JP2007131914A - Horizontal continuous heating furnace, and method for continuously annealing steel sheet using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a horizontal continuous heating furnace that is used in a horizontal continuous annealing apparatus which has a preheating oven, a horizontal continuous heating furnace and a reducing furnace, prevents heat from dissipating from the hearth roll, does not coarsen a crystal in the ends in a width direction of a steel sheet made from extra-low carbon steel even when the steel sheet is heated in the horizontal continuous heating furnace and then final-annealed in a reducing furnace of a subsequent stage, and secures product quality, and to provide a continuous annealing method using the same. <P>SOLUTION: The horizontal continuous heating furnace 3 has an opening 9 arranged so as to longitudinally penetrate a heat insulation wall 7 which is installed on a hearth 8 of the horizontal continuous heating furnace 3 and covers the undersurface of a hearth roll 5. The opening 9 has an open area ratio of 30 to 90%, which is expressed by the equation: open area ratio (%)=cross section of the opening in furnace width direction/total cross section of heat insulation wall in furnace width direction including the opening×100. The continuous annealing method includes adjusting a passing speed of the sheet and/or a burner output of a direct flame burner 6 so that a temperature difference between a central part and an end in a width direction of a steel sheet (A) can be 70°C or lower in an outlet of the horizontal continuous heating furnace 3. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a horizontal continuous heating furnace that heats and anneals a steel plate with a direct burner and a continuous annealing method using the same, and particularly, a horizontal continuous suitable for continuous annealing of a steel plate made of ultra-low carbon steel such as IF steel. The present invention relates to a heating furnace and a continuous annealing method using the same.

  As shown in the sectional view in the longitudinal direction of the furnace in FIG. 1, a horizontal continuous annealing apparatus 1 used for continuous annealing of a steel plate A is a preheating furnace 2 for preheating the steel plate A, and the preheated steel plate A is directly heated. A horizontal continuous heating furnace (hereinafter also simply referred to as a “heating furnace”) 3 that is heated at a temperature, and a reduction furnace 4 that is finally annealed while reducing the steel sheet A whose surface has been oxidized by direct fire heating by indirect heating. ing. Then, the combustion gas of the heating furnace 3 is sucked into the preheating furnace 2 by a fan (not shown) provided outside the furnace so as to be opposed to the sheet passing direction. In addition, as shown in FIG. 2, the heating furnace 3 includes a plurality of hearth rolls 5 that convey the steel plate A in a substantially horizontal direction, and a plurality of direct fire burners 6 that radiately heat the steel plate A from above and below (above the steel plate A). The installed one is referred to as the upper burner 6a, and the lower one is referred to as the lower burner 6b.) Since the hearth roll 5 is usually made of metal, it does not cause thermal breakage in the high-temperature heating furnace 3. So that it is water-cooled. In order to reduce heat loss through the surface of the water-cooled hearth roll 5 as much as possible, as shown in the sectional view in the heating furnace width direction of FIG. It is common to stand up (see Patent Document 1).

  However, when the steel plate A made of ultra-low carbon steel such as IF steel is annealed using the horizontal continuous annealing apparatus 1 having the heating furnace 3 having such a heat shield wall 7, at the plate width end portion of the steel plate A after annealing. The phenomenon that the crystal grains become coarse was observed, and it was found that the target product quality could not be ensured.

