Method for manufacturing slab, method for manufacturing steel sheet, and continuous casting machine

Abstract

Provided is a method for manufacturing a slab capable of suppressing the occurrence of cross-sectional defects in the slab without forcibly rolling down a cast piece at the final stage of solidification.　In the method for manufacturing a slab manufactured using a continuous casting machine, a cast piece formed by cooling of a mold is cooled by a plurality of secondary cooling devices, and a surface heat recuperation amount of the cast piece in a range where a central solid phase ratio is more than 0.0 and equal to or less than 1.0 is set to a predetermined target value or less.

Description

Slab manufacturing method, steel plate manufacturing method and continuous casting machine

[0001] The present invention relates to a slab manufacturing method, a steel plate manufacturing method, and a continuous casting machine that can suppress the occurrence of cross-sectional defects.

[0002] In recent years, in the field of automobiles, steel sheets containing large amounts of elements such as Si, Mn, and Ti have been used from the viewpoint of reducing the weight of vehicle bodies and improving workability. Steel sheets containing large amounts of elements such as Si, Mn, and Ti frequently suffer from the problem of splitting into two pieces during rolling, which has significantly reduced productivity. Here, splitting into two pieces refers to a phenomenon in which a crack occurs in the center of the thickness of a steel sheet, causing the steel sheet to split into two pieces.

[0003] Even if the cracking does not become apparent at the stage of hot-rolled steel sheet, if the hot-rolled steel sheet is shipped with an internal crack, the cracking may occur during pressing or bending. For these reasons, it is necessary to eliminate defects in steel sheets that are the cause of the cracking.

[0004] The cause of the splitting is thought to be oxidation of cross-sectional defects in the slab (cross-sectional cracks present on the cut surface or voids in the final solidification portion) in the heating furnace. As a technique for suppressing the splitting, Patent Document 1 discloses a continuous casting method for steel in which the slab is forcibly pressed down at the end of solidification to close the voids in the final solidification portion.

[0005] Japanese Patent Application Laid-Open No. 2003-94154

[0006] The method disclosed in Patent Document 1 can prevent cross-sectional defects from occurring in the slab. However, the reaction force generated when rolling down the slab increases, which causes a problem of shortening the lifespan of the rolls and segments.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing a slab that can suppress the occurrence of cross-sectional defects in the slab without forcibly reducing the slab in the final stage of solidification. Another object of the present invention is to provide a continuous casting machine capable of producing such a slab, and a method for producing a steel plate using such a slab.

[0008] The means for solving the above problems are as follows. [1] A method for producing a slab using a continuous casting machine, wherein a slab formed by cooling a mold is cooled in a plurality of secondary cooling devices, and the amount of surface recuperation of the slab when the center solid fraction is in the range of more than 0.0 and 1.0 or less is set to a predetermined target value or less. [2] The method for producing a slab according to [1], wherein the target value is 200°C. [3] The method for producing a slab according to [1] or [2], wherein the specific water amount of the secondary cooling device satisfies the following (A) and (B): (A) The specific water amount from immediately below the mold in the casting direction to a position where the center solid fraction is 1.0 is set to 1.8 L / kg-steel or less. (B) A specific water amount 1 from immediately below the mold in the casting direction to a position halfway between the position where the center solid fraction is 1.0 and a specific water amount 2 from the halfway position to the position where the center solid fraction is 1.0 satisfy the following formula (1): (specific water amount 1 - specific water amount 2) ≦ 1.0 L / kg-steel ... (1) [4] The method for producing a slab according to any one of [1] to [3], which calculates the surface recuperation heat amount in real time and controls the amount of secondary cooling water in the secondary cooling device in real time so that the surface recuperation heat amount is equal to or less than a predetermined target value. [5] A method for producing a steel plate, which comprises heating a slab produced by the method for producing a slab according to any one of [1] to [4] and hot rolling it to produce a steel plate. [6] A continuous casting machine comprising: a mold for cooling molten steel; multiple secondary cooling devices for cooling a slab formed by cooling the mold; and a control device for controlling the secondary cooling devices, wherein the control device controls the secondary cooling devices so that the surface recuperation heat amount of the slab is equal to or less than a predetermined target value when the center solid fraction of the slab is in the range of greater than 0.0 and equal to or less than 1.0. [7] The continuous casting machine according to [6], wherein the target value is 200°C. [8] The continuous casting machine according to [6] or [7], wherein the control device controls the secondary cooling device so as to satisfy the following (A) and (B): (A) the specific water amount from directly below the mold in the casting direction to a position where the central solid fraction is 1.0 is set to 1.8 L / kg-steel or less.(B) A specific water amount 1 in the casting direction from directly below the mold to a position halfway up to the position where the central solid fraction becomes 1.0, and a specific water amount 2 from the halfway position to the position where the central solid fraction becomes 1.0 satisfy the following formula (1): (specific water amount 1 - specific water amount 2) ≦ 1.0 L / kg-steel (1) [9] The continuous casting machine according to any one of [6] to [8], wherein the control device calculates the surface recuperation amount in real time and controls the amount of secondary cooling water in the secondary cooling device in real time so that the surface recuperation amount becomes equal to or less than a predetermined target value.

