Method for estimating temperature and oxide thickness of a steel strip

Abstract

A method for estimating the oxide thickness and temperature of a heated steel strip, said heated steel strip having undergone a heat treatment performed at a temperature from 100°C to 1100°C, comprising the following steps: 1. measuring at least two intensities of radiation emitted by said heated steel strip at different wavelengths in the range from 1 to 5 μm; 2. estimating the temperature T 估计 of said heated steel strip based on: said at least two measured intensities of radiation, and a reference intensity of radiation emitted at at least a reference wavelength by a reference steel strip having a determined oxide layer thickness; 3. estimating the emissivity coefficient ε 估计 of said heated steel strip using at least one of said measured intensities of radiation and estimated temperature T 估计 ; 4. estimating the oxide thickness Ox 估计 of said heated steel strip using said estimated emissivity ε 估计 .

Description

[0001] This invention relates to a method that allows for the estimation of the temperature and oxide layer thickness of a steel strip.

[0002] The steel strip undergoes several heat treatments to enhance its properties. In most of these treatments, the steel strip is heated to a predetermined temperature and then cooled more or less rapidly.

[0003] One of the most common heat treatments is annealing, which allows for increased ductility and reduced hardness of the steel strip. In this process, the strip is heated and held above its recrystallization temperature, then cooled. During annealing, the strip surface is gradually oxidized, typically forming an oxide layer. However, depending on factors such as annealing conditions (e.g., temperature, dew point, atmosphere, and steel grade), the oxide layer thickness varies from 0 nm to 200 nm. Typically, due to thermodynamic conditions, the oxide layer primarily consists of FeO.

[0004] Controlling the strip temperature and oxide layer thickness is crucial for ensuring good strip quality, process control, and adaptation to subsequent process steps. In annealing furnaces, this control is typically achieved using a pyrometer, which measures temperature using strip radiation.

[0005] However, variations in the oxide layer thickness affect temperature measurements taken by the pyrometer. It is acknowledged that a thicker oxide layer results in higher emissivity and therefore a stronger signal detected by the pyrometer. However, increasing steel temperature also leads to a larger detected signal. Therefore, the pyrometer cannot reliably detect the presence of an oxide layer, let alone its thickness. When the detected signal strength increases, it is impossible to determine whether it is due to an increase in temperature, an increase in oxide layer thickness, or both.

[0006] Therefore, temperatures measured by a pyrometer are unreliable because they do not account for changes in emissivity due to variations in oxide layer thickness. Consequently, a coefficient must be applied to the temperature given by the pyrometer, depending on the emissivity of the layer being measured. Several methods have been developed for estimating the temperature and emissivity of steel strips during annealing.

[0007] JP 09 033 464 discloses a method for online measurement of oxide scale thickness. It claims a six-step process comprising the following steps:

[0008] - Detecting infrared emission light in an annealing furnace.

[0009] - Determine the first emissivity temperature S1 at wavelength L1 between 12 μm and 20 μm, where emissivity is considered independent of oxide thickness.

[0010] - Determine the second emissivity temperature S2 at wavelengths L2 between 2.5 μm and 4 μm, where the emissivity depends on the oxide thickness.

[0011] - Determine the steel plate temperature based on the emissivity at L1 and S1.

[0012] - Calculate the emissivity e2 at L2 based on the determined steel plate temperature and emissivity temperature S2.

[0013] - Determine oxide thickness based on emissivity e2.

[0014] The reliability of such measurements is limited because even though the emissivity is almost constant in the 12 μm to 20 μm range, the percentage variation is not negligible and can lead to temperature measurement errors exceeding 50ºC. Furthermore, the emissivity in this wavelength range is particularly susceptible to parasitic currents under industrial conditions, which further reduces temperature reliability.

[0015] JP 11 324 839 discloses a method for accurately measuring the thickness of an oxide film formed on a steel plate. The method comprises the following two steps:

[0016] - Assuming the steel temperature is equal to the steel's soaking temperature,

[0017] - Measure the emissivity of the steel plate surface at multiple wavelengths between 2.5 μm and 10 μm.

