JP5131229B2 - Slab continuous casting method - Google Patents

Description

  The present invention relates to a continuous casting method for slabs including steel slabs.

  When metals such as steel are continuously cast, central segregation occurs in which impurities contained in the molten steel segregate at the center of the slab. Such central segregation of impurities significantly impairs the homogeneity of the final product and lowers the product quality. Therefore, a continuous casting method capable of reducing the central segregation has been devised.

  In the center segregation of the slab, the solidified shell of the slab solidifies and shrinks at the end of solidification, and as a result, the residual molten metal at the end of solidification flows to wash out the concentrated molten steel near the solid-liquid interface. It is caused by thickening. Therefore, in order to prevent the center segregation of the slab, it is important to remove the cause of the residual molten metal flow. To this end, the slab bulging between the rolls is minimized and the amount of solidification shrinkage of the solidified shell is equivalent. It is known that it is effective to reduce the slab by an amount to be reduced. For example, Patent Documents 1 and 2 disclose a method for lightly rolling a slab in continuous slab casting.

  In continuous casting of slabs, solidification progresses from the upper and lower long sides and the left and right short sides of the slab, and at the end of solidification, as shown in FIG. A shell 10 is formed, and an unsolidified portion 9 is left in the vicinity of the thickness center portion. In the vicinity of the short sides at both ends in the width direction, solidified shells are formed over the entire thickness direction, so in order to reduce the thickness of the slab by lightly reducing the slab by an amount commensurate with the amount of solidification shrinkage at the end of solidification. It is necessary to reduce the solidified shell formed near the short sides at both ends in the width direction.

  The slab is lightly reduced during continuous casting by applying a reduction force that overcomes the reduction reaction force when the solidified shell formed near the short side is reduced. The reduction reaction force from the slab is affected by the slab temperature at the slab part to be reduced. As shown in FIG. 9, as the slab temperature decreases, the reduction reaction force increases. Therefore, if the slab temperature is too low, it is difficult to lightly reduce the slab.

  The slab is cooled by spraying cooling water onto the long side surface of the slab using a spray nozzle in the secondary cooling zone from immediately below the continuous casting mold until solidification is completed. Since the corners of the long and short sides of the slab are most likely to be cooled, the temperature is likely to be lowered as compared to the long side surfaces excluding the corners. Therefore, when the slab is lightly reduced during continuous casting, the temperature of the solidified shell formed in the vicinity of the short side becomes too low, and it may be difficult to lightly reduce.

  In slab continuous casting, slabs having various widths are cast. As for the secondary cooling, as shown in FIG. 2 (c), when the secondary cooling spray nozzle is arranged so that the cooling water is spread over the entire long side of the slab with the widest width, the slab with the narrow width is formed. When casting, as shown in FIG. 2 (d), the cooling water from the spray nozzle arranged in the part protruding from the long side cools the short side of the slab, so it is compared with a wide slab. Then, the slab temperature near the shorter side is further lowered, and light reduction is more difficult.

  Regarding the amount of secondary cooling water in the secondary cooling zone, the required amount of cooling water is determined in order to keep the bulging amount of the slab solidified shell during casting within a predetermined range. Therefore, if the amount of secondary cooling water on the long side surface is reduced with the intention of increasing the temperature of the slab corner near the solidification completed part during casting, the amount of bulging during casting exceeds the allowable range, It cannot be used because it causes quality defects such as internal cracks.

  It is conceivable that the range in which the secondary cooling water is dispersed in the width direction of the slab is narrowed so that the secondary cooling water does not hit the corner of the slab. If the secondary cooling water spraying range in the width direction is determined so as to be optimized when casting the slab with the widest width, the slab corner portion is eventually supercooled when casting the narrow slab. On the other hand, if the secondary cooling water collision range is narrowed so that corner supercooling does not occur even when casting a narrow slab, secondary cooling water should not collide over a wide range including the end of the long side during wide casting. Thus, solidification in the vicinity of the width end portion is delayed as compared with the width center portion. As a result, the position of the solidified end becomes non-uniform in the slab width direction, and the shape of the line connecting the solidified end (crater end) exhibits a W shape. In this case, even if light reduction is performed, the optimum center segregation reduction cannot be performed, and the center segregation cannot be sufficiently improved.

  When casting various slab widths, there is a method to adjust the cooling in the slab width direction continuously and steplessly by moving the spray nozzle for the purpose of cooling while preventing overcooling of the corner of the slab. It is disclosed. In the method described in Patent Document 3, the position of the spray nozzle is moved in the vertical direction according to the width of the slab, and in narrow casting, water injection from the spray nozzle outside the width is stopped, This is a method of adjusting the flow rate.

  The method described in Patent Document 4 selects a spray nozzle that operates according to the width of the slab to be cooled, and from at least one of the spray nozzles on the inner side in the width direction and on the outermost side in the same width direction. Describes a secondary cooling method of continuous casting in which gas is blown onto a slab. Cooling water is not sprayed from the spray nozzle outside the width of the slab, and only gas is blown from the spray nozzle near the width end, so that water does not flow into the end or end face of the slab. It is said that the supercooling of the slab end due to heat removal from the water can be suppressed.

