| The development and application of the ultra-high strength hot stamping steel(UHS)is the important way of the automobile lightweight,energy saving and emission reduction.The door anti-collision beam is one of the key components of the automobile lightweight.It can improve the door stiffness in side impact and reduce the amount of collision intrusion,and effectively reduce the injury of collision to the members in the automobile.At present,the anti-collision components inside the door are mainly anti-collision rods made of 22 Mn B5 steel,with a tensile strength of about 1500 MPa to1600 MPa.Research and development of the higher-level UHS and its application in anti-collision parts,and mastering the internal relationships between microstructure evolutions and mechanical properties during hot stamping are of great significance to the optimization control of UHS hot stamping process and the improvement of the automobile material lightweight level.Taking the 1800MPa UHS produced by a steel plant(hereinafter referred to as UHS-1800)as the object,this paper studies the relationships between the hot stamping process,microstructure evolutions and mechanical properties,and formulates the optimum hot stamping austenitizing process,and carries out the trial production of the door anti-collision beam.The prediction model of the austenite grain growth and the hermal mechanical phase transformation model of door anti-collision beam are proposed.Then UHS-1800 is applied to the door bumper beam.The ultra-high strength hot stamping steel with 2000 MPa grade(hereinafter referred to as UHS-2000)is designed,and its microstructure evolutions and properties are studied.The main conclusions are as follows.(1)The effects of the austenitizing temperature and holding time on the microstructure evolutions and mechanical properties of UHS-1800 in hot stamping process are studied by thermodynamic calculations and experiments.With the increase of the austenitizing temperature and holding time,the martensite becomes coarser and the tensile strength decreases gradually.The suitable austenitizing process of UHS-1800 is determined: the temperature is 870 ℃ and the holding time is 3min.At this time,the tensile strength is 1882.76 MPa and the strength-ductility product is 13.07 GPa·%,which is 2.12 GPa·% higher than that of 22 Mn B5.When the austenitizing temperature increases from 870 ℃ to 990 ℃,the size of the martensite lath increases from 2.27μm to 3.44 μm,and the tensile strength decreases from 1871.59 MPa to 1801.42 MPa.When the holding time increases from 3min to 9min,the size of the martensitic lath increases from 2.37 μM to 3.05 μm,and the tensile strength decreases from 1853.15 MPa to 1788.91 MPa.(2)The austenitizing heat preservation in the hot stamping process is refined and defined as two stages of the heating and soaking.On the basis of considering the coarsening of the second phase particles formed by Nb,V and Ti and the pinning effect on the grain boundary at the same time,an austenitizing grain growth prediction model suitable for UHS-1800 hot stamping process is proposed.The model can quantitatively predict and calculate the influence of the austenitizing temperature and holding time on the austenite grain size.The maximum error between the predicted value of the model and the experimental measured value is 5.35 %,which provides a basis for the optimization and formulation of the hot stamping parameters of UHS.When the holding time is 5min and the austenitizing temperature increases from 900 ℃ to 990 ℃,the grain size of the original austenite increases from 12.60 μm to 20.88 μm.The second phase particle radius increases from 15.23 nm to 25.36 nm,and the grain size and the second phase particle radius change obviously.When the austenitizing temperature is930 ℃ and the holding time is increased from 3 min to 9 min,the grain size of the original austenite increases from 13.52 μm to 17.22 μm.The second phase particle radius increases from 15.72 nm to 20.50 nm,and the grain size and the second phase particle radius also increase.(3)The test device of the high-temperature dynamic friction coefficient of UHS is designed and assembled,and the coefficients of UHS-1800 are measured,and the influence law of the temperature and pressure on the coefficient of UHS-1800 is proved.Based on the real-time dynamic friction coefficient varying with temperature and pressure,the thermal force phase transition model of ultra-high strength door anti-collision beam during hot stamping is established by using Deform software,and the evolution behavior of temperature,stress and microstructure under different process conditions in the hot stamping process of door anti-collision beam is quantitatively studied.UHS-1800 door anti-collision beam is trial manufactured under the suitable austenitizing process conditions.The comparative experimental study shows that the tensile strength at different positions of the anti-collision beam reaches more than 1800MPa,and the total elongation and strength-ductility product at the corner(1# position)are relatively high.(4)Based on the JMat Pro calculation platform and the tensile strength test results of 22 Mn B5,22 Mn B5Nb V and UHS-1800,an prediction modified model of the tensile strength of UHS is proposed.The accuracy of JMat Pro software in predicting and calculating the tensile strength of UHS is improved.The maximum error of UHS-1800 tensile strength predicted by the model is 0.35%,which provides a basis for the design and development of other UHS.(5)Based on thermodynamic calculation and modified model prediction,2000 MPa UHS are designed and developed.The thermal phase transformation model of UHS-2000 hot stamping process is established,and the evolution behaviors of temperature and phase microstructures in UHS-2000 hot stamping process are simulated.The microstructures and properties of UHS-2000 are experimentally studied,and the effects of Nb and V alloy elements on the microstructures and properties of UHS-2000 are found out.Nb element can obviously refine the quenching microstructures.V element can improve the ability of anti microcrack.The addition of Nb and V elements is beneficial to grain refinement and mechanical properties improvement.The maximum tensile strength and strength-ductility product of the designed and developed UHS-2000 can reach 2030.62 MPa d 16.12 GPa·%,respectively.The results provide a theoretical basis for the composition design,microstructure and property control of UHS. |
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