Fin steel and preparation method thereof

Abstract

The present invention provides a fin steel and a preparation method thereof, wherein the composition of the fin steel includes C: ≤0.012%, Si: ≤0.02%, Mn: 0.2%-0.32%, P: ≤0.018%, S: ≤0.0095%, Al: ≤0.005%, N: 0.0035%-0.0070% by mass fraction, and the remainder is iron and inevitable impurities; the fin steel includes SiO2-MnO inclusions and composite inclusions, the composite inclusions are inclusions formed by MnO-SiO2-Al2O3 inclusions partially or completely encapsulating Al2O3-MnO inclusions, the size of SiO2-MnO inclusions in the molten steel of the fin steel is 0.5-2.5 μm, and the size of the composite inclusions is 0.5-12 μm. The above-mentioned fin steel reasonably controls the composition of inclusions in the molten steel and greatly reduces surface inclusion defects.

Description

Technical Field

[0001] The invention relates to the field of fin steel, and in particular to a fin steel and a preparation method thereof. Background Art

[0002] Fin steel is the raw material of heat exchange equipment. It is called fin steel because the appearance of the heat exchange unit is similar to fins. In terms of composition, fin steel is a kind of ultra-low carbon and low silicon semi-killed steel. The molten steel is not completely deoxidized, and the internal inclusions are very different from those of conventional ultra-low carbon steel. The surface of this steel needs to be aluminum-coated in the later processing, so the surface quality requirements are very high. No surface inclusions are allowed to avoid affecting the quality of aluminum coating processing. In actual production, the downgrade rate of fin steel due to surface inclusion defects is as high as 6.5%, which is a quality problem that needs to be solved urgently. Summary of the invention

[0003] The main purpose of the present invention is to provide a fin steel and a preparation method thereof, so as to solve the technical problem of high surface inclusion defects of the fin steel in the prior art.

[0004] To achieve the above object, the present invention provides a fin steel, wherein the composition of the fin steel includes C: ≤0.012%, Si: ≤0.02%, Mn: 0.2%-0.32%, P: ≤0.018%, S: ≤0.0095%, Al: ≤0.005%, N: 0.0035%-0.0070%, and the balance is iron and unavoidable impurities. The fin steel includes SiO2-MnO inclusions and composite inclusions, wherein the composite inclusions are formed by MnO-SiO2-Al2O3 inclusions partially or completely enveloping Al2O3-MnO inclusions. In the molten steel of fin steel, the size of the SiO2-MnO inclusions is 0.5-2.5 μm, the size of the composite inclusions is 0.5-12 μm, and the number of composite inclusions smaller than 2 μm accounts for more than 85%, and the number of inclusions larger than 10 μm accounts for less than 1.0%.

[0005] According to an embodiment of the present application, in the SiO2-MnO inclusion, the stoichiometric ratio of SiO2 to MnO is 1:1.5.

[0006] The present application also provides a method for preparing the above-mentioned fin steel, comprising the following steps:

[0007] After the molten steel is smelted in a converter, RH refining is performed. The RH refining step includes:

[0008] After RH decarburization is completed, ferrosilicon and ferromanganese are added in the first batch for composite pre-deoxidation, so that the Mn mass fraction in the molten steel after pre-deoxidation is 0.13~0.25%, the Si mass fraction is below 0.013%, and the free oxygen content is 280-380ppm.

[0009] The second batch of aluminum blocks was added to make the free oxygen content in the molten steel 10~30ppm.

[0010] If the Mn content in the molten steel is insufficient, ferromanganese is added in the third batch to make the Mn mass fraction in the molten steel reach 0.2%-0.32%.

[0011] The refined molten steel is continuously cast and then rolled to form the fin steel.

[0012] According to an embodiment of the present application, in the operation of the composite pre-deoxidation, the amount of the ferrosilicon added is 0.1-0.15 kg / ton of molten steel, the ferromanganese is high carbon ferromanganese, low carbon ferromanganese or metallic manganese, and the amount of the high carbon ferromanganese, the low carbon ferromanganese or the metallic manganese added is 0.6-1.8 kg / ton of molten steel, 0.5-1.6 kg / ton of molten steel, and 0.3-1.4 kg / ton of molten steel, respectively.

