Method for producing molten steel and method for producing cast sheet

Abstract

Provided is a method for producing molten steel with which it is possible to determine the addition amount of Mg that reduces Al2O3 inclusions to form fine MgO inclusions. In a method for producing molten steel, a metal Mg or an Mg-containing alloy is added to the molten steel deoxidized with Al in an addition amount determined on the basis of the ratio of the difference between the Mg concentrations of the molten steel before and after the addition of Mg and the oxygen concentration of the molten steel before the addition of Mg.

Description

TECHNICAL FIELD

[0001] The present application relates to a molten steel manufacturing method capable of reducing coarse Al2O3-based inclusions in molten steel to form fine MgO inclusions, and a casting slab manufacturing method. BACKGROUND

[0002] Non-metallic inclusions in steel can affect the quality of the steel product. In particular, in molten steel after aluminum (hereinafter also referred to as "Al") deoxidation, Al2O3-based inclusions generated are coarsened and clustered. Coarsened and clustered Al2O3-based inclusions become a cause of surface defects and the like of the steel product, and therefore, methods of controlling the morphology of non-metallic inclusions based on blowing of non-active gas into molten steel, floatation separation of molten steel in a ladle, or addition of elements have been studied.

[0003] Among them, as a method of controlling the morphology of non-metallic inclusions by addition of elements, it is known that when magnesium (hereinafter, referred to as "Mg") is added to molten steel, fine inclusions containing Mg are generated. In Non-Patent Literature 1, it is disclosed that the γ-grains of the steel product are refined by fine inclusions containing Mg, and high HAZ toughness can be achieved.

[0004] In order to generate fine inclusions containing Mg in steel, a method of adding Mg to molten steel has been proposed. In Patent Literature 1, a method of vaporizing metallic Mg and adding it to molten steel is disclosed. In Patent Literature 2, a method of adding a wire or rod containing Mg to molten steel together with a carrier gas is disclosed.

[0005] PRIOR ART DOCUMENTS PATENT LITERATURE Patent Literature 1: Japanese Patent Application Publication No. 2017-20064 Patent Literature 2: Japanese Patent Application Publication No. 2008-189975 SUMMARY

[0006] PROBLEMS TO BE SOLVED BY THE INVENTION In Non-Patent Literature 1 and Patent Literatures 1 and 2, the use of fine inclusions containing Mg and the method of adding Mg to molten steel are disclosed, but it is not disclosed how to determine the amount of Mg added to molten steel that can suppress the clustering of coarse Al2O3-based inclusions. Therefore, there is a problem that the amount of Mg added to molten steel cannot be determined in these methods. The present application is an invention made in view of the problems of the prior art, and aims to provide a molten steel manufacturing method capable of determining the amount of Mg that can reduce Al2O3-based inclusions to form fine MgO inclusions, and a casting slab manufacturing method using molten steel manufactured by the manufacturing method.

[0007] MEANS FOR SOLVING THE PROBLEMS The gist of the present application capable of solving the above problems is as follows.

[0008] [1] A method for producing molten steel, wherein metal Mg or an Mg-containing alloy is added to Al-deoxidized molten steel in an addition amount determined based on a ratio of a difference between Mg concentrations of the molten steel before and after the addition of Mg to an oxygen concentration of the molten steel before the addition of Mg.

[0009] [2] The method for producing molten steel according to [1], wherein the metal Mg or the Mg-containing alloy is added to the Al-deoxidized molten steel in a manner satisfying the following expression (1), ([T.Mg] - [T.Mg] B ) / [T.O] ≥ 0.5 (1) In the above expression (1), [T.Mg] is the Mg concentration (ppm) of the molten steel after the addition of Mg, [T.Mg] B is the Mg concentration (ppm) of the molten steel before the addition of Mg, and [T.O] is the oxygen concentration (ppm) of the molten steel before the addition of Mg.

[0010] [3] The method for producing molten steel according to [2], wherein the S concentration of the molten steel before the addition of Mg is less than 20 ppm.

[0011] [4] A method for producing a cast slab, wherein molten steel produced by the method for producing molten steel according to any one of [1] to [3] is poured into a mold of a continuous casting machine, and a cast slab is continuously cast by cooling in the mold.

