Steel sheet production method and steel sheet production device

Abstract

The present invention provides a steel sheet production method and a steel sheet production device that are each capable of suppressing the occurrence of uneven temperature caused by an alloy oxide. Provided is a steel sheet production method using a slab that contains, in terms of mass%, 0.2-3.0% Si and 1.0-4.0% Mn. The steel sheet production method comprises: a descaling step for removing a scale formed on the surface of a sheet bar by spraying water at a total energy density of 0.20-1.50 J / mm2 to the sheet bar obtained by subjecting the slab to hot-rolling; a rolling step for performing rolling with respect to the sheet bar subjected to the descaling step; a cooling step for cooling with cooling water until the surface temperature of the steel sheet becomes not higher than 500°C; and an iron oxide forming step for forming, during a period from the end of the descaling step to the start of the cooling step, an iron oxide on the surface of the steel sheet obtained by subjecting the sheet bar to rolling.

Description

Method for manufacturing steel plates and apparatus for manufacturing steel plates

[0001] The present invention relates to a method for manufacturing steel sheets using a slab containing, by mass%, Si: 0.2 to 3.0% and Mn: 1.0 to 4.0%, and to an apparatus for manufacturing steel sheets.

[0002] When hot-rolled steel sheets are manufactured, a slab heated to a high temperature is rolled to a predetermined size, and then cooled with cooling water in cooling equipment such as a runout table.

[0003] Cooling with cooling water is primarily performed to control the precipitates and transformation structure of steel sheets and to obtain the desired material properties such as strength and ductility. In particular, by precisely controlling the temperature when cooling is completed, variations in material properties can be suppressed.

[0004] Patent Document 1 discloses a method for manufacturing steel sheets by hot rolling, in which heated steel material is subjected to rough rolling and finish rolling, followed by descaling with high-pressure water and draining of the descaling water with low-pressure water.

[0005] Japanese Patent Publication No. 2003-126906

[0006] Incidentally, cooling using a runout table presents a problem of uneven temperature distribution on the steel surface. Furthermore, cooling using a runout table cannot be precisely terminated at the desired temperature. Such uneven temperature distribution on the steel may affect the subsequent properties of the steel plate.

[0007] In particular, when manufacturing high-strength steel sheets containing Si and Mn as components, there is a problem in that temperature unevenness is likely to occur due to the formation of Si and Mn oxides (hereinafter also referred to as alloy oxides) on the surface.

[0008] This invention has been made in view of the above-mentioned problems, and aims to provide a method for manufacturing steel sheets and a steel sheet manufacturing apparatus that can suppress the occurrence of temperature unevenness due to alloy oxides.

[0009] To solve the above problems, the present invention has the following features.

[0010] [1] A method for manufacturing a steel sheet using a slab containing, by mass%, Si: 0.2 to 3.0% and Mn: 1.0 to 4.0%, wherein the total energy density of a sheet bar obtained by hot rolling the slab is 0.20 to 1.50 J / mm². ２ A method for manufacturing a steel sheet, comprising: a descaling step of removing scale formed on the surface of the sheet bar by spraying the above water; a rolling step of rolling the sheet bar after the descaling step has been performed; a cooling step of cooling the steel sheet with cooling water until the surface temperature of the steel sheet is 500°C or less; and an iron oxide forming step of forming iron oxide on the surface of the steel sheet that has been rolled on the sheet bar between the end of the descaling step and the start of the cooling step. [2] The method for manufacturing a steel sheet according to [1], wherein the cooling step comprises: a first cooling step of ending the cooling of the steel sheet at a temperature higher than the transition boiling onset temperature; and a second cooling step of cooling the steel sheet with cooling water having a water density that causes nucleation boiling after the first cooling step has been performed. [3] The descaling step comprises: an energy density of 0.05 J / mm ２ A first descaling step involves spraying the above-mentioned water onto the seat bar, and an energy density of 0.15 J / mm². ２A method for manufacturing a steel sheet according to [1] or [2], further comprising: a second descaling step of spraying the above-mentioned water onto the sheet bar; [4] A method for manufacturing a steel sheet according to any one of [1] to [3], wherein the iron oxide formation step is performed for 7 to 25 seconds; [5] A method for manufacturing a steel sheet according to any one of [1] to [4], wherein the rolling step is performed with a total reduction rate of 85 to 96%; [6] A method for manufacturing a steel sheet according to any one of [1] to [5], wherein the cooling step comprises: a first cooling step in which the cooling of the steel sheet is completed at a temperature higher than the transition boiling onset temperature; and a second cooling step in which, after the first cooling step has been performed, the steel sheet is cooled with cooling water having a water density that causes nucleation boiling, wherein in the second cooling step, the cooling water is sprayed at a rate of 7 m / s or more; [7] A steel plate manufacturing apparatus comprising: a descaling unit that sprays water onto a sheet bar that has been hot-rolled from a slab; a rolling unit that performs rolling on the sheet bar; an iron oxide forming unit that forms iron oxide on the surface of the steel plate that has been rolled onto the sheet bar; and a cooling unit that cools the steel plate with cooling water, wherein the cooling unit comprises a first cooling unit and a second cooling unit that discharge the cooling water, arranged along the conveying direction of the steel plate; the second cooling unit comprises an upstream cooling unit arranged on the upstream side in the conveying direction and a downstream cooling unit arranged on the downstream side in the conveying direction; the upstream nozzle of the upstream cooling unit that discharges the cooling water is provided facing downstream in the conveying direction of the steel plate; and the downstream nozzle of the downstream cooling unit that discharges the cooling water is provided facing upstream in the conveying direction of the steel plate. [8] The steel plate manufacturing apparatus according to [7], wherein the shaft of the upstream nozzle and the shaft of the downstream nozzle are at an angle of 30° to 60° with respect to the conveying direction. [9] The steel plate manufacturing apparatus according to [7] or [8], wherein the upstream nozzle and the downstream nozzle are arranged in a plurality along the left-right direction when viewed from the conveying direction, and the shafts of the upstream nozzle and the shafts of the downstream nozzle are arranged to intersect each other.

