JP2009000697A - Laser beam welding method and laser welded product - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser beam welding method capable of increasing the strength of a joint without considerably increasing the cost or without considerably degrading the productivity. <P>SOLUTION: In the laser beam welding method of steel plates for generating a HAZ (Heat Affected Zone)-softened part in a heat affected zone of the laser beam welding, the laser beam welding is performed so as to establish either or both the relationship A/t≤1.4 between the distance A from a boundary part between a weld metal and the heat affected zone to the HAZ most-softened part and the plate thickness t, or the relationship B/t≤2.0 between the distance B between the center of the width of the weld metal and the HAZ most-softened part and the plate thickness t on the superposed surface of the steel plates. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a laser welding method and a laser welded product of a high-tensile steel plate.

  In recent years, the use of high-tensile steel sheets has been increasing for the purpose of reducing the weight of automobile bodies and improving collision safety. Conventionally, the tensile strength has been mainly up to about 590 MPa, but in recent years, so-called ultra-high strength steel plates of 780 MPa class or 980 MPa class are being applied.

  Ultra high strength steel sheets often use transformation strengthening and fine grain strengthening as the strengthening mechanism. The former achieves high strength by dispersing a hard structure such as a martensite phase in a ferrite matrix. On the other hand, the latter achieves high strength by refining the structure by imparting processing strain, optimizing heat treatment conditions, and the like.

When the steel sheet is welded, it is from a heat-affected zone (Heat Affected Zone; hereinafter, sometimes referred to as “HAZ”) than the original steel sheet (hereinafter, sometimes referred to as “base metal”). Is also known to produce a region of reduced hardness (hereinafter sometimes referred to as “HAZ softened portion”). So far, this HAZ softened portion has been considered to reduce the strength characteristics of the welded joint. As conventional techniques for solving this problem, for example, as disclosed in Patent Documents 1 and 2, most of the techniques focus on reducing the amount of softening by cooling the steel sheet during welding.
JP-A-2005-103568 JP2005-199297

  However, the above-described conventional technology uses a method of cooling the steel plate in advance with liquid nitrogen before welding or a method of closely contacting the cooling plate at the time of welding. There is a problem in terms of productivity. Accordingly, an object of the present invention is to provide a laser welding method capable of improving joint strength without causing a significant increase in cost and a significant reduction in productivity based on a completely different concept from the prior art.

  In a high-strength steel sheet using transformation strengthening or fine grain strengthening, even if any cooling means is taken, it is impossible to completely suppress the occurrence of HAZ softening itself. Therefore, the present inventors considered that it is important to clarify the influence on the joint strength, considering that HAZ softening is essentially inevitable. Then, for the case where the HAZ softened part is generated, detailed research was conducted on the correlation between the strength characteristics of the laser welded joint and the welded part shape (such as the position of the HAZ softened part) from both the experiment and numerical analysis. As a result, it has been found that the joint strength may be improved by optimizing the position of the HAZ softened portion.

The present invention is based on the above findings, and the gist thereof is as follows.
1st this invention is the laser welding method of the steel plate which produces a HAZ softening part in the heat affected zone of laser welding, Comprising: In the overlapping surface of a steel plate, it is a HAZ softest part from the boundary part of a weld metal and a heat affected zone. Between the distance A and the thickness t
A / t ≦ 1.4
Or between the distance B between the width center of the weld metal and the HAZ softened portion and the thickness t,
B / t ≦ 2.0
The laser welding method is characterized in that laser welding is performed so that either or both of these relationships are established.

In the laser welding method according to the first aspect of the present invention, an angle θ (degrees) formed by the thickness direction of the boundary portion between the weld metal and the heat affected zone in the overlapping surface of the steel plates,
θ ≦ 40 or 50 ≦ θ
Laser welding is preferably performed so that

  In the laser welding method of the first aspect of the present invention, the steel plate is preferably a high-tensile steel plate having a tensile strength of 590 MPa or more.

