Molten steel manufacturing method and cast steel manufacturing method

Abstract

Provided is a method for producing molten steel, the method enabling specifying the amount of Mg to be added to produce a fine MgO inclusion by reducing an Al2O3 inclusion.　In this method for producing molten steel, metallic Mg or an Mg-containing alloy is added to molten steel deoxidized by Al, in an amount specified on the basis of the ratio between the difference in the Mg concentration of the molten steel before and after Mg addition and the oxygen concentration of the molten steel before Mg addition.

Description

[Technical Field]

[0001] The present invention relates to a method for producing molten steel and a method for producing a slab, which are capable of reducing coarse Al2O3-based inclusions in molten steel to fine MgO inclusions. [Background technology]

[0002] Nonmetallic inclusions in steel affect the quality of steel. In particular, in aluminum (Al) deoxidized molten steel, the resulting Al2O3-based inclusions coarsen and form clusters. Since the coarsened and clustered Al2O3-based inclusions can cause defects on the steel surface, methods for controlling the morphology of nonmetallic inclusions are being investigated, such as blowing inert gas into molten steel, allowing the molten steel to stand in the ladle so that it floats up and separates, or adding elements.

[0003] Among these, as a method for controlling the shape of non-metallic inclusions by adding elements, it is said that adding magnesium (hereinafter referred to as "Mg") to molten steel generates fine inclusions containing Mg. Non-Patent Document 1 discloses that the fine inclusions containing Mg refine the gamma grains of the steel material, thereby achieving high HAZ toughness.

[0004] Methods of adding Mg to molten steel have been proposed to generate fine inclusions containing Mg in steel. Patent Document 1 discloses a method of vaporizing metallic Mg and adding it to molten steel. Patent Document 2 discloses a method of adding a wire or rod containing Mg to molten steel together with a carrier gas. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2017-20064 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-189975 Summary of the Invention [Problem to be solved by the invention]

[0006] Non-Patent Document 1 and Patent Documents 1 and 2 disclose the use of fine inclusions containing Mg and methods of adding Mg to molten steel, but do not disclose how to specify the amount of Mg to be added to molten steel that can suppress clustering of coarse Al2O3-based inclusions. Therefore, these methods have the problem of not being able to specify the amount of Mg to be added to molten steel. The present invention has been made in consideration of these problems in the prior art, and aims to provide a method for producing molten steel that can specify the amount of Mg to be added that can reduce Al2O3-based inclusions to fine MgO inclusions, and a method for producing a slab using molten steel produced by this method. [Means for solving the problem]

[0007] The gist of the present invention that can solve the above problems is as follows. [1] A method for producing molten steel, in which metallic Mg or a Mg-containing alloy is added to molten steel deoxidized with Al, in an amount determined based on the ratio of the difference in Mg concentration in the molten steel before and after the addition of Mg to the oxygen concentration in the molten steel before the addition of Mg. [2] The method for producing molten steel according to [1], wherein metallic Mg or an alloy containing Mg is added to molten steel deoxidized with Al so as to satisfy the following formula (1): ([T.Mg]-[T.Mg] B ) / [TO]≧0.5 (1) In the above formula (1), [T.Mg] is the Mg concentration (ppm) of the molten steel after Mg addition, and [T.Mg] B is the Mg concentration (ppm) of the molten steel before Mg addition, and [TO] is the oxygen concentration (ppm) of the molten steel before Mg addition. [3] The method for producing molten steel according to [2], wherein the S concentration in the molten steel before the addition of Mg is less than 20 ppm. [4] A method for producing a slab, comprising pouring molten steel produced by the method for producing molten steel according to any one of [1] to [3] into a mold of a continuous casting machine, cooling the slab in the mold, and continuously casting the slab. [Effects of the Invention]

[0008] According to the present invention, the ratio of the difference in Mg concentration before and after the addition of Mg to the oxygen concentration of the molten steel before the addition of Mg affects the particle size of Al2O3-based inclusions, and therefore, based on this ratio, it becomes possible to specify the amount of metallic Mg or Mg-containing alloy to be added that can reduce coarse Al2O3-based inclusions to fine MgO inclusions. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a cross-sectional schematic view showing an example of continuous casting equipment in which the method for producing a cast slab according to this embodiment can be implemented. [Figure 2] FIG. 2 is a graph showing the relationship between the value of the left side of equation (1) shown in Table 2 and the predicted maximum diameter of inclusions. DETAILED DESCRIPTION OF THE INVENTION

[0010] In order to confirm the amount of metallic Mg to be added that can control the morphology of coarse Al2O3-based inclusions in Al-deoxidized molten steel, the inventors conducted a test of adding Mg to Al-deoxidized molten steel and investigated the composition of non-metallic inclusions in the molten steel and the particle size of the inclusions.

