Tailoring the Processing Route to Optimize the Strength–Toughness Combination of Pearlitic Steel

The present study fine tunes the processing route of a eutectoid steel to shape an optimum strength–toughness combination through appropriate microstructural design. A fully lamellar coarse pearlite microstructure leads to poor strength and toughness. Hot deformation prior to the isothermal treatment breaks down the lamellar pearlite to a spheroidized structure. Moreover, reducing the hot deformation temperature not only refines the pearlite nodule size but also increases the spheroidized pearlite fraction, which thereby improves the toughness of the steel. However, no proportionate increase in yield strength was obtained due to insignificant change in the interlamellar spacing. Remarkable refinement in lamellar spacing and increase in the spheroidization amount was ensured when the hot deformation was carried out just below the eutectoid temperature, owing to the strain-induced pearlite transformation. The presence of a mixed microstructure of fine lamellar pearlite along with spheroidized pearlite constituents simultaneously improves both the yield strength and toughness of the steel. Optimum strength–toughness combination was attained when the hot deformation strain (just below the eutectoid temperature) was increased up to 45 pct. Subsequent increase in strain creates deformation bands and traces of strain-induced bainite in the microstructure, which again deteriorates the tensile elongation of the steel.