HIGH-YIELD RATIO-HIGH-STRENGTH STEEL SHEET HAVING AN EXCELLENT WORKABILITY, CAPABLE OF PROVIDING THE SAME WITH A TENSILE STRENGTH OF ABOVE 980 MPa AND A HIGH YIELD RATIO

摘要

PURPOSE: A high-yield ratio-high-strength steel sheet having an excellent workability is provided to provide the same with a tensile strength of above 980 MPa, a high yield ratio, and the excellent workability.;CONSTITUTION: A high-yield ratio-high-strength steel sheet having an excellent workability includes 0.06 to 0.12 mass% of C, below 0.12 mass% of Si, 2.0 to 3.5 mass% of Mn, 0.01 to 0.15 mass% of the total amount of at least one selected from a group of Ti, Nb, and V, 0.0003 to 0.005 mass% of B, below 0.05 mass% of P, below 0.05 mass% of S, 0.005 to 0.1 mass% of Al, below 0.015 mass% of N, and remnants Fe and inevitable impurities. The whole structure of metal for the steel sheet includes 5 to 15% of ferrite, 25 to 55% of martensite, 30 to 70% of the total amount of bainite and tempered martensite. The average crystal grain size of the structure is below 3.0 um. The tensile strength of the steel sheet is above 980 MPa.;COPYRIGHT KIPO 2013;[Reference numerals] (AA) Heating speed HR; (BB) Crack process (crack temperature T1, crack time t1); (CC) First cooling (first cooling speed CR1); (DD) Alloying temperature; (EE) Second cooling (second cooling speed CR2); (FF) Low temperature maintenance process (low temperature maintenance temperature T2) (low temperature maintenance time t2)