Fundamental Research On Microstructure-Mechancial Properties And Hydrogen-Trapped Behaviors Of Microalloyed TRIP-assisted Annealed Martensitic Steels

Advanced high strength steels,especially the ultra-high strength steels,have been increasingly used in various applications.Along with their wide uses hydrogen embrittlement presents a potential challenge to this kind of steels since hydrogen readily invades its matrix when they are subject to cold-working or in service under moist atmosphere.As a typical high strength high ductility steel in which Transformation Induced Plasticity(TRIP)effect can play its role,TRIP-assisted annealed martensitic(TAM)steel could be suitable for automotive applications such as structural parts,crash-deployed elements and interior reinforcement members,complying with the automotive light-weighting technologies.TAM steel is highly susceptible to hydrogen embrittlement due to its martensitic matrix in which the solubility of hydrogen atom is rather low.Therefore,TAM steel has not yet been gaining its desired market share in automotive section since it was firstly developed two decades ago.There are few publications highlighting further performance improvement for previous TAM steel through vanadium microalloying technology which has been widely applied in diversities of existing steel grades.Besides,few reported investigations has been conducted on how to improve hydrogen induced delayed fracture behaviors in TAM steel through niobium,vanadium or titanium microalloying,and clarifying hydrogen permeation behaviors in these possible microalloyed TAM steels as well.Thus,understanding hydrogen permeation behavior and hydrogen embrittlement sensitivity of microalloyed TAM steels is essential to enable their commercialization.In order to serve this purpose,the base chemistry of typical TRIP steel with 0.2%C-1.50%Si-2.0%Mn(wt.%)was selected,and 5 kinds of steels microalloyed by one of microalloying additions namely 0.05%Nb,0.05%Ti,and various V additions(0%,0.05%,0.10%and 0.20%)had been designed.Physical simulations,microstructural observations and conventional tensile tests had been conducted to investigate the evolution of microstructure and mechanical properties of TAM steels during heat treatments,to obtain the optimum heat treatment process parameters for typical TAM steels and influence of vanadium contents on the microstructure and mechanical properties of TAM steels.Hydrogen permeation experiments had been carried out to study the hydrogen diffusion behaviors in the designed TAM steels.The sensitivity to Hydrogen Induced Crack(HIC)of TAM steels with different microalloying additions such as Nb,Ti or V had been investigated by means of slow strain rate tensile(SSRT)tests along with electro-chemical hydrogen charging simultaneously,and Thermal-Desorption Analysis(TDA)had been carried out to quantitatively determine the amount of desorption hydrogen escaping from different kinds of hydrogen traps.Fracture morphology on both SSRT samples with/without electro-chemical hydrogen charging process had been analyzed,in combination with HIC sensitivity results,to serve the purpose of making efforts to reveal the mechanism of microalloying elements improving hydrogen induced fracture of studied steels.The main conclusions obtained from this study are as follows:(1)The study on the microstructure evolution and mechanical properties of TAM steels shows that:After the cold-rolled steel sheet is subject to the combined austenitizing+inter-critical annealing+partitioning heat treatment cycles of’austenitizing at 920℃ for 100s immediately quenched down to room temperature at cooling rate of 50℃/s,and inter-critical annealing at 800℃ for 100s then fast cooled down to 400℃,finally partitioning for 200s then quenched down to room temperature’,the resulted microstructure of heat-treated TAM steel mainly consists of annealed martensite and retained austenite,where retained austenite distributes between the annealed martensite laths.For three vanadium microalloyed TAM steels,the mean sizes of vanadium precipitates range between 7 to 11 nm,of which the average size of the precipitate in 0.098%V steel is the smallest at 7.30 ± 0.88 nm.The volume fractions of vanadium precipitates approximate 0.0011～0.0041%and no evidence of TiC particle has been found in all vanadium microalloyed TAM steels from TEM observations.Microstructure becomes refine and volume fraction of retained austenite slightly increases from 14.2%for TAM-R steel to 14.7%for TAM-V10 steel as vanadium content increasing from 0%to 0.098%.For 0.21%V TAM steel,the volume fraction of retained austenite decreases dramatically down to 10.7%.Carbon content in retained austenite decreases with the increase of vanadium addition.The C and V atom distribution from 3DAP investigation on TAM-V5 indicates that during intercritical annealing and partitioning processes the effective carbon redistribution takes place and thus improves austenite stabilization.The final C contents in the retained austenite are between 0.95%to 1.14%for three vanadium microalloyed TAM steels,which is significantly higher than the average composition of the matrix of 0.20%C.As the vanadium content is increased from 0 to 0.098%,both the yield strength and tensile strength of TAM steels show upward trend,while their total elongations continuously decline.When the vanadium content is further increased to 0.21%,the tensile strength drops to 1095MPa,but the total elongation slightly improves to 24.5%.The products of tensile strength and elongation(TS×T.EL)of the four kinds of vanadium-containing steels(including 0%V steel)are between 26.1-30.0 GPa·%,of which the highest TS×T.EL value of 30.0 GPa·%is achieved at 0.052%V steel,while the lowest value of 26.1 GPa·%is seen for 0.098%V steel.