Patent Description:
Recently, in the automotive field, there have been a demand for the weight reduction of vehicle bodies in order for gas mileage improvement and the reduction of CO<NUM> emission and a demand for the strengthening of vehicle body members in order for collision safety improvement. In addition, in order to meet such demands, high-strength steel sheets are used for vehicle body members, a variety of components, and the like.

In steps for manufacturing vehicle body members made of high-strength steel sheets and steps for attaching components made of high-strength steel sheets, mainly, resistance spot welding (hereinafter, also simply referred to as spot welding) is broadly used. For example, as basic structural members constituting vehicle bodies, lap weld joints of steel sheets are used, and these lap weld joints are manufactured by overlapping two steel sheet members having a hat shape and spot-welding the overlapping portions. <FIG> is a horizontal cross-sectional view showing a lap weld joint <NUM> of a steel sheet of the related art and is a view for describing a lap welding method of a steel sheet of the related art. In addition, <FIG> is a partial enlarged view of <FIG>. Meanwhile, in <FIG>, one of a pair of welding electrodes <NUM> is not shown.

As shown in <FIG>, in the lap welding method of the related art, two steel sheet members <NUM> which respectively have a pair of flange portions <NUM> and a pair of standing wall portion <NUM> that stands from these flange portions <NUM> and have a hat-like cross-sectional shape are overlapped with each other, and then the flange portions <NUM> of these steel sheet members <NUM> are interposed between the pair of welding electrodes <NUM> and are spot-welded, thereby forming solidified portions <NUM> (hereinafter, referred to as "nuggets") between the flange portions <NUM>.

As shown in <FIG> and <FIG>, the welding electrode <NUM> that is used for the spot welding has a cylindrical main body portion <NUM> and a taper portion <NUM> that tapers toward a tip end. Meanwhile, the diameter ϕ of the main body portion <NUM> is, for example, <NUM>, and the diameter ϕ' of a tip end surface 94a of the taper portion is, for example, <NUM>. In addition, the width w of the flange portion <NUM> of the steel sheet member <NUM> is, for example, <NUM> to <NUM>. In addition, the tip end surface 94a of the taper portion <NUM> comes into contact with the flange portion <NUM> of the steel sheet member <NUM>, whereby electric currents flow in the flange portion <NUM>, and the nugget <NUM> is formed. That is, the diameter ϕ' of a tip end surface 94a of the welding electrode <NUM> determines an electric conduction diameter and almost coincides with the maximum nugget diameter to be obtained.

In a case in which two steel sheet members <NUM> are spot-welded using the welding electrodes <NUM>, when the standing wall portion <NUM> of the steel sheet member <NUM> and the welding electrode <NUM> come into contact with each other, the standing wall portion <NUM> and the welding electrode <NUM> are electrically conducted to each other, and there is a concern that it may not be possible to weld the flange portions <NUM> of the steel sheet members <NUM>. Therefore, during the spot welding of the steel sheet members <NUM>, it is necessary to provide a gap for avoiding interference between the standing wall portion <NUM> and the welding electrode <NUM>. Furthermore, as described above, the welding electrode <NUM> has the taper portion <NUM> that tapers toward the tip end. Therefore, the nugget <NUM> is formed at a location a predetermined distance away from the standing wall portion <NUM>.

In the lap weld joint <NUM> obtained by the lap welding method of the related art, the nuggets <NUM> are formed at locations away from the standing wall portion <NUM> as described above, and thus, in a case in which a tensile stress acts thereon, the flange portions <NUM> of the two steel sheet members <NUM> easily deform in a direction in which the flange portions move away from each other (that is, torn-open deformation), consequently, stress focuses on an end portion of the nugget <NUM>, and the joint strength decreases. In addition, even in a case in which a torsional moment acts on the surrounding of a central axis line CL of the lap weld joint <NUM>, torn-open deformation is easily caused, and the torsional stiffness decreases.

Here, Patent Document <NUM> discloses a technique in which a quenching treatment is carried out on a portion <NUM> to <NUM> wide from the outer circumferential end of a nugget in order to increase the tensile shear strength of a spot welded joint. In addition, Patent Document <NUM> discloses a technique in which, when a weld bead is formed by laser-welding flanges of two steel sheet members, a to-be-welded location at which the formation of the weld bead is expected is tacked by means of spot welding or the like.

Patent Document <NUM> discloses a method of welding in which a plurality of steel sheet members are joined at an overlapped portion, and at least one of the plurality of steel sheet members contains martensite, the method including forming a spot-welded portion having a nugget in the overlapped portion and emitting a laser beam to form a melted and solidified portion crossing an end of the nugget.

However, in Patent Document <NUM>, the hardness in the vicinity of a nugget end portion is increased by quenching the portion <NUM> to <NUM> wide from the outer circumferential end of the nugget, and thus, in a lap weld joint that is obtained by spot-welding the overlapping portions of a flange portion of a hat-shaped steel sheet member and another steel sheet member, it is difficult to suppress the torn-open deformation. Therefore, in the technique of Patent Document <NUM>, it is difficult to improve the joint strength and torsional stiffness of the lap weld joint.

