WO1994009175A1 - Method of continuously carburizing metal strip - Google Patents

Abstract

This invention aims at providing a method of continuously carburizing a metal strip, which is capable of providing industrially optimum carburization conditions while attaining non-soot-generating atmospheric data, desired carburization concentration distribution and desired carburization rate, in a case where a strip passed through a carburization furnace is carburized continuously in a surface reaction rate-determining region in which the carbon concentration in a superficial layer of the strip has not yet reached an equilibrium level with respect to the time. The method consist of carburization concentration distribution (S7), on the basis of the carburization conditions including given specification data for the steel plate, furnace temperature and composition of the atmospheric gas, outputting the concentration of the components of the atmospheric gas, feed and discharge rates and other carburization conditions when the set carburization rate and an actual carburization rate are equal (S8-S15), and correcting the set carburization rate when a difference between the set carburization rate and an actual carburization rate is large, and correcting the strip feed rate while correcting the composition of the atmospheric gas when a difference between the predetermined carburization rate and set carburization rate is large (S9).

Description

 Specification

 Method of continuous carburizing of metal strip

 The present invention relates to a continuous carburizing method for continuous gas carburizing of a metal strip.For example, the present invention relates to a method for continuously carburizing a strip made of ultra-low carbon steel from an annealing furnace through a carburizing furnace. However, it is desired that the strip be passed at a passing speed set by operating conditions other than the carburizing treatment in a surface reaction rate-controlling region before the carbon concentration of the surface portion of the strip reaches the equilibrium concentration between the strip and the atmosphere gas. Atmosphere gas composition and composition gas concentration as an atmosphere specification where sting does not occur in order to achieve carburization with the amount of carburizing that occurs and to obtain the desired carburizing concentration distribution in steel. It is suitable for controlling furnace temperature, metal zone temperature, and passing speed. Background art

 For example, in the metal secondary processing industry such as the automobile industry, there is a demand for higher workability and higher strength for the metal sheet to be processed. Specifically, in the automotive industry, since it is necessary to reduce the weight of the vehicle body in order to pursue low fuel consumption due to global environmental issues that have recently become a problem, higher strength is maintained while maintaining the conventional deep drawability. Steel sheet is required

As evaluation indexes for such metal sheets, for example, ductility, deep drawability, aging, strength, secondary work brittleness, bake hardenability, spot weldability, and the like can be considered. In view of this, the deep drawability is particularly important, and when this deep drawability is evaluated by a Rankford value (hereinafter, r value: metal plate width distortion Z plate thickness distortion), carbon in steel (hereinafter, referred to as C) is evaluated. It is known that it is most advantageous to reduce the amount. In addition, the low carbon content improves the ductility (Elongation: E 1) and the normal temperature aging property (Aging Index: the lower the AI, the better). . However, on the other hand, as the amount of C in steel decreases, other evaluation indices deteriorate to a large extent. For example, the tensile strength (TS) is reduced due to reduced precipitates and reduced structural strength, and the secondary processing brittleness is poor due to reduced grain boundary strength. The bake hardenability deteriorates because the amount of solid solution C decreases. When the C content in steel is 5 O ppm or less, the grain growth rate is accelerated by heating by welding, and the spot weldability deteriorates due to coarsening of the heat-affected zone (HAZ).

 Therefore, as shown in Fig. 1, a metal strip made of ultra-low carbon steel is recrystallized and annealed by continuous annealing to obtain the above-mentioned ductility, deep drawability, and room-temperature delayed aging. In order to improve the tensile strength, secondary work brittleness, BH property, and spot weldability by making solid solution C exist in the surface layer by the continuous immersion treatment, the present applicant As shown in the figure, we have developed a continuous annealing carburizing facility described in Japanese Patent Application Laid-Open No. 4-88816.

 According to this continuous annealing and carburizing equipment, after performing a predetermined recrystallization annealing on the metal zone (strip A) from the pre-tropical zone 1 to the heating zone 2 or the soaking zone 3, the metal zone temperature in the carburizing zone 4 Carburizing is performed by controlling the atmosphere specifications, transfer speed (furnace time) and cooling conditions so that the surface carburized depth and concentration distribution can be set to desired values (form) while satisfying the material specifications of the metal strip. Enables continuous production of strips.

On the other hand, there is such a distribution of the carburized depth and the carburizing concentration of the metal strip surface layer portion _(those described in Japanese Patent Publication 5 4 3 1 9 7 6 discloses a method of controlling the form. The carburized depth and The method of controlling the carburizing concentration is as follows: During the carburizing period, a carburizing gas is spouted out at a predetermined flow rate to infiltrate the carbon into the surface of the metal band, and during the diffusion period following the carburizing period, the carburizing gas is exhausted and depressurized sufficiently Under the condition, the carbon permeated into the surface of the metal belt is diffused, and by controlling the carburizing period and the diffusion period, the carburizing concentration distribution form consisting of the carburizing depth and the carburizing concentration is controlled. According to the method for controlling the carburizing depth and the carburizing concentration, it is possible to prevent uneven carburization, which is likely to occur particularly in gas jet carburizing requiring a thin carburized layer (carburized skin).

 By the way, in actually setting the conditions of such continuous annealing carburizing equipment, the following problems were found.

(1) Regarding the carburizing rate, according to a report by Habara et al. (Yasukun, Yamashiro et al .: Journal of the Japan Institute of Metals 49 (1985) 7, 529), as shown in Fig. 3, the C content in the metal surface layer was somewhat high. If the carburizing time is long, the carburizing speed is the speed at which C diffuses into the metal structure after the C concentration in the surface layer reaches the equilibrium concentration between the strip and the atmospheric gas. Therefore, the time carburization gain region is generally called a diffusion-controlled region.On the other hand, as described above, the C content of the metal surface layer is extremely low and the carburization time is extremely short. If the carbon concentration is short, the C concentration in the surface layer does not reach the equilibrium concentration, so the carburizing speed is proportional to the direct reaction between the metal surface layer and carbon. It is known to be called a rate-limiting region.

Therefore, for example, the carburizing condition of the metal band is obtained from the specifications required for the metal intended to improve the secondary work brittleness resistance (Japanese Patent Application Laid-Open No. H3-193934). Since the concentration and carburizing depth are extremely small, in this case, it is necessary to perform carburizing treatment in the surface reaction rate-controlling region, and it is considered that the metal layer surface layer is always in equilibrium with the carburizing force of the atmospheric gas. in COZC 0 carbon potential (C potential) due to management of ₂ control, were found not to control the carburization of the metal strip.

(2) In general, the atmosphere gas composition under carburizing conditions can be obtained by chemical equilibrium.However, conventional solutions can list all possible gas-phase reactions, and from the equilibrium relationship between these individual reactions, a non-linear The gas composition was obtained by solving the simultaneous equations. However, it is extremely difficult to determine the exact limit of soot generation (sooting, inging) from the reaction equation of the gas phase system.

(3) Furthermore, there is a report from the above-mentioned leaf reaction rate on the above-mentioned surface reaction rate, but this report only discusses the carburization rate in CO gas only. In fact, a continuous carburizing operation cannot be deployed. Meanwhile, in the continuous annealing carburizing equipment as shown in Fig. 2, a predetermined annealing treatment is performed on the metal zone in the heating zones 2 and Z or the soaking zone 3, a predetermined carburizing process is performed in the carburizing zone 4, and each cooling zone 5 Therefore, in each heat treatment zone, it is necessary to control the temperature of a predetermined metal zone (hereinafter, also referred to as sheet temperature) by controlling the furnace temperature, for example. In each furnace constituting each heat treatment zone, the plate temperature is controlled mainly by heat transfer, but at the same time, the upper and lower limits of the in-furnace temperature (hereinafter also referred to as furnace temperature) itself are calculated by calculating the capacity of each furnace. Also exists. For example, in the IJ Tropical heating furnace and the Tropical soaking furnace, the upper limit of the furnace temperature is set based on the capacity of the furnace, and the aging factor between the radiant tube, furnace wall, hearth roll, etc. is taken into account. The in-furnace time (that is, heating time or soaking time) of the strip that satisfies the upper and lower limits of the sheet temperature from the heat balance is set. Then, the passing speed to satisfy the furnace time is set. Further, in the cooling furnace of each cooling zone, the heat transfer coefficient of the cooling gas jet or the like is used as the heat transfer coefficient.

 On the other hand, in such continuous annealing carburizing equipment, various operating conditions such as changing operating conditions at unsteady parts such as coil joints are mixed. In many cases, a high threading speed is controlled. However, no specific means has been proposed yet for setting the carburizing conditions in the carburizing furnace for the strip speed set from various operating conditions including the above-mentioned continuous annealing carburizing sheet temperature control. In particular, under the conditions where the threading speed is set, the means for controlling the physical properties and temperature in the carburizing furnace that achieves the carburizing amount to satisfy the specifications required for the steel sheet as described above are urgently required. Is desired.

 In order to remove such a restriction on the passing speed, it is conceivable to insert a looper between the heat treatment zones. However, continuous annealing equipment that requires a very large installation space and continuous carburizing Installing a looper, which also requires a large installation space, in a continuous annealing carburizing system with additional equipment is difficult to put into practical use in light of the actual problem.

