Mold for continuous casting, method for continuously casting cast pieces, and continuously casting for cast pieces

Abstract

To provide a mold for continuous casting capable of achieving both suppression of a corner crack of a cast pieces and stability of initial solidification.SOLUTION: In a mold for continuous casting of the present disclosure, an opening shape in a cross section orthogonal to a casting direction is defined by a pair of short sides, a free-side long side, a fixed-side long side, a free-side corner portion, and a fixed-side corner portion. The opening shape satisfies conditions of 10≤A1≤30, A1+A2≤30, and 1.00≤B1 / A1≤3.00. Here, A1 is a length (mm) from an intersection O1 to a connection point P1, A2 is a length (mm) from an intersection O2 to a connection point P2, B1 is a length (mm) from the intersection O1 to a connection point P3, the intersection O1 is an intersection of an extension line of a short side and an extension line of the free-side long side, the intersection O2 is an intersection of the extension line of the short side and an extension line of the fixed-side long side, the connection point P1 is a connection point of the free-side corner portion and the free-side long side, the connection point P2 is a connection point of the fixed-side corner portion and the fixed-side long side, and the connection point P3 is a connection point of the free-side corner portion and the short side.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present application discloses a mold for continuous casting, a method for continuous casting of a slab, and a continuously cast slab.

Background Art

[0002] In recent years, in order to improve the mechanical properties of steel materials, many alloy steels containing alloying elements such as Al, Nb, Ti, and B have been manufactured. On the other hand, with the addition of these alloying elements, defects such as transverse cracks occur on the surface layer of the slab produced in continuous casting, which causes problems in operation and product quality.

[0003] It is known that transverse cracks occur along the prior austenite grain boundaries on the surface layer of the slab. For example, transverse cracks occur due to the concentration of straightening stress on austenite grain boundaries embrittled by precipitation of AlN, NbC, etc., or film-like ferrite formed along the prior austenite grain boundaries. The straightening stress is applied at the straightening point in a curved or vertically bent continuous casting machine. Transverse cracks are particularly likely to occur in the temperature range near the phase transformation region from austenite to ferrite. Therefore, usually, the occurrence of transverse cracks is suppressed by controlling the continuous casting conditions so that the surface temperature of the slab at the straightening point does not fall within the embrittlement temperature range (temperature range where ductility decreases).

[0004] On the other hand, when continuously casting a general rectangular slab, the corners (corner portions) of the slab are cooled from both the long side surface and the short side surface. Therefore, the surface temperature of the corner portion of the slab becomes lower than the surface temperature of other portions of the slab, and it can inevitably be bent and straightened in the above-mentioned embrittlement temperature range. That is, when continuously casting a rectangular slab, corner cracks of the slab are likely to occur. In response to such problems, for example, Patent Documents 1 to 5 disclose techniques for casting a slab while chamfering the corner portion of the slab. By chamfering the corner portion of the slab, an extreme temperature drop of the corner portion is suppressed, and it becomes easier to control the temperature of the corner portion to be higher than the embrittlement temperature during straightening, and it is considered that corner cracks can be suppressed.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0006] In the prior art, there is room for improvement regarding achieving both suppression of corner cracking of the slab during continuous casting and stability of initial solidification.