[0009] According to the present invention, the occurrence of cross-sectional defects in a slab can be suppressed simply by controlling the secondary cooling of the continuous casting machine, without forcibly reducing the slab pressure at the final stage of solidification, thereby making it possible to suppress the occurrence of cross-sectional defects in a slab without shortening the lives of rolls and segments.

[0010] Fig. 1 is a schematic diagram showing a continuous casting machine capable of carrying out the slab manufacturing method according to this embodiment. Fig. 2 is a graph showing the relationship between the amount of surface recuperation and the tensile strain at the center of the thickness.

[0011] The present invention will be specifically described below through embodiments of the present invention. The following embodiments are preferred examples of the present invention, and the present invention is not limited to these embodiments.

[0012] 1 is a schematic side view of a continuous casting machine 100 capable of carrying out the slab manufacturing method according to this embodiment. The continuous casting machine 100 includes a mold 10, a tundish 12 installed above the mold 10, multiple pairs of strand support rolls 32 arranged in a row below the mold 10, multiple secondary cooling devices 42, 44, three thermal cameras 46, and a control device 48. A sliding nozzle 14 for adjusting the flow rate of molten steel 18 is installed at the bottom of the tundish 12, and a submerged nozzle 16 is installed below the sliding nozzle 14.

[0013] Molten steel 18 is poured into the mold 10 through the submerged nozzle 16. The molten steel 18 poured into the mold 10 is cooled and solidified by the mold 10, forming a solidified shell 20. This results in a cast 24 having the solidified shell 20 as its outer shell and an unsolidified layer 22 made of the molten steel 18 inside.

[0014] Below the mold 10, there are provided multiple pairs of strand support rolls 32, including support rolls 26, guide rolls 28, and pinch rolls 30. Of these, the pinch rolls 30 support the strand 24 and also serve as drive rolls for withdrawing the strand 24. In the gaps between adjacent strand support rolls 32 in the casting direction, there are provided multiple secondary cooling devices 42, 44, each equipped with a spray nozzle (not shown), such as a water spray nozzle or an air mist spray nozzle.

[0015] The slab 24 is cooled while being withdrawn by cooling water (hereinafter referred to as "secondary cooling water") sprayed from the spray nozzles of multiple secondary cooling devices 42, 44, reducing the internal unsolidified layer 22 and growing the solidified shell 20. Thereafter, the slab 24 is appropriately cooled, which advances the solidification of the unsolidified layer 22 until the slab 24 is completely solidified. Reference numeral 40 indicates the position at which the slab 24 has completed solidification.

[0016] A plurality of transport rolls 34 for transporting the cast strand 24 are provided downstream of the strand support rolls 32, and a strand cutter 36 for cutting the cast strand 24 into slabs 38 of a predetermined length is provided above the transport rolls 34. Once solidification is complete, the strand 24 is cut into slabs 38 of a predetermined length by the strand cutter 36. In this manner, slabs 38 are produced using the continuous casting machine 100.

