JP2004337985A - Water tank for water cooling of billet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a water tank for water cooling of a billet capable of reducing partial brightness unevenness and scab. <P>SOLUTION: The water tank for cooling the billet by immersing the billet in water is provided with a billet supporting section for supporting the billet in the water tank in such a manner that its broad surfaces turn into top and under surfaces and a water injector which injects the water to the under surface of the billet supported by the billet supporting section. The water injector is preferably so disposed that its water injection direction is made into a direction perpendicular to or diagonal with the under surface of the billet and at this time, the distance between the water injection position and the under surface of the billet is preferably set at 30 to 500 mm. By such water tank, the billet can be immersed into the water in such a manner that its broad surfaces turn into the top and bottom surfaces and the water can be injected to the under surface of the billet. As a result, the partial brightness unevenness and scab which occur when especially the continuously cast chromium steel billet containing 5 to 30wt% Cr is rolled down to a cold rolled steel sheet can be reduced. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to water cooling of steel slabs, and more particularly, to an apparatus suitable for rapidly quenching steel slabs at high temperatures such as continuous cast slabs manufactured in a continuous steel casting facility by immersing them in water.

  In the steel manufacturing process, molten steel that has been refined and adjusted to a predetermined component composition is made into a slab by a continuous casting method or an ingot-making method, and then hot-rolled or cold-rolled into a steel material having a predetermined shape. It is generally done. In order to avoid the transformation that deteriorates the surface quality and internal quality of the steel material, especially in the process of cooling the hot slab after solidification in such a process, and to avoid the precipitation of undesired precipitates, Pieces may be quenched in water.

  For example, if a continuous cast slab of stainless steel is allowed to cool as it is after casting, alloying elements such as chromium in the steel and carbon are combined into carbon in the cooling process, which selectively precipitates at crystal grain boundaries, A chromium deficient layer may be formed near the precipitate. When a slab containing such non-uniformity of components is rolled, particularly when hot rolling is further performed and then cold rolling, surface defects such as uneven gloss due to the non-uniformity of components described above may occur in the steel sheet. There is.

  In addition, periodic irregularities (oscillation marks) are formed on the surface of the continuous cast slab due to vertical vibration (oscillation) of the mold. Ni-enriched surface segregation formed in the recesses (valleys) of the oscillation marks becomes a grain pattern defect after rolling and pickling, which has been a problem.

In order to cope with the above-mentioned problem, the present applicant has previously described a method for producing a stainless steel slab in which a continuous cast slab is rapidly cooled at a predetermined cooling rate or more in Patent Document 1, and a continuous cast slab in Patent Document 2. A method for refining stainless steel slabs, in which slabs were quenched at a surface temperature of 400 ° C. or more, further subjected to shot blasting, and then heated to 1100 ° C. or more to remove scales of the slabs, was proposed. Patent Document 3 proposes a rapid cooling device for a high-temperature slab suitable for rapid cooling in water.
JP-A-6-87054 JP-A-4-266416 JP-A-7-100609

  However, the present inventors processed a continuously cast stainless steel slab by the method described in Patent Documents 1 and 2 using the rapid cooling device for slabs described in Patent Document 3 and heat it. When a stainless steel sheet was manufactured by cold rolling and cold rolling, a situation was encountered in which a part of the surface of the steel sheet was partially observed with surface defects such as uneven gloss and swarf.

  The present invention advantageously solves such problems that were not anticipated in the prior art, and can reduce as much as possible partial gloss unevenness and shedding generated when rolling to a cold-rolled steel sheet. It is an object of the present invention to provide a water tank for water cooling.

  In order to achieve the above object, first, the present inventors hot-rolled and cold-rolled a slab treated by the method described in Patent Document 1 or Patent Document 2 to obtain a stainless steel sheet. The cause of the surface defect occurring on one surface of the steel sheet was investigated in detail. As a result, (1) the case where only the quenching (water cooling) treatment was performed in the same manner as in the method described in Patent Document 1, and (2) the quenching (in the same manner as the method described in Patent Document 2). After the (water cooling) treatment, when a shot blasting treatment was further performed, it was confirmed that any surface defects such as uneven gloss and shading occurred in any of the cases.

