Continuous Cooling Transformation Data Research Articles

Numerical simulation and experimental investigation of tailored hot stamping of boron steel by partial heating

For the purpose of manufacturing the automotive body structure parts with tailored properties, the hot stamping process with partial heating was experimentally and numerically investigated. In this study, the deformation behaviors of the steel 22MnB5 with the fully and incompletely austenitized microstructure were studied by hot tensile testing and modeled by the modified Zerilli–Armstrong constitutive equations taking the lattice structure into account. The classical Kirkaldy–Venugopalan transformation kinetics equations were modified to be suitable for the investigated steel, and the diffusional phase transformation equations were calibrated by using the continuous cooling transformation data. By the virtue of user subroutine VUHARD in finite element program ABAQUS, the metallurgical-field coupled finite element model was established for simulating the hot stamping process with partial heating. A special fixture was developed to realize the partial heating process of circular sheet blank. The finite element simulation and experimental research were carried out for a cup-shaped part. Finally, the phase distribution, hardness variation, shape error, and thickness variation of the cup-shaped part were analyzed and discussed. The validity and accuracy of the constitutive model and the transformation kinetics model were verified by comparing the simulation results with the experimental data. Besides, the present research results also demonstrate the hot stamping parts with variable mechanical properties can be manufactured by changing the austenitization degree of sheet blank during the heating stage.

Optimization of the CCT Curves for Steels Containing Al, Cu and B

New continuous cooling transformation (CCT) equations have been optimized to calculate the start temperatures and critical cooling rates of phase formations during austenite decomposition in low-alloyed steels. Experimental CCT data from the literature were used for applying the recently developed method of calculating the grain boundary soluble compositions of the steels for optimization. These compositions, which are influenced by solute microsegregation and precipitation depending on the heating/cooling/holding process, are expected to control the start of the austenite decomposition, if initiated at the grain boundaries. The current optimization was carried out rigorously for an extended set of steels than used previously, besides including three new solute elements, Al, Cu and B, in the CCT-equations. The validity of the equations was, therefore, boosted not only due to the inclusion of new elements, but also due to the addition of more low-alloyed steels in the optimization. The final optimization was made with a mini-tab tool, which discarded statistically insignificant parameters from the equations and made them prudently safer to use. Using a thermodynamic-kinetic software, IDS, the new equations were further validated using new experimental CCT data measured in this study. The agreement is good both for the phase transformation start temperatures as well as the final phase fractions. In addition, IDS simulations were carried out to construct the CCT diagrams and the final phase fraction diagrams for 17 steels and two cast irons, in order to outline the influence of solute elements on the calculations and their relationship with literature recommendations.

A Simple Mathematical Model for Establishing Isothermal Transformation Kinetics from Continuous Cooling Data

A simple mathematical model for establishing isothermal transformation kinetics from continuous cooling transformation data is presented. A new regression function of k, which is a reaction parameter of the Johnson–Mehl–Avrami equation, is proposed. The isothermal transformation kinetics of austenite-pearlite transformation in a eutectoid steel is converted from continuous cooling transformation data using the regression function in combination with the inverse additivity rule. The converted isothermal transformation kinetics excellently agrees with experimental results irrespective of temperature.

A Simple Mathematical Model for Establishing Isothermal Transformation Kinetics from Continuous Cooling Data

A simple mathematical model for establishing isothermal transformation kinetics from continuous cooling transformation data is presented. A new regression function of k, which is a reaction parameter of the Johnson-Mehl-Avrami equation is proposed. The isothermal transformation kinetics of austenite-pearlite transformation in a eutectoid steel is converted from continuous cooling transformation data using the regression function in combination with the inverse additivity rule. The converted isothermal transformation kinetics excellently agrees with experimental results irrespective of temperature, showing a big improvement compared with a previous model.