In addition, in Patent Document <NUM>, on a plurality of tacked places formed along the longitudinal direction of the flange, a weld bead is formed by laser welding. Therefore, in Patent Document <NUM>, similar to Patent Document <NUM>, it is difficult to suppress the torn-open deformation.

The present invention has been made in consideration of the above-described circumstance, and an object of the present invention is to provide a lap welding method of a steel sheet and a lap weld joint of a steel sheet which are capable of improving joint strength and torsional stiffness in lap weld joints that are obtained by welding a steel sheet member having flange portions and standing wall portions to another steel sheet member.

In order to achieve the above-described object, the present invention employs the features of the claims which define the invention.

According to the present invention as recited in claim <NUM>, it is possible to improve joint strength and torsional stiffness in lap weld joints that are obtained by welding a steel sheet member having flange portions and standing wall portions to another steel sheet member.

<FIG> is a perspective view showing a lap weld joint <NUM> of a steel sheet according to the present invention (hereinafter, also simply referred to as the lap weld joint <NUM>). The lap weld joint <NUM> is obtained by spot-welding and laser-welding a pair of steel sheet members <NUM>. In the following description, first, the steel sheet member <NUM> will be described.

<FIG> is a perspective view showing the steel sheet member <NUM> in the lap weld joint <NUM>. As shown in <FIG>, the steel sheet member <NUM> includes a pair of flange portions <NUM> that is long in a direction, has a hat-shaped cross section that is perpendicular to a longitudinal direction, and is parallel to each other, a pair of standing wall portions <NUM> that substantially perpendicularly stands from the pair of flange portions <NUM>, and a transverse wall portion <NUM> that connects the pair of standing wall portions <NUM> and is parallel to the flange portions <NUM>. The steel sheet member <NUM> is manufactured by, for example, bending a steel sheet by means of press forming. That is, in the steel sheet member <NUM>, the flange portion <NUM> and the standing wall portion <NUM> are continuous with each other, the standing wall portion <NUM> and the transverse wall portion <NUM> are continuous with each other, and, particularly, there are no holes or the like formed on surfaces of the flange portions <NUM>, and thus it is possible to prevent a decrease in strength.

Meanwhile, in <FIG>, reference symbol X indicates a central axis line of the steel sheet member <NUM>. In addition, the direction of the central axis line X coincides with the longitudinal direction of the steel sheet member <NUM>.

The standing wall portion <NUM> of the steel sheet member <NUM> has an R portion <NUM> that is connected to the flange portion <NUM> of the steel sheet member <NUM> and has a predetermined curvature radius. The curvature radius of the R portion <NUM> is, for example, <NUM> to <NUM>.

The sheet thickness of the steel sheet member <NUM> is, for example, <NUM> to <NUM>. In addition, the width (the length of the flange portion <NUM> perpendicular to a sheet thickness direction and the longitudinal direction) of the flange portion <NUM> of the steel sheet member <NUM> is, for example, <NUM> to <NUM>.

The component composition of the steel sheet member <NUM> is not particularly limited and may be appropriately set so that mechanical characteristics suitable for uses can be obtained. Meanwhile, in a case in which the steel sheet member <NUM> contains <NUM>% by mass or more of carbon, the tensile strength significantly improves. Therefore, the content of carbon in the steel sheet member <NUM> is preferably <NUM>% by mass or more.

In addition, the steel sheet member <NUM> may have a surface-treated film(s) formed on both surfaces or on a single surface. The surface-treated film is, for example, a plated film, a coated film, or the like. Examples of the plated film include a zinc plate, an aluminum plate, a zinc/nickel plate, a zinc/iron plate, a zinc/aluminum/magnesium plate, and the like, and examples of a method for manufacturing plates include hot-dip plating, electroplating, and the like.

Next, the lap weld joint <NUM> according independent claim <NUM> will be described. <FIG> is a perspective view of the lap weld joint <NUM>, and <FIG> is a cross-sectional view of a cross section that is perpendicular to the longitudinal direction of the lap weld joint <NUM> and includes an end portion 110a of a nugget <NUM>. As shown in <FIG> and <FIG>, the lap weld joint <NUM> is long in the direction of the central axis line X and has a hollow cross section that is perpendicular to the central axis line X (longitudinal direction). In addition, the lap weld joint <NUM> includes a pair of steel sheet members <NUM> facing each other, a plurality of nuggets <NUM> that are formed by spot-welding flange portions <NUM> of the pair of steel sheet members <NUM>, and a plurality of weld beads <NUM> that are formed by laser-welding the flange portions <NUM> of the pair of the steel sheet members <NUM>. Meanwhile, in <FIG> and <FIG>, reference symbol 1X indicates the steel sheet member <NUM> disposed on the upper side, and reference symbol 1Y indicates the steel sheet member <NUM> disposed on the lower side.

