JP3427794B2 - Continuous casting method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method in which post-bulging reduction is performed which can reduce the occurrence of internal cracks, surface cracks and center segregation of a steel slab. In particular, the present invention relates to a continuous casting method for hypoperitectic steel slab, which is a material for thick steel plates in which center segregation is a serious problem.

[0002]

2. Description of the Related Art When a steel slab is manufactured by a continuous casting method, the occurrence of internal defects called center segregation often becomes a problem. The occurrence of this center segregation is a phenomenon in which the constituent elements such as C, Mn, S, and P in the molten steel are concentrated and positively segregate in the final solidified portion in the center of the slab in the thickness direction. This phenomenon is a particularly serious problem in thick steel plates, and is known to cause a decrease in toughness in the segregated portion and hydrogen-induced cracking.

The cause of such center segregation is that molten steel in which constituent elements are concentrated remains between dendrites at the final stage of solidification and solidifies as it is at the center of the slab in the thickness direction, and during solidification. This is because the molten steel moves macroscopically toward the solidification completion point of the final solidification portion due to the molten steel flow caused by swelling of the slab called contraction or bulging. Therefore, as a measure for preventing the center segregation, it is effective to reduce the movement of the concentrated molten steel between the dendrites and to prevent the localized accumulation of the concentrated molten steel.

As described above, when bulging occurs in the cast piece during casting, it is said that center segregation occurs. However, the center segregation is prevented by positively bulging the cast piece and then pressing it down. A method (hereinafter, this method is referred to as “continuous casting method in which rolling is performed after bulging”), for example, the following method has been proposed (see FIG. 1).

Liquid line crater end 8a of mold 1 and slab 2
A bulging force is applied to the solidification shell 2a between the liquid phase crater end 8a and the solid phase line crater end 8a.
Continuous casting method in which reduction is applied to the slab between and (Japanese Patent Laid-Open No. 60-6
(See Japanese Patent No. 254).

[0006] In continuous casting of a slab 2 having an aspect ratio of 1.6 or less, a plurality of sets of guide rolls (3a-
3n) the gap in the thickness direction of the slab of a pair of rolls (hereinafter, simply referred to as "roll gap") is made wider than the inner thickness of the lower end of the mold to cause bulging in the thickness direction of the slab, and at the rear of the other A method for producing a slab in which the slab is rolled down by 0.04 to 10% by the roll (see JP-A-60-21150).

Guide rolls (3a to 3n) arranged in the drawing direction from directly below the mold 1 gradually increase the roll gap in the thickness direction of the slab 2 to cause bulging of the slab, A continuous casting method of a cast piece in which the thickness is made 2 to 3 times the short side of the mold, and then lightly rolled down by a small-diameter roll near the crater end 8 (see JP-A-1-178355).

Bulging is caused at a position where the solid fraction in the center of the slab 2 is 0.1 or less, the maximum thickness of the slab is made 20 to 100 mm thicker than the short side length of the mold 1, and the solidification completion point 8 Immediately before, a reduction of 20 mm or more is applied to each pair of the reduction rolls 4a, and a continuous casting method is performed to reduce the amount corresponding to the bulging amount (JP-A-9-57410).
(See Japanese Patent Publication).

[0009] While the unsolidified thickness of the slab 2 is up to a position of 30 mm or more, bulging is caused in the slab, and the maximum thickness of the slab is increased by 10 to 50% of the short side length of the mold 1 to solidify. A continuous casting method in which at least a pair of reduction rolls is used to give a reduction with a reduction gradient of 80 mm / m or more per slab length until just before completion, and a bulging amount is reduced (see Japanese Unexamined Patent Publication No. 9-206903).

[0010]

When the continuous casting method of rolling down after bulging, which has been proposed from the above, is applied to a curved type continuous casting apparatus, cracks may occur on the surface of the slab depending on the type of steel. There is a problem that the maintenance man-hours increase. The curved type continuous casting apparatus is an apparatus that bends and draws a slab at a right angle from the longitudinal direction of the mold in which the drawing direction is substantially vertical. These include vertical bending type, vertical progressive bending type, arc bending type and multi-step arc bending type (3rd edition, Iron and Steel Handbook II, Iron and Steel Making, p. 624).

