Continuous-cast slab, method and apparatus of manufacturing the same, and method and apparatus of manufacturing thick steel plate

1. A continuous-cast slab of 0.1 to 0.3 in ratio D/W, the ratio D/W being a ratio of casting thickness D to casting width W, and 230 to 380 mm in casting thickness D, the continuous-east slab having a horizontally symmetrical granular equiaxed crystals at least in a center in a thickness direction, the continuous-cast slab comprising:

a first reduction dent and a second reduction dent that is narrower than the first reduction dent at least on one long side surface, the second reduction dent denting further from a bottom surface of the first reduction dent,

wherein a dent amount d 1 of the first reduction dent from an end surface of the continuous-cast slab is 0.08 to 1.1 mm, and a dent amount d 2 of the second reduction dent from the bottom surface of the first reduction dent is 1.2 to 12 mm, and/or a dent rate of the first reduction dent from an end surface of the continuous-cast slab, to the casting thickness D is 0.03 to 0.36%, and a dent rate of the second reduction dent from the bottom surface of the first reduction dent, to the casting thickness D is 0.6 to 4%.

2. The continuous-cast slab according to claim 1 , wherein

distance between either end of the first reduction dent and the end surface of the continuous-cast slab is 0.37×the casting thickness D to 1.0×the casting thickness D, and distance between either end of the second reduction dent and the end surface of the continuous-cast slab is 0.5×the casting thickness D to 1.2×the casting thickness D.

3. The continuous-cast slab according to claim 1 , wherein a maximum porosity volume is no more than 1.5×10 −4 cm 3 /g.

4. A method of manufacturing a continuous-cast slab, the method comprising:

a first step of forming a first reduction dent at least in one long side surface of the continuous-cast slab by carrying out reduction with first reduction rolls on the continuous-cast slab, the continuous-cast slab being 0.1 to 0.3 in ratio D/W, the ratio D/W being a ratio of casting thickness D to casting width W, and 230 to 380 mm in casting thickness D, the continuous-cast slab having a horizontally symmetrical granular equiaxed crystals at least in a center in a thickness direction; and

a second step of forming a second reduction dent that is narrower than the first reduction dent by carrying out further reduction on a bottom surface of the first reduction dent with second reduction rolls that are narrower than the first reduction rolls,

wherein in the first step, the reduction is carried out on the continuous-cast slab so that a dent amount d 1 of the first reduction dent from an end surface of the continuous-cast slab is 0.08 to 1.1 mm, and

in the second step, the reduction is carried out on the continuous-cast slab so that a dent amount d 2 of the second reduction dent from the bottom surface of the first reduction dent is 1.2 to 12 mm, and/or

wherein in the first step, the reduction is carried out on the continuous-cast slab so that a dent rate of the first reduction dent from an end surface of the continuous-cast slab, to the casting thickness D is 0.03 to 0.36%, and

in the second step, the reduction is carried out on the continuous-cast slab so that a dent rate of the second reduction dent from the bottom surface of the first reduction dent, to the casting thickness D is 0.6 to 4%.

5. The method according to claim 4 , wherein

the first reduction rolls are provided for a zone where a fraction solid is 0.3 to 0.7, and the second reduction rolls are provided for a zone where a fraction solid is 0.7 to 1.0, downstream from the first reduction rolls.

6. The method according to claim 4 , wherein

distance between either end of the first reduction dent and an end surface of the continuous-cast slab is 0.37×the casting thickness D to 1.0×the casting thickness D, and distance between either end of the second reduction dent and the end surface of the continuous-cast slab is 0.5×the casting thickness D to 1.2×the casting thickness D.

7. The method according to claim 4 , wherein

a maximum porosity volume of the continuous-cast slab manufactured through the first and second steps is no more than 1.5×10 4 cm 3 /g.

8. An apparatus of manufacturing a continuous-cast slab, the apparatus comprising:

first reduction rolls that shape an intermediate shaped product having a first reduction dent at least on one long side surface of the continuous-cast slab, the continuous-cast slab being 0.1 to 0.3 in ratio D/W, the ratio D/W being a ratio of casting thickness D to casting width W, the continuous-cast slab being 230 to 380 mm in casting thickness D, and having horizontally symmetrical granular equiaxed crystals at least in a center in a thickness direction; and

second reduction rolls that shape a second reduction dent denting further from a bottom surface of the first reduction dent of the intermediate shaped product and being narrower than the first reduction dent, the second reduction rolls having shapes narrower than the first reduction rolls and being arranged downstream from the first reduction rolls,

wherein the first reduction rolls are provided so that a dent amount d 1 of the first reduction dent from an end surface of the continuous-cast slab is 0.08 to 1.1 mm, and the second reduction rolls are provided so that a dent amount d 2 of the second reduction dent from the bottom surface of the first reduction dent is 1.2 to 12 mm, and/or the first reduction rolls are provided so that a dent rate of the first reduction dent from an end surface of the continuous-cast slab, to the casting thickness D is 0.03 to 0.36%, and the second reduction rolls are provided so that a dent rate of the second reduction dent from the bottom surface of the first reduction dent, to the casting thickness D is 0.6 to 4%.

9. The apparatus according to claim 8 , wherein

the first reduction rolls are provided for a zone where a fraction solid is 0.3 to 0.7, and the second reduction rolls are provided for a zone where a fraction solid is 0.7 to 1.0, downstream from the first reduction rolls.

10. The apparatus according to claim 8 , wherein

the first reduction rolls are provided so that distance between either end of the first reduction dent and the end surface of the continuous-cast slab is 0.37×the casting thickness D to 1.0×the casting thickness D, and the second reduction rolls are provided so that distance between either end of the second reduction dent and the end surface of the continuous-cast slab is 0.5×the casting thickness D to 1.2×the casting thickness D.

11. The apparatus according to claim 8 , wherein a maximum porosity volume of the continuous-cast slab is no more than 1.5×10 −4 cm 3 /g.

12. A method of manufacturing a thick steel plate comprising:

a continuous-cast slab manufacturing step of manufacturing a continuous-cast slab according to the method of manufacturing a continuous-cast slab of claim 4 ; and

a rolling step of rolling, within the range of 0.2 to 0.65 in maximum shape factor, the continuous-cast slab manufactured in the continuous-cast slab manufacturing step, the continuous-cast slab being no more than 2.5×10 4 cm 3 /g in maximum porosity volume.

13. The method according to claim 12 , wherein steel plate thickness to casting thickness D after the rolling step is ended is 50% to 80% by the rolling step.

14. The method according to claim 12 , wherein the steel plate thickness after the rolling step is ended is 150 to 300 mm by the rolling step.

15. An apparatus of manufacturing a thick steel plate, the apparatus comprising:

the apparatus of manufacturing a continuous-cast slab according to claim 8 ; and

a rolling mill that rolls the continuous-cast slab manufactured by the apparatus of manufacturing a continuous-cast slab,

wherein the rolling mill rolls, within the range of 0.2 to 0.65 in maximum shape factor, the continuous-cast slab of no more than 2.5×10 −4 cm 3 /g in maximum porosity volume.

16. The apparatus according to claim 15 , wherein the rolling mill makes steel plate thickness after said rolling 50% to 80% to casting thickness D.

17. The apparatus according to claim 15 , wherein the rolling mill makes steel plate thickness after said rolling 150 to 300 mm.