Patent Publication Number: US-11648626-B2

Title: Laser-welded lap joint, method for producing laser-welded lap joint, and automobile frame component

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is the U.S. National Phase application of PCT/JP2019/019837, filed May 20, 2019, which claims priority to Japanese Patent Application No. 2018-096825, filed May 21, 2018, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes. 
     FIELD OF THE INVENTION 
     The present invention relates to a laser-welded lap joint, to a method for producing the laser-welded lap joint, and an automobile frame component including the laser-welded lap joint. 
     BACKGROUND OF THE INVENTION 
     Resistance spot welding has conventionally been used for welding of automobile structural members having flange portions. However, resistance spot welding has the following problems: the welding is time consuming; a pitch cannot be reduced because the amount of heat generated decreases due to shunt current; and there are spatial limitations due to a gun of the welder. In recent years, to solve these problems, it has been contemplated to use laser lap welding in addition to the conventional resistance spot welding. The laser lap welding is a welding method in which a surface of a plurality of lapped steel sheets is irradiated with a laser beam to join the steel sheets together. 
     In the laser lap welding, a surface of a plurality of lapped steel sheets is irradiated linearly with a laser beam. The irradiated portion of the steel sheets irradiated with the laser beam is fused and solidified, and a fusion zone (weld zone) is thereby formed. In this manner, the lapped steel sheets are joined together, and a laser-welded lap joint can be obtained. However, one problem with the laser lap welding is that cracking is likely to occur at the terminal end side of the linear fusion zone. Once a crack occurs, the crack propagates over the entire length of the fusion zone. It is feared that the propagation of the crack formed in the weld metal may cause a reduction in static strength such as shear strength and peeling strength of the laser-welded lap joint portion and also a significant reduction in fatigue strength due to propagation of cracks from the initial crack. In recent years, high-tensile steel sheets with increased strength are being used for automobile body components, particularly frame components, in order to improve the strength and stiffness of automobile bodies, and a reduction in static strength and fatigue strength of joints due to cracks in weld zones is a serious problem. 
     Accordingly, various techniques have been disclosed as methods for preventing the occurrence and propagation of cracks in laser lap weld zones that are formed when lapped steel sheets are laser-welded. 
     For example, Patent Literature 1 discloses a technique for preventing weld cracking by disposing a lower steel sheet to be lap-welded so as to protrude and setting a welding start position to be spaced apart from an edge of a flange. Patent Literature 2 discloses a technique for preventing weld cracking by irradiating an edge portion of lapped surfaces with a laser beam obliquely. Patent Literature 3 and Patent Literature 4 disclose techniques for preventing weld cracking by reheating or welding an already welded portion or a peripheral portion of the welded portion. Patent Literature 5 discloses a technique for preventing the occurrence of weld cracking by welding lapped surfaces elliptically. 
     PATENT LITERATURE 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2007-229740 
     PTL 2: Japanese Unexamined Patent Application Publication No. 2008-296236 
     PTL 3: Japanese Unexamined Patent Application Publication No. 2012-240083 
     PTL 4: Japanese Unexamined Patent Application Publication No. 2012-240086 
     PTL 5: Japanese Unexamined Patent Application Publication No. 2017-113781 
     SUMMARY OF THE INVENTION 
     However, in the method described in Patent Literature 1, since the lower steel sheet to be lap-welded is disposed so as to protrude, the protruding portion is redundant, and the design of components is disadvantageously restricted. 
     In the method described in Patent Literature 2, the laser beam is applied obliquely. When a gap is present between lapped sheets, a fusion zone is not well formed on the lapped surfaces, and this results in incomplete penetration. Therefore, disadvantageously, it is difficult to obtain sufficient strength. 
     In the methods described in Patent Literature 3 and Patent Literature 4, an already welded portion or a peripheral portion of the welded portion is reheated or welded. Therefore, disadvantageously, additional welding time for reheating or welding is required. 
     In the method described in Patent Literature 5, lapped surfaces are welded elliptically. Therefore, the method is not applicable to weld cracking in linear weld zones. 
     Aspects of the present invention have been made in view of the foregoing problems, and it is an object to provide a laser-welded lap joint in which the occurrence of a crack in a terminal end portion of a fusion zone and the propagation of the crack can be prevented and which has good peeling strength and to provide a method for producing the laser-welded lap joint and an automobile frame component including the laser-welded lap joint. 
     The present inventors have conducted studies to solve the foregoing problems and obtained the following findings. 
     In accordance with aspects of the present invention, attention is given to (a) the full length of a fusion zone, (b) the length of a final weld zone, (c) the length, depth, and width of a crater in a terminal end portion of the laser lap weld zone, and (d) the thickness of the uppermost steel sheet of a plurality of lapped steel sheets. The inventors have found that, by controlling all the relations of the above (a) to (d), i.e., by forming a weld zone satisfying formulas (1) to (4) below by laser lap welding, the occurrence of a crack on the terminal end side of the weld zone and the propagation of the crack can be prevented. In accordance with aspects of the present invention, a fusion zone and a heat-affected zone are collectively referred to as a weld zone.
 
 L≥ 15.0  (1)
 
10.0≥ L 2≥2 l   c   (2)
 
 t 1≥2 d   c   (3)
 
 w   c   &gt;d   c   (4)
 
     Here, L is the full length (unit: mm) of the weld zone, and L 2  is the length (unit: mm) of the final weld zone. l c  is the length (unit: mm) of the crater in the final weld zone, and t 1  is the thickness (unit: mm) of the uppermost steel sheet in the lapped portion. d c  is the depth (unit: mm) of the crater in the final weld zone, and w c  is the width (unit: mm) of the crater in the final weld zone. 
     Moreover, the inventors have found that, by controlling the total thickness of the plurality of lapped steel sheets and the total size of gaps between the plurality of lapped steel sheets, stress concentration on the fusion zone on the lapped surfaces can be reduced and the peeling strength can be further improved. 
     Aspects of the present invention have been made based on the above findings and are summarized as follows. 
     [1] A laser-welded lap joint including a weld zone formed by laser lap welding in a lapped portion including a plurality of steel sheets lapped one over another, 
     wherein the weld zone includes a main weld zone that penetrates the steel sheets in the lapped portion and a final weld zone formed at one end of the main weld zone and having a crater, and 
     wherein the weld zone satisfies formulas (1) to (4):
 
 L≥ 15.0;  (1)
 
10.0≥ L 2≥2 l   c ;  (2)
 
 t 1≥2 d   c ;  (3)
 
 w   c   &gt;d   c ,  (4)
 
where L is the full length (unit: mm) of the weld zone, L 2  is the length (unit: mm) of the final weld zone, l c  is the length (unit: mm) of the crater in the final weld zone, t 1  is the thickness (unit: mm) of an uppermost steel sheet in the lapped portion, d c  is the depth (unit: mm) of the crater in the final weld zone, and w c  is the width (unit: mm) of the crater in the final weld zone.
 
     [2] The laser-welded lap joint according to [1], wherein the total size of a gap between the steel sheets in the lapped portion is 0% or more and 15% or less of the total thickness of the plurality of steel sheets. 
     [3] The laser-welded lap joint according to [1] or [2], wherein at least one steel sheet of the plurality of steel sheets has a chemical composition containing, in mass %: 
     C: more than 0.07% and 0.25% or less; 
     P+S: less than 0.03%; 
     Mn: 1.8% or more and 3.0% or less; and 
     Si: more than 1.2% and 2.5% or less, 
     with the balance being Fe and unavoidable impurities. 
     [4] The laser-welded lap joint according to any one of [1] to [3], further containing, in addition to the chemical composition, one or two selected from the following groups A and B: 
     group A: in mass %, one or two selected from Ti: 0.005% or more and 0.01% or less and Nb: 0.005% or more and less than 0.050%, and 
     group B: in mass %, one or two or more selected from Cr: 1.0% or less, Mo: 0.50% or less, and B: 0.10% or less. 
     [5] The laser-welded lap joint according to any one of [1] to [4], wherein at least one steel sheet of the plurality of steel sheets is a high-tensile steel sheet with a tensile strength of 980 MPa or more. 
     [6] A method for producing the laser-welded lap joint according to any one of [1] to [5], the method including: 
     lapping the plurality of steel sheets vertically one over another; and 
     performing laser welding in which a surface of the uppermost steel sheet in the lapped portion including the plurality of lapped steel sheets is irradiated with a laser beam to form the weld zone in the lapped portion to thereby join the plurality of lapped steel sheets together. 
     [7] The method for producing the laser-welded lap joint according to [6], wherein the laser welding includes: a main welding step of forming the main weld zone; and a final welding step of forming the final weld zone having the crater, and 
     wherein at least one of laser power, welding speed, a focal position, and a beam diameter in the final welding step is controlled under conditions in which formula (5) to (7) are satisfied such that the weld zone formed satisfies formulas (1) to (4):
 
 L≥ 15.0;  (1)
 
10.0≥ L 2≥2 l   c ;  (2)
 
 t 1≥2 d   c ;  (3)
 
 w   c   &gt;d   c ,  (4)
 
 P   i   ≥P   f ≥(¼) P   i ;  (5)
 
 v   i   ≥v   f ≥(¼) v   i ;  (6)
 
 f   i   ≤f   f ≤20.0,  (7)
 
where L is the full length (unit: mm) of the weld zone, L 2  is the length (unit: mm) of the final weld zone, l c  is the length (unit: mm) of the crater in the final weld zone, t 1  is the thickness (unit: mm) of the uppermost steel sheet in the lapped portion, d c  is the depth (unit: mm) of the crater in the final weld zone, w c  is the width (unit: mm) of the crater in the final weld zone, P i  is laser power (unit: kW) in the main welding step, P f  is the laser power (unit: kW) in the final welding step, v i  is welding speed (unit: m/min) in the main welding step, f f  is the welding speed (unit: m/min) in the final welding step, f i  is a focal position (unit: mm) in the main welding step, and f f  is the focal position (unit: mm) in the final welding step.
 
