Ae3 Temperature Research Articles

Cracking Behavior and High-Temperature Thermoplastic Analysis of 09CrCuSb Steel Billets

This paper characterizes the surface crack morphology and elements of 09CrCuSb steel continuous casting slabs and studies the high-temperature thermo-plasticity of continuous casting slabs using a Gleeble 3500 thermal simulator. Combining the macro/micro fracture morphology and microstructure characteristics, the formation rules of slab cracks are discussed. The research results show that the increase in Ae3 temperature caused by changes in alloy elements results in the precipitation of a thin, film-like coexisting ferrite along the grain boundaries at a certain temperature, as well as the element segregation behavior of low-melting-point alloy elements at the original austenite grain boundaries, which are the main factors inducing cracks in 09CrCuSb steel continuous casting billets. The plasticity of 09CrCuSb steel at high temperatures is poor at 800~900 °C. In continuous casting process control, it is necessary to try to avoid long-term stay of the billet at this temperature range.

Effect of Reheating Temperatures on Hot Ductility and Precipitation Behavior of Low‐Carbon Steels

The influence of reheating temperatures on hot ductility is investigated in two Ni‐, Mo‐, and Cu‐containing low‐carbon steels. The effects of solution treatment conditions at 1623, 1673 K, and remelting at 1803 K on the reduction in area (RA) are analyzed. The results show an RA trough in the range of 973–1173 K in all experimental conditions. At 973 K, which is close to the Ae3 temperature of both steel grades, ductility drop is caused by the formation of the thin ferrite film at the prior austenite grain boundary. Higher RA values are achieved after reheating to 1623 K, mainly due to smaller austenite grain size by the action of preformed precipitation throughout the matrix. In contrast, increased reheating temperatures result in relatively poor RA values because of the large austenite grain size due to the dissolution of preformed precipitate and reprecipitation at the grain boundaries during cooling. A comparison between two steel grades shows that better ductility at 973 K is achieved in low Ni‐containing steel due to the relatively thick ferrite film. The effects of alloy composition and preformed precipitate's existence on hot ductility are further discussed based on the matrix strengthening and reprecipitation behavior.

Dynamic Ferrite Formation and Evolution above the Ae3 Temperature during Plate Rolling Simulation of an API X80 Steel

Thermo-mechanically controlled rolling is a technique used to produce steel strips and plates. One of the steels widely used in the production of heavy plates for application in oil and gas pipelines is API X80. The hot rolling process of this family of steels consists of applying deformation passes at high temperatures, mainly above Ae3, inside the austenite phase field. It has been shown that during deformation, the phenomenon of dynamic transformation (DT) of austenite into ferrite leads to lower hot deformation resistance within the stable austenite region. In this investigation, hot torsion simulations of an industrial rolling process under continuous cooling conditions were used to monitor the formation of ferrite by DT. Stress–strain flow curves and equivalent mean flow stresses followed by sample characterization via optical and electron microscopy showed the inevitable formation of ferrite above the Ae3. The employed 10-pass deformation schedule was divided into 5 roughing and 5 finishing passes, thereby promoting an increased volume fraction of ferrite and decreased critical strain for the onset of DT and dynamic recrystallization (DRX). A microstructural analysis confirmed the formation of ferrite from the first roughing strain until the last finishing pass. The volume fraction of DT ferrite increased due to strain accumulation, an increased number of deformation passes and as the temperature approached the Ae3, leading to a characteristic torsion texture at the end of the simulation.

Physical Simulation Based on Dynamic Transformation Under Hot Plate Rolling of a Nb-Microalloyed Steel

In this work, the presence of dynamically formed ferrite above the Ae3 temperature during the physical simulation of hot rolling was presented. This unusual metallurgical process is known as dynamic transformation (DT). The metastable ferrite phase undergoes a reverse transformation when the temperature is held above the Ae3 by means of a diffusion process. These phenomena affect the rolling load during high-temperature plate rolling. Therefore, a linepipe X70 steel was studied under plate rolling with two-pass roughing and seven-pass finishing strains of 0.4 and 0.2, respectively, applied at strain rate of 1 s−1 and interpasses of 10, 20, and 30 s. The samples were cooling down during deformation, which mimics the actual industrial hot rolling. It was observed that the alloy softens as the hot rolling progresses, as depicted by flow curves and mean flow stress plots, which are linked to the combined effects of dynamic transformation and recrystallization. The former initially occurs at lower strains, followed by the latter at higher strains. The critical strain to DT was affected by the number of passes and temperature of deformation. Shorter interpass time allows higher amounts of ferrite to form due to higher retained work hardening. Similarly, the closer the deformation temperature to the Ae3 permits a higher DT ferrite fraction. The information from this work can be used to predict the formation of phases immediately after hot rolling and optimize models applied to the accelerated cooling.

