Computer Model of Phase Transformation From HotDeformed Austenite in Niobium Microalloyed Steels

Based on thermodynamics and kinetics, a new mathematical model was developed to calculate the CCT diagrams and the transformation kinetics in low carbon niobium steels, in which the effect of deformation on the degree of supercooling was taken into account. The undercooling caused by deformation is the major reason for the increase of the starting transition temperature during continuous cooling. The critical cooling rate of bainite formation is within 2-5 ℃/s for the studied niobium steels and deformation is suitable for the occurrence of pearlite. The ferrite volume fraction increases with the increase of the austenite boundary area, and decreases with the increase of the cooling rate. The calculated CCT diagrams and the volume fraction of each phase are in good agreement with the measurements.     

Modeling of Austenite Decomposition in Low SiMn TRIP Steel During Cooling

Transformation behavior in low carbon SiMn TRIP steel was investigated by means of microstructural observation and computer modelling. A transformation model in which transformation is controlled by carbon diffusion was suggested, which well described the volume fractional change of ferrite, pearlite, and bainite during continuous cooling. The influence of Si content and austenite grain size was thoroughly investigated. The calculated results indicated that Si retards pearlite transformation, accelerates polygonal ferrite transformation, refines the austenite grain, and increases the ferrite transformation rate.     

Effect of WarmRolled Pearlite Microstructural Features on Austenitic Transformation

The austenite transformation characteristics for various warmrolled pearlite during rapid heating were investigated. The results indicate that the start temperature (Ts) is sensitive to the microstructural feature of pearlite, whereas the dislocation plays an important role in the transformation rate; at the same time, the uniformity of austenite grains is more or less affected by the amount of spheroidized pearlite. A critical effect on the state of austenite grain is created through the influence of initial microstructures on the start temperature of transformation.     

Mechanism of Austenite Evolution During Deformation of UltraHigh Carbon Steel

The mechanism of transformation of austenite to cementite and pearlite during the deformation of ultrahigh carbon steel was discussed. The results indicate that the pearlite and cementite can be induced by deformation between Acm to Arcm. The transformation during deformation is still considered as a diffusioncontrolled process. With the increase of time and reduction, the pearlite fraction increased. At the beginning of the transformation, the pearlite was lamelliform. When the rate of reduction was increased to 70%, some of the induced lamellar pearlite was broken up under deformation.     

Characteristics of Strain-Induced Ferrite in Low Carbon Steel

Itiswellknownthatthestrain inducedtrans formationfromaustenitetoferritecanleadtograinrefinement .Thushighermechanicalpropertiessuchasstrengthandductilitycanbeobtained[1,2 ] .Someresearcheshavebeencarriedoutonthenucleationofstrain inducedferriteinthepastyears .PDHodgsonetalsuggestedthattheaustenitegrainsshouldbeascoarseaspossibletodecreasenucleationatgrainbo undaries[3 ] .PJHurleyetalconsideredthatthecel lularboundaryofdislocationsuppliesthesitesfornu cleationofstrain inducedferrite[4] .Yang…     

Effect of Dissolved and Precipitated Niobium in Microalloyed Steel on Deformation Induced Ferrite Transformation （DIFT）

 Hot deformation processing was designed to study the effects of niobium (Nb) on DIFT. A prestrain of 051 at 880 ℃ for different isothermal time was used for adjusting the deformed austenite constitution and Nb existing state, followed by a secondary heavy deformation at 780 ℃ for inducing the ferrite transformation. The volume fraction and grain size of deformation induced ferrite (DIF) obtained at different isothermal time between double hits were investigated. It was found that Nb dissolved in austenite is adverse to DIFT; however, the precipitation of Nb is beneficial to DIFT. As Nb plays the role in the conventional TMCP, Nb retards the recrystallization of deformed austenite and enhances the deformation stored energy in the multipass deformation, and in result, Nb promotes DIFT.     

Numerical Simulation of Austenite Recrystallization in CSP Hot Rolled CMn Steel Strip

An integrated mathematical model is developed to predict the microstructure evolution of CMn steel during multipass hot rolling on the CSP production line, and the thermal evolution, the temperature distribution, the deformation, and the austenite recrystallization are simulated. The characteristics of austenite recrystallization of hot rolled CMn steel in the CSP process are also discussed. The simulation of the microstructure evolution of CMn steel ZJ510L during CSP multipass hot rolling indicates that dynamic recrystallization and metadynamic recrystallization may easily occur in the first few passes, where nonuniform recrystallization and inhomogeneous grain size microstructure may readily occur; during the last few passes, static recrystallization may occur dominantly, and the microstructure will become more homogeneous and partial recrystallization may occur at relatively low temperature.     

Effect of Rare Earth Elements on Isothermal Transformation and Microstructures in 20Mn Steel

The effect of rare earth elements on the isothermal transformation and microstructures in 20Mn steel is in-vestigated by means of metallography and dilatometry.Rare earth elements decrease both the incubation periodof pro-eutectoid ferrite and the rate of pearlitic transformation.In addition,rare earth elements play a role ofreducing needle-like ferrite and the amount of pearlite,densifing the lamellar space of pearlite and enhcingsegregation of carbide in granular bainite.It is suggested that rare earth elements may decrease the interfacialenergy of grain boundary and interphase,hinder the diffusion of carbon atoms and form rare earth carbides withhigh melting point which reduce the carbon content in austenite.     

