JP3028995B2 - Method for continuous carburization of metal strip and sheet temperature control - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling the temperature of a metal strip in a carburizing furnace, such as a heat treatment furnace inside or outside of the carburizing furnace, while controlling the sheet temperature to obtain a desired sheet temperature. When performing continuous gas carburizing of a metal strip passed through a metal strip, a metal strip for setting a stripping speed in order to obtain a desired carburizing amount or carburizing concentration distribution within an atmosphere composition range in which sooting does not occur in the carburizing furnace. The present invention relates to a continuous carburizing control method and a sheet temperature controlling method for obtaining the desired sheet temperature at this passing speed, for example, by heating or equalizing a strip made of ultra-low carbon steel to a predetermined sheet temperature and re-heating the strip. When passing through a carburizing furnace for continuous gas carburization after crystal annealing, atmosphere gas composition, composition gas concentration, carburizing temperature, passing speed, etc. to obtain the carburizing amount or carburizing concentration distribution. While controlling the heating furnace and / or Those suitable for controlling the furnace temperature (furnace temperature) and fuel input of the furnace.

[0002]

2. Description of the Related Art In the metal secondary processing industry such as the automobile industry, for example, it is required that a metal plate to be processed has both higher workability and higher strength. Specifically, in the automobile industry, since it is necessary to reduce the weight of the vehicle body in order to pursue fuel efficiency due to global environmental issues that have recently become a problem, the conventional deep drawability for press-baked coated steel sheets and the like has been improved. Higher strength steel sheets are required after maintenance.

[0003] As evaluation indices of such a metal plate, 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 sheet width distortion / sheet thickness distortion), carbon in steel (hereinafter, referred to as C) is used. It is known that it is most advantageous to reduce the amount. In addition, the reduction of carbon improves ductility (Elongation: El) and delayed aging at normal temperature (Aging Index: lower the AI, the better). However, on the other hand, as the C content in the steel decreases, other evaluation indices generally deteriorate. For example, the tensile strength (Tensile Stre
ngth: TS), the grain boundary strength is reduced, the brittleness in secondary processing is deteriorated, and the amount of solid solution C is reduced, and the bake hardenability is deteriorated. If the C content in steel is 50 ppm or less, the grain growth rate is accelerated by heating by welding, and the heat-affected zone (Heat
Affected Zone (HAZ) deteriorates spot weldability due to coarsening.

On the other hand, in the case of press-coated steel sheets used in the secondary metalworking industry, baking coating is often performed after press working, so that the formability is exhibited during press working and baking is performed during baking coating. There is a demand for a high bake hardenable steel sheet which exhibits hardenability and improves strength. Of course, a normal temperature delayed aging property that can maintain the formability until the press working is required, and as a result, the steel sheet used is a high bake hardening type steel sheet having a normal temperature delayed aging property (low AI-high BH steel sheet). Is required.

Accordingly, 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 aging property. The presence of solid solution C in the surface layer by the treatment results in the tensile strength, secondary work brittleness, BH property,
In order to improve the spot weldability, the present applicant has developed a continuous annealing carburizing facility described in JP-A-4-88126 as shown in FIG.

According to the continuous annealing carburizing equipment, the heating zone 2
Alternatively, after performing a predetermined recrystallization annealing on the metal strip in the soaking zone 3, the temperature of the steel sheet, the specifications of the atmosphere, the transfer speed (furnace time),
By performing the carburizing process while controlling the cooling conditions, it is possible to continuously produce a metal strip having a desired surface carburization depth and concentration distribution while satisfying the material specifications of the metal strip.

Further, the present applicant has developed and proposed a useful steel plate described in Japanese Patent Application No. 4-95503 as the low AI-high BH steel plate. This steel sheet is subjected to an appropriate annealing treatment and carburizing treatment for a very low carbon steel containing a predetermined amount of a predetermined element to obtain a set solid solution C amount or carbon concentration distribution of a surface layer portion and an inner layer portion, Low AI-
It exhibits high BH properties. Specifically, as shown in FIG. 3, while the C concentration in the inner layer of the steel sheet is kept low, the C concentration in the surface layer is kept low.
It requires an annealing treatment and a carburizing treatment that greatly increase only the concentration.

[0008]

By the way, the continuous annealing carburizing equipment as shown in FIG. 2 in which the continuous carburizing equipment is added to the continuous annealing equipment which originally requires a very large installation space, has a very large installation. Requires space. On the other hand, when the specifications of the metal strip are determined from the specifications required for the metal sheet such as the low AI-high BH steel sheet, the carburizing concentration and the carburizing depth are extremely small, and the carburizing time, that is, the furnace time is short. It became clear. Therefore, in order to make the huge installation space of the continuous annealing carburizing equipment as described above as small as possible, it is desirable to shorten the effective carburizing furnace length related to the carburizing zone existing time so that the carburizing furnace is made as compact as possible. .

Further, as is known, there is a parameter called carburizing temperature in controlling the amount of carburizing into a metal strip. This is because as the temperature rises, the reaction rate at which C separated from the atmospheric gas is combined with Fe on the surface of the metal strip increases, and the rate at which C diffuses inward from the surface layer portion of the metal strip also increases. On the other hand, when this temperature is lowered, the concentration limit of free C, that is, the concentration of carbon monoxide (CO) at which sooting occurs is lowered, and hydrogen (H
₂ ) The concentration limit also drops. It is also known that the control factor of the carburizing amount control temperature is large in the control response time. However, it is necessary to continuously control the amount of carburizing of the strips according to each steel sheet under the conditions of short carburizing time as described above by passing a series of strips made of steel sheets with different specifications in a series. In the continuous industrial carburizing operation, if the control amount of the carburizing temperature is increased, the response time may not be enough to actually follow the carburizing control, or extreme overshoot may occur with respect to the target control amount. Therefore, there is a demand to reduce the control amount of the carburizing temperature as much as possible.

Here, the conditions for setting the carburizing temperature will be considered. From the material conditions, the carburizing temperature is preferably lower than the recrystallization temperature. On the other hand, under the condition that the carburizing furnace is made compact and a large processing capacity is obtained with a specified furnace length, the bonding reaction rate between Fe and free C and the internal diffusion rate of solid solution C (hereinafter, both are considered simultaneously) In this case, the carburizing speed is referred to as the carburizing speed, and the specific reason will be described later).
I would like to set the O concentration high and at the same time raise the carburizing temperature in order to increase the carburizing speed itself.

When the carburizing temperature is regulated to some extent in this way, the upper limit of the composition and concentration of the atmosphere gas is set from the sooting limit and the dew point limit. That is, the furnace presence time is set, and thereby the passing speed in the carburizing furnace is set. When controlling the carburizing concentration distribution required for the low AI-high BH steel sheet, the carburizing temperature related to the reaction rate and the diffusion rate of the surface of the metal band that controls the amount of solid solution C taken into account is determined by: The carburizing time related to the atmosphere composition and the diffusion time is obtained, and the sheet passing speed is set based on the carburizing time.

