OVERLAP-WELDED MEMBER, AUTOMOBILE PART, METHOD OF WELDING OVERLAPPED PORTION, AND METHOD OF MANUFACTURING OVERLAP-WELDED MEMBER

Abstract

The present invention provides an overlap-welded member in which an overlapped portion including plural steel sheet members is joined at a spot-welded portion, in which at least one of the plural steel sheet members contains martensite, and the spot-welded portion includes: a nugget formed through spot welding; a heat-affected zone formed in the vicinity of the nugget; the softest zone having the lowest Vickers hardness in the heat-affected zone; and a tempered area formed between a central portion of the nugget and the softest zone and made out of tempered martensite having Vickers hardness of not more than 120% in the case where Vickers hardness of the softest zone is 100%.

Claims

1. An overlap-welded member in which an overlapped portion including a plurality of steel sheet members is joined at a spot-welded portion, wherein at least one of the plurality of steel sheet members contains martensite; and the spot-welded portion includes: a nugget formed through spot welding; a heat-affected zone formed in the vicinity of the nugget; a softest zone having the lowest Vickers hardness in the heat-affected zone; and a tempered area formed between a central portion of the nugget and the softest zone and made out of tempered martensite having Vickers hardness of not more than 120% in a case where Vickers hardness of the softest zone is 100%.

2. The overlap-welded member according to claim 1, wherein the plurality of steel sheet members include a hot-stamped member.

3. An automobile part including the overlap-welded member according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0075] FIG. 1 is a diagram schematically illustrating distribution of hardness in a spot-welded portion in the case where conventional energization conditions are applied to a TRIP member.

[0076] FIG. 2A is a perspective view schematically illustrating a cross-tension test.

[0077] FIG. 2B is a diagram illustrating a fracture mode concerning a spot-welded portion with a cross-tension test, and is a sectional view illustrating a partial plug fracture.

[0078] FIG. 2C is a diagram illustrating a fracture mode concerning a spot-welded portion with a cross-section test, and is a sectional view illustrating a plug fracture.

[0079] FIG. 2D is a diagram illustrating an example of a correlation between tensile strength of a base metal and a cross-tension strength.

[0080] FIG. 3 is a diagram schematically illustrating a subsequent energization method.

[0081] FIG. 4 is a diagram schematically illustrating distribution of hardness in a spot-welded portion after the spot-welded portion is formed by overlapping test pieces according to a subsequent energization method illustrated in FIG. 3, and applying subsequent energization.

[0082] FIG. 5 is a diagram schematically illustrating changes of a HAZ in a spot-welded portion in the case where the spot-welded portion formed on a hot-stamped member is subjected to subsequent energization.

[0083] FIG. 6 is a diagram illustrating a schematic configuration of a spot-welded portion including a nugget according to an embodiment of the present invention.

[0084] FIG. 7 is a diagram illustrating a schematic configuration of a nugget and a HAZ in the same spot-welded portion.

[0085] FIG. 8 is a diagram explaining energization conditions in resistance spot welding according to an embodiment of the present invention.

[0086] FIG. 9A is a diagram schematically illustrating portions of a spot-welded portion according to an embodiment of the present invention where hardness is measured.

[0087] FIG. 9B is a graph showing a relationship between distances (mm) from a melting boundary of a nugget and Vickers hardness.

[0088] FIG. 10A is a diagram illustrating an analysis model under a short-time energization condition with a distance from a nugget end to the softest zone in HAZ being set to 0.75 mm.

[0089] FIG. 10B is a diagram illustrating an analysis model under a normal condition with a distance from a nugget end to the softest zone in HAZ being set to 1.5 mm.

[0090] FIG. 11 is a graph related to an analysis model of each spot-welded portion under (a) short-time energization condition, (b) normal condition, and (c) no HAZ softening, and illustrating equivalent plastic strain at Position 1 illustrated in FIG. 10A.

[0091] FIG. 12 is a graph related to an analysis model of each spot-welded portion under (a) short-time energization condition, (b) normal condition, and (c) no HAZ softening, and illustrating equivalent plastic strain at Position 2 illustrated in FIG. 10A.

[0092] FIG. 13A is a diagram illustrating a relationship between a thickness t and a distance D from a melting boundary of a nugget to a HAZ softening zone.

[0093] FIG. 13B is a diagram illustrating a relationship between cross-tension strength and distances D from the melting boundary of a nugget to a HAZ softening zone.

[0094] FIG. 14 is a graph showing behavior of nugget growth in the case where a short-time energization condition, a normal condition, and a two-step energization condition are applied to a hot-stamped member of an 1800 MPa class having a thickness of 1.6 mm.

[0095] FIG. 15 is a graph showing distribution of hardness in a spot-welded portion formed under the conditions shown in FIG. 14.

[0096] FIG. 16 is a diagram schematically illustrating changes of HAZ in spot-welded portions after single energization and after subsequent energization in the case where a short-time energization condition according to an embodiment of the present invention is applied to a hot-stamped member.

[0097] FIG. 17 is a diagram schematically illustrating changes in Vickers hardness in spot-welded portions after single energization and after subsequent energization in the case where a short-time energization condition according to an embodiment of the present invention is applied to a hot-stamped member.

[0098] FIG. 18 is a graph showing distribution of hardness in spot-welded portions after single energization in the case where a short-time energization condition according to an embodiment of the present invention and a normal energization condition are applied to a hot-stamped member of an 1800 MPa class having a thickness of 1.8 mm.

[0099] FIG. 19 is a graph showing distribution of hardness in spot-welded portions after subsequent energization in the case where a short-time energization condition according to an embodiment of the present invention and a normal energization condition are applied to a hot-stamped member of an 1800 MPa having a thickness of 1.8 mm.

[0100] FIG. 20 is a diagram schematically illustrating changes of distribution of hardness in a spot-welded portion obtained by forming the spot-welded portion under a short-time energization condition according to an embodiment of the present invention and a normal energization condition, and then, applying subsequent energization.

[0101] FIG. 21 is a perspective view illustrating an L-shaped test.

EMBODIMENTS OF THE INVENTION

[0102] The present inventors carried out thorough investigations on improving the peel strength in the case where plural steel sheet members including at least one steel sheet member containing martensite are joined at a spot-welded portion in an overlapped portion. As a result, it was found that, by applying single energization (short-time single energization) under a short-time energization condition in which an electric current value is increased and an energization period of time is shorter than conventional one, the HAZ hardened portion is reduced, and a distance between a nugget end and the softest zone in HAZ is reduced.

