Microstructural Development During Welding of Transformation Induced Plasticity (TRIP) Steels

摘要

The Advanced High Strength Steels (AHSS) are promising solutions for the production of lighter automobiles which help to reduce fuel consumption and increase passenger safety by improving crash worthiness. Transformation Induced Plasticity Steel (TRIP) is one class of AHSS which offers a high strength and toughness combination with excellent uniform elongation. However, the higher alloying content of these steel limits their weldability and the thermal cycle of a welding process destroy the carefully designed microstructure which results in inferior mechanical properties of the weld. The objective of this work is to improve our understanding of the response of the material to welding by (i) studying the microstructural evolution during weld thermal cycles employed in automotive manufacturing, (ii) applying specific heat treatments and (iii) examining the influence of thermal cycles on the final properties of the welded material. The experimental investigations show that the presence of strong oxidizing elements such as aluminium and silicon lead to the formation of non-metallic inclusions in the fusion zone. The partitioning of aluminium to the solidifying fusion and grain boundaries in the fusion zone lead to the stabilisation of ferrite in aluminium containing TRIP steels. In-situ synchrotron X-ray diffraction analysis are used to study the phase transformation kinetics and lattice parameter behaviour during heating and cooling thermal cycles representative of typical welding conditions. Results of these experiments indicate ambient temperature transformation of austenite after cooling.