  From the above, the present inventors presumed that surface defects such as partial gloss unevenness and bulge were caused by other than shot blasting.

  Next, the present inventors investigated which surface of the slab had a large number of surface defects of the steel sheet. As a result, it was found that there was no surface on the surface corresponding to the upper surface side of the slab, and that it occurred exclusively on the surface corresponding to the lower surface side of the slab.

  Then, the present inventors presumed that the above-mentioned defect was caused by either the process of continuous casting or the process of rapidly cooling (water cooling) a slab.

  Next, the present inventors turned the slab obtained by continuous casting upside down and water-cooled, then hot-rolled and cold-rolled into a cold-rolled steel sheet, and investigated the occurrence of defects on the steel sheet surface. . As a result, it has been found that defects occur only on the steel sheet surface corresponding to the lower surface side of the slab after the reversal. From this, it was presumed that the surface defect of the steel sheet was caused by a phenomenon in the water cooling treatment of the slab.

  Based on this finding, the present inventors presumed that the cooling of the lower surface side of the slab was insufficient or non-uniform during the water cooling treatment of the slab, and examined a method for improving this.

  First, as a method of strengthening and improving the cooling of the lower surface side in the quenching of the slab underwater, a method disclosed in Japanese Patent Application Laid-Open No. 55-147468, that is, a high-temperature slab is immersed in a cooling liquid and compressed gas is applied from below the slab. I tried a method of quenching while forcing out. This method is intended to prevent explosion sound and warpage when the slab is cooled in water, and although the present inventors have tried it, noise reduction and warpage prevention have been somewhat effective. However, no effect was observed in preventing surface defects of the cold-rolled steel sheet.

  Next, the inventors of the present invention corresponded to the correspondence between the surface shape of the slab after cooling and the state of occurrence of the dechromized layer, and the occurrence of steel sheet surface defects in the dechromized layer of the slab and the steel sheet obtained by hot rolling and cold rolling the slab. The correspondence with the position was examined in detail. As a result, a large amount of chromium carbide is deposited, and the dechromized layer is developed, especially in the parts where the slab depressions and oscillation marks are particularly deep, and surface defects occur especially in the steel sheet corresponding to that part. I found that I was.

  From this, the present inventors have found that bubbles and water vapor film of water vapor generated when the slab is rapidly cooled in water are stagnated in the cavities of the slab and particularly deep portions of the oscillation marks, and forcibly eject compressed gas. Such a water vapor film is not removed with sufficient agitation force, preventing the removal of heat from the relevant portion, or the gas itself that has been forcibly ejected stagnates on the lower surface side of the slab, and similarly heat transfer between the slab and water It was presumed that cooling was insufficient.

  Based on the above findings, the present inventors further strengthened the cooling of the lower surface side of the slab, in particular, to wash away the water vapor film stagnant on the lower surface of the slab, and to set a water tank so that water flows to the lower surface of the slab. The inventors have conceived of injecting the cooling water itself, and have finally completed the present invention.

  That is, the present invention provides, in a water tank for cooling a steel slab by immersing the steel slab, a steel slab support portion for supporting the steel slab inside the water tub so that the wide surface thereof is the upper and lower surfaces, and a steel slab support portion. And a water injection device for injecting water on the lower surface of the steel slab. In the present invention, it is preferable that the water injection direction of the water injection device is provided so as to be perpendicular or oblique to the lower surface of the steel slab. The distance between the lower surfaces of the billets is preferably 30 to 500 mm.

  According to the present invention, underwater cooling of a stainless steel continuous cast slab, due to insufficient cooling or uneven cooling of the lower surface side thereof, hot rolling, surface defects of the steel sheet after cold rolling are reduced as much as possible. It has a special industrial effect.

  Further, the present invention is not limited to stainless steel slabs, and can be suitably applied to a case where uneven cooling or insufficient cooling in water on the lower surface side of a slab may cause a problem in material, and the quality of steel material can be reduced. There is also an effect that it can be significantly improved.