     [8] An automobile frame component including the laser-welded lap joint according to any one of [1] to [5]. 
     According to aspects of the present invention, the occurrence and propagation of cracks in a terminal end portion of the weld zone formed by laser lap-welding the plurality of lapped steel sheets can be prevented. Therefore, the laser-welded lap joint produced can have good peeling strength. Since the laser-welded lap joint according to aspects of the present invention has excellent appearance, the laser-welded lap joint is preferable for structural components of automobiles, and an automobile frame component including the laser-welded lap joint according to aspects of the present invention can be produced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view showing an example of the laser-welded lap joint according to aspects of the present invention. 
         FIG.  2 (A)  is a schematic illustration showing a welding terminal end portion of a conventional laser-welded lap joint, and  FIG.  2 (B)  is a schematic illustration showing a welding terminal end portion in the laser-welded lap joint according to aspects of the present invention. 
         FIG.  3 (A)  is a cross-sectional view of the laser-welded lap joint in  FIG.  2 (A)  taken along line A-A, and  FIG.  3 (B)  is a cross-sectional view of the laser-welded lap joint in  FIG.  2 (B)  taken along line B-B. 
         FIG.  4 (A)  shows a top view and a cross-sectional view illustrating a weld zone in the conventional laser-welded lap joint, and  FIG.  4 (B)  shows a top view and a cross-sectional view illustrating a weld zone in the laser-welded lap joint according to aspects of the present invention. 
         FIG.  5    is a perspective view illustrating a welding method for the laser-welded lap joint according to aspects of the present invention. 
         FIG.  6 (A)  is a top view illustrating the position of the weld zone (fusion zone) in the laser-welded lap joint according to aspects of the present invention, and  FIG.  6 (B)  is a cross-sectional view taken along line C-C in  FIG.  6 (A) . 
         FIG.  7    is an illustration showing an example of a laser-welded lap joint in an Example according to aspects of the present invention. 
         FIG.  8    shows graphs of an example of laser lap welding conditions in a method for producing the laser-welded lap joint according to aspects of the present invention,  FIG.  8 (A)  showing the relation between laser power and welding time,  FIG.  8 (B)  showing the relation between welding speed and the welding time,  FIG.  8 (C)  showing the relation between a focal position and the welding time. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     The laser-welded lap joint according to aspects of the present invention, a method for producing the laser-welded lap joint, and an automobile frame component will be described with reference to the drawings. However, the present invention is not limited to the embodiments. 
     &lt;Laser-Welded Lap Joint&gt; 
     The laser-welded lap joint according to aspects of the present invention includes a weld zone formed by laser lap welding in a lapped portion including a plurality of lapped steel sheets. The weld zone includes a main weld zone penetrating the steel sheets in the lapped portion and a final weld zone formed at one end of the main weld zone and having a crater. The weld zone satisfies the following formulas (1) to (4):
 
 L≥ 15.0;  (1)
 
10.0≥ L 2≥2 l   c ;  (2)
 
 t 1≥2 d   c ;  (3)
 
 w   c   &gt;d   c .  (4)
 
