Thermomechanical processing and deformation behavior of dual phase steel

Abstract

The current work developed two grades of direct hot rolled ultralow carbon dual phase steel, ferrite bainite dual phase “FBDP” and ferrite martensite dual phase "FMDP” steel with high ductility values. The alloy design and production route cope with a compact slab production CSP plant. Ultralow carbon alloys were determined within the pre-peritectic region from phase diagrams plotted by Thermo-Calc 2020. The chemical compositions of the alloys are 0.045% C, 0.28 Si%, 0.91% Mn, and different Cr weight contents of 0.59%, 1%, 1.56%, and 2%. These alloys were used to develop FBDP steel. Analogous to "FBDP" steel, the martensitic dual phase steel is developed using ultralow carbon alloy and the only difference is the addition of niobium. The chemical composition of FMDP steel is (0.045 Wt.% carbon, 0.04 Wt.% niobium, 0.59 Wt.% chromium, 0.9 Wt.% manganese, 0.28 wt.% silicon) . These alloys were melted and cast into blocks, and then a certain thermomechanical schedule was applied using the actual data obtained from the log files of a CSP plant. The cooling schedule was designed with the aid of CCT and TTT curves plotted by JMatPro. The designed thermomechanical processing scheduled was performed by two methods. In the first method physical simulator "Baher TTS 820" is used. This device allows to perform physical simulation of direct hot rolled dual phase low carbon steel by varying the metallurgical temperatures and deformation parameters of to simulate compact slab production plant CSP. The second method is the pilot scale rolling and salt bath heat treatment, this route is used to develop FBDP steel under specific conditions determined from the results of physical simulation of the thermomechanical processing. The niobium was added to the FMDP steel to enhance the ductility of the martensite DP steel, since the current work aims to get high ductility values of DP steel, while lower carbon content is enough to get high ductile FBDP steel. The micro alloying affects the hot deformation flow behavior of the steel and the final microstructure. Consequently rheological curve characteristics under different rolling conditions are predicted using data of compact slab production log files and the phenomenological hyperbolic sinusoidal Arrhenius mathematical model. The Practical values of high strain rates reaches to 100 S^(-1) along with the practical deformations temperatures of an industrial hot strip mill plant are used. Mean flow stress, non-recrystallization temperature and microstructure grain size are investigated. The results show that hot rolled FBDP steel contains a substantial amount of bainite from 10 to 28% and negligible martensite. The increase in Cr led to an increase in the bainite volume fraction and an increase in the yield strength and tensile strength, while the elongation decreased slightly. The strain hardening exponent (n-value) reached values ranging from