Steel production method

Abstract

To provide a steel production method that can continuously cast steel while keeping both Al2O3 system inclusions and CaS, which are causative substances of nozzle occlusion, at a low level even in molten steel comprising a relatively large amount of S.SOLUTION: A steel production method comprises a step of performing continuous casting by supplying Al-added and Ca-treated molten steel having an S content of 0.010-0.035 mass% into a tundish 80, and supplying the molten steel 100 in the tundish to a mold 90 via a nozzle 82. When a cross-section of a sample obtained by collecting and solidifying the molten steel from the tundish is observed, a content of CaS is 15.0% or less in an inclusion comprising an oxide and CaS having a circle equivalent diameter of 1 μm or more, when the total content of CaO, SiO2, Al2O3, MgO and CaS in mass% is 100%.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present disclosure relates to a method for producing steel. [Background technology]

[0002] When continuously casting steel, the molten steel after primary refining, in which the steel is decarburized under atmospheric pressure in a converter or the like, is tapped into a ladle and subjected to deoxidation and composition adjustment (secondary refining) by adding alloys, etc. After impurity gases and the like are removed in a vacuum degassing device, the molten steel in the ladle is poured into a tundish to further remove inclusions, and then poured into a mold for continuous casting to be cast.

[0003] For example, when manufacturing automotive bar and wire steel, which is used as the raw material for gears, springs, shafts, etc., the submerged entry nozzle that supplies molten steel from the tundish to the mold in bloom or billet size continuous casting is prone to blockage of the flow passage due to inclusions that adhere and grow inside. Since these clogging materials can fall off during casting and cause product defects, it is important to reduce nozzle clogging. Nozzle clogging is mainly caused by Al, which has a high melting point. 2 O 3 The influence of the inclusions is large, so these inclusions are replaced by low-melting CaO-Al 2 O 3 A technique called "Ca treatment" is commonly used to modify the inclusions to suppress nozzle clogging.

[0004] For example, Patent Document 1 describes a method for Ca-treating molten steel that can improve the machinability and cold rolling property of Al-killed steel without impairing the fatigue resistance. The method describes a method for Ca-treating molten steel that can improve the machinability and cold rolling property of Al-killed steel by measuring [O] T Taking the above into consideration, a Ca treatment method for molten steel has been disclosed in which the amount of Ca added is controlled so as to satisfy the following formulas (1), (2), and (3). 0.50≦[Ca] / [O] T ≦1.00 (1) [Ca]≦40 (2) [O] T <= 40 (3) [Ca]: Calcium concentration in molten steel (ppm) [O]T : Total oxygen concentration in molten steel (ppm) [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 3-79713 Summary of the Invention [Problem to be solved by the invention]

[0006] In S-added steels, which are used for the purpose of improving machinability, such as bar and wire steel for automobiles, the Ca added for the purpose of improving the quality reacts with the S in the molten steel to produce CaS, which has a high melting point, and this also causes nozzle blockage.

[0007] This disclosure is based on the idea that even in molten steel containing a relatively large amount of S, Al, which is a cause of nozzle clogging, can be removed. 2 O 3 The object of the present invention is to provide a method for producing steel that can continuously cast steel while keeping both calcium sulphide inclusions and CaS at low levels. [Means for solving the problem]

