JP2017170466A - Method for production of weld structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for production of a weld structure hardly generating solidification crack even if a flange width is lessened compared to a spot weld structure in weld structure production including overlapping the flange of a metal plate member, for example a hat-shaped member, with the other metal plate member and then carrying out laser welding.SOLUTION: A method for production of a weld structure includes a process of laser-welding two or more metal plate members, and prior thereto, a process of forming a partial hardening part by applying a hardening treatment to a site of one side short in a distance from a weld line (region to be laser-welded) to the edge face of the metal plate member among the sites of one and the other sides of the metal plate member in the orthogonal direction of the weld line as a center.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a method for manufacturing a welded structure. More specifically, the present invention relates to a method for manufacturing a welded structure obtained by laser welding an overlapping portion of metal plate members. The “metal plate member” is used to mean a member formed and / or cut into a predetermined shape from a metal plate.

In an automobile, a long member having a cross-sectional hat shape (hereinafter simply referred to as a “hat type”), which is one embodiment of a metal plate member manufactured from a metal plate (typically, a steel plate and will be described below as a steel plate). A number of "members" are used. Such a hat-shaped member is usually overlapped with a steel plate member (for example, a closing plate or another hat-shaped member) that is another metal plate member by a flange, and is joined at the overlapping portion. The most commonly used method for joining in this case is resistance spot welding (hereinafter referred to as spot welding).
Recently, studies have been made to reduce the weight of the member by narrowing the flange width of the hat-shaped member by applying laser welding instead of spot welding. Specifically, it is as follows.

In spot welding, it is necessary to sandwich and pressurize the welded portion with an electrode. Further, if the welding position is too close to the end face of the flange, the molten metal is scattered (chile). Therefore, in spot welding, it is necessary to secure the width of the flange with a size of about 15 mm to 20 mm, which increases the weight.
On the other hand, according to laser welding, it is not necessary to apply pressure unlike spot welding, and the melt width can be suppressed to about 1 mm. Therefore, in laser welding, there is a possibility that the width of the flange can be reduced to, for example, about 5 mm to 10 mm.

However, generally, when laser welding is performed near the end face of the flange (generally, an area less than 10 mm from the end face), there is a high risk of solidification cracking in the weld line. Accordingly, there is a need for a welding method that does not cause solidification cracks even when the flange width is shortened.
For example, “Hirohira Ando,“ Evaluation Method for Hot Cracking Progression Mechanism and Hot Cracking Susceptibility by Rotational Deformation-Hot Cracking Phenomenon of Thin Aluminum Alloy (2nd Report) ”, Journal of the Japan Welding Society, Vol. Pp. 37-47 (1973) "indicates that solidification cracking during welding is a solidification brittle temperature region (Brittens Temperature region) in which the ductility in which a solid phase and a liquid phase coexist is reduced in the process of melting a molten metal. In the Range (BTR)), the increase in strain caused by the deformation of the steel plate edge (in the case of a hat-shaped member) by the heat of fusion exceeds the strain required for cracking (hereinafter referred to as “limit strain”). It is thought that it occurs in.

In view of this, as a method for preventing solidification cracking, there are a material approach by reducing the BTR and controlling the limit strain by optimizing the components of the weld metal, and a mechanical approach by suppressing the strain generated at the end of the steel plate.

  One possible material approach is to reduce the BTR by optimizing the components of the weld metal. Specifically, the reduction in BTR by optimizing the composition of the weld metal is based on the solidification temperature range between the liquid phase and the solid phase, which is generally considered to be one of the factors affecting solidification cracking. For binary systems, the amount in the weld metal such as C, P, and S, which is an element that widens the solidification temperature range even with a small amount of addition, is limited (for example, Patent Document 1 and Patent Document 2). However, in Patent Document 3, when the distance from the flange end is reduced to the limit of laser welding where the spot diameter is 0.6 mm (minimum value of distance 1.5 mm), the metal of Patent Document 1 and Patent Document 2 above. Even if it is composition of this, it is described that the solidification crack generate | occur | produces, It cannot be said that it is enough.

  Another material approach is the control of critical strain. The limit strain is controlled by increasing the limit strain by optimizing the components of the weld metal so that solidification cracks do not occur even if the generated strain increment remains unchanged. An example of this is the method shown in Patent Document 3 (laser welding method and laser welded joint), which controls the equiaxed crystal ratio by optimizing the components of the weld metal to prevent solidification cracking. Specifically, Ti, N, Si, or the like is utilized to generate titanium nitride (TiN) that becomes the nucleus of equiaxed crystals. However, with this technique, there is a possibility of an increase in material cost due to the use of Ti or the like, and there is a limit in relation to other performance.

