Ferrite/pearlite band formation in hot rolled medium carbon steel

Abstract

The in¯uence of the microsegregation of Mn, Si, and Cr on the austenite decomposition during isothermal transformations in hot rolled medium carbon steel has been studied by neutron depolarisation, electron probe microanalysis (EPMA), and optical microscopy. Eight specimens of the same alloy were held at 1173 K for 30 min and were rapidly cooled to different isothermal transformation temperatures. Two-dimensional EPMA maps of the specimen annealed at 1013 K showed that microsegregation of alloying elements in hot rolled steel is strongly related to the ferrite/pearlite band formation. The local variations in alloying element concentration lead to variations in local transition temperatures, which were calculated with the thermodynamic database MTDATA. Similar EPMA maps for the specimen transformed at 953 K demonstrate the presence of microchemical bands, while optical microscopy reveals the absence of microstructural bands. It is shown that the formation of microchemical bands is a prerequisite for band formation, but that the kinetics of the phase transformation determines the actual formation of microstructural bands. A quantitative model has been developed, which describes the observations in terms of the relative difference between ferrite nucleation rates in regions with a high and low local undercooling and the subsequent growth of the ferrite. The isothermal transformation experiments have led to generalised nucleation and growth criteria for the formation of microstructural bands.MST/5150

Microstructure and Mechanical Properties of J55ERW Steel Pipe Processed by On-Line Spray Water Cooling

“…For a steel pipe produced by air cooling, the zonal microstructure is very evident in comparison with the OSWC-processed steel pipes. The extent of zonal structure decreases with the transformation temperature due to the relative difference of ferrite nucleation rates [26]. In other words, the OSWC process can decrease the zonal structure of ferrite and pearlite because the rapid cooling reduces the transformation temperature of steels.…”

Ferrite/Pearlite Band Prevention in Dual Phase and TRIP Steels: Model Development

“…One of these models has been developed by Offerman et al 7) for a hot rolled medium carbon steel. In this model the ferrite nucleation rates are calculated in composition distinctive regions of Mn, Si and Cr; and based on experimental data, it is postulated that when the difference in ferrite nucleation rates exceeds 6-8 %, ferrite/pearlite banding occurs.…”

“…Consistent with previous experimental work, in the present model assumes that bands form for rϾ0.07. Such a difference in nucleation rates may be calculated as 7) ................ (6) where (dN/dt) N 1 and (dN/dt) N 2 are the changes in nucleation rate in nodes N1 and N 2, which are selected to maximise r; k B is the Boltzmann's constant, DG* N 2 and DG* N1 are the energy barriers for a nucleation at the compositions that characterise nodes N1 and N 2, respectively.Referring to Eq. (6), t is a scaling parameter which was taken as 0.0015 for all alloys.…”

“…The need of this parameter has been discussed elsewhere. (7) where z 1 , z 2 and z 3 are geometrical parameters that depend on the type of nucleation site in the austenite grain, 7) g ag ϭ0.6 J m Ϫ2 and g gg ϭ 0.85 J m Ϫ2 are the interfacial energies per unit area of the a/g interface and g/g grain boundary 7) and DG v is the driving force for volume nucleation. DG v was obtained from…”

Comportamento De Aço Estrutural Api 5l X65 Após Exposição Ao Hidrogênio: Uma Abordagem Baseada Na Fratura Dúctil

“…O controle da composição química dos aços tem como objetivo proporcionar maior homogeneidade microestrutural e melhorar as propriedades mecânicas da liga metálica. Como exemplo de microestrutura bandeada se pode citar a ferrita-perlita em aços de baixo e médio carbono forjados e laminados a quente (Geels, 2007;Offerman, et al, 2002).…”