[0103] Furthermore, it was also found that, with the reduction in the distance between the nugget end and the softest zone in HAZ, the stress acting at the time of load applied on a nugget end portion area in the peeling direction is alleviated, and the peel strength largely improves.

[0104] On the basis of the findings described above, in place of the subsequent energization with the conventional type that needs longer time, the present inventors developed a method that can improve the strength with single energization.

[0105] Furthermore, it was also found that, by reducing the distance between the nugget end and the softest zone in HAZ, and applying the subsequent energization, the nugget and the HAZ hardened portion are tempered, whereby it is possible to suppress hard portions being locally formed between the nugget end and the softest zone in HAZ. Thus, the peel strength of the spot-welded portion is improved as compared with the subsequent energization with the conventional type.

[0106] Below, the present invention made on the basis of the findings described above will be described in detail with reference to the drawings.

[0107] FIG. 6 is a sectional view illustrating a spot-welded portion, which illustrates the schematic configuration of a spot-welded portion 10 formed in an overlap-welded member used, for example, as an automobile part, according to an embodiment of the present invention.

[0108] The overlap-welded member according to this embodiment is formed by joining steel sheet members S1 and S2 through a spot-welded portion 10 as illustrated in FIG. 6.

[0109] As illustrated in FIG. 6, a nugget 12 is formed in an overlapped portion of the steel sheet members S1 and S2 through energization applied from a pair of electrodes 50, 50, which are used for resistance spot welding and to squeeze the steel sheet members S1 and S2 in the thickness direction between the pair of electrodes with the central line CL of electrodes 50 being the center.

[0110] As for molten metal generated through the energization, solidification grows in an area in the vicinity of the central line CL and in contact with the electrodes 50 toward the thickness direction due to heat dissipated to the electrodes 50, whereas, in an area distant from the central line CL of the electrodes 50, solidification grows toward the central direction of the nugget (toward the central line CL of the electrodes) in addition to toward the thickness direction.

[0111] As a result, the nugget 12 includes an area 12A where dendrite grows in the thickness direction, and an area 12B where dendrite grows so as to intersect the thickness direction.

[0112] In this specification, when the overlapped portion is viewed from the thickness direction, the nugget end 12E represents the outermost boundary (in other words, melting boundary portion of the nugget 12) that melts when the nugget 12 is formed, and the nugget end portion area 12B represents an area from a meeting portion 12C between the area 12A and the area 12B to the nugget end 12E.

[0113] FIG. 7 is a sectional view illustrating a spot-welded portion, which illustrates the spot-welded portion 10 where the overlapped portion is welded. The spot-welded portion 10 includes the nugget 12 formed through spot welding, and the HAZ 14 formed in the vicinity of this nugget 12 through spot welding.

[0114] The HAZ 14 includes a HAZ hardened portion 14H formed next to the nugget 12, and a HAZ softening zone 14T formed around the HAZ hardened portion 14H.

[0115] Furthermore, the softest zone 14L in HAZ having the lowest Vickers hardness is formed in the vicinity of the inner peripheral portion in the HAZ softening zone 14T.

[0116] The reference character D illustrated in FIG. 7 represents a distance between the nugget end 12E and the softest zone 14L in HAZ.

[0117] FIG. 8 is a diagram illustrating energization conditions in resistance spot welding according to this embodiment.

[0118] As illustrated in FIG. 8, in the case of a short-time energization condition C11 according to this embodiment, resistance spot welding is performed by first applying single energization in which an energization electric current I11, which is higher than an energization electric current I21 under a normal energization condition C21, is applied for an energization time T11, which is shorter than a conventional normal energization time T21.

[0119] The broken line in FIG. 8 indicates the first energization C21 (an electric current value I21 and an energization time T21) under a normal condition, where the electric current value I11>the electric current value I21, and the energization time T11 (cyc)<the energization time T21 (cyc).

[0120] Furthermore, in FIG. 8, the reason that the short-time energization condition C11 is illustrated from the intermediate stage in the normal energization condition C21 on the time axis is to match the completion times of energization.

[0121] In the short-time energization condition C11 according to this embodiment, as illustrated in FIG. 8, the molten metal generated at the time of forming the nugget 12 through energization is rapidly cooled after the single energization is completed, and temperatures thereof pass through the Ms point and are decreased to the Mf point or lower, so that martensite is formed.

[0122] Furthermore, by comparing a temperature curve H1 of the nugget 12 under the short-time energization condition C11 with a temperature curve H2 of the nugget under the normal energization condition C21, the joined portion is melted, and the nugget 12 is formed under the short-time energization condition C11 in a shorter period of time than those under the normal energization condition C21.

[0123] Thus, with the short-time energization condition C11, the excessive heat flow to the vicinity of the nugget 12 is suppressed, the size of the HAZ hardened portion is reduced, and the distance D between the nugget end 12E and the softest zone 14L in HAZ is reduced.

[0124] As a result, only with the single energization described above are the strains at the time of peeling concentrated on portions other than the nugget end portion area 12B, and the stress concentrated on the nugget end portion area 12B can be alleviated, whereby the peel strength improves.

[0125] It should be noted that the spot-welded portion 10 formed with single energization under the short-time energization condition C11 may be used as it is without applying additional processing. Furthermore, after a predetermined suspension period of time Ts elapses, it may be possible to apply subsequent energization (in other words, the second energization) under a subsequent energization condition C12 to the spot-welded portion 10 thus formed.

[0126] By applying the energization under the subsequent energization condition C12 (an electric current value 112 and an energization time T12) to the spot-welded portion 10 formed under the short-time energization condition C11 after energization is suspended for the suspension period of time Ts, the nugget 12 is heated to temperatures not less than a temperature (approximately 550 to 600 C.) at which tempering is possible and not more than Ac.sub.1, and then, is gradually cooled, whereby tempered martensite can be obtained without re-quenching the HAZ 14.

[0127] As described above, in the case of the short-time energization condition C11, the electric current value I11 is set so as to be larger than the electric current value I21 under the normal energization condition C21, and the energization time T11 is set so as to be shorter than the energization time T21 under the normal energization condition C21. Thus, temperatures of the nugget 12 are raised in a short period of time, and transference of the heat generated through energization to the vicinity thereof is not developed, whereby the HAZ 14 is less likely to become high temperature as compared with the normal condition.

