JP2008030123A - Mold for continuously casting steel difficult to develop heat crack at meniscus part - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a mold for continuously casting steel composed of a copper alloy, which can restrain erosion or alloying phenomenon caused by low melting point metal elements having melting points of not more than 700°C represented by impurities originated from molten steel such as zinc, aluminum, tin, lead, and cadmium, without damaging high-temperature strength of a copper material for the mold, and can prevent the occurrence of heat cracks at a meniscus part. <P>SOLUTION: The mold for continuously casting steel, which is composed of a copper alloy and is provided with an electromagnetic stirring device, is characterized in that the upper front surface or a part thereof in the inner part of the mold, directly contacting with the molten steel, is coated with copper plating. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a mold for continuous casting of steel, and more particularly to a steel continuous casting mold that employs an electromagnetic stirring method.

  In order to continuously rub the inner surface of the casting mold with the molten steel (casting piece) that is mainly solidified, as a countermeasure against wear of the lower part of the mold copper material, some coating material has been conventionally applied to the steel and continuous casting mold made of copper and copper alloy. A means for coating is used. The application of various surface coating materials to the mold copper material is effective in protecting the copper material and extending the life, and the quality of wear and corrosion resistance of the surface coating material determines the mold repair cycle. It is a fact. Even today, the demand for heat resistance, corrosion resistance, and wear resistance of the material coated on the mold has not stopped. For example, as a coating material for a mold, there has been a history of using chrome plating in the past period, but this is surprisingly short life, and 3-5 mm thick nickel plating (which can achieve longer life than chrome plating) For example, there is a history of conversion to Patent Document 1). Thereafter, in order to obtain a longer life, various surface coating materials such as nickel-iron alloy plating (see, for example, Patent Document 2), and alloy plating of cobalt and nickel having different alloy ratios (see, for example, Patent Document 3 or 4). Although the method is fundamentally different from plating, a nickel-chromium self-fluxing alloy sprayed coating (see, for example, Patent Document 5 or 6), nickel-boron alloy plating (see, for example, Patent Document 7), and the like have appeared.

  Depending on the required purpose, these coatings can be combined to form a multilayer structure, for example, nickel plating / nickel-phosphorus alloy plating / chromium plating from the lower layer to the upper layer, for example, upper nickel plating, lower layer on the same surface There is also a combination of nickel-chromium self-fluxing alloy spraying. Some of these are still used as the mainstream of coating materials. However, in the steel continuous casting mold, the fundamentally required characteristics are different between the upper part of the mold where molten steel is poured and the outlet side (lower half) of the cast piece where the solidified shell (cell) is growing. When the coating material is applied to all of the inner walls of the casting mold, the heat removal effect is hindered at the upper part of the casting mold. As a result, heat cracks are generated on the mold surface due to thermal fatigue, and the cracks propagate to the casting copper material.

  Propagation of heat cracks to the copper alloy results in an increase in the amount of copper material to be cut and removed during the reclaiming process of the surface of the mold copper material, leading to a reduction in the number of times the copper material itself can be regenerated. Therefore, in many cases, the above-described coating material is mainly applied to the lower half of the casting mold that eventually requires corrosion resistance and wear resistance. In other words, in the upper half of the mold, the solidification of the molten steel has not yet progressed, and the copper or copper alloy constituting the mold is used in an exposed state with the expectation of a heat removal effect rather than wear resistance. When a known coating material is applied, the thickness of the coating material may be intentionally different between the upper part and the lower part of the casting mold in order to avoid the occurrence of heat cracks. In the past, there was also a case where the upper part of the mold was coated with copper plating according to the thickness of the corrosion / abrasion-resistant film that coats the lower half of the casting mold, but the problem of interlayer swelling of the copper plating layer was not solved during casting. However, it has not been put into practical use (for example, see Non-Patent Document 1, Patent Document 8, or Patent Document 9).

