METHOD FOR MANUFACTURING THICK STEEL PLATE HAVING HIGH TENSILE STRENGTH OF 780 MPA GRADE AND SUPERIOR WELDABILITY AND LOW-TEMPERATURE TOUGHNESS

摘要

PROBLEM TO BE SOLVED: To provide a method for manufacturing a thick steel plate having a high tensile strength of 780 MPa grade and superior weldability and low-temperature toughness while adding no Ni to the plate and skipping tempering heat treatment in the manufacturing process.;SOLUTION: A slab includes, by mass%, 0.03-0.055% C, 0.6-1.5% Cr, 0.02-0.05% Nb, 0.005-0.015% Ti, 0.0005-0.0020% B and 0.0015-0.0060% N; has a Pcm value of 0.19 to 0.22%, which is a weld cracking parameter; and has a DI value of 1.70 to 2.50, which is a hardenability index. The method for manufacturing the thick steel plate comprises the steps of: heating the slab to 1,000 to 1,170°C; rough-rolling it at a cumulative reduction rate of 40 to 80% in a temperature range of 930°C or higher; finish-rolling it at 760°C or higher at a cumulative reduction rate of 50 to 70% in a range of 760 to 900°C; subsequently starting accelerated cooling from a temperature higher than 700°C at a cooling velocity of 8 to 80°C/sec; and stopping the accelerated cooling in a range between room temperature and 350°C.;COPYRIGHT: (C)2008,JPO&INPIT