JP4462052B2 - Continuous casting mold and steel continuous casting method - Google Patents

Description

  The present invention relates to a continuous casting mold and a continuous casting method of steel, and more specifically, suppresses non-uniform solidification near the solidification start position in the mold and prevents surface defects of a slab generated due to the non-uniform solidification. The present invention relates to a continuous casting mold that can be used, and a steel continuous casting method using the mold.

  When steel slab slabs are manufactured on a continuous casting machine, the molten steel is cooled in a water-cooled mold, and the solidified shell is drawn down while producing a solidified shell on the contact surface with the mold. If the cooling unevenness exceeds a certain level, the surface of the solidified shell tends to become uneven, and this unevenness of the surface leads to further unevenness of solidified shell growth. Cracking occurs. When such surface vertical cracks occur, a work for removing surface defect portions such as vertical cracks, that is, a so-called maintenance work, is required prior to conveying the slab to the rolling process. Such a tendency of occurrence of defects tends to increase remarkably with an increase in casting speed (= slab drawing speed), and the casting speed in a steel slab continuous casting machine is about 10 years ago compared to, for example, 10 years ago. The increase in speed by 1.5 to 2 times is also a factor in the increase in maintenance work today. In recent years, the same problem has become apparent even in a thin slab continuous casting machine. In strip casting and continuous casting of foil strips, surface defects cannot be treated and removed, which is a serious problem.

  Therefore, the non-uniform growth of the solidified shell accompanying the non-uniform cooling in the initial stage of solidification in the mold is a direct feed heating (also called “hot charge”) or direct feed rolling which is increasingly being applied to slab slabs due to its excellent economic efficiency. (Also referred to as “direct charge”), and at the same time, hinders high-speed casting, which is a requirement for improving productivity.

  Conventionally, as a method of preventing the occurrence of vertical cracks in continuous cast slabs due to this non-uniform cooling at the initial stage of solidification, uniform slow cooling is performed in the initial stage of solidification (hereinafter referred to as “initial solidification”) to grow the solidified shell It is thought that it is important to equalize and measures are being taken for that purpose.

  For example, Non-Patent Document 1 describes a technique for improving the surface properties of a slab by providing irregularities on the inner wall surface of a mold that comes into contact with a solidified shell.

  In Patent Document 1, as a casting mold for continuous cooling for the purpose of slow cooling, nickel plating is applied to the surface of the mold copper plate to a thickness of 500 μm or more, and the nickel plating layer is subjected to vertical groove processing by chemical corrosion, followed by nickel plating. Further, there is disclosed a continuous casting mold in which a hardened layer (nickel-phosphorous plating or chromium plating) having a thickness of 20 μm or more is further provided on the surface. The technical idea is that the heat transfer resistance is increased by the air layer formed in the flutes (recesses) on the surface of the mold so that it is cooled slowly. The formation of the hardened layer is presumed to be the purpose of maintaining the groove shape during use and maintaining the effect for a long time.

  In Patent Document 2, a recess having a depth of 100 to 1000 μm is provided on the mold surface in contact with the solidified shell, and melted mold powder enters and solidifies in the recess, and the heat transfer resistance due to the fixed mold powder layer. A method of slowly cooling by using the increase of the above is disclosed.

  Further, Patent Document 3 and Patent Document 4 disclose a continuous casting mold in which a surface of a mold copper plate is plated with a dissimilar metal or alloy having a thermal conductivity lower than that of copper. The grid-like plated portion is slowly cooled by utilizing the fact that the thermal conductivity is lower than that of the surrounding copper base material portion.

In the above-mentioned conventional methods for the purpose of slow cooling of the mold, in Patent Document 3 and Patent Document 4, the processed recesses are plated to have a smooth surface when used. This is a technique that requires machining work to provide a recess on the surface of the mold that comes into contact.
Nakai et al., Iron and Steel, Vol.73 (1987), p498-504 JP-A-61-129257 Japanese Patent Laid-Open No. 9-94634 JP-A-7-284896 Japanese Patent Laid-Open No. 10-29043

  However, the above prior art has the following problems.