Austenite Grain Refinement by Reverse α′→γ Transformation in Metastable Austenitic Manganese Steel

Microstructure of metastable austenitic manganese steel after reverse transformation treatment was investigated using optical microscopy,X-ray diffraction(XRD),electrical resistivity and hardness testing.Austenite grain refinement was successfully achieved by a two-step heat treatment.First,martensite was produced by cooling the solution-treated samples to-196 ℃.Then,the deep cryogenic treated samples were heated to 850 ℃ upon slow or rapid heating.The mean size of original austenite grain was about 400μm.But the mean size of equiaxed reversion austenite was refined to 50μm.Microstructure evolution and electrical resistivity change showed that martensite plates underwent tempering action upon slow heating,and the residual austenite was decomposed,resulting in the formation of pearlite nodules at the austenite grains boundaries.The refinement mechanism upon slow heating is the diffusion-controlled nucleation and growth of austenite.However,the reverse transformation upon rapid heating was predominated by displacive manner.The residual austenite was not decomposed.The plateα-phase was carbon-supersaturated until the starting of reverse transformation.The reverse transformation was accompanied by surface effect,resulting in the formation of plate austenite with high density dislocations.The refinement mechanism upon rapid heating is the recrystallization of displacive reversed austenite.     

Influence of Holding Time After Deformation on Bainite Transformation in Niobium Microalloyed Steel

  Using Gleeble 1500 system, the influence of holding time on bainite transformation in deformed niobium microalloyed steel during continuous cooling was analyzed, and the carbides in upper bainite were also systematically researched. The results show that the occurrence of the static recrystallization decreases the amount of bainite with an increase in the holding time and the emergence of retained austenite (RA) with the longer holding time. Two types of carbides were observed in upper bainite with regard to their precipitation sites. They either existed between the bainite ferrite laths or co existed with RA. The formation mechanism of two kinds of carbides was analyzed by combining TEM micrographs with the model.     

Effect of Rare Earths on Precipitation Kinetics of Niobium Carbide in Microalloyed Steel

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Effect of Boron on Delayed Fracture Resistance of MediumCarbon High Strength Spring Steel

The delayed fracture behavior of mediumcarbon high strength spring steel containing different amounts of boron (0000 5%, 0001 6%) was studied using sustained load delayed fracture test. The results show that delayed fracture resistance of boron containing steels is higher than that of conventional steel 60Si2MnA at the same strength level and it increases with the increase of boron content from 0000 5% to 0001 6%. The delayed fracture mode is mainly intergranular in the boron containing steels tempered at 350 ℃, which indicates that the addition of boron does not change the fracture character. However, the increase of boron content enlarges the size of the crack initiation area. Further study of phase analysis indicates that most boron is in solid solution, and only a very small quantity of boron is in the M3(C, B) phase.     

Effect of LowFrequency AC Power Supply During Electroslag Remelting on Qualities of Alloy Steel

The effect of frequencies of AC power supply on the quality of the electroslagmelted ingot is studied. The results show that with a decrease in the frequency, electromagnetic force becomes more violent, and the temperature in the slag bath becomes more homogeneous, and therefore, the depth of molten metal pool is decreased; electrochemical reactions occur with the decrease in the frequency, and the atomic oxygen electrolyzed dissolves in the molten metal pool; the nonmetallic inclusions, which are distributed dispersively in the ingot, have an increased content, and their size is approximately in the range of 2-3 μm.     

Effect of Microstructure of Cementite on Interphase Stress State in Carbon Steel

 The experiments related to stress states of ferrite and cementite in carbon steels were carried out including in situ four point bending and tensile test by X ray diffraction technique. Stresses in the cementite phase can be measured by conventional X ray diffraction instrument after a specific treatment on the specimen surface. In order to estimate the stress states in two phases, the X ray elastic constants of two phases in single phase state (PXEC) are determined by the experimental X ray elastic constants of them in composite state (CXEC). The effects of volume fraction and particle size of spheroidal cementite on the interphase stress state are estimated. The experimental results are in good agreement with the theoretical relationships reported in the previous studies.     

Morphology and Precipitation Kinetics of MnS in LowCarbon Steel During Thin Slab Continuous Casting Process

The morphology of manganese sulfide formed during thin slab continuous casting process in lowcarbon steel produced by compact strip production (CSP) technique was investigated. Using transmission electron microscopy analysis, it was seen that a majority of manganese sulfides precipitated at austenite grain boundaries, the morphologies of which were spherical or close to the spherical shape and the size of MnS precipitates ranged from 30 nm to 100 nm. A mathematical model of the manganese sulfide precipitation in this process was developed based on classical nucleation theory. Under the given conditions, the starting and finishing precipitation temperatures of MnS in the continuous casting thin slab of the studied lowcarbon steel are 1 189 ℃ and 1 171 ℃, respectively, and the average diameter of MnS precipitates is about 48 nm within this precipitation temperature range. The influences of chemical components and thermomechanical processing conditions on the precipitation behavior of MnS in the same process were also discussed.     