[0013] According to an embodiment of the present application, in the operation of the composite pre-deoxidation, the circulation time after the addition of ferrosilicon and ferromanganese is 10-15 minutes.

[0014] According to the implementation manner of the present application, the amount of the aluminum block added satisfies the following formula:

[0015] W Al =1.125 [O] / 10000 W 钢水 10 / 0.75+W 钢水 / y Al [Als] 10

[0016] Where: W Al Indicates the amount of aluminum added, unit: kg.

[0017] [O] represents the free oxygen content in molten steel before adding aluminum, unit: ppm.

[0018] W 钢水 Indicates the mass of molten steel, unit: ton.

[0019] y Al Indicates the yield of Al, unit: %.

[0020] [Als] represents the acid-soluble aluminum content in molten steel after RH refining, unit: %, and is set to a fixed value of 0.0042%.

[0021] According to the implementation mode of the present application, it also includes:

[0022] During the steel-tapping process of the converter smelting, lime and top slag modifier are added. After the RH refining is completed, the CaO / Al2O3 mass ratio in the refined slag is between 1.8 and 2.3, and the sum of the mass fractions of total iron and MnO is ≤14%.

[0023] According to the implementation manner of the present application, the amount of lime added is 2.5~3.3kg / ton of molten steel, the amount of top slag modifier added is 1.5~2.0kg / ton of molten steel, and the top slag modifier is Al40.

[0024] According to the embodiment of the present application, during the continuous casting process, argon is blown between the plates at the connection between the upper water inlet and the submerged lower water inlet, and argon is not blown between the stopper rod and the upper water inlet. The thickness of the cast billet obtained by the continuous casting is 230-250 mm.

[0025] According to an embodiment of the present application, a KR desulfurization step is also included before the converter smelting.

[0026] The above-mentioned fin steel reasonably controls the composition of inclusions in the molten steel, has few high-melting-point hard inclusions, and mainly contains low-melting-point inclusions, which significantly improves the stability of the fin steel liquid during the continuous casting process and greatly reduces surface inclusion defects. In addition, the low-melting-point inclusions deform and extend during the subsequent hot rolling process and will not destroy the continuity of the steel matrix, thereby improving the quality and performance of the steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0028] Figure 1 It is a schematic diagram of an immersed nozzle of a fin steel according to an embodiment of the present application after continuous casting, with no nodule on the inner wall;

[0029] Figure 2 is a SEM image of SiO2-MnO low melting point inclusions in the refined molten steel of Example 1;

[0030] Figure 3 yes Figure 2 Spectrum Figure 1 Energy spectrum analysis diagram at the marked location;

[0031] Figure 4 This is a SEM image of a composite inclusion of Al2O3-MnO inclusions partially encapsulated by the outer layer of Al2O3 inclusions in the refined molten steel of Comparative Example 1;

[0032] Figure 5 yes Figure 4 Spectrum Figure 1 Energy spectrum analysis diagram at the marked location;

[0033] Figure 6 yes Figure 4 Spectrum Figure 2 Energy spectrum analysis diagram at the marked location;

[0034] Figure 7 is a SEM image of composite inclusions (MnO-SiO2-Al2O3 inclusions all encapsulating Al2O3-MnO inclusions) in the molten steel of the fin steel of Example 1;

[0035] Figure 8 yes Figure 7 Spectrum Figure 1 Energy spectrum analysis diagram at the marked location;

[0036] Fig. 9 yes Figure 7 Spectrum Figure 2 Energy spectrum analysis diagram at the marked location;

[0037] Fig.10 This is a metallographic image of the fin steel of Example 1 after hot rolling, in which the SiO2-MnO low melting point inclusions are deformed and extended into short chain-shaped inclusions;

[0038] Fig.11 is a SEM image of the composite inclusions (MnO-SiO2-Al2O3 inclusions partially encapsulating Al2O3-MnO inclusions) of the finished fin steel of Example 1 after deformation due to rolling;

[0039] Fig.12 for Fig.11 Spectrum Figure 3 Energy spectrum analysis diagram at the marked location;

[0040] Fig.13 for Fig.11 Spectrum Figure 4 Energy spectrum analysis diagram at the marked location;

[0041] Fig.14 is the continuous casting stopper rod and liquid level curve in comparative example 1;

[0042] Fig.15 It is the continuous casting stopper rod and liquid level curve in Example 1.