  In other words, a certain effect is recognized at the beginning of use, but during long-time use and high-speed casting with a large thermal load, cracks are generated from the tip of the processing groove of the mold copper plate, and dissimilar metals and alloys filled in the processing portion. There was a problem that peeling from the boundary with the mold copper plate of the mold occurred and continuous use for a long time became impossible. In addition, when flow control of molten steel in a mold is performed with a moving magnetic field or an alternating magnetic field, if a magnetic material is used as a dissimilar metal or alloy applied to the mold surface, an eddy current is generated there, and the magnetic field is attenuated. For this reason, there has been a problem that the magnetic field generated on the back surface of the mold cannot efficiently penetrate into the molten steel in the mold.

  The present invention has been made in view of the above circumstances, and the object of the present invention is a mold capable of suppressing non-uniform initial solidification in the mold, and ensuring the suppression effect over a long period of time. It is to provide a continuous casting mold that can be maintained, and at the same time, to provide a continuous casting method of steel that can produce a slab having excellent surface properties using this continuous casting mold. That is.

The casting mold for continuous casting according to the first invention for solving the above-mentioned problems is used in the mold as a lubricant between the casting mold and the solidified shell on the mold surface on the side in contact with the solidified shell and having no recess. A coating layer made of TiN, which is a material that makes the wetting angle with the lubricant smaller than any of the wetting angle between the lubricant and copper, the wetting angle between the lubricant and chromium, and the wetting angle between the lubricant and nickel, It is characterized by being deposited in a regular pattern by a physical vapor deposition method (PVD method) .

The casting mold for continuous casting according to the second invention is characterized in that, in the first invention, the coating layer has a thickness of 2 μm to 20 μm.

According to a third aspect of the present invention, there is provided a steel continuous casting method characterized by casting molten steel using the continuous casting mold described in the first or second invention .

  According to the continuous casting mold according to the present invention having the above-described configuration, a material having a smaller wetting angle with the lubricant than copper, chromium, and nickel, in other words, better wettability with the lubricant. Since the coating layer made of a material that increases the contact area with the solidified shell is formed on the mold surface on the side in contact with the solidified shell, there is a portion where the coating layer of the mold is not formed and a portion where the coating layer is formed. It is possible to produce slabs with excellent surface properties without vertical cracks and the like, as if the amount of heat removal is different, the same effect as if a concave portion was formed in the mold was developed, and the initial solidification was made non-uniform. It becomes. In addition, since the coating layer is thin and there is no need to process the recess for forming the coating layer on the mold surface, there is no occurrence of cracking of the mold due to the recess, and it should be used stably over a long period of time. Can do.

  Hereinafter, an example in which the continuous casting mold according to the present invention is applied to a mold of a molten steel slab continuous casting machine will be described in detail with reference to the accompanying drawings. 1-3 is a figure which shows embodiment of this invention, Comprising: It is the front schematic diagram of the casting_mold | template copper plate which comprises the casting_mold | template which concerns on this invention. In addition, the mold of the molten steel slab continuous casting machine is usually a combination of a pair of opposed long-side copper plates and a pair of opposed short-side copper plates that are installed inside the long-side copper plate, A mold having a rectangular cross section is formed, and FIGS. 1 to 3 show examples of a long-side copper plate.

  In continuous casting of steel, molten steel is poured into a water-cooled mold and cooled, and a solidified shell is formed on the contact surface with the mold, and the solidified shell is continuously drawn below the mold to produce a continuous cast slab. is doing. The slab whose surface layer part is a solidified shell and whose inside is an unsolidified phase is cooled by water spray or air mist spray while being supported by a slab support roll below the mold, and eventually solidifies completely to the inside. In this continuous casting, in order to reduce the frictional resistance between the surface of the mold copper plate with copper as the base material and chromium or nickel or an alloy of these metals plated as necessary, and the solidified shell produced, Mold powder, rapeseed oil, etc. are used as lubricants.

