Study On Microstructure And Plasticity Of 1000MPa Hot Rolled Ultra-high Strength Steel

Economic development has made steel structures large-scale and integrated,and the use of high-strength steel can save raw materials.The use of high-strength steel can not only reduce the consumption of raw materials,but also meet the requirements of steel structure for good plastic toughness and welding performance.In this paper,the effects of two processes,as Q&P heat treatment and controlled rolling and controlled cooling,on the evolution of microstructure,mechanical properties and toughening of 1000 MPa highstrength longitudinally welded pipes were studied.In order to increase the elongation of straight seam welded pipe steel to more than15%,the C content was reduced on the basis of the chemical composition of Q&P steel and the F-Q&P process is adopted to hot-rolled low-carbon ultra-high strength straight seam welded pipe steel with and without B addition.The B-containing steel obtained the best comprehensive properties after final rolling at 840 ℃,relaxation at 700 ℃ for 4 s,and partitioning at 370 ℃ for 60 s.Although the segregation of B on the grain boundary changes morphology of ferrite and improves the strength and toughness of the steel,it leads to the cracking of the weld of the steel,which is found by the flattening test.In cooperation with a steel mill,the composition of microalloyed steel was designed and two-stage controlled rolling and ultra-fast cooling were used to obtain three kinds of straight seam welded pipe steels with different Ti contents that meet the performance requirements.One of the steels fails to meet the standard,but the performance of the other two steels meet the requirements and the elongation of the welded pipe is also about 15%.The strengthening mechanism of steel with higher Ti content is mainly bainite strengthening + precipitation strengthening of Ti C.The smaller size granular bainite and M/A are uniformly distributed in the microstructure and the toughness of the welding heat affected zone of the steel is excellent.After being controlled rolling,the yield strength and tensile strength of the sheet reach 938 MPa and 986 MPa,respectively.Its elongation reaches a maximum of 18.6%,and no cracks are found after being flattened.