Meanwhile, in the continuous annealing carburizing equipment as shown in FIG. 2, the heating zone 2 and / or the soaking zone 3 may be used other than the carburizing zone.
In the cooling zone 5,
6, it is necessary to perform a predetermined cooling treatment. Therefore, in each heat treatment zone (except for the carburization zone, these heat treatment zones are mainly used to control the sheet temperature, the holding time thereof, and the sheet temperature gradient generated from the both. However, it is necessary to perform a predetermined plate temperature control, for example, by controlling the furnace temperature. In each furnace constituting each of these plate temperature control zones, the plate temperature is controlled mainly by heat transfer, but at the same time, the upper and lower limit values of the furnace temperature itself also exist by the calculation of the capacity of each furnace. For example, in a heating furnace in a heating zone or a soaking furnace in a tropic zone, an upper limit of the furnace temperature is set based on the capacity of the furnace in order to obtain the maximum processing capacity, and the heat transfer between the radiant tube, the furnace wall, the hearth roll, and the like is performed. From the heat balance in consideration of the coefficient, the minimum furnace time (that is, the minimum heating time or the minimum soaking time) of the strip that satisfies the upper and lower limits of the sheet temperature is set. The maximum passing speed will be set. In the cooling furnace of each cooling zone, a heat transfer coefficient of a cooling gas jet or the like is adopted as the heat transfer coefficient.

Considering the maximum threading speed set in the carburizing zone and the maximum threading speed set in the other sheet temperature control zones, priority is given to the maximum threading speed set in the carburizing zone. In other words, if the maximum passing speed of the carburized zone is lower than the maximum passing speed of the other zone, the temperature control zones other than the carburizing zone, ie, the heating furnace, soaking furnace, cooling furnace, etc. No specific means for controlling the temperature has been proposed yet, and in particular, means for controlling the physical properties and temperature of a heating furnace or a soaking furnace which is indispensable for the annealing step is urgently desired.

Further, in actually setting the various conditions of such a continuous annealing carburizing facility, the following problems have been found. (1) Regarding the carburizing rate, according to a report by Habara et al. (Hashun Yu, Haruyama Shiro et al: Journal of the Japan Institute of Metals 49 (1985) 7,529)
As shown in FIG. 4, when the amount of C in the metal surface layer is high to some extent and the carburizing time is long, the carburizing speed is such that after the C concentration in the surface layer reaches the equilibrium concentration (that is, the equilibrium concentration), C becomes the metal structure. In general, it is proportional to the square root of time, and this time carburizing gain region is called a diffusion controlled region. On the other hand, as described above, the C amount in the metal surface layer portion is reduced. When the carburizing time is extremely low and the carburizing time is extremely short, the C concentration in the surface layer does not reach the equilibrium concentration, so that the rate of carburizing is proportional to the rate at which the metal surface layer directly reacts with carbon. It is known that the time carburizing gain region is called a surface reaction rate-limiting region.

Therefore, for example, from the specifications required for a metal intended to improve the resistance to secondary working brittleness, see Japanese Patent Application Laid-Open No.
When carburizing conditions of the metal band are determined, since the carburizing concentration and the carburizing depth are extremely small, in this case, it is necessary to perform carburizing treatment in a surface reaction rate-controlling region,
Considering that the equilibrium carbon concentration in the steel in the metal layer surface layer is in the same state, it has been found that the carbon potential (C potential) control by conventional management of CO / CO ₂ etc. cannot control the amount of carburization in the metal band. did. (2) In general, the atmosphere gas composition under carburizing conditions can be determined by chemical equilibrium. In the conventional solution method, all possible reactions are listed, and the gas composition is obtained by solving a non-linear simultaneous equation from the equilibrium relation of these reactions. However, it is extremely difficult to determine the exact limit of soot generation (suiting) from the reaction equation of the gas phase system. (3) In addition, there is a report from the above-mentioned Leaf et al. On the above-mentioned surface reaction rate, but this report only discusses the carburization rate of CO gas alone, and it is used as it is to continuously carburize a complex composition. You can't really deploy the operation.

The present invention has been developed in view of the above problems, and in particular, carburizing control for obtaining a desired amount of carburizing of a metal strip within an atmosphere composition range in which sooting does not occur in a carburizing furnace. It is another object of the present invention to provide a specific control method of the sheet temperature control performed inside and outside the carburizing furnace when the sheet passing speed is controlled in advance from the actual carburizing process.

[0017]

Means for Solving the Problems The present inventors have conducted intensive studies on the above problems and as a result, have developed the present invention based on the following findings. That is, for example, the occurrence of sooting can be suppressed in consideration of the material balance constraint that the amount of the element brought in by the original system is constant with respect to the element brought out of the atmosphere by the carburization reaction to the metal surface layer part. The equilibrium state was modeled by a thermodynamic (atmospheric composition) model relating to the carburizing temperature using the carbon monoxide concentration or the carbon monoxide concentration and the hydrogen concentration as parameters. At the same time, it is necessary to increase the carburizing speed in order to make the carburizing furnace compact and obtain the maximum processing capacity with the specified furnace length. Therefore, when the concentration of carbon monoxide is set in this way, the setting of the concentration of hydrogen, which has not been universally used, is determined based on the concentration of carbon monoxide. Specifically, the hydrogen concentration is set based on a relational expression that does not inhibit the carburizing reaction in the surface reaction rate-determining region described later.

On the other hand, for example, as described above, in order to control the amount of carburization into the metal zone in the surface reaction rate-limiting region before the carbon concentration in the metal surface layer reaches the equilibrium concentration, first, the surface reaction in the reaction speed region is controlled. We focused on the fact that a rate could be obtained and this reaction rate integrated over time. This time, that is, the carburizing time is determined by the passing speed. Then, while studying this surface reaction rate, the reaction rate was controlled by controlling the gas composition included in the carburizing reaction equation and the deoxygenation reaction equation that can be considered in the surface reaction between the metal strip and the atmospheric gas. Can be controlled. The most effective components for this gas composition are carbon monoxide and hydrogen. Particularly, when the supply / discharge flow rate of the atmospheric gas is small at high temperatures, the composition amount is small, but carbon dioxide and H ₂ O are also in the carburization reaction. It has been found that these compositions have an effect in terms of inhibiting the surface reaction rate, and further, it has been experimentally proved that the partial pressure of these compositions is a controlling factor of the surface reaction rate. Considering the dependence of the material reaction on temperature, a control factor called temperature is interposed in the coefficient of the surface reaction rate.

When the carburizing concentration distribution of the metal strip is required as in the case of the low AI-high BH steel sheet, the surface reaction rate and the carburizing time are controlled under the above-mentioned constant carburizing condition. That is, carburizing amount = surface reaction speed × carburizing time.
As described above, parameters for controlling the surface reaction rate include the above-described parameters of the carburizing atmosphere gas composition, the composition concentration, and the carburizing temperature. Therefore, specifically, for example, when increasing the C concentration in the surface layer portion of the metal strip (making the C concentration gradient steep), the carburizing time is shortened to increase the surface reaction rate, and the C concentration in the surface layer portion is lowered. In the case where the C concentration gradient is moderated, the carburizing time is lengthened to reduce the surface reaction rate.

