The Effect of TMCP on Microstructure and Nano-precipitation Behavior in High Silicon Containing Dual Phase Steels

ABSTRACT As the issues of energy become more important and the structural members in automobile become more complicated. The materials developed in the automobile industry have to be not only light and strong but also need great formability and weldability. Based on cost consideration and the convenience of controlling the microstructure and mechanical behavior of materials by heat treatment to adapt to the different strength, ductility demands in different part in automobile. The development of DP steels was never stopped since it was showed up in industrial application in 1970s. The performance and properties have been improved continually by the effort of scientists and engineers until now. Although dual phase steel has great mechanical properties and high work hardenability, there is a large strength difference between two phases which makes strain not uniformly distributed. This problem results in the crack formed at the interface of two phases when the steel suffered stress. Strengthening ferrite can reduce the strength difference between two phases and enhance yield strength, hole expansion ratio and the elongation of non-uniform plasticity deformation region. One of the trend in the DP development is that adding Nb,Ti,V to form nano-carbides to enhance ferrite strength and reduce the strength difference between two phases. Thermo-Mechanical control process (TMCP) is the most important process in the production line of hot-rolled strip. The purpose of well-designed controlled rolling process is not only for the thickness reason but also can refine the grain size and control microstructure to reach a better mechanical properties of materials. However, there is still a lot of complicated physical and metallurgical phenomenon to influence the properties and performance of materials after TMCP. Adding titanium to form nano-precipitates to strengthen ferrite is the core concept in this research. In addition, the parameters of s controlled rolling simulation and the silicon content effect on the microstructure and the behavior of nano-precipitates in ferrite will be discuss in this research. In the first section, the effect of austenitizing temperature will be discussed. Austenitizing temperature has a decisive influence on the behavior of precipitates of titanium carbides. Although lower austenitizing temperature can make smaller prior austenite grain and formed smaller ferrite grain about 2-4μm, it caused softer ferrite matrix. There are two possible reasons, first, coarser precipitates formed in the austenitizing process when austenitizing temperature is low; and second, the titanium carbide which existed in the material as received didn’t dissolve in the austenite matrix. These reasons inhibited the precipitation behavior of titanium carbides which is formed in the temperature of two phase region. Analyzing the size and distribution of carbides in ferrite by the observation of TEM and SEM. In addition, there is no interphase precipitation was observed in low austenitizing temperature condition. In the second section, the effect of temperature and time of hot rolling on the precipitation behavior will be discussed. The temperature is not controlled at specific numeric value in the actual rolling process, but the rolling temperature is important to the nano-precipitates strengthened dual phase steel. Larger precipitates formed in the high temperature rolling process has competitive relationship with smaller precipitates which is formed in the temperature of two phase region. In brief, the precipitation behavior of titanium carbides in the high temperature rolling process has a decisive influence on the final property of steels. The behavior of nano-precipitates under different rolling condition was analyzed by the microhardness test and TEM observation. This research found that the time during rolling process should be short and the rolling temperature should be high, or the hardness of ferrite will be low. In the third section, the effect of silicon content will be discussed. Silicon has a lot of interesting effect on the TMCP and heat treatment of dual phase steel, including raising Ar3, solid solution strengthening, accelerating the phase transformation of ferrite. Raising Ar3 makes austenite grain size smaller after materials be heated to austenitizing temperature. This effect also caused smaller grain size of ferrite. In the aspect of solid solution strengthening, 1wt% of silicon can offer 30HV hardness to ferrite based on past experience. As an element which is incompatible with carbon, silicon can accelerate the phase transformation of ferrite and promote the precipitation of carbide. Microhardness test and two stages hot deformation test which used to estimate softening ratio will be used to analyze the comprehensive influence of silicon in this research. It was found that high silicon caused the coarsening of Titanium carbide at the rolling stage which is unfavorable to the nano-precipitation strengthening. Key words: Transmission Electron Microscopy(TEM), Nano-precipitation, Dual phase steel, Silicon, TMCP