Method for producing molten steel and method for producing cast slab

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

Molten steel manufacturing method and cast steel manufacturing method

[0001] The present invention is directed to the removal of coarse Al particles from molten steel. ２ O ３ The present invention relates to a method for producing molten steel and a method for producing slabs that can reduce system inclusions to produce fine MgO inclusions.

[0002] Non-metallic inclusions in steel affect the quality of steel. In particular, in aluminum (hereinafter also referred to as "Al") deoxidized molten steel, the generated Al ２ O ３ The coarsened and clustered Al inclusions occur. ２ O ３ Since these systems can cause surface defects in steel products, methods of controlling the shape of nonmetallic inclusions are being investigated, such as blowing inert gas into molten steel, floating and separating the molten steel by leaving it still in the ladle, 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.

[0005] JP 2017-20064 A JP 2008-189975 A

[0006] Non-Patent Document 1 and Patent Documents 1 and 2 disclose the use of fine inclusions containing Mg and a method of adding Mg to molten steel, but do not disclose the use of coarse Al ２ O ３There is no disclosure of how to specify the amount of Mg to be added to molten steel that can suppress clustering of Al-based inclusions. Therefore, these methods have the problem that it is not possible to specify the amount of Mg to be added to molten steel. The present invention has been made in consideration of such problems in the prior art, and ２ O ３ The present invention aims to provide a method for producing molten steel that can specify the amount of Mg to be added to reduce system inclusions to form fine MgO inclusions, and a method for producing slabs using the molten steel produced by this method.

[0007] The gist of the present invention, which can solve the above problems, is as follows. [1] A method for producing molten steel, which comprises 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] A method for producing molten steel according to [1], which comprises adding metallic Mg or an Mg-containing alloy 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 the addition of Mg, and [T.O] is the oxygen concentration (ppm) of the molten steel before the addition of Mg. [3] A 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. [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 [1] to [3] into a mold of a continuous casting machine, cooling the molten steel in the mold, and continuously casting a slab.

[0008] According to the present invention, the ratio of the difference in Mg concentration before and after Mg addition to the oxygen concentration of the molten steel before Mg addition is Al ２ O ３ It affects the grain size of the Al-based inclusions. ２ O ３ It becomes possible to specify the amount of metallic Mg or Mg-containing alloy to be added that can reduce the system inclusions to fine MgO inclusions.

[0009] 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. 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.

[0010] The inventors have found that coarse Al particles are formed in molten steel deoxidized with Al. ２ O ３ In order to confirm the amount of metallic Mg added that can control the morphology of non-metallic inclusions, a test of adding Mg to molten steel deoxidized with Al was carried out, and the composition of non-metallic inclusions in the molten steel and the particle size of the inclusions were investigated.

[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. An energy dispersive X-ray analyzer (hereinafter also referred to as "EDS") equipped in this SEM was used to investigate the composition and particle size of the non-metallic inclusions in the molten steel due to the addition of Mg. As a result, the following findings (1) and (2) were obtained.

[0012] (1) The volume fraction of inclusions is almost unchanged before and after the addition of Mg. The fine MgO inclusions that are formed after the addition of Mg are the same as the Al inclusions that existed before the addition. ２ O ３ (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, the molten steel before Mg addition contains MgO mixed in from the refractory. ２ O ３ Not only MgO.Al ２ O ３ Inclusions are generated, grow, and become coarse. ２ O ３ Unless sufficient dissolved Mg is supplied to the molten steel by adding Mg to the inclusions, MgO.Al ２ O ３ The inclusions could not be completely reduced, resulting in fine MgO inclusions and coarse aggregated MgO.Al ２ O ３Inclusions become mixed in the molten steel.

