Method for manufacturing slab for non-oriented electromagnetic steel sheet

Abstract

This method for manufacturing a slab for a non-oriented electromagnetic steel sheet includes: a tapping step for tapping molten steel, which has not been deoxidized, from an electric furnace into a ladle of which a slag-contacting surface is lined with magnesia-based refractory material; a decarburization treatment step; a deoxidation treatment step for adding a metal Al-containing substance from above the ladle; a denitrification and desulfurization treatment step for stirring the molten steel in contact with the slag; and a casting step, wherein during a period from completion of the tapping step to completion of the decarburization treatment step, a CaO- and / or Al2O3-containing substance is added into the ladle such that at the start of the denitrification and desulfurization treatment step, the mass ratio C / A(-) of CaO to Al2O3 in the slag falls within the a range of 1.0 to 1.4, and at the start of the denitrification and desulfurization treatment step, the thickness of the slag is at least 100 mm.

Description

Manufacturing method for slabs for non-oriented electrical steel sheets

[0001] The present invention relates to a method for manufacturing a slab for a non-oriented electrical steel sheet.

[0002] Nitrogen is a potentially harmful component of metal materials. In conventional blast furnace and converter processes, nitrogen is removed from molten iron by adsorbing it onto the surface of carbon monoxide bubbles generated during the decarburization process. However, in electric arc furnace processes, scrap and reduced iron are melted, resulting in molten steel with a lower carbon concentration than molten iron. This limits the amount of decarburization, i.e., the amount of carbon monoxide generated, making it impossible to remove nitrogen to a low concentration. Therefore, Patent Document 1, for example, proposes a method for removing nitrogen from molten steel using slag.

[0003] Japanese Patent Application Laid-Open No. 2022-189514

[0004] When the nitrogen concentration is reduced by the method of Patent Document 1, the higher the slag composition, the more advantageous it is for the denitrification reaction. ２ O ３ The mass ratio C / A(-) is preferably in the range of 0.4 to 1.8, more preferably in the range of 0.7 to 1.7.

[0005] However, even if C / A(-) is within the above range, the closer to the upper or lower limit, the higher the molten steel temperature must be to cause slag formation. However, if the molten steel temperature is high, the refractory lining of the ladle, particularly on the surface that comes into contact with the slag, is likely to be melted and damaged, and there is a concern that the components in the refractory may have an adverse effect on the refining reaction and the properties of the steel product.

[0006] When producing slabs for non-oriented electrical steel sheets with a high Si concentration, the liquidus temperature of the steel is low. Therefore, if the molten steel temperature is too high, the temperature must be lowered to a temperature suitable for casting, resulting in waiting times and the need to use large amounts of coolant. Furthermore, if the molten steel is held in the ladle for a long time, the refractory lining of the ladle, particularly on the surface that comes into contact with the slag, is likely to be melted and damaged. This raises concerns that the components in the refractory may adversely affect the refining reaction and the properties of the steel. Furthermore, the use of a large amount of coolant raises concerns that impurities in the coolant may be mixed into the steel, adversely affecting the properties of the steel.

[0007] Furthermore, in the decarburization treatment of slabs for non-oriented electrical steel sheets, which is carried out after tapping from an electric furnace to reduce the C content to 0.0050% or less, the dissolved oxygen necessary for the decarburization treatment is present in the molten steel, and the slag also becomes oxidizing. On the other hand, since the Al concentration is high, when Al is added to the molten steel after the decarburization treatment, the slag is similarly deoxidized and becomes reducing slag.

[0008] In this way, the refractory lining the ladle is exposed to both oxidizing and reducing slag, but it is difficult to prepare a large amount of refractory that is highly resistant to corrosion by both oxidizing and reducing slag for the steelmaking process, due to the cost. Therefore, it is important to suppress the corrosion of the refractory under the operating conditions.

[0009] The present invention has been made in view of the above, and an object of the present invention is to provide a method for producing slabs for non-oriented electrical steel sheets, which can remove carbon, nitrogen, and sulfur from molten steel to low concentrations while suppressing melting damage to the refractory lining the ladle in an electric furnace process.

[0010] In order to solve the above-mentioned problems and achieve the object, the method for producing a slab for a non-oriented electrical steel sheet according to the present invention includes, in mass%, producing a slab for a non-oriented electrical steel sheet containing C: 0.0050% or less, Si: 2.0% to 4.5%, Mn: 0.1% to 2.0%, S: 0.0030% or less, Al: 0.15% to 2.5%, N: 0.0050% or less, and Ti: 0.0100% or less, the method comprising the steps of: (a) removing undeoxidized molten steel from an electric furnace; (b) a decarburization treatment step of stirring the molten steel under reduced pressure; (c) a deoxidation treatment step of adding a metallic Al-containing substance from above the ladle; (d) a denitrification and desulfurization treatment step of stirring the molten steel in contact with the slag; and (e) a casting step of continuously casting the molten steel, wherein the molten steel is subjected to continuous casting after completion of the tapping step and before completion of the decarburization treatment step. ２ O ３ The contained material is added to the ladle, and at the start of the denitrification and desulfurization treatment process, CaO and Al in the slag are ２ O ３The mass ratio C / A(-) is set to a range of 1.0 to 1.4, and the thickness of the slag is set to 100 mm or more at the start of the denitrification and desulfurization treatment step.

[0011] Further, in the manufacturing method of a slab for a non-oriented electrical steel sheet according to the present invention, when the decarburization treatment step is carried out in a ladle vacuum degassing apparatus, CaO and / or Al are not added during the period from the start of the decarburization treatment step to 40% of the decarburization treatment time. ２ O ３ A portion of the contained material is added to the ladle.

