JP2019173144A - Vertical continuous annealing furnace and annealing method - Google Patents

Abstract

To provide a vertical continuous annealing furnace and an annealing method capable of accurately controlling decarburization reaction by improving dew point controllability in the furnace.SOLUTION: The vertical continuous annealing furnace according to the present invention that is configured to heat-treat a steel strip while conveying it in a specific horizontal direction by looping the steel strip between a plurality of top rolls and a plurality of bottom rolls so as to meander in a vertical direction includes: the inside being divided into a plurality of sections along the specific horizontal direction and one or more partition plates having an opening so that an atmospheric gas in the furnace flows so as to counter flow of the steel strip meandering up and down; a plurality of dew point measuring devices arranged in each of the plurality of the sections; an injection mechanism of a high dew point gas (including water vapor) to adjust the dew point in each of the plurality of sections; a mechanism to control an amount of high dew point gas injection by the injection mechanism based on a required water vapor amount calculated from composition of the atmospheric gas and a water vapor amount to be consumed by the decarburization reaction which is predicted according to history of the dew point by the dew point measuring device and the temperature of the steel strip.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a vertical continuous annealing furnace and an annealing method.

  In order to obtain the desired quality by removing the carbon content in the steel sheet, it is important to control the dew point so that the atmospheric gas in the furnace contains water vapor and the humidity is controlled to an appropriate value.

  As a dew point control method, for example, a method of continuously supplying a non-oxidizing gas to a heating furnace during a heat treatment for annealing a steel sheet to reduce the dew point in the heating furnace has been proposed (Japanese Patent Laid-Open No. 9-256074). reference). In the method described in this publication, the dew point of the atmosphere in the heating furnace is measured, and the supply amount of the non-oxidizing gas is adjusted so that the dew point is within the target range.

Japanese Patent Laid-Open No. 9-256074

  However, there are dew point distributions and oxygen concentration distributions in the furnace, and even if controlled based on measured values, there is a possibility that the dew point temperature in the furnace will not be uniform due to the representativeness of the measurement points and response delays. . In particular, in the case of a vertical furnace, since there is a circulation flow in the furnace, it becomes more difficult to adjust the dew point distribution in the furnace.

  Further, in the case of controlling the decarburization reaction in the annealing furnace, the partial pressure of water vapor is low in the furnace under a low dew point atmosphere, and it is difficult to adjust the atmosphere in the furnace, and the control of the decarburization reaction becomes more difficult. Therefore, there is a possibility that the decarburization reaction cannot be accurately controlled when a large change occurs in the sheet passing speed due to the difference in the composition of the steel plates or the plate thickness.

  This invention is made | formed in view of such a situation, and it aims at providing the annealing method of the steel plate which improves the dew point controllability in a furnace and can control a decarburization reaction accurately.

  As a result of studying the amount of water vapor consumed in the decarburization reaction in the annealing furnace, the present inventors have determined that the amount of water vapor input to adjust the dew point of the atmospheric gas in the furnace is the amount of water vapor consumed in the decarburization reaction. It was found that it is necessary to consider.

  One embodiment of the present invention made to solve the above problems includes a plurality of top rolls and a plurality of bottom rolls disposed therein, and a strip-shaped steel plate between the plurality of top rolls and the plurality of bottom rolls. Is a vertical continuous annealing furnace constructed so as to meander in the vertical direction and heat-treat while conveying the steel plate in a specific horizontal direction, and the interior is divided into a plurality of sections along the specific horizontal direction. And one or a plurality of partition plates having an opening so that the atmospheric gas flowing in the furnace so as to face the steel plate meanders up and down, a plurality of dew point measuring devices arranged for each of the plurality of sections, A mechanism for injecting a high dew point gas (including water vapor) to adjust the dew point of the plurality of compartments, a required water vapor amount calculated from the composition of the atmospheric gas, a history of the dew point by the dew point measuring instrument, and the steel sheet Based on the consumption amount of water vapor by decarburization predicted by degrees, a vertical continuous annealing furnace and a mechanism for controlling the high dew point gas injection amount of the blow mechanism.

  The vertical continuous annealing furnace is divided into a plurality of sections along the specific horizontal direction, and has one or more openings so that the atmospheric gas flowing in the furnace faces up and down to face the steel plate By providing this partition plate, the circulation property of the atmospheric gas in the furnace can be improved. Also, based on the required water vapor amount calculated from the composition of the atmospheric gas, the dew point history by the dew point measuring instrument, and the amount of water vapor consumed by the decarburization reaction predicted by the temperature of the steel sheet, the high dew point gas blowing amount of the blowing mechanism is By providing a mechanism to control, the dew point in the furnace can be raised with high accuracy, so that the dew point controllability in the furnace can be improved and the decarburization reaction can be controlled with high precision. As a result, the desired quality of the steel sheet such as bendability Can be obtained.

  It is preferable that the dew point of the section upstream in the sheet passing direction is higher than the dew point of the section downstream in the sheet passing direction. The dew point of the upstream section in the sheet passing direction is higher than the dew point in the downstream section in the sheet passing direction, so that the water vapor consumed in the downstream section (the latter stage) in the passing direction is the upstream section (the previous stage). ) Can improve the controllability of the dew point in the furnace.

  The area ratio of the opening of the partition plate to the area in the furnace of the vertical furnace cross section at the position of the partition plate is preferably 1/10 or more and 1/4 or less. When the area ratio of the opening of the partition plate is in the above range, the flow rate of the atmospheric gas can be maintained in a favorable range, and the dew point controllability in the furnace can be improved.

  Another aspect of the present invention, which has been made to solve the above problems, includes a plurality of top rolls and a plurality of bottom rolls disposed therein, and a belt-like shape between the plurality of top rolls and the plurality of bottom rolls. One or a plurality of steel plates are arranged so as to meander in the vertical direction, and are heat-treated while transporting the steel plates in a specific horizontal direction, and have an opening so that the atmospheric gas flowing in the furnace meanders up and down An annealing method using a vertical continuous annealing furnace that divides the interior into a plurality of sections along the specific horizontal direction by a partition plate, the step of calculating a required water vapor amount from the composition of the atmospheric gas, Predicting the amount of water vapor consumed by decarburization reaction from the dew point history of the plurality of compartments and the temperature of the steel sheet, and the high dew of the one or more compartments depending on the required water vapor amount and the consumed water vapor amount. I.e., an annealing process and a step of controlling the gas blowing amount.

