Continuous Cooling Transformation Behavior Research Articles

Simulation Kinetics of Austenitic Phase Transformation in Ti+Nb Stabilized IF and Microalloyed Steels

In the present study, the influence of cooling rates (low to ultrafast) on diffusion controlled and displacive transformation of Ti-Nb IF and microalloyed steels has been thoroughly investigated. Mechanisms of nucleation and formation of non-equiaxed ferrite morphologies (i.e., acicular ferrite and bainitic ferrite) have been analyzed in details. The continuous cooling transformation behavior has been studied in a thermomechanical simulator (Gleeble 3800) using the cooling rates of 1-150 °C/s. On the basis of the dilatometric analysis of each cooling rate, continuous cooling transformation (CCT) diagrams have been constructed for both the steels to correlate the microstructural features at each cooling rate in different critical zones. In the case of the IF steel, massive ferrite grains along with granular bainite structures have been developed at cooling rates > 120 °C/s. On the other hand, a mixture of lath bainitic and lath martensite structures has been formed at a cooling rate of ~ 80 °C/s in the microalloyed steel. A strong dependence of the cooling rates and C content on the microstructures and mechanical properties has been established. The steel samples that were fast cooled to a mixture of bainite ferrite and martensite showed a significant improvement of impact toughness and hardness (157 J, for IF steel and 174 J for microalloyed steel) as compared to that of the as-received specimens (133 J for IF steel and 116 J for microalloyed steel). Thus, it can be concluded that the hardness and impact toughness properties are correlated well with the microstructural constituents as indicated by the CCT diagram. Transformation mechanisms and kinetics of austenitic transformation to different phase morphologies at various cooling rates have been discussed in details to correlate microstructural evolution and mechanical properties.

Continuous cooling transformation behavior and the kinetics of bainite formation in a bainitic–martensitic steel

Abstract The continuous cooling transformation diagram of a low carbon bainitic–martensitic steel was constructed using dilatometry and metallographic methods. It was found that as cooling rate increases, the structure changes from granular bainite to lath martensite. Three regions of different kinetic behavior were discerned for the bainitic–martensitic steel. One of the regions conformed to martensite formation and the other two comprised transformation to bainite. A non-isothermal type of Johnson–Mehl–Avrami–Kolmogorov kinetic equation of reaction rate was used to analyze the transformation behavior during continuous cooling of bainite formation. The Avrami exponent and activation energy values for different regions at cooling rates of 0.1 and 0.4 K s−1 varied from 1.5 to 4.7 and 71 to 84 kJ mol–1 respectively. Models obtained from such kinetic coefficients closely corresponded to experimental results.

Continuous cooling transformation behavior and impact toughness in heat-affected zone of Nb-containing fire-resistant steel

Simulated heat-affected zone continuous cooling transformation diagram was developed for advanced fireresistant steel. Over a wide range of cooling rates, corresponding to t8/5 from 6 s to 150 s, granular bainite was the dominant transformation constituent, while the morphology of less dominant martensite-austenite (M-A) constituent changed from film-like to block-type constituent; but the hardness remained similar to the average value of 190-205 HV (0.2). The start and finish transformation temperature was high at 700 °C and 500 °C, and is different from the conventional high strength low alloy steels. It is believed that the high-content (0.09 wt%) of Nb may promote bainite transformation at relatively high temperatures. Martenistic matrix was not observed at high cooling rate and the film-like M-A constituent and blocky M-A constituent with thin film of retained austenite and lath martensite were observed on slow cooling. Excellent impact toughness was obtained in the heat-affected zone with 15-75 kJ/cm welding heat input.

Effect of vanadium on dynamic continuous cooling transformation behavior of medium-carbon forging steels

Dynamic continuous cooling transformation (CCT) behavior of medium-carbon forging steels microalloyed with different V contents up to 0.29% was investigated by means of dilatometric measurement, microstructural observation and hardness measurement. The results showed that the dynamic CCT diagrams were similar and the main difference was that the fields of the transformation products were shifted to the right side of the diagrams with the increase of V content, and this effect was more noticeable with an addition of 0.29% V. The Ac1 and Ac3 temperatures showed increasing trends with increasing V content, while the critical cooling rates decreased with increasing V content. The increase of V content resulted in significant increase of hardness and this tendency was enhanced with increasing cooling rate until the formation of acicular ferrite (AF). A promising approach of remarkably improving the toughness of ferritic-pearlitic medium-carbon forging steels with suitable V addition and the introduction of AF without notable penalty on its strength level was suggested.

