Optimization of deformation and relaxation for enhancing precipitation strengthening in Ti–Mo microalloyed steel

This study systematically examined the effects of deformation and subsequent relaxation time in the non-recrystallization region at 900 °C on precipitation behavior and mechanical properties of the Ti-Mo steel. The primary objective was to optimize precipitation during controlled rolling and ferrite isothermal treatment to maximize strengthening effects and improve mechanical performance. Results showed that increasing strain refined the phase-transformation microstructure. However, strains exceeding 40% resulted in a significantly non-uniform ferrite grain size distribution. With respect to precipitation, strains below 20% exerted little influence on the morphology and number density of precipitates. With higher strain, intra-ferrite precipitates decreased, while particle growth and coarsening along dislocations intensified, weakening precipitation strengthening. Extending relaxation after 20% deformation barely influenced ferrite grain size but significantly increased the number and size of strain-induced precipitates. Consequently, the formation of nanoprecipitates during the subsequent ferrite isothermal transformation was suppressed, leading to coarser precipitates, weaker strengthening, and lower microhardness. To achieve the maximum strengthening effect in Ti-Mo steel, it was critical to control the strain level in the non-recrystallization region and increase the cooling rate to inhibit strain-induced precipitation. This practice ensured a uniform and fine dispersion of key elements within the ferrite matrix. By combining the beneficial effects of grain-refinement strengthening and precipitation strengthening, the optimized processing route for the Ti-Mo steel was determined as: 20% deformation at 900 °C, followed by rapid cooling to 625°C and isothermal holding for 5400 s, yielding a maximum microhardness of 322 ± 8.2 HV.