[0012] In addition, in the method for producing a slab for a non-oriented electrical steel sheet according to the present invention, the decarburization treatment step is carried out in an RH vacuum degassing apparatus.

[0013] In the method for producing a slab for a non-oriented electrical steel sheet according to the present invention, the molten steel temperature at the start of the denitrification and desulfurization treatment step is set to 1580°C or higher and 1620°C or lower.

[0014] Further, in the manufacturing method of a slab for a non-oriented electrical steel sheet according to the present invention, in the above-mentioned invention, in the denitrification and desulfurization treatment step, SiO in the slag is reduced during half the time from the start of the treatment to the end of the treatment. ２ The total concentration of MnO, T, and Fe is 3.0 mass % or less.

[0015] Furthermore, in the method for producing a slab for a non-oriented electrical steel sheet according to the present invention, in the denitrification / desulfurization treatment step, the molten steel after the addition of the metallic Al-containing substance is stirred at a stirring power density of 100 W or more per ton of molten steel.

[0016] Furthermore, the method for producing a slab for a non-oriented electrical steel sheet according to the present invention produces the slab for the non-oriented electrical steel sheet according to the above invention, further containing, by mass%, Mg: 0.0030% or less.

[0017] Further, in the method for producing a slab for a non-oriented electrical steel sheet according to the present invention, in the above-mentioned invention, CaO and / or Al is added during the period from the completion of the tapping step to the completion of the decarburization treatment step. ２ O ３The metal Al-containing substance is added to the ladle, and the MgO concentration in the slag before the addition of the metal Al-containing substance is set to 5 mass % or less.

[0018] Furthermore, in the method for producing a slab for a non-oriented electrical steel sheet according to the present invention, in the above-described invention, an MgO-containing substance is added to the ladle after the addition of the metallic Al-containing substance, so that the MgO concentration in the slag is 8 mass% or more.

[0019] Furthermore, in the method for producing a slab for a non-oriented electrical steel sheet according to the present invention, when the pre-denitrification and desulfurization treatment step is carried out in a ladle vacuum degassing apparatus, stirring is carried out at 5000 Pa or less for 5 minutes or more after the addition of the metallic Al-containing substance.

[0020] According to the method for producing a slab for a non-oriented electrical steel sheet of the present invention, carbon, nitrogen, and sulfur in molten steel can be removed to low concentrations while suppressing melting damage to the refractory lining the ladle in an electric furnace process.

[0021] Fig. 1 is a schematic diagram showing an example of a ladle vacuum degassing apparatus used in the method for producing a slab for a non-oriented electrical steel sheet according to an embodiment of the present invention. Fig. 2 is a schematic diagram showing an example of an RH vacuum degassing apparatus used in the method for producing a slab for a non-oriented electrical steel sheet according to an embodiment of the present invention.

[0022] A method for manufacturing a slab for a non-oriented electrical steel sheet according to an embodiment of the present invention will be described with reference to the drawings. Note that the components in the following embodiments include those that are easily replaceable by those skilled in the art, or those that are substantially identical.

[0023] [Embodiment 1] In a method for manufacturing a slab for a non-oriented electrical steel sheet according to embodiment 1, a slab for a non-oriented electrical steel sheet containing, in mass%, C: 0.0050% or less, Si: 2.0% to 4.5%, Mn: 0.1% to 2.0%, S: 0.0030% or less, Al: 0.15% to 2.5%, N: 0.0050% or less, and Ti: 0.0100% or less.

[0024] In the method for manufacturing a slab for a non-oriented electrical steel sheet according to the first embodiment, a tapping process, a decarburization process, a deoxidation process, a denitrification / desulfurization process, and a casting process are performed in this order. Each process will be described in detail below.

[0025] (Tapping process) In the tapping process, scrap, pig iron, and reduced iron are melted in an electric furnace, and the resulting undeoxidized molten steel is tapped from the electric furnace into a ladle lined with magnesia-based refractories on the surface that comes into contact with the slag. During this process, it is desirable to prevent slag from flowing into the ladle as much as possible, or to remove the slag after it has flowed into the ladle.

[0026] (Decarburization Treatment Step) In the decarburization treatment step, after the completion of tapping, the ladle is transferred to a vacuum degassing apparatus having an exhaust system, and the molten steel is stirred under reduced pressure. Specifically, in the decarburization treatment step, decarburization is performed by blowing an inert stirring gas into the molten steel in a reduced pressure atmosphere. As the inert stirring gas used in the decarburization treatment step, for example, Ar gas that does not contain nitrogen gas, etc., is desirable.

[0027] When a ladle vacuum degassing apparatus such as that shown in Fig. 1 is used as the vacuum degassing apparatus, a ladle 2 is placed in a vacuum tank 3, and an inert stirring gas is injected from a nozzle (bottom-blowing gas injection pipe) 4 installed at the bottom of the ladle 2. In addition to the above, the inert stirring gas may be injected through a lance immersed in the molten steel 1. In the figure, reference numeral 5 denotes slag, and reference numeral 21 denotes a magnesia-based refractory.

[0028] During the decarburization process, CO gas is generated from the molten steel 1. At that time, the CO gas is captured by the slag 5 formed on the molten steel 1, causing slag foaming, which increases the slag height, and may cause leakage from the upper end of the ladle 2. In order to prevent this slag foaming, CaO and / or Al are added to the slag between the start and end of the decarburization process. ２ O ３ The contained material is added from above into the ladle 2. This creates holes in the foamed slag 5, through which the CO gas trapped in the slag 5 escapes, thereby suppressing slag foaming.

