The effects of Nb and Mo addition on transformation and properties in low carbon bainitic steels

A low carbon steel microalloyed with Nb-V-Ti were heat treated by means of two-step quenching and partitioning (Q&P) process after partial austenitization. Combined use of SEM equipped with EBSD, XRD and TEM showed that the microstructure was composed of intercritical polygonal ferrite, tempered lath martensite, secondary twin martensite and carbon-enriched retained austenite, as well as microalloyed carbides (mainly VC) precipitated inside ferrite and martensite. When increasing partitioning time from 0 s to 3600 s, the volume fraction of retained austenite increased up to 6.5% with mean carbon content in austenite keeping a constant level. The retained austenite kept K-S orientation relationship with adjacent tempered lath martensite and neighboring secondary twin martensite in different martensite packet. As partitioning time increased up to 3600 s, the initial lath martensite tempered gradually to ferrite without any cementite formation.

The relationships of microstructure-mechanical properties in quenching and partitioning (Q&P) steel accompanied with microalloyed carbide precipitation

High Performance Ferrous Alloys

This chapter contains detailed theoretical analysis and experimental data on the relationship of structure parameters of dual-phase steels (volume fraction and hardness of martensite, ferrite grain size, structure morphology) and various mechanical properties such as yield and tensile strength, strain hardening, elongation. Various facture characteristics including resistance to crack initiation and propagation, as well as resistance to fatigue hydrogen embrittlement are summarized. The main features of intercritical annealing (enrichment of austenite by carbon and transformations in cooling affected by pre-existing austenite-ferrite interfaces) are discussed. Individual steps of the processing, including formation of austenite phase, its growth, decomposition, and final tempering of ferrite-martensite mixture, are described. This chapter summarizes various effects of steel composition on processes during the heating in the intercritical temperature range, including kinetics austenitization, recrystallization of initial structure and morphology of the formed austenite-ferrite mixture. Effects of chemical composition on tensile properties, behavior at aging, and tempering, are presented.

Dual-phase steels

This work discusses the development of the microstructure and mechanical properties of medium-carbon steel that contains silicon, aluminium and microadditions of Nb and Ti. Two cooling strategies were designed based on the thermodynamic equilibrium calculations and continuous cooling transformation diagram, which was determined for plastically deformed austenite. The cooling paths enabled the production of ferrite based and bainite based steels. The specimens were obtained via the thermomechanical rolling process with isothermal holding of steel at 450°C. Microstructure investigations were performed using light, scanning and transmission microscopy methods. The distribution and amount of retained austenite were determined using the electron backscatter diffraction technique, whereas transmission electron microscopy allowed the identification of the morphology of the γ phase. The amount of austenite and its carbon content were assessed using X-ray diffraction. Relations between microstructure and m...

Microstructure–property relationships in TRIP aided medium-C bainitic steel with lamellar retained austenite

In this study the concept of microalloying was applied to TRIP-aided bainitic–ferritic (TBF) sheet steel. A newly designed chemical composition was introduced including significantly reduced Si content. The impact of Nb on phase transformation, microstructure and mechanical properties was investigated. Nb addition was found to indirectly accelerate transformation kinetics through grain refinement resulting in pro-eutectoid ferrite and upper bainite formation. A change in morphology of the bainitic matrix from lath-like to globular was observed as a result of grain refinement. The amount of cementite was reduced and the amount of retained austenite increased through the addition of Nb. The changes in phase transformation and microstructure resulted in significantly improved mechanical properties comprising both elevated stretch formability and edge crack resistance.

K. Hausmann

Papers

The influence of Nb on transformation behavior and mechanical properties of TRIP-assisted bainitic–ferritic sheet steels