JP2006150400A - Continuously cast slab and producing method therefor, and method for producing hot rolled steel plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To restrain the development of surface flaw near the edge part of a hot-rolled steel plate obtained by applying a hot-rolling to a continuously cast slab produced with a continuous caster without bringing about a special increase of a process load. <P>SOLUTION: In a long side 5 direction of the cast slab 1 which casts into a mold in the continuous caster and has almost rectangular cross section of the long side 5 and the short side 7, two edge parts positioned at both sides of the center part 1a are not rolled down and the cast slab 1 is drawn out while guiding with the plurality of support roll pairs. In such a way, the continuously cast slab 1 having the almost rectangular cross section of the long side 5 and the short side 7 and provided with the center part 1a including the center 2 in this long side 5 direction and the edge parts 1b at both sides of the center part 1a, is produced so that an average thickness α of the center part 1a in the long side 5 direction and thicknesses β<SB>1</SB>, β<SB>2</SB>of the edge parts at the position separating 10-20mm from the edge surfaces in the long side 5 direction of the edge parts 1b, satisfy the relation (α-β<SB>1</SB>)≥4mm and (α-β<SB>2</SB>)≥4mm. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a continuous cast slab, a manufacturing method thereof, and a manufacturing method of a hot-rolled steel sheet. Specifically, the present invention can suppress the occurrence of surface flaws in the vicinity of the edge portion of a hot-rolled steel sheet obtained by hot rolling a continuous cast slab produced by a continuous casting machine. The present invention relates to a cast slab, a manufacturing method thereof, and a manufacturing method of a hot-rolled steel sheet.

  Usually, a hot-rolled steel sheet is obtained by heating a continuous cast slab produced by a continuous casting machine into a heating furnace, heating it to a predetermined temperature, extracting it from the heating furnace, and hot rolling the continuous cast slab. It is manufactured by winding it on a coil.

  In this way, wrinkles may occur at the corners of the coil of the hot-rolled steel sheet manufactured in the vicinity (near both edge parts in the plate width direction). Contributing to the rise. It has been considered that this soot is caused by the fact that the temperature drop at both edge portions of the continuous cast slab after being extracted from the heating furnace is more significant than the temperature drop at the center portion excluding both edge portions. For this reason, until now, the temperature of the continuous cast slab at the time of extraction from the heating furnace is set high, or only the two edge portions of the continuous cast slab are plasma-heated or induction heated before starting the hot rolling. Measures have been taken to raise the temperature. However, these measures inevitably involve an increase in the energy cost and electric cost of the heating furnace, leading to an increase in manufacturing cost.

  For this reason, for example, in Patent Document 1, a chamfering roll is arranged on the exit side of the mold of a continuous casting machine, and the continuous chamfering shape is chamfered by the chamfering roll, whereby the temperature during rolling at both edge portions of the slab is obtained. An invention is disclosed in which the occurrence of wrinkles in the vicinity of both edges of a coil is prevented by suppressing the decrease and suppressing the transformation from γ tissue to α tissue.

  However, in the invention disclosed in Patent Document 1, even if the facility management is performed with great care, the scale detached from the slab bites into this chamfering roll, and indentation flaws are generated at both edge portions of the slab. On the other hand, soot is induced, and it is difficult to stably maintain the effect of suppressing the generation of soot, and there is a risk of increasing the equipment cost. In addition, as a cause of wrinkles in the vicinity of both edge portions, surface flaws caused by surface cracks and surface irregularities of the slab can be cited, but in this invention, the scale generated in the cracked portions and the uneven portions is pickled by a subsequent pickling. May not be sufficiently removed and may cause serious surface defects after cold rolling. In particular, the scale formed after micro cracks during hot rolling bite into the inside by rolling, and when subjected to cold rolling without being removed by pickling, a long linear surface extending in the rolling direction Because of the occurrence of defects, which are defects, the yield of cold-rolled steel sheets is significantly reduced.

  In order to suppress this surface flaw caused by hot rolling, for example, Patent Document 2 discloses an invention that suppresses the occurrence of bald wrinkles by cleaning and removing surface defects (pinholes) of a slab made of stainless steel. Has been.

Patent Documents 3 and 4 disclose an invention that suppresses the occurrence of edge seam wrinkles of stainless steel in hot rolling by performing hot rolling on a slab having a shape in which the vicinity of the center in the width direction of the short side is recessed. Is disclosed.
JP 2001-18040 A Japanese Patent Laid-Open No. 2-15806 JP 58-138502 A JP-A-3-207551

  However, according to any of the inventions disclosed in Patent Documents 2 to 4, a hot-rolled steel sheet obtained by hot rolling a continuous cast slab manufactured by a continuous casting machine without particularly increasing the process load. It is impossible to suppress the occurrence of surface flaws in the vicinity of the edge portion.

