Technical field
[0001]
​PROBLEM TO BE SOLVED: to provide a continuous casting method of a cast slab, to provide a method for continuously casting a slab using a continuous casting machine of a curved or vertical bending type
​Background
[0002]
​In continuous casting, molten steel is injected into a tundish from a ladle, in addition, molten steel is injected into the mold from the tundish. A solidified shell is formed in an outer peripheral part of the molten steel in a mold, the cast slab in this state (the solidified shell and the inside molten steel) is pulled out of the mold. Thereafter, the cast slab is solidified to the inside by secondarily cooling in a spray band. The cast slab obtained in this manner is provided, is cut to an appropriate size, and after the temperature is set to an appropriate temperature by the mass re-heating, the ingot is divided and rolled
[0003]
​Depending on the cooling condition of the slab, cracking occurs on the surface of the slab at the time of batch reheating. Therefore, in order to prevent such cracking, a cooling method of the slab is devised. For example, in order to make the structure of a slab surface layer fine, cooling (tertiary cooling) is performed using a blue cooler which is a cooling device outside the continuous casting machine
[0004]
​Patent document 1 discloses a method of cutting a continuously cast slab into a predetermined length, and a method for cooling from a temperature region just above the ar3 point is described using a blue cooler. PROBLEM TO BE SOLVED: to solve the problem that in patent document 1, of the present invention water density is 5 × 10-4 to 4 × 10-3 m3／ SM2 (= 30-240L)／ minute／ m2) and the water volume density of the side surface and the lower surface of the slab is calculated, and the water volume density on the upper surface of the slab is made different from that of the upper surface of the cast slab, cracks occurring during cooling can be prevented

[0005]
​In patent document 2, a blue cooler is used, when cooling the cast slab at a temperature just above the ar3 point, the moving speed of the slab is set to 3 to 10 m／ minutes. PROBLEM TO BE SOLVED: to solve the problem of japanese patent application laid-open no. (2) so that the lower surface of the slab can be uniformly cooled
[0006]
​SOLUTION: the method of patent documents 1 and 2 includes, and it is intended that there is a tissue in which the gamma-grains have been miniaturized
[0007]
​On the other hand, in the patent document 3, the slab is rapidly cooled during the secondary cooling, the structure of the surface layer of the cast slab is reformed to a tissue having high temperature ductility, a cast slab having no crack on its surface is obtained
​Prior art literature
​Patent document
[0008]
​Patent document 1: japanese patent application laid-open no. 10-1719
​Patent document 2: japanese patent application laid-open no. 2005-40837
​Patent document 3: japanese patent application laid-open no. 2002-307149 [0009]
​However, any of patent documents 1 and 2 May be adopted, cracking may occur at the time of recuperation of the cast slab, and cracking occurs at the time of ingot rolling. When the cast slab is rapidly cooled, of the present invention a part of which is martensite and expanded at the time of condensation, thermal stress is generated between the surface layer and the inside of the slab at the time of batch reheating
[0010]
​Furthermore, in recent years, a method for extremely reducing the cooling capacity of the tertiary cooling has been proposed, a sufficient effect is not obtained
[0011]
​The corner part of the slab has a width direction (long side direction) in the width direction (long side direction) of the slab at the time of cooling,) and in two directions in the thickness direction (short side direction). Therefore, in the method of patent document 3, and quenching is performed only to modify the structure of the long side surface of the slab, the crack in the corner part tends to increase
[0012]
​SOLUTION: the present invention provides, and to provide a continuous casting method capable of manufacturing a cast slab in which surface crack is difficult to occur in a process from secondary cooling to minute rolling
​Means for solving the problem
[0013]
​The present inventors have cooling for modifying the structure of the slab during secondary cooling, and a corner part (a region within 20 mm from the apex and ridge of the slab in the present invention). SOLUTION: the same); cooling (first water-cooling step) of cooling a part other than the corner part of the slab; a cooling (second water cooling step) are divided into a plurality of categories ​Only corner part of cast piece, and after the end of the first water cooling step for cooling the slab so that the surface temperature becomes less than the ar3 point, and the entire surface of the long side surface of the slab including the corner part of the slab is condensed to a temperature equal to or higher than the ar3 point, and after the heat condensation step is performed, and the whole length of the long side face of the slab including the corner part of the slab is less than the ar3 point of the present invention temperature; and a second water-cooling step of cooling the second water-cooling step to a temperature after the end of the second water-cooling step, the corner part of the slab is held at a temperature less than the ar3 point, and the part other than the corner part of the slab is condensed to a temperature equal to or higher than the ar3 point ​As a result, a cast slab in which the whole surface