By the way, these carburizing times are represented by a simple calculation: carburizing time = furnace time = effective carburizing furnace length / sheet passing speed. Therefore, if the carburizing amount is given as a given condition from the specifications of the steel sheet and the carburizing time is set as described above from the requirement of the maximum processing capacity or the carburizing concentration distribution in the furnace, the carburizing time is Is set in order to achieve the following. If the set maximum sheet passing speed in the carburizing furnace is smaller than the maximum sheet passing speed set in other sheet temperature control zones, such as a heating zone and a soaking zone, the other sheet temperature control zones Is controlled by the maximum sheet passing speed in the carburizing furnace, so that the oven time (heating time, soaking time) of each sheet temperature control zone is set. For this furnace time,
In order to achieve the required sheet temperature, the furnace temperature is set from a heat balance or the like in consideration of each heat transfer coefficient. In order to achieve this furnace temperature, for example, in heating zones and
That is, the fuel injection amount may be set.

According to the method for continuously carburizing a metal strip and controlling the sheet temperature according to the first aspect of the present invention, the sheet temperature control for obtaining a desired sheet temperature for the metal strip inside and outside a carburizing furnace is performed. While performing, within the atmosphere composition range where sooting does not occur,
A method for controlling carburization and sheet temperature in the case of continuously carburizing a metal strip passed through a carburizing furnace, wherein a sheet passing speed is determined based on conditions in which sooting does not occur in the carburizing furnace and carburizing conditions. br /> set, the furnace temperature in the sheet temperature control zone as the plate temperature control amount for obtaining the desired sheet temperature in accordance with the sheet passing speed and /
Alternatively, the fuel injection amount is set.

The continuous carburizing and sheet temperature control method for a metal strip according to claim 2 of the present invention, the invention smell of claim 1
In setting the sheet passing speed from the conditions in which the sooting does not occur and the carburizing conditions, an atmosphere composition model formula and / or a carburizing method relating to a preset carburizing temperature in order to prevent the sooting in the carburizing furnace. Do not inhibit the reaction
Based on the relational expression , the carbon monoxide concentration or the carbon monoxide concentration and the hydrogen concentration for obtaining the target carburizing concentration distribution and the carburizing time are set, and the passing speed required to achieve the carburizing time is set. Is set.

The continuous carburizing and sheet temperature control method for a metal strip according to a third aspect of the invention, the invention smell of claim 1
In setting the sheet passing speed from the conditions in which the sooting does not occur and the carburizing conditions, an atmosphere composition model formula and / or a carburizing method relating to a preset carburizing temperature in order to prevent the sooting in the carburizing furnace. Do not inhibit the reaction
The carbon monoxide concentration or the carbon monoxide concentration and the hydrogen concentration are set based on the relational expression , and the carburizing is set based on this.
The present invention is characterized in that a carburizing time is set based on a maximum carburizing capacity in a furnace, and a sheet passing speed necessary for achieving the carburizing time is set.

According to a fourth aspect of the present invention, a method for continuously carburizing a metal strip and controlling a sheet temperature according to the fourth aspect of the present invention is directed to the second or third aspect.
In setting the hydrogen concentration, with respect to the carbon monoxide concentration set based on the atmosphere composition model formula for preventing the sooting, the relational formula set to not inhibit the carburizing reaction. It is characterized in that the hydrogen concentration is set on the basis of this.

[0025]

According to the continuous carburizing and strip temperature control method of the present invention, for example, the amount of carburizing into a strip satisfying the required specifications of a steel sheet is set, and the atmosphere composition model of the sooting generation limit with respect to the carburizing temperature is set. The maximum concentration of carbon monoxide and the maximum concentration of carbon monoxide are set by using the maximum concentration of carbon monoxide, or the atmosphere composition model formula and a relational expression which does not inhibit the carburizing reaction, if necessary. At this time, when the carburizing concentration distribution in the strip plate thickness direction is required, the carbon monoxide concentration or the carbon monoxide concentration that achieves the carburizing concentration distribution under the constant condition of the carburizing amount and the nominal hydrogen concentration. The carburizing time is set, and when the carburizing concentration distribution is not particularly required and only the carburizing amount is required, the atmosphere of the sooting occurrence limit is set.
Set the carburizing time from the maximum carburizing capacity in the carburizing furnace set based on the carbon monoxide concentration or the carbon monoxide concentration and the hydrogen concentration obtained from the atmosphere composition model formula, and achieve these set carburizing times To set the maximum threading speed in the carburizing furnace. When the maximum sheet passing speed in the carburizing furnace is smaller than the maximum sheet passing speed in the other sheet temperature control zones set by means other than the above, the sheet passing speed in the carburizing furnace is set to the other sheet passing speed. the sheet passing speed of the temperature control zone because the rate-limiting, in order to obtain a predetermined sheet temperature in each sheet temperature control zone at the strip running speed, the furnace temperature and example that put the heating zone and the soaking zone, you need The fuel input amount was set as the plate temperature control amount .

Thus, for example, in a reaction rate range in which the carburization rate follows a surface reaction rate greater than the diffusion rate from the surface layer portion of the metal strip to the inside, the amount of carburization into the metal strip from the specifications of the steel sheet is determined. Set the carburizing temperature to increase the carburizing reaction below the recrystallization temperature that is the carburizing condition, and set the atmosphere composition model formula of the sooting limit in the carburizing furnace at the carburizing temperature or this atmosphere composition model formula and the carburizing reaction. The control amount of the carbon monoxide concentration or the carbon monoxide concentration and the hydrogen concentration is calculated from the relational expression that does not hinder the control, and the control amount of the carbon monoxide partial pressure or the carbon monoxide partial pressure and the hydrogen partial pressure calculated based on this is calculated. If, to calculate the surface reaction rate per unit time based on the temperature-dependent coefficient of the reaction rate of carburization calculated from the atmosphere composition model formula related to the control amount of the carburization temperature, Either calculate the carburizing time by differentiating the carburizing amount with this surface reaction speed, or calculate the carburizing time to achieve the carburizing concentration distribution set according to the surface reaction speed, and use the effective carburizing time with these carburizing times. The maximum threading speed in the carburizing furnace is calculated by dividing the furnace length. When the maximum sheet passing speed in the carburizing furnace controls the sheet passing speed of the continuous annealing carburizing equipment, the furnace temperature of each of the sheet temperature control zones and the fuel injection are controlled by the sheet passing speed in the carburizing furnace. By performing the control to set the amount, etc., under the most efficient carburizing conditions, while obtaining the amount of carburization to the metal band surface layer portion that satisfies the specifications of the steel sheet, a predetermined sheet in each sheet temperature control zone You can get warmth.

