Studying On Cold-Rolled And Annealing Process Of Q345A

Q345A is a low alloy high strenth steel(HSLA). In this paper, Q345A hot-rolled steel sheet produced by Lianyuan Iron and Steel Co. on its CSP process is used as the raw material. Recrystallization, cold-rolled and annealling experiments are studied under labrary conditions. Also performance testing is carried on annealed samples. The recrystallization temperature of cold-rolled plate is measured by hardness testing as well as the effect of cold-rolled reduction on recrystallization temperature analyzed. The effect on microstructure, mechanical properties and stamping performance at different cold-rolled reduction rate (61%, 67%, 72%, 78% respectively) is studied. When the total cold-rolled reduction rate is 61%, the influence of different annealing temperature and time on microstructure together with properties are discussed, providing the reference for establishing reasonable cold-rolled and annealing process parameters. When annealing temperature changes, the annealing time is 4 hours. And when annealing time changes, the temperature is 720℃. The results show that:(1) The recrystallization temperature of Q345A is about 600℃. And the influence of cold-rolled reduction on the recrystallization temperature is not obvious. With the cold-rolled reduction rate increasing, ReL, Rm and ReL/Rm increase, A and the IE value decrease at the same time. The r value first increases and then reduces, reaches the maximum when the cold-rolled reduction rate is 72%.(2) When the annealing time is 4 hours, as the increasing of temperature, the ferrite grain grows up gradually, ReL and Rm reduce. A and the r value first increase and then decrease. ReL/Rm is 0.70 at 720℃annealing 4 hours. When the annealing temperature is 720℃, as the increasing of time, the ferrite grain grows up slowly. The content of spherical cementite reduces, but the pearlite slice increases. ReL and Rm reduce while A increases. ReL/Rm is 0.70 at 720℃annealing 4 hours or 16 hours. The r value increases with annealing time's extension in a certain range.(3) When the cold-rolled reduction rate is 61%, it can obtain the highest IE value, which is 10.81 at 720℃annealing 4 hours. The IE value is higher at 700℃or 750℃but lowest at 800℃annealing 4 hours. It doesn't present obvious regularity with the change of annealing temperature and time.(4) The strengthen mechanism of Q345A steel is mainly fine grain strengthening, second is precipitation strengthening. In order to achieve the goal of experiment, considering of saving energy and improving productivity, when the cold-rolled reduction rate is 61%, ReL>350MPa, Rm>500MPa, ReL/Rm=0.70, A>26%, r=1.169, the IE value is 10.81 are obtained at 720℃annealing 4 hours. It satisfies for high strength, as well as good stamping formability.