[0128] As a result, it can be considered that the width of the HAZ hardened portion 14H is narrow, and the distance D between the nugget end 12E and the softest zone 14L in HAZ is reduced.

[0129] As described above, by forming the nugget 12 under the short-time energization condition C11, the width of the HAZ hardened portion 14H can be made narrow, whereby the nugget 12 and the HAZ hardened portion 14H are sufficiently tempered.

[0130] Thus, it is possible to prevent high Vickers hardness portions from being formed between the nugget end 12E and the softest zone 14L in HAZ.

[0131] More specifically, since the HAZ hardened portion 14H is softened in a uniform manner, deformation becomes easy, and stress acting on the nugget end portion area 12B at the time of peeling is reduced, whereby it is possible to improve the peel strength.

[0132] As described above, by employing the short-time energization condition C11 and the subsequent energization condition C12, Vickers hardness between the softest zone 14L in HAZ and the nugget end 12E can be made to 120% or lower in the case where Vickers hardness in the softest zone 14L in HAZ is 100%, whereby toughness of the spot-welded portion 10 can be sufficiently secured.

[0133] Below, a relationship between distances (mm) from the melting boundary of the nugget 12 and Vickers hardness will be described with reference to FIG. 9A and FIG. 9B.

[0134] FIG. 9A and FIG. 9B are diagrams each illustrating a case where a first energization is applied to a hot-stamped member of a 1500 MPa class having a thickness of 1.6 mm under a (a) short-time energization condition and a (b) normal condition according to this embodiment. FIG. 9A is a sectional view illustrating a spot-welded portion, and FIG. 9B is a graph showing distribution of hardness in the spot-welded portion 10.

[0135] As for measurement of distribution of hardness, as illustrated in FIG. 9A, measurement is performed at a position located at a distance of of the thickness from the joining surface of the steel sheet members S1 and S2 toward the steel sheet member S1 and the inner side of the steel sheet member S1 by applying a load of 9.8 N at 0.5 mm pitches according to JIS Z 2244.

[0136] In the graph shown in FIG. 9B, the blank diamonds represent the short-time energization condition, and the blank circles represent the normal energization condition.

[0137] It should be noted that the energization time in the short-time energization condition is set to 9cyc, the energization time in the normal condition is set to 20cyc, and the electric current value is adjusted such that the nugget diameter is 4t (mm) (t represents a thickness).

[0138] From FIG. 9A and FIG. 9B, it can be understood that the distance D from the nugget end 12E to the softest zone 14L in HAZ is reduced by applying the first energization under the (a) short-time energization condition.

[0139] Below, equivalent strain in the case where energization is performed under the short-time energization and the normal condition will be described with reference to FIG. 10A, FIG. 10B, FIG. 11, and FIG. 12.

[0140] Equivalent plastic strains in the case of the (a) short-time energization condition, the (b) normal condition, and the (c) no HAZ softening are obtained through elasto-plastic FEM analysis under the short-time energization and the normal condition. Detailed descriptions will be made below.

[0141] FIG. 10A is a sectional view illustrating a spot-welded portion, which illustrates an analysis model of a test piece for single energization obtained by applying the first energization under the (a) short-time energization condition with the distance D from the nugget end 12E to the softest zone 14L in HAZ being set to 0.75 mm.

[0142] FIG. 10B is a sectional view illustrating a spot-welded portion, which illustrates an analysis model of a test piece for single energization obtained by applying the first energization under the normal condition with the distance D from the nugget end 12E to the softest zone 14L in HAZ being set to 1.5 mm.

[0143] It should be noted that, for the analysis models, the distribution of hardness in the HAZ softening zone is varied in a stepwise manner from the hardness of the softest zone to the hardness of the base metal portion on the basis of the measurement results shown in FIG. 9B.

[0144] In FIG. 10A and FIG. 10B, the Position 1 represents the softest zone 14L in HAZ, and the Position 2 represents the nugget end 12E.

[0145] For the analysis models, three patterns are used, which include a case where the (a) short-time energization condition in FIG. 10A is simulated, a case where the (b) normal condition in FIG. 10B is simulated, and a case where the (c) no HAZ softening is simulated.

[0146] FIG. 11 is a graph showing equivalent plastic strains at the Position 1 illustrated in FIG. 10A in the case where the analysis models formed by the spot-welded portions with the (a) short-time energization condition, the (b) normal condition, and the (c) no HAZ softening are subjected to cross-tension testing with a load with which the spot-welded portion in the case of the (b) normal condition is fractured through the cross-tension testing.

[0147] It should be noted that, in FIG. 11, the Position 1 in the analysis model with the (c) no HAZ softening is set to the same position as that with the (b) normal condition.

[0148] As shown in the graph in FIG. 11, the equivalent plastic strain at the Position 1 is approximately 0.032 with the (a) short-time energization condition, which significantly increases as compared with 0.013 with the (b) normal condition and approximately 0.018 with the (c) no HAZ softening.

[0149] FIG. 12 is a graph showing equivalent plastic strains at the Position 2 illustrated in FIG. 10A in the case where the analysis models formed by the spot-welded portions with the (a) short-time energization condition, the (b) normal condition, and the (c) no HAZ softening are subjected to cross-tension testing with a load with which the spot-welded portion in the case of the (b) normal condition is fractured through the cross-tension testing.

[0150] It should be noted that, in FIG. 12, the Position 2 in the analysis model with the (c) no HAZ softening is set to the same position as that with the (b) normal condition.

[0151] Furthermore, as shown in the graph in FIG. 12, the equivalent plastic strain at the Position 2 is approximately 0.010 with the (a) short-time energization condition, which decreases as compared with 0.0115 with the (b) normal condition and approximately 0.0118 with the (c) no HAZ softening.

[0152] However, at the position of HAZ softening with the (b) normal condition, the existence or absence of the HAZ softening has a limited effect on the equivalent plastic strain in the end portion area of the nugget, as compared with the (b) normal condition and the (c) no HAZ softening.

