Determining the ferrite phase fraction after heating or cooling of a steel strip

Abstract

A method for determining the ferrite phase fraction x.sub.a after heating or when cooling a steel strip (2) in a metallurgic system. Also, a device for carrying out the method. A method by which the ferrite phase fraction in the steel strip (2) can be determined online, quickly and easily, includes measuring a width w.sub.1 and a temperature T.sub.1 of the steel strip (2), wherein the steel strip (2) comprises a ferrite phase fraction x.sub.a 1 during the measurements; heating or cooling the steel strip (2); when heating the steel strip (2) a phase conversion at least in part occurs, a.fwdarw.y from the ferrite state a into the austenitic state y and when cooling the steel strip a phase conversion at least in part occurs, from the austenitic state y into the ferrite state a; measuring of a width w and a temperature T of steel strip (2) converted at least in part; determining the ferrite phase fraction of the formula (I), wherein T.sub.0 is a reference temperature and a.sub.a and a.sub.y are the linear heat expansion coefficients of ferrite and austenite.

Claims

1. A method for determining a ferritic phase fraction x.sub. of an at least partly converted steel strip after at least one of heating or cooling of the steel strip, comprising the following method steps in sequence: measuring a width w.sub.1 and a temperature T.sub.1 of the steel strip, wherein the steel strip has a ferritic phase fraction x.sub.1 during the measurement; heating or cooling the steel strip, wherein at least a partial phase conversion from a ferritic state into an austenitic state takes place in the steel strip during heating or at least a partial phase conversion from an austenitic state into a ferritic state takes place in the steel strip during cooling; measuring a width w and a temperature T of the at least partly converted steel strip and; determining the ferritic phase fraction x.sub. of the at least partly converted steel strip through $x_{}=\frac{\begin{array}{c}-w-w{x}_1{}_{}\left(T_1-T_0\right)-w{}_{}\left(T_1-T_0\right)+\\ w{x}_1{}_{}\left(T_1-T_0\right)+w_1+w_1{}_{}\left(T-T_0\right)\end{array}}{w_1\left[-{}_{}\left(T-T_0\right)+{}_{}\left(T-T_0\right)\right]},$ wherein T.sub.0 is a reference temperature and .sub. and .sub. are linear coefficients of thermal expansion of ferrite and austenite, respectively.

2. A method for determining a ferritic phase fraction x.sub. of an at least partly converted steel strip after heating of the steel strip comprising the following method steps in sequence: measuring a width w.sub.1 and a temperature T.sub.1 of the steel strip, wherein the steel strip is entirely in a ferritic state with x.sub.1=1 during the measurement; heating the steel strip, wherein at least a partial phase conversion from a ferritic state into an austenitic state takes place in the steel strip; measuring a width w and a temperature T of the at least partly converted steel strip; and determining the ferritic phase fraction X.sub. of the at least partly converted steel strip through $x_{}=\frac{-w-w{}_{}\left(T_1-T_0\right)+w_1+w_1{}_{}\left(T-T_0\right)}{w_1\left[-{}_{}\left(T-T_0\right)+{}_{}\left(T-T_0\right)\right]}.$ wherein T.sub.0 is a reference temperature and a.sub.a and a.sub.y are linear coefficients of thermal expansion of ferrite and austenite, respectively.

3. The method as claimed in claim 1, further comprising cooling the steel strip in a cooling zone before the measurement of the width w.sub.1 and of the temperature T.sub.1.

4. The method as claimed in claim 1, further comprising annealing the steel strip and then measuring the width w and the temperature T of the at least partly converted steel strip during or after the annealing.

5. The method as claimed in claim 4, further comprising setting an annealing duration and/or an annealing temperature during the annealing as a function of the ferritic phase fraction x.sub..

6. The method as claimed in claim 5, setting the annealing duration and/or the annealing temperature under open-loop or closed-loop control.

7. A method for determining a ferritic phase fraction x.sub. of an at least partly converted steel strip after cooling of the steel strip comprising the following method steps in sequence: measuring a width w.sub.1 and a temperature T.sub.1 of the steel strip, wherein the steel strip is entirely in an austenitic state with x.sub.1=0 during the measurement; cooling the steel strip, wherein at least a partial phase conversion from an austenitic state into a ferritic state takes place in the steel strip; measuring a width w and a temperature T of the at least partly converted steel strip; and determining the ferritic phase fraction x.sub. of the at least partly converted steel strip through $x_{}=\frac{-w-w{}_{}\left(T_1-T_0\right)+w_1+w_1{}_{}\left(T-T_0\right)}{w_1\left[-{}_{}\left(T-T_0\right)+{}_{}\left(T-T_0\right)\right]}.$ wherein T.sub.0 is a reference temperature and a.sub.a and .sub.y are linear coefficients of thermal expansion of ferrite and austenite, respectively.