In Patent Document 5, as a simulation device for a continuous casting machine, a heat transfer coefficient from a finite element model M _{j of a} cast slab that is continuously guided by rolls R _i and R _{i + 1} and sprayed with cooling water or the like to the atmosphere. hw is the surface temperature of the slab obtained from the heat transfer coefficient hw from the finite element model M _j-1 calculated last time, and the spray amount distribution which is the operational measurement data regarding the spray amount of the cooling water and the like. Thus, there is provided a method capable of accurately simulating the thermal behavior of a slab during casting.

JP 62-33048 A JP-A-62-275556 JP-A-7-195164 Japanese Patent Laid-Open No. 7-9100 JP-A-4-231158

  In a conventional continuous casting apparatus, a single flow rate control is performed for secondary cooling in a certain range in the casting length direction, and separate flow rate control is not performed near the center and end in the casting width direction. . Moreover, the spray nozzle installed between rolls is fixed by the cooling water piping to a spray nozzle, and control which moves a spray nozzle to the width direction is not performed. On the other hand, as the casting width is changed, the position of the spray nozzle is moved in the width direction, or the spray nozzle is moved in the vertical direction as described in Patent Document 3, and further described in Patent Documents 3 and 4. Thus, if it is attempted to stop only the water injection from the spray nozzle at the width end according to the casting width, it is necessary to increase the number of cooling water flow rate control systems, and the spray nozzles in the width direction or the vertical direction. It is necessary to newly install a movement control mechanism that moves.

  However, in the case of such a complicated mechanism, the cost of remodeling the equipment is high and the maintenance is difficult, and furthermore, the control performance cannot be maintained unless sufficient maintenance is performed, but sufficient maintenance is performed in the actual operation. Therefore, it is practically difficult to use equipment that requires sufficient maintenance. Accordingly, there is a problem in that the complicated mechanism as described above cannot substantially control stably.

  In the present invention, when continuously casting slabs of various casting widths while lightly reducing, the slab temperature near the end of the slab width direction near the solidification end is appropriately adjusted without having a complicated secondary cooling mechanism. An object of the present invention is to provide a continuous slab casting method capable of maintaining and reducing the center segregation of a cast slab.

That is, the gist of the present invention is as follows.
(1) For secondary cooling from directly below the mold to a range where the solid fraction _{s s} of the central part of the slab is 0.7, the specific water amount is set to 0.5 to 2.0 liters / kg and secondary cooling is performed. Radiation cooling from the short side surface of the slab is suppressed when the water collision range is the full width of the long side surface of the slab and the solid phase ratio f _s of the slab is at least 0.1 to 0.7. A slab long side surface with a roll in a range where the solid part ratio f _s at the center of the slab is at least 0.3 or more and 0.7 or less, and the amount of reduction per unit time is 0.5 mm / min. Or a reduction of 2 mm / min , the slab center solid phase rate f _s is 0.7, and the slab has a distance of ½ of the slab thickness from the widthwise end of the slab. Continuous slab casting method, characterized in that the thickness direction cross-sectional average temperature is controlled to be equal to or higher than a predetermined temperature. .
(2) The continuous casting method for a slab according to (1), wherein the predetermined temperature is 800 ° C.
(3) In the above (1) or (2), the heat retaining mechanism has a heat insulating plate facing the slab short side surface and disposed at a position where the distance from the slab short side surface is 70 mm or less. The continuous casting method of the slab as described.
(4) The slab continuous casting method according to (3) above, wherein the heat retaining mechanism has a heat insulating plate position changing device for changing the position of the heat insulating plate according to the width of the slab. .

In the present invention, the secondary cooling water collision range is set to the full width of the long side surface of the slab for the secondary cooling from directly under the mold to the range where the slab central solid fraction f _s is 0.7. Regardless of the case, the line connecting the coagulation ends does not exhibit a W shape. In addition, a heat retention mechanism for suppressing radiation cooling from the short side surface of the slab is provided in the range where the solid portion ratio f _s at the center part of the slab is at least 0.1 or more and 0.7 or less. In slab corners do not overcool. As a result, the average temperature in the thickness direction of the slab is predetermined at a distance of ½ of the thickness of the slab from the end in the width direction of the slab at a center solid phase ratio f _s of 0.7. it is possible to control so that the above temperature, when the center solid phase ratio f _s of the slab performs soft reduction to reduction the slab long side surface by the rolls in the range of at least 0.3 to 0.7 Thus, it is possible to reduce the light pressure reaction force and perform good light pressure casting.

It is a figure which shows the roll segment which has the heat retention mechanism of this invention, (a) is a side view, (b) is AA arrow sectional drawing, (c) is BB arrow sectional drawing. It is a conceptual diagram which shows the cross section of the slab secondarily cooled, (a) (b) has the heat retention mechanism of this invention, (c) (d) is a case where it does not have a heat retention mechanism, (e ) Is a diagram showing a cross-section of the slab near the solidification end. It is a conceptual diagram which shows the heat retention mechanism of slab, (a) is this invention, (b) (c) shows a comparative example. It is a figure which shows a secondary cooling pattern. It is a figure which shows the coagulation | solidification terminal part of the slab during casting of a wide material. It is a figure which shows the center segregation result of the slab width direction of a wide material. It is a figure which shows the solidification terminal part of the slab during casting of a narrow material. It is a figure which shows the center segregation result of the slab width direction of a narrow material. It is a figure which shows the relationship between slab temperature and rolling resistance.