(2)The study on hydrogen permeation behavior of different microalloyed TAM steels shows that:The additions of microalloying elements into steels strongly influence the diffusion behavior of hydrogen charged in TAM steel.Under electro-chemical hydrogen charging conditions at current density of 5mA/cm2,the time for hydrogen penetration through around 0.3mm thick test samples requires 383s for TAM-R steel without microalloying addition,while penetration time for TAM-V5,TAM-Ti and TAM-Nb steels are significantly prolonged to 523s,1502s and 902s,respectively.As the vanadium content in steel increases,both apparent hydrogen diffusion coefficient Da and steady-state hydrogen diffusion flux J∞ of vanadium microalloyed TAM steel gradually decrease,while the hydrogen concentration in the crystal lattice CHS increases.Based on the experimental curves of hydrogen permeation and calculations,corresponding mathematical models of the apparent hydrogen diffusion coefficient,the steady-state hydrogen diffusion flux,and the hydrogen concentration in crystal lattice as function of vanadium content have been established.(3)The study on the hydrogen induced ductility loss of different kinds of microalloyed TAM steels shows that:There are three complete deformation stages,ie,elastic deformation,uniform plastic deformation and non-uniform plastic deformation stages,existing in the stress-strain curve for TAM steel specimen during SSRT without hydrogen-charging process.For TAM steel specimen during SSRT along with hydrogen-charging process,however,there are elastic deformation stages in the stress-strain curves for all steels but no obvious non-uniform plastic deformation stage is presented.The tensile fractures of no hydrogen-charged specimen show ductile fracture pattern,and the phenomenon of obvious necking occurs adjacent to the fracture.The fracture morphology of TAM steel specimen during SSRT along with hydrogen charging process shows brittle fracture pattern with no evidence of necking,and their fracture modes are dominated by intergranular fracture accompanied by transgranular fracture where cracks pass through the M/A islands and propagate along the annealed martensite laths.Among the studied TAM steels,TAM-R steel gives the largest brittle sensitive area on the fracture surface.TAM-R steel without microalloying addition achieves the greatest ductility loss ratio caused by hydrogen effect,that is,91.1%.Among three different microalloyed steels TAM-V5,TAM-Ti and TAM-Nb,TAM-Ti steel shows the highest ductility of 5.45%and the lowest hydrogen induced ductility loss ratio ID of 78.8%after SSRT along with hydrogen charging,TAM-V5 steel(ID=83.4%)is next to the TAM-Ti steel and TAM-Nb steel(ID=87.3%)is the worst with respect to the effect of microalloying addition improving resistance to hydrogen induced embrittlement.Besides,both steels TAM-V10 and TAM-V20 achieve the best ID of as low as 76.2%among all six studied steels.(4)The study on hydrogen thermal desorption behavior of different microalloyed TAM steels shows that:For four kinds of TAM steels,namely TAM-V5,TAM-Ti,TAM-Nb and TAM-R,the thermal hydrogen desorption curves of samples without hydrogen charging have single hydrogen desorption peak zone,and their corresponding hydrogen desorption peak temperatures range in 400～410℃.Correspondingly,the hydrogen-charged specimens have a clear first peak zone and an apparent second peak zone during hydrogen desorption test,and there is also a third less-apparent peak zone,where the peak temperatures of the first peak zone are between 167℃ and 183℃,the second peak temperatures range 275～305℃,and the third peak temperatures approximate 400℃.The total hydrogen content escaping from hydrogen-charged specimen excesses 10 ppm,which is～40 times higher than that of non-hydrogen charging specimen.The first peak zone is related to the hydrogen desorption from high-density dislocations acting as reversible hydrogen traps in the martensitic matrix,and the corresponding thermal desorption hydrogen content is as high as 8.66-10.02 ppm.The second peak zone is attributed to the hydrogen desorption from microalloyed precipitates acting as irreversible hydrogen traps,and desorption hydrogen contents are only between 0.10 to 0.42 ppm for different microalloyed TAM steels.It is expected that the microalloyed precipitates in TAM steels can act as effective trap sites to capture hydrogen atoms,and then inhibit the movement and enrichment of diffusible hydrogen toward the the defects such as phase boundary and specially dislocation,reducing the possibility of stress concentration at the defect tips and thus contributing to lower sensitivity of hydrogen embrittlement in high strength microalloyed TAM steels.Generally speaking,when the amount of hydrogen captured by the microalloyed precipitates is higher,the hydrogen induced ductility loss ratio is lower.For TAM-Ti steel,the desorption hydrogen at～300℃ and～700℃ are from traps acted by TiC particles,and the total desorption hydrogen content in TAM-Ti steel is the highest among those studied steels.Regardless of whether these test samples are subject to hydrogen-charged process or not,there are hydrogen thermal desorption peak temperature zones approximating at 400～410℃ for all samples,and these peak zones are relatively weaker compared to the large peak zones with temperature ranging 167℃～183℃obtained from the hydrogen-charged samples.The desorption hydrogen rate at this weaker peak zone is extremely low,and the desorption hydrogen from this peak zone is attributed to the capturing effect by retained austenite in the TAM steel as hydrogen trap sites,where the measured desorption hydrogen content is less than 0.1 ppm.