<FIG> is an enlarged view of a portion indicated by reference symbol P in <FIG>. As shown in <FIG> and <FIG>, the nugget <NUM> is formed between the flange portions <NUM> by spot-welding the flange portion <NUM> of the steel sheet member 1X and the flange portion <NUM> of the steel sheet member 1Y and joins the flange portion <NUM> of the steel sheet member 1X and the flange portion <NUM> of the steel sheet member 1Y. In other words, the nugget <NUM> is formed on an overlapping surface of the flange portion <NUM> of the steel sheet member 1X and the flange portion <NUM> of the steel sheet member 1Y.

The weld bead <NUM> is formed between the flange portions <NUM> by welding the flange portion <NUM> of the steel sheet member 1X and the flange portion <NUM> of the steel sheet member 1Y by radiating laser beams from the upper side of the flange portion <NUM> of the steel sheet member 1X, and joins the flange portions <NUM> to each other. In other words, the weld bead <NUM> is formed from an external surface (among two surfaces in the sheet thickness direction, a surface facing the outside) of the flange portion <NUM> of the steel sheet member 1X to an inside of the flange portion <NUM> of the steel sheet member 1Y. Meanwhile, the weld bead <NUM> may or may not penetrate the external surface of the flange portion <NUM> of the steel sheet member 1X and an external surface of the flange portion <NUM> of the steel sheet member 1Y.

In addition, the weld bead <NUM> is formed in a region between an R stop 3a of the R portion <NUM> of the steel sheet member 1X and the nugget <NUM> as shown in <FIG>. Specifically, an end portion 120a of the weld bead <NUM> on the inside of the flange portion <NUM> in the width direction (an end portion of the weld bead <NUM> in the width direction which is closest to the R stop 3a) is located on the outside of the R stop 3a in the width direction in the flange portion <NUM>. In addition, an end portion 120b of the weld bead <NUM> on the outside of the flange portion <NUM> in the width direction (an end portion of the weld bead <NUM> in the width direction which is farthest from the R stop 3a) is located on the inside of an end portion 110a of the nugget <NUM> on the inside of the flange portion <NUM> in the width direction (an end portion of the nugget <NUM> which is closest to the R stop 3a) in the width direction in the flange portion <NUM>. Meanwhile, the end portion 120a of the weld bead <NUM> may be located on the inside of the end portion 110a of the nugget <NUM> in the width direction in the flange portion <NUM>, and the end portion 120b of the weld bead <NUM> may be located on the outside of the end portion 110a of the nugget <NUM> in the width direction in the flange portion <NUM>. That is, a part of the weld bead <NUM> may be formed on the nugget <NUM>.

Here, the R stop 3a will be described using <FIG>. Meanwhile, <FIG> is an enlarged view of a portion indicated by reference symbol Q in <FIG>. As shown in <FIG>, the R stop 3a is a transition place from the R portion <NUM> to the flange portion <NUM>. Specifically, on a surface of the flange portion <NUM>, a straight line Y1 is drawn from an end portion of the flange portion <NUM> toward the inside of the steel sheet member 1X in the width direction, and furthermore, a perpendicular line is drawn from an arbitrary point A on the R portion <NUM> of the steel sheet member 1X so as to intersect the straight line Y1. In addition, the point A on the R portion <NUM> at which a distance d between an intersection point B between the straight line Y1 and the perpendicular line and the point A reaches <NUM> is considered as the R stop 3a.

As described above, the weld bead <NUM> is formed between the R stop 3a and the nugget <NUM>, and thus a distance D1 (mm) between the R stop 3a and the nugget <NUM> becomes greater than a distance D2 (mm) between the R stop 3a and the weld bead <NUM>. Meanwhile, the distance D1 refers to a distance between an intersection point between a perpendicular line drawn from the end portion 110a of the nugget <NUM> so as to intersect the straight line Y2 and the straight line Y2 and the R stop 3a. That is, the distance D1 is the shortest distance between the R stop 3a and the nugget <NUM>.

In addition, the distance D2 refers to a distance between an intersection point between a perpendicular line drawn from the end portion 120a of the weld bead <NUM> so as to intersect the straight line Y2 and the straight line Y2 and the R stop 3a. That is, the distance D2 is the shortest distance between the R stop 3a and the weld bead <NUM>.

In addition, as described above, in the lap weld joint <NUM>, the end portion 120a of the weld bead <NUM> is located on the outside of the R stop 3a in the width direction in the flange portion <NUM>, and the end portion 120b of the weld bead <NUM> is located on the inside of the end portion 110a of the nugget <NUM> in the width direction in the flange portion <NUM>, and thus the distance D1 is greater than the distance D2 (D1>D2) and is greater than the sum of the distance D2 and a width W (mm) of the weld bead <NUM> (D1>D2+W). Meanwhile, in a case in which the end portion 120a of the weld bead <NUM> is located on the outside of the R stop 3a in the width direction in the flange portion <NUM>, and the end portion 120b of the weld bead <NUM> is located on the outside of the end portion 110a of the nugget <NUM> in the width direction in the flange portion <NUM>, the distance D1 is greater than the distance D2 (D1>D2) and is less than the sum of the distance D2 and the width W of the weld bead <NUM> (D1<D2+W).