It is an object of the present invention to provide a method for reducing center segregation without causing surface cracks in a slab in any continuous steel casting method using a curved continuous casting apparatus in a continuous casting method in which post-bulging reduction is performed. To provide.

[0012]

Surface cracking of a slab, which is caused by a continuous casting method in which rolling is performed after bulging, occurs when a specific carbon steel reaches a specific surface temperature. To prevent this, the surface temperature of the slab from the mold to the straightening point should be increased, but if the surface temperature of the slab is increased, central segregation increases. Therefore, the inventors have found that it is better to raise the surface temperature of the slab near the straightening point and lower the surface temperature of the slab at the reduction position, and completed the present invention.

The gist of the present invention resides in the continuous casting method in which the following post-bulging reduction is carried out.

A bulging is caused in the slab by widening the roll gap of a group of guide rolls arranged in a predetermined range between a position corresponding to the liquidus line crater end 8a and a position corresponding to the solidus line crater end 8 of the slab, Then, a continuous casting method in which a rolling amount corresponding to the amount of bulging is applied by at least a pair of rolling rolls 4a, and a slab straightening point 3i from directly below the mold 1 is used.
The surface temperature of the slab in the secondary cooling zone A to the 1000 ° C. or higher, a method for further molding a surface temperature of the slab as 1000 ° C. or less in the secondary cooling zone B from straightening point 3i to the position to start rolling .

The following method is one of the methods for adjusting the surface temperature of the slab as described above. Cooling strength in the secondary cooling zone A from directly below the mold 1 to the slab straightening point 3i, that is, the specific water content is 1.0 (liter / 1kg steel) or less, and the secondary cooling from the straightening point 3i to the position where the reduction starts. A method of cooling the slab with a specific water content between strips B of 0.2 to 1.5 (liter / 1kg steel).

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of investigating a slab cast by a continuous casting method in which rolling is performed after bulging, the inventors find that cracks are observed on the surface when a curved type continuous casting apparatus is used. That is, the carbon equivalent (C
p) is hypoperitectic steel from 0.1% to 0.2% by weight,
I found a thing. In the following description, the notation of the component content of steel is% means% by weight.

Cp = [% C] +0.02 [% Mn] +0.04 [% Ni] −0.01 [% Si] +0.02 [% Cr] +0.07 [% S] (a) and In order to prevent surface cracking of the slab, it is effective to raise the surface temperature of the slab at the straightening point of the curved portion to 1000 ° C or higher, but if the surface temperature of the slab in the reduction zone is high, center segregation will occur. I found it to be noticeable.

Therefore, according to the method of the present invention, when casting is performed by using a vertical bending type continuous casting apparatus to perform reduction of hypoperitectic steel after bulging, the surface temperature of the slab from the mold to the straightening point is increased to reduce the reduction area. It was decided to lower the surface temperature of the cast slab. These temperature adjustments are performed by adjusting the amount of cooling water.

FIG. 1 is a diagram schematically showing an example of a vertical curve type continuous casting apparatus for carrying out a continuous casting method in which the bulging is followed by reduction.

The continuous casting method of the present invention is a method in which the continuous casting equipment as shown in FIG. It should be noted that the method of the present invention can be carried out by a curved continuous casting apparatus or a vertical bending continuous casting apparatus. However, the principle of bulging and the subsequent rolling is basically the same in any type of continuous casting apparatus, and therefore the apparatus shown in FIG. 1 will be described as an example.

The continuous casting apparatus is provided with a group of guide rolls (3a to 3n) and a reduction roll device (4a to 4n) arranged in the drawing direction of the slab 2 from immediately below the mold 1. In the guide roll group, for example, the guide rolls 3d to 3n can widen the roll gap by 20 to 100 mm in the thickness direction of the slab. The reduction roll devices (4a to 4n) are composed of at least a pair of reduction rolls and can apply a reduction amount corresponding to the bulging amount.

In the present invention, as shown in FIG. 1, the guide rolls 3d to 3n are bulging zones, and the rolling rolls 4a to 4n.
Up to 4n is referred to as a rolling zone, the position of the guide roll 3c is referred to as a bending point, and the position of the guide roll 3i is referred to as a straightening point. Further, the range from just below the mold to the straightening point is called the secondary cooling zone A, and the range from the straightening point to the rolling start point is called the secondary cooling zone B.