     Here, L is the full length (unit: mm) of the weld zone, L 2  is the length (unit: mm) of the final weld zone, l c  is the length (unit: mm) of the crater in the final weld zone, t 1  is the thickness (unit: mm) of the uppermost steel sheet in the lapped portion, d c  is the depth (unit: mm) of the crater in the final weld zone, and w c  is the width (unit: mm) of the crater in the final weld zone. 
     An embodiment of the laser-welded lap joint  1  according to aspects of the present invention will be described using  FIGS.  1  to  4   .  FIG.  1    is a perspective view showing an example of the laser-welded lap joint  1  according to aspects of the present invention.  FIG.  2 (A)  is a schematic illustration showing a welding terminal end portion of a conventional laser-welded lap joint, and  FIG.  2 (B)  is a schematic illustration showing a welding terminal end portion of the laser-welded lap joint according to aspects of the present invention.  FIG.  3 (A)  is a cross-sectional view taken along line A-A shown in  FIG.  2 (A) , and  FIG.  3 (B)  is a cross-sectional view taken along line B-B shown in  FIG.  2 (B) .  FIG.  4 (A)  shows a top view and a cross-sectional view of a weld zone in the conventional laser-welded lap joint, and  FIG.  4 (B)  shows a top view and a cross-sectional view of a weld zone in the laser-welded lap joint according to aspects of the present invention. 
     Referring first to  FIG.  1   , the laser-welded lap joint  1  according to aspects of the present invention will be described. 
     In the laser-welded lap joint  1  according to aspects of the present invention, at least two steel sheets are lapped one over another. In the example shown in  FIG.  1   , two steel sheets, i.e., a steel sheet  2  having a substantially hat-shaped cross-sectional shape and including a vertical wall portion  2   a  and a flange portion  2   b  extending outward from an end of the vertical wall portion  2   a  and a flat panel-shaped steel sheet  3 , are used. The steel sheet  2  and the steel sheet  3  are lapped so as to face each other, and a region (flat region) of the flange portion  2   b  of the steel sheet  2  serves as a joint surface. The two lapped steel sheets  2  and  3  are joined in the flange portion  2   b  by laser lap-welding. In the laser lap welding, a fusion zone that penetrates at least one of the steel sheets  2  and  3  to join the steel sheets  2  and  3  together is formed. The fusion zone and a heat-affected zone around the fusion zone form a weld zone  4 . 
     The laser welding is performed by applying a laser beam  7  intermittently along the vertical wall portion  2   a  while the laser beam  7  is moved in a longitudinal direction. In this manner, as shown in  FIG.  1   , a plurality of weld zones  4  having a substantially linear surface shape are formed on the joint surface of the steel sheets  2  and  3 . In the example described in this case, the two steel sheets  2  and  3  are lapped to form the laser-welded lap joint  1  according to aspects of the present invention. However, three or more steel sheets may be lapped. 
     Referring next to  FIGS.  2  to  4   , the technical idea according to aspects of the present invention and the structure of the weld zones  4  will be described.  FIGS.  2 (A),  3 (A) , and  4 (A) show a weld zone  14  and its periphery in a conventional laser-welded lap joint, and  FIGS.  2 (B),  3 (B) , and  FIG.  4 (B)  show a weld zone  4  and its periphery in the laser-welded lap joint according to aspects of the present invention. 
     In a conventional laser lap welding method, a weld zone is formed by performing welding at constant power and constant speed. Therefore, the size of a keyhole that is a governing factor of the width of fusion is constant even in the second half of the welding, and the weld zone  14  shown in  FIG.  4 (A)  is generally formed. Specifically, the width W of the weld zone  14  is substantially constant from a start point S of the weld zone  14  to its end point E. In the lapped portion, the steel sheets  2  and  3  are fused by welding such that the fusion zone penetrates the entire steel sheets  2  and  3 . At the terminal end point E of the weld zone  14 , a crater  15  with a large cross-sectional area is formed in the upper steel sheet  2 . 
     In the laser-welded lap joint welded by the conventional laser lap welding method, the depth of the crater  15  in the terminal end portion of the weld zone  14  is larger than the thickness of the uppermost steel sheet. Specifically, as shown in  FIGS.  2 (A) and  3 (A) , the crater  15  penetrates the steel sheet  2 , and the depth d c  of the crater  15  is equal to or larger than the thickness of the steel sheet  2 . When the crater  15  has such a shape, tensile stress directed from the outer circumference of the fusion zone toward the outside (force in the direction of an arrow Fa shown in  FIG.  2 (A) ) is concentrated on a central portion, which is a final solidification zone, at the terminal end point E of the weld zone  14 . In this case, solidification cracking  16  may occur in the terminal end portion of the weld zone  14  and propagate therefrom. 
     However, in a laser lap welding method for forming the shape of the weld zone  4  in accordance with aspects of the present invention described later, the weld zone  4  is formed such that its width W is substantially constant from the starting edge S to a predetermined point and that the width W decreases gradually from the predetermined point toward the terminal end point E, as shown in  FIG.  4 (B) . The steel sheets  2  and  3  in the lapped portion are welded such that, in the first half of the welding until the predetermined point, the fusion zone penetrates the entire steel sheets  2  and  3 . However, in the second half of the welding after the predetermined point, the amount of the fused portion of the lower steel sheet  3  decreases. At the terminal end point E of the weld zone  4 , a small crater  5  is formed in the upper steel sheet  2 . 
     As shown in  FIGS.  4 (A) and  4 (B) , in contrast to the conventional crater  15 , the size of the crater  5  formed at the terminal end point E of the weld zone  4  is smaller (shallower) than the thickness of the uppermost steel sheet. Specifically, as shown in  FIGS.  2 (B) and  3 (B) , the depth d c  of the crater  5  is small, and the crater  5  does not penetrate the steel sheet  2 . In the case of such a shape, the concentration of tensile stress directed from the outer circumference of the fusion zone toward the outside (force in the direction of an arrow Fb in  FIG.  2 (B) ) on a central portion at the terminal end point E of the weld zone  4  (a central portion of a final solidification zone at the terminal end point E) is small. Therefore, the occurrence of solidification cracking in the terminal end portion of the weld zone  4  can be prevented. 
     In the laser-welded lap joint  1  according to aspects of the present invention, the surface size of the weld zone  4  is adjusted within a prescribed range based on the above-described technological idea. In particular, it is important to reduce the size of the crater  5  in the welding terminal end portion. 
     Specifically, as shown in  FIG.  4 (B) , the weld zone  4  includes a main weld zone  4   a  that penetrates the steel sheets in the lapped portion and a final weld zone  4   b  formed continuously with one end of the main weld zone  4   a  and having the crater  5 . The weld zone  4  satisfies the above formulas (1) to (4). 
     (Full Length L (mm) of Weld Zone  4 : L≥15.0 mm) 
     If the full length L of each weld zone  4  is shorter than 15.0 mm, the length L 2  (mm) of the final weld zone  4   b  described later is insufficient, and weld cracking occurs. Therefore, the full length L of the weld zone  4  is set to 15.0 mm or more (formula (1) above). Preferably, the full length L is 20.0 mm or more. The upper limit of the full length L of the weld zone  4  is not particularly specified. However, from the viewpoint of preventing an increase in the time for welding the components, the full length L is preferably 40.0 mm or less and more preferably 30.0 mm or less. 
     (Length L 2  (mm) of Final Weld Zone  4   b:  10.0≥L 2 ≥2l c ) 
     If the length l c  of the crater  5  in the final weld zone  4   b  is larger than ½ of the length L 2  of the final weld zone  4   b , the ratio of the crater  5  to the terminal end portion (the final weld zone  4   b ) of the weld zone  4  is large, so that the occurrence of weld cracking cannot be prevented. Therefore, the length l c  of the crater  5  in the final weld zone  4   b  is equal to or less than ½ of the length L 2  (mm) of the final weld zone  4   b . Specifically, the length L 2  of the final weld zone  4   b  is equal to or larger than 2 times the length l c  of the crater  5  in the final weld zone  4   b  (formula (2) above) and is preferably 7.0 mm or more. From the viewpoint of preventing the occurrence of weld cracking, the upper limit of the length L 2  of the final weld zone  4   b  is 10.0 mm or less and preferably 9.5 mm or less. 
     (Thickness t 1  (mm) of Uppermost Steel Sheet in Lapped Portion: t 1 ≥2d c ) 
     If the depth d c  of the crater  5  in the final weld zone  4   b  is larger than ½ of the thickness of the uppermost steel in the lapped portion (the uppermost one of the plurality of lapped steel sheets in the lapped portion), underfill on the surface of the steel sheets is significant. Therefore, the tensile stress concentration on the terminal end point E of the weld zone  4  increases, and weld cracking is likely to occur. When weld cracking occurs, fracture tends to occur from the weld cracking portion in a peeling strength evaluation test as shown in Examples according to aspects of the invention described later, and the target peeling strength according to aspects of the present invention may not be obtained. Therefore, the depth d c  of the crater  5  in the final weld zone  4   b  is set to be equal to or less than ½ of the thickness t 1  (mm) of the uppermost steel sheet in the lapped portion. Specifically, the thickness t 1  of the uppermost steel sheet in the lapped portion is set to be equal to or larger than 2 times the depth d c  of the crater  5  in the final weld zone  4   b  (formula (3) above). 
     In the following description, “the thickness t 1  of the uppermost steel sheet in the lapped portion” is as follows. In the case of the example shown in  FIG.  4 (B) , “t 2 ” in the figure corresponds to the above t 1 . 
     In accordance with aspects of the present invention, the lower limit of the depth d c  of the crater  5  in the final weld zone  4   b  is not particularly specified. However, from the viewpoint of actual operation, the depth d c  of the crater  5  in the final weld zone  4   b  is preferably equal to or less than ⅓ of the thickness t 1  (mm) of the uppermost steel sheet in the lapped portion. Specifically, the thickness t 1  of the uppermost steel sheet in the lapped portion is preferably equal to or more than 3 times the depth d c  of the crater  5  in the final weld zone  4   b  (3d c ≤t 1 ). More preferably, the thickness t 1  of the uppermost steel sheet in the lapped portion is 0.7 mm or more. 
     From the viewpoint of performing welding efficiently without lowering the laser power as much as possible, the depth d c  of the crater  5  in the final weld zone  4   b  is preferably equal to or more than 1/10 of the thickness t 1  (mm) of the uppermost steel sheet in the lapped portion. Specifically, the thickness t 1  of the uppermost steel sheet in the lapped portion is preferably equal to or less than 10 times the depth d c  of the crater  5  in the final weld zone  4   b  (10×d c ≥t 1 ). 
     (Width w c  (mm) of Crater  5  in Final Weld Zone  4   b : w c &gt;d c ) 
     Even when the depth d c  of the crater  5  in the final weld zone  4   b  is equal to or less than ½ of the thickness t 1  of the uppermost steel sheet in the lapped portion, if the depth d c  of the crater  5  in the final weld zone  4   b  is equal to or more than the width w c  of the crater  5  in the final weld zone  4   b , underfill on the surface of the steel sheet is significant. In this case, the concentration of tensile stress on the terminal end point E of the weld zone  4  increases, and weld cracking is likely to occur. When a weld crack occurs, fracture tends to be initiated from the weld crack in a peeling strength evaluation test as shown in Examples according to aspects of the invention described later, and the target peeling strength according to aspects of the present invention may not be obtained. Therefore, the depth d c  of the crater  5  in the final weld zone  4   b  is set to be less than the width w c  of the crater  5  in the final weld zone  4   b . Specifically, the width w c  of the crater  5  in the final weld zone  4   b  is larger than the depth d c  of the crater  5  in the final weld zone  4   b  (formula (4) above). Preferably, the width w c  of the crater  5  in the final weld zone  4   b  is 0.15 mm or more. 
     The lower limit of the depth d c  of the crater  5  in the final weld zone  4   b  is not particularly specified. However, from the viewpoint of actual operation, the depth d c  of the crater  5  is preferably 0.30 mm or more. 
     As the size of the crater  5  in the final weld zone  4   b  increases, the degree of underfill increases. Therefore, preferably, the width w c  of the crater  5  in the final weld zone  4   b  is set to be 2 mm or less. 
     As described above, in the weld zone  4  in accordance with aspects of the present invention, the main weld zone  4   a  and the final weld zone  4   b  are formed so as to satisfy the above prescribed ranges, and the effects obtained by reducing the size of the crater  5  can be maximized. Therefore, excessive stress concentration on the terminal end point E of the weld zone  4  can be prevented, and the occurrence of weld cracking can be prevented. In this case, even when the minimum value of the full length L of the weld zone  4  is short, i.e., 15.0 mm, the occurrence of weld defects at the end of the weld zone can be prevented. 
     The laser-welded lap joint  1  according to aspects of the present invention is configured as described above, and the target characteristics according to aspects of the invention can thereby be obtained. However, the following optional structural features may be added in addition to the above described structural features. 
     (Ratio of Total Size of Gaps (Mm) Between Steel Sheets to Total Thickness T (Mm) of Plurality of Steel Sheets: 0% or More and 15% or Less) 