[0018] - The oxide film thickness is determined based on the relationship between emissivity, oxide film thickness, and emissivity.

[0019] The reliability of this measurement is limited because, in industry, the target homogenization temperature can be different within the furnace. Furthermore, a temperature difference may exist between the homogenization temperature and the steel temperature during emissivity measurements.

[0020] Therefore, it is necessary to develop methods that allow for accurate and reliable determination of the temperature of steel strips to improve the reliability and accuracy of measurements of their oxide layer thickness.

[0021] This objective is achieved by providing a method for estimating the oxide thickness and temperature of a heated steel strip that has undergone heat treatment at a temperature ranging from 100ºC to 1100ºC, the method comprising the following steps:

[0022] 1) Measure the intensity of at least two radiations emitted by a heated steel strip at different wavelengths in the range of 1 μm to 5 μm.

[0023] 2) Estimate the temperature T of the heated steel strip based on the following: 估计 ,

[0024] -At least two measurements of radiation intensity, and

[0025] - At least two reference radiative intensities and at least two reference emissivity at different wavelengths for a reference steel strip with at least N oxide layer thicknesses from 0 nm to 200 nm and a known temperature.

[0026] 3) Use the measured radiation intensity and the estimated temperature T 估计 At least one of these can be used to estimate the emissivity coefficient ε of the heated steel strip. 估计 ,

[0027] 4) Use the estimated emissivity ε 估计 To estimate the oxide thickness Ox of the heated steel strip 估计 The method may further include the heated steel strip being operated; in step 1), measuring at least 10 radiation intensities emitted by the heated steel strip at different wavelengths in the 1 μm to 5 μm range; and in step 2), using the at least 10 radiation intensities to estimate T. 估计 In step 1), at least 20 radiation intensities emitted by the steel strip at different wavelengths in the 1 μm to 5 μm range are measured; and in step 2), the at least 20 radiation intensities are used to estimate T. 估计 At least two radiation intensities are measured with a wavelength difference of at least 0.1 μm, more preferably at least 0.5 μm, and even more preferably at least 1 μm; the heat treatment is performed at a temperature from 500ºC to 1100ºC, and in step 1), the wavelength range of the at least two measured radiation intensities is from 1 μm to 1.7 μm; the heat treatment is performed at a temperature from 100ºC to 500ºC, and in step 1), the wavelength range of the at least two measured radiation intensities is from 3 μm to 5 μm; any features of steps 1) to 4) are repeated for some points on the surface of the heated steel strip. A method for heat treatment of a heated steel strip performed in a furnace, wherein the method provided above for estimating the oxide thickness and temperature of the heated steel strip is performed, and T 估计 A method for controlling furnace temperature according to a heat treatment of heated steel strip performed in a furnace, wherein the furnace includes a heating section and a soaking section, and wherein the method described above is performed in the heating section, and T 估计 A method for controlling furnace temperature during a heating step and a heat treatment of steel strip, the heat treatment comprising a heating step and a soaking step, the heating step and the soaking step being performed in a furnace including burners having adjustable power along the width of the steel strip being heated, wherein a method according to the above-described method for estimating the oxide thickness and temperature of the steel strip being heated is performed during the heating step, and the estimated oxide thickness Ox 估计The heat treatment method, which involves adjusting the power of the burner along the heated steel strip and ensuring that the oxide thickness is uniform along the width of the heated steel strip, relates to the measurements performed using the methods provided above.

[0028] Other features and advantages of the invention will become apparent from the following detailed description.

[0029] To illustrate the invention, various embodiments and experiments with non-limiting examples will be described in particular with reference to the following drawings:

[0030] Figure 1 A process flow diagram is shown for measurement methods as known in the prior art and as claimed in this invention.

[0031] Figure 2 The steps of an embodiment of the present invention are shown.

[0032] Figure 3 It is a graph representing the relative brightness of steel strips with different oxide layer thicknesses as a function of wavelength.

[0033] Figure 4 Two temperature measurements are shown, one based on the prior art and the other based on the method of the present invention.