[0014] Therefore, the Mg concentration is determined by the difference between the Mg concentration in the molten steel before and after the addition of Mg (the difference between the Mg concentration in the molten steel after the addition of Mg and the Mg concentration in the molten steel before the addition of Mg). ２ O ３ It is a criterion for determining whether or not inclusions can be reduced. The oxygen concentration in molten steel is an indicator of the amount of inclusions present in the 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 an amount of metallic Mg to be added that is specified based on this ratio is added to the molten steel after Al deoxidation. ２ O ３ The range of ratio within which the inclusions can be reduced to fine MgO inclusions is determined in advance, and the amount of metallic Mg added is specified so that it falls within this range of ratio.

[0016] The range of the ratio between the difference in Mg concentration in molten steel before and after the addition of Mg and the oxygen concentration in the molten steel was determined by preparing molten steel after Al deoxidation in which the ratio was changed by changing the amount of metallic Mg added, and ２ O ３ In this way, the inventors have been able to predict the particle size of coarse Al inclusions based on the ratio of the difference in Mg concentration in molten steel before and after Mg addition to the oxygen concentration in molten steel. ２ O ３ The present invention was completed by finding that it is possible to specify the amount of metallic Mg to be added that can convert the sintered inclusions into fine MgO inclusions. Hereinafter, an embodiment of the present invention will be described using an example in which the present invention is applied to processes 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 performed on molten steel in a ladle after tapping from a converter and after Al deoxidation, on molten steel after RH vacuum degassing treatment, 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 performed between RH vacuum degassing treatment and continuous casting.

[0018] Mg can be added to molten steel either in the form of metal Mg or 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 an Mg-containing alloy is added to molten steel to achieve an appropriate Mg concentration in the steel, the dissolved Mg concentration in the molten steel becomes higher than the deoxidation equilibrium concentration, and coarse Al ２ O ３ The inclusions are reduced, resulting in the formation of coarse Al ２ O ３ The morphology of the Al inclusions is controlled to be fine MgO inclusions. ２ O ３ The above effect cannot be obtained even if Mg-containing oxides contained in slag, refractories, etc., which cannot reduce the inclusions, are added to the molten steel after Al deoxidation.

[0020] In the method for producing molten steel according to this embodiment, the amount of metallic Mg or 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 Mg-containing alloy is added to the molten steel. ２ O ３The above-mentioned range of ratios within which the inclusions can be reduced to fine MgO inclusions is determined in advance, and by measuring the Mg concentration and oxygen concentration of the molten steel before adding Mg, it becomes possible to specify the amount of metallic Mg or Mg-containing alloy to be added to the molten steel after Al deoxidation.

[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): ２ O ３ This makes it possible to reduce the inclusions and produce molten steel containing fine MgO inclusions.

[0022] ([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.

[0023] On the other hand, when the ratio of the difference in Mg concentration before and after the addition of Mg, 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 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 the addition of Mg 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 the addition of Mg is high, the reaction Mg + S → MgS occurs. MgS has a lower density than MgO, and when it is generated, it coarsens inclusions. Furthermore, MgS is easily oxidized, and reacts with oxides in the slag and refractories before casting, which also coarsens inclusions. For this reason, it is preferable to suppress the generation of MgS, and the S concentration in the molten steel before the addition of Mg is preferably 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 has 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 in the figure, 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 method for producing molten steel according to this embodiment, and contains coarse Al ２ O ３ The molten steel contains fine MgO inclusions produced by reducing the system 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 cast 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 cast slab 28a produced by such continuous casting contains coarse Al particles which may cause surface defects in the steel material. ２ O ３ Therefore, by carrying out the method for producing a slab according to this embodiment, it is possible to produce a slab 28a that can produce a steel material with reduced quality defects such as surface defects.

[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 the 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, CaO-Al ２ O ３ -SiO ２ The flux of this system was used as a desulfurization agent, and the temperature of the molten steel was raised by arc heating from a black smoke electrode, and the desulfurization agent was turned into slag. ３ The molten steel and the desulfurizing agent were stirred by blowing Ar gas as a stirring gas at a rate of 1 / hour, and the molten steel was adjusted to have the composition shown in Table 1 below.

