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Description
The transition from conventional steelmaking to fossil-free production promotes more continuous processing and reduces flexibility in chemical compositions, increasing the need to produce multiple strength grades from a single alloy. This study examines the feasibility of achieving several steel strength grades from one chemical composition by varying the cooling method. Two laboratory-melted low-carbon steels containing 0.045 C (in wt.%), 0.5 Si, 1.2 or 1.5 Mn and scrap-derived residuals of copper, nickel, and chromium were thermomechanically processed using three cooling routes: (1) air cooling (3.2 °C/s) to room temperature, (2) interrupted accelerated cooling (15 °C/s to 500 °C, then 0.027 °C/s), and (3) direct quenching (35 °C/s) to room temperature. Tensile testing, impact toughness testing, field emission scanning electron microscopy, electron backscatter diffraction, and dilatometry were used to characterize mechanical properties, microstructure, and phase transformation behavior. Direct quenching produced a microstructure consisting of 95% bainite and 5% martensite/austenite (M/A). With successful interrupted accelerated cooling, bainite and M/A fractions changed to 99.5% and 0.5%, respectively. The more complete bainitic transformation increased yield strength. A variation of 50 °C in finish cooling temperature significantly affected yield strength. An increase in Mn content had a negative effect on impact toughness. Generally, impact toughness increased when cooling rate increased due to the formation of a more complex microstructure. One composition can be used to produce 420, 460 and 500 MPa yield strength steel grades.