[0029] In addition, this CaO and / or Al ２ O ３It is more effective to add the contained substances when slag foaming is likely to occur, i.e., when CO gas generation is active. After careful investigation into this period, it was found that CO gas generation is active up to 40% of the decarburization treatment time. Therefore, when the decarburization treatment process is carried out using a ladle vacuum degassing device, it is recommended to add CaO and / or Al during the period from the start of the decarburization treatment process to 40% of the decarburization treatment time. ２ O ３ By adding some of the contained substances to the ladle 2, slag foaming can be effectively suppressed. ２ O ３ There is no particular restriction on the method of adding the ingredients, and they may be added all at once, in portions, or continuously.

[0030] 2 is used as the vacuum degassing apparatus, two immersion pipes 7 connected to a vacuum vessel 6 are immersed in the ladle 2, and with the vacuum vessel 6 maintained at a reduced pressure, an inert gas for circulating the molten steel is blown in from a circulating gas blowing pipe 8 of one of the immersion pipes 7. In addition to the above, the inert gas for circulating the molten steel may be blown in from a nozzle or injection lance installed at the bottom of the ladle 2 at the immersion position of the immersion pipes 7.

[0031] Furthermore, whether a ladle vacuum degasser or an RH vacuum degasser is used as the vacuum degasser, oxygen gas may be enriched by a top blowing lance, injection, or the like in order to increase the oxygen concentration in the molten steel in order to promote the decarburization reaction.

[0032] (Deoxidation Treatment Step) In the deoxidation treatment step, after the completion of the decarburization treatment, a metallic Al-containing substance is added from above the ladle to deoxidize the molten steel. Then, the process proceeds to the denitrification / desulfurization treatment step. The deoxidation treatment step may be performed as part of the denitrification / desulfurization treatment step.

[0033] The timing and number of times that the metallic Al-containing substance is added are not particularly limited, as long as it is added at least once between the completion of the decarburization treatment step and the completion of the denitrification / desulfurization treatment step. The decarburization treatment and the denitrification / desulfurization treatment may be performed in separate apparatuses, but since the apparatus for performing the denitrification / desulfurization treatment must be capable of reacting molten steel with slag, it is preferable to perform the treatment in the ladle vacuum degassing apparatus or ladle refining apparatus described above. The addition of the Al-containing substance after the completion of the decarburization treatment may be performed in either or both of the above apparatuses, but it is preferable to add some or all of the Al-containing substance at the start of the denitrification / desulfurization treatment.

[0034] (Denitrification / desulfurization treatment step) In the denitrification / desulfurization treatment step, the molten steel in contact with the slag is stirred. Specifically, the denitrification / desulfurization treatment is performed by blowing an inert gas for stirring into the molten steel.

[0035] The denitrification and desulfurization reaction starts after the molten steel is deoxidized by adding the metallic Al-containing substance. Therefore, before adding the metallic Al-containing substance, that is, at the start of the denitrification and desulfurization treatment, CaO and Al are present on the molten steel in the ladle. ２ O ３ It is desirable that the slag contains the CaO-containing material added before the deoxidation treatment and the Al generated by the deoxidation treatment. ２ O ３ However, the Al used to form the slag ２ O ３ If Al is insufficient, add Al before deoxidation. ２ O ３ The Al-containing material is added. ２ O ３ There is no particular specification for the timing of adding the contained substances, but it is desirable to add them when the slag is molten, so it is recommended to add them as early as possible, for example, when tapping steel from the electric furnace into the ladle. ２ O ３ It is desirable to add the CaO-containing substance. ２ O ３ The inclusion material may be calcium aluminate in premelt or premix form.

[0036] When performing denitrification and desulfurization, the higher the slag conversion rate, the more advantageous it is for the denitrification and desulfurization reaction. Therefore, the CaO and Al in the slag are ２ O ３ If the mass ratio C / A(-) is too high or too low, the slag conversion rate will be low. In order to ensure the slag conversion rate, it is effective to adjust this C / A(-) to, for example, 0.7 to 1.7.

[0037] However, if the slag turns into slag, the refractory material in contact with the slag is easily eroded. If the amount of erosion increases, the life of the ladle is shortened and costs increase. Furthermore, in the case of magnesia-based refractories, which are commonly used as refractories for ladles, the refractory is eroded and MgO is eluted into the slag. As shown in Patent Document 1, the elution of MgO into the slag adversely affects the denitrification process.

[0038] At the same time, if the refractory contains C or Ti oxides, these will also be eluted, with some of the C remaining as an impurity in the steel, and Ti oxides being reduced by Al in the steel and becoming an impurity in the steel, which will have an adverse effect on the electromagnetic properties of the steel sheet.

[0039] As a result of extensive research, it was found that, within the range of C / A(-) of 0.7 to 1.7, the higher the C / A(-), the smaller the corrosion of the magnesia-based refractory. It was also found that, even if MgO is eluted into the slag, in order to suppress the elution to a level that does not adversely affect the denitrification treatment, it is desirable to set the lower limit of C / A(-) to 1.0.

[0040] Furthermore, since the slag formation rate decreases when the temperature is low or when MgO is present in the slag, it is desirable to lower the liquidus temperature of the slag as much as possible when the C / A(-) ratio is high. Therefore, it is desirable to set the upper limit of C / A(-) to 1.4, and by further setting the upper limit to 1.2, it becomes possible to more effectively eliminate inhibiting factors.

