This invention relates to a butt welding method for joining steel sheets to one another.
In recent years, butt welding has come to be widely used in place of flash butt welding as a method of efficiently and accurately joining steel sheets with one another.
Butt welding is a type of resistance pressure welding method. In this method, as shown in FIGS. 1a and 1b, two steel sheets 2 to be welded are held between two confronting electrodes 1 which also serve as a means for applying pressure. The ends of the steel sheets 2 are made to abut under pressure and a large current is passed therebetween, as a result of which the end surfaces of the steel sheets 2 are joined to one another in an extremely short time.
However, when performing butt welding on steel sheets in the above-described manner, there is a problem in that it is easy for the abutting surfaces of the sheets to be unevenly heated in the widthwise direction. Namely, the outer portions in the widthwise direction of the abutting surfaces tend to be inadequately heated, and those portions of the abutting surfaces which are located somewhat farther inwards in the widthwise direction tend to be overheated. In extreme cases, as shown in FIG. 2, those portions which are overheated melt through, or there may be inadequate upsetting of the abutting surfaces. In FIG. 2, reference numeral 3 indicates locations of poor upsetting between two steel sheets 2 due to overheating. On the other hand, if the amount of heating of the steel sheets 2 is reduced in order to avoid the above-described problems due to overheating, the outer portions of the abutting surfaces to be joined end up being underheated, cold welding defects take place, and the outer portions of the sheets are poorly joined to one another.