A second aspect of the present invention is a laser welded product of a steel plate having a HAZ softened portion in a heat-affected zone of laser welding, wherein the HAZ is separated from the boundary between the weld metal and the heat-affected zone on the overlapping surface of the laser welded product. Between the distance A to the softest part and the thickness t,
A / t ≦ 1.4
Or between the distance B between the width center of the weld metal and the HAZ softened portion and the thickness t,
B / t ≦ 2.0
This is a laser welded product characterized in that either or both of these relationships are established.

In the welded product according to the second aspect of the present invention, an angle θ (degree) formed by the thickness direction of the boundary portion between the weld metal and the heat affected zone on the overlapping surface of the steel plates is
θ ≦ 40 or 50 ≦ θ
It is preferable that

  In addition, in the welded product according to the second aspect of the present invention, the steel plate is preferably a high-tensile steel plate having a tensile strength of 590 MPa or more.

  The third aspect of the present invention is a method for evaluating the joint strength of a welded part of a laser welded product obtained by laser welding with overlapping steel sheets, from the boundary between the weld metal and the heat-affected zone on the overlapped surface of the welded part. One or both of the distance A to the HAZ softest part and the distance B from the center of the width of the weld metal to the HAZ softest part are obtained, and the ratio (A / t) between the distance A and the thickness t This is a method for evaluating the joint strength of a welded part of a laser welded product, characterized in that the joint strength of the welded part is evaluated by either or both of the ratio of B and the plate thickness t (B / t).

  According to the present invention, the strength of the steel plate laser welded joint can be improved by optimizing the position of the HAZ softened portion and the shape of the weld metal. In addition, if such a welded joint is applied to an automobile part, it is possible to reduce the weight of the automobile body and improve the collision safety.

As described above, the present invention is directed to a steel plate that causes laser welding as a welding method and a HAZ softened portion during welding as a steel plate. The laser used for welding is not particularly limited as long as it is used for the method of joining by melting and solidifying the metal locally by irradiating the laser beam mainly on the metal as a heat source. However, for example, a CO ₂ laser or a YAG laser whose output is increasing as a laser oscillator can be given. In addition, examples of the steel plate that generates the HAZ softened portion at the time of welding include a high-tensile steel plate of 590 MPa class, an ultra-high-tensile steel plate of 780 MPa class or higher (including hot pressed material), and a material that has been cold worked.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a typical example of the relationship between the cross-sectional shape of the laser weld and the hardness distribution when the HAZ softened portion is generated. The upper part of FIG. 1 schematically represents the hardness distribution from the melting center to both sides, and the lower part schematically represents the cross-sectional shape from the melting center to both sides.

In the welded portion 100, the weld metal 1, HAZ 2, and base material 3 are positioned in this order in the both side directions centering on the melting center 7. Further, when attention is paid to the hardness distribution in the HAZ 2, there are a region that hardens to the same extent as the weld metal 1, a region where the hardness sharply decreases, and a HAZ softened portion 4 from the side close to the weld metal 1. The hardness distribution and the cross-sectional shape may be regarded as almost symmetrical about the melting center 7. For convenience in the following description, the width of the weld metal 1 at the plate-plate interface is the “joining width”, the distance from the melt boundary 5 to the most softened portion 6 at the plate-plate interface is the “most softening distance A”, and the welding center The distance from 7 to the softest part 6 is referred to as “the softest position B”. As you can see from the figure,
B = (junction width / 2) + A
Holds.

  Consider a case where a load in the peeling direction acts on the welded portion 100 as shown in FIG. 2 (this drawing shows a one-side peel-off type test piece 20). The flange portion 21 is equivalent to a cantilever beam having a welded portion as a fulcrum, and bending deformation occurs in the plate. Therefore, the closer to the weld bead 22, the higher the generated stress. For this reason, if it is the material which does not produce a HAZ softening part, it will normally destroy from the weld metal edge part which produces the highest stress as a starting point.

