JP2018039019A - Spot-welding method - Google Patents

Abstract

The present invention relates to a spot welding method capable of forming a high-quality spot welded joint by preventing cracking at the time of corona bond nugget on the overlapping surface of steel plates in spot welding of a plate assembly including steel plates to which zinc-based plating has been applied. provide. A method of spot welding to a member, wherein, during spot welding, the total thickness of a plurality of steel plates on which welding locations are superimposed is t [mm], and an electrode tip attached to the tip of a welding electrode is provided. When the angle between the normal of the contact surface and the normal of the steel plate surface is d [degree] (d> 0), the energization time is (5t + 2.8d + 2) / 50 [sec] or more. Spot welding method to do. [Selection] Figure 1

Description

  The present invention relates to a spot welding method for a plurality of steel plates including a steel plate on which zinc-based plating is performed.

  As a member for automobiles, a zinc-based plated steel sheet having excellent corrosion resistance is widely used from the viewpoint of increasing 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 plating base plates are used in galvanized steel sheets used for automobiles.

  Spot welding is mainly used for assembling automobile bodies and mounting parts. However, when spot welding is performed on a zinc-based plated high-strength steel sheet, there is a problem that cracks occur in the steel sheet in contact with the welding electrode.

  This crack is caused by the applied stress of the electrode and the thermal expansion and contraction of the steel sheet, and the welded part is subjected to tensile stress, and zinc melted on the surface of the welded steel sheet and copper of the electrode penetrates the steel sheet and increases the grain boundary strength. It is said to be a crack caused by so-called liquid metal embrittlement caused by lowering. In automobile bodies, there is a problem that the strength decreases when cracks in the welded portion are significant, and a technique for suppressing cracks in the welded portion by controlling the composition and structure of the steel sheet is known.

  In Patent Document 1, by adjusting the component composition of the steel sheet, the austenite phase generated at the time of spot welding is made into fine crystal grains, and has a metal structure that is complicated with the crystal grains of other phases. In addition, a technique has been disclosed in which the diffusion penetration path of molten zinc into the crystal grain boundary is complicated to make it difficult for the molten zinc to enter, thereby preventing liquid metal embrittlement cracking during spot welding.

  In Patent Document 2, the composition of the steel sheet is adjusted and the grain of the hot-rolled steel sheet is adjusted, assuming that the occurrence of cracks in the welded part may not be sufficiently suppressed only by making the grain boundaries complicated by controlling the structure of the steel sheet. The grain boundary erosion depth of the alloyed hot-dip galvanized steel sheet should be 5 μm or less by performing the Fe-based electroplating process on the cold-rolled steel sheet before the alloying hot-dip galvanizing treatment with the field oxidation depth being 5 μm or less. And the technique which suppresses generation | occurrence | production of the crack in the welding location of a galvannealed steel plate is disclosed.

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

JP 2006-265671 A JP 2008-231493 A Japanese Patent Laid-Open No. 05-277552

  Thus, although measures against cracks in spot welds have been studied, in spot welded joints that include steel sheets with zinc-based plating applied to the plate assembly, even when cracks at the contact points between the steel sheets and electrodes do not occur, Still, the desired tensile strength may not be obtained.

  As a result of investigations by the present inventors, it was confirmed that a crack was generated in the corona bond on the superposed surface of the steel plate in the spot welded joint where the desired tensile strength could not be obtained. Hereinafter, the internal crack of the corona bond is referred to as “corona bond internal crack”.

  In FIG. 1, the outline of the crack in a corona bond is shown. A crack generated in the region of the corona bond formed between the steel plates 1 is a corona bond internal crack 3.

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

  In spot welding, when the steel plate is tilted with respect to the electrode (FIG. 2), when an angle is generated by the deflection of the welding gun (FIG. 3), when using a welding gun with the electrode tilted (FIG. 4), etc. The electrode does not hit the steel sheet perpendicularly. In addition, when the space around the welded part is limited, the electrode is likely not to hit the steel plate perpendicularly.

  In view of such a situation, the present invention prevents the internal crack of the overlapping surface of the steel plates in the spot welding of the plate set including the steel plates to which the zinc-based plating has been applied, and can form a high-quality spot welded joint. It is an object to provide a welding method.

  The present inventors diligently studied a method for solving the above-mentioned problem.

