The Determining Role of Finish Cooling Temperature on the Microstructural Evolution and Precipitation Behavior in an Nb-V-Ti Microalloyed Steel in the Context of Newly Developed Ultrafast Cooling

Abstract

We have studied here the impact of finish cooling temperature on the microstructural evolution and precipitation behavior in Nb-V-Ti microalloyed steel through thermo-mechanical simulation in the context of newly developed ultrafast cooling system. The microstructural evolution was studied in terms of morphology and crystallography of precipitates using high-resolution transmission electron microscopy. At finish cooling temperature of 933 K and 893 K (660°C and 620°C), the microstructure primarily consisted of polygonal ferrite, together with a small amount of wedge-shaped acicular ferrite and lamellar pearlite, while, at 853 K and 813 K (580°C and 540°C), the microstructure consisted of lath bainite with fine interlath cementite and granular bainite with martensite/ austenite (M/A) constituent. In all the finish cooling temperatures studied, the near-spherical precipitates of size range~2 to 15 nm were randomly dispersed in ferrite and bainite matrix. The carbide precipitates were identified as (Nb,V)C with NaCl-type crystal structure. With a decrease in the finish cooling temperature, the size of the precipitates was decreased, while the number density first increased with a peak at 893 K (620°C) and then decreased. Using Ashby-Orowan model, the contribution of the precipitation strengthening to yield strength was~149 MPa at the finish cooling temperature of 893 K (620°C).In the last few decades, high-strength low-alloy (HSLA) steels have been widely used in buildings, bridges, and ships because of their potential to obtain high strength-toughness combination. [1] Superior mechanical properties were obtained through optimization of alloy design in conjunction with thermo-mechanical processing (TMCP). [2][3][4][5] The improvement in mechanical properties was a consequence of grain refinement together with microstructural control and precipitation strengthening. The carbide-forming elements include titanium and niobium that facilitate grain refinement and contribute to dispersion hardening through carbide precipitation in the matrix. The relative contribution of the microalloying elements is determined by the solubility of carbides in the microstructure. Titanium is beneficial because it combines with nitrogen at relatively high temperature, preventing grain growth, while niobium can effectively retard recovery and recrystallization during hot rolling leading to grain refinement. Moreover, these two elements can precipitate as carbides from the supersaturated ferrite solid solution or precipitate as interphase precipitates during austenite-to-ferrite transformation increasing strength. Vanadium is less effective in grain refinement but contributes to strength via precipitation hardening because of higher solubility in austenite as compared to niobium, leading to precipitation at lower temperature. [6] TMCP involving ultrafast cooling (UFC) technology developed by our laboratory is being currently applied to industrial production, [7][8][9] with the aim to reduce the consumption of alloying elements and make the s...

Precipitation behavior and kinetics in Nb-V-bearing low-carbon steel

“…At all holding time, the carbide size was determined to be $ 2-15 nm. The result of this study is in good agreement with the previous atom probe or neutron scattering results [10][11][12], indicating that the average size obtained by Moiré Fringe in HRTEM image is reasonably accurate. The average size of precipitates increased with holding time at 650°C.…”

Carbide Precipitation in Ferrite in Nb–V-Bearing Low-Carbon Steel During Isothermal Quenching Process

“…At all holding times, the carbide size was determined to be *2-15 nm. This is in good agreement with the previous atom probe or neutron scattering results [26][27][28], indicating that the average size obtained by Moiré fringe in HRTEM image is reasonably accurate. In addition, the volume fractions of the precipitates were determined to be *0.0207, 0.0483, and 0.0917%, respectively.…”

“…This method is also used in Refs. [26] and [31]. Figure 13a shows BF image of interphase precipitation, and the HRTEM image of the circled precipitate is displaced in Fig.…”

Titanium microalloyed steels

“…A series of papers by Misra et al [144,145,227,[229][230][231][232]] included investigations into titanium-containing steels and have made significant contributions to understanding the development of microstructures essential to attaining yield strengths eventually exceeding1000 MPa. This strength level permits reduced strip thickness in automotive weight reduction applications and in pipeline.…”