JP7073162B2 - Vertical continuous annealing furnace and annealing method - Google Patents

Description

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

In order to remove the carbon content in the steel sheet and obtain the desired quality, it is important to contain water vapor in the atmospheric gas in the furnace and to control the dew point to control the humidity 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 lower the dew point in the heating furnace has been proposed (Japanese Unexamined Patent Publication No. 9-257044). 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 Unexamined Patent Publication No. 9-256074

However, there is a dew point distribution and an oxygen concentration distribution in the furnace, and even if controlled based on the 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 the response delay. .. Especially in the case of a vertical furnace, it is more difficult to adjust the dew point distribution in the furnace because there is a circulating flow 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, it is difficult to adjust the atmosphere in the furnace, and it is more difficult to control the decarburization reaction. Therefore, if the plate passing speed changes significantly due to the difference in the composition of the steel plate or the plate thickness, the decarburization reaction may not be controlled accurately.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a method for annealing a steel sheet, which can improve the dew point controllability in a furnace and can accurately control the decarburization reaction.

As a result of examining the amount of water vapor consumed in the decarburization reaction in the bleaching furnace, the present inventors have found 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 aspect of the present invention made to solve the above problems includes a plurality of top rolls and a plurality of bottom rolls arranged inside, and a strip-shaped steel plate between the plurality of top rolls and the plurality of bottom rolls. This is a vertical continuous incinerator that is configured to meander in the vertical direction and heat the steel plate while transporting it in a specific horizontal direction. The inside is divided into a plurality of sections along the specific horizontal direction. One or a plurality of partition plates having openings so that the atmospheric gas flowing in the furnace so as to face the steel plate in the vertical direction, and a plurality of dew point measuring instruments arranged in each of the plurality of compartments. Predicted by the mechanism that blows high dew point gas (including water vapor) to adjust the dew point of the plurality of sections, the required amount of water vapor calculated from the composition of the atmospheric gas, the history of the dew point by the dew point measuring device, and the temperature of the steel plate. It is a vertical continuous bleaching furnace equipped with a mechanism for controlling the high dew point gas blowing amount of the blowing mechanism based on the amount of water vapor consumed by the decarburization reaction.

The vertical continuous annealing furnace is divided into a plurality of sections along the specific horizontal direction, and one or a plurality of openings are provided so that the atmospheric gas flowing in the furnace so as to face the steel plate is meandered up and down. By providing the partition plate, the circulation of the atmospheric gas in the furnace can be improved. Further, 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 water vapor consumption amount due to the decarburization reaction predicted by the temperature of the steel plate, the high dew point gas blowing amount of the blowing mechanism is determined. By providing a control mechanism, the dew point in the furnace can be raised accurately, so that the dew point controllability in the furnace can be improved and the decarburization reaction can be controlled accurately, and as a result, the desired quality of the steel plate such as bendability can be obtained. Can be obtained.

It is preferable that the dew point of the section on the upstream side in the plate-passing direction is higher than the dew point of the section on the downstream side in the plate-passing direction. Since the dew point of the section on the upstream side in the plate-passing direction is higher than the dew point of the section on the downstream side in the plate-passing direction, the water vapor consumed in the section on the downstream side in the plate-passing direction (rear stage) is on the upstream side in the plate-passing direction (front stage). ), So the controllability of the dew point in the furnace can be improved.

It is preferable that the area ratio of the opening of the partition plate to the area inside 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. When the area ratio of the openings of the partition plate is in the above range, the flow rate of the atmospheric gas can be maintained in a good range and the dew point controllability in the furnace can be improved.

Another aspect of the present invention made to solve the above problems includes a plurality of top rolls and a plurality of bottom rolls arranged inside, and a band shape between the plurality of top rolls and the plurality of bottom rolls. One or more having openings so that the atmospheric gas flowing in the furnace can meander up and down, which is configured to be constructed so that the steel plate of the above means meandering in the vertical direction and the steel plate is conveyed in a specific horizontal direction while being heat-treated. It is an annealing method using a vertical continuous annealing furnace that divides the inside into a plurality of sections along the specific horizontal direction by a partition plate, and is a step of calculating the required water vapor amount from the composition of the atmospheric gas, and the above 1 or The step of predicting the amount of water vapor consumed by the decarburization reaction from the history of dew points in a plurality of sections and the temperature of the steel plate, and the amount of high dew point gas blown into the above one or more sections are controlled by the required amount of water vapor and the amount of water consumed. It is an annealing method including a step of performing.