8. The method as claimed in claim 1, further comprising hot rolling the steel strip before the measurement of the width w.sub.1 and the temperature T.sub.1.

9. The method as claimed in claim 7, further comprising measuring the width w and the temperature T of the at least partly converted steel strip during or after the cooling of the steel strip in a cooling zone.

10. The method as claimed in claim 9, further comprising setting the cooling as a function of the ferritic phase fraction x.sub..

11. The method as claimed in claim 10, further comprising setting the cooling duration and/or the cooling intensity during cooling under open-loop control or closed-loop control.

12. A computer program product comprising a non-transitory computer readable storage medium, and a computer program comprised of computer program code stored on the medium, wherein the program code is programmed to cause a computer to control a performance of the method of claim 1, wherein the program code includes values for the width w.sub.1 and the temperature T.sub.1 of the steel strip before the at least partial phase conversion, for the width w and the temperature T of the steel strip after the at least partial phase conversion, and for physical parameters of the steel strip supplied by performing the method of claim 1; and the computer program has a computing module for computing the ferritic phase fraction x.sub. of the at least partly converted steel strip through $x_{}=\frac{\begin{array}{c}-w-w{x}_1{}_{}\left(T_1-T_0\right)-w{}_{}\left(T_1-T_0\right)+\\ w{x}_1{}_{}\left(T_1-T_0\right)+w_1+w_1{}_{}\left(T-T_0\right)\end{array}}{w_1\left[-{}_{}\left(T-T_0\right)+{}_{}\left(T-T_0\right)\right]}.$ wherein T.sub.0 is a reference temperature and a.sub.a and a.sub.y are linear coefficients of thermal expansion of ferrite and austenite, respectively.

13. An apparatus for determining a ferritic phase fraction x.sub. of an at least partly converted steel strip after at least one of heating or cooling of the steel strip, the apparatus carrying out the method as claimed in claim 1, the apparatus comprising: a first temperature measuring device for measuring a temperature T.sub.1 and a first width measuring device for measuring a width w.sub.1 of the steel strip, and being disposed before a cooling zone or a heating zone, the cooling zone being configured for cooling the steel strip or the heating zone being configured for heating the steel strip; a second temperature measuring device for measuring a temperature T and a second width measuring device for measuring a width w of the steel strip, and being disposed after the cooling zone or the heating zone; and a computing unit for determining the ferritic phase fraction x.sub. of the at least partly converted steel strip through $x_{}=\frac{\begin{array}{c}-w-w{x}_1{}_{}\left(T_1-T_0\right)-w{}_{}\left(T_1-T_0\right)+\\ w{x}_1{}_{}\left(T_1-T_0\right)+w_1+w_1{}_{}\left(T-T_0\right)\end{array}}{w_1\left[-{}_{}\left(T-T_0\right)+{}_{}\left(T-T_0\right)\right]},$ wherein T.sub.0 is a reference temperature and a.sub.a and a.sub.y are linear coefficients of thermal expansion of ferrite and austenite, respectively, wherein the computing unit is connected for signaling purposes to all of the first temperature measuring device, the first width measuring device, the second temperature measuring device and the second width measuring device.

14. The apparatus as claimed in claim 13, wherein the cooling zone has at least one cooling nozzle with a respective setting device for the cooling nozzle, or the heating zone has at least one heating element with a respective setting device for the heating element; and the computing unit is connected for signaling purposes to the setting device, so that the computing unit can set the ferritic phase fraction x.sub. of the at least partly converted steel strip.

15. The apparatus as claimed in claim 14, further comprising an open-loop control device or a closed-loop control device disposed between the computing unit and the setting device, wherein the open-loop control device or the closed-loop control device is connected for signaling purposes to the setting device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further advantages and features of the present invention emerge from the description given below of non-restrictive exemplary embodiments wherein, in the figures below:

(2) FIG. 1A shows a side view and FIG. 1B shows a floor plan of a part of a hot-rolling mill with a device for carrying out the inventive method.

(3) FIG. 2 shows a side view of a part of a hot-rolling mill with a variant of the device for carrying out the inventive method.

