Precipitation Behavior Study And Control On Microstructure And Properties Of Hot-rolled Ultra-high Strength Automobile Strip

With the establishment and implementation of the government regulations and politics for energy conservation, emission reduction and environmental protection, the pace of construction for resource-saving and environment-friendly society is speeding up. So energy conservation and emission reduction of automobile has become a key research subject for the automobile industry development. The main approaches of solving this problem are new energy automobile technology and lightweight technology. For the heavy-duty automobile accounted for30%of the automobile output of China, now lightweight technology is the fastest and most efficient measure of automobile energy conservation and emission reduction.70%of the materials for heavy-duty automobiles’body are steel, and the yield strength of the domestic carriage strip and crossbeam strip is between350～450MPa; however the yield strength700MPa grade ultra-high strength steel has been widely used in heavy-duty automobile manufacturing in abroad. So the adoption of the high strength thin steel plate to replace the original low strength thick steel plate can achieve the lightweight of body, reducing fuel consumption while increasing effective load capacity and transport efficiency, which has great significance for national economic sustainable development.With the support of the fundamental research funds for the central universities of china (N090607003), this paper took Nb-Ti microalloying ultra-high strength strip as research object, and developed the basic theoretical research on high temperature deformation behavior and continuous cooling transformation behavior of austenite, nano scale (Nb,Ti)C precipitation behavior and its thermo stability, and then700MPa grade carriage strip and780MPa grade crossbeam strip were successfully trial-produced on the1750mm semi-continuous hot rolling production line in Tianjin Iron Hot-rolled Plate Company. Meanwhile, the cold forming performance, toughness (impact toughness and fracture toughness), weldability, fatigue properties and wear resistance which are closely related to the application of the ultra-high strength strip were systematically studied. Major researches and innovative achievements in this paper are as follows:(1) By using the thermal simulation experiment technology, the study of austenitic high temperature deformation behavior of experimental steel was conducted by single pass compression experiments, and the effect of deformation process parameters on deformation resistance was analyzed, and the deformation resistance model of the experimental steel was established, and the influence laws of Nb and Ti on austenitic dynamic recrystallization behavior were studied. The results indicated:austenitic dynamic recrystallization behavior was inhibited by carbonitrides of niobium and titanium pinning dislocations and grain boundaries, as well as solute drag effect of solid solution niobium, so the austenitic recrystallization stop temperature was improved.(2) Using the thermal simulation experiment technology, the austenitic continuous cooling transformation behavior was studied by continuous cooling phase transformation experiments. The dynamic and static CCT curves at900℃of the experimental steel were established, and the roles of deformation, microalloy elements Nb and Ti in continuous cooling phase transformation were investigated.Deformation promoted ferritic transformation and refined ferritic grain size; due to pearlitic transformation was after ferritic transformation, so deformation indirectly impacted perlitic transformation through promoting ferritic transformation; the effect of the deformation on bainitic transformation was subject to condition whether ferritic transformation before bainitic transformation, When ferritic transformation exists, the systematic free energy was reduced, the stability of untransformed austenite was increased, so bainitic transformation was suppressed; when there is no ferritic transformation, the deformation promoted bainitic transformation. Solid solution Nb inhibited ferritic transformation, but solid solution Nb and Ti promoted bainitic transformation. When the nucleation of nano-scale (Nb,Ti)C was before ferritic transformation, the ferritic transformation was promoted, but which is not good for refining the ferrite grain.(3) Using the thermal simulation experiment technology, the effects of deformation, cooling rate and isothermal temperature on nano-scale (Nb,Ti)C precipitation behavior were revealed, and the effect mechanism between nano precipitation and ferrite phase transformation or bainite phase transformation was established. Series of isothermal heat treatment experiments was conducted, accordingly nanometer precipitation thermal stability was discussed.Deformation improved nucleation rate of nano-scale (Nb,Ti)C and refined the size of precipitation; it also promoted vacancy nucleation. Consequently, when cooling rate less than5℃/s,(Nb,Ti)C-depleted precipitate free zone (PFZ) was formed in subgrain boundaries of the original austenite, and the width of PFZ decreased with the cooling rate increasing. When the cooling rate reached15℃/s, the nucleation of precipitation was totally inhibited during cooling process. Due to there was a competition mechanism for austenitic defects between ferritic transformation and precipitation, these two transformation behaviors restrict each other. Bainitic transformation could effectively freeze austenitic defects, so promoted (Nb,Ti)C precipitation. Precipitation was impacted by nucleation driving force and atom diffusibility, so the peak temperature of nano-scale (Nb,Ti)C precipitation was550℃。 Nano-scale (Nb,Ti)C had excellent thermal stability between500～660℃.(4) The fine microstructure and precipitation were observed on transmission electron microscope, the sample include metal foil and extraction carbon replica. Precipitation of trial-produced ultra-strength automobile strip was analyzed, and the strengthening mechanism was determined.Microstructure of700MPa grade carriage strip and780MPa grade crossbeam strip were dominated by ultrafine ferrite and bainitic ferrite respectively. There was a large number of diffusion nearly circular precipitation (Nb,Ti)C in ferrite or bainitic ferrite matrix, and its size was between3-20nm, among which90%was below10nm and the nucleation mechanism was mainly dislocation nucleation. The strengthening mechanism of ultra-strength automobile strip was featured by precipitation strengthening and fine-grain strengthening, and the contribution amount of the former was about300MPa.(5) The results of three-point bending experiment and hole expanding experiment indicated experimental steels have excellent cold formability; toughness of experimental steels was determined by low-temperature impact experiment and CTOD experiment. The welding experiment of experimental steels was conducted by using of CO2gas shielded welding arc, and the effects of microstructure, M/A island etc on toughness of welded joint were explained, and the welding process of experimental steels was obtained.Ultra-strength automobile strip had excellent cold formability. Low temperature impact toughnesses of experimental steels and developed product in home and abroad were in the same level, which conform to the performance requirement of EN10149-2, and had good unstable crack propagation. Ultra-strength automobile strip had good weldability. Reducing the effective grain size, improving the large angle grain boundaries proportion, refining M/A islands size and reducing the inclusion content could improve impact toughness of welding joint.(6) Fatigue behavior of ultra-strength automobile strips were studied by the MTS-810material testing system, the S-N curve of experimental steels under the stress ratio R＝-1was established. The fatigue strength ratio of experimental steels (σ-1/Rm) was about0.6, which higher than general steel. The reasons of experimental steels having excellent anti-fatigue performance were ultrafine grain size, nanometer precipitation and granular or short bar carbides.