     In accordance with aspects of the present invention, the ratio of the total size G of gaps between steel sheets (hereinafter may be referred to as the total sheet gap) in the lapped portion including the plurality of lapped steel sheets to the total thickness T (mm) of the plurality steel sheets may be 0% or more and 15% or less. In the example shown in  FIG.  4 (B) , the gap (sheet gap) between steel sheets is present only between the steel sheets  2  and  3 . Let the total sheet gap be denoted by G mm, and let the total thickness T of the plurality of steel sheets be denoted by (t 2 +t 3 ) mm. Then 0%≤G/(t 2 +t 3 )≤15% holds. By controlling the ratio of the total sheet gap G (mm) to the total thickness T (mm) of the plurality of steel sheets within the above range, the degree of underfill is reduced, and the stress concentration on the weld zone formed on the lapped surfaces is reduced. Therefore, weld cracking can be prevented, and the peeling strength can be further improved. The ratio of the total sheet gap G (mm) to the total thickness T (mm) of the plurality of steel sheets is more preferably 5% or more and is more preferably 10% or less. 
     (Chemical Composition of Steel Sheets) 
     No particular limitation is imposed on the chemical composition of the steel sheets used for the laser-welded lap joint  1  according to aspects of the present invention. At least one steel sheet of the plurality of lapped steel sheets may have a chemical composition containing, in mass %, for example, C: more than 0.07% and 0.25% or less, P+S: less than 0.03%, Mn: 1.8% or more and 3.0% or less, and Si: more than 1.2% and 2.5% or less, with the balance being Fe and unavoidable impurities. % in the chemical composition means % by mass. 
     (C: More than 0.07% and 0.25% or Less) 
     When the content of C exceeds 0.07%, the effect of precipitation strengthening can be obtained. When the content of C is 0.25% or less, coarse carbide precipitates may not be formed, and desired high strength and desired workability can be obtained. Therefore, the content of C is preferably more than 0.07% and 0.25% or less. More preferably, the content of C is 0.10% or more and 0.21% or less. 
     (P+S: Less than 0.03%) 
     When the total of the content of P and the content of S (P+S) is less than 0.03%, ductility and toughness do not deteriorate, and the desired high strength and the desired workability can be obtained. Therefore, the total (P+S) of the content of P and the content of S is preferably less than 0.03%. 
     (Mn: 1.8% or More and 3.0% or Less) 
     When the content of Mn is 1.8% or more, sufficient hardenability can be obtained, so that coarse carbide precipitates are unlikely to be formed. When the content of Mn is 3.0% or less, susceptibility to grain boundary embrittlement increases, and toughness and resistance to low-temperature cracking are unlikely to deteriorate. Therefore, the content of Mn is preferably 1.8% or more and 3.0% or less. The content of Mn is more preferably 1.9% or more. The content of Mn is more preferably 2.7% or less and still more preferably 2.5% or less. 
     (Si: More than 1.2% and 2.5% or Less) 
     When the content of Si is more than 1.2%, the effect of increasing the strength of the steel by solid solution strengthening can be obtained sufficiently. When the content of Si is 2.5% or less, the heat-affected zone is less likely to be excessively hardened, and the toughness of the heat-affected zone and its resistance to low-temperature cracking are unlikely to deteriorate. Therefore, the content of Si is preferably more than 1.2% and 2.5% or less. More preferably, the content of Si is 1.3% or more and 1.5% or less. 
     (Balance: Fe and Unavoidable Impurities) 
     The balance of the chemical composition is Fe and unavoidable impurities. Examples of the unavoidable impurities include Al: 0.015 to 0.050% and N: 0.002 to 0.005%. 
     Moreover, to further improve the strength of the steel sheets and their peeling strength, one or two selected from the following group A and group B may be optionally contained in addition to the above-described chemical composition. 
     (Group A: In Mass %, One or Two Selected from Ti: 0.005% or More and 0.01% or Less and Nb: 0.005% or More and Less than 0.050%) 
     Ti and Nb precipitate as carbides or nitrides and have the effect of preventing austenite from coarsening during annealing. Therefore, when Ti and/or Nb is contained, it is preferable to contain at least one of them. When Ti and/or Nb is contained in order to obtain the above effect, it is preferable to contain Ti in an amount of 0.005% or more and/or Nb in an amount of 0.005% or more. Even though excessively large amounts of these elements are contained, the efficacy of the above effect is saturated, and this may rather be uneconomical. Moreover, the recrystallization temperature during annealing increases, and the metallographic structure after annealing may become nonuniform, so that stretch flangeability may deteriorate. In addition, the amount of precipitated carbides or nitrides may increase. In this case, an increase in yield ratio may occur, and deterioration in shape fixability may also occur. Therefore, when Ti and/or Nb is contained, the content of Ti is preferably 0.01% or less, and the content of Nb is preferably less than 0.050%. The content of Ti is more preferably less than 0.0080%. The content of Nb is more preferably less than 0.040%. 
     (Group B: In Mass %, One or Two or More Selected from Cr: 1.0% or Less, Mo: 0.50% or Less, and B: 0.10% or Less) 
     Cr, Mo, and B are elements having the effect of improving the hardenability of the steel. Therefore, at least one of these elements may be contained. However, even though excessively large amounts of these elements are contained, the above effect is saturated, and this may rather be uneconomical. Therefore, when Cr, Mo, and B are contained, the content of Cr is 1.0% or less, the content of Mo is 0.50% or less, and the content of B is 0.10% or less. The content of Cr is more preferably 0.50% or less. The content of Mo is more preferably 0.10% or less. The content of B is more preferably 0.03% or less and still more preferably 0.0030% or less. The content of Cr is preferably 0.01% or more. The content of Mo is preferably 0.004% or more. The content of B is preferably 0.0001% or more. 
     (Tensile Strength of Steel Sheets) 
     At least one steel sheet of the plurality of steel sheets used for the laser-welded lap joint  1  according to aspects of the present invention may be a high-tensile steel sheet having a tensile strength TS of 980 MPa or more. Even when at least one steel sheet is the above high-tensile steel sheet, the laser-welded lap joint  1  can have high joint strength, and the occurrence of weld defects can be prevented. Preferably, for example, at least one steel sheet of the plurality of steel sheets has the above-described chemical composition and has a tensile strength TS of 980 MPa or more. The plurality of steel sheets may be of the same type and may have the same shape, but different types of steel sheets or steel sheets with different shapes may be used. 
     (Thicknesses of Steel Sheets) 
     In accordance with aspects of the present invention, no particular limitation is imposed on the thicknesses t of the plurality of steel sheets to be subjected to laser lap welding. However, the thicknesses are preferably within the range of, for example, 0.5 mm≤t≤3.2 mm. Steel sheets with thicknesses within the above range can be used preferably for automobile exterior body panels and automobile frame components. The thicknesses of the plurality of steel sheets may be the same or different. 
     Specifically, in the laser-welded lap joint  1  shown in  FIG.  1    etc., it is preferable that the thickness t 2  of the upper steel sheet  2  satisfies 0.6 mm≤t 2 ≤1.2 mm and that the thickness t 3  of the lower steel sheet  3  satisfies 1.0 mm≤t 3 ≤2.5 mm. Alternatively, it is preferable that the thickness t 2  of the upper steel sheet  2  and the thickness t 3  of the lower steel sheet  3  fall within the ranges of 0.5 mm≤t 2 ≤3.2 mm and 0.5 mm≤t 3 ≤3.2 mm. 
     The “weld cracking” in accordance with aspects of the present invention is low-temperature cracking that occurs at the welding terminal end portion of the weld zone  4  and propagates from the welding terminal end point E to the welding starting edge S. The presence or absence of the weld cracking can be determined by cutting the weld zone  4  after welding and checking the presence or absence of cracking. The presence or absence of cracking can be checked by visual inspection. To check the presence of cracking more clearly, it is preferable to observe the cross section of the weld zone, for example, under an optical microscope at a magnification of about 10×. When a photograph of the cross section of the weld zone is taken, the weld zone is cut in a direction perpendicular to the welding direction at a position spaced 5 mm apart from the welding terminal end portion using a precision cutter. Weld cracking observed penetrates the weld zone  4  from the front surface to the back surface. 
     &lt;Method for Producing Laser-Welded Lap Joint&gt; 
     First, a method for producing the laser-welded lap joint  1  according to aspects of the present invention will be described using  FIG.  5   .  FIG.  5    is an illustration showing an example of the method for producing the laser-welded lap joint  1  according to aspects of the present invention. 
     The method for producing the laser-welded lap joint  1  according to aspects of the present invention is the method for producing the above-described laser-welded lap joint  1  and includes: lapping the plurality of steel sheets vertically; and then performing laser lap welding by applying a laser beam to the surface of the uppermost steel sheet in the lapped portion including the plurality of lapped steel sheets to form a weld zone  4  in the lapped portion to thereby join the plurality of lapped steel sheets together. 
     In according to aspects the present invention, one-side welding is performed on the plurality of lapped steel sheets. By performing one-side welding, space-saving can be achieved in an assembly line for automobile body components. In the one-side welding, it is preferable that laser lap welding is performed from the side on which a thicker steel sheet among the plurality of lapped steel sheets is disposed. In this manner, the occurrence of burn through can be prevented. When the steel sheets have the same thickness, laser lap welding may be performed from any side. 
     In the example shown in  FIG.  5   , the laser-welded lap joint  1  according to aspects of the present invention can be obtained by laser lap welding. Specifically, the steel sheets  2  and  3  are lapped, and the laser beam  7  is applied linearly to the surface of the outermost steel sheet  2  so as to form the weld zone  4  in the steel sheets  2  and  3 . 
     In the laser lap welding described above, the laser beam  7  is continuously applied while scanned linearly. For example, as shown in  FIG.  5   , the main weld zone  4   a  and the final weld zone  4   b  are formed continuously to form the weld zone  4 . In this case, by performing a main welding step of forming the main weld zone  4   a  and a final welding step of forming the final weld zone  4   b  continuously, the effect of the crater  5  with a reduced size can be maximized. This is preferable because excessive stress concentration on the terminal end point E of the weld zone  4  (see  FIGS.  2 (B) and  3 (B) ) can be prevented and the occurrence of cracking can be prevented. 
     The laser lap welding in accordance with aspects of the present invention includes the main welding step of forming the main weld zone  4   a  and the final welding step of forming the final weld zone  4   b  having the crater  5 . The main welding step is the first half of the welding, and main welding is performed to form a weld zone penetrating all the steel sheets in the lapped portion. In the final welding step subsequent to the main welding step, final welding is performed until the terminal end point E of the weld zone  4  while welding conditions described later are controlled for the purpose of reducing the size of the crater  5  formed at the terminal end point E of the weld zone  4 . Preferably, at least one of laser power, welding speed, a focal position, and a beam diameter in the final welding step is controlled such that the main weld zone  4   a  and the final weld zone  4   b  having the crater  5  form the weld zone  4  satisfying formulas (1) to (4) described above. In the main welding step, the welding is performed under constant conditions. 
     Examples of the laser beam that can be used include a fiber laser and a disk laser. It is preferable that the beam diameter is 0.3 to 0.8 mm, that the laser power is 2.0 to 5.0 kW, that the focal position is located in the range extending from the surface of the outermost steel sheet to a position 0 to 20 mm above the surface of the outermost steel sheet, and that the welding speed is 2.0 to 5.0 m/min. 
     During the formation of the main weld zone  4   a , it is more preferable to control the beam diameter within the range of 0.5 to 0.8 mm, the laser power within the range of 2.5 to 4.5 kW, the focal position within the range extending from the surface of the uppermost steel sheet to a position 20 mm above the surface of the uppermost steel sheet, and the welding speed within the range of 2.5 to 4.5 m/min. 
     During the formation of the final weld zone  4   b , it is preferable to control the beam diameter within the range of 0.2 to 0.6 mm, the laser power within the range of 0.5 to 3.0 kW, the focal position within the range extending from the surface of the uppermost steel sheet to a position 30 mm above the surface of the uppermost steel sheet, and the welding speed within the range of 2.0 to 4.0 m/min. 
     It is more preferable that the laser power, the welding speed, and the focal position are controlled such that the relations between the laser power P i , the welding speed v i , and the focal position f i  in the main welding step and the laser power P f , the welding speed v f , and the focal position f f  in the final welding step are as shown in graphs in  FIGS.  8 (A) to  8 (C) . Specifically, at least one of the laser power, the welding speed, the focal position, and the beam diameter in the final welding step can be controlled under the conditions satisfying formulas (5) to (7).
 