  In the annealing method, the interior is divided into a plurality of sections along the specific horizontal direction, and one or a plurality of partition plates having openings so that the atmospheric gas flowing in the furnace faces up and down to face the steel plate By using a vertical continuous annealing furnace provided with the above, it is possible to improve the circulation of the atmospheric gas in the furnace. Moreover, the dew point in the furnace is controlled by controlling the high dew point gas injection amount based on the required water vapor amount calculated from the composition of the atmospheric gas and the dew point history and the water vapor consumption due to the decarburization reaction predicted by the temperature of the steel sheet. Since it can raise accurately, the dew point controllability in a furnace can be improved and a decarburization reaction can be controlled accurately, As a result, desired quality of steel plates, such as bendability, can be obtained.

  The annealing method of the present invention improves the dew point controllability in the furnace and can accurately control the decarburization reaction.

It is a schematic diagram which shows the structure of the vertical type continuous annealing furnace of 1st Embodiment of this invention. It is a schematic diagram which shows a part of structure of the vertical continuous annealing furnace of 2nd Embodiment of this invention. It is a schematic diagram which shows a partial structure of the vertical continuous annealing furnace of 3rd Embodiment of this invention. It is a graph which shows the relationship between the temperature rising rate of soaking zone, steel plate temperature, decarburization rate, and the position of the longitudinal direction of a furnace. It is a graph which shows the relationship between the temperature rising rate of soaking zone, steel plate temperature, decarburization rate, and the position of the longitudinal direction of a furnace. It is a schematic diagram which shows other embodiment of arrangement | positioning of a gas supply nozzle. It is a schematic diagram which shows the structure of the vertical type continuous annealing furnace in an Example. It is a graph which shows the relationship between the dew point and the decarburization depth in an Example.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings as appropriate.

<Vertical continuous annealing furnace>
[First Embodiment]
The present invention relates to a vertical continuous annealing furnace suitable for a high dew point operation in which a decarburization in a furnace is increased to promote a decarburization reaction of a steel sheet in a hot dip galvanizing facility or the like. The vertical continuous annealing furnace includes a plurality of top rolls and a plurality of bottom rolls disposed therein, and a belt-shaped steel plate is interposed between the plurality of top rolls and the plurality of bottom rolls so as to meander in the vertical direction. It is configured to carry out heat treatment while conveying the steel sheet in a specific horizontal direction. The vertical continuous annealing furnace can be performed as one step of manufacturing a hot-dip galvanized steel sheet used for automobiles, ships, building materials, home appliances, and the like.

  The configuration of the first embodiment of the vertical continuous annealing furnace will be described with reference to FIG. The vertical continuous annealing furnace 1 includes a heating zone 5, a soaking zone 10, and a cooling zone 7, which are juxtaposed in this order, and decarburize steel sheets in the soaking zone 10. The heating zone 5 has a first heating zone 5A (front stage of the heating zone) and a second heating zone 5B (back stage of the heating zone). For example, a hot dip galvanizing bath is connected to the cooling zone 7 of the vertical continuous annealing furnace 1.

  In the vertical continuous annealing furnace 1, a plurality of top rolls 3 are arranged in the upper part of the furnace, and a plurality of bottom rolls 4 are arranged in the lower part of the furnace. The steel plate 2 is introduced into the first heating zone 5A from the steel strip inlet at the lower part of the first heating zone 5A, and is conveyed in the sheet passing direction indicated by the arrow P. The vertical continuous annealing furnace 1 is constructed such that a steel plate 2 is meandered between a plurality of top rolls 3 and a plurality of bottom rolls 4 so as to meander in a vertical direction, and the steel plate 2 is heat-treated while being conveyed in a specific horizontal direction. .

  When the steel plate 2 is folded back at 180 degrees starting from the top roll 3 and the bottom roll 4, the steel plate 2 is conveyed a plurality of times in the vertical direction within a predetermined band of the vertical continuous annealing furnace 1 to form a plurality of passes. The number of passes can be appropriately set according to the processing conditions. Moreover, in some top rolls 3 and bottom rolls 4, the direction of the steel plate 2 is changed to a right angle without folding back, and the steel plate 2 is moved to the next band. The steel plate 2 is conveyed in the order of the heating zone 5, the soaking zone 10 and the cooling zone 7 in the vertical continuous annealing furnace 1, and the steel plate 2 is annealed.

  In the vertical continuous annealing furnace 1, adjacent bands communicate with each other via a communication portion that connects the upper parts or the lower parts of each band. The first heating zone 5A and the second heating zone 5B communicate with each other via a throat 27 that connects the upper portions of the respective zones. The second heating zone 5B and the soaking zone 10 communicate with each other via a throat 26 that connects the lower portions of each zone. The soaking zone 10 and the cooling zone 7 communicate with each other via a throat 25 that connects the lower portions of each zone.

(Heating zone)
In the present embodiment, the second heating zone 5B is a direct-fired heating furnace. On the inner wall of the direct-fired heating furnace in the second heating zone 5B, a plurality of burners are arranged in a distributed manner facing the steel plate 2. Moreover, the combustion exhaust gas of the 2nd heating zone 5B is supplied inside the 1st heating zone 5A, and the steel plate 2 is preheated with the heat. Although the temperature inside the 2nd heating zone 5B is selected according to the kind and intended purpose of the steel plate 2, it is preferable to set it as 800-1200 degreeC, for example.

(Cooling zone)
In the cooling zone 7, the heated steel plate 2 is cooled. The cooling zone 7 is supplied with an atmospheric gas having a constant flow rate and a low dew point from the gas inlet G1 to gradually lower the furnace temperature. Examples of the atmospheric gas to be supplied include a gas having a composition composed of N ₂ and inevitable impurities and having a dew point of about −60 ° C.

