PROCESS FOR THE PRODUCTION OF A METALLIC STRIP OR SHEET

Abstract

The invention relates to a method for producing a metallic strip or sheet (1), in which the strip or sheet (1) is rolled in a multi-stand rolling mill (11) and is discharged downstream of the last roll stand (14) of the rolling mill (11) in the conveying direction (F), wherein the strip or sheet (1) is cooled in the multi-stand rolling mill (11) and/or downstream of the rolling mill (11) as viewed in conveying direction (F), wherein a temperature of the strip or sheet (1) is measured upstream of the last roll stand (14) of the rolling mill (11) as viewed in conveying direction (F). Based on this measured temperature, a temperature for the strip or sheet (1) at the exit (A) of the last roll stand (14) of the rolling mill (11), is then determined purely by calculation with the aid of a temperature calculation model, with which temperature further processes of the manufacturing method can be controlled or regulated after a comparison with a predetermined reference value.

Claims

1. A method for producing a metallic strip or sheet (1), in which the strip or sheet is rolled in a multi-stand rolling mill (11) and is discharged in the conveying direction (F) behind the last roll stand (14) of the rolling mill (11), wherein the strip or sheet (1) is cooled in the multi-stand rolling mill (11) and/or downstream of the rolling mill (11) as viewed in conveying direction (F), wherein a temperature (T2) of the strip or sheet (1) is measured upstream of the last roll stand (14) of the rolling mill (11) as viewed in the conveying direction (F), characterized by the steps: (i) calculating a temperature (TFM) for the strip or sheet (1) immediately at the exit (A) of the last roll stand (14) of the rolling mill (11) by means of a temperature calculation model on the basis of the temperature (T2) of the strip or sheet (1) measured upstream of the last roll stand (14) of the rolling mill (11), wherein said calculating step is carried out for a system formed by the material section of the strip or sheet (1) between the point at which the temperature (T2) is measured upstream of the last roll stand (14), and the exit (A) of the last roll stand (14), (ii) comparing the temperature (TFM) calculated for the strip or sheet (1) at the exit (A) of the last roll stand (14) of the rolling mill (11) with a predetermined reference value (TFM.sub.ref), and (iii) adjusting (controlling, preferably Rules) at least one process parameter for the strip or sheet (1), taking into—account the comparison of the calculated temperature (TFM) with the predetermined reference—value (TFM ref) according to step (ii), wherein, depending this process parameter the strip or sheet is processed, heated or cooled.

2. The method according to claim 1, characterized in that the temperature (TFM) calculated in step (i) is a surface temperature of the strip or sheet (1).

3. The method according to claim 1, characterized in that the process parameter is the temperature of an intermediate stand cooling (22) of the rolling mill (11) arranged upstream of the last roll stand (14), as seen in the conveying direction (F), the temperature of this intermediate stand cooling (22) being controlled, preferably regulated, in step (iii) taking into account the comparison according to step (ii).

4. The method according to claim 1, characterized in that the process parameter is the temperature of a preliminary strip cooling (24) arranged upstream of the rolling mill (11), as seen in the conveying direction (F), the temperature of this preliminary strip cooling (26) being controlled, preferably regulated, in step (iii) taking into account the comparison according to step (ii).

5. The method according to claim 1, characterized in that the process parameter is the temperature of an inductive heater (26) arranged upstream of the rolling mill (11), as seen in the conveying direction (F), the temperature of this inductive heater (26) being controlled, preferably regulated, in step (iii) taking into account the comparison according to step (ii).

6. The method according to claim 1, characterized in that the process parameter is the temperature of a furnace (28) arranged upstream of the rolling mill (11), as seen in the conveying direction (F), the temperature of this furnace (28) being controlled, preferably regulated, in step (iii) taking into account the comparison according to step (ii).

7. The method according to claim 1, characterized in that the process parameter is the operating position of a thermal insulation hood (30) arranged upstream of the last roll stand (14), as seen in the conveying direction (F), the thermal insulation hood (30) being opened or closed relative to the strip or sheet in step (iii), taking into account the comparison according to step (ii).

8. The method according to claim 1, characterized in that in step (iii) a laminar cooling device (18) arranged downstream of the last roll stand (14) of the rolling mill (11) as viewed in the conveying direction (F), is controlled, preferably regulated, taking into account the comparison according to step (ii).

9. The method according to claim 1, characterized in that in step (iii) a rapid cooling device (16) arranged immediately downstream of the last roll stand (14) of the rolling mill (11), as viewed in the conveying direction (F), is controlled, preferably regulated, taking into account the comparison according to step (ii).