[0012] Effects of the Invention According to the present application, the ratio of the difference between the Mg concentrations before and after the addition of Mg to the oxygen concentration of the molten steel before the addition of Mg has an effect on the particle diameter of Al2O3-based inclusions. Therefore, by determining the addition amount of metal Mg or an Mg-containing alloy based on the ratio, it is possible to determine the addition amount of metal Mg or an Mg-containing alloy that reduces large Al2O3-based inclusions to form fine MgO inclusions. BRIEF DESCRIPTION OF DRAWINGS

[0013] [ Figure 1 ] Figure 1 is a cross-sectional schematic view showing an example of a continuous casting apparatus capable of implementing the method for producing a cast slab according to the present embodiment.

[0014] [ Figure 2 ] Figure 2 is a graph showing the relationship between the value on the left side of expression (1) shown in Table 2 and the predicted maximum diameter of inclusions. DETAILED DESCRIPTION

[0015] The inventors of the present application conducted Mg addition tests in which Mg was added to Al-deoxidized molten steel in order to confirm the amount of metal Mg that can control the morphology of coarse Al2O3-based inclusions in the Al-deoxidized molten steel, and investigated the composition of non-metallic inclusions and the particle size of the inclusion morphology in the molten steel.

[0016] The Mg addition tests were conducted using a scanning electron microscope (hereinafter, referred to as "SEM") having a particle analysis function capable of detecting and measuring a large number of non-metallic inclusions in the sample molten steel. The composition and particle size of non-metallic inclusions in the molten steel under the influence of Mg addition were investigated using an energy dispersive X-ray analysis device (hereinafter, also referred to as "EDS") provided in the SEM. As a result, the following (1) and (2) were found.

[0017] (1) The volume fraction of inclusions hardly changed before and after Mg addition, and fine MgO inclusions generated after Mg addition were generated by reduction of Al2O3-based inclusions present before addition.

[0018] (2) Unless a certain amount of Mg is added with respect to the Mg concentration of the molten steel mixed from the refractory (MgO) into the molten steel and the oxygen concentration of the molten steel at the time before Mg addition, fine MgO inclusions cannot be obtained.

[0019] After Al deoxidation, in the molten steel before Mg addition, not only Al2O3 but also MgO-Al2O3 inclusions are generated, grow, and coarsen due to the MgO mixed from the refractory. With respect to the MgO-Al2O3 inclusions, unless a sufficient amount of dissolved Mg is supplied to the molten steel by Mg addition, the MgO-Al2O3 inclusions cannot be completely reduced to fine MgO inclusions, and exist in a state in which the fine MgO inclusions and the coarsened MgO-Al2O3 inclusions are mixed in the molten steel.

[0020] Therefore, as the Mg concentration, the difference between the Mg concentrations of the molten steel before and after Mg addition (the difference between the Mg concentration of the molten steel after Mg addition and the Mg concentration of the molten steel before Mg addition) becomes a criterion for whether the coarsened MgO-Al2O3 inclusions can be reduced. The oxygen concentration in the molten steel becomes an index of the amount of inclusions present in the molten steel.

[0021] Therefore, in the molten steel manufacturing method according to the present embodiment, metal Mg is added to the molten steel after Al deoxidization in such a manner that the ratio of the difference between the Mg concentrations of the molten steel before and after Mg addition to the oxygen concentration in the molten steel is an index, and the amount of metal Mg determined based on the ratio is added. Specifically, the range of the ratio at which the coarsened MgO-Al2O3 inclusions can be reduced to fine MgO inclusions is determined in advance, and the amount of metal Mg is determined in such a manner that the ratio becomes within the range.

[0022] The range of the ratio of the difference between the Mg concentrations of the molten steel before and after the Mg addition to the oxygen concentration in the molten steel can be determined in advance as follows: the Al-deoxidized molten steel in which the above ratio is changed by changing the amount of the metal Mg added is prepared, and the particle diameter of the Al2O3-based inclusions in the molten steel is predicted. Thus, the inventors of the present application have found that, by using the ratio of the difference between the Mg concentrations of the molten steel before and after the Mg addition to the oxygen concentration in the molten steel, it is possible to determine the amount of the metal Mg to be added to form the coarse Al2O3-based inclusions into the fine MgO inclusions, thereby completing the present application. Hereinafter, the embodiments of the present application will be described using an example in which the embodiments of the present application are applied to a process including a converter refining, a secondary refining, and a continuous casting. The following embodiments represent a preferred example of the present application, and are not limited to the embodiments in any way.