  However, the present inventors have found that in a material that generates a HAZ softened portion, the HAZ softened portion becomes a fracture starting point, and the joint strength may be unexpectedly greatly improved. The reason is considered as follows.

  When the HAZ softened part approaches the weld metal and the generated stress increases, the yield stress is exceeded earlier than the weld metal end part, and the plastic strain is concentrated. Therefore, the welded joint exhibits a deformed state that bends at two locations of the weld metal end and the HAZ softened portion. When such a deformed state is shown, stress and strain concentrations at the weld metal end are alleviated. As a result, the deformability of the welded joint up to breakage is improved, and the maximum load is also improved. Moreover, even if it finally breaks not with the HAZ softened portion but with the weld metal, as long as the deformed state bends in two steps, the same effect as in the case where the HAZ softened portion breaks can be obtained in improving joint strength.

  As described above, the above phenomenon occurs when the distance between the HAZ softened portion and the weld metal becomes smaller than a certain value. Based on the test results for various welding conditions and materials, the present inventors can determine that the ratio (A / t) of the maximum softening distance A to the sheet thickness t is 1.4 or less as shown in FIG. I found something good. In addition, the vertical axis | shaft of FIG. 3 is a relative value which set 1.0 as the maximum value of the joint strength of a 980 MPa class super high strength steel plate.

  Here, the reason why it is defined as “1.4 or less” as described above is that when the HAZ softened portion is further moved away, plastic strain does not occur in the portion, and the effect of improving the joint strength cannot be obtained. Preferably, “the ratio (A / t) between the maximum softening distance A and the sheet thickness t is 0.8 or less”.

  By adjusting the welding speed and the amount of defocus (DF), the ratio (A / t) between the maximum softening distance A and the plate thickness t can be made 1.4 or less.

The ratio (A / t) increases as the heat input increases. Therefore, in order to reduce the ratio (A / t), it is effective to increase the welding speed. For example,
A / t ≦ 1.4
Therefore, the welding speed is preferably 2 m / min or more. More preferable A / t ≦ 0.8
In order to achieve this, it is desirable to set it to 4 m / min or more.

  Further, as the defocus (DF) amount increases, the ratio (A / t) tends to increase. It is presumed that as the defocus amount increases, the laser irradiation area increases and the amount of heat input increases. Regarding the ratio (A / t), the influence of the above-described welding speed is larger, and it is difficult to independently define the appropriate value of the DF amount, but the tendency of the present invention obtained from the range of the data , The defocus (DF) amount is preferably about 10 mm or less. As will be described later, when the amount of DF is increased, the bonding width increases and the shear strength can be increased. In order to increase the shear strength, the DF amount is desirably 7 mm or more.

  Further, as shown in FIG. 4, the present inventors have found that there is a strong correlation with the joint peel strength for the softest position as well, and as a condition for ensuring the peel strength, “the softest position B and the thickness t The ratio (B / t) is 2 or less. " The reason why such a correlation is obtained is the same as in the case of the softest distance A. Here, the vertical axis in FIG. 4 is a relative value where the maximum value of the joint strength of the 980 MPa class ultra-high strength steel sheet is 1.0.

  By adjusting the welding speed and defocus (DF) amount, the ratio (B / t) between the softest position B and the plate thickness t can be made 2.0 or less.

  The ratio (B / t) increases with increasing heat input. Therefore, in order to reduce the ratio (B / t), it is effective to increase the welding speed. For example, the welding speed is preferably 2 m / min or more. More preferably, it is 4 m / min or more.

  Further, as the defocus (DF) amount increases, the ratio (B / t) tends to increase. It is presumed that as the defocus amount increases, the laser irradiation area increases and the amount of heat input increases. The amount of defocus (DF) is preferably about 10 mm or less.

