Continuous cooling transformation diagrams of HSLA steel for seamless tubes production

Abstract

The CCT and two DCCT diagrams were constructed for the HSLA steel containing Cr, V, Nb, and N microadditions, taking into account industrial processing parameters of this material in a seamless tubes rolling mill. The typical finish hot rolling temperature of 1173 K was used for the construction of the standard CCT diagram and the effect of previous austenite deformation at this temperature was evaluated in the DCCT diagram. Another DCCT diagram was developed after heating at 1553 K, followed by plastic deformation at 1173 K. The prior austenite grain size in the hot rolled material after heating at 1173 K was approx. 10 m, the heating of the as-cast material at 1553 K resulted in the prior austenite grain size over 200 m. The effect of the previous austenite deformation after low-temperature heating on the CCT diagram was negligible. The high-temperature heating showed a great influence on the austenite decomposition processes. The Ferrite-start temperature was significantly reduced at high cooling rates and the preferred decomposition of coarse grained austenite to acicular ferrite suppressed the bainite transformation at medium cooling rates. The developed DCCT diagrams can be used for the prediction of austenite decomposition products during the cooling of the seamless tubes from the finish rolling temperature. The CCT diagram can be utilized for the quality heat treatment of tubes.

Effects of Austenitization Temperature and Pre-Deformation on CCT Diagrams of 23MnNiCrMo5-3 Steel

“…Relatively low strain values of up to 25% had almost no effect in manganese–nickel and manganese low-alloy steels [ 52 ]. For the HSLA steel microalloyed with niobium and vanadium, the CCT and DCCT (strain e = 0.35) diagrams differed only slightly after austenitization at 900 °C [ 7 ]. At cooling rates below about 4 °C·s −1 , the ferrite-start temperatures were even lower in the case of pre-deformation.…”

“…Generally, the austenitization temperature can have a major effect on the kinetics of phase transformations during the cooling of the deformed structure. The prior grain size and the state of precipitates in HSLA steel are important [7], leading to a significant acceleration of the austenite → ferrite transformation at cooling rates above about 4 • C•s −1 after austenitization at 900 • C compared to preheating at 1280 • C. The effect of the very high temperature of austenitization (e.g., 1200 • C vs. 885 • C in [1]) is strong and results in the shift of the C-curves for diffusion controlled transformations towards the longer times. Experiments performed on 34CrMo4 steel showed a minor effect of a high austenitization and deformation temperature of 1200 • C on the DCCT diagram in comparison with the CCT diagram [22].…”

“…They are always constructed for the specific austenitization parameters and initial microstructure. The deformation continuous cooling transformation (DCCT) diagrams include the effect of pre-deformation, characterized mainly by the strain value [ 7 , 8 , 9 , 10 , 11 ].…”

Development of Coated Electrodes with Solid Wire and Flux-Cored Alloyed Wire for Microalloyed Steel Welding

“…This microstructure clearly shows a narrow longitudinal line of proeutectoid ferrite (PF), which was separated at the border between austenite grain and needles of acicular ferrite (AF). Austenite grains show spherical non-metallic inclusions from the basic coating, which serve as sites for the intragranular nucleation of needle-like ferrite [8,[12][13][14][15][16][17]. The volume of acicular ferrite (AF) within the MW of J55 microalloyed steel created by the electrode IHIS 2.5Ni Mo B (sample No.1.2) with (1.34% Mn + 2.850% Ni + 0.59% Mo) ranged from 56% to 79.44%, while the other forms of ferrite were proeutectoid ferrite and ferrite with secondary phase.…”