Ladle cover for vacuum refining process

Abstract: Ladles used for vacuum refining processes, for example a VOD process, are provided with covers to cover openings of ladles. The covers generally consist of refractories, which are required to have excellent resistance to thermal spalling due to heat cycles and not inhibit decarbonization during refining processes of molten steel. The ladle cover comprises a refractory having a carbon content 5 wt% or more. Preferably, a refractory has a carbon content 5 wt% or more is used for a central section of the ladle cover, and a refractory having a carbon content less than 5 wt% is used for peripheral sections of the ladle cover.

Description: The invention relates to covers that are placed on ladles to cover openings on the ladle. The ladles are used in vacuum refining processes, such as Vacuum Oxygen Decarbonization (VOD). 2. Description of the Related Art In VOD equipment for secondary refining of molten steel, a ladle is placed in a vacuum chamber under reduced pressure. The ladle is provided with a cover. The cover prevents spattering and deposition of molten steel or slag into the vacuum chamber. The spattering and deposition may be caused by bubbles from bubbling gas, decarbonization, deoxidation, or denitrodation in the ladle. The ladle cover also suppresses thermal radiation of a steel bath during a refining process. In general, a ladle cover is formed from refractories. A known ladle cover is made of a ceiling refractory formed of a combination of unburned MgO-Cr 2 O 3 with graphite, and is disclosed in Tables 13 and 19 of "Steel Handbook, Iron Making and Steel Making" 3rd edition, (page 712) (Maruzen). In the ladle cover of "Steel Handbook", a lance hole for a top blowing lance is made of graphite, where other sections are made of unburned MgO-Cr 2 O 3 . Unburned MgO-Cr 2 O 3 , which is a refractory of an insulation fire brick nature having a thermal conductivity of 1.5 kcal/mH°C, is provided over the entire ceiling, except at a periphery of the lance hole. In the "Steel Handbook" cover, the cover has a set radius. A circular area or section radially within 70 to 80% of a cover's center is rapidly heated by radiation heat from molten steel during refining periods. The section is also cooled during nonrefining periods to define a thermal cycle. Such repeated thermal cycles facilitate thermal spalling. Thus, the life of the refractory is shortened. Deterioration due to thermal spalling can be prevented by providing a spalling resistive material, for example graphite, over the entire ceiling. However, the use of graphite will cause a problem in processes that produce ultra low carbon steels. (In such a process, the graphite is dissolved and inhibits decarbonization.) The graphite lined on an inner surface of a ladle cover is consumed as a result of secondary combustion, which is unavoidably caused by top blowing oxygen in a space defined between the molten steel surface in the ladle and the ladle cover. This results in a shortened life of the refractory. A watercooling type ladle cover is disclosed in Japanese Laid Open Patent No. 6-10031 (JP 031). The JP 031 ladle cover is provided with watercooling tubes to continuously circulate cooling water so that the tube is thermally protected and has a very long life. The watercooling type ladle cover reduces production and maintenance costs of ladle covers. In the JP 031 watercooling type cover, the heat radiated from molten metal is conducted away from the cover by the cooling water in the watercooling tubes. The watercooling tubes are maintained at a low temperature during the process, so a temperature of the molten steel drastically decreases during the process. Thus, a large amount of heat must be added during the process to maintain molten steel. This results in a substantial and often uneconomical increases in production costs. From patent abstracts of Japan, volume 013, No. 423 (C-638) & JP-A-01162714 there is known a lining refractory brick for a converter composed of 5 - 10 weight percent carbon such as graphite and the balance MgO clinker. These bricks are reported to have an excellent thermal spalling resistance as the MgO and the C do not react with each other even at the high temperature of 1650°C. Furthermore, said conventional lining refractory brick for a converter avoids an increase in the carbon content of a melt by carbon pick up from the refractory bricks at the time of refining low carbon steels. It is an object of the present invention to provide a ladle cover that exhibits excellent durability against thermal spalling due to thermal heating cycles. Furthermore, it is an object of the invention to provide a ladle cover having a prolonged life due to an improved resistance to thermal spalling without resulting in a detrimental influence from decarbonization. The object is achieved by the subject matter of claim 1. Preferred embodiments and further improvements of the invention are defined in the depending subclaims. Other objections, advantages and salient features of the invention will become apparent form the following detailed description, which, taken in conjunction with the annexed drawing, discloses preferred embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view illustrating a ladle and a ladle cover; Fig. 2 is a schematic plane