[0043] The realization of the purpose, functional features and advantages of the present invention will be further explained in conjunction with the implementation methods and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0044] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0045] It should be noted that all directional indications (such as up, down, etc.) in the embodiments of the present invention are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0046] In addition, in the present invention, the descriptions such as "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" or "second" may explicitly or implicitly include at least one of the features.

[0047] Furthermore, the technical solutions between the various embodiments of the present invention may be combined with each other, but this must be based on the fact that they can be implemented by ordinary technicians in the field. When the combination of technical solutions is mutually contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0048] The present invention provides a fin steel, wherein the composition of the fin steel includes C: ≤0.012%, Si: ≤0.02%, Mn: 0.2%-0.32%, P: ≤0.018%, S: ≤0.0095%, Al: ≤0.005%, N: 0.0035%-0.0070%, and the remainder is iron and unavoidable impurities. The fin steel includes SiO2-MnO inclusions and composite inclusions, wherein the composite inclusions are formed by MnO-SiO2-Al2O3 inclusions partially or completely enveloping Al2O3-MnO inclusions. In the fin steel liquid, the size of the SiO2-MnO inclusions is 0.5-2.5 μm, the size of the composite inclusions is 0.5-12 μm, and the number of composite inclusions with a size less than 2 μm accounts for more than 85%, and the number of inclusions greater than 10 μm accounts for less than 1.0%.

[0049] The fin steel of the present application includes SiO2-MnO inclusions and composite inclusions. Among them, SiO2-MnO inclusions and MnO-SiO2-Al2O3 inclusions are both low-melting-point inclusions, and Al2O3-MnO inclusions are high-melting-point inclusions. Therefore, the state of the composite inclusions in the molten steel is that the liquid (or molten) MnO-SiO2-Al2O3 inclusions completely or partially wrap the solid Al2O3-MnO inclusions. Therefore, it can also be regarded as a liquid or near-liquid form in the molten steel, so it has good fluidity and is not easy to adhere to the stopper rod or the immersed nozzle. Moreover, since the outer layer of the composite inclusion is completely or partially wrapped by the liquid (or molten) MnO-SiO2-Al2O3, the shape of the composite inclusions in the molten steel is also relatively regular, such as Figure 7 As shown, it is spherical, so it is not easy to adhere to the stopper rod or the immersed nozzle. Therefore, when the fin steel of the present application is prepared, it is not easy to block the nozzle, and the continuous casting stability is improved.

[0050] In addition, due to the low melting point of MnO-SiO2-Al2O3 on the surface of the composite inclusions, MnO-SiO2-Al2O3 is liquid at high temperatures and deforms and extends during the subsequent hot rolling process without destroying the continuity of the steel matrix, thereby improving the quality and performance of the steel.

[0051] On the other hand, Al2O3-MnO inclusions are solid in the molten steel and have irregular shapes, which can easily block the water inlet, causing large fluctuations in the crystallizer liquid level and the entrainment of protective slag, or the blockages fall off and fall into the molten steel, resulting in more inclusions on the surface of the steel plate.

[0052] In general, the fin steel of the present application mainly comprises fine dispersed SiO2-MnO low melting point inclusions, and a small amount of MnO-SiO2-Al2O3 inclusions partially or completely encapsulating Al2O3-MnO high melting point inclusions.

[0053] Among all the inclusions, SiO2-MnO inclusions are the main ones, and the number of composite inclusions is relatively small. In the molten steel of fin steel, the size and proportion of each inclusion are as follows: the size of SiO2-MnO is 0.5-2.5μm, the size of composite inclusions is 0.5-12μm, and the proportion of composite inclusions with a size less than 2μm is more than 85%, and the proportion of inclusions larger than 10μm is less than 1.0%. The small number of large-sized composite inclusions further reduces the probability of clogging the nozzle, improves the stability of continuous casting, and further improves the deformation ductility of the composite inclusion surface, thereby improving the quality and performance of the steel.

