Effect Of Rapid Cycle Annealing On Microstructure And Mechanical Properties Of Cold Rolled Low Carbon Low Alloying Steel

The heating methods are the key technologies of the continuous heat treatment lines. The development and application of the technology for the continuous annealing lines can produce super-high strength steel (SHSS) at low cost; The application of rapid cycle annealing can reduce the length of the continuous annealing furnace to a large extent, refine the steel grain, upgrade the steel strength, and eventually bring a technical revolution to the continuous heat treatment. It is thus of great importance to study the effect of the water quenching and rapid cycle annealing on the microstructures and properties of the cold rolled low carbon and low alloying steel.In the present report, the cold rolled steel from BAOSTEEL are chosen to study the effects of normal continuous annealing, new rapid cycle annealing and overaging on the microstructures and properties of the Bake-Hardened Steels (340BH),B170P1, HSLA(B340LA)and the Transformation Induced Plasticity (TRIP) steel of TRIP590, to study the effects of rapid cycle annealing on the microstructures and properties of the cold rolled steel by optical microscope, thermal expansion simulation, X-ray diffraction, scanning electron microscope (SEM), optical electron microscopy, transmission electron microscopy (TEM) and the tester of tensile properties. The relationships among the above processing parameters, the steel microstructures and tensile properties are established, the related mechanisms are also discussed.Experiment results shows that:1) The increase of heating rate delayed the complete recrystallization temperature and time of BH steel. When heating speed is5℃/s,BH has completely recrystallization nearby780℃, and while heating speed increased to500℃/s, recrystallization temperature is between810℃and840℃. After both soaking20s at780℃, along with the increase of heating speed,the average intercept of grains reduced from8.39um of5℃/s to7.48um of500℃/s.2) With the increase of heating speed, the Ac1and AC3temperature of340BH, B170P1, B340LA, TRIP590steel get more higher. As the end temperature of the austenite transformation changes (Ac3) more significant, thus intercritical temperature get wider.3) After rapid heat treatment, BH organization mainly is ferrite and the precipitates distribution in grain inside.After soaking20s at780℃, the strength of BH steel reachs408MPa, higher than the362MPa of production. In addition,the elongation is45%, just above43%of simulated production.The mechanical properties of B170P1also have been markedly improved after rapid heat treatment.With corresponding,the strength of Low-alloy and high strength steel(B340LA) can be up to500Mpa after20s soaking at high temperature.While elongation is30%, make sure that B340LA have high plastic deformation. After the rapid annealing,TRIP steel showed excellent mechanical properties,with tensile strength shows650MPa-750MPa,total elongation ratio can reach nearly30%,and determined by XRD,there is8%retained austenite in microstruct-ure can be obtained.