JP2011047018A - Continuous heat treatment furnace for steel strip and method for operating the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous heat treatment furnace for a steel strip which can prevent the phenomenon that scale is transferred and baked to the surface of a hearth roll from the steel strip during rolling without reducing the heating efficiency therein, and to provide a method for operating the same. <P>SOLUTION: A continuous heat treatment furnace where the inside of a heating zone composed of a direct-fired furnace heating a steel strip by a gas burner and a radiation furnace heating the same by a radiant tube is provided with a plurality of hearth rolls supporting the steel strip is improved. Concretely, a non-water-cooled system and a water-cooled system are adopted to the hearth roll, further, the water-cooled system hearth roll is arranged only at the generation region of scale, and also, the surface temperature of the hearth roll is controlled to the one less than the sintering temperature of Fe oxide. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a continuous heat treatment furnace for a steel strip and a method of operating the same, and more particularly, a hearth roll disposed to support the steel strip in a continuous heat treatment furnace used for a steel strip subjected to continuous hot dip galvanizing. The present invention relates to a technique effective for preventing the scale from being transferred and seized on a surface from a steel strip.

  For example, when hot dip galvanizing is applied to a steel strip, it is necessary to heat, cool and heat the steel strip in advance in order to give the steel strip predetermined mechanical properties. In addition, a horizontal continuous heat treatment furnace that passes through in the horizontal direction is generally used.

  Among these, as shown schematically in FIGS. 3 (a) and 3 (b), the horizontal continuous heat treatment furnace includes a horizontal heating zone, a direct-fired furnace 3 that directly heats the steel strip 2 with a gas burner 1, and a radiant. A radiation furnace 5 heated by a tube 4 (abbreviation of Radiant Tube Heating, commonly referred to as RTH), a holding band (abbreviation of Holding Section, referred to as HS), and a gas injection cooling device 6 (of Jet Cooling) It is abbreviated as JC). The direct-fired furnace 3 is also called a non-oxidizing furnace (abbreviated as Non Oxidizing Furnace, commonly called NOF).

  The steel strip 2 that continuously passes through these furnaces (referred to as a through plate) is called a hearth roll 7 and is supported by arranging a large number of small-diameter cylindrical rotating bodies. This hearth roll 7 is made of stainless steel of SUS304, SUS430, and was previously used in a non-water-cooled type that was simply cylindrical, but now the reason will be described later. A water-cooled type formed of a jacket (not shown) capable of passing water is also used. Note that symbols such as “# 301” in FIG. 3B are numbers designated for the hearth roll 7.

  By the way, the steel strip 2 is heated and cooled when passing over the hearth roll 7 and is subjected to heat treatment (for example, annealing). Therefore, when trying to obtain a long annealing time, the length of the furnace body is increased. Becomes long. Therefore, in order to shorten the length of the furnace body, as shown in FIGS. 3A and 3B, the direct-fired furnace 3 having a high heating capacity is arranged on the entrance side of the heating zone.

  In the direct furnace 3, the steel strip 2 is directly heated to about 600 to 700 ° C. by the gas burner 1. The gas burner 1 is designed so that the ratio (air ratio) of COG gas, which is fuel, to air is 1.0 or less, and the steel strip 2 is not oxidized. Is weakly oxidizable, and the surface of the steel strip 2 is slightly oxidized in the threading plate to form a thin iron oxide (referred to as scale) layer. Therefore, as shown in FIG. 2, the scale layer 8 is transferred onto the surface of the hearth roll 7 and is seized (also referred to as a pickup) to form a hard rugged surface. Therefore, after that, a defect called a push rod occurs on the surface of the steel strip 2 that is supported by the hearth roll 7 and passed through, causing a problem that the surface quality of the product steel strip is deteriorated.

  In order to solve such problems, conventionally, the inside of the hearth roll 7 has been cooled to prevent pickup. However, when water cooling is performed, the surface temperature of the hearth roll 7 is lowered and the scale can be prevented from seizing, but the heat is also taken away and the atmosphere temperature in the furnace and the temperature of the steel strip 2 are also lowered. However, there is a demerit that the basic unit of fuel will rise if we maintain it.

