Continuous casting method for steel and method for producing hypo-peritectic steel

Abstract

Provided is a continuous casting method for hypo-peritectic steel in which the steel can be inhibited from cracking. This continuous casting method for steel includes an addition step in which a mold flux is added to a molten steel held in a casting mold of a continuous casting machine. The casting mold has, on a surface thereof, a coating which includes either one metal selected from among Ni, Cr, Co, and Fe or an alloy of two or more thereof in an amount of 20 mass% or larger and which contains a metal carbide and a metal nitride in an amount of 0.1 mass% or larger. The mold flux to be added in the addition step has a CaO / SiO2 mass ratio of 0.80-2.20.

Description

Continuous casting method for steel and method for producing subepidial steel

[0001] This invention relates to a continuous casting method for steel and a method for producing subpericrystalline steel.

[0002] In continuous casting, increasing the casting speed is essential for improving productivity. However, increasing the casting speed can lead to problems such as seizing of the solidified shell due to insufficient mold flux consumption and uneven flow of mold flux. Uneven flow of mold flux creates an air gap layer with low thermal conductivity between the mold flux and the mold, resulting in partially reduced heat removal from the molten steel. This creates areas with high and low heat removal from the molten steel, leading to uneven solidification and a problem of uneven thickness in the solidified shell. When uneven solidification occurs, surface defects such as longitudinal cracks appear in the cast slabs produced by continuous casting. Furthermore, in medium carbon steels such as subpericrystalline steel, in addition to the above problems during high-speed casting, the volume change due to δ / γ transformation during solidification becomes larger, so the problem of uneven solidification tends to be even more pronounced.

[0003] Patent Document 1 discloses a mold for continuous casting in which the molten steel contact surface of the mold is coated with an amorphous alloy made of an iron alloy or nickel alloy. According to Patent Document 1, applying this coating makes the mold highly wear-resistant, thereby enabling smooth continuous casting of steel and extending the lifespan of the mold.

[0004] Japanese Patent Publication No. 2001-105103

[0005] In the mold for continuous casting disclosed in Patent Document 1, the surface facing the casting space is coated with a molten layer of amorphous alloy, but this coating has low wettability with molten mold flux. As a result, under high-speed casting conditions, the molten mold flux cannot flow between the molten steel and the mold, causing stalemate between the mold and the solidified shell. Furthermore, even under normal casting conditions, the low wettability between the coating and the molten mold flux causes the molten mold flux to flow unevenly, forming an air gap layer, which leads to cracking due to uneven solidification of the molten steel.

[0006] This invention has been made in view of the above-mentioned problems, and aims to provide a continuous casting method for steel that can suppress the occurrence of cracks in steel, and a method for manufacturing subpericrystalline steel.

[0007] To solve the above problems, the present invention has the following features.

[0008] [1] A continuous casting method for steel, comprising an addition step of adding mold flux to molten steel contained in a mold of a continuous casting equipment, wherein the mold has a coating on its surface that contains 20% by mass or more of a metal or alloy consisting of two or more of Ni, Cr, Co and Fe, and 0.1% by mass or more of metal carbides and metal nitrides, and the mold flux added in the addition step is CaO / SiO ２ [1] A method for continuous casting of steel, wherein the mass ratio of is 0.80 or more and 2.20 or less. [2] The method for continuous casting of steel according to [1], wherein continuous casting is performed at a casting speed of 1.4 m / min or more. [3] The method for continuous casting of steel according to [1] or [2], wherein the thickness of the steel is 40 to 500 mm and the casting speed is 1.4 m / min or more and 10.0 m / min or less. [4] The method for continuous casting of steel according to [1] or [2], wherein the molten steel contains, by mass%, C: 0.00 to 0.30%, Si: 0.00 to 2.00%, Mn: 0.50 to 3.00%, P: 0.00 to 0.10%, S: 0.00 to 0.030%, Al: 0.00 to 0.10%, with the remainder being Fe and unavoidable impurities. [5] A method for producing subpericrystalline steel, using the continuous casting method for steel described in any of [1] to [4], wherein the components include, by mass%, C: 0.08 to 0.18%, Si: 0.10 to 0.30%, and Mn: 0.50 to 2.00%.

