Research And Development Of Low Cost And Ultra-fine Grain Weathering Steel

With the rapid development of national economy, only good corrosion resistance could not meet the needs as a kind of widely used steel. Improving combination property including the strength and toughness and reducing the cost of production is the new development direction. Ultra-fine grain weathering steel has high strength and toughness, as well as better corrosion resistance, and more and more attention has been paid to them. Under the background of National Secondly 973 Project, which is about the theory research for ferrite and pearlite weathering steel of alloying and microstructure controlling in industry atmospheric environment, the present work is to investigate the microstructure changing, grain refinement, the strengthening of bainite transformation for low cost Mn-Cu weathering steel during hot rolling process, to investigate corrosion resistance by dry/wet cyclic accelerated corrosion experiment and electro-polarization experiment. The main work is as follows:(1) The influence of the deformation temperature, strain value and strain rate of the weathering steel on the austenite dynamic recrystallization behavior and microstructure evolution were investigated. The result indicates that the typical dynamic recrystallization characters are showed when the samples are deformed with small strain rate and high deformation temperature. The activation energy of dynamic recrystallization was obtained. Meanwhile, the influence of deformation parameters on resistance of deformation were investigated and its mathematical model was established.(2) The behavior of austenite static recrystallization of the weathering steel during constant temperature after the hot deformation was investigated and the softening rate curves of the weathering steel were obtained. The influence of deformation temperature and pre-strain value on static recrystallization was discussed. The activation energy of static recrystallization was obtained and the static recrystallization kinetic model was established.(3) The thermal expansion method was taken to test the CCT curves on condition of different stain value. With the metallographic analysis, the influence of strain value and cooling rates on microstructure of the weathering steel was obtained. The influence of deformation parameters on ferrite transformation and bainite transformation was investigated. The results indicate that deformation make ferrite and perlite area be enlarged, bainite area be reduced, promoted the transformation of austenite to ferrite. The starting transformation temperature of austenite to ferrite and bainite increased with increasing strain value.(4) By the thermal simulation experiment, the influence of the deformation parameters on microstructure was investigated. Based on the thermal simulation experiment, the grain refining technology of the steel was investigated through the hot rolling test in the laboratory. The influence of the deformation parameters on the microstructure and mechanical property of the weathering steel was obtained, and the proper TMCP parameters were made. The experiment indicated that the strength and yield ratio of weathering steels increased and elongation percentage decreased a little with decreasing finish rolling and coiling temperature. The results indicate that the volume fraction of bainite increased and ferrite decreased, and the grain is refined and the strength increased with increasing Mn content.(5) Dry/wet cyclic accelerated corrosion experiment and electro-chemistry experiment were conducted on the weathering steels as well as SPA-H steel and Q345 steel, to investigate the corrosion resistance for weathering steels with different grain size. Meanwhile, the difference of rust layer structure between weathering steels and carbon steel was analyzed, so as to have a better understanding of the role of alloying elements on corrosion resistance, and the principle of corrosion resistance of weathering steel was investigated.(6) On the basis of thermal simulation and hot rolling experiment at laboratory, TMCP parameters were determined for container plate of weathering steel, then the industrial experiment was carried out on 1700 hot strip mill. Through analysis on corrosion resistance, the mechanical properties and microstructure of samples obtained under industrial experiment condition, the weathering steel strip was obtained that corrosion resistance and mechanical properties met the requirement. The results indicate that the TMCP parameters are rational schedule.