  Therefore, in order to suppress such a temperature drop, in the water-cooled roll that cools water through the cylindrical body of the hearth roll 7 and coats the inner wall of the body with a thickness of 0.5 to 3.0 mm. A water-cooled roll is disclosed (see, for example, Patent Document 1). There is also a technique for individually adjusting the outer peripheral surface temperature of the water-cooled hearth roll in accordance with the furnace atmosphere temperature. At that time, the outer peripheral surface temperature is adjusted to 750 to 800 ° C., but considering the heat loss, the lower limit is set to 750 ° C. (see, for example, Patent Document 2). Furthermore, a non-water-cooled hearth roll is disposed in the RTH furnace of the continuous heat treatment furnace, and a thermal barrier that blocks radiant heat from the radiant tube is installed between the non-water-cooled hearth roll and the lower radiant tube, A technique for preventing seizure of the scale without giving a large heat loss to the inside or the steel strip is also disclosed (see, for example, Patent Document 3).

  In this way, the use of water-cooled hearth rolls has taken into consideration that the heat loss of the steel strip due to cooling is large, and the heating efficiency of the heat treatment furnace is extremely reduced. It is devised.

  However, even with the use of the above-described technology, no satisfactory results have yet been obtained with respect to scale seizure and furnace fuel intensity. Further, the position of the water-cooled hearth roll is not based on a clear idea, but is merely arranged by trial and error, and there is room for further study.

JP 52-53711 A JP-A-9-241762 JP 2006-307296 A

  In view of such circumstances, the present invention is a continuous heat treatment furnace for a steel strip capable of preventing scale from being transferred and seized onto a hearth roll from a steel strip in a plate without lowering the heating efficiency of the continuous heat treatment furnace, and The purpose is to provide the method of operation. In particular, it is preferably applied to a horizontal continuous heat treatment furnace.

  The inventor has intensively studied to achieve the above object, and the results have been embodied in the present invention. That is, in the present invention, a plurality of hearth rolls supporting the steel strip are disposed in a heating zone in which an oxidation zone for heating the steel strip in an oxidizing atmosphere and a reduction zone for heating in a reducing atmosphere are sequentially provided. In the continuous heat treatment furnace of the steel strip, the region from the position of the oxidation zone where oxidation of Fe occurs to the position of the reduction zone where the oxidation amount by the oxidation zone becomes 0 by reduction of the scale by the reduction zone, A continuous heat treatment furnace for a steel strip, characterized in that a water-cooled hearth roll is disposed only in a portion where the steel strip temperature is equal to or higher than the temperature at which the Fe oxide is sintered into the hearth roll.

  In the present invention, a plurality of hearth rolls supporting the steel strip are arranged in a heating zone in which an oxidation zone for heating the steel strip in an oxidizing atmosphere and a reduction zone for heating in a reducing atmosphere are sequentially provided. In the continuous heat treatment furnace of the steel strip, the amount of oxidation by the oxidation zone determined by the following equation (1) is determined by the following equation (2) from the position of the oxidation zone where oxidation of Fe determined by the following equation (1) occurs. A water-cooled hearth roll is only applied to a portion where the steel strip temperature is equal to or higher than the temperature at which the Fe oxide is sintered into the hearth roll in the region up to the reduction zone position where the scale is reduced to 0 by reduction of the scale by the reduction zone. Is a continuous heat treatment furnace for steel strips.

Oxidation formula X ² = {7750exp [−2970 / RT ₁ ]} × to + (0.66T ₁ ) −186) ² × (72/16) (1)
Reduction formula Y = 9.55 × 10 ⁻²¹ × t _R ^0.19 × T ^6.98 × H ₂ ^1.26 × H ₂ O ^−0.03 × 10 ³ (2)
here,
X: oxidation amount (mg / m ² ), to: oxidation time (sec),
R: gas constant [= 1.99] (cal / mol K), T ₁ : oxidation temperature [= steel strip temperature] (K)
Y: reduction amount (mg / m ² ), t _R : reduction time (sec),
T: Reduction temperature [= steel strip temperature] (° C.), H ₂ : H ₂ concentration [= constant at 10 vol%],
H ₂ O: H ₂ O concentration “constant at 1 vol%”
In this case, the temperature at which the sintering occurs is preferably 713 ° C. Alternatively, the oxidation zone is preferably a direct furnace that heats the steel strip with a gas burner, and the reduction zone is preferably a radiation furnace that heats the steel strip with a radiant tube.

