Method for determining a decarburization indicator for a steel product, associated control and monitoring methods and systems

Abstract

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Description

Method for determining a decarburization indicator for a steel product, associated control and monitoring methods and systems

[0001] The technical field is that of steelmaking, more particularly that of characterizing and controlling surface oxidation, in particular decarburization.Technical background

[0002] During a steel processing operation, when a steel product is hot and is in an oxidizing atmosphere, like in a reheating furnace for instance, an iron oxide layer called scale forms at the surface of the steel product. Not iron only is oxidized, but carbon too. More precisely, the carbon initially contained in the steel is oxidized in the form of gaseous species (CO and CO2) and so, close to the steel surface (close to the steel-scale interface) the carbon content decreases (compared to a carbon content Cbuik in the bulk of the steel). This decarburization modifies the properties of the steel product, in particular its mechanical properties (stiffness, bendability of the final rolled product, ...). Depending on the application intended for the steel product, decarburization may be desirable, or not. Anyhow, it is observed that decarburization does not happen in any conditions. For instance, in some cases (for which the steel phase was almost entirely austenite), it has been observed that decarburization does not occur.

[0003] It would thus be useful to be able to predict if decarburization occurs or not, depending on the conditions (in particular depending on the temperature) in the environment of a steel product, and possibly also to determine how strong decarburization is, in these conditions.

[0004] Being able to predict if decarburization occurs or not (and how strongly) is useful in particular for real-time control of the steel processing operation (eg: for the product reheating), for allowing an operator to follow the impact of the in-course processing operation on the product properties, or for determining properties obtained for a steel product that has been processed, depending on the processing conditions during the processing operation.Summary

[0005] In this context, a method comprising the following steps is provided:- acquiring a temperature T of a steel product,- acquiring a bulk carbon content Cbuik for the steel product,- computing an equilibrium carbon activity ac,E at a steel-scale interface of the steel product, according to formula F1 :where:Pcoand PCO2 are the partial pressures of, respectively, carbon monoxide CO and of carbon dioxide CO2, at the steel-scale interface of the steel product,Pois a standard reference pressure, andKi and K2are equilibrium constants, respectively for the chemical reaction R1 and for the chemical reaction R2:FeO + C ^ Fe + CO (R1)C + CO2 2CO (R2)

[0006] This method comprises also determining a decarburization indicator, depending on ac,E and depending on a bulk carbon activity ac,B for the steel product which corresponds to said bulk carbon content. The decarburization indicator may be a binary indicator specifying that decarburization either occurs or does not occur. It may also be a continuous indicator specifying a decarburized depth or a denaturized thickness for the steel product, determined based on ac,E and on ac,B (or, in an equivalent way, determined based on Cbuik and on an equilibrium carbon content CE corresponding to ac,E). The decarburization indicator may also gather a binary indication (specifying that decarburization occurs or not) and a value for the decarburized depth or denaturized thickness.

[0007] The decarburization indicator specifies (either as a binary indication, or in the form of a non-zero value for the decarburization or denatured thickness) that the steel product undergoes or has undergone decarburization when: the equilibrium carbon activity ac,E is below a bulk carbon activity ac,B for the steel product which corresponds to said bulk carbon content Cbuik.

[0008] Using chemical reactions R1 and R2 allows to take into account the most significant mechanisms for carbon oxidation at the steel surface.

[0009] The expression of ac,E, specified by formula F1 , is derived from the chemical equilibrium equations for the chemical reactions R1 and R2, wherein the activities of Fe and FeO are assumed to be equal to 1 at the steel-scale interface:and soPc° which finally leads to formula F1 .

[0010] Yet, it is noted that, even after having selected the chemical reactions R1 and R2, the the chemical equilibrium equations for these reactions can be arranged in different ways thanformula F1 . For instance, ac E could also be expressed as — .D, or as — . (assuming K2po •pCO2K1po that the activities of Fe and FeO equal 1 , at the steel-scale interface). But among the different manners for expressing ac,E, the one corresponding to formula F1 turns out to be particularly useful, as it is based on the quantity Pco+pco2 (rather than Pco / Pco2, for instance). Indeed, the quantity Pco+Pco2 can be robustly estimated, without necessarily requiring accuratemeasurements of Pco and Pco2. For example, in a re-heating furnace supplied with natural gas (or with another fuel gas), the quantitypco+pco2 can be robustly estimated without requiring measurements of the atmosphere composition.

