Microstructure Evolution Of Longitudinal Profiled Steel Plate Under Controlled Rolling And Controlled Cooling Process

In the construction of ships and bridges,longitudinal profiled steel plate(LP steel plate)can optimize the structural design,reduce the amount of steel and the number of welding,and is a green and energy-saving steel product.However,the thickness of the LP steel plate varies continuously along the length,if the same cooling process as the equal-thickness steel plate is used,the microstructure and properties of different thickness parts of LP steel plate will be deteriorated.Therefore,it is particularly important to develop a process to obtain high performance uniformity of LP steel plate.Foreign JFE and other advanced steel companies have carried out research on LP steel plates earlier.The LP steel plates produced by them have stable performance and are used in Japan and European countries.However,the production process,especially the core controlled rolling and controlled cooling processes,are strictly confidential.Domestic Anshan Iron and Steel Company adopts the process of casting heterogeneous billet and direct air cooling after rolling to realize the production of LP steel plate with uniform performance,but it has the problems of high difficulty in casting,low performance and high cost.And there are few reports on the development of uniformity of LP steel plate by controlled rolling and controlled cooling processes,therefore,it is necessary to further strengthen process exploration and related research.Under this background,this thesis takes low carbon steel as the research object,explores the process of developing LP steel plate under different process paths.And the microstructure evolution law of LP steel plate in controlled rolling and controlled cooling process is studied.The main research conclusions are as follows:(1)Taking Taking low carbon microalloyed steel as the research object,the evolution law of microstructure of LP steel plate under different cooling paths was studied.The results show that the LP steel plate is ferrite and pearlite when the cooling rate is 0.5?2?/s.and the difference in hardness is large;when the cooling rate is 10?/s,the microstructure of the thin and thick parts is not uniform,and the large deformation of the thin part of the LP steel plate inhibits the production of bainite.When the final cooling temperature is increased to 700?,the microstructure of the LP steel plate is ferrite and pearlite at different cooling speeds,and the hardness difference is reduced.Comparing the difference of microstructure and hardness between the thin and thick parts of the LP steel plate under the conventional cooling process and the cooling path control process,the cooling path control process is beneficial to improve the uniformity of the microstructure and performance of the LP steel plate.(2)Taking the low carbon microalloyed steel as the research object,the evolution law of microstructure of LP steel plate under the heat-treatment online process(HOP process)was studied.The results show that the martensite or carbide between the bainite lath gradually changes from long strip to short rod and granular with the extension of isothermal time.As the tempering temperature increases,the bainite size decreases,and carbides between the slabs refine,reducing carbide aggregation of grain boundary.Comparing the difference of the microstructure and hardness of the LP steel plate after the conventional cooling process and the cooling path control process,it shows that the two processes are beneficial to improve the performance uniformity of the LP steel plate after HOP tempering treatment,but the difference in microstructure still exists under the conventional cooling process,while under the cooling path control process,the difference in microstructure and hardness is reduced.(3)Taking oxide metallurgy steel as the research object,the evolution law of microstructure of LP steel plate under different cooling paths in high temperature rolling and two-stage rolling experiment was studied.In the high temperature rolling experiment,the cooling rate is 5?/s and above,the final cooling temperature is 600?650?,the hardness of oxide metallurgical steel fluctuates less,and the hardness of non-oxide metallurgy steel increases gradually.In the twostage rolling experiment,the two steel grades can further refine the grain size after two-stage deformation,but the oxide metallurgy steel has a finer grain size and uniformity of microstructure properties than the oxide-free metallurgical steel.It is indicated that oxide metallurgy steel is beneficial to reduce the difference in the microstructure and performance of LP steel plate.