including the corner part of the slab is reformed is obtained, and it is possible to prevent surface cracking in a process from secondary cooling to minute rolling, based on such knowledge, the present invention is described below. In the following description, "ar3 point -900° c" means an ar3 point or more and less than 900° c. in addition, "x-y" means a numerical range, unless otherwise specified, x is x or more and y or less ​SOLUTION: a curved or vertical bending type of the present invention a method for continuously casting a slab using a continuous casting machine, a step in a secondary cooling zone in which cooling is performed from just under the mold with respect to the slab drawn out of the mold, a first water-cooling step and a first heat recovery step performed after the first water-cooling step, and a second water cooling step performed after the first condensation step, and a second heat recovery step which is performed after the second water cooling step,
​In the first water cooling step, and cooling water is supplied to the wide surface of the slab having a surface temperature of 1,000°c or higher, only a corner part which is a region within 20 mm from the apex and ridge of the cast piece, the surface temperature is less than the ar3 point and the surface temperature is less than the ar3 point, and the surface temperature of the part of the cast slab other than the corner part remains at an ar3 point or more, a step of cooling the slab,
​The first condensation step is performed, and the whole surface temperature of the slab including the corner part is equal to or higher than an ar3 point, a step of condensing the cast slab,
​The second water cooling step is performed, and the cooling water is supplied to the wide surface of the slab having a surface temperature of ar3 to 900° c, and the whole surface temperature of the slab including the corner part is less than the ar3 point, a step of cooling the slab,
​The second heat condensation step holds the surface temperature of the corner part at a temperature less than ar3 point, and the surface temperature of the part of the cast slab other than the corner part becomes equal to or higher than the ar3 point, a step of condensing the cast slab,
​Method for continuous casting of cast slab
[0015]
​Here, the "slab" in the present invention is, and has a thickness of 200 mm or more, in the slab in the present invention, a so-called "slab (slab))" and " bloom (bloom) ". When the cooling by the first water cooling step is started, of the present invention "1000°c or more" which is the surface temperature of the cast slab, " ar3 point to 900°c ", which is the surface temperature of the slab when the cooling by the second water cooling step is started, and the depth from the surface is 10 mm in the center in the width direction of the slab ​In addition, and whether the temperature is less than or equal to an ar3 point is controlled by cooling or recuperation, and the "surface temperature" of a part other than the corner part and the corner part of the cast slab is also set to the "surface temperature" of the part other than the corner part and the corner part of the cast slab, and the depth from the surface of the slab is 10 mm. these surface temperatures can be determined, for example, by calculation by coagulation heat transfer analysis. The "wide surface" is also a " wide surface ", and a long side (a side in the width direction of the slab) defining a cross section obtained by cutting a slab in a plane having a longitudinal direction of the slab as a normal direction) and short sides (sides in the thickness direction of the slab). In other words, the broad surface means the upper surface and the lower surface of the slab ​The "first water cooling step" and the "second water cooling step" in the present invention are also provided, from the upper surface side and the lower surface side of the slab, the cooling water is supplied to the entire surface of the wide surface of the slab when the slab is a slab slab, the cooling water is supplied to a part other than the corner part of the wide surface of the slab when the slab is a bloom slab, and the whole surface of the wide surface of the slab including the corner part of the slab is water-cooled
[0016]
​SOLUTION: an angle part cooled to a temperature of less than an ar3 point in a first water cooling step, at a temperature equal to or higher than the ar3 point in a first condensing process utilizing sensible heat and latent heat of unsolidified molten steel existing in the interior of the slab, the surface layer of only the corner part of the slab (the outermost surface of the slab) (5~10mm) a region having a thickness of 5 mm or less. The same applies to the following. Thus, a tissue in which the gamma grain boundary is unclear can be formed. The tissue is a mixed tissue of ferrite and pearlite ​More specifically, when the slab is cooled from the high temperature side to the lower temperature side than the ar3 point, and the ferrite is a solidified structure in a state in which the ferrite is formed in a grain boundary in the gamma grain boundary the structure has high temperature ductility. Here, in order to form a tissue in which the gamma grain boundary is unclear, after the temperature is set to a temperature less than the ar3 point, it is necessary to return the temperature to an ar3 point or more. SOLUTION: in the present invention, and the surface temperature of a part other than the corner part of the slab is a temperature equal to or higher than the ar3 point. Therefore, a first water-cooling step and a first heat-condensation step may be performed, a tissue which does not obscure the gamma grain boundary is not formed at a part other than the corner part of the slab ​SOLUTION: in the present invention, and the water volume density of the cooling water supplied to each of the upper surface and the lower surface of the slab is referred to as the water volume density of the cooling water supplied to each of the upper surface and the lower surface of the slab, and the amount of water supplied per unit time per unit surface area of the slab. In addition, "time for supplying cooling water" is " time for supplying cooling water ", and a time (cooling time) for supplying cooling water to each of the upper surface and the lower surface of the slab