Then, suppression of the generated strain can be considered as a mechanical approach. Several methods have been proposed for suppressing the generated strain.
First, there is a restraint by a jig. For example, in Patent Document 4, when the composition of the weld metal can cause cracking, the plate is pressed against the steel plate end near the irradiation position of the laser beam, and welding is performed while suppressing the expansion of the steel plate end. An invention for preventing solidification cracking is disclosed. However, the present invention requires a device for suppressing the expansion of the steel plate end portion during welding, and cannot be used for welding lines of small members or members of complicated shapes, and increases the number of welding work steps. However, it becomes a complicated work.

  Secondly, there is a method of suppressing distortion by adjusting welding conditions (amount of heat input, welding speed) and rigidity (plate thickness, distance from the flange end of the weld line, etc.). For example, Patent Document 5 describes that the presence or absence of solidification cracking depends on the distance (L) from the end, the welding speed (V), and the plate thickness (h). However, when the flange width is small, the welding speed cannot be increased, and no solution has been proposed.

  Thirdly, there is a method of cooling the flange end face proposed in Patent Document 6. Here, it is considered that cracking occurs due to strain generated due to a difference in thermal expansion between the base material side and the end face side at a higher temperature than the base material side. This method is considered to be particularly effective in conditions where heat removal is difficult (for example, when the flange end is short and the welding speed is low). On the other hand, if the flange end face is cooled, the temperature difference from the position of the weld line is increased and the driving force for rotational deformation is increased, so that depending on the welding conditions, distortion may be increased and cracking may be promoted. Further, when the welding speed is high, it is considered that heat is easily removed compared with the case where the speed is low, and the cooling effect is considered to be small. Also in this method, attention must be paid to the arrangement of the cooling device, which may increase the number of work steps.

JP 2007-229740 A JP 2009-255134 A JP 2013-128938 A JP 2008-18450 A JP 2009-285722 A JP 2009-56483 A

  As described above, solidification cracking at the time of welding is caused by deformation of the end of the steel sheet due to heat of fusion in the BTR where the ductility in which the solid phase and the liquid phase coexist is reduced in the process of melting the molten metal. Generated by adding strain. It is considered that solidification cracking occurs when the strain increment generated in the BTR exceeds a certain threshold. In the flange-type laser welding of a hat-type panel, the following two main factors affecting the strain increment in the BTR are as follows. Can be mentioned.

(1) Member rigidity (or resistance to rotational bending) determined by the size of the flange width (distance from the flange end of the welding position), etc .:
The greater the flange width and the greater the distance from the flange end to the welding position, the higher the rigidity against rotation bending (or resistance to rotation bending), and the more the deformation / strain is suppressed. That is, if only the plate width is increased under the same welding conditions, the rigidity against deformation increases, so the amount of rotational deformation of the specimen due to welding heat decreases. However, simply increasing the flange width is against the weight reduction of the member.

(2) Temperature distribution in the plate width direction due to heat input:
The non-uniform thermal expansion accompanying the non-uniform temperature rise of the weld line becomes the driving force for rotational deformation. It is well known that the driving force of rotational deformation works when there is uneven temperature distribution in the width direction of the plate (Kohei Ando, Shuji Nakata "Calculation on deformation of rectangular plate by heating", National Welding Society) It is considered that the strain increment can be suppressed by suppressing this rotational deformation, according to the 10th Annual Conference Outline, p.305 (Showa 47).
That is, when welding from A to B of the plate 1 as shown in FIG. 6 (a), a non-uniform temperature distribution is generated in the plate width direction. Rotational deformation occurs in the direction of C. Since the strength of the embrittled region of the weld line is extremely small, the weld metal in the embrittled region part can hardly suppress this rotational deformation. Cracks occur when the amount of strain applied to the embrittled region due to this rotational deformation exceeds the limit strain. When welding proceeds thereafter, the weld pool Y also advances as shown in FIG. 6B, and the fulcrum of the rotational deformation follows the same and moves to the point P ′. At this time, if the temperature distribution is in a quasi-steady state, the amount of strain applied to the embrittled region is considered to be constant over time, and in this case, cracks propagate along the weld line as shown in FIG. To do.
On the other hand, when the welding speed is low, the temperature distribution in the plate width direction is easily uniformized, and the driving force for rotational deformation becomes small. However, if the welding speed is simply lowered, the productivity of the member is deteriorated, and if the plate width is reduced, the rigidity is lowered, so that the generated strain cannot be suppressed and the possibility of occurrence of solidification cracks increases.

