JPH07227657A - Method for improving the internal quality of continuous cast slabs by large unsolidified reduction - Google Patents

Abstract

(57) [Summary] [Purpose] To prevent the formation of center segregation and center porosity during continuous casting of rectangular slabs. [Structure] The flatness ratio of the cast slab 1 is set to 1.1 or more. (1)
The rolling-down terminal 4 is provided at a position y that satisfies Expression 1 or Expression (2). The casting rate is adjusted so that the solid fraction in the center of the cross section of the cast piece 1 on the entry side of the rolling-down terminal 4 is 0.6 to 0.95. The radius of curvature in the axial direction of the slab of the part that presses down the slab 1 is 2
80 to 540 mm / using the pressing terminal 4 of 00 mm or more
5 to 40 at a reduction rate in the thickness direction of the cast piece 1 at a reduction rate of min
% The slab 1 is rolled down. ## EQU1 ## When 0 ≦ D <250, 0.02 × D ≦ y (m) ≦ 0.06 × D
(1) When 250 ≦ D 0.02 × D ≦ y (m) ≦ 0.067 × (D-250)
+15 (2) However, D: thickness of the slab (mm)

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention prevents continuous generation of hot and cold working raw materials by preventing the formation of internal defects such as center segregation and center porosity during continuous casting of rectangular slabs. It relates to a method enabling the production of slabs.

[0002]

2. Description of the Related Art As a method for improving segregation of continuously cast slabs, there have been conventionally used a low temperature casting method and a low speed casting method for refining a solidification structure to disperse the segregation, and an electromagnetic wave in a mold or a secondary cooling zone. Although stirring technology has been developed and has contributed to the improvement of center segregation, the effect cannot be said to be sufficient for steel types with severe segregation or segregation strict materials with severe segregation allowable levels, either alone or in combination.

On the other hand, Japanese Patent Publication No. 59-16862, Japanese Patent Publication No. 59-39225, and Japanese Patent Publication No. 62-34460.
JP-B No. 2-56982 and the like disclose a method at the final stage of solidification in which a cast piece is rolled down at the final stage of solidification to suppress the flow of concentrated molten steel due to solidification shrinkage to improve center segregation. ing. Under these end-stage light pressures at the end of solidification, central segregation can be greatly improved, but internal cracking and reverse V segregation may occur, which may worsen segregation, thus limiting the amount of reduction and pressing the center porosity. Is not enough. Furthermore, there is a possibility that an ideal reduction condition will be realized under light pressure and a slab with no center segregation can be manufactured, but it is extremely difficult to actually realize that condition in a stable manner, and it is extremely difficult to make a product. If slight segregation is a problem after processing, it cannot be handled under light pressure, and the ductility of the center of the material is increased as in the case of negative segregation at the center of the cross section of the material, improving hot and cold workability. It won't be much improved.

Further, Japanese Patent Laid-Open No. 61-132247,
JP-A-63-183765, or "iron and steel"
60th pp. 875-884 and "Iron and Steel" 78th 1st
Pages 794 to 1801 disclose a method for improving the center segregation by subjecting a cast piece at the final stage of solidification to a large reduction with a roll or a die. However, in order to improve the center segregation by squeezing the concentrated molten steel by these methods or to squeeze the concentrated molten steel to generate negative segregation in the central part, internal cracks may be generated or the concentrated molten steel may be squeezed out depending on the rolling conditions. If it is incomplete, concentrated molten steel will be trapped and conspicuous segregation will occur, which in turn will cause problems such as deterioration of the internal quality of the slab, and if the position to install the reduction terminal is inadequate In the final part, there is a considerable amount of the portion that cannot be used as a product due to the residual of the concentrated molten steel, and the yield is lowered, resulting in a large increase in manufacturing cost.

[0005]

Generally, the lower the C equivalent of a steel material, the lower the strength and the higher the ductility. Therefore, the center portion of the material is negatively segregated while preventing the occurrence of internal cracks and the capture of concentrated molten steel. Alternatively, by preventing segregation, it is possible to manufacture a material having excellent hot and cold workability. The present invention, when continuously casting a rectangular slab, while pressing the center porosity and preventing the occurrence of internal cracks and trapping of concentrated molten steel, generate negative segregation in the center of the slab, or significantly segregate The continuous cast slab having excellent hot and cold workability can be economically manufactured.

