DE69310897T2 - METHOD FOR CONTINUOUSLY CARBONING A STEEL TAPE - Google Patents

Description

The invention relates to a method for the continuous gas carburization of a metal strip. For example, during the continuous gas carburization of a strip made of a steel with an extremely low carbon content by passing the strip through from an annealing furnace to a carburizing furnace (for carburizing with a certain amount of carbon), the strip, which is passed through at a certain speed due to different process conditions than during carburizing, reaches the equilibrium concentration of the strip/gas atmosphere in an area determined by the surface reaction in front of a certain carbon concentration in an edge zone of the strip. In order to achieve a certain carbon concentration distribution in the steel, the composition of the gas atmosphere, the gas concentration, the surface temperature, the metal strip temperature, the passage speed, etc. are set as atmospheric factors that prevent soot formation.

The metalworking industry demands better formability and strength in the sheet metal to be processed. In particular, the automotive industry requires, from the point of view of body weight, In order to achieve low fuel consumption and reduce environmental pollution, steel sheets with higher strength and the same good deep-drawability as before have recently been used.

The evaluation factors for such sheets include, for example, an elongation index, deep drawability, an aging index, strength, strain aging, bake-hardening properties, welding behavior, etc. When evaluating deep drawability using the Lankford value (hereinafter referred to as r-value: sheet width stress/sheet thickness stress) in the case of a great importance of deep drawing behavior, it is known that a reduction in the carbon content in the steel is extremely advantageous and, in addition, as a result of a low carbon content, the elongation index (EI) and the cold hardening index (AI: the lower the index, the better) or the storage stability are improved. On the other hand, a lower carbon content of the steel impairs most of the other evaluation criteria. For example, if the structural strength is reduced due to reduced precipitates, the tensile strength (TS) is reduced, and strain ageing is affected because the grain boundary strength is reduced, and bake hardening is impaired because the amount of carbon in solid solution is reduced. In addition, if the carbon content is 50 ppm or less, coarse grain formation is promoted due to heat input during welding and spot weldability is impaired due to coarse grain formation in the heat affected zone (HAZ).

The patentee has developed a process for continuous annealing and carburizing, which is described in Japanese Laid-Open Patent Application Hei 4-88126 and shown in Fig. 2 and improves the tensile strength, strain aging or processing embrittlement, BH (bake hardening value) and spot weldability by continuously carburizing following continuous annealing of the strip made of an extremely low carbon steel, as shown in Fig. 1, so that the carbon is in solid solution in a surface zone and the elongation index, deep drawability and storage stability are adjusted by means of recrystallization and annealing.

In this continuous annealing and carburizing, after a preliminary recrystallization and annealing of a metal strip A in a preheating zone 1 and an annealing zone 2 or a uniform annealing zone 3, carburization takes place in a carburizing zone 4 in such a way that the strip temperature, the gas atmosphere and the throughput speed (furnace time) as well as the cooling conditions are set so that a metal strip with the desired values for the carburization depth and the concentration distribution can be achieved continuously while simultaneously guaranteeing the specific material properties.

On the other hand, Japanese Patent Publication No. 54-31976 describes a method for adjusting the carburization depth and concentration distribution in the edge zone of a metal strip. In this method for adjusting the carburization depth and the concentration distribution, a carburizing gas is fed in at a predetermined flow rate or quantity in order to introduce carbon into the surface zone of the metal strip and, during a diffusion phase following carburization, to subject the introduced carbon to diffusion in the edge zone of the metal strip under sufficiently reduced pressure of the extracted carburizing gas. The carbon concentration distribution resulting from the carburization depth and the carbon concentration is adjusted using the carburization and diffusion times. With this method of adjusting the carburization depth and the carbon concentration, it is possible to prevent uneven carburization, as occurs with gas jet carburization, particularly in the case of narrow carburization zones.

Under various conditions, it has been found that such continuous carburizing and annealing causes the following problems:

(1) Regarding the carburization rate, it is known from a report by Yo et al. (Yo kuun, HARUYAMA shiro et al.: Japan Metallic Society Journal 49 (1985) 7,529) that, as shown in Fig. 3, when the carbon content in the edge zone is high and the carburization time is long after the carbon equilibrium concentration between the strip and the gas atmosphere is reached, the carburization rate is normally proportional to the square root of time. This carburization range is called diffusion-controlled region because here the carburization rate is proportional to the diffusion rate of the carbon in the metal structure. On the other hand, if the amount of carbon in the edge zone is very small and the carburization time is very short, then because the carbon concentration in the edge zone does not reach the equilibrium concentration, the carburization rate is proportional to the reaction rate of the carbon directly at the metal surface. This time-carburization region is called the surface reaction-determined region.

Accordingly, if, for example, the carburizing conditions for a metal strip result from the specification (Japanese Laid-Open Publication Hei 3-199 344 etc.) of the metal strip or its property profile, which is to be improved with regard to its forming or processing embrittlement, then, because the carbon concentration and the carburizing depth are very low, it is necessary in this case to carburize in the area determined by the surface reaction. It was found that the amount of carburization in the metal strip cannot be adjusted by adjusting the carbon potential via the ratio CO/CO₂ etc., because this is based on the idea that the strip surface is always in a state of equilibrium with the carburizing capacity of the gas atmosphere.

(2) In addition, the composition of the gas atmosphere under the carburizing conditions can generally be adjusted using chemical equilibrium. In conventional methods, all conceivable reactions in the gas phase are taken into account and a gas composition is obtained by solving non-linear simultaneous equations from these equilibrium conditions for individual reactions. However, it is very difficult to obtain an exact limit for the formation of soot from the reaction equations in the gas phase.

(3) Furthermore, regarding the surface reaction rate mentioned above, the report by Yo et al., supra, only discusses the carburization rate of carbon monoxide, and it is impossible to apply this to an operational situation in continuous carburizing which requires a complicated composition.

In this respect, in the continuous annealing and carburizing process in a plant according to Fig. 2, it is necessary to carry out a temperature control of the metal strip (hereinafter also referred to as sheet temperature control) in the heat treatment zone concerned, for example by adjusting the furnace temperature, since it is necessary to carry out a certain heat treatment of the metal strip in the heating zone 2 and/or the uniform annealing zone 3, a certain Carburization and a certain cooling in each cooling zone 5 and 6. In each furnace which represents a heat treatment zone, the sheet temperature is primarily controlled by heat transfer, but at the same time upper and lower limits of the internal furnace temperature (hereinafter also referred to as furnace temperature) are given as a result of the calculated furnace power. For example, in the reheating furnace in the reheating zone and in a uniform annealing furnace in the uniform annealing zone, upper limits of the furnace temperature arise from the furnace power. In addition, the furnace time (i.e. also the warm-up time or uniform annealing time) of the strip, which takes the upper and lower limits into account, is derived from the heat balance which takes into account the heat transfer coefficient between a radiant tube, the furnace wall, the hearth rollers, etc. The result is a throughput speed of the strip which takes into account the residence time in the furnace. Likewise, in each annealing furnace in each cooling zone, a heat transfer coefficient of the cooling gas jet serves as the heat transfer coefficient in the above sense.

On the other hand, in such a continuous annealing and carburizing plant, various process conditions are superimposed, whereby the process condition changes at a non-stationary point, for example at a joint of the strips of two coils or the like, and in order to take these conditions into account, it is often necessary to change the strip speed, which has the highest response speed. However, no concrete measures have yet been taken to determine the various carburizing conditions. in the carburizing furnace with respect to the strip speed which is determined by various process conditions including the strip temperature in the above-mentioned continuous annealing and carburizing. It is therefore highly desirable to provide means for adjusting the physical properties and the temperature in the carburizing furnace in order to achieve the amount of carburization and to achieve the material properties required for the steel sheet as described above, particularly under conditions which dictate the strip speed.

In order to eliminate the limits on the throughput speed, a belt storage device could be installed between the relevant annealing zones. This is difficult from a practical point of view, since a belt storage device takes up a lot of space in the annealing section and the annealing section already requires a lot of space. This also applies to a continuous annealing and carburizing section, which is a continuous annealing section with an additional continuous carburizing section.

Furthermore, the tendency is to create more sensitive conditions than the material properties of the above-mentioned carburized sheet require. In order to meet the relevant requirements, it is necessary to influence and adjust the carbon concentration distribution in the edge zone, ie to create a concentration profile in the strip thickness direction of the carburized edge zone. In order to achieve bake hardening in body and electrical sheet after press forming, To carry out this process, the sheet must have properties that ensure high deformability during pressing in the form of a high elongation index EI and deep drawability in the form of a sufficient r value, while bake hardening increases the strength in the form of the BH value. At the same time, the steel sheet must have sufficient storage stability (AI) to ensure the deformation properties until the time of press forming. Accordingly, these steel sheets must have a low ageing index and a high bake hardening value (steel sheet with low AI and high EH) and be deep drawable. If one considers the concentration profile of the carburization in the steel, ie the distribution state required for the continuous annealing and carburizing of an ELC steel, it becomes necessary to greatly increase the carbon concentration in the surface zone and to set an optimal carbon gradient, but at the same time to keep the carbon concentration outside the surface zone in the direction of the sheet thickness at the level of the ELC steel. The method for controlling the carburization depth and the distribution of carbon concentration described in the above-mentioned Japanese Patent Publication No. 54-31967 does not take such a concentration profile into consideration, and therefore it is not possible to apply this method as such to adjust the concentration profile.

German patent application 21 52 440 relates to a process for carburizing steel without soot formation during annealing at a temperature below the final temperature. This document describes However, it is not a continuous process for treating steel and is therefore unsuitable for steel strip.

The German laid-open specification 21 52 439 also refers to a process for carburizing steel, in which the carbon content is automatically regulated to suppress soot formation using a temperature and a carbon potential signal. This citation also refers to a non-continuous process and does not describe any measures for the continuous treatment of steel strip.

The invention was made in view of the various problems mentioned above and has as its aim a process that allows a desired carburization and distribution of the carbon concentration to be achieved in steel strip without soot formation even when the strip travel speed is limited by process conditions outside of the carburization. The carburization takes place even at such a strip travel speed in the above-mentioned range determined by the surface reaction.