[0023] The molten steel tapped from the converter is collected in a ladle, and after being treated by the secondary refining, is cast by a continuous casting machine. The molten steel manufacturing method according to the present embodiment can be performed in any one of the molten steel in the ladle after being tapped from the converter and Al-deoxidized, the molten steel after the RH degassing treatment, or the molten steel in the tundish during the continuous casting. Among them, since the cleanliness of the molten steel is improved due to the backflow in the RH degassing layer, the molten steel manufacturing method according to the present embodiment is preferably performed during the period from after the RH degassing treatment to the continuous casting.

[0024] As the addition form of the Mg to the molten steel, any of the metal Mg or the Mg-containing alloy can be used. Among them, since the metal Mg has a high vapor pressure and a high reactivity with the molten steel, it is preferable to add the Mg-containing alloy containing Si and Al, which is stabilized, to the molten steel. As the addition method of the Mg to the molten steel, the existing secondary material addition method, such as the injection method or the wire feeding method, can be used.

[0025] On the other hand, the metal Mg or the Mg-containing alloy for obtaining a proper Mg concentration in the steel is added to the molten steel, and the dissolved Mg concentration in the molten steel becomes higher than the deoxidation equilibrium concentration, whereby the coarse Al2O3-based inclusions are reduced, and thus the coarse Al2O3-based inclusions are controlled in form to the fine MgO inclusions. Therefore, even if the Mg-containing oxide contained in the slag, the refractory, or the like, which cannot reduce the Al2O3-based inclusions, is added to the Al-deoxidized molten steel, the above effect is not obtained.

[0026] In the steel liquid manufacturing method according to the present embodiment, the amount of metallic Mg or Mg-containing alloy to be added to the steel liquid after Al deoxidation is determined based on the ratio of the difference between the Mg concentrations before and after the addition of Mg to the oxygen concentration in the steel liquid, and the determined amount of metallic Mg or Mg-containing alloy is added to the steel liquid. Thus, the range of the above ratio that enables reduction of MgO-Al2O3 inclusions to form fine MgO inclusions is determined in advance by grasping the relationship between the inclusion particle size and the above ratio through experiments or the like. As a result, by measuring the Mg concentration and the oxygen concentration of the steel liquid before the addition of Mg, the amount of metallic Mg or Mg-containing alloy to be added to the steel liquid after Al deoxidation can be determined.

[0027] Specifically, it is preferable to add metallic Mg or Mg-containing alloy to the steel liquid after Al deoxidation in a manner that satisfies the following equation (1). As a result, coarse Al2O3 inclusions in the steel liquid can be reduced, and a steel liquid containing fine MgO inclusions can be manufactured.

[0028] ([T.Mg] - [T.Mg B ) / [T.O] ≥ 0.5 (1) In the above equation (1), [T.Mg] is the Mg concentration (ppm) of the steel liquid after the addition of Mg, [T.Mg] B is the Mg concentration (ppm) of the steel liquid before the addition of Mg, and [T.O] is the oxygen concentration (ppm) of the steel liquid before the addition of Mg.

[0029] On the other hand, when the ratio of the difference between the Mg concentrations before and after the addition of Mg to the oxygen concentration in the steel liquid, i.e., the left side of the above equation (1), is greater than 3.0, the Mg concentration in the steel liquid increases, and thus Mg is aggregated, and the particle size of inclusions starts to become large. Therefore, it is preferable to add metallic Mg or Mg-containing alloy to the steel liquid after Al deoxidation in a manner that the ratio of the difference between the Mg concentrations before and after the addition of Mg to the oxygen concentration in the steel liquid becomes 3.0 or less.

[0030] Mg has not only a high affinity with oxygen but also a high affinity with S. Therefore, in the case where the S concentration in the steel liquid before the addition of Mg is high, the reaction Mg + S → MgS occurs. The density of MgS is lower than that of MgO, and in the case where MgS is generated, inclusions become coarse. In addition, MgS is easily oxidized, and reacts with oxides in the slag and refractories during the period until casting, and thus inclusions also become coarse. Therefore, it is preferable to suppress the generation of MgS, and the S concentration in the steel liquid before the addition of Mg is preferably less than 20 ppm.

[0031] Next, a method of manufacturing a cast piece using the steel liquid manufactured by the steel liquid manufacturing method according to the present embodiment will be described. Figure 1is a cross-sectional schematic view showing an example of a continuous casting apparatus capable of implementing the slab manufacturing method according to the present embodiment.

