| Strip casting process is the most important technology of material and metallurgical field in the 21st century.The process has connected the casting with rolling,which can produce the strip steel without the process of high-temperature heating,rough rolling and finish rolling,greatly shorten the length of the strip production line and reduce construction investment and production costs.At the same time,it can also significantly decrease energy consumption and harmful gas emission,which reduces the pollution to the environment,in line with China's sustainable development strategic objectives.Meet the strategic objective of our country's sustainable development.In this paper,three as-cast strips with different compositions were successfully fabricated via TRSC process in the laboratory.The solidification-annealing microstructure evolution and mechanical properties of whole process of strip casting,cold rolling and heat treatment of tested steel were investigated and characterized by OM,SEM,EPMA,TEM and tensile machine.The main work of the present paper is as follows:The microstructure formation characteristics via sub-rapid solidification of Fe-C-Si-Mn-Al and Fe-B-Cr system alloy steels were investigated.Based on its superiority in sub-rapid solidification and near-net-shape characteristics,there was an obvious difference between the solidification microstructures of as-cast strip with that produced by the conventional steel production route.In the thickness direction,all as-cast strips with different compositions were made up of columnar crystal in the surface and equiaxed crystal in the center,In the width direction,the ratio of columnar crystal and equiaxed crystal was really different at different positions.Additionally,at some positions there was angle between the growth direction of columnar crystal with normal direction,which proved that the as-cast strip had been deformed between the rollers gap.The microstructures and mechanical properties of the annealed sheet of 0.11C-1.55Mn-0.9Si-0.8Al structural steel were investigated.The microstructures of annealed sheet were made up of ferrite and martensite.The martensite was distributed on the ferrite matrix with the shape of island.With the temperature increasing from 850? to 950? the volume fraction of martensite increased.The tensile strength increased and the elongation was decreased with the increasing of annealing temperature,The best combination of tensile strength(763MPa)and elongation(20.7%)of annealed sheet was achieved after annealing at 850? for 5min.The product of strength and elongation is 15794.1MPa·%.The MnS and Al2O3 inclusions formed in the process of casting,size range of 0.2?2?m,which was bad for the mechanical properties and can not be eliminated by cold rolling and heat treatment process.Columnar crystal and carbon rich region microstructure between columnar crystals were inherited in the cold rolling and annealing process.The grain size of recrystallization ferrite and martensite were not uniform,solidification microstructure should avoid the appearance of large columnar grains.The microstructures and mechanical properties of the annealed sheet and tempered sheet of 0.20C-2.01Mn-1.2Si-1.5Al steel were studied.In the annealed sheet martensite was distributed on the ferrite matrix with the shape of chain,which obtained a low elongation in this study.From the microstructures of the fracture region,it was found that there were cracks originated at the interface,which proved that there was stress at here.The chain martensite was easy to break.However,the mechanical properties of the annealing sheet were improved by tempering.With the temperature increasing from 400? to 500?,and with the holding time increasing from 30min to 90min at 450? tempering,the tensile strength decreased while the elongation increased,but the change was not obvious.The segregation behaviors of B,Cr and Ni in the as-cast strip influenced by sub-rapid solidification were studied.It was noted that B could distinctly refine the as-cast strip solidification structure.The sub-rapid solidification could not control the segregation behaviors of B and Cr,and there were much more boride forming in the as-cast strip.Via solution treatment,the morphology of precipitation of boride could be changed,but it could not improve the mechanical properties of the as-cast strip.The strip could not cold roll at room temperature because of its bad forming property.However,warm rolling could improve the forming property of B as-cast strip,but at a high reduction rate the boride was broken and cracks were formed at the edge of sheet. |
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