JP2005224829A - Spot welding method for hot dip galvanized steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spot welding method for a hot dip galvanized steel sheet, in which the service life of an electrode is improved without damaging the appearance, workability or the like of the plated steel sheet, when a hot dip galvanized steel sheet is continuously subjected to spot welding. <P>SOLUTION: A hot dip galvanized steel sheet which has a hot dip galvanized layer with a coating weight of ≥30 g/m<SP>2</SP>per side, formed on at least one surface of the steel sheet and in which the base material of the steel sheet has Vickers hardness measured with a load of 50 g at 500°C is ≥70, is subjected to spot welding using an electrode with the shape of a truncated cone in which the size at the tip is ≤8 mmϕ, and the working face of the tip has a projecting curved face. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a spot welding technique for a hot-dip galvanized steel sheet in which a hot-dip galvanized layer is formed on the surface of the steel sheet, and in particular, the nugget diameter necessary for ensuring the strength of the welded portion can be stably obtained, and the electrode life is long. The present invention relates to a spot welding method.

  Conventionally, a steel sheet with a tensile strength of 270 to 440 MPa, which is used for the outer plate of automobile bodies, is formed with a hot dip galvanized layer with excellent corrosion resistance on the surface of the steel plate base material in order to improve the life of the vehicle body. Many hot dip galvanized steel sheets are used. However, this hot dip galvanized steel sheet has a problem that when spot welding is continuously carried out, the electrode is consumed rapidly and the number of continuous dots is small, that is, the electrode life is short. In particular, in the case of a hot-dip galvanized steel sheet that requires excellent workability (pressability), the welding current required to obtain an appropriate nugget diameter is increased, so that the electrode life is remarkably short.

  That is, since the melting point of the plated layer of the hot dip galvanized steel sheet is lower than the melting point of the alloyed hot dip galvanized steel sheet mainly composed of zinc and iron alloy, the plated layer easily melts and the current path of spot welding As a result, the welding current density decreases, and the welding current necessary for forming the nugget tends to increase. Also, since the electrode material copper and the zinc of the plating layer (zinc that is not alloyed with iron) form an alloy, welding between the electrode and the hot dip galvanized steel sheet is likely to occur. Exfoliation causes significant electrode wear. Further, in the hot dip galvanized hot dip galvanized steel sheet, in order to suppress the diffusion of Fe in the plated layer and suppress alloying, a larger amount of Al is contained in the plated layer than in the case of the galvannealed steel sheet. Therefore, an oxide layer that is formed in the case of an alloyed hot-dip galvanized steel sheet does not occur on the electrode surface. Therefore, it is said that the electrode life in spot welding of hot-dip galvanized steel sheets is significantly inferior to that of alloyed hot-dip galvanized steel sheets.

In order to solve the above problems, for example, a technique for improving the electrode life by applying an oxide film mainly composed of ZnO to the surface of an alloyed hot-dip galvanized steel sheet (see, for example, Patent Document 1). In addition, a technique using a steel sheet in which the Zn content and the Al ₂ O ₃ content on the surface of the alloyed hot-dip galvanized steel are specified (for example, see Patent Document 2) has been proposed. However, these techniques are intended to improve weldability only by improving the plating layer, and there is a problem that a great effect cannot be expected in the case of a hot-dip galvanized steel sheet in which the plating layer easily melts.

Moreover, although the technique (for example, refer patent document 3) which aims at the improvement of an electrode lifetime by reducing the amount of Al in a hot-dip galvanized layer is proposed, reduction of Al amount is the hard and brittle alloy layer. In order to promote the development, there is a problem that the plating properties are deteriorated.
JP 63-230861 A Japanese Patent Laid-Open No. 10-330902 Japanese Unexamined Patent Publication No. 04-021750

  As described above, the improvement of the electrode life of the hot dip galvanized steel sheet is still insufficient, and the electrode used for spot welding of the hot dip galvanized steel sheet must be frequently replaced in the assembly welding line of the white body of an automobile. I do not get. Therefore, in Japan, hot dip galvanized steel sheets are rarely used for automobile outer plates where spot welding electrode life is important. Therefore, in order to effectively use the hot dip galvanized steel sheet, a technique for improving the electrode life of spot welding is desired.

  An object of the present invention is to provide a spot welding method for a hot dip galvanized steel sheet that can improve the electrode life without impairing the appearance or workability of the hot dip galvanized steel sheet when spot galvanized steel sheets are continuously spot welded. There is to do.

  In order to solve the above-mentioned problems of the prior art, the inventors focused on the plating composition, steel plate base material characteristics, electrode composition and shape, etc. as factors affecting the electrode life of the hot dip galvanized steel sheet. The examination was done. As a result, the inventors have found that the electrode life can be improved by limiting the high-temperature hardness of the steel plate base material and making the electrode shape suitable, thereby leading to the development of the present invention.