  By the way, the problem of recent steel continuous casting has become quite clear, to suppress the segregation of blowholes and non-metallic inclusions due to gas and impurities inevitably mixed in the molten steel to improve the quality of the cast piece, For the purpose of uniform solidification of cast pieces, there are increasing examples of introducing electromagnetic stirring type casting molds. On the other hand, the electromagnetic stirrer is placed outside the casting mold, but in order to improve the permeability and increase the stirring effect of the molten steel, the thickness of the mold copper material is reduced and the thermal conductivity is intentionally reduced. Tend to use copper alloys. However, a decrease in the thermal conductivity of the mold copper material causes an increase in the surface temperature of the upper part of the mold, leading to the occurrence of heat cracks in the mold, and the mold must be regenerated after a short period of use. In other words, the active use of electromagnetic stirrers has brought new challenges to this technical field.

  More specifically, when molten steel is cast while performing electromagnetic stirring, the amount of heat applied to the mold is relatively high due to the flow motion of the copper alloy having low thermal conductivity and the molten steel associated with electromagnetic stirring, and the heat effect. Appears prominently in the vicinity of the meniscus portion. In other words, the gas components and impurities in the molten steel are forced to rise to the top due to the stirring effect by electromagnetic stirring, but in the impurities, the melting point of molten steel such as zinc, aluminum, tin, lead, cadmium, etc. 700 ° C or less A small amount of the impurity metal element having a low melting point is also mixed, and new molten steel is poured into the casting mold all the time, so that the impurity metal element is concentrated in the vicinity of the meniscus. And since these low melting point impurity metal elements are forcibly sprayed on the mold surface by electromagnetic stirring force, the surface of the mold copper material in a specific part near the meniscus or a protective film other than copper is coated. Sometimes, the surface receives an attack of these low melting point impurity metal elements to form an alloy layer, and the growth of the alloy layer is promoted as the number of castings increases. When this state is reached, the heat removal effect is further impaired, and the temperature in the vicinity of the meniscus portion further increases.

  Since the low melting point impurity metal element has a low elongation and forms an alloy layer having a lower melting point than copper, it acts in the direction of worsening the heat fatigue resistance. These combined factors combine to cause heat cracks in the casting mold at an early stage. Once a crack occurs, the copper material is gradually alloyed and eroded toward the inside of the copper material. As described above, the mechanism of heat crack generation in the electromagnetic stirring mold was found through detailed investigation of several used molds. However, heat cracks are generated by reducing the number of rechargeable casting molds and the number of times they can be regenerated. As a result, the cost of steel casting increases significantly due to an increase in the number of times of production of new copper plates.

Japanese Patent Publication 48-28255 Shoko 60-34639 Shoko 61-17581 JP-B 60-59999 JP-B 61-15758 JP-A-60-206552 Japanese Patent Publication 4-59064 Japanese Patent Publication 1-44425 Shoko 58-13257 4-2337 Japan Iron and Steel Institute 96th Performance Convention Performance Summary 160 pages

  A technique is disclosed in which, when a molten steel containing zinc as an impurity is coated with 10 to 1,000 μm of cobalt or a nickel alloy containing 10% or more of cobalt, there is a remarkable zinc diffusion preventing effect (for example, Patent Document 10). reference). However, impurities in the molten steel are not necessarily limited to zinc, and a trace amount of low melting point metal elements such as tin, aluminum, lead, cadmium and the like are contained in addition to zinc. In fact, elements such as zinc, aluminum, cadmium, etc. are always detected in addition to the elements constituting the casting mold in the sample collected from the vicinity of the meniscus where the heat crack of the used electromagnetic stirring mold has occurred. In addition, in continuous casting molds that are electromagnetically stirred, the heat removal effect can be reduced by applying the conventional technology of coating different types of metals that are inferior in heat conductivity and heat fatigue resistance (elongation) to the raw material. As a result, the temperature rises near the hot water surface (meniscus). In addition, since alloying with not only zinc but also the above-described low melting point impurity metal element occurs, it has been found that the decrease in elongation becomes rate-determining rather than the function of preventing zinc diffusion, and the occurrence of heat cracks has priority. Moreover, once cracks occur, erosion (alloying) of various low melting point impurity metal elements proceeds toward the inside of the mold copper material starting from the cracks, so that cobalt or cobalt-nickel alloy is simply coated. The present inventors have found that it is not necessary to prevent diffusion of a low melting point impurity metal element containing zinc.

