Continuous Cooling Process Research Articles

Secondary Phase Precipitation in Fe-22Mn-9Al-0.6C Low-Density Steel during Continuous Cooling Process.

Secondary phase precipitation in Fe-22Mn-9Al-0.6C low-density steel was investigated during a continuous cooling process with different cooling rates through a DIL805A thermal expansion dilatometer, and the changes in microstructures and hardness by different cooling rates were discussed. The results showed that the matrix of the Fe-22Mn-9Al-0.6C was composed of austenite and δ-ferrite; moreover, the secondary phases included κ-carbide, β-Mn and DO3 at room temperature. The precipitation temperatures of 858 °C, 709 °C and 495 °C corresponded to the secondary phases B2, κ-carbide and β-Mn, respectively, which were obtained from the thermal expansion curve by the tangent method. When the cooling rate was slow, it had enough time to accommodate C-poor and Al-rich regions in the austenite due to amplitude modulation decomposition. Furthermore, the Al enrichment promoted δ-ferrite formation. Meanwhile, the subsequent formation of κ-carbide and β-Mn occurred through the continuous diffusion of C and Mn into austenite. In addition, the hardness of austenite was high at 0.03 °C/s due to the κ-carbide and β-Mn production and C enrichment, and it was inversely proportional to the cooling rate. It can be concluded that the presence of κ-carbide, DO3 and β-Mn produced at the austenitic/ferrite interface when the cooling rate was below 0.1 °C/s resulted in κ-carbide and β-Mn precipitating hardly at cooling rates exceeding 0.1 °C/s, which provides a guideline for the industrial production of Fe-Mn-Al-C low-density steel in the design of the hot working process.

Study on Phase Transformation Behavior in Continuous Cooling Process

The continuous cooling transition curve (CCT curve) system expresses the influence of cooling rate on the starting point of transition, transformation rate and microstructure are studied. In order to obtain desired microstructures and properties, the appropriate process parameters can be selected according to the continuous cooling transition curve. The CCT curve is very similar to the actual production conditions, so it is of great significance for studying the process of hot stamping of experimental steel.

Continuous Cooling Kinetics Modeling for a Medium–High Carbon Spring Steel

Continuous cooling transformation (CCT) diagrams of steel are the basis to determine the optimum heat treatment technology. Combined with metallography, the CCT diagram of medium–high carbon spring steel 60Si2CrA was obtained by dilatometric measurement at cooling rates from 0.5 to 50 K/s. It is found that ferrite, pearlite, and martensite were available in the experimental range. Taking into account of the incubation time and volume fraction evolution of the new phase, a new phase transformation kinetics model is established to estimate the fraction of ferrite and pearlite during continuous cooling process (CCP). Material constants in the model were determined and optimized by minimizing the errors between experimental and predicted data. The kinetics model was validated and proved its efficiency in predicting the new phase fraction of 60Si2CrA during continuous cooling and will probably constitute a useful tool for guiding the production process.

Effects of Nb on Bainite Transformation Behavior and Mechanical Properties of Low‐Carbon Bainitic Steels

The bainite transformation behavior and the mechanical properties of low‐carbon bainitic steels with and without niobium (Nb) are investigated by two different heat‐treatment methods − isothermal transformation process (ITP) and continuous cooling process (CCP). The results indicate that for ITP, the kinetics of bainitic transformation for the Nb‐added sample is accelerated at the beginning of the isothermal holding process due to more nucleation sites followed by its further deceleration as a consequence of the formation of ferrite prior to austempering. For all ITPs, Nb addition resulted in a decrease in the final amount of transformed bainites. In addition, different from the generally accepted result, the tensile strength of the steel is degraded by Nb addition during both ITP and CCPs, and the addition of Nb did not improve the final comprehensive properties of the steels. Moreover, Nb addition caused an increase in the critical temperature of high‐temperature transformation (Ar3); however, it yielded no significant effects on critical temperatures of Ac1 and Ac3.

