Method for melting high-purity steel

Abstract

To provide a high-purity steel melting method having high desulfurization efficiency and preventing nozzle blockage.SOLUTION: The present invention provides a high-purity steel melting method for producing a steel comprising C: 0.10 mass% or more, Si: 0.8-2.0 mass%, and Al: 0.01 mass% or more by desulfurization refining of molten steel after tapping the molten steel. When Al and Si alloys are charged during the tapping of the molten steel, the Si alloy with Si: 60 mass% or more, C: 15-30 mass%, Ca: less than 0.20 mass%, and the remainder being impurities is charged in the range of 2.0-4.0 kg / t during tapping.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for melting high-purity steel with high desulfurization ability.

Background Art

[0002] Conventionally, in high-Si steel materials with high Si concentration such as ultra-high-tensile steel (super high-tensile) and special wire rods, high purity such as S reduction is often required. Therefore, in order to achieve high purity in molten steel desulfurization, it is known that reducing lower oxides such as FeO and MnO in the slag and performing slag modification is effective in improving desulfurization ability.

[0003] As a technique for improving desulfurization ability by modifying slag, Patent Document 1 discloses a method for melting high-cleanliness steel in which Al slag and CaCO3 are added to the ladle slag after tapping to perform slag modification. Further, Patent Document 2 discloses a technique of adding 0.3-0.4 kg / t of an Al modifier to the ladle slag after tapping. Furthermore, Patent Document 3 discloses a method for melting extra-low carbon and extra-low sulfur steel, in which burnt lime and Al are added to the ladle slag and circulated at 4000 Nl / min or more in an RH vacuum degassing facility to promote slag reaction.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the method described in Patent Document 1 utilizes stirring by CO2 gas generation from CaCO3, but the slag is cooled by the heat of decomposition of CaCO3, and the melting point of the slag rises as the amount of CaO in the slag increases, so there is a limit to the amount of CaCO3 that can be added while ensuring slag fluidity. Furthermore, the reducing agent is only scattered on the top surface of the ladle slag, and the added reducing agent cannot be sufficiently incorporated into the interior of the ladle slag. As a result, the generated CO2 gas is not necessarily generated at the bottom of the slag layer (near the slag-metal interface), which is effective for stirring the slag layer, but is also generated near the slag surface, which does not contribute to stirring the slag layer. In other words, the amount of CO2 gas generated does not sufficiently contribute to stirring the slag layer.

[0006] Furthermore, the method described in Patent Document 2 merely involves scattering an Al modifier on the upper surface of the ladle slag and does not promote agitation by gas using a gas-generating substance, resulting in an insufficient slag modification effect. Moreover, the method described in Patent Document 3 is a technique that promotes slag modification by agitating the ladle slag, but since the ladle slag cannot be directly agitated in an RH vacuum degassing facility, the slag modification effect is smaller compared to converter tapping and ladle refining.

[0007] Furthermore, in high-Si steel materials with a high Si concentration, FeSi alloy is often added to the molten steel to adjust the Si concentration. FeSi alloy contains a large amount of Ca as an impurity, and when Ca is mixed into the molten steel, it combines with oxygen in the molten steel and then with Al2O3 in the molten steel to form CaO-Al2O3 inclusions. When these inclusions are mixed into the molten steel, they adhere to the refractory material through the liquid phase, causing nozzle blockage in tundishes and other applications.

[0008] In view of the aforementioned problems, the present invention aims to provide a method for melting high-purity steel that has high desulfurization efficiency and can prevent nozzle blockage. [Means for solving the problem]

[0009] The inventors focused on SiC alloy as a slag modifier that reduces lower oxides such as FeO and MnO in slag and can prevent nozzle clogging. They noted that SiC alloy has a lower specific gravity than FeSi alloy, making it easier to dissolve near the slag surface, thus providing an effect of reducing FeO and MnO in the slag. Furthermore, the inventors found that because SiC alloy has a lower Ca concentration than FeSi alloy, it can suppress the formation of CaO-Al2O3 inclusions compared to adding only FeSi alloy, thereby significantly reducing the risk of nozzle clogging.

