MANUFACTURE OF NON-HEAT TREATED STEEL EXCELLENT IN DWTT PROPERTY

摘要

PURPOSE:To easily manufacture a non-heat treated steel excellent in DWTT properties by successively executing heating, rolling and cooling to a slab having a specified compsn. having prescribed content of Nb and Cr under specified conditions. CONSTITUTION:A slab contg., by weight, 0.005 to 0.15% C, 0.05 to 1.0% Si, 0.6 to 2.5% Mn, 0.005 to 0.08% Al, 0.005 to 0.1% Nb and 0.5 to 1.5% Cr or furthermore contg. at least one kind selected from 0.01 to 0.10% V, =1.0% Ni, =0.5% Mo, 0.005 to 0.1% Ti, 0.0003 to 0.0020% B, 0.001 to 0.010% Ca and 0.001 to 0.010% rare earth metal and the balance iron with inevitable impurities is heated in the temp. range of 1050 to 1250 deg.C. Next, =30% draft is applied thereto in the recrystallized gamma area of (the Ar3+150 deg.C) or above, and, after that, =50% draft is furthermore applied thereto in the unrecrystallized gamma area of (the Ar3+150 deg.C) to the Ar3. Then, it is subjected to accelerated cooling to the temp. range of 650 to 400 deg.C at 3 to 30 deg.C/sec cooling rate. In this way, the non-heat treated steel suitable for a pipeline transporting natural gas, etc., can be obtd.