Effects of Al addition on bainite transformation and properties of high-strength carbide-free bainitic steels

The effects of aluminum addition on bainite transformation and properties of carbide-free bainitic steels containing 0.22 wt.% carbon were investigated by two different types of heat treatment processes: continuous cooling process (CCP) and isothermal transformation process (ITP). The results indicate that for the CCP treatment, Al addition significantly promoted the ferrite and bainite transformation; however, it did not significantly increase the product of tensile strength and total elongation (PSE). For the ITP treatment, Al addition significantly promoted the kinetics of bainite transformation, and thus, more bainite was formed with Al addition; however, it was found that Al addition resulted in a decrease in tensile strength and an increase in elongation of the tested bainitic steels. Moreover, the effects of Al addition on comprehensive property were profoundly dependent on austempering temperatures. When the austempering temperature was higher (430 °C), PSE significantly increased with Al addition, whereas it decreased at the lower austempering temperature (400 °C). Therefore, it can be concluded that the effects of Al on properties of bainitic steels were more significant at higher austempering temperatures.

Effects of Ni and Cr on Cryogenic Impact Toughness of Bainite/Martensite Multiphase Steels

In the present research, the effects of Nickel (Ni) and Chromium (Cr) on cryogenic impact toughness (CIT) of low-carbon bainite/martensite multiphase steels [processed by two different cooling processes: isothermal transformation process (ITP) and continuous cooling process (CCP)] were investigated. It was found that due to the formation of carbides during isothermal treatment, the addition of Ni and Cr yielded no significant improvements in CIT. However, during CCP treatment, the addition of Ni manifested a considerable enhancement in CIT, whereas the addition of both Ni and Cr caused a decrease in CIT. Further, after ITP treatment, the microstructure of all steels consisted of bainite and martenite, while Ni + Cr steel contained the largest amount of bainite. The microstructures of the CCP-treated steels mainly also consisted of bainite and martensite, but no retained austenite and carbides were observed, thus resulting in a superior CIT.

Transformation Behavior and Properties of Carbide‐Free Bainite Steels with Different Si Contents

The bainite transformation behavior and properties of low carbon carbide‐free bainitic steels containing different silicon (Si) contents are investigated by two different types of heat treatment processes: isothermal transformation process (ITP) and continuous cooling process (CCP). The results indicate that for ITP and CCP, the transformation kinetics of bainite is retarded and the final bainite amount decreases with increasing Si content. However, both the strength and total elongation improve with the increase of Si content in the range of 1.0–2.0 wt%, resulting in an apparent increment in comprehensive property of bainitic steels due to the more film‐like RA and less carbides. It can be attributed to the increase of shear strength and stability of undercooled austenite and the formation of Cottrell atmosphere, as well as the solid solution strengthening of Si because of higher Si content. In addition, for the same samples, better mechanical properties can be achieved by a lower austempering temperature. Moreover, the increase of Si content resulted in an increase in the temperatures of Ac1 and Ac3.

In Situ Observation of the Lengthening Rate of Bainite Sheaves During Continuous Cooling Process in a Fe–C–Mn–Si Superbainitic Steel

The evolution of lengthening rate of bainite sheaves during continuous cooling process in a Fe–C–Mn–Si superbainitic steel was investigated by in situ observation on high-temperature laser scanning confocal microscope. The lengthening rates of bainite sheaves in three temperature ranges were calculated. The results indicate that the lengthening rate of bainite sheaves continuously decreases with the decrease of temperature during continuous cooling process. The lengthening rate of bainite sheaf depends on undercooling, transformation temperature, the diffusion of carbon atoms and the carbon content in parent austenite etc. The lengthening rate at high temperature is large due to the favorable carbon diffusion, smaller carbon content and less plastic deformation in untransformed austenite. Additionally, the microstructures after different isothermal holding temperatures were analyzed, indicating that the larger lengthening rate of bainite sheaves due to the high isothermal transformation temperature does not mean more amount of bainite transformation. Lower bainitic transformation temperature results in more and finer bainite plates.

Modeling of Austenite Decomposition during Continuous Cooling Process in Heat Treatment of Hypoeutectoid Steel with Cellular Automaton Method

The austenite decomposition during continuous cooling process in heat treatment of hypoeutectoid steel is studied with experimental method and numerical simulation. To investigate the phase transformation, quenching experiments are conducted and the microstructure evolution is investigated through metallographic examination. Dilatometric experiments are conducted as well and the experimental results are employed to analyze the phase transformation process during austenite decomposition. Subsequent studies of austenite decomposition in the steel are conducted using a numerical model based on cellular automaton (CA) method. In the simulation, the transformation of austenite into ferrite and pearlite is simulated, and the growth kinetics and average grain size of ferrite as well as the average interlamellar spacing of pearlite were obtained. The simulation results of the effects of cooling rate on the average grain size of ferrite and interlamellar spacing of pearlite agree well with the experimental results. Finally, integrated with a previously developed model for the austenization in heating process of heat treatment, the present model shows a capacity to investigate the effect of the prior austenite grain size on the final microstructure formed at the end of austenite decomposition.

