferior, so the upper limit of the electrode holding time is 0.4 sec. From the viewpoint of enhancing the effect of reducing the tensile stress, the pressure in step S4 should be 0.8×P or more with respect to the pressure P in step S2, that is, the pressure P at the time the nugget formation is completed. . Although the upper limit is not particularly limited, the applied pressure in step S4 may be 0.9×P, for example. The applied pressure P is not particularly limited, but is generally 2 to 8 kN.

EXAMPLES The present invention will be described in more detail below with reference to Examples, but the present invention is not limited to these Examples.

In the following examples, high-strength steel sheets were manufactured under various conditions, and resistance spot-welded joints according to embodiments of the present invention were manufactured using the obtained high-strength steel sheets by a plurality of manufacturing methods. The presence or absence of the occurrence of LME cracking was investigated.

[Example 1]
[Manufacturing of high-strength steel sheets]
% by mass, C: 0.22% or 0.24%, Si: 0.41%, Mn: 0.64%, P: 0.0190%, S: 0.0038%, Al: 0.717% , N: 0.0021%, Ti: 0.470%, and the balance: Fe and impurities. These steel slabs were placed in a furnace heated to 1220° C., held for 60 minutes for homogenization, taken out into the atmosphere, and hot rolled to obtain a steel plate having a thickness of 2.6 mm. The completion temperature of finish rolling in hot rolling was 910°C, and the steel was cooled to 550°C and coiled. Subsequently, the oxide scale of this hot-rolled steel sheet is removed from the surface layers of both sides of the steel sheet by pickling to a thickness of 10 μm on one side (the thickness of the internal oxide layer after pickling and before cold rolling is 0 μm), and the rolling reduction is 45%. was cold rolled to finish the plate thickness to 1.6 mm. Furthermore, when the cold-rolled steel sheet is annealed, specifically, when the temperature is raised to 860 ° C., the temperature range of 200 to 400 ° C. is controlled to an atmosphere with a dew point of -5 ° C. (comparative steel sheets 1 and 2 are 25 ° C.), The holding time in that temperature range was set to 30 seconds, and in addition, the temperature range of 740 to 900°C was controlled to have a dew point of 5°C, and the holding time in that temperature range was set to 120 seconds (35 seconds for comparative steel sheet 1). . Finally, the cold-rolled steel sheet is cooled under prescribed conditions and then alloyed hot-dip galvanized (GA) is applied under normal conditions to form a high-strength steel sheet with a zinc-based coating (high-strength steel sheets 1 and 2 and comparative Steel plates 1 and 2) were produced. Table 1 shows the properties of each high-strength steel sheet. The tensile strength was measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece was parallel to the rolling direction of the steel plate, and performing a tensile test according to JIS Z 2241:2011.

[Manufacturing of resistance spot welded joints: set of two of the same type]
Next, with respect to the high-strength steel plates shown in Table 1, resistance spot welded joints were produced by superimposing two identical steel plates and performing resistance spot welding. Normal (single-stage energization) in Table 1 means a process of pressing two superimposed high-strength steel sheets using a pair of opposing electrodes, and pressing these high-strength steel sheets while pressing between the electrodes. and forming a nugget and a corona bond by energizing the nugget. Other production methods 1-4 correspond to the production methods described herein. Detailed manufacturing conditions are as follows.

Two steel plates were arranged substantially horizontally, and a pair of electrodes were arranged so as to sandwich these steel plates. The electrode placed on one steel plate was used as a movable electrode, and the electrode placed under the other steel plate was used as a fixed electrode. By moving the upper electrode toward the lower electrode, the steel plates were pressed. . At the start of energization, the current value was instantaneously increased to a predetermined value, and thereafter the current value was kept constant until the nugget was completed. Other welding conditions are shown below. The striking angle is the angle between the axial direction of the movable electrode and the direction perpendicular to the surface of the steel plate. The clearance is the distance between the surface of the steel plate and the electrode, and is defined here as the distance between the tip of the lower electrode before welding and the surface of the steel plate closest to each other. Striking angle and clearance are disturbance factors in resistance spot welding, and are factors that cause LME cracking. By setting the striking angle and the clearance as follows, conditions were established in which LME cracking is relatively likely to occur. After forming the nugget, the applied pressure was maintained at P: 4 kN for the holding time shown in Table 1.
Welder: Servo pressurized stationary welder, single-phase AC (frequency 50 kHz)
Electrode: Dome Radius (DR) Cr-Cu
Shape of electrode tip: φ6mm, R40mm
Applied force P during energization and holding: 4 kN
Current value I at the completion of nugget formation: 8 kA (conditions for forming a nugget of 5.5 to 6.5 mm)
Energization time for main energization (when forming a nugget): 18 cycles (360 ms)
Hitting angle: 5°
Clearance: 0.3mm

