Research On The Mechanism Of Austenite Transformation Behavior And Microstructure Refinement Under Gradient Temperature Control Rolling

Medium plate is an important iron and steel material to support the development of the national economy.The demand for plate thickness and performance in large-span construction engineering steel,large ship steel,marine engineering steel,and heavy machinery steel is increasing.However,the current production of medium and heavy plates has problems such as insufficient deformation and permeability,too long rolling waiting time and affecting rolling efficiency.For that,this paper relies on the "13th Five-Year" national key R&D project "Highstrength,large-size,easy-weld marine engineering steel and application",based on gradient temperature control rolling process for microstructure evolution,phase transformation model,and mechanical properties for differential rolling.The research work and results of this thesis are as follows:(1)The continuous cooling phase transition behavior of the experimental steel and the cooling-returning reciprocating phase transformation fine grain behavior of the surface layer of the steel plate under the differential rolling process were studied to determine the phase composition law at different temperatures,so as to control the deformation temperature to optimize grain size and microstructure.(2)Investigate the evolution of the thickness of the steel plate to the microstructure and mechanical properties under the gradient temperature control rolling process.Compared with the conventional process,the gradient temperature control rolling process can promote the thickness refinement of the steel plate,reduce the formation of granular bainite structure,and form a fine-grained structure on the surface layer.The ferritic content of the experimental steel from the surface layer to the core can effectively promote the formation of residual austenite,optimize the morphology of M-A islands,reduce the size of M-A islands,and reduce the texture density.The thickness elongation of the differential rolling process is higher than that of the conventional rolling process.The tensile strength of the core and the 1/4 thickness of the differential rolling process is 582 MPa and 570 MPa,which are higher than the conventional process.It was found that the gradient temperature control rolling process can increase the proportion of large-angle grain boundaries in the thick microstructure and improve the impact toughness of the experimental steel.(3)The phase transition model and precipitation behavior of ferrocarbon alloys under gradient temperature control rolling process were studied.The cementite with higher precipitation density was obtained through gradient temperature control rolling process,and the thermodynamic and Fe-C phases were obtained respectively.The angle of the figure illustrates the driving force and law of carbon diffusion in the austenite side and ferrite side in the twophase interface under differential rolling conditions,and the carbon concentration distribution mechanism at different distances in the austenite grains.The results show that the decomposition driving force on the austenite side at the phase boundary is the largest,and that of the austenite in the experimental steel is the second,while the decomposition driving force on the ferrite side at the phase boundary is too low to decompose.However,it was found that under the normal carbon concentration,the precipitation of grain boundaries in the conventional process gradually decreases.The differential rolling process has a higher carbon concentration due to the effect of fine grains.(4)Differential rolling industrial trial production of 30 mm and 120 mm thick steel plates was completed using interpass cooling devices,which improved the internal quality and surface quality in the thickness direction,and improved the production efficiency of thick plates.