[0034] This invention relates to a method for estimating the oxide thickness and temperature of a heated steel strip subjected to heat treatment at temperatures ranging from 100ºC to 1100ºC, the method comprising the following steps:

[0035] 1) Measure the intensities of at least two types of radiation emitted by the heated steel strip at different wavelengths in the range of 1 μm to 5 μm.

[0036] 2) Estimate the temperature T of the heated steel strip based on the following: 估计

[0037] - The at least two measured radiation intensities, and

[0038] - Reference radiation intensity emitted by a reference steel strip having a defined oxide layer thickness for at least a reference wavelength.

[0039] 3) Using the measured radiation intensity and estimated temperature T 估计 At least one of the following is used to estimate the emissivity coefficient ε of the heated steel strip. 估计 ,

[0040] 4) Using the estimated emissivity ε 估计 To estimate the oxide thickness Ox of the heated steel strip. 估计 .

[0041] The heat treatment performed at a temperature from 100ºC to 1100ºC can be an annealing process that includes a heating step and a soaking step. Furthermore, after the heat treatment, the steel strip can be cooled and coated.

[0042] The steps of the method for which protection is sought are in Figure 1 As shown in the image.

[0043] In the first step of the process, the intensities of at least two types of radiation emitted by the heated steel strip at different wavelengths in the 1 μm to 5 μm range are measured using any suitable measuring device. For example, the first radiation intensity at a wavelength of 2 μm is measured, and the second radiation intensity at a wavelength of 4 μm is measured. The measuring device can be two spectrometers or a hyperspectral imaging device. Figure 1 In this process, the first step is represented by the following graph, which represents the radiation intensity as a function of wavelength that can be generated by the suitable measuring device.

[0044] The wavelength for measuring intensity is preferably no more than 5 μm, because the 5 μm to 8 μm range falls within the absorption region of air, and also because the larger the wavelength, the greater the estimation error regarding the temperature difference, as can be deduced from the following equation:

[0045] .

[0046] The radiation intensity at each wavelength detected by the recording device depends primarily on two factors: the emissivity and emissivity of the heated steel strip. In the following terminology, λ refers to the wavelength, T refers to the temperature of the steel strip, and Ox... TH This refers to the thickness of the oxide layer.

[0047] As explained by Planck's law, the emissivity of the steel strip, Radiance (λ, T), depends only on the temperature of the steel strip and the wavelength being measured.

[0048] Emissivity (λ, Ox) of steel strip of steel grade TH The intensity depends on the oxide layer thickness and wavelength. Therefore, the recorded intensity can be defined by equation (1):

[0049] (1)

[0050] In the second step, the objective is to accurately estimate the temperature of the heated steel strip using at least two reference radiation intensities at different wavelengths emitted by a reference steel strip with a known temperature and at least N oxide layer thicknesses ranging from 0 nm to 200 nm, and the at least two measured radiation intensities. The N oxide layer thicknesses are denoted as Ox. TH n.

[0051] N is an integer. Preferably, N is greater than 10. Even more preferably, N is greater than 25. Preferably, the step between the thicknesses of each reference oxide layer is 5 nm.

[0052] One method to achieve this will be described below. The terms of equation (1) can be divided by the radiation intensity of the reference steel strip to obtain equation (2).

[0053] (2)

[0054] Equation (3) can be easily derived from equation (2).

[0055] (3)

[0056] item equal . Rewritten as ,in, equal Where T is the temperature of the heated steel strip, T REF The reference temperature is the temperature of the steel strip, and C2 is a constant according to Planck's formula and equal to... , where h is Planck's constant and k is Boltzmann's constant.

[0057] It can be defined as equal to The linearized emissivity. The linearized emissivity can be approximated using an affine function by combining the at least two reference emissivityes at different wavelengths of a reference steel strip with at least N oxide layer thicknesses from 0 nm to 200 nm and the linearized emissivity.

[0058] For example, the affine function can have a slope "a" and a y-intercept "b", where a polynomial function is used to approximate 'a' and 'b'. For example, 'a' = a1 x Ox N 2 + a2 x Ox N + a3, and 'b' = b1 x Ox N 2 + b2 x Ox N + b3.