The inventors have intensively studied the above-mentioned problems and made the present invention based on the following findings. Specifically, in the problem of soot formation which occurs in the form of free carbon even when each component is quantitatively changed in a production system in the carburizing furnace, the total amount concerned remains constant when the level is considered on the basis of each element. And in the case of an isothermal, isotactic system, During a natural change, the Gibbs free energy increases and reaches a minimum when a state of equilibrium exists in the system between the gas atmosphere and the metal strip. Consequently, it is possible to limit or suppress the reaction towards the formation of free carbon (soot), since the equilibrium state can be reached in the furnace atmosphere if a composition of the gas atmosphere is selected in which the Gibbs free energy corresponds to the minimum value. However, it was found that it is not possible to calculate the actual equilibrium state of operational continuous carburizing, that is, the true soot limit, without taking into account the limiting conditions of material input and output, in which elements are carried out of the gas atmosphere by the metal strip by means of a reaction in the peripheral zone of the metal strip, while the element compounds introduced into the system are constant. Accordingly, with regard to the actual material introduction and discharge, not only the composition of the gas atmosphere, but also the inflow and outflow velocity of the gas in the furnace atmosphere, the throughput speed of the strip, the furnace temperature, the strip thickness and the strip width, etc. must be taken into account.

A first aspect of the present invention consists in a method for continuous carburization of a carburizing furnace with a belt passing through a specific atmosphere which essentially contains an element from the group: carbon, oxygen, hydrogen and nitrogen, in which soot formation is thereby prevented is achieved by adjusting the soot limit according to the CO/002 concentration by adjusting the amount of gases flowing into the carburizing furnace of the relevant carburizing atmosphere and the strip temperature.

(a) the total amount of carburization based on the formula for the rate (V) of the carburization reaction at the surface,

V = k&sub1; f 1 (pCO, pH 2 , θo) x ? f3 (pCO, pCO2), or

V = k&sub1; f1 (pCO, pH2, θo) -k2 f2 (pCO2 , pH2 O)

where α is a constant, k₁, k₂ are reaction rate constants, pCO is the partial pressure of carbon monoxide, pCO₂ is the partial pressure of carbon dioxide, pH₂ is the partial pressure of hydrogen and θo is the coating rate of absorbed oxygen, and based on the following model formula for carbon diffusion in steel:

dc/dt = D d²c/dX²,

calculated, where C is the carbon concentration in the steel, t is the time, D is the diffusion coefficient and X is the diffusion distance;

(b) appropriate ranges for the carburization temperature, the respective concentrations of CO, H₂, CO₂ and H₂O in the carburization atmosphere and the carburization time are selected so that the carburization amount corresponds to the target amount;

(c) during carburizing of the strip, the carburizing temperature, the respective concentrations of Co, H₂, CO₂ and H₂O in the carburizing atmosphere and the carburizing time are set within the limits according to (b).

A second aspect of the present invention relates to a method for continuously carburizing steel strip in a continuous furnace, in which variables are calculated by means of a computer on the basis of initial conditions from the material properties of the strip in the carburized state and the variables are adjusted to achieve a target carbon amount at which soot formation is suppressed by means of a soot limit value dependent on the CO/CO2 concentration and the steel temperature and by adjusting the flow rate or amount of each gas component, wherein

(a) a target carburizing amount (ΔC�0), the gas composition, the flow rate or amount of the gas, the carburizing temperature, the strip passing speed (LS) and the size of the steel strip serve as input conditions;

(b) the concentration of each gas component at which soot suppression is ensured is calculated and the total Gibbs free energy (F(x)) resulting from using the concentration of each component of the gas as a variable is taken to be a minimum under the condition that the amount of carbon in the gas introduced into the furnace and the amount of carbon formed during carburisation of the strip are constant;

(c) the surface reaction rate using the formula:

V = k&sub1; f 1 (pCO, pH 2 , θo) x ? f3 (pCO, pCO2), or

V = k&sub1; f1 (pCO, pH2, θo) -k2 f2 (pCO2 , pH2 O),

where α is a constant and k₁,k₂ are reaction rate constants, which are calculated using the equation:

ki = Ai exp(-Ei/RT)

where Ai is the frequency factor, Ei is the activation energy, R is the gas constant and T is the absolute temperature, and

the formulas as parameters the carbon monoxide partial pressure and the hydrogen partial pressure and additionally include the CO₂ partial pressure and the H₂O partial pressure, provided that the carburization amount is set within the range governed by the surface reaction in which the carbon concentration of the strip in its edge zone is below the concentration at which it reaches equilibrium with the gas, and furthermore, disturbance of the carburization reaction by CO₂ and H₂ formed during carburization is taken into account,

(d) the carburization amount (ΔC') of the strip is calculated by integration over the carburization time and the total area of the strip as well as the surface reaction rate (V) per unit time and unit area, each calculated according to (c);

(e) the carburization amount (ΔC') according to (d) is compared with the target carburization amount (ΔCo) and the composition of the gas, the strip travel speed and the carburization temperature, each calculated or predetermined, are changed individually or in parallel to repeat the process steps according to (b) to (d) if the difference between the calculated carburization amount (ΔC') and the target carburization amount (ΔCo) is not less than a predetermine value;

(f) the concentration of each gas component, the strip travel speed and the carburization temperature, each calculated or selected, are used when the difference between the calculated carburization amount (ΔC') and the target carburization amount (ΔCo) is less than the specified value; and

(g) based on the results of (f), the furnace temperature is set to a value of 700 to 950ºC, the carbon monoxide concentration to a value of over 0% but not more than 22%, the hydrogen concentration to a value of 0 to 30% as the gas composition and furnace temperature conditions under which no soot formation occurs in the furnace.

A third aspect of the invention relates to a method for continuously carburizing steel strip in a continuous furnace, in which variable control variables are calculated by means of a computer on the basis of inputs determined by the material properties of the strip in the carburized state, and the variables are set with regard to a target carburization amount and a certain distribution of the carbon concentration over the strip thickness, and soot formation is suppressed by means of a soot limit corresponding to the CO/CO2 concentration and the steel temperature and by adjusting the flow rate or amount of each gas component introduced into the furnace by

(a) a target carburization amount (ΔCo), the carbon concentration (C₁) at a certain distance (X₁) from the surface of the strip, the composition and the flow rate or amount of the gas, the carburization temperature, the strip speed (LS) and the size of the strip are input;

(b) the concentration of each gas component at which soot formation is suppressed and the total Gibbs free energy (F(x)) obtained by using the concentration of each component of the gas as a variable is minimum under the condition that the amount of carbon in the gas and the amount of carbon taken up during carburization of the strip are constant;

(c) the diffusion rate of carbon in solid solution in the steel strip (i.e. the carburization rate) according to the model formula for carbon diffusion in steel:

dC/dt = D d²c/dX²

is calculated, where C is the carbon concentration in the steel, t is the time, D is the diffusion coefficient and X is the diffusion distance and a carbon diffusion quantity for the diffusion into the strip is obtained;

(d) a carburization quantity (ΔC') in the steel strip is calculated by integration over the carburization time, the total area of the steel strip and - each calculated according to (c) - the diffused carbon quantity in the steel strip per unit time and the area;

(e) the carburization amount (ΔC') according to (d) is compared with the target carburization amount (ΔCo) and the composition of the gas, the belt speed and the carburization temperature, each calculated or selected, are changed individually or side by side to repeat the process steps according to (b) to (d) if the difference between the calculated carburization amount (ΔC') and the target carburization amount (ΔCo) is not less than a predetermined value;

f) the carbon concentration (C'₁) at a specific carburization depth (X₁), based on the strip surface, according to the model formula of carbon diffusion in steel:

dC/dt = D d²C/dx²

is calculated when the difference between the calculated carburization amount (ΔC') and the target carburization amount (ΔC�0) is less than a predetermined value;

(g) the carbon concentration (C'₁) at a determined carburizing depth (X₁) calculated in accordance with (f) is compared with the target carbon concentration (C₁) at the depth (X₁) entered in accordance with (a) and the composition of the gas, the belt speed, the carburizing temperature, each calculated or selected, are changed individually or in parallel and the process steps (b) to (f) are repeated if the difference between the calculated carbon concentration (C'₁) and the target carbon concentration (C₁) is not less than a predetermined value, and the composition of the gas, the belt speed, the carburizing temperature and the distribution of the carburizing concentration from the surface are used if the difference is less than the predetermined value; and

(h) on the basis of the result according to (g), the furnace temperature is set to a value of 700 to 950ºC, the carbon monoxide concentration to a value of over 0 to a maximum of 22% and the hydrogen concentration to a value of up to 30% as conditions for the composition of the gas and the furnace temperature at which soot formation does not occur inside the furnace, and the carburizing concentration is set to a certain carburizing concentration at least at one point at a depth of 10 to 250 µm in the peripheral zone according to the distribution of the carburizing concentration in the direction of the strip thickness.

In this way, in the continuous carburizing process according to the invention, by setting the parameters of the carburizing atmosphere, which include carbon, oxygen and nitrogen or carbon, oxygen, hydrogen and nitrogen, to values that do not lead to soot formation, the composition of the gas atmosphere and/or the furnace temperature is calculated on the basis of the formula of a thermodynamic model that is directed to an equilibrium state of the furnace atmosphere via a state in which the Gibbs free energy of the entire furnace atmosphere is at a minimum, taking into account the current material input and output for each element during continuous carburization in the carburizing furnace. In this way, it is possible to improve the carburizing potential of the furnace atmosphere and at the same time prevent the formation of soot, compared with the method in which the composition of the gas atmosphere and/or the furnace temperature are calculated based on an equilibrium state resulting only from the composition of the gas fed into the furnace and the furnace temperature without taking into account the input and output of each element in the furnace. In other words, it is possible to improve the process conditions in the form of a higher throughput rate by increasing the CO concentration in the gas atmosphere.

Furthermore, regarding the above-mentioned parameters of the gas atmosphere, the following conditions are recommended for industrial continuous carburizing, in which the furnace temperature is 700 to 950ºC, the carbon monoxide concentration is 0 to 22%, and the hydrogen concentration is 0 to 30%. In this case, nitrogen, which is considered to be an inactive gas in the atmosphere that dilutes the concentration of the atmosphere, can be used as an inactive gas similar to argon or the like.

Furthermore, in order to adjust the amount of carburization in the metal strip in the range determined by the surface reaction, in which the carbon concentration in the edge zone is equal to or lower than the equilibrium concentration between the metal strip and the gas atmosphere, it has been found that only the amount of carburization in this range, i.e. the surface reaction rate, needs to be determined and then integrated over time. This time, i.e. the carburization time, determines the speed of passage of the strip. Furthermore, during the study of the surface reaction rate, it was found that it is possible to control the reaction rate by adjusting the composition of the gas, which is contained in a formula for the carburization reaction between the metal strip and the gas atmosphere and also in a formula for the deoxidation reaction. In addition, it was found that carbon monoxide and hydrogen have the greatest influence on the gas composition. and in the case of lower inflow and outflow speeds or quantities of gas, particularly at high temperatures, carbon dioxide and H₂O are also effective in disrupting the carburizing reaction despite the small quantity. Furthermore, experiments have shown that with these compositions the partial pressures influence the speed of the surface reaction mentioned above. Furthermore, in view of the dependence of a material reaction on temperature, a factor known as the metal strip temperature is included in the speed coefficient of the surface reaction.