  Furthermore, the present invention is a method for operating a continuous heat treatment furnace for steel strips as described above, wherein the water flow rate to the hearth roll is adjusted so that the surface temperature of the water-cooled hearth roll is less than 713 ° C. It is also a method for operating a continuous heat treatment furnace for steel strip.

  In the present invention, in consideration of the amount of scale generated (also referred to as oxidation amount) and the amount of disappearance (also referred to as reduction amount) when passing through the heating zone, the sintered area of the scale in the heating zone is estimated, Since the water-cooled hearth rolls are arranged only in the area, not only the concept of the water-cooled hearth rolls is clarified, but also the number of water-cooled hearth rolls that cause heat loss is reduced compared to the conventional one. This makes it possible to minimize the influence of heat loss and to prevent a reduction in the heating efficiency of the heat treatment furnace.

  Furthermore, since the surface temperature of the water-cooled hearth roll arranged in the heating zone is adjusted to less than 713 ° C. which is the sintering temperature of the scale formed on the surface of the steel strip and transferred to the surface of the hearth roll, Prevents seizure (pickup) of the transferred scale to the hearth roll.

It is a figure explaining the arrangement | positioning of the sintering area | region of a scale in the heating zone determined by this invention, and a hearth roll. It is a figure which shows the state of the scale baked on the surface of the hearth roll. It is a schematic diagram explaining a horizontal type | mold continuous heat processing furnace, (a) has shown the whole furnace, (b) has shown the heating zone. It is a figure which shows the result of having investigated the influence which the oxidation temperature and the oxidation time exert on the oxidation amount in the oxidation experiment of the steel strip.

  Hereinafter, embodiments of the present invention will be described, including the background to the invention.

  First, the inventor transferred the scale (iron oxide) generated by the reaction between the surface of the steel strip and oxygen remaining in the furnace atmosphere of the direct-fired furnace to the hearth roll 7 and seized (see FIG. 3). ) Was further investigated. The investigation was conducted on the hearth roll 7 in the radiant furnace 5 where the scale was seized. As a result of chemical analysis of the scale, a reduced iron layer (Fe: 83 mass%, O: 10 mass%, Ca: 3 mass%) exists on the surface layer of the scale that is baked on the hearth roll 7, and its lower layer It was found that wustite (Fe: 71 mass%, O: 29 mass%) was present as iron oxide. The reason why the reduced iron layer exists is that the radiation furnace 5 is in a reducing atmosphere, and iron oxide formed in the direct-fired furnace 3 is reduced and returned to iron. And when the sintering temperature of the powder of main iron oxide was calculated | required by experiment separately, the result shown in Table 1 was obtained. The iron oxide directly baked on the hearth roll 7 was wustite as described above. From Table 1, in order to avoid the seizure, the surface temperature of the hearth roll 7 should be less than 713 ° C. I understood. This value is considerably lower than 750 ° C. described in Patent Document 2.

  In order to make the surface temperature of the hearth roll 7 lower than 713 ° C., a water-cooled hearth roll is indispensable. In this case, although not shown, the temperature is controlled by the amount of cooling water flowing into the jacket. Become. Moreover, when it becomes less than 713 degreeC, compared with the case of 750 degreeC of patent document 2, the calorie | heat amount taken away from the steel strip 2 will increase. Therefore, since the heating efficiency of the heating zone is reduced, it is desirable to reduce the reduction as much as possible. Therefore, in the present invention, the temperature to be managed is set to (713−α) ° C., and specifically, the opening / closing degree of the valve provided in the piping of the cooling water sent to the jacket is set to a thermometer so that α is minimized. More preferably, it is performed by automatically adjusting according to an instruction from.

