Apex Steel

Apex Steel
Debashish Banerjee - Planning
Hot Rolling Mills

The process capability mapping at Apex Steel

Reheating furnace

water blast scale remover bay

rougher mill


Reduction in furnace passage time

Analysis of productivity parameters

  1. Reduction in Furnace Passage Time
    1. Charge pre-heat to around 500-700 degrees Celsius to enable shortening of lead time to heat up the charge to the desired 1200 degrees Celsius and thereby help improve on the charge – discharge cycle time through the furnace.
    2. Current trends in the reheating furnace always accelerate the charge from levels of cold start (almost the room temperature of typically around 35 degrees Celsius) to a gradual level of 200-300 degrees Celsius in the initial 25% of the furnace length and then to levels of 500-650 degrees Celsius at the 45-65% mark before switching over to the optimum levels of 1200 degrees Celsius in the last 25% of the length of the furnace. In this gradual progress, the charge heat gets dissipated while simultaneously adding heat as the equilibrium conditions are never attained and hence the heat exchange is fundamentally flawed. c. Metallurgical changes occur in the configuration of both the charge and the refractory and need to be discussed in detail at this juncture to analyze the impact in the productivity and the efficiency of the reheating furnace. The thermal expansion is sporadic and sharp in the enthalpy system of the charge thereby causing abrupt disruptions in the configuration of the charge structure. The levels of crystalline and amorphous regions change sharply and so do areas of tg and the plasticity within the structure. The internal heat levels within the billets also do vary as per the metallurgical structure and consequently the resultant changes to breakaway plasticity is determined by the factors of thermal graph within the passage and the retention of heat within the furnace as a function of the thermal strains within the refractory lining of the walls. Creating the turnaround algorithm
    3. Configuration changes simulation Charge density at consistency points would envisage a grain distribution in the structural configuration of the run down billet prior to the rolling bay. That is the foundation for ensuring uniformity in the dimensional changes in plasticity during the rolling at given contact duration and with given surface friction conditions and the corresponding productivity levels.

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Illustrations in the following passages shall clarify the conceptual coordinates for bringing in fundamental changes in the reheating furnace


STATE 1 : Current states of the reheating furnace - Simulated Mapping

  • Assertion 1

    Conceptually, enlarged crystals get formed on spatial reduction in the initial stages of thermal expansion

  • Assertion 2

    The refractory dynamics is a direct link to the configuration of the thermal decomposition within the charge - since the key determinant for the enthalpy exchange is the ability of the "heat substrates" to achieve equilibrium over a smaller trajectory change rather than give in to prolonged effects resulting in abrupt disruptions in the structure and consequently only able to achieve near-equilibrium conditions rather than consistent and firm coordinates.

  • Assertion 3

    The charge moves in a larger trajectory of thermal changes and consequently lead to structural configurations that are relatively arrayed over larger differences internally - a veritable problem for the subsequent drawing and rolling in process; yet quantified only in the breaking work and the related tensile properties of the finished rolled product.

Charge configuration in the current states -simulated dynamics

Furnace properties on current states - simulated dynamics

Furnace properties on current states - simulated dynamics


STATE 2 :Preheating the charge to 300-400 degrees Celsius Simulated Dynamics

  • Assertion 4

    Preheating the charge configures the thermal decomposition of the compounds and hence align the structural matrices to predictable and consistent "heat substrates" - the essence of quickening the equilibrium heat and ensuring high thermal retention by the refractory lining. ENERGY EFFICIENCY AND PRODUCTIVITY are the key derivatives of these initiatives as is showcased in the illustration

1Charge configuration with preheated simulated conditions

Impact on the furnace properties -simulated dynamics of the preheated charge

Surface Temperature (ST) in the charge -preheated charge dynamics


STATE 3: Refractory - Improved metallurgy for higher thermal creep curve area

  • Assertion 5

    The refractory material is the key determinant - working on improving the modulus properties and metallurgical achievement of a significantly larger stress-strain curve area through the primary creep coordinates to the secondary creep and effectively changing the thermal fatigue curve of the wall - shall ensure significantly lower thermal strains and linear expansion leading to high retention of heat.

Charge dynamics with improved refractory metallurgy

Furnace properties in simulated "heat run" with improved refractory metallurgy

Surface Temperature (ST) in the charge -with improved refractory metallurgy and preheated charge