(Soaking)
In the soaking zone 10, by maintaining the atmospheric temperature in the furnace at a predetermined annealing temperature, the steel plate 2 is decarburized according to the following reaction formula while being heated up to the annealing temperature.
H ₂ O + C → CO + H ₂

  In the soaking zone 10, the steel plate 2 can be indirectly heated using, for example, an electric heater or a radiant tube (RT) as a heating means. The average temperature inside the soaking zone 10 is selected according to the type and purpose of use of the steel plate 2, but is preferably set to 700 to 900 ° C, for example.

The soaking zone 10 and the cooling zone 7 are supplied with a low dew point atmospheric gas from the cooling zone 7 through the throat 25 that connects the lower portions of each zone. An atmospheric gas with a low dew point such as N ₂ is also supplied from the gas inlet G2 at a constant flow rate. The atmospheric gas supply amount and the gas amount from the cooling zone are measured by a gas flow meter provided in the pipe.

  The soaking zone 10 is provided with a partition plate 14, which divides the inside of the furnace into a plurality of sections along the horizontal direction. Specifically, the soaking zone 10 is divided into two sections, a front stage 12 on the upstream side in the plate passing direction and a rear stage 11 on the downstream side. The partition plate 14 is provided in a state where one end is fixed to the furnace bottom wall and the other end is opened between the furnace bottom wall and the roll lower end portion on the side where the steel plate 2 of the top roll 3 is not wound. The partition plate 14 has an opening so that the atmospheric gas circulated in the furnace faces the steel plate 2 so as to meander up and down.

  As described above, the present inventors have found that the position of the opening has an appropriate position for making the gas in the furnace uniform by the flow simulation in the furnace. In addition, it has been found by simulation of the decarburization reaction that the dew point can be appropriately adjusted even when the stay time varies due to the influence of the sheet passing speed of the steel plate for each section by performing the dew point control for each section. In this way, by defining the introduction and discharge positions of the atmospheric gas in the furnace, the atmospheric gas circulation is improved and the uniformity of the atmospheric gas is improved, and dew point control is performed for each section, The dew point can be controlled appropriately according to the section.

  The partition plate 14 has only to have heat resistance in a temperature range to be used and strength not to be damaged by operation, and can substantially block the gas. For example, a known heat-resistant material such as a ceramic board is used.

  The area ratio of the opening of the partition plate to the area in the furnace of the vertical furnace cross section at the position of the partition plate is 1/10 or more and 1/4 or less. Here, the area ratio of the opening of the partition plate 14 is that which regulates the gas flow in the direction perpendicular to the cross section in the cross section relative to the area in the furnace of the vertical furnace cross section in the partition installation portion. It is the ratio of the area of no part. When the area ratio of the opening of the partition plate 14 is within the above range, the flow rate of the atmospheric gas can be maintained in a favorable range, and the dew point controllability in the furnace can be improved. Moreover, when the area ratio of the opening of the said partition plate is less than 1/10, there exists a possibility that the pressure loss of gas may become large too much and the flow volume of atmospheric gas required for a decarburization reaction cannot fully be ensured. On the other hand, if the area ratio of the opening of the partition plate exceeds 1/4, the atmosphere gas moves through the opening, and the action of restricting the direction of the flow of the atmosphere gas in the direction opposite to the plate direction may be reduced. There is.

  The vertical continuous annealing furnace includes a plurality of dew point measuring devices arranged in a plurality of sections. In the soaking zone 10, the dew point is constantly measured by the dew point measuring device R1 and the dew point measuring device R2 arranged for each section.

  The vertical continuous annealing furnace 1 includes a mechanism for blowing a high dew point gas (including water vapor) (not shown) in order to adjust the dew points of a plurality of sections. Specifically, the vertical continuous annealing furnace 1 is provided with a gas supply device (not shown) for supplying high dew point gas or water vapor. The gas supply device includes a gas supply nozzle. Gas supply nozzles 6 </ b> A and 6 </ b> B that supply high dew point gas along the flow path of the atmospheric gas and toward the surface of the steel plate 2 are disposed in the front stage 12 of the soaking zone 10. The gas supply nozzles 6 </ b> A and 6 </ b> B are arranged so that steam is blown from the upper wall of the furnace toward a part of the top rolls 3. Moreover, since the steel plate is oxidized when water vapor hits the steel plate, it is preferable to arrange the gas supply nozzle so as not to directly hit the steel plate. Therefore, the gas supply nozzles may be arranged between the top rolls 3 and the bottom rolls 4, or may be arranged at the atmospheric gas introduction port G2 to mix with the atmospheric gas and blow high dew point gas. Further, if the atmospheric dew point is maintained at 0 ° C., the dew point of the gas to be blown needs to be increased in consideration of the consumption of water vapor.

  Moreover, the vertical continuous annealing furnace 1 includes a control mechanism (not shown) that controls the high dew point gas blowing amount of the blowing mechanism. As the control mechanism, a conventionally known computer system can be used. With this control mechanism, the high dew point of the blowing mechanism is based on the required water vapor amount calculated from the composition of the atmospheric gas, the dew point history by the dew point measuring instrument, and the amount of water vapor consumed by the decarburization reaction predicted by the temperature of the steel sheet. A gas blowing amount is set, and the above-mentioned blowing amount of high dew point gas is blown into the furnace from the blowing mechanism. Control by this control mechanism includes, for example, blowing according to the required water vapor amount calculated from the composition of the atmospheric gas, the dew point history by the dew point measuring device, and the consumed water vapor amount by the decarburization reaction predicted by the temperature of the steel plate. Examples include control for adjusting the high dew point gas blowing amount of the mechanism and control for adjusting the high dew point gas blowing position, the blowing time, and the like. Further, the target dew point is set so that the final decarburization depth is examined according to the rigidity of the target steel sheet and this decarburization depth is achieved.

<Advantages>
Although the vertical continuous annealing furnace has a vertical structure in which the atmospheric gas is difficult to circulate uniformly in the furnace, it can improve the circulation of the atmospheric gas in the furnace. Also, based on the required water vapor amount calculated from the composition of the atmospheric gas, the dew point history by the dew point measuring instrument, and the amount of water vapor consumed by the decarburization reaction predicted by the temperature of the steel sheet, the high dew point gas blowing amount of the blowing mechanism is By providing a control mechanism, the dew point in the furnace can be raised with high precision, so that the dew point controllability in the furnace can be improved and the decarburization reaction can be controlled with high precision, and as a result, the desired quality of the steel sheet such as bendability can be improved. Obtainable.