Accordingly, in the method of the invention for continuously carburizing metal strip under carburizing conditions in which the carburization rate follows the surface reaction which is greater than the diffusion rate in the direction of the strip thickness, a temperature-dependent coefficient of the reaction rate in the surface reaction during carburizing is calculated by calculation, for example, based on a formula related to the metal temperature in the carburizing furnace, and the rate of the surface reaction during carburizing is calculated from this coefficient of temperature dependence and from a formula related to the partial pressure of carbon monoxide, or the partial pressure of carbon monoxide and the partial pressure of hydrogen and the carburization amount in the metal strip can be calculated from the rate of the surface reaction based on a formula related to the above-mentioned carburizing time As a result, it is possible to achieve the carburization amount corresponding to the specification of the steel sheet under highly effective carburizing conditions by setting the carburization amount in the metal strip as required by the sheet specification after carburizing and by parameters in accordance with the actual carburizing conditions which are included as variables in each formula. Furthermore, in the case where the inflow and outflow speed of the gas of the carburizing atmosphere at a high temperature is low, it is possible to set the carburization amount in the metal strip in the presence of CO₂ and H₂O by adding carbon dioxide and the associated partial pressure increase and the H₂O partial pressure as control variables, ie as parameters in the formula for the surface reaction rate, in order to take into account the influence of a disturbance of the carburizing reaction.

Furthermore, the concentrations of CO₂ and H₂O in the gas atmosphere can be reduced by increasing the inflow velocity of the gas atmosphere, and it is possible to increase them by reducing the inflow velocity of the gas into the furnace atmosphere.

If the speed of the surface reaction is integrated over time, this is done with the actual carburization time. This carburization time results from a simple calculation using the carburizing time = residence time in the furnace = furnace dimensions/throughput speed. Thus, if the throughput speed is reduced due to process conditions outside the carburizing process, the carburizing time is in turn determined by this throughput speed. This confirms that a certain amount of carburization can be achieved by adjusting the other control variables. In industrial carburizing, with regard to the relationship between the carburizing time and the throughput speed, only the composition of the gas atmosphere and the temperature of the metal strip need to be taken into account. In this case, if the reduced strip speed is within a certain range, it is also possible to add the carburizing time to the parameters of the above-mentioned formulas in order to ensure the accuracy of the adjustment.

Here, in the continuous carburizing according to the invention, for example, with regard to the necessary control of the carburization amount, even if the ranges of control of the strip temperature and carburization are the same or different as in cases where the heat treatment and carburization are carried out simultaneously and the carburization is carried out after the heat treatment by a certain reduction in temperature, the same control can be carried out taking into account, for example, the time aspect of the throughput speed. On the other hand, it has been found that whether the carbon concentration at a certain depth of the edge zone can be derived from a carbon diffusion model formula based on the so-called Fick's law which uses the carburizing time and the carburizing temperature as parameters. Experiments have confirmed this. Accordingly, in continuous carburizing of metal strip, it is possible to determine the carburizing time and strip temperature required for a certain carbon concentration at a certain distance from the strip surface by using the desired carbon concentration distribution with this carbon diffusion model formula. Moreover, for the low Al, high BH steel strip described above, the desired carbon concentration distribution requires a higher carbon concentration near the strip surface, that is, a narrower surface zone, and a lower carbon concentration in a zone further away from the strip surface, that is, at a greater distance from the strip surface. However, it was found that when the carbon concentration distribution in the metal strip is determined by the material specification of the carburized sheet, only the carbon concentration distribution at a distance of 10 to 250 µm from the metal strip surface needs to be adjusted. On the other hand, the carburization amount is also determined by integrating the carbon concentration distribution in the direction of the strip thickness. In addition, in the case of decarburization during cooling, Carbon concentration distribution curve, a maximum value of carbon concentration is at a depth of about 10 to 50 μm, and the carbon concentration decreases with increasing distance from the strip surface. Based on this, in the continuous carburizing of metal strip according to the invention, when the total carburization amount is constant, the carburization concentration is set at a point in a distance range of 10 to 50 μm based on the model formula for carbon diffusion, thereby achieving a culmination point of the carbon concentration distribution to finally determine the model formula for carbon diffusion. Even when the total carburization amount is different, the carbon concentration is set at a different point or multiple points in the distance range of 10 to 250 μm to finally determine the model formula for carbon diffusion. In this way, it is possible to set the strip temperature, the composition of the gas atmosphere and a carburizing time, which are the parameters of the carbon diffusion model formula, by calculating a carbon concentration distribution in which a carbon concentration exists at each point in the strip thickness direction that satisfies the above-mentioned carbon diffusion model formula and is within a predetermined tolerance range of a target value. Furthermore, assuming that the total carburizing amount is not fixed, it is also possible to set the carburizing amount by integrating a carburization concentration distribution in the direction of the strip thickness, which is obtained from the model formula for carbon diffusion. Furthermore, in the continuous carburization of metal strip according to the invention, it is of course possible to use the surface reaction rate of the above-mentioned area of carburization determined by the surface reaction.

Furthermore, in the continuous method for carburizing metal strip according to the invention, the carbon in solid solution in the edge zone of the metal strip is still in a state that allows diffusion or decarburization, and it is possible to keep the carbon in solid solution at a desired concentration by controlling the diffusion or decarburization of the carbon in solid solution by adjusting the strip temperature after carburizing, for example by means of the cooling rate of the steel sheet.

The invention is explained in more detail below with reference to the drawings.

Fig. 1 shows a diagram illustrating the heat treatment during continuous annealing and carburizing. Fig. 2 is a schematic representation of a continuous annealing and carburizing plant as it is suitable for controlling carburization according to the method according to the invention. for continuous carburizing of metal strip. Fig. 3 is a diagram illustrating the diffusion-determined region after the carbon concentration in the edge zone of the metal strip has reached the equilibrium concentration and the region determined by the surface reaction before the equilibrium concentration is reached. Fig. 4 is a flow chart of the algorithm containing the logic of the overall control in the continuous annealing and carburizing plant of Fig. 2. Fig. 5 shows the relationship between the temperature and a coefficient based on data obtained by changing the carburizing temperature with a view to calculating the temperature dependence of the rate coefficient of the surface reaction in the continuous annealing of metal strip according to the invention. Fig. 6 is a flow chart of the algorithm for controlling carburization by applying the continuous carburizing of metal strip according to the invention. Fig. 7 is a CO-H₂ diagram with the soot formation limit in the continuous carburizing of metal strip according to the invention. Fig. 8 shows the relationship between the calculated and the operationally measured value of the carburization amount from the algorithm of the example of Fig. 6. Fig. 9 shows a diagram to illustrate the various calculated carburization conditions with respect to a target amount by the algorithm of the example of Fig. 6. Fig. 10 relates to a diagram to illustrate various carburization conditions that are calculated to achieve a target amount under conditions where the belt speed was set by the algorithm of the embodiment of Fig. 6. Fig. 11 is a diagram showing an example of the distribution of carbon concentration in the case where the gas phase composition and the carburizing time were set by the algorithm of the embodiment of Fig. 6. Fig. 12 is a diagram showing an example of the distribution of carbon concentration in the case where the gas phase composition and the carburizing time were controlled by the algorithm of Fig. 6. Fig. 13 is a diagram showing an example of the distribution of carbon concentration in the case where the cooling rate after carburizing is set by the algorithm of the embodiment of Fig. 6. Fig. 14 is a graphical representation of the gas composition measured during operation and the composition calculated by the model formula for the composition of the gas atmosphere.

The following describes a best solution for implementing the invention.

Fig. 2 shows an example of a plant for the continuous annealing and carburizing of strip made of an ELC steel for implementing the inventive method for the continuous carburizing of metal strip.

According to Fig. 2, a strip A of an ELC steel enters an inlet station (not shown) with a discharge reel, a welding machine and a cleaning machine etc. and then into a preheating zone 1, a heating zone 2, a zone 3 for even-annealing, a carburizing zone 4, a first cooling zone 51, a second cooling zone 6 and an outlet station (not shown) with a shear and a reel etc. to carry out the control of the strip temperature as shown in Fig. 1.

In the heating zone 2, the strip, which is continuously fed from the input side and preheated in the preheating zone 1, is brought to the recrystallization temperature or a higher temperature, preferably to a furnace temperature of 850 to 1,000ºC, and the strip is heated so that it reaches a temperature of 700 to 950ºC. The heated strip A is kept at the recrystallization temperature or a higher temperature for a predetermined time in the equalization zone 3. It is possible to develop a (1,1,1) structure, which is advantageous for deep drawability.

Numerous radiation tubes are arranged in the vicinity of the strip path up and down over the hearth rollers in the heating zone 2 and the equalization zone 3, and a heating gas fed into the radiation tubes is burned there to adjust the internal temperature of the furnace (furnace temperature). By setting the inflow rate or quantities of the heating gas, an upper limit for the furnace temperature can be set with the aid of a host computer (not shown and described later) from the heat balance taking into account the heat transfer coefficient between radiant tubes, strip and hearth rollers, etc. This is done together with a throughput speed which ensures a residence time in the furnace (heating time, equalization annealing time) or in each heat treatment zone on the basis of a process model calculation, the upper and lower limits of the desired recrystallization temperature and an optimal path calculation in which an optimal time series of the throughput speed at the connection point between the coils is calculated, a calculation of the thermal roll deflection in which a maximum throughput speed is calculated by predicting and calculating the hearth roller deflection or the like. In the present case, the setting of the flow rate or amount of heating gas into the radiation tubes corresponds to the amount of heat required by the furnace, which takes into account the heat input and heat losses in the furnace, which is obtained by adding the heat losses in the exhaust gas and the radiation of the furnace body to the amount of heat supplied to the belt that removes heat when leaving the furnace. This is possible with the help of the host computer, not shown in connection with the control algorithm of the entire treatment line described below.

In the carburizing zone 4, in order to form a carburized phase in a surface layer of the strip A in which carbon in solid solution is present in a very thin layer (surface layer) of the strip surface, a strip temperature of 700 to 950°C and a strip speed are set by means of the host computer (not shown) so that the strip passes through the furnace for 10 to 120 seconds at a temperature of 700°C or more, preferably at the recrystallization temperature or lower. This is done so that the carburization amount (carburization reaction rate x carburization time) is constant over the strip length and deviations from the material characteristics are suppressed. In this respect, the furnace temperature control is carried out so as to avoid the problem that when the strip temperature is below 700°C, the carburization reaction rate and the heat treatment productivity are reduced, while when the furnace temperature is above 950°C, the material properties are deteriorated. The temperature control serves to maintain the carburizing conditions. In addition, it is known that the occurrence of soot, ie free carbon, on the strip surface impairs the cementation behavior and quality and has a detrimental effect on downstream processes. On the other hand, the carburizing reaction is impaired if the reaction in the furnace is directed in a predetermined direction, for example in the direction of the carburizing reaction. and as a result the dew point is raised, the carburizing reaction is impaired and the strip surface is oxidized and temper colors appear. For this reason, the physical properties in the furnace and the furnace temperature are strictly controlled in accordance with the algorithm setting the carburizing conditions, as will be described below.