[0010] The present invention is as follows: (1) A method for producing high-purity steel, comprising tapping molten steel and then desulfurizing and refining the molten steel to produce steel containing C: 0.10 mass% or more, Si: 0.8 to 2.0 mass% and Al: 0.01 mass% or more, A method for producing high-purity steel, characterized by adding Al and Si alloys during the tapping of molten steel, wherein the Si alloy consists of Si: 60 mass% or more, C: 15-30 mass%, Ca: less than 0.20 mass%, and the remainder being impurities, at a rate of 2.0-4.0 kg / t during tapping. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a method for melting high-purity steel that has high desulfurization efficiency and can prevent nozzle clogging. [Brief explanation of the drawing]

[0012] [Figure 1] This is a diagram illustrating the mechanism by which Si alloys dissolve. [Modes for carrying out the invention]

[0013] Embodiments of the present invention will be described below with reference to the drawings. This embodiment is a method for producing highly clean steel with C: 0.10 mass% or more, Si: 0.8 to 2.0 mass%, and Al: 0.01 mass% or more, and is a method for achieving high desulfurization capacity when producing highly clean steel such as high-Si steel such as ultra-high-tensile steel.

[0014] Herein, the method according to this embodiment is intended for Al-killed steel and can be effective regardless of the Al concentration in the molten steel. Furthermore, if the Al concentration is less than 0.01% by mass, the deoxidation of the molten steel is too weak to achieve desulfurization, and the effects of the present invention cannot be obtained. Therefore, the scope of the present invention is defined as Al: 0.01% by mass or more.

[0015] Next, the method for producing high-purity steel in this embodiment will be described. In this embodiment, when the molten steel that has been decarburized in the converter is tapped into the ladle, deoxidation and composition adjustment are performed during tapping. Specifically, metallic Al or Al alloy and Si alloy are added during tapping.

[0016] In molten steel desulfurization, slag modification to reduce lower oxides such as FeO and MnO in the slag is effective in improving desulfurization efficiency, and in this embodiment, a SiC alloy is used as the Si alloy.

[0017] Fig. 1 is a diagram schematically showing the reaction when FeSi and SiC are added during tapping from a converter. Fig. 1(a) shows the reaction when an FeSi alloy is used as the Si alloy. The FeSi alloy contains 0.5 mass% or more of Ca as an impurity. As shown in Fig. 1(a), when FeSi is added as the Si alloy during tapping of the molten steel 1, the FeSi alloy sinks deep into the bath, and Si and Ca dissolve. On the other hand, Al added during tapping as a deoxidizing element reacts with oxygen in the molten steel to form Al2O3. Then, the dissolved Ca also reacts with oxygen in the molten steel to form CaO, which combines with Al2O3 in the molten steel to form CaO - Al2O3 inclusions. When CaO - Al2O3 inclusions are formed, the melting point of the inclusions decreases, a small amount of liquid phase is generated, and it adheres to refractories etc. through the liquid phase, causing a risk of nozzle blockage in the tundish etc.

[0018] On the other hand, Fig. 1(b) shows the reaction when a SiC alloy is used as the Si alloy. The SiC alloy also contains Ca as an impurity, but the content is less than that of the FeSi alloy and is generally less than 0.20 mass%. Therefore, even when the SiC alloy is added, Ca dissolves in the molten steel, but the amount of CaO - Al2O3 inclusions formed can be kept lower compared to the case of adding the FeSi alloy, and the risk of nozzle blockage can be significantly reduced.

[0019] Also, as shown in Fig. 1(b), since the specific gravity of the SiC alloy is smaller than that of the FeSi alloy, when the SiC alloy is added during tapping of the molten steel 1, even if it temporarily sinks deep into the bath, it floats up before melting and melts near the slag 2 on the ladle. When the SiC alloy melts near the slag 2, FeO and MnO in the slag 2 are reduced by the dissolved Si and C through the following reactions. Si + 2FeO (or MnO) = SiO2 + 2Fe (or Mn) ···(1) C + FeO (or MnO) = CO↑ + Fe (or Mn) ···(2)

[0020] The above reaction reduces FeO and MnO in the slag, and when molten steel is subsequently desulfurized in the ladle, high desulfurization efficiency can be achieved. Furthermore, as can be seen from reaction equation (2), when a SiC alloy is added, CO gas is also generated by the reduction of lower oxides, so the slag can be stirred with CO gas, further promoting the reaction efficiency. In addition, since the reaction in reaction equation (2) occurs at the interface between the slag and the molten steel, the generated CO gas bubbles pass almost entirely through the interior of the slag, and almost all of the bubbles contribute to the stirring of the slag. Therefore, the slag stirring efficiency by gas is higher compared to the method described in Patent Document 1, where CaCO3 is sprinkled on the upper surface of the ladle slag.

[0021] Next, we will explain in detail the composition of the SiC alloy to be used as the Si alloy. In order to obtain the effects described above, a Si alloy is used in which Si: 60 mass% or more, C: 15-30 mass%, Ca: less than 0.20 mass%, and the remainder being unavoidable impurities.