[Second Embodiment]
Drawing 2 is a mimetic diagram showing composition of soaking zone 20 which is a part of a vertical type continuous annealing furnace of a 2nd embodiment of the present invention. In addition, the same structure as 1st Embodiment describes only the same referential mark, and abbreviate | omits description.

In the vertical continuous annealing furnace according to the second embodiment, both a throat 25 serving as an inlet for atmospheric gas in the soaking zone 20 and a throat 26 serving as an outlet are disposed in the upper part of the furnace. An atmospheric gas having a low dew point is supplied from the cooling zone 7 to the soaking zone 20 via a throat 25 that connects the soaking zone 20 and the cooling zone 7. Further, an atmospheric gas having a low dew point such as N ₂ is also supplied from the gas introduction port G4 at a constant flow rate. The soaking zone 20 is divided into two sections, a upstream stage 22 on the upstream side in the plate passing direction and a downstream stage 21 on the downstream side. One end of the partition plate 24 is fixed to the soaking zone furnace upper wall, and the other end is the bottom of the furnace. It is provided in an open state between the wall and the roll upper end on the side where the steel plate 2 of the bottom roll 4 is not wound.

  In the second embodiment, the soaking zone 20 into which high dew point gas (including water vapor) is blown to adjust the dew point is divided into a plurality of sections by the partition plate 24, and high dew point gas blowing and dew point control are performed for each section. Is called. Therefore, mixing of the atmospheric gas and the blown-in high dew point gas is promoted, and the dew point in the furnace becomes more uniform. Further, as in the first embodiment, the gas supply nozzle for supplying the high dew point gas is disposed along the flow path of the atmospheric gas and toward the surface of the steel plate 2, so that it affects the atmospheric gas in other compartments. The dew point can be controlled for each section without giving any value.

  As described above, since the consumption amount of water vapor varies depending on the steel plate temperature, in this embodiment, it is possible to measure the steel plate temperature for each section and adjust the additional input amount of steam. In the annealing furnace, the plate passing speed decreases as the plate thickness increases, and the decarburization depth increases as the plate passing rate decreases. Even in such a case, the controllability of the decarburization reaction can be improved by adjusting the input amount of water vapor for each section.

  The dew point of the front stage 22 that is a section upstream of the sheet passing direction is preferably higher than the dew point of the rear stage 21 that is a section downstream of the sheet passing direction. Since the dew point of the upstream stage 22 upstream of the sheet passing direction is higher than the dew point of the downstream stage 21 downstream of the sheet passing direction, water consumed in the downstream stage downstream of the sheet passing direction can be compensated by the upstream stage upstream of the sheet passing direction. Therefore, the controllability of the dew point in the furnace can be improved. In addition, even when the soaking zone stay time fluctuates due to changes in the plate passing speed, it is possible to adjust the decarburization depth by controlling the dew point with high accuracy.

<Advantages>
According to the second embodiment, the mixing of the flow of the atmospheric gas at the partition with the partition gas is promoted, the dew point in the furnace becomes more uniform, and the control accuracy of the water vapor distribution in the furnace is further improved. it can. Further, by controlling the dew point for each section, it is possible to accurately correct the dew point of the atmospheric gas that has fallen due to the decarburization reaction performed in the preceding stage downstream of the soaking zone in the plate passing direction.

[Third Embodiment]
Drawing 3 is a mimetic diagram showing composition of soaking zone 30 which is a part of a vertical type continuous annealing furnace of a 3rd embodiment of the present invention. In addition, the same structure as 1st Embodiment describes only the same referential mark, and abbreviate | omits description.

In the vertical continuous annealing furnace according to the third embodiment, both a throat 25 serving as an inlet for atmospheric gas in the soaking zone 30 and a throat 26 serving as an outlet are disposed at the furnace bottom. The soaking zone 30 is supplied with atmospheric gas having a low dew point from the cooling zone 7 through a throat 25 that connects the soaking zone 30 and the cooling zone 7. An atmospheric gas with a low dew point such as N ₂ is also supplied from the gas inlet G6 at a constant flow rate. The soaking zone 30 is divided into two sections, a front stage 32 on the upstream side in the plate passing direction and a rear stage 31 on the downstream side. One end of the partition plate 34 is fixed to the furnace bottom wall of the soaking zone 30 and the other end is the furnace. It is provided in an open state between the bottom wall and the lower end of the roll on the side where the steel plate 2 of the top roll 3 is not wound.

  In order to adjust the dew point, the soaking zone 30 into which high dew point gas (including water vapor) is blown is divided into a plurality of sections by the partition plate 34, and high dew point gas blowing and dew point control are performed for each section. Therefore, mixing of the atmospheric gas and the blown-in high dew point gas is promoted, and the dew point in the furnace becomes more uniform. Moreover, since the gas supply nozzle which supplies high dew point gas along the flow path of atmospheric gas and toward the surface of the steel plate 2 is arrange | positioned similarly to 1st Embodiment, it has influence on the atmospheric gas of another division. The dew point can be controlled for each section without giving.

  Also in 3rd Embodiment, it is good for the dew point of the front | former stage 32 which is a division of the sheet | seat direction upstream side to be higher than the dew point of the back | latter stage 31 which is a division of the sheet | seat direction downstream side. Since the dew point of the upstream stage 32 on the upstream side in the sheet passing direction is higher than the dew point on the downstream stage 31 on the downstream side in the sheet passing direction, water consumed in the downstream stage on the downstream side in the sheet passing direction can be compensated in the upstream stage in the upstream direction in the sheet passing direction Therefore, the controllability of the dew point in the furnace can be improved. In addition, even when the soaking zone stay time fluctuates due to changes in the plate passing speed, it is possible to adjust the decarburization depth by controlling the dew point with high accuracy.

