The performance of hydrogen embrittlement, fatigue properties and impact toughness of ultra high strength transformation induced plasticity aided steels with bainitic ferrite matrix (TBF steels) has been discussed. Some characteristics and deformation transformation mechanism of the retained austenite has also been discussed. It has been observed that mechanical stability and volume fraction of the interlath retained austenite phase in the TBF steels play an important role in increasing delayed fracture strength, fatigue limit and impact toughness.

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Fracture strength and toughness of ultra high strength TRIP aided steels

Effect of intercritical annealing on retained austenite characterization in textured TRIP-assisted steel sheet

Dual phase (DP) steels offer a combination of tensile properties such as low yield strength and a high tensile strength, which make them unique among advanced high strength steels (AHSS). They also exhibit high work hardening rates in the early stage of plastic deformation as well as good bake hardening (BH) potential. The utilization of thermo-mechanical-controlled processing (TMCP) by hot rolling is an industrial method for manufacturing DP steels. In this technique, the control of the rolling parameters and chemical compositions has a significant role on the final microstructure and mechanical behavior. This entry provides a detailed study of replication of hot rolling mill by TMCP aimed at further development of new categories of DP steels in terms of mechanical properties and bake hardenability via affordable addition of alloying elements together with optimization of the processing conditions. The influence of hot rolling parameters (deformation temperatures and amounts of strain), cooling conditions during γ!α transformation, amount of martensite as well as variation of alloying elements (C, Si, Mo, and Nb) on the microstructure development, mechanical properties, and BH behavior of DP steels is investigated. Additionally, this entry offers a detailed study of the local aging and BH behavior of DP steel. For this purpose, two methods were investigated to achieve local strengthening. These methods are local deformation by embossing and local heat treatment by laser. Furthermore, the influence of load paths and pre-straining on the BH behavior of the DP steel was studied. DP steels were pre-strained under uniaxial, biaxial, and plane strain conditions with different degrees of pre-strain.

Encyclopedia of Iron, Steel, and Their Alloys

Two Mo-Nb microalloyed transformation-induced-plasticity (TRIP) steels, with Al contents of 0.23 and 0.65, were subjected to several hot-rolling conditions designed to generate different ferrite morphologies and grain sizes. These structures were then cold rolled and TRIP annealed under different heat-treatment conditions. To further develop TRIP steel in terms of strength and ductility, stabilizing retained austenite by isothermal bainitic transformation was studied in detail. Microstructure observation and tensile tests were conducted, and volume fractions of retained austenite were measured. It was observed that increasing the aluminum content enhances the transformation rate and increases the total amount of bainite fraction at the expense of retained austenite. The latter effect enhances formability by increasing ductility. Furthermore, it was observed that the hot-rolling schedule, prior to cold rolling and heat treatment, has a decisive effect on structure refinement, which enhances the strength-ductility balance of the final product. To study the transformation behavior, dilatometer testing was conducted under conditions similar to that of the heat treatment. Thermodynamic calculations were used to verify the results.

On Factors Affecting the Phase Transformation and Mechanical Properties of Cold-Rolled Transformation-Induced-Plasticity–Aided Steel

The precipitation state, microstructure, retained austenite stability, and mechanical properties of cold-rolled Ti-microalloyed CMnAlSiP TRIP steel were investigated. The precipitation state was analyzed in each processing step by means of transmission electron microscopy, chemical analysis using the anodic dissolution method, and by model calculations. Ti additions refined the microstructure mainly by the pinning of austenite and ferrite grain boundaries by various Ti-containing precipitates. Ti additions also resulted in a refinement of the size of the retained austenite islands and caused a slight decrease of the volume fraction and carbon content of the retained austenite. The morphology of the retained austenite was also changed and the stability of the retained austenite decreased, but Ti addition still resulted in an adequate strength-ductility balance and a tensile strength close to 1 GPa.

Mechanical Properties of TRIP Steel Microalloyed with Ti

In this paper, a Mo-Nb microalloyed TRIP Steel was subjected to several heat treatments designed to generate different microstructures. These microstructures were then cold rolled and TRIP-annealed, and the resulting tensile properties and retained austenite characteristics were determined. The results reveal that prior heat treatment has a significant effect on the cold rolled and annealed behavior. Generally, an increasing volume fraction of proeutectoid ferrite prior to cold rolling and TRIP annealing leads to improved tensile ductility. This is due to increased work-hardening rates, which, in turn, correlate to increased carbon enrichment in the retained austenite. These results can clearly be used to optimize the hot rolling process to produce hot strip for the subsequent production of cold rolled Mo-Nb TRIP Steel.

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R. L. Donaberger

Papers

The Effect of Processing History on a Cold Rolled and Annealed Mo-Nb Microalloyed TRIP Steel