In the present invention, the determination of the control range of the secondary cooling control and the installation position of the heat retention mechanism for suppressing the cooling of the slab short side surface are performed using the solid phase fraction f _s of the slab as an index. Therefore, first, calculation of the center part solid phase ratio f _s of the slab will be described. Incidentally, the center solid fraction f _s indicating the light reduction range means the solid fraction at the center of the slab width and the center of the slab thickness.

The total width of the slab is W, the arbitrary position in the width direction of the slab is y, the arbitrary position in the casting direction of the slab is x, and the slab thickness at an arbitrary point (x, y) of the slab The solid fraction at the center of the direction is defined as the central solid fraction f _s (x, y). An example of how to obtain the central solid phase ratio f _s (x, y) in the slab thickness direction at an arbitrary point (x, y) of the slab will be described.

When performing heat transfer solidification calculation in a cross section (transverse cross section) perpendicular to the casting direction of the slab (hereinafter sometimes referred to as C cross section), as a boundary condition for the calculation, the inside of the mold is the following well-known formula (1) ( For example, the heat removal amount q to the mold can be determined by JP-A 2000-119726. In the following formula (1), x is a casting direction distance from the meniscus, Vc is a casting speed, and α and β are constants.
q = α × (x / Vc) ^β (1)

  Next, for heat transfer solidification calculation after the mold, heat removal by cooling water sprayed from the spray nozzle to the slab, heat removal by the continuous casting roll, and heat removal by radiation of the uncooled part are given as heat transfer coefficients. be able to. Here, the continuous casting roll means a roll disposed after the mold.

For example, the technique disclosed in Patent Document 5 can be applied to the calculation of the coefficient. For example, the heat transfer coefficient hw from the finite element model M _{j of the} slab guided by the rolls R _i and R _{i + 1 and} sprayed with cooling water to the atmosphere is calculated from the previously calculated finite element model M _j−1. It can be calculated based on the surface temperature of the slab determined from the heat transfer coefficient hw of the slab and the spray amount distribution which is actual measurement data regarding the spray amount of the cooling water. The slab is cooled by cooling water and air as actual measurement data sprayed on the elliptical spray region, and the measured data is also used for the spray amount distribution in the spray region.

Here, the heat transfer rates hr and ha of heat escaping from the slab to the rolls R _i and R _{i + 1} or from the air cooling part of the slab to the atmosphere are not affected by the cooling water and depend on the temperature of the atmosphere. Further, the heat transfer rate hm from the molten steel in the unsolidified portion or the slab to the mold is almost constant regardless of the surface temperature of the slab. On the other hand, the heat transfer rate hw of heat that escapes from the water-cooled portion of the slab to which the cooling water is sprayed to the atmosphere depends on the amount of cooling water and air sprayed on the water-cooled portion and the surface temperature of the water-cooled portion.

Furthermore, the heat transfer coefficient hw related to the finite element E corresponding to the position of the water-cooled part is defined by the following formula, and is included in the surface temperature T of the slab for each finite element E, the spray region of the spray nozzle, and the spray amount distribution. The cooling water amount W and the air amount A obtained for each finite element E are applied.
hw = hw (T, W, A) = α × T ^f × W ^g × A ⁿ (2)
However, in the above formula, α, f, g, and n are coefficients previously obtained by experiments or the like for each nozzle, and are recorded in a database or the like together with the spray area and spray amount for each nozzle, Calculate based on that.

  By the above calculation, the boundary condition of the slab surface at any casting direction position x and width direction position y can be obtained.

Next, using the above boundary conditions, the “heat transfer solidification calculation” can be performed from the temperatures at any casting direction position x and width direction position y by heat transfer calculation. The central solid phase ratio f _s (x, y) at the thickness center of the piece can be obtained.

Here, the central solid fraction f _s (x, y) is the solid fraction at the center of the thickness. When the liquidus temperature of the molten steel is T _LL and the solidus temperature is T _SL , The central solid fraction f _s (x, y) at the temperature T _m (x, y) at an arbitrary position (x, y) of the thickness center is expressed as f _s (x, y) = (T _LL −T _m (X, y)) / (T _LL −T _SL ). The temperatures of T _LL and T _SL can be obtained using the Hirai equation. It can also be experimentally determined in advance corresponding to a desired molten steel component. Regarding the calculation of T _LL and T _SL using the Hirai equation, for example, liquidus temperature T _LL : Sakai and Steel, Nippon Steel & Steel Association, Vol. 55, no. 3 (19690227) S85, with reference to the Japan Iron and Steel Institute, solidus temperature T _SL : Hirai, Kanamaru, Mori; Gakken 19 Committee, Fifth Solidification Phenomenon Council document, Solidification 46 (December 1968 ) To calculate each.