In the annealing method, the inside 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 so as to face the steel plate in the vertical direction meanders up and down. By using a vertical continuous annealing furnace equipped with the above, the circulation of atmospheric gas in the furnace can be improved. In addition, the dew point in the furnace is controlled by controlling the high dew point gas injection amount based on the required steam amount calculated from the composition of the atmospheric gas, the history of the dew point, and the water vapor consumption due to the decarburization reaction predicted by the temperature of the steel plate. Since it can be raised with high accuracy, the dew point controllability in the furnace can be improved and the decarburization reaction can be controlled with high accuracy, and as a result, the desired quality of the steel plate 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 continuous annealing furnace of 1st Embodiment of this invention. It is a schematic diagram which shows the structure of a part of the vertical continuous annealing furnace of the 2nd Embodiment of this invention. It is a schematic diagram which shows the structure of a part of the vertical continuous annealing furnace of the 3rd Embodiment of this invention. It is a graph which shows the relationship between the temperature rise rate of the soothing tropics, the steel plate temperature and the decarburization rate, and the position in the longitudinal direction of the furnace. It is a graph which shows the relationship between the temperature rise rate of the soothing tropics, the steel plate temperature and the decarburization rate, and the position in the longitudinal direction of the furnace. It is a schematic diagram which shows the other embodiment of the arrangement of a gas supply nozzle. It is a schematic diagram which shows the structure of the vertical 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 high dew point operation in which a dew point in a furnace is raised to promote a dew point reaction of a steel sheet in a hot dip galvanizing facility or the like. The vertical continuous annealing furnace is provided with a plurality of top rolls and a plurality of bottom rolls arranged inside, 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. It is configured to be passed and heat treated while transporting this 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, for example, 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 is provided with a heating zone 5, a soaking zone 10 and a cooling zone 7, and is juxtaposed in this order, and the steel plate is decarburized in the soaking zone 10. The heating zone 5 has a first heating zone 5A (first stage of the heating zone) and a second heating zone 5B (second 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 introduction port at the lower part of the first heating zone 5A, and is conveyed in the plate passing direction indicated by the arrow P. The vertical continuous annealing furnace 1 is configured to bridge a steel plate 2 between a plurality of top rolls 3 and a plurality of bottom rolls 4 so as to meander in the vertical direction, and heat-treat the steel plate 2 while transporting the steel plate 2 in a specific horizontal direction. ..

When the steel plate 2 is folded back at 180 degrees from the top roll 3 and the bottom roll 4, the steel plate 2 is conveyed in the predetermined band of the vertical continuous annealing furnace 1 a plurality of times in the vertical direction to form a plurality of paths. The number of paths can be appropriately set according to the processing conditions. Further, in some of the top rolls 3 and the bottom roll 4, the steel plate 2 is turned at a right angle without being folded 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 inside the vertical continuous annealing furnace 1, and the steel sheet 2 is annealed.

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

(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 dispersedly arranged facing the steel plate 2. Further, the combustion exhaust gas of the second heating zone 5B is supplied to the inside of the first heating zone 5A, and the heat of the combustion exhaust gas preheats the steel sheet 2. The temperature inside the second heating zone 5B is selected according to the type of the steel sheet 2 and the purpose of use, but is preferably 800 to 1200 ° C., for example.

(Cooling zone)
In the cooling zone 7, the heated steel sheet 2 is cooled. Atmospheric gas having a low dew point is supplied to the cooling zone 7 from the gas inlet G1 at a constant flow rate, and the temperature inside the furnace is gradually lowered. Examples of the supplied atmospheric gas include a gas having a composition consisting of N ₂ and unavoidable impurities and having a dew point of about −60 ° C.

(Tropical)
In the soaking tropic 10, by keeping the ambient temperature in the furnace at a predetermined annealing temperature, the decarburization reaction of the steel sheet is performed according to the following reaction formula while heating the inside of the steel sheet 2 to the annealing temperature.
H ₂ O + C → CO + H ₂

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

In the tropics 10, the tropics 10 and the cooling zone 7 are supplied with a low dew point atmosphere gas from the cooling zone 7 via a throat 25 connecting the lower portions of the zones. Further, the atmospheric gas having a low dew point such as _N2 is supplied from the gas inlet G2 at a constant flow rate. The amount of atmospheric gas supplied and the amount of gas from the cooling zone are measured by a gas flow meter provided in the piping.

The solitary tropics 10 is provided with a partition plate 14, and the partition plate 14 divides the inside of the furnace into a plurality of sections along the horizontal direction. Specifically, the solitary tropic 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. One end of the partition plate 14 is fixed to the bottom wall of the furnace, and the other end is provided in a state of being open between the bottom wall of the furnace and the lower end of the roll 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 flowing in the furnace meanders up and down so as to face the steel plate 2.