[Evaluation of LME cracking resistance]
Resistance spot welding was performed 10 times for each high-strength steel plate. The resistance spot-welded joint thus obtained was cut along a plane passing through the center of the nugget and perpendicular to the surface of the steel sheet, and the cross-section was appropriately prepared, and the presence or absence of cracks was confirmed with an optical microscope. The number of cracks in 10 tests (the number of cracks in 10 tests) was 3 or less, and was evaluated as a resistance spot welded joint capable of suppressing the occurrence of LME cracks during spot welding. Table 1 shows the results.

Referring to Table 1, in Comparative Example 1, since the requirement that the hardness of the second depth region is lower than the hardness of the first and third depth regions was not satisfied, deformation during spot welding resulted in It is believed that the strain introduced into the steel sheet could not be sufficiently absorbed by the second depth region, resulting in an excessive increase in strain in the first depth region. As a result, it is considered that molten zinc penetrated from the surface layer of the steel sheet to the inside, and the occurrence of LME cracking could not be sufficiently suppressed. In Comparative Example 2, although the hardness of the second depth region was controlled to be lower than the hardness of the third hardness region, the hardness of the second depth region was lower than the hardness of the first depth region. Since the requirement of being lower than the depth was not satisfied, the strain introduced into the steel plate by deformation during spot welding could not be sufficiently absorbed by the second depth region, and in the first depth region This is considered to have caused an excessive increase in strain. As a result, it is considered that molten zinc penetrated from the surface layer of the steel sheet to the inside, and the occurrence of LME cracking could not be sufficiently suppressed.

In contrast, in Examples 3 to 10 of the present invention, by controlling the hardness of the second depth region to be lower than the hardness of the first depth region and the hardness of the third depth region, It is believed that the second depth region was able to sufficiently absorb the strain introduced into the steel plate due to deformation during spot welding, and that an excessive increase in strain in the first depth region could be suppressed. be done. As a result, it was possible to sufficiently suppress the occurrence of LME cracks in all the invention examples. In particular, in Examples 4 to 7 and 10 of the present invention produced by production methods 1 to 3, the area ratio of the η phase in the corona bond on the lap surface and in the weld shoulder on the electrode-side surface was 20 area% or less. Therefore, no LME cracking was observed, and very high LME cracking resistance could be achieved. In addition, in Invention Example 6 in which the cooling time was provided in the manufacturing method among Invention Examples 3 to 10, the nugget had a double structure, whereas in all other Invention Examples, the nugget was single. had a structure.

[Example 2]
[Manufacturing of resistance spot welded joints: set of 2 or 3 sheets of different types]
In this example, two-piece or three-piece resistance spot-welded joints were manufactured according to the combinations shown in Table 2 using the high-strength steel sheet 2 of Example 1 and the low-strength steel sheet having a tensile strength of 603 MPa. Details of the manufacturing conditions are as shown in Example 1 and Table 2. Table 2 also shows the presence or absence of zinc-based plating (alloyed hot-dip galvanizing) on each steel sheet. Regarding the evaluation of LME cracking resistance, as in Example 1, the case where the number of cracks is 3 or less in 10 tests is passed, and the resistance spot welded joint that can suppress the occurrence of LME cracks during spot welding. evaluated. Table 2 shows the results.

Referring to Table 2, in all of Inventive Examples 11 to 13, the hardness of the second depth region is lower than the hardness of the first depth region and the hardness of the third depth region. By using No. 2, it was possible to sufficiently suppress the occurrence of LME cracking.