(4) FIG. 3 shows a schematic diagram of a temperature curve in a continuous annealer for intercritical annealing of a steel strip

DESCRIPTION OF EMBODIMENTS

(5) FIGS. 1A and 1B shows a rear part of a hot-rolling mill for manufacturing of a steel strip. In concrete terms a steel strip 2 made from a material CK60 with a thickness of 2 mm and a width of 1800 mm is produced in the hot rolling mill. The steel strip 2 is finish-rolled in the last roll stand 1 of the not completely shown finish-rolling train entirely in the austenitic state at a temperature of T.sub.FM=800 C. and leaves the last roll stand 1 at a transport speed of e.g. 6 to 8 m/s. Immediately after the finish-rolling the temperature T.sub.1 of the steel strip 2 is detected by a first temperature measuring device 4a, in concrete terms a pyrometer. At the same time the width w.sub.1 of the steel strip 2 is detected by a first width measuring device 5a, which is embodied here as a camera. Subsequently the steel strip 2 is cooled in a cooling zone 6, by which the austenitic phase fraction in the structure of the steel strip 2 is converted at least partly into ferritic phase fractions . The object of the invention is to determine the degree of the conversion .fwdarw. in the cooling zone 6 or after the cooling zone (e.g. before coiling in the coiler 3). To this end the steel strip 2 is moved in the direction shown by the arrow through the cooling zone 6 and is cooled during this process. The strip 2 is cooled by a number of cooling nozzles which have not been shown additionally. After the cooling zone 6 the temperature T and the width w of the steel strip 2 are detected by a second temperature measuring device 4b, in concrete terms a pyrometer or a thermal camera, and a second width measuring device 5b. Subsequently the steel strip is wound into a coil by the coiler 3.

(6) The knowledge of the temperatures and widths at at least two points of the strip before and after cooling, as well as under the prerequisite of an entirely austenitic initial state and the linear coefficient of thermal expansion for ferrite and austenite, enables the ferritic phase fraction x.sub. to be determined. This process will be outlined below:

(7) The width w of a steel strip as a function of the temperature T is given by w=w.sub.0[1+(TT.sub.0)], wherein w.sub.0 corresponds to the width of the steel strip at a reference temperature T.sub.0 of typically 20 C. and is the linear coefficient of thermal expansion. Naturally a higher-order polynomial approach can be used instead of the linear approach.

(8) Since the austenitic phase has a different linear coefficient of thermal expansion .sub. than a ferritic phase .sup.(i) with .sub..sup.(i), the width of a steel strip, which has a fraction x.sub..sup.(i) of a ferritic phase (i) and a fraction x.sub. of the austenitic phase , can be written in a mixed approach as follows

(9) $w=w_0\left[1+x_{}{}_{}\left(T-T_0\right)+\underset{i}{.Math.}{x}_{}^{\left(i\right)}{}_{}^{\left(i\right)}\left(T-T_0\right)\right]$

(10) If it is further assumed that only one ferritic phase (typically ferrite) is present in the steel strip during the cooling, then the previous expression is simplified to
w=w.sub.0[1+x.sub..sub.(TT.sub.0)+x.sub..sub.(TT.sub.0)]

(11) It is further known that the sum of the austenitic phase and all ferritic phases always amounts to 1, i.e.
x.sub.+.sub.ix.sub..sup.(i)=1

(12) If only one ferritic phase is present, x.sub. +x.sub.=1 applies.

(13) For the case with only one ferritic phase the following therefore applies
w=w.sub.0[1+(1x.sub.).sub.(TT.sub.0)+x.sub..sub.(TT.sub.0)]

(14) Thus the following applies for the ferritic phase fraction

(15) $x_{}=\frac{\frac{w}{w_0}-1-{}_{}\left(T-T_0\right)}{\left(T-T_0\right)\left({}_{}-{}_{}\right)}.$

(16) The width w.sub.1 of the steel strip at a temperature T.sub.1 during austenitic rolling is given by
w.sub.1=w.sub.0[1+.sub.(T.sub.1T.sub.0)]
wherein .sub. is the linear coefficient of thermal expansion of austenite.

(17) By combination of the last two equations the following applies

(18) $x_{}=\frac{w\left[1+{}_{}\left(T_1-T_0\right)\right]-w_1-w_1{}_{}\left(T-T_0\right)}{w_1\left[{}_{}\left(T-T_0\right)-{}_{}\left(T-T_0\right)\right]}.$

(19) In concrete terms, for w.sub.1=1.8 in and .sub.=1.Math.10.sup.5 1/K and .sub.=6.Math.10.sup.6 1/K, a ferritic phase fraction of x.sub.=20% is produced from the last equation at T=400 C. and a width of w=L7923 m.