[0011] The pressures primarily involved to determine ac,E are the pressures at the steel-scale interface. When scale has cracks or detaches from the steel at some places (which is the most usual case), the pressures Pco and Pco2 at the steel-scale interface can be set as equal to the partial pressures PCO aand PCo2,a of CO and CO2 in the environment of the steel product. In the (less frequent) case of a gas-tight scale, or in the case of a scale with a gas-tight coating on it, CO and CO2 produced by carbon oxidation accumulate at the steel-scale interface, and the quantity (pco+pco2) / p° can be set to 1. And for intermediate situations (partial gascontact between the atmosphere and the steel-scale interface), an intermediate value of (pco+pco2) / p°, comprise between (pco,a+pco2,a) / P° and 1 , can be employed.

[0012] So, using formula F1 for determining ac,E, based onpco+pco2, allows also for conveniently taking into account the gas-tight or on the contrary permeable nature of the scale.

[0013] The equilibrium carbon activity ac,E, determined as above explained, is useful for determining if decarburization occurs or not. It is useful also for computing the decarburized or the denatured thickness for the steel product. Indeed, carbon diffusion through the steel, from the bulk to the surface, it strongly influenced by the carbon content at the steel-scale interface (and thus the carbon activity at the steel-scale interface), as the driving force for this diffusion (and so for the growth of the decarburized depth) is all the higher than the carbon content at the steel-scale interface is low.

[0014] The method according to the instant technology may comprise one or several additional features, defined in claims 2 to 10, considered alone or in combination.

[0015] The instant technology also concerns an electronic device according to claim 1 1. It concerns also a computer program, whose execution on a computer makes the computer to execute the method according to anyone of claims 1 to 10 (the computer being possibly connected to sensors and / or actuators of steelmaking installation, or to a controller of the installation, or to one or more external storage devices, depending on the details of implementation of the method). It concerns also a non-volatile computer-readable storage medium comprising such a computer program.Detailed d esc r i p t i o n

[0016] The instant technology will now be described in more detail and illustrated by examples without introducing limitations, with reference to the appended figures.

[0017] Figure 1 is a is a schematic sectional side-view of the steel product.

[0018] Figure 2 represents a quantity Q, plotted against the temperature T of the steel product, a value of Q above the value of (pco+pco2) / P° indicating that decarburization occurs.

[0019] Figure 3 represents the quantity Q, plotted against the temperature T, for different operating conditions than in figure 2.

[0020] Figure 4 represents an equilibrium carbon content CE at the steel-scale interface, as a function of the temperature T, for a steel product coated with a gas-tight coating.

[0021] Figure 5 represents the equilibrium carbon content CE, as a function of the temperature T, in the case of a heating based on H2burning (with no coating), and based on CH4burning (with no coating), and also in the case in which a gas-tight coating is employed.

[0022] Figure 6 represents schematically an installation comprising a reheating furnace and a control and monitoring system for the reheating furnace.

[0023] Figure 7 represents schematically a system for determining a degree of oxidation of steel product based on a temperature path followed by the steel product.Decarburization

[0024] Figure 1 schematically represents some aspects of steel decarburization and scale growth mechanisms. Figure 1 is a schematic cross-section sideview of a steel product 2 (entirely in the austenite phase, in this exemplary case). The scale 21 forms at the surface of the steel 20, due to the presence of dioxygen O2(or possibly due to the presence of H20) in the atmosphere 3 where the steel product 2 is. Some of the chemical reactions occurring at the steel-scale interface, and at the scale-atmosphere interface are represented in figure 1. Some chemical species fluxes are schematically represented by arrows. Both Iron and carbon are oxidized. More precisely, the carbon initially contained in the steel is oxidized in the form of gaseous species (CO and CO2) and the carbon content at the steel surface (steel-scale interface) thus decreases, compared to the carbon content Cbuik in the bulk of the steel. The carbon profile 22 schematically represents the evolution of the carbon content C(x) as a function of depth x (depth relative to the initial position 23 of the steel surface), in an exemplary case for which the steel is assumed to be entirely in the form of austenite. As represented, the carbon content decreases from the in-depth content Cbuik to a lower content Csat the steelscale interface. The thickness of the carbon-lean region is the decarburized depth Ddec. It may be defined as the depth for which the carbon concentration is equal to Pdec.Cbuik, with Pdec a constant, for instance from 0.7 to 0.95.