  That is, as described above, the use of an electromagnetic stirrer in a continuous casting mold increases the temperature rise in the meniscus portion, and at the same time, a trace amount of low melting point impurities such as zinc, tin, aluminum, lead, cadmium, etc. contained in molten steel. The elements attack not only the surface of the mold copper material but also the inside, resulting in a shortened mold life. The present inventor has intensively studied on this problem and has completed the present invention.

  As described above, electromagnetic stirring molds tend to use thin copper alloys with slightly inferior thermal conductivity in order to improve permeability. Forced solidification by removing impurities and gases The stir of the molten steel is accompanied by a temperature rise near the meniscus, unlike a normal mold, and in particular, the agitated molten steel heat flow directly hits the upper surface of the mold near the meniscus, resulting in a marked singular point of temperature rise, The present inventor has discovered that heat cracks are concentrated on this singular point. Furthermore, the present inventor has found that the problems of the prior art can be solved at once by the means described below as a result of extensive trial and error studies including the following test examples and examples.

That is, the present invention
(1) In a continuous casting mold for steel with an electromagnetic stirrer made of a copper alloy, a continuous casting mold characterized in that the entire upper surface inside the mold or a part thereof in direct contact with molten steel is coated with copper plating,
(2) The continuous casting mold according to the above (1), wherein the upper part inside the mold is in a range of at least 300 mm downward from the upper end of the mold,
(3) The continuous casting mold according to (1) or (2), wherein the purity of the copper plating is 99.5% or more and the thickness of the copper plating is in the range of 0.3 to 10 mm. ,
(4) The continuous surface according to any one of (1) to (3) above, wherein the entire surface of the copper plating surface or a part thereof is further coated with chromium, nickel, cobalt, or an alloy made of these metals. Casting mold,
About.

  As mentioned above, if high-purity copper plating is applied to the upper surface of the inner surface of a continuous casting mold for electromagnetic stirring, where heat cracks are likely to occur, the high thermal conductivity and the effect of elongation cause the low melting point metal element in the molten steel. The attack is remarkably slow, has a lifespan more than three times that of the conventional mold, and has a great economic effect that shows a significant cost reduction of the steel casting cost.

  The copper alloy used in the mold of the present invention is not particularly limited, and those conventionally used in this technical field are appropriately used. For example, a copper material for chromium-zirconium-added precipitation hardening mold (preferably Cr: 0.5 to 1.5% by weight, Zr: 0.08 to 0.30% by weight), a chromium / zirconium / aluminum-added mold for electromagnetic stirring Copper materials (preferably Cr: 0.50 to 1.50% by weight, Zr: 0.08 to 0.30% by weight, Al: 0.7 to 1.1% by weight) and the like.

  The copper plating for protecting the upper inner surface of the mold base material made of the copper alloy may be a copper plating used in this technical field. The copper purity in copper plating may be a normal one, but is particularly preferably high purity. The high purity is usually 99.5% by weight or more, preferably 99.9% by weight or more. Moreover, the film thickness by plating is 0.3-10 mm normally, Preferably it is the range of 1-7 mm.

  In other words, the coating of high-purity copper plating avoids alloying to some extent in continuous use over 2,000 charges even though the degree of alloying due to attack of low melting point impurity metal elements is low. As a result, fine cracks may occur due to thermal fatigue. In addition, when the film thickness is too thin, the influence of the thermal conductivity of the mold base material having a low thermal conductivity becomes dominant and the heat removal effect is reduced. For this reason, a thickness of 0.3 mm or less is inappropriate. On the other hand, if the thickness is too large, the mold strength and magnetic permeability are lowered, which is not preferable in another sense.

  In addition, the coating range of high-purity copper plating is that the meniscus part fluctuates because the casting mold oscillates up and down, and impurities, gases, etc. in molten steel containing low melting point metal elements concentrate on singular points of the meniscus part, etc. In consideration of the above, it is sufficient if it is in the range of about 300 mm or less from the upper upper end of the mold toward the drawing side. However, if there is no problem in wear resistance, etc., it is about 1/2 (about half) of the total height of the mold. Also good. If the singular point where the molten steel heat flow concentrates is clear, it may be covered locally.