Means for Solving the Problems

[0007] As one of the means for solving the above problems, the present application discloses the following multiple aspects. <Aspect 1> A mold for continuous casting, wherein the opening shape of the mold for continuous casting in a cross-section orthogonal to the casting direction is a pair of short sides, a free-side long side, a fixed-side long side, a free-side corner portion connecting the short side and the free-side long side, and a fixed-side corner portion connecting the short side and the fixed-side long side, defined by and the opening shape satisfies the following conditions (1) to (3): 10 ≦ A1 ≦ 30 ···(1) A1 + A2 ≦ 30 ···(2) 1.00 ≦ B1 / A1 ≦ 3.00 ···(3) A1: Length from intersection point O1 to connection point P1 (mm) A2: Length from intersection point O2 to connection point P2 (mm) B1: Length from intersection point O1 to connection point P3 (mm) Intersection point O1: Intersection of the extension line of the short side and the extension line of the free-side long side Intersection point O2: Intersection of the extension line of the short side and the extension line of the fixed-side long side Connection point P1: Connection point of the free-side corner portion and the free-side long side Connection point P2: Connection point of the fixed-side corner portion and the fixed-side long side Connection point P3: Connection point of the free-side corner portion and the short side satisfying A mold for continuous casting. <Aspect 2> A mold for continuous casting according to Aspect 1, wherein the opening shape satisfies the following condition (4) or (5): A2 = B2 = 0 ··· (4) 1.00 ≤ B2 / A2 ≤ 3.00 ··· (5) B2: Length from intersection point O2 to connection point P4 (mm) Connection point P4: Connection point of the fixed-side corner portion and the short side satisfying A mold for continuous casting. <Aspect 3> A method for continuous casting of a slab, comprising supplying molten steel from a submerged nozzle to a mold, continuously withdrawing the slab from the lower end of the mold, and straightening the slab withdrawn from the mold, wherein the mold is a mold for continuous casting according to Aspect 1 or 2 A method for continuous casting of a slab. <Aspect 4> A continuously cast slab, wherein the cross-sectional shape perpendicular to the casting direction of the continuously cast slab is a pair of short sides, a free-side long side, a fixed-side long side, A free-side corner portion connecting the short side and the free-side long side, and a fixed-side corner portion connecting the short side and the fixed-side long side, which is defined by, and the cross-sectional shape satisfies the following conditions (1) to (3): 10 ≦ A1 ≦ 30 ··· (1) A1 + A2 ≦ 30 ··· (2) 1.00 ≦ B1 / A1 ≦ 3.00 ··· (3) A1: The length from the intersection point O1 to the connection point P1 (mm) A2: The length from the intersection point O2 to the connection point P2 (mm) B1: The length from the intersection point O1 to the connection point P3 (mm) Intersection point O1: The intersection point of the extension line of the short side and the extension line of the free-side long side Intersection point O2: The intersection point of the extension line of the short side and the extension line of the fixed-side long side Connection point P1: The connection point of the free-side corner portion and the free-side long side Connection point P2: The connection point of the fixed-side corner portion and the fixed-side long side Connection point P3: The connection point of the free-side corner portion and the short side satisfies, a continuously cast slab.

Advantages of the Invention

[0008] When a slab is continuously cast using the continuous casting mold of the present disclosure, it is easy to achieve both suppression of corner cracking of the slab and stability of initial solidification.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3A

Figure 3B

Embodiment for Carrying Out the Invention

[0010] Hereinafter, an embodiment of the mold for continuous casting and the method for continuous casting of a slab according to the present disclosure will be described. However, the mold for continuous casting and the method for continuous casting of a slab according to the present disclosure are not limited to the following embodiments.

[0011] 1. Mold for Continuous Casting FIGS. 1 and 2 schematically show the shape of a continuous casting mold 100 according to an embodiment. As shown in FIGS. 1 and 2, in the continuous casting mold 100 according to an embodiment, the opening shape of the continuous casting mold 100 in a cross-section orthogonal to the casting direction is a pair of short sides 10a, 10b, a free-side long side 10c, a fixed-side long side 10d, free-side corner portions 10ac, 10bc connecting the short sides 10a, 10b and the free-side long side 10c, and fixed-side corner portions 10ad, 10bd connecting the short sides 10a, 10b and the fixed-side long side 10d, defined by, and the opening shape satisfies the following conditions (1) to (3): 10 ≦ A1 ≦ 30 ··· (1) A1 + A2 ≦ 30 ··· (2) 1.00 ≦ B1 / A1 ≦ 3.00 ··· (3) A1: length from the intersection point O1 to the connection point P1 (mm) A2: length from the intersection point O2 to the connection point P2 (mm) B1: length from the intersection point O1 to the connection point P3 (mm) Intersection point O1: intersection of the extension lines of the short sides 10a, 10b and the extension line of the free-side long side 10c Intersection point O2: intersection of the extension lines of the short sides 10a, 10b and the extension line of the fixed-side long side 10d Connection point P1: connection point of the free-side corner portions 10ac, 10bc and the free-side long side 10c Connection point P2: The connection point between the fixed-side corner portions 10ad and 10bd and the fixed-side long side 10d Connection point P3: The connection point between the free-side corner portions 10ac and 10bc and the short sides 10a and 10b is satisfied.