[0006]

According to the present invention, when continuously casting a rectangular slab, the aspect ratio of the slab is set to 1.1 or more, and the formula (1) or (2) formula 2 is satisfied. A casting terminal is provided at a position y where the casting speed is adjusted to adjust the solidification rate of the cross-section center portion of the casting terminal on the entry side of the casting terminal to 0.6 to 0.95, and the casting shaft of the portion for pressing the casting piece. Radius of curvature is 20
80 to 540 mm / mi using a reduction terminal of 0 mm or more
A method for improving the internal quality of continuously cast slabs by large unsolidified rolling, characterized by rolling down slabs at a rolling rate of 5 to 40% at a rolling rate of n at a rolling rate of n. In addition, the height H
Has a protrusion that is 5% or more of the thickness of the slab, and the width W of the protrusion is larger than the unsolidified width defined by the solid fraction of 0.5 of the slab at the rolling position, and the width of the slab is It is preferable to use a rolling-down terminal having a ratio of 80% or less, and to apply an electromagnetic force to the molten steel in the continuous cast slab to stir the molten steel in the slab.

[0007]

## EQU2 ## When 0 ≦ D <250 0.02 × D ≦ y (m) ≦ 0.06 × D (1) When 250 ≦ D 0.02 × D ≦ y (m) ≦ 0.067 × (D-250) +15 (2) where D: thickness of slab (mm)

[0008]

The center segregation of the continuous cast slab is formed by the accumulation of concentrated molten steel between the trees at the center due to the flow of molten steel due to solidification shrinkage. In addition, Center Polo City
When solidification progresses at the center of the slab and the hot water supply property in the solid-liquid coexisting phase is lost, the volume due to solidification shrinkage is not compensated, and thus it is generated.

In order to improve the center segregation, it is sufficient to prevent the flow of the concentrated molten steel by applying a reduction in an amount commensurate with the amount of solidification shrinkage in the same manner as the conventional light reduction in the final stage of solidification. When a deformation such as a roll that significantly exceeds the amount of solidification shrinkage is applied to the unsolidified portion, the concentrated molten steel is squeezed out and a molten steel flow toward the meniscus side (upstream side) is induced, and the flow thickens the trees. Since molten steel is also sucked out, negative segregation is formed. As described above, in order to form the negative segregation at the center of the slab, a considerably larger amount of reduction is required than in the light reduction, and the shape of the reduction roll, the shape of the slab to be reduced, the installation position of the reduction terminal and the reduction are performed. At this time, the casting speed and the rolling conditions such as the rolling speed depending on the casting speed have a great influence on the quality of the slab. In particular, if the rolling position such as the position of the rolling-down terminal or the rolling-down speed is inappropriate, the concentrated molten steel is not fully squeezed out and is captured, or remarkable internal cracking occurs. Therefore, the present invention is effective in that various studies are conducted using a real machine test or a three-dimensional rigid-plastic analysis and a model experiment using plastisine for a method of obtaining a negative segregation by preventing the capture of concentrated molten steel and the occurrence of internal cracks. It was confirmed.

Generally, in a curved continuous casting machine or a horizontal continuous casting machine, the equiaxed crystals generated are settled and particularly settled toward the bottom surface of the slab, so that the cross-sectional shape of the unsolidified portion is flat especially in the width direction of the slab. It becomes the shape. When pressing the unsolidified portion having such a cross-sectional shape, it is easier to penetrate the deformation of the surface in the thickness direction than in the width direction of the cast piece, and it is easy to press-bond the solid-liquid interface. Therefore, solidification shrinkage compensation and concentrated molten steel can be efficiently squeezed out. In addition, the larger the flatness ratio of the cast piece (ratio of cast piece width / thickness), the larger the unsolidified width and the flattened cross-sectional shape of the unsolidified portion. Therefore, the deformation applied in the thickness direction of the cast piece is more internal. Easy to penetrate into

When rolling in the thickness direction of the slab, the flatness ratio is 1.
If it is less than 1, the cross-sectional shape of the unsolidified portion becomes semi-circular, and especially when the unsolidified thickness is large, it is difficult to crimp the solidification interface cleanly, and the concentrated molten steel tends to remain in the form of being trapped at the end of the unsolidified portion. However, when the flatness ratio is 1.1 or more, the cross-sectional shape of the unsolidified portion is flattened in the width direction, and the difficulty of crimping is considerably alleviated.

