Research On Hot Forming Technology For Tailored Performance Process Of Ultra-high Strength Steel

The automobile manufacturing industry is one of the important pillar industries of China’s national economy.At present,in line with the development trend of energy saving and emission reduction,promoting green development,achieving the goal of carbon peak and carbon neutral,such as automobile A and B pillars,bumpers and other components need to meet the requirements of high strength and light weight,although the amount of aluminum alloy,magnesium alloy,plastic and composite materials in automobile manufacturing is increasing,but ultra-high strength steel with its high weight reduction potential,high crash absorption energy,high fatigue strength,high formability and other advantages is still the basic structural material of automobile body.Although the use of aluminum alloys,magnesium alloys,plastics and composite materials in automobile manufacturing is increasing,ultra-high strength steel is still the basic structural material for automobile bodies because of its high weight reduction potential,high crash absorption energy,high fatigue strength and high formability.Hot forming technology can make ultra-high strength steel form martensite structure to ensure its sufficient strength and stiffness,and this technology has the advantages of low forming load and low rebound of parts.However,with the introduction of national automotive front impact,side impact,emissions and other mandatory regulations,energy saving,environmental protection and safety has become a necessary element of the vehicle components.Through the reasonable matching of component thinning and high strength,it can not only effectively reduce the weight of the body and reduce fuel consumption,but also ensure and improve the safety and comfort of the model.However,for the hot forming process,thinning and high strength are two factors that deteriorate the formability,which not only makes the body parts easy to crack during the forming process,but also easy to produce excessive rebound,while the uniform performance of ultra-high strength hot forming parts is poor in toughness and plasticity,which cannot achieve efficient crash energy absorption.In response to the demand for lighter weight,improved crash safety and lower manufacturing costs,Tailored Performance Process(TPP)has emerged and attracted great attention,with the potential to customize automotive parts to meet functional requirements.This technology is designed to meet the strength requirements of body safety and energy absorption,and to achieve different mechanical properties in different areas of the hot forming part,which is important for improving crash safety and reducing the mass of automotive components.The partition cooling tailored properties hot forming process is a plastic deformation process of material accompanied by tissue transformation.Compared with conventional hot forming,it couples drastic temperature field changes and microstructure transformation on top of stress-strain field,so it is a highly nonlinear problem with thermal,force and phase change coupling.A systematic study is carried out from various aspects such as theoretical research,process design,simulation evaluation and platform construction,etc.The main contents and innovations are as follows.(1)To construct a 38 Mn B5Nb microstructure field dynamic evolution model,which can accurately characterize the microstructure field evolution law at any moment under complex process,optimize and establish a non-diffusion type/diffusion type phase transition kinetic model,a rapid tempering hardness prediction model for martensite,and a gradient mixed microstructure hardness prediction model.(2)Propose the tailored Performance Process Diagram(TPPD),investigate the relationship between different cooling temperature,tempering temperature,tempering time and other process parameters with microstructure and microhardness with the help of experimental methods such as thermal expansion,in-situ observation,SEM,S/TEM,etc.,and design and formulate the process framework of the partition slow cooling tailored properties hot forming process(TPP-S)and the partition fast cooling tailored properties hot forming process(TPP-F).(3)To establish a unified viscoplastic ontology with coupled dynamic microstructure evolution and complex temperature gradients,characterize the high-temperature rheological behavior of materials in mixed microstructure states,explore the hightemperature rheological stress characteristics of 38 Mn B5Nb steel,and establish a multiphase and mixed tissue ontology model using the strain-compensated Arrhenius equation.Optimize the unified viscoplastic intrinsic model,develop an advanced intrinsic model parameter genetic optimization algorithm,and explore the optimization method of model computational complexity.(4)To build a multi-parameter jet heat transfer coefficient measurement platform with air-return structure,propose a comprehensive heat transfer capacity calculation method for complex flow field characteristics,prefer a jet cooling scheme for flow field characteristics,and propose an accurate measurement scheme and algorithm framework for the heat transfer coefficient of blank with inverse heat conduction problem(IHCP).(5)Establish a numerical simulation model for the whole process of TPP under multiphysics field coupling,provide technical support for the development of TPP advanced process equipment,and reveal the dynamic evolution law and characteristics of the whole process of TPP material temperature field,microstructure field,and stress-strain field.(6)Develop the first test platform for partition cooling advanced hot forming process,realize the preparation of TPP parts with high performance soft zone and very narrow performance transition zone,provide industrial means for the new generation of partition cooling tailored properties hot forming process,and provide new methods and new ideas for the fine tuning of ultra-high strength steel microstructure.The results of the gradient performance U-shaped parts evaluation show that: the core temperature of the TPP-S and TPP-F process blanks after initial cooling is 453.7 ℃and 201.5 ℃,respectively,and the thickness of the blank sidewall after forming is 1.91-1.93 mm,with a thinning rate of 4.5%.the soft region of the TPP-S(900 ℃)process is lamellar pearlite and carbide,the radius of the soft region is about 34 mm,and the Vickers hardness is less than 300 HV.Hard region microstructure slat and needle martensite,Vickers hardness greater than 550 HV,transition region width 24 mm.TPP-F(1000 ℃)process heart soft region for tempering a higher degree of martensite and carbide,parts soft region radius of about 6 mm,Vickers hardness of about 300 HV.hard region for martensite,Vickers hardness of 550 HV or more,transition region for tempering martensite and secondary the transition region is a mixture of tempered martensite and secondary martensite,and the process yields a very narrow transition region of 8 mm.In this paper,the basic theoretical and experimental research is carried out for the partition cooling tailored properties hot forming process,and the numerical results are in good agreement with the experimental results,which proves the correctness of the established phase change kinetic model,advanced intrinsic structure model and comprehensive heat transfer model,verifies the feasibility of the TPP-S and TPP-F processes for the production of lightweight automotive parts with tailored properties,and proposes a production-level the partition cooling tailored properties hot forming production line.The concept will provide a reference for the industrial application of partition cooling customized thermoforming technology and promote further innovation and improvement of advanced thermoforming technology.