​SOLUTION: a time for supplying water quantity density and cooling water in a first water cooling step and a second water cooling step, within the above-mentioned range, by cooling with a smaller amount of cooling water than before, it is easy to form a tissue in which the gamma grain boundary is unclear in the surface layer of a part other than the corner part and the corner part. thereby, the amount of cooling water used in the secondary cooling band can be made smaller than that of the conventional one, the surface crack can be prevented in a process from the secondary cooling to the minute rolling ​Here, the part to be water-cooled by the second water-cooling step in the longitudinal direction of the slab is, compared to a part to be a water-cooled object by the # t first water-cooling step, the temperature is low because it is located on the downstream side in the moving direction of the # t-cast slab. Therefore, in the second water cooling step, compared to the first water cooling step, the amount of cooling water used in # t can be reduced, a part other than the corner part of the slab can be cooled to a temperature less than the ar3 point
[0019]
​In the present invention, it is preferable that the time for condensing the slab in the first condensation step is two minutes or more
​In the present invention, it is preferable that the time for condensing the slab in the second heat recovery step is two minutes or more
[0020]
​In the first heat recovery step, for example, the time for condensing the cast slab is two minutes or more, substantially all over the entire width direction of the surface of the slab, the surface layer of the slab is easily condensed to a temperature equal to or higher than the ar3 point. In the second heat condensation step, for example, and the time for condensing the cast slab is set to be two minutes or more, the surface layer of a part other than the corner part of the slab is easily condensed to a temperature equal to or higher than the ar3 point. After cooling to a temperature of less than the ar3 point, at a temperature equal to or higher than the ar3 point, it is possible to form a tissue in which the gamma grain boundary is unclear, in such a form, the surface crack is easily prevented in a process from the secondary cooling to the minute rolling
[0021]
​FIG. 1 shows a water-cooled cast slab, and an example of the relationship between the elapsed time of # t and the temperature of the surface of the slab and the temperature inside the cast slab. The # t surface temperature is a temperature measured by a thermocouple installed on the surface of the slab, in the # t internal temperature, and a temperature measured by a thermocouple installed at a site of a depth of 22 mm from the surface of the slab. In this example, and when water-cooling which is 1123 k is stopped (indicated by an one-dot chain line t0). when two minutes have elapsed (indicated by an one-dot chain line t2). at the time of the lapse of three minutes (indicated by an one-dot chain line t3). the surface temperature of the cast slab is found to be condensed to an ar3 point or higher between them
​On the other hand, as shown in FIG. 1, the heat recovery time may be longer than three minutes, the effect of returning to an ar3 point or more is saturated. For this reason, it is preferable that the heat recovery time is, for example, 2 to 3 minutes
​Effects of the invention
[0022]
​SOLUTION: according to the present invention, a structure having high high-temperature ductility is formed over almost the whole area of the surface of the slab, the cast slab can be produced. Thereby, a process from secondary cooling to minute rolling is performed () for example, a secondary cooling step, a condensing step, a minute re-heating step, and a partial ingot rolling step) it is possible to prevent the occurrence of cracks on the surface of the slab ​Simple description of drawings
[0023]
​A water-cooled cast slab is provided, and shows an example of the relationship between the elapsed time, the surface of the slab, and the temperature of the inside
​FIG. 2 is a diagram for explaining a continuous casting method of the cast slab of the present invention
​FIG. 3 is a diagram showing a region including a position where tissue is observed in a slab cross section;
​FIG. 4 is a comparison example (1) and a cross section of a corner part of a slab to which the continuous casting method of the present invention is carried out is described
​FIG. 5 is a comparative example (6) is a diagram for explaining a cross section of a center part of a slab to which the continuous casting method of the present invention is carried out
​FIG. 6 is a comparison example (6) and a cross section of a corner part of a slab to which the continuous casting method of the present invention is carried out is described
