The present invention relates to a thin steel sheet manufacturing apparatus and a thin steel sheet manufacturing method.
　The present application claims priority based on Japanese Patent Application No. 2018-21347 filed in Japan on November 14, 2018, the contents of which are incorporated herein by reference.
Background technology
[0002]
　Thin steel sheets for automobiles and the like are manufactured by hot rolling or further cold rolling using slabs as a material. In recent years, thin steel sheets for automobiles have been required to be thin in order to reduce the weight, and thin steel sheets having a thickness of less than 1.2 mm are also required. When such a thin material is manufactured on a conventional rolling line, there is a problem that the rolling load increases and it becomes difficult to pass the top and bottom of the coil.
[0003]
　On the other hand, a line (hereinafter, TSCR: Thin Slab Casting and Rolling) in which a continuous casting apparatus for thin slabs and a rolling line are combined is known. This is a line in which the continuous casting of thin slabs and the hot rolling line are directly connected. It is more compact than the conventional process, and the slabs cast by continuous casting are rolled as they are without cutting, resulting in endless rolling. The feature is that it can be done. When manufacturing a thin thin steel sheet as described above, since the starting material is a thin slab, the rolling load can be reduced. Further, since the endless rolling is performed, the frequency with which the top and bottom of the coil pass through the plate during rolling can be extremely reduced. Therefore, it is possible to significantly reduce the problem of plate-passability in rolling. Therefore, stable production of thin steel sheets having a thickness of less than 1.2 mm can be expected.
[0004]
　Patent Document 1 describes TSCR, in which a thin slab is first cast in a casting apparatus, and the thin slab is subsequently rolled in one or more rolling lines using the primary heat of the casting process. , A method for producing strips by casting and rolling is disclosed. Here, the cast thin slab passes through a holding furnace and an induction furnace between the casting equipment and one or more rolling lines. The holding and induction furnaces start, stop or control depending on the mode of operation selected, namely the first mode of continuous production of strips and the second mode of operation of discontinuous production of strips. Or adjusted.
[0005]
　Patent Document 2 discloses a continuous manufacturing method for manufacturing strip steel or sheet steel from thin slabs, which is TSCR and is manufactured by a curved continuous casting method having a horizontal discharge direction. Here, after the continuous casting material is solidified, a thin slab is formed in the first forming step at a temperature higher than 1100 ° C. Induction heating is performed again to a temperature of about 1100 ° C. over the entire cross section of the thin slab with the best possible temperature compensation. The thin slab is formed at a rolling speed corresponding to each roll in at least one second forming step.
[0006]
　Patent Document 3 describes a method for continuously casting steel slabs, based on the dendrite primary arm spacing λ 0 at the center of the slab in the thickness direction when casting without reduction, and the thickness of the slab. the lambda and dendrite primary arm spacing lambda in a direction around 0 value lambda / lambda ratio of 0 such that from 0.1 to 0.8 immediately after the thickness direction center of the slab has solidified, pressure immediately before the Disclosed is a method for continuously casting slabs, which comprises performing reduction so that the reduction ratio, which is the value obtained by dividing the thickness of the slab by the thickness of the slab immediately after reduction, is 1.41 or more and 2.00 or less. ing.
Prior art literature
Patent documents
[0007]
Patent Document 1: Japanese Kohyo 2009-508691 JP
Patent Document 2: Japanese Kohyo 3-504572 JP
Patent Document 3: Japanese Patent 2015-6680 JP
Outline of the invention
Problems to be solved by the invention
[0008]
　As described above, by using TSCR, especially when manufacturing a thin steel sheet, it is possible to avoid the problem of increasing the rolling load and the problem of passing the top and bottom of the coil. On the other hand, thin steel sheets for automobiles are made of high-strength materials in order to prevent a decrease in rigidity due to thinning. The component system of the high-strength steel plate is a high alloy system (high Mn steel). Since the high alloy thin steel sheet has a remarkable segregation, there are problems in the deterioration of the material due to the segregation and the aesthetic appearance of the steel sheet surface. In the conventional rolling line, segregation diffusion can be performed by soaking the slabs produced by continuous casting. On the other hand, as described above, in TSCR, the cast slab is immediately rolled into a thin steel plate, so that there is a problem that segregation cannot be improved by the soaking process.
[0009]
　An object of the present invention is to provide a thin steel sheet manufacturing apparatus and a thin steel sheet manufacturing method capable of stably manufacturing a thin steel sheet which is a high alloy type and has little segregation by TSCR.
Means to solve problems
[0010]
　That is, the gist of the present invention is as follows.
(1) A continuous casting device for thin slabs having a slab thickness of 70 mm to 120 mm at the lower end of the mold, a holding furnace for keeping the cast slabs warm and / or heating, and a rolling stand for finish rolling in this order. In a thin steel plate manufacturing apparatus capable of continuously performing from continuous casting to passing through a holding furnace and finish rolling without cutting the slab, the position in the continuous casting apparatus is higher than the solidification completion position of the slab. It has a rolling roll on the downstream side, and the slab can be rolled by the rolling roll.
(2) In the above (1), the holding furnace may be either a furnace in which the slabs pass through an atmosphere kept at a high temperature, or a furnace in which the slabs are heated by induction heating.
(3) In the method for manufacturing a thin sheet steel using the thin sheet steel manufacturing apparatus according to (1) or (2) above, the casting speed of the thin slab at the lower end of the mold is set to 4 to 7 m / min, and after solidification is completed. Moreover, when the center temperature of the slab is 1300 ° C. or higher, the slab may be reduced by the reduction roll at a reduction rate of 30% or more.