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

The partition plate 14 may be any as long as it has heat resistance in the temperature range used and strength that does not break during operation and can substantially block gas, and 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 inside 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 regulates the flow of gas in the cross section in the direction perpendicular to the cross section with respect to the area inside the furnace in the vertical furnace cross section in the partition installation portion. It is the ratio of the area of the non-existent part. When the area ratio of the openings of the partition plate 14 is in the above range, the flow rate of the atmospheric gas can be maintained in a good range and the dew point controllability in the furnace can be improved. Further, when the area ratio of the openings of the partition plate is less than 1/10, the pressure loss of the gas becomes too large, and there is a possibility that the flow rate of the atmospheric gas required for the decarburization reaction cannot be sufficiently secured. On the other hand, if the area ratio of the opening of the partition plate exceeds 1/4, the atmospheric gas may move through this opening, and the action of restricting the flow direction of the atmospheric 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 instruments arranged in a plurality of sections. In the average tropics 10, the dew point is constantly measured by the dew point measuring device R1 and the dew point measuring device R2 arranged in each section.

The vertical continuous annealing furnace 1 includes a mechanism for blowing a high dew point gas (including steam) (not shown) in order to adjust the dew point 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 steam. The gas supply device includes a gas supply nozzle. Further, gas supply nozzles 6A and 6B for supplying high dew point gas toward the surface of the steel plate 2 along the flow path of the atmospheric gas are arranged in the front stage 12 of the soothing tropics 10. The gas supply nozzles 6A and 6B are arranged so that steam is blown from the furnace upper wall toward a part of the top rolls 3. Further, since the steel sheet is oxidized when the water vapor hits the steel sheet, it is preferable to arrange the gas supply nozzle so as not to directly hit the steel sheet. Therefore, the gas supply nozzles may be arranged between the top rolls 3 and the bottom rolls 4, or the gas supply nozzles may be arranged at the atmosphere gas introduction port G2 to mix with the atmosphere gas and blow the high dew point gas. Further, when trying to maintain the dew point of the atmosphere at 0 ° C., it is necessary to raise the dew point of the blown gas in consideration of the consumption of water vapor.

Further, the vertical continuous annealing furnace 1 includes a control mechanism (not shown) for controlling 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 amount of water vapor 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 plate. The gas blowing amount is set, and the high dew point gas of the blowing amount is blown into the furnace from the blowing mechanism. The control by this control mechanism is, for example, blown according to the required amount of water vapor 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 plate. Examples include control for adjusting the amount of high dew point gas blown by the mechanism, control for adjusting the blowing position of high dew point gas, blowing time, and the like. Further, regarding the target dew point, the final decarburization depth is examined according to the rigidity of the target steel sheet, and this dew point is set so as to be achieved.

<Advantage>
Although the vertical continuous annealing furnace has a vertical structure in which it is difficult for the atmospheric gas to circulate uniformly in the furnace, the circulation of the atmospheric gas in the furnace can be improved. Further, 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 water vapor consumption amount due to the decarburization reaction predicted by the temperature of the steel plate, the high dew point gas blowing amount of the blowing mechanism is determined. By providing a control mechanism, the dew point in the furnace can be raised accurately, so that the dew point controllability in the furnace can be improved and the decarburization reaction can be controlled accurately, and as a result, the desired quality of the steel plate such as bendability can be obtained. Obtainable.

[Second Embodiment]
FIG. 2 is a schematic view showing the configuration of the soothing tropics 20 which is a part of the vertical continuous annealing furnace of the second embodiment of the present invention. In the same configuration as in the first embodiment, only the same reference numerals will be described, and the description thereof will be omitted.

In the vertical continuous annealing furnace according to the second embodiment, both the throat 25 as the inlet and the throat 26 as the outlet of the atmospheric gas of the solitary tropics 20 are arranged in the upper part of the furnace. Atmospheric gas having a low dew point is supplied from the cooling zone 7 to the tropics 20 via a throat 25 connecting the tropics 20 and the cooling zone 7. Further, the atmospheric gas having a low dew point such as _N2 is supplied from the gas inlet G4 at a constant flow rate. The solitary tropic 20 is divided into two sections, a front stage 22 on the upstream side in the through plate direction and a rear stage 21 on the downstream side. The steel plate 2 of the bottom roll 4 is provided in an open state between the wall and the upper end of the roll on the side where the bottom roll 4 is not wound.

In the second embodiment, the solitary tropic 20 that blows high dew point gas (including steam) to adjust the dew point is divided into a plurality of sections by the partition plate 24, and the high dew point gas is blown and the dew point control is performed for each section. Will be. Therefore, the mixing of the atmospheric gas and the blown high dew point gas is promoted, and the dew point in the furnace becomes more uniform. Further, as in the first embodiment, since the gas supply nozzle for supplying the high dew point gas is arranged along the flow path of the atmospheric gas and toward the surface of the steel plate 2, it affects the atmospheric gas in other sections. The dew point can be controlled for each section without giving.

As described above, since the amount of steam consumed differs depending on the temperature of the steel sheet, in the present embodiment, the temperature of the steel sheet can be measured for each section and the amount of additional steam input can be adjusted. Further, in the annealing furnace, the thicker the plate thickness, the lower the plate passing speed, and when the plate passing speed decreases, the decarburization depth increases. Even in such a case, the controllability of the decarburization reaction can be improved by adjusting the amount of water vapor input for each section.

