JP2005152931A - Method for continuously casting medium carbon steel slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To efficiently provide a medium carbon steel slab excellent in surface quality without developing the surface defect caused by the longitudinal crack and deoxidation generated material, etc., even in the case of such high speed continuous casting as to exceed 20 m/min. <P>SOLUTION: Under condition of 150-240 mm short wall side length in the casting space in a mold and >2.0 m/min casting speed in a continuous casting facility, as mold flux, the flux containing at least two components selected from 0.5-8.0 mass% TiO<SB>2</SB>, 0.6-4.0 mass% MnO and 2.0-6.0 mass% Al<SB>2</SB>O<SB>3</SB>and satisfying in the range of 1.4-2.0 the ratio of CaO and SiO<SB>2</SB>in the remained components, is used. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to slab continuous casting of medium carbon steel, and in particular, by enabling high-speed casting without the occurrence of vertical cracks, which has been a concern in the past, it is possible to improve the productivity advantageously, and to obtain the obtained slab and the raw material. It is intended to improve the surface quality of the product plate.

In continuous casting for so-called medium carbon steel having a C content of 0.07 to 0.22 mass%, there is a problem that vertical cracks are likely to occur on the surface of the slab during continuous casting.
Thus, various studies have been made on the mechanism of occurrence of such vertical cracks and countermeasures therefor.
Longitudinal cracks tend to occur in medium carbon steel because it is in the peritectic transformation region with a C content of 0.07 to 0.22 mass%. Due to the occurrence of transformation stress in the solidification process of steel, It is clear that this is due to the fact that a large difference occurs in the fast part, that is, the nonuniformity of the growth of the solidified shell increases.

Here, the above non-uniformity of the solidified shell has a correlation with the initial heat radiation amount in the mold,
(1) Slow cooling in the mold,
(2) Eliminate the air gap that is the cause of non-uniform heat removal from the mold,
(3) It has already been known that it can be mitigated by suppressing variation in the thickness of the mold flux film between the mold and the solidified shell.

As a prior art regarding this point, for example, in Patent Document 1, as a method for producing a wide continuous cast slab having few surface flaws, by using an oil casting method and a mold flux casting method in combination, non-uniformity of molten steel in a mold is achieved. A technique for preventing surface vertical cracks due to cooling is disclosed.
Patent Document 2 uses a continuous casting mold in which a plurality of longitudinal grooves having appropriate shapes and dimensions are formed on the upper surface of the mold, and a medium carbon steel melt is supplied to the mold. A continuous casting technique is disclosed in which the solidified shell is uniformly cooled to avoid surface cracks by slowly cooling only at the upper part.

Further, as a prior art related to mold flux, Patent Document 3 discloses that the viscosity and crystallization temperature of the flux are set low in order to allow the mold flux to flow uniformly and avoid non-uniform cooling of the solidified shell. A technique that is applied only to the central portion of each is disclosed.
In Patent Document 4 and Patent Document 5, the basicity and solidification temperature of the mold flux are increased, the solid phase thickness in the mold flux layer between the mold and the slab is increased, and the radiant heat flux is decreased. A technique for preventing cracking has been proposed.

On the other hand, in Non-Patent Document 1, the basicity of the mold flux is maintained at about 1.2, ZrO ₂ is added at about 3%, and the solidification point is lowered by further reducing the radiant heat transfer and between the mold and the flux solidification layer. A technique is disclosed in which slow cooling is achieved by increasing contact thermal resistance to prevent cracking.

In addition, in continuous casting of stainless steel containing a certain amount or more of Ti, the physical properties of the flux change due to the concentration of TiO ₂ in the mold flux during casting, and the surface defects or lubrication of the steel sheet due to the flux itself is impaired. It is shown in Non-Patent Document 2 that it becomes a cause of restraint of the solidified shell to the mold due to the above.
The reporter is, as a solution to such a problem, it is proposed to reduce the ratio i.e. basicity of CaO and SiO ₂ 0.5 to 0.6.