(4) A method for manufacturing a thin sheet steel using the thin sheet steel manufacturing apparatus according to (1) or (2) above, wherein the casting speed of the thin slab at the lower end of the mold is 4 to 7 m / min and solidification occurs. After completion and when the center temperature of the slab is 1300 ° C or higher, the slab is reduced by the reduction roll at a reduction rate of 30% or more, and the slab is held at a temperature of 1150 ° C or higher and 1300 ° C or lower for 5 minutes or longer in the holding furnace. You can do it.
(5) In the above (3) or (4), the thin plate steel plate has C: 0.01% to 1.0%, Si: 0.02% to 2.00%, Mn: 0. 1% to 3.5%, P: 0.02% or less, S: 0.002 to 0.030%, Al: 0.0005 to 0.0500%, N: 0.002 to 0.010% and O : 0.0001 to 0.0150% may be contained, and the balance may have a chemical component consisting of Fe and impurities.
(6) In the above (5), the thin steel plate further has Ti: 0.005 to 0.030%, Nb: 0.0010 to 0.0150%, V: 0.010 to 0.150 in mass%. %, B: 0.0001 to 0.0100%, Cr: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Mo: 0. It may contain one or more of 01 to 1.00% and W: 0.01 to 1.00%.
The invention's effect
[0011]
　According to the present invention, when a thin steel sheet is manufactured on a line in which a thin slab continuous casting apparatus, a holding furnace that keeps and / or heats the slabs, and a rolling line are combined, a high alloy system is used and segregation is small. Thin steel sheets can be manufactured stably.
A brief description of the drawing
[0012]
[Fig. 1] Fig. 1 is a diagram showing an outline of a thin steel sheet manufacturing apparatus.
FIG. 2 is a partial cross-sectional view showing the vicinity of the machine end of the continuous casting apparatus.
Mode for carrying out the invention
[0013]
　As described in Patent Document 3, the segregation interval can be shortened by reducing the pressure immediately after the center of the slab thickness is solidified in the continuous casting apparatus under specific conditions, and even a short heat treatment can be performed. It is known that segregating elements can be diffused and detoxified. The same document also discloses a method of adding Bi, Sn and Te as a method of making the dendrite structure which is the segregation interval finer. In this document, studies are being conducted on continuous casting methods under conditions where the mold thickness is 200 mm or more and the casting speed is about 1 m / min.
[0014]
　As a method for stably producing high alloy thin steel sheets without segregation, a process that combines continuous casting (CC) capable of high-speed casting with a slab thickness of about 100 mm in a mold and compact hot rolling. Therefore, the optimum conditions of casting conditions, heating conditions and rolling conditions were investigated.
[0015]
　By reducing the slab immediately after solidification is completed in the continuous casting apparatus and keeping the slab after reduction at a high temperature in the heat treatment furnace, macrosegregation of the central part of the slab and between the dendrite trees The idea was to further reduce microsegregation.
[0016]
　Therefore, an experiment was conducted in which the slabs to be cast under the conditions A and B were rolled after solidification was completed and immediately after solidification while still hot in the machine of the continuous casting apparatus. After the solidification was completed, the slab was reduced at a reduction rate of 30 to 50% in a region where the center temperature of the slab was 1300 ° C. or higher. Then, after the slab is discharged from the continuous casting apparatus, the slab is immediately cut, and the cut slab is immediately charged into a holding furnace held at 1250 ° C., and the heat treatment for holding the slab in the furnace is performed for 10 minutes to 60 minutes. It was carried out in minutes. In the case of condition A, there are cases where there is no reduction and no heat treatment, cases where reduction is performed at a reduction ratio of 30% but no heat treatment, and cases where reduction is performed at reduction ratios of 30%, 40%, and 50%, and the heat treatment time is set at 1250 ° C. The central segregation ratio and the micro segregation ratio under each condition were determined by comparing with the case of performing 10 minutes and 60 minutes. In the case of condition B, there are cases where there is no reduction and no heat treatment, cases where reduction is performed at a reduction ratio of 30% but no heat treatment, and cases where reduction is performed at a reduction ratio of 30% and 50%, and the heat treatment time is 10 minutes and 60 minutes. The central segregation ratio and the micro segregation ratio under each condition were determined by comparing with the case. The center segregation ratio is measured by using EPMA to analyze the Mn concentration near the center of the thickness of the surface perpendicular to the rolling direction of the slab, and line analysis is performed in the thickness direction with a beam diameter of 50 μm. The Mn concentration distribution was measured, and the maximum concentration of Mn in the measurement range was determined. Then, the value obtained by dividing the value of the maximum concentration of Mn by the initial content of Mn (2.40% by mass) obtained from the chemical analysis at the molten steel stage was defined as the central segregation ratio. For the measurement of the microsegregation ratio, the same slab as the central segregation measurement was used, and line analysis was performed in the width direction at a slab thickness of 1/4. Then, from the distribution of Mn concentrated on the dendrite primary arm, the value obtained by dividing the value of the maximum concentration of Mn by the initial content of Mn obtained from the chemical analysis at the molten steel stage was defined as the microsegregation ratio. Here, the reduction ratio (%) by the reduction roll was determined as "(pre-compression slab thickness-reduction-reduction slab thickness) / reduction pre-slab thickness x 100".
[0017]
[table 1]

[0018]
　From Table 1, it was found that as the reduction ratio was higher and the heat treatment time was longer, both the central segregation ratio and the microsegregation ratio approached 1 showing segregation-free and improved. Further, it was found that the condition A for continuous casting of thin slabs has a greater effect of improving the segregation ratio than the condition B for continuous casting of conventional thick slabs.