  In addition to the technique disclosed in Patent Document 5 as heat transfer solidification calculation, the enthalpy method and equivalent specific heat method described in “Introduction to computer heat transfer, solidification analysis, written by Onaka, published by Maruzen Co., Ltd.” It can be applied or done.

  The present invention will be described with reference to FIGS.

  In a continuous casting apparatus to which the slab continuous casting method of the present invention is applied, as shown in FIG. 1, upper and lower rolls 2 are disposed on a roll segment 1 so as to sandwich a slab 5, and a slab 5 is formed by a spray nozzle 3. Secondary cooling is performed. In the heat retaining mechanism 11 of the present invention, the heat insulating plate 12 is disposed so as to face the short side surface 7 of the slab 5. The heat insulating plate 12 is connected to the heat insulating plate position changing device 13 and can be arranged so that the heat insulating plate 12 is always close to the short side surface 7 of the slab 5 even when the width of the cast slab changes. .

  The secondary cooling employed in the present invention may be water cooling in which only water is ejected from the spray nozzle, but is preferably air-water spray cooling in which air is sprayed together with water from the spray nozzle.

In the present invention, as shown in FIG. 2 (a), first, the secondary cooling from the area 22 immediately below the mold to the range where the slab center solid phase ratio f _s is 0.7, the collision of the secondary cooling water 4 occurs. The range is the full width of the slab long side surface 6. As was conventionally done, the secondary cooling water 4 does not collide with the entire width of the long side surface 6 of the slab without adopting cooling that does not cause the secondary cooling water to collide with the width end portion of the long side surface of the slab. The spray nozzle 3 is arrange | positioned so that it may, and the cooling water 4 is injected. As a result, regardless of the slab width, the line connecting the solidification ends does not exhibit a W shape, and solidification can be completed at a uniform position in the width direction.

  When the maximum casting width of the continuous casting machine targeted by the present invention is, for example, 2200 mm, the injection region of the secondary cooling water injected from the spray nozzle in the secondary cooling region covers the entire range in the width direction of 2200 mm. Even when the cast slab has a narrow width of, for example, 1400 mm, the injection region of the secondary cooling water is the same as that at the time of casting of the maximum casting width, and the injection region of 2200 mm is maintained.

As described above, the range in which the collision range of the secondary cooling water is the entire width of the long side surface of the slab is at least from the region 22 immediately below the mold to the range in which the solid phase fraction f _{s of the} slab becomes 0.7. Upon performing soft reduction of the slab during casting, the center solid phase ratio f _s is even attempts to soft reduction to a region of more than 0.7, the billet is too hard, the effect of soft reduction is not reflected. Therefore, the light pressure is performed in a region where the central solid phase ratio f _s is 0.7 or less. Thus, at least the center solid phase ratio f _s the range of the collision range of the secondary cooling water with the entire width of the slab long side surface and to the extent that the 0.7, slab width at the time of performing the soft reduction The solidification uniformity in the direction can be sufficiently ensured.

In the present invention, the amount of specific water is set to 0.5 to 2.0 liters / kg for the secondary cooling from directly under the mold to the range where the solid fraction f _{s at} the center of the slab is 0.7. This is a range that covers the specific water amount in a speed range of about 0.3 to 2.0 m / min, which is a casting condition for casting with a normal slab continuous casting equipment. If the specific water amount is too small, the ability to stably cool the slab may be insufficient and stable casting may not be possible. However, if the specific water amount is 0.5 liter / kg or more, the cooling capacity is sufficient. On the other hand, even if the specific flow rate exceeds 2.0 liter / kg, the effect of improving the cooling capacity of the slab is saturated, so the upper limit was made 2.0 liter / kg.

In the present invention, a heat retaining mechanism 11 for suppressing radiation cooling from the short side surface 7 of the slab is further provided in the range where the solid part solid fraction f _s of the slab is at least 0.1 or more and 0.7 or less.

As described above in the present invention, the center solid phase ratio f _s of the slab from immediately below the mold for secondary cooling to the extent that the 0.7, and the collision range of the secondary cooling water with the entire width of the slab long side surface. In the case of a conventional casting method, even when casting a slab having the maximum width (FIG. 2 (c)), by pouring secondary cooling water over the entire width of the long side surface of the slab, It was not possible to prevent the portion 8 from being overcooled. In the present invention, as shown in FIG. 2 (a), it is possible to prevent overcooling in the slab corner portion 8 by providing a heat retaining mechanism 11 for suppressing radiation cooling from the short side surface 7 of the slab. it can. As a result, when casting the maximum width slab first, even if secondary cooling water is poured over the entire width of the long side surface of the slab, the slab temperature at the corner of the slab can be kept high. .

