JP2009173994A - Method for producing al-less extra-low carbon steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the erosion of an immersion tube in an RH vacuum-degassing apparatus with slag, when an Al-less extra- low carbon steel is produced by using a converter and the RH vacuum-degassing apparatus. <P>SOLUTION: When the Al-less extra-low carbon steel having ≤0.001 mass% Al content is produced by decarburize-treating the molten steel 3 refined with the converter, with the RH vacuum-degassing apparatus 1; at the tapping-off time or after tapping-off the molten steel from the converter, CaO source and MgO source are added into a ladle accommodating the molten steel, and CaO content and MgO content in the slag 4 existing in the molten steel in the ladle, are adjusted to respectively 50-70 mass% and 10-30 mass%, and thereafter, the molten steel is refined with the RH vacuum-degassing apparatus. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for melting Al-less ultra-low carbon steel using a converter and an RH vacuum degassing apparatus, and more specifically, when refining an Al-less ultra-low carbon steel with an RH vacuum degassing apparatus. The present invention relates to a method of melting an Al-less ultra-low carbon steel while suppressing melting damage due to slag of a dip tube installed in a vacuum degassing apparatus.

  Electromagnetic steel sheets used for iron core materials generally have better magnetic properties as the crystal grain size increases, so the carbides, nitrides, sulfides, oxides, etc. in steel that are the cause of hindering crystal grain growth The non-metallic inclusions are reduced as much as possible. For example, in the refining process, C, N, and S in steel are reduced by decarburization treatment, denitrification treatment and desulfurization treatment in a secondary refining furnace, and in the component design of materials, Ti, V A component system that does not contain nitride-forming elements and sulfide-generating elements such as Al, Zr, and the like has been used. That is, an electromagnetic steel sheet having excellent magnetic properties is generally operated with an Al-less ultra-low carbon steel.

However, in Al-less ultra-low carbon steel, Al, which has a large oxide reduction effect, cannot be used as a deoxidizer, so Si and Mn, which have a lower affinity for oxygen than Al, must be used as a deoxidizer. Therefore, as compared with Al killed steel, not only the molten steel but also the slag present on the molten steel has a higher oxygen potential. In other words, the slag formed during the refining process of Al-less ultra-low carbon steel has a high concentration of FeO and MnO, which are components that increase the oxygen potential, and as a result, the slag itself liquefies, making it reactive to refractories. At the same time, the graphite-containing refractory promotes the oxidation of the antioxidant added to the refractory to prevent wear of the refractory. Moreover, since the molten steel temperature at the time of processing in the RH vacuum degassing apparatus of the Al-less ultra-low carbon steel is 1600 ° C. or higher, and depending on the steel type, it is 1650 ° C. or higher. Since it is accelerated and rich in reactivity, and because of high temperature, the saturation solubility of MgO and Al ₂ O _{3 in} the slag itself increases.

  The dip tube installed in the RH vacuum degassing apparatus is generally made of MgO refractory or MgO-C refractory, and the ladle with such slag is placed in the ladle. When molten steel is processed in an RH vacuum degassing apparatus to produce an Al-less ultra-low carbon steel, the outer periphery of the dip tube, which is a part in contact with the slag, is severely melted. Therefore, when processing Al-less ultra-low carbon steel with an RH vacuum degassing device, it is necessary to perform repair work such as MgO spray repair on the dip tube every time one charge is processed. This has been one of the causes of impairing device productivity.