[0012] Also, as shown in FIGS. 1 and 2, in the continuous casting mold 100 according to one embodiment, the opening shape satisfies the following condition (4) or (5): A2 = B2 = 0 ···(4) 1.00 ≦ B2 / A2 ≦ 3.00 ···(5) B2: The length (mm) from the intersection point O2 to the connection point P4 Connection point P4: The connection point between the fixed-side corner portions 10ad and 10bd and the short sides 10a and 10b may also be satisfied.

[0013] In the present application, the "opening shape of the continuous casting mold in a cross section orthogonal to the casting direction" refers to the opening shape at any position of the portion where the continuous casting mold and the slab can come into contact (the portion of the continuous casting mold that comes into contact with the slab and affects the shape of the slab). In the present application, for the opening shape of the mold in the portion where the continuous casting mold and the slab do not come into contact, it is not necessary to satisfy the above conditions (1) to (5).

[0014] In the present application, the "free side" refers to the inner side of the bend of the slab. That is, the "free-side long side" refers to the long side that can come into contact with the surface of the slab that becomes the inner side of the bend. Also, the "fixed side" refers to the outer side of the bend of the slab. That is, the "fixed-side long side" refers to the long side that can come into contact with the surface of the slab that becomes the outer side of the bend.

[0015] 1.1 Short sides As shown in FIGS. 1 and 2, a pair of short sides 10a and 10b each have a shape corresponding to the inner wall surface shape of a pair of short side molds 1a and 1b. For example, as shown in FIGS. 1 and 2, the shapes of the pair of short sides 10a and 10b are each linear. In FIG. 2, the lengths L1 and L2 of the short sides 10a and 10b (the length from connection point P3 to connection point P4), the length B1 at the free side corner portions 10ac and 10bc, and the lengths B2 at the fixed side corner portions 10ad and 10bd, the sums L1 + B1 + B2 and L2 + B1 + B2 may each be, for example, 100 mm or more and 600 mm or less, or 200 mm or more and 300 mm or less. The length L1 of the short side 10a and the length L2 of the short side 10b may be the same as each other or different from each other.

[0016] The short side molds 1a and 1b may be constituted by, for example, copper plates. The short side molds 1a and 1b may each have protruding portions 1ax, 1ay, 1bx, and 1by protruding inward of the mold at the ends in the short side direction, and the corner portions 10ac, 10bc, 10ad, and 10bd described later may be formed by the protruding portions 1ax, 1ay, 1bx, and 1by. The short side molds 1a and 1b may be sandwiched by a free side long side mold 1c and a fixed side long side mold 1d, and may also be configured to be movable along the long side direction. That is, in the continuous casting mold 100, the lengths L3 and L4 of the long sides described later may be variable.

[0017] 1.2 Long sides As shown in FIGS. 1 and 2, the free-side long side 10c and the fixed-side long side 10d each have a shape corresponding to the inner wall surface shape of the free-side long side mold 1c and the fixed-side long side mold 1d. For example, as shown in FIGS. 1 and 2, the shapes of the free-side long side 10c and the fixed-side long side 10d are each linear. The sum L3 + A1 + A1 of the length L3 of the free-side long side 10c (the length from one connection point P1 to the other connection point P1), the length A1 at one free-side corner portion 10ac, and the length A1 at the other free-side corner portion 10bc may be, for example, 600 mm or more and 2400 mm or less, or 700 mm or more and 1900 mm or less. The sum L4 + A2 + A2 of the length L4 of the fixed-side long side 10d (the length from one connection point P2 to the other connection point P2), the length A2 at one fixed-side corner portion 10ad, and the length A2 at the other fixed-side corner portion 10bd may be, for example, 600 mm or more and 2500 mm or less, or 1100 mm or more and 2200 mm or less.

[0018] The free-side long side mold 1c and the fixed-side long side mold 1d may each be constituted by, for example, a copper plate. As described above, the short-side molds 1a and 1b may be sandwiched between the free-side long side mold 1c and the fixed-side long side mold 1d, or the short-side molds 1a and 1b may be configured to be movable along the long side direction.

