JP2008260997A - Desulfurization method of molten steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a desulfurization method of molten steel in a secondary refining step in steelmaking, which can achieve a high desulfurization efficiency in the secondary refining step by inhibiting the decrease in the desulfurization performance due to the presence of Al<SB>2</SB>O<SB>3</SB>inclusions, can perform desulfurization at a region with low sulfur contents by using a reduced amount of a desulfurization agent, and can reduce refractory erosion. <P>SOLUTION: In the desulfurization method of the molten steel, when molten steel is tapped to a ladle from a converter after decarburizing and refining, ferrosilicon is charged in an amount of ≥2 kg in terms of Si, per 1t molten steel. In a vacuum degassing facility in the subsequent secondary refining step, Al is charged in an amount of ≥0.2 kg per 1t molten steel. The molten steel is circulated for 3-10 min, and the desulfurization agent is subsequently blown in together with Ar. Preferably, the desulfurization agent comprises CaO, CaF<SB>2</SB>, MgO and unavoidable impurities, wherein the MgO content is 10-40 mass% and the composition satisfies the relation: 1.0≤CaO/CaF<SB>2</SB>≤3.0. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for desulfurizing molten steel in a steelmaking secondary refining process.

  The desulfurization treatment when producing molten steel is generally performed in the hot metal pretreatment process, but since the slag and scrap are recovered from the slag and scrap in the next converter process, the secondary refining process is used in low-sulfur steel refining. But desulfurization is performed to reduce the sulfur to a level that meets the quality requirements of the steel.

Since the molten steel temperature at the time of secondary refining is 1550 to 1650 ° C., a CaO-based desulfurizing agent that does not easily decompose or evaporate at high temperatures is used. Since CaO has a high melting point of 2500 ° C. or more and is poor in reactivity when used alone, it is usually used together with a melting accelerator (enamelling agent) such as CaF ₂ (fluorite), Al ₂ O ₃ , or SiO ₂ . In particular, since CaF ₂ has a melting point as low as 1423 ° C., it has a large melting acceleration effect and does not lower the desulfurization ability of CaO. As an example using a fluorinating agent other than CaF ₂ , a desulfurization agent that defines a compounding ratio of CaO, Al ₂ O ₃ , MgO or the like as disclosed in Patent Document 2, for example, is disclosed.

The desulfurization reaction is represented by the following formula (1), and progresses more easily as the oxygen (dissolved oxygen) in the molten steel is lower. Therefore, it is usual to perform deoxidation treatment by adding metal Al before the desulfurization treatment, at the time of steel leaving the converter or at the initial stage of secondary refining.
CaO + S = CaS + O (1)

In addition, Al ₂ O ₃ inclusions are generated in the molten steel due to deoxidation at the time of steel extraction or secondary refining. Since the Al ₂ O ₃ inclusions often form coarse clusters, when combined with the desulfurizing agent particles, the concentration of Al ₂ O ₃ in the desulfurizing agent particles is 40 to 50% by mass or more, and the sulfide capacity is greatly reduced. . Therefore, in Patent Document 3, a method of blowing a desulfurizing agent after floating Al ₂ O ₃ inclusions after a predetermined time from Al input, and in Patent Document 4 for removing Al ₂ O ₃ inclusions before adding the desulfurizing agent. A method for blowing powder is disclosed.

JP 60-59011 A JP-A-61-106706 JP-A-62-205220 JP-A-62-196317

  However, in the method of blowing a desulfurization agent after a predetermined time has passed since Al input as in Patent Document 3, when Al is input at the time of steel output, dissolved oxygen decreases once immediately after Al input, but thereafter oxygen absorption from the atmosphere Therefore, the yield of Al is low and it is difficult to obtain a large deoxidation effect. In addition, when Al is added in the secondary refining process, the desulfurization agent is added after the molten steel is circulated for substantially 10 minutes or more.

When the powder for removing Al ₂ O ₃ inclusions is blown in advance as in Patent Document 4, since the appropriate range of usage is unknown, the amount of powder is insufficient relative to the amount of Al ₂ O ₃ inclusions. Cannot suppress a decrease in desulfurization ability, and if excessive, powder remains as inclusions. In addition, since a plurality of powders are used, a new equipment investment such as a tank is required.

When desulfurizing to a low sulfur region, a desulfurizing agent containing CaF ₂ as in Patent Document 1 is advantageous because a melting rate is faster than a desulfurizing agent not containing CaF ₂ as in Patent Document 2. A large amount of desulfurization agent not containing CaF ₂ and long-time treatment are not impossible, but are substantially difficult. Therefore, CaF ₂ is indispensable as a desulfurizing agent component, but on the other hand, it also has an action of eroding refractories. Therefore, when used in a large amount, the cost and non-operating time associated with refractory repair increase. In addition, the combination of the desulfurizing agent and the Al ₂ O ₃ inclusion not only inhibits the desulfurization but also increases the amount of the desulfurizing agent used, and promotes refractory damage due to CaF ₂ .

  In view of these problems, the present invention improves the desulfurization efficiency in the secondary refining process, so that even if the amount of desulfurizing agent used is less than that of the prior art, it can be desulfurized to a low sulfur region, and further, the refractory melting loss is reduced. It aims to provide a method.

  The present inventors have conducted intensive research to solve the problems of the prior art. As a result, by using Si deoxidation, the dissolved oxygen before the secondary refining process is stably controlled, Al is introduced in the subsequent secondary refining process, and a high desulfurization rate is achieved by blowing a desulfurizing agent after a predetermined time. I found a way to get it. This method can also reduce refractory melt damage.

The gist of the present invention is as follows.
(1) When the molten steel is discharged from the converter to the ladle after decarburization refining, 2 kg or more of ferrosilicon is added per 1 ton of molten steel in terms of Si, and in the vacuum degassing equipment in the subsequent refining process, 1 t of molten steel is added. A desulfurization method for molten steel, wherein 0.2 kg or more per unit is charged and the desulfurizing agent is blown together with Ar after circulating the molten steel for 3 minutes or more and 10 minutes or less.
(2) A desulfurization agent composed of CaO, CaF ₂ , MgO and unavoidable impurities, the composition satisfying 1.0 ≦ CaO / CaF ₂ ≦ 3.0, and MgO being 10% by mass or more and 40% by mass or less. The method for desulfurizing molten steel according to claim 1, wherein:

  According to the present invention, a high desulfurization rate can be obtained even if the amount of the desulfurizing agent used is reduced, and the refractory melt damage can be reduced.

  The best embodiment of the present invention will be described in detail.

  Since the molten steel that has been decarburized and refined in the converter contains 400 to 1000 ppm of oxygen, 5 kg or more of ferrosilicon is added per 1 ton of molten steel in terms of Si at the time of steel output. FIG. 1 shows the relationship between the amount of Si added (kg / t) and dissolved oxygen (ppm). It can be seen that the change in dissolved oxygen with respect to the amount of Si added is small in the region of 5 kg / t or more. That is, if 5 kg / t or more is added, dissolved oxygen can be stably controlled, and even if oxygen absorption from the atmosphere occurs, a rapid change in dissolved oxygen is unlikely to occur.

  The upper limit of the amount of ferrosilicon input is determined according to the final product Si. Although the present invention cannot be applied to a steel grade having a low final product Si, low-sulfur steel is often added with ferrosilicon to develop high strength like a high-strength steel plate for automobiles. The present invention exhibits a great effect.

  Next, Al is added and deoxidized in the secondary refining equipment. If vacuum degassing equipment such as RH is used as equipment, hydrogen pick-up due to moisture contained in the desulfurizing agent powder added to the molten steel after deoxidation can be prevented. FIG. 2 shows the relationship between the amount of added Al (kg / t) and dissolved oxygen (ppm). It can be seen that the dissolved oxygen is greatly reduced by the addition of 0.2 kg / t Al. Therefore, the lower limit of the amount of Al added is 0.2 kg / t. Even if the Al addition amount exceeds 0.6 kg / t, the further decrease in dissolved oxygen is only small and the cost is high, so the upper limit of the Al addition amount is preferably 0.6 kg / t.

When Al is added, the reactions of formulas (2) and (3) occur.
2 Al +3 O = Al ₂ O ₃ (2)
4 Al + 3SiO ₂ = 2Al ₂ O ₃ +3 Si (3)
Formula (2) is a deoxidation reaction of dissolved oxygen by Al, and Formula (3) is a reaction in which Al reduces SiO ₂ inclusions generated by adding ferrosilicon during steel output. Here, since the Al ₂ O ₃ inclusions produced by the reaction of the formula (2) form a cluster exceeding 100 μm, when combined with the desulfurizing agent particles, the desulfurization ability is greatly reduced. On the other hand, since the Al ₂ O ₃ inclusions produced by the reaction of formula (3) have a particle size of generally less than 50 μm, the concentration of Al ₂ O _{3 in the} particles hardly increases even when combined with the desulfurizing agent particles. . In the research by the inventors, the increase in the concentration of Al ₂ O ₃ in this case is 10% by mass or less, and at this level, Al ₂ O ₃ promotes the hatching of CaO and contributes to the improvement of the desulfurization rate. I found out. Thus, since it is necessary to reduce the SiO ₂ to make a minute Al ₂ O ₃ inclusion, if ferrosilicon and Al are added simultaneously at the time of steelmaking, Al ₂ O ₃ is generated as it is. The effect of the present invention is difficult to obtain.

Even if the same Al ₂ O ₃ inclusions are produced by the formula (2) and the formula (3), the influence on the desulfurizing agent particles is different. Try to separate. The circulation here means an operation of sucking and flowing molten steel from a ladle to a vacuum tank of a vacuum degassing facility, and for example, treatment with RH or DH is applicable thereto. FIG. 3 shows the relationship between the circulation time after the addition of Al and the Al ₂ O ₃ inclusion index. This index is expressed as a value relative to the amount of Al ₂ O ₃ inclusions immediately after addition of Al as 1. When Al is added for 3 minutes or more after circulation, coarse Al ₂ O ₃ inclusions are reduced to less than half, but when it is circulated for 10 minutes or more, minute Al ₂ O ₃ inclusions are also reduced to less than half.

FIG. 4 shows the relationship between the circulation time and the desulfurization rate. The desulfurization rate is defined by equation (4).
Desulfurization rate = ([Si] _i − [Si] _f ) / [Si] _i × 100 (4)
Here, [S] _i is the S concentration in the initial molten steel, and [S] _f is the S concentration in the end-point molten steel. Circulation time coalescence of coarse Al ₂ O ₃ inclusions and the desulfurization agent particles is less than 3 minutes for the 10 minutes than to decrease both desulfurization rate by a reduction in the fine Al ₂ O ₃ inclusions, Al after addition A suitable circulation time is 3 minutes or more and 10 minutes or less. Since the desulfurization reaction proceeds more easily at higher temperatures, long-term circulation is not preferable from the viewpoint of lowering the molten steel temperature. The upper limit of the circulation time is more preferably 5 minutes or less.

Although the reduction reaction of formula (3) does not occur completely with some inclusions and the inclusions may have an Al ₂ O ₃ —SiO ₂ composite composition, SiO ₂ is also composed of CaO in the same manner as Al ₂ O _3. Has hatching action. At this time, the sum of the Al ₂ O ₃ and SiO ₂ concentration increases should be 10% by mass or less, and the same effect can be obtained even with Al ₂ O ₃ —SiO ₂ composite inclusions.

The dip tube for sucking molten steel into the vacuum chamber is always in contact with the slag on the molten steel surface during processing, and is most easily damaged by using a desulfurizing agent containing CaF ₂ . However, since the present invention can obtain a high desulfurization rate even if the amount of the desulfurizing agent used is less than that of the prior art, it is possible to reduce the refractory melt damage.

Furthermore, the present invention has found that it is possible to increase suppress refractory erosion Using CaO-CaF ₂ -MgO based desulfurizing agent. This is because CaF ₂ is diluted by the presence of MgO, and the erosion action of the refractory is suppressed. However, when CaF ₂ is diluted, the melting rate of CaO is slowed and the desulfurization rate is likely to decrease, so that there is a suitable composition range.

First, the ratio of CaO and CaF ₂ is 1.0 ≦ CaO / CaF ₂ ≦ 3.0. When CaO / CaF ₂ <1.0, the CaF ₂ ratio is high, so that the refractory melts severely. When CaO / CaF ₂ > 3.0, hatching is insufficient and it is difficult to obtain a high desulfurization rate. MgO is 10 mass% or more and 40 mass% or less. If MgO is less than 10% by mass, the CaF ₂ dilution effect is difficult to obtain, and if it exceeds 40% by mass, hatching tends to be insufficient. The ratio of MgO is more preferably 20% by mass or more and 35% by mass or less.