It is preferable that the dew point of the front stage 22 which is the section on the upstream side in the through plate direction is higher than the dew point of the rear stage 21 which is the section on the downstream side in the through plate direction. Since the dew point of the front stage 22 on the upstream side in the plate-passing direction is higher than the dew point of the rear stage 21 on the downstream side in the plate-passing direction, the water vapor consumed in the rear stage on the downstream side in the plate-passing direction can be compensated by the front stage on the upstream side in the plate-passing direction. Therefore, the controllability of the dew point in the furnace can be improved. In addition, even when the staying time in the tropics fluctuates due to the change in the plate passing speed, the dew point can be controlled accurately to adjust the decarburization depth to an appropriate level.

<Advantage>
According to the second embodiment, the flow of the atmospheric gas is promoted by injecting steam into each section by the partition plate, the dew point in the furnace becomes more uniform, and the accuracy of controlling the water vapor distribution in the furnace is further improved. can. In addition, 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 front stage on the downstream side in the plate-passing direction of the tropics.

[Third Embodiment]
FIG. 3 is a schematic view showing the configuration of the soothing tropics 30 which is a part of the vertical continuous annealing furnace of the third embodiment of the present invention. In the same configuration as in the first embodiment, only the same reference numerals will be described, and the description thereof will be omitted.

In the vertical continuous annealing furnace according to the third embodiment, both the throat 25 as the inlet and the throat 26 as the outlet of the atmospheric gas of the soothing tropics 30 are arranged at the bottom of the furnace. Atmospheric gas having a low dew point is supplied from the cooling zone 7 to the tropics 30 via a throat 25 connecting the tropics 30 and the cooling zone 7. Further, an atmospheric gas having a low dew point such as _N2 is supplied from the gas inlet G6 at a constant flow rate. The solitary tropics 30 is divided into two sections, a front stage 32 on the upstream side in the through plate direction and a rear stage 31 on the downstream side, one end of the partition plate 34 is fixed to the bottom wall of the solitary tropics 30, and the other end is a furnace. The steel plate 2 of the top roll 3 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.

The solitary tropic 30 that blows high dew point gas (including steam) to adjust the dew point is divided into a plurality of sections by the partition plate 34, and the high dew point gas is blown and the dew point control is performed for each section. Therefore, the mixing of the atmospheric gas and the blown high dew point gas is promoted, and the dew point in the furnace becomes more uniform. Further, as in the first embodiment, since the gas supply nozzle that supplies the high dew point gas along the flow path of the atmospheric gas and toward the surface of the steel plate 2 is arranged, it affects the atmospheric gas in other sections. The dew point can be controlled for each section without giving.

Also in the third embodiment, it is preferable that the dew point of the front stage 32, which is the section on the upstream side in the plate-passing direction, is higher than the dew point of the rear stage 31, which is the section on the downstream side in the plate-passing direction. Since the dew point of the front stage 32 on the upstream side in the plate-passing direction is higher than the dew point of the rear stage 31 on the downstream side in the plate-passing direction, the water vapor consumed in the rear stage on the downstream side in the plate-passing direction can be compensated by the front stage on the upstream side in the plate-passing direction. Therefore, the controllability of the dew point in the furnace can be improved. In addition, even when the staying time in the tropics fluctuates due to the change in the plate passing speed, the dew point can be controlled accurately to adjust the decarburization depth to an appropriate level.

<Advantage>
According to the third embodiment, the flow of the atmospheric gas is promoted by injecting steam into each section by the partition plate, the dew point in the furnace becomes more uniform, and the accuracy of controlling the steam distribution in the furnace is further improved. can. In addition, 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 front stage on the downstream side in the plate-passing direction of the tropics.

<Annealing method>
The annealing method includes a plurality of top rolls and a plurality of bottom rolls arranged inside, and a strip-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. It is configured to heat-treat the steel plate while transporting it in a specific horizontal direction, and has an opening so that the atmospheric gas flowing in the furnace meanders up and down. This is an annealing method using a vertical continuous annealing furnace that divides into sections. The annealing method relates to an atmosphere gas supply method and a control method for performing a high dew point operation for promoting a dew point reaction of a steel sheet by raising the dew point in the furnace in a hot dip galvanizing facility or the like. The annealing method can be performed as one step of manufacturing a hot-dip galvanized steel sheet used for, for example, automobiles, ships, building materials, home appliances and the like.

The annealing method includes a step of calculating the required amount of water vapor from the composition of the atmospheric gas, a step of predicting the amount of water vapor consumed by the decarburization reaction from the history of the dew points of the one or a plurality of sections and the temperature of the steel plate, and the above. The present invention includes a step of controlling the amount of high dew point gas blown into the one or a plurality of sections according to the required amount of steam and the amount of steam consumed.