 P   i   ≥P   f ≥(¼) P   i   (5)
 
 v   i   ≥v   f ≥(¼) v   i   (6)
 
 f   i   ≤f   f ≤20.0  (7)
 
     Here, P i  is the laser power (unit: kW) in the main welding step, and P f  is the laser power (unit: kW) in the final welding step. v i  is the welding speed (unit: m/min) in the main welding step, and v f  is the welding speed (unit: m/min) in the final welding step. f i  is the focal position (unit: mm) in the main welding step, and f f  is the focal position (unit: mm) in the final welding step.
 
 P   i   ≥P   f ≥(¼) P   i   (5)
 
     If the laser power P f  (kW) in the final welding step is larger than the laser power P i  (kW) in the main welding step, the degree of underfill increases, and burn through may occur. More preferably, the laser power P f  in the final welding step is (P i ×0.5) or less. 
     If the laser power P f  in the final welding step is less than ¼ of the laser power P i  in the main welding step, the laser power is insufficient, and the steel sheets cannot be melted. Therefore, a sufficient weld line length cannot be obtained, and peeling strength may be insufficient. More preferably, the laser power P f  in the final welding step is (P i ×⅓) or more.
 
 v   i   ≥v   f ≥(¼) v   i   (6)
 
     If the welding speed v f  (m/min) in the final welding step exceeds the welding speed v i  (m/min) in the main welding step, the depth d c  of the crater  5  in the final weld zone  4   b  may increase. More preferably, the welding speed v f  in the final welding step is (v i ×0.8) or less. 
     If the welding speed v f  in the final welding step is less than ¼ of the welding speed v i  in the main welding step, the size of the crater  5  in the final weld zone  4   b  increases, and burn through may occur. More preferably, the welding speed v f  in the final welding step is (v i ×½) or more.
 
 f   i   ≤f   f ≤20.0  (7)
 
     If the focal position f f  (mm) in the final welding step is less than the focal position f i  (mm) in the main welding step, the depth d c  of the crater  5  in the final weld zone  4   b  may increase. More preferably, the focal position f f  in the final welding step is (f i ×1.2) or more. 
     If the focal position f f  in the final welding step exceeds 20.0 mm, the power density of the laser beam is insufficient, and the steel sheets cannot be melted. Therefore, a sufficient weld line length cannot be obtained, and the peeling strength may be insufficient. More preferably, the focal position f f  in the final welding step is 15.0 mm or less. 
     In the welding method according to aspects of the present invention, the steel sheets  2  and  3  used may be, for example, steel sheets having the above-described chemical composition and having a tensile strength TS of 980 MPa or more. As for the thicknesses t 2  and t 3  of the steel sheets  2  and  3 , 0.5 mm≤t 2 ≤3.2 mm holds, and 0.5 mm≤t 3 ≤3.2 mm holds. The sheet gap between the steel sheets  2  and  3  may be 0% or more and 15% or less of the total thickness of the steel sheets  2  and  3 . The reason that these steel sheets are used is the same as above. 
     Next, an example of a preferred welding position in the laser-welded lap joint  1  according to aspects of the present invention will be described using  FIG.  6   . 
       FIG.  6    shows illustrations of an example of a preferred position of the weld zone (fusion zone)  4  in the laser-welded lap joint  1  according to aspects of the present invention.  FIG.  6 (A)  is a top view showing the periphery of the weld zone  4  in the two steel sheets  2  and  3 , and  FIG.  6 (B)  is a cross-sectional view taken along line C-C in  FIG.  6 (A) . In the description of  FIG.  6   , the steel sheet  2  is referred to also as a flange portion  2   b , and the steel sheet  3  is referred to also as a frame component. 
     In the example shown in  FIGS.  6 (A) and  6 (B) , the position coordinate of an end of the contact portion between the flange portion  2   b  that is the upper steel sheet  2  and the frame component that is the lower steel sheet  3  is set to 0. In this coordinate system, the outer end side of the flange portion  2   b  is set to (−), and the vertical wall portion  2   a  side in a substantially hat shape (only part of the substantially hat shape is shown in  FIG.  6   ) is set to (+). In the flange portion  2   b  in the substantially hat shape, the thickness of a thickest portion of the steel sheet is denoted by t (mm). In this case, it is preferable to perform welding at a welding position X (mm) represented by formula (8) below by applying the one-side welding method. In this manner, an L-form tension-test piece shown in  FIG.  7   , including two lapped sheets, having a total thickness T of 2 to 5 mm, and having the flange portion  2   b  with a length of 50 mm can have a peeling strength of 1.2 kN or more.
 
−2 t≥X≥− 4 t   (8)
 
     The reason that X is set so as to satisfy formula (8) above will be described. 
     If the welding position X is closer to the contact end of the flange portion  2   b  than −2t, rupture tends to occur from the weld metal portion during the tensile test, and the peeling strength may also decrease. If the welding position X is farther from the contact end of the flange portion  2   b  than −4t, the moment applied to the weld zone  4  tends to increase, and the peeling strength may decrease. It is therefore preferable to set the welding position X so as to satisfy formula (8) above. 
     &lt;Automobile Frame Component&gt; 
     One example of a component for which the laser-welded lap joint  1  according to aspects of the present invention can be preferably used is an automobile frame component. In the automobile frame component shown in  FIG.  1   , the steel sheet  2  that is a frame component having a hat-shaped cross-sectional shape and the steel sheet  3  that is a panel component are used. In the automobile frame component, the flange portion  2   b  of the frame component (the steel sheet  2  shown in  FIG.  1   ) and the panel component (the steel sheet  3  shown in  FIG.  1   ) disposed so as to face the flange portion  2   b  are welded by the above-described welding method to form the weld zones  4 , and a closed cross section is thereby formed. 
     The automobile frame component according to aspects of the present invention is preferably used, for example, for center pillars, roof rails, etc. It is important in terms of collision safety that these components have sufficient peeling strength. A center pillar to which the automobile frame component according to aspects of the present invention is applied has sufficient peeling strength as described above. 
     As described above, in accordance with aspects of the present invention, a plurality of steel sheets including at least one high-tensile steel sheet are lapped, and a weld zone  4  is formed to weld and join the plurality of steel sheets together. In this manner, a laser-welded lap joint  1  with no weld defects formed on the front and back surfaces of the steel sheets can be obtained. 
     In the weld zone  4  in accordance with aspects of the present invention, the size of the crater at the end of the fusion zone can be reduced, and therefore the concentration of tensile stress on a central portion of the final solidification zone is reduced. In this case, the occurrence of cracking at the end of the fusion zone and propagation of the cracking can be prevented, and therefore the laser-welded lap joint  1  produced can have high peeling strength and excellent durability. 
     Moreover, even when the length of the fusion zone (15 mm) is shorter than that in conventional laser lap welding, the occurrence of weld cracking can be prevented. It is therefore expected that flexibility in the design of components is improved and the strength is improved by welding many portions necessary to have high peeling strength. 
     Moreover, since the laser-welded lap joint  1  according to aspects of the present invention has good appearance, it can be used preferably for structural members of automobiles. For example, by using high-strength steel sheets as the steel sheets to be joined, an automobile frame component can be obtained. By using such a laser-welded lap joint  1 , an automobile frame component etc. with high peeling strength can be obtained. 
     EXAMPLES 
     The operations and effects according to aspects of the present invention will be described by way of Examples. However, the present invention is not limited to the following Examples. 
     In the Examples, steel sheets having chemical compositions shown in Table 1 were used as test specimens. 
     The thickness of each steel sheet is 1.2 mm, 1.6 mm, or 2.0 mm, and its width is 50 mm. These steel sheets were used and bent into a shape with an L-shaped cross section as shown in  FIG.  7   . The L-shaped steel sheets  8  each have a long-side portion  8   a  and a short-side portion  8   b . The long-side portion  8   a  corresponds to the vertical wall portion  2   a  of the steel sheet  2  of the laser-welded lap joint  1  shown in  FIG.  1   , and the short-side portion  8   b  corresponds to the flange portion  2   b.    
     Two L-shaped steel sheets  8  of the same type and having the same thickness were used, and the short-side portions  8   b  of the steel sheets were lapped. Then laser lap welding was performed at a plurality of positions of the lapped portion (a flat portion in  FIG.  7   ) intermittently in a longitudinal direction to form weld beads (weld zones  4 ), and an L-shaped-test piece (hereinafter referred to as a test piece) was thereby produced. As for the size of the test piece, the size of the long-side portion  8   a  (the length of the vertical wall) was 120 mm, the size of the short-side portion  8   b  (the width of the test piece) was 50 mm, and the size of the lapped portion (the width of the flange) was 30 mm. The gap (sheet gap) G between the upper and lower L-shaped steel sheets  8  was appropriately adjusted within the range of 0 to 20% according to the thickness of the sheets. 
     The conditions for the weld zones  4  formed by the laser lap welding are shown in Tables 2-1 and 2-2. 
     As shown in  FIG.  7   , the coordinate of the welding position is represented in a coordinate system in which, in the lapped portion including the two L-shaped steel sheets  8 , a contact position closest to the vertical wall is set to 0. In this coordinate system, the outside of the lapped portion of the test piece is set to (−), and the vertical wall side of the test piece is set to (+). The welding position in this case is denoted by X, and the full length of each fusion zone (weld zone)  4  is denoted by L. The length of the terminal end portion (the final weld zone  4   b ) of each weld zone  4  is denoted by L 2 , and the length of the crater  5  in the final weld zone  4   b  is denoted by l c . The depth of the crater  5  in the final weld zone  4   b  is denoted by d c , and the width of the crater  5  in the final weld zone  4   b  is denoted by w c . The test was performed on various test pieces with different dimensional values. 
     A fiber laser with a beam diameter of 0.6 mm ϕ at the focal position was used for the laser welding. The laser power, the welding speed, and the focal position in each of the main welding step and the final welding step were controlled so as to satisfy the relations shown in the graphs in  FIGS.  8 (A)  to  8 (C), and the size of the crater  5  in the final weld zone  4   b  was thereby adjusted. A work distance was set to the surface of the uppermost steel sheet. The focal position in the laser lap welding was set to 0 on the surface of the upper most one of the lapped steel sheets (the surface of the upper L-shaped steel sheet  8  in the example shown in  FIG.  7   ), and the vertically upward direction relative to the L-shaped steel sheets  8  was set to positive. The welding was performed in air. 
     The peeling strength was measured by an L-form tension test. Specifically, steel sheets  8  bent into an L shape were lapped as shown in  FIG.  7    and lap laser-welded, and a tensile load was applied from opposite sides of the welded steel sheets. The tensile test was performed according to JIS 23136 at a speed of 10 mm/min. When the peeling strength was 1.2 kN or more, the test piece was regarded as having high joint strength and rated “pass.” 
     The occurrence of cracking can be determined by visual inspection and a liquid penetrant test. In the Examples, the occurrence of cracking was determined by visual inspection as described above. Specifically, each of the obtained test pieces was cut in a direction perpendicular to the welding direction at a position 5 mm from the center of the crater  5  in the final weld zone  4   b  toward the welding start side. The section was observed under an optical microscope at a magnification of 10×. One test piece was observed for each of the conditions. When weld cracking penetrating the weld zone  4  from the front surface to the back surface was found, the test piece was judged as having weld cracking. 
     The obtained judgement results for weld cracking and peeling strength are shown in Tables 2-1 and 2-2. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Steel 
                 Chemical composition (% by mass) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 type 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Ti 
                 Nb 
                 Cr 
                 Mo 
                 B 
                 Al 
                 N 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 A 
                 0.21 
                 1.5 
                 2.0 
                 0.004 
                 0.0007 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.031 
                 0.0025 
               
               
                 B 
                 0.12 
                 1.4 
                 2.0 
                 0.015 
                 0.002 
                 0.0017 
                 0.042 
                 — 
                 — 
                 — 
                 0.031 
                 0.0040 
               
