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Effect of Cooling Process on Microstructure and Properties of Hot Rolled Ferritic Bainite Dual Phase Steel

2016-07-05

For car manufacturers, the most important issue has been to use high-strength steel plates to ensure the car's safety and reduce the weight of the car, so as to ensure fuel efficiency and ensure the car's anti-collision capability. Duplex steel is widely used in the automotive field due to its low yield point, continuous yield, and high initial work hardening rate. It has been at the forefront of high strength low alloy steel development. Martensite duplex steel is easy to crack at the opening part during the forming process, its extended flangeability is not good enough, and the fatigue strength is low, so its application is limited. For this reason, bainite, which is more susceptible to plastic deformation than martensite, is used as a reinforcing structure to obtain a balance between strength, welding, fatigue, and stretch flange properties. Ferritic bainite duplex steel is a development of traditional duplex steel with good forming, welding and fatigue properties. In the ferritic bainite duplex steel, bainite as the second phase has a favorable influence on the flange tensile properties and fracture toughness as compared with martensite. Therefore, the study of high strength grade ferritic bainite duplex steel (FBDP steel) with high pore expansion performance has practical significance. The author designed a high-strength ferritic bainite dual-phase steel, measured the continuous cooling transformation curve of deformed austenite, and studied the influence of the cooling process after hot rolling on the microstructure and properties of the test steel.

A ferritic bainite duplex steel was designed. The continuous cooling transition (CCT) curve of the test steel was measured by Gleeble-3500 heat test, and the test steel was controlled and controlled. The cold test was conducted to study the effects of different cooling processes on the microstructure and properties of the test steel. The results show that the deformed CCT curve is divided into ferrite transformation zone and bainite transformation zone. After the hot rolling of the test steel, the ferrite bainite duplex structure can be obtained by different cooling methods. The three-stage cooling method has a smaller volume fraction of ferrite than that obtained by two-stage cooling, and the grain size is smaller. After finishing rolling at 840 °C, the water is cooled to 690 ° C, air cooling is about 8 s. The tensile strength of the precision stainless steel strip test steel reaches 765 MPa, the elongation is 20%, and the comprehensive performance is good.


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