Secondary refining method for aluminum silicon killed steel

Abstract

To provide a secondary refining method for an aluminum silicon-killed steel, capable of avoiding solid-liquid coexistence regions in a molten steel and preventing a nozzle of a tundish from being blocked by controlling oxygen given to the molten steel without adding Ca to the molten steel.SOLUTION: This invention relates to a secondary refining method for an aluminum silicon-killed steel in which Ca is not added after the secondary refining, wherein when a Ca input ratio A by alloy addition excluding Al is 0.001% or more during the period from the input of deoxidized Al to the molten steel 5 that has subjected to calcination refining to the end of secondary refining, and when oxygen is added to the molten steel 5 during the secondary refining, the formula for acid feed rate B (Nm3): 12.5×A (Ca input ratio (%))×M (molten steel weight (kg))+64.2<B<1,000 is satisfied.SELECTED DRAWING: Figure 5B

Description

[Technical Field]

[0001] The present invention relates to a method for secondary refining aluminum-silicon killed steel in which Ca is not added after secondary refining. [Background technology]

[0002] Conventionally, molten steel decarburized in a converter or the like is transported to a secondary refining process, where the molten steel is vacuum degassed. Vacuum degassing (RH treatment) mainly adjusts the composition of the molten steel and degasses the molten steel. For example, calcium in the molten steel is reduced to a target value or less. A technique for producing low-calcium steel is disclosed in Patent Document 1. Specifically, when adjusting the composition of molten steel by adding a metal or ferroalloy for adjusting the composition in the vacuum vessel, the necessary reflux time t0 from the final addition of the metal or ferroalloy for adjusting the composition is calculated from the calcium concentration, the calculated necessary reflux time t0 is ensured, and the molten steel is refluxed within a time period not exceeding 60 seconds beyond the necessary reflux time t0, after which the refining is completed. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-119656 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in Patent Document 1, the required reflux time t0 increases depending on the initial Ca concentration, so the required reflux time t0 becomes longer for high-component steels, resulting in a long processing time.In addition, the control range of the reflux time is narrow, so there is little flexibility in controlling degassing and other component control items, making this technology difficult to use. As part of measures to combat global warming, CO2 emission regulations are becoming stricter every year, and there is a demand for further weight reduction in automobiles. In addition, there is also a demand for improved automobile collision safety, and as a result, the proportion of high-tensile steel (high-tensile steel) used, which meets these needs for weight reduction and safety, is increasing year by year.

[0005] To produce high-tensile steel, it is not possible to do so by simply improving rolling and heat treatment techniques; it is essential to increase the amount of alloy added. As a result, high-tensile steel is becoming more highly alloyed every year, but this can cause problems such as clogging of the tundish nozzle during the continuous casting process. This nozzle clogging problem is caused by inclusions that exist in a solid-liquid state in the molten steel when casting high-Si, Mn steel. In other words, the liquid inclusions act as a binder, and the solid inclusions adhere to the inner wall of the nozzle, causing nozzle clogging. However, if only solid inclusions are present, they will only come into contact with the inner wall of the nozzle without adhering, and nozzle clogging will not occur.

[0006] Thus, in order to prevent nozzle clogging, it is necessary to avoid the solid-liquid coexistence region in the molten steel (to prevent solid-liquid coexistence). For example, methods for keeping the entire steel in the liquid phase and avoiding the solid-liquid coexistence region include using a high-Ca alloy and adding Ca wire after secondary refining. However, these methods result in a large heat load and place a heavy burden on the ladle refractory.

[0007] On the other hand, methods to avoid the solid-liquid coexistence region by maintaining the solid phase include using an alloy with a low Ca content or adding an alloy before deoxidation. However, with this method, the higher the element content of the steel (higher content of Si, Mn, etc.), the more necessary it is to use an alloy with higher purity. For this reason, the inventors of the present application noticed that in Al-killed steel (aluminum-silicon killed steel), adding oxygen to molten steel increases Al2O3. Having confirmed that the composition of inclusions in molten steel changes depending on the amount of OB (oxygen flow rate) during secondary refining (RH treatment), and that the composition of inclusions falls into the solid phase region when sufficient oxygen is added, they investigated the appropriate amount of oxygen flow rate according to the amount of Ca added.

