Study On The Influence Of Precipitated Relative Effective Grain Size In Ti - Containing Martensitic Steel

Low carbon martensitic steel has the lath martensite microstructure. It was widely used after heat treatment with high strength, good plasticity and toughness, and low notch sensitivity, heat sensitivity and excellent workability, good weldability and small deformation after heat treatment. Nowadays with the improved requirement for the application of steel, the combination of strength and toughness was paid more attention for the low carbon martensitic steel. Grain refinement is the effective way to improve both strength and toughness for the steel. The microalloying is an important means for refining grain size and the precipitation behavior of microalloyed elements plays a key role in the process of refining grains. Due to the complex multi-level microstructure in low carbon martensitic steel, it is important to control the effective grain size related to the strength and toughness, including the original austenite grain size, packet size and block size. But it is still unclear how the precipitated phase influences the microstructure controlling units respectively in the low carbon martensitic steel. The direct quenching+tempering+reheat quenching heat treatment process was applied for the steels with different Ti and Mo content in this paper. The metallographic microscope, scanning electron microscopy (SEM), electron back scattering diffraction (EBSD) and extraction phase analysis method was used to study the precipitation behavior of the precipitated phase in low carbon martensitic steel, which can reveal the mechanism of the influence of the precipitated phase on the effective grain size.After extraction phase analysis method to determined the precipitated phase quantitatively, it was found that after direct quenching+tempering+reheat quenching heat treatment process, the amount of Ti precipitation was linear relationship with the Ti content for the steel with Ti added only, and the Ti was completely precipitated in the steel. It had little influence on the amount of TiC precipitation for changing the tempering temperature. The (Ti, Mo) C was precipitated and the precipitation phase increased in the steel with Ti and Mo added and the ratio Ti/Mo decreased rapidly with the Mo element added. With increasing the tempering temperature, precipitation phase began to grow up in steel. When tempered at700℃, the ratio Ti/Mo was approximately1and the original austenite grain size was about6μm and the packet was5.92μm and the block was2.94μm and the grain boundary strengthening effect is obvious.By studying the relationship between the particle size of the precipition phase and effective grain size, it was found the original austenite grain size and the martensite lath size was associated with1-10nm precipitated phase, the martensite packet size was associated10-20nm precipitated phase.