JP2000317582A - Method for continuously casting beam blank - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a continuous casting method, with which a beam blank excellent in the internal quality can be produced without developing center segregation, center porosity and internal crack. SOLUTION: In the producing method of the beam blank 10 by using a rolling reduction apparatus 9 provided with horizontal rolls 2-1, 2-2 and vertical rolls 3-1, 3-2, executing the rolling reduction to the position including non-solidified part 8 in a cast slab 1 having cross section of rectangle-shape from the upper and lower direction and holding the bulge of the cast slab from the right and left direction, the rolling reduction is executed under condition, in which an internal draft ε shown with ε=(η/100)×(R/L) (wherein, η internal penetrance (%), R: the quantity of rolling reduction (mm) and L: the thickness of non- solidified part at the starting time of the rolling reduction (mm)) becomes 0.25-1.25.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a continuous casting method for producing a beam blank used as a material such as an H-section steel.

[0002]

2. Description of the Related Art With the demand for higher-rise buildings and improved seismic performance, it has been required to increase the strength and quality of H-section steel used for building materials and the like. In order to satisfy such characteristics required for the H-section steel, the beam blank, which is a material thereof, is required to have excellent internal quality free from component segregation, porosity, and internal cracks.

On the other hand, in the production of H-section steel, the purpose of reducing the production cost is to omit the slab rolling step of producing a beam blank of the material from the steel ingot, to improve the heating efficiency of the material in a heating furnace, and to reduce the cost of the material. Attempts have been made to reduce the number of rolling passes in the product rolling process of rolling. Therefore, a method of directly manufacturing a beam blank of a raw material by continuous casting, or a method of manufacturing by rolling down a slab or a bloom such as a bloom in a continuous casting machine has been proposed.

[0004] In Iron and Steel, 67 (1981) 8, p. 284, a method of directly producing a slab having a cross section of a beam blank by continuous casting has been proposed. However, in this method, center segregation and center porosity may occur near the center of the thickness of the slab.

The term "center segregation" refers to a state in which segregation components such as C, P, S, and Mn are concentrated near the center of the thickness of the slab. The molten steel in which the segregated components are concentrated with the progress of solidification moves and accumulates near the center of the thickness of the slab, which is the final solidified portion, due to shrinkage during solidification and the like, and is generated by solidification as it is. Also, the center porosity is generated because the molten steel in which the segregated component is concentrated does not easily flow in the final solidification part, and solidification is completed without replenishing the molten steel in which the segregated component is concentrated in a narrow gap caused by shrinkage. A hole. Therefore, as a countermeasure against these center segregation and center porosity, measures to reduce the movement and accumulation of molten steel with concentrated segregation components by rolling down the position near the final solidified part of the slab with a pair of rolling rolls are effective. It is a target.

In Japanese Patent Application Laid-Open No. 8-281301, a universal rolling mill is installed at the rear end of a bloom continuous caster having a rectangular cross section, and a beam blank is lowered by reducing the position where an unsolidified portion of a slab exists. Manufacturing methods have been proposed. However, in this method, when the rolling start time and the rolling amount are not appropriate, there is a problem that a center segregation or a center porosity occurs in the beam blank after the rolling, and further, an internal crack occurs.

[0007]

As described above, in the conventional continuous casting method in which the beam blank is manufactured by reducing the position where the unsolidified portion of the slab is present, the reduction start timing and the reduction amount are not appropriate. Another problem is that center segregation, center porosity, and internal cracks occur in the beam blank after rolling.

[0008] It is an object of the present invention to provide a continuous casting method capable of producing a beam blank excellent in internal quality, free of center segregation and center porosity and free of internal cracks.

[0009]

SUMMARY OF THE INVENTION The gist of the present invention is to reduce the position of an unsolidified portion of a slab having a rectangular cross section by using a rolling device having a horizontal roll and a vertical roll, from the vertical direction, In a method of manufacturing a beam blank by suppressing swelling of a slab from the left and right directions, an internal draft ratio ε represented by the following equation (1) is 0.25 to 1.25.
The method is a continuous casting method of a beam blank that is reduced under the following conditions.