[0153] More specifically, in the case of the normal condition, the HAZ softening zone 14T provides little effect as to reducing the strains to the nugget end portion area 12B at the time of peeling, and since the HAZ softening zone 14T approaches the nugget end portion area 12B, the strains concentrate on the HAZ softening zone 14T. As a result, it was found that the strains concentrated on the nugget end portion area 12B can be reduced. In other words, with this effect, the peel strength can be increased by using the short-time energization condition.

[0154] Below, conditions appropriate for the overlap-welded member obtained by joining, at the spot-welded portion, overlapped portions including plural steel sheet members, according to this embodiment will be described with reference to FIG. 13A and FIG. 13B.

[0155] FIG. 13A is a diagram illustrating a relationship between the thickness t (mm) of the overlapped portion and a distance D (mm) from a melting boundary (end of the nugget) of the nugget to the softest zone in HAZ.

[0156] In FIG. 13A, the blank circles represent a DP steel of a 980 MPa class with a conventional single energization.

[0157] Furthermore, the blank diamonds represent a hot-stamp steel of a 1500 MPa class with a conventional single energization.

[0158] Here, as for the thickness t (mm) in FIG. 13A, in the case where there is only one steel sheet member having the highest tensile strength of plural steel sheet members, t (mm) is the thickness of this steel sheet member, and in the case where there are plural steel sheet members having the highest tensile strength, t (mm) is the thickness of a steel sheet member having the thinnest thickness of all the steel sheet members.

[0159] As illustrated in FIG. 13A, in the case of the single energization with a conventional condition, the distance D between the nugget end 12E and the softest zone 14L in HAZ on the overlapping interface between two steel sheets is formed so as to fall in a range exceeding D (mm)=t.sup.0.2 (mm). At the time of the cross-tension test, in the case of a sheet set obtained by combining a steel type having low joint strength and a steel type having high joint strength, fracture tends to occur on the side of the low joint strength.

[0160] For example, in the case where the strength of the base metal is higher than a 780 MPa class, the cross-tension strength decreases with an increase in the strength of the base metal, and hence, fracture is more likely to occur as the strength of the base metal increases.

[0161] Furthermore, in the case of a sheet set having the same steel type but different thicknesses, fracture occurs on the side of the steel sheet having the thinner thickness.

[0162] For the reasons described above, the thickness t of a steel sheet member having the thinnest thickness is employed.

[0163] FIG. 13B is a diagram illustrating a relationship between the cross-tension strength and the distance D from the nugget end 12E of the spot-welded portion to the HAZ softening zone in the case where the nugget diameter in a hot-stamped member of a 1500 MPa class is 4t.

[0164] As illustrated in FIG. 13B, by setting the distance D (mm) between the nugget end 12E and the softest zone 14L in HAZ to t.sup.0.2 (mm) or shorter, the cross-tension strength increases to approximately 7 kN and is made stable, which makes it possible to make the fracture mode to be the plug fracture. Furthermore, by setting the distance D (mm) between the nugget end 12E and the softest zone 14L in HAZ to 0.75(t.sup.0.2) (mm) or shorter, the cross-tension strength increases to approximately 8 kN, and is made further stable, which makes the fracture mode to be the plug fracture. This is more favorable.

[0165] As described above, by reducing the distance D from the nugget end 12E of the spot-welded portion to the HAZ softening zone 14T, the cross-tension strength improves.

[0166] Furthermore, as for the hardness from the base metal toward the nugget end portion area 12B (including the nugget end portion area 12B) on the overlapping interface between these two steel sheet members, the hardness gradually decreases toward the nugget end 12E in a range where the maximum value of Vickers hardness relative to the softest zone 14L in HAZ is approximately 115%, or the hardness is equivalent to the hardness of the softest zone 14L in HAZ.

[0167] According to the overlap-welded member of this embodiment, by bringing the HAZ softening zone 14T close to the nugget end portion area 12B, the stress concentration on the nugget end portion area 12B serving as the starting point of the fracture within the nugget (interface fracture, partial plug fracture) is alleviated, whereby it is possible to improve the joint strength.

[0168] The effect of improving the joint strength becomes more apparent as the fracture mode changes from the fracture within the nugget (the interface fracture and the partial plug fracture) to the plug fracture.

[0169] In particular, as for a joint for which the plug fracture cannot be obtained because the toughness of the nugget 12 itself is not sufficient and the crack propagates into the nugget even if the stress concentration on the nugget end portion area 12B is alleviated, by applying subsequent energization to this joint in addition to optimization of the HAZ softening zone 14T, it is possible to obtain the effect of improving the joint strength stronger than the conventional one.

[0170] This mechanism has already been described above.

[0171] As described above, if the distance D (mm) from the nugget end 12E to the softest zone 14L in HAZ satisfies

Dt.sup.0.2Equation (1),

it is possible to sufficiently improve the joint strength.

[0172] Thus, with the overlap-welded member according to this embodiment, a condition is set such that the distance D from the nugget end 12E to the softest zone 14L in HAZ satisfies Equation (1) described above.

[0173] Furthermore, by making the distance D (mm) from the nugget end 12E to the softest zone in HAZ satisfy

D0.75(t.sup.0.2)Equation (1A),

the fracture mode can be more reliably made to be the plug fracture, which is preferable.

[0174] Below, a method of welding the overlap-welded portion using a resistance spot welding process and a tempering process will be described in detail.

[0175] (Resistance Spot Welding Process)

[0176] In the resistance spot welding process, a spot-welded portion 10 including a nugget 12, a HAZ 14 formed around this nugget 12, and the softest zone 14L having the lowest Vickers hardness in this HAZ 14 is formed through resistance spot welding at an overlapped portion formed by plural steel sheet members.

[0177] (Tempering Process)

[0178] In the tempering process, a tempered area made out of tempered martensite having the Vickers hardness of 120% or lower in the case where the Vickers hardness of the softest zone 14L is 100% is formed between the central portion of the nugget 12 formed through the resistance spot welding process and the softest zone 14L.

[0179] It is preferable to apply subsequent energization to form the tempered area. However, this formation is not limited to through the subsequent energization. It may be possible to use, for example, emission of laser beam to form the tempered area.

[0180] With the method of welding an overlapped portion according to this embodiment as described above, the tempered area having the Vickers hardness of 120% or lower in the case where the Vickers hardness of the softest zone 14L is 100% is formed between the central portion of the nugget 12 and the softest zone 14L.