[0059] In a similar manner, the linearization strength can be defined as equal to The linearized intensity can be approximated using an affine function by combining the at least two reference radiation intensities at different wavelengths emitted by a reference steel strip with at least N oxide layer thicknesses from 0 nm to 200 nm and at a known temperature.

[0060] For example, the affine function can have a slope "a" and a y-intercept "b". A polynomial function can be used to approximate 'a' and 'b'.

[0061] For example, 'a' = a1 x Ox N 2 + a2 x Ox N + a3, and b = b1 x Ox N 2 + b2 x Ox N + b3 +C T (T).

[0062] By combining Equation 3 with linearized intensity and emissivity, the following equation can be established:

[0063] (4)

[0064] So, C T (T) can be obtained by solving a system of equations. Solving the system of equations yields two pairs of equations related to C. T (T) The associated oxide thickness value, i.e., the temperature of the heated steel. Those skilled in the art can easily exclude pairs of incoherent values ​​by setting an acceptable range for the value. For example, oxide thickness values ​​that are negative or exceed a threshold (such as 500 nm) or steel temperatures that are higher than the steel's melting temperature can be considered impossible.

[0065] Allows finding the estimated temperature T of the heated steel plate. 估计 .

[0066] The more measurements of radiation intensity, the more reference radiation intensity, and the more reference emissivity there are, the more accurate the coefficients of the polynomial will be, and therefore the more accurate the estimated temperature will be.

[0067] Preferably, the reference steel strip and the heated steel strip have similar compositions or belong to the same steel grade. Even more preferably, the reference steel strip has the same composition as the heated steel strip.

[0068] As is widely known, based on Planck's law, the emissivity of an object can be calculated when its temperature is known. Therefore, in the third step, Planck's law and the estimated temperature T can be used. 估计 To estimate the emissivity of the heated steel strip. For example, equation (5) can be used to estimate the emissivity, where L is the luminance according to Planck's law. This is in Figure 1 As shown in the figure. The estimated emissivity is expressed as ε. 估计 .

[0069] (5)

[0070] More than one emissivity of the heated steel strip can be estimated by using more than one of the at least two measured radiation intensities.

[0071] In the fourth step, an abacus can be used to estimate the iron oxide thickness, which is plotted as a function of the steel strip's emissivity with respect to a given wavelength. Such a curve is... Figure 1 The graph is plotted in a diagram where the oxide layer thickness is plotted as a function of the emissivity of the FeO oxide with respect to a given wavelength.

[0072] The oxide thickness of the heated steel strip can be estimated by using more than one of the estimated emissivity values.

[0073] In this invention, measured values ​​and reference values ​​are used to estimate the temperature of the steel strip. In contrast, in the prior art, a predicted process temperature or two emissivity temperatures are used to estimate the temperature, such as... Figure 2 As shown. Furthermore, as... Figure 3 As shown, the assumption that emissivity is independent of oxide thickness for wavelengths between 12 μm and 20 μm is incorrect, where relative brightness is plotted as a function of wavelength for oxide thicknesses from 0 nm to 500 nm.

[0074] Therefore, the estimated temperature of the present invention is determined more accurately and reliably because the estimated temperature of the present invention takes into account the surface condition of the heated steel strip (e.g., true emissivity). Thus, the estimated temperature of the present invention also allows for improved estimation of the oxide layer thickness.

[0075] Preferably, the heated steel strip is in operation.

[0076] Preferably, in step 1), at least ten radiation intensities emitted by the heated steel strip at different wavelengths in the 1 μm to 5 μm range are measured, and in step 2), the at least ten radiation intensities are used to estimate T. 估计 Even more preferably, in step 1), at least 20 radiation intensities emitted by the steel strip at different wavelengths in the 1 μm to 5 μm range are measured, and in step 2), said at least 20 radiation intensities are used to estimate T. 估计 The higher the radiation intensity used, the more reliable the estimate will be.

