Research On The Heat Treatments Key Technique Of High-Strength Cold Rolled Dual-Phase Steel

Cold rolled Dual phase(DP) steel is characterized by a microstructure consisting of a dispersion of hard martensite particles in a soft ferrite matrix. Due to its combination of high strength, good ductility, low yield ratio, continuous yielding and high work-hardening ratio, dual phase steel has been considered to be ideal automobile component materials which require good formability to reduce the weight of vehicle for fuel economy.In this paper, based on the hot processing of cold rolled DP steel, the effect of hot rolling coiling temperature, the continuous annealing process with the gas jet cooling, the continuous annealing process with the water quench cooling, and galvanizing annealing process based on the quenching-tempering on the microstructure and mechanical properties of C-Mn and C-Mn-Nb dual phase steel was investigated by means of optical metallography(OM), scanning electronic microscope(SEM), transmission electronic microscope(TEM), X-ray diffractometer(XRD), Nanoindentation tests, Thermal Calc phase-diagram calculation and tensile experiments. At the same time, a flow stress calculation model was constructed to reflect the stress-strain relationship of continuous annealed microalloyed-DP steel.The coiling temperature of hot rolling determines the cold rolled steel microstructures to a great extent. The effect of coiling temperature on microstructure and mechanical properties of cold rolled DP steel was studied, and the results showed that the high coiling temperature can result in coarse ferrite grains which is beneficial for the decreasing of yield strength. On the other hand, the resulting Mn enrichment at the grain boundary can improve the hardenability of austenite during the course of gas cooling, and result in more martensite upon the gas jet cooling which can increase the tensile strength of DP steel. Therefore, the high coiling temperature can ensure the lower yield ratio and better ductility for the continuous annealed DP steel with gas jet cooling, and it is good for forming.The effect of continuous annealing process with gas jet cooling on microstructure and mechanical properties of cold rolled DP steel was studied, and the results showed that the proper soaking temperature and time are very important for the mechanical properties of DP steel, and the soaking should ensure the enough austenite, but the too much austenite is bad for the hardenability during the course of gas cooling. For the gas jet cooled DP steel, the martensite volume fraction is the key factor to the mechanical properties. During the course of cooling after soaking, the new ferrite must form at the slow cooling stage or the rapid cooling. For the present steel B, the optimum annealing process is: heated to 790℃and soaked for 80s, and slowly cooled to 650℃at a 10℃/s cooling velocity, and then cooled quickly at a 40℃/s cooling velocity to under 300℃, which can lead to the DP steel with a more than 800Mpa tensile strength, a more than 16% elongation and a less than 0.5 yield ratio.The effect of continuous annealing process with water quench cooling on microstructure and mechanical properties of cold rolled dual phase steel was studied, and the results showed that the slow cooling has little effect on the mechanical properties of C-Mn DP steel. However, for the C-Mn-Nb DP steel, due to exemption of precipitation strengthening, new ferrite will contribute to not only the increase of ductility, but also the improvement of microalloyed DP steel strength. Therefore, the slow cooling should be controlled properly so as to get more new ferrite and thus the better properties. For the present steel B, the optimum annealing process is: soaking at 810℃and water quenching at 630℃with a 10℃/s slow cooling velocity, which can lead to the DP steel with a more than 850Mpa tensile strength, a more than 15% elongation and a less than 0.6 yield ratio..To solve the galvanizability problem of DP steel, the new galvanizing process is put forward, and the effect of galvanizing annealing process based on the quenching-tempering on the microstructure and mechanical properties of cold rolled DP steel was studied, and the results showed that the strength decreases while total elongation increases with the increasing galvanizing/galvannealing time. Compared with galvanizing annealing, the tempering temperature of galvannealing is higher, and the tempering time of galvannealing is longer, so the decreasing of water quenched DP steel strength is more evident. The solute Nb is capable of not only suppressing recrystallisation of lathe martensite but also retarding the precipitation of solute C in the martensite, so it can improve the tempering resistance of water quenched DP steel and thus the mechanical properties of galvanized/galvannealed DP steel. The optimum galvanizing process is: the Nb-microalloyed steel B is heated, soaked and water quenched, then descaled in the hydrochloric acid solution, and reheated to 460℃～500℃and galvanized, which adopts the descaling to improve the galvanizability and employs minute Nb to enhance the tempering resistance of DP steel.The stress-strain relationship is closely related to the yield strength, tensile strength and work hardening behavior of material, and is the key parameter of engineering application and calculation. In this paper, a micromechanical model is constructed to reflect the stress-strain relationship of continuous annealed microalloyed-DP steel, and the flow stress of a continuous annealed microalloyed-DP steel has been calculated by using the stepwise approach of Tomota, where the Eshelby inclusion theory and the Mori-Tanaka mean field theory are combined. A comparison of stress-strain curves determined from the model and experimentally derived stress-strain curves for a C-Mn-Nb DP steel gives satisfactory agreement. The present model underlies the mechanical foundation for the DP steel theory and heat treatment process optimizations.