TRIP-TYPE TWO-PHASE MARTENSITIC STEEL AND ULTRAHIGH-STRENGTH-STEEL PROCESSED ARTICLE OBTAINED THEREFROM

摘要

Provided is a TRIP-type two-phase martensitic steel which is excellent in terms of strength/elongation balance and Charpy impact value and in which the matrix has been made to be two-phase martensite composed of a soft lath martensitic structure and a hard lath martensitic structure not by regulating forging temperature, forging reduction ratio, or the like but by controlling heat treatment conditions. The two-phase martensitic steel is characterized by containing 0.1-0.7% C, 0.5-2.5% Si, 0.5-3.0% Mn, 0.5-2.0% Cr, up to 0.5% Mo (including 0%), and 0.04-2.5% Al, with the remainder comprising Fe and incidental impurities, and by having a metallographic structure in which the matrix is composed of a soft lath martensitic structure and a hard lath martensitic structure. The two-phase martensitic steel is further characterized by being obtained by heating a raw steel material to the γ region, subsequently rapidly cooling the heated material to a temperature slightly higher than the martensite transformation initiation temperature (Ms point), and then subjecting the cooled material to an isothermal transformation treatment in the temperature range from the Mf point to [(Mf point)-100ºC].