REFERENCE SIGNS LIST 1 resistance spot welded joint 11 high strength steel plate 11′ steel plate 12 zinc-based plating 13 nugget 14 corona bond 15 lap surface 16 heat affected zone 17 weld 18 weld shoulder A electrode

Claims (9)

a plurality of superimposed steel plates;
A resistance spot welded joint comprising a nugget joining the steel plates and a weld having a corona bond and a heat affected zone formed around the nugget,
At least one of the plurality of steel plates is a high-strength steel plate having a tensile strength of 980 MPa or more,
The high-strength steel sheet has zinc-based plating, or a steel sheet adjacent to the high-strength steel sheet has zinc-based plating,
On the base material surface where the base material of the high-strength steel sheet is in contact with the zinc-based plating,
The hardness in a depth region of 10 to 60 μm from the base material surface is lower than the hardness in a depth region of 1 to 10 μm from the base material surface and the hardness of a depth region of 60 μm or more from the base material surface. A resistance spot weld joint characterized by: The surface of the base material of the high-strength steel sheet in contact with the zinc-based plating is located on the overlapping surface of the plurality of steel sheets,
Precipitates with a diameter of less than 0.1 μm are present in a depth region of 1 to 10 μm from the base material surface at a number density of 10 to 200/μm ² ,
2. The resistance spot welded joint according to claim 1, wherein the amount of dissolved C in a depth region of 10 to 60 μm from the base material surface is less than 0.20% by mass. The amount of the η phase of the zinc-based plating in the corona bond of the overlapping surfaces of the plurality of steel sheets where the base material surface of the high-strength steel sheet is in contact with the zinc-based plating is 20 area% or less. 3. A resistance spot welded joint according to claim 2, characterized in that. The base material surface of the high-strength steel sheet in contact with the zinc-based plating is located on the outermost surface of the plurality of superimposed steel sheets,
Precipitates with a diameter of less than 0.1 μm are present in a depth region of 1 to 10 μm from the base material surface at a number density of 10 to 200/μm ² ,
2. The resistance spot welded joint according to claim 1, wherein the amount of dissolved C in a depth region of 10 to 60 μm from the base material surface is less than 0.20% by mass. The amount of η phase of the zinc-based plating in the welded shoulder portion of the outermost surface of the plurality of steel sheets where the base material surface of the high-strength steel sheet is in contact with the zinc-based plating is 20 area% or less. 5. A resistance spot welded joint according to claim 4, characterized in that: A step of pressing a plurality of steel plates that are superimposed using a pair of electrodes facing each other;
forming a nugget and a corona bond by energizing between the electrodes while pressing the plurality of steel plates;
The step of continuously reducing the current value between the electrodes to 0 while maintaining the pressure on the plurality of steel plates, according to any one of claims 1 to 5. A method of manufacturing a resistance spot welded joint. A step of pressing a plurality of steel plates that are superimposed using a pair of electrodes facing each other;
forming a nugget and a corona bond by energizing between the electrodes while pressing the plurality of steel plates;
reducing the current value between the electrodes to 0 while maintaining the pressure on the plurality of steel plates, wherein the current value between the electrodes at the time when the nugget formation is completed is defined as I; When 1/2 of the total plate thickness in the unit mm of the steel plate is defined as tm,
When the current value between the electrodes is reduced to 0 or less than 0.04 seconds after the current value is reduced to 0, the current value between the electrodes is reduced from 0.9 × I to 0.3 × I The method for manufacturing a resistance spot welded joint according to any one of claims 1 to 5, characterized in that it is held at a constant value of 0.12 × tm or more in units of seconds within the range of up to. A step of pressing a plurality of steel plates that are superimposed using a pair of electrodes facing each other;
A step of preheating the steel plates by pre-energizing between the electrodes while pressurizing the plurality of steel plates;
forming a nugget and a corona bond by energizing between the electrodes while pressing the plurality of steel plates;
A method for manufacturing a resistance spot welded joint according to any one of claims 1 to 5, characterized in that it comprises A step of pressing a plurality of steel plates that are superimposed using a pair of electrodes facing each other;
forming a nugget and a corona bond by energizing between the electrodes while pressing the plurality of steel plates;
reducing the current value between the electrodes to 0 while maintaining pressure on the plurality of steel plates;
and holding the pressure at 0.8×P or more for 0.2 sec or more and 0.4 sec or less with the current value between the electrodes set to 0, where P is the completion of the formation of the nugget. The method for manufacturing a resistance spot welded joint according to any one of claims 1 to 5, characterized in that the pressure is applied at the time when the pressure is applied.

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