  By the way, in order to improve the cleanliness of steel, various measures such as reforming the slag in the ladle are taken, and measures are also taken to improve the cleanliness even in extremely low carbon steel. For example, Patent Document 1 discloses a flux mainly composed of CaO in desulfurizing molten steel refined in a converter using an RH vacuum degassing apparatus and then deoxidizing the molten steel to produce ultra-low carbon Si killed steel. During the treatment with the RH vacuum degassing apparatus so that the ratio (B / A) of the pure CaO mass (B) and the molten steel mass (A) to be treated is in the range of 0.001 to 0.01. It has been proposed to add the flux to the molten steel. By applying patent document 1, the added flux unites with the slag floating on the molten steel in the ladle, dilutes the iron oxide concentration of the slag, and lowers the oxygen potential of the slag. However, from the test and examination results of the present inventors, Patent Document 1 is effective in improving the cleanliness of Al-less ultra-low carbon steel, but only reduces the oxygen potential of the slag. It has been confirmed that the dip tube cannot be prevented from being melted during RH vacuum degassing of low carbon steel.

In addition, Patent Document 2 adds a slag modifier containing metal Al or an Al alloy when removing molten steel to a ladle when melting ultra-low carbon steel, (Mass% CaO) / (mass% Al ₂ O ₃ ) is set to 1 to 2, and then decarburization processing is performed in the RH vacuum degassing apparatus, and after the decarburization processing is completed, metal Al or Al alloy is added and decarburized. In addition to performing acid treatment, a flux mainly composed of CaO and having a particle diameter of 0.1 to 10 mm is represented by the relationship between the flux addition amount Y (kg / t) and the slag mass X (kg / t) in the ladle. It has been proposed to add to the molten steel in the vacuum chamber so that “0.1X + 1) ≦ Y ≦ (0.4X + 4)”. In Patent Document 2, the slag in the ladle is reformed twice at the time of steelmaking and at the time of RH vacuum degassing and finally reformed to a slag having a considerably high CaO concentration. High slag is formed at the end of the RH vacuum degassing refining, and the effect of suppressing the dip tube melting is small. Patent Document 2 is an Al killed steel, which is different from the Al-less ultra-low carbon steel that is the subject of the present invention.
JP 11-293329 A JP-A-9-49012

  As described above, in melting Al-less ultra-low carbon steel using a converter and an RH vacuum degassing device, a technique for preventing melting damage due to slag in the dip tube of the RH vacuum degassing device is desired. Despite this, there has been no effective means in the past, and dip tube repairs are made every time Al-less ultra-low carbon steel is treated with the RH vacuum degasser, and the productivity of the RH vacuum degasser As well as lowering the manufacturing cost, the manufacturing cost increased.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an RH vacuum degassing apparatus for melting Al-less ultra-low carbon steel using a converter and an RH vacuum degassing apparatus. Al-less ultra-low carbon that can prevent melting damage due to slag in the dip tube, and as a result, can improve the productivity of the RH vacuum degassing device and can greatly reduce the manufacturing cost compared to the conventional one. It is to provide a method for melting steel.

  The method for melting Al-less ultra-low carbon steel according to the first aspect of the present invention for solving the above-described problem is that the molten steel refined in the converter is decarburized with an RH vacuum degassing apparatus and has an Al content of 0. When melting Al-less ultra-low carbon steel of 001 mass% or less, a CaO source and an MgO source are added to a ladle containing molten steel at the time of or after the molten steel is left in the converter. The CaO content in the slag existing on the inner molten steel is adjusted to 50 to 70 mass%, the MgO content is adjusted to 10 to 30 mass%, and then refined with an RH vacuum degassing apparatus. is there.

  The method for melting Al-less ultra-low carbon steel according to the second invention is the molten steel in the first invention, after decarburizing with an RH vacuum degassing apparatus, and then adding Si source to the molten steel in the vacuum chamber. Is characterized by deoxidation treatment.

  The method for melting Al-less ultra-low carbon steel according to the third invention is characterized in that, in the first or second invention, quick lime is used as the CaO source and magnesia clinker is used as the MgO source. It is.