Japanese Patent Laid-Open No. 50-59229 JP-A-61-92756 JP 63-235054 A JP-A-8-197214 JP-A-5-277680 "CAMP-ISIJ, 6 (1993), p.283" "Steel Research No.324 (1987), P.10"

However, all of the above conventional techniques have the following problems.
Since the technology disclosed in Patent Document 1 needs to newly install equipment for oil casting in addition to equipment for mold flux casting which is currently common in steel types other than medium carbon steel, There was a problem that an increase and an increase in equipment cost were inevitable.
The technique disclosed in Patent Document 2 requires the use of a dedicated mold for casting medium carbon steel, which is disadvantageous in terms of manufacturing cost when considering downtime such as mold replacement. In addition, since the mold surface temperature is estimated to be 400 ° C. or higher, the mold itself and the plating layer are significantly deteriorated, which is difficult to realize in terms of mold life.
The technique disclosed in Patent Document 3 uses two types of mold flux with a boundary of about 200 mm from the center of the long side of the slab as a boundary. Although stable heat removal occurs and the effect of suppressing vertical cracks is recognized, the degree of improvement is very slight.

In the technique using the high freezing point flux disclosed in Patent Document 4, Patent Document 5 and Non-Patent Document 1, it is possible to perform casting with almost no problem at the beginning of casting. It was confirmed that the mold flux composition deviated from the initial composition by direct reaction with molten steel, and the uniform slow cooling effect due to uniform crystal formation could not be obtained. In particular, in high-speed casting with a casting speed exceeding 2.0 m / min, there was a tendency for vertical cracks to occur significantly due to uneven cooling.
From the above viewpoint, if the ratio (basicity) of CaO and SiO ₂ is set to a high value, for example, about 1.8, it is possible to maintain the crystallization characteristics after the change of the components. Since the temperature is too high to maintain the lubrication between the mold and the slab, the solidified shell breaks and breaks out, which is not practical as a countermeasure.

When the ratio of CaO and SiO ₂ , that is, the basicity is reduced to 0.5 to 0.6 according to the presentation of Non-Patent Document 2, the non-uniformity of mold flux inflow becomes remarkable as the casting speed increases, and cooling in the mold width direction is reduced. Since it became extremely uneven, satisfactory vertical crack prevention could not be achieved.
This is due to the fact that since the basicity is low, the inflowing mold flux does not crystallize and becomes glassy, and the contact with the mold becomes thermally conductive.

  The present invention advantageously solves the above problem, and effectively prevents the deviation of the mold flux composition from the initial composition due to the direct reaction with floating inclusions or molten steel, and enables uniform crystal formation. In addition, it effectively avoids trapping of deoxidation products, mold flux, etc. in the solidified shell, so that even in high-speed continuous casting exceeding 2.0 m / min, vertical cracks and the above-mentioned deoxidation products An object of the present invention is to propose a method for continuously casting slabs of medium carbon steel that can advantageously produce continuous cast slabs excellent in surface quality while advantageously preventing the occurrence of surface defects.

Now, as a result of repeating various experiments and examinations about problems peculiar to continuous casting of medium carbon steel in the peritectic transformation region, that is, prevention of surface cracking and improvement of lubricity of mold and slab, The following findings were obtained.
In other words, although depending on the deoxidation method, oxides caused by deoxidation products such as Al ₂ O ₃ , MnO, TiO ₂ or the like, or reduction products by SiO ₂ in the flux, whose composition changes significantly during casting and mold flux This suppresses the crystal formation of the mold flux itself and prevents the maintenance of uniform slow cooling characteristics as intended in the composition design, but the adverse effect is greatly promoted by the combination of a plurality of deoxidation products. In order to avoid this, after adding a certain amount of the above-mentioned oxide that suppresses crystal formation in advance, mainly the phase ratio of CaO and SiO ₂ , and further Na, F, The MgO concentration may be adjusted.

In order to prevent surface defects due to deoxidation products, mold flux, etc., under high-speed continuous casting exceeding 2.0 m / min, the short side length of the casting space in the mold of the continuous casting equipment is set to 150 ~. It is effective to control within the range of 240mm.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. When producing medium carbon steel slabs with a C content of 0.07 to 0.22 mass% using continuous casting equipment,
The short side length of the casting space in the mold of the above continuous casting equipment: 150 to 240 mm, casting speed: more than 2.0 m / min, as mold flux, TiO ₂ : 0.5 to 8.0 mass%, MnO: 0.6 to 4.0 mass% and Al ₂ O ₃ : Use a flux that contains at least two components selected from 2.0 to 6.0 mass% and that satisfies the ratio of CaO and SiO ₂ of the remaining components in the range of 1.4 to 2.0. A method for continuously casting slabs of medium carbon steel characterized by the above.