view of a ladle cover; Fig. 3 is a graph illustrating a correlation between carbon content in a refractory and a thermal impact resistance temperature differential; and Fig. 4 is a graph illustrating a correlation between carbon content in a refractory and a decarbonization rate. DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the invention is shown in Fig. 1. In Fig. 1, a ladle cover 1 is placed on a ladle 2 to cover an opening in the ladle 2. The ladle cover 1 is formed with a diskshaped body and has a lance hole 3 lined with a refractory, for example a refractory comprising graphite. A top blowing lance can be inserted in the lance hole 3. The lance hole 3 is, for example, positioned in the center of the cover 1. The periphery of the ladle cover 1 is encircled by a peripheral metal frame 4. The ladle cover 1 between the lance hole 3 and the peripheral metal frame 4 is lined with at least one refractory. The refractory may have any appropriate composition and may be another type of refractory, other than the refractory at the lance hole 3. Thermal spalling of refractories due to heat is most likely caused by irregularities in temperature during heating and cooling of the molten metal. When a thermal conductivity of the refractory is high, heat diffusion is promoted inside the refractory. Thus, temperature deviation in the refractory becomes smaller. To improve the resistance to thermal spalling due to heat, a higher thermal conductivity is desirable. In order to obtain a ladle cover 1 with excellent resistance to thermal spalling due to heat, the refractories should preferably have a carbon content approximately 5 wt% or more. Thermal conductivity of a refractory significantly varies with its carbon content. For example, in MgO refractories, thermal conductivities at 500 °C are 5 kcal/mH°C for a MgO refractory, 9 kcal/mH°C for a MgO-C refractory containing 5 wt% of carbon, 11 kcal/mH°C for a MgO-C refractory containing 10 wt% of carbon, and 16 kcal/mH°C for a MgO-C refractory containing 15 wt% of carbon. Similarly, thermal conductivities at 1,000 °C are 3.5 kcal/mH°C for a MgO refractory, 6.5 kcal/mH°C for a MgO-C refractory containing 5 wt% of carbon, 8 kcal/mH°C for a MgO-C refractory containing 10 wt% of carbon, and 16 kcal/mH°C for a MgO-C refractory containing 13 wt% of carbon. Thermal impact resistance temperature differential is an index of resistance to thermal spalling due to heat. The thermal impact resistance temperature differential of various materials was investigated to determine if a correlation existed between carbon content in MgO refractories and resistance to thermal spalling due to heat. In particular, a thermal impact resistance temperature differential between a room temperature and a temperature where breakage and/or cracks do not occur when a refractory at room temperature is rapidly exposed to a high temperature atmosphere with respect to carbon content of the refractory was investigated. Test results are shown in Fig. 3. Fig. 3 illustrates that thermal impact resistance temperature differential rapidly increases when carbon content in the refractories exceeds 5 wt%. Further, the thermal impact resistance temperature differential increases when carbon content in the refractories exceeds 20 wt%. The results indicate resistance to thermal spalling due to heat in a ladle cover comprising refractories can be improved by using refractories having a carbon content approximately 5 wt% or more. The results also indicate that resistance to thermal spalling can be further improved with a refractory having a carbon content approximately 20 wt% or more. When the carbon content in the refractories comprising the ladle cover increases, some carbon may drop off of the ladle cover during decarbonization. Thus, the carbon will enter molten steel and inhibit decarbonization. Therefore, average decarbonization rates for molten steel were investigated, using crucibles made of MgO-C refractories having different carbon contents. Test results are shown in Fig. 4. Fig. 4 illustrates that decarbonization rates do not rapidly decrease until the carbon content of the refractories is approximately 10 wt%. Since a lower limit for practical decarbonization rates is 80% of a decarbonization rate with a refractory containing less than 5 wt% of carbon, a refractory with a carbon content of approximately 20 wt% or less will permit practical decarbonization. The above test results indicate that resistance to thermal spalling due to heat in the ladle cover is improved by using a refractory having a carbon content approximately 5 wt% or more. The results also indicate a decrease in the decarbonization rate during the decarbonization is prevented by limiting the carbon content in the refractory to approximately 20 wt% or less. When a ladle cover is formed of two kinds of refractories, each having different carbon contents, a resistance to thermal spalling due to heat in the ladle cover can be improved without harmfully influencing decarbonization. For example, a radial inner section 5 of the ladle cover surrounding the lance hole 3 can be lined with a refractory containing approximately 5 wt% or more of carbon. A radial outer section 6 of the ladle cover surrounding the inner section 5 can be lined with a refractory containing less than approximately 5 wt% of