Influence of Mn Content and Hot Deformation on Transformation Behavior of CMn Steels

The hot deformation behaviors and the microstructural evolution of plain CMn steels with similar contents of C and Si but different contents of Mn have been investigated by compressive processing using Gleeble1500 mechanical simulator. Influence of Mn and hot deformation on continuous cooling transformation of steels has been studied. The experimental results showed that deformation in austenite region accelerated transformation process, and the extent is dependent on the hot deformation and cooling conditions. The hot deformation would promote transformation process, but the increase of transformation temperature is dependent on Mn contents. The results have also shown that the effect of deformation on ferrite transformation becomes more obvious with the increase of Mn content at relatively low cooling rate.     

Process Simulation and Microstructure Analysis of Low Carbon SiMn Quenched and Partitioned Steel

The quenching and partitioning (Q&P) process was experimentally investigated on the thermomechanical simulator (Gleeble3800). The microstructure and fracture mechanism of the sheets were investigated by means of TEM. It was found that the microstructure of quenched and partitioned steel consists of fine lath martensite and thin interlath austenite films. The optimum quenching temperature of producing the maximum amount of retained austenite after final quenching at room temperature was predicted by Matlab software package. It was found that the calculations by Matlab software can provide guidance for experimental processing design reliably. The volume fraction of retained austenite at room temperature was approximately 8%, which was measured easily by the software VC60++ programming. The results verified that quenched and partitioned steel possesses a good combination of strength and plasticity due to its fine microstructure. This steel exhibited high ultimate tensile strength (exceeding 1 000 MPa) and good elongation of 25%. The results showed that the fracture mechanism of the sheets is typical tough fracture under the condition of tensile failure.     

Effect of Heat Input and Niobium on the Microstructure and Toughness of Coarse Grain Heat-Affected Zone in HSLA Steels

 The influence of Nb on the coarse grain heat-affected zone (CGHAZ) microstructure, mechanical property and the transformation kinetics of HSLA steels under different heat inputs, has been investigated. When welded at higher heat inputs (100~60kJ cm-1), impact toughness values of the steel without Nb are much higher than the steel with Nb, and the lowest span is 153J at 60kJ cm-1. But only a little higher values are observed at lower heat inputs (40~30kJ cm-1), and the highest span is 68J at 30kJ cm-1. Dilatation studies indicate continuous cooling transformation start temperatures (Ts) of CGHAZ for the steel with Nb are approximately 15~30°C lower than the steel without Nb at all heat inputs. Under higher heat inputs Nb in solid solution suppress ferrite transformation and promote the formation of granular bainite which has detrimental effect on impact toughness. Under lower heat inputs higher Charpy impact energy values in the steel with Nb are associated with the formation of low carbon self-tempered martensite.     

Effects of AlMnCa and AlMnFe Alloys on Deoxidization of Low Carbon and Low SiliconAluminum Killed Steels

To confirm the effects of AlMnCa and AlMnFe alloys on the deoxidization and modification of Al2O3 inclusions, experiments of 4heat low carbon and low siliconaluminum killed steels deoxidized by AlMnCa and AlMnFe alloys were done in a MoSi2 furnace at 1 873 K. It is found that the 1# AlMnCa alloy has the best ability of deoxidization and modification of Al2O3 inclusions than 2# AlMnCa and AlMnFe alloys. Steel A deoxidized by 1# AlMnCa alloy has the lowest total oxygen content in the terminal steel, which is 37×10-6. Most of the inclusions in the steel deoxidized by 1# AlMnCa alloy are spherical CaOcontaining compound inclusions, and 891% of them are smaller than 10 μm. The diameter of the inclusion bigger than 50 μm is not found in the final steels deoxidized by AlMnCa alloys. Whereas, for the steels deoxidized by AlMnFe alloys, most inclusions in the terminal steel are Al2O3 or Al2O3MnO inclusions, and a few of them are spherical, and only 768% of them are smaller than 10 μm. Some inclusions bigger than 50 μm are found in the steel D deoxidized by AlMnFe alloy.     

Effect of MicroArea pH Value on Formation of Different Rust Phases of Weathering Steel in Initial Corrosion Period

 The formation of protective rust layer of atmospheric corrosion resistance was discussed for weathering steel. The Rust layers of experimental steels were made by using an accelerating industrial atmospheric corrosion test of dry-wet cyclic immersion. Furthermore XRD were used to measure and analyze the rust layers. The initial corrosion processes below thin water film were observed and the changes of micro-area PH value were measured. The results are as follows: The corrosion processes are obviously different between P-RE weathering steel and carbon steel. The surface of P-RE weathering steel rapidly forms α-FeOOH and the micro-area PH value is above 5.4 during the initial corrosion period. While the component of the rust layer for the carbon steel is mainly Fe3O4 and the micro-area PH value is below 3.8.