Vanadium Precipitation During Intercritical Annealing in Cold Rolled TRIP Steels

Abstract

Flat carbon steel manufacturers are currently developing new TRansformation Induced Plasticity (TRIP) steels as a response to strong demands for vehicle lightening and security reinforcement from the automobile sector. Compared to conventional high strength steels these advanced grades exhibit a very favourable compromise between strength and ductility and can therefore be produced in thinner, lighter gauge strips with equivalent functional properties. The excellent mechanical properties of TRIP steels are attributed to the high strain hardening coefficient generated by the progressive transformation of metastable retained austenite to martensite during plastic deformation. Further improvements in mechanical properties can be obtained by microalloying, especially with vanadium and nitrogen additions. In this paper we discuss the rather complex evolution of vanadium carbonitride V(C, N) precipitation during continuous annealing of cold rolled strip. Transmission electron microscopy (TEM) and selective chemical dissolution are used to characterise the precipitation state during interrupted intercritical annealing cycles. The experimental results are compared with calculations made using a recent kinetic precipitation model. We show that reasonable agreement can be achieved using a simple uncoupled model, however a complete description of the precipitation sequence during continuous annealing will require fully coupled kinetic models describing the interactions between cementite dissolution, the ferrite to austenite transformation and V(C, N) precipitation.KEY WORDS: cold rolled TRIP; microalloying; V(C, N) precipitation; kinetics modelling. ISIJ International, Vol. 47 (2007), No. 8, pp. 1168-1177 Points 1-4 and 6 require that the precipitating species should be highly soluble in austenite. Points 5, 7 and 8 require the maximum possible precipitation driving force to generate a high particle nucleation density. Concerning point 7, intragranular precipitation will dominate if the precipitation step occurs in a deformed matrix, for example during annealing after cold rolling. Finally, point 9 would seem to favour the precipitation of nitrides or carbonitrides rather than pure carbides, which would tend to reduce the amount of carbon available to stabilise the residual austenite during the bainitic transformation. Taking into account all of the above criteria, the microalloying element which best satisfies these criteria is vanadium. Precipitation of VanadiumThe equilibrium solubility products for microalloy carbides and nitrides in austenite show that VN is the nitride which has the greatest solubility in austenite and that VC is even more soluble in austenite and in ferrite.10) For the purposes of precipitation strengthening in TRIP grades, it is the precipitates formed in the soft ferrite phase that are the most effective. It is therefore desirable to maintain the V in solution in the hot coil and to precipitate during continuous annealing after cold rolling. Due to the short holding times available in commercial c...

Precipitation strengthening in high manganese austenitic TWIP steels

“…However, due to the small effect on the mechanical properties this was not checked. [33], the solubility of V in austenite is such that there will be little or no V precipitation during hot rolling and coiling as long as the coiling temperature is kept below *500 8C. This is confirmed by the low concentration of precipitated V measured in the hot bands (Table 4b) and also by TEM investigations on thin foils taken from hot strips.…”

Probing the Characteristic Deformation Behaviors of Transformation-Induced Plasticity Steels

“…However, in addition to RA, multiple phases are usually present in TRIP steels due to the thermomechanical processing, which renders the deformation of TRIP steels very complicated. So far great effort was devoted to optimizing the thermomechanical processing parameters for improved mechanical properties, [10][11][12][13][14] but the influence of strain-induced martensitic transformation in TRIP steels on the work-hardening behavior was not well understood. In particular, the micromechanical character of individual phase on the work-hardening process in TRIP steels was far from being well explored due to the complexity from the multiple phases.…”

Precipitation Behavior and Microstructural Evolution of Vanadium-Added TRIP-Assisted Annealed Martensitic Steel

“…It is found that the spherical VC precipitates disperse at the phase interfaces and they effectively pin the interfaces so that they cannot move. The VC precipitates are mainly precipitated at low temperature (FCC → BCC phase transformation) and prevent the austenite grains coarsening and improve the strength of the steel . However, the carbides at the phase interfaces can decrease the plasticity and toughness of the steel .…”