Thermomechanical processing of advanced high strength steels

Grain boundary segregation engineering and austenite reversion turn embrittlement into toughness: Example of a 9 wt.% medium Mn steel

A review of friction stir welding of steels: tool, material flow, microstructure, and properties

Third generation of AHSS with increased fraction of retained austenite for the automotive industry

In the present study, we fundamentally explore the reasons underlying differences in mechanical properties in hot-rolled 0.2C-1.6Al-6.1Mn-Fe TRIP steels subjected to different heat treatments. Comparing with austenite reverted transformation annealing (ART) process, quenching and tempering (QT [ART (UTS: 885–945MPa, TEL: 13–28%)]. In the ART process, long time annealing led to excessive C and Mn enrichment in austenite, which rendered austenite too stable and deteriorated TRIP effect. Furthermore, long time annealing reduced dislocation density and led to low work-hardening rate. The Q&T process enabled appropriate enrichment of elements and hence desired stability for significant TRIP effect to be observed. Thus, the steel quenched from 625 °C exhibited best combination of mechanical properties (UTS: 1038 MPa, TEL: 42%, UTS×TEL: 43.6 GPa%) because of significant contribution of TRIP effect and high dislocation density in austenite.

Microstructure-mechanical property relationship and austenite stability in medium-Mn TRIP steels: The effect of austenite-reverted transformation and quenching-tempering treatments

The work deals with dilatometric studies of a new-developed advanced high-strength bainitic 3Mn–1.5Al steel. Ferritic, bainitic and martensitic phase transformations are investigated in detail in respect of their temperature range forming and microstructures produced under various conditions of both continuous and isothermal cooling. The equilibrium temperatures of A
 e1 and A
 e3 and phase composition of the investigated steel were initially calculated whereas critical temperatures of A
 c1 and A
 c3 as well as the decomposition of retained austenite were determined upon heating. The major tests consisted of controlled cooling of undeformed or plastically deformed austenite using the dilatometer within the cooling rate range of 2–0.5 °C s−1. The effects of the cooling rate and deformation at temperatures of 900 and 1,050 °C on the phase transformation behaviour and microstructure were explained. The final experiment was carried out using a thermo-mechanical simulator under conditions of multi-step deformation and isothermal holding of the steel at 400 °C. Microstructural features were revealed using light microscopy and scanning electron microscopy techniques.

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Dilatometric study of phase transformations in advanced high-strength bainitic steel

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

The work addresses the phase equilibrium analysis and austenite decomposition of two Nb-microalloyed medium-Mn steels containing 3% and 5% Mn. The pseudobinary Fe-C diagrams of the steels were calculated using Thermo-Calc. Thermodynamic calculations of the volume fraction evolution of microstructural constituents vs. temperature were carried out. The study comprised the determination of the time-temperature-transformation (TTT) diagrams and continuous cooling transformation (CCT) diagrams of the investigated steels. The diagrams were used to determine continuous and isothermal cooling paths suitable for production of bainite-based steels. It was found that the various Mn content strongly influences the hardenability of the steels and hence the austenite decomposition during cooling. The knowledge of CCT diagrams and the analysis of experimental dilatometric curves enabled to produce bainite-austenite mixtures in the thermomechanical simulator. Light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to assess the effect of heat treatment on morphological details of produced multiphase microstructures.

https://www.mdpi.com/2075-4701/6/10/248/pdf

Phase Equilibrium and Austenite Decomposition in Advanced High-Strength Medium-Mn Bainitic Steels

Purpose: The aim of the research presented in the paper is to design the cooling routes after plastic deformation ensuring the multiphase structure with a high fraction of retained austenite on the basis of DCCT diagram for a new-developed Si-Al microalloyed TRIP steel. Design/methodology/approach: The CCT and DCCT diagrams were developed. Eight variants of the thermomechanical processing were designed on the basis of the DCCT diagram. The thermomechanical processing consisted of a multi-stage compression and various cooling strategies in the γ→α transformation range and isothermal holding temperature at a bainitic transformation region. Findings: The usefulness of DCCT diagram for designing the thermomechanical processing conditions for TRIP steels was demonstrated. The fraction of ferrite and retained austenite are highly dependent on a cooling path applied in the γ→α transformation region and a fraction of retained austenite especially on the isothermal holding temperature. The highest fraction of retained austenite as irregular grains located in a ferritic matrix and fine islands or interlath regions in bainitic regions were obtained at temperature of 400 and 450°C. Research limitations/implications: To determine precisely a fraction of retained austenite, the X-ray investigations has to be applied additionally to the image analysis. Practical implications: The designed cooling conditions are of great importance for the thermomechanical strategy for manufacturing the advanced high strength TRIP steels. Originality/value: The thermomechanical processing was carried out for a new group of TRIP steels with Si partially replaced by Al and containing microadditions of Nb and Ti.

W. Zalecki

Papers

Third generation of AHSS with increased fraction of retained austenite for the automotive industry

Dilatometric study of phase transformations in advanced high-strength bainitic steel

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

Phase Equilibrium and Austenite Decomposition in Advanced High-Strength Medium-Mn Bainitic Steels

Designing of cooling conditions for Si-Al microalloyed TRIP steel on the basis of DCCT diagrams