Research On The Structures-formation Mechanics Of C-Mn Dual Phase Steel And Development Of The Steel

Before the present work, a prediction was given by the author, according to a conjecture mechanics, which is that the dual phase steel could be made of C-Mn steel by using one-stage type continuous cooling on Compact Strip Production (CSP) line. Simply, the conjecture mechanics consider that the fast & largely nucleation ferrite grain suppress the remainder austenite-bainite transition in the middle temperature scale and make the remainder austenite change to martensite under the Ms point. Ferrite grain nucleation fast & largely is the important feature of the CSP line. During the subsequent industrial experiments, a 540MPa grade C-Mn dual-phase steel strip was developed successfully by using continuous cooling process on Baotou CSP line, based on the prediction of the author. The result of the prediction was verified certainly. However, the conjecture mechanics need detecting in logic or in theory by no means. Therefore, the topic of the paper is that the research on the structures-formation mechanics in the hot rolled C-Mn dual-phase steel produced by using continuous cooling process, i.e., the research on the over-cooling austenite transformation to the ferrite and martensite, but no bainite or only microscale, on the condition of continuous cooling.Firstly, a method for obtaining the transition kinetics curve was established, based on the mathematics relationship between the dilatation curve and the transition kinetics curve. Also, based on the transition kinetics curve, a new method, named Austin-Rickett formula Exponent (ARE) method, was constructed, which is used for analyzing or determining the critical temperature and the corresponding transition fraction among the different transformation course or the different stage in the same transformation course. By using the ARE method, the effects of the austenite grain size, cooling rate and carbon content on the pro-eutectoid transition were researched respectively. The phenomenon of the ferrite liberating quickly and massively was interpreted. The effect of the pro-eutectoid with the ferrite liberating quickly and massively on the subsequent transformation of the residue austenite was researched. Based on the CCT with the bainite zone'segregation', the dual-phase structures-formation mechanics in the hot rolled C-Mn dual-phase steel produced by using continuous cooling process was established. The validity of the mechanics was certified by a supplementary experiment.The reason why the dual phase steel was made of C-Mn steel by using continuous cooling on Baotou CSP line was explained by using the mechanics. The adaptability of the CSP line for producing dual-phase steels was analyzed. According to the mechanics, the High Rate Cooling (HRC) device had been installed on the CSP line. It is used for increasing the thickness specification of dual-phase steel and producing low cast C-Mn dual phase steel commercially.The method for obtaining the transition kinetics curve and the ARE method in the present work are the new means for studying the transition regularity on the condition of the continuous cooling. Also, the CCT with bainite zone 'segregation' is the theoretical basis on the aspect of transition kinetics for the development of C-Mn dual-phase steels using continuous cooling process.