Abstract
A new two‐step air cooling (TSAC) process following finish rolling is developed for producing a multiphase (ferrite–bainite–martensite [FBM]) microstructure in a medium carbon V‐microalloyed steel without any post‐forming heat treatment. In TSAC, step 1, designated as natural air cooling (NAC − 4.2 and 5.1 °C s−1); step 2, representing two different cooling methods, namely, forced air cooling (FAC − 7 and 7.71 °C s−1) and controlled air cooling (CAC − 1.63 and 1.75 °C s−1), are adopted from two critical transformation temperatures. Both cooling methods result in multiphase (FBM) microstructures which consist of polygonal ferrite (PF), bainite (B), martensite (M), retained austenite (RA), and vanadium carbonitride (V (C, N)) precipitates. In both steels, the fraction of RA lies in the range of 8–11%. The FAC steel exhibits a tensile strength (TS) in the range of 1652–1671 MPa and ≈4–7% of total elongation, whereas the CAC steel displays a maximum TS of 1100 MPa and 17% elongation at fracture. The FAC steel exhibits a higher work‐hardening rate compared to a CAC steel. A transmission electron microscopy (TEM) analysis of tested samples reveals that stable RA plays a crucial role in improving the work‐hardening capability of these multiphase steels.
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