Patent Document 2 discloses a horizontal continuous heat treatment furnace in which a partition wall 8 having an opening 12 penetrating in the longitudinal direction of the furnace is installed close to the top and bottom of the hearth roll 3 as shown in FIG. In addition, the partition wall 8 installed below the hearth roll 3 looks like an approximation of the thermal barrier according to the present invention. However, this partition wall 8 is installed at every other hearth roll so that the combustion gas can easily pass through the breathable solid 7 installed for the purpose of improving the heat transfer efficiency from the combustion gas to the steel plate 2. In addition, since it is not installed on all the hearth rolls, it is not intended to function as a thermal barrier that prevents heat loss through the hearth roll surface. Therefore, the invention described in Patent Document 2 is completely different from the invention of the present application in terms of the solution problem and effect of the invention, and is different from the technical idea of the present invention.
JP 2002-155313 A JP 59-47319 A

Therefore, the present invention relates to a horizontal continuous heating furnace used in a horizontal continuous annealing apparatus having a preheating furnace, a horizontal continuous heating furnace, and a reduction furnace, and a steel plate made of ultra-low carbon steel while preventing heat loss from the hearth roll. After heating, the horizontal continuous heating furnace that can ensure product quality without crystal coarsening at the width end of the steel sheet even if it is finally annealed in a subsequent reduction furnace, and continuous annealing using the same It aims to provide a method.

  When the present inventors annealed the steel plate A made of ultra-low carbon steel using the horizontal continuous annealing apparatus 1 having the heating furnace 3 provided with the heat insulating wall 6 as described above, the plate width end of the steel plate A after annealing. In order to elucidate the cause of the coarsening of crystal grains, the following verification experiment was conducted.

  First, the temperature distribution in the sheet width direction of the steel sheet A made of IF steel, which is a kind of ultra-low carbon steel, at each of the heating furnace 3 outlet and the reduction furnace 4 outlet of the actual horizontal continuous annealing apparatus 1 (hereinafter referred to as “sheet temperature distribution”). Also measured with a radiation thermometer. As a result of the measurement, as shown in FIG. 7, the temperature at the center of the plate width of the steel sheet A (hereinafter also referred to as “plate temperature”) is about 700 ° C. at the outlet of the heating furnace 3 and about 800 ° C. at the outlet of the reducing furnace 4. On the other hand, the plate temperature at the end of the plate width of the steel plate A reaches about 850 ° C. at the outlet of the heating furnace 3 and reaches about 950 ° C. at the outlet of the reduction furnace 4, and is about 150 ° C. at maximum from the plate temperature at the center of the plate width. I found it to be higher. In other words, IF steel, which is an extremely low carbon steel, has a higher recrystallization temperature than ordinary steel, so the annealing temperature must also be higher than that of ordinary steel. The part will be heated to a temperature up to 150 ° C. higher than that. In this way, the end of the plate width is heated in the heating furnace 4 so as to greatly exceed the target heating temperature, and the excess of the heating temperature is carried over into the reduction furnace 4 and finally about 950 ° C. at the reduction furnace 4 outlet. As a result of heating to such a high temperature, it was estimated that the crystal grains became coarse at the end of the plate width.

  Therefore, in order to verify this estimation, an annealing experiment simulating the heat pattern of the actual horizontal continuous annealing apparatus 1 was conducted in an annealing simulator using a batch type small heating furnace, and the annealing temperature and annealing time affecting the crystal grain size. The effect of was investigated. That is, a steel plate made of IF steel was heat-annealed under conditions in which the combination of the atmospheric temperature and the holding time was variously changed in an annealing simulator, and the ferrite crystal grain size was measured based on JIS G0552 for each steel plate after annealing.

  As is apparent from the measurement results summarized in FIG. 8, when the plate temperature (= atmosphere temperature) is less than 920 ° C., the ferrite grain size is 7.0 or more even if the holding time is extended, and the crystal grains are fine. However, when the plate temperature is 920 ° C. or higher, the ferrite crystal grain size becomes less than 7.0 and the crystal grains become coarse when the holding time exceeds a certain time.

  From the results of these verification experiments, in the actual horizontal continuous annealing apparatus 1, the central portion of the plate width is finally heated only to about 800 ° C., which is less than 920 ° C. at the outlet of the reduction furnace 4, whereas the plate width It can be concluded that the end portion was finally heated at over 920 ° C. to about 950 ° C. at the outlet of the reducing furnace 4 and as a result, coarsening of crystal grains occurred at the end portion of the plate width.

