Effect of Isothermal Annealing on Nano Ti-Carbides Precipitation

Transmission electron microscopy was used to investigate the effect of isothermal holding temperature and time on the nano Ti-precipitates.A holding temperature was varied systematically from 400℃ to 1200℃.The isothermal holding was continued for 30s,300s and 900s,respectively.Nano carbides of (Ti,Nb)C were precipitated significantly at 900℃.The size of carbides was approximately 10nm at 30s holding and increased to 20～30nm at 900s holding.Isothermal holding at 1000℃ showed the increased amount of carbides larger than 100nm.At 800℃,nano (Ti,Nb)C was not observed at 30s and it was examined at 300s.The size of nano (Ti,Nb)C was smaller than that of 900℃.As the isothermal temperature decreased to 700 ℃,the nano (Ti,Nb)C was only seen at 900s holding and the size of carbides was smaller than 10nm.Nano (Ti,Nb)C was disappeared at isothermal holding below 600℃.The kinetics of nano (Ti,Nb)C precipitation were studied as a function of isothermal holding temperature and time,respectively,using the precipitate growth equations.

The controlling factors on metallurgical properties of advanced high strength steel sheets

Recent progress in production process for advanced high strength steel sheets（AHSS） is described focussing on Bainitic ferrite steel,transformation induced plasticity（TRIP） steel and precipitation hardened steel by NANO sized precipitates are reviewed. In higher tensile strength（TS） grade,elongation（El）,hole expansion ratio（λ） and their balance are eagerly requested to improve by customers.Bainitic ferrite steel which has shear deformed lath with various distribution of iron carbides and retained austenite should be distinguished to produce different balance of El/λ. Chemical composition and heat cycle are reviewed to produce various kinds of metallurgical phase and mechanical property.5%Mn -2%Si TRIP steel is introduced in which a plenty of retained austenite at about 30%and larger absorbed energy（AE） more than 3 000 kgf/mm;TS·%El（gauge length:50 mm） can be obtained. That steel also showed excellent upper shelf energy in Charpy test of 140 J/cm;,as well as excellent property of 20%El and 1 400 MPa TS after forging. A new type of precipitation hardened steel having NANO sized（Ti,Mo） C in ferrite matrix is introduced.It satisfies the both of excellent El andλat the same time. Bending property and holes expansion property obey the same kind of deforming criteria which depends on the forming capability of work hardened parts.Homogeneity of structure is their controlling factor and a new parameterαwhich is obtained by gauge length dependency of total elongation is proposed.

Altered age-hardening behavior in the ultrafine-grained surface layer of Mg-Zn-Y-Ce-Zr alloy processed by sliding friction treatment

To gain insight into the ageing behavior of ultrafine grain(UFG)structure,the precipitation phenomena and microstructural evolutions of Mg-6 Zn-1 Y-0.4 Ce-0.5 Zr(wt.%)alloy processed by sliding friction treatment(SFT)were systematically studied using hardness texting,transmission electron microscopy(TEM)equipped with high-angle annular dark-field scanning(HADDF-STEM),X-ray diffraction(XRD)and XRD line broadening analysis.The microhardness of the SFT-processed(SFTed)sample initially decreases from 109.6 HV to 104.8 HV at ageing for 8 h,and then increases to the peak-ageing point of 115.4 HV at16 h.Subsequently,it enters the over-aged period.The un-SFTed sample,as the counterpart,follows a regular ageing behavior that increases from 89.9 HV to 99.6 HV when ageing for 12 h,and then drops.A multi-mechanistic model is established to describe the strengthening due to grain refinement,dislocation accumulation,precipitation etc.The analysis reveals that the temperature sensitive UFG structure has an obvious grain coarsening effect,which arouses the soft phenomenon in the early ageing stage.But precipitation hardening provides an excellent hardness enhancement for overcoming the negative influence and helping to reach the peak-aged point.In our microstructural observations,a lot of equilibrium ultrafine Mg Zn2 precipitates precipitate along dislocations because defects can provide the favorable conditions for the migration and segregation of solute atoms.