  According to the present invention, when an Al-less ultra-low carbon steel having an Al content of 0.001% by mass or less is refined by an RH vacuum degassing apparatus, the slag composition in the ladle is reduced to a CaO content of 50. -70 mass%, MgO content is adjusted in the range of 10-30 mass% in advance, so that the reactivity of the slag to refractories increases due to the melting point of the slag, and the slag is already sufficiently high in concentration MgO Due to the effect of suppressing the elution of MgO into the slag due to the inclusion of slag, it becomes possible to suppress the damaging of the dip tube in contact with the slag during the RH vacuum degassing process. As a result, it is no longer necessary to repair the dip tube every time the Al-less ultra-low carbon steel is processed by the RH vacuum degassing apparatus, and the productivity of the RH vacuum degassing apparatus is improved and the manufacturing cost is reduced. .

  The present invention will be specifically described below. The Al-less ultra-low carbon steel targeted in the present invention has a C content of 0.003% by mass or less, an Al content of 0.001% by mass or less, and other components as necessary. Steel containing Si, Mn and the like.

  The hot metal discharged from the blast furnace is received in a hot metal holding / conveying vessel such as a torpedo car or hot metal ladle and transferred to the next converter where decarburization and refining is performed under atmospheric pressure. During this conveyance, it is common to perform hot metal pretreatment such as preliminary desulfurization treatment or preliminary dephosphorization treatment on the hot metal. In the present invention, the hot metal preliminary treatment is performed according to the composition of the Al-less ultra-low carbon steel. Perform the process. That is, when a low level is required for the S content or P content of the Al-less ultra-low carbon steel, preliminary desulfurization treatment or preliminary dephosphorization treatment is performed. A conventional treatment method may be used for the preliminary desulfurization treatment and the preliminary dephosphorization treatment.

  This hot metal is charged into a converter and decarburized and refined by top blowing oxygen or bottom blowing oxygen. The decarburization and refining of hot metal in this converter is carried out by ordinary refining using quick lime or dolomite as a solvent medium. The C concentration in the molten steel at the end of decarburization refining in the converter is preferably 0.02 to 0.06% by mass. When decarburizing and refining to less than 0.02% by mass, oxidation of Mn in Fe and molten steel becomes remarkable, and the yield of Fe and Mn is lowered, leading to an increase in manufacturing cost. On the other hand, when the C concentration in the molten steel at the end of decarburization refining exceeds 0.06% by mass, the burden of decarburization refining in the RH vacuum degassing apparatus in the next process becomes heavy, and the processing time is extended. This is not preferable because the manufacturing cost is increased. Adjustment to the said component range of C concentration in molten steel at the time of completion | finish of decarburization refining is implemented by adjustment of the supply amount of oxygen gas for decarburization in a converter.

  After completion of decarburization refining in the converter, the molten steel obtained by decarburization refining is discharged from the converter into a ladle. At the end of steelmaking, a part of the slag in the converter is caught in the molten steel and flows out into the ladle and remains on the molten steel in the ladle, so the CaO content in the slag present on the molten steel is 50 to A CaO source and a MgO source are added into the ladle during or immediately after the steel output so that the content is 70% by mass and the MgO content is 10 to 30% by mass. In order to promote melting of the added CaO source and MgO source, the molten steel may be stirred after the addition by blowing a stirring gas into the molten steel.

  By making CaO content in slag 50-70 mass%, melting | fusing point of slag rises, liquefaction of slag is suppressed and the reactivity with respect to a refractory falls. Moreover, the density | concentration of FeO and MnO in slag which is a component which raises the oxygen potential of slag is diluted, the oxygen potential of slag falls, and the oxidation of the antioxidant added to a graphite containing refractory is suppressed. Dilution of the concentration of FeO or MnO in the slag also contributes to an increase in the melting point of the slag.

  Further, by setting the MgO content in the slag to 10 to 30% by mass, the MgO concentration in the slag becomes equal to or higher than the saturation solubility of MgO, and the MgO dissolves in the slag, that is, the refractory that comes into contact with the slag. Dissolution of MgO in the slag is suppressed. Incidentally, when the MgO content in the slag is equal to or higher than the saturation solubility, theoretically, the dissolution of MgO in the refractory into the slag becomes zero. MgO is a high melting point substance, and adding MgO to the slag also contributes to an increase in the melting point of the slag.