2. 2. The mold flux according to 1 above, wherein the mold flux further comprises a composition containing 4.0 to 12.0 mass% Na, 6.0 to 15.0 mass% F, and 0 to 2.0 mass% MgO in terms of Na ₂ O. Slab continuous casting method for medium carbon steel.

3. 3. The slab continuous casting method for medium carbon steel according to 1 or 2 above, wherein the casting speed is 2.4 m / min or more.

  According to the present invention, even in high-speed continuous casting exceeding 2.0 m / min, it is possible to advantageously prevent the occurrence of surface defects due to vertical cracks, deoxidation products, etc., and as a result, the surface quality is excellent. Medium carbon steel slabs can be produced efficiently.

Hereinafter, the present invention will be specifically described.
Now, the situation where a solidified shell is being formed in the vicinity of the so-called meniscus in the upper part of the continuous casting mold is as follows.
All the heat removal from the molten steel and the solidified shell in the mold is performed through the mold flux between the solidified shell and the mold. Then, a solid phase layer (solidified layer) is formed at the portion of the mold flux in contact with the mold, and a liquid phase is formed at the portion in contact with the solidified shell because the solidification temperature of the flux is exceeded. The heat transfer performed through such a flux layer is determined by the heat transfer resistance between the mold and the flux solidification layer, that is, the contact state between the two, based on the contribution rate.

  Therefore, by using an appropriate mold flux, a crystal phase is generated from the liquid phase on the mold side, and if the thermal resistance with the mold surface corresponding to the solidified layer irregularities of about 10 μm is formed, uniform gentle cooling can be achieved. Is done.

Therefore, the inventors have made extensive studies on what kind of flux is suitable as such a mold flux.
As a result, contrary to the conventional idea, it was found that it is preferable to previously contain an appropriate amount of Al ₂ O ₃ , MnO, TiO ₂ or the like, which was supposed to suppress the crystallization of the mold flux itself, in the flux. Obtained.

That is, the deoxidation product such as Al ₂ O ₃ , MnO, TiO ₂ or the reduction product by SiO ₂ does not always progress uniformly in the width direction. The crystallization characteristics of the mold flux deteriorate with the passage of time and the non-uniformity of cooling increases. However, if such Al ₂ O ₃ , MnO, TiO ₂ is contained in the mold flux in advance, these As a result, it has been found that the deterioration of crystallization characteristics is within an acceptable range from the beginning to the end of casting, and that no vertical cracks are generated.

Here, the appropriate content of Al ₂ O ₃ , MnO, and TiO _{2 in} the mold flux is as follows.
_{TiO 2: 0.5 ~8.0 mass%,} MnO: 0.6 ~4.0 mass% and _{Al 2 O 3: 2.0 ~6.0 mass} %
If the content of any of the above oxides is less than the lower limit, the crystallization characteristics of the mold flux are also deteriorated as the casting time elapses. On the other hand, if the upper limit is exceeded, the crystallization suppression effect of the mold flux is strong. Thus, it becomes difficult to crystallize the mold flux, and in any case, there is a disadvantage that the non-uniformity of cooling is increased.
Both TiO _2, MnO and Al ₂ O ₃ acts on the direction of inhibiting the crystallization of mold flux, initial if not each contain lower amounts (before use), used immediately after the start and TiO _2, When either MnO or Al ₂ O ₃ begins to concentrate, the crystallization behavior of the flux or the solidification rate changes greatly, and the designed heat transfer and lubrication characteristics cannot be obtained stably.