[0041] Thus, in the denitrification and desulfurization treatment process, at the start of the denitrification and desulfurization treatment process, CaO and Al in the slag ２ O ３ The mass ratio C / A(-) is set to be in the range of 1.0 to 1.4. Note that C / A(-) is determined by the amount of additives (CaO and / or Al) added during tapping, for example.２ O ３ The amount of additives (CaO and / or Al) added between the end of tapping and the end of decarburization treatment ２ O ３ The amount of the additive (containing substance) can be controlled.

[0042] Here, the temperature of the molten steel at the start of the denitrification / desulfurization treatment, i.e., after the addition of the metallic Al-containing substance, is desirably 1580°C or higher, since it is necessary to secure an amount of molten slag that can sufficiently guarantee the reactivity of the slag. On the other hand, after the end of the denitrification / desulfurization treatment, the temperature of the molten steel must be lowered to a temperature at which casting is possible by the time casting begins.

[0043] The steel of this embodiment has a high concentration of Si, and the liquidus temperature (TLL) of the steel, calculated by the following formula (1), for example, is about 30 to 50°C lower than that of steel not containing Si. Therefore, if the temperature of the molten steel at the start of the denitrification / desulfurization treatment is too high, there is a concern that the temperature may not be lowered sufficiently before the start of casting. Furthermore, in order to prevent impurities from being mixed into the refractory due to melting of the refractory lining the ladle, it is desirable to set the upper limit of the molten steel temperature at the start of the denitrification / desulfurization treatment to 1620°C.

[0044] TLL=1538-(55(%C)+80(%C) ２ +13(%Si)+4.8(%Mn)+4.3(%Ni)+1.5(%Cr))...(1)

[0045] Even if the temperature of the molten steel at the start of the denitrification and desulfurization treatment is set to 1580°C or higher and 1620°C or lower as described above, if the temperature cannot be lowered to the predetermined temperature by the time casting is started, steel material may be added to the molten steel as a coolant.

[0046] In addition, the lower the oxidation degree of the slag, the more advantageous it is for the denitrification and desulfurization reaction. Therefore, the following components, SiO, which increase the oxidation degree of the slag, are added. ２ , MnO, T. Fe(FeO, Fe ２ O ３ ) concentration in the slag is preferably low. ２ , MnO, T. Fe(FeO, Fe ２ O ３ ) is preferably 3.0 mass % or less.

[0047] Furthermore, because the steel of this embodiment has a high Al concentration, the above components are gradually reduced and decreased after the addition of the metallic Al-containing substance. However, if the reduction takes a long time, there is a concern that the denitrification / desulfurization treatment time will be extended or the denitrification / desulfurization will be insufficient. Therefore, by reducing the concentration to the above level within half of the available denitrification / desulfurization treatment time, i.e., half the time from the start to the end of the denitrification / desulfurization treatment, it is possible to proceed with the desired reaction in the latter half, even if there is an adverse effect on the first half of the denitrification / desulfurization reaction. As a result, it is possible to minimize the extension of the denitrification / desulfurization treatment time and the insufficient denitrification / desulfurization.

[0048] Furthermore, if the slag is pushed aside by the injection of the stirring inert gas during the formation of the slag, exposing a large area of ​​the molten steel, the molten steel may come into contact with the air in the atmosphere, which may inhibit the denitrification / desulfurization reaction. Therefore, it is desirable to set the thickness of the slag to 100 mm or more at the start of the denitrification / desulfurization treatment process.

[0049] There is no particular upper limit to the thickness of the slag, as long as it is thick enough to avoid CO gas generated during decarburization, swelling of the molten steel surface due to the blowing of inert gas, or overflow from the upper end of the ladle due to slag foaming. The thickness of the slag may vary depending on the amount of additives (CaO and / or Al) added during tapping, for example. ２ O ３ The amount of additives (CaO and / or Al) added between the end of tapping and the end of decarburization treatment ２ O ３ The amount of the additive (containing substance) can be controlled.

[0050] Regarding the amount of inert gas injected for stirring during the denitrification and desulfurization treatment, it is desirable to stir the molten steel after adding the metallic Al-containing substance at a stirring power density of 100 W or more per ton of molten steel. This promotes the denitrification and desulfurization treatment and also reduces the amount of SiO in the slag. ２ , MnO, T. Fe(FeO, Fe ２ O ３ The stirring power density ε of molten steel can be calculated, for example, by the following formula (2):

[0051] ε = 370.4 × G × T / W × (1 - 297 / T + ln (1 + ρ × g × H / P)) (2) where G is the flow rate of the blown gas (Nm ３ / s) T: Molten steel temperature (K) W: Molten steel amount (ton) ρ: Molten steel density (kg / m ３ ) g: Gravitational acceleration (m / s ２ ) H: Gas injection depth (m) P: Ambient pressure (Pa)

[0052] As long as the stirring power density of the molten steel is 100 W or more per ton of molten steel, there are no particular limitations on the method of blowing the stirring inert gas during the denitrification / desulfurization treatment, as with the decarburization treatment, and the atmospheric pressure may also be either reduced pressure or atmospheric pressure.

[0053] There is no particular restriction on the timing of adding the Si concentration adjusting alloy. However, if it is added before the end of the decarburization treatment, it will react with oxygen in the steel to produce SiO ２ is generated and is taken up into the slag, resulting in the formation of SiO ２ The concentration of SiO in the slag during the denitrification and desulfurization treatment increases. ２ Therefore, it is desirable to add the Si concentration adjusting alloy after the molten steel has been deoxidized by adding the metallic Al-containing substance.

