JP6780386B2 - Spot welding method - Google Patents

Description

The present invention relates to a method of spot welding a plurality of steel sheets including a zinc-based plated steel sheet.

As a member for an automobile, a galvanized steel sheet having excellent corrosion resistance is widely used from the viewpoint of high rust prevention of the vehicle body. From the viewpoint of weight reduction and high strength, zinc-based plated high-strength steel sheets using high-strength steel sheets as the original plating plates are used in the zinc-based plated steel sheets used for automobiles.

Spot welding is mainly used in the assembly of automobile bodies and the installation of parts. However, when spot welding is performed on a zinc-based plated high-strength steel sheet, there is a problem that the steel sheet in contact with the welding electrode is cracked.

In this crack, tensile stress due to the pressing force of the electrode, thermal expansion and contraction of the steel plate is applied to the welded part, and zinc melted on the surface of the steel plate at the welded part, copper of the electrode, etc. penetrate into the steel plate and increase the grain boundary strength. It is said that the cracks are caused by so-called liquid metal brittleness caused by lowering. In automobile bodies, there is a problem that the strength decreases when the welded portion is significantly cracked, and a technique for suppressing the cracked welded portion by controlling the composition and structure of the steel sheet is known.

In Patent Document 1, the composition of the steel plate is adjusted so that the austenite phase generated during spot welding is made into fine crystal grains and has a metal structure intricately intertwined with the crystal grains of other phases. , A technique has been disclosed that complicates the diffusion infiltration route of molten zinc into grain boundaries, makes it difficult for molten zinc to penetrate, and prevents liquid metal brittle cracking during spot welding.

Patent Document 2 states that it may not be possible to sufficiently suppress the occurrence of cracks in the welded portion simply by complicating the grain boundaries by controlling the structure of the steel sheet, and the composition of the steel sheet is adjusted to adjust the grain of the hot-rolled steel sheet. The grain boundary erosion depth of the alloyed hot-dip zinc-plated steel sheet should be 5 μm or less by setting the field oxidation depth to 5 μm or less and performing Fe-based electroplating on the cold-rolled steel sheet before the alloyed hot-dip zinc-plated steel sheet. So, a technique for suppressing the occurrence of cracks at a welded portion of an alloyed hot-dip zinc-plated steel sheet is disclosed.

Patent Document 3 discloses a technique for removing plating at the butt portion at the end of a strip in order to prevent embrittlement of liquid metal when manufacturing an electrosewn steel pipe from a steel plate plated with zinc or the like. ..

Japanese Unexamined Patent Publication No. 2006-265671 Japanese Unexamined Patent Publication No. 2008-231493 Japanese Unexamined Patent Publication No. 05-277552

Although countermeasures against cracks at spot welds have been studied in this way, even if cracks do not occur at the contact points between the steel plate and the electrodes in a spot welded joint containing a steel plate with zinc-based plating on the plate assembly. Still, the desired tensile strength may not be obtained.

As a result of investigation by the present inventors, it was confirmed that the spot welded joint in which the desired tensile strength could not be obtained had cracks inside the corona bond on the overlapping surface of the steel sheet. Hereinafter, the crack inside the corona bond is referred to as "crack inside the corona bond".

FIG. 1 shows an outline of internal cracks in the corona bond. The crack generated in the region of the corona bond formed between the steel plates 1 is the corona bond internal crack 3.

As a result of the investigation by the present inventors, when the spot welding electrode does not hit the steel plate vertically or there is a gap between the steel plates, the corona bond internal cracks are found in a steel plate having a strength of 780 MPa class steel plate or less. It was found that it is more likely to occur when a steel plate with higher strength is included.

In spot welding, when the steel plate is tilted with respect to the electrodes (Fig. 2), when an angle is generated due to the deflection of the welding gun (Fig. 3), when a welding gun with tilted electrodes is used (Fig. 4), etc. , The electrode does not hit the steel plate vertically. Further, even when the space around the welded portion is limited, the electrode tends not to hit the steel plate vertically.

In view of such circumstances, the present invention is a spot that can prevent internal cracking of the overlapping surfaces of steel plates and form a high-quality spot welded joint in spot welding of a plate assembly including a steel plate subjected to zinc plating. An object of the present invention is to provide a welding method.

The present inventors have diligently studied a method for solving the above problems.