Retransformation of dynamically induced ferrite during physical simulation of Steckel mill hot rolling

It has been well established that austenite can partially transform into ferrite during thermomechanical processing. The laboratory-scale strip and plate rolling simulations in the literature suggest that both forward and backward dynamic transformation (DT) can occur above the equilibrium Ae3 temperature. In this work, a five-pass Steckel mill rolling schedule (isothermal deformation) of an X70 high Nb steel was simulated using a torsion testing machine. Four different roughing schedules with a strain of 0.4 per pass were employed at 1100 °C before the application of finishing passes. A total of five finishing passes were simulated at 900 °C with pass strains of 0.3, strain rate of 1.0 s−1, and interpass time of 10 s, followed by isothermal holding varying from 1 to 270 s. The level of the flow curves depicted activation of softening phenomena, which was shown to be the formation of DT ferrite by means of optical and scanning electron microscopy images. The volume fraction of DT ferrite increases with increasing the number of roughing passes due to retained work hardening. Moreover, the DT ferrite retransforms back into austenite during the 10 s pass interval and holding after last strains, as shown by phase fraction measurement before and after each pass. Thus, the interpass time and holding intervals after the simulations play a significant role in reducing the amount of ferrite after the process. It seems that the presence of Nb in the alloy affects both the forward and backward transformation. The present investigation can be used to estimate the amount of formed and retransformed ferrite during Steckel mill operations in order to have a better metallurgical understanding of the process. This may result in better dimensional and quality control during the manufacturing of X70 pipeline steels.

Induced ferrite formation above the Ae3 during plate rolling simulation of a X70 steel

ABSTRACTPartial amount of austenite can be dynamically transformed into ferrite above the Ae3 temperature when it is being deformed. This happens by a displacive mechanism. On removal of the load, it retransforms back into the stable austenite by diffusional processes. Plate rolling simulation under continuous cooling conditions was carried out on a high Nb X70 steel. Pass strains of 0.2 together with interpass times of 10, 20 and 30 s were employed. The initial and final temperatures for the finishing simulation were 920 and 830 °C, respectively. The mean flow stresses (MFS`s) behaviour indicates that dynamic transformation (DT) and recrystallization (DRX) were taking place during straining. It is shown that ferrite is formed during the roughing passes and increases its volume fraction throughout the finishing rolling steps. The ferrite formation is favoured by strain accumulation, shorter time between passes and also when the temperature reaches the Ae3 line. The results obtained here can be used to design improved models for transformation on accelerated cooling.

Dynamic Transformation of Austenite at Temperatures above the Ae<sub>3</sub>

Under loading above the Ae3 temperature, austenite transforms displacively into Widmanstätten ferrite. Here the driving force for transformation is the net softening during the phase change while the obstacle consists of the free energy difference between austenite and ferrite as well as the work of shear accommodation and dilatation during the transformation. Once the driving force is higher than the obstacle, phase transformation occurs. This phenomenon was explored here by means of the optical and electron microscopy of a C-Mn steel deformed above their transformation temperatures. Strain-temperature-transformation (STT) curves are presented that accurately quantify the amount of dynamically formed ferrite; the kinetics of retransformation are also specified in the form of appropriate TTRT diagrams. This technique can be used to improve the models for transformation on accelerated cooling in strip and plate rolling.

Microstructure and Hardness Evolution during Deformation near Ae3 in a Cr–Mn–Ti Gear Steel

Developing appropriate thermomechanical processing routes for decreasing macrohardness of gear steels without heat treatment has a numerous futuristic applications in rolling industry. The effect of different deformation processes applied to Cr–Mn–Ti gear steel on the evolution of microstructure and hardness are studied by using a thermal simulator. It is found that imposing second‐pass deformation near Ae3 temperature inhibits occurrence of dynamic recrystallization (DRX), and increases stored deformation energy in austenite, which significantly promotes ferrite and pearlite transformation. As a result, the maximum volume fraction of ferrite and pearlite approaches ≈90%, and the corresponding macrohardness decreases to ≈215 HV. The difference of ferrite transformation between different deformation processes is analyzed using a thermodynamic equation by taking into account the stored deformation energy. The present calculations indicate that interval between Ae3 and deformation temperature can only be avoided at lower deformation temperature of second‐pass. This restricts static recovery or recrystallization, and quantities of dislocations that act as nucleation sites are retained before ferrite transformation.