[Required water vapor amount calculation process]
The required water vapor amount is calculated in the following steps.
(1) Setting the final decarburization depth First, set the final decarburization depth according to the rigidity of the target steel sheet. In the annealing method, the decarburization depth was analyzed by GDOES in the thickness direction of the steel sheet, and the carbon concentration deficient from the surface layer due to decarburization increased in the thickness direction to 90% of the base metal. The depth of.
(2) Setting the target dew point Since the decarburization reaction proceeds by the diffusion rate-determining of C, the decarburization depth x1 is proportional to the elapsed time (staying time in the furnace) t to the 1/2 power, and the following equation It is represented by (1).

Further, the diffusion coefficient kp is a function of the steel plate temperature T1 and becomes larger as the steel plate temperature T1 increases in the Arrhenius type. The diffusion coefficient kp is expressed by the following equation (2).

Here, const is a constant indicating a frequency factor. _PH2O is a saturated water vapor pressure. n is a constant, and in the present invention, n = 1.

In the formula using the carbon decarburization depth x1 according to the elapsed time t, the decarburization rate dx1 / dt is expressed by the following formula (3).

The final decarburization depth x2 is represented by the following equation (4).

From the history of the steel plate temperature T1, the amount of water vapor for obtaining the target final decarburization depth x2 is determined, and the target dew point is set. More specifically, from the equations (1), (2) and (3), the decarburization rate dx1 / dt and the elapsed time t when _PH2O is set to an arbitrary constant value with respect to the history of the steel plate temperature T1. Decarburization depth x1 according to the above is obtained. Then, the final decarburization depth is calculated from the equation (4), the calculation result of this decarburization depth is compared with the target decarburization depth, and the calculation result of the decarburization depth and the target decarburization depth match. The calculation will be repeated until the amount of water vapor to be applied is determined.

(3) Calculation of the amount of water vapor required to set the dew point in the furnace to the target dew point Next, the concentration (ppm) of the 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 formula (5).

The amount of steam required to set the dew point in the furnace as the target dew point during the decarburization reaction is expressed by the following formula (6).
Required water vapor amount = water vapor amount that raises the atmospheric dew point + decarburization reaction consumption ... (6)

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

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

Amount of water vapor due to decarburization reaction = amount of passing plate x amount of carbon in steel material x (depth of decarburization ÷ thickness of steel plate) ÷ 2 x 2 sides x 0.9 x amount of water vapor consumed by carbon ... (8)

In the above formula (8), the two surfaces are the front surface and the back surface of the steel sheet. Regarding 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. Further, the carbon concentration distribution in the thickness direction of the decarburized steel sheet is approximately 0% on the surface of the steel sheet, increases substantially linearly in the thickness direction, and increases to the concentration of the base metal. The decarburized amount is the carbon concentration reduced from the carbon concentration of the base metal. As described above, in the carbon concentration distribution in the thickness direction of the steel sheet, the carbon concentration increases substantially linearly, so that the carbon concentration is calculated as the area of the triangle, so that it is divided by 2 in the 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 the internal oxidation of the steel sheet may be taken into consideration. The amount of water vapor consumed by internal oxidation can be calculated, for example, as follows.
Water vapor consumption due to internal oxidation = Thickness of internal oxide layer x (Si content in steel material x 1.0 x 2 surfaces x Water vapor consumption of Si + Mn content in steel material x 1.0 x 2 surfaces x Mn Water vapor consumption) x plate amount ... (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 substantially linearly in the thickness direction. In the annealing method, the decarburization depth is defined as the depth at which the carbon concentration depleted from the surface layer due to decarburization increases in the thickness direction and becomes 90% of the base metal. On the other hand, in the internal oxidation, Si and Mn existing in the internal oxide layer are almost completely oxidized regardless of the thickness direction, so that the above formula (9) does not have a coefficient like the decarburization reaction formula (8). ..

[High dew point gas injection amount control process]
In this step, the amount of steam input is determined based on the required amount of steam calculated by the above-mentioned required steam amount calculation step and the amount of steam consumption predicted by the above-mentioned steam consumption prediction step, and the amount is high from the above-mentioned blowing mechanism into the furnace. Inject dew point gas. That is, the amount of steam input is
The amount of steam input = the amount of steam required to raise the dew point + the amount of steam consumed to achieve the target decarburization depth.