<FIG> is a plan view of the lap weld joint <NUM> and is a partial enlarged view of the flange portion <NUM> of the steel sheet member 1X. In addition, <FIG> is an enlarged view of a portion indicated by reference symbol S in <FIG>. As shown in <FIG> (that is, in a case in which the flange portion <NUM> of the steel sheet member 1X is seen in a plan view), a plurality of nuggets <NUM> and a plurality of weld beads <NUM> are respectively formed in series along the longitudinal direction of the flange portion <NUM> of the steel sheet member 1X. In addition, the plurality of weld beads <NUM> is located on the inside of the plurality of nuggets <NUM> in the width direction in the flange portion <NUM>, and faces the plurality of nuggets <NUM> in the width direction of the flange portion <NUM>.

As shown in <FIG>, in a plan view, the nugget <NUM> has, for example, a circular shape, an elliptical shape, an oval shape, or the like, and a diameter Dn thereof is, for example, <NUM>√t to <NUM>√t (mm). Meanwhile, the diameter Dn of the nugget <NUM> refers to the length of the nugget <NUM> in the longitudinal direction of the flange portion <NUM>. In addition, the t (mm) represents a thinner sheet thickness between the sheet thickness of the flange portion <NUM> of the steel sheet member 1X and the sheet thickness of the flange portion <NUM> of the steel sheet member 1Y.

The weld bead <NUM> has a linear shape that extends along the longitudinal direction of the flange portion <NUM> of the steel sheet member 1X, and a length L (mm) (the length in the longitudinal direction of the flange portion <NUM>) is equal to or greater than the diameter Dn of the nugget <NUM>. That is, the weld bead <NUM> is formed astride both end portions of the nugget <NUM> in the longitudinal direction of the flange portion <NUM>. Meanwhile, the length L of the weld bead <NUM> represents the total length of the weld bead <NUM>. In addition, the width W (the length in the width direction of the flange portion <NUM>) of the weld bead <NUM> is <NUM> to <NUM>.

The upper limit of the length L of the weld bead <NUM> is not particularly limited. But, from the viewpoint of joint strength and torsional stiffness, the upper limit of the length L is preferably greater.

In addition, the width W of the weld bead <NUM> is preferably <NUM> to <NUM>, in consideration of the efficiency of an operation for forming the weld beads.

According to the lap weld joint <NUM> described above, the weld bead <NUM> having the length L that is equal to or greater than the diameter Dn of the nugget <NUM> is formed in the region between the nugget <NUM> and the R stop 3a of the R portion <NUM> of the standing wall portion <NUM>, and thus it is possible to suppress torn-open deformation in the circumference of the nugget <NUM> against torsional moments around the central axis line X and tensile stress. In addition, the width W of the weld bead <NUM> is set to <NUM> to <NUM>, and thus it is possible to impart a sufficient strength for suppressing the torn-open deformation of the flange portion <NUM> to the weld bead <NUM>. Therefore, it is possible to improve torsional stiffness and joint strength.

Meanwhile, regarding the distance D1 and the distance D2 shown in <FIG> and <FIG>, a ratio D2/D1 of the distance D2 to the distance D1 is preferably <NUM>/<NUM> or less. In this case, the weld bead <NUM> comes close to the R stop 3a, and thus the torn-open deformation of the circumference of the nugget <NUM> is further suppressed, and torsional stiffness and joint strength can be further improved. In addition, from the above-described viewpoint, the end portion of the weld bead <NUM> on the inside in the width direction is more preferably formed at the R stop 3a (that is, D2=<NUM> (mm)). In this case, it is possible to further improve torsional stiffness and joint strength.

The lap welding method of a steel sheet according to the present invention the method of claim <NUM> for obtaining the lap weld joint <NUM> using the steel sheet members 1X and 1Y. First, as shown in <FIG> and <FIG>, the flange portion <NUM> of the steel sheet member 1X and the flange portion <NUM> of the steel sheet member 1Y are overlapped with each other so that the steel sheet members 1X and 1Y face each other.

Subsequently, the flange portion <NUM> of the steel sheet member 1X and the flange portion <NUM> of the steel sheet member 1Y are spot-welded in a state in which the flange portion <NUM> of the steel sheet member 1X and the flange portion <NUM> of the steel sheet member 1Y are overlapped with each other, thereby forming the plurality of nuggets <NUM> along the longitudinal direction of the flange portion <NUM>. At this time, the conditions of the spot welding and the like are not particularly limited, and, for example, it is possible to use a DR-type electrode having a diameter of approximately <NUM> and set the welding pressure to <NUM> to <NUM> kgf, the electric conduction time to <NUM> to <NUM>, and the electric conduction current to <NUM> to <NUM> kA. In addition, currents may be any of direct currents and alternating currents, and the current waveform may be any of single-phase current and multi-phase current.

In addition, regarding the diameter Dn of the nugget <NUM>, by evaluating the relationship between welding conditions and nugget diameters Dn to be obtained in advance using coupons (test pieces), it is possible to form the nuggets <NUM> having a desired diameter in the steel sheet members 1X and 1Y. Meanwhile, the diameter Dn of the nugget <NUM> can be evaluated by observing a cross section in the sheet thickness direction which includes the nugget <NUM>.