[0011] The Mg addition test was carried out using a scanning electron microscope (hereinafter referred to as "SEM") with a particle analysis function that can detect and measure a large number of non-metallic inclusions in the sample molten steel. The composition and particle size of the non-metallic inclusions in the molten steel due to the addition of Mg were investigated using an energy dispersive X-ray analyzer (hereinafter referred to as "EDS") attached to the SEM. As a result, the following findings (1) and (2) were obtained.

[0012] (1) The volume fraction of inclusions remains almost unchanged before and after the addition of Mg, and the fine MgO inclusions that form after the addition of Mg are formed by reducing the Al2O3-based inclusions that existed before the addition. (2) Fine MgO inclusions cannot be obtained unless a certain amount of Mg is added to the molten steel relative to the Mg concentration and oxygen concentration of the molten steel that were mixed into the molten steel from the refractory (MgO) before the addition of Mg.

[0013] After Al deoxidation and before the addition of Mg, not only Al2O3 but also MgO·Al2O3 inclusions are generated, grow, and coarsened due to the MgO mixed in from the refractory. If Mg is not added to the molten steel to supply a sufficient amount of dissolved Mg to the molten steel, the MgO·Al2O3 inclusions cannot be completely reduced, and the molten steel will become a mixture of fine MgO inclusions and coarse aggregated MgO·Al2O3 inclusions.

[0014] Therefore, the difference in Mg concentration in molten steel before and after Mg addition (the difference between the Mg concentration in molten steel after Mg addition and the Mg concentration in molten steel before Mg addition) is the criterion for whether coarse MgO·Al2O3 inclusions can be reduced. The oxygen concentration in molten steel is an indicator of the amount of inclusions present in molten steel.

[0015] Therefore, in the method for producing molten steel according to this embodiment, the ratio between the difference in Mg concentration in the molten steel before and after the addition of Mg and the oxygen concentration in the molten steel is used as an index, and metallic Mg in an amount specified based on this ratio is added to the molten steel after Al deoxidation. Specifically, the range of ratios within which coarse MgO Al2O3 inclusions can be reduced to fine MgO inclusions is determined in advance, and the amount of metallic Mg to be added is specified so that it falls within this range of ratios.

[0016] The range of the ratio between the difference in Mg concentration in molten steel before and after Mg addition and the oxygen concentration in the molten steel can be predetermined by preparing molten steel after Al deoxidation in which the ratio is changed by changing the amount of metallic Mg added, and predicting the particle size of Al2O3-based inclusions in the molten steel. Thus, the inventors discovered that the amount of metallic Mg added that can convert coarse Al2O3-based inclusions into fine MgO inclusions can be determined based on the ratio between the difference in Mg concentration in molten steel before and after Mg addition and the oxygen concentration in the molten steel, and thus completed the present invention. Hereinafter, an embodiment of the present invention will be described using an example in which it is applied to a process including converter refining, secondary refining, and continuous casting. The following embodiment shows a preferred example of the present invention, and the present invention is not limited to this embodiment.

[0017] Molten steel tapped from a converter is placed in a ladle, treated by secondary refining, and then cast by a continuous casting machine. The method for producing molten steel according to this embodiment may be carried out on molten steel in a ladle after tapping from a converter and deoxidizing with Al, on molten steel after RH vacuum degassing, or on molten steel in a tundish during continuous casting. However, because reflux in the RH vacuum degassing layer improves the cleanliness of the molten steel, the method for producing molten steel according to this embodiment is preferably carried out between RH vacuum degassing and continuous casting.

[0018] Mg can be added to molten steel either as metal Mg or as a Mg-containing alloy. However, since metal Mg has a high vapor pressure and is highly reactive with molten steel, it is preferable to add a stabilized Mg-containing alloy containing Si or Al to the molten steel. Mg can be added to molten steel using existing auxiliary material addition methods and addition equipment, such as the injection method and wire feeder method.