High Quality TMCP Production and Metallurgy of Niobium Bearing Steels

The technological and metallurgical advancements of value-added niobium (Nb) microalloyed thermo-mechanical controlled process (TMCP) plate steels continue to be developed for more demanding end user requirements. The market demand for reduced fuel consumption and CO2 emissions in the automotive and construction sectors have further increased the demand for these new and advanced higher quality Nb-bearing steel grades. Often, the transition from laboratory melted and TMCP hot rolled heats to the production scale requires some continuous casting, thermal and mechanical metallurgy adjustments from the laboratory results in order to accomplish proper industrial continuous casting and hot rolling processes. These advanced high strength steels are microalloyed with Nb, Mo and/or other elements which affect the austenite-ferrite transformation. Niobium enables achievement of substantial grain refinement when the plate is rolled with the proper reduction and thermal schedule. The effects of these microalloying elements on the continuous cooling transformation behavior must be carefully controlled during the reheating and rolling process to successfully achieve the desired mechanical properties. TMCP applications have been successfully developed in numerous product sectors with thickness exceeding 120 mm. Since the very fine grained microstructure improves toughness and increases the yield strength, this TMCP process enables the required tensile properties with the growing trend to leaner chemical composition designs (less than 0.10%C) and excellent toughness properties. The consequence of leaner chemical compositions, especially lower carbon content and lower carbon equivalent enhances mechanical properties, fabrication and weldability.

Continuous cooling transformation behaviour and bainite formation kinetics of new bainitic steel

In order to avoid the appearance of soft particles composed of ferrite or pearlite in the actual production of new bainitic steel, the phase transformation behaviour and bainite formation kinetics were investigated by DIL805A dilatometer, optical microscopy, scanning electron microscopy and Vickers-durometer. The results show that the soft particles cannot appear when the cooling rate exceeds 0.025 K s−1, and this condition can be ensured by direct spray cooling in production. The local activation energy decreases with increasing transformed bainite volume fraction (fb), and the average energy is about 136.7 kJ mol−1. The local Avrami exponent mainly lies between 0.5 and 3 in a wide fb range, indicating that the dominating mechanism of bainite formation is two-dimensional and one-dimensional growth.

Dynamic Continuous Cooling Transformation Behavior of A Novel Cu-bearing Pipeline Steel

Dynamic Continuous Cooling Transformation Behavior of A Novel Cu-bearing Pipeline Steel

Continuous cooling transformation behavior in the weld coarse grained heat affected zone and mechanical properties of Nb-microalloyed and HY85 steels

Continuous cooling transformation (CCT) diagrams were determined for weld simulated coarse grained heat affected zone (CGHAZ) of Nb-microalloyed and HY 85 steels using a Gleeble®3800 thermo-mechanical simulator. Samples were heated to a peak temperature of 1573K (1300°C) and cooled at different cooling rates from 0.5K/s to 120K/s. Microstructures, hardness, and impact toughness at 223K were determined corresponding to simulated samples of weld CGHAZ thermal cycles. Transformation temperatures AC3, AC1, Bs, Bf, Ms, and Mf were also determined using dilatometric analysis. At slow cooling rates pro-eutectoid ferrite and pearlite were observed in Nb-microalloyed steel and a mixture of bainitic ferrite and granular bainite was observed in HY 85 steel but no pro-eutectoid ferrite was observed. At moderate cooling rates, only bainite was observed in both the steels. At high cooling rates, lath martensite was observed for both the steels. A strong dependence of prior austenite grain size on mechanical properties has been found. A prior austenite grain size of approximately 30μm (80K/s) has shown impact toughness more than 100J and 198J for Nb-microalloyed steel and HY85 steel samples respectively. Based on microstructures, hardness, impact toughness, and transformation temperatures, CCT diagrams for the weld CGHAZ region were determined.

Effect of Continuous Cooling Rate on Transformation Characteristic in Microalloyed Low Carbon Bainite Cryogenic Pressure Vessel Steel

Continuous cooling transformation behavior of a low carbon bainite microalloyed cryogenic pressure vessel steel was analyzed to explore transformation mechanism of super-cooled austenite on MMS-300 thermomechanical simulator. Microstructure transformation laws of steel at various cooling rates were determined by means of dilatometric measurement and microstructure observation. The results showed that the bainite microstructure was formed in broad range of cooling rate 5–30 °C/s. The martensitic transformation existed above 50 °C/s and the martensitic microstructure was dominated above 150 °C/s. The martensitic transformation temperature was first increased and then decreased with the increase of cooling rate in the range of 1–30 °C/s and the final transformation of bainite and martensite would move toward higher temperature at higher cooling rate.