carbon. This arrangement is effective because the radial inner section 5 of the ladle cover 1 just above steel bath M is subject to severe heat cycles that may cause thermal spalling. When the radial inner section 5 of the ladle cover 1 is lined with a refractory having a carbon content approximately 5 wt% or more, the resistance to thermal spalling due to heat is improved. The radial outer section 6 is lined with a refractory having a carbon content less than approximately 5 wt%, so it barely acts as a carbon source. Thus, the ladle cover 1 has excellent resistance to thermal spalling due to heat, and does not inhibit decarbonization. Fig. 3 illustrates that a refractory having a carbon content approximately 20 wt% or more is preferable for the radial inner section 5. The area of the radial inner section 5 in the ladle cover 1 must be controlled, so decarbonization is not inhibited even if a refractory having a carbon content approximately 20 wt% or more is used. In Fig. 4, an area of the refractory having a carbon content approximately 5 wt% is (1X), and an area of the refractory having a carbon content approximately 20 wt% of carbon is X. The decarbonization rate can then be expressed by the equation: 108 × (1X) + 82 × X Since it is desirable to have a low decarbonization rate, preferably 80% of a decarbonization rate with a refractory having a carbon content less than 5 wt%, the decarbonization rate can be expressed by the equation: 108 × (1X) + 82 × X ≥ 109 × 0.80 From this equation, X ≤ 0.80. Accordingly, the area of the radial inner section 5 using a refractory having a carbon content 5 wt% or more is preferably limited to approximately 80% or less of the ladle cover 1. Further, a corresponding radius ratio of a radius of the radial inner section to the radius of the ladle cover 1 is limited to 90% or less. However, when the area of the radial inner section 5 drastically decreases, the resistance to thermal spalling due to heat at the periphery is significantly affected by radiant heat. Thus, it is preferable that radial inner section 5 have an area of 40% or more of the ladle cover 1, or a radius ratio i.e., a ratio of the radius of the radial inner section 5 to the radius of the ladle cover 1, approximately 65% or more. Since the lance hole 3 occupies at most approximately 10% of the cover ladle area, a ladle cover 1 where only the lance hole 3 is made of a high carbon content refractory is unsatisfactory. Accordingly, the radial inner section 5 of the ladle cover 1 formed with a refractory having a carbon content approximately 5 wt% or more preferably has an area of 40 to 80% of the cover, or has a radius ratio of 65 to 90%. Preferably, the radial inner section 5 of the ladle cover 1 has an area of 64 to 80% of the cover, or a radius ratio of 80 to 90%. Further, the carbon content of the refractory at the radial inner section 5 is preferably approximately 5 to 30 wt%. More preferably, the carbon content of the refractory of the radial inner section 5 is approximately 10 to 20 wt%, given the relation of resistance to thermal spalling due to heat and decarbonization rate. Various refractory integrated structures for the ladle cover can be used in accordance with the invention. Although diskshape block fabrication ladle cover is shown in Fig. 1 and Fig. 2, other shaped structures are contemplated by the invention. For example, a plurality of refractories with at least one projection and recess section fit to each other is contemplated herein, a plurality of independent ringshaped arches having different radii are formed from refractories is also possible in accordance with the invention. An example of the invention will now be described. With VOD equipment for secondary refining of molten steel, having a capacity of 160 tons. Vacuum refining processes were carried out with carbon concentrations of molten steel at 0.10 wt% to 30 ppm. An diskshape ladle cover 1, as shown in Fig. 1 and Fig. 2 was used with the VOD equipment. The specifications for the covers and carbon contents of MgObase refractories are illustrated in Table 1. Table 1 also illustrates the life of ladle covers until refractories in the ladle dissolved and the covers dropped out during decarbonization processes. Table 1 also illustrates average decarbonization times. Specifications Life until Refractories Drop out (heat) Average Decarbonization Time (min) Example 1 All MgO-C refractories containing 7 wt% of carbon 283 14 Example 2 MgO-C refractories containing 30 wt% of carbon for the section within 70% of the radius from the center, and MgO-Cr 2 O 3 base refractories for the residual section. 280 15 Example 3 MgO-C refractories containing 20wt% of carbon for the section within 90% of the radius from the center, and magnesia dolomitebase refractories for the residual section. 300 15 Example 4 All MgO-C base refractories containing 20wt% of carbon. 285 20 Comparative Example 1 All MgO-Cr 2 O 3 base refractories. 100 14 Comparative Example 2 MgO-C base refractories containing 15 wt% of carbon for the section within 10 % of the radius from the center, and magnesia dolomitebase refractories for the residual section. 120 14 As clearly indicated, Table 1 demonstrates that ladle covers, in accordance with the invention, have extremely prolonged life compared with Comparative Examples 1 and 2.