In the method of the present invention, the position y where the pressing terminal is provided is defined so as to satisfy the formula (1) or the formula (2) depending on the thickness D of the cast piece for the following reason. That is, when negative segregation is generated in the center of the slab by pressing, if the pressing terminal is provided at a position outside the range defined by the equation (1) or (2) with respect to the thickness D of the slab, the pressing terminal is The solid fraction at the center of the cross section of the cast piece on the inlet side is 0.6 to 0.
In order to obtain 95, it is necessary to increase the casting speed as compared with the case where the casting is installed at a position within the range defined by the formula (1) or the formula (2). In this case, even if the amount of reduction is constant, the reduction speed becomes faster, and accordingly, the flow velocity of the concentrated molten steel flowing to the meniscus side by the squeezing due to the reduction also increases. When the flow velocity of the concentrated molten steel to be squeezed out becomes high, the solid fraction is locally low at the boundary between the columnar crystal part on the upper surface side of the slab and the equiaxed crystal zone due to the discharge pressure of the concentrated molten steel especially in the curved continuous casting machine. A channel is formed. As a result, the concentrated molten steel passes through the channel exclusively and is squeezed out toward the meniscus side, and in the equiaxed crystal region deposited on the lower surface side of the slab, the flow becomes inactive, and the equiaxed crystal The squeezing of the concentrated molten steel that has entered between the crystals is incomplete and linear segregation is formed. In order to prevent the formation of this linear segregation, it is necessary to prevent the formation of a channel having a low solid fraction locally in the solid-liquid coexisting phase of the cast slab. It is effective to install a reduction terminal at a position within the range defined by the equation (2) and reduce the casting speed during reduction as much as possible to reduce the discharge pressure of the concentrated molten steel that is squeezed out.

When a reduction for squeezing the concentrated molten steel is applied to the continuous cast slab, if the reduction is continued at the end of the casting, the concentrated molten steel squeezed out overflows from the final casting part to the outside. Need to interrupt at. Therefore, there is a part where the concentrated molten steel that has been squeezed remains in the final casting part, and that part cannot be used as a product.Therefore, whether this method is economically feasible depends on how much this part can be reduced. It's a fundamental issue.

For example, by casting the same steel type one after another, the ratio of the portion that cannot be used as a product can be reduced, but such a measure cannot be adopted for steel types with a small production amount. Further, as the position where the pressure reducing terminal is installed is farther from the mold, the area that cannot be reduced due to the leakage of molten steel at the end of casting increases, so from that meaning as well, it is outside the range specified by formula (1) or (2). It is not preferable to provide the pressing terminal at the position. Further, regardless of whether negative segregation is formed or not, when the pressing terminal is provided at a position closer to the mold than the range defined by the formula (1) or (2) with respect to the thickness D of the slab, For this reason, it is disadvantageous in terms of cost. That is, the solid content in the center of the cross section of the cast piece on the side of the pressing terminal can be significantly improved by 0.6 to 0.
The casting speed for achieving 95 is considerably low, the high productivity which is the advantage of continuous casting cannot be maintained, and in addition, the molten steel temperature is lowered at the end of casting, and nozzle clogging is likely to occur.
Further, when the casting speed is lowered, the contact time between the rolling-down terminal and the slab is increased, and the heat load is increased, so that the life of the rolling-down terminal is greatly reduced.

[0015] The timing of rolling down is also an important operating factor for improving segregation without producing internal cracks by rolling down, or for squeezing out concentrated molten steel to form negative segregation in the center of the slab. In the present invention, the solid fraction of the central part of the cast slab to be rolled down, that is, the weight fraction of the solid phase in the central part of the cross section of the strand is 0.
6 to 0.95. The central segregation is formed by the central solid fraction of 0.95. Even if the central segregation is reduced at a time of exceeding 0.95, the reduction does not contribute to the improvement of the central segregation.
Negative segregation cannot be generated in the center of the slab by squeezing the concentrated molten steel. On the other hand, when the solid fraction of the central portion is 0.6 or less, the solidification interface is likely to be subjected to tensile deformation due to the large unsolidified portion, and it is impossible to prevent the occurrence of remarkable internal cracks.
The central solid fraction of the cast slab must be controlled to 0.6 or more.

