In order to study the effect of soaking process in critical zone of Q&P ( Queching and Partition) steel on the cementite precipitation during partitioning process

the PTT ( precipitation amount-temperature-time) curves of the cementite precipitation of steel Q&P are calculated by Avrami dynamic equation. Q&P steel is annealed in single phase zone and critical zone

and quenched after annealing by thermal dilatometer

the carbon content of martensite is calculated by measuring the diffraction spot of room temperature martensite with transmission electron microscopy to predict the carbon content of high temperature austenite. Different critical annealing Q&P process conditions are simulated by continuous annealing simulate machine. The size of precipitated cementite is measured by scanning electron microscopy. Combining heat treatment process and maturity rate of cementite

the starting time of precipitation is calculated. The absolute position of the 5% cementite precipitated volume fraction PTT curve is obtained. At the soaking condition of 846 °C

the nose temperature is 300 °C. At the condition of soaking at 811°Cthe nose temperature rises to 325 °C

and at the condition of soaking at 764 °C

the nose temperature rises to 400 °C. The precipitation of cementite can be avoided when the partition time is ≤450 s at the condition of soaking at 746 ~ 846°C