Influence of Nb Micro-alloying on TRIP Steels Treated by Continuous Cooling Process

TRIP (transformation induced plasticity) steels are low alloyed steels with multiphase microstructure consisting of ferrite, carbide-free bainite and retained austenite. They are typically produced by thermo-mechanical treatment, which involves the hold in bainite transformation region. The hold ensures enough bainite in the final microstructure and also helps to stabilize higher amount of retained austenite. Due to transformation induce plasticity effect; TRIP steels possess very good combination of high strength and high ductility. In response to industrial demands, C-Mn-Si and C-Mn-Si-Nb TRIP steels were subjected to thermo-mechanical treatment with continuous cooling which corresponded to real rolling mill processing of the steel with similar chemical compositions. Typical TRIP microstructures with 10-15% of retained austenite were achieved for both steels after optimization of cooling schedules. However, cooling by two different cooling rates had to be applied to C-Mn-Si steel to obtain the convenient microstructure. Beneficial effect of Nb micro-alloying on low sensitivity of TRIP steel to variations in cooling parameters has been found out. Mechanical properties of the most convenient microstructures were very promising, ultimate tensile strength reached 850MPa with ductility A5mm around 25%.

(Nb, Ti)C在轧后卷取中的析出及对铁素体相微观力学特征的影响

以复合添加Nb和Ti的微合金钢为研究对象,采用热模拟、显微硬度、透射电镜以及纳米压痕技术等方法对实验钢在连续冷却条件下和卷取过程中的冷却速率对组织演变及显微硬度的影响进行观察和分析,研究了（Nb,Ti）C在卷取中的析出规律及其对铁素体相微观力学性能的影响.结果表明,连续冷却和卷取过程中的冷却速率的增加都能促进Nb-Ti实验钢从铁素体＋珠光体组织向贝氏体组织转变,细化铁素体晶粒.在连续冷却条件下,实验钢的显微硬度随着冷却速率的增加逐渐升高,而在卷取过程中由于较小冷却速率能够促进（Nb,Ti）C在铁素体中的形核和长大使得铁素体中存在大量均匀弥散分布的纳米析出物,提高了基体的强度,因此随着卷取过程中冷却速率的增加实验钢的显微硬度呈现降低的趋势.Nb-Ti实验钢中铁素体相的纳米硬度为4.13 GPa,Young’s模量为249.3 GPa,普通C-Si-Mn钢铁素体相的纳米硬度为2.64 GPa,Young’s模量为237.4 GPa,纳米析出物对铁素体相的纳米硬度的贡献达到1.49 GPa.

Microstructure Transformation of Nb-V Microalloyed Steel during Continuous Cooling Process

To achieve reasonable rolling technology of the novel Nb-V composite microalloyed steel, the continuous cooling transformation (CCT) curve was established by thermal simulation experiment. Microstructure and microhardness at different cooling rates were characterized using an optical microscope (OM) and microhardness tester. The results indicate that the critical quenching speed of Nb-V microalloyed steel is about 23 °C/s. The start and finishing temperatures of phase transformation decreased with the rise of cooling rate. Widmannstatten (W) structure appears at lower cooling rate interval. Microstructure transfers into martensite (M) and bainite (B) with obviously refined grains in higher cooling rate interval. Microhardness improves with the increase of cooling rates. Microhardness value is greatly improved to 298.6 HV at the cooling rate of 11 °C/s, which could be related to the formation of lower bainite during phase transformation process. When the cooling rate is above 29 °C/s, microhardness values remain unchanged basically. This illustrates that the microstructure of Nb-V microalloyed steel consists of martensite and lower bainite.

Influence of Vanadium on Grain Growth of Deformation Induced Ferrite in Low Carbon Steel during Continuous Cooling Process

The effects of vanadium on the deformation induced ferrite transformation (DIFT) in low carbon steel during heavy deformation at the temperature slightly higher than Ar3 and on the coarsening of DIFT microstructure during the continuous cooling processes after deformation were investigated using a thermo‐simulator. The results show that vanadium has little effect on the volume fraction of DIFT microstructure under heavy deformation, whereas the deformation induced ferrite (DIF) grains are refined with increasing vanadium content. The steel containing a small amount of vanadium exhibits similar velocity of grain growth compared to vanadium free steel, while the vanadium remarkably inhibits grain growth in a steel containing a high amount of vanadium during the continuous cooling process. The diffusion activation energy of grain boundaries for all the tested steels is calculated and the influencing mechanism of vanadium on the grain growth during the continuous cooling process is discussed.