[0054] Furthermore, since the Ti concentration in the slab for non-oriented electrical steel sheet of this embodiment must be 0.0100% or less, it is necessary to minimize the inclusion of Ti. Specifically, the total amount of Ti in the slag before deoxidation of the molten steel with a metallic Al-containing substance, the amount of Ti in the additive for adjusting the composition added to the molten steel after deoxidation, and the amount of Ti in the coolant for adjusting the temperature is preferably 0.05 kg or less per ton of molten steel. Ti exists in the slag as an oxide, but since the steel of this embodiment has a high Al concentration in the steel, most of the Ti oxide in the slag is reduced and Ti is mixed into the steel. Therefore, the amount of Ti in the slag must also be adjusted within a suitable range.

[0055] Furthermore, although the corrosion of the magnesia-based refractory lining the surface of the ladle that comes into contact with the slag is minimized by adjusting the C / A(-) ratio in the slag, TiO as an impurity in the refractory ２ In addition, since refractories are not free from corrosion on surfaces other than those in contact with slag, the refractories used there should also contain TiO as an impurity. ２ It is desirable to avoid including as much as possible.

[0056] (Casting Process) In the casting process, molten steel is continuously cast to produce slabs.

[0057] [Embodiment 2] It is said that the properties of non-oriented electrical steel sheets deteriorate when the Mg concentration in the steel is high. However, when metallic Al is added to slag containing MgO, the MgO in the slag is reduced by the Al to generate Mg. This Mg vapor escapes into the gas phase, and when it enters the steel, the Mg concentration in the steel increases. Therefore, for example, a reference document (Japanese Patent No. 3893769) proposes a method of reducing the MgO concentration in the slag to prevent the generation of Mg vapor.

[0058] In the technology of the reference document, a high MgO concentration in the slag can cause Mg vapor generation, so the MgO concentration in the slag is set to 0 to 5 mass%. However, a low MgO concentration in the slag makes the magnesia-based refractory more susceptible to corrosion. Furthermore, in the technology of the reference document, the C / A(-) ratio in the slag is set to 0.5 to 0.7 to suppress Mg vapor generation when Al is added. As a result, a low C / A ratio reduces the slag slag conversion rate, inhibiting denitrification.

[0059] Therefore, in the second embodiment, a low-cost magnesia-based refractory is used as the refractory lining on the surface of the ladle that comes into contact with the slag, and a method for manufacturing a slab for a non-oriented electrical steel sheet is proposed, which suppresses the erosion of the refractory, promotes denitrification and desulfurization, and also suppresses an increase in the Mg concentration in the steel.

[0060] In the method for manufacturing a slab for non-oriented electrical steel sheet according to the second embodiment, a slab for non-oriented electrical steel sheet containing, in mass%, C: 0.0050% or less, Si: 2.0% or more and 4.5% or less, Mn: 0.1% or more and 2.0% or less, S: 0.0030% or less, Al: 0.15% or more and 2.5% or less, N: 0.0050% or less, Ti: 0.0100% or less, and Mg: 0.0030% or less is manufactured.

[0061] In the method for manufacturing a slab for a non-oriented electrical steel sheet according to the second embodiment, a tapping step, a decarburization step, a deoxidation step, a denitrification / desulfurization step, and a casting step are performed in this order. Note that the tapping step, the decarburization step, and the casting step are the same as those in the first embodiment, and therefore the other steps will be described in detail below.

[0062] (Deoxidation Treatment Step) The MgO concentration in the slag before the deoxidation treatment step is set to 5 mass% or less. By setting the MgO concentration in the slag before the deoxidation treatment step to 5 mass% or less, the amount of MgO reduced by the addition of Al is reduced, and therefore the Mg concentration in the steel does not become high. Note that if the MgO concentration in the slag after the tapping step from the electric furnace exceeds 5 mass%, the amount of CaO and / or Al may increase during the period from the completion of the tapping step to the completion of the decarburization treatment step. ２ O ３ The metal Al-containing substance is added to the ladle, whereby the MgO concentration in the slag before the addition of the metal Al-containing substance is set to 5 mass % or less.

[0063] (Denitrification / Desulfurization Treatment Step) As described above, by setting the C / A ratio of the slag to 1.0 to 1.4, refractory corrosion can be suppressed. However, since the slag is kept at a high slag conversion rate to promote the denitrification / desulfurization treatment, refractory corrosion is not completely eliminated. Therefore, an MgO-containing substance is added to the ladle at the beginning of the denitrification treatment (after the addition of the metallic Al-containing substance) to set the MgO concentration in the slag to 8% by mass or more. By increasing the MgO concentration in the slag in this way, it is possible to suppress the elution of MgO from the refractory into the slag, thereby suppressing refractory corrosion.

[0064] (Mg removal treatment) In the method for producing a slab for a non-oriented electrical steel sheet according to the second embodiment, in addition to the above steps, a Mg removal treatment is also carried out.

[0065] That is, when the denitrification / desulfurization treatment step is carried out in a ladle vacuum degassing apparatus, after the addition of the metallic Al-containing substance (after the denitrification / desulfurization treatment), stirring treatment is carried out at 5000 Pa or less for 5 minutes or more, thereby making it possible to reduce the Mg concentration in the steel.

[0066] In this case, when the decompression treatment is performed in a ladle vacuum degassing apparatus equipped with an exhaust system that passes through a bag filter, Mg vapor evaporated from the steel may become metallic Mg and adhere to the bag filter. As a result, when the exhaust system is restored to its original pressure after the decompression treatment, the Mg adhering to the bag filter may come into contact with the gas used for restoring pressure, which may ignite the Mg and cause a fire. In this case, for example, if possible, a switching valve may be used to route the exhaust gas to a line that does not pass through the bag filter, or the exhaust gas may be cooled so that the ambient temperature of the bag filter is maintained below the ignition temperature of Mg in the gas used for restoring pressure (air: 420°C, nitrogen: 490°C).