[0008] In view of the above problems, an object of the present invention is to provide a method for secondary refining of aluminum-silicon killed steel, which can avoid a solid-liquid coexistence region in the molten steel and prevent clogging of the nozzle of the tundish by controlling the oxygen imparted to the molten steel without adding Ca to the molten steel. [Means for solving the problem]

[0009] In order to achieve the above object, the present invention provides the following technical means. The method for secondary refining aluminum-silicon killed steel according to the present invention is a method for secondary refining aluminum-silicon killed steel in which Ca is not added after secondary refining, and in which the Ca input ratio A by adding alloys other than Al is set to 0.001% or more during the period from the time when deoxidizing Al is added to molten steel that has been subjected to decarburization refining until the end of secondary refining. 0.003% or less When oxygen is supplied to the molten steel during the secondary refining, the oxygen supply amount B (Nm 3 ) is characterized by satisfying the formula (1).

[0010] 12.5×A×M+64.2 <B<1000 ···(1) Where M: Weight of molten steel (kg) A: Ca input ratio (%) [Effects of the Invention]

[0011] According to the present invention, by controlling the amount of oxygen given to the molten steel without adding Ca to the molten steel, it is possible to avoid a solid-liquid coexistence region in the molten steel, and to prevent clogging of the nozzle of the tundish. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 10 is a diagram showing the deposition site and the composition of the deposits in the case of nozzle clogging. [Figure 2] FIG. 2 is a diagram showing the composition of inclusions in molten steel before secondary refining. [Figure 3] 1 is a diagram showing the relationship between the (CaO / Al2O3) composition analysis results by EPMA and PDA Ca / PDA Al. [Figure 4] FIG. 1 is a graph showing the relationship between RH oxygen supply rate (Nm3) and PDA Ca / PDA Al. [Figure 5A] This is a diagram showing the distribution of PDA Ca / PDA Al≧8.2 in the relationship between Ca input ratio A (%) and oxygen supply rate B / M (Nm3 / kg-steel). [Figure 5B] 1 is a graph showing the distribution of PDA Ca / PDA Al≧8.2 in the relationship between Ca input ratio A×molten steel weight M (kg) and oxygen supply rate B (Nm3). DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the method for secondary refining aluminum-silicon killed steel according to the present invention will be described with reference to the drawings. The embodiment described below is an example of a specific embodiment of the present invention, and the configuration of the present invention is not limited to this specific example. Molten steel 5, which has been decarburized in a converter or the like, is transported to the secondary refining process, where it undergoes processes such as vacuum degassing (RH treatment).The molten steel 5 is then transferred from the ladle 1 to the tundish 2 and poured into a mold 4 (continuous casting device) through a nozzle 3 of the tundish 2.

[0014] The RH type vacuum degassing treatment device is primarily used to adjust the composition of molten steel 5 and to degas the molten steel 5. Although not shown, the RH type vacuum degassing treatment device has a ladle 1 into which molten steel 5 is charged and a vacuum chamber that is placed in a vacuum state to degas the molten steel 5. Two immersion pipes are provided at the bottom of the vacuum chamber and immersed in the molten steel 5 in the ladle 1. A gas injection pipe is provided on one side of the immersion pipes to inject gas into the molten steel 5 flowing into the vacuum chamber. An exhaust port is provided at the top of the vacuum chamber to exhaust gas from the chamber to the outside.

[0015] In performing the RH vacuum degassing treatment, first, the immersion tube is immersed in the molten steel 5 in the ladle 1. Then, a gas such as argon gas or nitrogen gas is blown into the vacuum chamber from the gas blowing pipe, and the gas in the vacuum chamber is exhausted to the outside from the exhaust port to maintain a substantial vacuum inside the vacuum chamber, and the molten steel 5 is circulated between the vacuum chamber and the ladle 1. At this time, an alloy or the like is supplied to the molten steel 5 to adjust the composition of the molten steel 5.

[0016] The method for secondary refining aluminum-silicon killed steel according to the present invention is a method for secondary refining aluminum-silicon killed steel (high alloy steel) in which Ca is not added after secondary refining, and in which the Ca input ratio A by adding alloys other than Al is set to 0.001% or more during the period from after deoxidizing Al is added to molten steel 5 that has been subjected to decarburization refining until the end of secondary refining, and when oxygen is supplied to molten steel 5 during secondary refining, the amount of oxygen supplied B (Nm 3 ) satisfies equation (1).