In particular, when the supply / discharge flow rate of the atmospheric gas is small at high temperatures, in order to take into account the effect of inhibiting the carburizing reaction, for example, the carbon dioxide partial pressure is calculated in the above formula for calculating the surface reaction rate. and by adding H ₂ O partial pressure, it is possible to more accurately control the carburization of the metal strip in the carburizing conditions CO _2, H ₂ O is present. And in the continuous carburizing method of the metal strip of the present invention, for example, in order to perform necessary carburizing control, such as when heating and carburizing are performed simultaneously or when carburizing is performed by lowering the temperature somewhat after heating, When the temperature control and the carburizing control are performed in the same or different fields, similar control can be performed by, for example, considering the passing speed in a time-series manner.

[0028]

FIG. 2 shows an example of a continuous annealing and carburizing apparatus for strip made of ultra-low carbon steel in which the method for continuously carburizing a metal strip and controlling the sheet temperature according to the present invention is implemented. In the figure, an ultra-low carbon steel strip A is a not-shown entrance equipment having a coil rewinding machine, a welding machine, a washing machine, etc., a pre-tropical zone 1, a heating zone 2, a soaking zone 3, a carburizing zone 4, a first cooling zone 5. , The second cooling zone 6, a shearing machine, a winding machine, and other delivery-side equipment (not shown).

The heating zone 2 is for heating the strip A continuously fed from the inlet facility and preheated in the pre-tropical zone 1 to a temperature higher than the recrystallization temperature. ~ 1000 ° C and the temperature of strip A is 700-9
Heat the strip to 50 ° C. Then, the heated strip A is used for the time required in the solitary zone 3,
By maintaining the temperature at or above the recrystallization temperature, a {1,1,1} texture advantageous for deep drawability can be developed.

A large number of radiant tubes are arranged near the path of the strip that is passed up and down through the heating zone 2 and the leveling zone 3 through a hearth roll. The temperature inside the furnace (furnace temperature) is controlled by burning the fuel gas supplied to the furnace. In the present invention, the setting of the supply flow rate of the fuel gas is based on the heat demand in the furnace obtained from the heat balance in the furnace, which is obtained by adding the heat loss to the strip and the heat dissipated in the furnace body to the amount of heat to the strip that is passed through and carries the heat from the furnace. (Necessary) It is equivalent to the amount of heat, and is performed by a host computer (not shown) in accordance with the control logic of the entire line described later.

The carburizing zone 4 is provided with a carburizing furnace (not shown) in the carburizing zone 4 in order to form a carburizing layer in which the solute carbon (C) is present in an extremely thin portion (surface layer) of the surface of the strip A. The temperature in the furnace is controlled to 700 to 950 ° C by a computer, and the strip temperature (plate temperature) is 700 ° C or more.
The passing speed is controlled so that it is preferably below the recrystallization temperature and the strip passes through the carburizing furnace in 10 to 120 seconds. Incidentally, the furnace temperature control is performed in order to make the amount of carburizing (carburizing reaction speed) constant in the passing direction of the strip and to suppress the variation in the material. Also,
As is known, sooting, that is, free carbon [C] adhering to the surface of a steel sheet causes a deterioration in surface quality and a detrimental factor in a post-process. At the same time, the reaction in the furnace accelerates in a predetermined direction, for example, 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. Further, the furnace temperature is controlled in an important manner based on a carburizing condition setting logic described later.

In the present invention, the physical properties, carburizing temperature, and sheet passing speed, ie, carburizing time, in the carburizing furnace are regarded as physical quantities to be controlled (control quantities) in continuous carburizing, and are formed into strips by the host computer. From the required specifications such as the required carburized concentration distribution and carburized depth of the carburized layer,
For example, the necessary amount of carburizing is set as a given condition, various basic expressions relating to these control amounts which are set in advance, which will be described later, are appropriately selected, and each control amount for realizing the carburizing amount is calculated. These control amounts are set in consideration of the capacity and process of the system.

In this embodiment, the composition of the carburizing gas supplied to the carburizing furnace and the supply / discharge flow rate are determined by the host computer in such a manner that the free energy in the furnace is minimized in consideration of the material balance in the furnace described later. Is controlled in accordance with various conditions calculated based on the atmosphere composition model formula. The composition of the carburizing gas and the supply and discharge flow rates are controlled so as to suppress the increase in the CO ₂ and the dew point to prevent a reduction in the carburizing reaction rate and a temper color.

Incidentally, the strips in the carburizing furnace are passed through the hearth rolls 20 while moving up and down the furnace through hearth rolls 20. These hearth rolls 20 are required to maintain their rotation and roll crown in a predetermined state. For example, the vicinity of the bearing is cooled. In order to maintain the strength and abrasion resistance of the roll itself, a chrome Cr alloy is used for the hearth roll. However, when the carburizing atmosphere gas reaches the vicinity of the hearth roll, cooling is performed and sooting proceeds, so that C 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, which breaks or expands the crystal grains of the heat-resistant alloy used for the hearth roll, while reducing the amount of solid-dissolved Cr. As the roll is embrittled and oxidized, corrosive corrosion proceeds. When the hearth roll is exposed to a carburizing atmosphere gas as described above, experiments performed by the present inventors have revealed that the hearth roll must be replaced within two years. Therefore, in the present embodiment, the hearth roll chamber is separated from the carburizing atmosphere by the non-contact sealing device 21 to prevent the deterioration of the hearth roll, and the hearth roll chamber is weakened to such an extent that the deterioration of the hearth roll does not progress. The carburized state succeeded in preventing so-called decarburization, in which C is radiated 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 there is no problem with decarburization from the surface layer of the steel sheet during the time, the hearth roll chamber may be in a non-carburized atmosphere.

Although the structure of the sealing device 21 is not described in detail here, for example, the sealing device 21 has a three-layered sealing layer interposed between the hearth roll chamber and the carburizing atmosphere chamber. The carburizing atmosphere gas is blown out to the layer, the carburizing atmosphere gas is blown out to the seal layer on the carburizing atmosphere chamber side, and exhaust is performed from the intermediate sealing layer. The flow rate of each atmosphere gas is controlled so as to flow toward the intermediate seal layer side, and the circulating flow generated by the plate layer flow accompanying the passing of the strip is formed on the width direction end face of the strip in the seal layer. The exhaust port is configured to exhaust air.

Strip A delivered from the carburized zone 4
Is supplied to the first cooling zone 5. In the first cooling zone 5, since the solid solution C is fixed only in a very thin area of the surface in the surface layer portion of the strip, the strip after carburizing has a steel sheet temperature of 600 ° C or less, preferably about 500 to 400 ° C. Rapid cooling at a cooling rate of 20 ° C./sec. In the first cooling zone 5, a flow rate, a flow rate and a cooling roll of a cooling gas blown from a cooling gas jet to a strip conveyed in the cooling zone by the host computer are set so as to achieve the cooling condition. Temperature, winding angle, etc. are controlled.

The strip A sent from the first cooling zone 5 is then sent to 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. Thus, finally, a cold-rolled steel sheet for press forming with extremely low carbon in which solid solution C exists only in the surface layer can be obtained. 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.