Then, next, examination was made on the cause of the plate temperature at the plate width end portion becoming significantly higher than the plate temperature at the plate width central portion. As a result of various considerations, as possible causes,
(1) Nonuniformity of radiant heat from the direct fire burner 6 in the longitudinal direction of the burner (2) Narrowing down to two points of the drift of the combustion gas from the lower burner 6b due to the heat barrier 7, and in order to investigate the influence of these, A verification experiment was conducted.

  First, as for (1), as a result of measuring the plate temperature distribution in the plate width direction using a laboratory experimental apparatus that simulates only the positional relationship between the steel plate A of the actual heating furnace 3 and one direct fire burner 6, The plate temperature difference between the width end portion and the plate width center portion was as small as about 15 ° C., and it was judged that the influence of non-uniformity of radiant heat from the direct fire burner 6 was small.

  Next, with respect to (2), in the actual heating furnace 3, the upper burner 6a was ignited when both the upper burner 6a and the lower burner 6b were ignited under the condition that the plate passing speed was constant (case 1). However, the plate temperature distribution in the plate width direction was measured for each of the cases where the lower burner 6b was extinguished (case 2). As a result of the measurement, the plate temperature difference between the center portion of the plate width and the end portion of the plate width was about 150 ° C. in case 1 but reduced to about 50 ° C. in case 2. From this result, it was determined that the influence of forced convection heat transfer due to the drift of combustion gas from the lower burner 6b was great.

  That is, as described above, the combustion gas flow B from the lower burner 6b is sucked by the fan and finally flows toward the preheating furnace 2, but as shown in the schematic diagram of FIG. It is blocked by the wall 7, passes through a gap between a furnace side wall (not shown) (see reference numeral 10 in FIG. 3) and the plate width end of the steel plate A, and once exits above the steel plate A to the preheating furnace 2 side. Will head. Therefore, since the flow velocity of the combustion gas flow B is accelerated when passing through this gap, the amount of forced convection heat transfer from the combustion gas to the end of the plate width increases. As a result, the end portion of the plate width is overheated by an increase in the amount of forced convection heat transfer compared to the central portion of the plate width, and the plate temperature rises.

  Then, the present inventors have further studied based on the above findings and have completed the following invention.

The invention according to claim 1 is a preheating furnace for preheating a steel plate with a combustion gas of a horizontal continuous heating furnace described later, and a horizontal continuous heating furnace for heating the preheated steel plate with a hearth roll while heating it with a direct fire burner. A horizontal continuous heating furnace used in a horizontal continuous annealing apparatus having a reduction furnace that anneals while reducing the heated steel sheet, on a thermal barrier that stands on the hearth and covers the lower surface of the hearth roll, It is a horizontal continuous heating furnace characterized in that an opening portion penetrating in the longitudinal direction of the furnace having an opening ratio defined by the following formula is 30 to 90%.
Formula Opening ratio (%) = furnace width direction cross-sectional area of the opening / furnace width direction total cross-sectional area below the pass line × 100

  The invention according to claim 2 is a method of annealing the steel sheet after reducing the heated steel sheet in the reduction furnace after heating the steel sheet using the horizontal continuous heating furnace according to claim 1, The sheet passing speed and / or the burner output of the direct-fired burner is adjusted so that the temperature difference between the plate width center portion and the plate width end portion of the steel plate at the outlet of the horizontal continuous heating furnace is within 70 ° C. It is a continuous annealing method of the steel sheet.

  In the present application, the “plate width end portion of the steel plate” refers to a position closer to the center of the plate width by 15 mm from the plate width end surface of the steel plate.

  The invention described in claim 3 is a method of heating the steel sheet using the horizontal continuous heating furnace according to claim 1 and then continuously annealing the steel sheet after heating in the reduction furnace. Adjusting the plate passing speed and / or the burner output of the direct-fired burner so that the difference in ferrite crystal grain size between the plate width central portion and the plate width end of the steel plate after annealing is within 2.0. It is the continuous annealing method of the steel plate characterized.