               
                 C 
                 0.13 
                 1.3 
                 2.0 
                 0.014 
                 0.002 
                 — 
                 — 
                 0.019 
                 0.010 
                 0.0001 
                 0.028 
                 0.0050 
               
               
                 D 
                 0.13 
                 1.4 
                 2.1 
                 0.013 
                 0.002 
                 0.0050 
                 0.039 
                 0.020 
                 0.010 
                 0.0001 
                 0.028 
                 0.0033 
               
               
                 E 
                 0.18 
                 1.4 
                 2.7 
                 0.003 
                 0.0009 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.025 
                 0.0035 
               
               
                 F 
                 0.13 
                 1.4 
                 2.1 
                 0.013 
                 0.002 
                 0.0020 
                 0.001 
                 — 
                 — 
                 — 
                 0.033 
                 0.0040 
               
               
                 G 
                 0.13 
                 1.3 
                 2.2 
                 0.013 
                 0.001 
                 — 
                 — 
                 0.021 
                 0.020 
                 0.0002 
                 0.029 
                 0.0040 
               
               
                 H 
                 0.12 
                 1.4 
                 2.1 
                 0.014 
                 0.002 
                 0.0018 
                 0.045 
                 0.023 
                 0.020 
                 0.0002 
                 0.030 
                 0.0050 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2-1 
               
               
                   
               
             
            
               
                   
                   
                   
                   
                   
                 Thickness  
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                   
                   
                 t1 of  
                 Thickness t 
                   
                   
                   
                 Full 
                 Length 
                   
               
               
                   
                   
                   
                 Thickness 
                 Thickness 
                 uppermost 
                 of thickest 
                   
                   
                   
                 length 
                 L2 of 
                 Main 
               
               
                   
                   
                   
                 t2 of 
                 t3 of 
                 steel sheet  
                 steel sheet 
                   
                 Total 
                   
                 L of 
                 final 
                 welding  
               
               
                   
                   
                   
                 steel 
                 steel 
                 in lapped 
                 in lapped 
                 Total 
                 sheet 
                 Welding 
                 weld 
                 weld 
                 step 
               
               
                 Test 
                 Steel 
                 TS 
                 sheet 2 
                 sheet 3 
                 portion 
                 portion 
                 thickness 
                 gap G 
                 position 
                 zone 
                 zone 
                 Power 
               
               
                 No. 
                 type 
                 (MPa) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 T (mm) 
                 (mm) 
                 X (mm) 
                 (mm) 
                 (mm) 
                 Pi (kW) 
               
               
                   
               
               
                 1 
                 A 
                 980 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 15.0 
                 7.5 
                 3.6 
               
               
                 2 
                 A 
                 980 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.4 
                 −3.6 
                 15.0 
                 7.5 
                 3.6 
               
               
                 4 
                 A 
                 980 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 10.0 
                 7.5 
                 3.6 
               
               
                 5 
                 A 
                 980 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 15.0 
                 5.0 
                 3.6 
               
               
                 6 
                 A 
                 980 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 15.0 
                 7.5 
                 3.6 
               
               
                 7 
                 A 
                 980 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 15.0 
                 7.5 
                 3.6 
               
               
                 8 
                 A 
                 980 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 15.0 
                 7.5 
                 3.6 
               
               
                 9 
                 E 
                 1180 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 15.0 
                 7.5 
                 3.6 
               
               
                 10 
                 E 
                 1180 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.4 
                 −3.6 
                 15.0 
                 7.5 
                 3.6 
               
               
                 12 
                 E 
                 1180 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 10.0 
                 7.5 
                 3.6 
               
               
                 13 
                 E 
                 1180 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 15.0 
                 5.0 
                 3.6 
               
               
                 14 
                 E 
                 1180 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 15.0 
                 7.5 
                 3.6 
               
               
                 15 
                 E 
                 1180 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 15.0 
                 7.5 
                 3.6 
               
               
                 16 
                 E 
                 1180 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 15.0 
                 7.5 
                 3.6 
               
               
                 17 
                 B 
                 980 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.3 
                 −4.8 
                 15.0 
                 7.5 
                 4.8 
               
               
                 18 
                 B 
                 980 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.5 
                 −4.8 
                 15.0 
                 7.5 
                 4.8 
               
               
                 20 
                 B 
                 980 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.3 
                 −4.8 
                 10.0 
                 7.5 
                 4.8 
               
               
                 21 
                 B 
                 980 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.3 
                 −4.8 
                 15.0 
                 5.0 
                 4.8 
               
               
                 22 
                 B 
                 980 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.3 
                 −4.8 
                 15.0 
                 7.5 
                 4.8 
               
               
                 23 
                 B 
                 980 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.3 
                 −4.8 
                 15.0 
                 7.5 
                 4.8 
               
               
                 24 
                 B 
                 980 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.3 
                 −4.8 
                 15.0 
                 7.5 
                 4.8 
               
               
                 25 
                 F 
                 1180 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.3 
                 −4.8 
                 15.0 
                 7.5 
                 4.8 
               
               
                 26 
                 F 
                 1180 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.5 
                 −4.8 
                 15.0 
                 7.5 
                 4.8 
               
               
                 28 
                 F 
                 1180 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.3 
                 −4.8 
                 10.0 
                 7.5 
                 4.8 
               
               
                 29 
                 F 
                 1180 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.3 
                 −4.8 
                 15.0 
                 5.0 
                 4.8 
               
               
                 30 
                 F 
                 1180 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.3 
                 −4.8 
                 15.0 
                 7.5 
                 4.8 
               
               
                 31 
                 F 
                 1180 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.3 
                 −4.8 
                 15.0 
                 7.5 
                 4.8 
               
               
                 32 
                 F 
                 1180 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.3 
                 −4.8 
                 15.0 
                 7.5 
                 4.8 
               
               
                 33 
                 C 
                 980 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.4 
                 −6.0 
                 15.0 
                 7.5 
                 6.0 
               
               
                 34 
                 C 
                 980 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.7 
                 −6.0 
                 15.0 
                 7.5 
                 6.0 
               
               
                 36 
                 C 
                 980 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.4 
                 −6.0 
                 10.0 
                 7.5 
                 6.0 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Main welding 
                   
                   
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 step 
                 Final welding step 
                 Shape of crater 
                 Evaluation 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Speed 
                 Focal 
                 Power 
                 Speed 
                 Focal 
                   
                 Depth 
                 Width 
                   
                 Peeling 
                   
               
               
                   
                 Test 
                 vi 
                 position 
                 Pf 
                 vf 
                 position 
                 Length 
                 dc 
                 wcC 
                 Weld 
                 strength 
                   
               
               
                   
                 No. 
                 (m/min) 
                 fi (mm) 
                 (kW) 
                 (m/min) 
                 ff (mm) 
                 lc (mm) 
                 (mm) 
                 (mm) 
                 cracking 
                 (kN) 
                 Remarks 
               
               
                   
               
               
                   
                 1 
                 3.0 
                 0.0 
                 0.9 
                 1.5 
                 10.0 
                 3.10 
                 0.52 
                 0.67 
                 No 
                 1.51 
                 Inventive Example 
               
               
                   
                 2 
                 3.0 
                 0.0 
                 0.9 
                 1.5 
                 10.0 
                 3.13 
                 0.87 
                 0.73 
                 Yes 
                 0.78 
                 Comparative Example 
               
               
                   
                 4 
                 3.0 
                 0.0 
                 0.9 
                 1.5 
                 10.0 
                 2.92 
                 0.45 
                 0.72 
                 Yes 
                 0.81 
                 Comparative Example 
               
               
                   
                 5 
                 3.0 
                 0.0 
                 0.9 
                 1.5 
                 10.0 
                 3.05 
                 0.50 
                 0.71 
                 Yes 
                 1.56 
                 Comparative Example 
               
               
                   
                 6 
                 3.0 
                 0.0 
                 3.6 
                 1.5 
                 10.0 
                 2.89 
                 0.78 
                 0.69 
                 Yes 
                 1.61 
                 Comparative Example 
               
               
                   
                 7 
                 3.0 
                 0.0 
                 0.9 
                 3.0 
                 10.0 
                 5.32 
                 0.51 
                 0.63 
                 Yes 
                 1.28 
                 Comparative Example 
               
               
                   
                 8 
                 3.0 
                 0.0 
                 0.9 
                 1.5 
                 0.0 
                 2.89 
                 0.46 
                 0.42 
                 Yes 
                 1.35 
                 Comparative Example 
               
               
                   
                 9 
                 3.0 
                 0.0 
                 0.9 
                 1.5 
                 10.0 
                 2.96 
                 0.57 
                 0.73 
                 No 
                 1.52 
                 Inventive Example 
               
               
                   
                 10 
                 3.0 
                 0.0 
                 0.9 
                 1.5 
                 10.0 
                 3.17 
                 0.80 
                 0.74 
                 Yes 
                 0.77 
                 Comparative Example 
               
               
                   
                 12 
                 3.0 
                 0.0 
                 0.9 
                 1.5 
                 10.0 
                 2.92 
                 0.41 
                 0.67 
                 Yes 
                 0.90 
                 Comparative Example 
               
               
                   
                 13 
                 3.0 
                 0.0 
                 0.9 
                 1.5 
                 10.0 
                 3.11 
                 0.44 
                 0.68 
                 Yes 
                 1.72 
                 Comparative Example 
               
               
                   
                 14 
                 3.0 
                 0.0 
                 3.6 
                 1.5 
                 10.0 
                 3.14 
                 0.88 
                 0.72 
                 Yes 
                 1.93 
                 Comparative Example 
               
               
                   
                 15 
                 3.0 
                 0.0 
                 0.9 
                 3.0 
                 10.0 
                 4.97 
                 0.54 
                 0.74 
                 Yes 
                 1.43 
                 Comparative Example 
               
               
                   
                 16 
                 3.0 
                 0.0 
                 0.9 
                 1.5 
                 0.0 
                 3.21 
                 0.49 
                 0.43 
                 Yes 
                 1.67 
                 Comparative Example 
               
               
                   
                 17 
                 3.0 
                 0.0 
                 1.2 
                 1.5 
                 10.0 
                 2.95 
                 0.67 
                 0.84 
                 No 
                 1.28 
                 Inventive Example 
               