The mold powder uses an oxide such as CaO, SiO ₂ , Al ₂ O ₃ , MgO, or MnO as a base material, and Na ₂ O, K ₂ O, CaF ₂ for adjusting the physical properties of the base material to these base materials. , MgF ₂ , Li ₂ CO ₃ , oxides or fluorides or carbonates of alkali metals or alkaline earth metals such as cryolite, and carbon dioxide as a component adjusting material for CaO and SiO ₂ as the main components of the substrate Calcium and diatomaceous earth and carbon materials such as carbon black and artificial graphite are added to adjust the melting rate, and the mold powder added on the molten steel in the mold is melted to melt the mold and solidified shell. It flows into the gap between the two and exerts the function as a lubricant. In addition, the mold powder has a function as an antioxidant, a heat retaining agent, and an absorbent for oxides such as alumina floating from molten steel.

As shown in FIGS. 1-3, in the mold copper plate 1 which comprises the casting_mold | template concerning this invention, it contacts with molten steel, a molten steel is cooled, copper, chromium, A plurality of coating layers 4 made of a material having a smaller wetting angle with the above-mentioned lubricant than nickel, in other words, a material having better wettability with the lubricant, are arranged at equal intervals in the longitudinal and lateral directions or in a staggered manner. It is formed by the vapor deposition method so as to have the regular arrangement. The shape of the covering layer 4 may be any pattern such as a square shape, a circular shape, an elliptical shape, a rhombus shape, or a regular hexagonal shape, or may be a lattice shape. FIG. 1 is a diagram showing an example in which a quadrangular coating layer 4 is staggered on a chrome plating layer 3, and FIG. 2 shows a rectangular coating layer 4 on the chrome plating layer 3 in the vertical and horizontal directions. FIG. 3 is a diagram showing an example in which circular covering layers 4 are staggered on the chromium plating layer 3. From the experience, the size of the coating layer 4 and the interval between the coating layers 4 are about ₂ to 20 mm in size (D ₁ , D ₂ ) and the interval (L ₁ , L ₂ ) between the coating layers 4. The size may be in the range of 0.2 to 1.0 times the size (D ₁ , D ₂ ) of the coating layer 4. However, it may be out of this range.

  FIG. 4 shows a cross-sectional view of the mold copper plate 1 where these coating layers 4 are formed. As shown in FIG. 4, a chrome plating layer 3 is formed on the surface of a smooth copper base material 2 constituting the mold copper plate 1, and a coating layer 4 is formed on the chrome plating layer 3 in a regular pattern. Yes. As is clear from FIG. 4, when forming the coating layer 4, it is not necessary to perform machining for providing a recess on the surface of the copper base material 2 and the surface of the chromium plating layer 3. In FIGS. 1 to 3, a chromium plating layer 3 is provided on the surface of the copper base material 2 and a coating layer 4 is formed thereon, but nickel may be plated instead of the chromium plating layer 3. Moreover, you may vapor-deposit the coating layer 4 directly on the surface of the copper base material 2, without providing these plating layers.

  Currently, TiN is most suitable as a material having a smaller wetting angle with the lubricant than copper, chromium, and nickel. Hereinafter, a case where TiN is deposited as the coating layer 4 will be described as an example.

  If the thickness of TiN as the coating layer 4 is 1 μm, the effect is exhibited. However, in order to reduce the influence of unevenness during vapor deposition and maintain the effect for a long time, it is desirable to coat 2 μm or more. In order to increase the adhesion (peeling resistance, thermal shock resistance) of the TiN film, it is desirable to coat the Ti film by about 0.5 to 3 μm before depositing the TiN film. The Ti and TiN films may each be a single layer, but there is no problem if they are alternately coated in layers like Ti-TiN-Ti-TiN.

