JP2024004032A - Continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method which can suppress longitudinal cracks, bleeds and breakouts in the vicinity of corners.

SOLUTION: Provided is a continuous casting method in which, while molten steel is poured into a mold for continuous casting, a solidified shell obtained by solidifying the molten steel is pulled out to produce a slab with a local heat flux ratio between a local heat flux qn on a short side mold side at positions in which casting direction distances from a meniscus in the mold are the same and which are separated by a prescribed distance from one corner and a local heat flux qw on a long side mold side at positions separated by a prescribed distance from the one corner R=qn/qw being controlled to a range of 0.7 or more to 1.4 or less.

SELECTED DRAWING: Figure 2

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present invention relates to a continuous casting method that prevents slab cracking and breakout caused by cracking by regulating the local heat flux of a mold copper plate, and particularly relates to a continuous casting method for medium carbon steel.

In continuous steel casting, molten steel injected into a mold is cooled by a water-cooled mold, and the molten steel solidifies at the contact surface with the mold to form a solidified layer (referred to as a "solidified shell"). This solidified shell is continuously pulled out below the mold together with the unsolidified layer inside while being cooled by a water spray or air water spray installed downstream of the mold. The slab is then solidified to the center by cooling with water spray or air/water spray to produce a slab.

If the cooling within the mold becomes uneven, the thickness of the solidified shell will become uneven in the casting direction, the width direction of the slab, and the thickness direction of the slab. Stress due to contraction and deformation of the solidified shell acts on the solidified shell, and in the early stage of solidification, this stress is concentrated in the thin wall portion of the solidified shell. This stress causes cracks to occur on the surface of the solidified shell. This crack expands due to external forces such as subsequent thermal stress and bending stress and straightening stress caused by the rolls of the continuous casting machine, resulting in a large surface crack. When the non-uniformity of the solidified shell thickness is large, vertical cracks occur within the mold, and breakouts may occur in which molten steel flows out from the vertical cracks. Additionally, vertical cracks can also occur near the corners of the mold, and in this case, it has been found that not only does the quality of the slab deteriorate, but there is also a greater possibility of causing operational problems such as bleed and breakouts.

It is known that vertical cracks caused by this non-uniform solidification are more likely to occur in the subperitectic region of steels with a peritectic reaction in which the carbon content is in the range of 0.08 to 0.17 mass%. . Conventionally, in order to prevent surface cracking of steel types that involve the above-mentioned peritectic reaction (referred to as "medium carbon steel"), for example, as proposed in Patent Document 1, Patent Document 2, and Patent Document 3, a composition that is easy to crystallize has been proposed. Attempts have been made to slowly cool the solidified shell by increasing the thermal resistance of the mold powder layer or the mold by using mold powder and a mold that promotes slow cooling.

Further, Patent Document 4 discloses a continuous casting method for medium carbon steel that prevents slab cracking and breakout caused by cracking by monitoring the heat flux of a mold copper plate. Using the time change in the heat flux ratio between the short side heat flux and the long side heat flux, the taper ratio of the mold copper plate on the short side is increased or decreased so that the heat flux ratio becomes appropriate.

Japanese Patent Application Publication No. 2005-297001 Japanese Patent Application Publication No. 06-304719 Japanese Patent Application Publication No. 09-276994 Japanese Patent Application Publication No. 2009-090309

However, the above conventional technology has the following problems.
That is, in the techniques disclosed in Patent Documents 1 to 3, although slow cooling of the solidified shell is effective in suppressing vertical cracks in areas other than the vicinity of corners, the effect is small on vertical cracks in the vicinity of corners. Therefore, the problem is that vertical cracking, bleeding, and breakout cannot be sufficiently suppressed.

In addition, in the technology disclosed in Patent Document 4, the measured heat flux is the surface average heat flux obtained from the heat removal of the cooling water, so the local heat flux near the corner cannot be directly measured. First, there is the problem that it is not possible to accurately suppress vertical cracks near corners.

