Effects of Intercritical Annealing on Microstructure and Mechanical Properties of Low C-Mn Steels

The present study deals with intercritical annealing (IA) and water quenching (WQ) to develop a dual-phase microstructure in two low-carbon Mn-containing steels. The microstructure of steel 1 consists of polygonal ferrite (≈ 25 μm) and pearlite. In contrast, steel 2 shows slightly coarser ferrite (≈ 28 μm) and pearlite after hot rolling and air cooling. All hot-rolled samples after IA (750 °C, 30 min) and WQ exhibit equiaxed ferrite (≈16 μm) and martensite islands (≈ 4 μm). The cold-rolled (CR) samples after IA and WQ reveal elongated ferrite, fine lath martensite, and martensite/austenite (M/A) constituent. The higher hardness (298 HV) and strength (951 MPa) of the HR + CR + IA + WQ sample than the HR + IA + WQ sample (259 HV and 872 MPa, respectively) are attributed to high dislocation density and martensite island. The three-stage work hardening has been demonstrated by the Jaoult–Crussard analysis. The fractography reveals fine dimples (10–15 µm), flat facets, and few voids.