Low-yield-ratio high-strength hot-rolled steel sheet and method of manufacturing the same.

摘要

A low-yield-ratio high-strength hot-rolled steel sheet combining the advantages of precipitation-strengthened steels and structure-strengthened steels without the disadvantages of these conventional steels, and a manufacturing method characterized by hot-rolling this steel sheets. The steel sheet has a composition consisting essentially of about: 0.18 wt% or less of C; 0.5 to 2.5 wt% of Si; 0.5 to 2.5 wt% of Mn; 0.05 wt% or less of P; 0.02 wt% or less of S; 0.01 to 0.1 wt% of Al; 0.02 to 0.5 wt% of Ti and/or 0.03 to 1.0 wt% of NB, said Ti and Nb being present approximately in relation to C according to the following formula: C wt% >/= 0.05 + Ti wt%/4 + NB wt%/8; and the balance substantially Fe and incidental impurities; wherein the structure of the steel is composed of ferrite containing a precipitated carbide of Ti and/or Nb and martensite, with eventually also retained austenite.