  That is, by making the CaO content in the slag 50 to 70% by mass and the MgO content in the slag 10 to 30% by mass, the refractory slag in contact with the slag is significantly suppressed from being damaged by the slag. Is possible.

  When the CaO content in the slag is less than 50% by mass, the increase in the slag melting point is small, so the effect of adding the CaO source is small. On the other hand, when the CaO content in the slag exceeds 70% by mass, the slag melting point increases. Although the amount is ensured, the amount of CaO source added is too large, and this is not preferable because it leads to an increase in cost due to a decrease in molten steel temperature and an increase in the basic unit of CaO source. In addition, when the MgO content in the slag is less than 10% by mass, the MgO content in the slag is lower than the saturated solubility of MgO in the slag, and the MgO content in the slag exceeds 30% by mass. In addition, the saturation solubility of MgO has been reached, and no more MgO is required, which is not preferable because of an increase in cost due to an increase in the MgO source unit.

Quick lime (CaO) or limestone (CaCO ₃ ) can be used as the CaO source, and magnesia clinker (MgO) or dolomite (MgCO ₃ · CaCO ₃ ) can be used as the MgO source. Dolomite also functions as a source of CaO. The addition amount of the CaO source and the MgO source may be calculated based on the empirically grasped amount and composition of the slag flowing out from the converter to the ladle and based on the empirically grasped numerical value. In order to rapidly melt the CaO source and the MgO source into the slag, the CaO source and the MgO source are preferably powders having a particle size of about 20 mm or less.

  In the present invention, decarburization refining (also referred to as “vacuum decarburization refining”) is performed under reduced pressure in the RH vacuum degassing apparatus in the next step. Therefore, the higher the oxygen potential of the molten steel, the more preferable. Components that function as strong deoxidizers such as Si are not added. Moreover, even if it is added, it is oxidized by vacuum decarburization refining, and even if it is a necessary component of the Al-less ultra-low carbon steel, it does not remain in the molten steel, and it needs to be added again, which is meaningless. Since Mn also functions as a deoxidizer, it is preferable not to add Mn when steeling. In addition, even if it is a component required for Al-less ultra-low carbon steel, a component that causes oxidation loss in decarburization refining under reduced pressure is meaningless and is not added.

  Next, the ladle containing the molten steel is transported to the RH vacuum degassing apparatus, and vacuum decarburization refining is performed in the RH vacuum degassing apparatus. In FIG. 1, the example of the RH vacuum degassing apparatus used when implementing this invention is shown with a longitudinal cross-sectional schematic diagram. In FIG. 1, the code | symbol 4 is the slag to which the component was adjusted by adding CaO source and MgO source.

  As shown in FIG. 1, the RH vacuum degassing apparatus 1 includes a vacuum tank 5 including an upper tank 6 and a lower tank 7, an ascending-side dip pipe 8 and a descending-side dip pipe 9 provided below the lower tank 7. The upper tank 6 is provided with a duct 11 connected to an exhaust device (not shown), a raw material inlet 12, and an upper blowing lance 13 that is movable in the vertical direction inside the vacuum tank 5, The ascending-side dip tube 8 is provided with a circulating gas blowing tube 10. From the reflux gas blowing tube 10, Ar gas is blown into the rising side immersion tube 8 as the reflux gas. The upper blowing lance 13 is configured to be able to blow oxygen gas toward the molten steel 3 inside the vacuum chamber 5. Of course, only a rare gas is blown or a mixed gas of a rare gas and an oxygen gas can be blown. The ascending-side dip tube 8 and the descending-side dip tube 9 have a structure in which a metal core (not shown) is provided at a substantially central portion of the thickness, and MgO refractories or MgO-C refractories are applied to both sides thereof. (See JP 2005-264263 A).

