Impact Toughness and Tensile Properties Improvement through Microstructure Control in Hot Forged Nb-V Microalloyed Steel

Abstract

The influence of thermomechanical processing parameters such as reheating temperature, deformation temperature, deformation percent and cooling rate on achieving high impact toughness properties was studied in a Nb-V microalloyed steel to be used as forged parts in automotive applications. 15 mm long and 65 mm diameter billets were forged using a 20 MN mechanical press. Tensile and Charpy impact tests specimens were machined out of the central part of the forged billets. The microstructure of the specimens was examined for each experimental condition using optical microscopy. Phase identification and distribution was studied using X-ray diffraction and orientation image microscopy techniques. The results indicate that, increasing the reheating temperature above the dissolution temperature of (Nb)(C, N) improved the impact energy values. By increasing the cooling rate from 0.3 to 3°C/s both tensile strength and impact toughness were improved. High elongation percent was also observed on samples reheated at higher temperature and/or cooled with the higher cooling rates. The obtained mechanical properties were related to the characteristics of microstructural components including acicular ferrite, retained austenite, pearlite and ferrite.The interrelationship between thermomechanical processing parameters, microstructure development, and final mechanical properties were identified and optimized forging conditions to obtain high impact energy (Ͼ30 J) microalloyed forge steels were determined.KEY WORDS: microalloy steel; mechanical properties; acicular ferrite; forging.The growth of ferrite plates results in carbon enrichment of the remaining austenite, which may remain untransformed or transform to martensite, bainite or interlath carbides. Upon the application of strain, the untransformed austenite is converted to martensite, which would increase strain hardening and residual compressive stresses bringing higher resistance to necking and crack arrest properties, respectively. 24) In the acicular ferrite microstructure, refining the ferrite lath size, eliminating pearlite, minimizing the development of interlath carbides, and control of the amount and distribution of retained austenite are critical for the achievement of optimum strength and toughness properties. 7,9,25) In the present article the influence of thermomechanical process parameters on the above mentioned microstructural characteristics particularly with regard to the formation of acicular ferrite and the resultant mechanical properties of a Nb-V microalloyed steel is investigated. The overall objective of the study was the development of a forging process to produce high strength-high toughness (tensile strength Ͼ850 Mpa, impact energy at room temperature Ͼ30 J) microalloyed forged automotive safety parts such as axles, hubs and steering parts. Material and Experimental ProceduresThe material used in this investigation was a Nb-V microalloyed steel produced in an electric arc furnace equipped with semi-automatic charging system and oxy-gas burners. Sec...

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

“…The transformation of acicular ferrite is similar to that of bainite as shown in fig.2.4, it is form below the transformation temperature of pro-eutectoid ferrite and pearlite and above the martensite start temperature but the nucleation sites are different [15,[46][47][48][49][50]. Bainite is a nonlameller aggregate of carbides and plate shaped ferrite as shown in fig.2.4, while the transformation to pearlite reaction is essentially occur at high temperature.The nature of bainite changes as the transformation temperature is lowered.…”

“…However a full description of this behaviour for Nb − C, Nb − N and Nb−C−N system is given in Ref. [46,[57][58][59][60][61][62][63][64][65][66][67][68][69]. …”

“…The plates of acicular ferrite nucleate heterogeneously on small nonmetallic inclusion as shown in figure 2.40(a,b).and radiate in many different directions [15,49,98].The acicular ferrite is in fact intragranularly nucleate bainite [15,46,48,49,130,136,139,140,145].The morphologies of acicular ferrite and conventional bainite differ in that the former nucleates intragranularly at inclusions inside large austenite grains. The microstructure of acicular ferrite is less organized when compared with the ordinary bainite, the acicular ferrite have chaotic arrangement of plates and fine grains interlocked, such microstructure are better suited to deflect propagation cleavage cracks and therefore more desirable from toughness point of view in comparison to BS [15,46,56,131].The crystallographic data show highly misoriented plates nucleated on the same inclusion, propagation cracks are then deflected on each encounter with a differently oriented acicular ferrite plate.…”

“…The microstructure of acicular ferrite is less organized when compared with the ordinary bainite, the acicular ferrite have chaotic arrangement of plates and fine grains interlocked, such microstructure are better suited to deflect propagation cleavage cracks and therefore more desirable from toughness point of view in comparison to BS [15,46,56,131].The crystallographic data show highly misoriented plates nucleated on the same inclusion, propagation cracks are then deflected on each encounter with a differently oriented acicular ferrite plate. A full description of this behaviour is given by Gourgues et al [146].This give rise to superior mechanical properties especially toughness (improve toughness without compromising strength).…”

“…A full description of this behaviour is given by Gourgues et al [146].This give rise to superior mechanical properties especially toughness (improve toughness without compromising strength). In some recent studies, an improved toughness, observed in medium-carbon steels is associated with AF [56,98].In acicular ferrite microstructure, four points are critical for the achievement of optimum strength and toughness properties [46]:-1. Refining the ferrite lath size.…”