During the hot processing of DP steel, the cooling process is reasonably formulated to make the austenitized steel stay in the ferrite phase transformation zone for a long time, ensuring that 80% of the austenite phase transforms into iron. The prime phase is then quickly cooled to bypass the bainite formation zone and cooled below the Ms point, thereby forming a structure with a ferrite martensite phase.
Let’s take the more typical DP600 as a representative to discuss its production process.
The current production process of hot-rolled DP steel strips in the steel industry is divided into two types: low-temperature coiling and medium-temperature coiling. One is the low-temperature coiling process. The production principle of low-temperature coiled hot-rolled dual-phase steel is: most of the A-F phase transformation (about 80) is completed during the cooling process after final rolling, and in the subsequent rapid cooling stage Quickly cool the steel strip to below the M8 point and coil it to obtain the F plus M structure. The coiling temperature is generally lower than 300°C to avoid the formation of bainite, as well as the aging of ferrite and self-tempering of martensite (as shown in the figure). Its characteristic is that it reduces the content of alloy elements and reduces costs, but it requires strict control of process parameters, especially the cooling process after final rolling, which is difficult to control and requires the use of higher-power coiling equipment. At present, only Japan's Nippon Steel and Kawasaki Steel and other plants use this process, and few domestic companies use this process.
The second is the medium temperature coiling process. The principle of the medium temperature coiling type is to appropriately add Cr, Mo and other alloying elements to improve the stability of austenite within the conventional coiling temperature range. During its continuous cooling process , there is an austenite metastable phase-transformation-free interval in the temperature range between A_F and A-B transformation. The steel plate after final rolling first completes most of the A_F phase change (about 80) during the continuous cooling process, and is coiled without phase change within the "coiling window" temperature. After coiling, it is air-cooled to room temperature. The medium residual austenite transforms into martensite, and finally the F plus M two-phase structure is obtained. The coiling temperature of this rolling method is 500-600℃.