               
                   
                 18 
                 3.0 
                 0.0 
                 1.2 
                 1.5 
                 10.0 
                 3.18 
                 1.15 
                 0.82 
                 Yes 
                 0.84 
                 Comparative Example 
               
               
                   
                 20 
                 3.0 
                 0.0 
                 1.2 
                 1.5 
                 10.0 
                 2.99 
                 0.78 
                 0.85 
                 Yes 
                 0.79 
                 Comparative Example 
               
               
                   
                 21 
                 3.0 
                 0.0 
                 1.2 
                 1.5 
                 10.0 
                 3.06 
                 0.66 
                 0.94 
                 Yes 
                 1.58 
                 Comparative Example 
               
               
                   
                 22 
                 3.0 
                 0.0 
                 4.8 
                 1.5 
                 10.0 
                 3.00 
                 1.20 
                 0.81 
                 Yes 
                 1.35 
                 Comparative Example 
               
               
                   
                 23 
                 3.0 
                 0.0 
                 1.2 
                 3.0 
                 10.0 
                 4.88 
                 0.68 
                 0.94 
                 Yes 
                 1.76 
                 Comparative Example 
               
               
                   
                 24 
                 3.0 
                 0.0 
                 1.2 
                 1.5 
                 0.0 
                 2.74 
                 0.74 
                 0.64 
                 Yes 
                 1.25 
                 Comparative Example 
               
               
                   
                 25 
                 3.0 
                 0.0 
                 1.2 
                 1.5 
                 10.0 
                 3.19 
                 0.77 
                 0.94 
                 No 
                 1.64 
                 Inventive Example 
               
               
                   
                 26 
                 3.0 
                 0.0 
                 1.2 
                 1.5 
                 10.0 
                 3.07 
                 1.07 
                 0.83 
                 Yes 
                 0.62 
                 Comparative Example 
               
               
                   
                 28 
                 3.0 
                 0.0 
                 1.2 
                 1.5 
                 10.0 
                 2.88 
                 0.68 
                 0.95 
                 Yes 
                 0.82 
                 Comparative Example 
               
               
                   
                 29 
                 3.0 
                 0.0 
                 1.2 
                 1.5 
                 10.0 
                 3.11 
                 0.74 
                 0.93 
                 Yes 
                 1.83 
                 Comparative Example 
               
               
                   
                 30 
                 3.0 
                 0.0 
                 4.8 
                 1.5 
                 10.0 
                 3.09 
                 1.04 
                 0.90 
                 Yes 
                 1.56 
                 Comparative Example 
               
               
                   
                 31 
                 3.0 
                 0.0 
                 1.2 
                 3.0 
                 10.0 
                 5.11 
                 0.73 
                 0.84 
                 Yes 
                 1.93 
                 Comparative Example 
               
               
                   
                 32 
                 3.0 
                 0.0 
                 1.2 
                 1.5 
                 0.0 
                 2.80 
                 0.75 
                 0.73 
                 Yes 
                 1.14 
                 Comparative Example 
               
               
                   
                 33 
                 3.0 
                 0.0 
                 1.5 
                 1.5 
                 10.0 
                 3.04 
                 0.89 
                 1.19 
                 No 
                 1.63 
                 Inventive Example 
               
               
                   
                 34 
                 3.0 
                 0.0 
                 1.5 
                 1.5 
                 10.0 
                 2.99 
                 1.43 
                 1.25 
                 Yes 
                 0.93 
                 Comparative Example 
               
               
                   
                 36 
                 3.0 
                 0.0 
                 1.5 
                 1.5 
                 10.0 
                 3.23 
                 0.97 
                 1.17 
                 Yes 
                 0.76 
                 Comparative Example 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2-2 
               
               
                   
               
             
            
               
                   
                   
                   
                   
                   
                   
                 Thickness t 
                   
                   
                   
                 Full 
                 Length 
                   
               
               
                   
                   
                   
                   
                   
                 Thickness t1 
                 of thickest 
                   
                 Total 
                   
                 length  
                 L2 of 
                   
               
               
                   
                   
                   
                 Thickness 
                 Thickness 
                 of uppermost 
                 steel sheet 
                   
                 sheet 
                   
                 L of  
                 final 
                 Main welding step 
               
               
                   
                   
                   
                 t2 of steel 
                 t3 of steel 
                 steel sheet in 
                 in lapped 
                 Total 
                 gap 
                 Welding 
                 weld 
                 weld 
                 Power 
               
               
                 Test 
                 Steel 
                 TS 
                 sheet 2 
                 sheet 3 
                 lapped 
                 portion 
                 thickness 
                 G 
                 position 
                 zone 
                 zone 
                 Pi 
               
               
                 No. 
                 type 
                 (MPa) 
                 (mm) 
                 (mm) 
                 portion (mm) 
                 (mm) 
                 T (mm) 
                 (mm) 
                 X (mm) 
                 (mm) 
                 (mm) 
                 (kW) 
               
               
                   
               
               
                 37 
                 C 
                 980 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.4 
                 −6.0 
                 15.0 
                 5.0 
                 6.0 
               
               
                 38 
                 C 
                 980 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.4 
                 −6.0 
                 15.0 
                 7.5 
                 6.0 
               
               
                 39 
                 C 
                 980 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.4 
                 −6.0 
                 15.0 
                 7.5 
                 6.0 
               
               
                 40 
                 C 
                 980 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.4 
                 −6.0 
                 15.0 
                 7.5 
                 6.0 
               
               
                 41 
                 G 
                 1180 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.4 
                 −6.0 
                 15.0 
                 7.5 
                 6.0 
               
               
                 42 
                 G 
                 1180 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.7 
                 −6.0 
                 15.0 
                 7.5 
                 6.0 
               
               
                 44 
                 G 
                 1180 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.4 
                 −6.0 
                 15.0 
                 7.5 
                 6.0 
               
               
                 45 
                 G 
                 1180 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.4 
                 −6.0 
                 15.0 
                 5.0 
                 6.0 
               
               
                 46 
                 G 
                 1180 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.4 
                 −6.0 
                 15.0 
                 7.5 
                 6.0 
               
               
                 47 
                 G 
                 1180 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.4 
                 −6.0 
                 15.0 
                 7.5 
                 6.0 
               
               
                 48 
                 G 
                 1180 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.4 
                 −6.0 
                 15.0 
                 7.5 
                 6.0 
               
               
                 49 
                 A 
                 980 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 40.0 
                 10.0 
                 3.6 
               
               
                 50 
                 D 
                 980 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.3 
                 −4.8 
                 40.0 
                 10.0 
                 4.8 
               
               
                 51 
                 C 
                 980 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.4 
                 −6.0 
                 40.0 
                 10.0 
                 6.0 
               
               
                 52 
                 E 
                 1180 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 40.0 
                 10.0 
                 3.6 
               
               
                 53 
                 H 
                 1180 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.3 
                 −4.8 
                 40.0 
                 10.0 
                 4.8 
               
               
                 54 
                 G 
                 1180 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.4 
                 −6.0 
                 40.0 
                 10.0 
                 6.0 
               
               
                 55 
                 A 
                 980 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.0 
                 −3.6 
                 15.0 
                 7.5 
                 3.6 
               
               
                 56 
                 D 
                 980 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.0 
                 −4.8 
                 15.0 
                 7.5 
                 4.8 
               
               
                 57 
                 C 
                 980 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.0 
                 −6.0 
                 15.0 
                 7.5 
                 6.0 
               
               
                 58 
                 E 
                 1180 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.0 
                 −3.6 
                 15.0 
                 7.5 
                 3.6 
               
               
                 59 
                 H 
                 1180 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.0 
                 −4.8 
                 15.0 
                 7.5 
                 4.8 
               
               
                 60 
                 G 
                 1180 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.0 
                 −6.0 
                 15.0 
                 7.5 
                 6.0 
               
               
                 61 
                 A 
                 980 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 15.0 
                 7.5 
                 3.6 
               
               
                 62 
                 D 
                 980 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.3 
                 −4.8 
                 15.0 
                 7.5 
                 4.8 
               
               
                 63 
                 C 
                 980 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.4 
                 −6.0 
                 15.0 
                 7.5 
                 6.0 
               
               
                 64 
                 E 
                 1180 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 15.0 
                 7.5 
                 3.6 
               
               
                 65 
                 H 
                 1180 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.3 
                 −4.8 
                 15.0 
                 7.5 
                 4.8 
               
               
                 66 
                 G 
                 1180 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.4 
                 −6.0 
                 15.0 
                 7.5 
                 6.0 
               
               
                 67 
                 A 
                 980 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 15.0 
                 10.0 
                 3.6 
               
               
                 68 
                 D 
                 980 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.3 
                 −4.8 
                 15.0 
                 10.0 
                 4.8 
               
               
                 69 
                 C 
                 980 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.4 
                 −6.0 
                 15.0 
                 10.0 
                 6.0 
               
               
                 70 
                 E 
                 1180 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 15.0 
                 10.0 
                 3.6 
               
               
                 71 
                 H 
                 1180 
                 1.6 
                 1.6 
                 1.6 
                 1.6 
                 3.2 
                 0.3 
                 −4.8 
                 15.0 
                 10.0 
                 4.8 
               
               
                 72 
                 G 
                 1180 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 4.0 
                 0.4 
                 −6.0 
                 15.0 
                 10.0 
                 6.0 
               
               
                 73 
                 A 
                 980 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 2.4 
                 0.2 
                 −3.6 
                 15.0 
                 8.0 
                 3.6 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 Main welding step 
                 Final welding step 
                 Shape of crater 
                 Evaluation 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Speed 
                 Focal 
                 Power 
                 Speed 
                 Focal 
                 Length 
                 Depth 
                 Width 
                   
                 Peeling 
                   
               
               
                   
                 Test 
                 vi 
                 position 
                 Pf 
                 vf 
                 position 
                 lc 
                 dc 
                 wc 
                 Weld 
                 strength 
                   
               
               
                   
                 No. 
                 (m/min) 
                 fi (mm) 
                 (kW) 
                 (m/min) 
                 ff (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 cracking 
                 (kN) 
                 Remarks 
               
               
                   