10. The method according to claim 1, characterized in that the process parameter is the temperature of an intermediate cooling of the rolling mill (11) arranged upstream of the last roll stand (14), as seen in the conveying direction (F), the temperature of this intermediate cooling being controlled, preferably regulated, in step (iii) taking into account the comparison according to step (ii).

11. The method according to claim 1, characterized in that, within the temperature calculation model, a total enthalpy is determined as the total free molar enthalpy (H) of the system by means of the Gibbs energy (G) at constant pressure (p) according to the equation $H=G-{T\left(\frac{\partial G}{\partial T}\right)}_p,$ wherein H=the molar enthalpy of the system, G=the Gibbs energy of the system, T=the absolute temperature in Kelvin and p=the pressure of the system.

12. The method according to claim 1, characterized in that, within the framework of the temperature calculation model, the temperature distribution in the system and in particular at the exit (A) of the last roll stand (14) of the rolling mill (11) is calculated by means of the Fourier heat equation $\rho c_p\frac{\partial T}{\partial t}-\frac{\partial }{\partial s}\left(\lambda \frac{\partial T}{\partial s}\right)=Q$ wherein ρ=the density, c.sub.p=the specific heat capacity at constant pressure, T=the calculated absolute temperature in Kelvin, λ=the thermal conductivity, s=the associated location coordinate, t=the time and Q=the energy released in front of the rolling mill (11) or upstream of it during the phase transition from liquid to solid of the system.

13. The method according to claim 1, characterized in that, in the context of the temperature calculation model for a phase mixture, the Gibbs energy (G) of the overall system is calculated as the sum of the Gibbs energies of the pure phases and their phase fractions according to the equation
G=f.sup.lG.sup.l+f.sup.γG.sup.γ+f.sup.pαG.sup.pα+f.sup.eαG.sup.eα+f.sup.ecG.sup.ec wherein G=the Gibbs energy of the system, f.sup.i=the Gibbs energy share of the respective phase or of the respective phase share in the overall system and G.sup.i=the Gibbs energy of the respective pure phase or the respective phase fraction of the system.

14. The method according to claim 1, characterized in that the predetermined reference value (TFM.sub.ref) is determined with the aid of a microstructure model for setting desired material properties.

15. The method according to claim 14, characterized in that, based on the microstructure model, in case of a deviation of the predetermined reference value (TFM.sub.ref) from the calculated temperature (TFM), it is decided whether a quality degradation of the material is likely, and in case this is not likely, the calculated temperature (TFM) is then set as the new predetermined reference value (TFM.sub.ref).

16. The method according to claim 14, characterized in that the microstructure model for compensation of possible quality devaluations contains new reference values for a temperature (T3, T4) of the strip or sheet also at a position downstream of the last roll stand (14) of the rolling mill (11) and/or downstream of a laminar cooling device (18) arranged downstream of the last roll stand (14) of the rolling mill (11)—viewed in the conveying direction (F)—as well as associated cooling rates (CR23, CR34).

17. The method according to claim 14, characterized in that the microstructure model is formed by a data-based model based on the Kriging algorithm and/or from neural networks.

Description

[0037] The invention is explained in more detail below, wherein the attached Figures serve facilitating understanding. In these Figures,

[0038] FIG. 1 shows the Gibbs energy for pure iron,

[0039] FIG. 2 is a (constructed) phase diagram with Gibbs energies, 3 shows the course of the total enthalpy by Gibbs for a coal—fuel lean steel,

[0040] FIG. 4 shows a schematic simplified side view of a system with which a metallic strip or sheet is produced according to a method according to the invention,

[0041] FIG. 5 shows a temperature profile for the strip or sheet metal over the length of the system from FIG. 4, and

[0042] FIG. 6, 7 each show schematic simplified side views of a system according to an embodiment supplemented in comparison to FIG. 4, with which a metallic strip or sheet is produced according to a method according to the invention.

[0043] In the following, a preferred embodiment of a process according to the invention for producing a metallic strip or sheet 1 is explained with reference to FIGS. 1 to 7. It is noted that the drawing in FIG. 4, FIG. 6 and FIG. 7 is merely simplified and in particular shown without scale.

[0044] In the process according to the invention, a temperature calculation model is used with which a temperature that the produced metallic strip or sheet 1 possesses at an exit of a last roll stand of a rolling mill can be specifically calculated.