Further, the specifications of the carburized thin steel sheet tend to require more detailed conditions, and in order to satisfy such specifications, the distribution of the carburizing concentration of the surface layer of the metal strip, that is, It has become necessary to control and control even the profile of the carburizing concentration in the depth direction. For example, in the case of steel sheets used in vehicles and electrical equipment, baking coating is often performed after pressing, so that during press processing, the ductility E1 and deep drawability r value are exhibited and the formability is high, and during baking coating, It is necessary to have such properties that the bake hardenability BH is exhibited to improve the strength. At the same time, these steel sheets are also required to have normal-temperature delayed aging (low AI) that can maintain their formability until press working. Therefore, it is necessary for these steel sheets to be low-aging, high-bake-hardening steel sheets (low AI-high BH steel sheets) with deep drawability. When examining the profile of the carburizing concentration in steel, which is necessary when such a steel sheet is obtained by continuous annealing and carburizing of ultra-low carbon steel, that is, the distribution state, the carbon concentration in the inner layer in the thickness direction of the steel sheet is similar to that of the ultra-low carbon steel. While still low, the carbon concentration at the surface must be greatly increased to create an optimal C concentration gradient. But However, in the control method of the carburized depth and the distribution form of the carburized concentration described in Japanese Patent Publication No. 54-319796, such a carburized concentration profile is not considered, and this control method is used as it is. It cannot be applied to the control of carburizing concentration distribution. Disclosure of the invention

 The present invention has been developed in view of such problems, and in particular, the sheet passing speed is regulated by operating conditions other than the carburizing process, and the carburizing process performed at the sheet passing speed is performed in the surface reaction controlled speed range. It is an object of the present invention to provide a control method capable of obtaining a desired carburizing amount and a carburizing concentration distribution in a metal strip while preventing sooting even when the carburizing is performed.

 As a result of intensive studies on the above problems, the present inventors have developed the present invention based on the following findings. In other words, regarding the problem of sooting that occurs in the form of free C in a carburizing furnace, the total amount of each component in the carburizing furnace is constant, even if the amount of each component changes in the carburizing furnace, when considered at the elemental level. In the case of an isothermal and isobaric system, the free energy of the cast in the carburizing furnace is reduced by a change that occurs naturally, and the system is in an equilibrium state between the atmosphere gas and the metal band in the carburizing furnace. The cast free energy is at a minimum. Therefore, if the atmosphere gas composition that minimizes the cast free energy is determined, the equilibrium state of the furnace atmosphere can be determined, and the reaction in the direction in which free C (soot) is generated can be reduced or suppressed. However, unless there is a material balance constraint that the elementary elements brought into the source system be constant with respect to the elements brought out of the atmosphere gas by the reaction in the surface zone of the metal strip, the actual case of continuous carburization We focused on the fact that it was not possible to calculate the true equilibrium state, that is, the true sooting generation limit. Therefore, when considering the actual situation of this material balance, it is necessary to consider not only the composition of the atmosphere gas, but also the supply and discharge flow rate of the atmosphere gas, the passing speed of the metal strip, the furnace temperature, the sheet thickness, the sheet width, etc. Nanare.

Therefore, in the method for continuously carburizing a metal strip according to the present invention, in controlling the carburizing atmosphere parameters containing carbon, oxygen, nitrogen or carbon, oxygen, hydrogen, and nitrogen and generating no sooting, Considering the material balance of each element level in the actual case of carburizing, it is necessary to find the state in which the cast free energy of the entire furnace atmosphere is minimized. By calculating the atmosphere gas composition and Z or the furnace temperature based on the matured dynamics model formula that obtains the equilibrium state of the furnace atmosphere by the above, the material balance of each element level in the furnace is not considered This makes it possible to increase the potential of the atmosphere composition while preventing the occurrence of sooting, as compared with a case where these are calculated simply from the equilibrium state obtained from the supplied gas composition flow rate and the furnace temperature. In other words, it is possible to improve the actual operation capacity, such as increasing the CO passing rate in the atmospheric gas to increase the sheet passing speed. As the atmosphere specifications of conditions, the temperature in the furnace 7 0 0~9 5 0 ° C- concentration of carbon monoxide rather 0% CO concentration ≤ 2 2%, hydrogen concentration 0% ≤H ₂ concentration ≤ 3 The actual conditions of industrial continuous carburizing operation, such as 0%, were set. Since nitrogen in the atmosphere gas composition is considered to be an inert gas for diluting the concentration of the atmosphere gas, a similar inert gas such as argon Ar may be used.

Also, as described above, in order to control the amount of carburization into the metal band in the surface reaction rate-determining region where the carbon concentration in the metal surface layer is equal to or less than the equilibrium concentration between the metal band and the atmospheric gas, We obtained the amount of carburization, that is, the surface reaction rate, and focused on the fact that this reaction rate should be integrated over time. This time, ie, the carburizing time, is determined by the passing speed. In the course of studying this surface reaction rate, the reaction was controlled by controlling the gas composition included in the carburization reaction formula and the deoxygenation reaction formula considered in the surface reaction between the metal strip and the atmospheric gas. I found that I could control the speed. And this for the most effective in gas composition are carbon monoxide and hydrogen, is also carbon dioxide and H ₂ 0 is small composition amount when the supply / emissions small flow rate of the atmospheric gas in particular at a high temperature, It was found that inhibiting carburization reaction had an effect in some way, and it was experimentally proved that the partial pressure of these compositions was a controlling factor of the surface reaction rate. In addition, taking into account the dependence of the material reaction on the temperature, a control factor called metal zone temperature was interposed in the coefficient of the surface reaction rate.

Therefore, in the method for continuous carburization of a metal strip according to the present invention, in the carburization condition range in which the carburization rate follows a surface reaction rate greater than the diffusion rate from the surface layer of the metal strip to the inside, for example, regarding the metal temperature in the carburizing furnace, The temperature dependence coefficient relating to the surface reaction rate of carburization is calculated from the prediction formula, and the surface reaction rate of carburization is calculated from the temperature dependence coefficient and the prediction formula relating to the carbon monoxide partial pressure or the carbon monoxide partial pressure and the hydrogen partial pressure. And calculate the surface reaction rate The carburizing amount in the metal strip can be calculated based on the prediction formula relating to the carburizing time from the above, and conversely, the carburizing amount in the metal strip is set from the specifications required for the steel sheet after carburizing, and By setting the parameters appropriately according to the actual continuous carburization using the control variables interposed in each prediction formula as parameters, the specifications of the steel sheet can be adjusted under the most efficient carburizing conditions. A satisfactory amount of carburization into the metal strip can be obtained. In addition, especially when the supply and discharge flow rates of the atmospheric gas are small at high temperatures, the effect of inhibiting the carburizing reaction should be taken into account. and H ₂ 0 partial pressure control amount, i.e., more be added as a parameter, C 0 _2, H ₂ 0 it is possible to accurately control the carburizing amount to metallic band at carburizing conditions present.

Incidentally, C 0 ₂ and H ₂ 〇 concentration of the ambient gas in the composition may be reduced by increasing the introduced flow of the ambient gas, also increased by reducing the projection's flow rate of the atmospheric gas be able to.

 When the surface reaction rate is integrated over time, the actual carburizing time is used. This carburizing time is simply calculated as: carburizing time = furnace duration = effective carburizing furnace length Z passing speed. Therefore, as described above, when the sheet passing speed is regulated by operating conditions other than carburizing treatment, the carburizing time set from the sheet passing speed is regarded as being fixed, and other control is performed. It was confirmed that the desired amount of carburization can be controlled by controlling the factors. In the actual carburizing process, the correlation between the carburizing time and the passing speed may be determined by considering the composition of the atmosphere gas in the carburizing furnace and the temperature of the metal strip. In this case, when the regulated threading speed has a certain range, it is also possible to add the parameter of the prediction formula to the carburizing time in pursuit of further control accuracy.

 Here, in the continuous carburizing method of the metal strip of the present invention, for example, in order to perform necessary carburization control, heat treatment and carburization are performed simultaneously, or carburization is performed at a somewhat lower temperature after heat treatment. In addition, even when the field of the sheet temperature control and the case of the carburizing control are the same or different, the same control can be performed by, for example, considering the sheet passing speed in a time series.

On the other hand, the carburizing concentration at a predetermined depth in the surface layer of the metal strip is determined by the carbon diffusion based on the so-called Fick's law, where the carburizing time (including the diffusion time) and the carburizing temperature are all parameters. By paying attention to the fact that it can be obtained by a model formula, this was proved by experiments. ₍ Accordingly, in the method for continuous carburization of a metal strip of the present invention, a desired carburizing concentration distribution is applied to this carbon diffusion model formula. In this way, it is possible to set the carburizing time and the metal zone temperature to obtain the carburizing concentration at each depth position. The closer the surface is, that is, the shallower the surface layer, the higher the carburizing concentration, and the farther from the surface of the metal strip, that is, the deeper the surface layer, the lower the carburizing concentration. From the results, it was found that setting the carburizing concentration distribution condition of the metal strip could control the carburizing concentration distribution at a depth of 10 to 250 m from the surface of the metal strip. By integrating in the direction In addition, when the carburization concentration distribution is affected by decarburization in the cooling process, the maximum carburization concentration exists at a depth of approximately 10 to 50 / m. Accordingly, the carburizing concentration becomes smaller as the depth increases.Accordingly, according to the continuous metal carburizing method of the present invention, when the total carburizing amount is constant, the carburizing concentration distribution form is based on the carbon diffusion model formula. By setting the carburizing concentration at one point in the range of the depth 10 to 50 m in the sense of capturing the peak point of the above, the carbon diffusion model formula is determined, and even when the total carburizing amount is different, other one or more points By setting the carburizing concentration within the range of 10 to 250 m in the depth, the carbon diffusion model equation is determined.For example, the carburizing concentration at each point in the depth direction satisfying the carburizing concentration distribution form Calculates the carburizing concentration distribution state that falls within the predetermined allowable range of the target value, for example. It is possible to set the metal zone temperature, the atmosphere gas composition, and the carburizing time, which are the parameters of the carbon diffusion model formula, and even if the total carburizing amount is not set, this carbon diffusion model formula can be used. By integrating the obtained carburizing concentration distribution in the depth direction, it is possible to set the carburizing amount.Moreover, in the method for continuous carburizing of a metal strip according to the present invention, the surface reaction rate in the above-mentioned surface reaction-limiting region is determined. Of course, it is also possible to apply.