The molten steel 5 is injected into the mold 1 through the immersion nozzle 6 and is sprayed with water between the mold 1 and the spray nozzle group provided between the guide roll groups (3a to 3n) arranged below the mold 1. It is cooled in the secondary cooling zone A and the secondary cooling zone B by the cooling water injected from (not shown) to form the solidified shell 2a, which becomes the cast piece 2 and is pulled out by the pinch roll group 7.

In the method of the present invention, as shown in FIG. 1, since a predetermined range between the liquidus crater end 8a and the solidus crater end 8 is bulged, the roll in the thickness direction of the slab in that section is bulged. Expand the gap. This expansion is preferably performed stepwise (that is, the roll gap gradually increases from the top to the bottom). The slab 2 in this zone
While being pulled out while holding the unsolidified portion 2b inside, so-called bulging occurs, in which the central portion on the long side thereof swells. Thereafter, the slab is subjected to a bulging-corresponding amount of reduction in the vicinity of the solidus line crater end 8 by a reduction roll (4a to 4n), and then pulled out by a pinch roll. On this occasion,
The slab from immediately below the mold to the straightening point is slowly cooled to prevent cracks from occurring on the surface, and further strong cooling from the straightening point to the end point of the rolling is performed and the reduction is applied to prevent the occurrence of center segregation.

Next, the reason for defining the specific water content will be described.

(1) The surface temperature of the slab in the secondary cooling zone A is 1000 ° C.
About the above: Carbon equivalent (Cp) from 0.1%
Hypoperitectic steels up to 0.2% are steels that are highly susceptible to cracking with temperature, are brittle in the temperature range of 850 ° C to 1000 ° C, and cracks frequently occur on the surface of the slab. Therefore, in the case of a curved continuous casting machine, the temperature of the slab up to the straightening point should be at least 1000.
Adjust the amount of cooling water to keep the temperature above ℃. The smaller the specific water content, the higher the temperature of the slab, but the more severe the damage to the guide rolls. Therefore, under normal operating conditions, it is desirable that the specific water content be at least 0.2 (liter / 1 kg steel) or more. However, when the specific water content exceeds 1.0 (liter / 1kg steel), the surface temperature of the slab at the straightening point becomes lower than 1000 ° C, and in the case of hypoperitectic steel, the slab surface cracks. Therefore, the specific water content should be 1.0 (liter / 1kg steel) or less.

(2) Secondary cooling zone B to the position where the reduction starts
In the surface temperature of the slab about to 1000 ° C. or less: cast slab is cast in a curved type continuous casting apparatus, the generation of surface cracks by increasing the surface temperature of the said mold so as to correct point It can be prevented, but the center segregation increases.
This is because the surface temperature of the slab in the reduction zone is high, so the reduction permeability is small even if the reduction is performed after bulging, and the effect of reducing center segregation cannot be obtained.

A curved continuous casting machine shown in FIG. 1 was used, and the specific water content of the secondary cooling zones A and B was 0.05 to 2.25 (liter / steel 1 kg), and the short side length of the cast piece was 240 mm and the long side length was 2000 mm. A slab was cast, and the relationship between the reduction penetration rate, the maximum P segregation degree, and the surface temperature of the reduction section was investigated. The specific water amount in the secondary cooling zones A and B was changed by changing the cooling water amount while keeping the casting speed constant.

The rolling reduction permeability is a value obtained by dividing the rolling amount of the unsolidified portion inside the slab by the rolling amount of the surface of the slab. this is,
It can be determined by adding a tracer or the like and measuring the thickness of the unsolidified portion.

The maximum segregation degree of P was determined as follows.

A plate-like material having a thickness of about 10 mm and having a thickness in the casting direction is cut out from a cross section of the cast piece perpendicular to the casting direction, and a test piece of about 100 mm square is taken from the center of the plate-shaped material.

100μ in the thickness direction of the cast piece on the surface of the test piece is 200μ
Divide into m pitches, measure the phosphorus (P) concentration at the intersection with the EPMA method, and obtain the maximum P concentration (Pmax).

The ratio (Pmax / Pave) of the maximum P concentration (Pmax) and the P concentration (Pave) of the mother molten steel is determined and is taken as the maximum segregation degree.