  In the RH vacuum degassing apparatus 1 configured as described above, first, a ladle 2 containing the molten steel 3 whose upper surface is covered with the slag 4 is conveyed directly under the vacuum tank 5, and the ladle 2 is moved up and down. The ascending-side dip tube 8 and the descending-side dip tube 9 are immersed in the molten steel 3 accommodated in the pan 2. Next, Ar gas is blown into the inside of the rising side dip tube 8 from the reflux gas blowing tube 10 as a reflux gas, and the inside of the vacuum chamber 5 is exhausted by an exhaust device connected to the duct 11. Depressurize the inside. When the inside of the vacuum chamber 5 is depressurized, the molten steel 3 accommodated in the ladle 2 ascends the rising side dip tube 8 by the gas lift effect together with Ar gas blown from the reflux gas blow tube 10, and After flowing into the inside, a flow returning to the ladle 2 through the descending side dip pipe 9, that is, a so-called recirculation flow is formed and RH vacuum degassing is performed.

  When the inside of the vacuum chamber 5 is in a reduced pressure atmosphere, the CO gas partial pressure in the inner atmosphere of the vacuum chamber 5 is significantly smaller than that at the time of converter decarburization and refining performed under atmospheric pressure. When exposed to a reduced pressure atmosphere inside the steel, a reaction between C in the molten steel and dissolved oxygen occurs. That is, a decarburization reaction occurs, C contained in the molten steel 3 becomes CO gas and is discharged together with the exhaust gas from the vacuum tank 5 through the duct 11, and the molten steel 3 is subjected to vacuum decarburization refining. In this case, when the decarburization reaction is delayed due to a shortage of dissolved oxygen in the molten steel 3, oxygen gas or oxygen gas and rare gas from the top blowing lance 13 toward the molten steel 3 inside the vacuum chamber 5. The decarburization reaction can be promoted by spraying a mixed gas.

  In this way, if the vacuum decarburization refining is continued and the C content of the molten steel 3 reaches a predetermined value of 0.003% by mass or less, a deoxidizer such as Si or Mn is supplied from the raw material inlet 12 to the molten steel 3. Is added to deoxidize the molten steel 3. By adding a deoxidizing agent such as Si or Mn, the dissolved oxygen in the molten steel 3 rapidly decreases, and the decarburization reaction ends accordingly. Then, after adding the deoxidizer, the recirculation is continued for about several minutes. A component adjusting agent such as is introduced into the molten steel 3 from the raw material inlet 12 to adjust the components of the molten steel 3. After the component adjustment, the inside of the vacuum chamber 5 is returned to atmospheric pressure, and the RH vacuum degassing refining is completed.

  During this RH vacuum degassing refining, the outer walls of the ascending side dip tube 8 and the descending side dip tube 9 and the slag 4 are kept in contact with each other, but the composition of the slag 4 has a CaO content of 50 to 70% by mass and an MgO content. Is adjusted in the range of 10 to 30% by mass in advance, so that a decrease in reactivity of the slag 4 to the refractory due to an increase in the melting point of the slag 4 and suppression of elution of MgO into the slag 4 are suppressed. Therefore, the melting damage by the slag 4 of the ascending side dip tube 8 and the descending side dip tube 9 is suppressed.

  Hereinafter, the C content is 0.0024% by mass or less, the Al content is 0.001% by mass or less, the Si content is 0.4 to 0.7% by mass, and the Mn content is 0.1 to 0.3% by mass. An example in which an Al-less ultra-low carbon steel having a% and S content of 0.003 mass% or less is melted by applying the present invention will be described. The ascending side dip tube and the descending side dip tube of the used RH vacuum degassing apparatus are both constructed of MgO refractories.