[0045] Prior to explaining the temperature calculation model and its application in a system for the production or processing of a strip or sheet in more detail, general principles relating to the temperature calculation for a metallic strip or sheet are explained:

[0046] The basis of the temperature calculation is Fourier's heat equation (1), in which c.sub.p represents the specific heat capacity of the system, λ the thermal conductivity, p the density and s the spatial coordinate. T indicates the calculated temperature. The term Q on the right hand side accounts for energies released during the phase transformation (equation 2). In the transition from liquid to solid, this term denotes the heat of fusion, f.sub.s indicates the degree of phase transformation.

[00003]$\begin{array}{cc}\rho c_p\frac{\partial T}{\partial t}-\frac{\partial }{\partial s}\left(\begin{array}{c}{\lambda }^{\underset{¯}{\partial T}}\\ \partial s\end{array}\right)=Q& \left(1\right)\\ Q=\rho L\frac{\partial f_s}{\partial t}& \left(2\right)\end{array}$

[0047] Among the necessary input variables of the equation, the thermal conductivity and the total enthalpy are particularly important since these quantities significantly influence the temperature result. The thermal conductivity is a function of temperature, chemical composition and phase fraction and can be accurately determined experimentally.

[0048] The total enthalpy H or the molar enthalpy of a material portion or material section can be calculated using the Gibbs energy as follows (3):

[00004]$\begin{array}{cc}H=G-{T\left(\frac{\partial G}{\partial T}\right)}_p& \left(3\right)\end{array}$

[0049] with the molar Gibbs energy G of the system. For a phase mixture, the Gibbs energy of the total system can be calculated via the Gibbs energies of the pure phases as well as their phase fractions

G=f.sup.lG.sup.l+f.sup.γG.sup.γ+f.sup.pαG.sup.pα+f.sup.eαG.sup.eα+f.sup.ecG.sup.ec  (4)

[0050] with the phase fractions f.sup.φ of the phase φ and G.sup.φ the molar Gibbs energy of this phase. For the austenite, ferrite and liquid phase (φ), the Gibbs energy is given by

G.sup.ϕ=Σ.sub.i=1.sup.nx.sub.i.sup.ϕG.sub.i.sup.ϕ+RTΣ.sub.i=1.sup.nx.sub.i ln(x.sub.i)+.sup.EG.sup.ϕ+magnG.sup.ϕ  (5)

.sup.EG.sup.ϕ=Σx.sub.ix.sub.j.sup.aL.sub.i,j.sup.ϕ(x.sub.i−x.sub.j).sup.a+Σx.sub.ix.sub.jx.sub.kL.sub.i,j,k.sup.ϕ  (6)

.sup.magnG.sup.ϕ=RT ln(1+β)f(τ)  (7)

[0051] In equation (4), the terms each correspond to a single-element energy, a contribution for the ideal mixture, as well as a contribution for the non-ideal mixture and the magnetic energy (equation 7). If the Gibbs energy of the system is known, the molar specific heat capacity can be calculated therefrom:

[00005]$\begin{array}{cc}{c}_p=-{T\left(\frac{{\partial }^{2}G}{\partial T^2}\right)}_p& \left(8\right)\end{array}$

[0052] The parameters of the terms of equations (5)-(7) are listed in a Thermocalc and Matcalc database and can be used to determine the Gibbs energies of a steel composition. With the aid of a mathematical derivation, this yields the total enthalpy of this steel composition.

[0053] FIG. 1 shows the representation of the Gibbs energy for pure iron. This shows that the individual phases ferrite, austenite and the liquid phase assume a minimum for a certain characteristic temperature range at which these phases are stable.

[0054] FIG. 2 shows the phase boundaries of an Fe—C alloy with 0.02% Si, 0.310% Mn, 0.018% P, 0.007% S, 0.02% Cr, 0.02% Ni, 0.027% Al and variable C content. With the formulation of the Gibbs energy, it is possible to construct such a phase diagram with any chemical composition and show the stable phase fractions.

[0055] FIG. 3 shows the Gibbs total enthalpy curve for a low carbon steel as a function of temperature. The solidus and liquidus temperatures are also shown in the figure.

[0056] FIG. 4 shows a simplified schematic side view of a plant 10 set up for the application of the process according to the invention, with which a strip or sheet 1 is produced or processed in a conveying direction F.

[0057] The plant 10 includes a multi-stand rolling mill 11, which in the shown example has a first roll stand 12, a center roll stand 13 and a last roll stand 14. Located immediately downstream of the last roll stand 14 or at its exit A is a rapid cooling device 16, followed by a further cooling in the form of a laminar cooling device 18. At the end of the production line, a reel 20 is provided with which a finished strip 1 can be wound up.