  The “ferrite grain size” is a value measured by a test method defined in JIS G0552.

  Invention of Claim 4 is the continuous annealing method of the steel plate of Claim 2 or 3 with which the said steel plate consists of ultra-low carbon steel.

  According to the present invention, in a horizontal continuous heating furnace used in a horizontal continuous annealing apparatus having a preheating furnace, a horizontal continuous heating furnace, and a reduction furnace, an opening having a predetermined opening ratio is provided on the heat shield wall. While preventing the heat loss, it is possible to suppress the drift of the combustion gas from the lower burner and to reliably prevent overheating of the plate width end. As a result, after heating a steel plate made of ultra-low carbon steel in a horizontal continuous heating furnace, even if it is finally annealed in a subsequent reduction furnace, crystals are not coarsened at the width end of the steel plate, and the product quality Can be secured.

  The horizontal continuous annealing apparatus according to the embodiment of the present invention has a configuration of a preheating furnace 2 + a horizontal continuous heating furnace 3 + a reducing furnace 4 as in the conventional horizontal continuous annealing apparatus 1 shown in FIG. Except for the structure of the heat insulation wall, since it is common with the conventional horizontal continuous heating furnace 3 shown in FIG. 2, the description of the common portion will be omitted, and the structure of the heat insulation wall will be described in detail below.

  FIG. 3 is a vertical cross-sectional view in the furnace width direction showing an embodiment of the heat insulation wall according to the present invention. Like the conventional heat insulation wall, the heat insulation wall 7 covers the bottom surface of the hearth roll 5 so as to cover the lower surface. Stand up. The heat barrier 7 is provided with an opening 9 that penetrates in the longitudinal direction of the furnace and has an opening ratio of 30 to 90%. Here, the aperture ratio is a value defined by an aperture ratio (%) = a cross-sectional area in the furnace width direction of the opening 9 / a total cross-sectional area in the furnace width direction below the pass line × 100. The pass line is the level at the upper end of the hearth roll 5.

  Thus, by providing the opening 9 with a predetermined opening ratio penetrating in the longitudinal direction of the furnace in the heat barrier 7, the combustion gas from the lower burner 6 b passes through the opening 9 and flows to the preheating furnace 2 side. Therefore, the flow rate of the combustion gas passing through the gap between the furnace side wall 10 and the width end of the steel plate A is decreased, the gas flow rate is decreased, and the forced convection heat transfer amount from the combustion gas to the width end of the plate Is reduced, and overheating of the end portion of the plate width is prevented.

  Here, the reason why the opening ratio of the opening 9 is set to 30 to 90% is as follows. That is, if the opening ratio is less than 30%, the flow rate of the combustion gas passing through the opening 9 is too small, and the flow rate of the combustion gas passing through the gap between the furnace side wall 10 and the width end of the steel plate A is sufficiently high. This is because overheating at the end of the plate width cannot be sufficiently suppressed. On the other hand, the higher the opening ratio, the lower the flow rate of the combustion gas passing through the gap, which is preferable from the viewpoint of preventing overheating of the plate width end, but at least the portion covering the lower surface of the hearth roll 5 is Since it is necessary to leave, it was 90% or less.

  The heat barrier 6 with an opening ratio of 30 to 90% needs to support the weight of the heat barrier 7 itself in the high-temperature heating furnace 3 while covering the lower surface of the hearth roll 5 over the entire furnace width. As in the case of the thermal barrier without an opening, a refractory structure using bricks is adopted, and as shown in FIG. 3, the space below the arch is supported by a structure that approximates the arch bridge between the side walls 10 of the furnace. And the opening 9 and the arch structure may support the weight of the refractory structure itself. The figure shows an example in which one leg 11 rising from the hearth 8 is provided at the center of the furnace width, and arches are provided between the leg 11 and the furnace side walls 10 on both sides. It goes without saying that the number, shape, mounting position, etc. of the legs 11 and arches can be appropriately changed according to the furnace width and the brick material used. In addition, it is difficult to increase the aperture ratio beyond about 60% due to structural restrictions in brickwork, but if heat insulating materials such as glass wool and rock wool are used instead of bricks, the opening ratio is increased to 90%. It is easy.