[0019] 1.3 Corner portion As shown in FIGS. 1 and 2, in the opening shape of the continuous casting mold 100, corner portions 10ac, 10bc, 10ad, and 10bd exist at the four corners of each of the pair of short sides 10a and 10b and each of the free-side long side 10c and the fixed-side long side 10d. The shape of each of the corner portions 10ac, 10bc, 10ad, and 10bd may be any shape that satisfies the above conditions (1) to (3). As shown in FIGS. 1 and 2, the shapes of the free-side corner portions 10ac and 10bc and the fixed-side corner portions 10ad and 10bd may each coincide with the inner wall surface shape of the protruding portions 1ax, 1bx, 1ay, and 1by provided at the ends of the short-side molds 1a and 1b.

[0020] 1.3.1 Condition (1) The opening shape of the mold 100 for continuous casting satisfies the condition (1) that 10 ≦ A1 ≦ 30. That is, the length A1 from the intersection point O1 to the connection point P1 in Fig. 2 is 10 mm or more and 30 mm or less. In continuous casting, when the slab is straightened, tensile strain is applied to the free side surface of the slab, and corner cracking is likely to occur. On the other hand, since compressive strain is applied to the fixed side surface of the slab, corner cracking is less likely to occur. Therefore, in order to suppress corner cracking of the slab, for example, at the straightening point, it is considered effective to increase the chamfering at the free side corner portion of the slab so that the temperature of the free side corner portion of the slab exceeds the embrittlement temperature range (so that the temperature of the free side corner portion of the slab does not drop extremely). The chamfering at the free side corner portion of the slab can be controlled by the shapes of the free side corner portions 10ac and 10bc of the opening shape of the mold 100 for continuous casting. According to the new findings of the present inventor, by making the length A1 in the free side corner portions 10ac and 10bc 10 mm or more and satisfying the condition (3) described later, corner cracking of the slab can be suppressed. On the other hand, if the length A1 is excessively long, solidification of the slab corner portion becomes unstable and solidification cracking is likely to occur. According to the new findings of the present inventor, by making the length A1 30 mm or less and satisfying the condition (3) described later, such solidification cracking can be suppressed. The length A1 may be 15 mm or more, 20 mm or more, or 25 mm or more, and may also be 25 mm or less, 20 mm or less, or 15 mm or less. In the mold 100 for continuous casting, the length A1 in one free side corner portion 10ac and the length A1 in the other free side corner portion 10bc may be the same as each other or may be different.

[0021] 1.3.2 Condition (2) The opening shape of the mold 100 for continuous casting satisfies the condition (2) that A1 + A2 ≤ 30. That is, the sum of the length A1 from the intersection point O1 to the connection point P1 in FIG. 2 and the length A2 from the intersection point O2 to the connection point P2 is 30 mm or less. As described above, since A1 is 10 mm or more and 30 mm or less, A2 is 0 mm or more and 20 mm or less. According to the new findings of the present inventor, when the sum of the length A1 and the length A2 is 30 mm or less, the initial solidification is stabilized and the solidification cracking is more remarkably suppressed. A1 + A2 may be 10 mm or more, 15 mm or more, 20 mm or more, or 25 mm or more, and may be 25 mm or less, 20 mm or less, or 15 mm or less. In the mold 100 for continuous casting, the length A2 at one fixed-side corner portion 10ad and the length A2 at the other fixed-side corner portion 10bd may be the same as each other or different from each other.

[0022] 1.3.3 Condition (3) The opening shape of the mold 100 for continuous casting satisfies the condition (3) that 1.00 ≤ B1 / A1 ≤ 3.00. That is, the length B1 from the intersection point O1 to the connection point P3 in FIG. 2 is 1.00 times or more and 3.00 times or less the above length A1. If the length B1 is too short with respect to the length A1, the solidification of the slab corner portion becomes unstable and solidification cracking is likely to occur. Also, the closer the length B1 approaches zero, the less chamfering is performed at the free-side corner portions 10ac and 10bc, and it becomes difficult to appropriately maintain the corner temperature of the slab, and corner cracking of the slab is likely to occur. On the other hand, if the length B1 is too long with respect to the length A1, the solidification of the slab corner portion becomes unstable and solidification cracking is likely to occur. According to the new findings of the present inventor, if B1 / A1 is 1.00 or more and 3.00 or less, such solidification cracking can be suppressed. B1 / A1 may be 1.50 or more, 2.00 or more, or 2.50 or more, and may be 2.50 or less, 2.00 or less, or 1.50 or less. In the mold 100 for continuous casting, the length B1 at one free-side corner portion 10ac and the length B1 at the other free-side corner portion 10bc may be the same as each other or different from each other.