[0054] The above-mentioned fin steel reasonably controls the composition of inclusions in the molten steel, has few high-melting-point hard inclusions, and mainly contains low-melting-point inclusions, which significantly improves the stability of the fin steel liquid during the continuous casting process and greatly reduces surface inclusion defects. In addition, the low-melting-point inclusions deform and extend during the subsequent hot rolling process and will not destroy the continuity of the steel matrix, thereby improving the quality and performance of the steel.

[0055] In some embodiments, in the SiO 2 —MnO inclusion, the stoichiometric ratio of SiO 2 to MnO is 1:1.5.

[0056] The present application also provides a method for preparing the above-mentioned fin steel, comprising the following steps:

[0057] S100: After smelting the molten steel in a converter, RH refining is performed. The RH refining step includes:

[0058] S110: After RH decarburization, ferrosilicon and ferromanganese are added in the first batch for composite pre-deoxidation, so that the mass fraction of Mn in the molten steel after pre-deoxidation is 0.13-0.25%, the mass fraction of Si is below 0.013%, and the free oxygen content is 280-380ppm.

[0059] S120: The second batch of aluminum blocks is added to make the free oxygen content in the molten steel 10~30ppm.

[0060] S130: If the Mn content in the molten steel is insufficient, ferromanganese is added in the third batch to make the Mn mass fraction in the molten steel reach 0.2%-0.32%.

[0061] Before introducing step S100, a relatively complete preparation process of general fin steel is briefly introduced. The production process of fin steel is: KR desulfurization → converter smelting → RH vacuum smelting → slab continuous casting → heating of the cast billet in a heating furnace → rough rolling → finishing rolling → coiling. This application mainly improves some of the steps, and the remaining steps that have not been improved can be carried out according to the methods and process parameters of the existing process steps.

[0062] In step S100, molten steel is smelted in a converter and then RH refining is performed. RH refining is a molten steel vacuum cycle degassing method, which is an efficient secondary steelmaking process, mainly used to remove gas and inclusions in steel to improve the purity of steel. RH refining can decarburize and deoxidize molten steel.

[0063] In the decarburization process, carbon reacts with oxygen in the steel to produce CO (carbon monoxide) and CO2 (carbon dioxide), which are then extracted by the vacuum system to complete the decarburization. The reaction of carbon with oxygen consumes some of the oxygen, so part of the deoxidation work is also completed in this process.

[0064] After RH decarburization is completed, step S110 is performed. In the molten steel after decarburization, the first batch of ferrosilicon and ferromanganese is added for composite pre-deoxidation. The amount of the two added and the duration of the composite pre-deoxidation after addition are based on the molten steel meeting the following conditions after pre-deoxidation: after pre-deoxidation, the Mn content in the steel is controlled at 0.13-0.25%, the Si content is controlled below 0.013%, and the free oxygen content in the molten steel is 280-380ppm.

[0065] Among them, ferrosilicon is an iron alloy composed of iron and silicon. Ferrosilicon is an iron-silicon alloy made by smelting in an electric furnace using coke, steel scraps, and quartz (or silica) as raw materials. Since silicon and oxygen can easily combine to form silicon dioxide, ferrosilicon is often used as a deoxidizer in steelmaking. At the same time, since a large amount of heat is released when SiO2 is generated, it is also beneficial to increase the temperature of molten steel while deoxidizing.

[0066] Ferromanganese: An iron alloy composed of manganese and iron. In steelmaking, it is used as a deoxidizer and alloy additive. It is mainly classified into: high carbon ferromanganese (7% carbon), medium carbon ferromanganese (1.0-1.5% carbon), and low carbon ferromanganese (0.5% carbon).

[0067] Specifically, during the RH refining process of fin steel, when decarburization is completed, the free oxygen content in the molten steel is relatively high. At this time, ferrosilicon and ferromanganese are added for composite pre-deoxidation, and circulated for a certain period of time to ensure sufficient reaction, so that fine and dispersed SiO2-MnO low-melting point inclusions are generated in the molten steel.

[0068] In other existing processes, in order to control the extremely low Si content, ferrosilicon is not added during the smelting process of fin steel. In the present invention, a small amount of ferrosilicon is added in the high oxygen state of the molten steel, and the Si in the ferrosilicon is basically combined with oxygen to form inclusions. The actual Si addition in the molten steel is very small, and the amount of Si added is less than 0.002%.

[0069] After completing step S110, step S120 is performed to add the second batch of aluminum blocks at one time.