[0025] In Figure 1 , the initial position of the steel surface, before oxidation starts (before scale growth starts), is represented by the dashed-line 23. The thickness of consumed (oxidised) metal is noted Xoc. The total thickness of denatured steel (denatured by iron oxidation and by decarburization) is noted Ddenand is the sum of Xocand Ddec.Decarburization indicator; influence of the atmosphere

[0026] The instant technology concerns, inter-alia, a method comprising:- acquiring a temperature T of a steel product, such as the steel product 2 of figure 1 ,- acquiring the bulk carbon content Cbuik for the steel product,- computing an equilibrium carbon activity ac,E at a steel-scale interface of the steel product, according to formula F1 :where: o Pcoand PCo2areth© partial pressures of, respectively, carbon monoxide CO and of carbon dioxide CO2, at the steel-scale interface of the steel product, o Pois a standard reference pressure, and o Ki and K2are equilibrium constants, respectively for the chemical reaction R1 and for the chemical reaction R2:FeO + C ^ Fe + CO (R1)C + CO2 2CO (R2)

[0027] Here, the method comprises also a step of determining a decarburization indicator, depending on ac,E and depending on a bulk carbon activity ac,B for the steel product which corresponds to said bulk carbon content. The decarburization indicator may be a binary indicator specifying that decarburization either occurs or does not occur. It may also be a continuous indicator specifying a decarburized depth or a denaturized thickness for the steel product, determined based on ac,E and on ac,B. The decarburization indicator may also gather a binary indication (specifying that decarburization occurs or not) and a value for the decarburized depth or of the denaturized thickness.

[0028] The decarburization indicator specifies (either as a binary indication, or in the form of a non-zero value for the decarburization or denatured thickness) that the steel product undergoes, or has undergone decarburization when the equilibrium carbon activity ac,E is below a bulk carbon activity ac,B for the steel product which corresponds to said bulk carbon content Cbuik.

[0029] The steel product in question may be a steel semi-product such as a slab, a bloom, a billet, an ingot or a wire. It could also be a beam, a bar, a tube, a rail, or a steel sheet (to be coiled).

[0030] The steel product for which the decarburization indicator is determined, may, like here, undergo (or may have undergone) a steel processing operation, or part of a steel processing operation, the decarburization indicator being relative to said operation. The steel processing operation is for instance: a reheating in a reheating furnace (prior to hot rolling), or a transfer from a reheating furnace to a hot roll mill entrance (hot roll bite vicinity), or a continuous casting, or temperature-maintaining storage (using heat containment enclosures), between the continuous casting output and the reheating furnace input. More generally, it is a steel processoperation occurring at high temperature (typically above 600 °C, or even above 700°C) and in an oxidizing atmosphere (in practice, an atmosphere containing O2 and / or H2O).

[0031] From the point of view of an electronic device (such as a computer) executing this method, acquiring T and Cbuik means receiving data representative of these quantities.

[0032] In this method, the respective values of Ki and K2are determined depending on the temperature T of the steel product. The values of these chemical equilibrium constants are the one corresponding to the standard reference pressure Po(which is for instance equal to 100 kPa, or, possibly, to 1 Atm).

[0033] The carbon contents, Cbuik, CE, CS, and C(x), are each a carbon concentration in the steel. They may each be expressed as a mass fraction (weight of carbon relative to the total weight of the steel, in weight%), a mass concentration (in g / m3, for instance) or as molar concentration (in Mol / m3or in Mol / L, for instance).

[0034] Regarding the activities of carbon, ac,E and ac,B , they are directly related to the carbon contents CE and Cbuik. For instance, if CE and Cbuik are expressed in Mol / L, ac,E and ac,B can be approximated as being equal respectively to CE and Cbuik. And if CE and Cbuik are expressed in weight%, they can be readily converted in Mol / L and then be equated to ac,E and ac,B. Alternatively, a more elaborate relationship between ac,E and CE could be employed, to take into account the presence of other alloying elements in the steel (via interaction coefficients between alloying elements).

[0035] The pressures Pco and Pco2 primarily involved to determine ac,E are pressures at the steel-scale interface. When scale has cracks or detaches from the steel at some places (which is the most usual case), the pressures Pco and Pco2 at the steel-scale interface can be set as equal to the partial pressures PCO aand PCo2,a of CO and CO2in the environment of the steel product (that is, in the atmosphere surrounding the steel product). In the (less frequent) case of a gas-tight scale, or in the case of a scale with a gas-tight coating on it, CO and CO2produced by carbon oxidation accumulate at the steel-scale interface, and the quantity (Pco+Pco2) / P° can be set to 1 .