[0009] According to the steel continuous casting method of the present invention, the mold is coated on its surface with a coating containing a predetermined component. Furthermore, the mold flux added in the additive step is CaO / SiO ２The mass ratio is between 0.80 and 2.20. As a result, the formation of an air gap layer between the mold flux and the mold is suppressed, and uneven solidification of the molten steel can be suppressed. In addition, the molten mold flux, which acts as a lubricant in the mold, flows more easily between the molten steel and the mold, so that the solidified shell and the mold do not stick together. This makes it possible to suppress the occurrence of cracks in the steel.

[0010] This is a schematic cross-sectional view showing an example of a continuous casting apparatus having a mold according to this embodiment. This is an explanatory diagram showing the outline of the mold. This is a cross-sectional view of the mold.

[0011] The present invention will be described below through embodiments of the invention. Figure 1 is a schematic cross-sectional view showing an example of a continuous casting apparatus 10. The continuous casting apparatus 10 includes a mold 12, a tundish 14 installed above the mold 12, and a plurality of slab support rolls 16 arranged in a row below the mold 12. Although not shown in the figure, a ladle for holding 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. An immersion nozzle 20 is installed at the bottom of the tundish 14, and the molten steel 18 is poured into the mold 12 through the immersion nozzle 20. The molten steel 18 is cooled and solidified from the inner surface of the mold 12, forming a solidified shell 24. As a result, a slab 28 is formed with the solidified shell 24 as its outer shell and an unsolidified layer 26 made of molten steel 18 inside.

[0012] Multiple secondary cooling zones 30, each equipped with a spray nozzle (not shown), are installed in the gaps between adjacent slab support rolls 16 in the casting direction, extending from directly below the mold 12 along the casting direction. The slab 28 is cooled as it is pulled out by the cooling water sprayed from the spray nozzles of the secondary cooling zones 30. As the slab 28 is conveyed by the slab support rolls 16 and passes through the multiple secondary cooling zones 30, the solidified shell 24 is properly cooled, the solidification of the unsolidified layer 26 progresses, and the solidification of the slab 28 is completed.

[0013] Downstream in the casting direction, multiple conveyor rolls 17 are installed to continue transporting the cast slabs 28. Above the conveyor rolls 17, a cast slab cutting machine 32 is positioned to cut the cast slabs 28. After solidification is complete, the cast slabs 28 are cut into pieces of a predetermined length by the cast slab cutting machine 32.

[0014] Figure 2 shows an outline of the mold 12. The mold 12 is formed in a hollow cylindrical shape. The mold 12 has a pair of mold copper plates 13a arranged facing each other, and a pair of mold copper plates 13b sandwiched between the mold copper plates 13a and also arranged facing each other.

[0015] In the example shown in Figure 2, the pair of mold copper plates 13a and the pair of mold copper plates 13b are formed in the shape of rectangular plates. The mold copper plates 13a and 13b contain copper or a copper alloy. Examples of materials for the mold copper plates 13a and 13b include copper alloys containing chromium, zirconium, etc.

[0016] A pair of mold copper plates 13a are positioned on the front and back of the mold 12. A pair of mold copper plates 13b are positioned on the right and left sides of the mold 12. The mold 12 has an internal space enclosed by the pair of mold copper plates 13a and the pair of mold copper plates 13b. An immersion nozzle 20 is inserted into the internal space of the mold 12.

[0017] Here, arrow A in the figure indicates the casting direction. Arrow B in the figure indicates the width direction of the mold 12.

[0018] The mold copper plates 13a and 13b have recesses formed on their back surfaces that serve as cooling water channels (not shown). The mold 12 is cooled by passing cooling water through these cooling water channels.