  Furthermore, the present inventors have found that even if the values shown above are equal, the peel strength differs depending on the weld metal shape. Specifically, as shown in FIG. It was found that there is a tendency to depend on the angle θ that spreads from the outside to the outside. However, from the experimental results alone, it was difficult to quantitatively clarify this angular dependence, so a finite element analysis was performed for detailed examination.

  FIG. 6 shows the element division of the analysis model, and FIG. 7 shows the boundary conditions. All the models were two-dimensional plane strain elements, the bonding width was fixed to 1 mm, and the angle θ was changed in the range of 0 to 56 degrees. The left and right and vertical symmetry conditions are set for the molten metal, which corresponds to a U-shaped peel joint of a weld having a vertically symmetrical molten metal shape. In order to exclude the influence of strain concentration on the HAZ softened part and extract only the influence of the molten metal shape, the element sets are two types, “welded metal” and “HAZ”. The element dimensions were made almost equal for each model.

  Note that the steel type to be analyzed is a 980 MPa class ultra-high strength steel plate, and the stress-strain relationship used for the analysis is, for example, “Automotive Technology Society Proceedings” Vol. 36, No. 1 (2005), pages 205 to 210. As shown in the above, the tensile test technique for each of the parts was directly obtained by the ultra-small test piece tensile test technique developed by the present inventors.

  Based on this analysis result, the joint strength was predicted. Specifically, based on the same idea as the above-mentioned known paper, the joint strength at the time of failure was determined as “the joint breaks when the maximum value of the equivalent plastic strain in HAZ reaches the failure strain”.

  FIG. 8 shows the relationship between the joint strength at the time of breakage and the angle θ. Note that the vertical axis is dimensionless with a value when θ = 0 degrees, which has the best strength. As can be seen, the joint strength is lowest when the angle θ is 45 degrees. Therefore, the shape of the melting boundary should not be close to 45 degrees. In the present invention, a desirable range of the angle θ is “a range in which the joint strength does not decrease by 25% or more than when θ = 0 degrees”. When this is specifically obtained from FIG. 8, it is “40 degrees or less, or 50 degrees or more”.

  The reason why the joint strength is defined as “a range in which the joint strength does not decrease by 25% or more than when θ = 0 °” is as follows. As described above, in the present invention, the upper limit values of the maximum softening distance A and the maximum softening position B are defined based on the joint test results for various materials and welded part shapes. When the minimum strength value of the joint having a welded shape within the scope of the present invention was compared with the maximum strength value of the joint deviating from the present invention, the ratio was approximately 75%. From this, it is considered that when the strength is reduced to 75% or less, the effect of strain concentration on the HAZ softened portion is lost, and 75% is set as the strength to be secured.

  The reason why the joint strength decreases when the angle θ approaches 45 degrees is considered as follows. As shown in FIG. 1, the hardness of the weld metal 1 and the HAZ 2 is often almost continuous near the melting boundary. However, since the microstructures differ greatly from each other, it is considered that there is a difference in tensile strength characteristics (yield stress, ductility, etc.). Therefore, the strain tends to concentrate near the melting boundary due to this strength mismatch. Furthermore, as is generally known in plastic deformation of ductile materials, strain concentration tends to occur in the maximum shear strain direction inclined by 45 degrees with respect to the plate thickness direction. Therefore, when the angle between the same direction and the melting boundary is close, it is considered that the strain concentration on the part is promoted and the joint strength tends to decrease.

  For the above reasons, the shape of the weld metal is defined as “θ is 40 degrees or less, or 50 degrees or more”. The angle θ is also affected by the amount of DF, the welding speed, etc., but since various factors have a combined effect, it is difficult to independently define each appropriate range. From the scope of the data of the present invention, it has been found that the angle θ increases as the DF amount increases as a general tendency. Therefore, “10 mm or less” is recommended as a measure of the DF amount based on this tendency. Of course, even if this value is exceeded, it goes without saying that the range of θ defined in the present invention may be satisfied depending on the value of the welding speed or the like.