  And in the continuous continuous annealing apparatus 1 which has the heating furnace 3 which installed the heat barrier 7 which has such an opening part 9, when the steel plate A is continuously annealed, on the material of the steel plate A, board width, board thickness, etc. Accordingly, the plate temperature difference between the plate width end portion and the plate width center portion at the outlet of the heating furnace 3 is adjusted to 70 ° C., either or both of the plate passing speed and the burner output of the direct fire burner 6 in the heating furnace 3 are adjusted. Within the range, preferably within 60 ° C., and more preferably within 50 ° C. Thus, by limiting the plate temperature difference between the plate width end portion at the outlet of the heating furnace 3 and the plate width center portion as small as possible, the plate temperature difference at the outlet of the subsequent reduction furnace 4 is also reduced, and the plate width end portion is reduced. Is not heated excessively, and coarsening of crystal grains is effectively prevented.

  Alternatively, depending on the material, width, thickness, etc. of the steel plate A to be annealed, both the plate speed and the burner output of the direct fire burner 6 in the heating furnace 3 are adjusted, and the heating furnace 3 After passing, the difference in the ferrite crystal grain size between the plate width center portion and the plate width end portion of the steel plate A after annealing reduced in the reduction furnace 4 is within 2.0, preferably within 1.5, more preferably 1. It may be controlled to be within 0. Thus, product quality can be ensured more reliably by making the ferrite crystal grain size distribution in the plate width direction of the steel plate A after annealing as uniform as possible.

  Horizontal continuous annealing apparatus of an actual machine (as shown in FIG. 1, it has a configuration of a preheating furnace 2 + a heating furnace 3 + a reduction furnace 4, and the heating furnace 3 includes two hearth rolls 5 below each hearth roll 5. In which a heat insulating wall 7 is installed), using a conventional heat insulating wall without an opening (comparative example), and each heat insulating wall with an arch structure shown in FIG. Comparison of operation (invention example) after remodeling to provide a part was performed. In addition, the steel plate to be annealed was the same material (IF steel), plate width (1.6 mm), and plate thickness (1219 mm) before and after modification. In each of the comparative example and the inventive example, the burner output is sequentially reduced to 100, 80, and 40%, and the plate temperature at the center of the plate width at the heating furnace outlet is the target heating temperature at each burner output. The plate passing speed was adjusted to be 780 to 820 ° C.

  The operation results are shown in FIG. 4 and FIG. As shown in FIG. 4, the plate temperature difference between the plate width end (position 15 mm closer to the plate width center from the plate width end surface of the steel plate) and the plate width center is about 100 ° C. at a burner output of 100%. The burner output is reduced to about 70 ° C only by reducing the burner output to 40%. In the example of the invention, the burner output is already reduced to about 50 ° C at 100%, and when the burner output is reduced to 40%. It further reduced to about 30 ° C.

  Moreover, as shown in FIG. 5, after passing through the heating furnace, the ferrite crystal grain size of the plate width end portion of the annealed steel plate subjected to reduction annealing in the reduction furnace (position closer to the center of the plate width of 15 mm from the plate width end surface of the steel plate) is In the comparative example, when the burner output is 100% and 80%, it is 6.0 or less, which does not satisfy the acceptance criteria (7.0 or more) of the product, and 7.0 is finally obtained when the burner output is reduced to 40%. In the example of the invention, while the burner output 100% is 6.0, which is less than the product quality acceptance standard, the burner output is reduced to 80% by reducing the burner output to 80%. It was found that 7.0 was obtained. (Note that the ferrite crystal grain size at the center of the plate width after annealing was maintained at about 9.0 regardless of the burner output in both the comparative example and the invention example.)