As the plating solution for copper plating the upper inner surface of the mold, known ones such as a copper sulfate bath, a copper pyrophosphate bath, and a copper cyanide bath can be used, but heating is unnecessary and low pollution copper sulfate. A bath is particularly preferred. However, in order to obtain a smooth surface and a glossy surface in any bath, it is common to use an organic additive that can provide a smoothing effect. However, when an additive is used in combination, these decomposition products may be taken into the copper plating film, leading to a decrease in thermal conductivity and elongation. In such a case, the combined use of the additive is not preferable.
As the additive, additives used for copper plating can be appropriately used. For example, commercially available products such as polyacrylamide, glue, gelatin, thiourea, triethanolamine and the like are Epac H (Okuno Pharmaceutical Co., Ltd.), Kapaland 300 ( Nippon Schering Co., Ltd.).

  The electrolysis method for plating may be a normal electrolysis method, but PR electrolysis in which the anode and the cathode are periodically switched is particularly preferable for obtaining high purity and surface smoothness of copper in the plating film. I also understood that. Table 1 shows particularly preferable copper plating conditions for obtaining the high-purity copper plating-coated mold of the present invention. If the bath and plating conditions provide high purity, high thermal conductivity, and good elongation, this is not necessarily true. It is not limited to.

  As for the inner surface of the lower part, which is the remaining part other than the upper part of the mold, a known coating having corrosion resistance and wear resistance as in the past, such as nickel-iron alloy plating, cobalt-nickel alloy plating, nickel-chromium self-fluxing alloy What is necessary is just to select a sprayed coating etc. suitably.

  Although the example of the coating form by the high purity copper plating of the continuous casting mold of this invention is shown to (a)-(e) of FIG. 2, it is not necessarily limited to this. Furthermore, the casting mold of the present invention is not necessarily limited to a casting mold for electromagnetic stirring, and it goes without saying that it can be applied to all casting molds that frequently generate heat cracks due to adhesion and erosion of low melting point metal elements. .

  (Test Example 1) The inventor of the present invention provides a coating material that is excellent in thermal conductivity and heat fatigue resistance without impairing the magnetic permeability and high-temperature strength of the mold copper material, and that hardly forms an alloy with a low-melting-point metal. First of all, we investigated the thermal conductivity and elongation of the main materials to find out the thermal conductivity involved in the heat removal effect of the main materials and the materials that affect the thermal fatigue resistance. The results of Preliminary Test-1 are shown in Table 2. The thermal conductivity is a silver-added mold copper material, additive-added copper plating, high-purity copper plating, and the elongation characteristics related to heat fatigue is a silver-added mold. Copper materials, high-purity copper plating, etc. were found to be particularly excellent.

  However, it is unpredictable how the improvement in copper thermal conductivity will eventually behave against low melting point metal elements such as zinc and aluminum. Therefore, in order to compare and evaluate the adhesion and erosion of zinc, we prepared 80 mm sq. X 15 mm plate thickness chromium / zirconium-added cast copper material, silver-added cast copper material, and electromagnetic stirring chrome / zirconium / aluminum-added copper material. A test piece was obtained. In addition, as for the chromium, zirconium, and aluminum-added copper material, a sheet having a thickness of 14 mm is separately prepared, and nickel plating, cobalt-10% nickel alloy plating, nickel-5% iron alloy plating, additive are added on one side. The combined copper plating and high-purity copper plating were each coated to a thickness of 1 mm to obtain a test piece having a total plate thickness of 15 mm.

  These test specimens were preliminarily tested-2, cooling with water in the manner shown in FIG. 1, selecting zinc as a low melting point metal, and injecting about 500 ml of heat-melted with a crucible to 600 ° C. into the test surface, After leaving to cool to room temperature, qualitatively assessing whether the solidified zinc is peelable from the test surface, and at the same time, the degree of formation of an alloy layer with zinc from the cross-section of the specimen is measured by EPMA (Electron Probe Micro Analyzer : Electron beam microanalyzer). In any of the test pieces, the molten zinc injection surface was placed in a clean state by aligning the surface roughness to 1 to 2 μm and simultaneously performing alkaline degreasing and electrolytic cleaning. The results are shown in Table 3, but the silver-added mold copper material having excellent thermal conductivity with the material alone, and the high-purity copper plating coated on the chrome / zirconium / aluminum-added copper material for electromagnetic stirring, Good results were obtained regardless of the degree of formation of the alloy layer. This result experimentally suggests that a material having excellent thermal conductivity is effective in preventing the occurrence of heat cracks.