[0022] If the Si content is less than 60% by mass, the deoxidizing power of Si weakens, and the slag reduction capacity cannot be obtained, making it impossible to achieve high desulfurization capacity. Therefore, the Si content should be 60% by mass or more.

[0023] Furthermore, if the carbon content is less than 15% by mass, the generation of CO gas according to the above reaction equation (2) becomes weak, preventing sufficient stirring of the slag and reducing the reaction efficiency. As a result, high desulfurization capacity cannot be achieved. On the other hand, if the carbon content exceeds 30% by mass, the specific gravity of the alloy becomes too low, and the alloy does not sink into the molten steel but is incorporated into the slag unreacted. Therefore, high desulfurization capacity cannot be achieved either. Based on the above, the carbon content should be set to 15-30% by mass.

[0024] Furthermore, if the Ca content is 0.20% by mass or more, CaO-Al2O3 inclusions will be formed, increasing the risk of nozzle blockage. Therefore, the Ca content should be less than 0.20% by mass.

[0025] Next, the timing and amount of SiC alloy with the above composition will be explained. The SiC alloy with the above composition will be added during tapping, when the stirring effect of the tapping flow can be obtained. The molten steel during tapping has a very high dissolved oxygen concentration due to the oxygen blowing immediately beforehand, and Ca is oxidized and removed from the SiC alloy, so the formation of CaO-Al2O3 inclusions is unlikely.

[0026] In contrast, if a SiC alloy with the above-mentioned composition is added during ladle refining after tapping, the SiC alloy, which has a lower specific gravity, will be added from the top of the stationary slag, and will not be able to penetrate the molten steel. As a result, the SiC alloy will not dissolve, and high desulfurization capacity cannot be achieved. Furthermore, desulfurization is performed by applying gas agitation during ladle refining, and at that time, the Si alloy incorporated into the slag dissolves into the molten steel with a delay due to the agitation. However, at the ladle refining stage, deoxidation has progressed to some extent, and the amount of inclusions and dissolved oxygen in the molten steel is small, so the Ca from the SiC alloy that dissolves later reacts with inclusions such as Al2O3 in the molten steel with a high yield, making it easy for CaO-Al2O3 inclusions to be formed. As a result, the risk of nozzle blockage increases. For the reasons above, the Si alloy with the above-mentioned composition should be added during tapping.

[0027] Furthermore, the amount of SiC alloy with the above-mentioned composition added will be in the range of 2.0 to 4.0 kg / t. The total amount of Si alloy added will vary depending on the target composition, but any amount exceeding the amount of SiC alloy with the above-mentioned composition will be added as FeSi alloy. Therefore, if the amount of SiC alloy with the above-mentioned composition added is increased, the amount of FeSi alloy added will be reduced accordingly to ensure a Si margin.

[0028] If the amount of SiC alloy with the above composition is less than 2.0 kg / t, the slag modification effect described above cannot be sufficiently obtained. Also, because the proportion of FeSi alloy added becomes relatively high, the effect of suppressing the formation of CaO-Al2O3 inclusions cannot be sufficiently obtained. On the other hand, if the amount added exceeds 4.0 kg / t, excessive CO gas is generated, causing problems such as slag overflow due to slag forming during tapping. For these reasons, the amount of SiC alloy with the above composition added should be in the range of 2.0 to 4.0 kg / t.

[0029] As described above, it is preferable to maximize the proportion of the SiC alloy with the above composition within the total Si alloy, while keeping the amount of SiC alloy with the above composition within a range that does not exceed 4.0 kg / t. [Examples]

[0030] Next, embodiments of the present invention will be described. These conditions are merely examples of conditions for confirming the feasibility and effectiveness of the present invention, and the present invention is not limited to those described in these embodiments. The present invention can be implemented by various means to achieve the objectives of the present invention without departing from the spirit of the invention.

[0031] Molten iron tapped from the blast furnace was processed by converter blowing, and 340 tons of molten steel were tapped into a ladle. During tapping, metallic aluminum (Al) for deoxidation and alloys for adjusting the composition, including Si alloys such as FeSi alloy and SiC alloy, were added. After tapping, the molten steel was refined in the ladle with gas agitation, then went through a vacuum degassing process, and the molten steel was cast by continuous casting to obtain steel materials. When increasing the amount of SiC alloy added, the amount of FeSi alloy added was reduced by the amount of Si concentration increase. In addition, the increase in C concentration due to the addition of SiC alloy was adjusted by changing the type of Mn alloy, etc. Meanwhile, in the desulfurization process during ladle refining, Ar gas was used as the carrier gas, and a method of blowing powdered desulfurizing agent through an injection lance was adopted, and the amount of desulfurizing agent added was kept within the range of 1000 to 1500 kg.