Conventionally, when the slab width to be cast is narrow, as shown in FIG. 2 (d), the spray nozzle 3 for injecting cooling water to the long side surface 6 is larger than the slab width. The spray water from the spray nozzle located on the outside collides with the short side surface 7 of the slab, and the corner 8 of the slab was further cooled. In the present invention, since the center part solid phase ratio f _s of the slab is at least 0.1 or more and 0.7 or less, a heat retention mechanism is provided for suppressing radiation cooling from the slab short side surface. As shown in FIG. 2 (b), the heat retaining mechanism 11 functions to shield water injection from the spray nozzle located on the outer side of the slab 5 to the short side surface 7. Therefore, the secondary cooling water of the present invention has the function of causing the secondary cooling water to collide with the full width of the long side surface of the slab with the widest slab without stopping the water injection from the spray nozzle outside the width. Even in the casting of the narrow slab, it is possible to prevent overcooling of the slab corner portion 8 and to maintain the slab temperature of the slab corner portion 8 at a high temperature.

The heat retaining mechanism 11 of the present invention is arranged in a range where the center part solid phase ratio f _s of the slab is at least 0.1 or more and 0.7 or less. The heat retaining mechanism 11 may be installed in a range where the central part solid phase ratio f _s is less than 0.1. However, since the temperature of the slab is high in this range, the center part and the end part in the width direction of the slab are used. Since there is no large temperature difference, heat insulation is not always necessary. That is, it is only necessary to install the heat retaining mechanism 11 in the range where the central portion solid phase ratio f _s is 0.1 or more. However, in order to easily control the temperature in the vicinity of the width direction end of the slab, at least one or more upstream roll segments located upstream from the casting range in which light reduction starts, the end in the width direction of the slab It is preferable to provide a heat retaining mechanism for suppressing radiation cooling in the vicinity of the part. On the other hand, as described above, soft reduction is the center solid phase ratio f _s is performed in 0.7 following areas. Therefore, the temperature of the slab corner portion when performing light reduction can be sufficiently ensured by setting the range in which the heat retaining mechanism is provided to at least the range where the central portion solid phase ratio f _s becomes 0.7. It should be noted that, depending on factors such as the size of the roll segment, the center solid phase ratio f _s may be expanded to a range exceeding 0.7.

Further, in the present invention, the slab thickness direction cross section 24 at a distance of ½ the slab thickness from the slab width direction end portion where the slab center portion solid phase ratio f _s is 0.7. It is necessary to control the average temperature Ts so as to be equal to or higher than a predetermined temperature. The position of the cross section 24 in the thickness direction of the slab at a distance of ½ the thickness of the slab from the end in the width direction of the slab is as shown in FIG. As described above, soft reduction is the center solid phase ratio f _s is performed in 0.7 following areas. Therefore, it is necessary to sufficiently reduce the reduction force required for light reduction in the slab portion having a central solid phase ratio f _s of 0.7 at the most downstream side where light reduction is performed. In this part, it is necessary that the average temperature Ts of the cross section 24 in the thickness direction of the slab at a distance of ½ the thickness of the slab from the end in the width direction of the slab is equal to or higher than a predetermined temperature. This is because the average temperature Ts of the cross section 24 in the thickness direction of the slab at a distance of ½ the thickness of the slab from the end in the width direction of the slab is the slab temperature that most affects the light pressure reaction force.

  Here, the predetermined temperature means a slab temperature that is a reduction reaction force equal to or less than a maximum reduction force at the time of light reduction set according to a continuous casting facility. The predetermined cooling temperature is changed in advance so that the average temperature in the thickness direction of the slab at a distance of ½ the thickness of the slab from the end in the width direction of the slab becomes various temperatures. It can be determined by carrying out a casting test and measuring the rolling reaction force of the slab at that time.

  In the case of using a normally cast light reduction device that is normally used, it is preferable that the predetermined temperature is 800 ° C. and the cross-sectional average temperature Ts is 800 ° C. or higher. If the average cross-sectional temperature Ts is 800 ° C. or more, for example, when performing a light reduction of 0.8 mm of slab pressure per roll using a roll having a roll diameter of 330 mm during casting of a slab having a thickness of 300 mm, the reduction force Can be reduced to 100 tons or less per roll, and light reduction can be achieved by a normally used light reduction facility. For reference, FIG. 9 shows the relationship between the slab temperature and the rolling resistance when a slab having a uniform temperature is reduced by 0.5 mm using a roll having a roll diameter of 330 mm.

The average temperature Ts of the cross section 24 in the thickness direction of the slab at a distance of ½ the thickness of the slab from the end in the width direction of the slab can be obtained by calculation. In the heat transfer calculation for determining the center solid phase ratio f _s (x, y) at the center of thickness of the slab at the arbitrary casting direction position x and width direction position y, the cross section in the slab thickness direction at arbitrary x and y positions. Since the temperature distribution is calculated, the average temperature Ts of the cross section 24 in the thickness direction of the slab at a distance of ½ the thickness of the slab from the end in the width direction of the slab is calculated using the calculation result of the cross section temperature distribution. Can be requested. About the influence on the slab heat removal by providing the heat retaining mechanism 11 of the present invention, the amount of temperature increase of the cross-sectional average temperature of the desired slab cross section at the desired casting position is the case where the heat retaining mechanism 11 is not installed beforehand. The casting test was conducted, and the result of measuring the surface temperature of the slab was compared with the calculation result, so that the heat transfer coefficient was evaluated in advance, and the desired slab temperature was maintained by reflecting the calculation. It is possible to calculate the cross-sectional temperature at the cross-sectional position. Based on this temperature information, it is possible to calculate the central portion solid phase ratio f _s when the slab is kept warm and the cross-sectional average temperature Ts of a predetermined region.