The internal cracks in the corona bond are considered to be liquid metal embrittlement cracks that occur in the cooling process after the end of energization. As a result of the examination by the present inventors, the present inventors are (i) discharging Zn inside the corona bond to the outside of the corona bond and plating Fe in the steel plate inside the corona bond during spot welding. By interdiffusing the Zn of the above, it is possible to raise the melting point of the plating inside the corona bond, and as a result, the plating is solidified before the tensile stress that causes brittle cracking of the liquid metal is applied, and the inside of the corona bond is formed. Cracking can be suppressed, and (ii) Liquid metal brittle cracking occurs when the angle between the normal line of the contact surface of the electrode (chip) and the normal line of the steel plate surface deviates from 0 degrees, and the larger this angle, the more the steel plate Since the tensile stress that enters becomes large, it is necessary to raise the melting point of the plating in a wider range. To achieve this, the energization time of spot welding is lengthened as the angle is larger, so that the inside of the corona bond We found that cracking can be suppressed.

The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) In a method of spot welding to a member, the welded portion of the member is composed of a plurality of laminated steel plates, and the overlapped surface of the plurality of laminated steel plates is coated with zinc-based plating. In the case of spot welding, the total thickness of the plurality of steel plates in which the welded parts are overlapped is t [mm], and the normal line of the contact surface of the electrode tip attached to the tip of the welded electrode. A spot welding method characterized in that the energization time is (5t + 2.8d + 2) / 50 [sec] or more when the angle with the normal line of the steel plate surface is d [degree] (d> 0).

(2) The spot welding method according to (1) above, wherein the angle between the normal of the contact surface of the electrode tip attached to the tip of the welding electrode and the normal of the steel plate surface is 1 to 20 °.

(3) The spot welding method according to (1) or (2) above, wherein the difference in tensile strength between adjacent steel plates of the plurality of steel plates is 50 MPa or more.

(4) The spot welding method according to (1) or (2) above, wherein the difference in tensile strength between adjacent steel plates of the plurality of steel plates is 500 MPa or more.

According to the present invention, by raising the melting point of the plating inside the corona bond, the plating solidifies before the tensile stress that causes internal cracking is applied, so that the angle between the electrode axis and the surface of the steel sheet deviates from the vertical. Even if there is, it is possible to prevent the occurrence of internal cracks in the corona bond due to liquid metal cracks.

It is a figure which shows the outline of the crack in the corona bond when spot welding is performed on the galvanized steel sheet. It is a figure which shows the outline in the case where a steel plate is tilted with respect to a welding electrode. It is a figure which shows the outline when the welding gun is bent. It is a figure which shows the outline in the case of using the welding gun in which the electrode is tilted. It is a figure which shows the stress distribution of the welded part after welding when there is no hitting angle. It is a figure which shows the stress distribution of the welded part after welding when the striking angle is 5 degrees. (A) shows the stress distribution of the entire welded portion, and (b) shows an enlarged view of the stress distribution in the immediate vicinity of the corona bond.

In the spot welding method of the present invention (hereinafter referred to as "welding method of the present invention"), steel plates of a member including a portion in which a plurality of steel plates including one or more steel plates coated with zinc-based plating are overlapped are overlapped. When the welded part is the welded part, the total thickness of the plurality of steel plates in which the welded parts are overlapped is t [mm], the normal of the contact surface of the electrode tip attached to the tip of the welded electrode, and the surface of the steel plate. Welding is performed so that the energizing time is (5t + 2.8d + 2) / 50 [sec] or more when the angle with the normal line is d [degree] (d> 0).

Hereinafter, the background of the study leading to the welding method of the present invention and the welding method of the present invention will be described in detail.

Using zinc-based plated steel sheets, the present inventors conducted the following experiments to investigate the reasons for the occurrence of internal cracks in the corona bond that occur on the inner surface side where the steel sheets are overlapped in spot welding.

Spot welding was performed under various welding conditions using two steel sheets having various composition or steel grades, which were coated with zinc-based plating and not coated with plating. Then, among the obtained spot welded joints, the cross section in the plate thickness direction including the nugget of the spot welded joint having a low tensile strength was observed.

As a result, internal cracking of the overlapped surface of the steel sheet occurs when so-called TRIP steel, which utilizes work-induced martensitic phase transformation containing a large amount of alloy components, is used, and the ferrite phase and martensite phase are compounded with few alloy components. It was difficult to occur when the so-called DP steel was used.

Further, this crack may occur even when a high-strength steel sheet having a tensile strength of 780 MPa or more and a Ceq of 0.15% by mass or more is used. Ceq is shown in the following equation (1).