[0008] Means for solving the above problems include the following aspects. <1> The method includes a step of supplying molten steel having an S content of 0.010 to 0.035% by mass, to which Al has been added, and which has been treated with Ca, into a tundish, and supplying the molten steel in the tundish through a nozzle into a mold to perform continuous casting, When a cross section of a sample obtained by collecting the molten steel from the tundish and solidifying it was observed, it was found that the inclusions contained oxides and CaS and had a circle equivalent diameter of 1 μm or more, and contained CaO, SiO 2 , Al 2 O 3 The CaS content is 15.0% or less, calculated when the total content of MgO and CaS in mass% is taken as 100%. <2> Before the step of performing the continuous casting, A process for pouring molten steel tapped from a converter or an electric furnace into a ladle and subjecting it to ladle refining, the slag composition after the ladle refining process being, in mass%, CaO: 35.0 to 70.0%, SiO 2 : 0.0~10.0%, Al 2 O 3 : 20.0 to 40.0%, MgO: 5.0 to 20.0%, and 2 O 3 The ratio of the CaO content to the Al content [CaO] / [Al 2 O 3 A ladle refining process in which the ratio of the melting point of the ladle to the melting point of the steel is controlled to be 1.7 to 2.0. a RH vacuum degassing process in which the molten steel after the ladle refining process is refined by an RH vacuum degassing apparatus, the RH vacuum degassing process being performed while keeping the pressure in a vacuum tank at 5332.88 Pa or less and ensuring a reflux state, and the reflux number N defined by the following formulas (1) and (2) being a treatment time that satisfies the relationship of the following formula (3); N=t RH / (W / Q) ···(1) t RH : Processing time of RH vacuum degassing refining process (min) W: Molten steel amount (ton) Q: Reflux amount (ton / min) Q=11.4G RH 1 / 3 D 4 / 3 {ln(P 1 / P 0 )} 1 / 3 (2) G RH : Circulation gas volume (NL / min) D: Immersion tube inner diameter (m), P 1 : Pressure at gas injection point (Pa) P 0 : Surface pressure of molten steel in vacuum vessel (Pa) N≧8.3 (3) a Ca treatment step of adding Ca to the molten steel after the RH vacuum degassing refining treatment, the Ca treatment step being performed by adjusting the Ca basic unit added in the Ca treatment to control the ratio [Ca] / [TO] of the Ca content to the total O content in the molten steel to be 0.1 to 0.3; Including, <1> A method for producing a steel according to claim 1. Effect of the Invention

[0009] According to the present disclosure, even in molten steel containing a relatively large amount of S, Al, which is a substance that causes nozzle clogging, can be removed. 2 O 3 A method for producing steel is provided that allows continuous casting of steel while keeping both the content of calcium sulphide and calcium sulphide at low levels. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing an example of a schematic configuration of equipment for performing ladle refining treatment. [Diagram 2] FIG. 1 is a diagram showing an example of a schematic configuration of a facility for performing an RH vacuum degassing refining process using an RH vacuum degassing device. [Diagram 3] FIG. 1 is a diagram showing an example of a schematic configuration of equipment for performing Ca treatment. [Figure 4] FIG. 1 is a diagram showing an example of a schematic configuration of a facility for performing continuous casting. [Diagram 5] FIG. 1 is a graph showing the relationship between [Ca] / [TO] in molten steel in a tundish (TD) and the CaS concentration in inclusions. [Figure 6] FIG. 1 is a graph showing the relationship between [Ca] / [TO] and clogging coefficient difference TKS in molten steel in a tundish (TD). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] An embodiment that is an example of the present disclosure will be described. In this disclosure, the "%" indication of the content or concentration of each component means "mass %" unless otherwise specified. In addition, in this specification, a numerical range expressed using "~" means a range that includes the numerical values ​​written before and after "~" as the lower and upper limits. However, when the numerical values ​​written before and after "~" are followed by "more than" or "less than," the numerical range does not include these numerical values ​​as the lower or upper limit. In the numerical ranges described in this specification in stages, the upper limit value of a certain numerical range may be replaced by the upper limit value of another numerical range described in stages, and the lower limit value of a certain numerical range may be replaced by the upper limit value of another numerical range described in stages. The term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

[0012] Al, which is the cause of nozzle clogging 2 O 3 In order to improve the quality of inclusions and suppress the formation of CaS, 2 O 3 Increasing the Ca consumption rate is effective in improving the quality of inclusions, but the counter-reaction of CaS generation has been an issue. To address this issue, increasing FO (free oxygen: oxygen dissolved in molten steel) is thermodynamically effective in suppressing CaS generation during Ca treatment. The inventors of the present disclosure have achieved a certain level of improvement, such as improving nozzle clogging by reducing CaS, by taking measures to increase FO and reduce Ca consumption rate. On the other hand, the concentration of FO increases during LF treatment (ladle refining treatment), deteriorating the cleanliness of the molten steel, making it difficult to manufacture steel with even higher cleanliness using this method. Therefore, the inventors of the present disclosure have proposed a method for reducing the amount of Al, a substance that causes nozzle clogging, by increasing the purity of the molten steel. 2 O 3 In an environment where the formation of CaS is promoted as a reaction while reducing system inclusions, the idea was conceived of reducing the Ca basic unit in order to reduce the formation of CaS.