[0023] 1.3.4 Condition (4) or (5) The opening shape of the continuous casting mold 100 may satisfy the condition (4) where A2 = B2 = 0. Alternatively, instead of the above condition (4), the opening shape of the continuous casting mold 100 may satisfy the condition (5) where 1.00 ≤ B2 / A2 ≤ 3.00. As described above, since corner cracking due to straightening is unlikely to occur at the fixed-side corner portions of the slab, at the fixed-side corner portions 10ad and 10bd, the slab may not be chamfered. In the continuous casting mold 100, the length B2 at one fixed-side corner portion 10ad and the length B2 at the other fixed-side corner portion 10bd may be the same as or different from each other.

[0024] 1.3.5 Other conditions 1.3.5.1 Conditions (6) and (7) As described above, in the continuous casting mold 100, the lengths A1, B1, A2, and B2 may be designed such that the chamfering of the slab at the free-side corner portions 10ac and 10bc is the same as or greater than the chamfering of the slab at the fixed-side corner portions 10ad and 10bd. For example, the opening shape of the continuous casting mold 100 may satisfy the following conditions (6) and (7): A2 ≤ A1 ··· (6) B2 ≤ B1 ··· (7) It may satisfy one or both of them.

[0025] 1.3.5.2 Condition (8) As shown in FIGS. 1 and 2, the shapes of the free-side corner portions 10ac and 10bc may be linear, or as shown in FIGS. 3A and 3B, they may be curved or polygonal. For example, as shown in FIGS. 3A and 3B, the opening shape of the continuous casting mold 100 may satisfy the following condition (8): D1 ≤ C1 ≤ 1.5 × D1 ··· (8) C1: The length (mm) of the free-side corner portions 10ac and 10bc from the connection point P1 to the connection point P3 D1: The length (mm) of the line segment from the connection point P1 to the connection point P3 It may satisfy it.

[0026] When the free-side corner portions 10ac and 10bc are linear, the length C1 thereof is equal to the length D1 of the line segment P1P3. On the other hand, when the free-side corner portions 10ac and 10bc are curved or polygonal line-shaped, the length C1 thereof is longer than the length D1 of the line segment P1P3. When the length C1 of the free-side corner portions 10ac and 10bc is 1.5 times or less the length D1 of the line segment P1P3, the chamfering shape of the slab by the free-side corner portions 10ac and 10bc becomes more optimal, and the above-mentioned corner cracks and solidification cracks are more easily suppressed. C1 may be 1.4 times or less, 1.3 times or less, 1.2 times or less, or 1.1 times or less of D1.

[0027] The same applies to the shapes of the fixed-side corner portions 10ad and 10bd. That is, the shapes of the fixed-side corner portions 10ad and 10bd may be linear, curved, or polygonal line-shaped. For example, the length of the fixed-side corner portions 10ad and 10bd is 1.0 times or more the length of the line segment P2P4, and may be 1.5 times or less, 1.4 times or less, 1.3 times or less, 1.2 times or less, or 1.1 times or less.

[0028] 1.3.5.3 Condition (9) As described above, the shapes of the free-side corner portions 10ac and 10bc may be linear, curved, or polygonal line-shaped. For example, as shown in FIGS. 3A and 3B, the opening shape of the continuous casting mold 100 satisfies the following condition (9): 0≦E1≦0.3×A1 ···(9) E1: The maximum distance (mm) between the line segment from the connection point P1 to the connection point P3 and the free-side corner portion and may satisfy the condition.