[0067] Furthermore, in order to minimize the amount of residual Mg in the steel, it is preferable that the MgO concentration in the slag be low when adding the metallic Al-containing substance. On the other hand, in order to prevent the refractory from being eroded, the MgO concentration is preferably 8 mass% or more, and more preferably, the MgO concentration is adjusted by adding the MgO-containing substance to the ladle after the steel tapping step.

[0068] According to the manufacturing method of the slab for non-oriented electrical steel sheet according to the second embodiment described above, it is possible to suppress the melting loss of the refractory material, promote denitrification and desulfurization, and also suppress an increase in the Mg concentration in the steel.

[0069] Example 1 A method for manufacturing a slab for a non-oriented electrical steel sheet according to Example 1 of the present invention will be described. In this example, an example in which decarburization treatment is performed using a ladle vacuum degassing device will be described.

[0070] In this example, about 150 tons of molten steel with a C concentration of 0.02 to 0.06 mass% was first tapped from an electric furnace into a ladle in an undeoxidized state. ２ O ３ (Al ２ O ３ :99% by mass), CaO・Al ２ O３ (CaO: 55% by mass, Al ２ O ３ The ladle was filled with magnesia chrome bricks (MgO: 60 mass%, Cr: 45 mass%) to an inner diameter of 2.9 m. ２ O ３ :25% by mass, TiO ２ : 0.1% by mass) was used as the liner.

[0071] After tapping, vacuum decarburization was carried out for 8 to 25 minutes in a ladle vacuum degasser. ２ O ３ The amount of CaO added is determined based on the amount of Al generated when deoxidizing molten steel and slag with Al. ２ O ３ The amount of CaO and / or Al was estimated so that the slag C / A(-) was in the range of 1.0 to 1.4. ２ O ３ Decarburization treatment without adding was also carried out.

[0072] After the decarburization treatment, metallic Al was added in an amount appropriate for deoxidation and composition adjustment, and the molten steel was stirred by blowing inert gas for stirring. The ladle was then transported to a tundish for continuous casting, and the molten steel was poured into the tundish for casting. When approximately half of the molten steel in the ladle was poured into the tundish, a sample of the molten steel in the tundish was taken and submitted for analysis. Table 1 shows the treatment conditions and results.

[0073]

[0074] As shown in Table 1, Examples 1 to 10 achieved the target C, N, and S concentrations of 0.0050% or less, 0.0030% or less, and 0.0050% or less. Furthermore, in Comparative Examples 1 to 3, in which the slag C / A(-) ratio at the start of the denitrification / desulfurization treatment was outside the range of 1.0 to 1.4, and in Comparative Example 4, in which the slag thickness at the start of the denitrification / desulfurization treatment was less than 100 mm, the N concentration did not reach the target concentration. Furthermore, in Comparative Example 4, in which the stirring power density was low (less than 100 W per ton of molten steel) and the slag oxidation level during the denitrification / desulfurization treatment was high, the S concentration also exceeded the target concentration.

[0075] Furthermore, in Comparative Examples 5 and 6, in which CaO was not added during vacuum decarburization, slag foaming occurred during the decarburization treatment, which required a temporary reduction in the degree of vacuum, resulting in a longer treatment time than in the Examples and a drop in the molten steel temperature to the point where casting was not possible. Therefore, in Comparative Examples 5 and 6, although the target compositions for both C and N were achieved after the denitrification and desulfurization treatment, subsequent electrode heating in the LF device to heat the molten steel to a temperature suitable for casting caused the C and N contents to exceed the target upper limits.

[0076] Example 2 A method for manufacturing a slab for a non-oriented electrical steel sheet according to Example 2 of the present invention will be described. In this example, an example in which decarburization treatment is performed using an RH vacuum degassing device will be described.

[0077] In this example, about 150 tons of molten steel with a C concentration of 0.02 to 0.05 mass% was first tapped from an electric furnace into a ladle in an undeoxidized state. ２ O ３ (Al ２ O ３ :99% by mass), CaO・Al ２ O ３ (CaO: 55% by mass, Al ２ O ３ The ladle was filled with magnesia chrome bricks (MgO: 60 mass%, Cr: 45 mass%) to an inner diameter of 2.9 m. ２ O ３ :25% by mass, TiO ２ : 0.1% by mass) was used as the liner.

[0078] After tapping, the steel was subjected to vacuum decarburization treatment for 10 to 24 minutes in an RH vacuum degasser. After decarburization, the ladle was transferred to a ladle refining equipment, where metallic Al was added in an amount appropriate for deoxidation and composition adjustment, and an inert gas for stirring was blown into the molten steel for stirring. The ladle was then transferred to a tundish for continuous casting, and the molten steel was poured into the tundish for casting. When approximately half of the molten steel in the ladle was poured into the tundish, a sample of the molten steel in the tundish was taken and submitted for analysis. Table 2 shows the treatment conditions and results.

[0079]

[0080] As shown in Table 2, Examples 11 to 18 achieved the target values ​​for C, N, and S, i.e., C: 0.0050% or less, S: 0.0030% or less, and N: 0.0050% or less. In Comparative Examples 7 to 9, where the slag C / A(-) at the start of the denitrification / desulfurization treatment was outside the range of 1.0 to 1.4, N did not reach the target values, and in Comparative Examples 10 and 11, where the slag thickness at the start of the denitrification / desulfurization treatment was less than 100 mm, N and S did not reach the target values.