Furthermore, in the method for continuously carburizing a metal strip according to the present invention, the solid solution C existing in the surface layer of the metal strip in the carburizing step is still in a state where it can be diffused or decarburized. By controlling the cooling rate, it is possible to control the diffusion and decarburization of the solute C to fix the solute C to a desired carburized concentration distribution state. BRIEF DESCRIPTION OF THE FIGURES

In the accompanying drawings, Fig. 1 is a conceptual illustration of the heat treatment process performed in the continuous annealing carburizing equipment, and Fig. 2 is a continuous annealing subject to carburizing control using the continuous strip carburizing method of the present invention. FIG. 3 is a schematic diagram showing an example of a carburizing facility, and FIG. 3 is an explanatory diagram of a diffusion-limited region after the carbon concentration in the surface layer of the metal band reaches the equilibrium concentration and a surface reaction-limited region before the carbon concentration reaches the equilibrium concentration. FIG. 4 is a flowchart of an algorithm for constructing the overall line control logic performed in the continuous annealing carburizing equipment of FIG. 2, and FIG. 5 is a surface reaction in the continuous metal carburizing method of the present invention. Temperature coefficient correlation diagram of the data obtained by changing the carburizing temperature to calculate the temperature dependence coefficient of speed.Fig. 6 shows the logic for performing carburizing control using the continuous strip carburizing method of the present invention. Algorithmic algorithm to build one embodiment FIG. 7 is a graph showing CO--comparing the sting occurrence limit obtained by the continuous carburizing method of the metal strip of the present invention with the sting occurrence limit obtained without considering the material balance in the furnace. H ₂ characteristic diagram, FIG. 8 is a correlation diagram between an observed value and a calculated value obtained carburizing quantity by Arugorizu arm of the six-view embodiment, FIG. 9, the algorithm of the embodiment of FIG. 6 FIG. 10 is an explanatory diagram of various carburizing conditions calculated to obtain a target carburizing amount according to an embodiment of the present invention. FIG. 10 shows a target carburizing amount under a condition in which a threading speed is set by the algorithm of the embodiment of FIG. Fig. 11 is an explanatory diagram of the carburizing conditions calculated to obtain the carburized concentration distribution and the measured carburized concentration distribution obtained according to the carbon diffusion model equation using the continuous carburizing method of the metal strip of the present invention. Fig. 12 is an explanatory diagram showing an example of the correlation with Fig. 13 is an explanatory diagram showing an example of the carburizing concentration distribution obtained when the atmosphere gas composition concentration and the carburizing time are controlled by the algorithm of Fig. 13. Fig. 13 controls the cooling rate after carburizing by the algorithm of the embodiment of Fig. 6. FIG. 14 is an explanatory diagram showing an example of a carburizing concentration distribution obtained in the case of performing the above-described process. FIG. 9 is an explanatory diagram showing a result. BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 2 shows an example of a continuous annealing and carburizing facility for a strip made of ultra-low carbon steel in which the method for continuously carburizing a metal strip according to the present invention has been implemented. In this figure, the ultra-low carbon steel strip A is provided with a coil unwinder, a welding machine, a washing machine, etc. so as to satisfy the history and history of the sheet temperature control shown in FIG. The equipment, pre-tropical zone 1, heating zone 2, isotropical zone 3, carburizing zone 4, first cooling zone 5, second cooling zone 6, shearing equipment, winder, and other unshown equipment are passed through in this order.

 The heating zone 2 is for heating the strip A, which is continuously passed from the inlet side equipment and is preheated in the pre-tropical zone 1, to a temperature higher than the recrystallization temperature. The strip A is heated so that the temperature of the strip A becomes 700 to 950 ° C at ~ 100 ° C. Then, the heated strip A is maintained at a temperature higher than the recrystallization temperature for a necessary time in the soaking zone 3, whereby the {1, 1, 1} texture advantageous for deep drawing can be developed.

 A number of radiant tubes are arranged near the passing path of strip A, which passes through the heating zone 2 and the solitary zone 3 while moving up and down through the Haas mouth. The temperature inside the furnace (furnace temperature) is controlled by burning the fuel gas supplied to the flat tube. The setting of the supply flow rate of the fuel gas is performed by a host computer (not shown), which will be described later, sets an upper limit value of the furnace temperature based on a heat balance in consideration of a heat transfer coefficient between the radiant tube, the strip, the hearth roll, and the like. Process model calculation that satisfies the upper and lower limits of the desired recrystallization temperature, optimal route calculation that calculates the optimal time series of the passing speed at the joint between coils, and prediction calculation of the heart crown roll It is set together with the passing speed that achieves the in-furnace time (heating time, soaking time) in each heat treatment zone, based on thermal-crown calculations that calculate the maximum passing speed. Here, in the present embodiment, the supply flow rate of the fuel gas to the radiant tube is set in the furnace by adding the exhaust gas loss heat, the furnace body heat dissipation, etc. to the amount of heat to the strip that passes through and carries the heat from the furnace. The furnace requirement obtained from the heat balance (required) is the same as the calorific value, and it can be performed by a host computer (not shown) in accordance with the control algorithm for the entire line described later.

The carburizing zone 4 is formed by a host computer (not shown) in which the carburizing furnace in the carburizing zone 4 is formed by forming a carburized phase in which solute carbon (C) is present in an extremely thin portion (surface layer) of the surface of the strip A. The temperature of the metal zone is controlled at 700 to 950 ° C, and the strip is kept at a temperature of 700 ° C or more, preferably a recrystallization temperature or less. The passing speed is controlled so as to pass in 0 to 120 seconds. This control is performed in order to keep the carburizing amount (carburizing reaction speed x carburizing time) constant in the strip passing direction and to suppress variations in the material. Incidentally, in the furnace temperature control, if the strip temperature is less than 700, the carburizing reaction rate on the metal strip surface is reduced and the heat treatment productivity is reduced, and if the temperature in the furnace exceeds 950 ° C, transformation occurs. This is done to avoid the problem of material deterioration beyond the point and satisfy carburizing conditions. Further, as is known, if free carbon [C] adheres to the surface of the steel sheet, that is, deterioration of the chemical conversion treatment and the like, and quality deterioration and adverse effects in the post-process are caused. At the same time, the reaction in the furnace accelerates in a predetermined direction, for example, in the direction of the carburizing reaction. As a result, if the dew point rises, the carburizing reaction is hindered, and oxidation occurs on the strip surface, causing a temper color. And the furnace temperature is controlled critically based on the carburizing condition setting algorithm described later.

 The composition and supply / discharge flow rate of the carburizing gas supplied into the carburizing furnace are determined by the host computer based on thermodynamics (atmosphere) that minimizes the free energy in the furnace in consideration of the material balance in the furnace described later. (Composition) Controlled according to various conditions calculated based on the model formula. The composition and supply and discharge flow rates of the carburizing gas are controlled so as to prevent the sting and to suppress the rise in the dew point to prevent the carburizing reaction speed from lowering and the temper collar. Of course, the specification of the strip, such as the carburized concentration distribution and the carburized depth of the carburized layer formed on the strip to be described later, is given top priority, and the composition of the carburized gas and the It goes without saying that supply and discharge flow rates are calculated.

 The physical properties in the carburizing furnace, the furnace temperature, the metal zone temperature, the passing speed, ie, the carburizing time, and the atmosphere gas composition are regarded as the physical quantities to be controlled (control quantities) in the actual case of continuous carburizing. From the specifications such as the required carburized concentration distribution of the carburized layer to be formed on the strip, the carburized depth, etc., for example, the necessary carburized amount is set, and various basic formulas for these control amounts, which are set in advance, will be described later. Each control amount for realizing the carburization amount is calculated by appropriately selecting the control amount, and the control amounts are set in consideration of the capacity and process of other equipment.

By the way, the strip in the carburizing furnace goes up and down the furnace through the hearth rolls 10. These hearth rolls 10 are cooled, for example, in the vicinity of bearings, in order to maintain their rotation and roll crown in a predetermined state. Also mouth

—The chrome Cr alloy is used for the hearth roll to maintain the strength and wear resistance of the roll itself. However, when the carburizing atmosphere gas reaches the vicinity of the hearth roll, cooling is performed and sooting proceeds, so that carbon adheres to the hearth roll and then diffuses into the hearth roll. When this occurs, the Cr and C are combined to precipitate Cr carbide, thereby breaking or expanding the crystal grains of the heat-resistant alloy used for the hearth roll, while reducing the solid solution Cr. As a result, the hearth roll is embrittled and oxidized, so that pore-shaped corrosion proceeds. When the hearth roll is exposed to the carburizing atmosphere gas as described above, experiments by the present inventors have revealed that the hearth roll must be replaced within two years. Therefore, in this embodiment, the hearth roll chamber is separated from the carburizing atmosphere by a non-contact sealing device 11 to prevent the hearth roll from deteriorating, and the hearth roll is deteriorated in the hearth roll chamber. By making the carburizing state weak enough to prevent carburization, we succeeded in preventing so-called decarburization, in which C diffuses from the carburized surface layer while the strip passes through the separated hearth roll chamber. If the time for the strip to pass through the hearth roll chamber is extremely short and decarburization from the surface layer of the steel sheet during the time is not a problem, the hearth roll chamber may be set to a non-carburized atmosphere.