[0019] Figure 3 is a schematic cross-sectional view showing a part of the mold 12. The inner surfaces of the four mold copper plates 13a and 13b that come into contact with the molten steel 18 are covered with a coating 22. Here, the parts of the mold copper plates 13a and 13b that come into contact with the molten steel 18 are also called the molten steel contact surfaces.

[0020] The coating 22 is provided to suppress the uneven inflow of molten mold flux during the initial solidification phase of the molten steel 18. For this reason, it is sufficient that at least the molten steel contact surfaces of the mold copper plates 13a and 13b are covered with the coating 22.

[0021] The thickness of the coating 22 is preferably 50 μm or more and 10 mm or less. If the thickness of the coating 22 is less than 50 μm, it is undesirable from the viewpoint of the wear resistance of the coating 22. Also, if the thickness of the coating 22 is greater than 10 mm, the amount of heat removed from the mold 12 will decrease, which may increase the number of cracks in the cast slab 28 during casting, so this is undesirable. Furthermore, the thickness of the coating 22 is more preferably 0.1 mm or more and 0.3 mm or less. By setting the thickness of the coating 22 to 0.1 mm or more and 0.3 mm or less, the loss of the coating 22 due to wear and the decrease in the amount of heat removed from the mold 12 can be suppressed.

[0022] The method for applying the coating 22 to the mold copper plates 13a and 13b is not particularly limited, but for example, laser overlay welding can be used.

[0023] As the metal carbides, metal nitrides, and metal carbonitrides included in the coating 22, for example, one or more of TiC, SiC, ZrC, TiN, CrN, TiAlN, and TiCN can be used. Among these, it is preferable to use TiC, which is a metal carbide.

[0024] Furthermore, it is preferable that the content of metal carbides, metal nitrides, and metal carbonitrides in the coating 22 is 0.1% by mass or more and 80.0% by mass or less. If the content of metal carbides, metal nitrides, and metal carbonitrides in the coating 22 exceeds 80.0% by mass, cracks may occur in the coating layer of the coating 22, which is undesirable. Also, if the content of metal carbides, metal nitrides, and metal carbonitrides in the coating 22 is less than 0.1% by mass, the effect of improving wettability with the molten mold flux will be reduced, which is undesirable.

[0025] Furthermore, it is preferable that the coating 22 contains 20% by mass or more of a metal or alloy composed of two or more of Ni, Cr, Co, and Fe. If the amount of a metal or alloy composed of two or more of Ni, Cr, Co, and Fe in the coating 22 is less than 20% by mass, it becomes difficult to form the coating 22, which is undesirable. Since the coating 22 contains at least 0.1% by mass of metal carbides, metal nitrides, and metal carbonitrides, the upper limit of the amount of a metal or alloy composed of two or more of Ni, Cr, Co, and Fe in the coating 22 is 99.9% by mass.

[0026] Furthermore, the coating 22 may contain impurities that are inevitably mixed in, in amounts of 5% by mass or less. If the amount is 5% by mass or less, the presence of such impurities will not affect the wettability with the molten mold flux 19.

[0027] The continuous casting method for producing steel using the continuous casting equipment 10 described above, and the method for producing subpericrystalline steel will now be explained. First, mold flux 19 is added to molten steel 18, and the addition process is performed. The molten steel 18 is preferably so-called medium-carbon steel.

[0028] Specifically, the molten steel 18 preferably contains the following components in mass percent: C: 0.00 to 0.30%, Si: 0.00 to 2.00%, Mn: 0.50 to 3.00%, P: 0.00 to 0.10%, S: 0.00 to 0.030%, and Al: 0.00 to 0.10%.

[0029] In other words, it is preferable that the molten steel 18 contains, in mass percent, C: 0.00 to 0.30%, Si: 0.00 to 2.00%, Mn: 0.50 to 3.00%, P: 0.00 to 0.10%, S: 0.00 to 0.030%, and Al: 0.00 to 0.10%, with the remainder consisting of Fe and unavoidable impurities.

[0030] More preferably, the molten steel 18 contains, by mass%, as components: C: 0.08 to 0.17%, Si: 0.10 to 0.30%, Mn: 0.50 to 1.20%, P: 0.010 to 0.030%, S: 0.005 to 0.015%, Al: 0.020 to 0.040%.