[0019]
　The reason why the central segregation ratio and the micro segregation ratio were improved by the reduction immediately after the completion of solidification and the heat treatment immediately after the casting in the high-speed casting by continuous thin slab casting is considered as follows. That is, the reason why the central segregation ratio and the microsegregation ratio are improved by the reduction and heat treatment immediately after the completion of solidification is that the dislocations introduced at the time of reduction are the diffusion paths of the segregating elements, and it is possible that they diffused at high speed. It is also considered that the reason for the improvement of segregation is that the central segregation is extended in the longitudinal direction of rolling due to the reduction and the time until the central segregation is diffused is shortened by reducing the thickness. Such a diffusion mechanism is consistent with the improvement in the central segregation ratio at a reduction rate of 30% without active heat treatment in the holding furnace. Since the slab is compressed when the center temperature of the slab is 1300 ° C or higher, there is a certain amount of time that the temperature of the center of the slab is around 1300 ° C even after the reduction, and it is considered that segregating elements diffuse during this period. Be done. Similar to the central segregation, the microsegregation interval is shortened by the reduction of the microsegregation, so that the diffusion of the segregating elements is promoted and the segregation is improved.
[0020]
　In the thin slab continuous casting according to the present embodiment, the slab thickness at the lower end of the mold is 70 mm to 120 mm. The casting speed of the thin slab at the lower end of the mold is 4 to 7 m / min. By casting a thin slab with a thickness of 120 mm or less at a high speed of 4 m / min or more, the dendrite arm spacing immediately after the completion of solidification can be made finer, and the central segregation ratio and the micro segregation ratio immediately after the completion of solidification can also be reduced. On the other hand, for the reason of productivity, the lower limit of the slab thickness is set to 70 mm. Further, the upper limit of the casting speed is set to 7 m / min due to casting troubles such as breakout. In the continuous casting apparatus, after the solidified shell has passed through the mold, unsolidified reduction may be performed in the roll band to reduce the slab thickness.
[0021]
　The relationship between the slab 10 near the solidification completion portion, the support roll 7, and the reduction roll 4 in the machine of the continuous casting apparatus 1 will be described with reference to FIG. The inside of the continuous casting apparatus means the inside of the continuous casting apparatus 1 located on the upstream side 21 of the holding furnace 2, and means the portion of the continuous casting apparatus 1 upstream of the support roll 7 provided on the most downstream side 22. The slab 10 before the completion of solidification includes a solid phase portion 13, a solid-liquid coexisting phase 14, and a liquid phase portion 15 in this order from the surface. Here, the boundary between the solid phase portion 13 and the solid-liquid coexisting phase 14 is referred to as a solid phase line 16. The boundary between the solid-liquid coexisting phase 14 and the liquid phase portion 15 is referred to as a liquid phase line 17. As the slab 10 moves in the casting direction 20 from the upstream side 21 to the downstream side 22, solidification of the slab 10 progresses and the thickness of the solid phase portion 13 becomes thicker. The portion where the solid phase line 16 on the upper surface side and the lower surface side of the slab 10 intersects is the solidification completion position 11. The temperature at the center of the slab thickness decreases toward the downstream side of the solidification completion position 11.
　The reduction using the reduction roll 4 in the continuous casting apparatus is preferably performed on the slab 10 at a reduction ratio of 30% or more at a position where the core temperature of the slab is 1300 ° C. or higher after the completion of solidification. That is, the reduction rate in one pass of reducing the slab 10 by the set of reduction rolls 4 at one location of the casting line in the continuous casting apparatus may be 30% or more. It should be noted that the reduction may be performed by a plurality of sets of reduction rolls 4 at a plurality of locations on the casting line in the continuous casting apparatus. That is, the portion of the slab 10 in the casting direction 20 that is reduced by the reduction roll 4 is a position between the solidification completion position 11 and the central portion 1300 ° C. position 12. In other words, the manufacturing apparatus has a reduction roll 4 in the continuous casting apparatus, 22 on the downstream side of the solidification completion position 11 of the slab 10, and 21 on the upstream side of the central portion 1300 ° C. position 12. ing. The reduction roll 4 is located on the upstream side 21 of the support roll 7 which is the most downstream in the continuous casting apparatus. The reason why the reduction position is set after the completion of solidification is that internal cracks occur when the inside is not solidified and the reduction is performed. The reason why the slab center temperature is set to 1300 ° C. or higher at the rolling position is that the effect of improving the segregation ratio is exhibited under the rolling position at 1300 ° C. or higher. This requirement is usually achieved by rolling down the slab 10 during casting in a continuous casting apparatus. The reason why the slab 10 is reduced at a reduction ratio of 30% or more is that the improvement of the central segregation ratio and the micro segregation ratio can be clearly obtained.
　As described above, the manufacturing apparatus according to the present embodiment has a large reduction rate of 30% or more immediately after solidification of thin slabs having a slab thickness of 70 mm to 120 mm is completed in the holding furnace 2 on the upstream side 21. Since the pressure is reduced with, TSCR can stably produce a high alloy type thin steel sheet with less segregation.
[0022]
　Regarding the heat retention of the slab 10 in the holding furnace 2, it is preferable to keep the slab 10 at an atmospheric temperature in the furnace of 1150 ° C. or higher and 1300 ° C. or lower for 5 minutes or longer. This is because the improvement of the central segregation ratio and the micro segregation ratio can be obtained more clearly by holding at 1150 ° C. or higher for 5 minutes or longer. On the other hand, the upper limit of the holding temperature is set to 1300 ° C. because scales are generated and scale defects occur at higher temperatures.
[0023]
　However, even if it is not held in the holding furnace 2 for 5 minutes or more as described above, it is in the continuous casting apparatus having a slab thickness of 70 mm to 120 mm at the lower end of the mold and downstream of the solidification completion position 11 of the slab 10. When the slab 10 is reduced by using the reduction roll 4 installed in 22, the central segregation ratio and the micro segregation ratio of the slab 10 are improved.