Further, according to the present invention, the pressing terminals are 80 to 5
The slab is rolled down at a reduction rate of 40 mm / min in the thickness direction of the slab at a reduction rate of 5 to 40%. The rolling reduction referred to here is defined by the following equation 3, and the thickness of the cast piece is the value at the center of the width of the cast piece.

[0017]

[Equation 3]

When the rolling reduction is 5% or less, the deformation due to rolling does not sufficiently penetrate into the center of the slab, so that the center segregation and the center porosity cannot be prevented and the concentrated molten steel cannot be sufficiently squeezed out. On the contrary, when 40% or more of the reduction is applied by the reduction terminals, the slab spreads in the width direction, which causes internal cracks along the axial direction of the slab and rather deteriorates the internal quality.

When the reduction speed exceeds 540 mm / min, the discharge pressure of the concentrated molten steel squeezed out by the reduction increases, and channels having a low solid fraction are locally formed as in the case where the casting speed is high. As a result, linear segregation is formed, which adversely affects cold workability and the like, and the solid-liquid interface is rapidly deformed, so that internal cracks are likely to occur. On the contrary, in order to reduce the rolling speed to 80 mm / min or less, a) reduce the rolling amount, or b) use a rolling-down terminal capable of rolling down a considerably wide range, that is, take a large contact area, or
c) It is necessary to reduce the casting speed, the reduction amount is small in a), the reduction does not sufficiently penetrate into the interior, and improvement of the internal quality cannot be expected, and in b) and c) the equipment becomes large, casting The contact time between the strip and the pressing terminal is extended, the thermal load on the pressing terminal becomes extremely severe, and the life of the pressing terminal is significantly reduced.

In the internal quality improving method of the present invention, a pressing terminal having a radius of curvature in the axial direction of the cast piece of 200 mm or more at the portion for pressing the cast piece is used. One of the internal cracks that occurs when a continuous casting slab with an unsolidified portion is subjected to large reduction with a rolling terminal such as a roll, the solidified shell on the roll bite side causes bending deformation in the vertical section of the slab due to the reduction roll. There is internal cracking that occurs when the solidification interface is stretched by receiving. Therefore, the solidification rate of the central part is increased to reduce the unsolidified thickness, and the radius of curvature in the axial direction of the cast piece of the cast piece is set to 200 mm or more to reduce the strain due to the bending deformation of the solidified shell to reduce the internal stress. The occurrence of cracks can be suppressed.

In the present invention, the height H is used as the pressing terminal.
Has a projection that is 5% or more of the thickness of the slab, and the width W of the projection is greater than the unsolidified width defined by the solid fraction 0.5 of the slab at the rolling position, and the width of the slab is It is preferable to use a pressure-reducing terminal that is 80% or less.

When the flatness ratio of the continuous cast slab is small, the deformation applied from the surface of the slab is consumed for the advance and width expansion of the slab, and it is difficult for the deformation to penetrate into the solidification interface. In such a case, in order to reliably suppress the flow of the concentrated molten steel and suppress the formation of center segregation, or to reliably squeeze out the concentrated molten steel and generate negative segregation, to improve the internal quality, the solidified shell of It is necessary to adopt a rolling-down method that suppresses the advance and width expansion and allows the deformation applied from the surface to permeate into the solidification interface more efficiently. Regarding the method of efficiently permeating the deformation on the surface of the slab into the inside, as described in JP-B-48-41132, JP-B-50-3750, and JP-A-61-132247, the unsolidified portion is not formed. It is effective to use a terminal such as a roll provided with a protrusion at the position corresponding to the center of the width of the remaining slab to reduce the height of the protrusion, as described in Japanese Patent Publication No. 50-3750. It is not sufficient to make the roller diameter 10 to 25%, and the roller diameter 3.5 to 5 times the opening height. The width of the protrusion, the reduction rate (amount of reduction), and the timing of the above-described reduction (the reduction position). It is also indispensable to properly set the central solid phase ratio in the above) and the shape of the cast piece (the shape of the unsolidified portion).