[0081] As described above, according to the method for producing a slab for a non-oriented electrical steel sheet according to the present invention, it is possible to reduce carbon, nitrogen, and sulfur in molten steel to low concentrations while suppressing the melting loss of the refractory lining the ladle in an electric furnace process.

[0082] Example 3 A method for manufacturing a slab for a non-oriented electrical steel sheet according to Example 3 of the present invention will be described.

[0083] In this example, the ladle was made of magnesia carbon bricks (MgO: 89% by mass, C: 5% by mass, TiO ２ The denitrification treatment was carried out in a ladle refining apparatus. The experimental method will be described below in order.

[0084] (1) Approximately 130 tons of molten steel was tapped from an electric furnace into a ladle. During the tapping, CaO (CaO: 98 mass%), Al ２ O ３ (Al ２ O ３ :99% by mass), CaO・Al ２ O ３ (CaO: 55% by mass, Al ２ O ３ (2) After tapping, the ladle was transported to a ladle refining equipment. (3) Vacuum decarburization was carried out in the ladle refining equipment for 10 to 22 minutes. CaO and / or Al were removed 4 minutes after the start of vacuum decarburization. ２ O ３ At that time, Al generated when deoxidizing the molten steel and slag with Al in (5) described later was added. ２ O ３Taking the amount into consideration, CaO and / or Al are added so that C / A(-) is in the range of 1.0 to 1.4. ２ O ３ (4) The slag on the molten steel in the ladle was sampled and analyzed. (5) Metallic Al was added in an amount appropriate for deoxidation and composition adjustment. (6) MgO (MgO: 70 mass%, CaO: 8 mass%, SiO ２ 6% by mass of molten steel was added. (7) Slag on the molten steel in the ladle was sampled and analyzed. (8) Stirring was carried out for 20 minutes by blowing inert gas for stirring into the molten steel. (9) The ladle was transported to a tundish for continuous casting, and the molten steel was poured into the tundish for casting. Then, when about half of the molten steel in the ladle was poured into the tundish, a sample of the molten steel in the tundish was taken and subjected to analysis.

[0085] For some ladles, the same treatment was repeated five times, and then the amount of refractory corrosion (thickness of corrosion) in the slag contact area was measured. The above steps (3) and (8) were carried out under the conditions of claims 1 and 2. Table 3 shows the treatment conditions and results.

[0086]

[0087] In Table 3, Examples 19 to 23 are those in which the MgO concentration in the slag before deoxidation was 5% or less and MgO was added after deoxidation, and the other examples are Comparative Examples 12 to 15.

[0088] As shown in Table 3, in Examples 19 to 23, the MgO concentration in the slag before deoxidation was 5% or less, and therefore the Mg concentration in the steel was 0.0025% (25 ppm) or less. On the other hand, in Comparative Examples 12 and 13, the MgO concentration in the slag before deoxidation was more than 5%, and therefore the Mg concentration in the steel exceeded 0.0030% (30 ppm).

[0089] In Examples 19 to 23, the MgO concentration in the slag before deoxidation was 5% or less and MgO was added after deoxidation, so the refractory loss was 20 mm or less. On the other hand, in Comparative Examples 14 and 15, the MgO concentration in the slag before deoxidation was 5% or less, but MgO was not added after deoxidation, so the refractory loss exceeded 40 mm.

[0090] In addition, Examples 19 to 23 and Comparative Examples 12 to 15 achieved the target contents of C, N, S, and Ti: C: 0.0050% or less, N: 0.0050% or less, S: 0.0030% or less, and Ti: 0.0100% or less.

[0091] Example 4 Example 4 of the method for producing a slab for a non-oriented electrical steel sheet according to the present invention will be described.

[0092] In this example, the ladle was made of magnesia carbon bricks (MgO: 89% by mass, C: 5% by mass, TiO ２ The denitrification treatment was carried out in a ladle vacuum degassing apparatus equipped with an exhaust system that did not pass through a bag filter. The experimental method will be described below in order.

[0093] (1) Approximately 130 tons of molten steel was tapped from an electric furnace into a ladle. During the tapping, CaO (CaO: 98 mass%), Al ２ O ３ (Al ２ O ３ :99% by mass), CaO・Al ２ O ３ (CaO: 55% by mass, Al ２ O ３ :45% by mass), MgO (MgO: 70% by mass, CaO: 8% by mass, SiO ２ 6% by mass of additives (flux) were added to the ladle. (2) After tapping, the ladle was transported to a ladle vacuum degasser. (3) Vacuum decarburization was carried out in the ladle vacuum degasser for 9 to 20 minutes. 4 minutes after the start of vacuum decarburization, CaO and / or Al were removed. ２ O ３ At that time, Al generated when deoxidizing the molten steel and slag with Al in (4) described later was added. ２ O ３ Taking the amount into consideration, CaO and / or Al are added so that C / A(-) is in the range of 1.0 to 1.4. ２ O ３(4) Metallic Al was added in an amount appropriate for deoxidation and composition adjustment. (5) Slag above the molten steel in the ladle was sampled and analyzed. (6) Stirring was performed for 20 minutes by blowing inert gas for stirring into the molten steel. (7) The vacuum tank of the ladle vacuum degassing equipment was depressurized, and stirring was performed by blowing inert gas for stirring into the molten steel. (8) The ladle was transported to a tundish for continuous casting, and the molten steel was poured into the tundish for casting. Then, when about half of the molten steel in the ladle was poured into the tundish, a sample of the molten steel in the tundish was taken and submitted for analysis.