　The continuous casting apparatus 1 mainly includes a roll band that supports a slab 10 having a mold and an unsolidified portion. The roll band includes a roller apron, a support roll 7, and the like. The support roll 7 may be provided with a roll that is free to rotate, and is a pinch roll provided with a roll that is driven to rotate and can give a rotational torque so as to send the slab 10 in the casting direction 20. It may be. Some of the support rolls 7 may be pinch rolls. The pinch roll is usually arranged on the upstream side 21 of the reduction roll 4.
　The slab 10 after being completely solidified is usually rapidly discharged from the continuous casting apparatus 1. Therefore, even in the present embodiment in which the reduction roll 4 is provided in the continuous casting apparatus, the distance from the complete solidification position of the slab 10 to the end of the continuous casting apparatus 1 is about 3 to 5 m, and the casting speed is 4 to 7 m /. If it is min, the slab 10 is discharged to the outside of the apparatus within 1 minute.
[0024]
　Due to such a short time, the temperature at the center of the slab 10 is approximately 1300 ° C. or higher even on the outlet side of the continuous casting apparatus 1. Therefore, it is not always necessary to hold the slab 10 in a furnace kept at 1150 to 1300 ° C. for 5 minutes or more only for improving the central segregation ratio and the micro segregation ratio. However, in the present embodiment, the continuously cast slab 10 is quickly rolled without being cut. In this case, even immediately after being discharged from the continuous casting apparatus 1, the surface corners of the slab 10 are often at a low temperature, so that the slab cannot be rolled immediately, but the slab for rolling. Since it is heating, it is sufficient to raise the temperature in a short time. An induction heating device is known as a device suitable for such a heating purpose.
[0025]
　In the present embodiment, either or both of a holding furnace for keeping the cast slab 10 warm and a heating furnace for heating the cast slab 10 are collectively referred to as a "holding furnace". The present embodiment is characterized in that the continuous casting apparatus 1, the holding furnace 2, and the rolling stand 3 are arranged linearly in this order.
[0026]
　The temperature T C at the center of the slab thickness direction at each position in the casting direction 20 during casting can be obtained by one-dimensional heat transfer solidification analysis (calculation). The position where the temperature T C of the central portion coincides with the solid phase line temperature T S is defined as the solidification completion position 11. By the same analysis, the central portion 1300 ° C. position 12 can be determined. In the heat transfer solidification analysis, the enthalpy method, the equivalent specific heat method, or the like can be used.
[0027]
　The method for manufacturing a thin steel sheet according to the present embodiment can be carried out by using a thin steel sheet manufacturing apparatus as shown in FIG. That is, the thin sheet steel manufacturing apparatus includes a continuous casting apparatus 1 for thin slabs having a slab thickness of 70 mm to 120 mm at the lower end of the mold, a holding furnace 2 for keeping the cast slabs 10 warm and / or heating, and finish rolling. The rolling stand 3 for performing the above is arranged in this order, and the slab 10 can be continuously performed from continuous casting to passing through the holding furnace and finish rolling without cutting the slab 10. The thin steel plate manufacturing apparatus has a reduction roll 4 on the downstream side 22 of the solidification complete portion of the slab 10 in the continuous casting apparatus, and the slab 10 can be reduced by the reduction roll 4. The rolling roll 4 is a rolling mill that stretches and rolls by passing a slab 10 while pressing it between a rotating roll and a flat plate or between rotating rolls.
[0028]
　The reduction by the reduction roll 4 in the continuous casting apparatus 1 is performed at a position after the solidification of the slab 10 is completed. Therefore, the reduction roll 4 is arranged on the downstream side 22 of the solidification completion position 11 of the slab 10. Since the reduction roll 4 is arranged in the continuous casting apparatus near the machine end, reduction can be performed at an appropriate position. Here, the vicinity of the machine end means the end position of the continuous casting apparatus 1 or a position within 5 m from the end position. At this position, it can be reduced immediately after the central portion of the thickness of the slab 10 during casting solidifies. Further, by arranging the reduction roll 4 in the continuous casting apparatus, the slab 10 can be reduced when the center temperature of the slab 10 is 1300 ° C. or higher.
[0029]
　In the thin sheet steel manufacturing apparatus according to the present embodiment, as shown in FIG. 1, a continuous casting apparatus 1, a holding furnace 2, and a rolling stand 3 for finish rolling are arranged in this order. Then, this manufacturing apparatus continuously performs from continuous casting to passing through a holding furnace and finish rolling without cutting the slab 10. After finish rolling, the take-up device 6 winds the thin steel plate. In the conventional batch type rolling, there is a top and a bottom for each coil to be rolled, which has a problem at the time of sheet passing. However, in the present embodiment, the slab 10 is continuously rolled without being cut. Therefore, the problem of passing the board at the top and the bottom can be avoided. Further, since the slab 10 after continuous casting is a thin slab, the rolling load can be reduced even in the production of a thin steel plate having a plate thickness of less than 1.2 mm.
[0030]
　In the present embodiment, the holding furnace 2 has a function of keeping heat and / or heating the cast slab 10. The holding furnace 2 may be a furnace in which the slab 10 passes through the atmosphere held at a high temperature, that is, a furnace in which the atmosphere through which the slab 10 passes is held at a high temperature, and the slab 10 is heated by induction heating. It may be.
[0031]
　Regarding the rolling stand 3 for finishing rolling, there is no limit to the number of finishing stands. When producing a thin material having a plate thickness of 1.2 mm or less, it is desirable that the number of finishing stands is 5 or more.
