To unnecessitate bead cutting by executing edge preheating for heating both edge parts of an open tube to unmelting temp. region not lower than Curie point by induction heating, heating them to a melting temp. region, butt- joining them with squeeze rolls and making the area of weld bead discharged to the outside and inside of the tube not higher than a specified value.
Band steel is preheated to 400-650°C. The band steel is formed with forming rolls and made into an open tube. Edge pre-heating is executed by heating both edge parts from the edge faces to about 5 mm to the temp. region of 600-1000°C by induction heating. The edge preheating is executed under the atmosphere of oxygen content lower than that of the air or under the atmosphere whose dew point is ≤-10°C. Air cooling is executed for ≥0.5 sec. The edge parts are heated to the melting temp. region of the melting point to the melting point +50°C by high-frequency induction heating of ≤400 kHz. The edge parts are butt-joined with the squeeze rolls and the are (mm2) of the weld bead discharged to the outside and inside of the tube is defined as ≤0.1 × thickness (mm).
HASHIMOTO YUJI
ITAYA MOTOAKI
YORIFUJI AKIRA
ONISHI TOSHIO
TANAKA NOBUKI
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