[0013] In the method for producing steel according to the present disclosure, molten steel having an S content of 0.010 to 0.035% by mass, to which Al has been added, and which has been treated with Ca is supplied into a tundish, and the molten steel in the tundish is supplied into a mold through a nozzle to perform continuous casting. When the cross section of a sample obtained by collecting the molten steel from the tundish and solidifying it is observed, it is found that the inclusions containing oxides and CaS and having a circle equivalent diameter of 1 μm or more (sometimes referred to as "coarse CaS-containing inclusions" in this specification) contain CaO, SiO, and the like. 2 , Al 2 O 3 The CaS content (sometimes referred to as "CaS content of coarse CaS-containing inclusions" in this specification) is 15.0% or less, calculated when the total content of MgO and CaS in mass% is taken as 100%. By performing continuous casting while keeping the CaS content of the coarse CaS-containing inclusions in the molten steel in the tundish to 15.0% or less in this manner, nozzle clogging can be effectively suppressed.

[0014] (Method of measuring inclusions) The molten steel sample used to measure the size and composition of coarse CaS-containing inclusions is a bomb sample with a diameter of 30 mm and a height of 70 mm, which is taken from the tundish and allowed to cool and solidify. The measurement surface is the center of the height direction cross section in the region 15 to 25 mm from the bottom surface of the bomb sample. The size of the inclusions is measured and the elements are analyzed using a SEM-EDS (scanning electron microscope equipped with an energy dispersive X-ray analyzer) with an acceleration voltage of 20 kV and a magnification of 750 times. The measurement field of view is approximately 100 mm. 2 Specifically, the measurement is carried out according to the following procedure. First, the measurement surface of the bomb sample is scanned across the entire measurement field of view to check for the presence or absence of inclusions. When an inclusion is detected, the size of the inclusion is recognized and the center of gravity is calculated. EDX analysis (accelerating voltage 20 kV) is performed at this center of gravity. EDX quantitative analysis is performed at one point for one inclusion. The measurement area at one point varies depending on the average atomic weight and average density of the analysis site, but has a spread of approximately 1 to 5 μm in the planar and depth directions. The EDX detector used in this disclosure adopts Silicon Drift Detectors (SDD) to increase speed, and Be is used for the detection window. The electron gun adopts a thermal electron gun. The energy resolution is approximately 135 eV.

[0015] From the composition quantified by EDX elemental analysis, S is assumed to be preferentially MnS, followed by CaS, and the remaining metal elements, including Ca, are assumed to be oxides, and the stoichiometric calculations are carried out to determine CaO, SiO 2 , Al 2 O 3 The arithmetic mean value of the CaS content in each coarse CaS-containing inclusion is regarded as the CaS content of the molten steel sample. The CaS content of the coarse CaS-containing inclusions is preferably 12.0 mass % or less, and more preferably 10.5 mass % or less.

[0016] <Al 2 O 3 Refining method to simultaneously reduce both CaS and inclusions> In the present disclosure, the method for controlling the CaS content of the coarse CaS-containing inclusions in the molten steel sample taken from the tundish to 15.0% or less is not particularly limited. For example, before the molten steel is supplied to the tundish, the high melting point Al may be removed by a ladle refining process, an RH vacuum degassing refining process, and a Ca treatment. 2 O 3 A method that reduces the total amount of inclusions while suppressing the formation of CaS, which has a high melting point, is preferable. The main Al content in ladle refining (sometimes referred to as "LF processing" in this disclosure) is 100%. 2O 3 The inclusions are adsorbed into the slag and separated from the molten steel. The RH vacuum degassing refining process (sometimes referred to as "RH treatment" in this disclosure) in which the molten steel after the LF treatment is refined using an RH vacuum degassing device promotes the floating and separation of inclusions. Furthermore, when Ca is added to the molten steel after RH treatment, Al with a high melting point is added. 2 O 3 It reduces the total amount of inclusions and suppresses the formation of CaS, which has a high melting point. The specific method for each process will be described below.