The present invention has been made in view of the above circumstances, and its purpose is to propose a continuous casting method that can suppress vertical cracks, bleeds, and breakouts near corners during continuous casting. It is in.

The continuous casting method according to the present invention, which advantageously solves the above-mentioned problems, involves injecting molten steel into a mold for continuous casting, and pulling out a solidified shell in which the molten steel has solidified, to manufacture a slab. The local heat flux qn on the short side mold side at a position where the casting direction distance from the meniscus is the same and a predetermined distance away from one corner, and the length at a position a predetermined distance away from the one corner. It is characterized in that the local heat flux qw on the side mold side and the local heat flux ratio R = qn/qw are in the range of 0.7 or more and 1.4 or less.

In addition, the continuous casting method according to the present invention is as follows:
(a) the positions at which the local heat fluxes on the short-side mold side and the long-side mold side are measured are separated from the corner by a range of 5 mm or more and 20 mm or less;
(b) When introducing mold powder onto the surface of the molten steel poured into the mold, the mold powder contains CaO, SiO _{2 , Al 2 O 3 , Na 2 O and Li 2 O, and the mold powder contains CaO, SiO 2} , Al ₂ O ₃ , Na ₂ O and Li ₂ O The basicity expressed by the ratio of the CaO concentration to the SiO ₂ concentration (mass% CaO/mass% SiO ₂ ) is 1.0 or more and 2.5 or less,
(c) continuous casting of medium carbon steel containing C: 0.05 to 0.22% by mass;
It is thought that this could be a more preferable solution.

According to the continuous casting method according to the present invention, vertical cracking, bleed, and breakout near corners can be suppressed, making it possible to prevent slab quality problems and operational problems, which is industrially useful. be.

This figure shows the influence of the distance in the casting direction from the meniscus of the heat flux measurement point on the relationship between the local heat flux ratio between the short side and the long side near the corner of a continuous casting mold and the number of vertical cracks at the corresponding slab corner. It is a graph. The measurement point of heat flux is determined from the corner to the short side and to the long side, which gives the relationship between the local heat flux ratio between the short side and long side near the corner of a continuous casting mold and the number of vertical cracks at the corresponding slab corner. FIG. 7 is a graph showing the effect of lateral distance; FIG. It is a graph showing the relationship between the number of vertical cracks in slab corners and the basicity of mold powder. It is a graph showing the number of vertical cracks in slab corners under each test condition in Examples.

Embodiments of the present invention will be specifically described below. Furthermore, the following embodiments are intended to exemplify equipment and methods for embodying the technical idea of the present invention, and the configuration is not limited to the following. That is, the technical idea of the present invention can be modified in various ways within the technical scope described in the claims.

The inventors conducted extensive research with the belief that by keeping the local heat flux ratio between the short side and long side of the mold corner within a predetermined range, the non-uniformity of the solidified shell near the corners could be resolved. did.

First, a molded copper plate during continuous casting has the same corner parts at L = 50 mm, 250 mm, and 500 mm positions (downward) in the casting direction from the meniscus, and a position 5 mm away from the corner part on the long side. The local heat flux qw(5) and the local heat flux qn(5) at a position 5 mm away from the corner on the short side were measured. The relationship between the local heat flux ratio R (5, 5) = qn (5) / qw (5) and the slab corner vertical crack number density (m ^-2 ) at the position corresponding to the corner is shown in Figure 1. Shown in (a) to (c). Figure 1 (a) shows the case where L = 50 mm from the meniscus in the casting direction, Figure 1 (b) shows the case where L = 250 mm from the meniscus in the casting direction, and Figure 1 (c) shows the case where L = 250 mm from the meniscus in the casting direction. =500mm is shown.

