Recently developed ultra high-strength low alloy transformation-induced plasticity (TRIP)-aided steel with martensitic lath structure matrix or "TRIP-aided Martensitic steel; TM steel" possesses a high impact toughness. In this study, to apply the TM steel to some hot-forging parts, the effects of hot-forging on microstructure, retained austenite characteristics, tensile properties and toughness in the TM steels with chemical composition of 0.3-0.4%C, 1.5%Si, 1.5%Mn, 0.002%B, 0.02Ti, 0.05Nb (mass%) were investigated. The hot forging brought on an excellent combinations of tensile strength of 1500-2000 MPa or 0.2% offset proof stress of 1200-1560 MPa and Charpy impact absorbed value of 35-80 J/cm2 when partitioned at 250-350°C after quenching in oil. The combinations exceeded so much those of the conventional quench and tempering structural steels. From examinations of microstructure and retained austenite characteristics, it was found that the excellent combinations are mainly caused by (i) refined and uniform martensitic lath structure matrix with a small amount of carbide, (ii) increasing narrow martensite with high dislocation density and (iii) the increased stability of retained austenite, resulting from the FQP process.

An ultra high-strength TRIP-aided steel consisting of bainitic ferrite lath structure matrix and interlath retained austenite films possesses high toughness and fatigue strength, as well as high resistance against hydrogen embrittlement. In this study, to develop the ultra high-strength TRIP-aided steel for hot-forging parts, the effects of hot-forging and subsequent austempering process on the microstructure, retained austenite characteristics, tensile properties and impact toughness of 0.2% C-1.5% Si-1.5% Mn-Cr-Mo-B TRIP-aided steels were investigated. The process achieved an excellent combination of yield strength of 700-800MPa and Charpy impact absorbed value of 110-130 J/cm(2) in these steels, as well as a good balance of yield strength and total elongation when subjected to austempering at temperatures between 325 degrees C and 375 degrees C below martensite-start temperature (Ms). These combinations exceeded so much those of the steel subjected to the conventional austempering without hot-forging. From examinations of microstructure and retained austenite characteristics, it was found that these combinations are mainly caused by (i) refined mixed structure of bainitic ferrite and retained austenite without pro-eutectoid ferrite (ii) high uniformity of the mixed structure and (iii) the increased volume fraction of metastable retained austenite.

Ultra high-strength TRIP-aided steel consisting of bainitic ferrite matrix and interlath retained austenite films (TBF steel) possesses high toughness and fatigue strength, as well as high resistance against hydrogen embrittlement. In this study, to improve further these mechanical properties, the effects of hot forging and subsequent isothermal transformation holding process (FIT process) on microstructure, retained austenite characteristics, tensile properties and toughness of the TBF steel with chemical composition of 0.4%C, 1.5%Si, 1.5%Mn, 0.5%Cr, 0.2%Mo, 0.05%Nb and 0.5%Al (mass%) were investigated. The FIT process brought on an excellent combination of tensile strength of 1350-1550 MPa and Charpy impact absorbed value of 100-110 J/cm(2) in the developed TBF steel, exceeding so much that of SCM440 steel. The excellent combination was mainly caused by (i) refined mixed structure of bainitic ferrite and retained austenite and (ii) the increased mechanical stability of retained austenite due to the FIT process.

Ultra high-strength low-alloyed TRIT-aided steels possess high impact toughness, fatigue strength and hydrogen embfittlement performance, as well as good formability. So, it is expected that the TRIP-aided steels are applied to some automotive parts such as not only center pillar but also driving parts, spring and bolts. In the present study, the effects of ausforming conditions (temperature and strain) after austenitizing on tensile properties and toughness of TRIP-aided steels with bainitic ferrite matrix were investigated to develop a new type of ultra high-strength forging steel. TRIP-aided steel with chemical composition of 0.2%C, 0.5%Si, 1.5%Mn, 1.0%Al 0.02%Nb and 0.1%Mo achieved large total elongation and high impact toughness by ausforming to 0-50% strain at 600-900 degrees C, followed by austempering at 400 degrees C for 500s. This was mainly caused by refined microstructure and stabilized retained austenite.