FIG. 2 is a graph showing the relationship between the reduction penetration rate and the maximum P segregation degree of the slab. As is apparent from this figure, the maximum segregation degree of P decreases as the reduction penetration rate increases. If the maximum segregation degree of P is 2.5 or less, it can be said that the cast piece has no practical problem. Therefore, the rolling reduction may be 40% or more.

FIG. 3 is a diagram showing the relationship between the surface temperature and the reduced penetration rate. This figure is a diagram in which the relationship between the surface temperature of the reduced part of the cast slab and the reduction permeability when the cast slab is reduced is plotted. From this figure, it can be seen that the surface temperature of the pressure-reduced part of the slab needs to be 1000 ° C. or less in order to achieve a reduction penetration of 40% or more.

From FIGS. 2 and 3, it is possible to obtain a good cast product with less segregation by lowering the surface temperature of the straight part of the cast product and increasing the reduction permeability.

In order to lower the surface temperature of the slab, it is necessary to increase the specific water content of the secondary cooling zone B. For that purpose, there are methods such as increasing the amount of water, lengthening the cooling zone, and decreasing the casting speed.

Increasing the amount of water in (1) not only causes an increase in equipment cost but also causes surface defects such as heat cracking when the slab is excessively cooled, so it is necessary to adjust the amount of water to an appropriate range. .

It is difficult for the equipment to lengthen the cooling zone, and it is necessary to find the optimum length.

Since the lowering of the casting speed causes deterioration of productivity, it is desirable to increase the casting speed as much as possible. However, when the casting speed is increased, the surface temperature of the pressure lower part is increased due to the heat recovery to the surface of the slab that occurs in the range where the cooling water is not poured. In addition, since the limit of the casting speed is determined by the length of the casting equipment, if the length of the cooling range of the secondary cooling zone B and the specific water amount according to the casting speed at which normal operation is performed are determined, the appropriate pressure surface temperature Can be obtained.

FIG. 4 is a diagram showing the relationship between the specific water content of the secondary cooling zone B and the surface temperature of the pressure lowering part. This figure shows that the length of the cooling range of the slab to be cooled in the secondary cooling zone B is 5 m, the specific water amount is changed in the range of 0.05 to 2.25 (liter / 1 kg steel), and the surface temperature of the lower part is measured. It is the plotted figure. Here, the casting speed was kept constant at 1.0 m / min, and the slab temperature before entering the secondary cooling zone B was set to 1100 ° C.
The specific amount of water was adjusted.

As is clear from FIG. 4, by setting the specific water amount in the secondary cooling zone B to 0.2 (liter / 1 kg steel) or more,
The surface temperature of the slab in the lower part can be controlled to 1000 ° C or lower. However, if the specific water volume exceeds 1.5 (liter / 1kg steel),
Thermal cracking occurs on the surface of the slab. Therefore, the secondary cooling zone
The specific water content of B was 0.2 to 1.5 (liter / 1kg steel).

[0043]

EXAMPLE Using the vertical bending type continuous casting equipment shown in FIG.
Under the seven kinds of casting conditions shown in Table 1, 40 kg class steel for thick plates (C: 0.16 to 0.18% by weight) was manufactured. Bulging amount
The bulging was performed by a pair of reduction rolls having a diameter of 350 mm and a diameter of 350 mm.

The injection temperature of molten steel is 1545 ° C., and the degree of superheat is 20
℃ was made. The length of the short side is the inner cross-sectional dimension of the mold used.
The length is 250 mm and the long side length is 2000 mm. Casting speed is 1.0m / min
Constant. The specific water contents of the secondary cooling zones A and B were changed as shown in Table 1, respectively. The cooling range of the secondary cooling zone B was set to 5 m. In the table, the mark * means that the casting conditions are out of the range defined by the present invention.

[0045]

[Table 1]

For the evaluation, the surface properties and center segregation of the slab cast under the above conditions were investigated.

The surface texture was obtained by scouring the widthwise surface of the slab 3 mm in the thickness direction and visually observing the surface, and calculating the total surface flaw length (product of flaw length and number, mm). The surface flaw index (mm / mm ² ) was divided by the surface area. The case where the surface flaw index is 0 (zero) is defined as the range defined in the present invention.

As the center segregation, the maximum segregation degree of phosphorus (P) was determined by the above-mentioned method. The results are also shown in Table 1.