 Although the structure of the sealing device 11 is not described in detail here, for example, the sealing layer provided between the hearth roll chamber and the carburizing atmosphere chamber has a three-layer structure. Ejects the above-mentioned weakly carburizing atmosphere gas, ejects the above-mentioned carburizing atmosphere gas to the seal layer on the carburizing atmosphere chamber side, and exhausts gas from the intermediate seal layer. The flow rate of each atmosphere gas is controlled so as to flow toward the intermediate seal layer by controlling the flow rate, and a circulating flow generated by a plate surface airflow accompanying the passing of the strip is formed on an end face in the width direction of the strip in the seal layer. The exhaust port is configured to exhaust air.

The strip A sent from the carburized zone 4 is passed through the first cooling zone 5. In the first cooling zone 5, since the solid solution C carburized in the carburizing zone 4 is fixed only in a very thin area on the surface of the surface layer of the strip, the strip after carburizing is heated to a steel sheet temperature. Cool rapidly at a cooling rate of 5 ° C / sec. Or more until the force becomes 600 ° C or less, preferably about 500 ° C to 400 ° C. In the first cooling zone 5, the cooling gas flow rate, the flow rate and the cooling roll temperature, which are blown from the cooling gas jet to the strip conveyed in the cooling zone by the host computer, so that the cooling condition can be achieved. The winding angle is controlled.

 The strip A sent from the first cooling zone 5 is then passed through the second cooling zone 6. In the second cooling zone 6, gas cooling is performed to a steel sheet temperature of about 250 to 200 ° C. In this way, it is possible to finally obtain a cold rolled steel sheet for ultra-low carbon press forming in which the amount and form of solid solution C in the surface layer are controlled.

 Next, in the continuous annealing and carburizing equipment of the present embodiment, a configuration concept of total continuous annealing and carburizing control performed by the host computer will be described. In order to facilitate understanding, hereinafter, the temperature of the metal zone involved in the carburizing reaction is also referred to as the carburizing temperature, but it is clear from the above description that the substantial control factor is the furnace temperature. Would.

 First, as described above, in carburizing control in a carburizing zone, the amount of carburizing in the steel sheet is given as a condition for obtaining the target material, including the case where the carburizing concentration distribution in the steel sheet is required. For example, when a carburizing concentration distribution is required, the carburizing amount is set by integrating the distribution in the depth direction. The upper limit of the carburizing temperature is set to be lower than the recrystallization temperature due to the material conditions. On the other hand, in order to obtain the maximum treatment capacity of the carburizing furnace, it is necessary to increase the carburizing reaction speed based on the principle of carburizing amount = carburizing reaction speed x carburizing time. The higher the value, the better. This also helps to prevent the occurrence of stinging described later and raises the CO concentration upper limit.

In the present embodiment, the occurrence limit of the above-mentioned sooting can be obtained by a thermodynamic (atmosphere composition) model formula in consideration of the material balance. it is difficult to set a result that the C_〇 concentration and H. ₂ concentration involved in. Therefore, the present invention previously sets the carburizing reaction rate equation which does not inhibit the, for example, based on the CO concentrations obtained by the generated not atmosphere composition model formula of the soot, and H ₂ concentration using the equation Calculate You. In particular,

H ₂ concentration = ax (CO concentration)

 However,

 a: constant in the range 0≤a <5

It is represented by The constant a is specifically in fundamental equations of the surface reaction rate, which will be described later, it is set to a value to suppress the generation concentration of C 0 ₂ and H ₂ 0 to inhibit the reaction to a minimum, usually from 0.5 to 1. It is often set in the range of 0. That is, when this relational expression is satisfied, the carburizing reaction rate based on the surface reaction rate equation becomes the maximum.

 In this embodiment, the carburizing time for achieving a desired carburizing concentration distribution is set based on the set surface reaction rate. That is, when increasing only the C concentration in the surface layer to steepen the gradient with the C concentration in the inner layer, the carburizing reaction speed may be increased (the carburizing force is increased) to shorten the carburizing time. Conversely, when increasing the entire C concentration of the steel sheet to make the C concentration gradient between the inner layer and the surface layer gentler, the carburizing reaction rate should be reduced (the carburizing power is reduced) and the carburizing time should be increased. . The control of the carburizing reaction rate and the carburizing time satisfies the above-mentioned constraint condition of constant carburizing amount.

 On the other hand, as mentioned in the previous section on the heating zone and the tropical zone, the optimum strip speed is set in each plate temperature control zone other than the carburizing zone by calculating the capacity and process of each furnace. In consideration of the maximum stripping speed of each of these strip temperature control zones and the maximum stripping speed of the carburized zone, in continuous annealing carburizing equipment where strips are stripped in series, any stripping speed is equal to the entire equipment. It is necessary to judge whether the speed of the passing plate is limited. In this case, all specifications of the steel plate must be considered, and the specifications are given as absolute conditions.

From the above, when the maximum threading speed obtained in the carburized zone is larger than the minimum value of each maximum threading speed obtained in each of the sheet temperature control zones, the maximum threading speed of each of the temperature control zones is obtained. Must be set as the line passing speed, and the atmospheric conditions of the carburizing furnace that satisfies the carburizing amount at this passing speed need to be set again. In this case, since the carburizing time becomes longer, it is necessary to reset the direction to decrease the carburizing reaction rate, that is, to reduce the CO concentration and the H ₂ concentration in the atmosphere gas under the above-mentioned constraint condition of the constant amount of carburizing. In other words, the condition that does not cause sooting is necessarily satisfied. Conversely, if the minimum value of the maximum threading speed obtained in each of the sheet temperature control zones is equal to or greater than the maximum threading speed obtained in the carburized zone, the maximum threading speed of the carburized zone is set to the line threading speed. In order to satisfy the sheet temperature of each sheet temperature control zone at this sheet passing speed, it is necessary to reset the furnace temperature / fuel supply amount as the sheet temperature control amount.

 These control concepts are embodied in the algorithm shown in FIG. 4 performed by the host computer.

 In this calculation processing, first, in step S20, in the carburizing zone and each sheet temperature control zone, the maximum value of the passing speed that satisfies the heating, carburizing, and cooling specifications of various steel sheets is set using the upper limit of the installation capacity as a constraint. Set. Specifically, for example, in the heating zone 2 and the isotropy 3, the heat transfer between the radiant tube, the furnace wall, the strip, the hearth roll, and the like is performed based on a mathematical model based on the heat transfer theory. Based on the heat balance considered, a process model formula is set. Based on this process model formula, the target plate is set within the range of the furnace temperature and fuel gas supply flow rate or the capacity of the electric heating device that can be set on the equipment. Calculate the maximum value of the passing speed that satisfies the temperature (hereinafter referred to as the maximum passing speed).

 On the other hand, in carburizing zone 4, based on a mathematical model based on thermodynamics, which will be described in detail later, an atmosphere gas composition model in the carburizing furnace was set in consideration of the material balance in the carburizing furnace. From the composition model and the carburizing reaction rate equation, calculate the maximum passing speed that is less than or equal to the upper limit of the atmosphere gas composition (specifically, CO 2) that does not cause sting and that satisfies the target carburizing amount.

 In the cooling zones 5 and 6, based on the model formula considering the heat transfer between the cooling gas and the strip by the cooling gas jet, within the range of the cooling gas supply capacity and the target cooling rate and / or the target cooling end temperature. The maximum passing speed that satisfies is calculated.

 In the cooling zones 5 and 6, when a cooling roll system other than the gas jet system or a mist cooling system is used as the cooling system, heat transfer between the medium and the strip used in these cooling devices is taken into consideration. The same calculation may be performed using the model formula obtained.

 As described above, the calculated maximum passing speeds of the heat treatment zones including the carburized zone are compared, and the minimum value is set as the maximum passing speed of the entire line.

Next, the process proceeds to step S21, and the entire line set in step S20 is set. Using the maximum stripping speed, determine the set value of the controlled variable that satisfies the heating, carburizing and cooling specifications of the steel sheet in each of the thermomechanical zones including the carburized zone.

 Specifically, for example, in the heating zone 2 and the isotropy 3, the furnace temperature that satisfies the target plate temperature is set using the heat transfer model described in step S20. This furnace temperature may control the fuel gas supply flow rate or the load of the electric heating device by feedback control, and minimize the sheet temperature fluctuation at the seam of the coil of the steel sheet based on the process model calculation described above. The optimum time series of the fuel gas supply flow rate or the load of the electric heating device to be calculated may be calculated by the optimum route calculation, and the feed-forward control may be performed based on the calculated time series.