[0077] Preferably, at least two radiation intensities have a wavelength difference of at least 0.1 μm, more preferably at least 0.5 μm, and even more preferably at least 1 μm. Obviously, the larger the wavelength difference, the more accurate the temperature estimation will be.

[0078] Preferably, the heated steel strip and the reference steel strip have similar compositions. Preferably, the compositions of the heated steel strip and the reference steel strip have a mass ratio difference of up to 10%, more preferably up to 5%, and even more preferably up to 2% for each element. For example, for a maximum mass ratio difference of 10%, if the heated steel strip contains 5% silicon, the reference steel strip contains between 4.5% and 5.5% silicon.

[0079] Preferably, the steel strip is set at a temperature from 500ºC to 1100ºC. This temperature range allows for increased emissivity of the steel strip in the range of 1 μm to 5 μm, thereby improving measurement accuracy. Setting the steel strip temperature within this range is preferably done during the annealing process.

[0080] Even more preferably, the heat treatment is performed at a temperature from 500ºC to 1100ºC, and in step 1), the wavelength range of the at least two measured radiation intensities is from 1 μm to 1.7 μm. This wavelength range is advantageous for this temperature range because variations in the thickness of the oxidized layer strongly affect the emissivity compared to other wavelength ranges from 1 μm to 5 μm. Second, this range demonstrates that the measurement uncertainty of the estimated temperature has minimal impact on the estimated emissivity, as it is within the 1 μm to 5 μm range.

[0081] Preferably, the heat treatment is performed at a temperature ranging from 100ºC to 500ºC, and in step 1), the wavelength range of the at least two measured radiation intensities is from 3 μm to 5 μm. Such a range is advantageous because the variation in radiation intensity is greater in this temperature range than in the 1 μm to 3 μm range.

[0082] Preferably, steps 1) to 4) are repeated for some points on the surface of the heated steel strip. Even more preferably, steps 1) to 4) are performed for some points along the width and length of the heated steel strip. Performing steps 1) to 4) at some points on the surface of the heated steel strip allows for plotting the oxide layer thickness and temperature of the heated steel strip at different locations. Advantageously, the measurements are taken near the edge of the strip and near the middle of the strip width.

[0083] Preferably, the method includes the following steps: plotting the oxide thickness and temperature of the steel strip using estimated oxide thickness and estimated temperature at some points on the surface of the steel strip.

[0084] The present invention also relates to a method for heat treatment of a heated steel strip performed in a furnace, wherein the previously described method is performed, and the T 估计 Used to control furnace temperature.

[0085] Preferably, the furnace includes a heating section and a soaking section. The previously described method is performed in the heating section, and the T 估计 Used to control the furnace temperature during the heating step.

[0086] During the heating and soaking stages, a target temperature is set for the heated steel strip to obtain the desired properties. The method described above allows for more precise and reliable monitoring of the heated steel strip temperature. Therefore, the furnace temperature and the heat transferred to the heated steel strip can be altered to achieve the desired temperature (T). 估计 Matches the target temperature.

[0087] The present invention also relates to a method for heat treatment of steel strip, the heat treatment comprising a heating step and a soaking step, the heating step and the soaking step being performed in a furnace including a burner having adjustable power along the width of the steel strip being heated, wherein the previously described method is performed during the heating step, and the estimated oxide thickness Ox 估计 Used to change the power of the burner along the heated steel strip, and to make the oxide thickness uniform along the width of the heated steel strip.

[0088] Because some oxide thickness was estimated along the belt width, the variation in oxide thickness along the belt width can be estimated. The burner intensity can then be adjusted to make the oxide thickness uniform along the width of the heated steel belt.

[0089] Experimental results

[0090] Comparative experiments have been conducted to evaluate the reliability of the claimed method. In these experiments, the temperature of the heated steel strip was measured using three different techniques: a pyrometer, a wedge measurement, and the method according to the invention. The results are plotted in... Figure 4 middle.

[0091] It is known that wedge measurements are highly reliable for steady conditions when the temperature is approximately constant, but unreliable for unstable conditions when the temperature of the steel strip varies.