  Considering the saturation phenomenon of the effect of the TiN film and the shape of the recess, it is sufficient that the thickness of the TiN layer is about 20 μm. The TiN film can be formed by vacuum deposition, sputtering, or ion plating as long as it is a physical vapor deposition method (PVD method). It is desirable to deposit by the arc ion plating method (AIP method) in the ting method. However, it is not necessary to limit to this. The method for partially forming the coating layer 4 on the surface of the mold copper plate 1 is not particularly limited. (1) After vapor deposition on the entire surface, the deposited film is chemically treated with chemicals so as to have a predetermined shape and arrangement. There is a method of dissolving and removing the film, (2) forming a template that has a predetermined shape and arrangement, masking the surface of the mold copper plate 1 with the template, and then depositing TiN. It doesn't matter how.

  Even when a material other than TiN is vapor-deposited as a material having a smaller wetting angle with the lubricant than copper, chromium, and nickel, vapor deposition is performed along the above. The installation range of the coating layer 4 does not have to be the entire surface of the mold copper plate 1, and the coating layer 4 may be formed only in a range that affects the initial solidification. You may make it half. The mold copper plate 1 includes a mold long side copper plate and a mold short side copper plate, and a coating layer 4 is provided on either or both of them.

  The operation and effect when the molten steel is continuously cast using the mold according to the present invention having such a configuration and using mold powder as a lubricant will be described below.

  The molten steel in the continuous casting mold is covered with mold powder used as a lubricant, and the mold powder is melted by heat from the molten steel. The molten mold powder flows into the gap between the mold that vibrates with a predetermined amplitude and frequency (referred to as “oscillation”) and the generated solidified shell. That is, the mold surface is in contact with the molten steel or the solidified shell through a film-shaped mold powder layer (hereinafter referred to as “powder film layer”) formed by the flow of molten mold powder. Although this powder film layer is cooled and solidified with the passage of time on the mold side, it is continuously lowered at a speed slower than the drawing speed of the slab. The mold surface in the vicinity (hereinafter referred to as “meniscus”) is in contact with the powder film layer that has not yet solidified, that is, the molten mold powder.

  The amount of heat removed from the mold in contact with the molten mold powder is affected by the surface material of the mold. Mold surface material with poor wettability with molten mold powder, that is, material with a large wetting angle with molten mold powder, specifically, copper base material that has not been plated on the surface In the chrome-plated layer or nickel-plated layer, the amount of heat removed from the mold is low, and the wettability with the molten mold powder is relatively better than copper, chromium and nickel, that is, the molten mold powder. In the part of the coating layer 4 made of a material whose wetting angle is smaller than that of copper, chromium, or nickel, the amount of heat removed from the mold is relatively high. That is, by providing the coating layer 4, there is a difference in the amount of heat removed from the mold at each site on the mold surface near the meniscus. The portion of the powder film layer that contacts the mold is solidified, but is solidified in the contacted state, and thus the contact state continues even after solidification. Therefore, regardless of the thermal conductivity of the material constituting the mold surface, the low heat removal portion and the high heat removal portion are regularly mixed side by side in two dimensions.

  In such a situation where the low heat removal portion and the high heat removal portion are regularly mixed two-dimensionally next to each other, even in the case of a steel type that tends to cause non-uniform solidification, for example, medium carbon steel that peritectic solidifies, As described in Patent Document 4, since the portion of the low heat removal portion becomes solidified, the solidification shell is deformed at every interval of the low heat removal portion due to the solidification shrinkage accompanying the γ → δ transformation. As a result, the entire solidified shell is not greatly deformed, no air gap is generated, and uniform solidification is achieved. Thereby, the surface crack of a slab is suppressed and the increase in casting speed is achieved.

  In the continuous casting mold according to the present invention, since the coating layer 4 is formed by vapor deposition, it is possible to prevent peeling of the coating layer 4 and cracking of the mold, and to stably and uniformly over a long period of time. Initial solidification can be achieved. Moreover, since the coating layer 4 is ceramics which are mainly nonmagnetic materials, there is no problem when the flow control of the molten steel in the mold is performed with a moving magnetic field or an alternating magnetic field.