<Advantages>
According to the said 3rd Embodiment, the mixing which introduce | transduces a vapor | steam for every division with a partition plate with the flow of atmospheric gas is accelerated | stimulated, the dew point in a furnace becomes more uniform, and the precision of control of the water vapor distribution in a furnace is improved more it can. Further, by controlling the dew point for each section, it is possible to accurately correct the dew point of the atmospheric gas that has fallen due to the decarburization reaction performed in the preceding stage downstream of the soaking zone in the plate passing direction.

<Annealing method>
The annealing method includes a plurality of top rolls and a plurality of bottom rolls disposed therein, and a belt-shaped steel plate is bridged between the plurality of top rolls and the plurality of bottom rolls so as to meander in the vertical direction. The steel plate is configured to be heat-treated while being conveyed in a specific horizontal direction, and a plurality of interiors along the specific horizontal direction are formed by one or a plurality of partition plates having openings so that the atmospheric gas flowing in the furnace meanders up and down. It is the annealing method using the vertical continuous annealing furnace divided | segmented into this division. The annealing method relates to an atmospheric gas supply method and a control method for performing a high dew point operation for increasing the dew point in the furnace and advancing the decarburization reaction of the steel sheet in a hot dip galvanizing facility or the like. The said annealing method can be performed as one process of manufacture of the hot dip galvanized steel plate used for a motor vehicle, a ship, a building material, a household appliance, etc., for example.

  The annealing method includes a step of calculating a required water vapor amount from the composition of the atmospheric gas, a step of predicting a water vapor consumption amount due to a decarburization reaction from the history of the dew point of the one or more sections and the temperature of the steel plate, and And a step of controlling the high dew point gas blowing amount of the one or more sections according to the required water vapor amount and the consumed water vapor amount.

[Required water vapor amount calculation process]
The required amount of water vapor is calculated in the following steps.
(1) Setting of final decarburization depth First, the final decarburization depth is set according to the rigidity of the target steel plate. In this annealing method, the decarburization depth was determined by GDOES analysis in the thickness direction of the steel sheet, and the carbon concentration lacking from the surface layer due to decarburization increased in the thickness direction, resulting in 90% of the base material. The depth of the.
(2) Setting of the target dew point Since the decarburization reaction proceeds according to the diffusion-controlled C, the decarburization depth x1 is proportional to the ½ power of the elapsed time (stay time in the furnace) t. It is represented by (1).

  Further, the diffusion coefficient kp is a function of the steel plate temperature T1, and increases with an increase in the steel plate temperature T1 in the Arrhenius type. The diffusion coefficient kp is expressed by the following formula (2).

Here, const is a constant indicating a frequency factor. P _H2O is the saturated water vapor pressure. n is a constant, and n = 1 in the present invention.

  In the equation using the decarburization depth x1 of carbon corresponding to the elapsed time t, the decarburization speed dx1 / dt is expressed by the following equation (3).

  The final decarburization depth x2 is expressed by the following formula (4).

The amount of water vapor for obtaining the target final decarburization depth x2 is determined from the history of the steel plate temperature T1, and the target dew point is set. More specifically, the decarburization speed dx1 / dt and the elapsed time t when PH _{2 O} is an arbitrary constant value with respect to the history of the steel plate temperature T1 from the expressions (1), (2), and (3). The decarburization depth x1 according to is obtained. Then, calculate the final decarburization depth from the equation (4), compare the calculated decarburization depth with the target decarburization depth, and the calculated decarburization depth matches the target decarburization depth. The calculation is repeated until the amount of water vapor to be determined is determined.

(3) Calculation of required water vapor amount for setting dew point in furnace to target dew point Next, the concentration (ppm) of contained water is calculated from the dew point of the atmospheric gas. First, the saturated water vapor pressure is calculated from the dew point T2 (° C.) from the Tetens equation. Then, the concentration of the contained water is calculated from the calculated saturated water vapor pressure. The concentration H ₂ O (ppm) of the contained water can be calculated from the following equation (5).

The amount of water vapor required for making the dew point in the furnace the target dew point during the decarburization reaction is expressed by the following equation (6).
Required amount of water vapor = amount of water vapor to raise the atmospheric dew point + consumption of decarburization reaction (6)

When the gas amount from the cooling zone and the additional atmospheric gas are supplied to the soaking zone, the water vapor amount that raises the atmospheric dew point of the above equation (6) is expressed by the following equation (7).
Amount of water vapor that raises the atmospheric dew point = additional atmospheric gas supply amount × {f (target dew point) −f (supply gas dew point)}
+ Gas amount from cooling zone x {f (target dew point)-f (gas cooling zone dew point)} (7)

[Consumption water vapor amount prediction process]
The prediction of the amount of water vapor consumption is performed in the following steps. In this step, the amount of water vapor consumed by the decarburization reaction is predicted from the history of the dew point of one or more sections and the temperature of the steel sheet.

Amount of water vapor by decarburization reaction = Amount of plate to be passed x Steel material carbon amount x (Decarburization depth ÷ Steel plate thickness) ÷ 2 x 2 surfaces x 0.9 x Carbon water vapor consumption ... (8)

In said formula (8), 2 sides are the surface and back surface of a steel plate. In the water vapor consumption of carbon, as described above, 1 mol of H ₂ O is consumed for 1 mol of C (carbon) in the decarburization reaction. In addition, the carbon concentration distribution in the thickness direction of the decarburized steel sheet is approximately 0% on the steel sheet surface, increases substantially linearly in the thickness direction, and increases to the concentration of the base material. The amount of decarburization is the carbon concentration reduced from the carbon concentration of the base material. As described above, in the carbon concentration distribution in the thickness direction of the steel sheet, since the carbon concentration increases substantially linearly, in order to calculate the carbon concentration as a triangular area, it is divided by 2 in Equation (8).