[0017] 12.5×A×M+64.2 <B<1000 ···(1) Where M: Weight of molten steel (kg) A: Ca input ratio (%) The secondary refining method for aluminum-silicon killed steel according to the present invention will be described in detail below. The present invention is directed to a method for secondary refining aluminum-silicon killed steel in the case where Ca is not added after secondary refining.

[0018] That is, the present invention is directed to a case where a predetermined amount of oxygen is blown into the molten steel 5 to remove CaO from inclusions in the molten steel 5, thereby preventing clogging of the nozzle 3 provided at the bottom of the tundish 2. Therefore, the present invention does not cover a case where Ca is added to the molten steel 5 after secondary refining to make the inclusion composition so that the nozzle 3 does not clog. The nozzle 3 is clogged mainly due to components containing large amounts of Si and Mn.

[0019] The Al2O3 contained in the deposits causes them to adhere to the inner wall of the nozzle 3, resulting in clogging of the nozzle 3. Therefore, the present invention is directed to aluminum-silicon killed steel. Figure 5A shows the relationship between the Ca input ratio A (%) and the oxygen supply amount B / molten steel weight M (Nm 3 / kg-steel), the distribution of PDA Ca / PDA Al ≧ 8.2 is shown.

[0020] As shown in FIG. 5A, in the present invention, the Ca input ratio A due to the addition of alloys other than Al during the period from the addition of deoxidizing Al to molten steel 5 that has been subjected to decarburization refining until the end of secondary refining is set to 0.001% or more. Preferably, the Ca input ratio A is set to 0.001% or more and 0.003% or less (see FIG. 5A).

[0021] The present invention was obtained by examining Si and Mn-added steel, but Ca, which is contained in alloys other than Al and whose addition amount is proportional to the amount of oxygen sent, has the same effect as Si and Mn. Furthermore, Al is added for the purposes of deoxidation, composition adjustment, and heating of the molten steel 5, and the Al added first after decarburization refining is referred to as deoxidizing Al.

[0022] However, trace amounts of Al contained in additives for adjusting other components are not included in the above deoxidizing Al. In addition, if an alloy is added before deoxidizing Al is added, it is not included in the Ca input ratio A. When the Ca content of the Si and Mn additives is a1 (%), a2 (%), a3 (%), and the additive amounts are b1 (kg), b2 (kg), b3 (kg), the Ca input ratio A is given by the following formula:

[0023] A=(Σ(a×b)) / M Where M: Weight of molten steel (kg) For example, if Al deoxidation is performed before the start of secondary refining, the total amount of alloy added in secondary refining is defined as Ca input ratio A. On the other hand, if Al deoxidation is performed in secondary refining, the amount of alloy added after deoxidizing Al is defined as Ca input ratio A.

[0024] When the Ca input ratio A is in the range of 0.001% or more, the inclusions in the molten steel 5 contain a large amount of CaO, resulting in an inclusion composition that causes clogging of the nozzle 3. The inclusions exist in a solid-liquid state (a mixture of solid and liquid phases) in the molten steel 5. The liquid phase acts as a binder, making the inclusions more likely to adhere to the refractory material applied to the inner wall of the nozzle 3, resulting in clogging of the nozzle 3.

[0025] In this way, in order to prevent the nozzle 3 from being clogged due to the influence of the inclusion composition containing CaO, it is common to modify the composition by adding Ca to the molten steel 5 after secondary refining. (Reference: Iron and Steel Handbook, 5th edition, Volume 1). However, the above-mentioned method has many disadvantages (issues) compared to the measure of the present invention of preventing nozzle 3 from clogging without adding Ca, such as higher costs, a larger heat load, and melting damage to the refractory of the nozzle 3.

[0026] Figure 5B shows the relationship between the Ca input ratio A × molten steel weight M (kg) and the oxygen supply rate B (Nm 3 ) shows the distribution of PDA Ca / PDA Al ≧ 8.2. As shown in FIG. 5B, in the present invention, when oxygen is supplied to the molten steel 5 during secondary refining, the oxygen supply amount B (Nm 3 ) satisfies equation (1). 12.5 × A × M + 64.2 < B < 1000 ···(1) However, M: weight of molten steel (kg) A: Ca input ratio (%) Equation (1) is a condition that can prevent the clogging of nozzle 3 by modifying the inclusion composition to the composition on the solid phase region side and avoiding the solid-liquid coexistence region.

