Title: Microstructure conditioning in an advanced boron-containing complex phase steel heat treated within the Q&P process concept

Abstract

Emerging ferrous alloys are part of a recent group of newer and promising high-performance engineering materials. The development of novel alloy design and physical structure reflects the interest in improving the flexibility of material treatment in its production stages due phases’ constitution and transformation over microstructure evolution. In this way, the development and implementation of advanced high-strength steels (AHSS) represent being the best solution to meet high levels of safety and environmental care, demanded to the automotive industry. The objective of this research work is to evaluate the effect of quenching temperature above and below the martensitic start line transformation (Ms) on the microstructure evolution of a complex phase steel, belonging to the 3rd generation AHSS, micro-alloyed with boron when it is heat treated based on the Q&P process concept of one and two steps. For this purpose, an experimental single micro-alloyed with boron (60 ppm) complex phase steel was fabricated and hot and cold rolled. After that, the steel was homogenized and quenched in a salt bath at 420 and 380°C, respectively. Then, the steel was tempered following one step (isothermally condition) and two-steps (reheated at 600°C) process, respectively, for carrying out the partition process. Finally, structural, microstructural and mechanical characterization was carried out with the aim to determine steel microstructure conditioning. In general terms, outstanding results have been obtained regarding to the austenitic grain size refinement after the Q&P heat treatment. Microconstituents such as retained austenite, martensite, bainite, ferrite and some perlite were obtained, homogeneously distributed in the two-steps Q&P steel microalloyed with boron and quenched at 420°C. Furthermore, high values of mechanical resistance superior to 1300 MPa were obtained. All this when compared with the obtained results in the remaining studied conditions for the non-microalloyed and boron containing complex phase steel.