JP2938292B2 - Continuous carburizing of metal strip - 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 producing a metal strip which is passed through a carburizing zone in a carburizing furnace under continuous gas carburizing conditions, without causing sooting in the carburizing furnace. The present invention relates to a method for continuously carburizing a metal strip to achieve a carburized amount obtained from specifications such as a carburized concentration distribution to be performed. It is suitable for continuous carburizing method that can reduce the amount. For example, when a very low carbon steel sheet having different desired specifications is also formed into a series of strips, and the strip is passed through a carburizing furnace and continuously carburized, The present invention is suitable for controlling the control amounts of the atmosphere gas composition, the composition gas concentration, the carburizing temperature, the sheet passing speed, and the like in order to achieve the carburizing amount obtained from the specifications of each steel sheet in a time-series manner.

[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. A higher strength thin steel sheet is 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] Incidentally, the carburized thin steel sheet required in the metal secondary processing industry includes, in addition to the low AI-high BH property steel sheet, a secondary work brittleness improved steel sheet having improved fracture brittleness in secondary work. And high-strength steels (high-tensile steels) that have both excellent deep-drawability and high strength.

[0009]

In the meantime, as described above, it is necessary to produce various types of steel sheets requiring various specifications in small quantities in a small quantity. Therefore, carburizing conditions satisfying the specifications of each steel sheet must be set while continuously passing the strip. The carburizing conditions include, for example, the low AI-high BH
It is necessary to set various factors, including controlling the carburizing concentration distribution as seen in the heat-resistant steel sheet. The control amount necessary for setting the carburizing conditions, that is, the control amount of the carburizing amount to the steel sheet obtained from the specification data including the carburizing concentration distribution includes carburizing temperature, carburizing time, atmosphere gas composition, and composition gas concentration. Is mentioned. Of these, the carburizing time is set according to the passing speed in the furnace. Therefore, if these control amounts are listed in ascending order of the response time, the passing speed, the passing gas speed, the composition gas concentration, the atmosphere gas composition, and the carburizing temperature are in this order. Become.

In the places where the carburizing amount is to be changed, such as seams of steel sheets having different specifications, there are various control amounts for performing stable carburizing amount control in the preceding steel sheet and stable control in the succeeding steel sheet. Each control amount for performing the carburizing amount control may be different. If the field of stable carburization control of the preceding steel sheet is defined as the previous steady state, and the field of stable carburization control of the succeeding steel sheet is defined as the next steady state, the following steady state is satisfied. Therefore, in some cases, one or more of these control amounts must be changed from the previous steady state. Thus, the transition period from the previous steady state to the next steady state is defined as an unsteady state.

However, in the conventional continuous carburizing method,
There is a problem that specific means for carburizing amount control in the unsteady state is lacking. Specifically, under the unsteady state carburizing condition, which is the transitional period, since the former steady state and the next steady state are different, if the steel sheet is passed through the unsteady state as it is, the desired specification Steel sheets with different carburizing amounts are manufactured, and there are some defective materials out of specification. This tendency shows that the threading speed is high,
Or, it becomes remarkable as the change width of each control amount is large. Especially,
If the atmosphere composition with a long response time or the control change amount of the carburizing temperature is large, a large amount of material defects will occur.

Incidentally, of the control amounts of the carburizing conditions,
The following relationship exists between the carburizing temperature, the atmosphere gas composition, and the composition gas concentration. As the carburizing temperature rises, the reaction rate at which C separated from the atmospheric gas combines with Fe on the surface of the metal strip increases, and the rate at which C diffuses inward from the surface layer of the metal strip also increases. Carburizing speed increases. On the other hand, when the temperature in the carburizing furnace for achieving this carburizing temperature is lowered, the concentration limit of free C, that is, carbon monoxide (CO) at which sooting occurs, is lowered, and hydrogen (H) due to a dew point causing temper color or the like is reduced. ₂ ) The concentration limit also drops. Here, considering the setting conditions of the carburizing temperature, the carburizing temperature is desirably equal to or lower than the recrystallization temperature from the material conditions. Further, under the condition that a large processing capacity can be obtained with the determined effective carburizing furnace length, the bonding reaction rate of Fe and free C and the internal diffusion rate of solid solution C (hereinafter, the rate when both are considered simultaneously) In order to increase the carburization rate, the specific reason will be described later), set the CO concentration high within a range where sooting does not occur, and at the same time, increase the carburization temperature to increase the carburization rate itself. Want to.

When the carburizing temperature is regulated to some extent in this way, the upper limit of the composition of the atmosphere gas and the concentration of the composition gas is set from the sooting limit and the dew point limit. That is, the in-furnace 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 may be set based on this.

The control or setting of the carburizing conditions under conditions in which sooting does not occur is being performed in the steady state, but is not universally set in the unsteady state. Therefore, conventionally, in order to reduce the material defect, an intermediate interposition material called a threading material is used in a portion corresponding to the material defect. However, it is difficult to reduce the processing efficiency and the cost of the threading material is small. However, there is no increase in the overall processing cost.

[0015] Further, in actually setting the conditions of such a continuous annealing carburizing facility, the following problems were 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 Unexamined Patent Publication 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, in the setting control of the carburizing conditions in the unsteady state, the carburizing concentration distribution is controlled under conditions that do not cause sooting in the carburizing furnace. Metallic strips that satisfy the carburizing amount obtained from the specifications of the steel sheet before and after including as much as possible, shorten the length of the defective material area or the time required for it, and reduce the overall cost It is an object to provide a continuous carburizing method.

