Development Of 550/690MPa High Strength-toughness Construction Steels With Low Yield Ratio And Research Of Its Anti-fracture Performance

550/690 MPa high strength-toughness steels were designed by low-carbon composition and adding proper amount of alloy elements such as Mo, Cr, Cu, Nb, Ti and so on. In order to obtain bainite and proper amount of ferrite, and ensure that the steels have excellent mechanical properties, weldability and lower yield ratio. Thermo Mechanical Control Process was employed. The analyses were carried out to investigate the relationships of composition-technology-microstructure-properties, especially the influences of cooling process, tempering technology, the sizes and quantities of inclusions, the sizes of M/A inlands and void initiation on microstructure, mechanical properties, and anti-seismic performance.The chemical compositions, rolling and cooling process of steels were decided in the lab,s investigation. The steels were designed by low-carbon composition and proper Mo, Cr, Cu, Nb, Ti. At the first rolling stage, the rolling process was finished in the austenite recrystallization region completely, and finish rolling temperature was no more than 1000℃. At the second rolling stage,the rolling process was finished in austenite non-recrystallization region, and start rolling temperature was no more than 980℃, finishing temperature was controlled in the range of 800-880℃, the reduction ratio of last three passes was no less than 35%. The start cooling temperature was at 750-800℃, and finish cooling temperature was at 300-550℃.Industrial trial was carried out according to the above, the steels have high strength, high toughness, low yield ratio and good mechanical properties. The microstructures are bainite and a small amount of ferrite and M/A islands with high dislocation density, there are more small spherical precipitates in bainite and ferrite matrix. The microstructure characteristics could make the mechanical properties of steels excellent.The results of welding and thermal simulation show that the steels have a lower weld cold crack and good low temperature toughness in heat affected zone. With the increase of heat input energy, the toughness in coarse grain grain heat affected zone decreases, the microstructure of coarse grain heat affected zone coarsen gradually. When the heat input energy is no more than 40 k J/cm, the coarse grain heat affected zone has good low temperature toughness. So the steels have a good weldability and could be applied to the constructional engineering.Influence of tempering on microstructure and mechanical properties of steel with low yield ratio were analyzed. The results show that tempering temperature has a decisive influence on the microstructure and properties of steels with low yield ratio. Before tempering, the microstructure of steel is composed of fine lath and granular bainite, small fraction of ferrite and some M/A constituent. With increase of the tempering temperature, lath bainite merges gradually, lath width increases, M/A constituent is decomposed, and the tensile strength and impact toughness decrease, while the yield strength remains stable, so the yield ratio increases. The M/A constituent between laths or at the bainite/ferrite boundary exists in form of lump and stringer. The high strength steel with good toughness and low yield ratio is obtained at lower tempering temperature.Start cooling and finish cooling temperature have an obvious effect on the volume fractions and sizes of each phase in steels, thus affecting the mechanical properties of steels. When start cooling temperature decreases, the volume fractions of ferrite and M/A islands increase, the strength and yield ratio of steels decrease. Decreasing finish cooling temperature increases the volume fractions of M/A islands and refines bainite microstructures, and it could improve the strength of steels and increase yield ratio. The yield strength and the yield ratio decrease and the tensile strength has not an obvious regulation when decreasing start and finish cooling temperatures at the same time. We can know from the test result the volume fractions of ferrite are the most important effect factor for the yield ratio. in order to excellent mechanical properties, during industrial trial, the start cooling and finish cooling temperature should be 750-780℃ and 260-440℃ respectively.Appling a serials of tensile test of different necking degrees, quantities, sizes and volume fractions of void were tested, and the influence of the sizes of calcium treatment inclusions and M/A islands on mechanical properties, yield ratio and anti-seismic performance were obtained. The results show that the void initiation has a significant effect on mechanical properties. There is no relationship between yield strength and void initiation. The larger the size of inclusions is, the earlier voids occur, the lower tensile strength and uniform elongation are, the higher yield ratio is. With increase of the sizes of inclusions and M/A islands, the total elongation increases, the instability process is longer, and this enhances the seismic behavior of steels.