Ceq = [C] + [Si] / 30 + [Mn] / 20 + 2 [P] + 4 [S] ... (1)
However, [C], "Si", [Mn], [P], and [S] are the contents (mass%) of C, Si, P, and S.

Further, as a mode of the steel plate at the time of spot welding in which this crack occurs, it may occur when spot welding is performed in a state where the shaft of the welding electrode and the surface of the steel plate are off the vertical.

The spot welded part after welding was modeled, and FEM analysis (finite element method analysis) of the stress distribution was performed. First, when the axis of the welding electrode and the surface of the steel plate are vertical (when there is no striking angle) and when the axis of the welding electrode and the surface of the steel plate are at an angle of 5 degrees from the vertical (when the striking angle is 5 degrees). The stress distribution at the welded part after spot welding was calculated. FIG. 5 shows the stress distribution of the welded portion after welding when there is no striking angle. FIG. 6 shows the stress distribution of the welded portion after welding when the striking angle is 5 degrees. FIG. 6A shows the stress distribution of the entire welded portion, and FIG. 6B shows an enlarged view of the stress distribution in the immediate vicinity of the corona bond.

In FIGS. 5 and 6A, there is a portion 5 having a high welding residual stress on the surface of the steel sheet 1 in contact with the welding electrodes 4a and 4b. Further, in FIG. 6A, there is a portion 6 (a portion surrounded by a dotted line) where the welding residual stress is high just outside the corona bond on the overlapping surface of the steel sheets. FIG. 6B shows an enlarged view of a portion near the portion 6 having a high welding residual stress in the immediate vicinity of the corona bond. Such a portion 6 having a high welding residual stress in the immediate vicinity of the corona bond did not occur in the stress distribution of the welded portion after welding when there was no striking angle shown in FIG.

The portion 6 having a high welding residual stress directly outside the corona bond is in a compressed state in the process of being crushed by the welding electrode during spot welding, but becomes a tensile state when the welding electrode is separated, and further in the cooling process after the welding is completed. As shown in FIG. 6, it is considered that the tensile stress increases before the zinc-based plated metal solidifies.

From this, the present inventors have found that when the development of the corona bond is insufficient, the liquid phase plating remains inside the corona bond, and when the corona bond is not bonded, the tensile stress is generated inside the corona bond. High points (not shown) are generated, and the zinc-based plated metal melted and removed from the nugget formation position invades the grain boundaries of the steel sheet at the high tensile stress inside the corona bond, reducing the grain boundary strength. It was speculated that this would cause internal cracking of the corona bond.

In spot welding, it is basic to apply the spot welding electrode perpendicularly to the surface of the steel sheet. However, in the assembly of automobiles, for example, the shaft of the electrode and the surface of the steel plate in contact with the electrode are vertical, such as a place where the space around the welded part is limited and it is difficult to insert the welding gun, such as a structural member already assembled. Spot welding may occur without hitting.

In such a case, the present inventors have studied means for preventing cracks inside the corona bond. As a result, it was considered that the occurrence of internal cracks in the corona bond could be suppressed by raising the melting point of the zinc-based plating and solidifying the plating before the stress that causes liquid metal cracks was applied.

Therefore, a plate assembly is made of steel plates of various thicknesses and various tensile strengths, the angle between the steel plate surface and the electrode (chip) is intentionally tilted from 90 degrees, and the energization time of spot welding is variously changed for welding. did. After spot welding, the steel plate was cut so as to pass through the center of the spot welded nugget, and the cut surface was observed to check for internal cracks in the corona bond.

As a result, the total plate thickness of the plurality of steel plates in which the welded portions are overlapped is t [mm], and the angle between the normal of the contact surface of the electrode tip and the normal of the steel plate surface is d [degree], and the plate thickness is t. It was found that the larger the value and the larger the angle d, the longer the energization time for preventing internal cracking of the corona bond was required. As a result of regression analysis of many experimental results, it was found that the required energization time was (5t + 2.8d + 2) / 50 [sec] or more.

The larger the plate thickness t, the longer it takes to heat the steel sheet, the Zn inside the corona bond is discharged to the outside of the corona bond, and the Fe in the steel sheet and the Zn during plating are mutually diffused, and the Zn in the plating inside the corona bond is diffused. It is presumed that it takes time to lower the concentration and raise the melting point of the Zn—Fe alloy.