After the flange portion <NUM> of the steel sheet member 1X and the flange portion <NUM> of the steel sheet member 1Y are spot-welded together, these flange portions <NUM> are laser-welded, thereby forming the plurality of weld beads <NUM> having the length L that is equal to or greater than the diameter Dn of the nugget <NUM> and the width W that is <NUM> to <NUM> along the longitudinal direction of the flange portion <NUM> in the region between the R stop 3a of the R portion <NUM> of the steel sheet member 1X and the nuggets <NUM>.

At this time, the conditions of the laser welding and the like are not particularly limited, but a remote laser welding apparatus is preferably used. This is because the remote laser welding apparatus moves the laser beam at a high speed among welding points using a galvanometer mirror attached to the tip end of a robot arm and thus it is possible to significantly shorten the operation time of welding. In addition, as a laser oscillator, for example, a laser such as a CO<NUM> laser, a YAG laser, a fiber laser, a DISK laser, or a semiconductor laser can be used. In addition, the laser welding can be carried out under conditions of a laser output of <NUM> to <NUM> kW, a beam diameter on a light focus surface of <NUM> to <NUM>, and a welding rate of <NUM> to <NUM>/min.

In a case in which the steel sheet members 1X and 1Y are spot-welded together as described above, due to the restrictions of the spot welding (restrictions such as a necessity of avoiding the contact between welding electrodes and the standing wall portions <NUM> of the steel sheet members 1X and 1Y), it is necessary to form the nuggets <NUM> at locations a predetermined distance away from the R stop 3a. In contrast, in the laser welding, there are no restrictions as described above, and it is possible to weld the flange portion <NUM> of the steel sheet member 1X and the flange portion <NUM> of the steel sheet member 1Y at locations close to the R stop 3a. That is, since the flange portion <NUM> of the steel sheet member 1X and the flange portion <NUM> of the steel sheet member 1Y are welded together by means of laser welding, it is possible to form the weld beads <NUM> between the R stop 3a of the R portion <NUM> of the standing wall portion <NUM> and the nuggets <NUM>.

In addition, as described above, when the steel sheet members 1X and 1Y are welded together, first, spot welding is carried out. Additionally, as shown in <FIG>, in a state after spot welding and before laser welding, an uplift phenomenon attributed to the plastic flow of the steel sheet members 1X and 1Y (hereinafter, referred to as "sheet separation phenomenon") occurs in the circumference of the welding portion of the flange portion <NUM> of the steel sheet member 1X and the flange portion <NUM> of the steel sheet member 1Y, and, due to this sheet separation phenomenon, for example, a gap G of <NUM> to <NUM> is generated between the flange portion <NUM> of the steel sheet member 1X and the flange portion <NUM> of the steel sheet member 1Y. This gap G is relatively uniformly formed and thus contributes to the stabilization of welding conditions during the laser welding.

That is, in a case in which the steel sheet members 1X and 1Y on which galvanizing has been carried out are laser-welded together, there are cases in which zinc vapor generated by heating with laser beams causes the scattering (sputtering) of molten steel. However, even in a case in which the steel sheet members 1X and 1Y on which galvanizing has been carried out are used, during the laser welding, the gap G of approximately <NUM> to <NUM> is formed due to the sheet separation phenomenon, and thus the zinc vapor is discharged through the gap G, and it is possible to suppress the scattering (sputtering) of molten steel.

Meanwhile, in the vicinity of the nuggets <NUM>, the gap G is ensured due to the sheet separation phenomenon; however, in places away from the nuggets <NUM>, there are cases in which the flange portions <NUM> of the steel sheet members 1X and 1Y come into contact with each other or the gap G becomes small. Therefore, when the distance between the weld bead <NUM> and the nugget <NUM> is adjusted so as to be approximately <NUM> to <NUM>, it is possible to suppress scattering, which is preferable. In other words, in <FIG>, the distance D1 is preferably approximately <NUM> to <NUM> greater than the sum of the distance D2 and the width W.

As described above, according to the lap welding method of a steel sheet according to the present embodiment, the steel sheet members 1X and 1Y are spot-welded together and then laser-welded, and thus it is possible to form the weld beads <NUM> between the nuggets <NUM> and the R stop 3a. In addition, the steel sheet members 1X and 1Y are spot-welded together and then laser-welded, and thus, even in a case in which the steel sheet members 1X and 1Y on which galvanizing has been carried out are welded together, it is possible to suppress the scattering (sputtering) of molten steel due to the sheet separation phenomenon.