The composition and the inflow and outflow rates of the carburizing gas fed into the carburizing furnace are controlled in accordance with various conditions calculated by the host computer on the basis of a thermodynamic model formula (composition of the gas atmosphere) which minimizes the free energy in the furnace taking into account the material input and output, as will be described later. The composition and the inflow and outflow rates of the carburizing gas are controlled by suppressing a rise in the dew point so that soot formation does not occur, nor does it reduce the rate of the carburizing reaction and tarnish. It goes without saying that the primary priority is given to the specification factors of the strip including the carbon concentration distribution and the carburization depth, etc. of the carburizing layer formed on the strip surface, which will be described later. The composition and the inflow and outflow rates of the carburizing gas are calculated with regard to the above-mentioned flow rate and the furnace temperature.

The physical conditions in the furnace, the furnace temperature, the strip temperature, the strip travel speed, i.e. the carburizing time and the composition of the gas atmosphere are considered as physical variables (control variables) that require control during continuous carburizing. For example, the host computer determines the required carburizing amount from the material characteristics including the carbon concentration distribution and the carburizing depth, etc. of a desired carburizing layer on the strip surface, and each control variable relevant to the carburizing amount is calculated by a suitable selection of various basic formulas related to these preset control variables, as will be described below. The control variables are determined taking into account the capabilities and methods of other systems.

The strip is guided up and down over the hearth rolls 10 and is kept in a predetermined state to maintain the rolling properties and the roll deflection; for example, adjacent bearings or the like are cooled. To maintain the strength and wear properties of the rolls themselves, they are made of a chromium alloy. When the carburizing gas is introduced into the When the heat-resistant alloy of the hearth rollers is introduced into the vicinity of the hearth rollers, it is cooled and soot is formed, so that carbon, after adhering to the hearth rollers, diffuses into the interior of the hearth rollers. When this happens, chromium and carbon are combined and carbide precipitation occurs. As a result, the grain structure of the heat-resistant hearth roller alloy is broken or expanded. On the other hand, the hearth rollers become brittle and oxidized due to depletion of solid-solution chromium, so that pore corrosion progresses. In this regard, when the hearth rollers come into contact with the carburizing gas atmosphere, as the inventors have proved through experiments, the hearth rollers must be replaced within two years. According to the invention, therefore, a hearth roller chamber is separated from the carburizing atmosphere by a non-contact seal 11, so that destruction of the hearth rollers does not occur. Moreover, the interior of the hearth roller chamber is in such a slightly carburizing state that destruction of the hearth rollers does not occur. In this way, the so-called decarburization, in which carbon is taken up from the carburized surface layer while the strip is passing through the separated hearth roller chamber, was successfully prevented. If the passage time in the hearth roller chamber is very short and the decarburization of the surface zone of the strip is therefore not a problem in view of the passage time, then the interior of the Hearth roller chamber must have a non-carburizing atmosphere.

The structure of the seal 11 is not described in detail, but it may be, for example, a sealing layer between the hearth roller chamber and the carburizing chamber made up of three sub-layers, and the above-mentioned light carburizing gas may be blown into the sealing layer on the hearth roller chamber side while being exhausted from an intermediate layer. Moreover, the blowing direction and the flow rate of each gas are controlled so that the flow of each gas is controlled into the intermediate layer side and at the same time the circulating flow caused by the passing strip is discharged through an outlet on an end face of the sealing layer which is located in the range of the width of the strip.

The strip leaving the carburizing zone 4 first enters the first cooling zone 5. In order to fix the carbon from the carburizing zone which is in solid solution in the very thin surface layer of the strip, the strip is cooled rapidly after carburizing in the first cooling zone 5 to a temperature of 600ºC or below, preferably at a cooling rate of 5ºC/s, until about 500 to 400ºC is reached. In order to ensure the necessary cooling conditions in the cooling zone 5, the host computer sets the amount, the flow rate, the cooling roller angle as well as the wrap angle and the like of the gas directed from a cooling gas nozzle against the strip entering the cooling zone.

The strip leaving the cooling zone 5 enters a second cooling zone 6. In this cooling zone, gas cooling continues until the strip reaches a temperature of 250 to 200ºC. In this way, it is ultimately possible to produce a cold-rolled sheet made of an ELC steel suitable for press forming and in which the carbon in solid solution in the surface zone is controlled in quantity and shape.

The following explains the idea of a general control of continuous annealing and carburizing with the help of the host computer. For ease of understanding, the strip temperature resulting from the carburizing reaction is referred to as the carburizing temperature, although the above explanations make it clear that the main control variable is the furnace temperature.

As already mentioned, when carburizing in the carburizing zone, including setting a certain carbon concentration distribution in the strip, the carburization amount in the strip is specified as the target value of the material. If, for example, a certain carbon concentration distribution is required, the carbon amount is obtained by integrating the carbon distribution over the strip thickness. The upper limit of the carburizing temperature is determined by the recrystallization temperature or a lower temperature of the material in question. On the other hand, in order to utilize the maximum carburizing capacity of the carburizing furnace, it is necessary to increase the rate of carburization based on the principle of carburizing amount = rate of carburizing reaction x carburizing time; therefore, it is desirable to increase the carburizing temperature linked to the rate of carburizing reaction, which in turn is associated with an increase in the upper limit of the carbon monoxide concentration.

According to the invention, the soot limit can be determined using the thermodynamic model formula (composition of the furnace atmosphere) taking into account the material input and output; however, it is difficult to determine the carbon monoxide concentration and a hydrogen concentration in the furnace gas solely from the condition that no soot formation occurs. Therefore, the invention is based on a reference formula that does not affect the speed of the carburizing reaction and uses, for example, the carbon monoxide concentration from the model formula for the composition of a gas atmosphere that does not lead to soot formation as a reference value and calculates the hydrogen concentration using the reference formula, as follows:

H₂ concentration = a x CO concentration,

where a is a constant in size from 0 to less than 5 .

The constant a is calculated according to the basic formula for the surface reaction described below to be a value which minimizes the concentration of CO and H₂O and is normally in the range of 0.5 to 1.0. Accordingly, the rate of the carburizing reaction from the formula for the surface reaction rate takes a maximum when the reference formula is satisfied.

Furthermore, in this embodiment, the carburizing time is determined with respect to a certain distribution of the carbon concentration on the basis of the surface reaction rate determined as mentioned above. In other words, if the concentration gradient of the carbon in the inner part of the peripheral zone is to be made steep by increasing only the carbon concentration in the surface zone, only the speed of the carburizing reaction (increase in carburizing ability) needs to be increased and the time of carburizing needs to be reduced. Conversely, if the concentration gradient of the inner part of the peripheral zone is to be made gradual due to increasing the carbon concentration of the strip, only the carburizing time needs to be increased and the carburizing rate (decrease in carburizing ability) needs to be reduced. The control of the speed the carburization reaction and time satisfies the above-mentioned limiting condition that the carburization amount is constant.

On the other hand, as already described in connection with the heating zone and the equalization zone, in the relevant strip temperature control zones, except in the carburizing zone, an optimal strip speed is determined by means of a performance and process calculation of the furnaces concerned. If one considers the maximum strip speed in each of the temperature control zones and the maximum strip speed in the carburizing zone of the continuously operating annealing and carburizing plant, through which the strip gradually passes, then it depends which of the strip speeds determines the throughput speed of the entire plant. In this case, all the requirements of the sheet specification must be taken into account, and these continue to be considered as absolute conditions.

From the above description, it is clear that if the maximum strip speed in the carburizing zone is greater than the minimum of each maximum speed in each of the temperature control zones, then it is necessary to set the minimum of the maximum strip speed of each strip temperature control zone as the strip speed of the annealing and carburizing section and again determine the conditions of the atmosphere in the carburizing furnace which, at this passing speed, satisfies the above-mentioned carburizing amount. In this case, This occurs because the carburizing time is prolonged under the restrictive condition that the carburizing amount is constant, in turn in the direction of a reduced speed of the carburizing reaction, ie the CO and H₂ concentration in the gas atmosphere are reduced, and accordingly the condition that no soot formation should occur is met.

If, however, the minimum value of the maximum run speeds in each temperature-controlled zone is equal to or greater than the maximum strip speed in the carburizing zone, then the maximum strip speed of the carburizing zone serves as the run speed of the entire line and the furnace temperature and the amount of heating gas supplied must in turn be determined as control variables for the strip temperature in order to ensure the strip temperature in each of the temperature-controlled zones at the relevant run speed. The control is reflected in the algorithm shown in Fig. 4, which is executed by the host computer.

In the computer work, an upper power limit is initially set as a limiting condition in the carburizing zone and each temperature-controlled zone in stage S 20, and a maximum value of the strip speed is set, which corresponds to the requirements for heating and cooling of the steel strip. For example, for the heating zone 2 and the compensation zone 3, a Model formula for the process is determined based on the heat balance, taking into account the heat transfer between the radiant tubes, the furnace wall, the belt and the hearth rollers, etc. On the basis of this model formula for the process, a maximum value of the belt speed (hereinafter referred to as the maximum belt speed) is then determined within the range of the furnace temperature, the amount of heating gas or the heat output of an electrical heater (which is also possible), namely 50, so that the calculated maximum value ensures the specified belt temperature.

On the other hand, for carburizing zone 4, a model of the composition of the gas atmosphere in the carburizing furnace is created on the basis of a mathematical model formula based on the thermodynamics described below, taking into account the material input and output in the carburizing furnace. Using this model of the composition of the gas atmosphere and the formula for the speed of the carburizing reaction, a maximum belt speed is calculated that is equal to or greater than the upper limit of the composition of the gas atmosphere (in particular CO) and takes into account the desired amount of carbon.

Furthermore, a maximum belt speed is calculated for cooling zones 5 and 6 on the basis of a model formula that takes into account the cooling gas from the cooling gas jet and the heat transfer, which lies within the range of the possible cooling gas supply and takes into account the desired cooling rate or the desired final cooling temperature.

In this respect, for cooling zones 5 and 6, in case of using cooling rollers or cooling mist instead of gas cooling as the cooling medium, similar calculations can be carried out using a model formula that takes into account the respective medium and the heat transfer of the belt.