[0018]

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. In other words, even in the unsteady state, in order to achieve the amount of carburization of each steel sheet, when the control amount such as the carburizing temperature or the atmosphere gas composition having a long response time is changed and controlled in a complex manner, the fluctuations thereof are Chronologically,
The control amount having a shorter response time must be changed and controlled. Therefore, by setting the priority order and the control amount of the control of the four control amounts so that the length of the material defect portion deviating from the target value of the carburization amount becomes the shortest, the setting of the carburizing condition in the unsteady state is performed. Control can be universally possible. Specifically, when the furnace temperature is changed to obtain the next steady-state carburizing temperature, control is performed in the order of the atmosphere gas composition or the composition gas concentration, and the passing speed according to the actual result of the set furnace temperature fluctuation. Need to do. The result includes the present result and the stored past results.

However, in the actual case in a carburizing furnace, consideration must be given to preventing the occurrence of the sooting. The occurrence of sooting directly leads to the deterioration of the surface properties of the steel sheet, which is fundamentally different from the purpose of the present invention in which the length of a defective material portion or the required time is shortened.
Considering the above-mentioned relationship between the furnace temperature, the atmosphere gas composition, and the composition gas concentration for achieving the carburizing temperature, changes in the atmosphere gas composition and the composition gas concentration always reflect the actual results of the furnace temperature. It is necessary to perform control in chronological order while watching. Specifically, when it is necessary to increase the carburizing temperature by the next steady state, the furnace temperature must be raised before the carburizing temperature can be obtained. The composition gas concentration in the atmosphere gas composition must be set within a range in which no pitting occurs. Conversely, if it is necessary to lower the carburizing temperature by the next steady state, the composition gas concentration in the atmosphere gas composition should be changed prior to the decrease in the furnace temperature to obtain the carburizing temperature. The temperature in the furnace must be lowered within a range in which sooting does not occur according to the result of the composition gas concentration.

In summary, the control of the passing speed with a short response time is the most easily controllable element for obtaining the required amount of carburizing of the steel sheet.
By controlling the passing speed in accordance with the atmospheric conditions such as the atmospheric gas composition and the composition gas concentration, it is possible to reliably obtain a desired carburizing amount. When the change control of the carburizing temperature having the longest response time among the four control amounts is not performed, the change in the sheet passing speed may be performed only by looking at the actual result of the atmosphere gas composition or the composition gas concentration. Of course, in order to achieve the desired carburized amount as much as possible, conversely, setting each control amount in accordance with the variation of the passing speed is also included in the contents.

Further, in the continuous annealing and carburizing equipment as described above, the passing speed of a metal strip which is continuously passed through may be regulated depending on equipment other than the carburizing zone and operating conditions.
Unless the carburizing method can cope with the setting control of the unsteady state carburizing condition in such a case, it cannot be said that it is truly universal carburizing control. Therefore, it is necessary to incorporate such a condition into the logic as one of the setting conditions.

In order to set conditions under which the sooting does not occur irrespective of the steady state or the unsteady state, the following idea may be pursued. For example, an equilibrium state in which the occurrence of sooting can be suppressed by taking into account the material balance constraint that the amount of the element brought in by the original system is constant for the element brought out of the atmosphere by the metal band due to the carburization reaction on the metal surface layer Was made into a thermodynamic (atmospheric composition) model formula 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 obtain the maximum processing capacity with the specified furnace length, so it is necessary to increase the carbon monoxide concentration within a range where this sooting does not occur, In the case where the carbon monoxide concentration is set in this way, the setting of the hydrogen concentration, which has not been universalized, is conventionally performed based on the carbon monoxide concentration. 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, in order to control the amount of carburization in the metal zone in the above-mentioned steady state and unsteady state, for example, in the surface reaction rate-limiting region before the carbon concentration in the metal surface layer reaches the equilibrium concentration as described above, First, attention was paid to the fact that a surface reaction rate in the reaction rate range was obtained, and this reaction rate was to be 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.
When the discharge flow rate is small, the composition amount is small, but it is found that carbon dioxide and H ₂ O also have an effect in that the carburization reaction is inhibited. It was proved by experiment that it was a regulator. Considering the dependence of the material reaction on temperature, a control factor called temperature is interposed in the coefficient of the surface reaction rate.

In a steady or unsteady state, the low AI-
When the carburizing concentration distribution of the metal strip is required as in the case of a high-BH steel sheet, the surface reaction rate and the carburizing time are controlled under the above-mentioned constraint condition of the constant carburizing amount. That is, carburizing amount = surface reaction speed x
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, the C concentration in the surface layer portion of the metal strip is increased (C
When the concentration gradient is steep), the carburizing time is shortened and the surface reaction rate is increased. When the C concentration in the surface layer is lowered (the C concentration gradient is moderated), the carburizing time is increased. At the same time, the surface reaction rate is reduced.