Since the decarburization reaction affects the heating rate of the furnace, the steel plate temperature T1, and the decarburization depth at each point, the decarburization reaction at the central portion of the furnace becomes large. FIG. 4 is a graph showing the relationship between the temperature rise rate, the steel plate temperature, and the decarburization rate when the temperature is raised from normal temperature in the tropics and the position in the longitudinal direction of the tropics. The position in the longitudinal direction of the solitary tropics is defined with the upstream end in the through plate direction as 0 m. As shown in FIG. 4, the consumption of water vapor differs depending on the temperature of the steel sheet, and when the plate is heated from a low temperature in the heating zone, the decarburization reaction becomes large in the range of 600 ° C. or higher at the temperature of the steel sheet. The amount of water vapor consumed differs depending on the temperature of the steel sheet. It is necessary to add steam. However, if water vapor is simply added to the gas from the cooling zone according to the flow rate of the atmospheric gas, the dew point may rise too much and the steel sheet may be oxidized in a place where consumption is low.

Further, FIG. 5 shows the heating rate in the furnace, the steel plate temperature and the decarburization rate, and the longitudinal direction of the tropics when the temperature is raised from 500 ° C to 700 ° C in the tropics after the temperature is raised 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 with the upstream end in the plate-passing direction as 0 m. The total length of the furnace is 250 m. It can be seen that the decarburization rate is similarly increased in the central part of the furnace 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 tropics becomes large, and the consumption of water vapor in the central part of the furnace becomes large. The vertical continuous annealing furnace can control the dew point according to the conditions such as the temperature of the steel sheet and the position in the tropics.

<Advantage>
In the annealing method, the circulation of atmospheric gas in the furnace can be improved by using the above-mentioned vertical continuous annealing furnace. Further, by controlling the amount of high dew point gas blown into the furnace based on the required amount of water vapor calculated from the composition of the atmospheric gas, the history of the dew point, and the amount of water vapor consumed by the decarburization reaction predicted by the temperature of the steel plate. Since the dew point can be raised accurately, the dew point controllability in the furnace can be improved, and the decarburization reaction of the steel plate can be controlled accurately, and as a result, the desired quality of the steel plate such as bendability can be obtained.

[Other embodiments]
The above embodiment does not limit the configuration of the present invention. Therefore, the above-described embodiment can be omitted, replaced or added with components of each part of the above-described embodiment based on the description of the present specification and the common general technical knowledge, and all of them are construed to belong to the scope of the present invention. Should be.

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 upper wall of the furnace, but they are arranged in another place where the steel plate is not directly exposed to steam. You can also. FIG. 6 is a schematic diagram showing another embodiment of the arrangement of the supply nozzles. In the same configuration as in FIG. 1, only the same reference numerals are described, and the description thereof will be omitted. For example, as shown in FIG. 6, as a form of the gas supply nozzle of the uniform tropical 10, two gas supply nozzles are arranged so that the two nozzle openings face each other, such as the gas supply nozzle 6C and the gas supply nozzle 6D. , High dew point gas may be supplied by using two gas supply nozzles in one place.

In the above embodiment, one partition plate is provided in the tropics, but two or more partition plates may be provided. In this case as well, the plurality of partition plates each have an opening so that the atmospheric gas flowing in the furnace meanders up and down. Therefore, for example, when both the inlet and the outlet of the atmospheric gas are arranged at the bottom or the top of the furnace, an odd number of partition plates are provided, and the inlet and the outlet are staggered at the bottom and the top of the furnace. When arranged in, an even number of partition plates will be provided. By arranging the fixed side or the open side of the partition plate alternately on the left and right, a space meandering to the left and right by the partition plate is formed between the inlet and the outlet of the tropics. That is, inside the tropics, there is a flow path of atmospheric gas that meanders from the inflow port to the left and right, passes through the space between one partition plate and the other partition plate, and finally connects to the outflow port. It is formed. This makes it possible to improve the circulation of atmospheric gas and high dew point gas in the tropics and improve the controllability of the decarburization reaction.

In addition, an outlet may be provided for each tropical section.

Hereinafter, the present invention will be described in detail based on Examples, but the present invention is not limitedly interpreted based on the description of this Example.

Examples 1 to 3 and Comparative Examples 1 to 2 are tests in which a cold-rolled steel sheet is produced as a steel sheet to be annealed and annealed while blowing a non-oxidizing gas using a heating furnace in which a heat insulating material is arranged inside the outer wall. Comparative Example 2 was performed.