The maximum strip speeds calculated in the manner described above are then compared in each of the heat treatment zones, including the carburizing zone, and the lowest value is set as the maximum strip speed in the entire system.

Next, in step 21, using the maximum strip speed over the entire section from step S 20, a value of the control variables is determined for each heat treatment zone, including the carburizing zone, which takes into account the requirements of strip heating, carburizing and cooling.

In detail, for example, a furnace temperature is determined for the heating zone 2 and the equalization zone 3 using the heat transfer model of stage 5 20, which takes the desired strip temperature into account. This strip temperature can be adjusted by controlling the heating gas supply or the Adjust the power of an electric heater using the feedback method. Alternatively, the strip temperature can also be adjusted by calculating an optimal flow rate or power of an electric heater using a path optimization based on the process model calculation described above, which minimizes the fluctuations in the strip temperature at the connection point between two coils in the strip. Feedback control is then possible based on the result.

On the other hand, four cases are conceivable for the carburizing zone, in which the target value includes only the carburization amount or, together with the carburization amount, the desired form of distribution of the carbon concentration in the direction of the strip thickness. If only the carburization amount is specified, then using the model of the gas atmosphere described in connection with stage S 20 and the formula for the carburization reaction on the steel surface, a gas composition is calculated that takes into account the desired carburization amount. In the other case, if in addition to the carburization amount, the form of distribution of the carbon concentration in the direction of the strip thickness is specified, the model of the composition of the gas atmosphere and the formula for the speed of the carburization reaction is used to apply the diffusion model in the steel strip, taking into account not only the carburization time but also the cooling phase, and again the strip speed is set so that it within or below the maximum belt speed for the entire system determined in stage S 20, thus making it possible to determine the desired form of distribution of the carbon concentration in the direction of the belt thickness. At the same time, the composition of the gas atmosphere is calculated taking into account the desired amount of carburization. In this case, the newly determined belt speed corresponds to that of the adjoining part of the system. However, the logic of determining the belt speed is such that the form of distribution of the carbon concentration in the direction of the belt thickness satisfies the predetermined value of the belt thickness in stage S 21. In order to avoid changing the belt speed and re-determining it for other reasons, the belt speed ensuring the form of distribution of the carbon concentration in the direction of the belt thickness is preferably determined in stage S 23. This is done accordingly in the case of the embodiment in stage S 23.

For cooling zones 5 and 6, using the heat transfer model described in connection with stage S 20, the flow rate of the cooling gas jet is set by means of several revolutions of a fan or the like in such a way that it takes into account the specified cooling rate and time.

Next, in stage S 22, the thermal deflection of the hearth rollers in each heat treatment zone, including the carburizing zone, is predicted and calculated using a model of the strip temperature and the heat balance of the roller chamber, as well as a maximum throughput speed that lies within the limits for the generation of jets and the occurrence of bulges on the strip, i.e. the so-called thermal deflection calculation is carried out. If the calculated maximum strip speed exceeds that of the strip speeds calculated in stages S 20 and S 21, then the transition to stage S 23 takes place. If, however, the calculated maximum speed is below the maximum speed of the entire treatment section determined in stages S 20 and S 21, then the maximum belt speed obtained from the calculation of the thermal roller deflection serves as the new belt speed of the entire treatment section and is entered into the preceding stage S 21.

In stage S 23, a target belt speed is set taking into account special operations on the belt such as welding operations to join the belt of two coils, testing of the coil and the like or other operations (mainly problems) and first ensuring that the specified belt speed is equal to or lower than the maximum belt speed set in stages S 20 and S 22. This belt speed is then set for the entire treatment section.

Then, in stage S 24, taking into account the last determined strip speed, the control variables are calculated for the entire treatment section, which meet the requirements for heating, carburizing and cooling the strip in each treatment zone, including the carburizing zone. In this stage, the calculation is similar in content to stage S 21, although the strip speed is not determined and calculated due to the shape of the distribution of the carbon concentration in the direction of the strip thickness.

When explaining the logic relating to the carburizing zone 4, the description of the control of the strip temperature with regard to the specified strip temperature is omitted, although the control of the strip temperature in the carburizing zone can be regarded as identical in content to the control of the strip temperature in the heating zone 2 and the compensation zone 3.

The control of the carburizing atmosphere in the carburizing zone is explained in more detail below.

First, it is explained how, based on the material properties for the steel strip, a press-formable steel strip with low AI, high BH value and sufficient strength can be produced under the conditions of the invention in comparison to the conditions of conventional carburizing and using the parameters for the carburizing conditions.

In conventional carburizing, surface hardening takes place in order to improve wear properties and reduce the impact sensitivity of individual objects in the form of so-called thermal tempering, such as gears, shafts and bearings, etc. The carbon content of the starting material is at least 0.05% and the carburization amount is at least 0.1% with a carburization depth of at least 0.5 to 1.5 mm. The carburization time is accordingly long, from 1 to 5 hours. Under these conditions, the carbon concentration reaches the equilibrium concentration in relation to the carburization time and the carburization rate is located in the diffusion-determined region of the diagram in Fig. 3, in which the carburization rate follows the diffusion rate in the steel. Accordingly, the carburization rate is proportional to the square root of time. In this range of carburizing rate, it is necessary to adjust the carbon potential (C potential) of the gas atmosphere so that the diffusion rate in the steel is equal to the surface reaction rate and the equilibrium concentration of carbon in the steel takes a predetermined value. As an actual index for process control, the adjustment of the CO/CO₂ ratio is important.

In contrast, in the continuous strip carburizing according to the invention, the strip is a continuum of an ELC steel and the carburizing serves to improve the surface properties of the strip and the material properties as a whole. Consequently, the carbon content in the starting material is at most 200 ppm or less and the carburization depth is only 50 to 200 µm if the carburizing conditions for the strip are based on the data of the material specification (Japanese Laid-Open Patent Application Hei 3-199 344, etc.), which are aimed at reducing the sensitivity to embrittlement during further processing. Furthermore, the carburizing time is 120 seconds or less depending on the throughput speed. Under these conditions, since the carbon concentration in the surface layer of the steel strip does not reach the equilibrium concentration in view of the carburization time, as follows from the above-mentioned statements by Yo et al., the carburization rate is in the area determined by the surface reaction in Fig. 3, in which the carburization rate follows the reaction rate at the steel surface and the carburization rate is proportional to time. In the area governed by the surface reaction, since both the carburization amount and the carburization depth are not dependent on equilibrium, as the operational process shows, it is necessary not only to control the CO/CO₂ ratio by controlling the C potential in the with regard to the equilibrium concentration of carbon in the surface zone of the steel strip, but also to select the carburizing conditions in such a way that the carburizing amount determined on the basis of the material specification of the steel sheet takes into account numerous control variables in the furnace.

In addition, in the continuous annealing and carburizing operation, there are many cases, such as shown in the algorithm of Fig. 4, in which the strip speed is determined from the control of the strip temperature, which takes place in the heat treatment zones excluding the carburizing zone. In other cases, the strip speed must be controlled in view of its greatest response speed with respect to various process conditions. Accordingly, in the continuous carburizing according to the present invention, when the strip speed is limited in view of the process conditions in the continuous annealing and carburizing excluding the carburizing phase, the carburizing conditions specified by the material specification of the steel sheet at the above-mentioned strip speed must be selected so as to take into account the carburization amount.

The following describes the principles for constructing the logic in accordance with the control system used by the host computer to control the carburizing amount. The algorithm provided by the method according to the invention is explained.

First of all, in order to adjust the composition of the gas atmosphere in the range determined by the surface reaction, it is necessary to prevent the previously described formation of soot and at the same time to avoid an increase in the dew point. The basis for these conditions is as follows.

In general, the composition of the gas atmosphere under carburizing conditions can be determined from the chemical equilibrium. In conventional solutions, all conceivable reactions are listed and the gas composition is determined by solving non-linear simultaneous equations from the equilibrium relationship of the reactions. However, it is very difficult to obtain an exact limit for soot formation from the reaction formula of the gas phase system alone.

The invention is therefore based, as described below, on a thermodynamic model formula (composition of the gas atmosphere) and arrives at a composition of the gas atmosphere that prevents soot formation.

In the case of an isothermal and isostatic system, the Gibbs free energy decreases during a naturally occurring change and takes a minimum value in the equilibrium state in the system In order to obtain the equilibrium state of the gas atmosphere using the resulting Gibbs free energy as an objective function for the entire system with a variable concentration of each gas component of the system, it is only necessary to obtain the concentration of each gas component which assumes a minimum under the restrictive condition of the material input and output at which elemental components introduced by the original system are constant, in particular, under the restrictive condition that the composition of the gas atmosphere and the amount fed into the furnace as well as the amount of carbon discharged from the metal strip from the furnace as a result of carburization are constant. The concentration of the gas components reaches the equilibrium composition of the gas atmosphere in the furnace at a given furnace temperature and a given pressure in the furnace, and the amount of carbon forming soot is expressed in the logic described below as a kind of condensation.

When calculating the composition of the gas atmosphere, there are two assumptions. One of the two is that the gas is an ideal gas. The other assumption is that the condensation phase represented by the free carbon does not mix with the gas. Under these assumptions, the total free energy F(X) of a gas and a condensation can be expressed by the following equation (1) with reference to the free energy fgi of the i. type of a gas and the free Energy fgh of the h. type of condensation can be expressed as:

where n is the number of gases and p is the number of condensations.

In this regard, the free energy fgi of the 1st gas with respect to the gaseous product is expressed by the following equations (2) to (4) assuming that the number of moles of the individual gas is xgi with respect to the free energy cgi of the i. gas.

On the other hand, as regards the condensation product, since the influence of pressure and mixing is excluded, under the above-mentioned assumptions, the free energy fch of the h. condensation is expressed by the following equations (5) and (6), assuming that the number of moles of the condensation in question is xch with respect to the molar energy Cch of the h. condensation.

fch = Xch Cch (5)

Cch = (F/(R T))ch (6)

In equations (3) and (6), (F/(R T) is defined by the following equation (7):

(F/(R T))i = ((F - H298 )/T)i/R + ΔH0 f.298,i/RT (7)

Next, the input and output of substances in this system are considered. Even if each amount of a component in the operating system is changed, the total amount is constant if each element is considered as an atomic unit of carbon, hydrogen, nitrogen and oxygen in the components of the gas atmosphere. The input and output of substances is expressed in the following equation (8):

This is

j = 1,2, ..., m

agij: the number of atoms of the jth element in a molecule of the ith gas,

acjj: the number of atoms of the jth element in a molecule of the i. condensation,

bj: the set of the jth element in the system and

m: the number of different elements in the system.

As can be seen from Fig. 14 regarding the composition of the gas in the furnace, the calculated results are in good agreement with the operational measured values.