  That is, the invention disclosed in Patent Document 2 cannot prevent the occurrence of minute cracks that occur during hot rolling. For this reason, the increase in the process load accompanying the care removal of the surface defect of a slab is unavoidable.

  Moreover, in the invention disclosed by patent document 3, 4, since the surface crack and surface unevenness | corrugation of a slab cannot be eliminated, generation | occurrence | production of the micro crack in hot rolling cannot be prevented reliably.

The present invention has a substantially rectangular cross-sectional shape having a long side and a short side, and includes a center part including the center of the direction of the long side and edge parts on both sides of the center part, and relates to the direction of the long side. The average thickness α of the center portion and the thicknesses β ₁ and β ₂ of the edge portion at a position 10 to 20 mm away from the end surface in the direction of the long side of the edge portion are (α−β ₁ ) ≧ 4 mm and (α−β ₂ ) A continuous cast slab characterized by satisfying a relationship of ≧ 4 mm.

  From another point of view, the present invention provides a slab having a substantially rectangular cross-sectional shape having a long side and a short side by casting a molten metal into a mold of a continuous casting machine. The continuous cast slab according to the present invention described above is manufactured by pulling out the slab while guiding it with a plurality of pairs of support rolls without reducing the edge portions located on both sides. It is a manufacturing method.

  In the method for producing a continuous cast slab according to the present invention, a plurality of sets of support rolls can impart bulging to the slab at a position upstream of the slab bend return completion position in the continuous caster. It is desirable that the respective intervals are set so as not to contact the corner portion including the end portion in the width direction of the slab after the slab is bulged.

  In the method for producing a continuous cast slab according to the present invention, it is desirable that the intervals between the plurality of pairs of support rolls are set to the same distance. For example, the intervals between the plurality of pairs of support rolls are The distance is preferably the same as the distance between the long side molds of the mold.

  From still another aspect, the present invention provides the above-described continuous cast slab according to the present invention by bulging the long side of the slab having a rectangular cross section cast into the mold of the continuous casting machine and pulling it out. A hot-rolled steel sheet manufacturing method characterized in that a hot-rolled steel sheet is manufactured by performing hot rolling on the continuous cast slab.

  In these continuous cast slabs according to the present invention, a method for producing the same, and a method for producing hot-rolled steel sheets, “cross section” is used as a cross section in the normal sense of a continuous cast slab that is a long body. Specifically, it means a cross section perpendicular to the rolling direction when the continuous cast slab is rolled.

Further, in these continuous cast slabs according to the present invention, the manufacturing method thereof and the hot rolled steel sheet manufacturing method, “thickness α, β ₁ , β ₂ ” means continuous casting as illustrated in FIG. 1 described later. It means the thickness at the end face in the longitudinal direction of the slab 1. The thicknesses α, β ₁ and β ₂ may be measured by appropriate means, for example, using calipers or the like. In measuring the thicknesses α, β ₁ and β ₂ , it is effective to remove the scale on the surface of the continuous cast slab first for accurate measurement.

  In addition, the “center portion including the center in the direction of the long side” of the continuous cast slab means an area including the center in the direction of the long side (the width direction of the continuous cast slab). However, it is exemplified that the region is formed in the center when the continuous cast slab is virtually divided into approximately three equal parts in the long side direction (width direction).

  Furthermore, the “thickness of the center portion including the center in the direction of the long side” of the continuous cast slab means the thickness of the center portion in short, but the center portion actually has minute unevenness. Therefore, the thickness of the center portion varies depending on the measurement site and is often not constant. For this reason, in this invention, it is supposed that average thickness (alpha) of the several measurement point regarding the direction of the long side of this center part is used as the thickness of a center part. Needless to say, in the case of a continuously cast slab where the thickness of the center portion is constant, the value of one measurement point may be used.

  According to the present invention, since the thickness of the center portion of the continuous cast slab is 4 mm or more larger than the thickness of the edge portion, the continuous cast slab manufactured by the continuous caster is not accompanied by a special increase in process load. The occurrence of surface flaws in the vicinity of the edge portion of the hot-rolled steel sheet obtained by hot rolling can be suppressed or virtually eliminated.

  In addition, according to the present invention, since the continuous cast slab is bulged in the continuous casting machine, it is possible to suppress overcooling that occurs at the edge portion of the continuous cast slab and to suppress cracking of the continuous cast slab. Thereby, generation | occurrence | production of the flaw in the edge vicinity can be suppressed irrespective of the material of a continuous cast slab.

