Understanding the interplay between microstructure,texture and mechanical properties in an advanced quenched and partitioned (Q&P) steel

Abstract

The vehicle light weighting and crash safety have intensified research into advanced high-strength steels, particularly quenching and partitioning (Q&P) grades with martensite (M), retained austenite (RA) and bainite (B) constituents. The influence of Nb microalloying on microstructural evolution, texture, and mechanical performance in high-carbon Q&P steels processed via industrially relevant routes remains unclear. In this study, one high-carbon Nb-microalloyed steel underwent two distinct processing conditions: Sample S1 was initially hot-rolled and then air-cooled to room temperature (RT), whereas Sample S2 underwent a one-step Q&P-treatment at 200 °C for 30 min. Microstructure, texture, and mechanical properties were systematically characterised using optical microscopy, SEM, TEM, electron back-scattered diffraction (EBSD), XRD, and mechanical testing. S1, because of air-cooling to RT, comprised M, pearlite, and RA, while the Q&P produced a multiphase microstructure in S2 containing tempered and fresh martensite, B and RA. S1 exhibited a higher ultimate tensile strength (∼2000 MPa) but lower ductility (12%), whereas S2 attained an enhanced elongation (∼19%) at a reduced strength (1888 MPa). XRD analysis revealed a higher RA volume in S2 (14.1%) compared to S1 (7.6%). Analysis of texture shows a higher intensity of the rotated cube (RC, {001}<110>) component in S1 (15.8%) than in S2 (7.5%), whereas a higher transformed brass component (TrB, {111}<112>) in S2 (5.9%) than in S1 (0.1%). The higher strength of S1 can be attributed to the combined effects of refined lath M morphology, higher dislocation density and a stronger RC component, whereas a higher RA fraction and stronger TrB component contribute to the higher elongation in S2. The tensile toughness increases from 230 to 340 MJ/m³ after the Q&P treatment.