| With the increasing pressure of energy and environmental protection,the third generation novel automotive steels with high product of strength and elongation as well as low cost have become a research hotspot of automotive lightweight and new energy vehicle materials.Novel automotive steels and components require both ultra-high strength and good plasticity.At present,High-performance automotive steels rely on residual austenite to increase plasticity.However,This relatively single plasticizing mechanism requires high alloy content or/and complex preparation process,which leads to a series of technical difficulties in their large-scale industrial production promotion and application.Therefore,it is of great significance to study novel steel grades and corresponding strengthen-toughening mechanism to develop ultra-high strength and high product of strength and elongation automotive steels that are easy to produce and promote.In this paper,based on the existing industrial equipment and technical conditions,a novel automotive steel was designed,and its strengthen-toughening mechanism and flexible application of cold and hot forming were explored.The research work adopted experimental apparatus and analytical methods of software simulation calculation,optical microscope（OM）,scanning electron microscopy（SEM）,energy dispersive spectroscopy（EDS）,electron backscatter diffraction（EBSD）,transmission electron microscopy（TEM）,X-ray diffractometer（XRD）,vickers hardness test,room temperature tensile test,erikson cupping test and so on.In the research process,firstly,through literature analysis and software calculation,a novel automobile steel 22Mn2Cr was designed,and its phase transformation law was studied systematically;Under the continuous annealing process condition of existing industrial production,the strengthen-toughening mechanism of the 22Mn2Cr novel automobile steel plate with ultra-high strength as well as high product of strength and elongation was analyzed;The effects of annealing temperature,aging temperature and pre-crystallization before austenitization on the microstructure and properties of cold-rolled automotive steel sheets during continuous annealing were investigated,and the key process parameters for regulating the different matching of strength and plasticity of cold-rolled automotive steel plates were derived;The simulated hot forming experiments were carried out with continuous annealing cold rolled sheets as raw materials,the hot forming process parameters to obtain the optimum product of strength and elongation were given;the strengthen-toughening mechanism of hot-formed parts was analyzed and the resistance to surface oxidation,cold bending and welding properties of the novel automotive steel after hot forming were evaluated.The novel 22Mn2Cr automotive steel designed has excellent matching of strength and plasticity,and realized the flexible application of cold and hot forming.This research work provides scientific basis for the flexible application of cold and hot forming of 22Mn2Cr novel automotive steel,which promote the high-quality development of automobiles lightweight.The main work and results are as follows:（1）The composition design and phase transformation law of novel automotive steel were studied.Using 22Mn B5 as a reference,the novel 22Mn2Cr automotive steel was designed by using chromium to improve hardenability and the qualities of second-phase particles that can be formed,while considering the cost,using Cr-Mn alloying,and combined with software simulation calculation,the target value of composition was determined.The results of the study on its phase transformation law show that the Ac1,Ac3and Msof 22Mn2Cr tested steel is 879°C,727°C and397°C,respectively.Its critical cooling rate is 10°C/s,which has good hardenability.It provides a basis for making reasonable continuous annealing process plan and hot forming process plan to control the microstructure and properties of tested steel.（2）The strengthen-toughening mechanism of 22Mn2Cr novel automobile steel cold rolled sheet with ultra-high strength（tensile strength>1000 MPa）and high product of strength and elongation（>20 GPa%）under typical continuous annealing process was studied.The results show that the strengthen-toughening mechanism of 22Mn2Cr novel automobile steel is different from that mechanism mainly relying on the retained austenite to enhance plasticity,which is the compound effect of multiple reinforcing and plasticizing mechanism by the interaction between the second phase particles and dislocations,microstructure refinement,ultra-fine high dislocation density ferrite and multiphase structure regulated by granular bainite,martensite and a small amount of retained austenite.（3）The effect of continuous annealing process parameters and pre-recrystallization on the microstructure and properties of 22Mn2Cr novel automobile steel cold rolled sheet was studied.The results show that the effect of annealing temperature and aging temperature on the microstructure and properties of cold rolled steel sheet is obvious.Ultra-high strength and good plasticity matching can be achieved at higher partial austenitizing annealing temperature and lower complete austenitizing annealing temperature.At the same time,the high product of strength and elongation of cold rolled steel sheet can be further improved by proper pre-recrystallization before austenitizing during continuous annealing heating process.The continuous annealing cold rolled sheet with tensile strength of 1246 MPa and product of strength and elongation of 19.3 GPa%,and tensile strength of 1062 MPa and product of strength and elongation of 23.9 GPa%can be obtained by adjusting the appropriate continuous annealing process parameters and pre-recrystallization treatment in the continuous annealing process,which is obviously better than Q&P series steel.（4）The hot forming application of 22Mn2Cr novel automobile steel cold rolled continuous annealing sheet was studied.After optimized hot stamping forming process,the different positions of 22Mn2Cr steel hot-formed parts have reached the tensile strength above 1500 MPa,the product of strength and elongation is 17.6 GPa%～18.5 GPa%,and the product of strength and elongation is significantly superior to 22Mn B5.The strengthen-toughening mechanism is mainly the coupling effect of fine martensite obtained at low austenitizing temperature,the interaction between the dislocations and residual second phase particles in continuous annealing plate,and the solution carbon content in martensite.Furthermore,after hot forming,22Mn2Cr novel automotive steel has excellent surface oxidation decarburization and cold bending properties,and good weldability.The above research results show that:The designed and developed 22Mn2Cr novel automobile steel can obtain the mechanical properties of ultra-high strength and high product of strength and elongation under the traditional continuous annealing process and hot forming conditions.It can be used for cold forming and hot forming,and can realize the flexible application of cold and hot forming.It has significant application value to promote the development of lightweight automobile safety and green manufacturing in automobile industry. |
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