Study Of Rolling And Water-cooling Coupled Control Rolling Process

Hot-rolling technology has developed from the conventional TMCP(Thermo-Mechanical Control Process)to the new generation TMCP(referred as NG-TMCP)with ultra fast cooling as the key.Accelerated cooling was replaced by ultra fast cooling that increases cooling rate and cooling accuracy.Thus,grain refinement strengthening,precipitation strengthening and phase tranformation strengthening can been realized by controling cooling path precisely.However,NG-TMCP mainly improves the finishing temperature to reduce rolling loads in rolling stage,and the effect of water-cooling in this period should be investigated.Under this condition,rolling and water-cooling coupled control rolling process(RCCP)has proposed in this paper that introduces the water cooling into the rolling process,and realizes the precise control of the temperature of steel plate in the rolling process to couple rolling and water-cooling.In RCCP,the surface of the steel plate has cooling and reheating phenomenon,which is its unique characteristic and can provid new ideas for microstructure and properties control.In this paper,systematically studies the evolution of microstructure and texture in RCCP and its effect on performance,especially the effect of texture on toughness,the main works are as follows:(1)Based on the unique cooling and reheating phenomenon at the surface layer of steel billet in control rolling period in RCCP,a new process that accumulating deformation in austenite no-recrystallized regin followed by reheating to induce recrystallization was proposed.It has been found that recrystallization may occur during holding period of reheating to 900 ? and 950 ? below than Tnr.When reheating to 1000 ?,recrystalliation can be finished in reheating period.Grain size can be refined to about 20 ?m after crystallization.This process can refine austnite graine to a certain degree.To realize the refinement of austenite greatly,warm deformation of over-cooling austenite followed by reheating process should be used.(2)Via the cooling and reheating phenomenon at the surface layer of steel billet,cycle phase transformation and warm deformation was introduced in control rolling period to refine microstructure in the surface layer.The results showed that without warm deformation,the finest austenite grain can be refined to?10 ?m.When warm deformation was applied,the austenite grain can be refined to?5 ?m.Meanwhile,thin plate rolling tests were carried out to study the effect of cycle phase transformation and warm deformation.It was found that cycle phase transformation and warm deformation new process can refine ferrite grain and pearlite structure from 6.8?m to 4.7 ?m and 5.9?m to 1.3 ?m,respectively.Due to the difference of pricipitation behavior,the new process steel plate had a low strength compared with the coarse-grained steel plate with the controlled rolling process,the new process steel plate had excellent low temperature toughness with ductile-brittle transition temperature as low as—163 ? with a high toughness as 5Ni steel.(3)Water-cooling applied in control rolling period of RCCP increases interval cooling rate,reduces deformation temperature and changes deformation distribution in two-stage control rolling at center thickness.The study showed that the increase in interval cooling rate rolling from 0.1 ?/s to 10 ?/s,ferrite grain size can be refined from 14.0 ?m to 11.5 ?m,which as laeger as 17.9%.By reducing deformation temperature from 1100 ? to 1000 ?,austenite grain before deformation in austenite no-recrystallization region and ferrite grain can refined from 60.0 ?m to 34.1 ?m and 15.5 ?m to 8.8 ?m,respectively.Both the degree of refinement can be reach 43.2%.Ddistributing more deformation in the no-recrystallized region can promote the formation of ferrite.And the volum fraction of ferrite increased from about 40%to 75.7%with the deformation in austenite no-crystallization increased from 45%to 60%.The ferrite grain was refined from 11.9 ?m to 6.7 ?m.The yield strength of the tested plates was about 470 MPa,tnesile strength was about 570 MPa,but enlongation was increased from 24.8%to 30.3%,ductile brittle transition temperature was gradually reduced to-91 ?.(4)Surface layer with ultrafine grained steel plate was de-veloped by RCCP via cooling and reheating phenomenon,and the evolution of micro structure and texture and the effect on properties were investigated.The formation mechanism of ultrafine grained ferrite was mainly due to the dynamic recovery.The grain size of the ultrafine grained layer was about 3 ?m,and the thickness of the ultrafine grain layer was 28%of the thickness of the steel plate.The texture of ultrafine grained layer was dominated by rotating cubic {100}<011>and a small amount of Goss {110}<001>texture components.And the texture in the center thickness was the typical transformation texture of deformed austenite under plane strain.Ultrafine grained layer had high strength and low elongation.Ultrafine grained layer had a high toughness.(5)Based on the process feature of RCCP,warm rolling in dual phase region was proposed to develop high strength and high toughness steel plate.It was found that the elongated ultrafine grain structure was obtained with high ? texture component and y texture component.Reducing deformation temperature from 810 ? to 750 ?,ferrite grain size was reduced from 2.8 ?m to 1.6 ?m,yield strength was increased from 521 MPa to 611 MPa.Delamanation occured in impact fracture.Reducing deformation temperature would increase the temperature beginning to delimination,and reduce the platform energy value of the absorbed impact engery-temperature curve.The high toughness of develpoed steel plates is idential to that of 3.5Ni steel.Via this study,we can conclude that large cooling rate should be used to inhibit reheating in the cooling process in the development of surface layer with ultrafine grained steel plate,which can not only ensures the thickness of ultrafine layer,but also ensures a higher finish rolling temperature,so that the ultrafine grained layer can of excellent low temperature toughness.