  The steel slab that is the object of the present invention is a steel material for producing a final product by processing such as rolling or forging, and is a steel material having a shape in which a water vapor film is likely to stagnate on the lower surface side particularly during cooling in water. Is a piece. Specifically, a slab or bloom having a flat rectangular parallelepiped shape corresponds to this. The direct trigger of the present invention is the non-uniform precipitation of carbides in the continuous cast slab of stainless steel and the resulting defect of the stainless steel plate due to the dechromized layer.However, uneven cooling or insufficient cooling in water on the lower surface side of the slab. Does not particularly ask whether the steel type is stainless steel. Needless to say, the slab may be a slab manufactured by a pressure casting method or a slab obtained by forming an ingot once by an ingot-making method and then subjecting the ingot to slab rolling.

  In the present invention, the steel slab is immersed in water and cooled. This is because the amount of water that can come into contact with the steel slab at a time is much larger in underwater cooling than in spray cooling or the like, and the quenching effect is higher.

  In the present invention, the billet is immersed in water such that the wide surface of the billet becomes the upper and lower surfaces. Here, the wide surface of the billet refers to a surface having the largest area among a plurality of surfaces forming the outer periphery of the billet. Speaking of slabs, these are two surfaces perpendicular to the thickness direction. It is easily presumed that standing a slab and immersing it in water can prevent the water vapor film from staying on the lower surface of the slab. It is performed almost horizontally. For this reason, in order to stand and immerse the slab in the water, it is necessary to provide a slab raising and lowering device, and there is a problem that equipment costs increase.

  Here, the expression that the wide surface of the billet is the upper and lower surfaces may be substantially horizontal, and does not necessarily mean that the wide surface of the billet is strictly perpendicular to the vertical direction. For the purpose of the present invention, it is preferable that the billet be held slightly inclined in order to promote the flushing of water vapor from the underside of the billet. However, when handling a steel slab with a crane, tongs, or the like, if the steel slab is too steeply inclined, handling may be impeded. Therefore, it is desirable to keep the steel slab to such an extent that such a hindrance does not occur.

  The most important point in the present invention is to perform water injection so that water flows on the lower surface of the billet immersed in water. This water jet flushes the stagnant water vapor and other air bubbles and gaseous films on the lower surface of the steel slab due to the momentum of the injected water, causing direct contact heat transfer between the water and the steel slab and turbulence. To increase the heat transfer coefficient.

  It is especially important that even if the average flow velocity of water is sufficiently large to maintain the temperature of the slab surface below 100 ° C, the unevenness Is that a small part of can be made. In such a part, the surface temperature of the slab becomes 100 ° C or more, and boiling occurs to generate steam bubbles.

  From this viewpoint, it is important that the larger the water injection amount is, and that the water is injected from a position closer to the lower surface of the billet. However, even if the water injection amount is increased more than necessary, the heat transfer resistance inside the steel slab becomes relatively larger than the heat transfer resistance between water and the steel slab, and the conduction heat transfer rate inside the steel slab is limited. However, the effect of increasing the water injection amount is saturated.

From the above points, the experiment was repeated for steel slabs of various steel types and sizes, and it was clarified that the flow rate of water injection was preferably set to 10 to 150 l / m ² · min with respect to the area of the lower surface of the steel slab. became. If the flow rate of water is less than 10 l / m ² · min, non-uniform cooling may remain partially in a continuously cast slab having a deep oscillation mark or a steel slab having a poor flatness on the wide surface side. Moreover, the effect reaches a substantially saturated at a flow rate of greater than 150l / m ² · min, the increase in flow rate of greater than 150l / m ² · min is the increase in utilities costs, since to increase the equipment load of the pump and piping system Not preferred.

  The water injection direction may be a direction parallel to the lower surface of the steel slab or a direction perpendicular or oblique to the lower surface of the steel slab. Either may be employed. Preferably, from the viewpoint of causing a larger turbulence on the lower surface side of the slab and achieving a high cooling effect and an effect of removing bubbles, the slab is preferably performed in a direction perpendicular or oblique to the lower surface of the slab.

