Abstract
The mechanism of formation of longitudinal facial cracks during continuous casting of peritectic steels has been studied using a process modeling approach. First, the delta-to-gamma transformation was modeled, assuming carbon diffusion control. The problem of the moving boundary (delta/gamma interface) was solved by employing a one-dimensional finite-difference method. Second, a heat-transfer model for continuous casting of steel was developed and combined with the phase transformation model. Three heat-flux conditions (i.e., low, medium, and high) were obtained from literature data and assumed as the thermal boundary condition. The delta-to-gamma transformation rate was evaluated as a function of the heat-flux conditions. Finally, the results of the coupled model were adopted to calculate the stresses in the solid shell applying the commercial finite-element program, ABAQUS. The results showed that the transformation from delta to gamma is comparatively rapid due to the high diffusivity of carbon in this temperature range. The variation of the heat flux at the meniscus results in large changes of the transformation rate in the meniscus region. Based on the results of the stress calculations, it was concluded that, in order to generate a longitudinal crack on the solid shell surface, not only the tensile stress caused by rapid transformation (i.e., rapid cooling) but also the presence of hot spots is required. A threshold value of approximately 16 pct was obtained for the retardation of shell growth required to generate longitudinal cracks.
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Abbreviations
- C 0 :
-
initial carbon concentration in the steel, wt pct
- C γ :
-
carbon concentration in the gamma phase, wt pct
- C γ|δγ :
-
carbon concentration of the gamma phase at the delta-gamma interface, wt pct
- C γ (T):
-
equilibrium carbon concentration of gamma phase at temperature T, wt pct
- C δ (T):
-
equilibrium carbon concentration of delta phase at temperature T, wt pct
- C n :
-
concentration in node n of Murray-Landis grid
- C pa :
-
apparent specific heat capacity, J g−1 °C−1
- C p :
-
heat capacity of steel, J g−1 °C−1
- C eff p :
-
effective heat capacity (which takes into account of the latent heat), J g−1 °C−1
- CR :
-
cooling rate, °C s−1
- D γ :
-
diffusion coefficient of carbon in the gamma phase, cm2 s−1
- E :
-
Young’s modulus, GPa
- ΔH :
-
latent heat of solidification of steel, J g−1
- k :
-
thermal conductivity of steel, W m−1 °C−1
- L :
-
length of the steel at temperature T, mm
- L 0 :
-
length of the steel at solidus temperature, mm
- q((t)):
-
heat flux, kW m−2
- R:
-
gas constant, J K−1 mol−1
- R γ :
-
volume fraction of gamma phase, dimensionless
- R 0 γ :
-
initial volume fraction of the gamma phase, dimensionless
- S :
-
solid shell thickness for normal position, mm
- S HS :
-
solid shell thickness for hot spot, mm
- t :
-
time, s
- t τ :
-
transit time, s
- T :
-
temperature, °C
- T 0 :
-
temperature corresponding to a strain-free state, °C
- T abs :
-
absolute temperature, K
- T avg :
-
average temperature of the solid shell, °C
- T i :
-
initial temperature of steel, °C
- T liq :
-
liquidus temperature, °C
- T p :
-
peritectic temperature, °C
- T sol :
-
solidus temperature, °C
- T surf :
-
temperature of the shell surface, °C
- T*:
-
fictitious temperature for ABAQUS calculation (derived from Eq. [24]), °C
- v i :
-
displacement of node i in y direction, mm
- V :
-
velocity of the delta-gamma interface toward the delta phase, mm/s
- V c :
-
casting velocity, m/min
- V δ :
-
velocity of the delta-gamma interface due to the carbon diffusion in the delta phase, mm/s
- W :
-
slab width, mm
- x :
-
distance in x-direction, mm
- X n :
-
node position in Murray-Landis grid
- X S :
-
slab thickness, mm
- y :
-
distance in y-direction, mm
- z :
-
distance in z-direction, mm
- α :
-
thermal expansion coefficient, °C−1
- ɛ :
-
total strain, dimensionless
- ɛ th :
-
thermal strain, dimensionless
- ɛ tr :
-
strain due to transformation, dimensionless
- λ :
-
primary dendrite-arm spacing (grain size), μm
- ρ :
-
density of steel, g cm−3
- σ max :
-
maximum tensile stress at solid shell surface, MPa
- σ UTS :
-
UTS, MPa
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Konishi, J., Militzer, M., Samarasekera, I.V. et al. Modeling the formation of longitudinal facial cracks during continuous casting of hypoperitectic steel. Metall Mater Trans B 33, 413–423 (2002). https://doi.org/10.1007/s11663-002-0053-y
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DOI: https://doi.org/10.1007/s11663-002-0053-y