[0036] In practice, ac,E may be determined based on an information (which may be entered by an operator, using a human-machine interface) specifying that the steel product is either- coated by such a gas-tight coating (or that the scale if considered gastight, for the steel product considered), in which case (Pco+Pco2) / P° is set to 1 ,- or, on the contrary, that there is no such coating, in which case (Pco+Pco2) / Po is set to (PCo,a+Pco2,a) / Po.

[0037] For intermediate situations, that is for partial gas-contact between the atmosphere and the steel-scale interface, an intermediate value of (Pco+Pco2) / P°, comprise between(PcO,a+Pco2,a) / P° and 1 , can be employed. In this regard, according to an optional feature of the instant method, the value of (Pco+Pco2) / P° is determined based on a gas-contact indicator, the gas-contact indicator specifying to what extent there is a gas contact between the steel-scale interface and the atmosphere (i.e.: complete contact, or on the contrary no gascontact, or intermediate contact). The value of (Pco+Pco2) / P° is then set as to be from between the steel-scale interface and the atmosphere is complete.

[0038] Figure 2 gathers information about decarburization for the case of a scale with a gas contact between the steel-scale interface and the atmosphere. Figure 2 represents more specifically a quantity noted Q, which is Q = aC B. (K-, + (K1)2 / K2), as a function of the temperature T (in °C). Q(T) is represented in figure 2 for different values of Cbuik: each curve in figure 2 corresponds to Q(T) for a given value of Cbuik (expressed in weight%, for instance 0.002%, or 0.032%). The dashed line corresponds to the value of (Pco,a+Pco2,a) / P°in atypical heating furnace atmosphere, for a furnace heated by burning natural gas in the furnace atmosphere (this value being 0.08).

[0039] According to the above modelization, (Pco,a+Pco2,a) / P° equals aC E. (K-, + (K1)2 / K2). And so, when the curve Q(T) is above the dashed line, it means that ac,E is below ac,B, and thus that decarburization occurs. For a given value of the carbon content Cbuik (for instance, for Cbuik=0.016 weight%), the intersect of Q(T) with the dashed line gives the temperature above which decarburization occurs (about 900°C, for CbUik=0.016 weight%). For another atmosphere composition (with less, or on the contrary more CO and CO2), the value of the position of the dashed line would be different, and the limit temperatures, above which decarburization occur, would be modified accordingly.

[0040] Figure 2 illustrates how the above method can be employed for determining a limit temperature, above which decarburization occur, depending on the bulk carbon content Cbuik and the partial pressures of CO and CO2 in the atmosphere around the steel product.

[0041] Figure 2 illustrates also that for steels with low carbon content (less than 80.10-3weight% of carbone, for instance), decarburization occurs only at hight temperature, typically above 1000°C, while steel with moderate and high carbon content are subject to decarburization, even at rather moderate temperatures (around 600 or 700 °C).

[0042] Figure 3 is similar to figure 2, but in a case in which the steel product is covered with a gas-tight coating, like the ‘Stopoxy’ coating produced by the company Sidercoat, or the ‘Sieridur’ material from the company Sievering (which a type of refractory concrete), for instance. In this case, (Pco,a+Pco2,a) / P° can be approximated to 1 , as above explained. Figure 3 illustrates that, with such a coating, steels with a carbon content of about 0.1% or 0.2% experience decarburization only at high temperature, typically above 900°C.

[0043] Figure 4 represents the carbon content CE as a function of the temperature T, in the case of a coated steel product (co’apoco2’a= 1 )-The constant content Cbuik (equal to 0.5%, in this case) is represented by the horizontal line. The difference between the Cbuik and CE (which is the driving force for decarburization) is all the higher than the temperature T is high.

[0044] Figure 5 is similar to figure 4, but for CbUik=0.43% and it represents CE(T) in different case:- heating by H2burning (so, no CO or CO2in the atmosphere),- heating by CH4burning with no coating and gas-contact between the steel-scale interface and the atmosphere,- use of a gas-tight coating.

[0045] Figure 5 shows that decarburization always take place when using H2burning for heating, even at relatively low temperatures. Regarding decarburization, H2-based heating thus seems clearly detrimental compared to CH4burning, at first view. But in fact, at low temperature, the oxidation kinetics is slow; and for a practical heating cycle (from ambient temperature to about 1200°C), most of the decarburization occurs at high temperature, for which the difference between the H2and CH4burning is less significant.Determination of the decarburized depth Ddec.