[0094] The above (3) and (6) were carried out under the conditions of claims 1 and 2. Table 4 shows the treatment conditions and results.

[0095]

[0096] As shown in Table 4, in Examples 24 to 27, the atmospheric pressure after denitrification and desulfurization was 5000 Pa or less and the treatment time was 5 minutes or more, so the Mg concentration in the steel was 0.0020% (20 ppm) or less. On the other hand, in Comparative Examples 16 to 18, the atmospheric pressure after denitrification and desulfurization was more than 5000 Pa, so the Mg concentration in the steel was more than 0.0040% (40 ppm). Furthermore, in Comparative Examples 19 and 20, the atmospheric pressure after denitrification and desulfurization was 5000 Pa or less, but the treatment time was less than 5 minutes, so the Mg concentration in the steel was more than 0.0030% (30 ppm).

[0097] In addition, Examples 24 to 27 and Comparative Examples 16 to 20 achieved the target contents of C, N, S, and Ti: C: 0.0050% or less, N: 0.0050% or less, S: 0.0030% or less, and Ti: 0.0100% or less.

[0098] As described above, according to the method for manufacturing a slab for a non-oriented electrical steel sheet according to the present invention, in the electric furnace process, it is possible to promote denitrification and desulfurization while suppressing the melting loss of the refractories, and also to suppress an increase in the Mg concentration in the steel.

[0099] The method for manufacturing a slab for a non-oriented electrical steel sheet according to the present invention has been specifically explained above using the detailed description and examples for carrying out the invention, but the gist of the present invention is not limited to these descriptions and should be broadly interpreted based on the claims. It goes without saying that various changes and modifications based on these descriptions are also included in the gist of the present invention.

[0100] REFERENCE SIGNS LIST 1 molten steel 2 ladle 21 magnesia refractory 3 vacuum tank 4 nozzle (bottom gas injection pipe) 5 slag 6 vacuum tank 7 immersion pipe 8 reflux gas injection pipe

Claims

1. A method for producing a slab for a non-oriented electrical steel sheet containing, by mass%, C: 0.0050% or less, Si: 2.0% to 4.5%, Mn: 0.1% to 2.0%, S: 0.0030% or less, Al: 0.15% to 2.5%, N: 0.0050% or less, and Ti: 0.0100% or less, the method comprising: (a) a tapping step of tapping undeoxidized molten steel from an electric furnace into a ladle whose surface that comes into contact with slag is lined with a magnesia-based refractory; (b) a decarburization step of stirring the molten steel under reduced pressure; (c) a deoxidation step of adding a metallic Al-containing substance from above the ladle; (d) a denitrification / desulfurization step of stirring the molten steel in contact with slag; and (e) a casting step of continuously casting the molten steel. Between the completion of the tapping step and the completion of the decarburization treatment step, CaO and / or Al ２ O ３ adding a containing material into the ladle, and at the start of the denitrification and desulfurization treatment step, ２ O ３ a mass ratio C / A(-) of the slag to the slag of 1.0 to 1.4, and a thickness of the slag at the start of the denitrification and desulfurization treatment step is 100 mm or more.

2. When the decarburization treatment step is carried out using a ladle vacuum degassing device, CaO and / or Al are not present within 40% of the decarburization treatment time from the start of the decarburization treatment step. ２ O ３ The method for producing a slab for a non-oriented electrical steel sheet according to claim 1, wherein a part of the contained substances is added into the ladle.

3. The method for producing a slab for a non-oriented electrical steel sheet according to claim 1, wherein the decarburization treatment step is carried out in an RH vacuum degassing apparatus.

4. A method for manufacturing a slab for non-oriented electrical steel sheet according to any one of claims 1 to 3, wherein the molten steel temperature at the start of the denitrification and desulfurization treatment step is 1580°C or higher and 1620°C or lower.

5. In the denitrification and desulfurization treatment step, the SiO in the slag is removed within half the time from the start of the treatment to the end of the treatment. ２ The method for producing a slab for a non-oriented electrical steel sheet according to any one of claims 1 to 3, wherein the total concentration of MnO, Tb, Fe is 3.0 mass% or less.

6. A method for manufacturing a slab for non-oriented electrical steel sheet according to any one of claims 1 to 3, wherein in the denitrification and desulfurization treatment process, the molten steel after adding the metallic Al-containing substance is stirred at a stirring power density of 100 W or more per ton of molten steel.

7. A method for manufacturing a slab for non-oriented electrical steel sheet according to any one of claims 1 to 3, which further contains, by mass%, Mg: 0.0030% or less.

8. After the steel tapping step is completed, CaO and / or Al is added to the steel during the period from the end of the steel tapping step to the end of the decarburization treatment step. ２ O ３ 8. The method for producing a slab for a non-oriented electrical steel sheet according to claim 7, wherein an Al-containing substance is added into the ladle, and the MgO concentration in the slag before the addition of the Al-containing substance is 5 mass % or less.

9. A method for producing a slab for non-oriented electrical steel sheet according to claim 8, wherein an MgO-containing substance is added to the ladle after the addition of the metallic Al-containing substance, so that the MgO concentration in the slag is 8 mass % or more.

10. A method for manufacturing a slab for non-oriented electrical steel sheet according to claim 7, wherein when the denitrification and desulfurization treatment process is carried out in a ladle vacuum degassing apparatus, a stirring treatment is carried out at 5000 Pa or less for 5 minutes or more after the addition of the metallic Al-containing substance.