               
               
                   
                 37 
                 3.0 
                 0.0 
                 1.5 
                 1.5 
                 10.0 
                 2.91 
                 0.83 
                 1.25 
                 Yes 
                 1.63 
                 Comparative Example 
               
               
                   
                 38 
                 3.0 
                 0.0 
                 6.0 
                 1.5 
                 10.0 
                 2.84 
                 1.29 
                 1.28 
                 Yes 
                 1.50 
                 Comparative Example 
               
               
                   
                 39 
                 3.0 
                 0.0 
                 1.5 
                 3.0 
                 10.0 
                 4.99 
                 0.87 
                 1.22 
                 Yes 
                 1.46 
                 Comparative Example 
               
               
                   
                 40 
                 3.0 
                 0.0 
                 1.5 
                 1.5 
                 0.0 
                 3.10 
                 0.93 
                 0.93 
                 Yes 
                 1.73 
                 Comparative Example 
               
               
                   
                 41 
                 3.0 
                 0.0 
                 1.5 
                 1.5 
                 10.0 
                 2.96 
                 0.82 
                 1.21 
                 No 
                 1.44 
                 Inventive Example 
               
               
                   
                 42 
                 3.0 
                 0.0 
                 1.5 
                 1.5 
                 10.0 
                 2.79 
                 1.39 
                 1.18 
                 Yes 
                 0.83 
                 Comparative Example 
               
               
                   
                 44 
                 3.0 
                 0.0 
                 1.5 
                 1.5 
                 10.0 
                 3.17 
                 0.85 
                 1.23 
                 Yes 
                 0.55 
                 Comparative Example 
               
               
                   
                 45 
                 3.0 
                 0.0 
                 1.5 
                 1.5 
                 10.0 
                 2.97 
                 0.82 
                 1.24 
                 Yes 
                 1.33 
                 Comparative Example 
               
               
                   
                 46 
                 3.0 
                 0.0 
                 6.0 
                 1.5 
                 10.0 
                 2.88 
                 1.30 
                 1.24 
                 Yes 
                 1.37 
                 Comparative Example 
               
               
                   
                 47 
                 3.0 
                 0.0 
                 1.5 
                 3.0 
                 10.0 
                 4.78 
                 0.85 
                 1.17 
                 Yes 
                 1.83 
                 Comparative Example 
               
               
                   
                 48 
                 3.0 
                 0.0 
                 1.5 
                 1.5 
                 0.0 
                 3.01 
                 0.89 
                 0.82 
                 No 
                 1.36 
                 Comparative Example 
               
               
                   
                 49 
                 3.0 
                 0.0 
                 0.9 
                 1.5 
                 10.0 
                 2.89 
                 0.58 
                 0.86 
                 No 
                 4.73 
                 Inventive Example 
               
               
                   
                 50 
                 3.0 
                 0.0 
                 1.2 
                 1.5 
                 10.0 
                 3.30 
                 0.67 
                 0.99 
                 No 
                 4.36 
                 Inventive Example 
               
               
                   
                 51 
                 3.0 
                 0.0 
                 1.5 
                 1.5 
                 10.0 
                 3.72 
                 0.86 
                 1.24 
                 No 
                 5.21 
                 Inventive Example 
               
               
                   
                 52 
                 3.0 
                 0.0 
                 0.9 
                 1.5 
                 10.0 
                 2.84 
                 0.53 
                 0.72 
                 No 
                 4.13 
                 Inventive Example 
               
               
                   
                 53 
                 3.0 
                 0.0 
                 1.2 
                 1.5 
                 10.0 
                 3.21 
                 0.76 
                 1.18 
                 No 
                 3.98 
                 Inventive Example 
               
               
                   
                 54 
                 3.0 
                 0.0 
                 1.5 
                 1.5 
                 10.0 
                 3.56 
                 0.88 
                 1.34 
                 No 
                 4.72 
                 Inventive Example 
               
               
                   
                 55 
                 3.0 
                 0.0 
                 0.9 
                 1.5 
                 10.0 
                 3.20 
                 0.32 
                 0.78 
                 No 
                 1.42 
                 Inventive Example 
               
               
                   
                 56 
                 3.0 
                 0.0 
                 1.2 
                 1.5 
                 10.0 
                 2.93 
                 0.24 
                 1.12 
                 No 
                 1.56 
                 Inventive Example 
               
               
                   
                 57 
                 3.0 
                 0.0 
                 1.5 
                 1.5 
                 10.0 
                 2.84 
                 0.41 
                 1.36 
                 No 
                 1.25 
                 Inventive Example 
               
               
                   
                 58 
                 3.0 
                 0.0 
                 0.9 
                 1.5 
                 10.0 
                 3.14 
                 0.25 
                 0.56 
                 No 
                 1.34 
                 Inventive Example 
               
               
                   
                 59 
                 3.0 
                 0.0 
                 1.2 
                 1.5 
                 10.0 
                 2.99 
                 0.28 
                 1.03 
                 No 
                 1.42 
                 Inventive Example 
               
               
                   
                 60 
                 3.0 
                 0.0 
                 1.5 
                 1.5 
                 10.0 
                 2.80 
                 0.43 
                 1.31 
                 No 
                 1.63 
                 Inventive Example 
               
               
                   
                 61 
                 3.0 
                 0.0 
                 1.8 
                 0.8 
                 20.0 
                 3.16 
                 0.55 
                 0.93 
                 No 
                 1.63 
                 Inventive Example 
               
               
                   
                 62 
                 3.0 
                 0.0 
                 2.4 
                 0.8 
                 20.0 
                 2.94 
                 0.53 
                 1.20 
                 No 
                 1.29 
                 Inventive Example 
               
               
                   
                 63 
                 3.0 
                 0.0 
                 3.0 
                 0.8 
                 20.0 
                 3.05 
                 0.92 
                 1.67 
                 No 
                 1.53 
                 Inventive Example 
               
               
                   
                 64 
                 3.0 
                 0.0 
                 1.8 
                 0.8 
                 20.0 
                 2.79 
                 0.43 
                 1.00 
                 No 
                 1.38 
                 Inventive Example 
               
               
                   
                 65 
                 3.0 
                 0.0 
                 2.4 
                 0.8 
                 20.0 
                 2.82 
                 0.67 
                 1.18 
                 No 
                 1.58 
                 Inventive Example 
               
               
                   
                 66 
                 3.0 
                 0.0 
                 3.0 
                 0.8 
                 20.0 
                 3.42 
                 0.82 
                 1.53 
                 No 
                 1.53 
                 Inventive Example 
               
               
                   
                 67 
                 3.0 
                 0.0 
                 1.8 
                 0.8 
                 20.0 
                 3.13 
                 0.53 
                 0.89 
                 No 
                 1.21 
                 Inventive Example 
               
               
                   
                 68 
                 3.0 
                 0.0 
                 2.4 
                 0.8 
                 20.0 
                 3.01 
                 0.45 
                 1.53 
                 No 
                 1.29 
                 Inventive Example 
               
               
                   
                 69 
                 3.0 
                 0.0 
                 3.0 
                 0.8 
                 20.0 
                 2.89 
                 0.60 
                 1.83 
                 No 
                 1.25 
                 Inventive Example 
               
               
                   
                 70 
                 3.0 
                 0.0 
                 1.8 
                 0.8 
                 20.0 
                 3.27 
                 0.46 
                 0.93 
                 No 
                 1.22 
                 Inventive Example 
               
               
                   
                 71 
                 3.0 
                 0.0 
                 2.4 
                 0.8 
                 20.0 
                 2.82 
                 0.52 
                 1.39 
                 No 
                 1.30 
                 Inventive Example 
               
               
                   
                 72 
                 3.0 
                 0.0 
                 3.0 
                 0.8 
                 20.0 
                 3.36 
                 0.64 
                 1.78 
                 No 
                 1.23 
                 Inventive Example 
               
               
                   
                 73 
                 3.0 
                 0.0 
                 1.8 
                 0.8 
                 20.0 
                 3.02 
                 0.51 
                 1.61 
                 No 
                 1.25 
                 Inventive Example 
               
               
                   
               
            
           
         
       
     
     As can be seen from Tables 2-1 and 2-2, the peeling strength of each of the test pieces in the Inventive Examples was 1.2 kN or more, and no weld defects occurred. 
     In test pieces Nos. 2, 10, 18, 26, 34, and 42 in Comparative Examples, the total sheet gap G was larger than 15% of the total thickness T, and the depth d c  of the crater  5  in the terminal end point E of the weld zone  4  was large (the value was large), so that weld cracking occurred. 
     In Nos. 4, 12, 20, 28, 36, and 44, the full length L of the weld zone  4  was shorter than 15.0 mm, so that weld cracking occurred. 
     In Nos. 5, 13, 21, 29, 37, and 45, the length L 2  of the final weld zone  4   b  was small, so that weld cracking occurred. 
     In Nos. 6, 14, 22, 30, 38, and 46, the depth d c  of the crater  5  in the terminal end point E of the weld zone  4  was large (the value was large), so that weld cracking occurred. 
     In Nos. 7, 15, 23, 31, 39, and 47, the length l c  of the crater  5  in the terminal end point E of the weld zone  4  was large, so that weld cracking occurred. 
     In Nos. 8, 16, 24, 32, 40, and 48, the width w c  of the crater  5  in the terminal end point E of the weld zone  4  was small, so that weld cracking occurred. 
     As described above, in each of the Inventive Examples in which the laser lap welding was performed according to aspects of the present invention described above, a good laser-welded lap joint having the target characteristics according to aspects of the present invention was obtained. However, in each of the Comparative Examples not satisfying the welding conditions according to aspects of the present invention, a good laser-welded lap joint was not obtained. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  laser-welded lap joint 
               2  steel sheet 
               3  steel sheet 
               4  weld zone 
               4   a  main weld zone 
               4   b  final weld zone 
               5  crater 
               7  laser beam 
               8  steel sheet 
               14  weld zone 
               15  crater 
               16  crack 
             L full length of weld zone 
             L 2  length of final weld zone 
             l c  length of crater 
             d c  depth of crater 
             w c  width of crater 
             W width of weld zone 
             G sheet gap