[0017] The three thermal cameras 46 are installed between the secondary cooling devices 42 and 44 and between the secondary cooling devices 44. The three thermal cameras 46 measure the surface temperature of the slab 24 in a range wider than the range where the center solid fraction of the slab 24 is greater than 0.0 and less than 1.0, and generate temperature information indicating the surface temperature of the slab 24. The thermal cameras 46 output the generated temperature information to the control device 48. In this way, by measuring the surface temperature of the slab 24 in a range wider than the range where the center solid fraction of the slab 24 is greater than 0.0 and less than 1.0, the surface temperature in that range can be determined even if the range where the center solid fraction is greater than 0.0 and less than 1.0 varies.

[0018] In this embodiment, the central solid fraction of the slab 24 refers to the solid fraction at the center of the thickness of the slab excluding the ends in the width direction of the slab, but may be represented by the solid fraction at the center of the width direction and the thickness direction of the slab. The central solid fraction is an index showing the progress of solidification, and is expressed in the range of 0.0 to 1.0, with a central solid fraction of 0.0 meaning that the slab has not solidified and a central solid fraction of 1.0 meaning that the slab has completely solidified.

[0019] The control device 48 is, for example, a general-purpose computer such as a workstation or a personal computer. The control device 48 acquires temperature information indicating the surface temperature of the slab 24 from the three thermal cameras 46. If there is an area in which the surface temperature of the slab 24 cannot be measured by the thermal cameras 46, the surface temperature of that area may be supplemented with a heat transfer heat calculation value using the surface temperature measured by the thermal cameras 46.

[0020] The control device 48 uses heat transfer calculations to determine the center solid fraction of the slab 24 in a range greater than 0.0 and equal to or less than 1.0. The center solid fraction of the slab 24 is calculated by calculating the temperature at the center from the surface temperature of the slab 24 using heat transfer calculations. If the temperature at the center is lower than the liquidus temperature, the center solid fraction is set to 1.0, and if the temperature at the center is higher than the solidus temperature, the center solid fraction is set to 0.0. When the temperature at the center of the slab 24 is between the liquidus temperature and the solidus temperature, the center solid fraction can be determined, for example, from an iron-carbon binary phase diagram using the lever rule, and is set to a value calculated using the following equation (2):

[0021] Center solid fraction=(C0-CL) / (CS-CL) (2) In the above formula (2), C0 is the carbon concentration (mass%). CS is the carbon concentration (mass%) at which the temperature at the center of the slab 24 intersects with the solidus line in the iron-carbon binary phase diagram. CL is the carbon concentration (mass%) at which the temperature at the center of the slab 24 intersects with the liquidus line in the iron-carbon binary phase diagram.

[0022] The above-described method for calculating the center solid fraction is merely an example, and other methods may be used to calculate the center solid fraction of the slab 24. The control device 48 determines the surface temperature increase values ​​at, for example, ¼, ½, and ¾ positions in the width direction of the slab 24 within the specified center solid fraction range of greater than 0.0 and equal to or less than 1.0, and specifies the largest temperature increase value among these as the surface recuperation amount. The control device 48 controls the amount of secondary cooling water sprayed from the multiple secondary cooling devices 42, 44 so that the specified surface recuperation amount is equal to or less than a predetermined target value for the surface recuperation amount. This makes it possible to suppress the occurrence of cross-sectional defects in the slab.

[0023] The control device 48 may determine in real time the surface temperature rise values ​​at ¼, ½, and ¾ positions in the width direction of the slab 24 within the specified range, and specify the largest temperature rise value among these as the surface reheating amount in real time. The control device 48 may control in real time the amount of secondary cooling water sprayed from the multiple secondary cooling devices 42, 44 so that the specified surface reheating amount is equal to or less than a predetermined target value for the surface reheating amount. This can further suppress the occurrence of cross-sectional defects in the slab.

[0024] Figure 2 is a graph showing the relationship between the amount of surface recuperation and the tensile strain at the thickness center. The horizontal axis of Figure 2 represents the amount of surface recuperation (°C) in the range where the central solid fraction of the slab 24 is greater than 0.0 and less than or equal to 1.0. The vertical axis of Figure 2 represents the tensile strain at the thickness center of the slab 24. The tensile strain was determined by elastic-plastic analysis using FEM analysis software Abaqus, taking into account the temperature change of the slab 24 during secondary cooling. The boundary conditions for the heat transfer calculation were determined using a relationship between the amount of secondary cooling water and the heat transfer coefficient.

