Kinetics of austenite growth and bainite transformation during reheating and cooling treatments of high strength microalloyed steel produced by sub-rapid solidification

First,strip cast samples of high strength microalloyed steel with sub-rapid solidification characteristics were prepared by simulated strip casting technique.Next,the isothermal growth of austenite grain during the reheating treatment of strip casts was observed in situ through confocal laser scanning microscope(CLSM).The results indicated that the time exponent of grains growth suddenly rise when the isothermal temperature higher than 1000℃.And the activation energy for austenite grain growth were calculated to be 538.0 kJ/mol in the high temperature region(above 1000℃)and 693.2 kJ/mol in the low temperature region(below 1000℃),respectively.Then,the kinetics model of austenite isothermal growth was established,which can predict the austenite grain size during isothermal hold very well.Besides,high density of second phase particles with small size was found during the isothermal hold at the low temperature region,leading to the refinement of austenite grain.After isothermal hold at different temperature for 1800 s,the bainite transformation in microalloyed steel strip was also observed in situ during the continuous cooling process.And growth rates of bainite plates with different nucleation positions and different prior austenite grain size(PAGS)were calculated.It was indicated that the growth rate of the bainite plate is not only related to the nucleation position but also to the PAGS.

INFLUENCE OF HOT DEFORMATION ON CONTINUOUSCOOLING BAINITIC TRANSFORMATION IN A LOWCARBON STEEL

The influence of hot deformation conditions on continuous cooling bainitic transformation has been investigated for a low carbon microalloyed steel. The CCT diagrams show that deformation in unrecrpstallized austcnite ation can accelerate transformation process. Bainitic transformation in intermediate transformation temperature region is prominent, and the proeutectoid polygonal ferrite transformation at evelated high temperature is suppressed. According to optical and TEM analyses, low carbon bainitic ferrite is characterized by granular and lathlike ferrite, based on the cooling rate and deformation conditions. For nondeformation, groaps of coarse parallel ferrite lath form from the prior austenite grain boundaries with the same crystallographic orientation. For heavy deformaton, cell structure within the austenite grains due to the high dislocation density formed, which provides more nucleation sites for bainite ferrite. So deformation can discontinue the growth of ferrite laths and decrease the length of ferrite laths.

Pre-bainitic Transformation in Fe-Ni Alloy

The behaviour of the pre bainitic transformation in Fe Ni alloy was investigated by using SAM, X ray diffraction and TEM. The results show that there is segregation of Ni atoms in austenite and that bainite forms in depleted regions of Ni atoms.

THERMODYNAMICS OF BAINITIC TRANSFORMATION IN Cu-24 at.-% Al ALLOY

Two driving forces △G^((β_1)+α')and △G^(β_1→β_2+α)for the bainitic transformation have been calcu- lated for Cu-24 at.-% Al alloy.Results reveal △G^(β_1→α')>0 within the transformation tem- perature range 600—750 K.Therefore,the bainitic transformation cannot be initiated by the β_1→α' shearing mechanism.On the other hand,the driving force △G^(β_1→β_2+α)to produce 5% of the α becomes negative only when the composition of the α satisfies x_(Al)~α≤0.204 at 700 K or x_(Al)~α=0.209 at 750 K.So the bainitic transformation can only proceed by β_1→β_2+α diffusional reaction with different diffusion amount at different temperatures.

THEORY OF BAINITIC TRANSFORMATION: A MODEL WITH COUPLED FIRST-ORDER PHASE TRANSFORMATIONS

The Ginzbury-Landau theory for bainitic transformation was devised, which contains two first-order phase transformations, one being reconstructive represented by the diffusional proeutectoidal precipitation of ferrite, and the other the displacive transformation. It provides a coupled mechanism for the formation of bainite. With the numerical simulation results, a diffusion-induced nucleation and a diffusion-accompanied growth of displacive transformation were suggested. This theory can be helpful to over- throw the thermodynamic difficulty of displacive transformation above the Ms temperature, and also helpful to understand the Bs temperature, the partial supersaturation, the single variation of bainitic carbides, and the incomplete-reaction phenomenon of bainitic transformation, etc..

A MODEL FOR BAINITIC TRANSFORMATION KINETICS WITH PARABOLIC NUCLEATION RATE

Based on the assumptions of parabolic variation of nucleation rate versus time and transformation kinetics depending mainly on nucleation rate, a different model for bainitic transformation kinetics in hypoeutectoid steels was established. And this model was proved to be effective in the description of bainitic transformation by comparison with the result of conventional Avrami equation.

