Effect of Cooling Path on Microstructure and Mechanical Properties of Ti–Zr Microalloyed Low-Carbon Steel

Abstract This study investigates the effects of post-rolling cooling methods (quench cooling, air cooling, and furnace cooling) on the microstructure and properties of Ti–Zr microalloyed steel via laboratory rolling tests. The microstructure is transformed from the lath martensite/bainite to the ferrite-dominated microstructure by decreasing the cooling rate and increasing the average grain size to 3.55 μm in furnace-cooled steel. Concurrently, transmission electron microscopy-based analysis reveals that the precipitate volume fraction increases from 0.038% to 0.106%, with the average size decreasing from 33.5 nm to 6.8 nm, enhancing the precipitation strengthening effect from 23 MPa to 121 MPa. Quench cooling forms a multiphase microstructure (martensite/bainite + ferrite), which reduces the yield ratio, while dislocation tangling in martensite decreases plasticity. Notably, quench-cooled steel exhibits high impact energy (151 J) despite low elongation. This is attributed to synergistic toughening by high dislocation density (1014 m−2) and retained austenite. Furnace-cooled steel balances strength (yield—716.5 MPa, tensile—787.6 MPa), elongation (18.9%), and 0 °C impact energy (133 J), thereby demonstrating cooling rate-controlled synergy between strengthening–toughening mechanisms in Ti–Zr microalloyed steel.