Papers by Keyword: Bainite Transformation

Abstract: We present calculations of austenite to bainite phase transformation start for different cooling paths and for different steel compositions and a method to estimate the cooling water required to cool a steel strip to desired temperatures during water cooling line after industrial hot rolling. We also quantitatively compare how different alloying elements affect the phase transformation activation energy and the time required for the transformation to start and proceed to the extent that it can be detected with dilatometer. This analysis can be used for aid when designing suitable cooling paths for hot rolled steel products. The calculations of the activation energy can be used as input in more detailed microstructure models.

Abstract: A Q550 high strength steel was selected at two different cooling rates through ultra-fast cooling process, and its microstructures and strengthening mechanisms were analyzed. The results show the bainite transformation temperature of the steel decreased with the increasing of cooling rate.The ultra-fast cooling process can improve the performance of Q550 compared with the conventional cooling process, and the yield strength, tensile strength and elongation are 600MPa, 755MPa and 19%, and - 20 °Cimpact energy is 253J, and good strength and toughness are obtained under ultra-fast cooling process. The microstructure of this steel is bainite and good strength and toughness are caused by the refinement of bainite and fine precipitates. Ultra-fast cooling technology improves the strength and toughness of this steel effectively.

Abstract: The amount of carbon in solid solution in bainitic ferrite at the early stage of transformation has been directly determined by atom probe tomography at 200 °C, taking advantage of the extremely slow transformation kinetics of a novel nanocrystalline steel. Results demonstrated that the original bainitic ferrite retains much of the carbon content of the parent austenite providing strong evidence that bainite transformation is essentially displacive in nature.

Abstract: Dilatometric measurements were used to design the processing parameters of two types of bainitic steels. The first type is a hypoeutectoid ultra fine bainite steel, for which the dilatometer was used to locate the temperature at which cementite is completely dissolved during intercritical annealing (TC). The intercritical annealing temperatures are then selected will above TC. To obtain the martensite start temperatures (MS), the steel is quenched to the room temperature (RT) from these selected temperatures and then the bainite transformation temperatures were selected to be well above MS. The dilatometer was then used to monitor the bainite transformation kinetics from which the required time frames for cessation of the bainitic reactions were estimated. In the second type, bimodal bainite had been produced in thermo-mechanically processed TRIP-steel. A deformation dilatometer is used to perform three deformation-steps before slow cooling to form approx. 30% polygonal ferrite. The material was then rapidly cooled to the first bainite formation temperature. During this step, the dilatometer was used to monitor the bainite reaction from which the required time for 50% decomposition of austenite is estimated. The martensite start of the undecomposed austenite was located by quenching to RT. The second bainite transformation step was then performed well above the new MSII to form a second generation of finer bainite.

Abstract: In steel wire processing it is difficult to reach a homogenous structure throughout the cross-section of the wire particularly in greater diameters. One alternative for producing a homogenous structure is to find a cooling path with a wide transformation temperature range. Fully austenite steel wire rolled at high temperatures can be decomposed into ferritic-martensitic dual phase structure using relatively slow cooling rates. Test materials were low alloyed low carbon steels with variations in alloying elements. Gleeble-1500 thermomechanical simulator was utilised to study the effect of cooling rate on decomposition of austenite after deformation. The microstructures were studied with an optical microscope. In certain low alloyed steels slow cooling rates eliminate the bainite transformation and instead martensite is formed. The final microstructure depends mainly on the carbon content but also on the amount of other alloying elements and their effects on the austenite phase.

Abstract: Low carbon-manganese wrought steels with addition of Ti-Al-N have been treated in order to obtain acicular ferrite structure. The microstructure of fine acicular ferrite nucleated intragranularly on Ti(C,N)+AlN and Ti(C,N)+AlN+MeS inclusions has showed high strength and toughness at low temperatures.

Abstract: New conceptual TMCP process for manufacturing high strength steel plates, which is applied an on-line heat treatment immediately after accelerated cooling (ACC), was developed. Transformation and precipitation behavior in the new TMCP process was investigated and compared with those in conventional ACC process and quenching and tempering process (Q+T). In the ACC process and Q+T process, microstructures were consisted of bainitic ferrite and second phase, such as cementite or martensite-austenite constituent (MA). And fine carbides, which were formed randomly, were observed in Q+T steel. On the other hand, in the new TMCP process polygonal ferrite was observed in addition to bainitic ferrite and cementite, and two kinds of precipitation forms, random precipitation and row precipitation, were observed. It was found that ferrite transformation is promoted during heating after accelerated cooling, which brings row precipitation of fine carbides. Furthermore, Control of the formation of MA this new TMCP process. In the conventional ACC process, MA constituents are formed from carbon enriched untransformed austenite during air cooling after ACC, and formation of MA is hard to prevent for higher strength steels. On the other hand, carbon enrichment to untransformed austenite can be prevented by carbide formation during on-line heat treatment after ACC. It was demonstrated that homogeneous microstructure with very low amount of MA constituents was achieved by the new TMCP process. And, absence of brittle phase brought excellent resistance to hydrogen induced cracking in NACE sour environment. In this paper, details of the metallurgical and mechanical feature of this new TMCP steel were discussed, and application to sour resistant linepipe steel was introduced.

Abstract: The platelike bainitic ferrite growth rates were calculated by two modified diffusional models. Good agreements between experimental and theoretical results are found in Fe-0.59C wt.%, Fe-0.81C wt.% and Fe-0.478C-4.87Ni wt.% alloys. A slowing down effect due to the alloying element Mo is emphasized in Fe-0.69C-1.8Ni-0.8Mo wt.% alloy. However, the experimental data are lower than theoretical ones about two orders in Fe-C-8.7Ni wt.% alloys. According to the discussion of the results, it is suggested that the bainite transformation mechanism may relates to steel composition and transformation temperature.