[0032] In this test, ladle refining was adopted as the desulfurization refining method, and the desulfurization rate in ladle refining, the presence or absence of nozzle blockage, and the occurrence of slag overflow due to slag forming during tapping were evaluated. Regarding the desulfurization rate, the sulfur concentration in the molten steel was analyzed by sampling the molten steel at tapping and before casting. A score of ○ was given if the desulfurization rate, defined by the following formula (3), was 70% or higher, and a score of × was given if it was less than 70%. Note that in formula (3), [S] 出鋼時 This represents the sulfur concentration in the molten steel at the time of tapping, and [S] 鋳造前 This represents the sulfur concentration in the molten steel before casting after the vacuum degassing process. Desulfurization rate=100×([S] 出鋼時 -[S] 鋳造前 ) / [S] 出鋼時 ...(3)

[0033] Regarding nozzle blockage, if one channel of molten steel was completely cast, it was evaluated as no nozzle blockage occurred (○), and if one channel of molten steel could not be completely cast, it was evaluated as nozzle blockage occurred (×). Regarding slag overflow, if no slag overflow occurred, it was evaluated as ○, and if slag overflow occurred, it was evaluated as ×. If all three evaluations were satisfactory, it was judged that the effects of the invention were achieved, and if even one of these three evaluations was unsatisfactory, it was judged that the effects of the invention were not achieved. The results are shown in Table 1. Note that Table 1 shows the composition of the molten steel before casting, but the final steel material had the same composition.

[0034] [Table 1]

[0035] Examples No. 1 to No. 3 showed good desulfurization rates, no nozzle clogging occurred, and no slag overflow occurred during tapping.

[0036] On the other hand, comparative example No. 4 is an example where the Si concentration in the SiC alloy was too low. The Si deoxidizing power was insufficient, and the slag modification effect was not obtained sufficiently, resulting in a low desulfurization rate. Comparative example No. 5 is an example where the carbon concentration in the SiC alloy was too low. Due to the low amount of CO gas generated, the stirring effect of the slag was low, resulting in insufficient slag modification and a low desulfurization rate. Comparative example No. 6 is an example where the carbon concentration in the SiC alloy was too high. Because the specific gravity of the SiC alloy was too low due to the high carbon concentration, a large portion of the SiC alloy was incorporated into the slag unreacted. As a result, the slag did not undergo a modification effect, and the desulfurization rate was low.

[0037] Comparative example No. 7 is an example where the Ca concentration in the SiC alloy was too high. Because it contained a large amount of Ca impurities, a large amount of CaO-Al2O3 inclusions were formed, causing nozzle blockage. Comparative example No. 8 is an example in which a SiC alloy was added to stationary slag after tapping. Because the SiC alloy, which has a low specific gravity, was added from the top surface of the stationary slag, the SiC alloy did not dissolve, and the slag modification effect was not obtained, resulting in a low desulfurization rate. In addition, although the SiC alloy dissolved into the molten steel by gas stirring during desulfurization, there was little dissolved oxygen at this stage, so the Ca in the SiC alloy reacted with inclusions such as Al2O3 in the molten steel at a high yield, resulting in the formation of many CaO-Al2O3 inclusions and nozzle clogging.

[0038] Comparative example No. 9 is an example where too little SiC alloy was added. Because the amount of SiC alloy was insufficient, the slag modification effect was not obtained, resulting in a low desulfurization rate. In addition, because the amount of FeSi alloy had to be increased due to the small amount of SiC alloy added, a large amount of CaO-Al2O3 inclusions were generated, causing nozzle clogging. Comparative example No. 10 is an example where a large amount of SiC alloy was used. Due to the excessive amount of SiC used, a large amount of CO gas was generated during tapping, resulting in slag overflow problems during slag forming.

[0039] Comparative example No. 11 is an example where the Al concentration of the molten steel was less than 0.01% by mass. Due to the insufficient Al concentration, the deoxidizing power of the molten steel was insufficient, making desulfurization extremely difficult, and resulting in a very low desulfurization rate. [Explanation of Symbols]

[0040] 1 Molten steel 2 slags

Claims

[Claim 1] A method for producing high-purity steel, comprising tapping molten steel and then desulfurizing and refining the molten steel to produce a steel material containing C: 0.10 mass% or more, Si: 0.8 to 2.0 mass% and Al: 0.01 mass% or more, A method for producing high-purity steel, characterized by adding Al and Si alloys during the tapping of molten steel, wherein the Si alloy consists of Si: 60% by mass or more, C: 15-30% by mass, Ca: less than 0.20% by mass, and the remainder being impurities, at a rate of 2.0 to 4.0 kg / t during tapping.

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