[0031] Further, it is preferable that the molten steel 18 contains components called so-called hypo-peritectic steel among medium-carbon steels. That is, according to the present invention, even hypo-peritectic steel, which was originally a difficult region for casting, can be cast with good quality and at a high casting speed.

[0032] Specifically, the hypo-peritectic steel contains, by mass%, as components: C: 0.08 to 0.18%, Si: 0.10 to 0.30%, Mn: 0.50 to 2.00%.

[0033] Further, the hypo-peritectic steel may contain, by mass%, P: 0.00 to 0.10%, S: 0.009 to 0.030%, Al: 0.00 to 0.10%.

[0034] The mold flux 19 added in the addition step contains SiO ２ , Al ２ O ３ and CaO as main components. The mold flux 19 may contain MgO, Na ２ O, LiO ２ etc.

[0035] It is more preferable that the mass ratio of CaO / SiO ２ of the mold flux 19 is 0.80 or more and 2.20 or less. In particular, when manufacturing hypo-peritectic steel, the mass ratio of CaO / SiO ２ is 0.70 or more and 2.20 or less, and preferably 0.80 or more and 2.20 or less. In addition, even if a mold flux 19 with a low basicity with a mass ratio of CaO / SiO ２ of 0.80 or more and less than 1.00 is used, high-quality hypo-peritectic steel can be manufactured. Also, the viscosity of the mold flux 19 is preferably 0.03 Pa·s or more and 3.0 Pa·s or less.

[0036] When the addition process is performed, the upper surface of the molten steel 18 in the mold 12 is covered by the molten mold flux 19 (hereinafter also referred to as the molten mold flux 19).

[0037] At this time, it is preferable to perform continuous casting at a casting speed of 1.4 m / min or more. The casting speed is preferably 1.4 m / min or more and 10.0 m / min or less, more preferably 1.4 m / min or more and 5.0 m / min or less, and even more preferably 1.4 m / min or more and 3.0 m / min or less. Also, the casting speed is preferably 1.6 m / min or more and 10.0 m / min or less, more preferably 1.6 m / min or more and 5.0 m / min or less, and even more preferably 1.6 m / min or more and 3.0 m / min or less. In particular, when producing hypoeutectoid steel, it is preferably 1.4 m / min or more and 3.0 m / min or less, more preferably 1.4 m / min or more and 2.5 m / min or less. Also, it is preferably 1.6 m / min or more and 3.0 m / min or less, more preferably 1.6 m / min or more and 2.5 m / min or less.

[0038] The casting speed is not particularly limited, but for example, it can be adjusted by adjusting the rotation speed of the strand support roll 16. Also, in addition to the rotation speed of the strand support roll 16, the casting speed can also be adjusted by adjusting the amount of molten steel 18 discharged from the immersion nozzle 20, adjusting the oscillation behavior of the mold 12, etc. The casting speed is adjusted by a system using at least one of these multiple parameters.

[0039] The casting speed is determined according to, for example, the thickness dimension of the steel. When continuous casting is performed at the above casting speed, for example, the thickness of the steel is preferably 40 to 500 mm, more preferably 200 to 400 mm, and even more preferably 200 to 300 mm. At such a casting speed, by adjusting the thickness of the steel within the above range, the wettability of the mold flux can be utilized to the maximum extent, and the occurrence of cracking in the steel can be reduced. In particular, when the slab thickness becomes thinner, the speed at which the slab solidifies becomes faster, so the theoretical casting speed increases. As the casting speed increases, there is a risk of deterioration in the surface quality of the slab and an increased risk of sticking due to uneven inflow of the mold flux. In this embodiment, since the wettability between the mold and the mold flux is high, the occurrence of such problems can be suppressed.

[0040] The molten steel 18 is injected into the mold 12 through the immersion nozzle 20 in a state covered with the molten mold flux 19. The injected molten steel 18 is cooled by the mold 12. As a result, the molten steel 18 at the interface between the mold 12 and the molten steel 18 solidifies, and a solidified shell 24 is formed.