[0025] As shown in Figure 2, it was confirmed that when the surface reheating amount exceeds 200°C in the range where the central solid fraction is greater than 0.0 and equal to or less than 1.0, the tensile strain generated at the solid-liquid interface in the thickness center of the slab 24 increases. This tensile strain occurs when the surface layer of the slab 24, which has thermally expanded due to large reheating, normally deforms isotropically. However, because the width direction and casting direction of the slab 24 are constrained, buckling deformation occurs in the thickness direction, and the strain is generated in the thickness center due to the tension caused by this deformation. The inventors discovered that the tensile strain generated by this reheating causes cross-sectional defects in the slab 38.

[0026] Therefore, in the continuous casting machine 100 according to this embodiment, the control device 48 controls the amount of secondary cooling water sprayed from the multiple secondary cooling devices 42, 44 so that the surface recuperation amount is 200°C or less in a range in which the center solid fraction is greater than 0.0 and equal to or less than 1.0. This prevents tensile strain from occurring in the central thickness portion of the slab 24 at the final stage of solidification, thereby preventing cross-sectional defects from occurring in the produced slab 38. 200°C is an example of a target value for the surface recuperation amount, and this target value is determined in advance using the graph of FIG. 2 showing the relationship between the surface recuperation amount and the tensile strain in the central thickness portion.

[0027] In the continuous casting machine 100 according to this embodiment, the secondary cooling device 42 is provided in the casting direction from directly below the mold 10 to a position halfway up to the position where the center solid fraction is 1.0. The secondary cooling device 44 is provided in the casting direction from the halfway position to the position where the center solid fraction is 1.0. To reduce the surface recuperation amount in the range where the center solid fraction is greater than 0.0 and equal to or less than 1.0, it is necessary to either reduce the amount of secondary cooling water in the upstream secondary cooling device 42 to weaken the cooling, or increase the amount of secondary cooling water in the downstream secondary cooling device 44 to strengthen the cooling. An extensive investigation into this secondary cooling balance confirmed that a cooling balance can be achieved and the surface recuperation amount of the slab 24 can be kept 200°C or less by controlling the amounts of secondary cooling water in the secondary cooling devices 42 and 44 so as to satisfy the following conditions (A) and (B).

[0028] (A) The specific water amount of the secondary cooling devices 42, 44 provided in the casting direction from directly below the mold 10 to the position where the central solid fraction is 1.0 is set to 1.8 L / kg-steel or less. (B) The specific water amount 1 of the secondary cooling device 42 and the specific water amount 2 of the secondary cooling device 44 satisfy the following formula (1).

[0029] (Specific water volume 1 - Specific water volume 2)≦1.0L / kg-steel...(1)

[0030] The specific water amounts 1 and 2 of the secondary cooling devices 42 and 44 are preferably 0.4 L / kg-steel or more, which reduces the risk of internal cracking or breakouts occurring in the slab 24. The specific water amounts of the secondary cooling devices 42 and 44 can be calculated using the following equation (3).

[0031] Specific water amount (L / kg-steel) = Water amount per unit time in a specified range (L / min) ÷ (slab width (m) × thickness (m) × casting speed (m / min) × density (7800 kg / m ３ )) ... (3)

[0032] From these results, it is preferable that the control device 48 controls the secondary cooling devices 42, 44 so as to satisfy the above (A) and (B). Specifically, it is preferable that the control device 48 controls the amount of secondary cooling water sprayed from the secondary cooling devices 42, 44 so as to satisfy the above (A) and (B). This makes it possible to suppress the amount of surface reheating of the slab 24 to 200°C or less when the center solid fraction is in the range of greater than 0.0 and not more than 1.0, thereby suppressing the occurrence of cross-sectional defects in the slab 38 produced.

[0033] As described above, by carrying out the slab manufacturing method according to this embodiment, it is possible to suppress the occurrence of cross-sectional defects in the manufactured slab 38 without forcibly reducing the rolling of the slab at the final stage of solidification. Then, by heating the slab in a heating furnace and hot rolling it to produce a steel plate, it becomes possible to manufacture a steel plate in which double cracking is suppressed.