Hereinafter, the best mode for carrying out the continuous cast slab according to the present invention, the manufacturing method thereof and the hot rolled steel sheet manufacturing method will be described in detail with reference to the accompanying drawings.
The present inventor has the shape of a continuous cast slab produced by a continuous casting machine and the frequency of surface flaws generated in the vicinity of the edge portion of a hot-rolled steel sheet obtained by hot rolling the continuous cast slab. The relationship was examined in detail. As a result, in brief, a continuous casting casting having a substantially rectangular cross-sectional shape having a long side and a short side, a center part including the center in the direction of the long side, and edge parts on both sides of the center part. If the shape of the piece is a new shape that does not exist in the past, the thickness of both edge portions is 4 mm or more thinner than the thickness of the center portion, even if hot rolling is performed on this continuous cast slab, It has been found that the occurrence of surface flaws in the vicinity of the edge portion of the obtained hot-rolled steel sheet can be suppressed or virtually eliminated.

FIG. 1 is an explanatory view showing the shape of the end face in the longitudinal direction of the continuous cast slab 1 in the present embodiment. 1 indicates the average thickness (mm) of the center portion 1a including the center 2 in the long side direction (width direction) of the continuous cast slab 1, and the symbols β ₁ and β ₂ respectively indicate the center. The thickness of edge part 1b, 1c in the position 10-20 mm away from the end surface regarding the direction of the long side of edge part 1b, 1c of the both sides of the part 1a is shown.

And in this continuous cast slab 1, both (α−β ₁ ) ≧ 4 mm and (α−β ₂ ) ≧ 4 mm are satisfied.
FIG. 2 compares the shape of the end face in the longitudinal direction of the conventional continuous cast slab 3 with the shape of the end face in the longitudinal direction of the continuous cast slab 1 in the present embodiment described with reference to FIG. It is explanatory drawing shown.

As shown in FIG. 2, the conventional continuous cast slab 3 has a shape of an end face obtained by preventing bulging on the long side 4 during continuous casting, that is, (α−β ₁ ) <2 mm and (α−β ₂ ) <2 mm. In this conventional continuous cast slab 3, the long side 4 is not bulged so that the short side 6 is bulged, and two concave portions 6a and convex portions 6b are formed on the short side 6. Many fine cracks 8 are generated inside.

On the other hand, the continuous cast slab 1 of the present embodiment has a shape of an end face obtained by positively bulging the long side 5 during continuous casting, that is, (α−β ₁ ) ≧ 4 mm, as will be described later. (Α−β ₂ ) ≧ 4 mm.

  FIG. 3 is a graph showing the relationship between (α−β) (mm) and the coil edge vicinity surface flaw occurrence index (−). As shown in the graph in the figure, by setting (α−β) (mm) to 4 mm or more, the surface flaw occurrence index near the coil edge can be made substantially zero.

  Moreover, since the continuous casting slab 1 of this Embodiment bulges the long side 5, the deformation | transformation of the slab shape in the corner part vicinity of the short side 7 and the long side 5 is suppressed. For this reason, the minute crack 8 confirmed in the conventional continuous cast slab 3 does not occur inside the short side 7.

  These continuous cast slabs 3 and 1 were subjected to hot rolling and wound into a coil, and the occurrence of surface flaws in the vicinity of the edge portion of the coil was investigated. As a result, in the vicinity of the edge portion of the coil made of the continuous cast slab 1 of the present embodiment as a raw material, it is caused by a large number of fine cracks 8 generated under the surface of the conventional continuous cast slab 3. The surface flaw hardly occurred and was significantly reduced as compared with the conventional coil made of the continuous cast slab 3.

FIG. 4 is an explanatory diagram schematically showing a generation mechanism of so-called edge seam wrinkles.
Further, in the conventional continuous cast slab 3, as shown in FIG. 4, the surface layer portion of the long side 4 of the continuous cast slab 3 is wider than the center part of the continuous cast slab 3 at the initial stage of hot rolling. When the central part of the continuous cast slab 3 projects in the width direction as the rolling progresses, the initial widened part remains as an edge seam flaw near the edge of the coil. .

On the other hand, according to the continuous cast slab 1 in this Embodiment, it was confirmed that generation | occurrence | production of this edge seam flaw is also reduced.
Furthermore, when the continuous cast slab 1 that is a material for hot rolling was confirmed, in the case of a medium carbon material having a carbon concentration of about 0.12% by mass, surface cracks were also generated at the edge of the continuous cast slab 1. It was confirmed that it was reduced as compared with the conventional continuous cast slab 3.