  In the present invention, the shape of the welded portion that can improve the peel strength of the joint is defined. However, in the case of an actual automobile member, the shear strength may be important as well as the peel strength. The present inventors have clarified that the shear strength is almost unaffected by the position of the softened region and is almost uniquely determined by the joining width. In order to increase the bonding width, it is effective to increase the DF amount, and specifically, it is desirable to set it to 7 mm or more. Simultaneously satisfying the lower limit value of the DF amount, the upper limit value of the desirable DF amount and the lower limit value of the welding speed shown up to all terms is a guideline for achieving both peeling strength and shear strength.

  Next, a method for evaluating the joint strength of the welded portion of the laser welded product, which is the third aspect of the present invention, will be described. This evaluation method is a method for evaluating the joint strength of a welded part of a laser welded product obtained by superposing steel sheets and performing laser welding. In the overlapped surface of the welded part, the HAZ maximum is measured from the boundary between the weld metal and the heat affected part. One or both of the distance A to the softened part and the distance B from the center of the width of the weld metal to the HAZ softest part is obtained, and the ratio (A / t) between the distance A and the plate thickness t, The joining strength of the welded portion is evaluated by either or both of the ratio (B / t) to the plate thickness t.

  In the evaluation, it is necessary to specify the lengths of the distance A and the distance B. For this purpose, the melting boundary 5 of the welded product and the part of the HAZ most softened portion 6 (see FIG. 1) are, for example, as follows. What is necessary is just to specify.

(Identification method of melting boundary)
It is specified by corroding the cross section perpendicular to the welding bead traveling direction with picric acid + surfactant reagent to cause a melting boundary to appear.

(Identification method of HAZ softest part)
Using a micro Vickers meter, an indentation is applied with a load of 1.96 N, a pitch of 0.2 mm or 0.3 mm, and specified from the hardness distribution.

  As test materials, 780 MPa class DP steel and 980 MPa class DP steel, which are transformation strengthened steels, were used. Here, “DP” is an abbreviation for Dual Phase (two-phase), and is a material in which a hard martensite phase is dispersed in a ferrite matrix phase. Both plate thicknesses are 1.2 mm. In the table of test results to be described later, the respective materials are abbreviated as “780DP” and “980DP”.

  The same steel types were overlapped and welded with a YAG laser. The laser processing point output was two types of 4.5 kW and 5.0 kW, and various weld shapes were obtained by changing the defocus (DF) amount and the welding speed. Moreover, it evaluated also about the case (test No. 6-8) welded, cooling the vicinity of a welding part using a water-cooling jig (circulating cooling water inside).

  The joint type is a peel joint 90 shown in FIG. The plate width was 60 mm, but the weld bead length was 55 mm and the toe portion was provided. Moreover, in order to prevent the displacement of a welding part to the load-axis perpendicular | vertical direction and to reduce the difference in the deformation state for every steel type, the same test piece was piled up and loaded simultaneously.

  The joint test results are shown in Table 1. In the table, welding conditions, welded part shapes (joining width, maximum softening distance, maximum softening position, values obtained by dividing each by the plate thickness, angle θ), joint strength, and fracture position are shown.

  Here, the bonding width and angle were measured by corroding the cross section perpendicular to the welding bead traveling direction with picric acid + surfactant reagent and causing a melting boundary to appear. Moreover, the softest distance and the softest position were determined from the hardness distribution by applying an indentation with a load of 1.96 N, 0.2 mm or 0.3 mm using a micro Vickers meter. Furthermore, with respect to the fracture position, “melting boundary” indicates that the fracture occurred at a portion other than the HAZ softened portion such as a weld metal or a melting boundary.

  Test No. in which the softest distance and the softest position exceed the upper limit values defined in the present invention. 1 and no. It can be seen that the joint strength of other test results is superior to 14. In addition, Test No. In 2 to 7, 10 to 13, and 15, although the HAZ softened portion is not finally broken, the strain is concentrated on the HAZ softened portion, so that the effect of improving the joint strength is obtained.