Further, as the amount of water vapor consumed in the furnace, in addition to the amount of water vapor consumed by the decarburization reaction, the amount of water vapor consumed by internal oxidation of the steel sheet may be considered. The amount of water vapor consumed by internal oxidation can be calculated as follows, for example.
Water vapor consumption due to internal oxidation = thickness of internal oxide layer x (Si content in steel material x 1.0 x 2 surface x water vapor consumption of Si + Mn content in steel material x 1.0 x 2 surface x Mn (Water vapor consumption) x Amount of passing plate ... (9)
As described above, in the case of the decarburization reaction, the carbon concentration distribution in the thickness direction of the decarburized steel sheet is approximately 0% on the surface of the steel sheet and increases approximately linearly in the thickness direction. In the annealing method, the carbon concentration deficient from the surface layer due to decarburization increases in the thickness direction, and the depth at the position where it becomes 90% of the base material is defined as the decarburization depth. On the other hand, in the internal oxidation, Si and Mn present in the internal oxide layer are oxidized almost entirely regardless of the thickness direction, so the above equation (9) does not have a coefficient like the equation (8) of the decarburization reaction. .

[High dew point gas injection control process]
In this step, the amount of steam input is determined based on the required water vapor amount calculated in the required water vapor amount calculating step and the consumed water vapor amount predicted in the consumed water vapor amount predicting step. Add dew point gas. That is, the amount of steam input is
Water vapor input amount = water vapor amount required for increasing dew point + water vapor amount consumed to achieve the target decarburization depth.

  Note that the decarburization reaction affects the temperature rise rate of the furnace, the steel plate temperature T1, and the decarburization depth at each point, so that the decarburization reaction at the center of the furnace becomes large. FIG. 4 is a graph showing the relationship between the temperature rise rate, the steel plate temperature, the decarburization rate, and the position in the longitudinal direction of the soaking zone when the temperature is raised from room temperature in the soaking zone. The position in the longitudinal direction of the soaking zone is determined by defining the upstream end in the sheet passing direction as 0 m. As shown in FIG. 4, the consumption of water vapor varies depending on the steel plate temperature, and when the plate is heated from a low temperature in the heating zone, the decarburization reaction increases in the range of 600 ° C. or higher at the steel plate temperature. The amount of water vapor consumption varies depending on the steel plate temperature. When the steel plate is heated from a low temperature in the heating zone, the decarburization reaction is large in the portion where the steel plate temperature is 600 ° C. or higher, and the amount of water vapor consumption is large. It is necessary to add water vapor. However, if water vapor is simply added to the gas from the cooling zone in accordance with the flow rate of the atmospheric gas, the dew point may increase excessively in a place where consumption is low, and the steel sheet may be oxidized.

  Further, FIG. 5 shows the temperature rise rate in the furnace, the steel plate temperature, the decarburization rate, and the longitudinal direction of the soaking zone when the temperature is raised from 500 ° C. to 700 ° C. in the soaking zone after raising the temperature to 500 ° C. in the heating zone. It is a graph which shows the relationship with a position. The position in the longitudinal direction of the furnace is determined by setting the upstream end in the sheet passing direction to 0 m. The total length of the furnace is 250 m. Similarly, it can be seen that the decarburization rate increases in the center of the furnace even when the temperature is raised to 500 ° C. in the heating zone in advance. That is, since the decarburization reaction affects the temperature and the decarburization depth at each point, the decarburization reaction in the central part of the soaking zone increases, and the consumption of water vapor increases in the central part of the furnace. The vertical continuous annealing furnace can perform dew point control according to the situation such as the temperature of the steel sheet and the position in the soaking zone.

<Advantages>
The annealing method can improve the circulation of the atmospheric gas in the furnace by using the vertical continuous annealing furnace. Also, by controlling the amount of water vapor consumed by the decarburization reaction predicted by the required water vapor amount and dew point history calculated from the composition of the atmospheric gas and the temperature of the steel sheet, the high dew point gas injection amount is controlled. Since the dew point can be raised with high accuracy, the dew point controllability in the furnace can be improved, and the decarburization reaction of the steel plate can be controlled with high accuracy. As a result, the desired quality of the steel plate such as bendability can be obtained.

[Other Embodiments]
The said embodiment does not limit the structure of this invention. Therefore, in the above-described embodiment, the components of each part of the above-described embodiment can be omitted, replaced, or added based on the description and common general knowledge of the present specification, and they are all interpreted as belonging to the scope of the present invention. Should.

  In the first embodiment, as shown in FIG. 1, the gas supply nozzles 6A and 6B for the high dew point gas are arranged on the furnace wall, but they are arranged at other places where the steam is not directly applied to the steel plate. You can also. FIG. 6 is a schematic view showing another embodiment of the arrangement of the supply nozzles. In addition, the same structure as FIG. 1 describes only the same referential mark, and abbreviate | omits description. For example, as shown in FIG. 6, as a form of the gas supply nozzle of the soaking zone 10, two gas supply nozzles are arranged so that the two nozzle ports face each other like the gas supply nozzle 6C and the gas supply nozzle 6D. The high dew point gas may be supplied using two gas supply nozzles in one place.

  In the above embodiment, one partition plate is provided in the soaking zone, but two or more partition plates may be provided. Also in this case, each of the plurality of partition plates has an opening so that the atmospheric gas flowing in the furnace meanders up and down. Therefore, for example, when both the inlet and outlet of the atmospheric gas are arranged at the furnace bottom or top, an odd number of partition plates are provided, and the inlet and outlet are staggered at the furnace bottom and top. In this case, an even number of partition plates are provided. By arranging the fixed side or the open side of the partition plate alternately on the left and right sides, a space meandering left and right by the partition plate is formed between the soaking zone inlet and the outlet. That is, inside the soaking zone, there is an atmosphere gas flow path that connects to the final outlet through the space between one partition plate and the other partition plate while meandering from the inlet to the left and right. It is formed. Thereby, the circulation of atmospheric gas and high dew point gas in the soaking zone can be improved, and the controllability of the decarburization reaction can be improved.

  Moreover, you may make it provide an outflow port for every soaking zone.

  EXAMPLES Hereinafter, although this invention is explained in full detail based on an Example, this invention is not interpreted limitedly based on description of this Example.

  A cold rolled steel sheet is produced as a steel sheet to be annealed, and a heating furnace in which a heat insulating material is disposed on the inner side of the outer wall is used, and Examples 1 to 3 and Comparative Examples 1 to 3 are subjected to annealing while blowing a non-oxidizing gas. Comparative Example 2 was performed.

