Microstructure And Properties Control And Hydrogen Induced Delayed Cracking Behavior Of Ultrahigh Strength Hot-stamped Steel

Under the background of peak carbon dioxide emissions and carbon neutrality,the development of automobile lightweight urgently needs to develop ultrahigh strength automobile steel and improve its service performance.As one of the safety parts with the highest strength level in automobile parts,hot-stamped steel effectively solves the problem that high-strength automobile steel is challenging to be cold-stamped.However,as the strength of hot-stamped steel increases,it inevitably faces the issues of poor plasticity and hydrogen-induced delayed cracking during its service.Additionally,the higher the strength level,the more severe the hydrogen embrittlement(HE)sensitivity.The HE of high-strength automobile steel often happens suddenly under unpredictable circumstances,which might lead to severe accidents.Therefore,the interaction between microstructure,mechanical properties and hydrogen-induced delayed cracking behavior of hot-stamped steel was intensively studied.And the strengthening mechanism and HE mechanism are revealed.Finally,hot-stamped steel has high strength,sufficient plasticity and excellent resistance to HE.It is of great engineering significance to realize the automobile lightweight and promote the steel and automobile industry to achieve the goal of double carbon.The composition optimization design is based on the chemical composition system of 1500-2000 MPa hot-stamped steel with the purpose of high strength,excellent plasticity and high resistance to HE.The nucleation and growth of austenite and the transformation from austenite to martensite are directly observed by high temperature laser confocal microscope.The thermal expansion equipment is used to study the transformation law and calculate the transformation point of the experiment steel,which provides guidance for setting important heat treatment process parameters.Study on different heat treatment process parameters to realize the microstructure and properties control,and obtain the process parameters of excellent comprehensive properties of the experiment steel.Scanning electron microscopy,transmission electron microscopy,electron backscatter diffraction,X-ray diffractometer and other microstructural characterization techniques to systematically analyze the microstructure and strength mechanism of ultrahigh strength hot-stamped steel.The preparation of ultrahigh strength hot-stamped steel is realized,which provides reference for the development of other advanced high strength steels.The main research contents and conclusions are as follows:The experiment steel(40MnCr2NbV)has excellent hardenability with the critical cooling rate(0.7℃/s)for transforming to martensite after completing austenitizing.The microstructure of hot-rolled steel is composed of ferrite and a large amount of lamellar pearlite,which has a tensile strength of 817 MPa,a yield strength of 485 MPa,and a total elongation of 16.4%.The microstructure of cold-rolled steel consists of deformed and broken pearlite and fibrous ferrite.The research on the bell-type annealing process of experiment cold-rolled steel plate uses the annealing temperature of 730℃ and keeps for 4 h.The microstructure of the annealed plate is composed of spherical carbide,ferrite and intergranular martensite,and its yield strength is significantly reduced to 295 MPa,the tensile strength is 655 MPa,and the total elongation is 26.0%.Low yield strength,high tensile strength and excellent total elongation are conducive to the pre-cold stamping of hotstamped steel substrates.The microstructure and properties of ultrahigh strength hot-stamped steel were studied with different quenching methods with water quenching(WQ),flat die quenching(DQ),oil quenching(OQ)and air cooling(AQ).The microstructures of WQ and DQ specimens are mostly block quenched martensite,showing the plasticity is low.The quenching speed of OQ is relatively slow.And the microstructure of the OQ sample is composed of a large amount of tempered martensite and a small amount of quenched martensite by elaborately controlling auto-tempering.The best comprehensive mechanical properties are obtained:tensile strength of 2428 MPa,yield strength of 1457 MPa,and total elongation of 8.4%without additional tempering.With the decrease in quenching speed,the microstructure of the AQ sample is composed of tempered martensite(the volume fraction is 91%)and massive retained austenite(the volume fraction is 9%).Subsequently,the strengthening mechanism of commercial 2000 MPa(34MnBV)and experiment steel was studied.The dislocation strengthening accounts for 41%(34MnBV)and 62%(40MnCr2NbV)of the yield strength of the hot-stamped steel.The dislocation strengthening is the major strengthening mechanism,followed by second-phase strengthening.Carrying out tempering heat treatment process to control the microstructure and properties of the as-quenched experiment steel.The dislocation density gradually decreases and the number of carbides gradually increases with the increase of tempering temperature.The morphology of carbides coarsens from needle-like to rod-like,which consumes part of the solid solution carbon content in the martensite,leading to a downward trend in tensile strength.Additionally,when the tempering temperature rises to 300℃,the HE resistance is significantly improved with the fracture strength and plastic loss rate after pre-hydrogen charging reduced to 1.3%and 13.2%,respectively.The HE resistance of the sample shows an increasing trend with the increase in tempering temperature,and the dislocation density shows a decreasing trend.It is concluded that reducing the dislocation density of ultrahigh strength hot-stamped steel can reduce its HE sensitivity.The hydrogen-induced delayed cracking behavior of ultrahigh strength hot-stamped steel was studied by hydrogen permeation,slow strain rate tensile test after pre-hydrogen charging and hydrogen thermal analysis tests.And we proposed measures to improve the HE sensitivity of ultrahigh strength hot-stamped steel.The behavior of hydrogen induceddelayed cracking of hot-stamped steel is mainly affected by the diffusion and aggregation of diffusible hydrogen,which is bounded by dislocations and grain boundaries.The results show that the movement rate of dislocations in the hot-stamped steel is far less than the diffusion rate of hydrogen atoms.It shows that the movable dislocations could carry hydrogen atoms to move and gather at grain boundaries,promoting the generation and propagation of hydrogen induced cracks.The movement of dislocations was pinned by refining grains and the second phase particles,which could increase the resistance to HE of hot-stamped steel.In the 40MnCr2NbV steel,the activation energy of grain boundary and dislocation bound hydrogen atoms in the experiment steel is 12.7 kJ/mol.And the activation energy of the irreversible hydrogen trap is 109.0 kJ/mol,mainly related to the V-rich(V,Nb)C precipitates with an average particle diameter of 13.1±0.3 nm.The Nb,V and C form nanoscale carbides could improve the uniformity of hydrogen atom distribution by refining the microstructure and increasing the grain boundary area.And pin dislocations hinder the movement of the "hydrogen-dislocation Cottrell atmosphere" to prevent the rapid diffusion of hydrogen atoms.Meanwhile,act as irreversible hydrogen trap to directly hinder the diffusion and aggregation of hydrogen atoms.Therefore,the hot-stamped steel has highstrength plasticity and excellent resistance to hydrogen-induced delayed cracking.