Thus, in the present embodiment, (α−β ₁ ) ≧ 4 mm and (α−β ₂ ) ≧ 4 mm by bulging the long side 5 of the rectangular continuous cast slab 1 and not bulging the short side 7. By satisfying the relationship, partial remarkable deformation of the slab shape in the entire area of the short side 7 and the long side 5 is suppressed, and thereby, it occurs inside the edge portion of the coil after hot rolling inside. The fine cracks 8 that cause surface flaws are not generated.

In addition, the shape from the center 2 of the center 1a of the continuous cast slab 1 of this Embodiment to the end surface of edge part 1b, 1c does not require a limitation in particular, ((alpha)-(beta) ₁ )> = 4mm and ((alpha)-). β ₂ ) ≧ 4 mm as long as the relationship is satisfied, as described above, from the gist of the present invention that the partial significant deformation of the slab shape in the entire area of the short side 7 and the long side 5 is suppressed. It is desirable that the thickness changes smoothly from 2 to the edge.

The continuous cast slab 1 of the present embodiment is configured as described above. Next, a method for producing the continuous cast slab 1 will be described.
As described above, in order to manufacture the continuous cast slab 1 of the present embodiment that satisfies the relationship of (α−β ₁ ) ≧ 4 mm and (α−β ₂ ) ≧ 4 mm, solidification of the continuous cast slab is performed. When the shell has reached an appropriate thickness, for example, immediately after the casting side of the continuous casting machine, the casting time is about 3 minutes. By enlarging and setting a value 4 mm or more larger than the interval between the inner surfaces on the side, the long side 5 other than the corner portion of the continuous cast slab 1 is actively bulged by a predetermined amount, and the support roll is moved out of the mold. It is effective to set the interval between the support rolls so as not to contact the surface of the corner portion of the continuous cast slab 1 from the side. For example,
(A) In continuous casting, the roll interval of a plurality of pairs of support rolls arranged below the mold of the continuous casting machine is set wider than the distance between the inner surfaces of the long side molds of the mold, for example, the mold The roll interval of the first-stage support roll pair immediately below is set to be, for example, 4 mm wider than the distance between the long-side molds at the lower end of the mold, and the interval between the second-stage support roll pair is cast. Set to narrow sequentially according to the coagulation shrinkage that occurs in, or
(B) It is exemplified that the roll interval of the plurality of pairs of support rolls is kept constant with respect to all the support roll pairs immediately after the first stage of the continuous casting mold and the continuous cast slab 1 is not crushed. .

  And in this Embodiment, a hot-rolled steel plate is manufactured by performing hot rolling using the continuously cast slab 1 manufactured in this way as a raw material, and winding up on a coil. The conditions for hot rolling and coiling may be based on well-known and conventional conditions, and since no limitation is required, further explanation is omitted.

By producing a hot-rolled steel sheet in this way, the effects listed below can be achieved.
FIG. 5 is an explanatory view schematically showing a state of deformation of the material to be rolled when hot-rolled steel sheets are manufactured by performing hot rolling on the continuous cast slab 1 of the present embodiment.

  First, as shown in FIG. 5, the amount of plastic deformation generated by the reduction in hot rolling without chamfering the corner portion of the continuous cast slab that has been conventionally performed in order to suppress the occurrence of edge seam flaws. Difference between the surface layer portion and the center portion of the continuous cast slab 1 is reduced. For this reason, according to this Embodiment, it becomes possible to suppress generation | occurrence | production itself of the edge seam in a hot-rolled steel plate.

  Moreover, according to this Embodiment, since the corner part of the continuous casting slab 1 does not contact a support roll inside a continuous casting machine, the continuous casting slab 1 of the continuous casting slab 1 by overcooling of the corner part of the continuous casting slab 1 is obtained. It is possible to suppress the occurrence of surface cracks.

By these, the surface flaw which generate | occur | produced in the vicinity of the edge part of the coil of a hot-rolled steel plate can be reduced significantly.
Thus, according to the present embodiment, the cross-sectional shape of the continuous cast slab, which is a raw material for hot rolling, the thickness of the central part in the width direction of the long side is the thickness of both edge parts in the width direction of the long side. Since the thickness is at least 4 mm or more with respect to the thickness, even if this continuous cast slab is hot-rolled and wound on a coil, the occurrence of coil surface flaws is not accompanied by a special increase in process load. It can be surely prevented.