  From the results of Table 3, the good result of the copper material for the silver-added mold is that the actual cast mold copper material can be converted to this, but there is a problem of permeability for the electromagnetic stirring effect. The thermal conductivity, that is, the electrical conductivity is good, which is not preferable for the magnetic permeability. Furthermore, the temperature rise in the vicinity of the meniscus is not preferable in that the high-temperature strength is low and the material is easily deformed by heat. From the standpoint, it is essential to prevent alloying by attack of low melting point impurity metal elements, electromagnetic stirring efficiency, and to ensure high temperature hot strength in a mold thinner than usual, while maintaining high temperature strength as a mold In order to achieve the purpose of preventing the occurrence of heat cracks, the conclusion of the present invention is that the coating of high-purity copper plating is most preferable.

  (Test Example 2) In the casting mold for slab, the two long sides sandwich the side surfaces of the two short sides, and the short side is moved back and forth (inside and outside) during casting according to the slab width. . That is, since the side surface of the short side slides on the surface of the long side, a sliding flaw is often generated in a range corresponding to the moving amount of the short side, and the cast piece may be bitten and lead to a breakout. Therefore, for the purpose of preventing the occurrence of sliding wrinkles, sometimes the upper part of the mold is not left as it is, and is often thinly coated with a different metal other than copper, such as nickel, nickel alloy, chrome plating or the like. The same applies to the casting mold of the present invention. In other words, the sliding range of the short side is limited, and even an electromagnetic stirrer mold is usually outside the range directly affected by the heat flux of molten steel. Formation and erosion are relatively difficult to occur, and almost no heat cracks are observed. On the other hand, the copper-coated portion of the mold coated with the high-purity copper plating of the present invention is basically preferably in a state where the copper is exposed, but it does not hinder the heat removal effect of the high-purity copper plating. If it is a thin film specification, even if it coat | covers the dissimilar metal other than copper, for example, the zinc diffusion prevention film which has cobalt or cobalt as a main component, it can function as it is.

  In order to test the effectiveness of high-purity copper plating with an actual continuous casting mold, a chromium-zirconium-aluminum-added copper material for an electromagnetic stirring casting mold (Cr: 1.0%, Zr: 0.2%, Al: 0.8 %)), Prepared 4 sets of 2,000mm width x 900mm height x about 30mm thickness, and plated under the following conditions. Mounted on. The results are shown in Table 5, and it was found that the heat cracking resistance of the electromagnetic stirring mold coated with high-purity copper plating according to the present invention was tremendous and showed durability more than 3 times that of the conventional mold. The plating conditions are shown in Table 6.

＊１）高純度銅めっきが被覆されていない部分は、鋳型銅材が露出している。
＊２）高純度銅以外のめっきは、公知方法に従って行った。

* 1) The mold copper material is exposed in the part that is not coated with high-purity copper plating.
* 2) Plating other than high-purity copper was performed according to a known method.

It is the figure which looked at the test piece used by the preliminary test, and the inject | poured molten zinc from the side surface. It is a figure which shows the coating pattern by copper plating of the continuous casting mold inner surface of this invention.

Explanation of symbols

1. 1. Injected molten zinc 2. SUS304 molten zinc outflow prevention frame Waterway 4. 4. Cooling water Test piece 6. 6. High purity copper plating coating part 7. Well-known corrosion / abrasion-resistant coating covering part 8. Base metal copper exposed portion Nickel plating coating

Claims (2)

The upper part of the mold that is in direct contact with the molten steel of the steel continuous casting mold with an electromagnetic stirrer that has no history of heat cracking and is made of a copper alloy has a purity of 99.000 in the range of at least 300 mm from the upper end of the mold downward. A method for suppressing the occurrence of heat cracks in a continuous casting mold, characterized in that the thickness of the plating is 1 to 7 mm with 5% or more copper plating. 2. The method for suppressing the occurrence of heat cracks in a continuous casting mold according to claim 1, wherein the entire surface of the copper plating surface or a part thereof is further coated with chromium, nickel, cobalt, or an alloy thereof.

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