[0032] The continuous casting apparatus 10 has a mold 12, a tundish 14 provided above the mold 12, and a plurality of slab support rollers 16 arranged and disposed below the mold 12. Although not shown, a ladle that houses molten steel 18 is provided above the tundish 14, and the molten steel 18 is injected from the bottom of the ladle to the tundish 14. The molten steel 18 is a molten steel containing fine MgO inclusions generated by reducing coarse Al203-based inclusions, which is manufactured by the molten steel manufacturing method according to the present embodiment.

[0033] An immersion nozzle 20 is provided at the bottom of the tundish 14, and the molten steel 18 is injected to the mold 12 via the immersion nozzle 20. The molten steel 18 is cooled by heat dissipation from the inner surface of the mold 12, and solidifies to form a solidified shell 24. Thus, a slab 28 is formed that has the solidified shell 24 as an outer shell and has an un-solidified layer 26 formed from the molten steel 18 inside.

[0034] A plurality of secondary cooling zones 30 provided with spray nozzles (not shown) are provided in the gap between the slab support rollers 16 adjacent in the casting direction, from the bottom of the mold 12 in the casting direction. The slab 28 is cooled by the cooling water sprayed from the spray nozzles of the secondary cooling zones 30 while being drawn. During the passage of the slab 28 through the plurality of secondary cooling zones 30 while being carried by the slab support rollers 16, the solidified shell 24 is appropriately cooled to cause solidification of the un-solidified layer 26, and the solidification of the slab 28 is completed.

[0035] A plurality of carrying rollers 17 for continuously carrying the slab 28 are provided downstream in the casting direction. A slab cutter 32 for cutting the slab 28 is disposed above the carrying rollers 17. The slab 28 after the completion of solidification is cut by the slab cutter 32 into slabs 28a of a prescribed length.

[0036] The coarse Al203-based inclusions that are the cause of surface defects and the like of steel products are not contained in the slab 28a manufactured by such continuous casting. Therefore, by implementing the slab manufacturing method according to the present embodiment, it is possible to manufacture a slab 28a that can manufacture a steel product with reduced quality defects such as surface defects.

[0037] Example Next, an embodiment confirming the effectiveness of inclusion morphology control in the steel manufacturing method according to this embodiment will be described. In this embodiment, metallic Al is added to 250t of molten steel contained in a ladle after tapping from the converter to deoxidize the molten steel, followed by desulfurization. In the desulfurization process, a CaO-Al2O3-SiO2 flux is used as the desulfurizing agent, and the desulfurizing agent is slag-forming by heating the molten steel with an electric arc from a graphite electrode. 100-150 Nm is blown into the molten steel from an injection lance immersed in the molten steel. 3 Ar gas is used as a stirring gas at a rate of / h to stir the molten steel and desulfurizing agent, thereby adjusting the molten steel to the composition shown in Table 1 below.

[0038] [Table 1] For the desulfurized molten steel, an RH vacuum degassing device was used for degassing, adjustment of the molten steel composition, and flotation and separation of inclusions based on stirring. The vacuum degassing refining time was adjusted in each experiment to ensure that the oxygen concentration in the molten steel was below 20 ppm. Furthermore, the desulfurization time was adjusted in each experiment to ensure that the sulfur concentration in the molten steel was below 30 ppm.

[0039] Mg was added to the molten steel whose composition was thus adjusted. As the form of Mg addition, an alloy containing Mg and Si (hereinafter referred to as "MgSi alloy") was used, and the alloying method employed was wire feeding. The MgSi alloy was an alloy containing 30% by mass of pure Mg, 60% by mass of pure Si, and the balance being Fe and unavoidable impurities. It consisted of an iron-coated wire (0.04 cm thick) internally filled with the MgSi alloy. After RH degassing treatment, a specified amount of MgSi alloy wire was added to the molten steel in a ladle at 1580–1620°C to produce molten steel with varying Mg addition amounts.

[0040] Molten steel collected from the ladle after the addition of MgSi alloy was quenched to prepare samples for confirming the morphology of inclusions. For these samples, a SEM with particle analysis capabilities was used, with each particle size measured at 40 mm. 2 The particle size distribution of inclusions was measured at 30 locations. The maximum particle size of each distribution was used as the estimated area of ​​the casting sheet, and the predicted maximum diameter of the inclusions was calculated using 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 on the left side of the above equation (1), and the predicted maximum particle size (μm) of the inclusions in these examples are shown in Table 2 below. In Table 2, [T.Mg] Bis the Mg concentration (ppm) of the molten steel before Mg addition, [T.O] is the oxygen concentration (ppm) of the molten steel before Mg addition, [T.Mg] is the Mg concentration (ppm) of the molten steel after Mg addition, and S concentration is the S concentration (ppm) of the molten steel before Mg addition.

