The Effect Of Cooling Way On The Microstructure And Mechanical Property Of High Rise Building Steel Q345GJ

Frequent occurrences of large earthquakes produce serious losses to people's life and property, therefore, the earthquake resistance of building steel becomes the core issue of public concern. However, low yield ratio high rise building steel can absorb the great energy of earthquake by more plastic deformation, delay the occurrence of final destruction. This thinking leads to the application of low yield ratio steel in the building structure. Aiming at the subject, the State Key Lab of Rolling and Automation of Northeastern University and Shouqin Co.,Tiansteel Co.are developing together the high rise building below Q390GJ. Based on Q345GJ, this paper studies the technological conditions for the formation of low yield ratio and provides optimized process scheme for large industrialized production.The main works involved are as follows:(1)When single press test was taken, the influence of the deforming temperature, deforming degree and deforming rate to the stress of the steel were researched, and the mathematical model of resistance of deformation was established. The behavior of dynamic softening of the steel to be tested in austenitic area was also researched. The activation energy of dynamic recrystallization in peak state and steady state are 245.448 kJ/mol and 166.994 kJ/mol.(2) The heat expansion method was taken to test the CCT curves on deformed condition undeformed condition and different heating temperature. The effect of deformation,cooling speed and heating temperature on the microstructure of experimental steel could be got from the metallographic. The influence of deform processing parameter to ferrite transformation and bainite transformation was also researched. The result shows deformation promoting the transformation from austenite to ferrite.Deformation has double effect to the bainite transformation:at lower cooling rate, the deformation promotes bainite transformation; at lower cooling rate, the deformation inhibits bainite transformation.(3)The simulation test of cooling way after hot deformation and controlled rolling and controlled cooling test in experiment were carried out, the effect of finish rolling temperature range, start cooling temperature, finish cooling temperature, cooling rate and heat treatment process on the microstructure and mechanical properties of tested steels were studied. The result shows that the recommended processing parameters for industrial production are:(1) the reheating temperature is 1200℃;(2) two stages controlled rolling is adopted, in which the first stage rolling startes at 1100～1150℃and the second stage rolling startes at 950℃,the final rolling temperature is 850℃;(3)time waiting treatment after final rolling, start cooling temperature is 740～770℃,finish cooling temperature is 600～650℃,cooling rate is 5～12℃/s.(4) The formation mechanism of low yield ratio was analyzed; the effect of finish rolling temperature range, finish cooling temperature,cooling rate, content of rough phase, ferrite grain size and heat treatment process on the yield ratio were studied. The transmission electron microscope, scanning electron microscope and electron backscattered diffraction were used to study the grain density of ferrite,the lamellar thickness of pearlite and the grain boundary misorientation of ferrite.