[0010] ε = (η / 100) × (R / L) ··· (1) _{where, η = Σ (a n ×} S n) × (D / 210) b R: rolling reduction (mm) L : The thickness of the unsolidified portion in the slab thickness direction (m
m) eta: Internal penetration (%) n: 0,1 or _{2 a n: a 0, a} 1 or _{a 2 a 0 = 23.5, a} 1 = 0.1, a 2 = 0.025 S: Area ratio of unsolidified portion at the start of reduction with respect to total area of cross section of cast slab (%) D: Diameter of horizontal roll (mm) b = 0.031 × {C content of steel (% by weight)} + 0.49 The internal reduction ratio ε is represented by the ratio of the reduction amount R of the slab in consideration of the internal permeability η to the thickness L of the unsolidified portion at the start of the reduction, as shown in the above equation (1). When the internal reduction ratio ε is a large value, the reduction amount R of the slab is larger than the thickness L of the unsolidified portion at the start of the reduction, and conversely, when the internal reduction ratio ε is a small value, the reduction starts. When compared to the thickness L of the unsolidified part at the time,
A small amount of reduction R will be applied.

[0011] The start of rolling means the time when the rolling force starts to be applied to the slab, and is the moment when the rolling roll comes into contact with the slab. The thickness L of the unsolidified portion at the start of rolling is the thickness of the unsolidified portion in the slab thickness direction between the solidified shells on both sides exceeding the solid phase ratio of 0.99.

The internal permeability η means the ratio of the amount of reduction applied to the slab from the outside to the unsolidified portion, and can be expressed by an equation obtained by regression calculation based on experimental results. That is, the internal permeability η is determined by the area ratio S of the area A _L of the unsolidified portion at the start of the reduction to the area A ₀ of the slab cross section before the start of the reduction, the diameter D of the horizontal roll, and the C content of the steel. It can be expressed in quantity. In the above equation (1), n, an and b are obtained by regression calculation of equations and numerical values suitable for explaining the phenomenon.

Here, the area ratio S of the unsolidified portion at the start of the reduction in the cross section of the slab is expressed by the following equation.

S (%) = (A _L / A ₀ ) × 100 The present inventors have solved the above-mentioned problem of the present invention as follows. That is, in the method of the present invention, when the beam blank is manufactured by reducing the position where the unsolidified portion of the slab is present, the reduction is performed under the condition that the internal reduction ratio ε is 0.25 to 1.25.

When the value of the internal draft ratio ε is a small value such as less than 0.25, the rolling start timing is too early, the thickness of the unsolidified portion at the start of rolling is large, and the amount of rolling is reduced. This corresponds to the case where the thickness is smaller than the thickness of. Under such a reduction, an unsolidified portion having a large thickness remains inside the slab after the reduction. Therefore, center segregation and center porosity occur with the progress of the subsequent solidification. Further, as described later, an internal crack may occur in the slab. Immediately after the start of the rolling, a tensile force acts on the portion of the solidified shell facing the unsolidified portion, and a crack is generated in the solidified shell, and the molten steel with the segregated component concentrated is sucked into the cracked portion. However, since the amount of reduction is small, the sucked concentrated molten steel is not squeezed out by the completion of the reduction but solidifies as it is. Cracks occur in the sucked portion of the concentrated molten steel.

When the value of the internal draft ratio ε is a large value exceeding 1.25, the rolling start timing is too late, the thickness of the unsolidified portion at the start of rolling is small, and the reduction amount is small. This corresponds to the case where the thickness is larger than the thickness. Under such reduction, the reduction load of the reduction roll becomes too large, and it becomes difficult to pull out the slab. Even if the slab is pulled out, the target shape and dimensions of the beam blank may not be obtained.

The internal draft ratio ε is 0.25 to 1.25.
By rolling down under the following conditions, it is possible to prevent internal cracks of the slab during rolling down. This is because when the stress state of the solidification interface receiving the reduction becomes a compression state, the concentrated molten steel sucked into the cracked portion of the solidified shell generated immediately after the start of the reduction is squeezed out by the time the reduction is completed.

As described above, the rolling start timing and the rolling reduction are set within appropriate ranges, and the thickness of the unsolidified portion at the time of the rolling start,
By setting the amount of reduction and the ratio of the amount of reduction to the thickness of the unsolidified portion under appropriate conditions, a beam blank free from center segregation or center porosity and free from internal cracks can be obtained.