[0029] When the free-side corner portions 10ac and 10bc are linear, the above maximum interval E1 becomes 0. On the other hand, when the free-side corner portions 10ac and 10bc are curved or polygonal line-shaped, the maximum interval E1 exceeds 0. By the maximum interval E1 being 0.3 times or less the length A1, the chamfering shape of the slab by the free-side corner portions 10ac and 10bc becomes even more optimal, and the above corner cracking and solidification cracking are more easily suppressed. The maximum interval E1 may be 0.2 times or less or 0.1 times or less the length A1. Note that the maximum interval E1 is the interval between the free-side corner portions 10ac and 10bc and the line segment P1P3, and refers to the maximum value of the intervals in the direction orthogonal to the line segment P1P3.

[0030] The same applies to the shapes of the fixed-side corner portions 10ad and 10bd. That is, in the fixed-side corner portions 10ad and 10bd, the maximum interval between the line segment from the connection point P2 to the connection point P4 and the fixed-side corner portions 10ad and 10bd is 0 mm or more, and may be 0.3 times or less, 0.2 times or less, or 0.1 times or less the length A2.

[0031] 2. Continuous casting method of slab The continuous casting method of a slab according to one embodiment is supplying molten steel from a submerged nozzle to a mold 100, continuously drawing out the slab from the lower end of the mold 100, and straightening the slab drawn out from the mold 100, and includes the mold 100 being the continuous casting mold of the present disclosure characterized by this. In the present embodiment, the basic configuration of the continuous casting apparatus may be the same as the conventional one except that the continuous casting mold of the present disclosure is used, and the continuous casting conditions may also be the same as the conventional ones. Since the conditions other than the continuous casting mold are self-evident in the technical field, detailed description is omitted here.

[0032] Furthermore, the mold 100 for continuous casting of the present disclosure may be employed as a mold in a curved continuous casting machine or as a mold in a vertical bending continuous casting machine. In any continuous casting machine, where solidification cracking and corner cracking at the straightening point are problems, continuous casting is performed using the mold for continuous casting of the present disclosure, thereby suppressing solidification cracking and corner cracking.

[0033] 3. Continuously cast slab The technology of the present disclosure also has a side surface as a continuously cast slab. When a continuously cast slab is obtained using the mold 100 for continuous casting of the present disclosure, the cross-sectional shape perpendicular to the casting direction of the continuously cast slab is a shape corresponding to the opening shape of the mold 100 for continuous casting of the present disclosure. For example, in a continuously cast slab according to an embodiment, the cross-sectional shape perpendicular to the casting direction is a pair of short sides, a free-side long side, a fixed-side long side, a free-side corner portion connecting the short side and the free-side long side, and a fixed-side corner portion connecting the short side and the fixed-side long side, defined by, and the cross-sectional shape satisfies the following conditions (1) to (3): 10 ≦ A1 ≦ 30 ··· (1) A1 + A2 ≦ 30 ··· (2) 1.00 ≦ B1 / A1 ≦ 3.00 ··· (3) A1: length from intersection point O1 to connection point P1 (mm) A2: length from intersection point O2 to connection point P2 (mm) B1: length from intersection point O1 to connection point P3 (mm) Intersection point O1: intersection of the extension line of the short side and the extension line of the free-side long side Intersection point O2: intersection of the extension line of the short side and the extension line of the fixed-side long side Connection point P1: connection point of the free-side corner portion and the free-side long side Connection point P2: connection point of the fixed-side corner portion and the fixed-side long side Connection point P3: connection point of the free-side corner portion and the short side It may also satisfy the following.

[0034] 4. Supplementary In addition, the mold 100 for continuous casting of the present disclosure may have an opening shape that continuously or intermittently changes from its inlet to its outlet. For example, the mold 100 for continuous casting may be provided with a taper. As far as the inventor has confirmed, the effects brought about by satisfying the above conditions (1) to (3) are similarly exhibited regardless of the amount of taper or multiple tapers in the mold.

Examples

[0035] Hereinafter, the present invention will be further described while showing examples, but the present invention is not limited to the following examples. The present invention can adopt various conditions as long as it does not deviate from the gist and can achieve its purpose.

[0036] 1. Continuous casting conditions Using a curved continuous caster, slabs with a width of 1100 - 2200 mm and a thickness of 250 mm were produced. Here, by providing a protrusion at the end of the short-side copper plate of the mold, chamfered slabs were produced. Regarding the opening shape of the mold, A1, A2, B1, and B2 shown in FIG. 2 were changed to examine the optimal shape. When A1 = A2 = B1 = B2 = 0, the opening shape of the mold is rectangular.

