Gas forming of ultra-high strength steel hollow part using air filled into sealed tube and resistance heating

•Gas forming process using air filled into sealed tubes was developed.•By applying die-quenching, a formed part had very high strength.•Dimensional accuracy of formed part was improved by increase in internal pressure.•To increase hardness, formed tube was cooled with air blowing during holding.•Oxidation of formed part was prevented by forming in a case filled with CO2 gas.

A new gas forming process of ultra-high strength steel hollow parts using air filled into sealed tubes and resistance heating was developed to omit the subsequent heat treatment. In this process, a sealed quenchable steel tube was rapidly resistance-heated to improve the formability. By applying die-quenching for holding at the bottom dead centre of a press, the formed part had very high strength, a hardness of 450 HV10 equivalents to a tensile strength of 1500 MPa. In addition, the dimensional accuracy of the formed part was improved by the increase in internal pressure for heating and compression of air filled into the sealed tube. To increase the hardness, the formed tube was cooled with air blowing during holding at the bottom dead centre and the corner of the die was optimised as to be in contact with the tube. The oxidation on the outer surface of the formed part was prevented by forming in a case filled with CO2 gas.

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