[0058] Between the first roll stand 12 and the center roll stand 13, an unspecified intermediate stand cooling system is provided for the rolling mill 11.

[0059] In the illustration of FIG. 4, arrow “F” indicates a conveying direction (from left to right in the drawing plane) in which a strip or sheet 1 is moved in the plant 10 or passes through the rolling mill 11 with the mentioned roll stands 12-14.

[0060] The system 10 includes several temperature measuring devices to measure the temperature of the strip or sheet at various points. These temperature measuring devices include: a first pyrometer P1, arranged upstream of the first roll stand 12 as viewed in conveying direction F; a second pyrometer P2, which is arranged between the second roll stand 13 and the last roll stand 14 (and thus upstream of the last roll stand 14 as viewed in conveying direction F); a third pyrometer P3 arranged between the rolling mill 11 and the laminar cooling device 18, as viewed in the conveying direction F; and a fourth pyrometer P4 arranged between the laminar cooling device 18 and the coiler 20.

[0061] With regard to the second pyrometer P2, which is arranged upstream of the last roll stand 14 as viewed in conveying direction F, it is separately emphasized that it is used to measure a temperature T2 which the strip or sheet 1 possesses prior to entering the last roll stand 14. Similarly, the temperatures measured by the pyrometers P1, P3 and T4, are hereinafter respectively designated T1, T3 and T4.

[0062] The use of the rapid cooling device 16 results in the strip or sheet 1 being cooled between the second pyrometer P2 (=T2) and the third pyrometer P3 (=T3) at a cooling rate CR23. The same applies to the area between the third pyrometer P3 (=T3) and the fourth pyrometer P4 (=T4), in which the laminar cooling device 18 is used for cooling at a cooling rate CR34.

[0063] The system 10 further includes a computing and control device, hereinafter briefly referred to as the control device, designated by “100” in FIG. 4, and symbolized in simplified form by a rectangle. The control device 100 is equipped with the temperature calculation model. The temperature calculation model can have or be based on a DTR or DSC (Dynamic Temperature Control/Dynamic Solidification Control). The calculation is carried out using a finite difference method.

[0064] The vertical arrows shown in the illustration of FIG. 4 between the plant 10 and the rectangle for the control device 100, symbolize the interactions between individual components of the plant 10 and the control device 100. Specifically, the arrows pointing upwards in each case illustrate that the temperatures measured by the pyrometers P1-P4 in each case are input into the control device 100 and processed therein in terms of signal technology. The arrows pointing downwards in each case symbolize that the associated components of the plant 10 can be controlled or regulated by the control device 10—this relates to the intermediate stand cooling (between the first roll stand 12 and the central roll stand 13), the last roll stand 14, the rapid cooling device 16 and/or the laminar cooling device 18, for example with regard to the supply of a coolant quantity to these components.

[0065] Using the aforementioned temperature calculation model, a temperature TFM present for the strip or sheet 1 immediately at the exit A of the last roll stand 14 is computationally determined based on or starting from the temperature T2 that was measured by the second pyrometer P2 upstream of the last roll stand 14, and input to the control device 100 as explained. This calculation is carried out according to the finite difference method for a system of the strip or sheet 1 formed by the material section of the strip or sheet 1 situated between the point at which the second pyrometer P2 is arranged and the exit A of the last roll stand 14. As explained above, in order to calculate this temperature profile or temperature TFM, the Fourier heat equation is solved. The boundary conditions in the rolling mill 11 (e.g. temperature output to air via radiation and convection as well as to the rolls of the last roll stand 14) and in the cooling section (temperature output to water cooling, air and roller table) are taken into account. Also taken into account is the heat generated by phase transformation, which can occur either in the rolling mill 11 or in the cooling section.

[0066] The various temperatures T1-T4 which occur along the length of the plant 10 for a strip or sheet 1 produced with the plant are shown in the diagram of FIG. 5 with a corresponding curve. The diagram also shows the calculated temperature TFM (at the exit A of the last roll stand 14) and the cooling rates CR23 and CR 34 explained above.

[0067] Following computation of the temperature TFM, the computed temperature is then compared by the control device 100 with a predetermined reference value TFM.sub.ref. Taking this comparison into account, a cooling water supply for the strip or sheet 1 is then suitably adjusted, i.e., controlled or regulated, by means of the control device 100, if necessary. The control (or regulation) of the cooling water supply may have the purpose to make a temperature of the strip or sheet 1 at the exit A of the last roll stand 14 to correspond with the predetermined reference value TFM.sub.ref, and/or to suitably adjust the further temperatures T3 (for pyrometer P3) and/or T4 (for pyrometer P4).