  It is thought that the crack in the corona bond is a liquid metal embrittlement crack generated in the cooling process after the end of energization. As a result of the study by the present inventors, the present inventors have (i) discharging the Zn inside the corona bond to the outside of the corona bond and performing the plating with Fe in the steel plate inside the corona bond during spot welding. It is possible to increase the melting point of the plating inside the corona bond by interdiffusion of Zn in the corona bond, and as a result, the plating is solidified before applying the tensile stress that causes liquid metal embrittlement cracking. (Ii) When the angle between the normal of the contact surface of the electrode (chip) and the normal of the surface of the steel sheet deviates from 0 degrees, liquid metal embrittlement cracking occurs. Since the tensile stress entering the region increases, it is necessary to increase the melting point of the plating over a wider range. To achieve this, the energizing time for spot welding is increased as the angle increases. In, coronavirus bond cracks found that can be suppressed.

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

  (1) A method of spot-welding a member, wherein the welded portion of the member is composed of a plurality of superposed steel plates, and the superposed surfaces of the superposed steel plates are covered with zinc-based plating In the case of spot welding, the total thickness of a plurality of steel plates on which the welding points are overlapped is t [mm], and the normal of the contact surface of the electrode tip attached to the tip of the welding electrode A spot welding method characterized in that the energization time is set to (5t + 2.8d + 2) / 50 [sec] or more, where d [degree] (d> 0) is an angle with the normal to the steel sheet surface.

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

  (3) The spot welding method according to (1) or (2), wherein a 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), wherein a 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 increasing the melting point of the plating inside the corona bond, the plating solidifies before the tensile stress that causes internal cracking is applied, so the angle between the electrode shaft and the steel sheet surface is out of the vertical direction. Even if it exists, generation | occurrence | production of the crack in a corona bond by a liquid metal crack can be prevented.

It is a figure which shows the outline of the crack in a corona bond at the time of performing spot welding to a zinc-based plated steel plate. It is a figure which shows the outline in case a steel plate inclines with respect to a welding electrode. It is a figure which shows the outline when the deflection | deviation has arisen in the welding gun. It is a figure which shows the outline at the time of using the welding gun in which the electrode inclined. It is a figure which shows the stress distribution of the welding location after welding when there is no hitting angle. It is a figure which shows the stress distribution of the welding location after welding in case a hit angle is 5 degree | times. (A) shows the stress distribution of the whole welding location, (b) shows the enlarged view of the stress distribution in the vicinity of the corona bond.

  The spot welding method of the present invention (hereinafter referred to as “welding method of the present invention”) is a method of superimposing steel plates of a member including a portion where a plurality of steel plates including one or more steel plates coated with zinc-based plating are superimposed. T is the total thickness of a plurality of steel plates on which the welding locations are superposed, the normal of the contact surface of the electrode tip attached to the tip of the welding electrode, and the steel plate surface When the angle with respect to the normal line is d [degrees] (d> 0), welding is performed so that the energization time is (5t + 2.8d + 2) / 50 [sec] or more.

  Hereinafter, the background of the examination that led to the welding method of the present invention and the welding method of the present invention will be described in detail.

  The inventors of the present invention conducted the following experiment and investigated the reason for the occurrence of internal cracks in the corona bond that occurred on the inner surface side where the steel plates were superposed in spot welding using a zinc-based plated steel plate.

  Spot welding was performed under various welding conditions using two steel sheets of various component compositions or steel types coated with zinc-based plating and not coated with plating. And the cross section of the plate | board thickness direction containing the nugget of the spot welded joint from which the tensile strength becomes low among the obtained spot welded joints was observed.

  As a result, internal cracks in the overlapped surface of steel sheets occur when using so-called TRIP steel that utilizes work-induced martensitic phase transformation that contains a large amount of alloy components, and there are few alloy components and a composite of ferrite and martensite phases. When so-called DP steel was used, it was difficult to occur.

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

  Moreover, as a steel plate aspect in the spot welding in which this crack occurs, it may occur when spot welding is performed in a state where the axis of the welding electrode and the steel plate surface are out of the vertical.

  The spot welding spot after welding was modeled, and FEM analysis (finite element method analysis) of stress distribution was performed. First, when the axis of the welding electrode and the steel plate surface are vertical (when there is no hitting angle), and when the axis of the welding electrode and the steel plate surface are at an angle of 5 degrees from the vertical (when the hitting angle is 5 degrees) The stress distribution at the welded spot after spot welding was obtained by calculation. FIG. 5 shows the stress distribution of the welded portion after welding when there is no strike angle. FIG. 6 shows the stress distribution in 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 vicinity of the corona bond.