[0027] In addition, the method of adding Ca to the molten steel 5 is to modify the inclusion composition to the composition on the liquid phase region side, but the present invention is to avoid it from the composition of the solid-liquid coexistence region by a method different from the above. Also, for the examples and comparative examples shown later, although it is the amount of acid injection B in the degassing refining (RH treatment), in order to modify the inclusion composition in the molten steel 5, the process of acid injection is not limited to degassing refining.

[0028] As described above, the lower limit of the acid injection amount B (12.5 × A × M + 64.2 (Nm 3 ) < B) is obtained from the range of PDA Ca / PDA Al. Also, for the upper limit of the acid injection amount B (B < 1000 Nm 3 ), it is set because there are demerits such as the treatment time due to acid injection becoming long and the load on the refractories installed in the RH vacuum chamber increasing.<00001​​​​​​​​​​​​​​​​​​​​​​​​​ [Table 3]

[0033] FIG. 1 shows the adhesion site and the composition of the adhesion when the nozzle 3 is clogged. When adding Si or Mn to steel, elemental metals or alloys are used, but these contain impurities. Among these impurities, Ca affects the composition of inclusions in the molten steel 5, which poses a risk of clogging the nozzle 3 before the continuous casting process. Furthermore, high-tensile steel contains large amounts of these elements, making it prone to clogging the nozzle 3.

[0034] As shown in FIG. 1, for example, when the purpose is to produce high-tensile steel, observing the cross section of a clogged nozzle 3 reveals that inclusions with CaO as a binder are attached. At this time, since the inclusions in the molten steel 5 are located in a solid-liquid coexistence region, in the case of high-tensile steel containing a large amount of Si and Mn, it is necessary to avoid this solid-liquid coexistence region in order to carry out stable continuous casting.

[0035] Methods for avoiding the solid-liquid coexistence region include increasing the amount of Ca added to the molten steel 5 to modify the inclusion composition to a composition on the liquid phase region side, or reducing the amount of Ca added or removing Ca from the molten steel 5 to modify the inclusion composition to a composition on the solid phase region side. Methods for modifying the inclusion composition to a liquid phase region and avoiding the solid-liquid coexistence region include using a high-Ca alloy and adding Ca wire after secondary refining. However, the former method (using a high-Ca alloy) makes it difficult to perform Al heating. Furthermore, the latter method (adding Ca wire) requires heating equivalent to the cooling amount caused by adding the Ca wire, which not only increases the heat load but also raises the risk of refractory melting, resulting in high costs.

[0036] On the other hand, methods for modifying the composition of inclusions to a solid phase region and avoiding the solid-liquid coexistence region include using low-Ca alloys and adding alloys before deoxidation. However, with this method, the higher the element content of the steel (steel with a high content of Si, Mn, etc.), the more pure the alloy must be, which increases the cost. For this reason, in the present invention, measures are taken to prevent nozzle 3 from clogging by modifying the inclusion composition to a composition closer to the solid phase region, thereby avoiding the solid-liquid coexistence region, without relying on component adjustment using a high-purity alloy.

[0037] Therefore, the present invention focuses on the fact that CaO is removed when the molten steel 5 contains a large amount of oxygen. Figure 2 shows the composition of inclusions in molten steel 5 before secondary refining when a 0.9% Si alloy containing 0.3% Ca is added to molten steel 5 after tapping and then Al deoxidation is performed. However, the average composition of the inclusions is in the Al2O3 region. Furthermore, because the amount of CaO is small, the addition of the Ca-containing alloy can be ignored.

[0038] From the above, it can be seen that the amount of Ca-containing alloy added after deoxidation has a large effect on the composition of inclusions in the molten steel 5. In secondary refining, alloys must be added according to the required specifications, so steel types with large amounts of alloy added have a large amount of Ca input, resulting in a large amount of CaO in inclusions. Figure 3 shows the results of (CaO / Al2O3) composition analysis by EPMA and the relationship between PDA Ca / PDA Al.