  Therefore, in view of the above problems, the present invention reduces the flange width in comparison with spot welding when manufacturing a welded structure in which a metal plate member, for example, a flange of a hat-shaped member is overlapped with another metal plate member for laser welding. It is an object of the present invention to provide a method for manufacturing a welded structure that hardly causes solidification cracks.

  One aspect of the present invention is a welding line centered on a weld line when a laser welded part is a weld line before the laser welding process of laser welding two or more metal plate members. A step of forming a partially quenched portion by performing a quenching process on one side portion of the metal plate member in one direction and the other side of the metal plate member in a direction perpendicular to each other, where the distance from the welding line to the end surface of the metal plate member is short. The manufacturing method of the welding structure which has these.

  In the method for manufacturing a welded structure, in the laser welding step, two or more metal plate members can be overlapped and laser welded at the overlapped portion.

  In the manufacturing method of the welded structure, the yield stress of the partially quenched portion can be higher than the yield stress of the portion not subjected to the quenching process by 250 Mpa or more.

  In the manufacturing method of the welded structure, 60% or more of the distance between the weld line and the end face of the metal plate member on the side including the partially quenched portion among the weld line and the end face of the metal plate member. Quenching treatment can be applied to the area.

  In the method for manufacturing a welded structure, the distance between the weld line and the end surface can be within 8 mm on the side including the partially quenched portion between the weld line and the end surface of the metal plate member.

  In the manufacturing method of the welded structure, the thickness of the metal plate member may be 0.5 mm or more and 3.2 mm or less.

  According to the present invention, in manufacturing a welded structure in which a metal plate member, for example, a flange of a hat-shaped member is overlapped with another metal plate member and laser-welded, even if the flange width is reduced compared to spot welding, The occurrence of solidification cracks can be suppressed.

1 is an external perspective view showing an outline of a hat-type welded structure 10. FIG. It is the figure which expanded and represented a part of flange 11c and the welding line 13. FIG. It is a figure explaining one scene of the process of a welding method. It is a figure explaining one scene of welding. It is a figure explaining the evaluation method by an Example. FIG. 6A is one explanatory diagram schematically showing the mechanism of solidification cracking, and FIG. 6B is another explanatory diagram schematically showing the mechanism of solidification cracking.

  FIG. 1 is a perspective view showing the appearance of a hat-type welded structure 10 which is one form of a welded structure obtained by the method for manufacturing a welded structure of the present embodiment. FIG. 2 is an enlarged view of a part of the flange 11c. In FIG. 1 and FIG. 2, the width direction, the longitudinal direction, and the height direction are shown together as necessary.

The hat-type welded structure 10 includes a hat-type member 11 and a closing plate 12.
The hat-shaped member 11 is formed of a steel plate and is one of metal plate members. The hat-shaped member 11 has a web piece 11a, a wall piece 11b extending from both ends of the web piece 11a, and a flange 11c provided at the end of the wall piece 11b in a cross section orthogonal to the longitudinal direction, and is formed into a so-called hat shape. Has been. In the flange 11c, a partially quenched portion 11d, which is a partially quenched portion, is disposed at the end in the width direction (the portion indicated by hatching in FIG. 2).

The partially quenched portion 11d preferably has a yield stress higher by 250 MPa or more than the portion that is not partially quenched. Thereby, the effect by this invention can be made more reliable. Further, since the effect increases as the increase in yield stress increases, the upper limit is not particularly limited, but can be set to, for example, 1500 MPa or 1000 MPa.
Further, the partially quenched portion 11d has 60% or more of the partially quenched portion 11d when the distance in the width direction (the size of IIa in FIG. 2) from a weld line 13 to be described later to the end of the flange 11c is 100%. It is preferable to occupy in the range of 100% or less.
Here, the yield stress of the partially quenched portion can be measured by collecting a test piece from the member. The size of the test piece shall comply with JIS Z 2201, and the test shall be performed according to JIS Z 2241. The test is performed three times and the average value of the yield stress is used. However, when the test piece for 3 times cannot be extract | collected from a member, you may use the average value of 1 value or 2 times.