  5 and 6 (a), there is a portion 5 where the welding residual stress is high on the surface of the steel sheet 1 in contact with the welding electrodes 4a and 4b. Moreover, in Fig.6 (a), there exists the location 6 (location enclosed with a dotted line) where welding residual stress is high just outside the corona bond of the overlapping surface of a steel plate. FIG. 6B shows an enlarged view of a portion in the vicinity of the portion 6 having a high welding residual stress in the vicinity immediately outside the corona bond. Such a portion 6 with high welding residual stress in the vicinity of the corona bond directly outside does not occur in the stress distribution of the welded portion after welding when there is no striking angle shown in FIG.

  The portion 6 having a high residual welding stress immediately 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 end of welding. As shown in FIG. 6, it is considered that the tensile stress is increased before the zinc-based plated metal is solidified.

  Accordingly, 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 joined, there is a tensile stress inside the corona bond. Zinc-based plated metal that has been melted and removed from the nugget formation position invades the crystal grain boundaries of the steel sheet where the tensile stress is high inside the corona bond, and lowers the grain boundary strength. It was inferred that this would cause a crack in the corona bond.

  In spot welding, it is fundamental to apply an electrode for spot welding perpendicularly to the surface of the steel sheet. However, when assembling automobiles, for example, the space around the welded part is limited as in the case of already assembled structural members, and it is difficult to insert a welding gun. Spot welding may occur without contact.

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

  Therefore, we made a plate assembly with steel plates of various plate thicknesses and various tensile strengths, intentionally inclined the angle between the steel plate surface and the electrode (tip) from 90 degrees, and varied the energization time of spot welding, and welding did. After spot welding, the steel sheet was cut so as to pass through the center of the nugget of spot welding, and the cut surface was observed to check for cracks in the corona bond.

  As a result, the total thickness of the plurality of steel plates on which the welded portions are superimposed 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 thickness t It was found that the energization time for preventing cracks in the corona bond needs to be increased as the angle d increases and the angle d increases. As a result of regression analysis of many experimental results, it was found that the necessary energization time was (5t + 2.8d + 2) / 50 [sec] or more.

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

  Moreover, since it is thought that the tensile stress which arises in a steel plate becomes large, so that the angle d is large, since it is necessary to raise melting | fusing point of a Zn-Fe alloy in a wide range, so that the angle d is large, it is necessary to lengthen energization time. Presumed to be.

  The present invention has been made through the above 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. A commonly used electrode may be used. Moreover, if the contact surface of a welding electrode and a steel plate is also a normal range, there will be no problem. In addition, the spot welding electrode is not perpendicular to the steel plate, and 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 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 is a dome radius tip having a tip diameter of 6 to 8 mm, the applied pressure is 150 to 600 kgf, the energization time is 5 to 100 cycles (power supply frequency 50 Hz), and the energization is performed. The current is 4 to 15 kA.

  The energization time of the spot welding, the total thickness of a plurality of steel plates overlaid with welding locations, t [mm], 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 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, the Zn inside the corona bond is discharged out of the corona bond, It becomes possible to promote the mutual diffusion of Zn during plating in Fe and the corona bond. Thereby, the Zn concentration of the plating is lowered, and the melting point of the plating is increased. As a result, since the plating is already solidified and solid at the timing when the stress that causes liquid metal embrittlement cracking occurs, the occurrence of cracks in the corona bond can be suppressed.

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

  The member is not particularly limited as long as the member is composed of at least a plurality of steel plates on which the welding locations are overlapped, and the overlapping surface of at least one of the steel plates is covered with zinc-based plating. For example, spot welding is performed with a plurality of steel plates in which zinc-plating is coated on the overlapping surfaces of all spot-welded steel plates or steel plates in which zinc-plating is coated on the overlapping surfaces of spot-welded steel plates. A plurality of steel plates including a steel plate that is not coated with zinc-based plating is exemplified.

  In addition, in a steel plate in which the overlapped surface to be spot welded is coated with zinc-based plating, the surface opposite to the overlapped surface of the steel plate to be spot welded, that is, the contact surface with the welding electrode is coated with plating. Or may not be coated. However, in consideration of the corrosion resistance of the spot welded joint, it is preferable that the contact surface with the welding electrode is also coated with plating.

