A HIGH STRENGTH STEEL HAVING EXCELLENT LOW TEMPERATURE TENACITY AND EXCELLENT TENACITY IN THE PORTION AFFECTED BY WELDING-HEAT AND A METHOD FOR MANUFACTURING THE HIGH STRENGTH STEEL AND A HIGH STRENGTH STEEL PIPE

摘要

The present invention provides an ultra-high-strength steel pipe excellent in weldability on site and a method for producing the steel pipe by improving the reliability of the low temperature toughness of a steel to which elements to enhance hardenability are added for furthering high-strengthening and also improving toughness at a weld heat affected zone subjected to double or more layer welding and, in the method, the steel is made to consist of a structure composed of bainite and/or martensite by containing prescribed amounts of C, Si, Mn, P, S, Ni, Mo, Nb, Ti, Al and N, and, as occasion demands, one or more of B, V, Cu, Cr, Ca, REM and Mg, and regulating C, Si, Mn, Cr, Ni, Cu, V and Mo, those being elements to enhance hardenability, by a specific relational expression. The diameter of prior austenite grains may be regulated in a prescribed range. The method includes the steps of heating a casting to a temperature not lower than the Ac3 point, hot rolling it, and thereafter cooling the resulting hot-rolled steel plate at a prescribed cooling rate. IMAGE