[0039] Here, in examining the conditions under which clogging of the nozzle 3 occurs, the ratio of PDA Ca / PDA Al was used. As shown in Figure 3, in the region of CaO / Al2O3<0.15, there is a correlation with PDA Ca / PDA Al analyzed by countback. The countback analysis was carried out using an optical emission spectrometer (manufactured by Shimadzu Corporation), model number PDA-6000 or PDA-7000.

[0040] Next, the region where clogging of the nozzle 3 occurs was examined. Table 4 shows the relationship between the average Ca input (PDA Ca / PDA Al) and the clogging status of nozzle 3 for steel type α.

[0041] [Table 4]

[0042] As shown in Table 4, when PDA Ca / PDA Al ≥ 8.2, clogging of nozzle 3 occurs. Based on this, we defined PDA Ca / PDA Al < 8.2 as the range in which clogging of nozzle 3 does not occur, and investigated the range of operating conditions that can meet this requirement. Therefore, as a method for reducing CaO in secondary refining, attention was focused on the fact that Al2O3 increases when oxygen is added to molten steel 5 in Al-killed steel. As a means for adding oxygen to molten steel 5 in this secondary refining, for example, OB (a method of blowing oxygen into molten steel 5) is used to heat molten steel 5.

[0043] Figure 4 shows the RH oxygen supply rate (Nm 3 ) and the relationship between PDA Ca / PDA Al. As shown in Figure 4, the oxygen supply rate was 140 Nm 3 This results in PDA Ca / PDA Al<8.2. However, not only does the amount of Ca input vary greatly depending on the steel type, but the element concentrations before secondary refining also vary greatly depending on the converter blowing conditions and the amount of slag mixed in. Therefore, it is necessary to add alloys based on the element concentrations before secondary refining and the required specifications.

[0044] Therefore, using steel types α and β in which nozzle 3 was clogged, the amount of oxygen supply required according to the Ca input in secondary refining was investigated. Figure 5B shows the relationship between the Ca input ratio A × molten steel weight M (kg) and the oxygen supply rate B (Nm 3 ) shows the distribution of PDA Ca / PDA Al ≧ 8.2. As shown in Figure 5B, PDA Ca / PDA Al ≥ 8.2 was achieved only when the oxygen supply rate was low, and as the Ca input increased, PDA Ca / PDA Al ≥ 8.2 tended to be achieved even at higher oxygen supply rates.

[0045] For this reason, the range where PDA Ca / PDA Al<8.2 (the range that satisfies (1) and (2)) was defined as the range of the present invention. (1) Ca input A: A ≥ 2.5 kg (2) Oxygen supply amount B: B>12.5×A×M+64.2 Within the above range, the continuous casting process can be carried out without causing clogging of the nozzle 3.

[0046] According to the secondary refining method for aluminum-silicon killed steel of the present invention, by controlling the oxygen imparted to the molten steel 5 without adding Ca to the molten steel 5, it is possible to avoid a solid-liquid coexistence region in the molten steel 5 and prevent the nozzle 3 of the tundish 2 from clogging. It should be noted that the embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive.

[0047] In particular, in the embodiments disclosed herein, matters not explicitly stated, such as operating conditions, operating conditions, various parameters, dimensions, weights, volumes of components, etc., do not deviate from the scope of ordinary practice by a person skilled in the art, and values ​​that can be easily assumed by a person ordinary skilled in the art are used. [Explanation of symbols]

[0048] 1 ladle 2 tundishes 3 nozzles 4 Mold (continuous casting equipment) 5. Molten Steel

Claims

[Claim 1] A method for secondary refining aluminum-silicon killed steel in which Ca is not added after secondary refining, comprising: The Ca input ratio A by adding alloys other than Al during the period from after deoxidizing Al is added to the molten steel that has undergone decarburization refining until the end of secondary refining is set to 0.001% or more and 0.003% or less, When oxygen is added to the molten steel during the secondary refining, the oxygen supply amount B (Nm ３ 2. A method for secondary refining aluminum-silicon killed steel, characterized in that formula (1) regarding the content of aluminum and silicon is satisfied. 12.5×A×M+64.2<B<1000...(1) where M is the weight of molten steel (kg) A: Ca input ratio (%)

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