Further, it is considered that the larger the angle d, the larger the tensile stress generated in the steel sheet. Therefore, the larger the angle d, the wider the melting point of the Zn—Fe alloy needs to be raised. Therefore, it is necessary to lengthen the energization time. Presumed to be.

The present invention has been made through the above-mentioned examination process. Hereinafter, the flow of the welding method of the present invention will be described.

The welding electrode used for spot welding of the present invention is not particularly limited. Commonly used electrodes may be used. Further, if the contact surface between the welding electrode and the steel plate is also within the normal range, there is no problem. Further, the spot-welded electrode does not hit the steel plate perpendicularly, and the normal of the contact surface of the electrode tip attached to the tip of the weld electrode and the normal of the steel plate surface form an angle of about 1 to 20 °. However, the present invention is applicable.

The conditions for spot welding are not particularly limited. For example, the electrode has a dome radius type tip diameter of 6 to 8 mm, a pressing force of 150 to 600 kgf, an energizing time of 5 to 100 cycles (power frequency 50 Hz), and energization. The current is 4 to 15 kA.

The energizing time for spot welding is the total thickness of multiple steel plates that are overlapped at the welded part, t [mm], and the normal line of the contact surface of the electrode tip attached to the tip of the weld electrode and the normal line of the steel plate surface. By setting the angle of d [degree] to (5t + 2.8d + 2) / 50 [sec] or more, the inside of the corona bond is kept at a high temperature, Zn is discharged to the outside of the corona bond inside the corona bond, and the inside of the steel sheet is discharged. It is possible to promote mutual diffusion of Fe and Zn during plating in the corona bond. As a result, the Zn concentration of the plating is lowered and the melting point of the plating is raised. As a result, the plating is already solidified at the timing when the stress at which the liquid metal embrittlement cracks occur, and since it is a solid, the occurrence of cracks inside the corona bond can be suppressed.

The upper limit of the energizing time is not particularly defined, but if the energizing time is too long, the advantage of spot welding, which is welding in a short time, is lost, so the practical upper limit is about 2 sec.

The member is not particularly limited as long as it is composed of at least a plurality of steel plates in which welded portions are overlapped, and the overlapped surface of at least one of the steel plates is coated with zinc-based plating. For example, it is spot-welded to a plurality of steel sheets in which the overlapping surfaces of all spot-welded steel sheets are coated with zinc-based plating, or to steel sheets in which the overlapping surfaces of spot-welded steel sheets are coated with zinc-based plating. Examples thereof include a plurality of steel sheets including a steel sheet in which the steel sheet is not coated with zinc-based plating.

Further, in a steel sheet in which the superposed surface to be spot-welded is coated with zinc-based plating, the surface opposite to the superposed surface of the spot-welded steel sheet, that is, the contact surface with the welding electrode is coated with plating. It may or may not be coated. However, considering the corrosion resistance of the spot welded joint, it is preferable that the contact surface with the weld electrode is also coated with plating.

The type of zinc-based plating of the zinc-based plating-coated steel sheet used in the present invention is not particularly limited. The present invention is applied to steel sheets coated with any of zinc-containing hot-dip galvanizing, alloyed hot-dip galvanizing, electrogalvanizing, zinc / nickel plating, zinc / aluminum / magnesium-based plating, etc. can do.

Although two steel plates are shown in FIGS. 2 to 4 as the plurality of steel plates to be spot welded, three or more steel plates may be used depending on the form of the structural parts to be joined. .. The thickness of each steel plate to be spot welded is not particularly limited. The welding method of the present invention is suitable for welding a steel plate having a plate thickness of 0.5 to 3.0 mm. Further, the total thickness of the plurality of steel plates is not particularly limited. The welding method of the present invention is suitable for welding a steel plate having an overall plate thickness of 1.0 to 7.0 mm.

Further, the steel plate may have at least a plate-shaped portion at least in part and may have a portion in which the plate-shaped portions are stacked on each other, and the entire steel plate does not have to be a plate. Further, the plurality of steel plates are not limited to those composed of separate steel plates, and for example, one steel plate formed into a predetermined shape such as a tubular shape may be superposed.

Further, the steel plate of the member to be spot welded is not particularly limited in terms of composition, metal structure, and the like. However, when a TRIP steel sheet or a high-strength steel sheet having a Ceq of 0.15% by mass or more is used, internal cracks in the corona bond are likely to occur. Therefore, the present invention is particularly suitable for spot welding of such steel sheets. Suitable.