Effects of Chemical Composition and Austenite Deformation on the Onset of Ferrite Formation for Arbitrary Cooling Paths

We present a computational method for calculating the phase transformation start for arbitrary cooling paths and for different steel compositions after thermomechanical treatments. We apply the method to quantitatively estimate how much austenite deformation and how many different alloying elements affect the transformation start at different temperatures. The calculations are done for recrystallized steel as well as strain hardened steel, and the results are compared. The method is parameterized using continuous cooling transformation (CCT) data as an input, and it can be easily adapted for different thermomechanical treatments when corresponding CCT data is available. The analysis can also be used to obtain estimates for the range of values for parameters in more detailed microstructure models.

Phase field modeling of the simultaneous formation of bainite and ferrite in TRIP steel

In transformation-induced plasticity (TRIP) steels, Si and/or Al delay cementite precipitation during bainite transformation leading to a transformation product that is usually referred to as carbide-free bainite. The present work proposes an approach to calibrate the parameters of a multi-phase field model to simulate the austenite decomposition into ferrite and bainite in a TRIP steel. Ferrite nuclei are introduced at austenite grain boundaries and suitable interfacial mobilities are selected to reproduce experimental ferrite formation kinetics. Bainite nucleation occurs for a sufficiently high undercooling at available interface sites (i.e. austenite grain boundaries, austenite-ferrite interfaces) and/or within austenite grains. For simplicity, the formation of carbide-free bainite is considered and a suitable anisotropy approach is proposed for the austenite-bainite interface mobility. The proposed model is benchmarked and validated with experimental continuous cooling transformation data.

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.

Influence of Composition and Prior Deformation on Phase Transformation Temperatures and Hardness in Direct Quenching Using Physical Simulation

Abstract For conventional reheated and quenched (RQ) steels, the level of alloying necessary to achieve a given hardenability and hardness can be estimated from calculated Jominy curves or ideal critical diameters (DI) given in ASTM A255-10(2014). However, for thermomechanically rolled direct quenched (DQ) steels, little data are available. In this study, the accuracy of the ASTM approach was estimated by designing an experiment to study the main effects of seven alloying elements (C, Mn, Cr, Ni, Mo, Nb, and V) at two levels with eight boron steels based on an eight-run resolution III partial factorial designed experiment. Continuous cooling transformation (CCT) diagrams covering cooling rates of 1.5°C/s–48°C/s were determined using Gleeble simulations with or without controlled deformation below Tnr. The effects of deformation below Tnr and the alloying elements were clearly revealed. In general, low temperature straining of the austenite led to higher levels of hardness than quenching from unstrained austenite. The start of the bainite transformation Ar3 (bainite) was modelled as a function of chemical composition and cooling rate. The data in the CCT diagrams were used to derive equivalent ideal critical diameters (DIB) for strained and unstrained prior austenite. The hardenability index DIB calculated from experimental Jominy curves generally agreed well with those of CCT data for unstrained austenite. In order to apply the ASTM A255 approach to the calculation of DIB, boron factors (BF) for steels with alloy factors (AFs) greater than 26 were estimated by extrapolating the ASTM data. However, this approach did not give satisfactory predictions for either strained or unstrained austenite. Preliminary analysis indicated that Cr, Mo, and V might be less effective at increasing hardenability than implied from their AFs. New formulae were given to allow estimations of the hardness to be expected in connection with direct quenching.

The Effect of Solute Nb on the Austenite-to-Ferrite Transformation

Niobium is a widely used micro-alloying element in steels that can retard the austenite-to-ferrite transformation primarily by solute drag when Nb remains in solution. It is critical to develop quantitative models to predict the effect of Nb on the transformation kinetics. In the present work, dedicated continuous cooling transformation (CCT) studies were performed for a low-carbon steel microalloyed with 0.047 wt pct Nb starting from fully recrystallized austenite states with the same grain size but different amounts of Nb in solution. The austenite-to-ferrite transformation kinetics is described from a fundamental perspective by assuming a mixed-mode reaction including solute drag of Nb. Using the solute drag model of Fazeli and Militzer, the intrinsic interface mobility, trans-interface diffusivity of Nb, and its binding energy to the interface have been determined from the CCT data. The interfacial parameters are critically analyzed and compared with independent measurements of diffusion and grain boundary segregation.