Furthermore, the present invention will be described more specifically with reference to examples.
Using an existing continuous casting machine, molten steel with [C] = 0.05% by mass is continuously cast at a casting speed of 2.0 m / min, and has a thickness of 270 mm and a width of 1600 mm. Continuous cast slabs having a rectangular surface shape (continuous cast slab) were produced by the conventional method and the method of the present invention, respectively.

  In the conventional method, the interval between the first-stage support roll pairs located immediately below the mold of the continuous casting machine is set to be the same as the dimension of the lower end of the mold, and the intervals between the second-stage and subsequent support roll pairs are continuously set. Continuous casting was carried out 10 times, setting so as to become narrower sequentially according to the solidification shrinkage of the cast slab.

  On the other hand, in the method 1 of the present invention, the interval between the first-stage support roll pairs is set to be 4 mm wider than the dimension at the lower end of the mold, and the interval between the second-stage and subsequent support roll pairs is continuously cast. Continuous casting was performed 5 times, setting so as to narrow sequentially according to the solidification shrinkage of the pieces.

Furthermore, in the method 2 of the present invention, the continuous rolls were performed five times by setting the interval rolls of all the support roll pairs in the first and subsequent stages to be constant from the beginning to the same dimension as the lower end of the mold.
The present invention method 1, 2 and the shape of the longitudinal end faces of the continuous casting slabs produced by each of the conventional method (thickness in FIG. _{2 α, β 1, β 2} ) was measured by calipers.

As a result, the continuous cast slab obtained by the conventional method has an end face shape with (α−β ₁ ) <2 mm and (α−β ₂ ) <2 mm, and this continuous cast slab is used as a raw material. When hot rolling was performed by a commonly used method, the occurrence rate of edge seam wrinkles was 1.4%.

On the other hand, the continuous cast slabs obtained by the methods 1 and 2 of the present invention all satisfy the relationship of (α−β ₁ ) ≧ 4 mm and (α−β ₂ ) ≧ 4 mm. When hot rolling was carried out using a conventional cast slab of steel, the rate of edge seam wrinkles was reduced to 0.1%, and the rate of edge seam wrinkles was improved by about 1.3%. I was able to.
there were.

It is explanatory drawing which shows the shape of the end surface of the longitudinal direction of the continuous cast slab in embodiment. It is explanatory drawing which shows the shape of the end surface of the longitudinal direction of the conventional continuous cast slab, and the shape of the end surface of the continuous direction of the continuous cast slab in this Embodiment. It is a graph which shows the relationship between ((alpha)-(beta)) (mm) and coil edge vicinity surface flaw generation | occurrence | production index (-). It is explanatory drawing which shows typically the generation | occurrence | production mechanism of what is called an edge seam wrinkle. It is explanatory drawing which shows typically the mode of a deformation | transformation of the to-be-rolled material when rolling with respect to the continuous cast slab of embodiment, and manufacturing a hot-rolled steel plate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Continuous cast slab 1a Center part 1b, 1c Edge part 2 Center 3 Conventional continuous cast slab 4, 5 Long side 6, 7 Short side 6a Convex part 6b Concave part 8 Crack

Claims (6)

The center portion having a substantially rectangular cross-sectional shape having a long side and a short side, a center portion including a center in the direction of the long side, and edge portions on both sides of the center portion, and the center portion in the direction of the long side The average thickness (α) of the edge portion and the thickness (β ₁ , β ₂ ) of the edge portion at a position 10 to 20 mm away from the end surface in the direction of the long side of the edge portion are (α−β ₁ ) ≧ 4 mm. And (α−β ₂ ) ≧ 4 mm. A cast piece having a substantially rectangular cross-sectional shape having a long side and a short side is formed by casting a molten metal into a mold of a continuous casting machine, and the edge portions located on both sides of the center portion are not crushed in the direction of the long side. Then, the continuous cast slab according to claim 1 is manufactured by pulling out the slab while being guided by a plurality of pairs of support rolls. The plurality of sets of support rolls can impart bulging to the slab at a position upstream of the slab bend return completion position in the continuous casting machine, and the edge portion after the slab is bulged. The method for producing a continuous cast slab according to claim 2, wherein the intervals are set so as not to contact with each other. 3. The method for producing a continuous cast slab according to claim 2, wherein intervals between the plurality of pairs of support rolls are set to the same distance. 5. The method for producing a continuous cast slab according to claim 4, wherein an interval between the plurality of pairs of support rolls is the same value as a distance between long-side molds of the mold. A continuous cast slab according to claim 1 is manufactured by bulging the long side of a slab having a rectangular cross section cast into a mold of a continuous casting machine and pulling it out. A method for producing a hot-rolled steel sheet, comprising producing a hot-rolled steel sheet by hot rolling.

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