  And, as the burner output is higher, the plate passing speed can be increased, so instead of the thermal barrier without the opening of the comparative example, by adopting the thermal barrier provided with the opening having the predetermined opening ratio of the invention example, It was confirmed that productivity could be greatly improved while ensuring the product quality of the steel sheet after annealing.

It is a furnace longitudinal direction sectional view showing a schematic structure of a horizontal type continuous annealing apparatus. It is a furnace longitudinal direction longitudinal cross-sectional view which shows schematic structure of a horizontal type continuous heating furnace. It is a width direction longitudinal cross-sectional view which shows schematic structure of the horizontal type continuous heating furnace which concerns on embodiment. It is a graph which shows the relationship between the burner output of a direct-fired burner, and the board temperature difference of a board width edge part and a board width center part in a horizontal continuous heating furnace. It is a graph which shows the relationship between the burner output of the direct-fired burner of a horizontal continuous heating furnace, and the ferrite crystal grain size of the board width end part after annealing annealed by the reduction furnace. It is a width direction longitudinal cross-sectional view which shows schematic structure of the conventional horizontal type | mold continuous heating furnace. It is a graph which shows the plate | board temperature of the plate width edge part and plate | board width center part in each exit of the conventional horizontal continuous heating furnace and a reduction furnace. It is a graph which shows the influence of plate temperature and holding time which has on a ferrite crystal grain size. It is a fragmentary perspective view which illustrates notionally the flow of the combustion gas in the conventional horizontal type continuous heating furnace.

Explanation of symbols

1: Horizontal continuous annealing device 2: Preheating furnace 3: Horizontal continuous heating furnace (heating furnace)
4: Reduction furnace 5: Hearth roll 6: Direct fire burner 6a: Upper burner 6b: Lower burner 7: Heat barrier 8: Furnace 9: Opening 10: Furnace sidewall 11: Leg A: Steel plate B: Combustion gas flow

Claims (4)

A preheating furnace that preheats the steel sheet with the combustion gas of the horizontal continuous heating furnace described later, a horizontal continuous heating furnace that heats the preheated steel sheet with a hearth roll while heating it with a direct-fired burner, and the heated steel sheet is reduced. A horizontal continuous heating furnace used in a horizontal continuous annealing apparatus having a reduction furnace for annealing while
An opening portion penetrating in the longitudinal direction of the furnace having an opening ratio of 30 to 90% defined by the following formula is provided on a heat insulating wall standing on the hearth and covering the lower surface of the hearth roll. Horizontal continuous heating furnace.
Formula Opening ratio (%) = furnace width direction cross-sectional area of the opening / furnace width direction total cross-sectional area below the pass line × 100   A steel plate at the outlet of the horizontal continuous heating furnace, wherein the steel plate is heated using the horizontal continuous heating furnace according to claim 1 and subsequently annealed while reducing the heated steel plate in the reduction furnace. A continuous annealing method of a steel sheet, characterized by adjusting a plate passing speed and / or a burner output of the direct fire burner so that a temperature difference between a plate width center portion and a plate width end portion is within 70 ° C.   A steel sheet is heated using the horizontal continuous heating furnace according to claim 1, and subsequently subjected to continuous annealing while reducing the heated steel sheet in the reduction furnace, and the sheet width of the steel sheet after annealing. A continuous annealing method for a steel sheet, characterized by adjusting a sheet passing speed and / or a burner output of the direct-fired burner so that a difference in ferrite crystal grain size between a center part and a sheet width end part is within 2.0.   The method for continuous annealing of a steel sheet according to claim 2 or 3, wherein the steel sheet is made of ultra-low carbon steel.

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