[0019] On the other hand, when metallic Mg or a Mg-containing alloy is added to molten steel to achieve an appropriate Mg concentration in steel, the dissolved Mg concentration in the molten steel becomes higher than the deoxidation equilibrium concentration, and coarse Al2O3-based inclusions are reduced, thereby controlling the morphology of the coarse Al2O3-based inclusions to fine MgO inclusions. Therefore, the above-mentioned effect cannot be obtained even if Mg-containing oxides contained in slag, refractories, etc., which cannot reduce Al2O3-based inclusions, are added to molten steel after Al deoxidation.

[0020] In the method for producing molten steel according to this embodiment, the amount of metallic Mg or an Mg-containing alloy to be added to the molten steel after Al deoxidation is determined based on the ratio between the difference in Mg concentration before and after Mg addition and the oxygen concentration in the molten steel, and the determined amount of metallic Mg or an Mg-containing alloy is added to the molten steel. Therefore, the range of the ratio within which MgO·Al2O3 inclusions can be reduced to fine MgO inclusions is determined in advance by conducting experiments or the like to understand the relationship between the particle size of the inclusions and the ratio. This makes it possible to determine the amount of metallic Mg or an Mg-containing alloy to be added to the molten steel after Al deoxidation by measuring the Mg concentration and oxygen concentration of the molten steel before Mg addition.

[0021] Specifically, it is preferable to add metallic Mg or an alloy containing Mg to the molten steel after Al deoxidation so as to satisfy the following formula (1): This reduces coarse Al2O3-based inclusions in the molten steel, making it possible to produce molten steel containing fine MgO inclusions.

[0022] ([T.Mg]-[T.Mg] B ) / [TO]≧0.5 (1) In the above formula (1), [T.Mg] is the Mg concentration (ppm) of the molten steel after Mg addition, and [T.Mg] B is the Mg concentration (ppm) of the molten steel before Mg addition, and [TO] is the oxygen concentration (ppm) of the molten steel before Mg addition.

[0023] On the other hand, when the ratio of the difference in Mg concentration before and after Mg addition, which is the left side of the above formula (1), to the oxygen concentration in the molten steel, exceeds 3.0, the Mg concentration in the molten steel increases, causing the Mg to aggregate and the particle size of inclusions to begin to increase. For this reason, it is preferable to add metallic Mg or a Mg-containing alloy to the Al-deoxidized molten steel so that the ratio of the difference in Mg concentration before and after Mg addition to the oxygen concentration in the molten steel becomes 3.0 or less.

[0024] Mg has a high affinity not only for oxygen but also for S. Therefore, if the S concentration in the molten steel before Mg addition is high, the reaction Mg + S → MgS occurs. MgS has a lower density than MgO, and if it is formed, it coarsens inclusions. Furthermore, MgS is easily oxidized, and reacts with oxides in the slag and refractories before casting, coarsening inclusions. For this reason, it is preferable to suppress the formation of MgS, and it is preferable that the S concentration in the molten steel before Mg addition be less than 20 ppm.

[0025] Next, a method for producing a slab using molten steel produced by the method for producing molten steel according to this embodiment will be described. Fig. 1 is a cross-sectional schematic diagram showing an example of continuous casting equipment in which the method for producing a slab according to this embodiment can be implemented.

[0026] The continuous casting equipment 10 includes a mold 12, a tundish 14 installed above the mold 12, and a plurality of strand support rolls 16 arranged in a row below the mold 12. Although not shown, a ladle for accommodating molten steel 18 is installed above the tundish 14, and the molten steel 18 is poured into the tundish 14 from the bottom of the ladle. The molten steel 18 is produced by the molten steel production method according to this embodiment, and contains fine MgO inclusions formed by reducing coarse AlO-based inclusions.

[0027] An immersion nozzle 20 is installed at the bottom of the tundish 14, and molten steel 18 is poured into the mold 12 through the immersion nozzle 20. The molten steel 18 solidifies as it is cooled and heat is removed from the inner surface of the mold 12, forming a solidified shell 24. This results in the formation of a slab 28 having the solidified shell 24 as its outer shell and an unsolidified layer 26 made of the molten steel 18 inside.