  Depending on the actual application, the hat-shaped member 11 may be straight in the longitudinal direction, or may be curved, or the cross-section may be widened or narrowed.

  On the other hand, the closing plate 12 is one of metal plate members, and is a substantially smooth steel plate that overlaps the partially quenched portion 11d. Further, it is preferable that the width direction end portion of the closing plate 12 is also a part of the partially quenched portion 11d, following the example of the hat-shaped member 11.

And the closing plate 12 is arrange | positioned so that it may pass between the two flanges 11c among the hat type members 11, and may overlap with the partial hardening part 11d of the hat type member 11, and this closing plate 12 and the flange 11c are piled up. Among the overlapped portions, a weld line 13 is provided on the inner side (opposite to the end face) of the partially quenched portion 11d, and both are joined by the weld line 13. In this embodiment, the weld line 13 is a weld line by penetration welding that penetrates the flange 11c and the closing plate 12 in the plate thickness direction.
The weld line 13 is formed by laser welding and extends along the longitudinal direction of the flange 11c. The present invention is a welding method for forming the weld line 13, and welding is performed at a high speed while shortening the distance from the end of the flange 11c shown by IIa in FIG. 2 to the weld line 13 as compared with the conventional laser welding. However, cracking can be suppressed, and as a result, the width of the flange 11c shown by IIb in FIG. 2 can be made smaller than in the case of spot welding. Although not particularly limited, as shown in FIG. 2, the width IIb of the flange 11c can be set to 15 mm or less. In addition, the distance between the weld line 13 and the end surface shown by IIa in FIG. 2 can be 8 mm or less, more preferably 3 mm or less.

Here, the steel plate used for the hat-shaped member 11 is not particularly limited, and is not particularly limited as long as it has a composition and a thickness that can be overlapped and laser-welded. For example, the thickness is preferably 0.5 mm or more and 3.2 mm or less. Moreover, you may equip the surface with well-known plating layers, such as zinc type plating and aluminum type plating. Further, instead of a steel plate, another metal plate such as an aluminum alloy plate can be used.
Depending on the actual application, the shape of the hat-shaped member 11 may be straight in the longitudinal direction, may be curved, or may have a cross-sectional shape changing in the longitudinal direction. The present invention may be applied to any of them. Moreover, it may replace with the closing plate 12 and may weld with another hat-type member and other shape metal plate members. Or it is applicable also to the welding structure which piles up and welds three or more metal plate members.

  Next, the hat-type welded structure 10 as described above can be manufactured, for example, as follows. FIG. 3 shows an explanatory diagram. FIG. 3 is a view from the same viewpoint as FIG. 1, and shows the width direction, the longitudinal direction, and the height direction together.

First, a hat-shaped member 11 ′ for welding is prepared. The welding hat-type member 11 ′ is a member on which the weld line 13 is not yet formed with respect to the hat-type member 11 described above. Here, a partially quenched portion 11d is formed on the welding hat-shaped member 11 ′.
The partially quenched portion 11d can be formed by performing a partial quenching process. Specifically, it is a process of increasing strength (increasing hardness) by causing transformation of the structure of the material by a process including heating and cooling, such as carburizing and induction hardening. The heating temperature may be set as appropriate so that the strength difference is equal to or greater than a predetermined value. However, when the metal plate is steel, it is preferable to heat up to Ac3 point or higher so that martensite precipitates.

  As shown in FIG. 3, the closing plate 12 is disposed so as to pass between the two flanges 11c of the welding hat panel 11 ′ as shown in FIG. 12 and the flange 11c are overlapped, whereby the partially quenched portion 11d and the end of the closing plate 12 (in this embodiment, the portion that has been partially quenched at the end of the closing plate 12) are overlapped.

Next, as shown in FIG. 4, the flange 11c and the closing plate 12 stacked on the flange 11c are welded so as to penetrate in the thickness direction. That is, laser irradiation is performed, and the irradiated portion is moved in the longitudinal direction as indicated by an arrow IVa in FIG. 4 to perform welding. A known laser welding apparatus can be used as an apparatus for welding. The type of laser is not particularly limited as long as it is a laser that is usually used for laser welding of a steel plate, and examples thereof include a CO ₂ laser, a YAG laser, and a fiber laser. In addition, the spot diameter (laser irradiation diameter of the steel plate) in laser welding is not particularly limited, but is preferably 0.5 mm or more and 1.0 mm or less, and the obtained weld line width is usually about 1 mm.