[0041] The molten mold flux 19 flows between the mold 12 and the molten steel 18 and serves as a lubricant. By the molten mold flux 19 serving as a lubricant flowing between the mold 12 and the molten steel 18, the sticking between the mold 12 and the solidified shell 24 is suppressed.

[0042] Here, the inner surfaces of the mold copper plates 13a and 13b on the side in contact with the molten steel 18 are covered with the coating 22. Further, continuous casting is performed at the above casting speed with the mold flux 19 having a mass ratio of CaO / SiO ２ of 0.80 or more and 2.20 or less added.

[0043] This improves the wettability between the inner surfaces of the mold copper plates 13a and 13b and the molten mold flux 19 compared to conventional methods, further suppressing the uneven inflow of the molten mold flux 19. As a result, it is possible to suppress the occurrence of air gaps due to the uneven inflow of the molten mold flux 19 in the initial stages of solidification of the molten steel 18, and the resulting uneven growth and longitudinal cracking of the cast slab due to uneven thermal resistance between the mold and the molten steel. Furthermore, the risk of smearing between the mold 12 and the solidified shell 24 can also be reduced.

[0044] Furthermore, by reducing the thermal conductivity during the initial stages of cooling the molten steel 18 compared to the final stages, it becomes possible to cool the molten steel 18 more gradually. As a result, it becomes possible to improve the occurrence of longitudinal cracks in the cast slab.

[0045] (Test Example 1: Verification of Solidification Shell Thickness) For Invention Examples A1 to C1 and Conventional Examples A1 to C1, the uniformity of the solidification shell thickness was evaluated under the conditions of subpericrystalline steel and a casting speed of 3.0 m / min. The uniformity of the initial solidification shell thickness determines the uniformity of the slab surface as solidification progresses and is related to the rate of longitudinal cracking in the slab.

[0046] Evaluating the initial solidification shell thickness in product slabs is difficult. Therefore, in the experiment, the evaluation was performed using a solidification shell generated to simulate the initial stage of casting. The copper block used in the experiment had a rectangular top view. The copper block was coated with two layers on opposite sides: one containing TiC and Inconel, and the other containing Ni and Co.

[0047] The basicity of the four mold fluxes, A to D, ranged from 0.86 to 2.10, respectively. These mold fluxes A to D were used to create a molten state by floating them in molten subpericrystalline steel. Copper blocks were immersed in this molten steel, and the thickness of the solidified shell formed on each surface of the copper block was measured.

[0048] The non-uniformity of the solidified shell was defined as the standard deviation of the widthwise thickness at the height center of the generated solidified shell divided by its thickness. Furthermore, the non-uniformity of the TiC and Inconel surfaces was divided by the non-uniformity of Ni and Co to determine the "index." An index of 1 or less indicates that the solidified shells of the TiC and Inconel surfaces are more uniform. The index was calculated for these four types of mold fluxes. The results are shown in Table 1.

[0049]

[0050] As shown in Table 1, the index was 1 or less for all four types of mold fluxes A to D. Therefore, it was shown that the solidified shells on surfaces using TiC and Inconel were uniform for all of the mold fluxes A to D. Thus, it was found that by using a mold flux with a basicity within the predetermined range in a mold coated within the predetermined range described above, the uniformity of the initial solidification is higher than in conventional methods.

[0051] (Test Example 2: Verification of Local Heat Flux Fluctuation Rate) Test casting of steel was performed to verify the fluctuation rate of the local heat flux. The composition of the steel produced was C: 0.002 to 0.174 mass%, Mn: 0.52 to 1.82 mass%, and Si: 0.00 to 0.30 mass%.

[0052] An example of the invention involved constructing a mold using a copper mold plate coated with TiC and Inconel. The coating consisted of 10% by mass of TiC and 90% by mass of Inconel. A conventional example involved constructing a mold using a copper mold plate coated with Ni and Co. The mold flux used in the test was CaO / SiO2. ２ Three types were used with mass ratios of 0.86, 1.55, and 2.10.

