Effect Of Gradient Temperature Rolling On Deformation And Microstructure Of Heavy Plate

The most prominent problem in the rolling of heavy plate is the serious deformation inhomogeneity between its surface and center.The deformation inhomogeneity also induces the difference in microstructure and mechanical properties between the surface and the center.To improve that,the current paper proposed a novel process named gradient temperature rolling(GTR).The process makes a temperature distribution which is?800? in the plate surface and?1100? in its center.The temperature distribution leads that the resistance of plate center is lower than that of its surface so that improve the central deformation.A rigid-plastic finite element model of GTR was built for thick plate.The difference in the strain and stress between GTR and uniform temperature rolling(UTR)and its influence on austenite recrystallization was analyzed.A simplified method was proposed to calculate the rolling force of GTR in practical production and its maximum error is 6%compared with the results of FEM.A simulation rolling experiment was performed using the plane strain reduction.The FEM results were verified through the austenite recrystallization.60 mm thick plates were rolled using laboratorial rolling mill with 400 mm diameter roller to investigate the effect of GTR on central microstructure and deformation of heavy plate.The rule of central crack closing was investigated by FEM model.The central interface evolution was studied by the cladding rolling.The microstructural development and the mechanical properties with verity cooling rates after GTR were researched.The type and morphology of precipitation was analyzed under the two processes.The results of finite element method and plane strain experiment showed that the central strain of the plate rolled by GTR was increased and more homogeneous compared with the UTR plate.It can be concluded that GTR can refine the austenite grain in central area through the calculation and experiment.The central austenite grain size of GTR plate was decreased to 34.2 ?m from 83.9?m of UTR plate.The effect of GTR on central deformation is interacting with reduction and thickness.If the reduction and thickness are too large or too small,the effect of GTR is not obvious.60 mm Q345B thick plates were rolled by GTR and UTR in laboratory.The results showed that the central recrystallization grain size of GTR plate was 25.2?m while that of UTR plate was 69.3 ?m,which further proved that GTR can increase strain and refine the austenite grain size in the center of heavy plate.Additionally,GTR plates had more uniform and equiaxed ferrite grain because of the finer austenite grain.There were however different kinds of ferrite in UTR plate.The impact energies of GTR plate along rolling and lateral directions were increased to 70.9 J and 75.9 J,respectively.Under the condition of air cooling after rolling,the ferrite grain size of E40 plate rolled by GTR was respectively 10.8 ?m,11.9 ?m,and 12.6 ?m in surface,quarter and half thickness.That of UTR plate was respectively 10.2 ?m,15.31 ?m,and 17.1 ?m in corresponding positions.GTR can decrease the effective grain size and increase the proportion of high-angle grain boundary in water cooling plate.The E40 heavy plate rolled by GTR followed air cooling showed the best integrated mechanical properties,particularly in the low temperature toughness.With the increase of cooling speed after rolling,E40 plate had a microstructure consisted of granular bainite,acicular ferrite,and quasi-polygonal ferrite.Because of the small total reduction,austenite was difficult to be segmented by acicular ferrite,which leads to a lot of coarsen area remained in matrix.The toughness of GTR plate with water cooling decreased compared with that GTR plate with air cooling.The central defect of GTR plate is easier to be closed than that of UTR plate because of the increase of central strain and stress.The results of FEM showed that the central crack of 120 mm thick plate can be completely closed by 11%reduction in single-pass GTR while the crack was closed by 14%reduction in UTR.The central crack of GTR plate was completely closed a pass ahead of that of UTR plate in entire rolling process.The clad rolling experiment illustrated that the un-weld area of GTR plate was 745 mm2.The oxide layer of GTR plate's interface was discontinuous,while the oxide layer of the interface of UTR plate was continuous and the un-weld area exceeded 5000 mm.The volume fraction of second-phase particles in experimental steel was less than 0.5%shown by Thermo-calc simulation.The main particles were TiN and MnS precipitated during casting,and Nb(C,N).Before rolling,there were fine Nb(C,N)in the surface of GTR plate,and corase composite precipitation based on TiN and MnS in its 1/4 and 1/2 thickness.Precipitation in GTR plate was coarser than that in UTR plate owing to much longer time for growth.However,the effect of GTR on the precipitation of heavy plate need to be further researched.