Effect of Nitrogen Addition on Continuous Cooling Transformation Behavior of Vanadium Microalloyed Steels

Effect of nitrogen addition on continuous cooling transformation behavior of vanadium microalloyed steels was investigated. Therefor, CCT curves of three experimental steels with different vanadium and nitrogen content were measured by thermal dilatometer; the microstructural evolution of the steels with the varying cooling rates was characterized; their precipitation behavior was tracked, and planer lattice misfit degree of the precipitates with the ferrite matrix was calculated. The results show that ferrite transformation is promoted by nitrogen addition, and the starting temperature of transformation and the critical cooling rate for full bainite transformation are increased as well. For cooling rates in a range0.8- 1.6℃/s, the microstructure of steels with low nitrogen consists of granular bainite + lath like bainite,while acicular ferrite also exists; during or after the γ-α transformation vanadium compounds in low nitrogen steels precipitate mainly as VC, the quantity of which increases with the increasing vanadium content. However, for the steel rich in nitrogen, vanadium compounds precipitate as VN in austenite at high temperature. The lattice misfit degree of ferrite with the precipitates of austenite, VC and VN, which occurred at 900℃are 6.72%, 3.89% and 1.55% respectively. It indicates that VN precipitates act as preferential nucleation sites for ferrites and promote the ferrite transformation.

Continuous Cooling Transformation Behavior and the Phase Transformation Model in Low Carbon High Strength Sheet Steel

Phase transformation of austenite continuous cooling process in low carbon high strength sheet steel has been researched by DIL805 thermal mechanical simulate. The Austenite continuous cooling transformation (CCT) diagram of steel has been determined by dilatometry and metallography. With the increase of cooling rate, ferritic transformation, perlitic transformation, bainite transformation and martensitic transformation have produced in the organization. Mathematical equations of phase transformation point-cooling rate and phase variable-cooling rate have been established and phase transformation model of high fit degree has been gained by regression calculation. The results show that calculated value and experimental value are nearly similar, so the phase transformation model is feasible.

Continuous Cooling Transformation Behavior of a Ti Addition Steel

The continuous cooling transformation (CCT) behavior of a Ti attached steel was studied through thermal simulation tests, and the influences of different cooling rates on the microstructure and transformation were investigated. The results show that the microstructure changes with the cooling rate, and the CCT curve of studied steel is plotted, which indicates that the solid-state phase transformation mainly consists of four regions. The CCT diagram made it possible to predict the microstructures of studied steel with different cooling rates.

Continuous Cooling Transformation, Microstructure and Mechanical Properties of High-Strength Low-Alloy Steels Containing B and Cu

This study investigated the continuous cooling transformation, microstructure, and mechanical properties of highstrength low-alloy steels containing B and Cu. Continuous cooling transformation diagrams under non-deformed and deformed conditions were constructed by means of dilatometry, metallographic methods, and hardness data. Based on the continuous cooling transformation behaviors, six kinds of steel specimens with different B and Cu contents were fabricated by a thermomechanical control process comprising controlled rolling and accelerated cooling. Then, tensile and Charpy impact tests were conducted to examine the correlation of the microstructure with mechanical properties. Deformation in the austenite region promoted the formation of quasi-polygonal ferrite and granular bainite with a significant increase in transformation start temperatures. The mechanical test results indicate that the B-added steel specimens had higher strength and lower upper-shelf energy than the B-free steel specimens without deterioration in low-temperature toughness because their microstructures were mostly composed of lower bainite and lath martensite with a small amount of degenerate upper bainite. On the other hand, the increase of Cu content from 0.5 wt.% to 1.5 wt.% noticeably increased yield and tensile strengths by 100 MPa without loss of ductility, which may be attributed to the enhanced solid solution hardening and precipitation hardening resulting from veryfine Cu precipitates formed during accelerated cooling.