[0068] FIG. 6 shows a further embodiment of the plant 10 which, compared with the embodiment of FIG. 4, additionally includes the components inductive heating 26, furnace 28 and/or thermal insulation hood 30. As can be seen, these components 26, 28, 30 are each arranged upstream of the rolling mill 11, as viewed in the conveying direction F of the strip or sheet, with the strip or sheet 1 being able to be guided through these components. The arrows extending from the control device 100 towards these components 26, 28 and 30, illustrate that the inductive heater 26, the furnace 28 and/or the thermal insulation hood 30 can be controlled or regulated by means of the control device 100, namely, as explained above, as a function of the calculated temperature TFM and the comparison with the predetermined reference value TFM.sub.ref made therewith. In this way, a temperature for the strip or sheet 1 is specifically influenced or increased.

[0069] The thermal insulation hood 30 operates as a device that thermally insulates the strip or sheet 1. Opening or closing the thermal insulation hood 30 allows influencing the degree of thermal insulation for the strip or sheet 1 on a roller table. By controlling the thermal insulation hood 30 with the control device 100, the thermal insulation hood 30 is opened or closed accordingly, or also caused to assume an intermediate position, whereby the temperature for the strip or sheet 1 is influenced in dependence on the respective position of the thermal insulation hood 30 11.

[0070] In the embodiment of FIG. 7, a preliminary strip cooling 24 is provided for the plant 10 upstream of the rolling mill 11, viewed in the conveying direction F of the strip or sheet 1, which preliminary strip cooling 24 can also be controlled or regulated by means of the control device 100, as indicated by the arrow. Depending on the calculated temperature TFM and the comparison with predetermined reference value TFM.sub.ref, a coolant quantity for this preliminary strip cooling 24 is then controlled or regulated in order to influence or reduce the temperature of the strip or sheet 1 in a targeted manner.

[0071] In the illustrations of FIGS. 4, 6 and 7, “22” symbolizes an intermediate stand cooling, which can also be controlled or regulated by means of the control device 100, namely by adjusting the amount of coolant supplied and/or by the number of spray nozzles used.

[0072] In another embodiment of the process according to the invention, it can be provided that in the control device 100 or for the temperature calculation model stored therein, corresponding reference values T1ref, T2ref, T3ref, T4ref are also specified for the temperatures T1, T2, T3 and T4 on the basis of a microstructure model to enable achieving optimum properties. Alternatively, the reference values would have to be determined on the basis of empirical values or measurement and production data. These could be models based on neural networks, the Kriging algorithm or the like.

[0073] In the case of deviations of T2 from T2ref, it can also be decided with the aid of the microstructure model that this deviation does not result in a quality degradation of the strip 1 to be produced. For this case, the measured value for the temperature T2 then becomes the new target value for this strip, with new target values being calculated accordingly for T3 and T4. In addition, the cooling rates CR23 and/or CR34 can be changed to achieve the same characteristics due to the changed temperature profile. The same applies to deviations of T3 from T3ref or T4 from T4ref.

[0074] It is also possible to make this decision using a data-based empirical model based on the available measurement and production data. These can for example include models based on neural networks, the Kriging algorithm or the like.

[0075] The temperature calculation can be carried out via the Gibbs energies and the enthalpy. In this respect, reference is made to the above explanations of equations (1)-(8).

LIST OF REFERENCE SYMBOLS

[0076] 1 strip or sheet [0077] 10 plant [0078] 11 rolling mill [0079] 12 first roll stand (of rolling mill 11) [0080] 13 middle roll stand (of rolling mill 11) [0081] 14 last roll stand (of rolling mill 11) [0082] 16 rapid cooling device [0083] 18 laminar cooling device [0084] 20 reel [0085] 22 inter-stand cooling [0086] 24 preliminary strip cooling [0087] 26 inductive heating [0088] 28 furnace [0089] 30 thermal insulation hood [0090] 100 computing and control device [0091] A exit (of the last roll stand 14) [0092] F direction of conveyance (for the strip or sheet 1) [0093] P1 first pyrometer [0094] P2 second pyrometer [0095] P3 third pyrometer [0096] P4 fourth pyrometer [0097] T1-T4 temperatures of the strip or sheet 1, at the measuring point of the pyrometer P1-P4