  In this case, the shorter the distance from the water injection position to the lower surface of the billet, the smaller the attenuation of the flow velocity of water from the water injection position to the lower surface of the billet. The linear flow velocity can be increased, which is preferable from the viewpoint of washing out bubbles and cooling the billet. However, if this distance is too small, the water flow that has collided with the underside of the billet and has been inverted will interfere with the water flow to be injected, increasing the pressure loss at the water injection position. As a result, the load on the equipment of the pump and the piping system is significantly increased. Also, as in the case of increasing the amount of water, the heat transfer resistance inside the slab is relatively larger than the heat transfer resistance between water and the slab. Therefore, the effect of reducing the distance is no longer saturated. In consideration of these, it is preferable to set the distance from the position of the water injection to the lower surface of the billet to 30 to 500 mm. When the distance between the water injection position of the water injection device and the lower surface of the billet is less than 30 mm, the effect reaches saturation and the equipment load is unnecessarily increased.

  On the other hand, increasing the distance between the water injection position and the lower surface of the billet decreases the flow velocity of water when reaching the lower surface of the billet, and requires increasing the depth of the water tank, which increases equipment costs. If the distance between the water injection position and the lower surface of the slab exceeds 500 mm, non-uniform cooling may remain partially in a continuously cast slab having a deep oscillation mark or a slab having a poor flatness on a wide surface side.

  Next, the water cooling method of the above-mentioned steel slab will be described in the case where the steel slab is a chromium-containing steel cast slab. The subject here is a continuous cast slab of chromium-containing steel containing 5 to 30 wt% of Cr, which is likely to cause surface defects when rolled to a steel sheet. In a continuous cast slab of chromium-containing steel containing 5 to 30 wt% of Cr, chromium carbide precipitates particularly during the cooling process, and surface defects are likely to occur when the steel sheet is rolled due to the chromium carbide. As the type of continuous casting, a vertical type, a vertical bending type, a fully curved type, a horizontal type, and the like are known, but the present invention is not particularly limited to such a type.

  In the present invention, the surface temperature of the chromium-containing steel slab to be cooled by immersion in water is set to 500 ° C. or higher before cooling. When the slab surface temperature is less than 500 ° C, a large amount of chromium carbide is precipitated on the slab surface layer, so that even with the water cooling method of the present invention, it is possible to sufficiently reduce the surface defects of the rolled steel sheet. difficult.

  Next, a specific method for setting the surface temperature of the slab before cooling to 500 ° C. or higher will be described.

  In the continuous casting of steel, molten steel is first poured into an internal water-cooled mold with both ends open, and after solidifying the outside, it is continuously drawn out by guide rolls and sprayed with cooling water to cool ( This is called secondary cooling) to completely solidify the inside. After being completely solidified, it is cut to a predetermined length by a flame of oxygen and a hot gas (this is called a torch cut) to obtain a slab. The surface temperature of the slab after torch cutting differs depending on the manner of the secondary cooling. Also, the slab surface temperature changes by cooling to the atmosphere depending on the elapsed time of the slab after torch cutting.

  Therefore, in the present invention, it is desirable to adjust the conditions of the secondary cooling, the casting speed, the elapsed time from the torch cutting to the start of the submersion cooling in water, and adjust the surface temperature of the slab before cooling to 500 ° C. or more.

  Thus, the slab whose surface temperature has been adjusted to 500 ° C. or higher is immersed in water, and cooled by the above-described method for cooling a slab until the slab surface temperature becomes 400 ° C. or lower.

  By immersing in water in this way (quenching), the slab is cooled from a high temperature range of 500 ° C or higher where chromium carbide does not precipitate on the surface layer to a temperature range of 400 ° C or lower where chromium carbide does not precipitate at the grain boundaries, Grain boundary precipitation of chromium carbide can be avoided. During this cooling, the slab may be cooled until the center of the slab becomes 400 ° C or less, but in this case, the slab must be immersed in water for a long time, which impairs productivity. become.

  Therefore, if the slab is cooled by immersion in water, the slab is taken out during the cooling, and post-treatment is performed, the time required for immersion cooling in water is reduced, and the productivity is improved.

  In general, a slab that is being cooled while being immersed in water has a temperature distribution in which the surface has a low temperature and the inside has a higher temperature. If a slab having such a temperature distribution is taken out of water and left in the air, spontaneous cooling to the air occurs, while heat moves from a high temperature portion inside to a low temperature portion on the surface. For this reason, the surface temperature of the slab rises, shows a peak at a certain point in time, and then falls slowly, resulting in a recuperation phenomenon.