Continuous Cooling Transformation Behavior of X70 Pipeline Steel

The continuous cooling transformation behaviors were researched on X70 pipeline steel through two pass deformation and non-deformed austenite using Gleeble-3500 thermal mechanical simulator, and static continuous cooling transformation curve and dynamic continuous cooling transformation curve were measured through thermal dilation method and metallographic method. The influence of cooling rate and deformation parameters on microstructure was analyzed. The results show that the hot deformation accelerates the acicular ferrite and polygonal ferrite phase transformation, increases the starting transformation temperature and the finishing transformation temperature significantly, and shifts the CCT curve moving upward to the left side corner. Acicular ferrite is obtained in practice using accelerated cooling rate after deformation Acicular ferrite can be obtained in wider range of cooling rates, and microstructure and island structure is finer through hot deformation.

Continuous cooling transformation behaviors of CLAM steel

The continuous cooling transformation (CCT) behaviors of CLAM (China Low Activation Martensitic) steel were studied, the CCT diagram was constructed, and the influence of cooling rates on the microstructures was also investigated. The microstructures were investigated using optical microscopy (OM) and microhardness tests were also carried out. The results showed that CLAM steel possessed high hardenability and there were ferrite and martensite transformation regions only. The maximum cooling rate to form ferrite microstructure was found to be 10–12K/min. In order to obtain fully ferrite microstructure, the cooling rate should be lower than 1K/min. The CCT diagram also gave relevant parameters such as the transformation temperatures, i.e., Ac1, Ac3, Ms and Mf were 1124K, 1193K, 705K and 593K, respectively. The diagram made it possible to predict the microstructures and properties of CLAM steel with different cooling rates.

Influence of Deformation Temperature on γ-α Phase Transformation in Nb–Ti Microalloyed Steel during Continuous Cooling

The dynamic continuous-cooling-transformation (CCT) diagrams of Nb–Ti microalloyed steel for four different deformation temperatures of 850, 900, 950 and 975°C were plotted by means of a combined method of dilatometry and metallography. The influence of deformation temperature on continuous cooling transformation behavior and transformation microstructure was elucidated. The reason that the polygonal ferrite transformation field is shifted to the right and moved down as the deformation temperature is increased and the progress of transformation for lowest continuous cooling rate of 0.5°C/s was obviously retarded at the lowest deformation temperature of 850°C was discussed in details based on transformation kinetics and strain-induced precipitation kinetics, respectively. Moreover, the polygonal ferrite grain size for lowest continuous cooling rate of 0.5°C/s can be significantly refined from approx. 15.4 μm to 9.4 μm as the deformation temperature is reduced from 975°C to 850°C. In addition, the empirical equation to calculate γ-α phase transformation start temperature was established, and the calculated Ar3 temperatures are in good agreement with measured ones.

Effects of Carbon on Continuous Cooling Transformation Behavior and Heat Treatment Characteristics of Flake Graphite Cast Iron

Effects of Carbon on Continuous Cooling Transformation Behavior and Heat Treatment Characteristics of Flake Graphite Cast Iron

Effects of Chromium on Continuous Cooling Transformation Behavior and Heat Treatment Characteristics for Low Alloyed Ductile Iron

Effects of Chromium on Continuous Cooling Transformation Behavior and Heat Treatment Characteristics for Low Alloyed Ductile Iron

Effects of Carbon on Continuous Cooling Transformation Behavior and Heat Treatment Characteristics for Spheroidal Graphite Cast Iron

Effects of Carbon on Continuous Cooling Transformation Behavior and Heat Treatment Characteristics for Spheroidal Graphite Cast Iron

Continuous Cooling Transformation Behavior and Kinetic Models of Transformations for an Ultra-Low Carbon Bainitic Steel

The aim was to investigate transformation behavior and transformation kinetics of an ultra-low carbon bainitic steel during continuous cooling. Continuous cooling transformation (CCT) curves of tested steel were measured by thermal dilatometer and metallographic structures at room temperature were observed by optical microscope. Then transformation kinetic equation of austenite to ferrite as well as austenite to bainite was established by analyzing the relationship of lnln[1/(1 −f)] and lnt in the kinetic equation on the basis of processed experimental data. Finally, the measured and calculated kinetic behaviors of the steel during continuous cooling were compared and growth patterns of transformed ferrite and bainite were analyzed. Results showed that calculated result was in reasonable agreement with the experimental data. It could be concluded that the growth modes of transformed ferrite and bainite were mainly one dimension as the Avrami exponents were between 1 and 2.