Dual-phase steel (referred to as DP steel) is a high-performance steel with low yield ratio, high strength, easy molding, and high initial work hardening index, such as martensite and other second-phase particles distributed in the ferrite matrix. Hot-dip galvanizing is a process method in which after the pretreatment of the metal (matrix) material to be plated, it is immersed in a low-melting-point zinc liquid or a zinc liquid alloy to obtain a protective coating. Hot-dip galvanized dual-phase steel technology refers to the dual-phase steel process technology in which the microstructure obtained after annealing, hot-dip galvanizing and alloying is mainly ferritic plus martensite, and the surface is covered with a galvanized layer. Specifically:

1. Annealing Continuous hot dip galvanizing of steel strips uses on-line continuous annealing. Annealing process is the key to producing high performance hot dip galvanized products. The annealing process of continuous hot dip galvanized steel sheet includes preheating, heating and homogenization core cooling process. Preheating can avoid the deformation of the steel strip due to the rapid temperature increase of the steel strip, which is very beneficial to the thin shape of the product. In the cooling process, the slow cooling and then rapid cooling are adopted to avoid the warping of the plate due to the excessive cooling speed, and to meet the requirements for obtaining the cooling rate of the two-phase structure. Sometimes, after the cooling section, an equalization section is provided to provide a buffer zone between the galvanizing and cooling sections, so that the steel strip has a better plate shape and a more uniform temperature to enter the zinc pot for galvanizing, and the galvanizing quality is improved. .

2. When the production process of hot-dip galvanized and alloyed dual-phase steel is cooled from the two-phase zone, a certain amount of ferrite is obtained, and then it is required to cool rapidly so that the undercooled austenite is transformed into martensite. A ferritic matrix-based martensitic microstructure dual phase steel was obtained.

Hot-dip galvanizing requires galvanizing and infiltration at 460°C, followed by hot-dip galvanizing at about 500°C. That is, the coiled steel sheet is immersed in a zinc plating bath, and after the tank is discharged, it is heated to 500° C. to form an alloy thin film of zinc and iron. Therefore, the production of hot-dip galvanized dual-phase steel, the cooling rate is relatively low, the degree of undercooling is small, it is difficult to get the ideal dual-phase steel organization. The key technology to solve this problem is to add alloying elements.

The added alloying elements are mainly Mo, and C, Si, Mn and other elements have an adverse effect. The C mass fraction is very effective in lowering the transition temperature of the Ms point. However, in order to obtain a well-plasticized dual-phase steel structure, the martensite structure of the dual-phase steel type should be mainly a tough low-C multiple dislocation type, not a high-C twin martensite. The C mass fraction should be less than 0.1%.

Secondly, silicon and manganese are effective alloying elements to reduce the critical cooling rate for obtaining martensite, but if added excessively, the surface of the steel strip is easily oxidized during annealing, which reduces the galvanized wettability and causes plating defects such as a leakage plating point and can damage the coating. Because of the spot weldability of galvanized steel, the amount of addition is also limited. In order not to affect the anti-pulverization performance, phosphorus also has to maintain a low mass fraction.