[0028] In the gaps between adjacent strand support rolls 16 in the casting direction, multiple secondary cooling zones 30, each equipped with spray nozzles (not shown), are installed along the casting direction from directly below the mold 12. The strand 28 is cooled as it is withdrawn by cooling water sprayed from the spray nozzles in the secondary cooling zones 30. While the strand 28 is transported by the strand support rolls 16 and passes through the multiple secondary cooling zones 30, the solidified shell 24 is appropriately cooled, solidification of the unsolidified layer 26 progresses, and solidification of the strand 28 is completed.

[0029] Downstream in the casting direction, a plurality of transport rolls 17 are installed for continuing to transport the slab 28. A slab cutter 32 for cutting the slab 28 is disposed above the transport rolls 17. After solidification is complete, the slab 28 is cut into slabs 28a of a predetermined length by the slab cutter 32.

[0030] The slab 28a produced by continuous casting in this manner does not contain coarse Al2O3-based inclusions that cause surface defects in steel materials, etc. Therefore, by implementing the slab production method according to this embodiment, it is possible to produce a slab 28a that can produce steel materials with reduced quality defects such as surface defects. [Example]

[0031] Next, an example will be described in which the effect of inclusion morphology control in the molten steel manufacturing method according to this embodiment was confirmed. In this example, metallic Al was added to 250 t of molten steel contained in a ladle after being tapped from a converter to deoxidize the molten steel, and then the molten steel was desulfurized. In the desulfurization of the molten steel, a CaO-Al2O3-SiO2-based flux was used as a desulfurization agent, and the molten steel was heated by arc heating from a black smoke electrode to convert the desulfurization agent into slag. An injection lance immersed in the molten steel was used to inject 100 to 150 Nm 3 The molten steel and the desulfurization agent were stirred by blowing Ar gas as a stirring gas at a rate of 1 / h, and the molten steel was adjusted to have the composition shown in Table 1 below.

[0032] [Table 1]

[0033] The molten steel after desulfurization was degassed using an RH vacuum degasser, and the molten steel composition was adjusted and inclusions were floated and separated by stirring. The vacuum degassing refining treatment time was adjusted so that the oxygen concentration in the molten steel was 20 ppm or less for each experiment. Furthermore, the desulfurization treatment time was adjusted so that the sulfur concentration in the molten steel was 30 ppm or less for each experiment.

[0034] Mg was added to the molten steel whose composition was adjusted in this way. An alloy containing Mg and Si (hereinafter referred to as "MgSi alloy") was used as the added form of Mg, and the wire feeder method was used as the alloy addition method. The MgSi alloy was an alloy containing 30 mass% Mg purity, 60 mass% Si purity, and the remainder Fe and unavoidable impurities, and was an iron-coated wire (sheath thickness 0.04 cm) filled with the MgSi alloy. A predetermined amount of MgSi alloy wire was added to molten steel in a ladle at 1580 to 1620°C after RH degassing treatment, and molten steel with different amounts of added Mg was produced.

[0035] After adding the MgSi alloy, the molten steel in the ladle was collected and quenched, and a sample was prepared to confirm the inclusion morphology. The sample was analyzed by scanning electron microscope (SEM) with particle analysis capabilities, measuring 40 mm at a time. 2 The particle size distribution of inclusions was measured at 30 points within the measurement range, and the predicted maximum diameter of inclusions was determined from the maximum particle size of each distribution obtained by using the maximum particle size estimated area as the cross-sectional area of ​​the slab using the method of extreme value statistics. The Mg concentration (ppm) of the molten steel before Mg addition, the oxygen concentration (ppm) of the molten steel before Mg addition, the Mg concentration (ppm) of the molten steel after Mg addition, the S concentration (ppm) of the molten steel before Mg addition, the value of the left side of the above equation (1), and the predicted maximum particle size (μm) of inclusions for these examples are shown in Table 2 below. In Table 2, [T.Mg] B is the Mg concentration (ppm) in the molten steel before Mg addition, [TO] is the oxygen concentration (ppm) in the molten steel before Mg addition, [T.Mg] is the Mg concentration (ppm) in the molten steel after Mg addition, and S concentration is the S concentration (ppm) in the molten steel before Mg addition.