[0017] [Ladle refining process] The molten steel tapped from a converter or electric furnace is placed in a ladle and subjected to ladle refining treatment (LF treatment). An example of the schematic configuration of equipment for performing ladle refining is shown in Figure 1. In Figure 1, 10 is a ladle, 12 is a top cover, 14 is an electrode, 16 is a slag raw material hopper, 18 is an alloy material hopper, 20 is a material supply line, 100 is molten steel, and 110 is slag. For example, molten steel 100 is tapped from a converter or electric furnace (not shown) and contained in a ladle 10, and a stirring gas 22 such as argon gas is blown into the ladle 10 from a bottom blowing plug 24 to stir the molten steel 100. Slag raw materials and alloy materials are added from each of the hoppers 16, 18 through a material supply line 20, and the compositions and temperatures of the molten steel 100 and slag 110 can be adjusted to target values ​​by arc heating using an electrode 14. The added slag raw materials are melted to form slag 110 having a desired composition. The reaction between the slag 110 and the molten steel 100 controls the shape of inclusions in the molten steel and desulfurizes the molten steel.

[0018] In the LF treatment of the present disclosure, the slag composition after the LF treatment is, in mass%, CaO: 35.0 to 70.0%, SiO 2 : 0.0~10.0%, Al 2 O 3 : 20.0 to 40.0%, MgO: 5.0 to 20.0%, and Al 2 O 3The ratio of the CaO content to the Al content [CaO] / [Al 2 O 3 ] is adjusted to 1.7 to 2.0. 2 O 3 ] is 1.7 to 2.0, which is high Al 2 O 3 This is the range where the adsorption capacity is ensured while the slag liquid phase ratio does not deteriorate. 2 O 3 By controlling the high level of [CaO] / [Al 2 O 3 ] is less than 1.7, Al 2 O 3 The adsorption capacity is insufficient, and Al is removed from the steel. 2 O 3 When the ratio exceeds 2.0, the liquid phase ratio increases and the adsorption capacity of the inclusions deteriorates, and Al is removed from the steel. 2 O 3 Inclusions in the system cannot be sufficiently removed.

[0019] The slag composition after the LF treatment and the Al content in the molten steel can be measured by spark discharge optical emission spectrometry. The slag composition after the LF treatment is preferably CaO, SiO 2 , Al 2 O 3 and MgO in total is 90.0 mass% or more, CaO: 35.0 to 70.0%, SiO 2 : 0.0~10.0%, Al 2 O 3 : 20.0-40.0%, and MgO: 5.0-20.0%, [CaO] / [Al 2 O 3 ] is 1.7 to 2.0.

[0020] To adjust the slag composition to the target after LF treatment, for example, CaO is added by adding quicklime, and SiO 2 By adding silica as necessary, Al 2 O 3 is the Al generated by deoxidation during LF treatment. 2 O 3Taking into consideration the above, alumina is added as necessary, and MgO is added by adding lightly burnt dolomite, taking into consideration the elution from the ladle refractories, to control the slag composition so that each falls within the above range.

[0021] The chemical composition of the steel produced by the steel production method according to the present disclosure is 0.010 to 0.035% S. With an S content of 0.010% or more, a steel material with high machinability can be obtained. If the S content exceeds 0.035%, a large amount of CaS is generated by Ca treatment even when each treatment is performed according to the method according to the present disclosure, which tends to cause nozzle clogging in the tundish. There are no particular limitations other than the S content, but the method for producing steel according to the present disclosure is particularly suitable for producing automotive bar and wire steel materials, which require machinability, and is preferably applied to the production of steel material having a chemical composition including, for example, C: 0.15-0.60%, Si: 0.05-1.00%, Mn: 0.20-2.00%, Al: 0.010-0.050%, and S: 0.010-0.035%.