According to the method for continuously carburizing a metal strip according to the first aspect of the present invention, a series of metal strips are formed from metal sheets having different desired specifications, and the metal sheet is formed in a carburizing furnace of the carburizing zone. In the continuous carburizing operation of a metal strip, which is continuously passed through a plate and subjected to continuous gas carburization, the seam of the metal strip and the carburization amount of the metal strip should be changed, and In a continuous carburizing method for a metal strip in an unsteady state in which the desired specifications of the metal sheet are different from those of the metal sheet, a transition is made from the previous steady state of the previous metal sheet to the next steady state of the next metal sheet. ,
During the unsteady state, one or more of the control amounts such as the atmosphere gas composition, the composition gas concentration, the carburizing temperature, and the passing speed are controlled under the condition that the sooting does not occur. It is characterized in that the carburizing amount is controlled in an unsteady state by controlling so as to satisfy the carburizing amount obtained from different specification data.

In the method for continuously carburizing a metal strip according to a second aspect of the present invention, in performing the carburizing amount control in the unsteady state, each control amount in the next steady state sandwiching the unsteady state is set. When any one or more of the control variables are changed from the previous steady state to satisfy the control variable, a furnace for obtaining the changed carburizing temperature under the condition that the sooting does not occur. The method is characterized in that the atmosphere gas composition or the composition gas concentration is set in consideration of the fluctuation of the internal temperature.

In the method for continuously carburizing a metal strip according to a third aspect of the present invention, when the carburizing temperature is increased, the furnace temperature is preceded by an increase in the furnace temperature.
The present invention is characterized in that the concentration of carbon monoxide gas or the concentration of carbon monoxide gas and the concentration of hydrogen gas in the atmosphere gas composition are increased within a range where sooting does not occur. In the method for continuously carburizing a metal strip according to claim 4 of the present invention, when the carburizing temperature is decreased, the carbon monoxide gas concentration or the carbon monoxide gas concentration and the hydrogen gas concentration in the atmosphere gas composition are reduced. Prior to this, the furnace temperature is lowered within a range in which sooting does not occur in accordance with the results of the atmospheric gas composition or the composition gas concentration.

In the method for continuously carburizing a metal strip according to a fifth aspect of the present invention, in controlling the carburizing amount in the unsteady state, each control amount in the next steady state sandwiching the unsteady state is set. In the case where one or more of the control amounts are changed from the previous steady state in order to satisfy the control amount, and the carburizing temperature is not changed, under the condition of the carburizing temperature, The present invention is characterized in that the atmosphere gas composition or the composition gas concentration is changed within a range in which sooting does not occur according to the actual result of the furnace temperature.

In the method for continuously carburizing a metal strip according to a sixth aspect of the present invention, in controlling the carburizing amount in the unsteady state, the control amounts of the atmosphere gas composition, the composition gas concentration, the carburizing temperature and the like are changed. Regardless of whether the control is performed or not changed, the passing speed is controlled so as to satisfy the desired amount of carburization of each metal plate according to the results of these control amounts.

[0030] The method for continuous carburizing of a metal strip according to claim 7 of the present invention is based on the practice of the continuous carburizing operation of the metal strip.
In the case where the passing speed is regulated, other control amounts are controlled so as to satisfy the carburizing amount desired for each of the metal plates under the condition that the sooting does not occur. is there.

[0031]

In the method for continuously carburizing a metal strip according to the present invention, for example, each control amount of the carburizing condition in a steady state in each steel sheet is set as follows. Including the reaction rate range according to the surface reaction rate where the carburizing rate is larger than the diffusion rate from the metal layer surface layer to the inside, the carburizing amount from the specification data of the steel sheet to the metal strip is set as a given condition, Set the carburizing temperature to increase the carburizing reaction below a certain recrystallization temperature, and determine the ambient gas composition from the basic formula of the sooting limit in the carburizing furnace at the carburizing temperature or the basic formula and the relational expression that does not inhibit the carburizing reaction. The control amount of each of the constituent gas concentrations, particularly the carbon monoxide concentration or the carbon monoxide concentration and the hydrogen concentration, is calculated, and the carbon monoxide partial pressure or the carbon monoxide partial pressure and the hydrogen partial pressure calculated based on the calculated amounts are controlled. Calculating the carburizing rate per unit time based on the amount and the temperature-dependent coefficient relating to the carburizing reaction rate calculated from the basic formula relating to the control amount of the carburizing temperature,
The carburizing time is calculated by differentiating the carburizing amount with the carburizing speed, or the carburizing time for achieving the carburizing concentration distribution set according to the surface reaction speed is calculated, and the effective carburizing furnace is calculated based on the carburizing time. Calculate the passing speed in the carburizing furnace by dividing the length.

In the present invention, in the unsteady state between the steady state of each steel sheet set as described above, any one of the control amounts of the atmosphere gas composition, the composition gas concentration, the carburizing temperature, and the sheet passing speed. One or more of them is controlled so as to satisfy the carburizing amount obtained from different specifications of the metal plate under the condition that the sooting does not occur. In controlling the carburizing amount in the unsteady state, when changing the carburizing temperature having a long control response time, the atmosphere gas composition or the composition gas in consideration of the fluctuation in the furnace temperature for obtaining the carburizing temperature in the next steady state. Set the density. Here, specifically, when raising the carburizing temperature, the rise in the furnace temperature is preceded, and the basic formula and the relational expression of the sooting limit described above are used according to the actual result of the furnace temperature. The concentration of carbon monoxide gas or the concentration of carbon monoxide gas and the concentration of hydrogen gas in the atmosphere gas composition which greatly affects the occurrence of sooting are increased. On the other hand, when the carburizing temperature is decreased, the carbon monoxide gas concentration or the carbon monoxide gas concentration and the hydrogen gas concentration in the atmosphere gas composition are preceded by a decrease in the atmosphere gas composition or the composition gas concentration. The furnace temperature is lowered accordingly. This makes it possible to prevent sooting in the carburizing furnace from occurring.