[0181] Furthermore, in the resistance spot welding process described above, the nugget 12 may be formed with an energization time T expressed in the following manner, where t (mm) is the thickness, and cyc (second) is a period of time for one cycle of energization in the resistance spot welding.

5tcycT(5t+4)cycEquation (2)

[0182] In general, in the spot welding, with an increase in the thickness, the energization time increases, and the distance D from the nugget end 12E to the softest zone 14L in HAZ tends to increase. However, with the satisfaction of this Equation (2), the nugget can be stably formed, and the distance D (mm) between the nugget end 12E and the softest zone 14L in HAZ can be more reliably formed so as to be not more than t.sup.0.2.

[0183] In other words, it is possible to stably improve the peel strength at the spot-welded portion.

[0184] It should be noted that, as for the thickness t (mm), in the case where there is only one steel sheet member having the highest tensile strength of plural steel sheet members, t (mm) is the thickness of this steel sheet member, and in the case where there are plural steel sheet members having the highest tensile strength, t (mm) is the thickness of a steel sheet member having the thinnest thickness of all the steel sheet members.

[0185] (Preheat Energization Process)

[0186] As described above, by applying spot welding while satisfying the energization time specified in this embodiment, this spot welding is effective from the viewpoint of the HAZ softening zone 14T. On the other hand, the appropriate electric current range reduces as compared with the conventional energization condition.

[0187] In this respect, the present inventors found that it is preferable to perform a preheat energization process before the resistance spot welding process described above is performed, in terms of being able to bring the softest zone in HAZ closer to the end portion area of the nugget as compared with the conventional technique while maintaining the appropriate electric current range (margin of electric current to the splash occurring current) equivalent to the conventional condition.

[0188] Here, the above-described effect obtained by performing the preheat energization process will be described with reference to FIG. 14 and FIG. 15.

[0189] FIG. 14 is a graph showing behavior of nugget growth in the case where the short-time energization condition (9cyc), the normal condition (20cyc), and a two-step energization condition (energization time in the first step: 11cyc, welding electric current: 4 kA, and energization time in the second step: 9cyc) are applied to a hot-stamped member of an 1800 MPa class having a thickness of 1.6 mm.

[0190] Furthermore, FIG. 15 is a graph showing distribution of Vickers hardness based on distances from the nugget end of the spot-welded portion formed under the conditions shown in FIG. 14.

[0191] As shown in FIG. 14 and FIG. 15, by applying two-step energization including the preheat energization and the main energization, it is possible to bring the position of HAZ softening closer to the nugget end portion area 12B as compared with the conventional technique while maintaining the appropriate electric current range almost equivalent to that of the conventional technique.

[0192] Below, energization conditions for the preheat energization process will be described in detail.

[0193] In the preheat energization process, energization with preheat electric current I (kA) is applied to the overlapped portion in a manner such that energization time T.sub.1 (second), a period of time cyc (second) for one cycle of energization, and a thickness t (mm) satisfy

5tcycT.sub.1(5t+8)cyc(Equation 3).

[0194] Then, in the case where the preheat energization process is performed, the nugget is formed by, after the preheat energization process, applying energization with welding electric current I.sub.0 (kA), which is less than or equal to the splash occurring current, to the overlapped portion so as to satisfy

5tcycT.sub.2(5t+4)cycEquation (4),

where T.sub.2 (second) is an energization time and cyc (second) is a period of time for one cycle of energization in the resistance spot welding.

[0195] Here, a relationship between the preheat electric current I (kA) and the welding electric current I.sub.0 (kA) satisfies

0.3I.sub.0I0.7I.sub.0Equation (5).

[0196] In the preheat energization process described above, the energization time T.sub.1 (second) is longer than or equal to 5tcyc, and the preheat electric current I (kA) is more than or equal to 0.3I.sub.0, in other words, is more than or equal to 30% of the welding electric current I.sub.0 in the resistance spot welding process for forming the nugget. Thus, the preheating effect is sufficient, and it is possible to secure a desired appropriate electric current range, which is preferable.

[0197] Furthermore, the energization time T.sub.1 (second) is less than or equal to (5t+4)cyc, and the preheat electric current I (kA) is less than or equal to 0.7I.sub.0, in other words, is less than or equal to 70% of the welding electric current I.sub.0 in the resistance spot welding process for forming the nugget. Thus, it is possible to reduce the distance D from the nugget end 12E to the softest zone 14L in HAZ, which is preferable.

[0198] Then, in the resistance spot welding process performed after the preheat energization, the energization time T.sub.2 is set to be not shorter than 5tcyc and not longer than (5t+4)cyc. Thus, it is possible to sufficiently form the nugget, and it is possible to make the distance D (mm) between the nugget end 12E and the softest zone 14L in HAZ to be not longer than t.sup.0.2. This makes it possible to stably improve the peel strength in the spot-welded portion.

[0199] Furthermore, by adjusting the energization time such that the D (mm) is shorter than or equal to 0.75(t.sup.0.2), it is possible to more reliably obtain the spot-welded portion whose fracture mode is the plug fracture, and it is possible to improve the peel strength.

[0200] By applying the tempering process described above (for example, tempering with the subsequent energization) to the thus obtained overlap-welded portion so that the nugget end portion area 12B is tempered, it is possible to form, between the central portion of the nugget 12 and the softest zone 14L, the tempered area formed by the tempered martensite having the Vickers hardness of 120% or less in the case where the Vickers hardness of the softest zone 14L is 100%.

[0201] Thus, it is possible to manufacture the overlap-welded member having the nugget diameter same as the conventional one, exhibiting excellent strength, and increased joint strength.

[0202] In order to obtain the effect as described above, it is necessary to adjust the short-time energization condition such that the nugget end 12E and the softest zone 14L in HAZ are brought closer to each other, and then, apply the tempering process to make the Vickers hardness of the tempered area to be not more than 120% of the Vickers hardness of the softest zone 14L. However, in order to obtain the effect in a more favorable manner, it is preferable to make the Vickers hardness of the tempered area to be not more than 115% of the Vickers hardness of the softest zone 14L, and it is more preferable to make the Vickers hardness of the tempered area to be not more than 110% of the Vickers hardness of the softest zone 14L.

[0203] It should be noted that the lower limit value of the Vickers hardness of the tempered area is not specified.

[0204] If the tensile strength of the base material is stronger than or equal to a 980 MPa class, the interface fracture or the partial plug fracture is more likely to occur in the overlap-welded member, and the joint strength tends to decrease.