  Furthermore, test no. In Nos. 2 to 4, test nos. Using a cooling jig under the same welding conditions. A joint strength equivalent to 6 to 8 is obtained, and according to the present invention, it can be seen that high strength can be realized without cooling.

  As described above, according to the present invention, it is possible to perform laser welding on a material that generates a HAZ softened portion while suppressing a decrease in joint strength, thereby reducing the weight of an automobile body and improving collision safety. The industrial effects are extremely large.

It is a figure which shows the typical example showing the relationship between the laser weld part cross-sectional shape in the case of producing a HAZ softening part, and hardness distribution. It is a figure which shows the test piece of a one-side peeling type. It is a figure which shows the relationship between peeling strength and (maximum softening distance) / (plate thickness). It is a figure which shows the relationship between peeling strength and (most softened position) / (plate thickness). It is a figure which shows a melting boundary spreading toward the exterior from the inside of welding. It is a figure which shows the element division of an analysis model. It is a figure which shows an analysis model dimension and boundary conditions. It is a figure which shows the relationship between peeling strength and angle (theta) which a fusion boundary and a plate | board thickness direction make. It is a figure which shows the peeling joint used in the Example.

Explanation of symbols

A Softest distance B Softest position 1 Molten metal 2 Heat affected zone (HAZ)
DESCRIPTION OF SYMBOLS 3 Base material 4 HAZ softening part 5 Melting boundary 6 Softest part 7 Melting center 20 Test piece 21 Flange part 22 Weld bead 90 Peeling joint 100 Welding part

Claims (7)

A method of laser welding a steel plate in which a HAZ softened portion is generated in a heat affected zone of laser welding, and a distance A and a thickness from a boundary portion between the weld metal and the heat affected zone to a HAZ softened portion on the overlapping surface of the steel plates between t and
A / t ≦ 1.4
Or between the distance B between the width center of the weld metal and the HAZ softened portion and the thickness t,
B / t ≦ 2.0
A laser welding method characterized by performing laser welding so that either or both of these relationships are established. Furthermore, in the overlapping surface of the steel plates, an angle θ (degree) formed by the thickness direction of the boundary portion between the weld metal and the heat affected zone is as follows:
θ ≦ 40 or 50 ≦ θ
The laser welding method according to claim 1, wherein laser welding is performed so that The laser welding method according to claim 1, wherein the steel plate is a high-tensile steel plate having a tensile strength of 590 MPa or more. A laser welded product of a steel plate having a HAZ softened part in a heat-affected zone of laser welding, and a distance A from the boundary between the weld metal and the heat-affected zone to the HAZ softest part on the overlapping surface of the laser welded product And the plate thickness t,
A / t ≦ 1.4
Or between the distance B between the width center of the weld metal and the HAZ softened portion and the thickness t,
B / t ≦ 2.0
A laser welded product characterized in that either or both of these relationships are established. In the overlapping surface of the steel plates, the angle θ (degrees) formed by the thickness direction of the boundary portion between the weld metal and the heat affected zone is:
θ ≦ 40 or 50 ≦ θ
The laser welded product according to claim 4, wherein The laser welded product according to claim 4 or 5, wherein the steel plate is a high-tensile steel plate having a tensile strength of 590 MPa or more. A method for evaluating the joint strength of a welded part of a laser welded product obtained by laser welding by superimposing steel sheets, from a boundary part between a weld metal and a heat-affected part to a HAZ most softened part on the superposed surface of the welded part. One or both of the distance A and the distance B from the center of the width of the weld metal to the HAZ most softened portion is obtained, the ratio (A / t) between the distance A and the plate thickness t, and the distance B A method for evaluating the joint strength of a welded part of a laser welded product, wherein the joint strength of the welded part is evaluated based on either or both of the ratio (B / t) to the plate thickness t.

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