[0036] [Table 2]

[0037] Figure 2 is a graph showing the relationship between the value of the left side of equation (1) shown in Table 2 and the predicted maximum diameter of inclusions. As shown in Figure 2, the value of the left side of equation (1) ([T.Mg] - [T.Mg] B ) / [TO]) affects the predicted maximum diameter of inclusions. Therefore, the amount of metallic Mg or Mg-containing alloy to be added that can reduce Al2O3-based inclusions to fine MgO inclusions can be determined based on the value of the left side of the above equation (1), i.e., the ratio of the difference in Mg concentration before and after Mg addition to the oxygen concentration in the molten steel before Mg addition.

[0038] As can be seen from Figure 2, the range of the ratio between the difference in Mg concentration before and after Mg addition, which allows coarse Al2O3-based inclusions to be reduced to fine MgO inclusions, and the oxygen concentration of the molten steel before Mg addition is 0.5 or greater. Therefore, by creating Table 2 and Figure 2 above in advance, the range of the ratio between the difference in Mg concentration before and after Mg addition and the oxygen concentration of the molten steel before Mg addition, which allows Al2O3-based inclusions to be sufficiently reduced, is determined in advance. By measuring the Mg concentration and oxygen concentration before Mg addition, it becomes possible to identify the amount of metallic Mg or Mg-containing alloy to be added, which allows coarse Al2O3-based inclusions to be reduced to fine MgO inclusions. Then, by adding the identified amount of metallic Mg or Mg-containing alloy to Al-deoxidized molten steel, it becomes possible to produce molten steel in which coarse Al2O3-based inclusions in the molten steel after Al deoxidation are reduced to fine MgO inclusions.

[0039] As shown in Figure 2, when the value of the left side of the above equation (1) was 0.5 or greater, the maximum particle size of the inclusions became significantly smaller. This result confirmed that by adding metallic Mg so as to satisfy the above equation (1), it is possible to reduce the coarse Al2O3-based inclusions contained in molten steel after Al deoxidation and turn them into fine MgO inclusions.

[0040] From the results of Experiments Nos. 1, 2, 4, and 5, it was confirmed that the predicted maximum grain size of inclusions tends to gradually decrease as the value of the left side of Equation (1) above increases. On the other hand, the predicted maximum grain size of inclusions in Experiment No. 3 was smaller than that in Experiment No. 1, where the value of the left side of Equation (1) was larger. This result is thought to be due to the fact that the S concentration in the molten steel before Mg addition was lower in Experiment No. 3 than in Experiment No. 1, which resulted in the predicted maximum grain size of inclusions being smaller than that in Experiment No. 1. From these results, it was confirmed that it is preferable for the S concentration in the molten steel before Mg addition to be low (less than 20 ppm), and that this allows the maximum grain size of inclusions to be reduced. [Explanation of symbols]

[0041] 10 Continuous casting equipment 12 Mold 13 Cast copper plate 14 Tundish 16. Casting strip support roll 17 Transport roll 18 Molten Steel 19 Molten mold flux 20 Submerged Entry Nozzle 22 Coating 23 Different substance filling section 24 Solidified shell 26 Unsolidified layer 28 Castings 30 Secondary cooling zone 32 Slab cutting machine

Claims

1. A method for producing molten steel, comprising adding metallic Mg or an Mg-containing alloy to molten steel deoxidized with Al, in an amount specified based on the ratio between the difference in Mg concentration in the molten steel before and after the addition of Mg and the oxygen concentration in the molten steel before the addition of Mg.

2. 2. The method for producing molten steel according to claim 1, wherein metallic Mg or an alloy containing Mg is added to molten steel deoxidized with Al so as to satisfy the following formula (1): ([T.Mg]-[T.Mg] Ｂ ) / [T.O]≧0.5・・・(1) In the above formula (1), [T. Mg] is the Mg concentration (ppm) of the molten steel after Mg addition, and [T. Mg] Ｂ is the Mg concentration (ppm) of the molten steel before Mg addition, and [T.O] is the oxygen concentration (ppm) of the molten steel before Mg addition.

3. The method for producing molten steel according to claim 2, wherein the S concentration in the molten steel before the addition of Mg is less than 20 ppm.

4. A method for producing a slab, comprising pouring the molten steel produced by the method for producing molten steel according to any one of claims 1 to 3 into a mold of a continuous casting machine and cooling the slab in the mold to produce a continuous cast slab.

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