[0046] In the embodiment described here, the determination of the decarburization indicator comprises determining the decarburized depth Ddecthat results from the steel processing operation the steel product is undergoing (or that he has undergone).

[0047] The steel processing operation starts at an initial time ti and ends at a final time tf. Ddecmay be the decarburized depth at a current time (between ti and tf), for instance for a real-time monitoring of the steel product decarburization. Ddecmay be the decarburized depth at the final time tf (for predicting the expected decarburized depth at the end of the operation, for instance).

[0048] The decarburized depth Ddecis determined based on a thermal path T(t) followed by the steel product during the steel processing operation. This determination is based on the carbon content Cs at the steel-scale interface, which in turn depends on the equilibrium carbon content CE above mentioned. Indeed, the carbon profile C(x) depends directly on Cs and Cbuik, with a Fick-law diffusion between these boundary values.

[0049] Cs is not systematically equal to CE, due to the effects of kinetics. The relationship between Cs and CE is set by the balance between the oxidation kinetics and the diffusion of carbon.

[0050] Cs May be determined, based on CE, using formula F2 below, for instanceCs=(Cbuik CE) FWagner(y) + CE(F2)being the diffusion coefficient for carbon in steel (at the temperature considered).

[0051] Alternatively, another modelization than F2 could be employed for determining Cs, this other modelization being for instance such that Cs:- takes into account the bulk carbon content Cbuik, the equilibrium carbon content at a steel-scale interface CE and the oxidation-decarburization competition ratio y,- is from CE to Cbuik,- approaches CE when y approaches zero.

[0052] In this respect, it is noted that the ratio y above reflects the balance between scale growth, or in other words oxidation (represented by Xoc), and carbon diffusion in the steel (represented by 2 [Dt). In this regard, when y is small compared to 1 (for instance, below 0.1 ), Cs is almost equal to CE (that is, it is set by chemical equilibrium between carbon in solid form, and carbon in the form of CO and CO2). On the contrary, when y is not small, the value of Cs depends directly on the balance between the kinetics of oxidation and diffusion (via the value of y).

[0053] The computation of Ddec over time t, given the value of Cs(t) (determined from CE ), as above explained), can then be achieved in different ways.

[0054] For instance, it could be achieved by computing at each time step an updated carbon content profile C(x), said profile being computed by numerically solving a 1 -dimensionnal Fick equation for carbon diffusion, with a frontier condition being that, at Xoc(t), the carbon content is Cs.

[0055] The evolution of Ddeccould also be computed based on the following formula:whereXdecis a distance between the initial position of the steel surface 23, prior to oxidation, and an end 24 of a zone that is decarburized, in the steel product, at time t (see figure 1 ). Xdecis determined based on carbon-diffusion computation. The value of Ddecis then deduced form the value of Ddec.

[0056] AXdecmay be determined for instance according to formula F4 below (which is a discretised version of an analytical solution for isothermal decarburization):where erfc1is the inverse function of erfc function.

[0057] More generally, AXdecis computed as equal to 4. D. At. k where the correction coefficient k depends on y (and, possibly, on Cbuik, Cs and CE).

[0058] AXgCmay be determined as explained further below.

[0059] The value of Ddecis then deduced form the value of £>ec(determined using F3). It is noted that computing elementary increases of Xdec(and of X%c), rather than computing AXdec=2 k. D——.At, is beneficial from a numerical point of view. Indeed, it avoids having evaluations errorson Xdec impacting the evaluation of the increment of decarburized depth. And it avoids also possible numerical errors arising from the computation of 1 / Xdec for small values of Xdec(and the same for the computation of

[0060] It may be noted that, in this document, by equal, it is meant equal within 10% or better, or equal within 5% or better, or even equal within 2% or better (or possibly equal within 1 %).

[0061] In formula F3, Ddecis determined taking into account Xoc, which reflects how much the scale has grown. And indeed, the scale growth influences directly the evolution of the decarburized depth Ddec, because the scale grows by consuming steel at the scale-steel interface (steel whose iron is oxidized). And if the scale grows quickly compared to the decarburization process (i.e.: compared to carbon diffusion in steel), the decarburized steel near the steel-scale interface is consumed faster than it forms and the decarburized depth Ddecdecreases with time (which is reflected by A£)ecbeing negative), or remain negligible.