[0092] exist Figure 4 It can be clearly seen that the temperature stabilized for about 12 minutes, and the temperature estimated using the protected method was closer to the temperature measured by the wedge measurement than the temperature measured by the pyrometer.

[0093] Therefore, the method claimed provides a more accurate approach.

Claims

1. A method for estimating the oxide thickness and temperature of a heated steel strip, the heated steel strip having undergone heat treatment performed at a temperature from 100ºC to 1100ºC, the method comprising the steps of: 1) Measure at least two radiation intensities emitted by the heated steel strip at different wavelengths in the range of 1 μm to 5 μm. 2) Estimate the temperature T of the heated steel strip based on the following: 估计 , -The radiation intensity measured by at least two measurements, and - At least two reference radiative intensities and at least two reference emissivity at different wavelengths for a reference steel strip with at least N oxide layer thicknesses from 0 nm to 200 nm and a known temperature. 3) Using the measured radiation intensity and estimated temperature T 估计 At least one of the following is used to estimate the emissivity coefficient ε of the heated steel strip. 估计 , 4) Using the estimated emissivity ε 估计 To estimate the oxide thickness Ox of the heated steel strip. 估计 .

2. The method according to claim 1, wherein, The heated steel strip is in operation.

3. The method according to claim 1, wherein, In step 1), at least 10 radiation intensities emitted by the heated steel strip at different wavelengths in the 1 μm to 5 μm range are measured, and in step 2), the at least 10 radiation intensities are used to estimate T. 估计 .

4. The method according to claim 3, wherein, In step 1), at least 20 radiation intensities emitted by the steel strip at different wavelengths in the 1 μm to 5 μm range are measured, and in step 2), the at least 20 radiation intensities are used to estimate T. 估计 .

5. The method according to claim 2, wherein, In step 1), at least 10 radiation intensities emitted by the heated steel strip at different wavelengths in the 1 μm to 5 μm range are measured, and in step 2), the at least 10 radiation intensities are used to estimate T. 估计 .

6. The method according to claim 5, wherein, In step 1), at least 20 radiation intensities emitted by the steel strip at different wavelengths in the 1 μm to 5 μm range are measured, and in step 2), the at least 20 radiation intensities are used to estimate T. 估计 .

7. The method according to any one of claims 1 to 6, wherein, The at least two radiation intensities are measured with a wavelength difference of at least 0.1 μm.

8. The method according to claim 7, wherein, The at least two radiation intensities are measured with a wavelength difference of at least 0.5 μm.

9. The method according to claim 8, wherein, The at least two radiation intensities are measured with a wavelength difference of at least 1 μm.

10. The method according to any one of claims 1 to 6, wherein, The heat treatment is performed at a temperature ranging from 500ºC to 1100ºC, and in step 1), the wavelength range of the at least two radiation intensities measured is from 1 μm to 1.7 μm.

11. The method according to any one of claims 1 to 6, wherein, The heat treatment is performed at a temperature ranging from 100ºC to 500ºC, and in step 1), the wavelength range of the at least two measured radiation intensities is from 3 μm to 5 μm.

12. The method according to any one of claims 1 to 6, wherein, Repeat steps 1) to 4) for some points on the surface of the heated steel strip.

13. A method for heat treatment of a heated steel strip performed in a furnace, wherein, Perform the method according to any one of claims 1 to 12, and the T 估计 Used to control furnace temperature.

14. The heat treatment method according to claim 13, wherein, The furnace includes a heating section and a soaking section, wherein the method according to any one of claims 1 to 12 is performed in the heating section, and the T 估计 Used to control the furnace temperature during the heating step.

15. A method for heat treatment of steel strip, the heat treatment comprising a heating step and a soaking step, the heating step and the soaking step being performed in a furnace including burners having adjustable power along the width of the steel strip being heated, wherein, The method according to any one of claims 1 to 12 is performed during the heating step, and the estimated oxide thickness Ox 估计 Used to adjust the power of the burner along the heated steel strip and to make the oxide thickness uniform along the width of the heated steel strip.

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