| With the rapid development of steel framed buildings, upgrade and update of construction steels are imperative in order to overcome problems such as resource, environment and energy, etc. Therefore, it is very necessary to continuously develop green construction steels with excellent mechanical property, good weldability and low cost. Moreover, technological independent innovation is also of significant importance to meet sustained increasing market demands.According to780MPa grade low yield ratio construction steel, excellent comprehensive properties (high strength, low yield ratio, high toughness and plasticity, and so on) could be obtained by means of the microstructural control from ferrite/pearlite (Q235-Q460) to non-equilibrium bainite/martensite. A new technological process for780MPa grade low yield ratio construction steel was studied in this paper by using transformation control as the means of obtaining proper composite microstructure, and NG-TMCP and the intercritical heat treatment as the processing routes. The main original works of this paper are present as follows:(1) Based on microstructural evolution and characteristic of super-cooled austenite under the continuous cooling condition, the process of "controlled rolling+relaxation+UFC" for780MPa grade low yield ratio construction steel was presented creatively.The increase of deformation shortened the incubation period of bainite transformation, and CCT curve was shifted upward correspondingly. In austenite non-recrystallization region, the decrease of deformation temperature was beneficial to the formation of new phase and the refinement of crystal grain. With the increase of continuous cooling rate, the general trend of microstructure evolution was:PF+GS/GBâ†'GS/GBâ†'GS/GB+LBâ†'LB. And the bainite starting temperature Bs showed decreasing tendency, meanwhile, the microstructure was transformed into lath structure with the more fine substructure and higher hardness. Increasing deformation or decreasing deformation temperature promoted the formation and refinement of M-A island and leaded to the disorder distribution of M-A islands with irregular shape or circle granular. Characteristics of carbon-poor area and carbon-rich area were the important factors to determine the formation of bainite ferrite and M-A island. Based on this, the novel "controlled rolling+relaxation+UFC" production process for780MPa grade low yield ratio construction steel was presented creatively in this paper.(2) Microstructure evolution behaviors with HOP and "controlled rolling+relaxation+UFC" were studied by the thermal simulation testing machine, and effects of related technological parameters on microstructure characteristics were also analyzed and discussed.In HOP process, bainite transformation amount could be regulated by finished cooling temperature and holding time. Increasing the heating rate of on-line tempering was beneficial to obtain high density dislocations and finely dispersed precipitates, and contributed to the formation of abundant M-A islands with small size. In "controlled rolling+relaxation+UF’ process, with the decrease of finished cooling temperature, the microstructure was transformed into lath substructure, and the volume fraction of M-A island increased at first, then decreased. The increase of cooling rate contributed to the formation of lath bainite ferrite, the decrease of M-A island content, and the refinement of effective grain size. Increasing final deformation temperature or decreasing deformation improved the orderly arrangement of bainite ferrite laths, and made prior austenite grain boundaries more clear. Large carbon-poor area and carbon-rich area were formed preferentially during relaxation process after deformation in the non-recrystallization, which contributed to obtain low yield ratio due to the composite microstructure including bainite ferrite and M-A island. Subsequently, the application of UFC avoided excessively coarse substructure, by which the microstructure was strengthened and toughened.(3) The relationship among process parameter, microstructure and comprehensive performance was investigated by hot rolling experiments based on "controlled rolling+relaxation+UFC" new technology, and the control mechanism of low yield ratio and microstructure characteristics of M-A island were explored.A new process of "controlled rolling+relaxation+UFC" was put forward to manufacture780MPa grade low yield ratio construction steel. And comprehensive mechanical properties of the steel plates with12mm,20mm and40mm thickness could meet the related standard completely. The phase transformation driving force was raised by the application of UFC due to the increase of chemical potential fluctuation. UFC technology was found to be beneficial to enhance transformation strengthening and refine microstructure. Plenty of M-A islands could be formed in the central area of40mm thickness steel plate during an approximate continuous cooling process caused by UFC, and then the multiphase including M-A hard second phase and bainite ferrite soft matrix was achieved ultimately. Otherwise, the microstructure near the surface was mainly composed of lath bainite, and a small amount of M-A islands by means of granular bainite were present in microstructure. UFC technology ensured high strength and low yield ratio for the thick steel plates.(4) Effects of process parameters on microstructure and mechanical properties were investigated according to RQ-IQ-T and TMCP-IQ-T, and the influence mechanisms of intercritical quenching on yield ratio and Charpy impact fracture behavior were also clarified.A composite microstructure of banded ferrite and tempered martensite was obtained by adding the intercritical quenching to the conventional heat treatment. When intercritical quenching temperature was780℃for30min, comprehensive mechanical properties could come up to requirements of780MPa grade low yield ratio construction steel. However, the microstructure was composed of massive ferrite and tempered martensite for TMCP-IQ-T under the most favorable intercritical quenching conditions (780-800℃for30min). The reversed austenite formed during tempering was one of the most important characteristics, which nucleated and grew in the fresh martensite laths obtained from intercritical quenching process. The reversed austenite had the coherent orientation relationship with the matrix, and the result was in agreement with K-S relationship. Two crack propagation path models were observed in banded ferrite:along the long axis direction, and across the grains and corresponding interfaces. However, cracks in massive ferrite propagated in a transgranular mode. The higher impact toughness for RQ-IQ-T was attributed mainly to the more ferrite-martensite interfaces with high-angle misorientation, which exhibited effective resistance to the crack propagation.(5) Aiming at typical steels with various properties, the relationships among microstructure, mechanical property, strain-aging sensitivity and low-cycle fatigue were investigated, and main influence factors of seismic behavior and mutual concurrent effects were analyzed and discussed.Plenty of dislocations were pinned by interstitial solute atoms through Cottrell atmosphere during the strain aging treatment, which caused the increase of strength and the decrease of plasticity. The low-cycle fatigue life was mainly determined by plasticity of the steel, moreover the strength was another important factor. Under strain amplitudes of4%and6%, excellent cyclic hardening and cyclic stabilization could be easily obtained for the steels with high strength and low yield ratio. High-strain low-cycle fatigue performance, which substituted for the static loading strength, could be used as a basis of seismic design for steel buildings in earthquake regions. And the optimum comprehensive performances including strength, toughness and ductility acquired by reasonable chemical composition and proper microstructural control were indispensable for construction steels. |
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