From experiments using various steel plate braids, the present inventors have found that when the difference in tensile strength between adjacent steel plates is 50 MPa or more, internal cracking in the corona bond is likely to occur, and further, this difference is 500 MPa or more. At that time, it was confirmed that internal cracks in the corona bond were more likely to occur.

When the tensile strength of one of the adjacent steel plates was low, the steel plate having the lower tensile strength was significantly deformed when the spot weld electrode pressed the steel plate. It was speculated that if the steel sheet was significantly deformed, the sensitivity to internal cracks in the corona bond would increase. Therefore, the present invention is particularly suitable for a plate assembly in which steel plates having a difference in tensile strength as described above are adjacent to each other.

The method for confirming the presence or absence of internal cracks in the corona bond is not particularly limited, and a method of cutting in the plate thickness direction so as to include the nugget and observing the cross section or a tensile test of the spot welded joint is carried out. It is possible to use a method of determining whether or not a predetermined tensile strength can be obtained. Alternatively, since internal cracks may not be observed depending on the cutting position of the cross section in the plate thickness direction including the spot welded portion, the cracks may be confirmed by performing an X-ray transmission test.

Internal cracks in the corona bond occur in the corona bond on the surface of the steel sheet coated with plating or the surface of the steel sheet coated with plating and the steel sheet overlapped via plating.

The welding method of the present invention has been described above. The welding method of the present invention is suitable for spot welding of galvanized steel sheets for automobiles, but the present invention is not limited thereto. As long as the gist of the present invention is not deviated and the object of the present invention is achieved, it can be applied to members of various vehicles, general machines, home appliances, ships and the like.

Next, an example of the present invention will be described. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is described in this one condition example. It is not limited. In the present invention, various conditions can be adopted as long as the gist of the present invention is not deviated and the object of the present invention is achieved.

The zinc-based plated steel plates shown in Table 1 are laminated by the plate sets shown in Tables 2 and 3, and the laminated steel plates are sandwiched between the dome radius type electrodes having a tip diameter of 6 mm from both sides. A test piece was prepared by spot welding with the energizing time and energizing current shown in Tables 2 to 3 while pressing with the pressing force shown in 3.

The striking angle d in Tables 2 and 3 means the angle between the normal of the contact surface of the electrode tip attached to the tip of the welding electrode and the normal of the surface of the steel sheet. Further, Wt [sec] indicates a value obtained by the formula: (5t + 2.8d + 2) / 50.

Regarding the prepared test piece, the presence or absence of internal cracks in the corona bond was confirmed. The crack was confirmed by cutting the test piece in the plate thickness direction so as to include the nugget and confirming the cross section thereof. The results are shown in Tables 2-3.

As shown in Tables 2 and 3, it was confirmed that the occurrence of internal cracks in the corona bond can be suppressed by carrying out the energization time of the present invention according to the plate thickness and angle.

According to the present invention, spot welding is performed with the normal of the contact surface of the electrode tip attached to the tip of the welding electrode and the normal of the steel plate surface at an angle of 1 to 20 degrees and the electrode tilted with respect to the steel plate. Even if it is performed, cracking of the liquid metal due to melting of the plating can be suppressed, so that the occurrence of cracking inside the corona bond can be prevented. Therefore, the present invention has high industrial applicability.

1 Steel plate 2 Nugget 3 Internal crack 4a Welding electrode 4b Welding electrode 5 Where the welding residual stress on the surface of the steel plate is high 6 Where the welding residual stress directly outside the corona bond is high

Claims (4)

It is a method of spot welding to a member.
The welded portion of the above member is composed of a plurality of stacked steel plates.
The plurality of laminated steel sheets include one or more steel sheets whose overlapping surfaces are coated with zinc-based plating.
At the time of spot welding, the total thickness of multiple steel plates with overlapping welded parts is t [mm], and the larger of the sharp angles formed by the axes of the two weld electrodes with the normal line of the steel plate surface is d. A spot welding method characterized in that the energization time is (5t + 2.8d + 2) / 50 [sec] or more when [degree] (d> 0) is set. The spot welding method according to claim 1, wherein the larger angle of the acute angles formed by the axes of the two welding electrodes with the normal of the steel sheet surface is 1 to 20 °. The spot welding method according to claim 1 or 2, wherein the difference in tensile strength between adjacent steel plates in the plurality of stacked steel plates is 50 MPa or more. The spot welding method according to claim 1 or 2, wherein the difference in tensile strength between adjacent steel plates in the plurality of stacked steel plates is 500 MPa or more.

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