[0019]

FIG. 1 is a view showing an example of a continuous casting apparatus for carrying out the method of the present invention. Here, the slab 1 having a rectangular cross section including the unsolidified portion 8 continuously drawn from the mold 4 is water-cooled in the secondary cooling zone 5,
After passing through the guide roll pair 6, it is pulled out by the pinch roll 7. Then, a horizontal roll 2 that can be rolled down from above and below
-1, 2-2 and vertical rolls 3-1 and 3-2 (3-2) capable of suppressing the swelling of the slab from the left and right directions.
Is not visible in the drawing), the slab 1 is reduced by the reduction device 9 provided with the device. The pressed slab 1 becomes the beam blank 10.

When the slab 1 is pressed down in the vertical direction and the bulge is pressed down in the horizontal direction, the slab 1 may be pressed down and pressed down simultaneously as shown in FIG. The slab 1 may be lowered with an interval such that the 2-2 and the vertical rolls 3-1 and 3-2 for pressing from the left and right directions are adjacent to each other in the casting direction.

The use of a rolling device having a horizontal roll and a vertical roll is used when a horizontal roll is pressed down near the center of the width of the long side of the slab to form a slab having a rectangular cross section into a beam blank. This is because the swelling of the short side of the slab is suppressed by a vertical roll.

FIG. 2 is a diagram showing a positional relationship between the internal state of the slab and the horizontal roll and the vertical roll at the start of rolling. The symbol A _L indicates the cross-sectional area of the unsolidified portion of the slab at the start of rolling, and the symbol A ₀ indicates the total cross-sectional area of the slab before starting the rolling. Reference numeral 11 is a line having a solid phase ratio of 0.99. The solid phase ratio can be determined from the temperature at each position of the slab by heat transfer solidification analysis, and from the results and the liquidus temperature and solidus temperature specific to the steel. The rolling device 9 is used for the slab 1
Horizontal roll 2 for rolling down the position where the unsolidified portion 8 is present
1, 2 and a vertical roll 3 for suppressing the swelling of the slab
-1, 3-2. Horizontal roll 2-D
The thickness of the unsolidified portion at the start of rolling is L
Is reduced.

As shown in FIG. 2, the horizontal length of the horizontal roll is preferably shorter than the width of the unsolidified portion of the slab in the width direction. This is because the rolling load of the rolling roll may be small. The length of the vertical roll in the height direction is preferably longer than the thickness of the slab. This is because the swelling of the slab can be stably suppressed.

FIG. 3 is a diagram showing the internal state of the slab during rolling and the positional relationship between the horizontal roll and the vertical roll. This shows a situation in which a slab having a rectangular cross section is substantially formed into a beam blank.

In the method of the present invention, the position where the unsolidified portion of the slab having a rectangular cross section is present is reduced under the condition that the internal reduction ratio ε represented by the above equation (1) is 0.25 to 1.25. I do.

FIG. 4 is a diagram showing the effect of the internal draft ratio ε on the center segregation and the center porosity. The results of a test in which the thickness of the unsolidified portion at the start of reduction was 30 mm and 60 mm, and the value of the internal reduction ratio ε was changed are shown. It can be seen that the center segregation and the center porosity are remarkably improved when the internal draft ratio ε is 0.25 or more. When the internal reduction ratio ε is less than 0.25, it can be seen that center segregation and center porosity occur in the beam blank after the reduction.

FIG. 5 is a diagram showing the effect of the internal draft ratio ε on the occurrence of internal cracks in the slab. The results of a test in which the thickness of the unsolidified portion at the start of reduction was 30 mm and 60 mm, and the value of the internal reduction ratio ε was changed are shown. Internal cracks
The internal rolling ratio ε is less than 0.25, and the length of the internal crack is maximized when the internal rolling ratio ε is around 0.2. By setting the internal draft ratio ε to 0.25 or more,
It can be seen that the occurrence of internal cracks in the slab during rolling can be prevented. The stress state of the solidification interface receiving the reduction becomes a compressed state, and the concentrated molten steel can be sufficiently squeezed out to the upstream side in the casting direction, so that the occurrence of internal cracks can be prevented.