On the other hand, when the carburizing temperature is not changed, under the condition of the carburizing temperature, depending on the actual result of the furnace temperature,
Atmosphere gas composition or composition gas concentration within a range where sooting does not occur, which is set by the basic formula or relational expression of the sooting limit, specifically the carbon monoxide gas concentration or carbon monoxide gas concentration in the atmosphere gas composition And the hydrogen gas concentration.

In this manner, regardless of whether or not each of the control amounts of the atmosphere gas composition, the composition gas concentration, and the carburizing temperature is changed, the most control amount among the control amounts is determined according to the actual results of these control amounts. By controlling the passing speed with a short response time, the carburizing amount desired for each metal plate can be achieved as much as possible. In addition, the achievement referred to here is
It is possible to employ both the detected current results and the stored past results. Therefore, it is possible to set the carburizing condition in the non-stationary state in real time according to the present results, and to set the carburizing state in the unsteady state calculated from the past results to feed forward.

Further, in the method for continuously carburizing a metal strip according to the present invention,
In the case where the stripping speed is restricted by the practice of continuous carburizing operation of the metal strip, the carburizing amount desired for each of the metal sheets is satisfied even in an unsteady state under the condition that the sooting does not occur. Control other control amounts. Specifically,
Since the carburizing time is calculated from the regulated passing speed, the carburizing temperature, the atmosphere gas composition, and the composition gas concentration can be set by means reverse to the above.

By the way, supply of atmospheric gas especially under high temperature
When the discharge flow rate is small, for example, by adding the partial pressure of carbon dioxide and the partial pressure of H ₂ O to the basic formula of the surface reaction rate, CO ₂ , It is possible to more accurately control the amount of carburization of the metal band under carburizing conditions in which H ₂ O is present.

[0037]

FIG. 2 shows an example of a continuous annealing and carburizing apparatus for strip made of ultra-low carbon steel, which has been subjected to the method for continuous carburizing of a metal strip according to the present invention. Each ultra-low carbon steel sheet including a steel sheet having a different specification as desired in the drawing is formed into a series of strips by an unillustrated entry-side facility having a coil unwinder, a welding machine, a washing machine, and the like. Are pre-tropical zone 1, heating zone 2, level zone 3, carburizing zone 4, first cooling zone 5, second cooling zone 6, shearing machine,
The sheets are passed in the order of a delivery device (not shown) such as a winder.

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 provided near the path of the strip that is passed through the heating zone 2 and the leveling zone 3 while moving up and down via hearth rolls. The temperature inside the furnace (furnace temperature) is controlled by burning the fuel gas supplied to the furnace. In the present embodiment, the setting of the supply flow rate of the fuel gas is determined by the heat balance in the furnace 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 passes through the plate and carries the heat from the furnace. Is determined to be equivalent to the required (required) heat quantity, 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 carburized 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, the carburizing temperature, and the passing speed, that is, the 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, concentration and supply / discharge flow rate of the carburizing gas supplied to the carburizing furnace are determined by the host computer by the free energy in the furnace in consideration of the material balance in the furnace described later. Is controlled according to various conditions calculated based on an atmosphere composition model formula that minimizes The composition, concentration and supply / discharge flow rate of this carburizing gas
The control is performed so as to suppress the rise of the CO ₂ and the dew point to prevent the carburization reaction speed from decreasing and the temper color.

Incidentally, the strip in the carburizing furnace is passed through the hearth roll 20 while moving up and down the furnace through the hearth roll 20, and these hearth rolls 20 are required to maintain the rotation property and the 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 will not be described in detail here, for example, the sealing device 21 has a three-layer structure 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 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. In addition to the carburizing control according to the present invention, the sheet temperature control performed in other heat treatment zones also needs control including the steady state and the unsteady state as described above. It is an explanation of a configuration concept of a logic that enables control of a field.

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 obtain the maximum treatment capacity with the specified effective carburizing furnace length, it is necessary to increase the carburizing reaction rate based on the principle of carburizing amount = carburizing reaction speed x carburizing time described above. The higher the carburizing temperature related to the speed, the better, which also prevents the occurrence of sooting described later and raises the upper limit of the CO concentration.

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 embodiment, a relational expression that does not inhibit the carburization reaction rate is set in advance, and for example, C is obtained by the atmosphere composition model expression that does not cause sooting.
Based on the O concentration, the H ₂ concentration is calculated using this relational expression. 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, only the C concentration in the surface layer is increased to increase the C concentration in the inner layer.
When the gradient with the concentration is to be 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.

As described above in the heating zone and the soaking zone, the optimum sheet passing speed is set in each sheet temperature control zone by calculating the capacity and process of each furnace. The above are the conditions for the steady state sheet temperature control and carburizing control of the steel sheet in each heat treatment zone. On the other hand, in the unsteady state where the next steel sheet shifts to the steady state, in the heat treatment zone (sheet temperature control zone) other than the carburizing zone, the sheet passing speed must be set according to the results of the fluctuations in the furnace temperature. Furthermore, in the carburizing zone, the atmosphere gas composition or the atmosphere composition that is set from the composition gas concentration is set according to the actual results of fluctuations in the furnace temperature. Speed must be set. This performance includes the detected current performance and the stored past performance, but in any case, the length of the material defect occurring in the unsteady state or the required processing time is minimized. In addition, feedforward control in which carburization control and sheet temperature control are set in advance must be performed. This is performed by the optimum loop calculation in this embodiment.