First, in the carburizing control in the carburizing zone as described above, the carburizing amount is given as a condition for obtaining the target material, including the case where the carburizing concentration distribution in the steel sheet is required. The upper limit of the carburizing temperature is set to be equal to or lower than the recrystallization temperature from the material conditions. On the other hand, in order to reduce the size of the carburizing furnace and obtain the maximum carburizing capacity with a specified furnace length, it is necessary to increase the carburizing reaction rate based on the principle of the above-described carburizing amount = carburizing reaction speed x carburizing time. From this necessity, the higher the carburizing temperature involved in the carburizing reaction rate, the better. This prevents the occurrence of sooting, which will be described later, and reduces CO2.
It also leads to raising the concentration upper limit.

In this embodiment, as described above, the generation limit of sooting is determined by thermodynamics (atmospheric composition) in consideration of the material balance.
Although it can be obtained by a model formula, it is difficult to set the CO concentration and the H ₂ concentration related to the atmosphere composition only under the condition that the sooting does not occur. Therefore, in the present invention, a relational expression that does not hinder the carburization reaction rate is set in advance, and for example, CO 2 obtained by the atmosphere composition model expression that does not cause sooting is obtained.
The H ₂ concentration is calculated using this relational expression based on the concentration. Specifically, H ₂ concentration = a × (CO concentration) where a is a constant in the range of 0 ≦ a <5. The constant a is specifically set to a value that minimizes the concentration of generated CO ₂ and H ₂ O that hinder the reaction in a basic formula of the surface reaction rate described later, and is usually 0.5 to
It is often set in the range of 1.0. That is, when this relational expression is satisfied, the carburizing reaction rate based on the surface reaction rate equation becomes the maximum.

When the surface reaction rate is set, a carburizing time for achieving a desired carburizing concentration distribution is set. That is, in the case where only the C concentration in the surface layer portion is increased to make the gradient with the C concentration in the inner layer portion steep, 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 reduce the C concentration gradient between the inner layer portion and the surface layer portion, the carburizing reaction speed may be reduced (the carburizing force is reduced) to increase the carburizing time. The control of the carburizing reaction speed and the carburizing time satisfies the above-mentioned constraint condition of constant carburizing amount.

On the other hand, as mentioned in the above-mentioned heating zone and soaking zone, the optimum sheet passing speed is set in each sheet temperature control 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 rate of the carburized zone, in a continuous annealing carburizing facility in which strips are passed in series, any stripping speed is the same as that of the entire facility. It must be decided whether to control the plate speed. In this case, all specifications of the steel sheet must be considered, and the specifications are given as absolute conditions.

From the above, when the maximum threading speed obtained in the carburizing zone is larger than the minimum value of each maximum threading speed obtained in each sheet temperature control zone, the maximum threading speed of each sheet temperature control zone is obtained. It is necessary to set the minimum value of the sheet speed as the line passing speed, and to set again the atmospheric conditions of the carburizing furnace that satisfies the carburizing amount at the sheet passing speed. In this case, since the carburizing time becomes longer, the direction of decreasing the carburizing reaction rate under the condition of a constant carburizing amount, that is, the CO concentration in the atmosphere gas, the H ₂
In this case, the density is set in a direction of lowering the density, and the condition for preventing sooting is necessarily satisfied.

Conversely, when the minimum value of the maximum threading speed obtained in each of the sheet temperature control zones is equal to or higher than the maximum threading speed obtained in the carburizing zone, the carburization that is the essence of the present invention is performed. It is necessary to set the maximum stripping speed of the strip as the line stripping speed, and to reset the furnace temperature and fuel supply rate as the strip temperature control amount in order to satisfy the strip temperature of each strip temperature control zone at this stripping speed. is there.

The logic shown in FIG. 5 executed by the host computer embodies these control concepts. In this logic, first, in step S20, the carburized zone,
The upper and lower limits of the furnace temperature required to satisfy the specifications of each steel sheet are set from the capacity limits of each furnace including each sheet temperature control zone.

Next, the process proceeds to step S21, where the sheet temperature control amount in each sheet temperature control zone and the carburizing control amount in the carburizing zone are set. Specifically, for example, based on a mathematical model based on the theory of heat transfer in the heating zone 2 and the soaking zone 3, a heat transfer considering the heat transfer coefficient between the radiant tube, furnace wall, strip, hearth roll, and the like is considered. A process model formula is set from the balance, and a process model calculation for performing feedback control by calculating a furnace temperature, a fuel gas supply flow rate, etc. that satisfies a target plate temperature based on the process model formula, and a steel plate, that is, a coil. Calculate the optimal time series of the fuel gas supply flow rate that minimizes the plate temperature fluctuation at the seam of the seam, and based on this, perform the optimal route calculation for presetting and feeding forward control when passing the target coil, and based on these To set the maximum threading speed for each furnace.

On the other hand, based on the carburizing model formula modeling the sooting conditions and carburizing conditions of the carburizing zone 4 described in detail later, the target carburizing amount set from the specifications of the steel sheet or the carburizing amount is satisfied. Atmospheric gas composition and carburizing temperature for obtaining the carburizing concentration distribution to be obtained are obtained, and under the conditions, the maximum passing speed in the carburizing zone is set. Next, proceeding to step S22, the heat crown of the hearth roll in each heat treatment furnace including the carburizing furnace is predicted and calculated by a sheet temperature model or the like, and the maximum sheet passing speed at which the roll crown is within the meandering limit of the strip is determined. The calculated thermal crown is calculated. This makes it possible to achieve the maximum throughput of the furnace in a stable operation range in which the meandering of the strip is suppressed.

Next, the process proceeds to step S23, where the maximum sheet passing speed in each furnace set in step S21 is compared, and the minimum value of the maximum sheet passing speed of each sheet temperature control zone is set to the maximum value of the carburized zone. If it is smaller than the passing speed, the minimum value of the maximum passing speed of the sheet temperature control zone is used.If the maximum passing speed of the carburized zone is equal to or less than the minimum value of the maximum passing speed of each sheet temperature control zone. Sets the maximum threading speed of the carburized zone to the threading speed of the entire line, respectively.

Next, the process proceeds to step S24, in which a control amount resetting calculation for resetting the control amount of each heat treatment furnace based on the passing speed of the entire line set in step S23 is performed. Here, in describing the carburizing control performed in the carburizing zone, for example, the aforementioned low AI-high B
Based on the specifications of the strip required to obtain a steel sheet having high press formability and strength, such as H steel sheet, how carburizing conditions in the present embodiment are compared with conventional carburizing conditions What is the level and what items are needed to satisfy the carburizing conditions will be described.