Next, regarding the necessary conditions of the composition of the gas atmosphere during operational carburizing, the carbon balance in the furnace is represented by the following equations (9) and (10). In this respect, equation (10) is a function calculated from the property profile of the material and the speed of the surface reaction.

Wgi = Wsc + Wgo (9)

Wsc = ξ (V,t,w,LS)) (10)

Here,

Wgi: the mass of carbon in the gas fed into the furnace,

Wsc: the mass of carbon discharged from the belt from the furnace,

Wgo: the mass of carbon removed from the furnace by the gas,

V: the rate of the surface reaction,

t: the carburizing time and

w: the bandwidth.

In this way, by calculating the parameters of the gas atmosphere on the basis of a thermodynamic model formula (composition of the gas atmosphere) taking into account the material input and output during continuous carburization in the carburizing furnace, it is possible to improve the carburizing capacity of the gas atmosphere in comparison with the parameters of the gas atmosphere that result without taking into account the material input and output in the furnace, and at the same time to reliably prevent soot formation. Accordingly, the operational performance can be improved by, for example, increasing the throughput speed with the help of a higher CO concentration in the gas atmosphere.

Next, the basics of adjusting the carburization amount, which is the core of the invention, are explained.

The surface reaction when using carbon monoxide as a gas atmosphere is given by the following equations (11) to (13).

CO [C] + O (11)

CO + O T CO₂ (12)

Fe + [C] T Fe - C (diffusion in steel) (13)

According to the above-mentioned report by Yo et al., the carburizing conditions do not reach the equilibrium state if the carbon concentration in the edge zone of the sheet is very low. and the duration of carburization is very short. For this reason, it can be assumed that since the reaction rate in equation (13) is greater than the release reaction of the adsorbed oxygen according to equation (2), this reaction is a rate-determining reaction and the rate of the surface reaction V in this surface reaction-determining region can be expressed by the following equation (14):

V = k PCO (PCO/(PCO + (ac/K))), (14),

where

k : a reaction constant, Pco: the CO partial pressure, ac: the carbon activity and K: the equilibrium constant.

However, equation (14) does not take into account the influence of hydrogen. As far as the reaction equation regarding hydrogen is concerned, the following equation (15) replaces the reaction equation (12):

CO + H2 + 2O T CO&sub2; + H2 O (15)

Furthermore, the following equation (16) applies to the resulting CO₂.

H&sub2; + CO2 H2O + CO (16)

Based on these reaction equations and considering the fact that hydrogen promotes the carburizing reaction, the rate V of the basic surface reaction is given by the following equation (17):

V = k, f&sub1; (PCO, PH2 ?o), (17),

where θo is the rate of formation of a coating of adsorbed oxygen.

Furthermore, when the concentration of carbon monoxide and hydrogen in the gas atmosphere generated by carburization is high (for example, CO/CO2 ≤ 50), the carburizing reaction is disturbed by the reaction according to the following equations (18) and (19):

V = k&sub1;f&sub1; (PCO, PH2 ?o) x ? f&sub3; (PCO, PCO2 ) (20)

V k1 f1 (PCO, PH2 , θo) - k2 f&sub2; (PCO2 , PH2 O), (21)

where α: constant and k₁ and k₂: are rate constants of the reaction.

The rate constants k₁ and k₂ can be determined according to the following equation (22):

k&sub1; = Ai exp ( - Egg/RT) (22)

where Ai : the frequency factor, Ei : the activation energy, R: the gas constant and T: the absolute temperature.

Since the frequency factor Ai, the activation energy Ei and the gas constant R are constants, the rate constants k₁ and k₂ are calculated from experimentally obtained values under the condition of different absolute temperatures T. Fig. 5 shows the experimentally determined constant k₁ of the reaction rate.

If it is only necessary to consider the carbon monoxide concentration, for example if the inflow velocity of the gas is high, then equation (14) can be used as a formula for the velocity of the surface reaction.

Next, the diffusion of carbon in solid solution is explained, where the diffusion in steel is taken as a model to obtain the desired distribution of carbon concentration. The diffusion state of carbon in steel is represented by the model formula for carbon diffusion of the following equation (23) based on Fick's law:

dC/dt = D d²C/dx² (23),

where C : the carbon concentration in the steel, t: the time, D: the diffusion coefficient and X: the diffusion distance.

The diffusion coefficient D results from the Arrhenius formula of the following equation (24), which in the embodiment is approximately represented by operational measurement data.

D = exp (a T⁻¹ + b), (24)

where T : the carburizing temperature, A: a proportionality coefficient and b: a constant.

Accordingly, the carburization amount in the steel sheet can be calculated according to equations (17) or (21) or (22) and (23). That is, under the assumption of a constant carburization amount, when the carburization concentration at one point of the desired carbon concentration distribution is taken out, the above-mentioned carbon diffusion model formula is used, and even if the carburization amount is different, when the carburization concentration is taken out at two or more points of the desired carbon concentration distribution. Furthermore, as described above, when the passage speed is limited by the process conditions other than carburization, because the carburization time t is set to a value obtained by dividing the effective length L of the carburizing furnace by the passage speed LS, the calculated value is applied in integrating equation (23) over the carburization time.

Fig. 6 shows a flow chart of the algorithm for determining the carburizing conditions, by which the above-mentioned calculations are carried out step by step using a program previously stored in the host computer. Under this carburizing condition, the specification of the steel sheet after carburizing, i.e. the carburization amount in the strip, which results from the desired distribution of the carbon concentration, coincides with the carburization amount in the strip calculated from the reduced amount of carbon in the gas atmosphere in the embodiment.

First, in the S 1 step, from the specified conditions given as the specification of the steel sheet after carburizing, the conditions such as the composition of the gas atmosphere, the flow rate or amount of the inflowing gas, the carburizing temperature and the passing speed of the strip, as well as from the steel sheet factor and the distribution of the carbon concentration, the conditions such as the carbon concentration C, at a selected distance X, from the strip surface are read in. In addition, the strip speed is represented here by LS, and this is a parameter that is subsequently modified.

Next, in stage S 2, a carburization amount ΔC in the steel sheet is determined based on the sheet specification, and a carbon amount per Time unit that the sheet is discharged from the carburizing furnace is calculated.

Then, in stage S 3, the above-mentioned model formula of the composition of the gas atmosphere is determined on the basis of the gas atmosphere read in stage S 1.

Then, in stage S 4, the amount of carbon discharged from the carburizing furnace by the strip is calculated in accordance with the model formula for the composition of the gas atmosphere in stage S 3, taking into account the concentration of each component of the gas atmosphere.

In stage S 5, the speed of the surface reaction of the steel strip is then calculated based on equation (17).

Finally, in stage S 6, the carburization rate in the steel and the amount of carbon diffused into the steel are calculated based on equation (23).

Then, if the time for the carburizing treatment has expired, step S 7 is carried out and the speed of the surface reaction or the amount of carbon diffused per unit time and area, calculated in step S 5 or S 6, is calculated over the treatment time and the entire surface of the steel strip and the carburization amount ΔC' in the steel strip is calculated.

In stage S 8, the absolute value of the difference between the specified carburization amount ΔC and the calculated carburization amount ΔC' is then checked to see whether it is smaller than a specified value a or not, and if the absolute difference value is smaller, the specified value a goes into stage S 10 or, if not, into stage S 9.

In stage S 9, based on the above-mentioned carburization amount, the specified carburization amount is corrected using the following equation (25) and enters stage 5 3:

ΔC = ΔC + (ΔC' - ΔC) x b, (25),

where b : is a constant.

Accordingly, the transition to stage 10 takes place when the total amount of carbon discharged from the carburizing furnace and the total amount of carburization are equal to each other, i.e. when the material input and output of the carburizing furnace are fulfilled in stage S 10.

In step S 10, the absolute value of the difference between the predetermined carburization amount ΔC₀ and the specified carburization amount ΔC is checked to see whether it is smaller than a predetermined value or not, and if the absolute value of the difference is smaller than the specified value d, the transition to stage S 12 takes place, if not to stage S 11.

In step S 11, in order to obtain the carburization amount specified based on the assumed carbon concentration distribution condition, any one or more parameters of the gas flow rate of the gas atmosphere, the composition of the gas atmosphere, the belt speed and the carburization temperature are changed, and step S 2 follows. Here, if the belt speed LS is correct, in order to correct the difference between the specified carburization amount ΔC₀ and the specified carburization amount ΔC, the belt speed LS, if it needs to be corrected, only needs to be calculated, for example, based on the following equation (26):

LS = LS + (ΔC - ΔC0 ) x d', (26),

where d' : is a constant.

In step S 12, a carbon concentration C'₁ at a selected distance X from the strip surface is calculated in accordance with the model of diffusion in the steel defined in step S 6.

Then, in step S 13, it is checked whether the absolute value of the difference between the carbon concentration C₁ read in step S 1 at the selected distance X from the strip surface and the carbon concentration C'₁ calculated in step S 12 at the selected distance X from the strip surface is less than a predetermined value e or not and, if the absolute value of the difference is less than the predetermined value, the transition to step S 15 takes place, and if not, to step S 14.

In stage S 14, in order to obtain the carburization amount determined based on the condition for the distribution of the carbon concentration, one or more of the parameters of the composition of the gas atmosphere, the strip temperature and the carburization temperature are changed and proceed to stage S 2.

In the step S 15, each specified value of the concentration of the components of the gas atmosphere or the belt speed or the carburizing temperature is output as a result of the above-mentioned calculation in accordance with the purpose of control, and at the same time, the calculated results such as the total carburizing amount, the average carburizing amount and the carbon distribution, etc. are output. The program is then complete.

In the flow diagram of Fig. 6, the gas velocity or quantity as described above, on the feeding side represents a variable control variable for changing the CO₂ and H₂O concentration in the furnace atmosphere. As far as the control variables are concerned, the composition of the furnace atmosphere should be included, similarly to the amount of CO + H₂ fed into the furnace.

The diagram in Fig. 7 shows the soot limit for each temperature in the form of a solid line, which was calculated according to the program taking into account the material input and output under operational conditions, i.e. a belt speed LS of 200 mpm, a belt thickness D of 0.75 mm, a belt width of 140 mm and a gas quantity of 100 Nm³/h. The diagram in Fig. 7 also shows the upper dew point limit in the form of a broken line. Furthermore, the dash-dotted line shows the soot limit if the material input and output are not taken into account, and finally the hatching in the diagram in Fig. 7 indicates an operational working range for carburization.