  Furthermore, the inventor has conceived that a water-cooled hearth roll is disposed only in a region where scale seizure occurs (sintered region) in order to reduce the heat loss of the steel strip and improve the thermal efficiency. did. That is, the present invention relates to the steel strip temperature in the region from the position of the oxidation zone where oxidation of Fe occurs to the position of the reduction zone where the amount of oxidation by the oxidation zone becomes 0 by reduction of the scale by the reduction zone. Is a continuous heat treatment furnace for a steel strip in which a water-cooled hearth roll is disposed only in a portion where the temperature is higher than the temperature at which the Fe oxide is sintered to the hearth roll. Subsequently, the sintered area of the scale was estimated.

  First, an oxidation zone heated in an oxidizing atmosphere, preferably the amount of oxidation of the steel strip 2 in the direct-fired furnace 3, that is, the amount of scale generated, and a reduction zone heated in a reducing atmosphere, preferably in the radiation furnace 5 An experiment was carried out to determine the amount of residual scale from the amount of reduction in iron, that is, the disappearance of the scale that returns the generated scale to iron. It investigates the influence of the oxidation temperature and oxidation time on the amount of oxidation and the influence of the steel strip temperature in the radiation furnace 5, the hydrogen concentration in the atmosphere and the reduction time on the amount of scale reduction. It is obtained as a formula representing the amount of oxidation and the amount of reduction.

In the oxidation experiment, the influence of the oxidation temperature and the oxidation time on the oxidation amount in a gas atmosphere of CO / CO ₂ = 0.11, O ₂ = 0.015 vol% was examined, and FIG. 4 was obtained. From FIG. 4, the oxidation amount of the steel strip 2 increases in proportion to the oxidation temperature and the oxidation time, but the increase amount of the oxide decreases as the temperature increases and the time increases. In other words, it is clear that the oxidation rate follows the parabolic law referred to as “kinetics”. Thus, when the experimental data was arranged with the horizontal axis (1 / RT) and the vertical axis X ² (experimental value), the following equation (1) was obtained.

On the other hand, the data obtained in the reduction experiment of the scale by heating in the atmosphere of H ₂ = 10 vol%, H ₂ O concentration = 1 vol% and the balance N ₂ —H ₂ was arranged by regression analysis to obtain the following formula (2). Therefore, when the reduction amount is subtracted from the oxidation amount, the amount of scale (oxidation amount) remaining during heating in the radiation furnace 5 is obtained.

Oxidation formula X ² = {7750exp [−2970 / RT ₁ ]} × to + (0.66T ₁ ) −186) ² × (72/16) (1)
Reduction formula Y = 9.55 × 10 ⁻²¹ × t _R ^0.19 × T ^6.98 × H ₂ ^1.26 × H ₂ O ^−0.03 × 10 ³ (2)
here,
X: oxidation amount (mg / m ² ), to: oxidation time (sec),
R: gas constant [= 1.99] (cal / mol K), T ₁ : oxidation temperature [= steel strip temperature] (K)
Y: reduction amount (mg / m ² ), t _R : reduction time (sec),
T: Reduction temperature [= steel strip temperature] (° C.), H ₂ : H ₂ concentration [= constant at 10 vol%],
H ₂ O: H ₂ O concentration “constant at 1 vol%”
The amount of oxidation was determined by preparing a calibration curve by fluorescent X-ray analysis from the wet analysis result of oxygen in the oxidized steel sheet, and quantifying oxygen by fluorescent X-ray analysis using this calibration curve.

The oxidation amount: X depends on the oxidation time: to in the direct-fired furnace 3 and the temperature T ₁ of the steel strip 2, and this T ₁ also varies depending on the oxidation time. Therefore, in the actual calculation, it is assumed that the temperature changes after 1 second of to, and first, X (oxidation amount) is calculated with the to of the equation (1) being constant at 1 second and the plate temperature being a certain value. Then, the plate temperature is raised and X is calculated again, and the subsequent calculation value is added to the previous calculation value and integrated to obtain the second oxidation amount. Thereafter, such calculation is sequentially repeated to reach the maximum oxidation amount in FIG.