The conventional carburizing technique is carried out for the purpose of surface hardening for improving the wear resistance and impact resistance of discontinuous materials made of so-called tempered steel such as gears, shafts and bearings.
Therefore, the C content in the material is 0.05% or more, the required carburizing amount is 0.1% or more, and the carburizing depth is 0.5 to 1.5 mm.
As described above, the required time for carburizing extends to 1 to 5 hours. Under such conditions, the C concentration in the surface layer of the steel sheet has reached an equilibrium concentration with respect to time. Therefore, as shown in FIG. 4, the carburization rate is a diffusion-limited region in the steel that follows the diffusion rate into the steel. The 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 atmosphere 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. Therefore, CO / CO ₂ management is important as an actual operation management index. 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, and is performed for the purpose of improving the surface characteristics of the strip and improving the material of the steel sheet itself. Will be Therefore, for example, when the carburizing conditions of the metal strip are obtained from the specifications (for example, JP-A-3-199344) required for the metal intended to improve the secondary work brittleness resistance, the present embodiment shows that The required amount of C is 20 ppm, the required amount of carburizing is 200 ppm or less, the carburizing depth is 50 to 200 μm, and the carburizing time depends on the sheet passing speed is 120 seconds or less. Under such conditions, the C concentration in the surface layer portion of the steel sheet does not reach the equilibrium concentration with respect to time.
As shown in (1), the carburizing rate is a surface reaction rate-determining region according to the reaction rate of the steel surface, and the carburizing rate is proportional to time itself. Since the carburizing amount and the carburizing depth are non-equilibrium in this surface reaction rate-controlling region, CO / CO is simply controlled by C potential control as an actual operation control index so that the equilibrium C concentration in the surface layer in the steel is simply obtained. It is necessary to set the carburizing conditions so as to obtain the required amount of carburizing determined from the required specifications of the steel sheet, taking into account not only the control of No. ₂ but also the large number of controlled variables in the furnace. Hereinafter, a basic principle of constructing the logic according to the logic processed by the host computer in order to control the carburizing amount in the present embodiment will be described.

First, when controlling the composition of the atmosphere gas in the above-mentioned surface reaction rate-controlling region, it is necessary to prevent the occurrence of sooting and to suppress the rise of the dew point as described above. To reason. Generally, the atmosphere gas composition under carburizing conditions can be determined by chemical equilibrium. In the conventional solution method, all possible reactions are listed, and the gas composition is obtained by solving a non-linear simultaneous equation from the equilibrium relation of these reactions. However, it is extremely difficult to determine the exact limit of soot generation (suiting) only from the reaction formula of the gas phase system.

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 and isobaric system, the Gibbs free energy decreases with a change that occurs spontaneously, and the Gibbs 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 Gibbs free energy of the entire system obtained using the concentration of each component gas in the production system as a variable, and this is a substance in which the elemental components brought into the original system are constant. Each component is set so that it becomes the minimum value under the constraint condition of the balance, specifically, under the constraint condition that the atmosphere gas composition and supply amount supplied into the furnace and the amount of C taken out of the furnace into the metal zone by carburization are constant. The gas concentration may be determined. This component gas concentration becomes the equilibrium composition of the atmospheric gas at the given furnace temperature and furnace pressure,
The sooting C amount is expressed as one of the condensed species in the logic described below.

In calculating the composition of the atmosphere gas, two assumptions are set. 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. Total free energy F (X) between the condensing species and gas species based on this assumption, i-th gas species in free energy f ^g _i, the following equation (1) with respect to h-th condensed species of the free energy f ^c _h Given by

[0053] Here, n: number of gas species, p: number of condensed species.

Here, the free energy f ^g _i of the i-th gas species with respect to the gas product is represented by the fact that the number of moles of the gas species is x ^{g with} respect to the molar energy C ^g _i of the i-th gas species.
_i is given by the following equations (2) to (4). _{^{f g i = x g i (}} C g i + ln (x g i / X)) ......... (2) C g i = (F / (R · T)) g i + lnP ......... (3) On the other hand, the condensation product, since the influence of pressure and mixing is removed based on the assumption, h th free energy f ^c _h condensation species, h th condensed species molar energy C ^c
The following number of moles of the condensing species as x ^c _h against _h 5
Equation 6 gives

[0055] ^{_{f c h = x c h ·}} C c h ......... (5) C c h = (F / (R · T)) C h ......... (6) In addition, the three equations in equation (6) (F / (RT)) is the following 7
Defined by an expression. (F / (R · T)) _i = ((F−H ₂₉₈ ) / T) _i / R + ΔH ⁰ _{f, 298, i} / RT (7) Next, the material balance in this system is considered. 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 O in the atmospheric gas component is constant in terms of atomic units. This material balance equation is expressed by the following eight equations.

[0056] However, j = 1,2, ........., m a g ij: i -th of j th element contained in the gas species of the molecule number of atoms a ^c _ij: j-th included in the i-th condensed species of the molecule B _j : amount of j-th element present in the system m: number of elemental species present in the system

Here, in this 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 determined. We decided to converge and obtain the optimal solution. 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 formulas 9 and 10. Equation 10 is a function calculated based on the specifications of the steel sheet and the surface reaction rate.

W ^g _I = W ^s _C + W ^g _O (9) W ^s _C = ξ (V, t, w, LS)) (10) where W ^g _I : enters the furnace C mass in atmospheric gas W ^s _C : C mass taken off by the strip W ^g _O : C mass in atmospheric gas exiting from the furnace V: Surface reaction rate, t: Carburizing time, w: Plate width.

In this manner, the generation of the above-mentioned atmosphere 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 ensures the generation of sooting. While preventing, 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 atmospheric gas to increase the sheet passing speed.

Next, the principle of carburizing amount control constituting the main part of this embodiment will be described. The surface reaction when CO is used as the atmosphere gas can be considered as the following formulas 11 to 13. CO⇔ [C] + O (11) CO + O → CO ₂ (12) Fe + [C] → Fe—C (diffusion in steel) (13) According to the above-mentioned leaves, the steel sheet surface layer When the C concentration in the part is extremely low and the carburizing time is extremely short, the carburizing condition does not reach an equilibrium state, and therefore the reaction rate of the equation (13) is faster than the desorption reaction of the adsorbed oxygen of the equation (12). Assuming that the reaction was rate-determining, the surface reaction rate V in this surface reaction-limiting region was expressed by the following equation (14).

V = k · PCO (PCO / (PCO + (ac / K))) (14) where, k: reaction rate constant, PCO: partial pressure of CO gas, ac: carbon activity, K: equilibrium Indicates a constant. However, the effect of H ₂ is not considered in the above equation (14). As the reaction formula for H ₂ ,
The reaction represented by the following formula 15 is considered with respect to the reaction formula represented by formula 2.

CO + H_Two+ 2O → CO_Two+ H_TwoO ............ (15) The generated CO_TwoTo the following 16
Equivalence is conceivable. H_Two+ CO_Two⇔H_TwoO + CO (16) Based on these reaction formulas,_TwoPromotes carburizing reaction
Fruity, CO_Two, H _TwoO can inhibit carburization reaction
Understand. Therefore, in this embodiment, the surface reaction rate V is
It was expressed by an equation.

V = k ₁ · f ₁ (PCO, PH ₂ , θ _O ) −k ₂ · f ₂ (PCO ₂ , PH ₂ O) (17) where θ _{O is} the coverage of adsorbed oxygen, k ₁ and k ₂ : reaction rate constants, and the reaction rate constants k ₁ and k ₂ can be set by the following equation 17 ′.