In detail, the diagram in Fig. 7 with the soot limit taking into account the material input and output on the one hand, compared with the soot formation limit without taking into account the material input and output on the other hand, shows that both the CO and H₂ concentration increase. Thus, the carburization rate increases with increasing concentration. On the other hand, if one follows the course of the curves for the soot limit, the CO and H₂ concentration is greater the higher the carburizing temperature is. Accordingly, the overall effectiveness of carburizing also depends on the temperature. Conversely, if the belt speed is high, the degree of freedom of action is also great and allows the furnace temperature to be increased as far as the material properties allow. In this way, the range of permissible conditions for continuous carburizing in operation is increased. Of course, even if the operating condition is at the soot limit without taking into account the material input and output, soot formation does not occur. However, the degree of freedom in choosing the operating conditions is reduced and the range of possible process conditions is narrowed.

Furthermore, the diagram in Fig. 8 shows the relationship between the carburization amount calculated according to the program, ie in the case of a change in each control variable on the one hand, and the carburization amount measured during operation on the other. The diagram shows that the calculated carburization amount and the actually measured value essentially coincide. This means that the choice of the carburization rate, ie the speed of the surface reaction, and the choice of the temperature dependency coefficient are correct. Furthermore, it can be seen that, as long as the choice of the speed of the surface reaction is correct, the continuous carburization according to the invention can be used in a wide range in which the Carburization rate follows the rate of surface reaction, which is greater than the diffusion rate.

Furthermore, concrete calculation examples of the control variables for the carburization quantity calculated according to the program are explained in more detail on the basis of the diagram in Fig. 9.

Based on the sheet properties read in stage S 1, for example the sheet thickness, the specified carburization quantity, as shown in Fig. 9, and at the same time the tolerance range of the sheet thickness were determined in stage S 2. Furthermore, the intended carburization temperature was determined in stage S 1 based on the material properties of the steel sheet.

Accordingly, in stages S 3 and S 4, the CO and H₂ concentrations were set as conditions for the gas atmosphere to suppress soot formation.

Assuming that in practice the control accuracy of the concentration of the gas components is + 0.3%, the target carburizing time and at the same time the tolerance range for deviations in the carburizing time were determined according to equations (17) to (23) in the stages S 3 to S 11, as illustrated in the diagram in Fig. 9.

Then, with reference to the length of the carburization zone, the throughput speed is as follows:

Belt speed = length of the carburizing zone in the furnace/carburizing time

In stage S 12 the intended belt speed and its tolerance range are determined and output.

In this way, the carburizing time (belt speed) was determined at the time of determining the carburizing amount and the composition of the gas atmosphere in the loop of stages S 10 and S 11.

As described above, according to the invention, in the range with the carburization rate determined by the speed of the surface reaction, it is possible to optimize the various carburization conditions required by the sheet properties for the necessary carburization amount with respect to the overall process conditions and to fully automate the control measures which are based on experiments in conventional processes.

Furthermore, the diagram in Fig. 10 shows the values calculated according to the program for the case of a strip speed limited by the process conditions with the exception of carburization. Calculation examples of the control variables for the carburization quantity are explained in more detail.

For example, the specified carburization quantity was determined as a target in stage S 2 based on the sheet properties read in stage S 1, such as the sheet thickness or the like. In addition, the carburization temperature was determined as a target value in stage S 1 based on the material properties of the steel sheet. Furthermore, the carburization time was calculated by dividing the furnace length effective for carburization by the belt speed.

Next, in steps S 3 and 5 4, the upper limits for the CO and H₂ concentrations as conditions of the gas atmosphere for suppressing soot formation were determined.

In contrast, in steps S 3 to S 9 of the flow sheet, the formula for the rate of the surface reaction and the model formula for the diffusion in the steel were determined and, based on these formulas, the CO, H₂, CO₂ and H₂O concentrations required to achieve the required carburization amount were determined.

Accordingly, as shown in the diagram of Fig. 10, the composition of the gas atmosphere is controlled, for example, such that the CO concentration in the gas atmosphere increases as the predetermined carburization amount increases, while in the In the case of a smaller predetermined carburization quantity or an increase in the carburization time, the gas atmosphere is controlled in such a way that, for example, the CO concentration in the gas atmosphere is reduced.

In this regard, it is possible to change the composition of the gas leaving the carburizing furnace at the carburizing temperature, for example, in terms of the CO + H₂ concentration, the ratio of the amount of CO and the amount of H₂ in the gas fed into the carburizing furnace, and the total amount of gas in terms of the CO₂ and H₂O concentrations.

As already described above, the invention allows the various conditions for the carburization amount resulting from the sheet properties to be optimized, taking into account all the process conditions, even in the case of a given limitation of the strip speed. Accordingly, it is possible to optimize the control measures based on experiments in conventional processes.

In the following, with reference to the diagrams in Fig. 11 to 13, calculation examples are explained in which the desired distribution of the carbon concentration in the sheet was calculated using the model formula for carbon diffusion based on Fick's law. According to the algorithm in Fig. 6, the carburization amount per Unit area was determined by integrating the carbon concentration distribution in the sheet thickness direction, and the carbon diffusion model formula was derived from the desired carbon concentration distribution under the restrictive condition of satisfying the carburization amount. Then, a tolerance range for a target value at each point in the sheet thickness direction and the carburization temperature and time were determined, which are parameters of the model formula selected so that the carburization concentration profile calculated from the carbon diffusion model formula is within the tolerance range.

However, within the concentration distribution of the diagram of Fig. 11, there is a culmination at a distance of 10 to 50 µm from the strip surface, from which the carbon concentration decreases continuously up to a distance of 250 µm. The reason for this is that initially at a point in the immediate vicinity of the strip surface with the highest carburization concentration, decarburization progresses during cooling after the furnace seal. Accordingly, in order to bring the course of the carbon concentration distribution into agreement with the model formula for carbon diffusion, only the carbon concentration at two or more points needs to be determined in the form of the carbon concentration distribution in the distance range of 10 to 250 µm. Preferably, in order to achieve the culmination point of the carbon concentration, the carbon concentration is set at one point in the distance range of 10 to 50 µm and at another point in the distance range of 10 to 250 µm. In the case of a constant carburization amount, the model formula for carbon diffusion can be set directly or once when the carbon concentration is set at only one point under conditions that various conditions such as the speed of the surface reaction and the carburization temperature and time and the like are set.

The diagram of Fig. 11 shows the relationship between the distance from the strip surface, ie the carburization depth (µm), and the carbon concentration in the steel (carburization in ppm) in the form of the curve and the operational measured values under the carburizing conditions that the carburizing time (treatment time in seconds) is t₁, t₂, t₃ and the CO concentration (%) is a₁, a₂, a₃ and the H₂ concentration is b₁, b₂, b₃ and the carburizing temperature T (ºC) is constant. In this case, however, the carburizing time t₁ = t₂ ≠ t₃ and the CO concentration is a₁ = a₃ ≠ a₂ and the H₂ concentration is b₁ = b₂ = b₃. In connection with the diagram in Fig. 11, when determining the measured values for the carbon concentration, a sample was placed in hydrofluoric acid in order to dissolve the surface and to calculate the amount of carbon in solid solution from the weight ratio between the amount of carbon and the amount of iron dissolved within a given time. It could also be estimated by measuring the depth of a special microstructure of the steel, which is determined by the carbon concentration (depending on it).

Next, the graph of Fig. 12 shows the results of experiments on the influence of carburizing time on the model formula for diffusion in steel. The experiments were conducted under the condition that the carburizing time T (°C) and the total carburizing amount ΔC (ppm) are constant. The solid curve in the graph refers to the case that carburizing took place under the condition that the CO concentration (%) was a₄, the H₂ concentration (%) was b₄ and the carburizing time (treatment time in seconds) was t₄, while the dashed curve refers to the case that the CO concentration (%) was a₅, the H₂ concentration (%) was b₅ and the carburizing time (treatment time in seconds) was t₅. In this case, however, the carburizing time t₅ ≠ 3 t₄,, the CO concentration a₄ > a₅ and the H₂ concentration b₄ > b₅. As already mentioned, the higher the CO concentration and the H₂ concentration, the higher the carburization rate. Furthermore, the longer the carburization takes, the greater the amount of carburization inside the edge zone. Accordingly, it can be seen from the diagram in Fig. 12 that if the gradient of the carbon concentration in the inner part of the edge zone due to an increase in the carbon concentration only in the part of the edge zone close to the surface, only the carburisation time needs to be reduced by increasing the speed of the carburisation reaction (increasing the carburising capacity) and conversely, if the aim is to flatten the gradient of the carbon concentration between the inner part of the edge zone and its part close to the surface by increasing the carbon concentration in the sheet as a whole, only the carburisation time needs to be increased by reducing the speed of the carburisation reaction (reducing the carburising capacity).

Next, the diagram of Fig. 13 shows the possibility of controlling the distribution of carbon concentration in the method according to the invention by controlling the strip temperature after carburizing, in particular by means of the cooling rate. In the diagram of Fig. 13, the solid curve, assuming that the carburizing temperature T (°C), the carburizing time t (sec), the CO concentration a₆ (%) and the H₂ concentration b₆ (%) are constant, relates to the case where cooling takes place at a cooling rate of T, (°C/s), and the dashed curve relates to the case where cooling takes place at a cooling rate of ΔT₂ (°C/s) and the cooling rates are in the relationship: ΔT₁ < ΔT₂. As can be seen from the diagram of Fig. 13, Because the diffusion of dissolved carbon into the interior of the sheet is abruptly interrupted due to the higher cooling rate, only the carbon concentration in the part of the edge zone near the surface increases and the gradient of the carbon concentration in the inner part of the edge zone becomes steeper. Conversely, at a lower cooling rate, due to the diffusion of dissolved carbon inwards, the carbon concentration in the part of the edge zone near the surface is low and the gradient of the carbon concentration in the inner part of the edge zone is flatter.

This example is in contrast to the case where the carburized strip is rapidly cooled in the first cooling zone and carbon diffusion is terminated. However, the invention allows the state of carbon diffusion to be manipulated by annealing, soaking and cooling the strip after carburization. Instead of or in addition to the first cooling zone, a temperature-controlled zone can therefore also be used.

Furthermore, in this embodiment, as described in detail, unlike the case where using the algorithm of Fig. 6, under the condition that the carburizing temperature is predetermined based on the material properties and the CO concentration and the H₂ concentration are predetermined taking the soot limit into account, the carburizing time (strip running speed) is lastly changed to produce a predetermined amount of carbon And in the case where, when the algorithm of Fig. 6 is applied, under the condition that the upper limits of the carburizing temperature and the carburizing time are selected based on the conditions for the distribution of the carbon concentration and the upper limits of the CO concentration and the H₂ concentration are selected based on the course of the soot boundary, the carburizing time (strip speed) and the composition of the gas atmosphere are lastly changed to achieve a predetermined distribution of the carbon concentration in the direction of the strip thickness and a predetermined carburizing amount; and in the case where, when the algorithm of Fig. 6 is applied, under the condition that the carburizing time is determined based on the strip speed determined by the process conditions other than carburizing and the carburizing time is determined based on the material properties, the composition of the gas atmosphere is lastly changed to achieve a predetermined carbon amount. Including the above-mentioned cases, the following control variables are also to be considered as examples of the above-mentioned control variables:

1) With the composition of the gas atmosphere constant, the carburizing temperature and/or the carburizing time are changed.