[0205] This embodiment is effective to the steel sheet member having the HAZ softening made as a result of spot welding. However, it is preferable to apply this embodiment to a high-tensile steel sheet having the base metal with a tensile strength of a 980 MPa class or higher.

[0206] In particular, in the case of the hot-stamped member, the base metal is full martensite. Thus, the amount of softening of HAZ is large, and the effect obtained by this embodiment is significant.

[0207] Furthermore, as for the overlap-welded member according to this embodiment, there is no limitation on the thickness of, the type (for example, DP, TRIP, and so on) of, and the existence or absence of plating of each steel sheet member in the overlapped portion formed by two or more steel sheet members.

[0208] Furthermore, in Example described later, although description will be made of a sheet set obtained by overlapping two steel sheets with the same type, application is not limited to this sheet set. The effect can be obtained in the case of a sheet set with different materials, or a sheet set with three or more sheets.

[0209] FIG. 16 is a diagram illustrating a schematic configuration of the spot-welded portion 10 formed with the energization condition according to this embodiment in the case where hot-stamped members are used as the steel sheet members S1 and S2. More specifically, (a) in FIG. 16 is a sectional view illustrating a spot-welded portion after a short-time energization is applied, and (b) in FIG. 16 is a sectional view illustrating a spot-welded portion after a subsequent energization is applied. (c) in FIG. 16 is a graph showing distribution of Vickers hardness after the single energization and after the subsequent energization.

[0210] Furthermore, FIG. 17 is a diagram schematically illustrating changes in Vickers hardness in the spot-welded portion after the single energization and after the subsequent energization.

[0211] By applying the short-time energization, the distance D between the nugget end 12E and the softest zone 14L in HAZ is reduced to approximately 1 mm as illustrated in (a) in FIG. 16.

[0212] Since the distance D between the nugget end 12E and the softest zone 14L in HAZ through energization under a conventional normal condition is approximately 1.5 mm, the distance is significantly reduced as compared with the conventional one.

[0213] As a result, it is possible to alleviate the stress concentration in the vicinity of the nugget end 12E.

[0214] Moreover, by further applying the subsequent energization, it is possible to sufficiently temper the HAZ hardened portion 14H in the dot-hatched area illustrated in (b) in FIG. 16.

[0215] As described above, in the case where the spot-welded portion 10 is formed by applying the energization condition according to the present invention to the hot-stamped members S1 and S2, the Vickers hardness of the HAZ hardened portion 14H is almost equal to that of the nugget 12 by applying the short-time single energization as illustrated in (a) and (b) in FIG. 17.

[0216] Furthermore, by applying the subsequent energization, the nugget 12 and the HAZ hardened portion 14H are sufficiently tempered as illustrated in (c) in FIG. 17, and the hardness in terms of Vickers hardness between the softest zone 14L in HAZ and the nugget end 12E is similar to that of the softest zone 14L in HAZ, or the maximum value of the hardness thereof is approximately 115% of the softest zone 14L in HAZ. Thus, the stress in the nugget end portion area 12B is sufficiently alleviated.

[0217] As a result, it is possible to improve the peel strength of the spot-welded portion 10 of the hot-stamped members. Note that the entire nugget 12 does not necessarily have to be tempered, provided that the nugget end portion area 12B is tempered.

[0218] As described above, by tempering the HAZ hardened portion 14H, the maximum hardness in terms of Vickers hardness between the nugget 12 and the softest zone 14L in HAZ is made fall in a range of less than or equal to approximately 120% of the hardness of the softest zone 14L in HAZ as illustrated in (c) in FIG. 16.

[0219] As a result, the toughness of the nugget 12 and the HAZ hardened portion is improved, whereby it is possible to improve the peel strength.

[0220] It should be noted that, even if the entire nugget 12 is not tempered, the joint strength improves, provided that the maximum Vickers hardness between the nugget end 12E and the softest zone 14L in HAZ is less than or equal to 120% of the Vickers hardness of the softest zone 14L in HAZ, preferably is less than or equal to 115%, more preferably is less than or equal to 110%.

[0221] It should be noted that it is preferable that: the short-time energization condition be adjusted; the nugget end and the softest zone in HAZ be brought closer to each other; and the maximum value in terms of Vickers hardness between the central portion of the nugget 12 and the softest zone 14L in HAZ after the subsequent energization is applied be made to be 115% relative to the softest zone 14L in HAZ.

[0222] Furthermore, it is more preferable that: the short-time energization condition be adjusted; the nugget end and the softest zone in HAZ be brought close to each other; and the maximum value in terms of Vickers hardness between the nugget and the softest zone in HAZ after the subsequent energization is applied be made to be 110% relative to the softest zone 14L in HAZ.

[0223] FIG. 18 is a graph showing distribution of hardness in the spot-welded portion through single energization in the case where the short-time energization condition according to this embodiment and a normal energization condition are applied to a hot-stamped member of an 1800 MPa class having a thickness of 1.8 mm.

[0224] Furthermore, FIG. 19 is a graph showing distribution of hardness in the spot-welded portion after the subsequent energization is applied in the case where the short-time energization condition according to this embodiment and a normal energization condition are applied to a hot-stamped member of an 1800 MPa class having a thickness of 1.8 mm.

[0225] In FIG. 18, the blank diamonds represent the distribution of hardness in a spot-welded portion in the case where the spot-welded portion is formed with a main energization employing a short-time energization condition with an energization time being 9cyc (second). Furthermore, the blank circles represent the distribution of hardness in a spot-welded portion in the case where the spot-welded portion is formed with a main energization employing a normal condition with an energization time being 22cyc (second).

[0226] In FIG. 19, the blank diamonds represent the distribution of hardness in a spot-welded portion in the case where the spot-welded portion is formed with a main energization employing a short-time energization condition with an energization time being 9cyc (second), and then, tempering is performed through a subsequent energization, and the blank circles represent the distribution of hardness in a spot-welded portion in the case where the spot-welded portion is formed with a main energization employing a normal condition with an energization time being 22cyc (second), and then, tempering is performed through a subsequent energization.

[0227] First, as shown in the graph in FIG. 18, with the short-time energization condition indicated by the blank diamonds, the distance from the nugget end to the softest zone in HAZ is reduced, as compared with that of the normal condition indicated by the blank circles.