Influence of strain reversal on dynamic transformation in microalloyed steels deformed above the Ae3 temperature

In the present work, the effect of strain path reversals on dynamic transformation (DT) above Ae3 temperature was studied using an API grade X-70 microalloyed steel deformed by torsion with single and multiple strain path reversals. The results revealed the important role played by strain path reversals on influencing the evolution of austenite grain boundaries through inhomogeneous deformation, therefore, affecting DT behaviours. In addition to flow stress–strain analysis and microstructure investigation, finite element method combined with 3D digital materials representation approach was used to gain insights into the effects of deformation with strain path reversals on the development of microstructural features in the prior-austenite grains.

Dynamic Transformation during the Simulation of Plate Rolling in an X70 Steel

A five-pass rolling schedule is simulated by means of torsion testing. The first and last pass temperatures are 920 and 860 °C with 15° of cooling between passes. All of the rolling is carried out above the Ae3 temperature of 845 °C that applies to this steel. Interpass times of 10, 20, and 30 s are employed, which correspond to cooling rates of 1.5, 0.75, and 0.5 °C s−1, respectively. Samples were quenched before and after the first, third, and fifth passes in order to determine the amount of dynamic ferrite produced in a given pass. The data also allow for estimation of the amounts of ferrite that retransform to austenite during the time between passes. The volume fractions of ferrite and martensite (prior austenite) were determined by optical metallography. The amount of dynamic ferrite formed and retained increased with pass number. The volume fraction of ferrite that retransformed increased with pass number, averaging about 4, 6, and 9% for the 10, 20, and 30 s interpass times, respectively. The simulations indicate that ferrite is unavoidably produced during plate rolling and that the microstructure present at the initiation of accelerated cooling does not consist solely of austenite.

Hot ductility of medium carbon steel with vanadium

Hot ductility of medium carbon steel containing 0.52wt% of carbon and 0.11wt% of vanadium was investigated using a hot tensile test performed up to fracture. The hot ductility was evaluated by measuring the reduction of area of the fractured specimens, which were strained at a variety of test temperatures in a range of 600–1100°C at a strain rate of 2×10−3/s. The hot ductility was excellent in a temperature range of 950–1100°C, followed by a decrease of the hot ductility below 950°C. The hot ductility continued to drop as the temperature was lowered to 600°C. The loss of hot ductility in a temperature range of 800–950°C, which is above the Ae3 temperature, was due to V(C,N) precipitation at austenite grain boundaries. The further decline of hot ductility between 700°C and 750°C resulted from the transformation of ferrite films decorating austenite grain boundaries. The hot ductility continued to decrease at 650°C or less, owing to ferrite films and the pearlite matrix, which is harder than ferrite. The pearlite was transformed from austenite due to relatively high carbon content.

Simulation of the growth of austenite during continuous heating in low carbon iron alloys

The growth of austenite from martensite or carbon-supersaturated ferrite matrix during continuous heating, which accompanies carbon diffusion in the growing austenite, was studied by DICTRA and linearized gradient approximation extended to include soft impingement of diffusion fields in the matrix. While the austenite growth is controlled by carbon diffusion in ferrite at an early stage, it is controlled by diffusion in austenite at the intermediate and late stages. At a low heating rate, the austenite-finish temperature Af is almost equal to the Ae3 temperature of the alloy, whereas at a high heating rate, Af exceeds the Ae3 to a progressively larger extent with the increasing heating rate and matrix grain size. At a very high heating rate, i.e., 105–106 °C/s, the mobility of α/γ interface is likely to have a significant influence on the growth of austenite, and the untransformed ferrite matrix is transformed in a massive mode. These results are in accordance with the earlier observation of austenite formation during rapid heating in a low carbon iron alloy.

Effect of Mn and Si on the dynamic transformation of austenite above the Ae3 temperature

In the mechanical activation model of dynamic transformation, the stress applied to the austenite is considered to be the driving force for the transformation. Here the yield stress of the newly formed Widmanstätten ferrite that replaces the austenite and that must continue to be deformed is taken into account as an additional obstacle to the transformation. This is estimated from hardness data. In this way, the driving force is redefined to consist only of the softening that takes place during the process and therefore only of the difference between the current flow stress of the work-hardened austenite and the yield stress of the fresh Widmanstätten ferrite that takes its place. According to this model, this driving force is opposed by the chemical free energy difference between the austenite and the Widmanstätten ferrite as well as the lattice dilatation work and shear accommodation work that accompany the transformation. In order to assess the effects of Mn and Si addition on the transformation, the high temperature flow stress data obtained by Wray (1984) by means of tensile tests on four Fe–Mn–Si alloys are employed to determine how these alloying additions affect the driving and opposing forces. The present type of model provides a more detailed analysis of the phenomenon than any previously available model and in this way shows that Mn addition is expected to lead to only a small decrease in the temperature range over which displacive ferrite can form, while Si addition is predicted to have a much larger effect on this interval.