[0037] The steel type to be cast is a medium carbon steel with C: 0.08 mass%, Si: 0.2 mass%, Mn: 1.5 mass%, P: 0.01 mass%, S: 0.003 mass%, Ni: 0.4 mass%, Nb: 0.01 mass%, Ti: 0.01 mass%, and continuous casting was performed at a casting speed of 1.3 m / min.

[0038] 2. Evaluation conditions Regarding the slabs produced as described above, the occurrence situation of cracks was evaluated. The occurrence situation of cracks was used as a comprehensive evaluation and was the total of the indices for each classified crack form. The crack indices were two types: "corner crack index" and "solidification crack index".

[0039] 2.1 Corner Crack Index For the "Corner Crack Index", in the steady state part of each casting condition, a slab about 10 m in length was cut out for evaluation. Specifically, about 100 mm from the end of the long side surface on the free side of the slab was ground with a 1 mm surface grinder from the surface, and the ground surface was subjected to a dye penetrant inspection to visually observe the presence or absence of cracks. When cracks were observed, grinding was further performed by 1 mm or 2 mm, and the dye penetrant inspection was performed again. Grinding and the dye penetrant inspection were repeated until the cracks disappeared, the crack depth was measured, and the degree of corner cracking was indexed according to the following criteria.

[0040] 0: No corner cracks were observed 1: Corner cracks with a crack depth of 2 mm or less were observed 2: Corner cracks with a crack depth exceeding 2 mm and less than or equal to 5 mm were observed 3: Corner cracks with a crack depth exceeding 5 mm were observed

[0041] 2.2 Solidification Crack Index For the "Solidification Crack Index", a C-section sample including the slab end taken from the above-mentioned steady state part was etched with picric acid to reveal the dendrite structure, and the length of solidification cracks along the dendrite trunks near the surface (50 mm from the surface of the slab to the inside) was measured and evaluated. The degree of solidification cracking was indexed according to the following criteria.

[0042] 0: No solidification cracks exist 1: The total length of the observed solidification cracks is 10 mm or less. 2: The total length of the observed solidification cracks exceeds 10 mm and is 30 mm or less. 3: The total length of the observed solidification cracks exceeds 30 mm.

[0043] 3. Evaluation Results Table 1 below shows the mold opening shapes and crack evaluation results for each of No. 1 to 10.

[0044]

Table 1

[0045] For No.1 and 2, although corner cracks did not occur, solidification cracks occurred. For No.1 and 2, since the ratio B1 / A1 of the length B1 to the length A1 in the free-side corner part of the mold opening shape was inappropriate, an air gap was generated between the mold and the solidification shell during the solidification progress of the slab, resulting in unstable solidification, and there was a possibility that the solidification shell could not be uniformly formed in the mold.

[0046] For No.3 and 4, although solidification cracks did not occur, corner cracks occurred. For No.3 and 4, the length A1 in the free-side corner part of the mold opening shape was short, the chamfering of the free-side corner part of the slab became small, the temperature decreased in the free-side corner part of the slab, the temperature of the free-side corner part during straightening became in the embrittlement temperature range, and it is considered that cracks occurred due to the tensile stress applied during straightening.

[0047] For No.5 and 6, although corner cracks did not occur, solidification cracks occurred. For No.5 and 6, since the sum A1 + A2 of the length A1 in the free-side corner part of the mold opening shape and the length A2 in the fixed-side corner part was too large, an air gap was generated between the mold and the solidification shell during the solidification progress, and there was a possibility that the solidification shell could not be uniformly formed in the mold.

[0048] On the other hand, for No.7 to 10, both corner cracks and solidification cracks could be significantly suppressed. From the results of No.7 to 10, it can be said that by using a continuous casting mold having the following configurations (I) and (II) in the continuous casting of slabs, it is possible to achieve both prevention of corner cracks in the slab and stability of initial solidification.