Thus, the average temperature Ts of the cross-section in the thickness direction 24 of the slab at a distance of 1/2 of the slab thickness from the end portions in the width direction of the center solid phase ratio f _s is strip cast a 0.7 of the slab Is controlled at a required timing during continuous casting, the slab heat transfer calculation is performed at a necessary timing during continuous casting, the temperature Ts is calculated based on the calculation result, and the Ts is equal to or higher than the predetermined temperature. Control may be performed so that For example, the predetermined temperature is 800 ° C., and the cross-sectional average temperature Ts calculated by taking into account the change of the heat transfer coefficient that changes due to the installation of the heat retaining mechanism under the conditions of casting by the method of the present invention is higher than 800 ° C., which is a preferable temperature range. In the case of lowering, the specific water amount may be decreased for each cooling zone within the range of 0.5 to 2.0 liter / kg, and the cross-sectional average temperature Ts may be controlled to be 800 ° C. or higher.

  Conversely, when the cross-sectional average temperature Ts is equal to or higher than a predetermined temperature (for example, 800 ° C.), as shown in FIG. 9, since the rolling resistance becomes smaller, the light rolling in the light rolling facility is performed without any trouble. Therefore, the upper limit of the cross-sectional average temperature is not particularly specified. However, if the average cross-sectional temperature exceeds 1200 ° C. depending on the continuous casting equipment, bulging on the short side may increase. In such a case, it is preferable to set the temperature to 1200 ° C. or less. Moreover, in order to prevent bulging more while performing more stable reduction, it is more preferable that the cross-sectional average temperature is in the range of 900 to 1100 ° C.

  For example, when the cross-sectional average temperature exceeds 1200 ° C., which is a preferable upper limit temperature, the temperature can be lowered to 1200 ° C. or less by controlling the heat retaining mechanism 11 of the present invention. For example, the heat insulating plate 12 and the slab short side Control can be performed by increasing the distance to the surface 7 and reducing the effect of heat retention on the short side surface. In addition, the distance between the heat insulating plate 12 and the short side surface 7 is not changed, and the specific water amount is increased within the range of 0.5 to 2.0 liter / kg for each cooling zone to enhance cooling. Also good.

In the present invention, light rolling is performed to roll down the long side surface of the slab with a roll in a range where the solid part solid fraction f _s of the slab is at least 0.3 or more and 0.7 or less. By setting the center part solid phase ratio f _s to at least 0.3 or more at the light reduction start position, it is possible to sufficiently exhibit the light reduction effect and reduce the center segregation of the slab. On the other hand, since the slab becomes too hard in the region where the solid fraction of the central part f _s exceeds 0.7 and the effect of light reduction is not reflected, the solid fraction of the central part f _s is 0.7 or less under light compression. Do in the area.

  Next, based on FIGS. 1-3, the heat retention mechanism 11 used with the continuous casting method of this invention is demonstrated in detail.

The heat retaining mechanism 11 includes a heat insulating plate 12 that faces the slab short side surface 7 and is disposed close to the slab short side surface 7. The heat insulating plate 12 is spaced from the slab short side surface 7 and faces the slab short side surface 7. The shorter the distance between the slab short side surface 7 and the heat insulating plate 12, the better the heat insulating effect. By setting the distance between the two to 70 mm or less, preferably 50 mm or less, a good heat insulating effect can be exhibited. As the heat insulating material constituting the heat insulating plate 12, a steel plate may be used since it is for preventing radiant heat transfer, but a material selected from heat insulating materials mainly composed of refractory materials such as SiO ₂ , Al ₂ O ₃ , and MgO. Is preferably used.

  As shown in FIG.3 (b), although the shape of a heat insulation board is made into a U shape and a part of long side surface near slab width end part is covered with a heat insulation board, it is also considered, but this invention It is preferable that the shape of the heat insulating plate is not a U-shape. That is, as shown in FIG. 3A, it is preferable to flatten the shape of the heat insulating plate 12 facing the slab short side surface 7. If the shape of the heat insulating plate is U-shaped and covers a part of the long side surface, the heat retention capacity is improved, but as shown in FIG. 14 is born. This is because when scale is deposited, scale indentation occurs particularly on the lower side of the slab, resulting in scale indentation and product defects.

  The heat retaining mechanism 11 of the present invention preferably has a heat insulating plate position changing device 13 for changing the position of the heat insulating plate 12 according to the width of the slab. As the width of the cast slab 5 changes, the position of the heat insulating plate 12 is changed by the heat insulating plate position changing device 13, and the distance between the heat insulating plate 12 and the slab short side surface 7 is always in a suitable range. Can be contained within.