Formation of fully pearlitic microstructure in medium carbon steel

Pearlitic microstructure can be obtained when austenite of eutectoid composition is slowly cooled from high temperature to below the Ae3 temperature. It is also possible to induce such structure in hypo-eutectoid composition of austenite through proper heat treatment as well. However, in the current work, one hypo-eutectoid steel during very slow cooling only produced completely pearlitic microstructure which was not expected from a steel with such composition. The calculations carried out considering orthoequilibrium condition actually predicted the presence of about 37% ferrite in the microstructure. Further calculations considering different equilibrium modes and kinetics of transformation indicates that the composition and thermal condition of the steel under consideration was such that proeutectoid ferrite formation could not start before the composition reaches to the Negligible Partitioning Local Equilibrium phase boundary which further coincides with the area described by Hultgren extrapolation and thus, the steel completely transforms to pearlite even with hypo-eutectoid composition during very slow cooling.

Numerical prediction on microstructure evolution for multi-stage hot forging of micro-alloyed piston

Purpose – The purpose of this paper is to build a flow stress model and microstructure evolution models which can be used to fulfill the multi-physics prediction of hot forging process, in this way the process design can be virtually verified and optimized. This is especially crucial for micro-alloyed steel forging which microstructure determines the component properties, since the downstream quenching is usually not needed. Design/methodology/approach – First, hot compression tests have been completed; second, experimental data are used to build the flow stress model and models for microstructure evolution; third, programming has been finished to integrate the proposed models into the commercial finite element method (FEM) code; fourth, case study is conducted to simulate multi-stage hot forging process of micro-alloyed steel F38MnV piston; and fifth, simulation results are validated by experiment. Findings – First, simulation results in grain size and phase volume fraction are in well agreement with experimental ones; second, the austenite grain is dramatically refined by the dynamic recrystallization in pre-forging process and static recrystallization in the two intervals has no obvious change during the following final forging and cooling above the Ae3 temperature; third, during the cooling process below the Ae3 temperature, ferrite and pearlite transformation begin from the thin skirt to the thick skirt and piston bottom because of different cooling speeds at different areas. Originality/value – First, flow stress model, dynamic recrystallization model, static recrystallization model, austenite grain growth model and phase transformation models are established for a micro-alloyed steel; second, the multi-physics FEM simulation of multi-stage hot forging of industrial piston has been conducted and verified by experiment, which show good agreement.

Occurrence of Dynamic Alpha-Phase Nucleation in Ti-5553 during Hot Deformation

This paper discusses the microstructural changes during hot deformation, mainly dynamic nucleation and growth of a precipitates, occurring in the b metastable titanium alloy Ti-5553 (Ti–5Al–5Mo–5V–3Cr–0.5Fe). The effect of process variables on flow response and microstructure evolution during hot working of the alloy with initial bimodal microstructure was established using isothermal hot-compression tests. Testing was conducted at 4 strain rates between 0.001 and 1 s−1 and 5 temperatures between 720 °C and 850 °C, on material with prior b grain size of 450 μm, a nodular size of 3 µm and a known primary a-phase fraction. All flow curves exhibited a peak stress followed by moderate flow softening in the two-phase domain. Flow softening was interpreted in terms of deformation heating and substructure or texture evolutions. The dependence on strain rate and temperature of the kinetics of dynamic a-phase nucleation during straining is complex and appears to be of second-order importance compared to the effects of strain. This suggests that the nucleation and growth of a phase in the temperature range between 720 °C and 990 °C results from a mixed-mode displacive-diffusional transformation, similar to the austenite / ferrite transformation above the Ae3 temperature reported by some authors.