[0022] [RH vacuum degassing refining process] The molten steel after the LF treatment is subjected to the RH vacuum degassing refining treatment (RH treatment) in which it is refined using an RH vacuum degassing device. FIG. 2 shows an example of a schematic configuration of a facility for performing RH vacuum degassing refining treatment using an RH vacuum degassing apparatus. The RH vacuum degassing apparatus 40 mainly includes a vacuum vessel 42 and two immersion pipes (a suction pipe 44A and a discharge pipe 44B) connected to the vacuum vessel 42. In the RH treatment, Ar gas (reflux gas) 50 is blown in from the side of the suction pipe 44A, and the molten steel 100 is sucked up into the vacuum vessel 42 by utilizing its buoyancy. In the vacuum vessel 42, impurity gases such as hydrogen are removed from the molten steel 100, and the molten steel 100 is discharged from the discharge pipe 44B into the ladle 10. In FIG. 2, 26 is a slag raw material hopper, 28 is an alloy material hopper, 30 is a material supply line, 34 is a top blowing lance, and 46 is an exhaust pipe. The pressure in the vacuum vessel 42 can be reduced through the exhaust pipe 26, and oxygen 60 can be blown from the top blowing lance 34 to the molten steel in the vacuum vessel as necessary to reduce the amount of C in the molten steel. By circulating the molten steel 100 between the ladle 10 and the vacuum vessel 42, impurity gases in the molten steel 100 are gradually removed, and by strongly stirring the molten steel 100, inclusions tend to aggregate and float to the surface.

[0023] In the RH treatment, the pressure in the vacuum chamber 42 is set to 40 Torr (5332.88 Pa) or less, a reflux state is ensured, and the treatment time is set so that the reflux count N defined by the following formulas (1) and (2) satisfies the relationship of the following formula (3). Note that 1 Torr=133.322 Pa. N=t RH / (W / Q) ···(1) In equation (1), t RH represents the processing time of the RH vacuum degassing refining process (min), W represents the amount of molten steel (tons), and Q represents the amount of reflux (tons / min). The reflux amount Q is calculated by the following formula (2). Q=11.4G RH 1 / 3 D 4 / 3 {ln(P 1 / P 0 )} 1 / 3 (2) In formula (2), G RH is the amount of reflux gas (NL / min), D is the inner diameter (m) of the immersion tubes 44A and 44B, P 1 is the pressure (Pa) at the gas injection point (depth position of the immersion tube) L, P 0 represents the surface pressure of the molten steel in the vacuum vessel (Pa). N≧8.3 (3)

[0024] The amount of molten steel W (tons) in equation (1) is the total amount of molten steel 100 contained in ladle 10 during RH treatment, and the reflux amount Q (tons / min) is the total amount of molten steel 100 in ladle 10 that is refluxed between ladle 10 and vacuum vessel 42 through the two immersion pipes 44A, 44B per minute. The reflux amount Q can be calculated by the formula (2). RH(NL / min) is the total amount of Ar gas 50 blown in from the side of the suction tube 44A per minute. The inner diameters of the two immersion tubes 44A and 44B are the same, and the immersion tube inner diameter D (m) is the inner diameter of each of the immersion tubes 44A and 44B. Pressure P at gas injection point (depth position of immersion tube) L 1 (Pa) is the pressure at the position where the Ar gas 50 is blown into the side of the suction tube 44A, and is measured by a pressure gauge. Also, the surface pressure of the molten steel in the vacuum vessel P 0 (Pa) is the pressure on the surface of the molten steel 100 sucked into the vacuum vessel, and is measured by a vacuum gauge. For example, if the inner diameter D of the immersion tubes 44A and 44B is 0.6 m, and the pressure P 1 When the pressure is 760 Torr (101325 Pa), the amount of reflux gas G RH can be set in the range of 1300 to 2800 NL / min.