[0070] Due to the limited deoxidation capacity of Si and Mn elements, the free oxygen content in the molten steel before adding the aluminum block is still 280-380ppm. At this time, the aluminum block is added at one time, most of which will generate Al2O3 cluster inclusions, which are very easy to float up and remove, and the other part will generate Al2O3-MnO high melting point inclusions. The molten steel is violently mixed in the RH vacuum chamber, which promotes the collision of inclusions. When the solid Al2O3-MnO inclusions collide with the liquid SiO2-MnO inclusions, due to the effect of interfacial tension, the solid particles tend to be drawn into the liquid inclusions, that is, the outer layer of the Al2O3-MnO inclusions is completely wrapped by the MnO-SiO2-Al2O3 inclusions.

[0071] Next, step S130 is performed. If the Mn content in the molten steel is insufficient, ferromanganese is added in a third batch so that the mass fraction of Mn in the molten steel reaches 0.2%-0.32%.

[0072] That is, if the Mn content in the molten steel is insufficient and does not reach 0.2%-0.32%, then an appropriate amount of ferromanganese is added to the molten steel in the third batch to adjust the Mn content of the molten steel to 0.2%-0.32%. If the Mn content in the molten steel reaches 0.2%-0.32% after steps S110 and S120, step S130 is not required.

[0073] S200: The refined molten steel is continuously cast and then rolled to form the fin steel.

[0074] After the molten steel is refined in the ladle, it is transferred to the crystallizer via the tundish, and a continuous casting operation is performed in the crystallizer to obtain a cast billet. In some embodiments, the cast billet is heated in a heating furnace, and is subjected to rough rolling, finish rolling, and coiling to obtain fin steel.

[0075] During continuous casting, there are very few pure high-melting-point inclusions in the molten steel, and there is no obvious nodule on the inner wall of the submerged nozzle. There will be no deviation flow that causes large fluctuations in the liquid level of the crystallizer, which in turn causes slag rolling. At the same time, there are also very few high-melting-point inclusions attached to and gathered on the inner wall of the nozzle that are washed into the crystallization by the molten steel to form surface inclusion defects. Therefore, the surface quality of the fin steel can be significantly improved.

[0076] The above-mentioned preparation method of fin steel reasonably controls the composition of inclusions in molten steel, so that there are very few pure high-melting-point inclusions in the molten steel, thereby improving the surface quality of the fin steel.

[0077] In some embodiments, in the composite pre-deoxidation operation, the amount of ferrosilicon added is 0.1-0.15 kg / ton of molten steel, the ferromanganese is high carbon ferromanganese, low carbon ferromanganese or metallic manganese, and the amount of high carbon ferromanganese, low carbon ferromanganese or metallic manganese added is 0.6-1.8 kg / ton of molten steel, 0.5-1.6 kg / ton of molten steel, and 0.3-1.4 kg / ton of molten steel, respectively.

[0078] In this embodiment, the composite pre-deoxidation reaction can be made more complete, and the Mn, Si and free oxygen contents in the molten steel can be better controlled to reach the target range.

[0079] In some embodiments, in the composite pre-deoxidation operation, the circulation time is 10-15 minutes after the addition of ferrosilicon and ferromanganese. The circulation time of 10-15 minutes can fully mix the molten steel, ensure sufficient reaction, and better control the Mn, Si and free oxygen content in the molten steel to reach the target range.

[0080] In some embodiments, the amount of the aluminum block added satisfies the following formula:

[0081] W Al =1.125 [O] / 10000 W 钢水 10 / 0.75+W 钢水 / y Al [Als] 10.

[0082] Where: W Al Indicates the amount of aluminum added, unit: kg.

[0083] [O] represents the free oxygen content in molten steel before adding aluminum, unit: ppm.

[0084] W 钢水 Indicates the mass of molten steel, unit: ton.

[0085] y Al Indicates the yield of Al, unit: %.

[0086] [Als] represents the acid-soluble aluminum content in molten steel after RH refining, unit: %, and is set to a fixed value of 0.0042%.

[0087] In some embodiments, it also includes:

[0088] During the steel-tapping process of the converter smelting, lime and top slag modifier are added. After the RH refining is completed, the CaO / Al2O3 mass ratio in the refined slag is between 1.8 and 2.3, and the sum of the mass fractions of total iron (Fe TOT) and MnO is ≤14%.

