Papers by Keyword: TRIP-Aided Steel

Abstract: To attain the aim of weight reduction and safety improvement of vehicles, some high strength steel sheets have been developed and investigated. TRIP-aided steel sheets with transformation-induced plasticity (TRIP) of the retained austenite have high strength and ductility, and excellent hydrogen embrittlement resistance. In previous study, as high strength TRIP-aided steel for forging parts, the volume fraction of retained austenite in the TRIP-aided steel could be increased by hot forging with austempering. Similarly, our research group reported that the thermomechanical process of hot rolling following by austempering could also increase the amount of retained austenite in the TRIP-aided steel sheet. The tensile properties and formabilities of TRIP-aided steel sheet subjected to the thermomechanical rolling just before austempering possess obvious advantages compared with those of TRIP-aided steel sheet without thermomechanical rolling process (with only austempering). These excellent mechanical properties may be caused by the finely dispersed retained austenite and refined bainitic ferrite and/or martensite brock by thermomechanical rolling process.

Abstract: Low alloy TRIP steel is expected to be applied to automobile bodies because of its high strength, high ductility, and excellent impact properties and press formability. It has been reported that the low alloy TRIP steel of hydrogen embrittlement resistance is improved by utilizing the hydrogen storage characteristics of highly stable retained austenite. Therefore, for the purpose of increasing the volume fraction of retained austenite, it was produced at various cooling rates below the martensite transformation start temperature. As a result, the volume fraction of retained austenite increased, and then the effect of hydrogen embrittlement decreased. The matrix phase and retained austenite is refined with decrees of the cooling rate. It is considered that the size and surface area of the retained austenite also affected the improvement of hydrogen embrittlement resistance.

Abstract: Multiphase steels consisting of retained austenite and martensite/bainite microstructures such as TRIP, low-temperature-bainite, and Q&P steels are attractive candidates for the new-generation of AHSS. These steels exhibit a remarkable combination of strength and toughness which is essential to meet the objective of weight reduction of engineering-components, while maintaining the compromise of tough-safety requirements. Such good mechanical properties are due to the enhanced work hardening rate caused by austenite-to-martensite transformation during deformation and the strengthening contribution of martensite/bainite. The retained austenite can thermally decompose into more thermodynamically stable phases as a consequence of temperature changes, which is referred to as the thermal stability of retained austenite. TRIP-aided steel is an effective candidate for automotive parts because of safety and weight reduction requirements. The strength–ductility balance of high strength steel sheets can be remarkably improved by using transformation induced plasticity behavior of retained austenite. In manufacturing hot rolled TRIP-aided sheet steels, austenite transforms into bainite during the coiling process. Because black hot coils cool slowly after the coiling process, they are exposed at about 350–450°C for a few hours or days. Therefore, the metastable residual austenite can be decomposed into other phases. This decomposition of residual austenite can produce serious deteriorate of mechanical properties in hot rolled TRIP-aided sheet steels. The present work identified the decomposition behavior and study the thermal stability of retained austenite in the TRIP-aided steel with bainitic/ferrite matrix depending on coiling temperatures and holding times by means of DSC and XRD analysis.

Abstract: Various high strength steel sheets for weight reduction and safety improvement of vehicles have been developed. TRIP-aided steel with transformation induced plasticity of the retained austenite has high strength and ductility. Conventional TRIP-aided steels are subjected to austempering process after austenitizing. Generally, elongation and formability of TRIP-aided steel are improved by finely dispersed retained austenite in BCC phase matrix. The finely dispersed retained austenite and grain refinement of TRIP-aided steel can be achieved by hot rolling with heat treatment. Therefore, the improvement of mechanical properties of TRIP-aided steel is expected from the manufacturing process with hot rolling and then isothermal transformation process. In this study, thermomechanical heat treatment is performed by combining hot rolling and isothermal holding as the manufacturing process of TRIP-aided steel sheets. The complex phase matrix is obtained by hot rolling and then isothermal holding. Although the hardness of the hot rolled and isothermal held TRIP-aided steel is decreased, the volume fraction of retained austenite is increased.

Abstract: With the aim of increasing the volume fraction and stability of the retained austenite characteristics in a transformation-induced plasticity (TRIP)-aided steel with wider lath-martensite structure matrix, the effects of varying the post-hot-working cooling rate of a 0.2%C-1.5%Si-1.5%Mn-1.0%Cr-0.05%Nb (mass%) steel on the retained austenite characteristics were investigated. When, after hot-working at 950°C, the steel was cooled to room temperature from 430°C above the martensite-start temperature using cooling rates lower than 3°C/s, the steel attained a higher volume fraction of metastable retained austenite and lower volume fractions of a finely dispersed martensite-austenite complex phase, carbide, and pro-eutectoid ferrite, although the volume fraction of bainitic ferrite increased. This was associated with a marked carbon-enrichment in the untransformed austenite, which was mainly due to the promoted bainitic ferrite, the initial lath martensite, and the refined prior austenitic grain.

Abstract: The effects of variations in the rate of post-austenitization cooling of a 0.2%C-1.5%Si-1.5%Mn-1.0%Cr-0.2%Mo-1.5%Ni-0.05%Nb (mass%) transformation-induced plasticity (TRIP)-aided steel with a lath martensite structure matrix on the Charpy impact toughness were investigated, with the aim of improving the material properties for automotive body applications. When cooled at 1.2°C/s after austenitization, the TRIP-aided steel showed a higher upper-shelf Charpy impact absorbed value (90 J/cm²) and a lower ductile-brittle fracture appearance transition temperature (−126°C), compared with the values determined (82 J/cm², −98°C) for steel cooled at 53.5°C/s. The lower cooling rate yielded a higher volume fraction and carbon concentration of metastable retained austenite, finer martensite-austenite constituents, and a lower carbide fraction in the wide lath martensite structure in the TRIP-aided steel. These improved microstructural characteristics resulted in superior impact toughness.

Abstract: The effects of microalloying on the fracture toughness of 0.2%C1.5%Si1.5%Mn 0.05%Nb (mass%) transformation-induced plasticity-aided steel with a lath-martensite structure matrix were investigated. When 0.002% B or 1.0% Cr was added to the base steel, the steel achieved a fracture toughness that was as high as that of 18%Ni maraging steel. Based on our results, the high fracture toughness was essentially caused by (i) a matrix with a softened lath-martensite structure, low carbide content and low carbon concentration; (ii) the effective plastic relaxation of localized stress concentration by the strain-induced transformation of metastable retained austenite of about 3 vol% in the martensite-austenite constituent or phase.

Abstract: 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.

Abstract: Fatigue properties of TRIP-aided annealed martensitic steels with chemical composition of 0.2%C, 1.5%Si, 1.5%Mn, 0-1.0%Cr, 0-0.2%Mo, 0-0.05%Nb, 0-18ppmB was examined for application of automotive diesel engine common rail. The steels achieved extremely higher notch fatigue limits and lower notch-sensitivity than the conventional structural steels, especially in steel with boron or without chromium and molybdenum. This was associated with (i) the TRIP effect of a large amount of stable retained austenite and (ii) strain-induced transformed hard martensite which suppressed the crack initiation and growth due to plastic relaxation.

Abstract: 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/cm2 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.