[Example 1]
(Steel plate)
As the steel sheet to be annealed, a cold-rolled steel sheet was prepared. Specifically, first, the raw materials are melted and cast, and slabs containing 0.2% by mass of carbon, 1.8% by mass of silicon, 2.1% by mass of manganese, and the balance of iron and unavoidable impurities are added. It was made and this slab was hot rolled. Next, the obtained rolled material was pickled to completely remove the oxide scale on the surface, and cold rolling was further performed to produce a thin steel sheet 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 tropics of Example 1, Example 3 and Comparative Examples 1 to 2, a vertical continuous annealing furnace 50 having an internal volume of 600 m ³ shown in FIG. 7 was used. In the soaking tropic 40 of the vertical continuous annealing furnace 50, both the throat 25 which is the inlet and the throat 26 which is the outflow port of the atmospheric gas of the soaking tropic 20 are arranged at the bottom of the furnace. Atmospheric gas having a low dew point is supplied to the tropics 40 from the cooling zone 7 via a throat 25 connecting the tropics 40 and the cooling zone 7. Further, an atmospheric gas having a low dew point such as _N2 is supplied from the gas inlet G9 at a constant flow rate. The solitary tropics 40 is divided into two sections, a front stage 42 on the upstream side in the plate-passing direction and a rear stage 41 on the downstream side, and the steel plate 2 is wound over two top rolls 3. One end of the partition plate 44 is fixed to the bottom wall of the tropical 40, and the other end is provided in a state of being open to the side where the steel plate 2 of the two top rolls 3 is not wound.

Then, nozzles (not shown) that directly blow steam into each of the front stage 42 and the rear stage 41 of the soothing tropics 40 are installed, the dew point in the furnace is measured while adjusting the steam flow rate, and the decarburization reaction of the steel plate 2 is advanced. rice field. The temperature of the steel plate at the entrance of the tropics 40 is 700 ° C. The board was passed at a speed of 60 m / min. Dew point measurement positions are indicated by R1 and R2. Table 1 shows the conditions of the atmospheric gas and other atmospheric gases introduced from the cooling zone.

Further, in the second embodiment, as in the second embodiment, the configuration of the vertical continuous annealing furnace 50 shown in FIG. 7 has both a throat serving as an inlet and an outlet for a tropical atmosphere gas. It is arranged in the upper part of the furnace, one end of the partition plate is fixed to the upper wall of the above-mentioned tropical furnace, and the other end is between the bottom wall of the furnace and the upper end of the roll on the side where the steel plate of the bottom roll is not wound. Changed to be provided in an open state.

The procedure for calculating the input water vapor amount in Example 1 is as follows.
(1) Setting the target final decarburization depth The target final decarburization depth was set to 80 μm.
(2) Setting of target dew point Based on the above formula, the target dew point for obtaining the target final decarburization depth of 80 μm under the condition of the steel plate temperature of 700 ° C. was set to the dew point of 0 ° C. (6028 ppm).
(3) Calculation of the amount of water vapor required to set the dew point in the furnace to the target dew point 1200 Nm ³ / h of atmospheric gas from the cooling zone (dew point -35 ° C, water vapor amount 304 ppm) and low dew point input atmospheric gas (dew point-) The amount of water vapor required to bring the dew point 0 ° C (6028 ppm) to the atmospheric gas consisting of 50 ° C and 60 ppm water vapor) 400 Nm ³ / h is
2000 × 6028- (1200 × 304 + 800 × 60) = 11.64Nm ³ / h = 9.36kg / h
Since the latter stage requires the amount of water vapor to raise the dew point of the gas from the cooling zone and the input atmosphere gas, the amount of water vapor to raise the dew point of the atmosphere gas in the latter stage is
(1600 × 6028- (1200 × 304 + 400 × 60)) / 100/10000 / 22.4 × 18 = 7.44 kg / h
Will be.
In the previous stage, it is only necessary to raise the dew point of the atmospheric gas in the previous stage, so the amount of water vapor for raising the dew point of the atmospheric gas in the previous stage is
400 x ( 6028-60) / 100/10000 / 22.4 x 18 = 1.92 kg
Will be.
(4) Calculation of water consumption from the conditions of Example 1
From the plate amount (kg / h) = plate thickness (mm) x plate width (mm) x plate speed (m / min) x 60 x density (kg / m ³ )
28260 kg / h = 1 x 1000 x 60 x 60 x 7850
Will be.
Next, the carbon content of the steel material is 0.2% by mass, the target decarburization depth is 80 μm, and the decarburization amount (kg / h) = the through plate amount (kg / h) × the carbon content of the steel material (%) × (decarburization depth ÷). Steel plate thickness) ÷ 2 × 2 surfaces (concentration distribution) × 0.9, the amount of decarburization is
28260 × 0.002 × (80/1000) ÷ 1 ÷ 2 × 2 ÷ 1 × 0.9 = 4.07 kg / h.
The carbon content used in the decarburization reaction is calculated from the target decarburization depth and the carbon content of the steel material.
Since the decarburization reaction is C + H ₂ O = CO + H ₂ ,
The amount of water vapor due to the decarburization reaction = the amount of decarburized / the molecular weight of carbon × the molecular weight of _H2O , and the amount of water vapor due to the decarburization reaction is
4.07 (kg / h) ÷ 12 (kg / kmol) x 18 (kg / kmol) = 6.10 (kg / h)
Will be.
Therefore, the amount of water vapor consumed is 9.49 kg / h, which is the total value of the consumption amount of 6.10 kg / h due to the decarburization reaction and the consumption amount of 3.39 kg / h due to the internal oxidation, considering the consumption amount due to internal oxidation. It becomes h.
The reaction amount ratio is calculated separately, and it 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 x 0.62 = 5.89 kg / h
Water consumption in the latter stage: 9.49 kg / h x 0.38 = 3.60 kg / h
Will be.
(5) Calculation of the amount of steam input The amount of steam input is 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 = 18.85kg / h
Will be. also,
Amount of water vapor input in the previous stage: 1.91 + 5.89 = 7.80 kg / h
Amount of water vapor input in the latter stage: 7.44 + 3.61 = 11.05 kg / h
Will be.

