Hui Weijun

STUDY ON THE MARTENSITE IN LOW CARBON CrNi3Si2MoV STEEL TREATED BY Q&P PROCESS

The martensite in low carbon CrNi3Si2MoV steel treated by QP(quenching and partitioning)process was characterized by means of SEM,TEM,EBSD and nano intender.The effect of martensite on uniaxial tension behaviors was discussed.The results showed that initial martensite phase formed at the first quenching step,whose carbon content was lowered due to its carbon diffusion into untransformed austenite during partitioning step.However,the martensite phase formed at the final quenching step consisted of only one single set of packet with lath thickness about 0.1-0.2μm, which was thinner than that of the initial martensite lath.It was found that the carbon content and hardness of the martensite formed in the final quenching step were higher than those of initial martensite,which deformed cooperatively with other phases and played a role of strengthening phase during deformation process.In addition,large sized carbonitride and oxide precipitations induced nucleation and expansion of crack during deformation process.

HYDROGEN ABSORPTION BEHAVIOR OF 1500 MPa- GRADE HIGH STRENGTH STEEL 42CrMoVNb

Hydrogen absorption behaviors of a newly developed 1500 MPa-grade high strength steel 42CrMoVNb at different austenitizing temperatures and tempering temperatures were studied using cathodic charging and hydrogen thermal desorption analysis,which were also compared with commercial structural steel 42CrMo.The results show that the hydrogen escape peak temperatures (θ_p) in hydrogen evolution curves of hydrogen charged 42CrMoVNb specimens are between 200℃to 300℃both at as-quenched condition and as-tempered condition.The absorbed hydrogen content of 42CrMoVNb specimen increases slowly with increasing tempering temperature up to 500℃.When the tempering temperature exceeded 500℃,the absorbed hydrogen content increases sharply and reaches its peak,6.6×10~(-6),for the specimen tempered at 600℃,which is 5 times as much as that of the as-quenched specimen.Thereafter the absorbed hydrogen content declines sharply as the tempering temperature was gone up sequentially.When the specimen was tempered at 400℃,the absorbed hydrogen content decreases slightly with austenitizing temperature increasing,and in the microstructure no fine dispersed(V,X)C carbide precipitated,while when the specimen was tempered at 600℃, the absorbed hydrogen content increases sharply with austenitizing temperature increasing,and more fine dispersed(V,X)C precipitated.These results indicate that fine dispersed(V,X)C precipitate could be regarded as a strong hydrogen trap,and the trap activation energy,E_a,is equal to about 28.7 kJ/mol,which was obtained by change heating rate.

HYDROGEN INDUCED DELAYED FRACTURE BEHAVIOR OF A LOW-CARBON Mn-B TYPE ULTRA-HIGH STRENGTH STEEL SHEET AFTER HOT STAMPING

In consideration of the light weight and the impact safety of cars, in recent years, hot stamping has been increasingly applied to the manufacture of car parts, which has driven forward the increasing use of ultra-high strength steel sheet in the automobile industry. However, when the tensile strength of steel is more than about 1200 MPa, the steel sheet is very susceptible to hydrogen induced delayed fracture. Up to now, some research has been carried out on delayed fracture behavior of both high strength and ultra-high strength steel sheet. However, the delayed fracture behavior of a low-carbon Mn-B type steel sheet widely used for hot stamping has not been paid enough attention and fully studied. There are also a scarcity of reports on hydrogen induced delayed fracture behavior of this steel sheet after hot stamping. With the development of the hot stamping, the evaluation of delayed fracture behavior of ultra-high strength steel sheet for hot stamping, especially the delayed fracture behavior of steel sheet after hot stamping has become very urgent. For these reasons, hydrogen induced delayed fracture behavior of a low-carbon Mn-B type ultra-high strength hot stamping steel sheet at both the hot stamped status and the common quenched and tempered status was studied by means of constant load delayed fracture test and hydrogen thermal desorption spectrometry (TDS) analysis. The results show that both the critical delayed fracture stressc and delayed fracture life of