Microstructure Evolution in a Low Carbon Nb–Ti Microalloyed Steel

Abstract

The micro mechanical properties, stability and three-dimensional (3D) morphology of intragranular ferrite was studied in a low carbon microalloyed steel utilizing nano indentation, three-dimensional reconstruction techniques along with LEO1 450 scanning electronic microscopy. The mixed microstructures of intragranular ferrite, granular bainite and lath or plate-like bainite was obtained in a Nb-Ti microalloyed steel, which was water cooled immediately after relaxation for a fixed time as hot deformation ended. The elastic modulus and hardness increased in the sequence of intragranular ferrite, granular bainite and lath or plate-like bainite by approximately 15 GPa and 0.6 GPa, respectively. On the contrary, the tempering treatment of the specimens showed that the stability of the mixed microstructures decreased in the sequence of intragranular ferrite, granular bainite and lath or plate-like bainite. The results of stability, elastic modulus and hardness indicated that intragranular ferrite was formed prior to bainite transformation. The intragranular ferrite thus effectively sectioned the prior austenite into many small zones and thus the bainite transformed at lower temperatures was restricted in the small zones. It is likely that the formation of lath or plate-like intragranular ferrite prior to bainite transformation played an important role in the refinement of mixed microstructures.KEY WORDS: microalloyed steel; intragranular ferrite; nano indentation; three-dimensional morphology.Optical microstructure was observed with an Olympus microscope. SEM observation was conducted at a LEO1450. Small load was applied for nano indentation because the intragranular ferrite in this study was thin and divided bainite. In order to compare the micro mechanical properties of intragranular ferrite, granular bainite and lath or plate-like bainite, the same load was applied for these microstructures. The nano indentation was applied with load 10 mN and conducted at National Metallic Materials Laboratory at University of Science and Technology Beijing. The elastic modulus and hardness of the microstructures was calculated from the measured force-depth curves using an empirical function, which was established utilizing single crystal metals.For serial sectioning the specimens were polished with an automatic grinder polisher under a fixed polishing load and for a fixed time. The polished surface of each layer was slightly etched with 3 % nital. Fiducial hardness indents were applied with a Vickers hardness indenter at a 200 g load for the purpose of alignment and measurement of the depth of removal. The depth of a removed layer was calculated by measuring the diagonal length of the hardness indents before and after each polishing. Approximately 0.50 mm was polished away each time. A series of optical micrographs were obtained with a digital CCD camera at a magnification of 400 in each polishing cycle. The images were then aligned horizontally. After the masking of ferrite was performed, a stack of 2D slices of ferrite image wa...

Austenite decomposition in medium carbon microalloyed steels: Mechanism, structure and properties

“…[147], TiN particles served as nucleation sites for proeutectoid ferrite preferentially and also accelerated its formation in low-carbon alloy steels. TiN is regarded as an effective nucleation sites for intragranular ferrite owing to low interfacial energy between ferrite and TiN.…”

“…In some studies it is suggested that, for titanium containing steel, the MnS is found to be precipitate on titanium oxides and stimulaing the nucleation of acicular ferrite [15].The formation of bainite or intragranular ferrite will found [131] dependend on the amount of sulphur added to the steel. So small concentration of sulphur (S ≈ 0.005 wt %) can sometimes stimulate the nucleation of bainite and resulting decrease in absorved energy [131,165].Moreover, the formation of intragranular ferrite plate IFP is promoted by increasing amount of sulfur (0.06-0.07 wt% S) due to increase the number of MnS particles as an effect off sulfur addition , resulting an increase in absorved energy (Fig.2.50-C), due to increase the resistance to the propagation of brittle fracture which is dominant in fracture mode at lower temperature, however as the amount of sulfur addition in large quantities, the MnS particles coarsened and acts as crack initiation points, there by promoting brittle fracture [46,131].Some researches indicate that the intragranular ferrite is caused by the formation of Mn depleted zone around inclusions [147].The precipitation of…”

“…MnS at oxide was reported to promote the formation of intragranular ferrite [147].Recent experimental results clearly demonstrate that intragranular ferrite plates can easy nucleate on VN [166].This has been related to the atomic matching between (100)vn // (100)α planes which allow the growing ferrite to maintain coherent, low energy interfaces with respect to vavadium nitride, in order to obtain intragranular ferrite the active VN particles must first form in austenite [166]. …”

“…The second alternatives a strain-induced precipitation of VN in austenite during hot rolling in the temperature range of VN precipitation. It is clarified that the addition of both vanadium and nitrogen is indispensible for the formation of intragranular ferrite plates [49,131,147].The presence of high V and N in steel that lead to vanadium nitride VN can precipitate on manganess sulfide particles (MnS) before austenite is transformed to ferrite acts as nucleation sites of intragranular ferrite [56,131,166].As the nitrogen content is increased the chemical driving force for precipitation is increased and the VN particles become coarser and more numerous, thereby increasing the nucleation rate of intragranular ferrite [166] shows an intergranular ferrite nucleated at MnS+V(C,N) complex precipitates.…”

“…TiN is regarded as an effective nucleation sites for intragranular ferrite owing to low interfacial energy between ferrite and TiN. Morikage et al [147] also reported that the critical size of a TiN particle for the formation of ferrite decreased with the amount of undercooling in low carbon steels, that is ≈140 nm at 750C and ≈ 50 nm at 650C. The intragranular nucleation of acicular ferrite can be promoted by TiN particles.…”