  Therefore, in the case of a chromium-containing steel slab containing 5 to 30 wt% Cr, even if the slab immersed in water and being cooled is taken out and allowed to stand, if the peak temperature at the time of reheating does not exceed 400 ° C, Thus, precipitation of chromium carbide can be avoided.

  Further, according to the findings of the present inventors, when a chromium-containing steel slab is rolled into a steel sheet, surface defects are caused by extremely fine precipitates and abnormal structures in the outermost layer up to 1% of the slab thickness. If the precipitation of chromium carbide can be avoided at least in this range, the occurrence of surface defects due to the precipitation of chromium carbide can be prevented.

  Therefore, in the present invention, when cooling a chromium-containing steel slab containing 5 to 30% by weight of Cr, the cooling time for immersing the slab in water is determined by taking the slab out of the water and leaving it to stand. The maximum reheat temperature at a position within 1% of the thickness was set so as not to exceed 400 ° C. FIG. 6 schematically shows a situation in which the surface temperature of the slab is restored by the cooling time during which the slab is immersed in water. In case 1, the cooling time for immersion in water is insufficient, and the surface temperature of the slab exceeds 400 ° C. due to reheating. In case 2, the cooling time for immersion in water is appropriate, and the surface temperature of the slab due to reheating is suppressed to 400 ° C. or less.

  Note that the temperature distribution in the slab is usually difficult to measure, so it is better to estimate it by heat transfer calculation. Although heat transfer calculation may be performed in three dimensions, as shown in FIG. 7, it is convenient to perform two-dimensional calculation on a representative cross section at the central position in the longitudinal direction of the slab (1 / 2L, L: slab length). The degree of coincidence with actuality is also high, which is preferable. Because, the position where the temperature becomes the highest due to recuperation is the longitudinal center of the slab, and in the longitudinal central position, the heat transfer in the longitudinal direction is almost zero, and in the cross section at the longitudinal central position, This is because even if the heat transfer calculation in the two-dimensional direction is performed, the deviation from the actual one is small. Here, as an initial condition, it is assumed that the slab internal temperature before immersion cooling in water is equal to the surface temperature and uniform. As the boundary condition during immersion in water, the heat transfer coefficient of forced convection using the flow velocity of water is used. Further, the heat transfer calculation after withdrawing from the water uses the heat transfer coefficient of natural convection in the atmosphere. By performing the numerical calculation in this manner, it is possible to estimate the temperature distribution in the slab at the time of immersion cooling in water and thereafter at the time of reheating, and to estimate the temperature history at the 1% thickness position below the surface layer, which is a problem. Can be.

  The slab cooled by being immersed in water for a predetermined time is suppressed in the precipitation of chromium carbide under the surface layer, and there is no formation of a dechromized phase that causes surface defects.Therefore, by using such a slab, Thus, a steel sheet having very few surface defects can be obtained.

  Next, a cooling water tank suitable for carrying out the method for cooling a billet of the present invention will be described. One embodiment of the cooling water tank is shown in FIGS.

  The steel slab cooling water tub 1 of the present invention is a water tub for cooling a steel slab by immersing the steel slab in water, and a steel slab supporting portion 2 for supporting the steel slab inside the water tub 1 so that its wide surface is horizontal. And a water injection device 3 for injecting water on the lower surface of the steel slab 4 supported by the steel slab support portion 2.

  The basic shape of this water tank is preferably a water tank whose upper surface is open so that a steel slab can be taken in and out from above, as disclosed in, for example, JP-A-8-253807 and JP-A-7-100609. Applicable to With such a water tank, the steel slab can be immersed in the direction as it was produced by the continuous casting facility or the slab rolling facility, that is, in such a direction that the wide surface becomes the upper and lower surfaces. There is no need to limit the other shapes of the aquarium. In addition, it is preferable that a plurality of steel slabs can be immersed side by side in consideration of the time required for cooling the slabs and the production rate.