Local heat flux was calculated using thermocouple temperature measurements embedded in the copper plate. When the local heat flux ratio R (5, 5) is smaller than 0.7, the local heat flux on the short side becomes too small compared to the long side, and the shell thickness on the long side becomes thicker than on the short side. . Therefore, the solidified shell on the short side near the corner is pulled toward the long side, and the gap between the solidified shell and the mold on the short side near the corner becomes large. As a result, it is thought that a solidification delay occurs in the solidified shell on the short side near the corner, making vertical cracks more likely to occur. On the other hand, when the local heat flux ratio R (5, 5) is larger than 1.4, the local heat flux on the short side becomes too large than on the long side, and the solidified shell thickness on the short side becomes smaller than that on the long side. growing. Therefore, the solidified shell on the long side near the corner is pulled toward the short side, and the gap between the solidified shell and the mold on the long side near the corner becomes large. As a result, it is thought that a solidification delay occurs in the solidified shell on the long side near the corner, making vertical cracks more likely to occur. Therefore, by setting the local heat flux ratio R (5, 5) in the range of 0.7 to 1.4, continuous casting can be performed without increasing the solidified shell thickness difference between the long side and short side near the corner. It is possible to suppress the occurrence of vertical cracks.

Next, have the same corner part at a position L = 50 mm away from the meniscus in the casting direction (downward), and n, w = 5 mm, 20 mm, 50 mm, and 100 mm away from the corner on the short side and long side, respectively. The heat flux ratio R(n, w)=qn(n)/qw(w) at the position was measured. n and w represent the distance (mm) from the corner of the short side and long side, qn (n) represents the local heat flux at a position n away from the short side, and qw (w) represents the distance from the corner of the long side represents the local heat flux at a position w away from . The relationship between the local heat flux ratio R (n, w) and the vertical crack number density (m ⁻² ) of the slab corner at the position corresponding to the corner was investigated, and the results are shown in FIG.

The local heat flux ratio R (n, w) at positions n, w = 50 mm and 100 mm away from the corner on the short side and long side, respectively, is in the range of 0.95 to 1.05, and vertical cracks at slab corners are observed. It was found that the correlation with number density was weak. In other words, the number density of vertical cracks at the corner of the slab may be about 0.2 m ^-2 or more than 1.0 m ^-2 in some cases. In addition, the local heat flux ratio R(n, w) at positions w, n = 5 mm and 20 mm away from the corner on the short side and long side, respectively, shows a strong correlation with the number density of longitudinal cracks at the slab corner. Understood. In other words, when the local heat flux ratio R=qn/qw is in the range of 0.7 or more and 1.4 or less, the number density of longitudinal cracks at the slab corner portion is good at about 0.2 m ⁻² or less. On the other hand, outside this range, the number density of vertical cracks at slab corners frequently exceeds 0.8 m ^-2 . Further, if w and n are less than 5 mm from the corner to the short side and the long side, respectively, the measurement will be performed near the corner, and it will be impossible to insert the thermocouple into the mold copper plate, so measurement will not be possible. Therefore, regarding the positions at which the local heat flux ratio R(n, w) is measured, the short side mold side is located at the same casting direction distance from the meniscus in the mold and is a predetermined distance away from one corner; In addition, by setting the long side of the mold at a predetermined distance from the corner and setting the local heat flux ratio R = qn/qw in the range of 0.7 to 1.4, the number of vertical cracks at the slab corner can be reduced. Density can be reduced. Preferably, the position where the local heat flux is measured is located away from the corner on the short side mold side and the long side mold side in a range of 5 mm or more and 20 mm or less. Note that the local heat flux ratio R (n, w) is determined by cooling between the short side mold side and the long side mold side, such as changing the taper of the short side of the mold, changing the cooling water amount balance between the long side and short side of the mold, etc. It can be selected as appropriate by changing the conditions. Furthermore, by changing the vibration waveform and frequency of the mold that is vibrated in the casting direction, the amount of mold powder consumed can be changed, and the conditions for cooling the slab in the mold can be changed.