Next, in consideration of the maximum sheet passing speed of each of the sheet temperature control zones and the maximum sheet passing speed of the carburized zone, in the continuous annealing carburizing equipment in which the strips are successively passed, It is necessary to judge whether or not will control the passing speed of the entire equipment. In this case, all specifications of the steel sheet must be considered, and the specifications are given as absolute conditions.

As described above, when the maximum passing speed obtained in the carburizing zone is larger than the minimum value of the maximum passing speed obtained in each of the temperature control zones, the maximum passing speed of each of the temperature control zones 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, if the minimum value of the maximum passing speed obtained in each of the sheet temperature control zones is equal to or higher than the maximum passing speed obtained in the carburizing zone, the maximum passing speed of the carburizing zone is set to a line. In order to satisfy the plate temperature of each plate temperature control zone at this passing speed, it is necessary to reset the furnace temperature and the fuel supply amount as the plate temperature control amount. The logic shown in FIG. 5 executed by the host computer embodies these control concepts.

In this logic, first, in step S20, the upper limit value of the furnace temperature necessary to satisfy the specifications of each steel sheet is set from the capacity limits of each furnace including the carburizing zone and 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 a carburizing model formula that models the sooting conditions and carburizing conditions of the carburizing zone 4 described in detail later, a process model calculation that enables feedback control in a steady state and feedforward in an unsteady state are performed. Calculate the optimum route that enables control, and calculate the atmosphere composition and carburizing temperature to obtain the target carburizing amount set from the specifications of each steel sheet or the carburizing concentration distribution that satisfies the carburizing amount. Set the maximum threading speed in the carburized zone under the conditions.

Next, the process proceeds to step S23, in which 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 crown is set so that the roll crown is within the meandering limit of the strip. A thermal crown calculation for calculating the plate speed is performed. 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 S24, where the maximum sheet passing speed in each furnace set in step S22 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 S25, 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 S24 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 performed for the purpose of surface hardening for improving the wear resistance and impact resistance of discontinuous materials such as gears, shafts, bearings and the like made of so-called tempered steel.
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 the surface characteristics of the strip and the material of the steel plate itself are improved. Performed for purpose. For this reason, for example, specifications required for a metal intended to improve the resistance to secondary working embrittlement (Japanese Unexamined Patent Publication No. 3-19934)
No. 4, etc.) to determine the carburizing conditions for the metal strip,
In this embodiment, the amount of C in the material is 20 ppm, the required amount of carburizing is 200 ppm or less, and the carburizing depth is 50 to 200 μm.
And the carburizing time is 120
Seconds or less. Under such conditions, the C concentration in the surface layer of the steel sheet does not reach the equilibrium concentration with respect to time. Therefore, as shown in the report of the above-mentioned leaves, the carburization rate is the reaction rate of the steel surface as shown in FIG. , And the carburizing rate is proportional to time itself. Since the carburization amount and the carburization depth are in non-equilibrium state in this surface reaction rate-controlling region, as an actual operation control index, the equilibrium C of the surface layer in the steel is simply used as in the past.
CO / C by C potential control so that the concentration becomes
In addition to controlling O ₂ , it is necessary to set carburizing conditions in consideration of a large number of control amounts in the furnace so as to obtain a required carburizing amount determined from specifications of steel sheets.
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, including the carburizing control in the unsteady state according to the present invention, as described above, the occurrence of sooting is prevented and the dew point is raised. Although it is necessary to control, these state generation mechanisms are inferred as follows. 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

[0063] Here, n: the number of gas species, p: the 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

[0065] ^{_{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.

[0066] 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 the above equation 8 and the above equation 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. 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 : atmosphere entering the furnace C mass in gas W ^s _C : C mass taken off by strip W ^g _O : C mass in atmospheric gas emitted from furnace V: Surface reaction rate, t: Carburizing time, w: Plate width.

In this manner, the generation of the atmosphere is calculated based on the thermodynamic (atmosphere composition) model formula in consideration of the actual material balance of the continuous carburizing in the carburizing furnace, so that the sooting can be reliably generated. 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 the carburizing amount control constituting the main part of the present embodiment including the unsteady state of the present invention will be described. The surface reaction when CO is used as the atmosphere gas is as follows.
Equations 1 to 13 can be considered. 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: indicates an equilibrium constant. However, said 1
Equation 4 does not take into account the effect of H ₂ . As a reaction formula relating to H _2, a reaction represented by the following formula 15 with respect to the reaction formula represented by the above formula 12 can be considered.

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
Recognize. 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 , k ₂ : reaction rate constants, and the reaction rate constants k ₁ , 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, and R is
Gas constant, T: indicates absolute temperature. Note that the frequency factors A _i ,
Since both the activation energy E _i and the gas constant R are constants, the reaction rate constants k ₁ and k ₂ were calculated from experimental values under various conditions of the absolute temperature T.

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,
X: Indicates the diffusion distance. In addition, the said diffusion coefficient D was decided to be displayed approximately by actual measurement data. Therefore, the above 1
The carburizing amount of the steel sheet can be calculated by the equations (7) and (18).