Claims: A ladle cover comprising a refractory material for a metallurgical vessel, characterized in that a radial inner section (5) of said ladle cover (1) comprises a first refractory and that a radial outer section (6) of said ladle cover comprises a second refractory, said first and second refractories each having a different carbon content, wherein at least one of said refractories has a carbon content of 5 weight percent or more. A ladle over according to claim 1, wherein said first refractory has a carbon content of 20 weight percent or less. A ladle cover according to claim 1, wherein a lance hole (3) is formed in said ladle cover (1), wherein said lance hole (3) is preferably positioned in the center of said ladle cover. A ladle cover according to claim 1, wherein said ladle cover is disked shaped. A ladle cover according to claim 1, wherein the refractory is a MgO-C refractory. A ladle cover according to claim 1, wherein said first refractory has a carbon content of 5 weight percent or more and less than 20 weight percent, and the second refractory has a carbon content of less than 5 weight percent. A ladle cover according to claim 1 and 3, wherein the first refractory of said radial inner section (5) has a carbon content of at least 5 weight percent and the second refractory of said radial outer section (6) has a carbon content of less than 5 percent, wherein said lance hole (3) or its lining, is lined with a refractory having a carbon content of at least 5 weight percent. A ladle cover according to claim 7, wherein said cover (1) has generally a round periphery and defines a ladle radius, said radial inner section (5) defining a radial inner section radius and a first cover area and a radius ratio is defined by a ratio of the radial inner section radius and the ladle radius, the radial inner section defining a radius ratio of approximately 90 percent, a second cover area comprising the ladle cover other than the first area, the first cover area comprising a refractory having a carbon content of 5 weight percent or more and the second cover area comprising a refractory having a carbon content less than 5 weight percent. A ladle cover according to claim 8, wherein the first cover area defines a radius ratio between 65 and 90 percent, comprising a refractory having a carbon content of 5 weight percent or more and less than 30 weight percent, and other areas of the ladle cover comprising a refractory having a carbon content less than 5 weight percent. A ladle cover according to claim 8, wherein the first ladle cover area defines between 64 percent and 80 percent of the ladle. A ladle cover according to claim 7, wherein the refractory has a carbon content of 20 weight percent or less. A ladle cover according to claim 7, wherein the ladle cover is disked shaped. A ladle cover according to claim 7, wherein the refractory is a MgO-C refractory.

Inventors: KANATANI TOSHIO, TAMURA NOZOMU, WASHIO MASARU, YAMADA SUMIO, KANATANI, TOSHIO, TAMURA, NOZOMU, WASHIO, MASARU, YAMADA, SUMIO, Kanatani, Toshio, c/o Kawasaki Steel Corp., Tamura, Nozomu, c/o Kawasaki Steel Corp., Washio, Masaru, c/o Kawasaki Steel Corp., Yamada, Sumio, c/o Kawasaki Steel Corp.
Applicants: KAWASAKI STEEL CO, KAWASAKI STEEL CORPORATION
Application year: 1996
Main classification: B22D41
IPC codes: C21C7, F27D1, B22D41