Composition Design And Optimization Of Controlled Rolling And Cooling Process For Q460C Steel

The prices of niobium and vanadium alloy are rising unceasingly, which leading to high cost of high strength low alloy steel. So how to ensure the mechanical properties of steel with as little as possible or without high-priced micro-alloying elements has become a hot research focus at home and abroad. Q460C steel is a typical high strength low alloy steel, which has high strength, well plasticity and toughness, also has widely used in coal mining hydraulic support, armored face conveyor and other equipments. It is one of the important plate species of Jinan Iron and Steel Group(Jinan Steel for short).The paper expands the research in view of higher cost of alloys, higher strength but unqualified plasticity and toughness of Q460C steel. Contrast and analysis microstructure characteristics of Q460C plate, including qualified, qualified after treatment and unqualified; explore the relationship among the alloy elements, microstructure and performance, optimize adding content of alloy elements; determine the recrystallization curve and CCT curve of optimized Q460C steel, and propose the optimization solutions of controlled rolling and controlled cooling; finally, the industrial experiment was put forward based on alloy composition and TMCP parameters which were determined by research. Conclusions through system research are as follows:(1) Several Q460C plates produced by Jinan Steel on a mass scale are unqualified mainly because of plasticity and toughness. Banded bainite structure appear from 1/4 thickness to the center of unqualified samples, which is the important reason of wooden pattern layered tensile fracture, also the root cause of unqualified elongation and impact toughness.(2) Microalloy carbonitrides are mainly TiN, Nb(C,N), V(C,N). TiN has higher solution and precipitation temperature, mostly square or nearly square, with larger size, also complex precipitate with Nb(C,N) and V(C,N), its main function is to suppress the coarsening of grains during austenitizing and intervals of rolling passes. Nb(C,N) and V(C,N) mainly precipitate on dislocation, grain boundary and sub-boundary, with oval, round or irregular shape, they can effectively pin the movement of dislocation, also have significant grain refinement effect. The precipitate reaction temperature can be appropriately reduced in order to control the precipitation amount at grian boundary and sub-boundary.(3) Q460C steel of Nb-Ti alloys without V alloy is optimized designed in view of producing practice for Jinan Steel. After analysis and calculation, the main mechanical properties and welding performance of optimized Q460C steel are predicted.(4) The static recrystallization curve, true stress-true strain curve and CCT curve of optimized Q460C steel are analysised. On the basis of theoretical calculations, the conclusions are obtained that partially recrystallized during rough rolling of optimized Q460C steel, defomed austenite experience fewer recrystallization frequency. The reduction is small on the initial rolling in order to reduce the volume fraction of thick recrystallized grains, the first four passes reduction can be set below 8.5mm, a big strain should be taken at last passes in order to refine the austenite grain, the finishing rolling temperature can be controlled at 1020℃. In the finishing rolling stage, the finishing rolling temperature can be controlled at 830～850℃. The cooling rate differences according to the thickness of plates, can be controlled at 5～10℃/s, the finishing cooling temperature can be controlled at about 670～690℃, direct air cooling to room temperature after controlled cooling.(5) The paper identifies the alloy composition and TMCP process parameters of Q460C steel in view of producing practice for Jinan Steel. The industrial experiment is put forward based on optimized alloy composition and TMCP parameters of Q460C steel. The results show that, the process is smooth, and alloy cost is reduced by 20%.