 On the other hand, in the case of carburizing zone 4, the target value is only the carburizing amount, or the target value of the C concentration distribution in the steel sheet thickness direction is specified together with the carburizing amount. If only the target carburization amount is specified, the atmosphere gas composition that satisfies the target carburization amount is calculated using the atmosphere gas composition model described in step S20 and the carburization reaction rate equation on the steel sheet surface. On the other hand, if the target value of the C concentration distribution in the thickness direction of the steel sheet is specified along with the target carburization amount, not only the carburization time but also the cooling time, together with the atmosphere gas composition model and the carburization reaction rate equation on the steel sheet surface. Using a diffusion model in steel that also considers the period, the C concentration distribution pattern in the target steel sheet thickness direction can be adjusted within the range where the threading speed is equal to or less than the maximum threading speed of the entire line set in step S20. At the same time as resetting to the set sheeting speed, calculate the atmosphere gas composition that satisfies the target carburizing amount. The threading speed reset here is set as the threading speed of the entire line after this step. Also, in this embodiment, the logic for setting the passing speed so that the C concentration distribution form in the thickness direction of the steel sheet satisfies the target value has been described in step S21. In order to prevent the setting from being changed due to factors, it is desirable to set the passing speed that satisfies the C concentration distribution in the thickness direction of the steel sheet in step S23 described later. 23.

 In the cooling zones 5 and 6, using the heat transfer model described in step S20, the wind speed of the cooling gas jet is adjusted by the fan speed so as to satisfy the target cooling speed and the target cooling end temperature. Set.

Next, proceeding to step S22, the heat roll of each heat treatment zone including the carburized zone is removed. The maximum crowning speed is calculated by predicting the crown by using the sheet temperature model and the heat balance model of the roll chamber, and calculating the maximum sheet passing speed so that the roll crown is within the meandering limit of the strip ス ト リ ッ プ the limit of buckling. Perform crown calculation. If the maximum threading speed calculated here is higher than the maximum threading speed of the entire line set in the steps up to step S21, the process proceeds to the next step S23. If the maximum threading speed calculated here is smaller than the maximum threading speed of the entire line set in the steps up to step S21, the maximum threading speed obtained by this thermal crown calculation Is reset to the threading speed of the entire line, and the flow shifts to step S21.

 In step S23, if the target threading speed has been specified in advance for operational reasons such as coil seam welding and coil inspection, and for other reasons (mainly trouble), the designation is made. After checking that the set passing speed is equal to or less than the maximum passing speed of the entire line set in steps S20 to S22, the passing speed of the entire line is set to the designated passing speed. I do.

 Next, proceeding to step S24, the control amount satisfying the heating, carburizing and cooling specifications of the steel sheet in each heat treatment zone including the carburizing zone with respect to the finally set threading speed of the entire line. Calculate and set. The content of the calculation in this step is the same as that in step S21, but the setting calculation of the sheet passing speed based on the C concentration distribution in the thickness direction of the steel sheet is not performed.

 In the explanation of the logic, the description of the sheet temperature control for satisfying the target temperature in the carburized zone 4 is omitted, but the sheet temperature control of the carburized zone 4 is described in the explanation of the logic. It can be considered that it is the same content as the plate temperature control in the heating zone 2 and the average tropical zone 3.

 Next, carburizing atmosphere control performed in the carburizing zone will be described.

 First, based on the specifications of the strip required to obtain a steel sheet with high press formability and strength, such as the low AI / high BH steel sheet described above, the carburizing conditions in this example were The level of the power compared to the carburizing conditions and the items required to satisfy the carburizing conditions will be described.

Conventional carburizing technology involves the use of so-called tempered steel such as gears, shafts and bearings. This is performed for the purpose of surface hardening to improve the wear resistance and impact resistance of the sequel. Therefore, the C content in the material is 0.05% or more, the required carburization amount is 0.1% or more, and the carburization depth is 0.5 to 5 mm or more, so the carburizing time is 1 to It can last up to 5 hours. Under these conditions, the C concentration in the surface layer of the steel sheet reaches an equilibrium concentration with respect to time.Therefore, as shown in Fig. 3, the carburization rate is a diffusion-limited region in steel that follows the diffusion rate into steel. However, its carburizing rate is proportional to the square root of time. In this carburizing speed range, it is necessary to control the carbon potential (C potential) of the atmospheric gas so that the diffusion rate in the steel is equal to the surface reaction rate so that the equilibrium C concentration in the steel at the surface layer of the steel sheet becomes a predetermined value. There are, management C_〇_ZC_〇 ₂ is important as an actual operation management indicators.

On the other hand, in the continuous carburizing of the strip as in the present embodiment, the strip is a continuous body made of the ultra-low carbon steel described above, and the purpose is to improve the surface characteristics of the strip and to improve the material of the steel sheet itself. Done. Therefore, for example, if the carburizing conditions of the metal strip are determined from the specifications (for example, Japanese Patent Application Laid-Open No. 3-193934) required for the metal intended to improve the secondary work brittleness resistance, In the example, the C content in the material is 20 ppm, the required carburizing amount is 200 ppm or less, the carburizing depth is 50 to 200 m, and the carburizing time depends on the passing speed is 1 20 seconds or less. Under these conditions, the C concentration in the surface layer of the steel sheet does not reach the equilibrium concentration with respect to time, so as shown in the report of the leaves mentioned above, the carburization rate was reduced as shown in Fig. 3. This is a surface reaction rate-determining region according to the reaction rate, and the carburizing rate is proportional to time itself. Since the carburization amount and carburization depth are in non-equilibrium state in this surface reaction rate-determining region, the actual operation management index is simply C potential control by C potential control so that the equilibrium C concentration in the surface layer in the steel is obtained. 〇 / C_〇 not only manage _2, taking into account the number of controlled variables in the furnace, so as to obtain a carburized amount determined from the required steel specifications specifications, is necessary to set the carburizing condition is there.

In the actual continuous annealing and carburizing operation, there are various cases including the case where the passing speed is set from the actual sheet temperature control performed in the heat section other than the carburizing zone as in the algorithm shown in FIG. In many cases, the speed at which the response speed is the fastest is controlled based on the operating conditions. Therefore, in the continuous carburizing method of the present invention, except for the carburizing treatment, If the stripping speed is regulated by the continuous annealing and carburizing operation conditions, the carburizing condition that satisfies the carburizing amount of the metal belt surface layer from the required specifications of the steel sheet is set under the stripping speed. I do.

 Here, in this embodiment, the basic principle of constructing the logic based on the algorithm processed by the host computer to control the carburizing amount will be described.

 First, in controlling the composition of the atmosphere gas in the surface reaction rate-limiting region, it is necessary to prevent the occurrence of sooting and to suppress the dew point rise as described above. Infer as follows.

 Generally, the atmosphere gas composition under carburizing conditions can be determined by chemical equilibrium. In the conventional solution, all possible reactions are listed, and the gas composition is obtained from the equilibrium relation of these reactions by solving a system of nonlinear equations. However, it is extremely difficult to find the exact limit of soot generation (sooting) from the gas-phase reaction equation alone.

 Therefore, in the present embodiment, a thermodynamic (atmosphere composition) model formula was considered as follows, and an atmosphere gas composition for preventing occurrence of sooting was obtained.

 In the case of an isothermal, isobaric system, the cast free energy decreases in a change that occurs naturally, and the cast free energy of the system takes a minimum value in an equilibrium state. Therefore, in order to determine the equilibrium state of the atmosphere gas, the objective function is the cast free energy of the whole system obtained using the concentration of each component gas in the production system as a variable, and the elemental components brought into the original system are constant. Under the constraint of the material balance, specifically, under the condition that the composition and amount of atmospheric gas supplied into the furnace and the amount of C taken out of the furnace into the metal zone by carburization are constant, the minimum value is reached. What is necessary is just to obtain the concentration of each component gas. This component gas concentration becomes the equilibrium composition of the atmosphere gas at the given furnace temperature and furnace pressure.

C content is expressed as one of the condensed species in the mouthpiece described below.

In calculating the composition of the atmospheric gas, two assumptions are made. One is that the gas should be an ideal gas. Another is that the condensed phase represented by free C cannot be mixed with gas. Under this assumption, the total free energy F (X) of the gas species and the condensed species is the free energy f ^g i of the i-th gas species and the free energy of the h-th condensed species. Energy: It is given by the following equation with respect to f ^e _h .

F (X) = ∑ i ^g i + ∑ f ^c h (1) i * 1 h-1

 n: number of gas species, ρ: number of condensed species

Is shown.

Here, the free energy f ^s i of the i-th gas species with respect to the gas product is represented by the following 2 with the number of moles of the gas species being x ^e i with respect to the molar energy C ^s i of the i-th gas species. Equations 4 to 4 are given.

f ^g i = x ^g ; (C ^s i + 1 n (x ^K i / X)) (2)

C ^g i = (F / (RT) i + 1 n P (3)

X = ∑ xi (4) i 1 On the other hand, for condensed products, the effects of pressure and mixing are excluded under the above assumption, so the free energy f ^e _h of the h-th condensed species is The molar number of the condensed species is given by the following equations (5) and (6) as x ^e _h for the molar energy C ^e _h of

4 L o

fh ~ 1ιh

C ^ch _h (F / (R · T)) ^c _h (6) In the above formulas (3) and (6), (FZ (R · T)) is defined by the following formula (7).

_{+ ΔΗ ° f, 298. I} / RT (7) Next consider the material balance in the system. Even if the amount of each component of the production system changes, the total amount of each element, that is, carbon C, hydrogen H, nitrogen N, and oxygen 中 in the atmospheric gas component is constant in terms of atomic units. This material balance equation is expressed by the following eight equations.

 n p,

∑ i-] a ^e ϋ · χ ^κ i + h∑ -a ^c _h j D j

1-x ^c _h = (8) where

j = 1, 2,, m a ⁸ : Number of atoms of j-th element contained in molecule of i-th gas species

a ^c : Number of atoms of element j number 5 contained in the molecule of the ith condensed species

 b j: amount of j-th element in the system

 m: Number of elemental species present in the system

Is shown.

