Authors: Kelvin Y Xie Lan Yao Chen Zhu Julie M Cairney Chris R Killmore Frank J Barbaro James G Williams Simon P Ringer
Publish Date: 2011/02/15
Volume: 42, Issue: 8, Pages: 2199-2206
Abstract
The microstructure and corresponding tensile properties of both plain and Nbmicroalloyed grades of ultrathin cast strip UCS low alloy steel produced using the CASTRIP® process were studied Both ascast and hotrolled strip cast steels with various levels of Nb microalloying were manufactured and investigated in this study Hot rolling had little effect on the yield strength of Nb microalloyed UCS specimens for a given chemical composition but resulted in a slightly finer microstructure The effect of Nb microalloying was significant and this is attributable to the promotion of finer tougher austenite transformation products such as bainite and acicular ferrite at the expense of large polygonal ferrite grains A fine dispersion of Nb solute clusters was observed in all Nbcontaining steels following hot rolling and it is suggested that this also contributes to the observed strengtheningThe CASTRIP1 process is a revolutionary new twinroll strip casting method of producing steel sheet directly from the liquid Using this technique thin ~1 to 15 mm strip steel can be produced with significantly less energy time and floor space while maintaining higher production levels compared to conventional slab casting techniques1 2 3 Initially only plain carbon steels were produced using this particular strip casting method but the production of higher strength grades using microalloyed steels has more recently been established following extensive plant trials and fundamental investigations This study was undertaken as part of the supporting research for the manufacture of microalloyed ultrathin cast strip UCS steels The effect of microalloying additions such as Nb on steel microstructure was hitherto unknown for this UCS steel A unique feature of UCS steel is that the solidification process occurs within 200 to 300 ms compared to conventional slab casting in which several minutes are required for complete solidification The aim of this study is to provide new information about the microstructureproperty relationships in microalloyed grades of these exciting new steels which are expected to form an entirely new sheet steel product category with the potential to replace both hotrolled and coldrolled steel sheet in many applications To this end a series of UCS steels produced by the CASTRIP process with varying levels of Nb content were manufactured for study in both ascast and hotrolled statesIt is most noteworthy that the effects of hot rolling on steel microstructures and mechanical properties were investigated by many researchers and a very large body of knowledge is available in this area4 5 6 7 8 It is generally accepted that hot rolling leads to finer ferrite grains This is because hot rolling recrystallizes austenite grains resulting in smaller austenite grains and subsequently finer ferrite grains Furthermore hot rolling refines ferrite grain size by introducing strain fields within any nonrecrystallized austenite grains providing extra nucleation sites for ferrite to nucleate and grow intragranularly in addition to along prior austenite grain boundaries Hot rolling was also found to retard the ferrite grain growth in these steels via mechanical stabilization5 6 7 8 whereby high density dislocations introduced by rolling act as obstacles to the interface migration of ferrite into austenite These result in a finer grain structure and consequently better strength for the product In respect to the effect of Nb microalloying on highstrength lowalloy steels many studies have been carried out9 10 11 12 13 14 15 An important and widely reported effect in both theoretical and experimental studies is the decrease in the transformation start temperature thus favoring the formation of lowtemperature transformation products such as acicular ferrite and bainite It also has been demonstrated that Nb precipitates strongly retard austenite recrystallization1516 Furthermore Nb additions are known to impart improved strength to these steels through the formation of nanoscale NbC or NbNC precipitates which retard the movement of dislocations within the grains during deformation9 10 11 12 13 14 In this research we provide the first comprehensive evaluation of the effects of hot rolling and Nb microalloying on the detailed microstructureproperty relationships in UCS steels
Keywords: