JP5671801B2 - Converter refining method - Google Patents

Description

  The present invention relates to a converter refining method for refining steel using an upper bottom blow converter.

  When refining hot metal using a converter, among the impurities in the hot metal to be removed by refining, only S is removed by refining with hot metal desulfurization, and Si, P, and C are refined once in the converter. It was removed.

  Thereafter, a hot metal preliminary dephosphorization method was developed, and Si and P in the hot metal were also removed by a preliminary treatment using a torpedo car or a hot metal ladle, and only decarburization refining was performed in the converter. In addition, a method of using two converters, performing dephosphorization refining in the first converter, and then performing decarburization refining in the second converter was also adopted.

  However, in preliminary refining using hot metal transfer containers such as torpedo cars and hot metal ladle, the capacity of the container is small and it is difficult to carry out strong stirring refining. Has the disadvantages of using more flux than necessary and requiring a long time for refining. Although this disadvantage can be improved by using a converter vessel for dephosphorization, it requires two converters including decarburization, which increases the equipment cost and increases the heat loss. And scrap melting capacity decreases.

In view of this, a method has been developed in which an upper bottom blow converter is used to perform dephosphorization refining in the same converter, then to remove slag generated by dephosphorization refining, and then decarburization refining. As described in Patent Document 1, in the first dephosphorization refining, the dephosphorization reaction proceeds to almost equilibrium between the molten iron and the flux by appropriately applying the bottom blowing stirring power. In the range of ˜1450 ° C., CaO / SiO _{2 in} the slag after treatment is 0.6 to 2.5 depending on the temperature, and the target dephosphorization amount is sufficiently reached. In addition, when removing slag after dephosphorization, it is preferable to discharge slag in a forming state. Therefore, bottom blowing gas is continuously blown, and slag can be slag in a short time while preventing the slag from being scattered by the pre-furnace fender. It can be discharged.

  In Patent Documents 2 to 4, when performing dephosphorization refining, slag removal, and decarburization refining in the same converter, the slag produced by decarburization refining is left in the converter and the next heat is generated. A method of using the decarburization slag of the previous heat as a dephosphorization refining flux of the next heat is disclosed. In addition, a gas injection port is provided not only in the furnace bottom but also in the furnace belly, and when the converter is tilted and slag is discharged after dephosphorization refining, bubbles are blown into the slag by gas injection from the furnace belly to form the slag. It is described that smooth fluid waste is performed while iron loss is minimized.

Japanese Patent No. 3239197 Japanese Patent Laid-Open No. 4-72007 Japanese Patent No. 2607328 Japanese Patent No. 2558292

  By using a top-bottom blow converter, converter refining with dephosphorization, slag removal, and decarburization refining in the same converter reduces overall refining time and reduces the amount of flux required for dephosphorization. It became possible to reduce heat loss in refining. However, with this method, it is difficult to extremely reduce the P concentration in the refined molten steel, and the extremely low phosphorus steel of P standards, for example, P ≦ 0.010% steel is stably melted. Had the problem that it was difficult to do.

  The present invention provides a converter refining capable of stably melting even the extremely strict P-standard extremely low phosphorus steel while enjoying the above-mentioned merit by performing dephosphorization refining and decarburization refining in the same converter. It aims to provide a method.

In the conventional converter refining method that uses a top-bottom blowing converter and performs dephosphorization refining, slag removal, and decarburization refining in the same converter, all the slag removal after dephosphorization refining is present in the converter. This slag cannot be removed, and the next decarburization refining is started while a part of the slag containing a large amount of P ₂ O ₅ remains. It was found that demelting and refining increased the molten metal temperature and caused rephosphorization from the slag to the molten metal, which increased P concentration in the molten steel after decarburization and refining.

  In order to completely remove the slag after dephosphorization and perform decarburization and refining, after dephosphorization and refining, the slag remains in the converter, the molten metal is discharged out of the furnace, and then the slag is discharged into the converter. It is necessary to charge the molten metal again, but when dephosphorization and decarburization are performed in the same converter, it takes a long time for refining, and when dephosphorization and decarburization are performed in another converter, the equipment costs Will take. In addition, heat loss increases because the hot water is discharged in the middle.

In contrast, after the first dephosphorization and subsequent slag removal, before the decarburization and refining, the flux is added to perform the second dephosphorization, and then the slag is removed. It was found that by performing decarburization refining, the P concentration in the molten steel after completion of decarburization refining can be sufficiently reduced to an extremely low P steel level. The second dephosphorization refining can further reduce the P level in the molten metal, and the P ₂ O ₅ level in the slag after the second dephosphorization refining is higher than the P ₂ O ₅ level in the slag after the first dephosphorization refining. This is because even if slag remains in the converter after slag removal, it is possible to keep the phosphorus recovery during decarburization refining low.

This invention is made | formed based on the said knowledge, The place made into the summary is as follows.
(1) When refining steel with a P concentration of 0.010% by mass or less using a top-bottom blow converter, the hot metal was charged into the converter in the first step and the flux was used in the second step Hot metal dephosphorization is performed by top bottom blowing refining, the converter is tilted in the third step, and part or all of the slag generated in the second step is discharged, and flux is added in the fourth step to add the top bottom of the converter Hot metal dephosphorization is performed by blowing refining, the converter is tilted in the fifth step, and part or all of the slag generated in the fourth step is discharged, and decarburization is performed by blowing the top and bottom of the converter in the sixth step. And, as the flux added in the fourth step, CaO-containing flux and SiO ₂ -containing flux are used, and the slag CaO / SiO ₂ (mass ratio) at the end of the fourth step is set to 1.5 to 2.0, The molten metal temperature at the end of the four steps is 1400 ° C. or lower and the C concentration in the molten metal is 3.0. Converter refining method which is characterized in that the amount% or more.
(2) As a flux used in the second step, a flux to be added to the converter at the start of the second step, a flux generated in the sixth step in the previous heat before the first step, and a part or all of the flux in the converter One or both of the remaining ones are used. The converter refining method according to (1) above, wherein one or both of them are used.
(3) After completion of the sixth step, in the seventh step, the converter is tilted to leave the molten steel while leaving the slag, and the estimated P ₂ O ₅ component of the slag in the converter and the steel P component target of the next heat Based on one or both of the values, as the eighth step, either one of the process of leaving the entire amount of slag in the converter, leaving only a part and discharging the other, or discharging the whole amount is selected. Then, the converter refining method according to the above (1) or (2), wherein the process proceeds to the first step of the next heat.

  In the present invention, when refining steel using a top-bottom blow converter, first dephosphorization and subsequent slag removal are performed, then flux is added to perform second dephosphorization, and then slag Since removal is performed and then decarburization refining is performed, the P concentration in the molten steel after completion of decarburization refining can be sufficiently reduced to an extremely low P steel level.

It is a conceptual diagram which shows the flow of the process of this invention. It is a figure which shows distribution of P density | concentration after a converter process about the example of this invention and a comparative example.

  The steel refining of the present invention is performed using an upper bottom blowing converter. Oxidation refining is advanced while stirring the molten metal with top blowing oxygen, and stirring of the molten metal during refining is enhanced with bottom blowing gas. Further, when removing the slag by tilting the converter, the slag can be formed by continuously blowing the bottom blowing gas, and the slag can be quickly discharged from the furnace port while the molten metal remains in the converter.

  Next, the present invention will be described in detail with reference to FIG.

  In the first step, the hot metal is charged into the upper bottom blowing converter 1 using the hot metal ladle 2, and in the second step, hot metal dephosphorization is performed by converter bottom bottom blowing refining using flux. The top blow refining is performed by the top blow lance 7. When scrap is used together with hot metal as a charging main material, scrap charging is also performed prior to hot metal charging in the first step. The flux used in the second step is to add the flux raw material into the furnace after the hot metal is charged, to leave the slag after decarburization refining of the previous heat in the converter, or to add the flux to the remaining slag of the previous heat. It can be selected from methods of adding a flux raw material to obtain a flux.

The hot metal charged into the converter in the first step contains P and C as impurities, and usually contains Si. Therefore, in the dephosphorization refining in the second step, first, Si in the hot metal is burned by deoxidation with oxygen gas, desiliconization proceeds, and then dephosphorization reaction proceeds. When a CaO-containing flux such as quick lime or calcined dolomite is added as a flux material in the second step, this becomes a CaO source in the slag. SiO ₂ produced by the desiliconization reaction of the hot metal becomes the SiO ₂ source in the slag. When the preheated decarburized slag is left as a flux, both the CaO and SiO ₂ are contained in the decarburized slag.

In order to rapidly advance the dephosphorization reaction in the hot metal dephosphorization in the second step, the post-treatment temperature in the second step is adjusted to a range of 1200 to 1450 ° C. Further, the CaO / SiO ₂ mass ratio in the treated slag is adjusted to a range of 0.6 to 2.5 according to the temperature. The lower the temperature, the more dephosphorization can proceed at a lower CaO / SiO ₂ ratio. In addition, T. If the Fe concentration is too low, dephosphorization is hindered due to insufficient oxygen potential, and if it is too high, dephosphorization is still hindered because the concentration of basic components in the slag is diluted. When the Fe concentration is in the range of 10 to 30% by mass, good dephosphorization can be performed. Furthermore, the dephosphorization reaction can be rapidly advanced by sufficiently stirring the molten metal with the bottom blowing gas by using the top bottom blowing converter.

  The converter 1 is tilted in the third step, and part or all of the slag generated in the second step is discharged. The slag 9 is discharged to the slag pan 4 on the slag pan cart 6. In order to prevent rebound after the next step, it is preferable to discharge as much slag as possible in the third step. Therefore, the bottom blowing gas is continuously blown during the slag discharge, and the slag is formed by the bottom blowing gas to promote the slag discharge. Here, when the molten metal temperature at the end of the second step is less than 1200 ° C., the slag is not sufficiently hatched, so that the waste in the third step does not proceed and the waste rate does not reach 60%. On the other hand, if the molten metal temperature at the end of the second step exceeds 1450 ° C., the rejection rate does not reach 60%. If it is the range of 1200-1450 degreeC, while being able to perform a dephosphorization reaction favorably in a 2nd process, the slag discharge | emission in a 3rd process can be performed favorably.

  In the present invention, after the dephosphorization refining in the second step and the slag discharge in the third step are completed, the process is not immediately shifted to the decarburization refining, but the dephosphorization refining as the fourth step and the slag discharge as the fifth step. Followed by decarburization refining as a sixth step.

In the conventional converter refining method in which decarburization refining is performed immediately after the dephosphorization refining in the second step and the slag removal in the third step, in the slag removal after the dephosphorization refining, The slag cannot be removed, and the next decarburization refining is started while a part of the slag containing a large amount of P ₂ O ₅ remains. As the molten metal temperature increased during decarburization refining, rephosphorization from the slag to the molten metal occurred, which increased the P concentration in the molten steel after completion of decarburization refining.

On the other hand, in the present invention, after dephosphorizing in the second step and slag removal in the third step, before decarburizing and refining, a flux is added as the fourth step and the second dephosphorizing is performed. Then, it is found that the P concentration in the molten steel after decarburization refining can be sufficiently reduced to the extremely low P steel level by performing slag removal in the fifth step and then decarburizing and refining as the sixth step. It was. Fourth with a can be further reduced melt in P level by dephosphorization refining process, the slag in P ₂ O ₅ levels after dephosphorization refining fourth step slag P ₂ O ₅ after dephosphorization refining in the second step This is because, since it is lower than the level, even if slag remains in the converter after the removal of slag, it is possible to keep the recovery from decarburization refining low.

  In addition, since the dephosphorization is performed in two stages of the second process and the fourth process, the P concentration in the molten metal before the decarburization and refining of the sixth process is lower than that in the conventional one-stage dephosphorization. It can be. Therefore, in the decarburization refining in the sixth step of the present invention, it becomes possible to reduce the recovery P prevention flux basic unit added at the time of decarburization refining as compared with the conventional one-step dephosphorization refining. In addition, since light dephosphorization is sufficient for the dephosphorization refining in the fourth step of the present invention, a small amount of flux for dephosphorization is sufficient. As a result, despite the fact that the present invention performs the dephosphorization in two stages, the total flux unit used is kept to the same level as the flux unit used in the conventional one-step dephosphorization. Was found to be possible.

In order to allow the dephosphorization reaction to proceed rapidly in the hot metal dephosphorization in the fourth step, the post-treatment temperature in the fourth step is adjusted to a range of 1200 to 1450 ° C. The lower the post-treatment temperature in the fourth step, the more advantageous for dephosphorization. The post-treatment temperature in the fourth step is preferably 1400 ° C. or lower. Further, the CaO / SiO ₂ mass ratio in the treated slag is adjusted to a range of 0.6 to 2.5 according to the temperature. Since the hot metal desiliconization reaction is completed in the second step, in order to adjust the CaO / SiO ₂ mass ratio in the slag to a suitable range in the fourth step, a CaO source is added as a flux, and usually SiO 2 _Two sources also need to be added. The lower the temperature, the more dephosphorization can proceed at a lower CaO / SiO ₂ ratio. In addition, T. If the Fe concentration is too low, dephosphorization is hindered due to insufficient oxygen potential, and if it is too high, dephosphorization is still hindered because the concentration of basic components in the slag is diluted. When the Fe concentration is in the range of 10 to 30% by mass, good dephosphorization can be performed. Furthermore, the dephosphorization reaction can be rapidly advanced by sufficiently stirring the molten metal with the bottom blowing gas by using the top bottom blowing converter.

When the P concentration in the hot metal charged into the converter in the first step is about 0.100% by mass, and the P concentration in the molten steel after the refining of the converter is reduced to 0.010% by mass or less, the second When the molten P concentration after completion of dephosphorization in the process is about 0.030 to 0.040 mass%, and the molten P concentration after completion of dephosphorization in the fourth process is about 0.020 to 0.025 mass%. preferable. In this case, the CaO / SiO ₂ mass ratio in the slag and the T. When the Fe concentration and bottom blowing stirring power are in the above preferred ranges, the melt temperature at the end of the second step is about 1300 to 1350 ° C., and the melt temperature at the end of the fourth step is about 1340 to 1400 ° C., the second step, The basic unit of flux in the fourth step can be minimized and good dephosphorization can be performed.

That is, as a flux to be added in the fourth step, a CaO-containing flux and a SiO ₂ -containing flux are used, and the slag CaO / SiO ₂ (mass ratio) at the end of the fourth step is set to 1.5 to 2.0. The molten metal temperature at the end is preferably 1400 ° C. or lower.

At this time, the flux basic unit in the second step is about 0 to 20 kg / ton in terms of CaO as the CaO source, and the flux basic unit in the fourth step is about 5 to 10 kg / ton in terms of CaO as the CaO source. In the fourth step, about 2 to 5 kg / ton is added as a SiO ₂ source in terms of SiO ₂ . As the CaO source to be added as the flux, quick lime, limestone, calcined dolomite and the like can be used. Moreover, soft silica, olivine, and Fe—Si can be used as the SiO ₂ source added as a flux.

  The converter is tilted in the fifth step, and part or all of the slag generated in the fourth step is discharged. The slag discharge in the fifth step can be performed in the same manner as the slag discharge in the third step.

After the slag discharge in the fifth step, the converter is upright, and in the sixth step, decarburization refining is performed by top bottom blowing refining of the converter. Flux such as the minimum required quick lime and light calcined dolomite according to the waste rate, furnace melt condition, and target P concentration in the fifth step is added, and decarburization blown to the target end point C concentration. Due to the flux added in the decarburization refining, the dephosphorization reaction usually proceeds also in the decarburization refining in the sixth step. As described above, the P concentration in the molten metal before decarburization refining is lower than that in the conventional one-step dephosphorization refining, and in the fourth step generated slag remaining in the converter before decarburization refining. Since the P ₂ O ₅ concentration is lower in the present invention than in the prior art, in the present invention, it is possible to reduce the flux intensity added in the sixth step.

  In the dephosphorization refining in the fourth step, the molten phosphorus temperature at the end of refining is suppressed to 1400 ° C. or lower in order to advance the dephosphorization reaction. On the other hand, after the decarburization refining in the sixth step, the molten steel temperature needs to be raised to about 1650 to 1730 ° C. Therefore, in order to raise the temperature in the sixth step, it is necessary to keep C in the molten metal without burning as much as possible in the dephosphorization refining in the fourth step. It is preferable that the C concentration in the molten metal at the end of the fourth step is 3.0% by mass or more. In addition, carbon in the molten metal inevitably combusts in the dephosphorization refining in the fourth step, and therefore it is preferable that the C concentration in the molten metal at the end of the dephosphorizing refining in the second step is 3.5% by mass or more. When the heat source is insufficient in the decarburization refining in the sixth step, a carbon source can be added to the furnace to make a heat source. A metal silicon source such as Fe-Si or a metal aluminum source such as an aluminum alloy may be added.

  After finishing the decarburization refining in the sixth step, the converter is tilted to take out the molten steel in the converter into the ladle while leaving the slag. Molten steel 8 is accommodated in the ladle 3 on the ladle cart 5. This step is called the seventh step.

When steel production is completed, slag remains in the converter. In the eighth step, the residual slag is processed. The converter may be tilted, and the entire amount of slag 9 in the converter may be discharged to the slag pan 4. Alternatively, the entire amount of slag 9 in the converter may be left in the converter 1 and then the process may move to the first step of the next heat. Furthermore, only a part of the slag in the converter may be left and the others may be discarded, and the part of the slag may be left, and then the process may move to the first step of the next heat. Based on one or both of the estimated P ₂ O ₅ component of the slag in the converter and the steel P component target value of the next heat, leave the entire amount of the slag in the converter, or leave only a part and reject the others You can choose either to do it or to reject the whole amount. When the steel P component target value of the next heat is low, the slag remaining amount in the converter can be increased. Moreover, even when the estimated P ₂ O ₅ component of the slag in the converter is low, the amount of slag remaining in the converter can be increased.

  When the entire amount of slag in the converter is discharged in the eighth step of the previous heat, the flux added to the converter at the start of the second step is used as the flux used in the second step of the heat this time. In addition, when part or all of the slag is left in the converter in the eighth step of the preheating, the flux generated in the sixth step of the preheating before the first step is used as the flux in the second step. In this case, a part or all of the one left in the converter is used.

The present invention was applied to the production of varieties having a target P concentration of 0.010% by mass or less using an upper-bottom blowing converter with a molten metal amount of 320 tons. The process of the present invention is as shown in FIG. As a comparative example, a process in which the fourth process and the fifth process in FIG. 1 were omitted was used. In both the inventive example and the comparative example, both the case where the slag was left in the converter and the case where the entire amount was discharged without being left were carried out as the eighth step. The top blowing acid speed was 1.5 to 4.5 Nm ³ / t / min.

Example 1
Table 1 shows the temperature and main components of the molten iron and the melt temperature, the melt component, and the slag component after the second step, after the fourth step, after the fourth step, and after the sixth step for the inventive example and the comparative example. In the slag component of Table 1, the unit of CaO / SiO ₂ is not mass% but unitless. Table 2 shows the amount of main raw material, the amount of added flux in the second step, the fourth step, and the sixth step, the total amount of added flux, and the amount of slag for the inventive example and the comparative example. In the flux addition amount of Table 2, the total CaO equivalent addition amount in the added flux is T.P. CaO and total SiO ₂ equivalent addition amount SiO ₂ is used.

  In Tables 1 and 2, no. The heat of 1-7 is a comparative example, No.1. Heats of 8 to 14 are examples of the present invention, and the average values of the 7 heats are also shown.

In both the inventive example and the comparative example, 80 to 90% of the charged main raw material is hot metal, and the rest is scrap. Further, quick lime, limestone and light calcined dolomite are used as the flux as the CaO source, and Fe—Si, olivine and soft silica are used as the flux as the SiO ₂ source.

  As is apparent from the molten P concentration after the sixth step in Table 1, Example No. of the present invention. 8 to 14 all achieved the target with a P concentration of 0.010% by mass or less, and the average P concentration of 7 heats was 0.009% by mass. The target P concentration could not be achieved in 4 heats, and the average P concentration in 7 heats was also 0.011% by mass.

  Further, as apparent from the description in the total column of Table 2, the present invention adds the fourth step and increases the total number of T.P. CaO was able to obtain a result that the inventive example was lower than the comparative example. Regarding the molten steel yield indicating the iron loss of the entire refining, no difference was found between the inventive examples and the comparative examples.

(Example 2)
Using the same conditions as those of the inventive example and comparative example of Example 1, a product having a product target P concentration of 0.010% by mass or less was produced. The results are shown in FIG. As is clear from FIG. 2, it is clear that the P concentration in the inventive example is significantly reduced as compared with the comparative example.

DESCRIPTION OF SYMBOLS 1 Top bottom blowing converter 2 Hot metal ladle 3 Ladle 4 Slag pan 5 Ladle cart 6 Slag pan cart 7 Top blowing lance 8 Molten metal (molten metal, molten steel)
9 Slag

Claims (3)

When refining steel with a P concentration of 0.010% by mass or less using an upper bottom blowing converter, molten iron is charged into the converter in the first step, and the upper bottom blowing of the converter using flux in the second step. Hot metal dephosphorization is performed by refining, the converter is tilted in the third step, and part or all of the slag generated in the second step is discharged. Hot metal dephosphorization, tilting the converter in the fifth step to discharge part or all of the slag produced in the fourth step, decarburizing by top bottom refining of the converter in the sixth step, and
As the flux to be added in the fourth step, a CaO-containing flux and a SiO ₂ -containing flux are used, and the slag CaO / SiO ₂ (mass ratio) at the end of the fourth step is set to 1.5 to 2.0, and the fourth step is ended. A converter refining method, wherein the molten metal temperature is 1400 ° C. or lower and the C concentration in the molten metal is 3.0 mass% or higher.   As the flux used in the second step, the flux added to the converter at the start of the second step, the flux generated in the sixth step in the previous heat before the first step, and a part or all of it left in the converter One or both of these are used, The converter refining method of Claim 1 characterized by the above-mentioned. After completion of the sixth step, in the seventh step, the converter is tilted to leave the molten steel while leaving the slag, and one of the estimated P ₂ O ₅ component of the slag in the converter and the steel P component target value of the next heat Or, based on both, as the eighth step, select either the process of leaving the entire amount of slag in the converter, leaving only a part and discharging the other, or discharging the whole amount, The converter refining method according to claim 1, wherein the process proceeds to the first step of the next heat.

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