In the present unclaimed embodiment, a case in which the plurality of weld beads <NUM> is formed so as to face the plurality of nuggets <NUM> as shown in <FIG> has been described. However, as shown in <FIG>, the weld beads <NUM> may be formed so as to face every other nugget <NUM>. In other words, in a case in which the flange portion <NUM> is seen in a plan view, the nuggets <NUM> facing the weld bead <NUM>, and the nuggets <NUM> not facing the weld bead <NUM> are alternately present in the longitudinal direction of the flange portion <NUM>. In this case, the number of the weld beads <NUM> can be decreased, and thus it is possible to improve the efficiency of laser welding operation. Meanwhile, depending on the number of the nuggets <NUM>, the weld beads <NUM> may be formed so as to face every two other nuggets <NUM>.

In addition, as shown in <FIG>, one weld bead <NUM> may be formed so as to face all of the nuggets <NUM>. However, compared with the modification example shown in <FIG>, the present embodiment (refer to <FIG>) is capable of further decreasing the thermal deformation of the steel sheet members 1X and 1Y caused by welding since the total volume of the weld beads <NUM> becomes smaller. In addition, in the present embodiment (refer to <FIG>), in the case of being seen in the longitudinal direction of the flange portion <NUM>, the plurality of weld beads <NUM> is formed at intervals, and portions having a high strength and portions having a low strength are alternately present, and thus it is possible to improve impact safety in a case in which the lap weld joint <NUM> is applied to automotive bodies. Therefore, from the above-described viewpoints, the plurality of weld beads <NUM> preferably faces the plurality of nuggets <NUM> as in the present embodiment (refer to <FIG>). Furthermore, as the number of the weld beads increases, the stiffness of the member tends to be saturated, and thus, even when the plurality of weld beads <NUM> is provided at intervals as in the present embodiment (refer to <FIG>), it is possible to obtain an effect of both joint strength improvement and member stiffness improvement as long as the weld beads have a length that is equal to or longer than a certain value.

In addition, a case in which linear weld beads <NUM> are formed as shown in <FIG> has been described. However, as shown in <FIG>, a weld bead <NUM> having a U shape in a plan view may be formed. In this case, it is possible to further relax stress concentration in end portions in which welding begins and ends.

In addition, as shown in <FIG>, a weld bead <NUM> having a wavy shape in a plan view may be formed. In this case, it is possible to further increase the joint area, and thus it is possible to further improve joint strength.

In addition, as shown in <FIG>, a weld bead <NUM> having an elliptical shape in a plan view may be formed. In this case, similar to the modification example of <FIG>, it is possible to further relax stress concentration in end portions in which welding begins and ends.

In addition, a case in which the steel sheet member 1X and the steel sheet member 1Y which have a hat-shaped cross section as shown in <FIG> and <FIG> are welded together has been described. However, as shown in <FIG>, the steel sheet member 1X and a planar steel sheet <NUM> may be welded together.

In addition, as shown in <FIG>, a steel sheet member <NUM> having one flange portion <NUM>, one standing wall portion <NUM>, and one transverse wall portion <NUM> parallel to the flange portion <NUM> may be welded to the steel sheet <NUM>.

In addition, as shown in <FIG>, the steel sheet member 1X and a steel sheet member 1X' having a size different from that of the steel sheet member 1X may be welded together so that the flange portions <NUM>, the standing wall portions <NUM>, and the transverse wall portions <NUM> thereof overlap each other.

Next, a lap weld joint <NUM> which is not part of the present invention will be described.

<FIG> is a horizontal cross-sectional view (a cross-sectional view perpendicular to the longitudinal direction) showing the lap weld joint <NUM> according to the present embodiment. In the first embodiment, a case in which the lap weld joint <NUM> is constituted of two steel sheet members 1X and 1Y has been described. In contrast, in the present embodiment, the lap weld joint <NUM> is constituted of the steel sheet members 1X and 1Y and, furthermore, a steel sheet member <NUM> having a hat-liked shape and a thinner sheet thickness than the steel sheet members 1X and 1Y as shown in <FIG>.

Regarding steel sheet components constituting automotive bodies, in steel sheet components made of three or more steel sheet members, there are cases in which the sheet thickness of the steel sheet member that is disposed on the outermost side is thinner than the sheet thicknesses of other steel sheet members (the case of a high sheet thickness ratio). In this case, nuggets that are formed by means of spot welding propagate from the center of the total sheet thickness, and thus, on the overlapping surface between the thin steel sheet member disposed on the outermost side and another steel sheet member disposed on the inside of the above-described thin steel sheet member, nuggets are incapable of easily propagating.

As shown in <FIG>, the lap weld joint <NUM> can be obtained by spot-welding and laser-welding three steel sheet members 1X, 1Y, and <NUM> in the same manner as in the lap welding method of a steel sheet according to the first embodiment. In addition, as shown in <FIG>, in the lap weld joint <NUM>, the flange portion <NUM> of the steel sheet member 1X, the flange portion <NUM> of the steel sheet member 1Y, and a flange portion <NUM> of the steel sheet member <NUM> are overlapped with one another, the nugget <NUM> is formed by spot welding, and the weld bead <NUM> is formed by laser welding between an R stop 33a of an R portion <NUM> and the nugget <NUM> on the R stop 33a side.

