Control of the Austenite Recrystallization in Niobium Microalloyed Steels

Abstract

Abstract. The use of heavy gauge steel sheets for structural applications very often requires a combination of high yield strength and adequate toughness. The most cost effective way to realize a high yield strength and a high ductility in a low alloyed steel is grain refinement. In industrial practice, this refinement is realized by controlled processing. This process consists of controlling the slab reheating temperature, applying a large amount of hot deformation below the nonrecrystallization temperature (T nr ) and accelerated cooling. A better knowledge of T nr could optimize the process and the best mechanical properties could be reached against the lowest cost. T nr can be raised by the addition of microalloying elements such as Nb. Nb can retard the static recrystallization of austenite at low temperatures either by solute drag or by precipitation pinning. In this study, the recrystallization behavior of five Nb-microalloyed model alloys with various Nb contents, was evaluated by double hit compression tests. Further, the precipitation state of the materials was investigated experimentally by Inductively Couples Mass Spectroscopy and X-ray Diffraction. The construction of recrystallization-time-temperature diagrams and precipitation-timetemperature diagrams showed that both mechanisms, i.e. recrystallization and precipitation, strongly influence each other.

Flow Stress Behaviour and Static Recrystallization Characteristics of Hot Deformed Austenite in Microalloyed Medium-Carbon Bainitic Steels

“…For instance, the time t 50 increases from 3.5 s at 1175 • C to 30 s at 1075 • C. However, it can be noticed that the stress remained relatively high at the three lowest temperatures, 1000, 950 and 900 • C, denoting that the softening was incomplete. Furthermore, at 950 and 900 • C, the stress relaxation curves displayed plateaus (constant stress periods), or even a slight increase in the stress level after about 30-40 s, followed by a drop after about 300-400 s. It is well known that strain-induced precipitation results in an increase in stress level during stress relaxation manifesting as plateaus in the curves or even an increase in stress [6,9,11,[26][27][28].…”

“…The SRX kinetics of hot deformed austenite in C-Mn steels (with Mn < 1.5%, Si < 0.25%; hereinafter, all concentrations are in wt.%) as a function of chemical composition including the effects of V, Ti and Nb have been reported by numerous authors, for example, [6][7][8][9][10][11][12]. Medina and Quispe [13] have published experimental recrystallization-precipitation-timetemperature diagrams for various V and Nb microalloyed steels illustrating the start and finish temperatures of SRX as well as precipitation at different temperatures.…”

“…Furthermore, they pointed out the mechanisms that discretely caused the retardation at low as well as high temperatures; the solute drag effect being the main reason at high temperatures and the presence of fine precipitates at low temperatures. Vervynckt et al [9] also examined the influence of different levels of Nb-microalloying on the SRX kinetics in low-carbon steels at 900 • C, and according to their study, 0.02% Nb alloying caused delay in the SRX kinetics only due to the solute drag effect, whereas a higher Nb alloying (0.04-0.16% Nb) resulted in both precipitation as well as the solute drag effect, respectively. The hindering effect of V and Nb on SRX kinetics in low alloy steels was also reported by Esterl et al [10].…”

Niobium in Microalloyed Rail Steels

“…Some articles [34,138,142] have reported equations for austenite recrystallisation kinetics in non-microalloyed pearlitic rail compositions. Laboratory methods like double-hit deformation testing, multipass hot torsion or, multipass plane-strain compression testing, stress relaxation testing, tension-compression testing etc [90,134,[143][144][145][146][147][148][149][150] can be utilised to determine T nR experimentally. The advantage of multistep hot torsion [90] or plane-strain compression [150] testing is that single experiments can approximate the T nR for a given deformation schedule as shown in Figure 8…”

Influência Da Temperatura De Reaquecimento E Do Tempo Entre Passes Na Evolução Microestrutural De Aços Inoxidáveis Austeníticos

“…Entretanto devido ao alto teor de níquel presente no F138 e sua baixa resistência na condição recozida e susceptibilidade a corrosão localizada [1], um aço com alto nitrogênio e nióbio, o ISO 5832-9, tem sido utilizado como uma alternativa para o F138. É bem conhecido que há vários fatores determinantes no condicionamento da microestrutura e que as propriedades finais dos materiais dependem do efeito combinado da composição química e do processamento termomecânico [2]. Estes fatores determinam as condições de recristalizações, que poderá de fato, controlar o tamanho de grão final e consequentemente às propriedades mecânicas finais.…”