K _i = A _i · exp (−E _i / RT) (17 ′) where A _{i is a} frequency factor, E _{i is} activation energy, R is a gas constant, and T is an absolute temperature. . Note that the frequency factor A _i , the activation energy E _i ,
Since all gas constants R are constants, the reaction rate constant k
₁ and k ₂ were calculated from experimental values under various absolute temperature T conditions.

In this embodiment, when only the CO concentration needs to be considered, for example, when the flow rate of the supplied gas in the atmosphere is large, the above equation (14) may be used as the surface reaction rate equation. Next, the diffusion of solute carbon in steel is considered. The diffusion state of C into steel is expressed by the following equation (18).

DC / dt = D · d ² C / dX ² (18) where C: C concentration in steel, t: time, D: diffusion coefficient, and X: diffusion distance. In addition, the said diffusion coefficient D was decided to be displayed approximately by actual measurement data. Therefore, the above equations (17) and (18)
The amount of carburizing of the steel sheet can be calculated by the formula.

Here, when the passing speed is regulated based on the operating conditions of the carburizing treatment as described above, the value obtained by differentiating the effective carburizing furnace length L from the target carburizing time t in the above equation (18) with this carburizing time t is obtained. The plate speed becomes L _S. The above operations are sequentially performed by a program stored in the host computer in advance, and the amount of carburization into the strip given by the specification data of the steel sheet after carburization and the amount of carbon reduction in the strip calculated from the amount of C reduction in the atmosphere gas are calculated. FIG. 6 is a flowchart showing the logic for setting the carburizing condition that matches the carburizing amount. Toes
This logic corresponds to step S of the logic of FIG.
21 or, if necessary, executed in step S24
It is a minor program.

First, in step S1, conditions such as the composition of the atmosphere gas, the flow rate of the input gas, the carburizing temperature and the sheet passing speed, and the specifications of the steel sheet are read from the condition settings given as the specifications of the steel sheet after carburizing. Next, the process proceeds to step S2 to calculate a set carburizing amount ΔC for the steel sheet from the steel sheet specification and the steel sheet specification.

Next, the process proceeds to step S3 to set the atmosphere composition model formula from the composition of the atmosphere gas read in step S1. Next, the process proceeds to step S4 to calculate the concentration of each component of the atmosphere gas according to the atmosphere composition model formula set in step S3. Next, step S5
Then, the surface reaction rate of the steel sheet is calculated based on the above equation (17).

Next, the process proceeds to step S6, in which the carburizing rate into the steel is calculated based on the above equation (19), and the amount of C diffusion into the steel is calculated. Next, when the carburizing process time has elapsed, the process proceeds to step S7, and the diffusion C amount into the steel per unit time and unit area calculated in step S6 is integrated with the process time and the total steel sheet surface area. Amount of carburizing into steel sheet Δ
C ′ is calculated.

Next, the routine proceeds to step S8, where it is determined whether or not the absolute value of the difference between the set carburizing amount ΔC and the carburizing amount ΔC 'is smaller than a predetermined value a. If it is smaller than a, the process proceeds to step S10; otherwise, the process proceeds to step S9. In step S9, the set carburizing amount is corrected based on the carburizing amount based on the following equation (20), and the process proceeds to step S3.

ΔC = ΔC + (ΔC′−ΔC) × b (20) where b represents a constant. In step S10, the calculation results such as the concentration of the atmospheric gas components, the total carburization amount, the average carburization amount, the carburization distribution from the steel sheet surface, the sooting C amount, and the passing speed obtained as a result of the above calculation are output to the program. To end.

The sooting generation limit obtained by taking into account the material balance at each carburizing temperature calculated by this program is shown by a solid line in FIG. In the figure, the broken line indicates the upper limit of the dew point. The dashed line indicates the sooting limit determined without considering the material balance. In the same figure, the shaded portion represents the operation range in the actual carburizing operation.

As is apparent from the figure, in the sooting generation limit determined in consideration of the material balance, the CO concentration and the H ₂ concentration are higher than the sooting generation limit determined without considering the material balance. Become. That is, the carburizing speed is improved accordingly. On the other hand, as the carburizing temperature increases, both the CO concentration and the H ₂ concentration accompanying the generation limit of sooting increase. This means that the overall carburizing operation efficiency also depends on the temperature, and conversely, when increasing the sheet passing speed, there is a margin of operation such as raising the furnace temperature as much as the material allows. As a result, the setting range of various conditions in the actual case of continuous carburization is further expanded. Of course, sooting does not occur even if the operating range is set along the sooting generation limit obtained 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 the carburizing conditions calculated by this program, that is, the carburizing amounts when the control amounts are changed, and the actually measured carburizing amounts. As is clear from the figure, the calculated value of the carburized amount and the measured value are in very good agreement. This means that the setting of the carburizing rate, that is, the setting of the surface reaction rate, and the setting of the temperature dependence coefficient thereof are correct. As long as the setting of the surface reaction rate is correct, the continuous carburizing method of the present invention reduces the carburizing rate. This means that a wide range of applications is possible in areas that follow a surface reaction rate greater than the diffusion rate.

Next, a specific calculation example of the carburizing control calculated by this program will be described. 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.

Next, step S3 and step S3 are performed.
Atmospheric gas condition to prevent sooting in step 4
Concentration, the upper limit of the concentration of H ₂ is set. On the other hand, in the flow of steps S3 to S9, the surface reaction rate equation,
Formulas for model of diffusion in steel are set, and from these formulas, CO concentration, H ₂ concentration, CO concentration necessary to achieve the target carburization amount
₂ Concentration, H ₂ O concentration, and carburizing time are set, and at the same time, a passing speed that achieves a desired carburizing time is newly set with respect to the set passing speed.

Therefore, according to this logic, even when the carburizing concentration distribution is not required, as well as when the carburizing concentration distribution is required, sooting is not generated while satisfying the carburizing amount given from the specification data of the steel sheet. Each of the carburizing control amounts is set so that the maximum carburizing capacity is obtained under the conditions. Next, when the passing speed is regulated by the carburizing control of the logic of FIG. 6, for example, the calculation process of FIG.
A specific calculation example of the sheet temperature control performed in the heat treatment zone other than the carburizing zone in step S24 will be described with reference to FIG.

According to the sheet temperature control program shown in FIG. 5, the furnace temperature is reset based on the calculation of the capacity of each heat treatment furnace and the specifications of the steel sheet, and at the same time, the target sheet temperature and the sheet temperature required for the annealing treatment. The fluctuation tolerance is reset. On the other hand, when the passing speed is regulated by the carburizing control program and the passing speed accuracy is ± 0.06 m / min., The control map of the sheet temperature and the annealing time with respect to the set furnace temperature shown in FIG. On the basis of this, the sheet temperature control time (target annealing time) and the allowable range of the annealing time fluctuation are set in accordance with the sheet passing conditions at the sheet passing speed.