 Here, in the present embodiment, an atmosphere composition model equation linearized from Equations 8 and 1 is set by a program stored in the host computer, and a solution obtained from this atmosphere composition model equation is set. We decided to converge and obtain the optimal solution.

 The gas composition in the carburizing furnace was calculated according to this atmosphere composition model formula. Fig. 14 shows the calculation results and the actual measurement results.

 As is clear from Fig. 14, the calculated results of the gas composition in the carburizing furnace are in good agreement with the measured values.

 Next, in considering the necessary conditions of the atmosphere gas composition in actual continuous carburizing, the C balance in the furnace was given by the following equations (9) and (10). Equation 10 is a function calculated based on the specifications of the steel sheet and the surface reaction rate.

W ^E , = W ^S c + W ^g . (9)

W ^s c ξ (V, t, w, LS) (10)

 1 C mass in atmospheric gas entering the furnace

W ^s c C mass removed to strip

W ^s o C mass in atmospheric gas from furnace

 V: Surface reaction rate, t: Carburizing time, w: Sheet width

Is shown.

In this way, by calculating the above-mentioned atmosphere parameters based on the thermodynamic (atmosphere composition) model formula in consideration of the material balance in the actual case of continuous carburization in the carburizing furnace, it is possible to reliably prevent sooting from occurring. However, the carburizing power of the atmosphere composition can be increased as compared with the atmosphere specifications obtained without considering the material balance in the furnace. Therefore, for example, it is possible to improve the actual operation capability such as increasing the CO concentration in the atmosphere gas to increase the sheet passing speed. Next, the principle of carburizing amount control constituting the main part of the present embodiment will be described. The surface reaction when CO is used as the atmospheric gas can be thought of as the following formulas 11 to 13.

 CO ^ [C] +0 (11)

C0 + 0 → C0 ₂ (12)

F e + [C] → F e-C (diffusion in steel) (13) According to the above-mentioned leaves, if the C concentration in the surface layer of the steel sheet is extremely low and the carburizing time is extremely short, the carburizing conditions will be in equilibrium. Since the reaction rate of Equation 13 is faster than the desorption reaction of adsorbed oxygen of Equation 12, it is assumed that this reaction is a rate-limiting reaction. It was expressed by 14 formulas.

 V = k ■ PCO (PCO / (PC0 + (a c / K))) (14)

 k: reaction rate constant, PC0: partial pressure of CO gas, ac: carbon activity, K: equilibrium constant.

However, the effect of H ₂ is not taken into account in the above equation (14). The reaction equation for H _2, reaction is considered to be represented by the following 1 Equation 5 to the reaction formula represented by 1 2 formula.

C_〇 + H ₂ + 2_Rei → C_〇 ₂ + H ₂ 〇 (15) Further, the reaction represented by the following 1 6 expression is considered with respect to the generated C0 _2.

H ₂ + C 0 ₂ ^ H ₂ 0 + C〇 (16) Based on these reaction equations, H ₂ has the effect of accelerating the carburization reaction. It was expressed by seven equations.

V = k]-f, (PCO, ΡΗ ₂ , θ ₀ ) (17)

 0. ： Adsorption oxygen coverage

Is shown.

Note that if the concentration of C_〇 ₂ or H ₂ 〇 in the atmospheric gas generated by the carburizing high (e.g. C0ZC0 ₂ ≤ 5 0), Te cowpea the reaction represented by 1 8 expression or 1 9 formula below Carburizing reaction is inhibited. C + C〇 ₂ ^ 2 CO (18)

C; H ₂ 0 C〇 + H ₂ (19) Therefore, in the present embodiment, the surface reaction rate V is expressed by the following equation (20) or (21) in consideration of these carburizing reaction inhibitory factors.

V = k, · f, (PCO, PH ₂ , 6> o) x α · fa (PC0, PC0 ₂ )

 ··… '… (20)

V = k, · f, ( PCO, PH 2, 6> o) -k 2 - f 2 (PC0 2, PH 2 0)

 (21) However,

Constant, k,, k ₂ : reaction rate constant

And the reaction rate constant, k ₂ , can be set by the following equation (22).

 ki · exp (-E i RT) (22) where

A: frequency factor, E;: activation energy, R: gas constant, T: absolute temperature. Since frequency factor Ai, activation energy Ei, any gas constant R is a constant number, the reaction rate constant k,, k ₂ was calculated from experimental values under the condition of various absolute temperature T. Figure 5 shows the reaction rate constant k, obtained by the experiment. In the present embodiment, when only the CO concentration needs to be considered, for example, when the supply gas flow rate in the atmosphere is large, the above equation (14) may be used as the surface reaction rate equation.

 Next, in this example, the diffusion of solute carbon in steel modeled to obtain a desired carburizing concentration distribution will be described. The diffusion state of C into steel is expressed by the following carbon diffusion model equation based on Fick's law, as shown in Equation 23 below.

^{dC / dt = D · d 2} C / dX 2 (23) where

 C: C concentration in steel, t: time, D: diffusion coefficient, X: diffusion distance

Is shown.

The diffusion coefficient D is also set by the Arrhenius equation represented by the following equation (24). In this example, the diffusion coefficient D is approximately displayed by actual measurement data. D = exp (aT + b) (24) where

 T: carburizing temperature, a: proportional coefficient, b: constant

Shows

Therefore, the amount of carburizing to the steel sheet can be calculated by the above-mentioned formula (17), formula (21), formula (22) and formula (23). This means that if the carburizing concentration is set at one point of the desired carburizing concentration distribution under the condition of constant carburizing amount, the above-mentioned carbon diffusion model equation is set, and even when the carburizing amount is different, the carburizing concentration distribution at two or more points is different. If the concentration is set, it means that the carbon diffusion model formula is set. In addition, when the stripping speed is regulated from operating conditions other than carburizing as described above, the carburizing time t in the above equation 23 is the value obtained by dividing the effective carburizing furnace length L by the stripping speed L _s. Therefore, this calculated value is used when time-integrating the above equation (23) with the carburizing time.

 The above calculation is sequentially performed by a program stored in the host computer in advance, and the specifications of the carburized steel sheet, that is, in this embodiment, the desired carburized concentration, the carburized amount of the strip given from the cloth, and the atmosphere The algorithm for setting the carburizing conditions, which matches the amount of carburizing on the strip calculated from the amount of C reduction in the gas, is shown in the flowchart of FIG.

 First, in step S1, the condition of the steel sheet after carburizing is set, the composition of the atmosphere gas, the flow rate of the input gas, the carburizing temperature and passing speed, the steel sheet specifications and the carburizing concentration distribution of the steel sheet are used to determine the surface of the steel sheet. The conditions such as the C concentration C, at the specified depth X, are read. Also, here, for example, the threading speed is set to L S, and it is corrected in a flow that is performed later. Parameter.

 Next, the process proceeds to step S2, where the set carburizing amount A C for the steel sheet is set based on the steel sheet specification and the steel sheet specification, and the C amount per unit time taken out of the carburizing furnace by the strip is calculated.

 Next, the process proceeds to step S3, where the atmosphere composition model formula is set from the composition of the atmosphere gas read in step S1.

Next, proceeding to step S, the atmosphere in consideration of the C amount taken out of the carburizing furnace by the strip according to the atmosphere composition model formula set in step S3. Calculate the concentration of each component of the gas.

 Next, proceeding to step S5, the surface reaction rate of the steel sheet is calculated based on the equation (17) .t) o

 Next, the process proceeds to step S6, in which the carburization rate in the steel is calculated based on the above equation 23, and the amount of C diffusion into the steel is calculated.

 Next, when the carburizing time has elapsed, the process proceeds to step S7, and the surface reaction rate per unit time and unit area calculated in step S5 or step S6 described above or the rate of reaction into steel is calculated. The amount of diffusion C is integrated with the processing time and the total surface area of the steel sheet to calculate the amount of carburization AC 'to the steel sheet.

 Next, proceeding to step S8, it is determined whether or not the absolute value of the difference between the set carburizing amount ΔC and the calculated carburizing amount ΔC is smaller than a predetermined value a, and the absolute value of the difference between the two is determined. If is smaller than the predetermined value a, the process shifts to step S10. Otherwise, the process shifts to step S9.

 In step S9, the set carburizing amount is corrected based on the carburizing amount based on the following equation 25, and the process proceeds to step S3.

 A C = A C + (A C-A C) x b (25) where

 b: Constant

Is shown. Therefore, if the total C amount taken out of the carburizing door by the strip is equal to the total C amount carburized, that is, if the material balance in the carburizing furnace is satisfied, the process proceeds to step S10.

 In step S10, it is determined whether the absolute value of the difference between the target carburization amount △ (. And the set carburization amount AC is smaller than a predetermined value d, and the absolute value of the difference between the two is greater than the predetermined value d. If smaller, the process proceeds to step S12. Otherwise, the process proceeds to step S11.

In step S11, at least one of the parameters of the atmosphere gas flow rate, the atmosphere composition, the passing speed, and the carburization temperature is changed in order to obtain the set carburization amount set from the carburization concentration distribution condition. The process moves to step S2. Here, for example, the predetermined carburizing amount ΔC. And LS to compensate for the difference between To correct this, the corrected threading speed LS may be calculated based on, for example, the following equation (26).