Characteristics of PbTiO3 thin films on Pt/Ti/SiO2/Si by continuous cooling process

In order to introduce a new deposition process for ferroelectric thin film, the deposition temperature was continuously cooled down from 580°C to 400°C during the deposition which we call “continuous cooling process (CCP)”. X-ray diffraction patterns showed that the PbTiO3 thin films deposited by the CCP and at 480°C had polycrystallinity, but at substrate temperatures of 400°C and 580°C had poor crystallinity. Scanning electron microscopy of the CCP-deposited film surface showed larger granular-like micrograins than that of the film deposited at 480°C and smaller than that of the film at 580°C. While there was no other phase formation at the PbTiO3-Pt interface in the CCP-deposited film, resulting in a sharp interface, there was severe interface reaction at the PbTiO3-Pt and the Pt-Si in the film deposited at 580°C, resulting in an abrupt interface. Atomic force microscopy under ambient conditions showed smoother surface of the film by the CCP than that of the films at 580°C. Furthermore, the film by the CCP had higher packing density than that of the film at 480°C. Besides enhancement of the structural properties, the CCP deposition appeared to have improved the electrical properties such as dielectric constant, dissipation factor, leak current density and polarization. In the case of the film by the CCP, polarization-electrical field measurement showed the saturation polarization of 27 μCcm−2, remanent of 14 μCcm−2 and coercive of 150 kV. These results indicate that the CCP in metalorganic chemical vapour deposition has a possibility for fabrication of PbTiO3 ferroelectric thin films.

Improvement of the crystallinity in PbTiO 3 films grown on indium tin oxide-coated glass by metalorganic chemical vapor deposition using the continuous cooling process

Metalorganic chemical vapor deposition of ferroelectric PbTiO 3 thin films in indium-tin-oxide-coated glass using Pb(tmhd) 2, Ti(OC 3H 7) 4, and N 2O via pyrolysis with several methods was performed in order to produce PbTiO 3 films with high-magnitude dielectric constants, a low dissipation factor, and high transmittance. Scanning electron microscopy showed that the surfaces of the PbTiO 3 films grown by the continuous cooling process (CCP) had the best surface morphology. The stoichiometry of the PbTiO 3 films was observed by Auger electron spectroscopy. Room-temperature current-voltage and capacitance-voltage ( C-V) measurements clearly revealed a metal-insulator-metal behavior for the PbTiO 3 insulators. The dielectric constant and the dissipation factor of the PbTiO 3 thin film grown by the CCP determined at a frequency of 10 kHz were 156 and 0.015, respectively. Optical transmittance measurements showed that the PbTiO 3 grown by the CCP had a transparency, which was approximately 85% of the transparency in the visible wavelength range. These results indicate that the PbTiO 3 thin films grown on indium-tin-oxide-coated glass by CCP have promising applications as insulator films for thin film flat-panel-electroluminescent displays.

Structural and optical properties of PbTiO 3 films grown on indium tin oxide-coated glass by metalorganic chemical vapor deposition using the continuous cooling process

The growth of ferroelectric PbTiO 3 thin films on indium tin oxide (ITO)-coated glass by metalorganic chemical vapor deposition, using Pb(tmhd) 2, Ti(OC 3h 7) 4, and N 2O via thermal pyrolysis with several methods was performed to produce high quality PbTiO 3 films. Scanning electron microscopy showed that the surface of the PbTiO 3 films grown by continuous cooling process (CCP) had a smooth surface without any indications of defects and that the interface between the PbTiO 3 film and ITO-coated glass was abrupt. Atomic force microscopy showed that the PbTiO 3 film grown by the CCP had a smooth surface consisting of hemispherical grains, and optical transmittance measurements showed that the PbTiO 3 grown by the CCP had approximately 80% of the transmittance in the visible wavelength range. The dielectric constant and dissipation factor of the PbTiO 3 film grown by the CCP are the largest and smallest values among those grown by several methods, respectively. These results indicate that the PbTiO 3 thin film grown on ITO-coated glass by CCP can be used for thin-film flat panel-electroluminescent display applications.