As a result of examination by the present inventor, in order to surely squeeze the concentrated molten steel when other conditions satisfy the above conditions, a protrusion is provided at a portion corresponding to the width center of the cast slab, When the height H of the protrusion is 5% or more of the thickness of the slab, deformation due to rolling is efficiently permeated into the solidification interface, center segregation is prevented, or concentrated molten steel is squeezed out.
It was effective to generate negative segregation in the center of the slab. This is because by providing a protrusion with a height of 5% or more of the thickness of the slab, the central portion of the width of the slab where the unsolidified portion remains can be preferentially reduced, and the temperature is low in the width direction of the slab and the deformation resistance This is because it is possible to avoid the reduction of the portion having a large area, and to suppress the advancement and width expansion of the solidified shell by avoiding the reduction of the portion, so that the deformation can be more efficiently permeated into the solidification interface.

If the width W of the protrusion is smaller than the unsolidified width defined by the solid fraction of 0.5 of the cast piece at the rolling position,
The solidification interface is pressure-bonded only in the central part of the slab, and concentrated molten steel is likely to remain at the end of the unsolidified part on the short side of the slab, and due to the widening of the slab and bending deformation in the transverse section of the slab, etc. Internal cracks are also likely to occur due to the solidification interface having low ductility being pulled by the slab width method. On the other hand, if the width W of the protrusion is 80% or more of the width of the slab, the advance and width expansion of the solidified shell become large for the reasons described above, and the deformation applied to the surface of the slab hardly penetrates into the solidification interface. Become.

Further, as shown in FIG. 2, the height H of the protrusion is
Is 5% or more of the thickness of the continuous cast slab 1 and the rolling reduction is 10 to
Within the range of 50%, the continuous casting slab 1 may be rolled down at a large reduction rate such that the portion of the rolling down roll 4 other than the protrusion contacts the continuous casting slab 1. Also in this case, the central portion of the width of the slab where the unsolidified portion 2 remains is started to be pressed down from the upstream side by the projection, and the advance of the slab due to the pressing down is considerably suppressed until the portion other than the projection comes into contact. Therefore, the permeability of the uncoagulated portion 2 under pressure is secured as such, and the uncoagulated portion 2
The pressure-bonding property and the squeeze-out property of concentrated molten steel are not significantly impaired. .

Further, it is preferable to combine electromagnetic stirring to the molten steel in the continuously cast slab to stir the molten steel in the slab. When large-scale reduction of a continuous cast slab with unsolidified part remains, the equiaxed crystal structure of the solidification structure is less susceptible to cracking than the columnar structure, and internal cracking due to rolling is less likely to occur As the equiaxed crystals exist in the widest possible range, it is easy to realize uniform squeezing of concentrated molten steel in a wide range of the solid-liquid coexisting phase, and the appearance of linear segregation due to channel formation in the solid-liquid coexisting phase. Since it is easy to prevent, it is preferable to apply an electromagnetic force to the molten steel in the continuous cast slab to stir the molten steel in the slab to promote equiaxed crystallization of the solidification structure.

[0027]

EXAMPLE An S45C slab was produced under the conditions shown in Tables 1 and 2 using a curved continuous casting machine, and the slab was rolled to obtain 40.
A JIS No. 4 type tensile test piece with a φ product and a parallel portion of 5φ was sampled, and the ductility of the product center portion was evaluated by squeezing. For the sake of comparison, the same investigation was carried out for a cast piece which was cast under substantially the same casting conditions and was not rolled, and a cast piece which was rolled under a condition outside the range. Further, in order to clarify the yield improving effect of the present invention, a comparison of the casting yield between the case of the present invention and the case where the casting was performed at a position of 22.2 m from a meniscus not satisfying the formula (1) or the formula (2) was also performed. .