[0062] Regarding the computation of A%dc, as far as a scale layer is present at the steel surface, it can be computed as X%c= Kp. t (parabolic scale growth regime), with Kp= where Kp0is a rate coefficient, PO2 is the dioxygen partial pressurein the atmosphere and Qpis an activation energy. Possibly, if the scale layer is not fully developed (i.e.: does not cover completely the steel product), that is for short times after oxidation has started, AXdccan be computed according to Xo= Ktt (linear scale growth regime)

[0063] Tests have been carried out for a reheating operation in a reheating furnace, for a steel product (in this case a billet, with a 18cm*18cm section) made in a steel that is a leaf spring grade (EN grade: 51 CrV4; grade number: 1 .8159). For this test, the temperature T is measured thanks to temperature sensors fitted on the steel product. The test was carried out in situ, in a reheating furnace of a working steel-making line, both in a case of an uncoated steel product, and in a case of steel product coated with Sieridur (with the same atmosphere composition for both cases). The final value of the decarburized depth Ddec, determined as above described, for the case without coating, is 0.944 mm, in good agreement with the directly measured value, which is 0.94 mm. While for the case with a coating, the value of Ddec, determined as above described, is 0.894 mm, while the directly measured value is 0.92 mm. These results illustrate that the influence, on decarburization, of a coating covering the product is well taken into account thanks to the instant method for determining the carbon activity at the steel-scale interface.Process monitoring and control

[0064] Figure 6 schematically represents an installation 1 for reheating steel products, such as slabs or billets. The installation 1 comprises a reheating furnace 30 and a monitoring and control system 10 for monitoring and controlling the steel products reheating.

[0065] The reheating furnace 30, for instance a walking beam furnace, has an input port 31 , to input the steel products, and an output port 32 for outputting the heated steel products. The reheating furnace 30 comprises actuators for controlling the reheating process, such as electrovalves for controlling fuel gas fluxes, electric power supplies for adjusting electric power supplied to electric heaters, or electric motors or servomotors for controlling the displacement of the steel products, from the input port to the output port.

[0066] One or more of these actuators is controlled by control data 18 output by the monitoring and control system 10. The control data 18 may take the form of control signals suitable to directly control the actuators, or may take the form of setpoints for controlling the actuators through the intermediary of low-level automation modules (such as PID control loops, for instance).

[0067] The installation 1 comprises one or more temperature sensors, arranged to measure the temperature T of the steel product 2 considered, before or during its reheating. It may be a temperature sensor placed upstream of the input port 31 and measuring an ambient temperature upstream of the reheating furnace. It may also be a temperature sensor such as a pyrometer arranged in the reheating furnace and measuring a (surface) temperature of the steel product at a given moment during its reheating. The reheating furnace 30 may comprise multiple such temperatures sensors.

[0068] Temperature-related signals 17 are acquired by the monitoring and control system 10. They comprise at least the temperature(s) measured by the one or more temperature sensors. Here, they comprise also data representative of heating powers, for instance data representative of fuel gas fluxes, or representative of an electric power supplied. The temperature-related signals 17 mays also comprise data representative of a position over time of the steel product 2 considered.

[0069] The monitoring and control system 10 comprises: one or more memories, including a non-transitory memory; one or more processors; a communication interface, such as a network of bus interface card, for acquiring the temperature-related signals 17 and for outputting the control data 18. Here, the electronic device comprises also a Human-Machine Interface 12 (including for instance one or more screens or indicators and input devices like a keyboard, buttons or selectors). The monitoring and control system 10 may be a programmable electronic unit, based for instance on an FPGA (Field Programmable Gate Array) or other programmable circuit. It may also be a computer (for instance a server), or a computer system. All or part of the monitoring and control system 10 may be implemented in a distributed manner (somehow “virtually”), using so-called “cloud” resources (computing and storing resources distributed among distinct physical systems in a network).

[0070] The monitoring and control system 10 is configured to determine, here by numerical simulation, a thermal path TP followed by the steel product 2 considered during its reheating.This determination is carried out based on the temperature-related signals 17. The thermal path TP may be represented by the temperature T(t) over time t, for the steel product 2, during its reheating, at least from the initial time ti (entry into the furnace) to the instant time t. Here, the thermal path comprises also predictions of the temperature to come for the steel product 2, that is T(t) from ti to the final time tf (time expected for the output of the steel product, out of the furnace). Here, the thermal path TP is determined by a thermal path module 13 of the monitoring and control system 10.