[0049] (I) The opening shape of the continuous casting mold in a cross-section perpendicular to the casting direction is defined by a pair of short sides, a free-side long side, a fixed-side long side, a free-side corner part connecting the short side and the free-side long side, and a fixed-side corner part connecting the short side and the fixed-side long side. (II) The opening shape satisfies the following conditions (1) to (3). 10 ≤ A1 ≤ 30 ···(1) A1 + A2 ≤ 30 ···(2) 1.00 ≤ B1 / A1 ≤ 3.00 ···(3) A1: Length from intersection point O1 to connection point P1 (mm) A2: Length from intersection point O2 to connection point P2 (mm) B1: Length from intersection point O1 to connection point P3 (mm) Intersection point O1: Intersection of the extension line of the short side and the extension line of the free-side long side Intersection point O2: Intersection of the extension line of the short side and the extension line of the fixed-side long side Connection point P1: Connection point of the free-side corner part and the free-side long side Connection point P2: Connection point of the fixed-side corner part and the fixed-side long side Connection point P3: Connection point of the free-side corner part and the short side

Explanation of symbols

[0050] Mold for continuous casting 1a Short-side mold 1b Short-side mold 1c Free-side long-side mold 1d Fixed-side long-side mold 10a Short side 10b Short side 10c Free-side long side 10d Fixed-side long side 10ac Free-side corner part 10bc Free-side corner part 10ad Fixed-side corner part 10bd Fixed-side corner part

Claims

1. A mold for continuous casting, wherein the opening shape of the mold for continuous casting in a cross-section orthogonal to the casting direction is a pair of short sides, a free-side long side, a fixed-side long side, a free-side corner portion connecting the short side and the free-side long side, and a fixed-side corner portion connecting the short side and the fixed-side long side, defined by, and the opening shape satisfies the following conditions (1) to (3): 10 ≦ A1 ≦ 30... (1) A1 + A2 ≦ 30... (2) 1.00 ≦ B1 / A1 ≦ 3.00... (3) A1: The length from the intersection point O1 to the connection point P1 (mm) A2: The length from the intersection point O2 to the connection point P2 (mm) B1: The length from the intersection point O1 to the connection point P3 (mm) Intersection point O1: The intersection point of the extension line of the short side and the extension line of the free-side long side Intersection point O2: The intersection point of the extension line of the short side and the extension line of the fixed-side long side Connection point P1: The connection point of the free-side corner portion and the free-side long side Connection point P2: The connection point of the fixed-side corner portion and the fixed-side long side Connection point P3: The connection point of the free-side corner portion and the short side satisfies, a mold for continuous casting.

2. The mold for continuous casting according to Claim 1, wherein the opening shape satisfies the following condition (4) or (5): A2 = B2 = 0... (4) 1.00 ≦ B2 / A2 ≦ 3.00... (5) B2: The length from the intersection point O2 to the connection point P4 (mm) Connection point P4: The connection point of the fixed-side corner portion and the short side satisfies, a mold for continuous casting.

3. A method for continuous casting of a slab, comprising supplying molten steel from a submerged nozzle to a mold, continuously withdrawing the slab from the lower end of the mold, and straightening the slab withdrawn from the mold, and the mold is the mold for continuous casting according to Claim 1 or 2, a method for continuous casting of a slab.

4. A continuously cast slab, wherein the cross-sectional shape of the continuously cast slab orthogonal to the casting direction is a pair of short sides, a free-side long side, a fixed-side long side, a free-side corner portion connecting the short side and the free-side long side, and a fixed-side corner portion connecting the short side and the fixed-side long side, defined by, and the cross-sectional shape satisfies the following conditions (1) to (3): 10 ≦ A1 ≦ 30... (1) A1 + A2 ≦ 30... (2) 1.00 ≦ B1 / A1 ≦ 3.00... (3) A1: The length from the intersection point O1 to the connection point P1 (mm) A2: The length from the intersection point O2 to the connection point P2 (mm) B1: The length from the intersection point O1 to the connection point P3 (mm) Intersection point O1: The intersection point of the extension line of the short side and the extension line of the free-side long side Intersection point O2: The intersection point of the extension line of the short side and the extension line of the fixed-side long side Connection point P1: The connection point of the free-side corner portion and the free-side long side Connection point P2: The connection point of the fixed-side corner portion and the fixed-side long side Connection point P3: The connection point of the free-side corner portion and the short side satisfies Continuous casting slab

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