When the internal draft ratio ε exceeds 1.25, the draft load of the draft roll becomes large, and it becomes difficult to pull out the slab. Even if the slab can be extracted, a beam blank having the desired shape and dimensions cannot be obtained.

Therefore, the internal draft ratio ε is 0.25 to
1.25. More desirable is 0.4-0.7
It is.

FIG. 6 is a diagram showing the effect of the diameter of the horizontal roll on the occurrence of internal cracks in the cast slab and the state of reduction of the horizontal roll. It can be seen that internal cracking occurs when the internal draft ratio ε is less than 0.25, regardless of the diameter of the horizontal roll. Further, it can be seen that, regardless of the diameter of the horizontal roll, when the internal draft ratio ε exceeds 1.25, the draft load becomes large, making it difficult to pull out the slab.

The diameter of the horizontal roll is 200 to 1200 mm
Is desirable. When the diameter is less than 200 mm, the value of the internal permeability η becomes small, so that the effect of improving center segregation and center porosity is small. If it exceeds 1200mm,
The size of the screw-down device increases, and the equipment cost increases.

As shown in FIG. 2, the roll shape of the horizontal roll is round at both ends and the other portions are flat, and the roll shape of the vertical roll is flat to suppress the swelling. The roll width is desirably wider than the thickness of the slab.

Although the number of the pressing devices is not particularly limited, it is usually preferable to provide only one pressing device because providing the pressing devices in multiple stages increases the size of the equipment.

[0034]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Using a continuous casting machine having an apparatus configuration shown in FIG.
A test was conducted in which a bloom having a cross-sectional shape of 300 mm in thickness and 650 mm in width was cast, and a position where an unsolidified portion of the bloom was present was pressed down by a horizontal roll and a vertical roll to produce a beam blank.

The steel used had a C content of 0.10 to 0.1.
With 2% by weight of carbon steel, 0.
Water cooling was performed under the conditions of a specific water volume of 1 to 2.0 liter / kg · steel. The casting speed was changed in the range of 0.8 to 2.0 m / min to change the thickness of the unsolidified portion at the start of rolling.

The thickness of the unsolidified portion was obtained by calculation by one-dimensional radial unsteady heat transfer analysis, and was investigated by adding FeS to molten steel in a mold during casting. The calculation and the thickness of the unsolidified portion actually measured from the sulfur print of the cross section sample of the slab were in good agreement.

The rolling device provided with a horizontal roll and a vertical roll was one unit, and the horizontal roll was disposed at a position 24 m from the meniscus of molten steel. The diameter of the horizontal roll is 300,
Three types of 600 and 900 mm, the width of the horizontal roll is 430 mm, and the corner of the horizontal roll has a radius of 5 to 5 mm.
It was rounded to 50 mm. The amount of reduction by the horizontal roll was changed in the range of 40 to 120 mm.

The diameter of the vertical roll was 400 mm, the body width of the vertical roll was 350 mm, and the roll shape of the vertical roll was flat. When rolling down the long side of the bloom with a horizontal roll, hold down the short side of the bloom with a vertical roll,
The short side of the bloom was prevented from expanding.

In each casting test, after the casting speed became constant, a beam blank having a length of 2 m was sampled in the casting direction, and 20 cross-sectional samples were cut out at intervals of 100 mm in the casting direction.

The state of occurrence of center segregation was examined for five cross-sectional samples. At the center of the width of the cross section sample,
In addition, a cut was taken by drilling from one center of the thickness with a drill blade having a diameter of 5 mm, and the C content was analyzed. The state of occurrence of center segregation was evaluated based on the degree of center segregation C / C ₀ , which is the ratio of the ladle analysis value C ₀ of C to the obtained C content C.

The state of occurrence of center porosity was examined for ten cross-sectional samples. The number and shape of the generated center porosity were visually observed, and the vertical and horizontal dimensions of each center porosity were measured. The shape of the center porosity was approximated to a circle or an ellipse, one center porosity area was determined from the measured vertical and horizontal dimensions, and these were summed to determine the total center porosity area. The ratio of the total area of the center porosity to the cross-sectional area of the cross-sectional sample was defined as the center porosity area ratio (%), and the state of occurrence of the center porosity was evaluated.