(20) FIG. 2 shows a further side view of a rear part of another hot-rolling mill for manufacturing a steel strip 2. The measured temperature values T.sub.1 and T of the first and second temperature measuring devices 4a and 4b, as well as the measured width values w.sub.1 and w of the first and second width measuring devices 5a and 5b are shown symbolically in this diagram. The measured values T.sub.1, T, w.sub.1 and w are processed in a computing unit 9, wherein, taking into consideration further physical parameters of the steel, the actual value of the ferritic phase fraction x.sub. is determined. On the one hand the actual value is shown in an output unit 12 embodied as a display, on the other hand the actual value is supplied to a closed-loop control device 11 which, by a required-actual comparison with a required value of the sum of the ferritic phase fraction x.sub., calculates a closed-loop control deviation not shown. Depending on the closed-loop control deviation the closed-loop control device outputs at least one setting value u, which under actual circumstances is supplied to an electric motor M as setting device 8. Depending on the setting value u the motor M changes its speed, which in turn influences the pressure of the cooling medium, which is fed by the centrifugal pump 14 to the individual cooling nozzles 7 of the cooling zone 6. Through this arrangement it is insured that the actual value of the sum of the ferritic phase fractions in the steel strip 2 largely corresponds to the required value, and does so essentially independently of transient changes in the operational control of the hot-rolling mill. The two width measuring devices 5a, 5b are embodied in this form of embodiment as so-called line-scan cameras below the strip 2. Not shown are the two blowers embodied as compressed air nozzles in the pyrometers 4a, 4b.

(21) FIG. 3 shows as an example a schematic diagram of the temperature management in a so-called continuous annealer for manufacturing a TRIP steel cold-rolled strip. In the input area of the system the width w.sub.1 and the temperature T.sub.1 of the steel strip 2 present in an initial state A are measured. This is done by a first width measuring device 5a and a first temperature measuring device 4a. In the initial state A the steel strip 2 contains ferritic and perlitic phase fractions. Subsequently the steel strip 2 is introduced into the heating zone 15 embodied as an annealer, wherein the steel strip is heated up. In the heating zone 15 the steel strip is heated by a number of burners 16 disposed over the longitudinal extent of the heating zone, through which the ferritic structure fractions convert partly into an austenitic structure. During the annealing the steel strip is present in an intermediate state B, which is characterized by the coexistence of ferritic and austenitic phases. The annealing temperature is set during a defined passage speed through the continuous annealer so that the actual austenite fraction in the steel strip before cooling corresponds as precisely as possible to the required value. At the end of the heating zone 15 the width w and the temperature T of the steel strip 2 present in the intermediate state B are again measured; this is done by the second width measuring device 5b and the second temperature measuring device 4b. The actual austenite fraction is determined in accordance with the method for determining the ferritic phase fraction x.sub. after the heating of a steel strip, taking into consideration w, w.sub.1, T, T.sub.1, wherein the sum of the austenitic phase and all ferritic phases always amounts to 1. Subsequently the steel strip 2 is cooled in a rapid cooling zone 6, so that a preferred ferritic-bainitic (if possible with a martensitic residual fraction) structure with residual austenite islands is set in the cooled steel strip 2. Immediately after the end of the cooling zone 6 the width w.sub.2 and the temperature T.sub.2 of the steel strip 2 present in the end state C are measured; this is done by the third width measuring device 5c and the third temperature measuring device 4c. The phase fractions in the cooled steel strip 2 are determined in accordance with the method for determining the ferritic phase fraction x.sub. after the cooling of a steel strip, taking into consideration w.sub.1, T.sub.1, w.sub.2 and T.sub.2.

(22) Although the invention has been illustrated and described in greater detail by the preferred exemplary embodiments, the invention is not restricted by the disclosed examples and other variations can be derived herefrom by the person skilled in the art, without departing from the scope of protection of the invention.

LIST OF REFERENCE CHARACTERS

(23) 1 Roll stand 2 Steel strip 3 Coiler 4 Temperature measuring device 4a, 4b, 4c First, second and third temperature measuring devices 5 Width measuring device 5a, 5b, 5c First, second and third width measuring devices 6 Cooling zone 7 Cooling nozzle 8 Setting device 9 Computing unit 11 Closed-loop control device 12 Output unit 14 Centrifugal pump 15 Heating zone 16 Burner Linear coefficient of thermal expansion T Temperature u Setting value w Width x Phase fraction x.sub. Ferritic phase fraction x.sub. Austenitic phase fraction A Initial state: Ferrite and perlite B Intermediate state: Intercritical area (ferrite and austenite coexistence) C End state: Ferrite, bainite, martensite and residual austenite