[0025] The reflux amount Q is calculated by the formula (2), and the RH treatment time t is set so that the reflux number N calculated by the formula (1) is 8.3 or more. RH By setting the RH treatment time (min), the MgO-Al generated by the LF treatment was 2 O 3 This promotes the floating and separation of inclusions such as MgO-Al, and ensures cleanliness. This improves the deterioration of cleanliness caused by the above-mentioned LF treatment. When the number of reflux times N is less than 8.3, MgO-Al 2 O 3 In order to promote the floating and separation of the inclusions, it is more preferable that the number of reflux cycles N is 10 or more. On the other hand, although there is no upper limit to the number of reflux times N, as the number of reflux times N increases, the RH treatment time becomes longer, the productivity decreases, and it becomes difficult to obtain an effect commensurate with the number of reflux times N, so it is more preferable that the number of reflux times N is 17 or less. Although it depends on the amount of molten steel W and the amount of reflux Q, for example, the RH treatment time t RH The total reflux amount during RH treatment is set to 2500t to 5000t, promoting the floating and separation of inclusions.

[0026] [Ca treatment] Ca treatment is performed to add Ca to the molten steel after the RH treatment. A calcium wire (Ca wire), a calcium silicon wire (CaSi wire), or the like can be used to add Ca. Figure 3 shows an example of a schematic configuration of equipment for performing the Ca treatment. Ca is added by charging a Ca-containing wire 70 into the molten steel 100 in a ladle 10.

[0027] In Ca treatment, the Ca basic unit added by the Ca-containing wire is adjusted to control the ratio (mass ratio) [Ca] / [TO] of the Ca content [Ca] to the O content (total oxygen content: [TO]) in the molten steel to 0.1 to 0.3. When the [Ca] / [TO] ratio in the molten steel is less than 0.1, the unmodified Al is not obtained when the molten steel after Ca treatment is fed to the tundish. 2 O 3 On the other hand, if the [Ca] / [TO] ratio exceeds 0.3, a large amount of CaS is generated, and nozzle clogging is likely to occur. Therefore, in the Ca treatment, the [Ca] / [TO] ratio is controlled to 0.1-0.3 to suppress the generation of CaS during the Ca treatment and to suppress the Al inclusions in the molten steel, which are deoxidation products during the RH treatment. 2 O 3 The low melting point inclusion CaO-Al 2 O 3 The Ca content and O content in the molten steel are measured by spark discharge optical emission spectrometry. In addition, by controlling inclusions in the LF process, the CaO-Al 2 O 3 The reason for avoiding the formation of CaO-Al is that it is difficult to obtain the effect of flotation and separation during RH treatment, and it is difficult to ensure the cleanliness of the molten steel. 2 O 3 The reason for promoting the formation of these inclusions is that they have a low melting point and are less likely to adhere to the submerged entry nozzle during casting, thereby suppressing nozzle clogging.

[0028] As a refining technique prior to continuous casting, the above-mentioned method can be used to optimize the composition and amount of inclusions in molten steel, thereby making it possible to both prevent nozzle clogging in the tundish and produce high-cleanliness steel. Furthermore, the above-mentioned elucidation of the conditions for suppressing nozzle clogging during casting and the refining technique for satisfying the conditions for suppressing nozzle clogging are universal to molten steel and can be applied to both the converter process and the electric furnace process.

[0029] [Continuous casting] After the Ca treatment, the molten steel is fed into a tundish, and the molten steel in the tundish is fed into a mold through a nozzle to perform continuous casting. Fig. 4 shows an example of a schematic configuration of equipment for performing continuous casting. Molten steel 100 in a ladle 10 is fed into a tundish 80, and is further fed from the tundish 80 into a mold 90 through a submerged nozzle 82. A slab 200 is pulled out from the lower end of the mold 90, and is supported and transported by a plurality of rolls 92, and is cooled by cooling water sprayed from a spray (not shown), thereby performing continuous casting. The cast slab 200 transported while being cooled is cut to a predetermined length by a cast slab cutter (not shown) and transported to equipment for the next process as a steel slab such as a billet or bloom.

[0030] Through the above steps, steel with a high degree of cleanliness (for example, the CaS content of coarse CaS-containing inclusions is 15.0% or less) can be produced. EXAMPLES

[0031] The method for producing steel according to the present disclosure will be described in more detail below with reference to examples, although these examples do not limit the method for producing steel according to the present disclosure.