  Incidentally, regarding the center segregation quality of the slab, a sample of the slab was collected, and an etch print of the full width of the C cross section perpendicular to the casting direction (for example, “Tsubaki to Steel: Nippon Steel and Steel Association Vol. 68, No. 4 ( 19198305) S217 "), and the segregated particle size in a predetermined range above and below the thickness center position can be evaluated. Moreover, using a measuring device such as EPMA (Electron Probe Micro-analyzer) on the basis of the component value of the segregation component in the steel, a high concentration portion that is 1.3 times or more of this value is mapped. The thickness may be evaluated.

  The present invention was applied using a vertical bending slab continuous casting apparatus having a machine length of 32 m from directly below the mold 22 to the machine end 23. The width of the maximum width slab to be cast is 2200 mm, and cooling of the secondary cooling zone employs air-water spray cooling.

The type to be cast is a medium charcoal material for thick plates, the maximum width 2200 mm and the narrow width 1800 mm are used as the cast slab width, the cast thickness is 300 mm, and the casting speed is 0.90 m / min. The center part solid phase ratio f _{s of the} slab is 0.1 at a position of about 23 m from directly below the mold, and f _s is 0.7 at a position of about 31 m from directly below the mold.

As the heat retaining mechanism 11 of the present invention, the structure shown in FIGS. 1 and 2A and 2B was used. The short side surface 7 of the slab is kept warm by the heat insulating plate 12 in the range where the solid part solid fraction f _{s of the} slab is 0.1 or more and 0.7 or less. The material of the heat insulating plate 12 is a SiO ₂ —Al ₂ O ₃ refractory, and the heat insulating plate 12 is changed by the heat insulating plate position changing device 13 so that the distance between the heat insulating plate 12 and the slab short side surface 7 becomes 50 mm. The position of was adjusted.

  Three patterns shown in FIGS. 4A to 4C were used as the secondary cooling region pattern. FIG. 4A shows a pattern in which the secondary cooling water is jetted over the entire width of the maximum casting width of 2200 mm over the entire machine length, and is referred to as “full width”. 4B and 4C are referred to as “width cut 1” and “width cut 2”, respectively.

Light reduction was performed by rolling down the long side surface of the slab with a roll in the range where the solid fraction f _{s at} the center of the slab was 0.3 or more and 0.7 or less. The light reduction pattern continuously reduces the unsolidified slab during casting, and the amount of reduction per unit time from the point when the center solid phase ratio f _s of the slab reaches a temperature corresponding to 0.3. In the region up to the time when the solid phase rate f _s reaches a temperature corresponding to 0.7, 0.5 mm / min to 2 mm / min is set, and after that, no reduction is performed in the region until the center of the slab reaches the solidus temperature. Pattern was used.

The average temperature Ts of the cross-section in the thickness direction 24 of the slab center solid phase ratio f _s is strip cast at a distance of 1/2 of the slab thickness in the width direction of the end of the piece cast a 0.7, The temperature at an arbitrary position corresponding to ½ of the slab thickness from the end in the width direction of the slab at the casting position at f _s = 0.7 was calculated by heat transfer solidification and obtained as an average value.

  About the center segregation quality of the cast slab, a slab sample having a length of 10 cm was collected from a slab of a full width of a steady part where light reduction was stable, and an etch print of a full width of a C section perpendicular to the casting direction was collected. The segregated particle diameter in the range of 20 mm above and below the thickness center position was evaluated. The segregation particle size was evaluated by evaluating the thickness of segregation evaluated by etch printing, and obtaining the maximum segregation particle size (mm) and the standard deviation (mm) of the maximum segregation particles.

  Table 1 shows casting patterns and casting results.

  No. Nos. 1 to 4 have a casting width of 2200 mm. 5 to 8 has a casting width of 1800 mm. The heat retention mechanism 11 of the present invention is provided in the present invention No. There are only 4 and 8, and the heat insulation mechanism is not provided for other levels. As a secondary cooling pattern, Comparative Example No. “Full width” was adopted for Examples 1 and 5 and Invention Examples 4 and 8, “Width Cut 1” was adopted for Comparative Examples 2 and 6, and “Width Cut 2” was adopted for Comparative Examples 3 and 7.

  The predetermined temperature (the lower limit temperature enabling light reduction) for Ts of the slab was 800 ° C. from the reduction capability of the light reduction equipment used this time.

  Comparative example No. which adopted the full width as the secondary cooling pattern. Consider 1 and 5. Comparative Example No. For No. 1, the temperature Ts was as low as 770 ° C., and it was not possible to reach the predetermined roll interval set so as to be an appropriate reduction amount. Even if it is the maximum width, since the secondary cooling water is injected over the entire width, the temperature of the slab corner portion is lowered. Comparative Example No. 5 is a narrow-width slab, the temperature Ts is even lower at 760 ° C., and it is in a state where it cannot reach the predetermined roll interval set to be an appropriate reduction amount as in Comparative Example 1, and the roll interval The fluctuation of was large. This is because the secondary cooling water also collides with the short side surface of the slab, so that the cooling of the corner portion is further supercooled. Therefore, Comparative Example No. The casting results of Nos. 1 and 5 could not reach the predetermined light reduction amount, so a good center segregation situation could not be obtained, and the standard deviation of the maximum segregation particle size was large and the variation of the center segregation was further increased. .

