Role of Microalloying Elements on Hardenability and Grain Refinement of Wrought Boron-Containing Steel

In response to increasing demands for cost-effective and energy-efficient manufacturing, there is growing interest in developing lean steel compositions that maintain sufficient hardenability and grain refinement while reducing reliance on expensive alloying elements and multi-step heat treatments. Niobium (Nb), titanium (Ti), and boron (B) are key microalloying elements that play critical roles in refining microstructure and enhancing the hardenability of low-alloy steels. Trace additions of boron (10–30 ppm) significantly increase hardenability by segregating to austenite grain boundaries and suppressing ferrite nucleation, enabling martensite transformation at lower cooling rates. Niobium contributes to grain refinement by retarding austenite recrystallization and promoting Nb(C,N) precipitation, which acts as a pinning agent. Boron nitrides (BN), which compromise boron’s effectiveness, is typically suppressed by titanium due to its higher affinity for nitrogen. Titanium is added in excess of the stoichiometric requirement to ensure full nitrogen fixation. However, excess Ti and N promote TiN coarsening during hot rolling and increase brittleness due to non-metallic inclusions. Understanding the interaction of these microalloying elements with thermal history is essential for tailoring steel properties. This study investigates vacuum induction melted (VIM) 15B35 steel with varying carbon, niobium, and boron contents to evaluate microstructure evolution and hardenability under industrially relevant thermomechanical processing, including austenitization at 1200 °C and final rolling at 1100 °C. Jominy end-quench testing assessed hardenability. EBSD characterized prior austenite grain structure, while confocal laser scanning microscopy identified low-melting boron-rich phases. CALPHAD-based thermodynamic modeling predicted phase stability. Deformation dilatometer compression tests analyzed flow stress. Mechanical properties (toughness, hardness) were also evaluated. Dilatometric continuous cooling transformation (CCT) diagrams were constructed to study phase transformation behavior. Results demonstrate that with optimized B (10 ppm), Nb (150 ppm), and lower-end carbon content typical of 15B35 steel, exceptional hardenability and grain refinement can be achieved without requiring additional heat treatments.