[0032] <Example 1> (LF processing) After primary refining in a converter, 270-290t (tons) of molten steel (target steel composition: C: 0.10-0.60%, Si: 0.01-3.0%, Mn: 0.1-3.0%, Al: 0.010-0.100%, S: 0.008-0.100%) was tapped into a ladle and transferred to a ladle refining facility (LF) for ladle refining treatment (LF treatment). In the LF treatment, the slag composition was adjusted using quicklime, silica, alumina, and lightly burned dolomite, and the molten steel composition was adjusted using various alloys.

[0033] (RH treatment) After the LF treatment, the ladle was moved and vacuum degassing treatment (RH treatment) was performed in an RH vacuum degasser. The immersion depth of the RH vacuum degasser's immersion tube in the molten steel was not changed from the start to the end of the treatment, and was maintained at a constant height relative to the ladle. The pressure in the vacuum chamber was kept below 40 Torr (5332.88 Pa), and Ar gas (reflux gas) was blown in from the side of the suction tube to ensure a reflux state of the molten steel between the ladle and the vacuum chamber, and RH treatment was performed.

[0034] (Ca treatment) After the RH treatment, CaSi wire was added to the molten steel in the ladle to adjust the ratio of the Ca content to the O content in the molten steel, [Ca] / [TO].

[0035] (Continuous Casting) The molten steel in the ladle after Ca treatment was fed into a tundish and continuous casting was carried out.

[0036] The conditions for the LF treatment, RH treatment, and Ca treatment are shown in Table 1. The vacuum chamber pressure P 0 " means "surface pressure of molten steel in vacuum vessel."

[0037] [Table 1]

[0038] (CaS content of coarse CaS-containing inclusions) The molten steel in the tundish during continuous casting was collected and cooled to prepare a bomb sample, and the size (equivalent circle diameter) and composition of the inclusions in the bomb sample were measured by the method described above. The CaO, SiO 2 , Al 2 O 3 The average CaS content was calculated based on the total content of MgO and CaS taken as 100%.

[0039] [evaluation] The nozzle clogging of the tundish by the molten steel in each example was evaluated by the clogging coefficient difference TKS. The clogging coefficient difference TKS is a value calculated by the following formula A, and is an index for evaluating the nozzle clogging of the tundish. The smaller the TKS, the less likely the nozzle clogging of the tundish will occur. TKS = Tz (at the end of casting) - Tz (at the start of casting) Formula 1

[0040]

number

[0041] Calculate Tz at the start and end of casting using equations 2 to 4. Substitute the results into equation 1 to calculate TKS for each CH (charge received in the tundish). In the above equations, the meanings of each variable are as follows. Note that MD refers to the mold of the continuous casting machine. Q c :Theoretical molten steel flow rate [t / min] Q p : Actual molten steel flow rate [t / min] α: Flow characteristic coefficient [-] η SN : Sliding nozzle (SN) opening S:SN pore area [m 2 ] g: Gravitational acceleration [m / s 2 ] H: molten steel head [m] ρ Fe : Molten steel density [t / m 3 ] V c:Casting speed [m / min] W:MD short side width [m] L:MD long side width [m] When nozzle clogging material accumulates, the molten steel flow is interrupted by the clogging material, so the actual molten steel flow rate Qp becomes smaller than the theoretical molten steel flow rate Qc, and Tz becomes larger.

[0042] If the clogging coefficient difference TKS was 8.0 or less, it was judged as good, and if it exceeded 8.0, it was judged as poor.

[0043] The molten steel components (S, TO, Ca / TO), coarse inclusions, and clogging coefficient difference TKS using molten steel samples are shown in Table 2. Underlines indicate that the information is outside the scope of this disclosure. Note that TD stands for tundish, and the "average composition of TD coarse inclusions" is the average composition of coarse CaS-containing inclusions measured using molten steel samples taken from the tundish.