  The billet support portion 2 supports the wide surface of the billet 4 so as to be the upper and lower surfaces, and can hold the lower surface of the billet 4 at a distance from the bottom of the water tank, and further sprays water onto the lower surface of the billet. The structure is not particularly limited as long as the injection device 3 can be installed and a water channel through which the injected water can flow can be secured. For example, a structure in which a member different from the water tank is attached to the bottom of the water tank by laying a rail on the bottom of the water tank 1 or welding the steel plate 2d vertically as shown in FIG. A structure in which the bottom of the water tank is partially protruded may be used, or a structure in which the steel slab 4 can be held by the support member 2a or the support member 2b from the side wall 1a of the water tank or the upper part of the water tank as shown in FIGS. Is also good. FIG. 4 shows an example in which the billet supporting portion 2 is attached to the side wall of the water tank, and FIG. 5 shows an example in which the billet supporting portion 2 is suspended on the upper end of the side wall of the water tank. 4 and 5, the illustration of the water injection device is omitted. In addition, although many other variations are conceivable, as long as the basic technical idea of the present invention is used and an effect equivalent thereto is obtained, the technical scope of the present invention or a range equivalent thereto, regardless of slight differences. It belongs to.

  Further, a water injection device 3 for injecting water so that water flows on the lower surface of the steel slab 4 supported by the steel slab support portion 2 is provided. One example of the water injection device is shown in FIGS. The water injection device 3 includes a water injection nozzle 3a for injecting water onto the lower surface of the billet 4, a water supply pipe 3b for supplying water to the water injection nozzle 3a, and a water supply pipe support 3c for supporting the water supply pipe 3b. The injection water (cooling water) supplied from the water supply pipe 3b is injected to the lower surface of the billet 4 via the water injection nozzle 3a. The water injection nozzle 3a is not particularly limited in its form, but includes an underwater injection nozzle, an injection slit for injecting water in a film form from a slit, an arrangement in which an injection hole is simply provided in a water supply pipe, and an opening in a side wall of a water tank. A device that sprays water therefrom is suitable, and further variations thereof are conceivable. The water supply pipe 3b is held by a water supply pipe support 3c.

  The direction in which water is sprayed may be parallel to the lower surface of the slab, or may be perpendicular or oblique to the lower surface of the slab. Either may be employed. Preferably, the injection is performed perpendicularly (FIG. 2) or obliquely (FIG. 3) with respect to the lower surface of the steel slab from the viewpoint of causing greater turbulence on the lower surface of the steel slab and achieving a high cooling effect and a bubble removing effect. Is preferably provided with a water injection nozzle 3a.

  In this case, it is preferable that the distance h from the water injection position to the lower surface of the billet is 30 to 500 mm. The reason is as described above. In the case of FIG. 3, the distance h between the water injection position and the lower surface of the billet may be measured along the neutral axis in the injection direction.

[Example 1 of the present invention]
A SUS 304 stainless steel slab (200 mm thick, 9.0 mm long) immediately after being cast by a continuous casting facility and torch cut into a water tank (length 10 m × width 10 m × water depth 1.2 m) schematically shown in FIGS. 1 and 2. m, a width of 650 to 1600 mm, and a slab surface temperature of 850 ° C.) were immersed in water so that the wide surface of the slab was substantially horizontal, and cooled with water. During water cooling, water was injected from the water injection device 3 to the lower surface of the slab to flow the water. The distance h between the water injection position of the water injection device 3 and the lower surface of the slab (slab) was 130 mm, and the flow rate of the injected water was 50 l / m ² · min. The water tank is designed so that a plurality of slabs can be arranged side by side in consideration of the time required for cooling the slab and the production speed. In the present invention, ten slabs were simultaneously cooled with water. Further, a steel slab supporting portion 2 formed by welding a plurality of steel plates having a thickness of 20 mm in a standing state is provided at the bottom of the water tank so that the lower surface of the slab (steel slab) 4 can be held apart from the bottom of the water tank. It is.

After quenching in water until the center temperature of the slab is 400 ° C or less, pull it out of the water tank, and then heat these slabs (10 pieces) in a slab heating furnace. It was made into a stainless steel plate, and was further subjected to 2B + BA finishing to obtain Example 1 of the present invention. The surface condition of these steel sheets was investigated. As a result, no unevenness or luster was observed on both sides of the obtained stainless steel sheet.
[Example 2 of the present invention]
As in Example 1 of the present invention, a SUS 304 stainless steel slab (200 mm thick, 9.0 m long, immediately after being cast and torch cut by a continuous casting facility using a water tank schematically shown in FIGS. 1 and 2, A slab having a width of 650 to 1600 mm and a slab surface temperature of 850 ° C.) was immersed in water so that the wide surface of the slab was substantially horizontal, cooled with water for 20 minutes, and then pulled out of the water tank. The water injection conditions were the same as those of the first embodiment of the present invention.