In a case in which the steel sheet members having a high sheet thickness ratio are welded together as shown in <FIG>, there are cases in which the nuggets <NUM> formed by spot welding propagate from the center of the total sheet thickness and the nuggets do not propagate or barely propagate on the overlapping surface of the flange portion <NUM> of the steel sheet member <NUM> having a thin sheet thickness which is disposed on the outermost side and the flange portion <NUM> of the steel sheet member 1X having a sheet thickness that is thicker than the above-described sheet thickness.

However, for the lap weld joint <NUM>, similar to the lap welding method of a steel sheet according to the first embodiment, spot welding and laser welding are sequentially carried out, and thus the weld beads <NUM> formed by laser welding are formed astride the steel sheet members 1X, 1Y, and <NUM>. Therefore, in the lap weld joint <NUM>, it is possible to obtain a sufficient joint strength even when there are portions in which the nuggets <NUM> do not sufficiently propagate on the overlapping surfaces of the steel sheet members 1X, 1Y, and <NUM>.

In addition, in the lap weld joint <NUM>, similar to the case of the first embodiment, spot welding is firstly carried out, and thus the sheet separation phenomenon attributed to the plastic flow of the steel sheet members occurs in the circumferences of the welding portion of the flange portion <NUM> of the steel sheet member 1X, the flange portion <NUM> of the steel sheet member 1Y, and the flange portion <NUM> of the steel sheet member <NUM>. In addition, a gap G is generated between these flange portions due to the sheet separation phenomenon. Therefore, even in the lap weld joint <NUM>, in a case in which the steel sheet members 1X, 1Y, and <NUM> on which galvanizing has been carried out are laser-welded together, gaps G of substantially <NUM> to <NUM> are formed due to the sheet separation phenomenon, and thus zinc vapor is discharged through these gaps G, and it is possible to suppress the scattering (sputtering) of molten steel.

Steel sheet members were prepared by forming steel sheets having a sheet thickness of <NUM> and a tensile strength of <NUM> MPa into an L-like shape or a hat shape. In order to use as tensile test pieces, for two steel sheet members having an L-like shape, the flange portions thereof were overlapped with each other and spot-welded together. In addition, in order to use as torsional stiffness test pieces, as shown in <FIG>, for two steel sheet members having a hat shape, the flange portions thereof were overlapped with each other and spot-welded together. In the spot welding, the flange portions of the two steel sheet members were interposed and pressed at a welding pressure of <NUM> kN in a DR-type electrode having a diameter of <NUM> so that nugget diameters reached <NUM>, and the spot welding was carried out at an electric conduction current of <NUM> kA for an electric conduction time of <NUM> cycles. In addition, the spot welding was carried out at a pitch of <NUM>.

Next, the flange portions were welded together using a fiber laser in a remote laser welding apparatus having a galvanometer mirror. In addition, regarding the shape and disposition of weld beads in the laser welding, the shape and disposition of weld beads shown in <FIG> were made. In addition, the width of the weld bead was adjusted by changing the welding rate while fixing the process point output of the laser to <NUM> kW. In Table <NUM>, the distance D1 from an R stop to the nugget (refer to <FIG>), the distance D2 from the R stop to the weld bead, the length L of the weld bead, the pitch P of the spot welding, and the width W of the weld bead are shown. In addition, in each of Test Nos. <NUM> to <NUM>, a tensile test piece and a torsional stiffness test were produced.

In Table <NUM>, Test No. <NUM> indicates a comparative example of a case in which only the spot welding was carried out (that is, a case in which the laser welding was not carried out). Test No. <NUM> indicates an invention example of a case in which D2 was zero, that is, the end portion of the weld bead closest to the R stop 3a was formed on the R stop 3a. <NUM> and <NUM>, <NUM> and <NUM>, and <NUM> indicate invention examples of a case in which D1 was greater than the sum of D2 and W (D1>D2+W) and the end portion 120b of the weld bead <NUM> was formed on the inside of the end portion 110a of the nugget <NUM> in the width direction as shown in <FIG>. Test No. <NUM> indicates an invention example of a case in which D1 was greater than D2 (D1>D2) and was less than the sum of D2 and W (D1<D2+W) and a part of the weld bead was formed on the nugget.

On the other hand, Test No. <NUM> indicates a comparative example of a case in which D1 was less than D2 (D1<D2), that is, the weld beads were not formed in a region between the nuggets and the R stop. In addition, Test No. <NUM> indicates a case in which the diameter Dn (refer to <FIG>) of the nugget was <NUM>, the length L of the weld bead was <NUM>, and thus L was less than Dn, that is, a comparative example. In addition, Test No. <NUM> indicates a comparative example in which the width W of the weld bead was <NUM> and failed to satisfy the range (W=<NUM> to <NUM>) of the present invention.

In addition, the joint strength and the torsional stiffness of the produced test pieces were measured. The joint strength (the maximum load) was obtained by pulling out both ends of the tensile test piece using a tensile tester and breaking the tensile test piece. Meanwhile, the torsional stiffness was obtained by fixing one end of the torsional stiffness test piece and obtaining the relationship between the torsional moment loaded to the other end and the torsional angle measured at the other end.