Advanced notes on the simulation series for States-1 through 3

  • The enthalpy states in the furnace have significant influences on the properties of the rolling in as much as the drawing of the billets in the red-hot semi-molten state is a function of the relative regions of crystalline concentrates and the trade-off with the malleable proportions; the consistency in the configurations within the clusters being the key differentiating elements for improvements in both the aspects of line productivity and cutting edge quality in the areas of tensile properties and metallurgical strength of the finished products.
    • The first in the line of importance in this furnace dynamics is the charge configuration for metallurgy and internal heat. The billets are usually an assortment with varying degrees of compressive strength and cluster orientation of the molecules thereby inherently defining wide bandwidths of internal heat across the cross-section of the charge itself. The thresholds of internal heat provide a significant challenge for the furnace heat to change the levels in the given timeline thereby forcing the line foreman to regulate the flow of the charge to allow heat to escalate to desired or sub-optimal levels. The treatise above is an attempt to simulate the possibilities with an increasing of the charge thresholds for the internal enthalpy at the start of the reheating process in the furnace. The key enthalpy threshold-change related derivatives in this initiative are:
    • The preheating is designed to augment the sensible heat levels to 300-400 degrees Celsius implying proportionate levels of increase in the internal latent heat and thereby preparing the substrates for quicker assimilation of the external enthalpy percolation into the structure and allowing a significantly higher trajectory graph area for the quantum of heat changes – a driver derivative to ensure consistency and narrower bandwidths of crystalline and malleable portions trade-off in the substrates of the charge. This is an important consequence that influences drawing and rolling properties.
    • Structural studies for the charge configurations and the changes within the furnace span are required to be analyzed and correlated with the enthalpy configuration to predict the properties of a given heat in the aspects of rolling and drawing quality in the value chain. The simulation herein attempts to matrix the variables and the parameters that link intrinsically to create major nodes of influences although the influences are largely hidden owing to subtle impact on the structural derivatives of the billets and the subsequent impact on the drawing and rolling in the value chain. The three major parameters in the simulation chart are:
      • 1. Charge configuration changes with Variables – Internal Heat, tg or glass transition temperature, plasticity and the resultant crystalline trade-off as a function of plasticity.
      • 2. Refractory dynamics with Variables – thermal strain, linear expansion and heat retention
      • 3. Surface temperature dynamics with Variables – surface temperature in the charge through the heat span
    • Mathematical expression of the factors of metallurgy influencing the charge characteristics is an important functionality of the simulation initiative herein:
      • Functionality of internal heat at the different coordinates of the heat span are purely metallurgical in the derivatives and would be influenced by the nature of the billet and the structural orientation of the material. Consequently, the mathematical expression is a function of the relative levels of thermal decomposition within the clusters at the beginning of the heat and right through the duration of the run through the furnace (heat span). The graphical area described by the coordinates of the internal heat are indicators of the quantum of enthalpy changes going into the process of establishing the thermal decomposition; conceptually, higher the area described within a given heat span, greater shall be the levels of decomposition achieved and higher shall be the consistency in getting the trade-off between the crystalline and malleable zones within the clusters – veritably the major determinants for drawing and rolling in and influencing significantly the tensile properties and the creep characteristics of the finished rolled products. The major determinants for productivity and quality are the equilibrium coordinates of the heat quantum. Mathematically, a threshold heat quantum needs to be exchanged to achieve the equilibrium for the structural orientation of the charge in a given heat span; essentially implying that the metallurgy is an independent variable; the timeline is a constant and the quantum of heat required to achieve the equilibrium threshold is the variable dependent on the metallurgy. Thus the expression could be
        (m) = f(q) + t0
        where :
        (m) = summation of the metallurgical coordinates inclusive of the spatial configuration within each given material in the charge cluster
        f(q) = functional dependent variable – the enthalpy changes required during the heat span
        t0 = the heat span determined by the desired productivity needs for a given charge to dwell in the reheating furnace
      • The furnace dynamics as determined by the quality of the refractory lining is another key determinant and shall have the following expressions of the elements leading to the mathematical statement:
        • The modulus properties of stress-strain curve in the thermal plane in the primary creep zone and the resulting area of the primary creep curve – the major determinant influenced by the heat absorption and retention trade-off of the material – silica rich or alumina rich refractory or a judicious hybrid with the selection of granules to define the porosity of the refractory compound – a factor that influences creep curve configurations through external contaminants and residual interferences. Higher areas in the primary creep curve and the relative insensitivity to the thermal range as implied through the thermal modulus of the refractory compound are the major lookout properties for the selection of the furnace.
        • The secondary creep elasticity as defined by the coordinates as well as the nature of the curve for the fatigue zones are the other key determinants and shall require sustain evaluation under high density cycles of thermal peaks and troughs in simulated lower heat spans to evaluate the suitability of a refractory compound. Intense laboratory evaluation of the stress-strain curves; the determination of the secondary creep point and the mapping of the fatigue curve are all necessary to home in on the right choice for the compound to be used for the refractory lining.
        • The elements for the mathematical expression shall be the heat retention property – an independent variable, the thermal strain and the linear expansion – the twin properties that define the dependent variable grid and with n0 – the number of thermal cycles in a given heat span being the constant. Thus the expression shall be:
          h = f(ln(s)) + no
          h = the heat retention variable; essentially assumed an independent function since it is a derived function of a cluster of variables. This is a summation of the enthalpy exchanges and hence is the quantum of heat left retained as a derivative at the end of the heat span.
          f(ln(s)) = the function of the thermal strain and the linear expansion as a logarithmic inverse function to express the inherent relationship between the two variables in relation to the derived property of the quantum of heat retained.
          no = the estimated number of thermal stress cycles established in the heat span; herein assumed a constant for the given furnace conditions and the lead time established by the operative for discharging.
  • The surface temperature on the charge as a function of flame characteristics in the last quarter of the heat span is a critical variable and hence requires a detailed mapping of the causal links to evaluate and monitor the performance indicators. However, notwithstanding the popular concepts doing the usual rounds in the hot rolling industry, the role of the preceding concepts in defining the substrate preparation for the roughing mills drawing and eventual rolling cannot be negated. In this section, we shall examine the elements defining the flame characteristics and the related influences on the surface temperatures of the charge critically.

Blowing power of the fan is a critical element that is the determinant for the heat distribution in and around the flame in the discharge zone of the reheating furnace. The following case study of the blower motor dynamics in a reputed hot rolling mill shall illustrate the issues related to the dynamics of heat distribution.

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