​FIG. 7 is an example (1) and a cross section of a corner part of a slab to which the continuous casting method of the present invention is carried out is described
​System for carrying out the invention
[0024]
​An embodiment of the present invention is described below. The following forms are examples of the present invention, the present invention is not limited to the following modes. SOLUTION: in a secondary cooling band for cooling a slab drawn out of a mold, a cooling mode and a condensate form are specifically specified
[0025]
​FIG. 2 is a diagram for explaining a continuous casting method of a slab according to the present invention. SOLUTION: as shown in fig. 2, (s2); a second water cooling step (S3); and a second heat recovery step (S4). s1 to s4 are steps included in the secondary cooling band
[0026]
​&T; first water cooling steps (S1) & gT;
​The first water cooling step (hereinafter referred to as "s1") is sometimes referred to as " s1 ". the cooling water is supplied to the wide surface of the slab having a surface temperature of 1,000°c or higher, only the corner part of the slab, the surface temperature is less than the ar3 point and the surface temperature is less than the ar3 point, and the surface temperature of the part of the cast slab other than the corner part remains at an ar3 point or more, a step of cooling the slab
[0027]
​As described above, in the present invention, the texture of the corner part of the slab is improved, and the tissue modification of a part other than the corner part of the # t-cast slab is separately performed, and after the tissue modification of the corner part of the # t-cast slab is performed, and the tissue of a part other than the corner part of the # t-cast slab is reformed. # Ts1 represents only the corner part of the slab # t of # t and a step of cooling necessary for performing the # t tissue modification. SOLUTION: in order to perform tissue modification in the present invention, it is necessary to temporarily cool down to a temperature of less than the ar3 point ​S1 is a step of cooling necessary for reforming the tissue of the corner of the slab, s1 is a part to be cooled to a temperature less than the ar3 point, and the surface temperature of a part other than the corner part of the cast slab is set to be lower than the surface temperature of the part other than the corner part of the cast slab, at a temperature equal to or higher than the ar3 point. That is, in s1, so that the surface temperature of a part other than the corner part of the slab remains at an ar3 point or more, and the surface temperature of the corner part of the slab is less than the ar3 point, cooling water is supplied to the slab to cool the slab

claims
1​.A method for continuously casting a slab using a continuous casting machine of a curved or vertical bending type,
​SOLUTION: in a step of a secondary cooling band in which cooling is performed from just under a mold to a slab drawn out from a mold, a first water-cooling step and a first heat recovery step performed after the first water-cooling step, a second water-cooling step performed after the first condensation step, and a second heat recovery step which is performed after the second water cooling step,

​In the first water cooling step, and cooling water is supplied to the wide surface of the slab having a surface temperature of 1,000°c or higher, only a corner part which is a region within 20 mm from a vertex and a ridge of the slab, the surface temperature is less than the ar3 point and the surface temperature is less than the ar3 point, and the surface temperature of the part of the slab other than the corner part remains at an ar3 point or more, a step of cooling the slab,
​The first recuperation step is performed, and the whole surface temperature of the slab including the corner part is equal to or higher than an ar3 point, a step of condensing the slab,
​The second water cooling step is performed, and the cooling water is supplied to the wide surface of the slab having a surface temperature of ar3 to 900° c, and the whole surface temperature of the slab including the corner part is less than an ar3 point, a step of cooling the slab,
​The second heat condensation step holds the surface temperature of the corner part at a temperature less than ar3 point, and the surface temperature of the part of the slab other than the corner part becomes equal to or higher than the ar3 point, a method for continuously casting the cast slab, which is a step of condensing the slab
​[ Claim 2 ]
​The water content density of the cooling water supplied to the slab in the first water cooling step is 170 to 290 l／ minute／ m2, and a time for supplying the cooling water to the slab in the first water cooling step is 0.95 to 4.0 minutes, a claim (1) the continuous casting method of the cast slab
​[ Claim 3 ]
​The water content density of the cooling water supplied to the slab in the second water cooling step is 170 to 290 l／ minute／ m2, and a time for supplying the cooling water to the slab in the second water cooling step is 0.95 to 4.0 minutes, a claim (1) or a method for continuously casting the cast slab as described in (2)
​[ Claim 4 ]
​The time for condensing the slab in the first condensation step is two minutes or more (1~3) the continuous casting method of the cast slab according to any one of the above items (1) to (3)
​[ Claim 5 ]
​The time for condensing the slab in the second heat recovery step is two minutes or more (1~4) the continuous casting method of the cast slab according to any one of the above items (1) to (3)