In Table <NUM>, the joint strength, the joint strength ratio, the torsional stiffness, and the torsional stiffness ratio are shown. The joint strength ratio and the torsional stiffness ratio are respectively ratios to the joint strength and the torsional stiffness of the case of Test No. <NUM> (that is, the case of the spot welding alone). In addition, joint strength ratios and torsional stiffness ratios of <NUM> or higher were determined as a pass.

In Test Nos. <NUM> to <NUM>, <NUM>, <NUM>, and <NUM>, the constitution of the present invention was fully satisfied, and thus the joint strength ratios and the torsional stiffness ratios were <NUM> or higher. That is, it could be confirmed that, compared with Test No. <NUM> in which only the spot welding was carried out, the joint strength and the torsional stiffness could be improved. Furthermore, in Test Nos. <NUM> to <NUM>, <NUM>, <NUM>, and <NUM>, it could be confirmed that D2/D1 was <NUM> or lower and the joint strength ratios and the torsional stiffness ratios became higher.

On the other hand, in Test No. <NUM>, the weld beads were not formed between the R stop and the nuggets, and thus the joint strength ratio and the torsional stiffness ratio were lower than <NUM>. In addition, in Test No. <NUM>, the length L of the weld bead was shorter than the diameter Dn of the nugget, and thus the torsional stiffness ratio was lower than <NUM>. In addition, in Test No. <NUM>, the width W of the weld bead was less than <NUM>, and thus the joint strength ratio was lower than <NUM>.

Hitherto, the embodiments of the present invention have been described, but the above-described embodiments are proposed as examples, and the scope of the present invention is not limited only to the above-described embodiments. The above-described embodiments can be carried out in a variety of different forms and can be omitted, substituted, and modified in various manners within the scope of the invention described in the claims.

For example, in the lap weld joints <NUM> and <NUM>, the type, the component composition, and the sheet thickness may be fully or partially identical among the respective steel sheet members or may be different among the respective steel sheet members.

In addition, for example, in the lap weld joint <NUM>, the disposition of the nuggets and the weld beads may vary in every flange portion, or it is also possible to, in a flange portion, divide the welding place into a plurality of sections and vary the disposition of nuggets and weld beads in every section.

In addition, for example, in the modification example of the first embodiment, a case in which one weld bead <NUM> is formed astride a plurality of nuggets <NUM> has been described (refer to <FIG>). However, a plurality of weld beads <NUM> extending astride a plurality of nuggets <NUM> may be formed.

In addition, for example, in the modification example of the first embodiment, a case in which the shape of the weld bead <NUM> is an elliptical shape in a plan view has been described (refer to <FIG>). However, the shape of the weld bead <NUM> may be a circular shape.

In addition, for example, in the modification example of the first embodiment, a case in which the steel sheet members 1X and 1X' which have a hat-shaped cross section are welded together has been described (refer to <FIG>). However, a lap weld joint constituted of three steel sheet members may be produced by overlapping the steel sheet <NUM> (refer to <FIG>) to these steel sheet members 1X and 1X' from the lower side of the steel sheet member 1X'.

Claim 1:
A lap welding method of a steel sheet for overlapping and welding a first steel sheet member (1Y, <NUM>) and a second steel sheet member (1X) having a flange portion (<NUM>) that is overlapped with the first steel sheet member and a standing wall portion (<NUM>) that stands from the flange portion, the standing wall portion (<NUM>) of the steel sheet member having an R portion (<NUM>) that is connected to the flange portion (<NUM>) of the steel sheet member and has a predetermined curvature radius, the method comprising:
spot welding in a state in which the flange portion is overlapped with the first steel sheet member, thereby forming a plurality of nuggets (<NUM>) between the first steel sheet member and the flange portion, along a longitudinal direction (X) of the flange portion; and
after the spot welding, laser welding a region between an R stop (3a) of the standing wall portion and the nugget, thereby forming a plurality of weld beads (<NUM>) along the longitudinal direction of the flange portion, the R stop (3a) being a transition place from the R portion (<NUM>) to the flange portion (<NUM>) determined by the following:
on a surface of the flange portion (<NUM>), a straight line (Y1) is drawn from an end portion of the flange portion (<NUM>) toward the inside of the steel sheet member in a width direction, and furthermore, a perpendicular line is drawn from a point A on the R portion <NUM> of the steel sheet member (1X) so as to intersect the straight line (Y1), wherein the point A on the R portion (<NUM>) at which a distance d between an intersection point B between the straight line (Y1) and the perpendicular line and the point A reaches <NUM> is considered as the R stop (3a),
wherein, the plurality of weld beads is formed at intervals in the longitudinal direction of the flange portion,
in the weld bead, a dimension (L) in a longitudinal direction of the flange portion is equal to or longer than a diameter (Dn) of the nugget, and a width dimension (W) is <NUM> to <NUM>, and
an end portion (120b) of the weld bead (<NUM>) in a width direction of the flange portion which is farthest from the R stop (3a) is located further inside than an end portion (110a) of the nugget (<NUM>) which is closest to the R stop (3a) in the width direction.