The present invention developed on the basis of the above findings has a Vickers measured at a load of 50 g at 500 ° C. of a steel plate base metal having a hot-dip galvanized layer with a basis weight per side of 30 g / m ² or more on at least one steel plate surface. Spot-welding method for hot-dip galvanized steel sheet having a hardness of 70 or more and spot welding using a frustoconical electrode having an electrode tip diameter of 8 mm or less and a convex curved surface at the electrode tip It is.

  According to the present invention, a sufficient electrode life can be obtained even if the hot-dip galvanized steel sheet is continuously spot welded. Furthermore, since the hot dip galvanized steel sheet and the galvannealed steel sheet can be constructed on the same assembly welding line, it greatly contributes to the improvement of productivity.

In spot welding of hot dip galvanized steel sheets, the melting point of the plating layer is low, so the energization path is expanded and the welding current required to obtain an appropriate nugget diameter is increased. Forms an alloy that easily wears away with zinc in the plating layer, so that the electrode life is remarkably shortened, and the reduction of the Al content in the plating layer is effective in improving the electrode life, but promotes alloying. However, as described above, the deterioration of the plating properties is caused. Therefore, we examined hot-dip galvanized steel sheets and electrodes that provide good electrode life even when there is a large amount of Al, such that the amount of Al in the plating layer exceeds 0.23 g / m ² in basis weight per side. It was. As a result, the above-described configuration of the present invention has been conceived. The present invention will be described below.

Electrode shape: frustoconical shape and tip diameter of 8 mmφ or less The inventors believe that it is effective to suppress electrode wear and improve nugget formation ability in order to extend the electrode life. The shape of the electrode effective for reducing the wear rate was investigated. As a result, the electrode used for spot welding of hot-dip galvanized steel sheet must be a truncated cone shape as shown in FIG. 1 instead of the spherical shape (dome radius shape) that has been used conventionally. I understood. The reason for this is that with a spherical electrode, the electrode easily sinks into the hot dip galvanized steel sheet, and the current-carrying area increases easily, resulting in a decrease in current density. This is because a large welding current is required to cover, and as a result, the wear rate of the electrode is increased. On the other hand, in the truncated conical electrode, the electrode sinks less, and the diameter expansion when worn becomes small, so that the wear of the electrode can be suppressed to a small value.

  Further, the conventional frustoconical electrode has a tip angle indicated by θ in FIG. 1 of 90 to 120 °. However, when θ is 90 ° or more, for the same reason as described above, the current-carrying area due to electrode wear has a large amount of change (increase), that is, the current-carrying area is inferior in stability. More preferably. The tip diameter of the truncated cone must be 8 mmφ or less. This is because when the tip diameter exceeds 8 mm, the pressure applied to press the electrode against the hot dip galvanized steel sheet is insufficient with a normal welding machine. On the other hand, the minimum diameter is more preferably 5 mmφ or more in order to obtain a good nugget diameter by welding a hot-dip galvanized steel sheet having a thickness of about 0.8 mm.

  Furthermore, in the present invention, the electrode tip serving as the contact surface is preferably a convex curved surface and a spherical surface with a radius of curvature of 30 to 100 mm from the viewpoint of ease of processing. This is because the contact surface of the electrode when the electrode is replaced is not necessarily set parallel to the surface of the hot dip galvanized steel sheet. In many cases, the contact area between the galvanized steel sheet and the hot dip galvanized steel sheet is small, and a good nugget diameter cannot be obtained. Therefore, it is necessary to reset the electrode many times, which is one of the factors hindering productivity in a continuous production line. In this respect, when the contact surface of the electrode tip is a convex curved surface, the above-mentioned problem can be solved, so that a welded assembly having a nugget diameter that is expected from the beginning can be obtained by only one electrode replacement. Because it can. When the radius of curvature of this spherical surface is less than 30 mm, the curvature is too large, and it becomes difficult to ensure a sufficient contact area between the electrode and the hot dip galvanized steel sheet. On the other hand, when the upper limit of 100 mm is exceeded, the effect of avoiding re-setting of the electrode is the same as when the electrode tip is flat, and the problem cannot be solved. The upper limit is preferably set to a value close to the sum of the cap and the adapter of the welder. In a general-purpose welder, the sum is about 100 mm. Therefore, when the contact surface of the electrode tip is a spherical surface, it is preferable that the radius of curvature is 30 to 100 mm.