[0028]

[Table 1] Note) Unsolidified width of fs0.5 at the rolling position: 30 to 70m
m

[0029]

[Table 2]

In Table 3, the casting yield and the C segregation degree at the center of the slab were investigated with a 5φ drill. As a result, internal cracks were evaluated by a sulfur print on the cross section of the slab, and the results of observation of center porosity with X-rays were obtained. , And the results of evaluating the ductility of the central part of the product by a tensile test on the product. The internal cracking score means that the larger the numerical value, the more the internal cracking occurs. In the case where the reduction rate is 0% without reduction, “-” is displayed in each column for the reduction position, the reduction roll, the central solid fraction and the reduction rate.

[0031]

[Table 3]

As is clear from Table 3, in the examples, the portion where the concentrated molten steel accumulates in the final casting part is greatly reduced to prevent the casting yield from being deteriorated, and in terms of quality, the occurrence of internal cracks is almost prevented. However, the ductility of the center of the product was remarkably improved by crimping the center porosity and negatively segregating the center of the material.

When the roll 5 having a radius of curvature of 200 mm or less is rolled, internal cracking is severely generated,
Further, when the pressing roll was provided with protrusions, deformation tended to penetrate into the inside of the slab due to the pressing, and the degree of negative segregation tended to increase. Also, comparison of A-4 and A-5, or B-1
From the comparison between B-2 and B-2, the casting yield and the C segregation degree were better as the electromagnetic stirring was applied to promote the equiaxed crystallization of the solidified structure.

[0034]

EFFECTS OF THE INVENTION According to the present invention, continuous casting slabs in which unsolidified portions remain are largely reduced to prevent the formation of internal defects such as center segregation and center porosity without lowering the casting yield. It is possible to economically manufacture a continuous cast slab having excellent hot and cold workability. It is possible to prevent the occurrence of cracks during hot forging due to Chevro cracks and cuppy disconnection during cold extrusion and drawing that were generated due to insufficient ductility of the steel core, and tougher hot work than before, Cold working is possible. Furthermore, since internal defects such as center segregation and center porosity at the slab stage disappear, it becomes possible to omit the soaking diffusion treatment and to manufacture steel materials with sufficient mechanical properties at a smaller rolling ratio. Therefore, it is possible to omit the agglomeration process.

[Brief description of drawings]

FIG. 1 is a diagram showing a situation in which reduction is applied only by a protrusion of a roll according to the present invention.

FIG. 2 is a diagram showing a situation in which reduction is applied to a roll other than a protrusion according to the present invention.

[Explanation of symbols]

 1 Continuous cast slab 2 Unsolidified portion 3 Interface with solid phase ratio of 0.5 4 Rolling roll W Width of protrusion L Diameter of protrusion H Height of protrusion w Unsolidified width of solid fraction 0.5

Claims (3)

[Claims] 1. When continuously casting a rectangular slab, the flatness ratio of the slab is set to 1.1 or more, and the formula (1) or (2) is used.
A rolling-down terminal is provided at a position y that satisfies Equation 1, and the casting speed is adjusted so that the solid fraction of the cross-section center portion of the rolling-inlet-inserted side is 0.6 to 0.95 and the rolling down is performed. It is characterized in that the slab is pressed down in the thickness direction of the slab at a reduction rate of 5 to 40% at a reduction rate of 80 to 540 mm / min using a reduction terminal having a radius of curvature in the axial direction of the slab of 200 mm or more. Method for improving the internal quality of continuous cast slabs by large reduction of solidification. ## EQU1 ## When 0 ≦ D <250 0.02 × D ≦ y (m) ≦ 0.06 × D (1) When 250 ≦ D 0.02 × D ≦ y (m) ≦ 0.067 × (D-250) +15 (2) where D: thickness of the slab (mm) 2. The height H of the pressing terminal is 5 of the thickness of the slab.
%, And the width W of the projection is greater than the unsolidified width defined by the solid fraction of 0.5 of the slab at the rolling position and 80% or less of the width of the slab. The method for improving the internal quality of a continuously cast slab according to claim 1, wherein a large amount of unsolidified reduction is used. 3. The internal quality of the continuous cast slab according to claim 1, wherein the molten steel in the continuous cast slab is mixed with an electromagnetic force to stir the molten steel in the slab. How to improve.