[0089] The purpose of this step is to control the RH refined slag system with a certain inclusion adsorption capacity and low oxidizability. The refined slag has a certain fluidity, which is conducive to the adsorption and removal of inclusions on the one hand, and the Al in the modifier can partially remove oxygen from the slag on the other hand, reducing the subsequent secondary oxidation of the molten steel caused by the excessively high oxygen potential in the slag.

[0090] In some embodiments, the amount of lime added is 2.5~3.3kg / ton of molten steel, the amount of top slag modifier added is 1.5~2.0kg / ton of molten steel, and the top slag modifier is Al40.

[0091] In some embodiments, during the continuous casting process, argon is blown between the plates at the connection between the upper water inlet and the submerged lower water inlet, and argon is not blown between the stopper rod and the upper water inlet. The thickness of the cast billet obtained by the continuous casting is 230-250 mm.

[0092] During continuous casting, the free oxygen content in the fin steel liquid is still 10-25ppm, which is semi-boiling steel. Therefore, argon is only blown between the plates at the connection between the upper water inlet and the immersed lower water inlet to prevent secondary oxidation caused by the inhalation of air. The stopper rod and the upper water inlet are not blown with argon to reduce the fluctuation of the liquid level, and at the same time, the pore defects of the casting are greatly reduced. On the other hand, the present invention controls the type of inclusions in the fin steel to be mainly SiO2-MnO low-melting point inclusions, and a small amount of Al2O3-MnO high-melting point inclusions are also wrapped by MnO-SiO2-Al2O3 low-melting point inclusions. Even if argon is not blown on the stopper rod and the upper water inlet, the water inlet will not be blocked. Figure 1 .

[0093] Hard inclusions with high melting points will destroy the continuity of the steel matrix. In the present invention, the inclusions in the fin steel are controlled to be low melting point inclusions with high plasticity, which can deform and extend in the subsequent hot rolling process. Fig.10 and Fig.11 , thereby improving the quality and performance of steel.

[0094] In some embodiments, a KR desulfurization step is also included before the converter smelting.

[0095] The above scheme is further described below in conjunction with specific embodiments, which are described in detail as follows:

[0096] The production process of fin steel is: KR desulfurization → converter smelting → RH refining → slab continuous casting → heating of the cast ingot in a heating furnace → rough rolling → finishing rolling → coiling. The composition of the finished fin steel is as follows by mass fraction: C: ≤0.012%, Si: ≤0.02%, Mn: 0.2%-0.32%, P: ≤0.018%, S: ≤0.0095%, Al: ≤0.005%, N: 0.0035%-0.0070%, and the balance is iron and unavoidable impurities.

[0097] The composition of the finished fin steel products of each embodiment and comparative example is shown in Table 1. The composition of the finished fin steel products of comparative example 1 and embodiment 1 is the same, and the composition of the finished fin steel products of comparative example 2 and embodiment 2 is the same.

[0098] Table 1 Composition of fin steel of each embodiment and comparative example

[0099]

[0100] In the production process, the difference between the comparative examples and the embodiments is that the alloy addition during the RH refining process is different and the components in the refined slag when the RH furnace is discharged are different, and the comparative examples only add lime when the converter is tapped, while the embodiments add lime and top slag modifier. The remaining steps are the same.

[0101] In the process of tapping the converter, lime and top slag modifier are added to the top slag in the embodiment, while only lime is added to the top slag in the comparative example. The components of the refined slag when the RH furnace is tapped are shown in Table 2. The slag melting points under the conditions of embodiments 1 to 4 are low, but the CaO / Al2O3 of comparative examples 1 and 2 are very high, the slag fluidity and adsorption capacity of inclusions are poor, and the sum of total iron (Fe TOT) and MnO in the slag is relatively high, which is easy to cause secondary oxidation of molten steel, which is not conducive to the cleanliness of the steel.

[0102] Table 2 Composition of refined slag when leaving RH furnace

[0103]

[0104] The alloy addition conditions and continuous casting pouring status during RH refining are shown in Table 3, where the amount of aluminum block added is determined according to the free oxygen content in the molten steel before aluminum addition.