[0032]
　A descaling device 5 is usually arranged between the holding furnace 2 and the rolling stand 3 for finish rolling.
[0033]
　In a line configuration with a general TSCR heat-retaining furnace, it is common to charge the slabs after continuous casting into the heat-retaining furnace, soak the heat, and then perform finish rolling. No rolling is done in front. This is because when the pressure is reduced in front of the heat-retaining furnace, the plate passing speed in the heat-retaining furnace increases, so the time spent in the heat-retaining furnace is shortened, and the heat-retaining furnace is extended to homogenize the temperature. This is because it has been thought that In the present embodiment, unlike the above idea, reduction is performed in the continuous casting apparatus aiming at segregation diffusion. According to the conventional wisdom, it was expected that the time spent in the heat-retaining furnace would be shortened due to the reduction of pressure, which would be disadvantageous for segregation diffusion and temperature homogenization. However, as described in detail above, the center segregation ratio and the microsegregation ratio of the slab after reduction are achieved by reducing the slab at a temperature of 1300 ° C. or higher, preferably at a reduction ratio of 30% or higher after the completion of solidification. It was found that segregation diffuses even if the holding time in the subsequent holding furnace is short. Further, if the center temperature is as high as 1300 ° C. or higher and the reduction rate is 30% or more under the reduction in the continuous casting apparatus, the average temperature of the cross section of the steel sheet is homogenized by the reduction, and the temperature can be homogenized even by a short heat treatment. Is enough.
[0034]
　That is, according to the present embodiment, it is possible to provide a method for producing a high alloy-based thin steel sheet with less segregation in TSCR that cannot be soaked.
[0035]
　The preferable component composition of the thin sheet steel used in the method for producing the thin sheet steel of the present embodiment will be described.
　The thin steel plate of the present embodiment has C: 0.01% to 1.0%, Si: 0.02% to 2.00%, Mn: 0.1% to 3.5%, P: in mass%. 0.02% or less, S: 0.002% to 0.030%, Al: 0.0005% to 0.0500%, N: 0.002% to 0.010% and O: 0.0001% to 0 It is preferable that the content is .0150% and the balance has a chemical component consisting of Fe and impurities.
[0036]
　C: 0.01% to 1.0%
　C is contained to increase the strength of the high-strength steel sheet. However, if the C content exceeds 1.0%, the weldability deteriorates. On the other hand, if the C content is less than 0.01%, the strength decreases.
[0037]
　Si: 0.02% to 2.00%
　Si is an element necessary for suppressing the formation of iron-based carbides in steel sheets and increasing strength and moldability. However, if the Si content exceeds 2.00%, the steel sheet becomes brittle and the ductility deteriorates. On the other hand, if the Si content is less than 0.02%, the strength decreases.
[0038]
　Mn: 0.1% to 3.5%
　Mn is added to the steel sheet of the present embodiment in order to increase the strength of the steel sheet. However, if the Mn content exceeds 3.5%, even in this embodiment, a coarse Mn-concentrated portion is formed in the central portion of the steel sheet thickness, and there is a concern that embrittlement is likely to occur. Further, if the Mn content exceeds 3.5%, the weldability also deteriorates. Therefore, the Mn content is preferably 3.5% or less. From the viewpoint of weldability, the Mn content is more preferably 3.00% or less. On the other hand, if the Mn content is less than 0.1%, the effect of improving central segregation and microsegregation cannot be clearly enjoyed. From this point of view, the Mn content is preferably 0.1% or more, more preferably 0.5% or more.
[0039]
　P: 0.02% or less
　P tends to segregate in the central portion of the thickness of the steel sheet and embrittles the welded portion. If the P content exceeds 0.02%, there is a concern that the welded portion will be significantly embrittled even in this embodiment.
[0040]
　S: 0.002% to 0.030%
　S adversely affects weldability and manufacturability during casting and hot spreading. Further, since Ti is combined with Ti to form sulfide, which prevents Ti from becoming a nitride and indirectly induces the formation of Al nitride, the upper limit of the S content is set to 0.030%. Is preferable. Even if the lower limit of the S content is not particularly defined, the effect of improving the segregation ratio is exhibited. Since setting the S content to less than 0.002% entails a significant increase in manufacturing cost, the lower limit of the S content is set to 0.002%.
[0041]
　Al: 0.0005% to 0.0500%
　Al, when added in a large amount, forms a coarse nitride, lowers the drawing value at low temperature, and lowers the impact resistance characteristics. Therefore, the upper limit of the Al content is set. It is preferably 0.050%. The Al content is more preferably 0.035% or less in order to avoid the formation of coarse nitrides. The lower limit of the Al content is not particularly specified, and the effect of improving the segregation ratio is exhibited, but the Al content of less than 0.0005% is accompanied by a significant increase in manufacturing cost. Further, Al is an element that is also effective as a deoxidizing material, and from this viewpoint, the Al content is preferably 0.005% or more, and more preferably 0.010% or more.
[0042]
　N: 0.002% to 0.010%
　N forms a coarse nitride that is the starting point of fracture at a low temperature and lowers the impact resistance characteristics, so it is necessary to suppress the addition amount. When the N content exceeds 0.010%, this effect becomes remarkable. Therefore, the range of the N content is preferably 0.010% or less. From this viewpoint, the content of N is more preferably 0.0040% or less, and further preferably 0.0030% or less. The lower limit of the N content is not particularly specified, and the effect of improving the segregation ratio is exhibited. However, if the N content is less than 0.002%, the manufacturing cost is significantly increased.