As is clear from Table 1, the test numbers 1 to 3 of the present invention are good because the surface flaw index of the slab is 0 and the maximum segregation degree of P is in the range of 1.0 to 2.3.

On the other hand, the test numbers and in the comparative examples have a large specific water content in the secondary cooling zone A of 1.8 and 1.2 (liter / 1 kg steel), so that the surface flaw index of the slab is 8 and 13 (mm / mm ² ) and large. Further, in the test number of the comparative example, since the specific water amount in the secondary cooling zone B is as small as 0.1 (liter / kg steel), the maximum segregation degree of P is as high as 7.4.

As is clear from the above test, when the cooling conditions of the secondary cooling zone A or the secondary cooling zone B deviate from the range defined by the present invention as in the comparative example, surface slabs or center segregation occur in the slab. Occurs, and a good cast piece cannot be obtained.

[0052]

According to the method of the present invention, after the slab is bulged, when the slab is reduced by a bulging-equivalent amount, the specific water content in the secondary cooling zone from directly below the mold to the slab straightening point. By adjusting the length of the cooling range and the specific water amount between the straightening point and the position where the reduction starts, the center segregation of the slab can be prevented. According to the method of the present invention, even if hypoperitectic steel is cast with bulging and then reduced in a curved continuous casting apparatus, a slab without surface cracks and center segregation can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing an example of a vertical-curve continuous casting apparatus that carries out a continuous casting method in which rolling is performed after bulging.

FIG. 2 is a diagram showing a relationship between a reduction penetration rate and a P maximum segregation degree of a slab.

FIG. 3 is a diagram showing a relationship between a surface temperature and a reduced penetration rate.

FIG. 4 is a diagram showing the relationship between the specific water content of the secondary cooling zone 2 and the slab surface temperature.

[Explanation of symbols]

1. Mold 2. Slab 3. Guide roll 4. Rolling device 5. Molten steel 6. Immersion nozzle 7. Pinch roll 8. Solid phase crater end 8a. Liquid phase crater end

─────────────────────────────────────────────────── ─── Continuation of front page (56) Reference JP-A-11-188469 (JP, A) JP-A-8-90182 (JP, A) JP-A-58-58968 (JP, A) JP-A-8- 197193 (JP, A) JP 10-52743 (JP, A) JP 8-132203 (JP, A) JP 4-238660 (JP, A) JP 10-43850 (JP, A) JP-A-8-224650 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B22D 11/128 350 B22D 11/124 B22D 11/16 B22D 11/22

Claims (4)

(57) [Claims] 1. A slab is formed by widening a gap in the slab thickness direction of a group of guide rolls arranged in a predetermined range from a position corresponding to the liquidus crater end to a position corresponding to the solidus crater end of the slab. A continuous casting method in which bulging is caused and then a rolling amount corresponding to the bulging is applied by at least a pair of rolling rolls, and the surface temperature of the slab in the secondary cooling zone from directly below the mold to the slab straightening point is 1000 A continuous casting method characterized in that the surface temperature of the slab is 1000 ° C or lower in the secondary cooling zone from the straightening point of the slab to the position where the reduction is started. 2. A slab is formed by widening a gap in the slab thickness direction of guide roll groups arranged in a predetermined range from a position corresponding to the liquidus crater end of the slab to a position corresponding to the solidus crater end. This is a continuous casting method in which bulging is caused and then reduction is performed by at least a pair of reduction rolls with a reduction amount equivalent to bulging, and the specific water amount in the secondary cooling zone from directly below the mold to the slab straightening point is 1.0 (liter / liter / 1kg steel) or less to cool the slab and further cool the slab to a specific water content of 0.2 to 1.5 (liter / 1kg steel) in the secondary cooling zone from the straightening point of the slab to the position where the reduction starts. Continuous casting method. 3. The slab is hypoperitectic steel,
The continuous casting method according to claim 1 or 2. 4. The carbon equivalent (C) calculated by the following formula (a)
p) is 0.1% to 0.2% by weight of steel
The connection according to any one of claims 1 to 3, characterized in that
Continuous casting method. Cp = [% C] +0.02 [% Mn] +0.04 [% Ni] -0.01 [% Si] +0.02 [%
Cr] ＋0.07 [% S] ・ ・ ・ (a)