Research On The Control Of Stretchflange Performance Of High Strength Steel And Its Precipitate Mechanism

As one of the main materials for automobile lightweight,highstretch-flange steel has been paid more and more attention in recent years.It has been widely used in automobile chassis structural parts and walking parts with high strength?high elongation ratioand good stretch-flange performance.This study is based on the joint research and development project of Wuhan Iron and Steel Company and Wuhan University of Science and Technology to develop 590 MPa and 780 MPa highstrength and high stretch-flange steel.This article summarized the development history at home and abroad,composition and process design characteristics of high stretch-flange steels,the factors affecting the performance of stretch-flange,and the development direction of high stretch-flange steels.The ways to improve the stretch-flange performance of steel plate were investigated focusing on steel purity,microstructure and processing methods.In this paper,the effect of steel on high stretch-flange steel was investigated and it is determined that there is a genetic relationship between the center segregation of the slab and the band structure of the steel plate,resulting in a decrease of the performance of stretch-flange of the steel.The adverse effect of the TiN inclusions on the the performance of stretch-flangeof the steel plate and crack initiation and propagation was investigated.The three-dimensional morphology and three-dimensional structure of TiN were observed and the formation mechanism was discussed.The influencing factors of TiN inclusion precipitation time,precipitation location and TiN size were calculated by thermodynamic formula,which provided guidance for improving the performance of highstrength and high stretch-flange performance.According to the results of steel research,the composition design of FB590 and FB780 was adopted,and the high temperature thermoplasticity of high Nb and Ti steel was studied to provide guidance for its continuous casting process.The CCT curve was plotted based on F-master,and the cooling process was thermally simulated.The twostage cooling process was determined to obtain fine grain and phase transformation strengthening effects,which provided guidance for the hot rolling cooling process.The effect of the micorstructure and the processing method on the hole expansion ratio and cracking mechanism was studied.The results show that the stretch-flange performance of the F+Bstructure with the same composition and tensile strength was 2.8 times than that of the F+M providing guidance for the microstucture design.The hole expansion rate of the steel for the center hole processed by waterjet/drilling was 3.3 times than that by the punching method,providing guidance for the use of material processing.The research on cracking mechanism shows that TiN inclusions in steel rupture themselves or crack at the inclusion/matrix interface and become the source of cracks.Factors such as grain refinement and turning in the vicinity of round holes caused by processing are not conducive to preventing crack propagation.In order to obtain high-strength and high stretch-flange steels with strength of 780 MPa or even higher,the effects of the heating and coiling processes on the second phase solid solution and precipitate and the austenite grain were investigated.The results show that the combined effect of Nb-Ti increases the total solution temperature of Nb by about 50°C.The size and number of the second phase particles in the steel are also different due to different coiling processes,and the precipitation strengthening effect is also different.The effect of Mo element on the size,quantity,composition and formation mechanism of the second phase precipitates were discussed.Mo element was beneficial to obtain NbTi-Mo composite precipitates with smaller size and larger quantity which laid foundation of FB780 with stable stretch-flange performance.According to the above research results,an ultra-high-strength and high stretchflange steel was invented.Through the composition and process design,single quasipolygonal ferrite or quasi-polygonal ferrite+acicular ferrite structure was obtained,which overcomes the deformation asynchronous phenomenon caused by the hardness difference between the F / B two-phase structure in the traditional high-stretch flange steel and improves the stretch flange performance.The experimental conclusions were verified on a large production line.The obtained FB590 and FB780 have stable strength and good stretch-flange flange performance.They are used in the manufacture of fuel tank brackets and beams of commercial vehicles with good formability and significant weight reduction effects.