Continuous Cooling Transformation Curve Research Articles

Effect of Nb and V on the continuous cooling transformation of undercooled austenite in Cr–Mo–V steel for brake discs

The increasing speed of trains necessitates the development of brake-disc materials that meet more stringent requirements. Therefore, Nb and V have been added to Cr–Mo–V steel to improve its thermal fatigue performance when used in brake discs. In this paper, the influences of Nb and V on the static continuous cooling transformation (CCT) behaviors of undercooled austenite were studied. The microstructures, hardness, and dislocation densities at different cooling rates and with the addition of different alloying elements were also investigated. The results show that the transformation products of ferrite, granular bainite, lower bainite, and martensite form under different cooling conditions. With increasing Nb and V contents, the CCT curves are shifted to the left, ferrite and bainite transformations are promoted, and the critical cooling rate of total martensite formation is increased. The added V mainly forms V-rich M8C7 precipitates and reduces the dissolved C content; therefore, the Ac1, Ac3, and Ms-point temperatures increase. Moreover, the stability of retained austenite is also reduced; its content therefore decreases. Compared with V, the effect of added Nb is weaker because of its smaller content. However, the addition of Nb improves the hardness at lower cooling rates because of the precipitation of fine NbC particles and refining of the microstructure.

Effect of Vanadium on the Phase Transformation Behavior of Ti–Mo Microalloyed Ultra‐High Strength Steel

Vanadium is a vital microalloying element in the precipitation hardening of hot‐rolled high strength steel. Its effects on the continuous cooling phase transformation behavior, microstructure, and properties of Ti–Mo and Ti–Mo–V microalloyed ultra‐high strength steels are studied by the thermal simulator, scanning electron microscopy (SEM), high‐resolution transmission electron microscopy (HRTEM), energy dispersive spectrometer (EDS), and electron backscatter diffractometer (EBSD). The dynamic continuous cooling transformation curves are obtained which show that vanadium can restrain the ferrite‐pearlite and bainite transformation, refine the microstructure, and harden the steel matrix. It is also found that the strain‐induced precipitation of nano‐sized (Ti, Mo) C and (Ti, V, Mo) C particles, with NaCl type structure, exists in Ti–Mo and Ti–Mo–V steel, respectively, as identified by the HRTEM and EDS. And the lattice constants of (Ti, Mo) C and (Ti, V, Mo) C are 0.432 and 0.430 nm, respectively. The higher hardness of Ti–Mo–V steel is attributed to the grain refinement and precipitation hardening by the nano‐sized (Ti, Mo, V) C particles.

A general approach on the modelling of incubation in ferrite transformation using non-isothermal kinetics data for 22MnB5 steel

Abstract The most challenging part, in modelling tailored hot stamping processes, is the variable cooling transformation that occurs in real industrial processes. In this study, analytical equations are first fitted to both experimental isothermal and continuous cooling transformation data. Then, an optimized fitting method with a weight coefficient is introduced that considers two transformation data to provide a more accurate transformation prediction. Finally, the generalized calculated result of incubation time using Rios's proposed method based on optimized continuous cooling transformation curves is contrasted against one calculated with a modified generalized calculated method. The results show that the consideration of the current temperature and cooling rate increases the accuracy of incubation time predictions as the gradient of cooling rate increases.

Effects of Phase Transformation in Hot Stamping of 22MnB5 High Strength Steel

In automotive industry, hot forming of high strength steels offers the possibility to obtain significant reduction of weight without affecting the structural performances of the car-body-in-white. The sheets used in the hot stamping process are heated above the austenitization temperature, and, then, formed and quenched in cooled dies by using a minimum cooling rate of 27°C/s in order to obtain the direct martensitic transformation from the initial austenite. However, improper transfer times from the furnaces to the dies may be unacceptable with regard to the phase changes to be realized during the quenching. Furthermore, strain induced effects influence the phase transformations, so making the process difficult to design and control in case of new part shapes: variations of the applied strains determine changes in the Continuous Cooling Transformation (CCT) curves and, consequently, the cooling rate becomes fundamental to control the material phase transformations, in order to obtain the desired mechanical properties. The paper investigates the effects of the soaking time and cooling rate in the nose area of the CCT curves, where the time represents a critical aspect to control the phase changes in the microstructure. It was found that the phase transformations at the critical temperature of 550°C determine an increase of the flow stress that depends on the specific cooling rate during the dies quenching and on the percentage of bainitic transformation.

