Influence of the Prior Austenite Microstructure on the Transformation Products Obtained for C-Mn-Nb Steels after Continuous Cooling.
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In the present work, the ferrite grain sizes after transformation from non-deformed (recrystallized) and work-hardened austenite have been considered. Two C-Mn-Nb steels have been deformed by multipass torsion tests carried out at temperatures, both above and below the determined Tnr (non-recrystallization temperatures) for these steels. The influence of the prior austenite grain size, the retained strain and the cooling rate on the final ferrite microstructure has been analysed. Austenite grain sizes varying from 28 to 125 μm with retained strains ranging from 0 (non-deformed austenite) to 2 and cooling rates of 1 and 5 s-1 have been considered. It has been observed that higher cooling rates tend to produce acicular microstructures, and that this effect is enhanced at the lower retained strains. It has been shown that the reduction of austenite grain size and an increase of retained strain, provide a higher density of nucleation sites, which leads to a refinement of the ferrite. A similar effect is produced by increasing the cooling rate from 1 to 5 s-1.Keywords:
Recrystallization (geology)
Acicular ferrite
Continuous cooling transformation
Acicular
The microstructural evolution of the high Nb X80 pipeline steel in Continuous Cooling Transformation (CCT) by Gleeble-3500HS thermal mechanical simulation testing system was studied, the corresponding CCT curves were drawn and the influence of some parameters such as deformation and cooling rate on microstructure of high Nb X80 pipeline steel was analyzed. The results show that as cooling rate increased, the phase transformation temperature of high Nb X80 steel decreased, with the microstructure transformation from ferrite-pearlite to acicular ferrite and bainite-ferrite. When cooling rate was between 20°C/s and 30°C/s, the microstructure was comparatively ideal acicular ferrite, thermal deformation accelerates phase transformation notably and made the dynamic CCT curves move upward and the initial temperature of phase transformation increase obviously. Meanwhile the thermal deformation refined acicular ferrite and extended the range of cooling rate accessible to acicular ferrite.
Acicular ferrite
Continuous cooling transformation
Acicular
Atmospheric temperature range
Isothermal transformation diagram
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This article has studied the microstructure and hardness under the different intensity match the Nb-Cr X80 steel of SMAW joint,the results indicated that under different intensity match the filling pass microstructure through the following weld bead heat treatment,has broken the welded joint column crystals shape,weld joint microstructure is the acicular ferrite;coarse-grained zone microstructure is granular bainite+board ferrite banding.The cap weld microstructure is acicular ferrite;coarse-grained zone microstructure is board ferrite banding+granular bainite.The cap welded joint does not look like the root pass and the fill pass receives the heat treatment of the following bead,the high hardness microstructure of ferrite and the granular bainite increase obviously,caused hardness of the cap allover big.
Acicular ferrite
Acicular
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Microstructure and properties of pipeline steels with different Nb contents under the same processes were studied by thermal simulation test.The effect of alloy element Nb on transformation temperature,microstructure and microhardness was analyzed.The results show that with the increasing of Nb content,the transformation temperature is decreased during continuous cooling of super-cooling austenite,at the low cooling rate such as 1 ℃/s,the initial transformation temperature is decreased about 70 ℃/s and final transformation temperature is decrease over 100 ℃/s.The granular bainite and acicular ferrite appear in microstructure obviously.Nb promotes low-temperature bainite microstructure and grain refinement,at the same time,the microhardness of the materials is also increased slightly.
Continuous cooling transformation
Acicular ferrite
Acicular
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Transformation kinetics of AS- rolled Si- Mn- Cr- Mo dual phase steels has been obtained in Si- Mn-Cr steels containing 1.53%Mn and 0.6%Cr.That is,there is a metastable austenite region about temperature interval 100℃ on CCT diagram. It is found again that relative amount of polygonal ferrite in microstructure of continuous cooling transformation increases as cooling rates decrease,and that of bainite decreases.Acicular martensite arises in microstructure when cooling rates decrease to the certain extent.
Acicular ferrite
Continuous cooling transformation
Acicular
Metastability
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In order to study the impact of cooling rate on microstructure and mechanical properties in the low carbon bainitic steel of 600 MPa grade,and explore the ways and methods of obtaining the best performance,this paper investigated the microstructure of the 600 MPa grade steel is investigated by OM and TEM observations.After the steel is curled,the effect of cooling rate in the air on the microstructures in the coil plate head,the plate and the plate tail are researched. The results show that for 600 MPa grade steel,the microstructure is mainly composed by acicular ferrite,lath ferrite,granular bainite,lath bainite,and M/A. The higher cooling rate in the coil head and tail leads to more acicular ferrite,while more lath ferrite and granular bainite are obtained due to lower cooling rate in the middle section of the coil. Thus the strength of the coil head and tail is higher than that in the middle.
Acicular ferrite
Lath
Acicular
Carbon steel
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Beta ferrite
Acicular ferrite
Acicular
Continuous cooling transformation
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The austenitic transformation kinetics of quenched and tempered 22CrMnNiMo steel was investigated by dilatometric measurement at different cooling rates of 0.1∼20 K/s. It was found according to the dilatometric curves that the bainite transformation process of super-cooled austenite exhibited incompleteness during continuous cooling. The incomplete characteristics under different cooling rates were analyzed by determining both changes of ferrite fraction and formation rates with decreasing temperature. It indicated that the three-stage transformation during austenite decomposition was gradually transferred to a distinct two-stage process with increasing the cooling rate. On the basis of microstructural observation and transformation kinetic models, it was further demonstrated that the incompleteness of bainite transformation occurring at low cooling rate was mainly ascribed to the formation of displacive-controlled acicular ferrite, while that during intermediate or fast cooling was attributed to martensite transformation.
Acicular ferrite
Continuous cooling transformation
Isothermal transformation diagram
Acicular
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Continuous cooling transformation
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Acicular ferrite
Continuous cooling transformation
Acicular
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Acicular ferrite
Continuous cooling transformation
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Beta ferrite
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