[0034] The present invention is not limited to the above embodiment and various modifications can be made. While the continuous casting machine 100 according to the present embodiment has been described as having three thermal cameras 46, this is not limiting. After secondary cooling conditions are set that allow the surface recuperation rate of the slab 24 to be equal to or less than the target value when the center solid fraction is in the range of greater than 0.0 and less than or equal to 1.0, the continuous casting machine 100 does not need to have a thermal camera 46. However, it is preferable to continue measuring the surface temperature using the thermal camera 46 even after the secondary cooling conditions are set. This allows changes in secondary cooling due to clogging of the spray nozzles in the secondary cooling devices 42, 44, for example, to be detected, and allows for proactive measures to be taken to prevent the surface recuperation rate of the slab 24 from exceeding the target value.

[0035] Although the continuous casting machine 100 has been described as having three thermal cameras 46, the number of thermal cameras 46 is not limited to three, and it is sufficient that the continuous casting machine 100 has at least one thermal camera 46. The control device 48 may acquire the surface temperature of the slab 24 in the range of the center solid fraction of the slab 24 greater than 0.0 and equal to or less than 1.0, using the surface temperature measured by the thermal camera 46 and a heat transfer calculation value using the measured surface temperature.

[0036] Next, an example will be described in which a slab was produced by controlling the secondary cooling of the secondary cooling devices 42, 44 of the continuous casting machine 100, the slab was heated, and the occurrence of splitting of the hot-rolled steel plate into two pieces was confirmed.

[0037] In the examples, the surface temperatures of slabs with a central solid fraction in the range of more than 0.0 and not more than 1.0 were measured, and the surface temperature rises at 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the slab were calculated using the measured surface temperatures. The largest temperature rise among these was taken as the surface temperature recuperation amount.

[0038] The component compositions of the steel types used in the examples, whether or not there was a change in secondary cooling, the amount of surface recuperation in the range of the central solid fraction greater than 0.0 and equal to or less than 1.0, the presence or absence of cross-sectional defects, and the incidence of double-lamination are shown in Table 1 below. The incidence of double-lamination in Table 1 indicates the percentage (%) in which double-lamination occurred in steel plates produced by heating each slab to 1200°C in a heating furnace and hot rolling it. The target value of the amount of surface recuperation in the range of the central solid fraction greater than 0.0 and equal to or less than 1.0 was set to 200°C based on the results in Figure 2.

[0039]

[0040] The specific water amounts of the secondary cooling devices 42 and 44, the specific water amount 1 of the secondary cooling device 42, the specific water amount 2 of the secondary cooling device 44, and the specific water amount difference (specific water amount 1 - specific water amount 2) are shown in Table 2 below.

[0041]

[0042] Test Nos. 1 to 3, 5, and 7 to 9 are test examples in which the surface recuperation amount in the range of the central solid fraction greater than 0.0 and not greater than 1.0 was greater than the target value of 200°C. In these test examples, tensile strain occurred near the center of the thickness of the slab 24, causing cross-sectional defects in the slab. For this reason, the incidence of double cracking in the steel plates obtained by heating and hot-rolling the slabs produced in these test examples was higher than in test examples in which the surface recuperation amount was 200°C or less.

[0043] Test Nos. 4, 6, 10, and 11 are test examples in which the secondary cooling conditions were changed to control the surface recuperation amount in the range of the center solid fraction greater than 0.0 and equal to or less than 1.0 so that it was equal to or less than the target value of 200° C. In Test Nos. 4, 6, 10, and 11, the amount of secondary cooling water in the secondary cooling devices 42 and 44 was controlled so as to satisfy the following (A) and (B).

[0044] (A) The specific water amount of the secondary cooling devices 42, 44 provided in the casting direction from directly below the mold 10 to the position where the central solid fraction is 1.0 is set to 1.8 L / kg-steel or less. (B) The specific water amount 1 of the secondary cooling device 42 and the specific water amount 2 of the secondary cooling device 44 satisfy the following formula (1).