[0044] [Table 2]

[0045] In Nos. 1 to 5 (invention examples), the CaS content of the coarse CaS-containing inclusions in the molten steel samples was 15 mass % or less, and the clogging coefficient difference TKS was 8.0 or less. On the other hand, in No. 6 (comparative example), the CaS content of the coarse CaS-containing inclusions in the molten steel sample exceeded 15 mass %, and the clogging coefficient difference TKS exceeded 8.0. By performing continuous casting under the operating conditions of No. 1 to 5, clogging of the submerged entry nozzle of the tundish is significantly suppressed.

[0046] Fig. 5 is a diagram showing the relationship between [Ca] / [TO] in the molten steel in the tundish (TD) and the CaS concentration in the inclusions, and Fig. 6 is a diagram showing the relationship between [Ca] / [TO] in the molten steel in the tundish (TD) and the clogging coefficient difference TKS. The meanings of the symbols are as follows: ○: Molten steel that meets the conditions of this disclosure □: Molten steel that was not treated with Ca △: Molten steel that was subjected to Ca treatment but did not meet the conditions of this disclosure (CaS content of coarse CaS-containing inclusions: over 15.0%)

[0047] In the molten steel satisfying the conditions of the present disclosure, despite having been subjected to Ca treatment, the CaS content of the coarse CaS-containing inclusions is suppressed to 15.0% or less, and the clogging coefficient difference TKS is at the same level as that of molten steel that has not been subjected to Ca treatment, indicating that nozzle clogging is unlikely to occur. [Explanation of symbols]

[0048] 10 ladle 40 Vacuum degassing equipment 42 Vacuum chamber 44A,44B dip tube 70 Ca-containing wire 80 Tundish 82 Submerged Nozzle 90 Mold 100 Molten Steel 110 Slug 200 Cast pieces

Claims

1. The method includes a step of supplying molten steel having an S content of 0.010 to 0.035% by mass, to which Al has been added, and which has been Ca-treated, into a tundish, and supplying the molten steel in the tundish through a nozzle into a mold to perform continuous casting, When a cross section of a sample obtained by collecting the molten steel from the tundish and solidifying it was observed, it was found that the inclusions contained oxides and CaS and had a circle equivalent diameter of 1 μm or more, and contained CaO, SiO ２ , Al ２ O ３ A method for producing steel, wherein the CaS content is 15.0% or less, calculated when the total content of MgO and CaS in mass% is taken as 100%.

2. Before the step of performing the continuous casting, A process for pouring molten steel tapped from a converter or an electric furnace into a ladle and subjecting it to ladle refining, the slag composition after the ladle refining process being, in mass%, CaO: 35.0 to 70.0%, SiO ２ :0.0~10.0%, Al ２ O ３ : 20.0 to 40.0%, MgO: 5.0 to 20.0%, and the Al ２ O ３ The ratio of the CaO content to the Al content is [CaO] / [Al ２ O ３ ] is controlled to be 1.7 to 2.0; a RH vacuum degassing process in which the molten steel after the ladle refining process is refined by an RH vacuum degassing apparatus, the RH vacuum degassing process being performed while keeping the pressure in a vacuum tank at 5332.88 Pa or less and ensuring a reflux state during the RH vacuum degassing process, and the reflux number N defined by the following formulas (1) and (2) being a processing time that satisfies the relationship of the following formula (3); N=t ＲＨ / (W / Q) ・・・(1) t ＲＨ : Processing time of RH vacuum degassing refining process (min) W: Molten steel amount (ton) Q: Reflux amount (ton / min) Q=11.4G ＲＨ １／３ D ４／３ {ln(P １ / P ０ )} １／３ ・・・(2) G ＲＨ : Circulation gas volume (NL / min) D: Immersion tube inner diameter (m), P １ : Pressure at gas injection point (Pa) P ０ :Surface pressure of molten steel in vacuum vessel (Pa) N≧8.3 (3) a Ca treatment step of adding Ca to the molten steel after the RH vacuum degassing refining treatment, in which a Ca basic unit added in the Ca treatment is adjusted to control a ratio [Ca] / [T.O] of the Ca content to the total O content in the molten steel to 0.1 to 0.3; The method of claim 1 comprising:

Images