  Regarding the casting results with a casting width of 2200 mm, Comparative Example No. 2, 3 and Invention Example No. Contrast 4 is performed.

Calculate the center solid fraction f _s by the solidification heat transfer calculation based on the casting conditions, and calculate the casting length of the solidification end portion (crater end), and define the position of f _s = 0.7 as the crater end, The relationship with the position in the slab width direction is shown in FIG. As is clear from this, Comparative Example No. In contrast to the W-shaped shapes for the samples Nos. 2 and 3, the invention example No. In No. 4, the solidification end length was kept uniform in the width direction.

  As shown in Table 1, Comparative Example No. 2, 3 and Invention Example No. In all cases, the temperature Ts was 800 ° C. or higher, and there was no problem in performing light reduction. On the other hand, with respect to the center segregation results, Example No. of the present invention. Only 4 gave good results. The distribution of the center segregation results in the width direction of the slab was observed and shown in FIG. In FIG. 6, the distance index from the short side of the horizontal axis is that the left end 0 indicates the work side short side position, the right end 1 indicates the drive side short side position, and the center 0.5 indicates the width center. . “Frequency” on the vertical axis indicates that when a slab sample having a length of about 1 m is taken from a predetermined casting position, and 10 pieces of etch preplets having a C width of a full width are taken at a pitch of 10 cm and investigated, The position where the maximum segregation particle size exists in the range of 20 mm above and below the thickness center position is plotted. As shown in FIG. Regarding “2” and “3”, the “frequency” was high in the vicinity of the end near the short side surface in the width direction. Invention Example No. As for No. 4, the crater end shape is uniform in the width direction as shown in FIG. 5, and it is presumed that a good center segregation result could be obtained as shown in FIG.

  Regarding the casting results with a narrow casting width of 1800 mm, comparative example No. 6, 7 and Invention Example No. A comparison of 8 is made.

  For a narrow casting width of 1800 mm, the crater end position was determined in the same manner as in the case of the casting width of 2200 mm, and the relationship with the position in the slab width direction is shown in FIG. As is clear from this, Comparative Example No. 6 and 7 are W-shaped, whereas the present invention example No. In No. 8, the solidification end length was kept uniform in the width direction.

  As shown in Table 1, Comparative Example No. 6,7, Invention Example No. In all cases, the temperature Ts was 800 ° C. or higher, and there was no problem in performing light reduction. On the other hand, with respect to the center segregation results, Example No. of the present invention. Only 8 gave good results. Similar to FIG. 6, the distribution of the center segregation results in the width direction of the slab was observed and shown in FIG. Comparative Example No. For “6” and “7”, “Frequency” was high near the end near the short side surface in the width direction. Invention Example No. For No. 8, good center segregation results could be obtained. As shown in FIG. 7, Comparative Example No. 1 in which “width cut 1” and “width cut 2” were selected as the secondary cooling region pattern. 6 and 7, the crater end shape is W-shaped. For No. 8, the crater end shape is uniform in the width direction, and as a result, it is estimated that good center segregation results could be obtained.

1 Roll Segment 2 Roll 3 Spray Nozzle 4 Cooling Water 5 Slab (Slab)
6 Long side surface 7 Short side surface 8 Corner 9 Unsolidified portion 10 Solidification completed portion 11 Insulation mechanism 12 Insulating plate 13 Insulating plate position changing device 14 Scale deposition 21 Secondary cooling region 22 Immediately under mold 23 Machine end 24 Width direction of slab Section in the thickness direction of the slab at a distance of half the slab thickness from the end of the

Claims (4)

For secondary cooling from just below the mold to the range where the solid phase fraction f _{s at} the center of the slab is 0.7, the specific water amount is 0.5 to 2.0 liters / kg, and the secondary cooling water collides. Heat retention for suppressing radiative cooling from the short side surface of the slab when the range is the full width of the long side surface of the slab and the solid phase ratio f _s of the slab is at least 0.1 to 0.7. A mechanism is provided, and the slab long side surface is rolled with a roll in a range where the solid phase ratio f _s at the center of the slab is at least 0.3 to 0.7 , and the reduction amount per unit time is 0.5 mm / min to 2 mm / The slab thickness direction cross section at a distance of ½ of the slab thickness from the end of the slab in the width direction, with a solid phase ratio f _s of 0.7 at the center of the slab A method for continuously casting a slab, characterized in that the average temperature is controlled to be equal to or higher than a predetermined temperature.   The said predetermined temperature is 800 degreeC, The continuous casting method of the slab of Claim 1 characterized by the above-mentioned. The said heat retention mechanism has a heat insulation board arrange | positioned in the position which opposes a slab short side surface, and the distance from a slab short side surface is 70 mm or less, The continuous of the slab of Claim 1 or 2 characterized by the above-mentioned. Casting method.   The said heat retention mechanism has the heat insulation board position change apparatus for changing the position of the said heat insulation board according to the width | variety of a slab, The continuous casting method of the slab of Claim 3 characterized by the above-mentioned.

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