[0105] Table 3 Alloy addition during RH refining and continuous casting pouring status

[0106]

[0107] The continuous casting stopper rod and liquid level curves of Example 1 and Comparative Example 1 are as follows: Fig.14 and Fig.15 shown.

[0108] Figure 1 This is a physical picture of the submerged nozzle of the fin steel of Production Example 1 after the continuous casting is completed, and there is no nodule on the inner wall of the nozzle.

[0109] The molten steel after refining in Example 1 and Comparative Example 1 was analyzed by scanning electron microscope. Figure 2 This is a SEM image of SiO2-MnO low-melting-point inclusions in the refined molten steel of Example 1. Figure 3 yes Figure 2 Spectrum Figure 1 Energy spectrum analysis diagram at the marked position. Figure 3 The energy spectrum analysis diagram can determine Figure 2 The inclusions shown are SiO2-MnO inclusions. Figure 3 The cts in the figure stands for signal strength, which indicates the unit of the ordinate, and the unit of the abscissa is keV (kilo-electron volt). The units of other energy spectrum analysis diagrams are the same and will not be described in detail.

[0110] Figure 4 This is a SEM picture of the composite inclusions of Al2O3-MnO inclusions partially wrapped in the outer layer of Al2O3 inclusions in the molten steel after refining in Example 1. Figure 5 yes Figure 4 Spectrum Figure 1 Energy spectrum analysis diagram at the marked location. Figure 6 yes Figure 4 Spectrum Figure 2 Energy spectrum analysis diagram at the marked position. Figure 5 and Figure 6 The energy spectrum analysis diagram can determine Figure 4 The inclusions shown are composite inclusions in which the outer layer of the Al2O3 inclusion partially encloses the Al2O3-MnO inclusion.

[0111] Figure 7 This is a SEM image of the composite inclusions (MnO-SiO2-Al2O3 inclusions all encapsulating Al2O3-MnO inclusions) in the molten steel of the fin steel of Example 1. Figure 8 yes Figure 7 Spectrum Figure 1 Energy spectrum analysis diagram at the marked location. Fig. 9 yes Figure 7 Spectrum Figure 2 Energy spectrum analysis diagram at the marked position. Figure 8 and Fig. 9 The energy spectrum analysis diagram can determine Figure 7 The inclusions shown are composite inclusions in which MnO-SiO2-Al2O3 inclusions completely enclose Al2O3-MnO inclusions.

[0112] The metallographic images and energy spectrum analysis images of the inclusions in other embodiments are similar to the corresponding images in Example 1. The metallographic images and energy spectrum analysis images of the inclusions in other comparative examples are similar to the corresponding images in Comparative Example 1.

[0113] It can be seen that in the molten steel of the fin steel, the inclusion types in the comparative example are mainly SiO2-MnO low melting point inclusions, and the others are high melting point Al2O3 inclusions and Al2O3-MnO composite inclusions; the inclusion types in the embodiment are mainly SiO2-MnO low melting point inclusions, and a small amount is a composite inclusion in which the MnO-SiO2-Al2O3 low melting point inclusions completely encapsulate the Al2O3-MnO high melting point inclusions.

[0114] Fig.10 This is a metallographic image of the fin steel of Example 1 after hot rolling, in which the SiO2-MnO low melting point inclusions are deformed and extended into short chain-shaped inclusions. Fig.11 This is the SEM image of the composite inclusions (MnO-SiO2-Al2O3 inclusions partially encapsulate Al2O3-MnO inclusions) in the finished fin steel of Example 1 due to rolling deformation (the two ends are deformed, and the hard Al2O3-MnO inclusions in the middle are not easily deformed). Fig.12 for Fig.11 Spectrum Figure 3 Energy spectrum analysis diagram at the marked location. Fig.13 for Fig.11 Spectrum Figure 4 Energy spectrum analysis diagram at the marked location.

[0115] In summary, by adopting the technical scheme of the present invention, the stopper rod and liquid level curve are stable during the continuous casting process, there is no obvious nodule phenomenon on the inner wall of the nozzle, and multi-furnace continuous casting is successfully achieved. The downgrading rate of fin steel due to surface inclusion defects is reduced from 6.5% to below 1.0%, which greatly improves the quality of the product.

