Evolution Of Steel Property In Continuous Casting And Its Ab Initio Investigations For Fe Matrix At Elevated Temperatures

Steel is a multi-components alloy with the matrix of iron,it is an important engineering material with wide applications.The efficient continuous casting process is essential for the production of steel,the property and the quality of continuously casting steel thus play a fundamental role for the following steel production processes as well as the quality of final steel products.The property of steel at elevated temperatures directly depends on the microstructure associated with various thermo-mechanical histories,and it basically determines by the intrinsic properties of primary phases which have different electronic structures.Fundamental understanding of temperature-dependent properties from electronic structure calculations,in combination with the quantitatively investigation of property and microstructure evolution for steel,should be a foundation for the control of microstructure,property and quality of steel based on quantum calculations for high-temperature processes like continuous casting process.Thus,with the knowledge of thermal histories occurring in continuous casting process,the property and the microstructure evolution of steels are quantitatively investigated at various thermal histories in the thesis.Additionally,temperature-dependent properties of paramagnetic Fe,including magnetism,elastic and plastic parameters and lattice expansion,are calculated using first-principles method.The physical essence of their temperature-dependence is consequently revealed.The main results of the thesis can be summarized as follows:?1?The influence of cooling rate on the temperature-dependent hot ductility,strain-stress curve and peak stress/strain of continuously casting steel is investigated.Based on the hot ductility measurements at a wide range of cooling rate,i.e.100⁶00 °C min-1,for two different steels,the width of the low ductility trough is obtained as a function of cooling rate,i.e.??-Aln?C_R?+B.Increasing the cooling rate,the ductility trough expands to both low and high temperatures.However,the cooling rate indicates relatively small influence on the strain-stress curve,peak stress and peak stress of steels at elevated temperatures.The failure mechanism of void coarsening induced by the defamation induced ferrite or proeutectoid ferrite?DIF or PF?in combination with the second particles at grain boundaries has little change with varying the cooling rate as well.Due to the precipitation of DIF?or PF?and the magnetic transitions,obvious transitions of temperature coefficients of peak strain and stress are consequently observed at around Ae3?or Ar3?and TC.?2?Quantitative analysis models of austenite transformation are established,and the influence of cooling rate is discussed.Using the linear thermal expansion measurements at cooling rates from 5 to 300 °C min-1 for two different steels,an empirical model is established using the regression analysis to describe the relationship among the critical temperatures of austenite transformation in continuous cooling?Ar3 and Ar1?,at equilibrium and the cooling rate,that is Ar???=Ae-exp?B+ClC_R?.Based on the different lattice structures of primary phases in austenite transformation,a method using the linear thermal expansion measurements to compute the phase fraction evolution in austenite transformation is proposed.The investigations of microstructure evolution for two steels indicate that both the fraction of ?-ferrite and the phase transformation rate increase with increasing the cooling rate,while the critical temperatures of austenite transformation apparently declines at cooling rates less than 100 °C min-1.The quantitative investigations of austenite transformation above lay the foundation for the control of the surface temperature and microstructure state,so as to optimize the property and quality of continuously casting steels.?3?The influence of temperature reversion and its reversion rate on steel properties are investigated,and the temperature-dependence is understood from ab initio calculations.With the temperature reversion process in continuous casting,steels are subjected to double austenite????-ferrite transformations in the undercooling and temperature reversion processes,which leads to the ?-ferrite existing at around Ae₃.The hot ductility trough and transitions of the temperature-dependent peak strain and stress consequently shift toward high temperatures by around 50 °C.However,the temperature reversion rate ranging from 60 to 300 °C min-1 indicates small influence on both the fracture mechanism and the temperature-dependence of properties of steels.From the ab initio calculations at different magnetic-elastic,volume-elastic and volume-magnetic couplings,due to the competitive contributions from magnetism and volume expansion in PM ?-Fe,the elastic constant c' and the Young's modulus E of ?-Fe is insensitive to temperature,which also explains the transition of temperature coefficient of peak stress in austenite-?-ferrite transformation.?4?The important contributions from thermal spin fluctuations to the temperature-dependent elastic properties of PM Fe are calculated.Based on series ab initio calculations performed with EMTO,a simple but accurate method is proposed to establish the thermo-induced spin fluctuations for PM Fe by using the partition function including the Jacobin term.Furthermore,a temperature-dependent quadratic mean magnetic moment msf is introduced to represent spin fluctuation distributions at elevated temperatures.Compared to the predictions from spin fluctuation distributions,the mean magnetic moment exhibits the same high accuracy for the ab initio calculation of elastic constants for PM Fe.With the consideration of lattice expansion and thermo-magneto couplings,the single-crystal elastic constants c' and c44,and their temperature-dependences are calculated and the influence of thermal spin fluctuations on the magnetism and elastic constants of PM Fe has been quantitatively discussed.In comparison with ?-Fe,a relatively strong thermo-magneto coupling is revealed in PM ?-Fe,which leads to relatively large effects on both the elastic constants and their temperature coefficients.For ?-Fe,c' and its softening coefficient with temperatures declines by around 25 % and 22 %,respectively.?5?The thermal lattice expansion of PM Fe is calculated using first-principles method,and the influence of thermo-magneto coupling on lattice expansion and other temperature-dependent properties is investigated.A method is proposed to calculate the lattice expansion at elevated temperatures using the self-consistent calculations of Helmhotltz,which includes the thermo-magneto,magneto-elastic and lattice vibration couplings evolved from ab initio calculations.With the calculations of the mean magnetic moment msf and single-crystal elastic constants as a function temperature and volume,both the thermal lattice expansion and intrinsic properties of PM ?-Fe and ?-Fe including Young's modulus,shear modulus,poisson's ratio,Debye temperature and bulk modulus are accurately predicted at elevated temperatures.Furthermore,the important contribution from thermal spin fluctuations to the lattice expansion of PM ?-Fe and ?-Fe is pioneered in the thesis.The comparatively weak thermo-magneto coupling in ?-Fe indicates small influence on its lattice expansion,while the volume of ?-Fe?represented by Wigner-Seitz radius?obviously declines by around 0.018 Bohr due to the relatively strong thermo-magneto coupling induced by spin fluctuations.Taking thermal spin fluctuations into consideration,ab initio predictions indicate high accuracy for both PM ?-Fe and ?-Fe as compared with experimental measurements.