In this case, if the allowable range of the sheet temperature fluctuation and the allowable range of the annealing time fluctuation do not match on the control map of the sheet temperature and the annealing time, the set furnace temperature is changed and the changed setting is changed. In order to achieve the furnace temperature, a control amount such as a fuel flow rate to the radiant tube, that is, a plate temperature control amount such as a heating amount is changed. That is, this corresponds to the continuous annealing and carburizing control in a steady state achieved by the logic of FIG. 5, and so-called feedback control is performed. On the other hand, in the unsteady continuous annealing carburizing control in the logic of FIG. 5, the response speed is calculated by directly calculating the control amount of the fuel flow rate that determines the furnace temperature by the process model calculation and the optimal route calculation. The so-called feed-forward control, which reduces or suppresses the time constant error of the furnace temperature control having a large value, enables the optimal plate temperature control.

In the present embodiment, C is used in the surface reaction.
Although the case where the surface reaction rate is calculated taking into account only the influence of O, H ₂ , CO ₂ and H ₂ O has been described in detail, as described above, the influence of other atmosphere gas compositions, for example, the influence of heavy hydrocarbons, is taken into account. Alternatively, the surface reaction rate may be calculated. Further, in the present embodiment, the thermodynamic model formula considering the material balance is linearized, and the equilibrium state is calculated by converging the solution. However, the means for calculating the equilibrium state is not limited to this. Absent.

Further, in this embodiment, particularly, a strip made of ultra-low carbon steel is continuously annealed and carburized, and before the carburizing concentration reaches the equilibrium concentration, the carburizing speed is controlled by the surface reaction speed. Although only carburizing control was described in detail,
Needless to say, the continuous annealing and continuous carburizing method of the present invention can be applied to a diffusion-controlled region where the carburizing speed is controlled by the diffusion speed from the surface of the metal strip to the inside.

[0083]

As described above, according to the method for continuously carburizing a metal strip and controlling the sheet temperature according to the present invention, the atmosphere model formula of the sooting occurrence limit with respect to the carburizing temperature and / or the carburizing reaction.
Based on a relational expression that does not hinder, the carburizing amount given from the specification data of the material is used as a constraint, for example, carbon monoxide or carbon monoxide that satisfies the desired carburizing concentration distribution or is obtained from the maximum carburizing capacity in the carburizing furnace. Concentration of carbon oxide and hydrogen
In addition to setting the partial pressure and setting the carburizing time to set the sheet passing speed to achieve this carburizing time, in order to obtain a predetermined sheet temperature in each sheet temperature control zone with this sheet passing time, for example, In order to set the furnace temperature in the heating zone and the soaking zone and the necessary fuel input amount, the amount of carburization to the surface layer of the metal strip that satisfies the specifications of the steel sheet under the most efficient carburizing conditions And a predetermined plate temperature can be obtained in each plate temperature control zone.

[Brief description of the drawings]

FIG. 1 is a conceptual explanatory view of a heat treatment step performed in a continuous annealing carburizing facility.

FIG. 2 is a schematic configuration diagram showing one embodiment of a continuous annealing carburizing facility to be subjected to carburizing control using the continuous strip carburizing and sheet temperature control method of the present invention.

FIG. 3 is an explanatory diagram of a carburizing concentration distribution in a sheet thickness direction required for a low AI-high BH steel sheet or the like.

FIG. 4 is an explanatory diagram of a diffusion-controlled region after the carbon concentration in the surface portion of the metal band reaches an equilibrium concentration and a surface reaction-limited region before the carbon concentration reaches the equilibrium concentration.

FIG. 5 is a flowchart showing the logic of the overall line control performed in the continuous annealing carburizing equipment of FIG. 2;

FIG. 6 is a flowchart illustrating an example of a logic for performing carburizing control using the continuous strip carburizing and sheet temperature control method of the present invention.

FIG. 7 is a CO-H ₂ characteristic diagram comparing a sooting generation limit obtained by the carburizing control logic of FIG. 6 with a sooting generation limit obtained without considering the material balance in the furnace.

FIG. 8 is a correlation diagram between a calculated value of carburized amount obtained by the logic of FIG. 6 and an actually measured value.

FIG. 9 is an explanatory diagram of sheet temperature control conditions calculated to obtain a target sheet temperature according to the embodiments of FIGS. 5 and 6;

[Explanation of symbols]

 1 is a pre-tropical zone 2 is a heating zone 3 is a solitary zone 4 is a carburizing zone 5 is a first cooling zone 6 is a second cooling zone A is a strip

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Jun Morozumi 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. No. 1 town Kawasaki Steel Engineering Co., Ltd. (58) Field surveyed (Int. Cl. ⁷ , DB name) C23C 8/22 C21D 9/52 101 C21D 11/00 101

Claims (4)

(57) [Claims] 1. A metal sheet passing through a carburizing furnace within an atmosphere composition range in which sooting does not occur, while performing a sheet temperature control for obtaining a desired sheet temperature for a metal strip inside and outside a carburizing furnace. A method for controlling carburization and sheet temperature when continuously carburizing a belt, wherein a sheet passing speed is set from a condition in which sooting does not occur in the carburizing furnace and a carburizing condition, and the sheet passing speed is set in accordance with the sheet passing speed. A sheet temperature control amount for obtaining the desired sheet temperature ;
A method for continuously carburizing a metal strip and controlling a sheet temperature, wherein the furnace temperature and / or the amount of fuel input in the sheet temperature control zone are set by setting the temperature. 2. When setting a sheet passing speed from the conditions in which sooting does not occur and the carburizing condition, an atmosphere composition model formula relating to a carburizing temperature set in advance so that sooting does not occur in the carburizing furnace; / Or carburizing reaction
The carbon monoxide concentration or carbon monoxide concentration and hydrogen concentration and the carburizing time for obtaining the target carburizing concentration distribution are set based on the relational expression that does not inhibit the carburizing time, and the necessary carburizing time for achieving this carburizing time is set. The method for continuously carburizing a metal strip and controlling a sheet temperature according to claim 1, wherein the sheet speed is set. 3. An atmosphere composition model formula relating to a predetermined carburizing temperature in order to prevent sooting from occurring in the carburizing furnace when setting the sheet passing speed from the conditions in which sooting does not occur and the carburizing conditions. / Or carburizing reaction
A carbon monoxide concentration or a carbon monoxide concentration and a hydrogen concentration are set based on a relational expression that does not hinder, and a carburizing time is set from a maximum carburizing treatment capacity in a carburizing furnace set based on this, and this carburizing time is The continuous strip carburization of a metal strip and the sheet temperature control method according to claim 1, wherein a sheet passing speed required to achieve the target is set. 4. When setting the hydrogen concentration, a relationship set so as not to inhibit a carburizing reaction with respect to a carbon monoxide concentration set based on an atmosphere composition model formula for preventing the sooting. The method for continuously carburizing a metal strip and controlling a sheet temperature according to claim 2 or 3 , wherein the hydrogen concentration is set based on an equation.