[0116] The above technical solutions of the present invention are only preferred embodiments of the present invention, and the patent scope of the present invention is not limited thereto. All equivalent structural changes made by using the contents of the present invention specification and drawings under the technical concept of the present invention, or directly / indirectly applied in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. A fin steel, characterized in that: The composition of the fin steel includes C: ≤0.012%, Si: ≤0.02%, Mn: 0.2%-0.32%, P: ≤0.018%, S: ≤0.0095%, Al: ≤0.005%, N: 0.0035%-0.0070%, and the balance is iron and inevitable impurities; the fin steel includes SiO2-MnO inclusions and composite inclusions, and the composite inclusions are inclusions formed by MnO-SiO2-Al2O3 inclusions partially or completely encapsulating Al2O3-MnO inclusions; in the molten steel of the fin steel, the size of the SiO2-MnO inclusions is 0.5-2.5μm, the size of the composite inclusions is 0.5-12μm, and the number of inclusions with a size less than 2μm in the composite inclusions accounts for more than 85%, and the number of inclusions larger than 10μm accounts for less than 1.0%.

2. The fin steel according to claim 1, characterized in that: In SiO2-MnO inclusions, the stoichiometric ratio of SiO2 to MnO is 1:1.

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3. A method for preparing fin steel according to claim 1 or 2, characterized in that: The following steps are involved: After molten steel is smelted in a converter, RH refining is performed; wherein the RH refining step includes: After RH decarburization, ferrosilicon and ferromanganese are added in the first batch for composite pre-deoxidation, so that the mass fraction of Mn in the molten steel after pre-deoxidation is 0.13-0.25%, the mass fraction of Si is below 0.013%, and the free oxygen content is 280-380ppm; The second batch of aluminum blocks is added at one time, so that the free oxygen content in the molten steel is 10~30ppm; If the Mn content in the molten steel is insufficient, ferromanganese is added in the third batch to make the Mn mass fraction in the molten steel reach 0.2%-0.32%; The refined molten steel is continuously cast and then rolled to form the fin steel.

4. The preparation method according to claim 3, characterized in that: In the operation of the composite pre-deoxidation, the amount of ferrosilicon added is 0.1-0.15 kg / ton of molten steel, the ferromanganese is high carbon ferromanganese, low carbon ferromanganese or metallic manganese, and the amount of high carbon ferromanganese, low carbon ferromanganese or metallic manganese added is 0.6-1.8 kg / ton of molten steel, 0.5-1.6 kg / ton of molten steel, and 0.3-1.4 kg / ton of molten steel, respectively.

5. The preparation method according to claim 3, characterized in that: In the operation of the composite pre-deoxidation, the circulation time after the addition of ferrosilicon and ferromanganese is 10-15 minutes.

6. The preparation method according to claim 3, characterized in that: The amount of aluminum block added satisfies the following formula: W Al =1.125 [O] / 10000 W 钢水 10 / 0.75+W 钢水 / y Al [Als] 10; Where: W Al Indicates the amount of aluminum added, unit: kg; [O] represents the free oxygen content in molten steel before adding aluminum, unit: ppm; W 钢水 Indicates the mass of molten steel, unit: tons; y Al Indicates the yield of Al, unit: %; [Als] represents the acid-soluble aluminum content in molten steel after RH refining, unit: %, and is set to a fixed value of 0.0042%.

7. The preparation method according to claim 3, characterized in that: Also includes: During the steel-tapping process of the converter smelting, lime and top slag modifier are added. After the RH refining is completed, the CaO / Al2O3 mass ratio in the refined slag is between 1.8 and 2.3, and the sum of the mass fractions of total iron and MnO is ≤14%.

8. The preparation method according to claim 7, characterized in that: The amount of lime added is 2.5-3.3 kg / ton of molten steel, the amount of top slag modifier added is 1.5-2.0 kg / ton of molten steel, and the top slag modifier is Al40.

9. The preparation method according to claim 3, characterized in that: During the continuous casting process, argon is blown between the plates at the connection between the upper water inlet and the submerged lower water inlet, but argon is not blown between the stopper rod and the upper water inlet; the thickness of the cast billet obtained by the continuous casting is 230-250mm.

10. The preparation method according to claim 3, characterized in that: A KR desulfurization step is also included before the converter smelting.

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