  When taking 400 t of molten steel decarburized and refined in a converter into a ladle, ferrosilicon having a Si concentration of 75% was introduced. Next, Al was introduced and circulated in RH, which is a secondary refining process, and then desulfurized by blowing a powder desulfurizing agent from a lance immersed in a ladle using Ar as a carrier gas.

The S concentration before RH treatment was 30 to 40 ppm, the temperature during treatment was 1580 to 1620 ° C., and the blowing rate of the desulfurizing agent was 50 kg / min. This process was performed for 5 consecutive charges at the level shown in Table 1. After the continuous treatment, the inner diameter change of the RH dip tube (Al ₂ O ₃ —MgO refractory) was measured to determine the amount of erosion per charge. Levels 1 to 11 are invention examples, and levels 12 to 15 are comparative examples. In the table, “*” indicates that the present invention is outside the scope of the present invention.

  Table 2 shows the test results in the case of performing desulfurization treatment by adding the desulfurization agent shown in levels 1 to 15 using each molten steel treated in Table 1 above, as an average value of 5 charges. The desulfurization treatment was carried out until the S concentration became 10 ppm or less. However, in Comparative Examples 12 and 13, the progress of the desulfurization was slow, and there was a concern that the molten steel temperature was abnormally lowered and the dip tube was abnormally damaged due to the long-time treatment. The treatment was terminated when the desulfurizing agent basic unit reached 3.8 kg / t for 30 minutes.

In Examples 1 to 11 as examples of the present invention, a high desulfurization rate of 70% or more was obtained. In particular, at levels 4 to 7 where the desulfurizing agent composition satisfies 1.0 ≦ CaO / CaF ₂ ≦ 3.0 and MgO is 10% by mass or more and 40% by mass or less, the amount of dip tube erosion is less than 3 mm per charge. Was suppressed. The results are shown in FIGS. In Levels 1 to 3 or Levels 8 to 9, although the dip tube erosion amount slightly increased, it was less than 5 mm per charge. In levels 10 and 11, 3.5 to 3.7 kg / t of desulfurizing agent basic unit was required until the S concentration was 10 ppm or less, but the dip tube erosion amount was less than 3 mm per charge.

  On the other hand, since level 12 as a comparative example did not input ferrosilicon at the time of steel output, level 13 as a comparative example had a small amount of Al input at RH, so that the S concentration did not reach 10 ppm and the dip tube Melted more than 8 mm per charge. Levels 14 and 15, which are comparative examples, both reached an S concentration of 10 ppm or less, but the former was too short for the circulation time after the Al was charged, and the latter was 3.0 kg because the circulation time after the Al was too long. A desulfurizing agent of at least / t was required, and the dip tube melted 6.5 to 6.9 mm per charge.

It is a figure which shows the relationship between Si addition amount per 1t of molten steel, and dissolved oxygen. It is a figure which shows the relationship between the amount of Al addition per 1t of molten steel, and dissolved oxygen. It is a figure which shows the relationship between the circulation time after Al addition, and an inclusion index. It is a figure which shows the relationship between the circulation time after Al addition, and a desulfurization rate. Is a diagram showing the relationship between CaO / CaF ₂ and the desulfurization rate and dip tube erosion amount. It is a figure which shows the relationship between MgO, a desulfurization rate, and the amount of dip pipe erosion.

Claims (2)

  When the molten steel is discharged from the converter to the ladle after decarburizing and refining, 5 kg or more of ferrosilicon is added per 1 ton of molten steel in terms of Si. A desulfurization method for molten steel, wherein 2 kg or more is charged and the desulfurization agent is blown together with Ar after circulating the molten steel for 3 minutes or more and 10 minutes or less. The desulfurizing agent is composed of CaO, CaF ₂ , MgO and inevitable impurities, the composition satisfies 1.0 ≦ CaO / CaF ₂ ≦ 3.0, and MgO is 10 mass% or more and 40 mass% or less. The method for desulfurization of molten steel according to claim 1, characterized in that it is present.

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