Dynamic Ferrite Transformation Behaviors in 6Ni-0.1C Steel

Phase transformation from austenite to ferrite is an important process to control the microstructures of steels. To obtain finer ferrite grains for enhancing its mechanical property, various thermomechanical processes followed by static ferrite transformation have been carried out for austenite phase. This article reviews the dynamic transformation (DT), in which ferrite transforms during deformation of austenite, in a 6Ni-0.1C steel recently studied by the authors. Softening of flow stress was caused by DT, and it was interpreted through a true stress–true strain curve analysis. This analysis predicted the formation of ferrite grains even above the Ae3 temperature (ortho-equilibrium transformation temperature between austenite and ferrite), where austenite is stable thermodynamically, under some deformation conditions, and the occurrence of DT above Ae3 was experimentally confirmed. Moreover, the change in ferrite grain size in DT was determined by deformation condition, i.e., deformation temperature and strain rate at a certain strain, and ultrafine ferrite grains with a mean grain size of 1 μm were obtained through DT with subsequent dynamic recrystallization of ferrite.

Nucleation Behavior Analysis of Intragranular Acicular Ferrite in a Ti-killed C-Mn Steel

By using a Gleeble 3500D thermo-mechanical simulator, the nuclcation behavior of intragranular acicular ferrites (IAF) was studied in a Ti-killed C-Mn steel. During continuous cooling transformation, the allotriomorphic ferrite (AF) and ferrite side plate (FSP) microstructures grew more rapidly with the temperature decreasing from 800 to 650 °C, and the IAF microstructure was dominant within austcnitc grain with further cooling to 600 °C. The diffusion bonding experiment and the effect of C, Mn and Si concentrations on the Ae3 temperature by thermodynamic calculation confirm that Ti2O3 itself absorbs neighboring Mn atoms to form Mn-depleted zone (MDZ), which promotes the nucleation of IAF microstructure effectively. High temperature holding tests indicate that the nucleation potential of IAF microstructure was lowered in the Ti-killed C-Mn steel when it was treated at high temperature (1250 °C) for a longer time, which is attributed to the saturated absorption degree of Mn atoms by titanium oxide.

Influence of Isothermal Treatment on MnS and Hot Ductility in Low Carbon, Low Mn Steels

Hot ductility tests were used to determine the hot-cracking susceptibility of two low-carbon, low Mn/S ratio steels and compared with a higher-carbon plain C-Mn steel and a low C, high Mn/S ratio steel. Specimens were solution treated at 1623 K (1350 °C) or in situ melted before cooling at 100 K/min to various testing temperatures and strained at 7.5 × 10−4 s−1, using a Gleeble 3500 Thermomechanical Simulator. The low C, low Mn/S steels showed embrittlement from 1073 K to 1323 K (800 °C to 1050 °C) because of precipitation of MnS at the austenite grain boundaries combined with large grain size. Isothermal holding for 10 minutes at 1273 K (1000 °C) coarsened the MnS leading to significant improvement in hot ductility. The higher-carbon plain C-Mn steel only displayed a narrow trough less than the Ae3 temperature because of intergranular failure occurring along thin films of ferrite at prior austenite boundaries. The low C, high Mn/S steel had improved ductility for solution treatment conditions over that of in situ melt conditions because of the grain-refining influence of Ti. The higher Mn/S ratio steel yielded significantly better ductility than the low Mn/S ratio steels. The low hot ductility of the two low Mn/S grades was in disagreement with commercial findings where no cracking susceptibility has been reported. This discrepancy was due to the oversimplification of the thermal history of the hot ductility testing in comparison with commercial production leading to a marked difference in precipitation behavior, whereas laboratory conditions promoted fine sulfide precipitation along the austenite grain boundaries and hence, low ductility.

Characterizations of Dynamic Strain-induced Transformation in Low Carbon Steel

Dynamic strain-induced transformation of the low carbon steel Q235 at 770 ‐ C and 850 ‐ C leads to flne ferrite grains. The microstructure characterization and mechanism of the flne ferrite grain were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscattered difiraction (EBSD) technique. The results show that strain-induced microstructure is the mixed microstructure of ferrite and pearlite, with cementite randomly distributed on ferrite grain boundaries and the grains interiors. EBSD images of grain boundaries demonstrate that high angle grain boundaries (HAGBs) are dominant in both of the deformation induced microstructures occurring below and above Ae3, with only a few low angle grain boundaries (LAGBs) existing in the grain interiors. It implies that the dynamic strain-induced transformation (DSIT) happens above and below Ae3 temperature and has the same phase transition mechanisms. The reflnement of ferrite is the cooperative efiect of DSIT and continuous dynamic recrystallization (CDRX) of ferrite. Besides, DSIT is deemed as an incomplete carbon difiusion phase transition through the analysis of microstructure and the previous simulated results. The strengths of the Q235 steel with reflned ferrite and pearlite structure get doubled than the initial state without treated by DSIT and the residual stress in the reflned structure is partly responsible for the ductility loss.