Here, in the case where the stripping speed is regulated based on the operation 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 equation (18) with the 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 conditions that match the carburizing amount.

First, in step S1, the conditions of the steel sheet after carburizing are set, the composition of the atmosphere gas, the flow rate of the input gas, the carburizing temperature and the sheet passing speed, the steel sheet specifications, and the carburizing concentration distribution of the steel sheet are used. Designated depth X from surface
Reads _one of the C concentration C _1, such as conditions of. 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, where the atmosphere composition model formula is set 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 speed into the steel is calculated based on the above equation (18), 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 is a constant. In step S10, it is determined whether the absolute value of the difference between the target carburizing amount ΔC ₀ and the set carburizing amount ΔC is smaller than a predetermined value d, and if the absolute value of the difference is smaller than the predetermined value d. Move to step S12,
If not, 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 carburizing temperature is changed to obtain the set carburizing amount set from the carburizing concentration distribution condition. , The process proceeds to step S2. In the step S12, to calculate the C concentration C _'1 of the specified depth X ₁ from the surface of the steel sheet according to the steel in the diffusion model set in step S6.

[0084] Next, the process proceeds to step S13, setting of the read elaborate designated by the steel sheet surface depth X ₁ in the step S1 C
And concentration C _1, the absolute value is a predetermined value e of the difference between the C concentration C _'1 of the specified depth X ₁ from the calculated surface of the steel sheet in the step S12
It is determined whether the difference is smaller than the predetermined value e. If the absolute value of the difference is smaller than the predetermined value e, the process proceeds to step S15; otherwise, the process proceeds to step S14.

In step S14, at least one of the parameters 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 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 clear from the figure, in the sooting generation limit obtained in consideration of the material balance, both the CO concentration and the H ₂ concentration are higher than the sooting generation limit obtained 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 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 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 steady state 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, not only when the carburizing concentration distribution is not required but also when the carburizing concentration distribution is required, soot 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 as to obtain the maximum processing capacity under the conditions. next,
The configuration concept of the carburizing condition setting of the present embodiment for executing the unsteady state carburizing amount control of the present invention and the specific logic thereof will be described. Prior to the description, the carburizing amount control logic shown in FIG. 6 is capable of preventing occurrence of sooting and obtaining a desired carburizing amount in both a steady state and an unsteady state. Therefore, the unsteady state carburizing condition setting logic described later can be higher or lower than the carburizing amount control logic depending on the case. The logic for setting the carburizing condition in the unsteady state in this embodiment is for setting the passing speed at which the target carburizing amount is finally attained. Is not always satisfied. Therefore, the unsteady state carburizing condition setting logic may constitute a part of the optimum route calculation in step S21 of the total line control logic of FIG. 5, or may be a premise of the total line control logic. It may be located at a higher level as a condition. For the above reasons, in the present embodiment, a loop is formed with all the logics including the unsteady-state carburizing condition setting logic described below, and an optimal solution is obtained from all the setting calculations.

As described above, as control amounts of the carburizing amount obtained from the specification data of the steel sheet including the carburizing concentration distribution, the passing speed (carburizing time), the atmosphere gas composition or the composition gas is listed in ascending order of the response time. Atmosphere composition set from concentration,
Carburization temperature. In the unsteady state where the next steel sheet transitions to the steady state obtained as described above, in order to achieve the desired carburization amount with these control amounts fluctuating, it is necessary to reduce the response time in the order of shorter response time. Easiest to control.

However, in the unsteady state, when the carburizing temperature and the atmosphere composition, which are the control amounts for preventing the occurrence of sooting in the furnace, fluctuate, the deterioration of the surface properties of the carburized thin steel sheet is prevented. Therefore, the condition that does not cause this sooting must be given priority. In order to achieve the desired amount of carburizing under conditions in which sooting does not occur when the carburizing temperature and the atmosphere composition fluctuate in this way, it is necessary to take into account the actual results of those controlled amounts, including the present and past, and Needs to be set.

Here, even in such an unsteady state, in the carburizing amount control logic shown in FIG. 6, for example, in step S1, the controller controlling the furnace temperature in order to attain the carburizing temperature changes from the furnace. By reading the results of the internal temperature and the results of the atmosphere composition from the controller that controls the atmosphere composition, and performing the carburizing amount control logic from time to time at predetermined time intervals, for example, when only the passing speed is changed, Even when the atmosphere composition and the sheet passing speed are changed under the condition that the furnace temperature is constant, the desired carburized amount can be achieved as much as possible under conditions where sooting does not occur.

However, when the furnace temperature fluctuates in order to change the carburizing temperature throughout the unsteady state, the control priority varies from the condition where sooting does not occur. That is, as described above, under the carburizing temperature set by the furnace temperature, the C is set to exhibit the maximum processing capacity.
In the case of O concentration or H ₂ concentration, sooting occurs when the furnace temperature is lowered. Therefore, when the furnace temperature is lowered, it is necessary to precede the reduction of the CO concentration and the H ₂ concentration in the atmosphere composition. On the other hand, when the furnace temperature is to be increased, the CO concentration and the H ₂ concentration must be increased in proportion to the increase in the furnace temperature.