[0053] The steels of the inventive examples A1 to C1 and the conventional examples A1 to C1 were manufactured according to the embodiments shown in Table 2 below.

[0054] For Invention Examples A1 to C1 and Conventional Examples A1 to C1, continuous casting was performed using a total of six casting speeds: 1.5 m / min, 1.6 m / min, 1.7 m / min, 1.8 m / min, 1.9 m / min, and 2.0 m / min.

[0055] The mean and standard deviation of the local heat flux in the width direction were calculated. Local heat flux q [W / m] ２ The expression q is given by q = (T_TC - T_W) / H, where T_TC is the temperature of the thermocouple [°C], T_W is the temperature of the cooling water [°C], and H is the thermal resistance of the entire copper mold plate [m ２ It is / (W・℃)).

[0056] The rate of change of the local heat flux, σ / q, was calculated using the standard deviation σ of the obtained local heat flux. The results are shown in Table 3.

[0057]

[0058] As shown in Table 3, at a casting speed of 1.5 m / min, no difference in the local heat flux fluctuation rate σ / q was observed between the inventive examples A1-C1 and the conventional examples A1-C1. In inventive examples A1-C1, the fluctuation rate was 15% or less in all cases at a casting speed of 1.6 m / min. Therefore, it is estimated that the possibility of cracking is low in all inventive examples A-C.

[0059] (Test Example 3: Verification of Cracking in Subepidactic Steel) Cracking of subepidactic steel obtained by continuous casting was verified. The conditions of continuous casting and the CaO / SiO of the mold flux were examined. ２ The mass ratio is the same as in Test Example 1, so the explanation is omitted. In this test example, continuous casting was performed at a specific casting speed between 1.5 and 2.0 (m / min), as shown in Table 4.

[0060]

[0061] In all of the invention examples A2 to C2, the crack occurrence rate was lower than that of the conventional examples A2 to C2. In particular, the mold flux CaO / SiO ２As the mass ratio decreased, the rate of cracking tended to increase, except for Invention Example B2 and Conventional Example B2. Invention Examples A2 to C2 were found to have a lower cracking rate than Conventional Examples A2 to C2. From this, it is thought that in Invention Examples A2 to C2, the mold flux flowed in uniformly, contributing to uniform solidification within the mold, resulting in a reduction in longitudinal cracking compared to Conventional Examples A2 to C2.

[0062] 10 Continuous casting equipment 12 Mold 13a Mold copper plate 13b Mold copper plate 22 Coating

Claims

1. A continuous casting method for steel, comprising an addition step of adding mold flux to molten steel contained in a mold of a continuous casting apparatus, wherein the mold has a coating on its surface that contains 20% by mass or more of a metal or alloy consisting of two or more of Ni, Cr, Co, and Fe, and 0.1% by mass or more of metal carbides and metal nitrides, and the mold flux added in the addition step is CaO / SiO ２ A continuous casting method for steel, wherein the mass ratio of is 0.80 or more and 2.20 or less.

2. The continuous casting method for steel according to claim 1, wherein continuous casting is performed at a casting speed of 1.4 m / min or more.

3. The continuous casting method for steel according to claim 1 or 2, wherein the thickness of the steel is 40 to 500 mm, and the casting speed is 1.4 m / min or more and 10.0 m / min or less.

4. The continuous casting method for steel according to any one of claims 1 to 3, wherein the molten steel comprises, in mass percent, C: 0.08 to 0.17%, Si: 0.10 to 0.30%, Mn: 0.50 to 1.20%, P: 0.010 to 0.030%, S: 0.05 to 0.015%, and Al: 0.020 to 0.040%.

5. A method for producing subpericrystalline steel, comprising using the continuous casting method for steel described in any one of claims 1 to 4, wherein the subpericrystalline steel contains, by mass%, C: 0.08 to 0.18%, Si: 0.10 to 0.30%, and Mn: 0.50 to 2.00%.

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