  As described above, by using the continuous casting mold according to the present invention, it is possible to stably secure and maintain uniform initial solidification over a long period of time, and as a result, continuous casting having excellent surface properties. It becomes possible to manufacture a slab stably over a long period of time.

  A template is manufactured so that TiN as a coating layer is equally spaced in the vertical and horizontal directions, and this is placed on the surface of the long copper plate, and a TiN film having a thickness of about 3 μm is regularly arranged by the AIP method. Vapor deposited. The arrangement pattern of the covering layers is shown in FIG. 3 as a square staggered arrangement (hereinafter referred to as “pattern A”) shown in FIG. 1, a square arrangement shown in FIG. 2 (hereinafter referred to as “pattern B”), and FIG. There were three types of circular staggered arrangement (hereinafter referred to as “pattern C”). The vapor deposition range was 50 mm on the upper side and 200 mm on the lower side with respect to the meniscus position in the casting direction of the mold, and vapor deposition was performed on the entire mold width in the mold width direction. Further, for comparison, a mold having chrome plating on the entire surface (conventional example 1), a mold plated with nickel that reproduces Patent Document 4 in a lattice shape (conventional example 2), and a mold having a TiN film deposited on the entire surface (conventional example 2). A comparative example was also prepared.

Using these molds, medium carbon steel having a carbon content of 0.08 to 0.13 mass% (hereinafter referred to as “%”) was molded powder (composition: SiO ₂ ; 35%, Al ₂ O ₃ 6.4%, CaO; 26%, MgO; 1.8%, F; 2.9%, Na ₂ O; 4.8%, 1300 ° C. viscosity = 4.1 Poise), pulling speed 2 Casting was performed at a rate of 3 m / min to produce a slab slab having a thickness of 220 mm and a width of 1500 mm to 1700 mm. The casting amount is 500 charges when the amount of molten steel per charge is 280 tons. Also, the mold surface condition was observed.

  The wetting angle between the mold surface material and the molten mold powder having the above composition was 17 ° for TiN and 44 ° for chromium and nickel. The wetting angle was 1 g (100 mesh) of mold powder powder containing no free carbon, which was solidified after being melted on a SUS304 substrate coated with TiN, chromium and nickel in an Ar atmosphere using a horizontal tubular electric furnace. ) Was molded into a tablet, the furnace temperature was controlled at a heating rate of 10 ° C./min, and the measurement was performed at 1140 ° C.

  Table 1 shows the installation conditions of the coating layer on the mold surface and the test results. As is clear from Table 1, by using the mold of the present invention, the vertical cracks on the slab surface and the mold surface can be stably improved in any of Pattern A, Pattern B, and Pattern C. Was confirmed.

FIG. 3 is a schematic front view of a part of the long-side copper plate of the mold constituting the mold according to the present invention. FIG. 3 is a schematic front view of a part of the long-side copper plate of the mold constituting the mold according to the present invention. It is a front schematic diagram of a part of a mold long side copper plate constituting a mold according to the invention. It is sectional drawing of the casting_mold | template long side copper plate shown in FIGS.

Explanation of symbols

1 Copper mold plate 2 Copper base material 3 Chromium plating layer 4 Coating layer

Claims (3)

On the mold surface that is in contact with the solidified shell and does not have a recess, the wetting angle of the lubricant used in the mold as the lubricant between the mold and the solidified shell, the wetting angle between the lubricant and copper, and the lubricant That the coating layer made of TiN, which is the material that makes the wetting angle between the chrome and chromium, and the wetting angle between the lubricant and nickel , is deposited in a regular pattern by physical vapor deposition (PVD method). Features a continuous casting mold. The continuous casting mold according to claim 1, wherein the coating layer has a thickness of 2 μm to 20 μm. A continuous casting method of steel, wherein molten steel is cast using the continuous casting mold according to claim 1 .

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