[0041] [Table 2] Figure 2 is a graph showing the relationship between the value of the left side of the formula (1) shown in Table 2 and the predicted maximum diameter of the inclusions. As shown in Figure 2 , it is known that the value of the left side of the above formula (1) (([T.Mg] - [T.Mg] B ) / [T.O]) affects the predicted maximum diameter of the inclusions. Therefore, based on the value of the left side of the above formula (1), i.e., the ratio of the difference between the Mg concentrations before and after Mg addition to the oxygen concentration of the molten steel before Mg addition, the amount of addition of metallic Mg or Mg-containing alloy that reduces Al2O3-based inclusions to form fine MgO inclusions can be determined.

[0042] It is known from Figure 2 that the range of the ratio of the difference between the Mg concentrations before and after Mg addition to the oxygen concentration of the molten steel before Mg addition, which reduces Al2O3-based inclusions to form fine MgO inclusions, is 0.5 or more. Therefore, the above Table 2, Figure 2 is prepared in advance, and the range of the ratio of the difference between the Mg concentrations before and after Mg addition to the oxygen concentration of the molten steel before Mg addition, which sufficiently reduces Al2O3-based inclusions, is determined in advance. Thus, by measuring the Mg concentration and the oxygen concentration before Mg addition, the amount of addition of metallic Mg or Mg-containing alloy that reduces Al2O3-based inclusions to form fine MgO inclusions can be determined. Furthermore, by adding the determined amount of metallic Mg or Mg-containing alloy to the molten steel after Al deoxidation, the production of molten steel in which Al2O3-based inclusions contained in the molten steel after Al deoxidation are reduced to form fine MgO inclusions can be achieved.

[0043] As shown in Figure 2 , if the value of the left side of the above formula (1) is 0.5 or more, the maximum particle diameter of the inclusions is significantly reduced. From this result, it is confirmed that by adding metallic Mg in a manner that satisfies the above formula (1), Al2O3-based inclusions contained in the molten steel after Al deoxidation can be reduced to form fine MgO inclusions.

[0044] From the results of Experiment Nos. 1, 2, 4, 5, it can be confirmed that the predicted maximum particle diameter of the inclusions tends to gradually decrease as the value on the left side of the above formula (1) becomes larger. On the other hand, the predicted maximum particle diameter of the inclusions in Experiment No. 3 is smaller than that in Experiment No. 1 in which the value on the left side of formula (1) is larger. For this result, it is considered that the S concentration of the molten steel before Mg addition in Experiment No. 3 is lower than that in Experiment No. 1, and thus the predicted maximum particle diameter of the inclusions in Experiment No. 3 is smaller than that in Experiment No. 1. From this result, it can be confirmed that it is preferable that the S concentration of the molten steel before Mg addition be low (less than 20 ppm), and thus the maximum particle diameter of the inclusions can be reduced.

[0045] BRIEF DESCRIPTION OF DRAWINGS 10 continuous casting apparatus 12 mold 13 mold copper plate 14 tundish 16 cast slab support roll 17 transfer roll 18 molten steel 19 molten flux 20 immersion nozzle 22 coating layer 23 inhomogeneous substance filling portion 24 solidified shell 26 un-solidified layer 28 cast slab 30 secondary cooling zone 32 cast slab cutting machine

Claims

1. A method of producing molten steel, in which a metal Mg or an Mg-containing alloy is added to Al-deoxidized molten steel in an addition amount determined based on a ratio of a difference between Mg concentrations of the molten steel before and after the addition of Mg to an oxygen concentration of the molten steel before the addition of Mg.

2. The molten steel production method as recited in claim 1, wherein The metal Mg or the Mg-containing alloy is added to the Al-deoxidized molten steel in such a manner 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, [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 molten steel production method as recited in claim 2, wherein the S concentration of the molten steel before the addition of Mg is less than 20 ppm.

4. A method of manufacturing a cast sheet, wherein, A molten steel produced by the method of producing molten steel according to any one of claims 1 to 3 is poured into a mold of a continuous casting machine, and a cast slab is continuously cast by cooling in the mold.

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