[0045] (Specific water volume 1 - Specific water volume 2)≦1.0L / kg-steel...(1)

[0046] As shown in Table 1, no cross-sectional defects occurred in the slabs produced in Test Nos. 4, 6, 10, and 11, in which the secondary cooling water flow rate was controlled. These slabs were heated to 1,200°C in a heating furnace and hot-rolled to produce steel plates, and no double-cracks occurred. Based on these results, a target value for the amount of surface recuperation that can suppress the occurrence of tensile strain in the center of the thickness of the slab 24, in which the central solid fraction is in the range of greater than 0.0 and less than or equal to 1.0, is determined in advance, and the amount of secondary cooling water in the secondary cooling devices 42 and 44 is controlled so that the amount of surface recuperation is equal to or less than the target value. This suppresses the occurrence of cross-sectional defects in the produced slabs, and it was confirmed that double-cracks can be suppressed in the steel plates produced by hot-rolling the slabs.

[0047] REFERENCE SIGNS LIST 10 Mold 12 Tundish 14 Sliding nozzle 16 Submerged nozzle 18 Molten steel 20 Solidified shell 22 Unsolidified layer 24 Strand 26 Support roll 28 Guide roll 30 Pinch roll 32 Strand support roll 34 Transport roll 36 Strand cutter 38 Slab 40 Solidification completion position 42 Secondary cooling device 44 Secondary cooling device 46 Thermo camera 48 Control device 100 Continuous casting machine

Claims

1. A method for producing slabs using a continuous casting machine, in which a cast piece formed by cooling a mold is cooled in multiple secondary cooling devices, and the amount of surface recuperation of the cast piece when the center solid fraction is in the range of greater than 0.0 and not more than 1.0 is set to a predetermined target value or less.

2. The method for manufacturing a slab according to claim 1, wherein the target temperature is 200°C.

3. The method for manufacturing a slab according to claim 1 or 2, wherein the specific water amount of the secondary cooling device satisfies the following (A) and (B): (A) The specific water amount from directly below the mold in the casting direction to the position where the central solid fraction becomes 1.0 is set to 1.8 L / kg-steel or less. (B) The specific water amount 1 from directly below the mold in the casting direction to a position halfway to the position where the central solid fraction becomes 1.0, and the specific water amount 2 from the halfway position to the position where the central solid fraction becomes 1.0, satisfy the following formula (1): (specific water amount 1 - specific water amount 2) ≦ 1.0 L / kg-steel... (1) 4. A method for manufacturing a slab described in any one of claims 1 to 3, wherein the surface heat recovery amount is calculated in real time, and the amount of secondary cooling water in the secondary cooling device is controlled in real time so that the surface heat recovery amount is below a predetermined target value.

5. A method for producing a steel plate, comprising heating a slab produced by the method for producing a slab according to any one of claims 1 to 4 and hot rolling the slab to produce a steel plate.

6. A continuous casting machine comprising: a mold for cooling molten steel; a plurality of secondary cooling devices for cooling a slab formed by cooling the mold; and a control device for controlling the secondary cooling devices, wherein the control device controls the secondary cooling devices so that the amount of surface recuperation of the slab when the central solid fraction of the slab is in the range of greater than 0.0 and not more than 1.0 is equal to or less than a predetermined target value.

7. A continuous casting machine according to claim 6, wherein the target value is 200°C.

8. The continuous casting machine according to claim 6 or 7, wherein the control device controls the secondary cooling device so as to satisfy the following (A) and (B): (A) the specific water amount from immediately below the mold in the casting direction to the position where the central solid fraction becomes 1.0 is set to 1.8 L / kg-steel or less; (B) the specific water amount 1 from immediately below the mold in the casting direction to a position halfway to the position where the central solid fraction becomes 1.0, and the specific water amount 2 from the halfway position to the position where the central solid fraction becomes 1.0 satisfy the following formula (1): (specific water amount 1 - specific water amount 2) ≦ 1.0 L / kg-steel ... (1) 9. A continuous casting machine as described in any one of claims 6 to 8, wherein the control device calculates the surface heat recovery amount in real time and controls the amount of secondary cooling water in the secondary cooling device in real time so that the surface heat recovery amount is equal to or less than a predetermined target value.