[0071] The monitoring and control system 10 is configured also to determine the control data 18 for controlling the heating route followed by the steel products, based on the thermal path TP and based on one or more heating targets, such as a final temperature target to be reached, or such as a minimum time to be spent over an austenitization threshold temperature. The control data 18 may be determined according to an optimization procedure, so as to minimize an energy consumption, or to comply with the heating targets for the highest number of steel products currently being reheated. Here, the control data 18 are determined by a control module 14 of the monitoring and control system 10.

[0072] The monitoring and control system 10 is configured also to determine the decarburization indicator for the steel product 2, at the current time t and / or at the final time tf, based on the thermal path TP. Here, the decarburization indicator is determined by a decarburization module 11 of the monitoring and control system 10. The value of (Pco+Pco2), employed to determine the decarburization indicator, maybe preset (and stored in a memory of the monitoring and control system 10), or received (input by an operator, or received from an atmosphere composition analyser), or computed based on received data relative to combustion conditions (kind of fuel gas, total combustion or not) and / or to gas-contact between the steel and the atmosphere.

[0073] The decarburization indicator is transmitted to the Human-Machine Interface 12, which outputs it (displays it). More particularly, in this embodiment, the Human-Machine Interface 12 outputs Ddec(tf) and / or Ddec(t).

[0074] The decarburization indicator (which comprises Ddec(tf), here) is also transmitted to the control module 14. The control module is configured to determine the control data 18 taking into account the decarburization indicator and a decarburization target. For example, the control data 18 is determined taking into account Ddec(tf) and a maximum decarburized depth Ddec,max, to avoid that Ddec(tf) overcomes Ddec,max. To this end, if Ddec(tf) turns out to be higher than DdeC,max, the heating may be controlled so as to have a late heating (to limit the decarburization) rather than early heating, for instance. The control data 18 may be determined by adding a penalty in the optimization procedure, so as to penalize cases for which Ddec(tf) overcomes Ddec,max. Alternatively, when the decarburization indicator is a binary indicator, thedecarburization target may also be a binary indication, specifying that decarburization is accepted or not, for the steel processing operation.

[0075] The different modules presented above (1 1 , 13 and 14) may each take the form of a dedicated electronic unit (distinct one from each other), or of a dedicated group of instructions (a distinct program or sub-program). On the contrary, these different modules may be implemented together, as a single electronic unit. More generally, the different functionalities of the control and monitoring system 10 could be arranged, distributed in different modules or on the contrary gathered differently than in figure 6.

[0076] Monitoring a steel processing operation, and even controlling it, based on the decarburization indicator (eg: based on the decarburized depth predicted for the steel product), has been presented above for a reheating operation but it can be applied similarly to other steel processing operations.

[0077] Besides, in alternative embodiments, the control features above described could be omitted, the installation comprising an electronic monitoring system, rather than the control and monitoring system above described.Steel product non-direct characterization

[0078] The instant technology concerns also a non-direct characterization of a steel product, possibly achieved after the steel processing operation, based on a thermal path TP followed by the steel product during this operation, and the quantity (Pco+Pco2) during this operation (whose value may be obtained or preset as above described, in reference to figure 6). In this case, the decarburization indicator comprises the decarburized depth Ddec(tf) and / or the denatured depth Dden(tf), resulting from the steel processing operation, which is computed by a computer 1 T (figure 7) or electronic device (based on the above-described method for determining Ddec(tf)), and transmitted to an industrial production database where it is recorded, associated to an individual identifier (such as a serial or production number) for the steel product.

[0079] The thermal path TP may be a thermal path directly measured during the steel processing operation. It may also be a thermal path derived, by numerical simulation, from temperature-related signals (like above explained in terms of process monitoring).

[0080] By industrial production database, it is meant a database where product features (dimensions, steel grade, measured or modelized ultimate tensile strength, etc...) are stored, associated to an individual identifier of the product considered, for different steel semiproducts.Computer Assisted Design of a steel processing operation

[0081] The instant method, in which a decarburization indicator is determined for a steel product undergoing a steel processing operation, can also be used for determining, in a Computer Aided Design way, processing conditions, in particular a thermal path for the steelprocessing operation, and possibly an atmosphere composition around the steel product, such that the decarburization indicator is in accordance with a decarburization target (such as a target range or a maximum value for the decarburized depth).

[0082] This thermal path can be determined, for instance, by executing the method for determining the decarburization indicator iteratively, for different candidate thermal paths, until a candidate thermal path leading to the decarburization target is found.