 LS = LS + (AC— △ (.) X d '(26) where

 d ": constant

Is shown.

 In the step S12, the C concentration C 'at the designated depth X, from the steel sheet surface, is calculated according to the diffusion model in steel set in the step S6.

 Next, proceeding to step S13, the setting C concentration C of the specified depth X, from the steel sheet surface read in step S1, and the specified depth from the steel sheet surface calculated in step S12, It is determined whether the absolute value of the difference between the X and the C concentration C 'is smaller than a predetermined value e.If the absolute value of the difference between the two is smaller than the predetermined value e, the process proceeds to step S15. Otherwise, go to step S14.

 In the step S14, at least one of each parameter of the atmosphere composition, the passing speed, and the carburizing temperature is changed in order to obtain the set carburizing amount set from the carburizing concentration distribution condition. Move to

 In step S15, the set values of the concentration of the atmospheric gas component or the passing speed or the carburizing temperature obtained as a result of the above calculation are output according to the purpose of control, and the total carburized amount and the average carburized amount are output. Outputs the calculation results such as the amount and distribution of carburization from the steel sheet surface, and terminates the program.

In the flowchart of FIG. 6, the ambient gas flow amount which is the input condition is a control amount for changing the C0 ₂ and H ₂ 0 concentration in the atmosphere gas as described above, as a regulator Is considered to be included in the atmosphere gas composition in the same way as the C〇 + H ₂ flow rate charged into the furnace.

Using this program, the passing conditions in the industrial continuous carburizing operation are as follows: passing speed LS = 200 mpm, plate thickness D = 0.75 mm, plate width W2 140 0 mm, supply gas amount = 10 At 0 ONm ³ / hr, the sooting occurrence limit obtained by taking into account the material balance at each carburizing temperature calculated by this program is shown by the solid line in Fig. 7. In the figure, the broken line indicates the upper limit of the dew point. Dot-and-dash lines do not take material balance into account. This shows the limit of sooting that was found. The shaded area in the figure indicates the operation range in actual carburizing operation.

The occurrence limit of the scan one coating obtained by considering the mass balance As is apparent from the drawing, CO concentration concentration of H ₂ becomes higher than the occurrence limit of sooting determined without considering material balance . That is, the carburizing speed is improved accordingly. On the other hand, the higher the carburizing temperature, the higher the CO and H ₂ concentrations associated with the sooting limit. This means that the overall carburizing operation efficiency also depends on the temperature, and conversely, when increasing the stripping speed, allowance for operation such as raising the furnace temperature as much as the material allows. Therefore, the setting range of various conditions in the actual case of continuous carburizing will be expanded. Of course, sooting does not occur even if the operating range is set along the sooting limit determined without considering the material balance in the furnace, but the operating margin is reduced by that much, and various conditions are set. The range narrows.

 FIG. 8 shows the correlation between each carburizing condition calculated by this program, that is, the carburizing amount when each of the control amounts is changed, and the actually measured carburizing amount. As is evident from the figure, the calculated and actual measured values of carburization amount agree very well. This means that the setting of the carburizing speed, that is, the surface reaction speed, and the setting of the temperature dependence coefficient are correct. As long as the setting of the surface reaction speed is correct, the continuous carburizing method of the present embodiment is used. Means that a wide range of applications is possible in the region where the carburization rate follows the surface reaction rate greater than the diffusion rate.

 Further, a specific calculation example of the control amount for carburizing amount control calculated by this program will be described with reference to FIG.

 Here, for example, a predetermined (target) carburizing amount is set in step S2 from the steel sheet specifications such as the thickness data read in step S1 as clearly shown in FIG. The tolerance has been set. In step S1, the target carburizing temperature was set based on the material conditions of the steel sheet.

Accordingly, the CO concentration and the H ₂ concentration are set as the atmosphere gas conditions for preventing sooting in the steps S3 and S4.

Assuming that the control accuracy of the concentration of the atmosphere gas component is 0.39 in the actual machine, the above equation (17) calculated in the flow of steps S3 to S11 is used. Equation 23 sets the target carburizing time and the allowable range of carburizing time fluctuation as shown in Fig. 9.

 Next, for the carburizing zone furnace length, the passing speed is

 Passing speed = carburizing zone furnace length Z carburizing time

In step S12, the target threading speed and its allowable range are set and output.

 When the carburizing amount and the atmosphere gas composition are set as described above, the carburizing time (sheet passing speed) is set in the loop of steps S10 and S11.

 As described above, in the present embodiment, in the region where the carburizing rate is determined by the surface reaction rate, the optimum conditions are determined in consideration of the overall carburizing conditions to obtain the carburizing amount set from the sheet specifications. The control can be completely automated, which previously relied on experience.

 Further, a specific calculation example of the control amount for carburizing amount control calculated by this program when the sheet passing speed is restricted by operating conditions other than the carburizing process will be described with reference to FIG.

 Here, for example, a predetermined (target) carburizing amount is set in step S2 from the steel sheet specifications such as the thickness data read in step S1. In step S1, the target carburizing temperature was set based on the material conditions of the steel sheet. Further, the carburizing time is calculated by dividing the effective carburizing furnace length by the passing speed read in step S1.

Then, CO concentration, the upper limit of the concentration of H ₂ is set as the atmospheric gas condition to prevent sooting in the step S 3 and step S 4.

Surface reaction rate equation in the flow of the step S 3~S 9 contrast, in steel diffusion model formula are set, CO concentration, H ₂ concentration required to achieve the target carburizing quantity from these equations, C 0 ₂ concentration, H ₂ 0 concentration is set.

 Therefore, if the target carburizing amount is large or the carburizing time is short as shown in Fig. 10, the atmosphere gas composition is controlled so that the CO concentration in the atmospheric gas becomes large, and the target carburizing amount becomes small. Or, if the carburizing time is long, the atmosphere gas composition is controlled so that, for example, the C〇 concentration in the atmosphere gas becomes small.

A method for controlling the composition of the atmosphere gas discharged from the carburizing furnace at the carburizing furnace temperature As, for example CO + H ₂ concentration may be changed to CO flow amount and flow rate of H ₂ ratio of the atmosphere gas flow rate supplied to the carburizing furnace, the concentration of C_〇 ₂ or H ₂ 0 is supplied Kiri囲What is necessary is just to change the total gas gas flow rate.

 As described above, in the present embodiment, even when the threading speed is regulated in advance, the carburizing conditions for obtaining the carburizing amount set from the sheet specifications are set to the optimum conditions in consideration of the overall operating conditions. These controls, which previously depended on experience, can now be fully automated.

 Next, a description will be given of a calculation example in which a desired carburizing concentration distribution of the carburized thin steel sheet is calculated by a carbon diffusion model formula based on the Fick's law with reference to FIGS. 11 to 13. FIG. According to the algorithm of FIG. 6, the carburizing amount per unit area is set by integrating the carburizing concentration distribution in the depth direction, and the desired carburizing concentration is obtained under the constraint conditions that satisfy the carburizing amount. A carbon diffusion model formula is set based on the distribution form, an allowable range is set for the target value at each point in the depth direction, and the carburized concentration profile calculated from the carbon diffusion model formula falls within this allowable range. Thus, the carburizing temperature and the carburizing time, which are the parameters of the model formula, are set.

 By the way, in the case of carburizing concentration distribution shown in Fig. 11, there is a peak of carburizing concentration at a depth of about 10 to 50 m from the surface of the metal strip, and at a deeper depth up to a depth of 250 m, Gradually the carburizing concentration decreases. This is because decarburization proceeds in the vicinity of the surface of the surface layer where the carburizing concentration is inherently the highest, assuming the above-mentioned seal and cooling. Therefore, in order to match the form of the carburizing concentration distribution with the above-mentioned carbon diffusion model formula, the carburizing concentration at two or more points in the form of the carburizing concentration distribution at a depth of 10 to 250 m from the surface is determined. Preferably, to capture the peak point of the carburizing concentration, one point in the range of 10 to 50〃m in depth, and one or more points in the range of 100 to 250〃m It is desired to set However, when the carburizing amount is constant as described above, the carbon diffusion model equation can be obtained by setting the carburizing concentration at only one point under the conditions such as the surface reaction rate, carburizing temperature, and carburizing time. Will be set uniquely.

Here, carburizing time (treatment time, se) is at t 2, t _3, CO concentration (%) of a], a _2, a _3, H ₂ concentration (%) is b,, b _2, b ₃ , The carburizing temperature is T (° C) Under constant carburizing conditions, the correlation curve and the measured data from the distance from the metal strip surface obtained by this model formula, that is, the depth (/ m) and the carbon concentration in steel (carburizing concentration, p pm) were obtained. It is shown in Figure 11. However, the carburizing time t, = t ₂ ≠ t 3, the C 0 concentration a, = a ₃ ≠ a ₂ , and the H ₂ concentration b] = b ₂ = b ₃ . In Fig. 11, the actual measurement of the carburizing concentration was calculated by adding the specimen to hydrofluoric acid and dissolving it from its surface, and calculating the amount of solute carbon from the weight ratio of the amount of dissolved C and the amount of Fe over a predetermined dissolution time. However, it may be estimated by measuring the depth of a specific structure of the steel, which is determined (dependent) by the carburizing concentration.