[Example 1]
(steel sheet)
A cold rolled steel sheet was prepared as the steel sheet to be annealed. Specifically, first, a raw material is melted and cast, and a slab containing 0.2% by mass of carbon, 1.8% by mass of silicon and 2.1% by mass of manganese, with the balance being iron and inevitable impurities. The slab was produced and hot rolled. Next, the obtained rolled material was pickled to completely remove the oxide scale on the surface, and further cold rolled to produce a thin steel plate having a plate width of 1000 mm and a plate thickness of 1.0 mm.

  In the heating zone, a reduction burner was used to heat the steel sheet without oxidizing it.

As the soaking zone of Example 1, Example 3, and Comparative Examples 1 and 2, vertical type vertical annealing furnace 50 having an internal volume of 600 m ³ shown in FIG. 7 was used. In the soaking zone 40 of the vertical continuous annealing furnace 50, both a throat 25 serving as an inlet for atmospheric gas in the soaking zone 20 and a throat 26 serving as an outlet are disposed at the bottom of the furnace. An atmospheric gas having a low dew point is supplied from the cooling zone 7 to the soaking zone 40 via a throat 25 that connects the soaking zone 40 and the cooling zone 7. An atmospheric gas with a low dew point such as N ₂ is also supplied from the gas inlet G9 at a constant flow rate. The soaking zone 40 is divided into two sections, an upstream front stage 42 and a downstream rear stage 41 in the sheet passing direction, and the steel plate 2 is wound around the two top rolls 3. The partition plate 44 is provided with one end fixed to the furnace bottom wall of the soaking zone 40 and the other end opened to the side on which the steel plate 2 of the two top rolls 3 is not wound.

  Then, a nozzle (not shown) for directly blowing water vapor into each of the front stage 42 and the rear stage 41 of the soaking zone 40 is installed, the dew point in the furnace is measured while adjusting the water vapor flow rate, and the decarburization reaction of the steel plate 2 is advanced. It was. The steel plate temperature at the entrance of the soaking zone 40 is 700 ° C. The sheet was passed at a feeding speed of 60 m / min. Dew point measurement positions are indicated by R1 and R2. Table 1 shows the conditions of the atmospheric gas introduced from the cooling zone and other atmospheric gases.

  For Example 2, the configuration of the vertical continuous annealing furnace 50 shown in FIG. 7 includes both a throat that serves as an inlet and a throat that serves as an inlet for atmospheric gas in the soaking zone as in the second embodiment. Arranged in the upper part of the furnace, one end of the partition plate is fixed to the soaking zone furnace upper wall, and the other end is between the furnace bottom wall and the roll upper end on the side where the steel plate of the bottom roll is not wound It has been changed so that it is provided in an open state.

The calculation procedure of the input water vapor amount in Example 1 is as follows.
(1) Setting of target final decarburization depth The target final decarburization depth was set to 80 μm.
(2) Setting of target dew point Based on the above-described formula, the target dew point for obtaining a target final decarburization depth of 80 µm under the condition of a steel plate temperature of 700 ° C was set to 0 ° C (6028 ppm).
(3) Calculation of the required water vapor amount to make the dew point in the furnace the target dew point Atmospheric gas from the cooling zone (dew point -35 ° C, water vapor amount 304 ppm) 1200 Nm ³ / h and low dew point input atmospheric gas (dew point- 50 ° C., water vapor amount 60 ppm) The amount of water vapor necessary to bring the atmospheric gas consisting of 400 Nm ³ / h to a dew point of 0 ° C. (6028 ppm) is
2000 × 6028− (1200 × 304 + 800 × 60) = 11.64 Nm ³ /h=9.36 kg / h
Since the latter stage requires a water vapor amount for raising the dew point of the gas from the cooling zone and the input atmosphere gas, the water vapor amount for raising the dew point of the latter atmosphere gas is
(1600 × 6028− (1200 × 304 + 400 × 60)) / 100/10000 / 22.4 × 18 = 7.44 kg / h
It becomes.
The first stage only needs to raise the dew point of the previous stage atmospheric gas, so the amount of water vapor to raise the dew point of the previous stage atmospheric gas is:
(400 × 6028-60) /100/10000/22.4×18=1.92 kg
It becomes.
(4) Calculation of water vapor consumption From the conditions of Example 1,
Through plate amount (kg / h) = plate thickness (mm) × plate width (mm) × plate passing speed (m / min) × 60 × density (kg / m ³ )
28260 kg / h = 1 × 1000 × 60 × 60 × 7850
It becomes.
Next, steel material carbon amount 0.2 mass%, target decarburization depth 80 μm and decarburization amount (kg / h) = through plate amount (kg / h) × steel material carbon amount (%) × (decarburization depth ÷ From steel plate thickness) ÷ 2 x 2 surfaces (concentration distribution) x 0.9, the amount of decarburization is
28260 × 0.002 × (80/1000) ÷ 1 ÷ 2 × 2 ÷ 1 × 0.9 = 4.07 kg / h.
The amount of carbon used in the decarburization reaction is calculated from the target decarburization depth and the steel material carbon amount.
Since the decarburization reaction is C + H ₂ O═CO + H ₂ ,
Water vapor amount by decarburization reaction = decarburization amount / carbon molecular weight × H ₂ O molecular weight.
4.07 (kg / h) ÷ 12 (kg / kmol) × 18 (kg / kmol) = 6.10 (kg / h)
It becomes.
Therefore, the amount of water vapor consumed is 9.49 kg / h which is the sum of the consumption of 6.10 kg / h due to decarburization and the consumption due to internal oxidation of 3.39 kg / h in consideration of the consumption due to internal oxidation. h.
The reaction amount ratio is calculated separately, and can be calculated as 62% for the first stage and 38% for the second stage.
Water consumption in the previous stage: 9.49 kg / h × 0.62 = 5.89 kg / h
Subsequent water vapor consumption: 9.49 kg / h × 0.38 = 3.60 kg / h
It becomes.
(5) Calculation of the amount of steam input The amount of steam input is expressed as the sum of the amount of steam consumed to achieve the target decarburization depth and the amount of steam required to raise the dew point. ,
9.36 + 5.89 + 3.60 = 18.85 kg / h
It becomes. Also,
Amount of steam input in the previous stage: 1.91 + 5.89 = 7.80 kg / h
Second stage water vapor input: 7.44 + 3.61 = 11.05 kg / h
It becomes.