[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 steam required to raise the dew point is charged as the amount of steam input, and in Comparative Example 2, the first stage and the second stage are the same under the condition of a plate thickness of 2.3 mm in which the plate passing speed decreases. The target dew point of was set.

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

As shown in Table 1, for Examples 1 to 3, it was possible to obtain a decarburization depth in a range close to the target decarburization depth of 80 μm. The water vapor blown into the furnace is cold and heavy, so it flows from top to bottom. The dew points in the previous stage, which have a large amount of water vapor, tend 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 first stage where the amount of input water vapor is large tends to be uniform, but the circulation of water vapor in the latter stage is slightly inferior. It is assumed that the dew point in the latter stage will easily vary. Therefore, it is calculated that the final decarburization depth of Example 2 varies slightly.

On the other hand, as the final amount of steam input, Comparative Example 1 in which only the amount of steam required to raise the dew point is charged, and Comparative Example 2 in which the same target dew point is set in the front stage and the rear stage using a steel plate having a plate thickness of 2.3 mm. The dew point depth was significantly different from the target dew point depth of 80 μm. In Comparative Example 1, since the amount of water vapor input was not included in the amount of water vapor consumed in the decarburization reaction, the dew point was lowered as the water vapor was consumed by the decarburization reaction. It is probable that the decarburization reaction did not proceed sufficiently. Further, in Comparative Example 2, since the plate thickness was 2.3 mm, the plate passing speed was lowered, and it is considered that the decarburization depth was as deep as 138 μm.

FIG. 8 is a graph showing the relationship between the dew point and the decarburization depth for each plate passing speed in Example 2, Example 3, and Comparative Example 1. Comparing 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, 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 having a plate thickness of 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., it is possible to accurately control the target decarburization depth to 80 μm 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.

1,50 Vertical continuous annealing furnace 2 Strip-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 Normal tropics 11, 21, 31, 41 Rear stage 12, 22, 32, 42 Front stage 14, 24, 34, 44 Partition plate 25, 26 Throat G1, G2, G4, G6, G9 Gas inlet R1, R2 Dew point measuring instrument

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 the steel plate is placed in a specific horizontal direction. A vertical continuous annealing furnace configured to heat-treat while transporting in the direction.
The inside 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 meanders up and down so as to face the steel plate.
A plurality of dew point measuring instruments arranged in each of the above-mentioned plurality of sections and measuring the dew point of the section ,
A mechanism for blowing a high dew point gas or water vapor having a higher dew point than the atmospheric gas in order to adjust the dew points of the plurality of sections.
High dew point gas blowing amount of the blowing mechanism based on the required water vapor amount calculated from the composition of the atmospheric gas and the water vapor consumption amount due to the decarburization reaction predicted by the dew point of each section and the staying time and temperature of the steel plate. A vertical continuous steaming furnace equipped with a mechanism to control. The vertical continuous annealing furnace according to claim 1, wherein the dew point of the section on the upstream side in the through plate direction is higher than the dew point of the section on the downstream side in the through plate direction. The vertical continuous annealing furnace according to claim 1 or 2, wherein the area ratio of the opening of the partition plate to the area inside the furnace in the vertical direction furnace cross section at the position of the partition plate is 1/10 or more and 1/4 or less. It is provided with a plurality of top rolls and a plurality of bottom rolls arranged inside, and a strip-shaped steel plate is laid between the plurality of top rolls and the plurality of bottom rolls in a vertical meandering manner, and the steel plate is placed in a specific horizontal direction. The inside is divided into a plurality of sections along the specific horizontal direction by one or a plurality of partition plates which are configured to be heat-treated while being conveyed in a direction and have openings so that the atmospheric gas flowing in the furnace meanders up and down. It is an annealing method using a vertical continuous annealing furnace.
The process of calculating the required amount of water vapor from the composition of the above atmospheric gas, and
A process of predicting the amount of water vapor consumed by the decarburization reaction from the dew point of each section and the staying time and temperature of the steel sheet, and
A quenching method comprising a step of controlling the amount of high dew point gas or water vapor having a dew point higher than that of the atmospheric gas in the plurality of sections according to the required amount of water vapor and the amount of water consumed.

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