Research On Q370qE-HPS Steel Controlled Rolling And Controlled Cooling Process

Since the beginning of new century,the bridge construction sector of our country has been boasting the change for each passing day,and the development of bridge technology has also been enjoying the compliment from traditional overseas bridge powers.Most of modern bridges feature long span,and big capacity to endure high-speed and heavy-load vehicle,therefore,bridge steel entails high strength for long span and light weight purpose,low yield ration and good low-temperature toughness for safety sake,as well as excellent weldability for applicability of engineering design.Throughout the development of bridge steel in our country,low carbon,low alloy and low carbon high strength steel grades lead the trend,among which Q370qE is the widely used grade.However,some issues,such as inferior weldability,unstable low-temperature toughness and unguaranteed normalizing strength of thick plate,remain unsolved.Therefore,this paper studied and developed new generation low-carbon microalloy bridge steel Q370qE-HPS by lowering carbon content,adding microalloy elements such as Nb,Ti,etc.,and utilizing TMCP process to optimize steel properties,to replace traditional Q370qE and yield good economic benefits.On the basis of theoretical calculation,and taking overall consideration of technical requirements of strength,low-temperature roughness and yield ratio,this paper designed ferrite grain size and pearlite content in Q370qE-HPS steel.Since microstructure control is essential to achieve required mechanical properties,such as yield strength,tensile strength,low-temperature roughness and yield ratio.The ferrite grain size shall be controlled within the range of 7-10?m,and the pearlite content shall be limited within the range of 3.5%-13.5%By studying the slab reheating process,to make microalloy element Nb play a role in TMCP process,at least 80%Nb solid solution in the process of reheating shall be guaranteed,meanwhile,complete recrystallization of austenite in high temperature zone shall be guaranteed,the austenite grain size shall be limited under 100?m.The soaking temperature of Q370qE-HPS slab in reheating furnace shall be designed at 1200±20? and reheating temperature shall be no more than 1250?.According to static recrystallization and dynamic CCT experiment,complete recovery and recrystallization can be acquired in Q370qE-HPS steel when the temperature is higher than 1000? and the waiting time between two passes is more than 10s,which can be taken as rough rolling process.When the temperature is lower than 925? and he waiting time between two passes is less than 20s,no recrystallization takes place,avoiding occurrence of mixed crystal,which can be taken as finish rolling process.A microstructure of homogeneous fine ferrite and pearlite can be acquired when the cooling rate of controlled cooling after rolling is around 5?/s and the final cooling temperature is more than 580?.After studying the changes of microstructure and properties of Q370qE-HPS steel with different finish rolling temperature,start cooling temperature,final cooling and self-tempering temperature,as well as different controlled cooling rate,the results indicate that:(1)Given that other process parameters keep unchanged,with the decrease of finish rolling temperature,the ferrite grain size continuously shrinks,pearlite content also continuously reduces,while yield strength and tensile strength gradually increase.According to the design,perfect microstructure and mechanical properties can be acquired with start cooling temperature at 880-840?.(2)Given that other process parameters keep unchanged,with the decrease of start cooling temperature,the ferrite grain size continuously augments,pearlite content also continuously increases,while yield strength and tensile strength gradually decrease.According to the design,perfect microstructure and mechanical properties can be acquired with start cooling temperature at 810-770?.(3)Given that other process parameters keep unchanged,with the decrease of self-tempering temperature,the microstructure transforms from equiaxial ferrite plus pearlite to acicular ferrite and bainite,and fine grain size can be acquired,yield strength and tensile strength gradually increase.According to the design,perfect microstructure and mechanical properties can be acquired with self-tempering temperature at 630-710?.(4)Given that other process parameters keep unchanged,with the increase of controlled cooling rate,the microstructure transforms from equiaxial ferrite plus pearlite to acicular ferrite and bainite,and fine grain size can be acquired,yield strength and tensile strength gradually increase.According to the design,perfect microstructure and mechanical properties can be acquired with controlled cooling rate at 4-6?/s.