Effects of Chromium, Vanadium and Austenite Deformation on Transformation Behaviors of High-strength Spring Steels

The phase transformation behavior during continuous cooling of high-strength spring steels containing different amounts of Cr was studied. Furthermore, the effects of combining Cr with V as well as austenite deformation on the transformation kinetics were investigated in the method of dilatometry and metallography hardness. The results showed that, with the increase of Cr, the pearlite transformation field was enlarged, the ferrite transformation field was narrowed, and the entire phase field shifted to the right. With the addition of V, the start transformation temperature of undercooling austenite (Ar3) was gradually increased, but the ferrite and pearlite transformation fields were not affected. Besides, the minimum critical cooling rate of martensitic transformation was also reduced. In addition, the dynamic continuous cooling transformation (CCT) curve moves to the top left compared with the static CCT curve. The transformed microstructures showed that the addition of V and the deformation not only refined the overall transformed microstructures but also reduced the lamellar spacing of pearlite. The alloying elements Cr and V promoted the Vickers hardness. However, the effect of Cr on the Vickers hardness of martensite was stronger and the influence of V on that of pearlite was stronger. Moreover, the Vickers hardness affected by the austenite deformation was more complex and strongly depended on the transformed microstructures.

A Modified Approach to Modeling of Diffusive Transformation Kinetics from Nonisothermal Data and Experimental Verification

An inverse model is proposed to construct the mathematical relationship between continuous cooling transformation (CCT) kinetics with constant rates and the isothermal one. The kinetic parameters in JMAK equations of isothermal kinetics can be deduced from the experimental CCT kinetics. Furthermore, a generalized model with a new additive rule is developed for predicting the kinetics of nucleation and growth during diffusional phase transformation with arbitrary cooling paths based only on CCT curve. A generalized contribution coefficient is introduced into the new additivity rule to describe the influences of current temperature and cooling rate on the incubation time of nuclei. Finally, then the reliability of the proposed model is validated using dilatometry experiments of a microalloy steel with fully bainitic microstructure based on various cooling routes.

Microstructural transformation and precipitation of an ultra-high strength steel under continuous cooling

We investigated phase transition and precipitation of ultra-high strength steel (UHSS) in a new “short process” with controlled rolling and controlled cooling. Thermal expansion test combined with metallographic observation was used to research the continuous cooling transformation (CCT) curve. Moreover, the microstructural transformation and precipitation law was revealed by morphological observation and alloying elements by electron probe micro-analyzer (EPMA). Transmission electron microscopy (TEM) was utilized to analyze the composition and grain orientation of microstructure. The study showed that the measured critical transformation temperatures of Ac1 and Ac3 were 746 and 868 °C, respectively. The CCT curve indicated that the undercooled austenite was transformed into proeutectoid ferrite and bainite with HV 520 in a broad range of cooling rate of 1 °C•s−1. When subjected to a cooling rate of 1 °C•s−1, the undercooled austenite was divided into small-sized blocks by formed martensite. With further increase of cooling rate, micro-hardness increased dramatically, the microstructure of specimen was mainly lathe bainite(LB), granular bainite(GB), lath martensite (LM) and residual austenite. By diffraction test analysis, it was identified that there was K-S orientation relationship between martensite and austenite for {110}α//{111}γ, {111}α//{101}γ. EPMA clearly showed that carbon diffused adequately due to staying for a long time at high temperature with a lower cooling rate of 2 °C•s−1. Phase transition drive force was lower and the residual austenite existed in the block form of Martensite austenite island (M-A). With the increase of cooling rate to 10 °C•s−1, the block residual austenite reduced, the carbon content of residual austenite increased and α phase around the residual austenite formed into a low carbon bainite form.

Effect of titanium on the microstructure and hardness uniformity of non-quenched and tempered prehardened steel for large-section plastic mould

Two non-quenched and tempered (NQT) prehardened plastic mould steels, differing by titanium content, were produced to have good hardness uniformity in large sections (485 × 970 and 465 × 1325 mm2). The effect of titanium on continuous cooling transformation curves, hardness uniformity and microstructure (grain size, phase constituents) were studied. It was found titanium had refined the grain size of NQT steel by the pinning effect of Ti (C, N) at grain boundaries, and titanium also effectively postponed the nucleation of proeutectoid ferrite in intercritical region. Using finite element method, the cooling rate from core to surface along longitude direction of the steels was predicted. The results indicated that the NQT steel with titanium had better process adaptability than that of NQT steel without titanium. Compared with production of NQT steel without titanium, production for NQT steel with titanium can largely save the production cost by shortening the production cycle and reducing the resources.

