| With the rapid development of automobile industry,the proportion of advanced high-strength steel in car body is higher and higher.The hot-rolled complex phase steel has high strength and good elongation,which can meet the requirements of mechanical properties of automobile parts.In addition,hot-rolled complex phase steel also has excellent hole expanding performance,that is,extended flange performance,which makes hot-rolled complex phase steel have the ability to manufacture complex parts conditions.Due to the complex and fine microstructure of complex phase steel,it is difficult to control stably in the production process,so the application of complex phase steel is greatly limited.At the same time,there is a lack of systematic research on the initiation and propagation of cracks in the process of hole expanding.In this paper,the hot rolling process and expanding performance of 800MPa grade hot rolled complex phase steel are systematically studied from the aspects of alloy composition design,controlled rolling and cooling parameters,precipitation behavior of second phase particles,influencing factors of hole expanding performance and cracking mechanism of expanding cracks,so as to provide reference for industrial production.The main work and results are as follows.(1)According to the strength requirements of hot-rolled complex phase steel,the experimental steel based on C-Mn-Si element,supplemented with Al element and micro alloy elements such as Ti,Nb and V was designed.The static and dynamic CCT curves of the test steel were obtained by continuous cooling transformation law.The results show that the composite structure mainly composed of granular bainite can be obtained in a wide range of cooling rate and temperature by reasonable composition design,which can provide reference for subsequent hot rolling process.(2)The effects of finishing rolling temperature and coiling temperature on microstructure and properties of hot rolled complex phase steel were studied.The results show that with the increase of finishing rolling temperature,the microstructure gradually transforms into granular bainite and quasi polygonal ferrite,and lath bainite appears in the microstructure at lower finishing temperature;with the increase of coiling temperature,the microstructure of test steel changes from granular bainite and quasi polygonal ferrite to polygonal ferrite and carbide.The strengthening mechanism changes from fine grain strengthening and dislocation strengthening to precipitation strengthening.(3)The equilibrium dissolution and precipitation behavior of the second phase particles in austenite and bainite were systematically analyzed.The results show that TiN is mainly precipitated in the high temperature stage before rolling,and TiC precipitates gradually with the decrease of temperature;a large amount of NbN and a small amount of NbC and VN are precipitated in NbV series complex phase steel during hot rolling,and VC and NbC precipitate a little in cooling stage after rolling.A large number of nano precipitates precipitated regularly in Ti and NbV complex phase steels.(4)The effect of different microstructure on the hole expanding performance was studied.The results show that the increase of bainite ratio is beneficial to increase the hole expansion rate,and the high density dislocation and large proportion of M/A islands in the grain boundary reduce the formability of the steel.KAM data analysis shows that the strain is concentrated near the M/A island.With the increase of coiling temperature,the selectivity of bainitic ferrite is enhanced,and the hole enlargement rate is also increased.(5)The results show that the proportion of small angle grain boundary increases with the increase of deformation;the grains in the favorable position begin to glide and deform first,while the grains in the non favorable position rotate first and then slide deformation;the microstructure is elongated during deformation,and there are slip bands in the grains. |
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