[0228] As shown in the graph in FIG. 19, in the case where the subsequent energization is applied, in the spot-welded portion formed by applying the normal condition at the time of the main energization, hard portions exist between the nugget and the softest zone (at a position approximately 1 mm from the end portion area of the nugget) because the softest zone in HAZ formed at the time of the main energization is far from the nugget end portion.

[0229] On the other hand, in the spot-welded portion formed by applying the short-time energization condition at the time of the main energization, the softest zone in HAZ formed at the time of the main energization is brought close to the end portion area of the nugget, and hence, it is possible to make the Vickers hardness of the nugget and the HAZ extending from the base metal to the nugget end portion area 12B (including the nugget end portion area 12B) to be 120% or lower of the hardness of the softest zone in HAZ.

[0230] In other words, in the case where the subsequent energization is applied, the strain concentration in the vicinity of the nugget end 12E can be alleviated if the distance from the nugget end 12E to the softest zone in HAZ formed at the time of the main energization is reduced.

[0231] As described above, reducing the energization time for forming the nugget 12 is effective from the viewpoint of bringing the position of the HAZ softening closer to the end portion area of the nugget to improve the joint strength.

[0232] FIG. 20 is a diagram schematically illustrating changes in HAZ of the spot-welded portion after the single energization and the subsequent energization in the case where the short-time energization condition according to this embodiment is applied to a DP member or a TRIP member.

[0233] (a) in FIG. 20 is a sectional view illustrating a spot-welded portion.

[0234] As illustrated in (b) in FIG. 20, in the case where the DP members or TRIP members are used as steel sheet members S1 and S2, the HAZ hardened portion 14H has the Vickers hardness almost equal to the nugget 12 in a state where the short-time single energization is applied.

[0235] In this case, in the nugget 12 and the HAZ hardened portion 14H, the distribution of hardness is different from the hot-stamped member illustrated in FIG. 17 in that they are significantly harder than the base material of the DP member or TRIP member.

[0236] Furthermore, as illustrated in (c) in FIG. 20, by applying the subsequent energization, the nugget 12 and the HAZ hardened portion 14H are sufficiently tempered, and the hardness in terms of Vickers hardness between the softest zone 14L in HAZ and the nugget end 12E becomes approximately 115% of the softest zone 14L in HAZ, whereby the stress in the nugget end portion area 12B is sufficiently alleviated.

[0237] As a result, it is possible to improve the peel strength in the spot-welded portion 10 in the case of the DP member or TRIP member.

[0238] It should be noted that the entire nugget 12 does not have to be necessarily tempered, provided that the nugget end portion area 12B is tempered.

[0239] It should be noted that the long dashed double-short dashed line illustrated in (c) in FIG. 20 indicates the distribution of hardness before the subsequent energization is applied.

[0240] It should be noted that, in the embodiment described above, descriptions have been made of a case where the HAZ hardened portion 14H is tempered through subsequent energization. However, the nugget and the HAZ hardened portion may be tempered, for example, by laser emission after the spot-welded portion 10 is formed through the short-time single energization.

EXAMPLES

[0241] Below, it is confirmed that bringing the softest zone in HAZ close to the nugget end portion is effective to improve the joint strength in the peeling direction.

[0242] By reducing the energization time, it is possible to bring the softest zone in HAZ close to the nugget end portion. However, from the viewpoint of formation of the nugget, if the energization time is reduced, the appropriate electric current range (in general, the electric current range from an electric current value with which the nugget diameter of 4t can be obtained, to occurrence of splash) becomes narrow.

[0243] Then, a study was made to achieve both controlling the position of HAZ softening and obtaining the appropriate electric current range, using a two-step energization in which steel sheets are heated through preliminary energization with an early-stage of energization being low electric current, and then, a nugget is expanded with a short-time high electric current.

[0244] Investigations were made on a hot-stamped member of an 1800 MPa class having a thickness of 1.6 mm under welding conditions shown in Table 1. Condition (1) corresponds to a short-time single energization condition, (2) corresponds to a conventional single energization condition, and (3) corresponds to a two-step energization condition.

[0245] As can be understood from the behavior of nugget formation illustrated in FIG. 14 and the distribution of hardness in the spot-welded portion illustrated in FIG. 15, with the two-step energization, it is possible to obtain an appropriate electric current range equivalent to that with the conventional single energization and bring the softest zone in HAZ close to the nugget end portion.

TABLE-US-00001 TABLE 1 First energization Second energization Compression force Time Electric current Time Electric current Retention time Electrode (kN) (cyc) (kA) (cyc) (kA) (cyc) Condition (1) Cu - 1% Cr 3.92 9 3.0-8.5 10 Condition (2) Dome type 20 3.0-8.5 Condition (3) Diameter of top 11 4.0 9 3.0-8.5 end: 6 mm

[0246] The welding electric current was adjusted so that the nugget diameter of 4t (mm) can be obtained. As for the subsequent energization condition, a condition effective in improving the peel strength, in other words, a condition with which the end portion area of a nugget can be softened was selected.

[0247] By using a steel sheet of a 980 MPa class, hot-stamped members of a 1500 MPa class, and hot-stamped members of an 1800 MPa class, each of which has a thickness in a range of 1.6 mm to 2.0 mm, the cross-tension strength and the L-shape tension strength were investigated. Table 2 shows welding conditions used. I.sub.0 represents an electric current value in the main energization process.

TABLE-US-00002 TABLE 2 Preliminary Subsequent energization energization process Main energization process Compression Electric process Suspension Electric Retaining force Time current Time time Time current time Condition Electrode (kN) (cyc) (kA) (cyc) (cyc) (cyc) (kA) (cyc) A Cu - 1% Cr 3.92 10t + 4 10 B Dome type 10t + 4 60 20-50 0.6I.sub.0-0.8I.sub.0 A1 Diameter of 5t A2 top end: 6 5t + 3 0.5I.sub.0-0.7I.sub.0 5t B1 mm 5t 60 20-50 0.6I.sub.0-0.8I.sub.0 B2 5t + 3 0.5I.sub.0-0.7I.sub.0 5t 60 20-50 0.6I.sub.0-0.8I.sub.0

[0248] Condition a corresponds to a conventional single energization condition, and condition b corresponds to a conventional subsequent energization condition. Condition A1 corresponds to a short-time single energization condition, and condition A2 corresponds to a two-step energization condition. As for conditions B1 and B2, subsequent energization was performed for the conditions A1 and A2, respectively. In each of the welding conditions, welding electric current was adjusted so that the nugget diameter of 4t can be obtained.

