Synopsis Spurred by the growing demand for heavy wall UOE pipes with excellent toughness, recent progress by the 4-wire submerged arc welding (SAW) method is reported from the aspects of productivity and weld quality. The 4-wire SAW method is found to bring about 1030 % improvement in productivity and to result in a remarkable suppression of slag inclusion under optimized welding conditions, f or example, in current phase and current ratio. It is also confirmed that the speedup by the 4-wire

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... method does not deteriorate weld quality. Special attentions are paid on the weld quality of heavy wall pipes to meet the recent market situation. This was because there are two problems, i.e., the formation of coarse microstructures of low toughness owing to slow cooling rate and the deterioration of external bead appearance. To cope with them, firstly, the control of nitrogen, oxygen and boron contents in weld metal must be conducted. The optimum ratio between nitrogen and boron contents for excellent toughness is found to be B -0.7 • [N] ±15 (ppm) for [N] <80 ppm. Secondly, a fused type flux is developed, which has excellent performances in both lowering oxygen content in weld metal to 250300 ppm and suppressing the deterioration of bead appearance. Charpy V-notch Energy more than 100 J at -60 °C has, then been achieved based on above studies. * Partly presented to the 109th ISIJ Meeting Research Article