FIG. 9 is a flowchart showing a concrete carburizing condition setting logic when the carburizing temperature is changed. In this embodiment, each controller for controlling the composition of the atmosphere and the temperature in the furnace (furnace temperature) always controls while reading the results in order to change the desired control amounts as set. In each step, it is considered that the same calculation as the carburizing amount control logic is performed in a subroutine.

FIG. 9A is a logic for lowering the furnace temperature to lower the carburizing temperature. First, in step S30, material conditions such as carburizing concentration distribution and sheet thickness are read. Next, the process proceeds to step S31, in which a target furnace temperature for achieving the target carburizing temperature obtained from the steady state logic is set. At the same time, steps S30 to S3 are performed.
In step 2, the target carburizing amount is set based on the material conditions.

From the step S31, the process proceeds to the step S31.
33, the furnace temperature fluctuation up to the target furnace temperature is predicted in a time series, and under this furnace temperature fluctuation, the change of the atmosphere composition such as the atmosphere gas composition and the composition gas concentration is calculated. A suitable atmosphere composition. The process proceeds from step S33 to step S34, and proceeds to step S3.
The atmosphere composition controller that has received the control signal of the atmosphere composition set in step 3 controls the flow rate of each atmosphere composition gas while reading the results of the atmosphere composition to achieve the time-series atmosphere composition change.

From step S34, the process proceeds to step S34.
In step 35, a change in the furnace temperature is calculated while reading the results of the atmosphere composition, thereby setting a time-series furnace temperature. Step S35 is followed by step S
Then, the furnace temperature controller, which has received the furnace temperature control signal set in step S35, controls the fuel gas flow rate while reading the results of the furnace temperature, etc. Achieve the change.

From the steps S32 and S36, the process proceeds to a step S37 to calculate a passing speed for achieving the target carburizing amount according to the atmosphere composition results and the furnace temperature results, and output the calculated speed. And exit the program.
FIG. 4B is a logic for raising the furnace temperature for raising the carburizing temperature. First, in step S40, material conditions such as carburizing concentration distribution and plate thickness are read.

Next, the flow shifts to step S41, in which a target furnace temperature for achieving the target carburizing temperature obtained from the logic in the steady state is set. At the same time, step S42
Then, the target carburizing amount is set from the material conditions.
From step S41, the process proceeds to step S43, in which furnace temperature fluctuations up to the target furnace temperature are predicted in time series, and a change in furnace temperature is calculated, thereby setting a time-series furnace temperature.

From step S43, step S
Going to 44, the furnace temperature controller, which has received the furnace temperature control signal set in step S43, controls the fuel gas flow rate while reading the actual results of the furnace temperature, etc. Achieve the change. Step S44
Then, the process proceeds to step S45 to change the atmosphere composition while reading the results of the furnace temperature, thereby setting the time-series atmosphere composition.

From the step S45, the next step S
In step 46, the atmosphere composition controller, which has received the control signal of the atmosphere composition set in step S45, controls the flow rate of each atmosphere composition gas while reading the actual results of the atmosphere composition, and performs Achieve the change. The process then proceeds to step S47 from steps S42 and S46, and calculates a sheet passing speed for achieving the target carburizing amount in accordance with the atmosphere composition results and the furnace temperature results, and outputs and outputs the program. To end.

A specific calculation example of the control of the control amount performed by the unsteady state carburizing condition setting logic will be described with reference to FIG. FIG. 10 shows a case where the carburizing temperature, that is, the furnace temperature is lowered according to the logic shown in FIG. 9a in an unsteady state between two types of steel sheets having different carburizing amounts with a constant thickness Dmm, in this case, between the A steel sheet and the B steel sheet. Show. Furnace temperature is the primary curve falls between times t ₂ from the setting time t ₁ in FIG. If the H ₂ concentration is constant under the conditions of the furnace temperature fluctuation, the CO concentration in the atmosphere, which indicates the limit of occurrence of sooting, is indicated by the hatched area in the figure. changing the curve of the atmosphere CO concentration in the furnace temperature fluctuation between t ₁ -t ₂ is set.

Here, since the passing speed in the steady state of each steel sheet is set in advance, the time t _1-
If the sheet passage rate of the two steady-state rose primary curved manner in a non-steady state between t _2, carburizing amount to each steel sheet this time t ₁
It varies as indicated by a broken line in FIG between -t _2, between this variation do not satisfy any of the carburizing quantity, the so-called material failure. In other words, the threading material is required for this length.

On the other hand, in this embodiment, the furnace temperature and atmosphere
To achieve the target carburization amount according to the results of the gas composition
In order to change and control the passing speed, the passing speed is specifically shown in the figure.
As shown by the solid line, the time t₁To t_ThreeCO concentration changes between
At the time t _ThreeFrom time t_FourPossible between
Increase as fast as possible, at time t_FourFrom time t_TwoBetween the CO
At the same time t_TwoNext
Steady state threading speed. This allows carburizing of each steel sheet
The amount varies as shown by the solid line in the figure. In this case, each steel plate
The material defect length that deviates from the target carburization amount of
Rapidly increasing time t_Three-T_FourIt ’s only short between
The required threading material length is also reduced.

According to this embodiment, the processing amount for 20 minutes corresponding to the time t ₁ -t ₂ in the unsteady state is improved by 20% compared with the conventional case, and the material defect rate during this period is 80%.
From 1% to 10.4%, about 1/8. In this embodiment, CO, H ₂ , CO ₂ and H ₂ O are used in the surface reaction.
Although the case where the surface reaction rate is calculated in consideration of only the influence of the above is described in detail, as described above, the surface reaction rate may be calculated in consideration of the influence of other atmospheric gas compositions, for example, the influence of heavy hydrocarbons. Good.