Next, FIG. 12 shows the results of an experiment on the effect of carburizing time in the steel diffusion model formula. Carburization temperature T ° C constant in the figure, under the conditions of total carburizing quantity AC ppm-constant, CO concentration (%) of a _4, H ₂ concentration (%) is b _4, carburizing time (treatment time, sec.) There the case of performing the carburization at atmospheric conditions t ₄ in solid lines, CO concentration (%) of a _5, H ₂ concentration (%) is b _5, carburizing time (treatment time, se) is at ambient conditions t ₅ The case of carburizing is shown by the broken line. However, the carburizing time t ₅ ^ 3 t 4, CO concentration a _4> a _5, H ₂ is a concentration of b ₄ »b _5. As described above, the higher the CO and H ₂ concentrations, the higher the carburizing reaction speed, and the longer the carburizing time, the larger the amount of carburization into the inner layer. Therefore, as is clear from the figure, in the present embodiment, when only the C concentration in the surface layer is increased and the gradient with the C concentration in the inner layer is made steep, the carburization reaction rate is increased (the carburizing power is increased). In other words, if the carburizing time should be shortened, and if the C concentration gradient between the inner layer and the surface layer is moderated by increasing the entire C concentration of the steel sheet, the carburizing reaction rate should be reduced (carburizing). It can be seen that the carburizing time should be increased by reducing the power.

Next, an embodiment in which the carburizing concentration distribution is controlled by controlling the sheet temperature after the carburizing step, specifically, by controlling the cooling rate will be described with reference to FIG. Carburization temperature T ° C constant in the figure, carburizing time tse constant, Rei_0 concentration of 3 _6% - constant, H ₂ concentration b _e% - at constant conditions, cooling rate .DELTA..tau, the case where the cooling in ° C / se a solid line indicates the case where the cooling at a cooling rate ΔΤ ₂ ° C / sec. with a broken line, cooling rate .DELTA..tau, a <.DELTA..tau _2. As is clear from the figure, the higher the cooling rate, the more quickly the solid solution C diffuses into the interior, so that the gradient of the C concentration in the inner layer becomes steeper when only the C concentration in the surface layer increases. Conversely, the lower the cooling rate, the more solid solution C diffuses inside, so the C concentration in the surface layer is low. In addition, the C concentration gradient with the inner layer becomes gentle.

 In this embodiment, the case where the strip subjected to the predetermined carburizing treatment in the carburizing zone is quenched by the first cooling zone to fix the carbon diffusion is described in detail, but in the present invention, the strip after the carburizing is heated. It is possible to control the carbon diffusion state by heating, soaking, and cooling. For this purpose, Z or a sheet temperature control zone may be provided instead of the first cooling zone.

In the present embodiment, the carburizing temperature is set from the material conditions and the CO concentration and the H ₂ concentration are set in advance from the sooting generation limit using the algorithm of FIG. If it carburizing time to obtain the amount of (sheet passing speed) finally modified is set an upper limit of carburizing temperature and carburizing time of carburized concentration distribution condition and scan from a coating of occurrence limit CO concentration and concentration of H ₂ of When the carburizing time (peeling speed) and the atmosphere gas composition are finally changed to obtain the carburizing concentration distribution and the carburizing amount in the predetermined steel sheet thickness direction under the condition where the upper limit is set in advance, except for carburizing treatment When the carburizing time is determined based on the sheet passing speed set from the operating conditions and the carburizing temperature is set based on the material conditions, and the atmosphere gas composition is finally changed in order to obtain the specified amount of C under the condition where the carburizing temperature is set More about As described above, each of the following control examples of the above-described control factors, including these, can be considered.

 1) When the atmosphere composition is constant, change the carburizing temperature and carburizing time individually or simultaneously.

2) When the carburizing temperature is constant, change the CO partial pressure or H ₂ partial pressure or C〇 + H ₂ partial pressure and carburizing time of the atmosphere composition individually or simultaneously.

3) When the carburizing time is constant, change the CO or H ₂ partial pressure or CO + H ₂ partial pressure and the carburizing temperature of the atmosphere composition individually or simultaneously.

 4) Change all control factors simultaneously or individually.

 The method of selecting these control factors is not limited to any one, and can be expanded in any case under various given conditions. ,

Further, in the surface reaction, in this embodiment CO, H _2, C 0 ₂ and has been described in detail for the case where considering only the effect of H ₂ 0 calculates the surface reaction rate, other ambient gas, as described above Calculate the surface reaction rate taking into account the composition, e.g., the effects of heavy hydrocarbons It may be issued.

 In this embodiment, the thermodynamic model equation taking into account the material balance is linearized, and the solution is converged to calculate the equilibrium state. However, the means for calculating the equilibrium state is not limited to this. is not.

 Further, in this embodiment, particularly, only the case where the strip made of ultra-low carbon steel is continuously annealed and carburized in the above-mentioned surface reaction rate-determining region is described in detail. However, the present invention can be applied to other metal strips.

Claims

The scope of the claims

 1. When the metal strip is continuously passed through a carburizing furnace for carburizing, the material balance in the carburizing furnace between the carburizing gas supplied into the carburizing furnace and the carbon fixed and taken out of the metal strip. A method for continuously carburizing a metal strip, comprising controlling the atmosphere gas composition or the furnace temperature in the carburizing furnace to the atmosphere gas composition or the furnace temperature at which sooting does not occur based on the method.

 2. The atmosphere gas composition and / or the furnace temperature are controlled in a region where the carbon concentration in the surface portion of the metal band is equal to or less than the concentration at which the metal band and the atmosphere gas equilibrate. 2. The method for continuous carburization of a metal strip according to 1.

 3. When continuously carburizing a metal strip passed through a carburizing furnace, an atmosphere gas composition containing at least carbon, oxygen, nitrogen or an element of carbon, oxygen, hydrogen, nitrogen and sooting does not occur. In controlling the furnace temperature, the equilibrium state of the furnace atmosphere is determined by obtaining a state in which the cast free energy of the entire furnace atmosphere is minimized based on the material balance of each element in the carburizing furnace. A method for continuously carburizing a metal strip, wherein the control amount of the composition of the atmosphere gas or the temperature in the furnace is calculated based on the obtained thermodynamic model equation.

4. As a condition of the atmospheric gas composition and the furnace temperature, the furnace temperature within 7 0 0-9 5 0, the carbon monoxide concentration is 0% <CO concentration ≤ 2 2%, hydrogen concentration 0% ≤H ₂ 4. The method for continuous carburization of a metal strip according to claim 3, wherein a condition of a concentration ≤ 30 is applied.

 5. When the metal strip is passed through the carburizing furnace and continuously carburized, the temperature of the metal strip, the composition of the atmosphere gas and the carburizing time are used as control variables, and these control variables are controlled to control the metal during carburization. Carburizing treatment should be performed in a reaction region in which the carbon concentration in the surface layer is equal to or less than the equilibrium concentration between the metal band and the atmospheric gas, and the carburization rate of the surface layer is higher than the diffusion rate from the surface layer to the inside. Characterized by continuous carburizing of metal strips.

6. In controlling each of the above control variables, a carburization reaction rate prediction equation in which the metal zone temperature and the carbon monoxide partial pressure, or the metal zone temperature, the carbon monoxide partial pressure, and the hydrogen partial pressure are all set in parallel, A prediction formula for the carburizing amount determined by the carburizing time is set in advance, and each control amount is set based on any or a combination of these prediction formulas. Item 6. A method for continuously carburizing a metal strip according to Item 5.

7. The method for continuous carburizing of a metal strip according to claim 6, wherein the carburizing amount is controlled by adding a partial pressure of carbon dioxide and / or a partial pressure of H ₂ ◦ to the control amount in the prediction formula.

8. When a metal strip is passed through a carburizing furnace for continuous gas carburization, the temperature and / or atmosphere gas composition of the metal strip is used as a control amount, and the strip is regulated by operating conditions other than carburizing. The control amount of the atmosphere gas composition and the Z or metal zone temperature is set based on a prediction formula set in advance to obtain a target carburizing amount for the carburizing time determined by the speed. Continuous carburizing method for metal strips described in 6 or 7

9. The continuous carburizing method of a metal strip according to claim 8, wherein the carburizing time is added as a parameter within a range of a regulated threading speed.

 10. When the metal strip is passed through a carburizing furnace for continuous gas carburization, the metal strip temperature, the atmosphere gas composition and the carburizing time are used as control variables to control the desired carburizing concentration distribution in the thickness direction. A method for continuously carburizing the metal strip, wherein the control amounts are set based on a metal strip-thickness carbon diffusion model formula based on Fick's law set in advance.

 11. The method for continuously carburizing a metal strip according to claim 10, wherein the temperature of the metal strip and the partial pressure of carbon monoxide and the carburizing time, or the temperature of the metal strip and the partial pressure of carbon monoxide and the hydrogen partial pressure and the carburizing time. And a method for continuously carburizing a metal strip, wherein

12. The method for continuously carburizing a metal strip according to claim 11, wherein a partial pressure of carbon dioxide and a partial pressure of H ₂₀ are added to the control amount as parameters.

 13. The carburizing concentration distribution form is to set the target carburizing concentration at one or more points in the range of 10 to 250 m depth of the metal layer surface or the metal structure form determined by the carburizing concentration. 13. The method for continuous carburizing of a metal strip according to claim 10, wherein the metal strip is carburized.

 14. The method for continuously carburizing a metal strip according to any one of claims 10 to 13, wherein after the gas carburizing, the temperature of the metal strip is controlled to control the distribution of carburizing concentration in the thickness direction of the metal strip. .