Modelling Phase Transformation Kinetics in Fe–Mn Alloys

A unified model is proposed for the austenite-to-ferrite transformation kinetics in binary Fe–Mn alloys that accounts for solute drag of Mn. To aid the model development, continuous cooling transformation (CCT) tests were conducted for an interstitial-free steel that can be considered as Fe–0.1%Mn alloy. The experimental transformation data are supplemented with literature data for Fe–1%Mn and Fe–2%Mn alloys to establish a CCT database for Fe–Mn alloys. The austenite-to-ferrite transformation kinetics is described from a fundamental perspective by assuming an interface-controlled reaction and including solute drag of Mn. Using the solute drag model of Fazeli and Militzer, intrinsic interface mobility, trans-interface diffusivity of Mn and its binding energy have been determined from the CCT data. The interfacial parameters are critically analyzed and compared with independent measurements of diffusion and grain boundary segregation.

Modelling Simultaneous Formation of Bainitic Ferrite and Carbide in TRIP Steels

The addition of 1.5–2 wt% Si is a commonly used alloying approach for TRIP steels. Si delays cementite precipitation during bainite transformation thereby enabling that an adequate amount of austenite can be retained at room temperature due to sufficient carbon enrichment. However, the degree of cementite prevention and thus the fraction of retained austenite depend on the employed processing parameters and steel chemistry. The present work proposes a modelling framework to quantify the delayed carbide precipitation during bainite formation. A nucleation-growth based model describes the simultaneous formation of bainitic ferrite and cementite precipitation for various continuous cooling scenarios. The retarding effect of Si on cementite precipitation is explicitly accounted for. The fraction of bainite and the carbon content of the remaining austenite, which determines the Ms temperature of remaining austenite, can be tracked along non-isothermal processing paths. The proposed model is evaluated using continuous cooling transformation data for a 0.19C–1.5Mn–1.6Si–0.2Mo (wt%) steel.

Kinetics of the bainite transformation

A theory is developed for the evolution of bainite as a function of time, temperature, chemical composition and austenite grain size. The model takes into account the details of the mechanism of transformation, including the fact that nucleation begins at the austenite grain surfaces, and that the growth of a sheaf occurs by the repeated nucleation of small platelets. Predictions made using the model are shown to compare well against published isothermal and continuous cooling transformation data.

HAZ Hardness and Microstructure Predictions of Arc Welded Steels—II. CCT Diagram Method

A prediction method for heat affected zone (HAZ) hardness and microstructure based on the use of continuous cooling transformation (CCT) diagrams is proposed and examined. The computer model uses temperature results obtained from an earlier developed finite element model and CCT data to predict HAZ hardnesses and microstructures. The hardness results are in good agreement with those from other experimental and analytical methods. The martensite volume fractions, however, are somewhat lower but are expected to be closer to the actual values. The use of CCT diagrams is beneficial for predicting the complete HAZ microstructure comprising martensite, bainite, ferrite and pearlite. Résumé Une méthode prédictive de la dureté et la microstructure de la ZAT, basée sur l'utilisation de diagrammes de transformation en refroidissement continu (TRC), est proposée et examinée. Le modèle informatique utilise les résultats de température obtenus à partir d'un modèle d'éléments finis déjà existent et les données de TRC pour prédire les duretés et les microstructures de la ZAT. Les duretés sont en accord avec d'autres résultats obtenus par méthode analytique ou expérimentale. Les fractions volumiques de martensite sont toutefois plus faibles mais semblent par contre être plus représentatives des valeurs réelles. L'utilisation des diagrammes TRC est bénéfique lors de la détermination de la microstructure complète de la ZAT, constituée de martensite, bainite, ferrite et perlite.