Precipitation of microalloyed carbides in medium-manganese low-carbon steels subjected to high-temperature Q&P process

• Main Authors: Meng-Hsuan Chiang, 江孟軒
• Other Authors: Hung-Wei Yen
• Format: Others
• Language: en_US
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/2y2gvb

碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 105 === In conventional quenching and partitioning (Q&P) processing, carbon partitions into retained austenite and stabilizes it during partitioning heat treatment at 300-500°C. The current research aimed at developing quenching and high temperature partitioning (Q&HTP) processing, in which manganese and carbon both partitioned into and stabilized retained austenite during high temperature partitioning heat treatment. To achieve this goal, new heat treatment processing and newly-designed alloys were developed. High temperature partitioning accompanied with austenite reversion was carried out at intercritical annealing temperature, producing fine austenite phase uniformly distributed in tempered martensite matrix. However, during intercritical annealing, martensite lath coalescence and dislocation annihilation would reduce the strength of Q&HTP steels. To compensate the softening effect, niobium and vanadium carbides were introduced into newly-designed alloy to enhance strength by grain refinement and precipitation strengthening. In present work, the increments of yield strength due to precipitation strengthening were about 100-200 MPa. Also, the effect of aluminum containing was investigated in this research. The optimum temperature of Al-free and Al-containing Q&HTP steels was 690 °C and 710 °C for 5 min heat treatment. However, the mechanical properties and microstructures of two steels were strongly related to the annealing temperature. The effect of precipitation strengthening in Al-containing steels were about 30-40 MPa higher than in Al-free steels.