Effect of precipitate-controlled microstructures on machinability of continuously cooled low-carbon high-strength low-alloy steel

In this study, the effects of manganese sulfide (MnS) and boron nitride (BN) modifications on microstructural evolution, mechanical properties, and machinability were evaluated in a low-carbon, boron-containing high-strength low-alloy (HSLA) steel produced by continuous cooling. Machinability was assessed through dry turning tests. Thermodynamic calculations and microstructural analyses showed that MnS and BN precipitated during solidification and remained partially undissolved at the applied annealing and austenitization temperatures. Continuous cooling resulted in predominantly bainitic microstructures with martensitic constituents. However, BN formation reduced the effective boron (B) content in solid solution, thereby limiting hardenability and promoting coarser microstructures, while MnS formation decreased the manganese (Mn) content in solid solution, leading to reduced hardness and strength. To isolate the effect of mechanical properties on machinability, cutting forces were normalized with respect to the reference steel. The results showed that MnS shortened chip length and reduced cutting forces, which varied between 144 and 368 N, whereas BN reduced tool–chip friction, resulting in cutting temperatures in the range of 159–575 °C. The combined MnS-BN modification provided the lowest cutting forces and temperatures, together with more stable tool wear behavior. These findings demonstrate that precipitation engineering through MnS and BN modifications provides an effective pathway for tailoring machinability in continuously cooled boron-containing high-strength steels, while the proposed hardness-normalized framework enables objective comparison independent of intrinsic mechanical property variations.