  The temperature transition at a position of 1% of the thickness of the slab from the slab surface after being pulled out of the water was predicted in advance by a two-dimensional heat transfer calculation. As a result, it was found that immersion cooling in water had to be performed for 15 minutes or more in order to make the maximum reheating temperature 400 ° C or less. Therefore, the time for cooling by immersion in water was set to 20 minutes.

After being pulled out of the water tank, these slabs (10 pieces) were heated in a slab heating furnace, subjected to hot rolling and cold rolling to form a 1.0 mm-thick stainless steel sheet, and further subjected to 2B + BA finishing. And As a result of investigating the surface condition of these steel sheets, no scabbing or unevenness in gloss was observed on both sides of the obtained stainless steel sheet.
[Example 3 of the present invention]
The slabs (2 pieces) manufactured under the same conditions as in Example 2 of the present invention were heated in a slab heating furnace, and then subjected to hot rolling and cold rolling to form a 0.5 mm thick stainless steel sheet, and further subjected to 2B + BA finishing. Inventive Example 3. As a result of examining the surface condition of these steel sheets, gloss unevenness was not found, but the ridges were found at a surface defect rate of 0.2%. The surface defect rate (%) is represented by (length of coil having defect) / (total length of stainless steel coil) × 100%.
(Comparative example)
On the other hand, when water was cooled in the water tank, compressed air (supply pressure: 5 kgf / mm ² ) was injected instead of water injection from the water injection nozzle, and cooled to obtain Comparative Example 1. The other conditions were the same as those of Example 1 of the present invention. No unevenness or luster was observed on the surface corresponding to the upper surface of the slab of the stainless steel plate of Comparative Example 1, but a surface defect of 1.8% was observed on the surface equivalent to the lower surface of the slab including the uneven glossy portion and the unevenness. Was. Here, the surface defect rate (%) was defined as (length of coil having defect) / (total length of stainless steel coil) × 100%.

  In addition, when water was cooled in the water tank, water was not injected from the water injection nozzle, and cooling was performed. The other conditions were the same as in Example 1 of the present invention. As a result, no unevenness or uneven luster was observed on the surface corresponding to the upper surface of the slab of the stainless steel sheet of Comparative Example 2, but 2.0% of surface defects including the uneven glossy portion and the unevenness were observed on the lower equivalent surface of the slab. Admitted.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic explanatory drawing which shows the structure of the cooling water tank which is one Example of this invention. BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic sectional drawing which shows the structure of the water injection apparatus in the cooling water tank which is one Example of this invention. BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic sectional drawing which shows the structure of the water injection apparatus in the cooling water tank which is one Example of this invention. It is an outline sectional view showing an example of the billet support part in the water tank for cooling of the present invention. It is an outline sectional view showing an example of the billet support part in the water tank for cooling of the present invention. It is a schematic diagram which shows the change of the slab surface temperature when a slab is immersed in water and pulled out of the water during cooling. It is explanatory drawing which shows the position of the typical cross section at the time of calculating the temperature distribution in a slab by heat transfer calculation. It is explanatory drawing which shows the definition of the warpage rate of a slab.

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 1 Water tank 1a Side wall 2 Steel piece support part 2a Support member 2b Support member 2d Steel plate 3 Water injection device 3a Water injection nozzle 3b Water supply pipe 3c Water supply pipe support 4 Steel slab (slab)
6 water

Claims (3)

  A water tank for immersing and cooling a steel slab, a steel slab support portion for supporting the steel slab such that the wide surface thereof is the upper and lower surfaces inside the water tub, and a lower surface of the steel slab supported by the steel slab support portion And a water injection device for injecting water to the steel slab.   The water bath for steel slab water cooling according to claim 1, wherein the water injection direction of the water injection device is provided to be perpendicular or oblique to the lower surface of the steel slab.   The water tank for cooling billets according to claim 2, wherein a distance between a water injection position of the water injection device and a lower surface of the billet is 30 to 500 mm.

Images