Ti \ Mo Micro-alloyed Steel Lath Martensite Precipitation, Microstructure And Mechanical Properties Of

Traditionally, the way to improve strength of martensitic steel by increase carbon content, the solid solution elements and circulating quenching technique. which makes weldability decrease and increase in cost. TMCP and micro-alloying technology can effectively improve the strength and toughness, and ensure a better economy.The lath martensitic steel with TiC precipitation hardening C-Mn was studied in this paper. The second precipitation, microstructure and mechanical property were tested by phase analysis, X-ray diffraction (XRD), metallographic microscope (OM), scanning electron microscope (SEM), spectroscopy (EDS), transmission electron microscopy (TEM), tensile and impact toughness test. The precipitation laws of Ti, Mo and their effects on the microstructure and mechanical properties were analyzed.Precipitated phase is almost TiC which are evenly distributed in Ti micro-alloyed lath martensitic steel. Under water quenching after hot rolling, only about57%of the Ti element are precipitated and have larger size precipitate phase, then by tempering and heat treatment, the Ti element are precipitated entirely, and nanoscale (1-18nm) TiC precipitate phase ratio increase significantly. What’s more, TiC was further refined, especially large-size (200-300nm) precipitation sharp reduction by Mo addition. Mo atoms partially substitute for Ti atoms in the TiC location, or attach to the TiC precipitation for obtaining composite precipitates. Compared with recrystallized area rolling, the unrecrystallized area rolling have these advantages, the amount of precipitation of Ti increase from57%to75%, the proportion of nanoscale (1-18nm) TiC precipitation was increased, while large particles precipitation unchanged. Further reducing rolling temperature, the large particles (200-300nm) TiC precipitate reduce, and the amount of precipitation of TiC and nanoscale (1-18nm) TiC precipitate phase ratio continues to increase. By air cooling after hot rolling, the mount of1-60nm sized Tic precipitate phase decreased and1-60nm sized Tic precipitate phase increased, especially200-300nm sized increased drastically.There is12μm grain refinement under tempering-rheating process while7μm after the direct quenching, fine-grain strengthening increments90Mpa, precipitation strengthening to180Mpa, and the solution strength loss of approximately70Mpa by the carbon element, sample yield strength increased by approximately46Mpa. Ti-Mo composite can achieve the biggest grain refinement effect, up to15μm, and fine grain strengthening increased by136Mpa, the precipitation hardening200Mpa, and the solid solution strength loss is about22Mpa, leading to yield strength increased by approximately40Mpa, consistent with the experimental results. Corresponding organization has also been refined to increase the percentage of large-angle, larger impact toughness value.With the rolling temperature decrease, the original austenite grain becomes flatten, the more small-angle grain boundaries, the dislocation density increases and the lath width becomes narrowed. But the effective grain size becomes larger, the yield strength and impact toughness is improved. Compared with the air-cooled after hot rolling, re-quenched martensite steel has a higher strength and toughness value under water-cooling.