Furthermore, we investigated the chemical composition of the mold powder, which affects the initial solidification within the mold. The mold powder of this embodiment contains CaO, SiO ₂ , Al ₂ O ₃ , Na ₂ O and Li ₂ O, and the ratio of the CaO concentration to the SiO ₂ concentration in the mold powder (mass%CaO/mass%SiO The basicity CaO/SiO ₂ expressed by ₂ ) was in the range of 0.7 to 2.9. The local heat flux ratio R( The relationship between the basicity of mold powder CaO/SiO ₂ at 5, 5) = 0.9 and the vertical crack number density (m ^-2 ) of the slab corner at the corresponding corner position was investigated, and the results are shown in Figure 3. . It is preferable that the basicity CaO/SiO ₂ of the mold powder is 1.0 or more, since the difference in solidified shell thickness near the corners can be further suppressed by slow cooling within the mold. Note that when the basicity CaO/SiO ₂ of the mold powder exceeds 2.5, although the number density of vertical corner cracks is low, slow cooling within the mold is promoted too much, and the solidified shell at the exit side of the mold is This results in insufficient thickness. Therefore, there is a possibility that the incidence of breakout directly under the mold may increase. Therefore, it is preferable that the basicity expressed as the ratio of CaO concentration to SiO ₂ concentration (mass% CaO/mass% SiO ₂ ) in the mold powder is 1.0 or more and 2.5 or less.

The continuous casting method of the present embodiment is applied to the continuous casting of medium carbon steel with a C content of 0.05 to 0.22 mass%, where vertical cracks are likely to occur due to solidification shrinkage, etc., to reduce vertical cracks at slab corners. , is suitably used. In particular, it is suitably used for continuous casting of medium carbon steel in the subperitectic region with a C content of about 0.08 to 0.17 mass % and accompanied by a peritectic reaction.

300 tons of molten steel in a ladle, which had been subjected to oxygen blowing in a converter and RH vacuum degassing treatment, was continuously cast under the conditions shown in Table 1. As shown in Table 1, the local heat flux R (n, w) is the thermoelectric flux at a position w (mm) away from the same corner on the long side mold side and at a position n (mm) away from the short side mold side. It was determined from the pair of temperature measurements. Further, the position in the casting direction from the meniscus where the local heat flux was measured was L=50 mm. The basicity of the mold powder, CaO/SiO ₂ , was cast under the conditions shown in Table 1. The C content of the molten steel was 0.20% by mass. Further, as a conventional example (Test No. 1), continuous casting was performed using the conditions that R (5, 5) = 0.6 and the basicity CaO/SiO ₂ of the mold powder was 0.90. Continuous casting was carried out under the conditions of the invention examples (Test Nos. 2 to 13), comparative examples (Tests No. 14 to 17), and conventional examples (Test No. 1) shown in Table 1. Figure 4 shows the results of measuring the number density (m ^-2 ) of vertical cracks in the surface. As is clear from FIG. 4, it was found that the number density of cracks on the slab surface could be significantly reduced in the invention example.

According to the continuous casting method of the present invention, vertical cracking, bleed, and breakout near the corners can be suppressed, making it possible to prevent slab quality problems and operational problems, which is industrially useful. .

Claims (4)

When producing slabs by injecting molten steel into a mold for continuous casting and pulling out the solidified shell in which the molten steel has solidified,
A local heat flux qn on the short side mold side at a position where the casting direction distance from the meniscus in the mold is the same and a predetermined distance from one corner, and a local heat flux qn at a position a predetermined distance from the one corner. A continuous casting method in which the local heat flux qw on the long side mold side and the local heat flux ratio R=qn/qw of are in the range of 0.7 or more and 1.4 or less. The continuous casting method according to claim 1, wherein the positions at which the local heat fluxes on the short-side mold side and the long-side mold side are measured are separated from the corner by a range of 5 mm or more and 20 mm or less. When pouring mold powder onto the surface of the molten steel poured into the mold, the mold powder contains CaO, SiO ₂ , Al ₂ O ₃ , Na ₂ O and Li ₂ O, and the CaO concentration in the mold powder The continuous casting method according to claim 1, wherein the basicity expressed by the ratio of CaO and SiO ₂ concentration (mass% CaO/mass% SiO ₂ ) is 1.0 or more and 2.5 or less. The continuous casting method according to claim 1, wherein medium carbon steel containing C: 0.05 to 0.22% by mass is continuously cast.

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