In situ observation of the effect of Al N particles on bainitic transformation in a carbide-free medium carbon steel

The bainitic transformation of the steels with different mass fractions of N, ~0.002% and 0.021%, was observed in situ by using high-temperature metalloscope. Micrometer-and nanometer-sized aluminum nitride(AlN) particles were found in the steel with 0.021% N.Grain boundaries, the interior of the grains, and Al N particles were used as initial nucleation sites of bainitic ferrite, and bainitic ferrite subunits served as new nucleation sites to induce secondary nucleation. The lengthening rate of bainitic ferrite varied at different nucleation sites, which was controlled by the repeated nucleation and growth of bainitic subunits. The Al N particles not only provided several nucleation sites, but also increased the autocatalytic effect on the transformation, further shortening the incubation period, promoting the bainitic transformation, and refining the bainitic microstructure.

Cyclic stress induced phase transformation in super-bainitic microstructure

The effects of cyclic stress loading on the microstructual evolution and tensile properties of a medium-carbon super- bainitic steel were investigated. Experimental results show that the cyclic stress can induce the carbon gathering in austenite and phase transformation from film-like retained austenite to twin martensite, which will obviously enhance the tensile strength and the product of tensile strength and ductility. The higher the bainitic transformation temperature, the lower the transformation rate of the retained austenite. The amount and thickness of the film-like retained austenite play an important role during the cyclic stress induced phase transformation.

CHARACTER OF CARBON-DEPLETED REGIONS IN UNDERCOOLED AUSTENITE AND THERMODYANMICS OF BAINITIC TRANSFORMATION

The chemical driving forces for the phase transformation from austenite in three different car- bon-depleted regions into ferrite with the identical composition have been derived respectively. The starting temperature of bainitic transformation,B_s,has also been calculated according to the shearing mechanism of bainitic transformation in carbon-depleted regions of austenite. The maximum driving forces for banitic transformation at B_s in both 3% Cr and the CrMo steels were obtained.It was shown that the proposed shearing mechanism of the bainitic trans- formation is thermodynamically possible.

Combined Reconstructive and Displacive Mechanisms Involved in Bainitic Transformation

TEM observation and analysis have been conducted on bainitie transformation with and without the influence of externally applied tensile sress for alloyed steel 35MV7. Recrystallizafion was found to occur within the bainitic structures transformed at 450 ℃ in cases of both with and without the application of external stress, and coupling between the reconstructive and displacive mechanisms is expected, due to the relatively high holding temperature and high dislocation density yielded with the displacive mechanism. RecrystaUization was not observed within the bainitic structures transformed at lower temperature of 350 ℃, both with and without the application of stress; However, for the stressed specimen, the structure with very fine subgrains was found to be preserved and not reconstructed thermically, due to the low temperature and short holding time.

Relationship between Relaxation Internal Friction Peak at about 200℃(f≈1Hz)and Bainitic Transformation In a CuZnAl Alloy

The present investigation showed that the re- laxation peak at about 200℃ (f≈1Hz) was related to bainitic transformation in a CuZnAl alloy.The peak decreased when bainite precipitated.The activation energy of the peak was the same as the apparent activation energy of bainitic transforma- tion.According to the relations between the peak and the bainitic transformation,the mechanism of the peak has been discussed.

PHASE TRANSFORMATION UNIT OF BAINITIC FERRITE AND ITS SURFACE RELIEF IN LOW AND MEDIUM CARBON ALLOY STEELS

The lath-or plate-shaped bainitic ferrite of low and medium carbon alloy steels consists of packets of ferrite sublaths which are composed of many finer and regular ferrite blocks.They are uniform shear growth units of bainitic phase transformation.No carbide is precipitated from them.The bainitic O-carbides are precipitated from γ-α interface or carbon-rich austenite.The mode of arrangement of the units in ferrite sublath packet is in uni-or bi-di- rection.Single surface relief is produced by the accumulation of uniform shear strains with all the ferrite units arranged unidirectionally in a sublath packet,while tent-shaped surface relief is formed by the integration of the uniform shear strains of two groups with ferrite units piling up in two directions and growing face to face;whereas if they grow back to back,the integra- tion will be responsible for invert-tent-shaped surface relief.The interface trace between two groups of ferrite units in a sublath packet is shown as“midrib”.

A Crystallography Study of Bainitic Transformation in a Cu-Zn-Al Alloy

The crystallographic nature of initial isothermal phase transformation in Cu-26Zn-4Al (wt-％) was investigated. The kinetic transforma- tion curve, morphology, crystal structure, substructure, orientation relationship and twin relationship of bainite plates have been studied by means of optical and transmission electron microscopy. The experimental results showed that the characteristics of initial 8→bainite transformation are not exactly consistent with that found in martensite transformation, for example, orientation relationship between matrix and bainite does not exist in martensite transformation.