[0083] Once the thermal path, or more generally the operation conditions that enable to obtain the decarburization target are identified, the steel processing operation may be executed according to these operating conditions.

[0084] The determination of such operating conditions, and then control of a steel processing installation accordingly, can be executed by an electronic device or system configured to this end.

Claims

CLAIMS1 . A method for determining a decarburization indicator for a steel product (2), the method comprising:- acquiring a temperature T of the steel product,- acquiring a bulk carbon content (Cbuik) for the steel product,- computing an equilibrium carbon activity ac,E at a steel-scale interface of the steel product, according to formula F1 :where: o Pcoand PCO2 are the partial pressures of, respectively, carbon monoxide CO and carbon dioxide CO2, at the steel-scale interface of the steel product, o Pois a standard reference pressure, and o Ki and K2are equilibrium constants, respectively for the chemical reaction R1 and for the chemical reaction R2:FeO + C ^ Fe + CO (R1)C + CO2 <■■> 2CO (R2)- the decarburization indicator specifying that the steel product undergoes decarburization when the equilibrium carbon activity ac,E is below a bulk carbon activity ac,B for the steel product which corresponds to said bulk carbon content (Cbuik)-2. A method according to claim 1 wherein, when the steel product is uncoated except for a scale covering the steel of the steel product, the quantity (PCo + Pcoz) / Po is set to p +P co, a co2,at0 compUteequilibrium carbon activity ac,E, Pco, a and Pco2,a being Po respectively the partial pressures of carbon monoxide CO and carbon dioxide CO2in an atmosphere around the steel product.

3. A method according to claim 1 or 2 wherein, when a scale which covers the steel of the steel product is covered by a gas-tight coating, the quantity (PCo + Pco2) / po is set to 1 to compute the equilibrium carbon activity ac,E-4. A method according to anyone of claims 1 to 3, wherein the quantityPc°+Pco2jsP 0 computed:depending on partial pressures PCo,a and Pco2,a> of carbon monoxide CO and carbon dioxide CO2 respectively, in an atmosphere around the steel product, and depending on a gas-contact indicator specifying to what extent there is a gas contact between the steel-scale interface and the atmosphere,- so as to be fromPcOa^Pco2 ato 1 , and to be all the closer toPcOa^Pco2 athan the gas■ 0 °0 contact between the steel-scale interface and the atmosphere is complete.

5. A method according to anyone of the preceding claims, comprising a gradual determination, time step by time step, of a decarburized depth (Ddec) for the steel product, or of a denatured thickness (Dden) for the steel product, determined depending on the equilibrium carbon activity ac,E and on the bulk carbon activity ac,B, or depending on an equilibrium carbon content (CE) corresponding to ac,E and depending on the bulk carbon content (Cbuik), and wherein the decarburization indicator specifies the decarburized depth, or the denatured thickness.

6. A method according to anyone of the preceding claims wherein the temperature T of the steel product (2) is measured, or is derived from at least one temperature measurement (17) and using a thermal model for a thermal evolution of the steel product.

7. A method according to claims 5 and 6, comprising:- acquiring thermal path data, representative of a thermal path (TP) followed by the steel product (2) during a steel processing operation,- determining the decarburized depth (Ddec) or the denatured thickness (Dden) for the steel product, based on said thermal path, and recording said decarburized depth or denatured thickness in an industrial production database.

8. A method according to claim 6 or 7, the method being executed while the steel product (2) is undergoing a steel processing operation, the method comprising adjusting one or more process setpoints (18) employed for controlling said processing operation, depending on the decarburization indicator (Ddec) and on a decarburization target (Ddec,max).

9. A method according to anyone of claims 6 to 8, the method being executed while the steel product is undergoing a steel processing operation, the method comprising displaying the decarburization indicator using a human-machine interface (12).

10. A method for processing a steel product, the method comprising:- acquiring a decarburization target for a steel processing operation achieved using a steel processing installation, - determining processing conditions for said steel processing operation such that a decarburization indicator for the steel product for said processing conditions is in accordance with the decarburization target, the decarburization indicator being determined according to the method of anyone of claims 1 to 5,- executing the steel processing operation according to said process conditions,11. An electronic device (10; 11; 11’) comprising at least a processor and a memory, configured for executing the method according to any of claims 1 to 10.

12. A computer program, comprising instructions whose execution on a computer make the computer to execute the method according to any of claims 1 to 10.