2) At constant carburizing temperature, the CO partial pressure or the H₂ partial pressure or the Partial pressures of carbon monoxide and hydrogen in the gas atmosphere as well as the carburizing time are changed individually or together.

3) When the carburizing time is kept constant, the CO partial pressure or H2 partial pressure or the carbon monoxide and hydrogen partial pressures in the gas atmosphere and the carburizing temperature are changed individually or together.

4) All control variables are changed individually or simultaneously.

The choice of control variables is not limited to any one. Rather, all control variables can be used in any case.

Unlike the case described, where the rate of the surface reaction was calculated taking into account the influence of CO, H₂, CO₂ and H₂O, the rate of the surface reaction can also be calculated taking into account the influence of ethane (bi-carbon hydride).

In the present case, the equilibrium state was calculated by linearizing the thermodynamic model formula, taking into account the substance input and output and converging their solutions. However, the calculation methods are not limited to this.

Furthermore, the invention was described in detail only for the case where a strip made of an ELC steel was continuously carburized and annealed in the area determined by the surface reaction. However, the invention is also suitable for other reaction-determined areas or the case where only carburization is required or other strip qualities are to be carburized.

Claims (8)

1. Process for the continuous carburization of a steel strip in a continuous furnace with an atmosphere containing essentially carbon, oxygen, hydrogen and nitrogen individually or in conjunction with one another, in which soot formation is suppressed by means of a soot limit value dependent on the CO/CO2 concentration and the steel temperature and the flow rate of the gas fed into the carburizing furnace is adjusted and (a) the total carburization amount based on the formula for the rate (V) of the carburization reaction at the surface V = k&sub1; f 1 (pCO, pH 27 ? o) x ? f3 (pCO, pCO2), or V = k&sub1; f1 (pCO, pH2, θo) -k2 f2 (pCO2 , pH2 O), where α is a constant, k₁, k₂ are reaction rate constants, pCO is the partial pressure of carbon monoxide, pCO₂ is the partial pressure of carbon dioxide, pH₂ is the partial pressure of hydrogen and θo is the transfer rate of absorbed oxygen, and on the basis of the following model formula for carbon diffusion in steel: dC/dt = D d²C/dx², is calculated, where C is the carbon concentration in the steel, t is the time, D is the diffusion coefficient and X is the diffusion distance and (b) suitable ranges for the carburization temperature, the respective concentrations of CO, H₂, CO₂ and H₂O in the carburization atmosphere and the carburization time are selected so that the carburization amount corresponds to the target amount, and (c) during carburizing of the strip, the carburizing temperature, the respective concentrations of CO, H₂, CO₂ and H₂O in the carburizing atmosphere and the carburizing time are adjusted within the limits according to (b). 2. Method according to claim 1, characterized in that the method step (b) is followed by a further method step in which the flow rate of the gas, the CO concentration and the H₂ concentration in the gas are adjusted within the range suitable according to (b) for CO, H₂, CO₂ and H₂O so that the calculated concentration for CO, H₂, CO₂ and H₂O in the carburizing atmosphere 1. one by the formulas: Wgi = Wsc + Wgo Wsc = ε(V, t, w, LS) where Wgi is the amount of carbon in the gas fed into the furnace, Wsc is the amount of carbon discharged with the belt, Wgo is the amount of carbon contained in the furnace exhaust gas, V is the rate of the surface reaction, t is the carburizing time, w is the belt width and LS is the belt speed, and 2. the Gibbs free energy according to the formula: under the condition that the amount of free carbon (condensed carbon) at a given temperature is zero, is minimized, where n is the number of individual gases and p is the number of individual condensations. 3. A method according to claim 1 or 2, wherein the strip temperature and/or the composition of the gas serve as control variables and the control variables gas composition and/or strip temperature are adjusted on the basis of a prediction formula which is set in advance in order to achieve a target carburization amount in a certain carburization time. which depends on the throughput speed limited by a process condition other than carburization. 4. Method according to claim 3, in which the distribution of the carbon concentration, the carbon concentrations of at least one point in a depth range of 10 to 250 µm of the edge zone of the strip or the metal structure determined by the carbon concentration serve as targets. 5. Method according to claim 3 or 4, in which a temperature control of the strip takes place after gas carburizing in order to adjust the carbon concentration in the direction of the strip thickness. 6. A method for continuously carburizing steel strip in a continuous furnace, in which variables are calculated by means of a computer on the basis of initial conditions of a specification of the strip in the carburized state and the variables are adjusted to achieve a target carbon amount at which soot formation is suppressed by means of a soot limit value dependent on CO/CO₂ concentration and steel temperature and by adjusting the flow rate of each gas component, wherein (a) a target carburization amount (ΔCo), the gas composition, the gas flow rate, the carburization temperature, the strip travel speed (LS) and the Dimensions of the steel strip serve as input conditions; (b) the concentration of each gas component at which soot suppression is ensured is calculated and the total Gibbs free energy (F(x)) resulting from using the concentration of each component of the gas as a variable is taken to be a minimum under the condition that the amount of carbon in the gas introduced into the furnace and the amount of carbon formed during carburisation of the strip are constant; (c) the surface reaction rate using the formula: V = k&sub1; f 1 (pCO, pH 2 , θo) X ? f3 (pCO, pCO2), or V = k&sub1; f1 (pCO, pH2, θo) -k2 f2 (pCO2 , pH2 O), is calculated, where α is a constant, k₁, k₂ are reaction rate constants, which can be calculated using the equation: ki = Ai exp(-Ei/RT) where Ai is the frequency factor, Ei is the activation energy, R is the gas constant and T is the absolute temperature, and the formulas include as parameters the carbon monoxide partial pressure and the hydrogen partial pressure and additionally the CO₂ partial pressure and the H₂O partial pressure, provided that the carburization amount is set within the range governed by the surface reaction in which the carbon concentration of the strip in its edge zone is below the concentration at which it reaches equilibrium with the gas, and furthermore, a disturbance of the carburization reaction by CO₂ and H₂ formed during carburization is taken into account, (d) the carburization amount (ΔC') of the strip by integration over the carburization time and the total area of the strip as well as - each calculated according to (c) - the surface reaction rate (V) per unit time and the unit area; (e) the carburization amount (ΔC') according to (d) is compared with the target carburization amount (ΔCo) and the composition of the gas, the speed of the strip and the carburization temperature are changed individually or in parallel, each calculated or specified, in order to repeat the process steps according to (b) to (d) if the difference between the calculated carburization amount (ΔC') and the target carburization amount (ΔCo) is not less than a predetermined value; (f) the concentration of each gas component, the strip travel speed and the carburization temperature, each calculated or selected, are used when the difference between the calculated carburization amount (ΔC') and the target carburization amount (ΔCo) is less than the specified value; and (g) based on the results of (f), the furnace temperature is set to a value of 700 to 950ºC, the carbon monoxide concentration to a value of over 0% but not more than 22%, the hydrogen concentration to a value of 0 to 30% as the gas composition and furnace temperature conditions under which no soot formation occurs in the furnace. 7. A method for continuously carburizing steel strip in a continuous furnace, in which variables are calculated by means of a computer on the basis of inputs given by a specification of the strip in the carburized state, and the variables are adjusted with respect to a target carburization amount and a certain distribution of carbon concentration over the strip thickness, and soot formation is suppressed by means of a soot limit value corresponding to the CO/CO₂ concentration and the steel temperature and by adjusting the flow rate of each gas component introduced into the furnace, (a) a target carburization amount (ΔCo), the carbon concentration (C₁) at a certain depth (x,) from the surface of the strip, the composition and the flow rate of the gas, the carburization temperature, the strip speed (LS) and the size of the strip are input; (b) the concentration of each gas component at which soot formation is suppressed and the total Gibbs free energy (F(x)) obtained by using the concentration of each component of the gas as a variable is minimum under the condition that the amount of carbon in the gas and the amount of carbon taken up during carburization of the strip are constant; (c) the diffusion rate of a carbon in solid solution in the steel strip (i.e. the carburization rate) according to the model formula for carbon diffusion in steel: dC/dt = D d²C/dx² is calculated, where C is the carbon concentration in the steel, t is the time, D is the diffusion coefficient and X is the diffusion distance and a carbon diffusion quantity for the diffusion into the strip is obtained; (d) a carburization amount (ΔC') in the steel strip is calculated by integration over the carburization time, the total area of the steel strip and - each calculated according to (c) - the diffused carbon amount in the steel strip per unit time and the area; (e) the carburization amount (ΔC') according to (d) is compared with the target carburization amount (ΔCo) and the composition of the gas, the belt speed and the carburization temperature, each calculated or selected, are changed individually or side by side to repeat the process steps according to (b) to (d) if the difference between the calculated carburization amount (ΔC') and the target carburization amount (ΔCo) is not less than a predetermined amount; (f) the carbon concentration (C'₁) at a specific carburization depth (X₁), based on the strip surface, according to the model formula of carbon diffusion in steel: dC/dt = D d²c/dx² is calculated when the difference between the calculated carburization amount (ΔC') and the target carburization amount (ΔC�0) is less than a predetermined value; (g) the carbon concentration (C'₁) at a determined carburizing depth (X₁) calculated in accordance with (f) is compared with the target carbon concentration (C₁) at the depth (X₁) entered in accordance with (a) and the composition of the gas, the belt speed, the carburizing temperature, each calculated or selected, are changed individually or in parallel and the process steps (b) to (f) are repeated if the difference between the calculated carbon concentration (C'₁) and the target carbon concentration (C₁) is not less than a predetermined value, and the composition of the gas, the belt speed, the carburizing temperature and the distribution of the carburizing concentration from the surface are used if the difference is less than the predetermined value; and (h) on the basis of the result according to (g), the furnace temperature is set to a value of 700 to 950ºC, the carbon monoxide concentration to a value of over 0 to a maximum of 22% and the hydrogen concentration to a value of up to 30% as conditions for the composition of the gas and the furnace temperature at which soot formation does not occur inside the furnace, and the carbon concentration at least at one point at a depth of 10 to 250 µm in the peripheral zone is set to a certain carbon concentration according to the distribution of carbon concentration in the direction of the strip thickness. 8. A method for continuously carburizing a steel strip according to claims 1 to 7, in which a carburizing furnace is part of a continuous annealing furnace.