Further, the ratio (basicity) of CaO and SiO ₂ needs to be controlled in the range of 1.4 to 2.0.
This is because if the basicity is less than 1.4, when the mold flux flows between the mold and the slab and forms a liquid phase and solid phase film, the solid phase does not crystallize sufficiently and the mold is unevenly distributed. Longitudinal cracks are easily generated, and if it exceeds 2.0, crystallization is promoted more than necessary, and the liquid phase necessary for lubrication cannot be maintained. This may cause a breakout.

Furthermore, uniform slow cooling characteristics can be improved by including appropriate amounts of Na, F, and MgO in the mold flux.
Na: 4.0 to 12.0 mass% in terms of Na ₂ O
Na acts in the direction of promoting crystallization of the mold flux, and the effect becomes remarkable from 4.0 mass%. However, if it exceeds 12.0 mass% and is contained excessively, the freezing point of the mold flux is lowered, and the described effect of uniform cooling cannot be obtained.
F: 6.0 to 15.0 mass%
F also has the same effect as Na. That is, by containing 6.0 mass% or more, crystallization is promoted. On the other hand, if it exceeds 15.0 mass%, the solidification temperature and viscosity are lowered, and desirable heat transfer and lubrication characteristics cannot be obtained.
MgO: 0 to 2.0 mass%
Unlike the two components described above, MgO has an action of inhibiting the crystallization of the mold flux. For this reason, it is preferable to make it contain in 2.0 mass% or less.

  By the way, the present invention assumes high-speed casting with a casting speed of over 2.0 m / min. By doing so, it is possible to advantageously avoid deoxidation products, mold flux, etc. (hereinafter simply referred to as foreign matter) being trapped by the solidified shell, and as a result, surface defects caused by such foreign matter are advantageously avoided. It was also investigated that it could be prevented.

Hereinafter, the process of elucidating this point will be described.
(1) Decrease in foreign matter trapping locations By increasing the casting speed Vc, the formation of meniscus initial solidified shells, so-called “nails”, is remarkably suppressed. This is because the solidified shell thickness at the same depth below the molten metal surface becomes thinner as Vc increases, so the force pressed against the mold side due to the influence of the molten steel static pressure is due to the heat shrinkage of the shell depending on the solidified shell thickness. This is because the nail is larger than the force of the nail to fall to the side. In addition, as the slab thickness decreases, the absolute value of shell shrinkage in the thickness direction (= slab thickness x temperature difference x linear expansion coefficient) decreases, so that the collapse to the molten steel side is further suppressed. The fall-suppressing effect becomes even more pronounced.

(2) Suppression of foreign matter adsorption Along with solidification, an interfacial tension gradient occurs due to segregation of the solute concentrated at the solidification interface, and this force causes a phenomenon that foreign matter is easily attracted and trapped at the solidified shell interface. . For this reason, attempts have been made to reduce the concentration of S, Ti, etc., which have a particularly large influence as a solute element that increases the force for attracting and capturing foreign matter. However, there is a problem that manipulating the components leads to cost increase (lower S) and material deterioration (Ti reduction).
In the present invention, by increasing the casting speed Vc, it is possible to suppress an increase in the force for sucking / capturing foreign matter into the solidified shell. That is, in the high-speed casting as in (1), the amount of solidification of the meniscus portion is further reduced, so that the amount of segregation is also reduced, and hence the interfacial tension gradient that is a suction force for foreign matters is also reduced. As a result, the amount of foreign matter adsorbed and captured on the solidified shell side is also suppressed.

(3) Decrease in trapping thickness of foreign matter When continuous casting is performed under the conditions of the present invention, foreign matter is trapped by the shell within 20 mm below the molten metal surface, and the trapping depth becomes shallower as the casting speed increases, and the casting speed Vc> 2.0. At m / min, the capture depth h from the slab surface is 1 mm or less.
Below this depth, even if the foreign matter is trapped by the shell, the foreign matter is dropped and removed together with the oxide scale on the surface of the slab in the process of becoming a product through the subsequent hot rolling → cold rolling process. Therefore, it is possible to obtain a defect-free product without performing slab maintenance. Note that when the casting speed is 2.4 m / min or more, the claw depth is 0.7 mm or less, that is, the foreign matter capturing thickness h is also less, so the casting speed is more preferably 2.4 m / min or more.