[Examples 2 to 3 and Comparative Examples 1 to 2]
A steel plate having a plate width of 1000 mm and a plate thickness of 2.3 mm was passed at a plate speed of 26 m / min or 60 m / min. Table 1 shows the conditions of each example and comparative example. In Comparative Example 1, only the amount of water vapor necessary for increasing the dew point is input as the amount of input water vapor, and Comparative Example 2 is the same in the former stage and the latter stage under the condition of a plate thickness of 2.3 mm where the plate passing speed is reduced. Set the target dew point.

(Evaluation)
Table 1 shows the target decarburization depth and the final decarburization depth obtained in Examples and Comparative Examples.

  As shown in Table 1, about Example 1- Example 3, the decarburization depth of the range close | similar to 80 micrometers of target decarburization depth was able to be obtained. Note that the steam blown into the furnace is cold and flows from top to bottom because it is heavy, but in Example 1 in which both the inlet and outlet of the atmospheric gas are arranged at the bottom of the furnace, the dew point and input at the latter stage The dew point in the previous stage with a large amount of water vapor tends to be uniform. On the other hand, in Example 2 in which both the inlet and outlet of the atmospheric gas are arranged in the upper part of the furnace, the dew point in the former stage with a large amount of input water vapor tends to be uniform, but the circulation of water vapor is slightly inferior in the latter stage. It is assumed that the dew point of the latter stage is likely to vary. Therefore, it is calculated that the final decarburization depth of Example 2 varies slightly.

  On the other hand, in Comparative Example 1 in which only the amount of water vapor necessary for increasing the dew point was input as the input water vapor amount, and in Comparative Example 2 in which the same target dew point was set in the former stage and the latter stage using a steel plate having a plate thickness of 2.3 mm. The decarburization depth was greatly different from the target decarburization depth of 80 μm. In Comparative Example 1, since the amount of water vapor consumed in the decarburization reaction is not included in the input water vapor amount, the dew point has decreased with the consumption of water vapor by the decarburization reaction. It is thought that the decarburization reaction did not progress sufficiently. Further, in Comparative Example 2, it is considered that the decarburization depth became as deep as 138 μm because the plate passing speed decreased because the plate thickness was 2.3 mm.

  FIG. 8 is a graph showing the relationship between the dew point and the decarburization depth for each plate speed in Example 2, Example 3, and Comparative Example 1. When Example 1 having a plate thickness of 1.0 mm and Comparative Example 3 having a plate thickness of 2.3 mm under the same dew point distribution in the furnace were compared, the decarburization depth of Comparative Example 2 in which the plate passing speed was reduced. Is getting deeper. On the other hand, the solid line of Example 3 in which the plate thickness is 2.3 mm shows the decarburization depth when the dew point of the rear stage is fixed at −20 ° C. and the dew point of the front stage is changed. In Example 3, it was shown that by setting the dew point at the latter stage to −20 ° C., the target decarburization depth of 80 μm can be accurately controlled even if the plate thickness is 2.3 mm.

  The vertical continuous annealing furnace and annealing method according to the present invention can be suitably used for, for example, hot-dip galvanized steel sheets used in automobiles, ships, building materials, home appliances, and the like.

DESCRIPTION OF SYMBOLS 1,50 Vertical type continuous annealing furnace 2 Strip | belt-shaped steel plate 3 Top roll 4 Bottom roll 5 Heating zone 5A 1st heating zone 5B 2nd heating zone 6A, 6B, 6C, 6D Gas supply nozzle 7 Cooling zone 10, 20, 30, 40 Soaking area 11, 21, 31, 41 Rear stage 12, 22, 32, 42 First stage 14, 24, 34, 44 Partition plate 25, 26 Throat G1, G2, G4, G6, G9 Gas inlet R1, R2 Dew point measuring device

Claims (4)

A plurality of top rolls and a plurality of bottom rolls arranged inside are provided, and a strip-shaped steel plate is laid between the plurality of top rolls and the plurality of bottom rolls so as to meander in the vertical direction, and this steel plate is placed in a specific horizontal direction. A vertical continuous annealing furnace configured to heat-treat while being conveyed in a direction,
One or a plurality of partition plates having an opening so that the atmosphere gas that circulates in the furnace so as to meander up and down is divided into a plurality of sections along the specific horizontal direction,
A plurality of dew point measuring devices arranged for each of the plurality of sections;
A mechanism for blowing a high dew point gas (including water vapor) to adjust the dew points of the plurality of compartments;
Based on the required water vapor amount calculated from the composition of the atmospheric gas, the history of the dew point by the dew point measuring device, and the amount of water vapor consumed by the decarburization reaction predicted by the temperature of the steel sheet, the high dew point gas blowing amount of the blowing mechanism is A vertical continuous annealing furnace equipped with a control mechanism.   The vertical continuous annealing furnace according to claim 1, wherein the dew point in the section upstream in the sheet passing direction is higher than the dew point in the section downstream in the sheet passing direction.   The vertical continuous annealing furnace according to claim 1 or 2, wherein an area ratio of an opening of the partition plate to an area in the furnace of a vertical cross section at the position of the partition plate is 1/10 or more and 1/4 or less. A plurality of top rolls and a plurality of bottom rolls arranged inside are provided, and a strip-shaped steel plate is laid between the plurality of top rolls and the plurality of bottom rolls so as to meander in the vertical direction, and this steel plate is placed in a specific horizontal direction. The interior is divided into a plurality of compartments along the specific horizontal direction by one or a plurality of partition plates which are configured to heat-treat while being conveyed in a direction and have an opening so that the atmospheric gas flowing in the furnace meanders up and down. An annealing method using a vertical continuous annealing furnace,
Calculating the required water vapor amount from the composition of the atmospheric gas,
Predicting the amount of water vapor consumed by decarburization reaction from the history of the dew point of the one or more sections and the temperature of the steel sheet;
And a step of controlling a high dew point gas blowing amount in the one or more sections according to the required water vapor amount and the consumed water vapor amount.

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