[0043]
　O: 0.0001% to 0.0150%
　O forms a coarse oxide and causes a starting point of fracture at a low temperature, so it is necessary to suppress the content. When the O content exceeds 0.0150%, this effect becomes remarkable. Therefore, it is preferable to set the upper limit of the O content to 0.0150% or less. From this viewpoint, the content of O is more preferably 0.0020% or less, and further preferably 0.0010% or less. The lower limit of the O content is not particularly specified, and the effect of improving the segregation ratio is exhibited, but the O content of less than 0.0001% is accompanied by a significant increase in manufacturing cost.
[0044]
　The thin steel sheet of the present embodiment may selectively further contain the following elements. That is, the thin steel plate further has Ti: 0.005% to 0.030%, Nb: 0.0010 to 0.0150%, V: 0.010 to 0.150%, B: 0. 0001 to 0.0100%, Cr: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Mo: 0.01 to 1.00% , W: 0.01 to 1.00% of 1 type or 2 or more types may be contained. The main effect according to the present embodiment is improvement of central segregation and micro segregation, and the effect is not particularly affected by the inclusion of the following elements.
[0045]
　Ti: 0.005% to 0.030%
　Ti is an element that forms fine nitrides by hot rolling under appropriate conditions and suppresses the formation of coarse Al nitrides, and is an element that suppresses the formation of coarse Al nitrides at low temperatures. It reduces the starting point of destruction and improves impact resistance. In order to obtain this effect, the Ti content is preferably 0.005% or more. On the other hand, when the Ti content exceeds 0.030%, the moldability of the soft portion in the steel sheet deteriorates due to the precipitation of fine carbonitride, and on the contrary, the drawing value at a low temperature is lowered. From the viewpoint of moldability, the Ti content is preferably 0.0120% or less, more preferably 0.0100% or less.
[0046]
　Nb: 0.0010% to 0.0150%
　Nb is an element that forms fine nitrides by hot rolling under appropriate conditions and suppresses the formation of coarse Al nitrides, and is an element that suppresses the formation of coarse Al nitrides at low temperatures. Reduce the starting point of destruction. In order to obtain this effect, the Nb content is preferably 0.0010% or more, the Nb content is more preferably 0.0030% or more, and the Nb content is further preferably 0.0050% or more. preferable. On the other hand, when the Nb content exceeds 0.0150%, the moldability of the soft portion in the steel sheet deteriorates due to the precipitation of fine carbonitride, and on the contrary, the drawing value at low temperature is lowered. The content is preferably 0.0150% or less. From the viewpoint of moldability, the Nb content is more preferably 0.0120% or less, and further preferably 0.0100% or less.
[0047]
　V: 0.010% to 0.150%
　V is an element that forms fine nitrides by hot rolling under appropriate conditions and suppresses the formation of coarse Al nitrides, and is an element that suppresses the formation of coarse Al nitrides at low temperatures. Reduce the starting point of destruction. In order to obtain this effect, the V content needs to be 0.010% or more, the content is preferably 0.030% or more, and more preferably 0.050% or more. On the other hand, when the V content exceeds 0.150%, the moldability of the soft portion in the steel sheet deteriorates due to the precipitation of fine carbonitride, and on the contrary, the drawing value at low temperature is lowered. The content is preferably 0.150% or less. From the viewpoint of moldability, the V content is more preferably 0.120% or less, and further preferably 0.100% or less.
[0048]
　B: 0.0001% to 0.0100%
　B is an element that forms fine nitrides by hot rolling under appropriate conditions and suppresses the formation of coarse Al nitrides, and is an element that suppresses the formation of coarse Al nitrides at low temperatures. Reduce the starting point of destruction. In order to obtain this effect, the B content is preferably 0.0001% or more, the B content is preferably 0.0003% or more, and even more preferably 0.0005% or more. .. Further, B is an element effective for suppressing phase transformation at high temperature and increasing the strength, and may be further added. However, when the content of B exceeds 0.0100%, the processability in hot weather is achieved. The content of B is preferably 0.0100% or less, because the content of B is impaired and the productivity is lowered. From the viewpoint of productivity, the content of B is more preferably 0.0050% or less, and further preferably 0.0030% or less.
[0049]
　Cr: 0.01% to 2.00%
　Cr is an element effective for suppressing phase transformation at high temperature and increasing the strength, and may be added in place of a part of C and / or Mn. If the Cr content exceeds 2.00%, the workability in hot water is impaired and the productivity is lowered. Therefore, the Cr content is preferably 2.00% or less. The lower limit of the Cr content is not particularly specified, and the effect of improving the segregation ratio is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by Cr, the Cr content should be 0.01% or more. Is preferable.
[0050]
　Ni: 0.01% to 2.00%
　Ni is an element effective for suppressing phase transformation at high temperature and increasing the strength, and may be added in place of a part of C and / or Mn. If the Ni content exceeds 2.00%, the weldability is impaired. Therefore, the Ni content is preferably 2.00% or less. The lower limit of the Ni content is not particularly specified, and the effect of improving the segregation ratio is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by Ni, the Ni content should be 0.01% or more. Is preferable.
[0051]
　Cu: 0.01% to 2.00%
　Cu is an element that increases the strength by being present in steel as fine particles, and can be added in place of a part of C and / or Mn. If the Cu content exceeds 2.00%, the weldability is impaired. Therefore, the Cu content is preferably 2.00% or less. The lower limit of the Cu content is not particularly specified, and the effect of improving the segregation ratio is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by Cu, the Cu content should be 0.01% or more. Is preferable.
[0052]
　Mo: 0.01% to 1.00%
　Mo is an element effective for suppressing phase transformation at high temperature and increasing the strength, and may be added in place of a part of C and / or Mn. If the Mo content exceeds 1.00%, the workability in hot water is impaired and the productivity is lowered. From this, the Mo content is preferably 1.00% or less. The lower limit of the Mo content is not particularly specified, and the effect of improving the segregation ratio is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by Mo, the Mo content should be 0.01% or more. Is preferable.