[0249] As for the joint strength, measurement was performed according to cross-tension test based on JIS Z3137 (1999) in the case of a cross joint, and measurement was performed with a test illustrated by a schematic diagram of FIG. 21 in the case of an L-shaped joint.

[0250] More specifically, in L-shaped tensile testing, bent portions of two test pieces each formed by bending a steel sheet into an L shape were overlapped with each other as illustrated in FIG. 21, and were joined by forming a spot-welded portion 10 having a nugget 12 formed in the overlapped portion through resistance spot welding; then, the overlapped portion was pulled in a direction of peeling; and a strength of the spot-welded portion 10 until fracture was measured as the joint strength.

[0251] First, by using a DP steel sheet of a 980 MPa class and hot-stamped members of a 1500 MPa class and an 1800 MPa class, each of which has a thickness in a range of 1.6 mm to 2.0 mm, the peel strength of overlap-welded members and fracture mode thereof were investigated. Table 4 shows welding conditions. t is the thickness of the steel sheet, and I.sub.0 was adjusted so that the nugget diameter 4t (mm) can be obtained in each of the sheet sets. Note that the distance D is a distance from the nugget end portion area to the softest zone in HAZ.

[0252] Table 3 shows effects of improvement of joint strength under the conditions A1 and A2. Table 3 is a table explaining Example related to the single energization.

TABLE-US-00003 TABLE 3 Distance Joint Thickness t t.sup.0.2 Welding D strength Fracture Steel type Joint shape (mm) (mm) condition (mm) (kN) mode Note 980 MPa Cross 2.0 1.15 a 1.5 9.8 Partial plug Comparative fracture Example A1 1.0 12.5 Partial plug Example of present fracture invention 1500 MPa 1.6 1.10 a 1.3 5.6 Partial plug Comparative fracture Example A1 0.8 8.2 Plug fracture Example of present invention A2 1.0 8.3 Plug fracture Example of present invention 2.0 1.15 a 1.5 7.2 Interface Comparative fracture Example A1 1.0 8.6 Partial plug Example of present fracture invention 1800 MPa 1.6 1.10 a 1.3 3.2 Partial plug Comparative fracture Example A2 1.0 3.9 Partial plug Example of present fracture invention 2.0 1.15 a 1.5 4.8 Interface Comparative fracture Example A1 1.0 5.5 Interface Example of present fracture invention 1800 MPa L shape 1.8 1.12 a 1.3 1.2 Interface Comparative fracture Example A1 0.8 1.5 Interface Example of present fracture invention

[0253] As shown in Table 3, with any steel type and any thickness, the spot-welded portions formed through the short-time energization with the condition A1 and the preliminary energization and the short-time energization with the A2 improve the joint strength, as compared with the conventional single energization with the condition a. It can be considered that this is because of the effect in which the stress concentration on the nugget end portion is alleviated by bringing the position of HAZ softening closer to the nugget end portion.

[0254] Furthermore, as for the L-shaped tensile strength concerning the hot-stamped member of an 1800 MPa class, by applying the short-time energization with the condition A1, the joint strength improves by approximately 25%, as compared with the case of the condition a.

[0255] Furthermore, by using the DP steel sheet of a 980 MPa class, and hot-stamped members of a 1500 MPa class and an 1800 MPa class, each of which has a thickness in a range of 1.6 mm to 2.0 mm, the peel strength of overlap-welded members and fracture mode thereof were investigated. Table 4 shows welding conditions. t is the thickness of the steel sheet, and I.sub.0 was adjusted so that the nugget diameter 4t (mm) can be obtained in each of the sheet sets. Note that the distance D is a distance from the nugget end portion area to the softest zone in HAZ.

[0256] Table 4 shows effects of improvement of joint strength under the conditions B1 and B2. Table 4 is a table explaining Example in the case where the subsequent energization is applied.

TABLE-US-00004 TABLE 4 Distance Joint Thickness t t.sup.0.2 Welding D strength Fracture Steel type Joint shape (mm) (mm) condition (mm) (kN) mode Note 980 MPa Cross 2.0 1.15 b 1.5 15.9 Plug fracture Comparative Example B1 1.0 18.1 Plug fracture Example of present invention 1500 MPa 2.0 1.15 b 1.5 14.5 Partial plug Comparative fracture Example B1 1.0 18.5 Plug fracture Example of present invention 1800 MPa 1.6 1.10 b 1.5 5.7 Partial plug Comparative fracture Example B1 1.0 8.7 Plug fracture Example of present invention B2 1.0 8.7 Plug fracture Example of present invention 2.0 1.15 b 1.5 5.2 Interface Comparative fracture Example B1 1.0 7.3 Partial plug Example of present fracture invention 1800 MPa L shape 1.8 1.12 b 1.5 2.2 Interface Comparative fracture Example B1 1.0 4.7 Plug fracture Example of present invention

[0257] As shown in Table 4, with any steel type and any thickness, the joint strength improves, as compared with the conventional single energization condition with the condition a. It can be considered that this is not only because the nugget end portion is tempered and the toughness improves as with the conventional subsequent energization technique, but also because of the effect in which the distribution of hardness appropriate for alleviating the stress concentration on the nugget end portion can be obtained.

[0258] Furthermore, as for the L-shaped tensile strength concerning the hot-stamped member of an 1800 MPa class, by applying the short-time energization and the subsequent energization with the condition B1, the fracture mode changes from the interface fracture to the plug fracture, and the joint strength improves by approximately 114%, as compared with the case of the condition b.

INDUSTRIAL APPLICABILITY

[0259] According to the present invention, it is possible to improve the peel strength of a spot-welded portion of an overlap-welded member in which plural steel sheet members, at least one of which contains martensite, are joined at an overlapped portion, and the overlapped portion is joined at the spot-welded portion. Therefore, the present invention is industrially applicable.

BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS

[0260] 10 spot-welded portion [0261] 12 nugget [0262] 12B nugget end portion area [0263] 12C meeting portion [0264] 12E nugget end [0265] 14 HAZ [0266] 14H HAZ hardened portion [0267] 14T HAZ softening zone [0268] 14L the softest zone in HAZ