Continuous Cooling Phase Transformation Rules of High Nb X80 Pipeline Steel

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.

The Effect of Nb on the Continuous Cooling Transformation Curves of Ultra-Thin Strip CASTRIP© Steels

The effect of Nb on the hardenability of ultra-thin cast strip (UCS) steels produced via the unique regime of rapid solidification, large austenite grain size, and inclusion engineering of the CASTRIP© process was investigated. Continuous cooling transformation (CCT) diagrams were constructed for 0, 0.014, 0.024, 0.04, 0.06 and 0.08 wt% Nb containing UCS steels. Phase nomenclature for the identification of lower transformation product in low carbon steels was reviewed. Even a small addition of 0.014 wt% Nb showed a potent effect on hardenability, shifting the ferrite C-curve to the right and expanding the bainitic ferrite and acicular ferrite phase fields. Higher Nb additions increased hardenability further, suppressed the formation of ferrite to even lower cooling rates, progressively lowered the transformation start and finish temperatures and promoted the transformation of bainite instead of acicular ferrite. The latter was due to Nb suppressing the formation of allotriomorphic ferrite and allowing bainite to nucleate at prior austenite grain boundaries, a lower energy site than that for the intragranular nucleation of acicular ferrite at inclusions. Strength and hardness increased with increasing Nb additions, largely due to microstructural strengthening and solid solution hardening, but not from precipitation hardening.

Microstructure Evolution and Alloying Features of a Developed High Strength and High Toughness Weld Metal Used for Pipeline Steels

ABSTRACT Lei, X.; Wu, K.; Wang, H., and Yu, L., 2015. Microstructure evolution and alloying features of a developed high strength and high toughness weld metal used for pipeline steels. The microstructure evolution and continuous cooling transformation curves of the simulated heat-affected zone of a developed high strength and high toughness weld metal was investigated by means of thermal simulation on a Gleeble 3,500. This weld metal was picked up from the welding joint of a high strength and high toughness pipeline steel which was welded with commercial welding materials. Results showed that fine acicular ferrite was the main microstructure during a wide range of cooling rates. Actual welding experiment was also carried out with the heat input of 21 kJ / cm. The yield strength of the weld joint in weld metal is 875 MPa. The average impact toughness are about 104 and 183 J at –30° C in weld metal and the fusion line, respectively. The observed results indicated that the developed weld metal had superior ...

Phase Transformation Behavior of Niobium Containing Microalloyed Steel with Predeformation and Continuous Cooling

The continuous cooling phase transformation behavior of niobium microalloyed steel was studied by Thermecmastor-Z thermomechanical simulator; the continuous cooling transformation curves (CCT) were established. The change of microstructure under different cooling rates was observed, and the influence of deformation in austenite non-recrystallization region on transformation was discussed. Based on these work, it was possible to know that the phase transformation is retarded and the ferritic grain is refined dramatically as the cooling rate increasing. The deformation in austenite non-recrystallization region caused deformation stored-energy, which improved the grain refinement of transformation to some extent, but not significant.

Microstructure transformation of X70 pipeline steel welding heat-affected zone

The continuous cooling transformation curve of heat-affected zone (HAZ) of X70 pipeline steel was measured by Gleeble-3500 thermal mechanical simulator, optical microscope (OM) and hardness analysis. The microstructure transformation rule at different cooling rates and solution behaviors of microalloy carbonitride during heating process of simulated specimens were investigated. When the cooling rate changes from 10 to 20 °C·s−1, microstructures at HAZ are identified as granular bainite, lathy bainite, and quasi-polygonal ferrite. This microstructure is featured with fine ferrite grains, martensite/austenite islands dispersed, high-density dislocations, and fine carbonitride particles, resulting in improving the strength and toughness of HAZ. With the cooling rate increasing to above 40 °C·s−1, the microstructure is predominantly coarse lathy bainite with clear primary austenite grain boundary. While the cooling rate decreases to below 1 °C·s−1, a fairly small amount of pearlite can be observed at the boundaries. The strength and toughness of HAZ are deteriorated because of coarse grains among these microstructures. Most of microalloy carbonitrides in HAZ could be dissolved in the matrix during heating process. A few of TiN particles existing as residues in the matrix can prevent austenite grain from growing, and then improve the strength and toughness of HAZ.

Research on Continuous Cooling Transformation Curve of a C-Si-Mn Steel

The CCT curve of a C-Si-Mn steel was investigated. The hot simulation tests at different cooling rates were conducted on Gleeble-1500 simulator and microstructures were observed. Based on transformation temperatures recorded by dilation curves and microstructures, the CCT curve diagram for tested steel was plotted. The effects of cooling rate on microstructure and transformation temperature were studied. The results provide the theoretical basis for determination of hot processing and heat treatment technology of C-Si-Mn steels.