(4) Decrease in foreign matter trapping probability The residence time of the solidified shell within 20 mm below the molten metal surface where foreign matter is easily trapped by the solidified shell becomes shorter as the casting speed increases. Therefore, even if the amount of foreign matter is the same, the probability of being trapped by the solidified shell is reduced. For example, in the case of Vc = 3.0 m / min, the probability of trapping foreign matter is halved compared to the case of Vc = 1.5 m / min.

  Therefore, in the present invention, the casting speed Vc is limited to more than 2.0 m / min, preferably 2.4 m / min or more.

(5) Prevention of short side bulging (reason for upper limit on short side length of casting space in mold)
If the short side length becomes too thick, when the casting speed Vc exceeds 2.0 m / min, problems of slab shape and breakout due to short side bulging occur. In this respect, when the short side length is small or Vc is small, bulging due to the molten steel static pressure on the short side of the slab after exiting the mold can be suppressed, and the risk of occurrence of breakout is low.
However, when the short side length (ie, slab thickness) exceeds 240 mm, the secondary flow rate increases due to electromagnetic brake braking due to the increase in the molten steel jet velocity from the immersion nozzle discharge hole due to the increase in the slab thickness even when the casting speed is 2.4 m / min. Therefore, it becomes difficult to suppress the delay in the growth of the short side shell, the short side bulging at the lower end of the mold becomes remarkable, and the risk of breakout (bulging amount ≧ 10 mm) increases.
If the short side length (ie, slab thickness) exceeds 240mm, the reverse flow or secondary flow from the short side of the molten steel jet will promote disturbance of the molten metal surface for the same reason as above. Flux entrainment and biting are likely to occur, and the increase in slab thickness tends to cause stagnation of molten steel in the meniscus area, particularly in the vicinity of the immersion nozzle, increasing slab surface defects and product defects. .

(6) Reason for the lower limit of the short side length of the casting space in the mold If the short side length (slab thickness) D of the casting space in the mold is less than 150 mm, it is not preferable for the following reasons.
That is, if the slab cross-sectional area becomes too small, the amount of fluctuation of the molten metal surface becomes large for the same variation of casting amount due to the problem of molten metal surface controllability, and the nail depth of 1 mm or more caused by the water wrinkles The occurrence frequency of (1) cannot be obtained. In addition, the mold flux is likely to be caught or bitten due to the fluctuation of the molten metal surface. Furthermore, the outer diameter of a general immersion nozzle is determined by the wall thickness (20 mm or more) determined from durability, throughput: 5.4 t / min (150 mm thickness, 2200 mm width, Vc: 2.1 m / min or more) to 14.5 t / min ( 240 mm thickness, 2200 mm width, Vc: ~ 3.5 m / min) is determined by the sum of the inner diameters (70 to 130 mm) determined from the viewpoint of ensuring. Here, if the short side length (slab thickness) D is too small, the distance between the outer wall of the immersion nozzle and the long side solidified shell becomes too narrow (<20 mm), the flow between them becomes uneven, and vertical cracks occur. Cause. In extreme cases, the solidified shell contacts and adheres to the nozzle, leading to breakout. Therefore, the short side length (slab thickness) D is 150 mm (inner diameter: 70 mm + outer wall total thickness: 40 mm (20 × 2) + distance between the outer wall of the immersion nozzle and the long side solidified shell: 40 mm (20 × 2)) or more. There is a need.

  Therefore, in the present invention, the short side length (slab thickness) of the casting space in the mold is limited to the range of 150 to 240 mm.

The long side length (slab width) of the casting space in the mold is not particularly limited, and may be a length for a normal cold-rolled steel sheet, and is preferably about 900 to 2200 mm.
The height of the mold in the vertical direction is not specified, but a solidified shell with a thickness sufficient to prevent bulging of the slab that has passed through the mold even when cast at a casting speed of more than 2.0 m / min. Since it needs to be generated, it is preferable that the thickness is about 800 mm to 1000 mm.

Further, the steel type targeted by the present invention is a so-called medium carbon steel having a C content of 0.07 to 0.22 mass%.
Incidentally, the typical composition of medium carbon steel other than C is as follows.
Si: 0.02 to 0.75 mass%, Mn: 0.3 to 2.1 mass%, P: 0.001 to 0.095 mass%, S: 0.001 to 0.020 mass%, Al: 0.01 to 0.065 mass%, Ti: 0.001 to 0.30 mass%.