  With the above welding method, the rotational deformation of the steel sheet can be suppressed, and it is not necessary to restrain the steel sheet or to contact the jig, so that welding can be easily performed. Further, for example, solidification cracking can be suppressed even when the welding speed is increased at a portion where the distance from the end face is small.

  In this embodiment, since it is a hat-type welded structure, a wall piece 11b is formed on the opposite side of the end of the flange 11c across the weld line 13, and this has the same function as the partially quenched portion 11d ( Suppresses rotational deformation of the steel sheet). Therefore, even if it is a form which is not a hat-type welded structure or a hat-type welded structure, a partially quenched portion may be provided at both ends of the weld line depending on circumstances. However, the present invention is not limited to this, and it is only necessary that a partially quenched portion is formed at the end portion of the portion on the side where the distance to the end face is short among the one and the other portions separated by the weld line.

  As a form of the present invention other than this, it is not always necessary that the partially quenched portions are formed for all the members at the overlapping end portion within a range in which solidification cracking can be prevented. For example, the case where the width | variety of one metal plate member overlaid is sufficiently wide, or the case where it is a steel component which hardly produces a solidification crack, etc. are assumed.

  In the examples, the presence / absence of solidification cracks was evaluated using the conditions in which a partially quenched portion was provided against the conditions under which solidification cracks were generated by a normal welding method by simulation. Although this example was evaluated with a model that irradiates a single steel plate with a laser, strictly speaking, it differs from welding, but this evaluation also applies to welding when multiple metal plate members are stacked. Is possible. In the embodiment, as shown in FIG. 5, the characteristics different from those of other flat portions are applied to a 1300 MPa class hot stamp material as described later as a partially quenched portion with respect to one 1 mm thick steel plate model 30. did. Then, it was evaluated whether or not solidification cracking occurred at a site through which heat input to the heating portion assuming laser irradiation of laser welding passed.

The heat input was assumed to be a laser emitted from a laser welding machine, and the heat input was 1100 W and the width of the irradiation range was 2 mm. The heat input portion was moved 3 mm from the end face of the steel plate model 30 in parallel with the end face. The moving speed is 21 mm / sec. As the material of the steel plate model, the physical property values of a 1300 MPa class hot stamp material were used.
The partially quenched portion 31 has the shape shown in FIG. 2 and is in a range of 2 mm from the end compared to other portions. More specifically, the material properties of the martensite and austenite structures were applied in a region 2 mm wide from the end. On the other hand, as a result of applying the material properties of the structure of ferrite and pearlite to other portions, the yield stress of the partially quenched portion was about 600 (597) Mpa higher than the yield stress of the portion not subjected to the quenching treatment.
On the other hand, the comparative example did not form a partially-quenched portion while maintaining heat input with the same shape and the same conditions as the example.

  As a result, in the comparative example, intermittent cracking (solidification cracking) occurred in the portion through which the heating unit passed. On the other hand, in the Example, the result that a crack did not generate | occur | produce was able to be obtained.

10 Hat-type welded structure (welded structure)
11 Hat-shaped member (metal plate member)
11c Flange 11d Partial quenching part 12 Closing plate (metal plate member)
13 Welded part

Claims (6)

Laser welding two or more metal plate members;
Before the laser welding step, when the laser welded portion is a weld line, the one side and the other side of the metal plate member in the orthogonal direction of the weld line with the weld line as the center And a step of quenching a portion on one side where the distance from the weld line to the end face of the metal plate member is short to form a partially quenched portion.   The method for manufacturing a welded structure according to claim 1, wherein in the laser welding step, the two or more metal plate members are overlapped and laser welded at the overlapped portion.   The method for manufacturing a welded structure according to claim 1 or 2, wherein a yield stress of the partially quenched portion is 250 Mpa or more higher than a yield stress of a portion not subjected to quenching treatment. Among the weld line and the end face of the metal plate member, on the side including the partially quenched portion,
The manufacturing method of the welding structure in any one of Claim 1 to 3 which performs the said hardening process to the area | region of 60% or more among the distances between the said weld line and the end surface of the said metal plate member. Among the weld line and the end face of the metal plate member, on the side including the partially quenched portion,
The method for manufacturing a welded structure according to any one of claims 1 to 4, wherein a distance between the weld line and the end face is within 8 mm.   The manufacturing method of the welding structure in any one of Claim 1 to 5 whose thickness of the said metal plate member is 0.5 mm or more and 3.2 mm or less.

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