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.

Influence on the Microstructure of Pinion Steel with Different Cooling Rates after Normalizing

According to continuous cooling transformation curve of pinion steel, heat treatment simulation tests of 8 series were developed. The results show that the microstructure of pinion steel depends on the cooling rate after normalizing. Furnace cooling according to 30°C/h after normalizing can get the typical F+P organizations; Converter cooling after normalizing mixed with some bainite organizations; Furnace cooling with the break after normalizing can get clear stripped bainite organizations.

Microstructures and Mechanical Properties of X100 Pipeline Steel Strip

Dynamic CCT (continuous cooling transformation) curves and effects of microstructures and M/A islands of X100 hot rolled strip on mechanical properties were studied by means of a thermal simulator, a scanning electron microscope (SEM), a transmission electron microscope (TEM), a digital micro-hardness tester and Image-Pro Plus analysis software. The results show that high contents of C and Mo can make transformation lines of acicular ferrite and bainite shift rightward. High Mo tested strip has higher strength and micro-hardness and lower toughness than the other one because lath bainite appears instead of quasi-polygonal ferrite. Tested strip with granular bainite, lath bainite and M/A islands has better mechanical properties and in the microstructures lath bainite content is around 36.5% and M/A islands are fine and disperse in the matrix.

Upgrade Rolling Based on Ultra Fast Cooling Technology for C-Mn Steel

By measuring the expansion curves of a C-Mn steel at different cooling rates by using an MMS-300 thermomechanical simulator, continuous cooling transformation curves were obtained. The new process “ultra fast cooling+laminar cooling” was simulated and the effects of ultra fast cooling ending temperature on microstructure had also been investigated. The hot rolling experiment was done by adopting “high temperature rolling+forepart ultra fast cooling” technologies at laboratory scale. The results revealed that ultra fast cooling can delay the decrease of dislocation density and refine ferrite grains. Diversity control of the microstructure and phase transformation strengthening can be realized by changing the ultra fast cooling ending temperature. With the decrease of ultra fast cooling ending temperature, the strength and toughness increase, but plasticity does not decrease obviously. The new technique can improve the yield strength by over 50 MPa. Therefore, the upgrade of mechanical properties of C-Mn steel can be realized by using “high temperature rolling+ultra fast cooling+laminar cooling” technique. Compared with “low temperature rolling with large deformation degree” technique, this new technology can decrease the rolling force and increase the production efficiency.

Investigation on grain refinement and precipitation strengthening applied in high speed wire rod containing vanadium

To obtain necessary information for the simulation of high speed wire production process, the effect of grain refinement and precipitation strengthening on two high speed wire rod steels with different vanadium and nitrogen contents was investigated by continuous cooling transformation (CCT) characteristics. CCT curves were constructed by the dilatometer test and microscopic observation. Results showed that the formation of intra-granular ferrite (IGF) could refine grain remarkably and accelerate the ferrite transformation. Schedules for high speed wire production process focused on the effect of cooling rate. Ferrite grain was refined by increasing cooling rate and the formation of IGF. The microhardness calculation revealed that the steels were strengthened mostly by a combined effect of grain refinement and precipitation hardening. Degenerated pearlite was observed at lower transformation temperature and the fracture morphology changed from cementite lamellar to nanoscale cementite particle with increasing cooling rate. Based on the analysis above, an optimal schedule was applied and the microstructure and microhardness were improved.

Effects of titanium on ferrite continuous cooling transformation curves of high-thickness Cr-Mo steels

The effect of Ti on the ferrite-phase transformation in the middle portion of high-thickness Cr-Mo steel vessels was studied. The phase diagrams and ferrite continuous cooling transformation (CCT) curves were calculated thermodynamically, and dilatometry tests were performed to determine the start and finish times of the ferrite transformation. When the Ti concentration was 0.015 mass%, ∆(Fs-Ff) of ferrite CCT curve decreased owing to an increase in the concentration of Mn dissolved as a result of (Mn, Ti) oxide formation. When the Ti concentration was 0.03 mass% or greater, the ferrite CCT curves shifted considerably to the right along the time axis owing to an increase in Ti oxide formation and the precipitation of Ti4C2S2, both of which affect the concentration of Mn dissolved in the austenite matrix. As a result, a completely bainitic structure was obtained when the Ti concentration was 0.03 mass% or greater.