The composition of the cast molten steel includes C: 0.10 mass%, Mn: 0.20 mass%, Si: 0.3 mass%, P: 0.01 mass%, S: 0.01 mass%, and Al: 0.01 mass%, and the balance is substantially The composition of Fe. In addition, the degree of superheated molten steel in the tundish was 21 ° C. As the mold flux, those shown in Table 1 and Table 2 were used. The balance of the flux is CaO and SiO ₂ indicated by the basicity ratio in the table. The casting speed was 1.5 to 3.0 m / min, but the results were obtained at 2.1 and 2.4 m / min, where slab lengths that were sufficiently comparable under each condition were obtained.

The survey items are as follows.
The crystallization of the mold flux, which is considered to be causally related to the occurrence of vertical cracks, was investigated, and the ratio of crystals in the flux film was determined by the X-ray diffraction method.
When using a flux having a high composition ratio of CaO and SiO ₂ as in the present invention, the solidified shell is restrained in the mold due to poor lubrication at the beginning of casting, and sometimes breaks out. As the percentage of the number of slabs per 1000 m, it indicates how much traces of the solidified shell are present on the slab surface. Moreover, the frequency of occurrence of vertical cracks is shown as a percentage of the number of vertical cracks per 1000 m of slab.

First, a description will be given using Table 1. In the table, as a comparative example, the case where the composition ratio (basicity) of CaO and SiO ₂ and the amounts of TiO ₂ , MnO, and Al ₂ O ₃ of the mold flux are larger and smaller than the applicable range of the present invention are listed. .

As shown in Table 1, when the composition ratio of CaO and SiO ₂ was less than 1.4 (Comparative Example 1), a sufficient vertical crack prevention effect was not obtained. In addition, when the composition ratio of CaO and SiO ₂ exceeded 2.0 (Comparative Example 2), it was confirmed that restraint of the solidified shell occurred at the initial stage of casting although the effect of suppressing vertical cracking was obtained.
In contrast, in Invention Examples 1 and 2, stable casting and suppression of vertical cracks were realized. Similarly, as shown in Invention Examples 3 to 34, when the amount of TiO ₂ , MnO, Al ₂ O ₃ in the mold flux satisfies the conforming range, there is no frequency of occurrence of vertical cracks, at least compared. The remarkable improvement effect is shown with respect to Examples 1-34.

Next, it demonstrates using Table 2. FIG. In the table, as a comparative example, Na ₂ O, F, and MgO are listed as being larger or smaller than the applicable range of the present invention.

In Comparative Examples 35 to 74 in which Na ₂ O, F, and MgO deviate from the applicable range of the present invention, since the ratio of crystals in the solidified phase was small and a uniform cooling effect was not obtained, the occurrence frequency of vertical cracks remained high. Met.
On the other hand, in the inventive examples 35 to 42, the crystal ratio in the flux film was higher than that in the comparative example, and stable casting and suppression of vertical cracks could be realized.

Claims (3)

When producing medium carbon steel slabs with a C content of 0.07 to 0.22 mass% using continuous casting equipment,
The short side length of the casting space in the mold of the above continuous casting equipment: 150 to 240 mm, casting speed: more than 2.0 m / min, as mold flux, TiO ₂ : 0.5 to 8.0 mass%, MnO: 0.6 to 4.0 mass% and Al ₂ O ₃ : Use a flux that contains at least two components selected from 2.0 to 6.0 mass% and that satisfies the ratio of CaO and SiO ₂ of the remaining components in the range of 1.4 to 2.0. A method for continuously casting slabs of medium carbon steel characterized by the above. _{2. The} composition according to claim 1, wherein the mold flux further comprises a composition containing 4.0 to 12.0 mass% Na, 6.0 to 15.0 mass% F, and 0 to 2.0 mass% MgO in terms of Na ₂ O. Continuous carbon steel slab casting method.   The slab continuous casting method for medium carbon steel according to claim 1 or 2, wherein the casting speed is 2.4 m / min or more.