  The kind of the zinc-based plating of the steel sheet coated with the zinc-based plating used in the present invention is not particularly limited. The present invention applies to any steel sheet coated with zinc, such as hot dip galvanizing, zinc galvanizing, electrogalvanizing, zinc / nickel plating, zinc / aluminum / magnesium plating, etc. can do.

  In FIG. 2 to FIG. 4, two steel plates are described as a plurality of steel plates to be spot welded, but three or more steel plates can be used depending on the form of the structural parts to be joined. . The plate 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 overall plate thickness of the plurality of steel plates is not particularly limited. The welding method of the present invention is suitable for welding steel plates having a total plate thickness of 1.0 to 7.0 mm.

  Moreover, the steel plate should just have a plate-shaped part in at least one part, and a plate-shaped part has a part by which each other is piled up, and the whole may not be a board. Further, the plurality of steel plates are not limited to those composed of separate steel plates, and for example, a single steel plate formed into a predetermined shape such as a tubular shape may be superimposed.

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

  From experiments using various steel sheet assemblies, the present inventors have found that when the difference in tensile strength between adjacent steel sheets is 50 MPa or more, cracks in corona bonds are likely to occur, and further, this difference is 500 MPa or more. Then, it was confirmed that cracks in the corona bond were more likely to occur.

  When the tensile strength of one of the adjacent steel plates was low, when the spot welding electrode pressed the steel plate, the steel plate having the lower tensile strength was greatly deformed. It was speculated that if the steel sheet was greatly deformed, the sensitivity to corona bond cracking 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 occurrence of 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, and conducting a tensile test of the spot welded joint Thus, it is possible to use a method of determining whether or not a predetermined tensile strength can be obtained. Or, depending on the cutting position of the cross section in the plate thickness direction including the spot welded portion, the internal crack may not be observed. Therefore, an X-ray transmission test may be performed to confirm the crack.

  The crack in the corona bond is generated in the corona bond on the overlapping surface of the steel sheet coated with plating or on the surface of the steel sheet coated with plating and the steel sheet overlapped through 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 to this. As long as the object of the present invention is achieved without departing from the gist of the present invention, the present invention can be applied to members of various vehicles, general machines, home appliances, ships, and the like.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

  The steel plates to which zinc-based plating shown in Table 1 is applied are overlapped with the plate assemblies shown in Tables 2 to 3, and the overlapped steel plates are sandwiched by dome radius type electrodes having a tip diameter of 6 mm from both sides. While pressing with the pressurizing force described in 3, spot welding was performed with the energizing time and energizing current described in Tables 2 to 3 to prepare test pieces.

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

  About the produced test piece, the presence or absence of the crack in a 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. The results are shown in Tables 2-3.

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

  According to the present invention, 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 degrees, and spot welding is performed in a state where the electrode is inclined with respect to the steel plate. Even if it is carried out, liquid metal cracking due to melting of the plating can be suppressed, so that occurrence of cracks in the corona bond can be prevented. Therefore, the present invention has high industrial applicability.

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Nugget 3 Internal crack 4a Welding electrode 4b Welding electrode 5 The place where the welding residual stress of a steel plate surface is high 6 The place where the welding residual stress right outside corona bond is high

Claims (4)

A method of spot welding to a member,
The welded portion of the member is composed of a plurality of stacked steel plates,
The plurality of superposed steel plates include one or more steel plates whose overlapping surfaces are coated with zinc plating,
At the time of spot welding, the total thickness of a plurality of steel plates on which the welding locations are superimposed is t [mm], and 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 A spot welding method, wherein the energization time is (5t + 2.8d + 2) / 50 [sec] or more when the angle is d [degrees] (d> 0).   2. The spot welding method according to claim 1, wherein an angle between a normal line of a contact surface of an electrode tip attached to a tip of the welding electrode and a normal line of a steel plate surface is 1 to 20 °.   The spot welding method according to claim 1 or 2, wherein a difference in tensile strength between adjacent steel plates in the superposed plurality of steel plates is 50 MPa or more.   The spot welding method according to claim 1 or 2, wherein a difference in tensile strength between adjacent steel plates in the superposed plurality of steel plates is 500 MPa or more.

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