On the other hand, the reduction amount: Y takes into account the temperature rise state of the reduction temperature: T (steel strip temperature) in the radiant furnace 5, and the reduction time: t _R is changed at a plurality of levels to reduce the reduction amount at each reduction time. It is obtained and subtracted from the maximum oxidation amount to obtain the remaining scale amount (oxidation amount), and the same calculation is repeated until the remaining scale amount (oxidation amount) becomes zero. The steel plate surface scale amount in FIG. 1 is estimated by such calculation.

  From FIG. 1, the inventors have determined that “the region where the scale is present on the surface of the steel strip 3 and the radiation furnace 5 and the scale is present on the surface of the steel strip may cause scale scoring on the hearth roll 7”. . The region was estimated to be a region between the hearth roll of # 305 and the hearth roll of # 311 as is apparent from the result of FIG. Further, in this region, it was considered that only the portion where the steel strip temperature is equal to or higher than the temperature at which the scale (Fe oxide) is sintered into the hearth roll is seized on the hearth roll 7. As described above, since the main component of the scale is wustite, the temperature at which the Fe oxide is sintered is more preferably 713 ° C.

  Therefore, the inventor arranges the water-cooled type only in the sintered portion of the Fe oxide among the hearth rolls arranged in the entire heating zone, and the non-water-cooled type in other positions. It is concluded that the arrangement is the best in suppressing the heat loss of the steel strip, and this idea is embodied in a continuous heat treatment furnace to be the present invention. This state is shown in FIG. 1 by representing a water-cooled hearth roll with a circle and a non-water-cooled hearth roll with a circle.

  Conventionally, the concept for the arrangement of water-cooled hearth rolls has been unclear, and there has been a tendency for the number to be arranged to be excessive. However, according to the present invention, the arrangement of water-cooled hearth rolls becomes appropriate, and Improvement in the heating efficiency of the steel strip 2 can be achieved by reducing the number of installed steel sheets.

  Moreover, in this invention, the plate | board speed of the steel strip 2 is not specifically limited. This is because the present invention can be applied to the steel strip 2 other than the material for galvanization. However, when the steel strip 2 is a material for galvanization, it is preferable that the plate passing speed is 30 to 80 m / min. This is because it is applied to continuous heat treatment before galvanization and can prevent almost 100% of defects caused by scale baking.

  In addition, as an operation method of the steel strip continuous heat treatment furnace, the inventor sets the surface temperature of the water-cooled hearth roll disposed only in the region defined above to less than 713 ° C, more preferably (713-α) ° C. In addition, it is preferable to adjust the amount of water flow to the hearth roll 7 so that α is minimized. The basis for this is described above and is omitted here.

  In the horizontal continuous heat treatment furnace shown in FIG. 3 (b) attached to a continuous hot dip galvanizing facility (line) for applying hot dip galvanizing to the steel strip 2, the steel strip 2 before plating was annealed as heat treatment. The coiled steel strip as a raw material had a width of 600 to 1600 mm and a plate thickness of 0.4 to 2.0 mm. The coil was unwound with an uncoiler and passed through, and annealed at a target steel strip temperature of 890 ° C. At that time, the sheet feeding speed of the steel strip 2 was set to 40 m / min. In addition, 22 hearth rolls are arranged in the heating zone (length: 80 m) of the horizontal continuous heat treatment furnace, most of which are non-water cooled, but the arrangement position and number of water-cooled ones are changed, The test operation was attempted by adjusting the surface temperature of the water-cooled hearth roll to adjust the amount of cooling water flow.

  In applying the present invention, the arrangement position of the water-cooled hearth roll is determined by the traveling direction position in the direct-fired furnace 3 where the generation of the scale on the steel strip 2 starts as described above, and the above equation (1). The oxidation amount of the steel strip 2 was determined by the position in the traveling direction in the radiation furnace 5 where the reduction amount of the scale determined by the above equation (2) becomes zero.