Kinetics of bainite-to-austenite transformation during continuous reheating in low carbon microalloyed steel

A dilatometer was used to study the kinetics of bainite-to-austenite transformation in low carbon microalloyed steel with the initial microstructure of bainite during the continuous reheating process. The bainite-to-austenite trans- formation was observed to take place in two steps at low heating rate. The first step is the dissolution of bainite, and the second one is the remaining bainite-to-austenite transformation controlled by a dissolution process. The calculation result of the kinetics of austenite formation shows that the two steps occur by diffusion at low heating rate. However, at high heating rate the bainite-to-austenite transformation occurs in a single step, and the process is mainly dominated by shear. The growth rate of austenite reaches the maximum at about 835℃ at different heating rates and the growth rate of austenite as a function of temperature increases with the increase in heating rate.

The influence of stress and strain on kinetics of bainite transformation

The influence of stress and strain on the kinetics and transformation plasticity of bainite transformation has been studied using a 26Cr2Ni4MoV steel by thermal simulation experimental method. It has been found that as applied stress increases the maximum volume fraction of bainite increases for applied stress lower than the yield stress but decreases for applied stress higher than the yield stress. This is believed to be related with the strengthening of parent phase due to pre-deformation. Experimental results indicate that applied stress accelerates bainite transformation. When applied stress is lower than the yield stress, the parameter of transformation plasticity increases with applied stress while it becomes a constant for stress larger than yield stress.

A Rationalization of Sympathetic Nucleation-Ledgewise Growth Theory of Bainite Transformation in Fe-C Alloys

Based on the classic diffusion controlled nucleation and goth theory, the sympathatic nucleation- ledgewise growth mechanism of bainite was studied theoretically for low carbon Fe-C alloys. The rationality of the occurrence of sympathetic nucleation on the terraces of ledges compating with lateral ledge growth and other sites nucleation was demonstrated by the present work quantita- tively The calculations indicated that low reaction temperatures and high carbon concentrations may favor the sympathetic nucleation, thus accounting for the formation of multilayer structures of bainite.

DRIVING FORCE OF BAINITE TRANSFORMATION FOR Fe-C-Mn ALLOYS

The regular solution model developed by Hillert and Stanffanson is applied to calculate the driving force for transformations with three possible mechanisms in the intermediate tempera- ture range between proeutectoid and martensite transformation.Focus is made on the thermodynamical discussions for both diffusional and shear mechanisms for bainitic transfor- mations in different temperature ranges.

TRANSFORMATION OF UPPER BAINITE IN 55SiMnMo STEEL DURING TEMPERING

The transformation of upper bainite of normalized 55SiMnMo steel during tempering has been studied by optical microscopy,X-ray diffraction,transmission electron microscopy and differential scanning calorimetry.Ferrite plate surface is parallel to ferrite{110}and is the coherent interface for α and γ,the crystallographic relationship is(111)_γ,∥(011)_α.The small plate surface of Fe_3C precipitated during tempering is parallel to(100).The strengthening mechanism for carbon-rich austenite in normalized condition is primarily uniform solid so- lution hardening,however,in the tempered state,it is strengthened chiefly by non-uniform one.

Dynamic Observation of Bainite Transformation in a Fe-C-Mn-Si Superbainite Steel

The dynamic observations of bainitic transformation in a Fe-C-Mn-Si superbainite steel were conducted on a high temperature laser scanning confocal microscope. It is indicated that the mutual intersection of bainite sheaves often occurs during growth of bainite ferrite, resulting in an interlocked bainite microstructure. Moreover, bainite transformation is promoted by higher austenization temperature and the longer and finer bainite platelets are obtained. Further, The average growth rate of bainite after austenization at 1 100 ℃ is calculated as 5.8 μm·s -1. In situ observation investigation makes it possible to identify bainite transformation in real time during isothermal holding.

Effects of External Stresses on the Carbide Precipitation in Bainite Transformation

TEM observation of carbide precipitates in bainite has been carried out for steel 40CMD8. Specimens transformed at 350 ℃ and 450 ℃ without external stress contain respectively the lower and the upper bainite. No fundamental change in carbide precipitation is observed in specimen transformed at 350 ℃ under stress of 128 MPa. The very fine cementite is found precipitated within the upper bainite plates in specimen transformed at 450 ℃ under stress of 144 MPa, which can be explained as the result of the difficulty of carbon diffusing out of the massive ferrite plate within a much shortened transformation time due to the external stress. For all the specimens examined, Bagaryatskii orientation relationship is found existing between the carbides and ferrite matrix.