Research On 2000 MPa Grade High Produce Of Strength And Elongation Of Press Hardening Steel

The automotive body steels have been developed toward ultra-high strength,high product of strength and elongation and easy forming due to high safety,low emission and cost.However,as well know the formability of steel is become bad by increasing it's strength.Therefore,the problems,such as cracking,rebound and the parts failing to satisfy the size requirement,were prone to occur in the cold forming process.Under this background,the hot stamping steels have developed in order to solve above problems.The researchs on hot stamping steel at home and abroad mainly focuses on the composition,structure and hot stamping process of 1500 MPa grade products.From the published data of hot stamping stees indicated that it's product of strength and elongation is just 5¹0 GPa·%,which it can not meet demand of lighweigthing and impacting absorbed energy related to safety.For higher strength products,new hot stamping steel with product of strength and elongation 15 GPa·%,there are few public report on the composition,hot stamping process,strengthening and toughening mechanism.According to the thermodynamic calculation and literature analysis,new hot stamping steel with different carbon and manganese contents was designed.The austenite transformation law,hot stamping process parameter and strengthening and toughening mechanism are studied in the laboratory.The optimal composition and the energy-saving hot stamping process of the new hot stamping steel were proposed.The press hardening steel developed in this work not only has an ultra-high strength of 1800²100 MPa,but also has the product of strength and elongation of 15 GPa·%or more.The problems of high manufacturing cost and poor formability of the third-generation advanced high strength steel can be solved by using this new steel,but also meets the requirements of high crash safety and energy-absorbency of the automobile industry,which is of great significance to the improvement of automobile body safety and industrial upgrading.The continuous cooling transformation curve?CCT curve?and the supercooled austenite isothermal transformation curve?TTT curve?of the experimental steels with different carbon contents were measured by Formast-F automatic dilatometer.The results showed that as the carbon content increases from 0.30%to 0.34%,the starting temperature A_c3 of the transformation of pro-eutectoid ferrite to austenite decreased from793°C to 788°C,and the martensite starting temperature M_s decreased from 369°C to353°C,which was consistent with the thermodynamic calculation.Moreover,with the increase in carbon content,the ferrite,pearlite and bainite transformations of the experimental steels are significantly delayed.And under the same supercooling,the incubation period of the isothermal phase transformation of the supercooled austenite in the steel with a carbon content of 0.34%is longer.The critical cooling rate of the press hardening steel developed in this work is 10°C/s,and the steel has good hardenability.The microstructure evolution and performance characteristics of the two experimental steels with different carbon contents prepared by different hot stamping processes were systematically studied by means of scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,optical microscopy?OM?,electron backscattered diffraction?EBSD?and X-ray diffraction?XRD?.The results showed that in order to generate sufficient kinetic conditions to drive austenite grain growth,complete carbide dissolution and homogenization of elements during the hot stamping process,for the studied steel with 0.30%carbon,the heating conditions should be holding at 900°C for 5 min,and above,while holding at 850°C or higher for 5 min and above is sufficient for the studied steel with 0.34%carbon.Through the grain refinement effect of niobium containing precipitates,the austenite grain size is ensured to be in the range of 11 to 12grade,which lays a foundation for achieving fine martensite lath bundles and isotropic bundles after mold press quenching.The study on the mechanism showed that carbon content plays a decisive role in the strength and hardness of the martensite of the hot-stamped steel developed in this work,and the manganese content should be optimized.The optimal manganese content should be controlled at about 1.22%.It is necessary to adopt the appropriate hot stamping process system and reasonable composition design.In addition,by a combination of phase transformation strengthening,grain boundary strengthening,solid solution strengthening,fine martensite lath bundles,a large number of small-angle grain boundaries,and low?coincidence site lattice?CSL?grain boundary control,the hot-stamped steel with an ultra-high strength of 1800 to 2100 MPa and a high product of strength and elongation of 15 GPa·%or more is achieved.Compared with the conventional 1500 MPa grade press hardening steel,the new grade press hardening steel developed in this work,0.34%C-1.22%Mn composition system with a certain amount of other alloying elements,has ultra high strength and high product of strength and elongation with the more energy-saving hot stamping process,i.e.holding at 850°C for 5⁸ min,while the conventional 1500 MPa grade press hardening steel should be held at 930⁹50°C for 5⁸ min during the hot stamping process.To meet the requirement of our country's green,energy saving.