  In addition, the water flow rate was adjusted by continuously measuring the surface temperature of each water-cooled hearth roll, setting the target temperature, and automatically opening and closing the water piping valve. These test results were evaluated based on the presence or absence of scale seizure on the surface of the water-cooled hearth roll and the amount of fuel consumption as the heating efficiency.

Scale seizure evaluation criteria △: Mild seizure generation on # 309, # 311 hearth rolls ○: No seizure occurrence Fuel basic unit evaluation criteria large (inferior) △>○> ◎ small (excellent)
Table 2 collectively shows the test results according to the arrangement position and the number of water-cooled hearth rolls. From Table 2, it is clear that when the present invention is applied, there is no scale seizure and a reduction in fuel consumption can be achieved even though the number of water-cooled hearth rolls is reduced by three.

DESCRIPTION OF SYMBOLS 1 Gas burner 2 Steel strip 3 Direct-fired furnace 4 Radiant tube 5 Radiation furnace 6 Gas injection cooling device 7 Hearth roll 8 Scale layer 9 Arrow which shows the advancing direction of a steel strip

Claims (5)

Continuous heat treatment of a steel strip in which a plurality of hearth rolls supporting the steel strip are arranged in a heating zone in which an oxidation zone for heating the steel strip in an oxidizing atmosphere and a reduction zone for heating in a reducing atmosphere are sequentially provided. In the furnace,
In the region from the position of the oxidation zone where oxidation of Fe occurs to the position of the reduction zone where the amount of oxidation due to the oxidation zone becomes 0 due to the reduction of the scale by the reduction zone, the steel strip temperature is the hearth of the Fe oxide A continuous heat treatment furnace for a steel strip, wherein a water-cooled hearth roll is disposed only in a portion where the temperature at which sintering to a roll occurs or higher. Continuous heat treatment of a steel strip in which a plurality of hearth rolls supporting the steel strip are arranged in a heating zone in which an oxidation zone for heating the steel strip in an oxidizing atmosphere and a reduction zone for heating in a reducing atmosphere are sequentially provided. In the furnace,
From the position of the oxidation zone where oxidation of Fe determined by the following formula (1) occurs, the oxidation amount by the oxidation zone determined by the following formula (1) is reduced to 0 by the reduction of the scale by the reduction zone determined by the following formula (2). A water-cooled hearth roll is disposed only in a portion where the steel strip temperature is equal to or higher than the temperature at which sintering of Fe oxide to the hearth roll is performed in the region up to the position of the reduction zone. A continuous heat treatment furnace for strips.
Oxidation formula X ² = {7750exp [−2970 / RT ₁ ]} × to + (0.66T ₁ ) −186) ² × (72/16) (1)
Reduction formula Y = 9.55 × 10 ⁻²¹ × t _R ^0.19 × T ^6.98 × H ₂ ^1.26 × H ₂ O ^−0.03 × 10 ³ (2)
here,
X: oxidation amount (mg / m ² ), to: oxidation time (sec),
R: gas constant [= 1.99] (cal / mol K), T ₁ : oxidation temperature [= steel strip temperature] (K)
Y: reduction amount (mg / m ² ), t _R : reduction time (sec),
T: Reduction temperature [= steel strip temperature] (° C.), H ₂ : H ₂ concentration [= constant at 10 vol%],
H ₂ O: H ₂ O concentration “constant at 1 vol%”   The continuous heat treatment furnace for steel strip according to claim 1 or 2, wherein a temperature at which the sintering occurs is 713 ° C.   The said oxidation zone is a direct-fired furnace which heats a steel strip with a gas burner, The said reduction zone is a radiant furnace which heats a steel strip with a radiant tube. Continuous heat treatment furnace for steel strip. It is an operation method of the continuous heat treatment furnace of the steel strip according to any one of claims 1 to 4,
A method for operating a continuous heat treatment furnace for a steel strip, wherein the amount of water passing through the hearth roll is adjusted so that the surface temperature of the water-cooled hearth roll is less than 713 ° C.

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