Sulfur printing was performed using the remaining five cross-sectional samples, and the occurrence of internal cracks was investigated.
Cracks were visually observed, and the degree of occurrence of the cracks was evaluated as none, slight, or remarkable.

Further, the state of the bloom pulling operation during the bloom reduction was observed, and it was evaluated whether or not pulling out was impossible. Tables 1 and 2 show test conditions and test results.

[0044]

[Table 1]

[0045]

[Table 2]

Test No. of the present invention example 1 to No. In 5,
The value of the internal draft ratio ε is set to 0.39 within the range specified in the present invention.
The pressure was reduced at 0.60. The center segregation degree C / C ₀ of the obtained beam blank was 0.98 to 0.99, and no center segregation occurred. In addition, the center porosity area ratio was as small as 0.01 to 0.02%, and further, there was no occurrence of internal cracks, and a beam blank of good internal quality was obtained.
Also, the pulling out work was well.

Test No. of Comparative Example In 6, the internal reduction ratio ε
Was tested at a small value of 0.19 outside the lower limit of the conditions specified in the present invention. The center segregation degree C / C ₀ is 1.5, remarkable center segregation occurs, the center porosity area ratio is as high as 2.8%, and internal cracks are remarkably generated, resulting in poor internal quality. It was a beam blank. The drawing operation was successful.

Test No. of Comparative Example 7 and No. 7 In No. 8, the internal draft ratio ε was tested at large values of 1.28 and 2.43 outside the upper limit of the conditions specified in the present invention. In each case, the rolling load of the horizontal rolls became large, making it difficult to pull out the slab, and the casting operation was stopped halfway.

[0049]

According to the method of the present invention, it is possible to produce a beam blank which is free from center segregation and center porosity and free from internal cracks and has excellent internal quality. Further, the manufacturing cost of the H-section steel can be reduced.

[Brief description of the drawings]

FIG. 1 is a view showing one example of a continuous casting apparatus for carrying out a method of the present invention.

FIG. 2 is a diagram showing the internal state of a slab and the positional relationship between a horizontal roll and a vertical roll at the start of rolling.

FIG. 3 is a view showing an internal state of a slab during rolling and a positional relationship between a horizontal roll and a vertical roll.

FIG. 4 is a diagram showing the effect of the internal draft ratio ε on center segregation and center porosity.

FIG. 5 is a diagram showing the effect of the internal draft ratio ε on the maximum length of internal cracks.

FIG. 6 is a diagram showing the influence of the diameter of a horizontal roll on the occurrence of internal cracks in a slab and the state of reduction load of the horizontal roll.

[Explanation of symbols]

1: Slabs 2-1 and 2-2: Horizontal rolls 3-1 and 3-2: Vertical rolls 4: Mold 5: Secondary cooling zone 6: Guide roll pair 7: Pinch roll 8: Non-solidified portion 9: Reduction Apparatus 10: Beam blank 11: Line of solid phase ratio 0.99 A _L : Cross-sectional area of unsolidified portion of slab at the start of rolling A ₀ : Cross-sectional area of slab before starting of rolling L: Not at the start of rolling Solidification part thickness

Claims (1)

[Claims] 1. Using a rolling device having a horizontal roll and a vertical roll, a position where an unsolidified portion of a slab having a rectangular cross section is present is lowered from the vertical direction, and the swelling of the slab is suppressed from the left and right directions. In the method of manufacturing a beam blank according to (1), the internal draft ratio ε represented by the following equation (1) is set to 0.1.
A continuous casting method for a beam blank, wherein the reduction is performed under a condition of 25 or more and 1.25 or less. ε = (η / 100) × (R / L) ··· (1) _{where, η = Σ (a n ×} S n) × (D / 210) b R: rolling reduction (mm) L: reduction start Thickness of unsolidified part in the direction of slab thickness (m
m) eta: Internal penetration (%) n: 0,1 or _{2 a n: a 0, a} 1 or _{a 2 a 0 = 23.5, a} 1 = 0.1, a 2 = 0.025 S: Area ratio of unsolidified portion at the start of reduction with respect to total area of cross section of cast slab (%) D: Diameter of horizontal roll (mm) b = 0.031 × {C content of steel (% by weight)} + 0.49