Next, the inventors have studied a hot dip galvanized steel sheet as a material to be welded and found that the following conditions must be satisfied in order to improve the nugget forming ability.
First, galvanized basis weight: 30 g / m ² or more per side First, the galvanized steel sheet that satisfies the conditions of the present invention must have a basis weight of 30 g / m ² or more. If the basis weight is less than 30 g / m ² , the corrosion resistance as a hot dip galvanized steel sheet is insufficient. Preferably, it is 50 g / m ² or more. The surface on which the plating layer is formed is generally both surfaces of the two surfaces of the steel plate, but may be only one surface depending on the application.

High temperature hardness at 500 ℃: Hv (50g) ≧ 70
In order to form a nugget by spot welding, it is more advantageous to use a base steel plate having a high tensile strength, in which the electrode does not easily sink. That is, it is considered that a high-strength steel plate is more advantageous for improving the electrode life. However, according to the inventors' investigation, it was found that in the case of a hot dip galvanized steel sheet having a tensile strength of 440 MPa or less, which is close to that of mild steel, the strength at room temperature and the electrode life are not necessarily correlated.

  Therefore, a hot-dip galvanized steel sheet coated on both sides with a tensile strength at room temperature of 270 to 440 MPa class using a truncated cone-shaped electrode (nominal diameter: 16 mm, tip angle θ: 90 °, electrode tip diameter: 5 mmφ). As an object, the effect of the high temperature strength of the steel sheet base metal on the electrode life was investigated. In addition, all the hot dip galvanized steel sheets used had good plating properties such as no hard and brittle alloy layers developed. Here, the high-temperature strength is evaluated by hardness at 500 ° C., and this high-temperature hardness has a cross-section of a steel plate base material as a measurement surface, and a high-temperature hardness meter is used in an Ar atmosphere furnace at 500 ° C. The Vickers hardness measured at a load of 50 g was used. The details of the measurement conformed to JIS Z 2252. The electrode life was evaluated as good (◯) when the number of continuous hits was 2000 or more and bad (●) when less than 2000. The results are shown in FIG. As is apparent from this figure, it was found that if the high temperature hardness at 500 ° C. is 70 or more, a good electrode life with 2000 or more consecutive dots can be obtained.

  A hot-dip galvanized plate of A to H shown in Table 1 using a truncated cone electrode satisfying the conditions of the present invention having the shape shown in FIG. 1 and having a nominal diameter of 16 mm, a tip angle θ of 90 °, and an electrode tip diameter of 5 mmφ. The spot welding experiment of the steel plate was conducted and the number of continuous hits was measured. In the experiment, two hot-dip galvanized steel sheets were stacked, pressurizing force: 2000 N, pressurizing time: 30 cycles (1 cycle is 1/50 second, the same applies hereinafter), energizing time: 10 cycles, holding time: 5 cycles, dot Interval: The condition was 2 seconds or less. In addition, the electrode life is determined based on the nugget diameter measured by peeling off the welded part of the nugget diameter measurement specimen (25 × 80mm) prepared separately for every 100 striking points. Was determined by the cumulative number of hit points immediately before the lower limit of 4√t (t: plate thickness). That is, the number of continuous hits was evaluated in units of 100 hits. As a reference example, a welding experiment using a conventional dome radius electrode (nominal diameter: 16 mm, electrode tip R: 40 mm) was also conducted. R is defined by JIS C 9304.

  Table 2 summarizes the results of the welding experiment. In the case of Nos. 1 to 5 in which hot dip galvanized steel sheets No. A to E satisfying the high temperature hardness Hv (50 g) ≧ 70 at 500 ° C. which is the condition of the present invention are welded using electrodes satisfying the conditions of the present invention A good electrode life of 2000 or more was obtained. On the other hand, in the case of No. 6 to 8 where hot galvanized steel sheets of No. F to H with high-temperature hardness Hv (50 g) <70 are welded, the number of continuous hit points does not reach 2000, which is insufficient. Electrode life. On the other hand, in the case of Nos. 9 to 13 using the dome radius electrode, the number of continuous hits did not reach 2000 even with the hot-dip galvanized steel sheet satisfying the conditions of the present invention.

  The present invention can be applied to the field using spot welding including an assembly welding line of an automobile.

It is a figure which shows the electrode shape which satisfy | fills the conditions of this invention. It is a graph which shows the relationship between the high temperature hardness of a steel plate base material, and an electrode lifetime.

Claims (1)

Hot-dip galvanized steel sheet having a hot-dip galvanized layer with a basis weight per side of 30 g / m ² or more on at least one steel sheet surface and having a Vickers hardness of 70 or more measured at a load of 50 g at 500 ° C. A spot welding method for hot-dip galvanized steel sheets, characterized in that spot welding is performed using a frustoconical electrode having an electrode tip diameter of 8 mm or less and an electrode tip having a convex curved surface.

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