[0053]
　W: 0.01% to 1.00%
　W is an element effective for suppressing phase transformation at high temperature and increasing the strength, and may be added in place of a part of C and / or Mn. If the W content exceeds 1.00%, the workability in hot water is impaired and the productivity is lowered. Therefore, the W content is preferably 1.00% or less. The lower limit of the W content is not particularly specified, and the effect of improving the segregation ratio is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by W, the W content should be 0.01% or more. Is preferable.
[0054]
　The balance may be iron and impurities.
Example
[0055]
　As shown in FIG. 1, a continuous casting apparatus 1 for thin slabs having a slab thickness of 100 mm at the lower end of a mold, a holding furnace 2 for heating the cast slabs 10, and a rolling stand 3 for finish rolling are used. The thin sheet steel was manufactured by using a thin sheet steel manufacturing apparatus capable of continuously performing the slab 10 from continuous casting to passing through a holding furnace and finish rolling without cutting. This manufacturing apparatus is inside the continuous casting apparatus 1, and has a reduction roll 4 having a roll diameter of 720 mm at a terminal position thereof. The mold size is 100 mm thick x 1500 mm wide. The casting speed is 5.0 m / min. The rolling speed of the rolling roll 4 is the same as the casting speed. The reduction rate is as shown in Table 3. The reduction position was set to the position where the thickness center temperature at the center of the slab width determined by the heat transfer solidification analysis was the temperature shown in Table 3 after the completion of solidification.
　When the holding furnace 2 of the type that keeps the cast slab 10 warm is used, the side of the holding furnace of the type that cuts the cast slab 10 to a predetermined length and heats it when the rolled slab 10 comes out from the continuous casting apparatus 1. In the holding furnace 2 installed in, only the plate passing speed obtained from the rolling reduction rate when it is assumed that the slab 10 was not cut and the furnace length when the holding furnace 2 is assumed to be 180 m are installed. After the slab 10 is put in, the slab 10 is returned to a predetermined line of a thin steel sheet manufacturing apparatus that can continuously perform the above-mentioned continuous casting, passing through a holding furnace, and finish rolling without cutting the slab 10. A thin steel plate was manufactured. In this case, since the slab 10 has been cut once, batch rolling is performed, but the slab 10 can be rolled without any problem. The temperature inside the holding furnace 2 was set to 1200 ° C. Table 3 shows the slab thickness and slab speed (holding furnace passing speed) at the machine end of the continuous casting apparatus 1 and the heat treatment time (holding furnace residence time) in the holding furnace 2.
[0056]
　In the test, the steel grade components shown in Table 2 were cast to produce a hot-rolled steel sheet (thin sheet product) having a plate thickness of 1.8 mm after finish rolling. Table 3 shows a list of test conditions and thin plate product quality.
[0057]
[Table 2]

[0058]
[Table 3]

[0059]
　The segregation degree of the steel sheet obtained by the above rolling was measured. The solute element to be measured was Mn. EPMA was used to analyze the Mn concentration, and line analysis was performed in the thickness direction of the steel sheet with a beam diameter of 50 μm to measure the Mn concentration distribution in the steel sheet, and the maximum concentration of Mn in the measurement range was determined. The value obtained by dividing the value of the maximum concentration of Mn by the initial content of Mn obtained from the chemical analysis at the molten steel stage was taken as the Mn segregation degree.
[0060]
　In addition, a sample for hole expansion test is cut out from a hot-rolled steel sheet, and a hole expansion test is performed in accordance with JIS Z 2256: 2010 (hole expansion test method for metal materials) to set the hole expansion limit value "λ (%)". Calculated. As a comprehensive evaluation, those with a hole expansion rate of 50% or more were evaluated as ◯, and those with a hole expansion rate of 50% or less were evaluated as x.
[0061]
　In Examples 1 to 4 of the present invention, the slab 10 is cut immediately after rolling at each reduction rate at the terminal position in the continuous casting apparatus 1, and the slab 10 is temporarily charged into a holding furnace 2 of a type that keeps the slab 10 warm. This is an example of a thin sheet steel sheet (thin sheet product) that has been rolled to a predetermined thickness by a descaler and finish rolling after the holding time described in 3.
　Example 5 of the present invention is a thin steel plate manufactured by continuously performing from continuous casting to passing through a holding furnace and finish rolling without cutting the slab 10 using a holding furnace 2 (induction heating furnace) for heating slabs. Is an example of.
　In Comparative Example 1, the slab was not rolled down at the terminal position in the continuous casting apparatus, the slab was cut, and the slab was once charged into a holding furnace 2 of a type that keeps the slab warm, and after the holding time shown in Table 3. This is an example of a thin steel plate which has been rolled to have the same thickness as Examples 1 to 5 of the present invention.
　The evaluation (* 1) of Example 1 of the present invention means that even if the reduction rate immediately after solidification is small and the hole expansion rate is 50% or less, it is superior to Comparative Example 1.
　The evaluation (* 1) of Example 5 of the present invention means that it is clearly superior to Comparative Example 1 even if there is no holding time in the holding furnace 2. The reason for this is that in addition to the 30% reduction at the end position in the continuous casting machine, it takes about 5 minutes from the end of the continuous casting machine to the entrance of the rolling stand 3 where finish rolling is performed via an induction heating furnace. It is probable that the segregation element diffused during that time. As confirmed and shown in Table 1 above, the central segregation and the micro segregation are improved by reducing the slab 10 cast by the continuous casting apparatus 1 of the thin slab in the continuous casting apparatus. it is conceivable that. Therefore, even if the slab holding time in the holding furnace 2 is not sufficiently secured, the quality of the thin sheet steel rolled by using induction heating is higher than that of Comparative Example 1 in which the slab is held in the holding furnace 2 for 60 minutes. It was confirmed that it can be equal to or higher than that.