In the present embodiment, the thermodynamic model equation taking into account the material balance is linearized, and the solution is converged to calculate the equilibrium state of the atmosphere composition. However, the present invention is not limited to this.
Further, in this embodiment, particularly, continuous annealing and carburizing of a strip made of ultra-low carbon steel to control carburization in a surface reaction limited region where the carburizing rate is controlled by the surface reaction rate before the carburizing concentration reaches the equilibrium concentration. Although only the details have been described in detail, it goes without saying that the continuous carburizing method of the present invention can be applied to a diffusion-controlled region in which the carburizing rate is controlled by the diffusion rate from the surface of the metal strip to the inside.

[0109]

As described above, according to the method for continuously carburizing a metal strip according to the present invention, the carburizing temperature, the composition of the atmosphere gas, and the concentration of the composition gas are determined in the unsteady state in which the transition from the previous steady state to the next steady state occurs. , By setting the control amount of the sheet passing speed in accordance with the actual results of the control amounts of each other so as to achieve a desired carburizing amount within a range in which sooting does not occur.
Since the length of the defective material or the length of the pass-through member used therebetween can be shortened, a substantial reduction in the processing efficiency can be suppressed, and the overall operating cost can be reduced.

[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 an example of a continuous annealing carburizing facility to be subjected to carburizing control using the metal strip continuous carburizing 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 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 shows a logic for setting carburizing conditions by the method for continuously carburizing a metal strip in an unsteady state according to the present invention, wherein (a) is a flow chart when the furnace temperature is lowered,
(B) is a flowchart in the case of raising the furnace temperature.

10 is an explanatory diagram showing an example of a correlation between the content of time change control of each control amount obtained by the logic of FIG. 9 and the amount of carburization achieved thereby.

[Description of Signs] 1 is pre-tropical 2 is heating zone 3 is solitary 4 is carburized zone 5 is first cooling zone 6 is second cooling zone A is strip

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-6-158268 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C22C 8/22

Claims (7)

(57) [Claims] 1. A continuous carburizing of a metal strip which is continuously subjected to continuous gas carburization by continuously forming a metal sheet having different desired specifications into a series of metal strips and passing the metal sheet into a carburizing furnace of the carburizing zone. Unsteady state in which the specifications of the desired metal sheet differ between the previous and next metal sheets, such as where the amount of carburization of the metal sheet at the seam or in the metal strip should be changed during operation. In the continuous carburizing method of the metal strip of the above, transition from the previous steady state on the previous metal sheet to the next steady state on the next metal sheet. During the unsteady state, the atmosphere gas composition, the composition gas concentration, the carburizing temperature , One or more of the control amounts such as the passing speed, etc., are controlled so as to satisfy the carburizing amount obtained from different specifications of the metal plate under conditions where sooting does not occur. Continuous carburization of metal strip characterized by unsteady state carburization control Method. 2. In performing the carburizing amount control in the unsteady state, each control amount in the next steady state sandwiching the unsteady state is set, and any one of the control amounts is set to satisfy the control amount. When changing one or more than the above steady state, the atmosphere gas composition or the composition gas in consideration of the fluctuation of the furnace temperature for obtaining the changed carburization temperature under the condition where the sooting does not occur. The method for continuously carburizing a metal strip according to claim 1, wherein the concentration is set. 3. When the carburizing temperature is raised, the furnace temperature is preceded by an increase in the furnace temperature. 3. The method for continuously carburizing a metal strip according to claim 1, wherein the concentration of carbon gas, the concentration of carbon monoxide gas, and the concentration of hydrogen gas are increased. 4. When the carburizing temperature is lowered, prior to the reduction of the concentration of carbon monoxide gas or the concentration of carbon monoxide gas and the concentration of hydrogen gas in the composition of the atmosphere gas, the composition of the atmosphere gas or the concentration of the composition gas is reduced. 3. The method for continuously carburizing a metal strip according to claim 1, wherein the furnace temperature is lowered within a range in which sooting does not occur according to the results of the above. 5. In performing the unsteady state carburizing amount control, each control amount in the next steady state sandwiching the unsteady state is set, and any one of the control amounts is set to satisfy the control amount. In the case where one or two or more are changed from the previous steady state, and the carburizing temperature is not changed, the range in which sooting does not occur under the conditions of the carburizing temperature according to the actual temperature in the furnace. The method for continuously carburizing a metal strip according to claim 1, wherein the composition of the atmosphere gas or the concentration of the composition gas is changed in the inside. 6. A method for controlling the amount of carburization in the unsteady state, wherein the control amounts of the atmosphere gas composition, the composition gas concentration, the carburizing temperature, and the like are changed or not. 6. The method for continuously carburizing a metal strip according to claim 1, wherein the passing speed is controlled so as to satisfy a desired amount of carburization of each of the metal plates according to the following. 7. When the passing speed is regulated by the actual carburizing operation of the metal strip, the carburizing amount desired for each metal plate is satisfied under the condition that the sooting does not occur. The method for continuously carburizing a metal strip according to any one of claims 1 to 5, wherein the other control amounts are controlled as described above.