[0032]

[0033] The desulfurized molten steel was subjected to degassing using an RH vacuum degassing apparatus, adjustment of the molten steel composition, and floating and separating of inclusions 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% pure Mg and 60 mass% pure Si, with the remainder consisting of 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] The molten steel sampled from the molten steel in the ladle after the addition of the MgSi alloy was rapidly cooled, and a sample was prepared to confirm the morphology of inclusions. ２ The particle size distribution of inclusions was measured at 30 points within a measurement range of 1000 m. 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 the addition of Mg, [T.O] is the oxygen concentration (ppm) in the molten steel before the addition of Mg, [T.Mg] is the Mg concentration (ppm) in the molten steel after the addition of Mg, and the S concentration is the S concentration (ppm) in the molten steel before the addition of Mg.

[0036]

[0037] 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. As shown in FIG. 2, the value of the left side of equation (1) ([T.Mg] - [T.Mg] Ｂ Therefore, it can be seen that the ratio of [T.O] / [T.O] affects the predicted maximum diameter of inclusions. Therefore, the Al concentration can be calculated based on the value of the left side of the above equation (1), that is, the ratio of the difference in Mg concentration before and after Mg addition to the oxygen concentration of the molten steel before Mg addition. ２ O ３ It is possible to determine the amount of metallic Mg or Mg-containing alloy to be added, which can reduce the system inclusions to fine MgO inclusions.

[0038] From Fig. 2, coarse Al ２ O ３ It can be seen that the range of the ratio between the difference in Mg concentration before and after the addition of Mg, which can reduce Al-based inclusions to fine MgO inclusions, and the oxygen concentration of the molten steel before the addition of Mg, is 0.5 or more. ２ O ３The range of the ratio between the difference in Mg concentration before and after the addition of Mg, which allows sufficient reduction of the Al-based inclusions, and the oxygen concentration of the molten steel before the addition of Mg is determined in advance. ２ O ３ It is possible to specify the amount of metallic Mg or Mg-containing alloy to be added that can reduce coarse Al inclusions contained in the Al-deoxidized molten steel by adding the specified amount of metallic Mg or Mg-containing alloy to the Al-deoxidized molten steel. ２ O ３ This makes it possible to produce molten steel in which the system inclusions are reduced to form fine MgO inclusions.

[0039] As shown in Figure 2, when the value of the left side of the formula (1) above is 0.5 or more, the maximum particle size of the inclusions becomes significantly smaller. From this result, it can be seen that by adding metallic Mg so as to satisfy the formula (1) above, the coarse Al particles contained in the molten steel after Al deoxidation can be reduced. ２ O ３ It was confirmed that the inclusions could be reduced to 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, in which the value of the left side of Equation (1) was larger. This result is thought to be because the S concentration of the molten steel before the addition of Mg was lower in Experiment No. 3 than in Experiment No. 1, and as a result, the predicted maximum grain size of inclusions in Experiment No. 3 was smaller than that in Experiment No. 1. From these results, it was confirmed that it is preferable for the S concentration of the molten steel before the addition of Mg to be low (less than 20 ppm), and this makes it possible to reduce the maximum grain size of inclusions.

[0041] REFERENCE SIGNS LIST 10 Continuous casting equipment 12 Mold 13 Mold copper plate 14 Tundish 16 Strand support roll 17 Conveyor roll 18 Molten steel 19 Molten mold flux 20 Submerged nozzle 22 Coating 23 Different substance filled section 24 Solidified shell 26 Unsolidified layer 28 Strand 30 Secondary cooling zone 32 Strand 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 determined 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. 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 is added, and [T.O] is the oxygen concentration (ppm) of the molten steel before Mg is added.

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

4. A method for producing slabs, 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, cooling the slabs in the mold, and continuously casting the slabs.

Images