[0062]
　Under the condition that the slab is cut after continuous casting and maintained in the holding furnace 2 for a long time, segregation can be alleviated and the hole expansion rate can be reduced if the heat treatment time is secured for 360 min without reducing the slab immediately after solidification. It turned out to improve. However, in TSCR, since the slab is continuously processed without being cut, such a heat treatment cannot be performed, and the feasibility is low.
[0063]
　From these comparative investigation results, the continuous casting apparatus 1 for thin slabs, the holding furnace 2 for keeping or heating the cast slabs 10 and the rolling stand 3 for finish rolling are arranged in this order, and from continuous casting. When a thin sheet steel is manufactured using a sheet steel sheet manufacturing device that can continuously perform the process of passing through a holding furnace and finish rolling without cutting the slab 10, the slab 10 is rolled down at the terminal position of the continuous casting device 1. It was found that the higher the rate and the longer the heat treatment time, the less central segregation and microsegregation can be produced.
[0064]
　Further, in Example 5 of the present invention, as a result of continuously performing the slab 10 from continuous casting to passing through a holding furnace and finish rolling without cutting the slab 10 to produce a thin steel plate, the sheet passability in the rolling stand 3 for finish rolling is performed. There was no problem in producing a 1.8 mm thick hot-rolled steel sheet from a high Mn steel containing 2.6% by mass of Mn. It was also confirmed that a hot-rolled steel sheet having a thinner thickness such as 0.8 mm can be manufactured by the same method. The effect of improving the plate-passability when rolling this high Mn steel can be obtained by installing the holding furnace 2 having a furnace length of 180 m between the continuous casting apparatus 1 and the rolling stand 3 according to the present invention. Examples 1 to 4 can be enjoyed in the same manner as in Example 5 of the present invention.
Industrial applicability
[0065]
　According to the present invention, when manufacturing a thin steel sheet by TSCR, it can be applied to a thin steel sheet manufacturing apparatus and a thin steel sheet manufacturing method capable of stably manufacturing a thin steel sheet which is a high alloy type and has less segregation.
Code description
[0066]
　1 Continuous casting equipment
　2 Holding furnace
　3 Rolling stand
　4 Rolling down
　5 Descaling equipment
　6 Winding equipment
　7 Support roll
10 Shards
11 Solidification completion position
12 Central part 1300 ° C position
13 Solid phase part
14 Solid-liquid coexisting phase
15 Liquid phase part
16 Solid phase line
17 Liquid phase line
20 Casting direction
21 Upstream side
22 Downstream side
The scope of the claims
[Claim 1]
　A continuous casting device for thin slabs having a slab thickness of 70 mm to 120 mm at the lower end of the mold, a holding furnace for keeping the cast slabs warm and / or heating, and a rolling stand for finish rolling are arranged in this order. In a thin sheet steel manufacturing apparatus capable of continuously performing from continuous casting to passing through a holding furnace and finish rolling without cutting slabs, in the
　continuous casting apparatus on the downstream side of the solidification completion position of the slabs. A thin steel sheet manufacturing apparatus having a rolling roll and capable of rolling a slab by the rolling roll.
[Claim 2]
　The thin steel sheet manufacturing apparatus according to claim 1, wherein the holding furnace is either a furnace in which slabs pass through an atmosphere kept at a high temperature or a furnace in which the slabs are heated by induction heating. ..
[Claim 3]
　The method for manufacturing a thin steel sheet using the thin steel sheet manufacturing apparatus according to claim 1 or 2,
　wherein the casting speed of the thin slab at the lower end of the mold is 4 to 7 m / min, and after solidification is completed and casting is performed. A method for producing a thin steel sheet, characterized in that a slab is reduced at a reduction rate of 30% or more by the reduction roll when the center temperature of one side is 1300 ° C. or more.
[Claim 4]
　The method for manufacturing a thin steel sheet using the thin steel sheet manufacturing apparatus according to claim 1 or 2,
　wherein the casting speed of the thin slab at the lower end of the mold is 4 to 7 m / min, and after solidification is completed and casting is performed. When the center temperature of one piece is 1300 ° C. or higher, the rolling roll reduces the slab at a reduction rate of 30% or more, and the
　holding furnace holds the slab at a temperature of 1150 ° C. or higher and 1300 ° C. or lower for 5 minutes or longer. A method for manufacturing a thin steel plate.
[Claim 5]
　The thin steel plate has C: 0.01% to 1.0%, Si: 0.02% to 2.00%, Mn: 0.1% to 3.5%, P: 0.02 in mass%. % Or less, S: 0.002 to 0.030%, Al: 0.0005 to 0.0500%, N: 0.002 to 0.010% and O: 0.0001 to 0.0150%. The method for producing a thin steel plate according to claim 3 or 4, wherein the balance has a chemical component composed of Fe and impurities.
[Claim 6]
　Further, the thin steel plate is Ti: 0.005 to 0.030%, Nb: 0.0010 to 0.0150%, V: 0.010 to 0.150%, B: 0.0001 to 0 in mass%. .0100%, Cr: 0.01-2.00%, Ni: 0.01-2.00%, Cu: 0.01-2.00%, Mo: 0.01-1.00%, W: The method for producing a thin sheet steel according to claim 5, wherein the thin plate steel sheet contains one or more of 0.01 to 1.00%.