  The hot metal discharged from the blast furnace is received by a torpedo car, and the received hot metal is transferred from the torpedo car to a ladle-type converter charging pot, and a CaO-based desulfurizing agent is added to the converter charging pot and rotated. The hot metal and the desulfurizing agent were stirred by immersing the stirring blade to perform a preliminary desulfurization treatment. After that, hot metal was charged into the converter and decarburization refining of the hot metal was performed. Molten steel components at the time of steel leaving the converter are C: 0.04 mass%, Si: trace, Mn: 0.12 mass%, P: 0.03 mass%, S: 0.002 mass%, and the molten steel temperature is 1670 ° C.

  After completion of decarburization and refining in the converter, quick lime having a particle size of 20 mm or less as a CaO source is added to the ladle to 3.0 kg per ton of molten steel, and grains are used as the MgO source when steel is discharged from the converter to the ladle. A magnesia clinker having a diameter of 10 mm or less was added into a 2.0 kg ladle per ton of molten steel. Then, the ladle was conveyed to the RH vacuum degassing apparatus, and first, vacuum decarburization refining was performed. When the C content in the molten steel reached 0.001% by mass by vacuum decarburization refining, the Fe-Si alloy was charged from the raw material inlet to finish the vacuum decarburization refining, and then continued to reflux for several more minutes. Thereafter, Fe—Si alloy and Fe—Mn alloy were added to adjust the components, and RH vacuum degassing refining was completed. During RH vacuum degassing, slag was collected from the ladle and analyzed for slag composition. Further, after the RH vacuum degassing refining, the erosion state of the ascending side dip tube and the descending side dip tube was analyzed based on the image of the CCD camera.

As a result, the slag composition was CaO: 56.1% by mass, SiO ₂ : 19.1% by mass, MgO: 15.3% by mass, Al ₂ O ₃ : 9.5% by mass, Almost no erosion is observed in the descending dip tubes, and it is possible to treat the Al-less ultra-low carbon steel continuously for 5 charges without performing repairs based on the erosion status of these dip tubes. Was confirmed. On the other hand, in the conventional case where the CaO source and the MgO source were not added to the pan, the slag composition was CaO: 33.1% by mass, SiO ₂ : 31.2% by mass, MgO: 13. 4% by mass and Al ₂ O ₃ : 22.3% by mass. Both the ascending side dip tube and the descending side dip tube are severely damaged, and the ascending side dip tube and the descending side each time the Al-less ultra-low carbon steel is processed. It is necessary to repair the dip tubes, and it was confirmed that the erosion damage of these dip tubes is greatly suppressed by the present invention.

It is the longitudinal cross-sectional schematic of the RH vacuum degassing apparatus used when implementing this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 RH vacuum degassing apparatus 2 Ladle 3 Molten steel 4 Slag 5 Vacuum tank 6 Upper tank 7 Lower tank 8 Rising side immersion pipe 9 Lowering side immersion pipe 10 Recirculation gas blowing pipe 11 Duct 12 Raw material inlet 13 Upper blowing lance

Claims (3)

  When molten steel refined in a converter is decarburized with an RH vacuum degasser to produce an Al-less ultra-low carbon steel with an Al content of 0.001% by mass or less, Or after adding steel, a CaO source and a MgO source are added to a ladle containing molten steel, and the CaO content in the slag present on the molten steel in the ladle is 50 to 70% by mass, and the MgO content is 10 to 10. A method for melting Al-less ultra-low carbon steel, which is adjusted to 30% by mass and then refined with an RH vacuum degassing apparatus.   2. The molten Al-less ultra-low carbon steel according to claim 1, wherein after decarburizing with an RH vacuum degassing apparatus, the molten steel in the vacuum chamber is added with a Si source to deoxidize the molten steel. Manufacturing method.   The method for melting an Al-less ultra-low carbon steel according to claim 1 or 2, wherein quick lime is used as the CaO source and magnesia clinker is used as the MgO source.

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