[0011] According to the steel sheet manufacturing method of the present invention, a steel sheet is manufactured using a slab containing, by mass%, Si: 0.2 to 3.0% and Mn: 1.0 to 4.0%. The steel sheet manufacturing method involves a descaling step in which water is sprayed onto a sheet bar that has been hot-rolled from the slab to remove scale formed on the surface of the sheet bar. A rolling step is then performed on the sheet bar that has undergone the descaling step. Furthermore, an iron oxide formation step is performed in which iron oxide is formed on the surface of the steel sheet that has been rolled from the sheet bar. As a result, the alloy oxide formed on the surface of the steel sheet is spread out on the steel sheet and then covered by the iron oxide. Therefore, since the alloy oxide is not exposed, the occurrence of temperature unevenness due to alloy oxide is suppressed, and it becomes possible to stably manufacture steel sheets with less variation in material properties.

[0012] This is an explanatory diagram showing the outline of a steel sheet manufacturing apparatus. This is an explanatory diagram showing the outline of a cooling section. This is an explanatory diagram showing the arrangement of the upstream and downstream cooling sections of the second cooling section. This is an explanatory diagram showing the angle formed by the nozzle axis of the upstream nozzle and the nozzle axis of the downstream nozzle of the downstream cooling section with respect to the conveying direction. This is an explanatory diagram showing the nozzle axis of the upstream nozzle and the nozzle axis of the downstream nozzle of the downstream cooling section as viewed from above. This is an explanatory diagram showing how cooling water is sprayed. This is a flow chart of the steel sheet manufacturing method. This shows a cross-section of a sheet bar that has been hot-rolled on a slab. This is an explanatory diagram showing the descaling process in step S01 of Figure 7. This is an explanatory diagram showing the rolling process in step S03 of Figure 7. This is an explanatory diagram showing the iron oxide formation process and the cooling process in step S04 of Figure 7. This is an explanatory diagram showing the heat flux and boiling morphology in runout cooling. This is an explanatory diagram showing the change in heat flux and boiling morphology when the water density of the cooling water is increased for runout cooling conditions. This graph shows the relationship between the transition boiling onset temperature and the nucleation boiling onset temperature when the water density of the cooling water is changed.

[0013] The present invention will be described below through embodiments of the invention. Figure 1 shows an overview of a steel sheet manufacturing apparatus. As shown in Figure 1, the steel sheet manufacturing apparatus 100 includes a heating furnace 10 for heating a slab, and a descaling unit 20 for spraying water onto the slab and the sheet bar that has been roughly rolled onto the slab.

[0014] The steel sheet manufacturing apparatus 100 includes a width reduction section 30 that performs width reduction on a slab, and a rolling section 40 that performs rolling on the width-reduced slab. The steel sheet manufacturing apparatus 100 also includes an iron oxide forming section 50 that forms iron oxide on the surface of the steel sheet rolled on a sheet bar, and a cooling section 60 that cools the steel sheet with cooling water. A sheet bar refers to a steel sheet that has undergone rough rolling and is not yet finished rolling.

[0015] In the steel plate manufacturing apparatus 100, a slab containing Si: 0.2 to 3.0% and Mn: 1.0 to 4.0% by mass is used for manufacturing the steel plate. The slab preferably contains 0.3 to 2.0% Si, and more preferably 0.4 to 1.5%. The slab also preferably contains 1.2 to 3.5% Mn, and more preferably 1.5 to 3.0% Mn.

[0016] The slab preferably has a composition of components consisting of Si and Mn, as well as carbon C, phosphorus P, sulfur S, nitrogen N, and aluminum Al, with the remainder being Fe and unavoidable impurities. Having such a composition of components in the steel sheet enables the realization of a material with a tensile strength of 780 MPa or more. Furthermore, having such a composition of components in the slab allows for the formation of Si and Mn oxides (hereinafter also referred to as alloy oxides) on the slab's surface. The composition of components other than Si and Mn in the slab is, for example, C: 0.04-0.2%, P: 0.0-0.1%, S: 0.00-0.02%, Al: 0.0-1.0%, and N: 0.00-0.01%.

[0017] When the Si content of the slab is less than 0.2%, the amount of alloy oxides formed on the surface of the sheet bar by selective oxidation of alloy components such as Si and Mn from the base material is small. Therefore, the alloy oxides on the surface of the sheet bar are easily removed by the descaling unit 20, and uniformity of cooling in the water cooling system can be ensured even with conventional technology. Furthermore, the upper limit for the Si content of the slab is 3.0% because it may reduce the hole-expanding properties of the hot-rolled steel sheet.

[0018] When the Mn content of the slab is less than 1.0%, the amount of alloy oxide generated on the surface of the sheet bar is small. Therefore, the alloy oxide on the surface of the sheet bar is easily removed by the descaling section 20, and uniformity of cooling in the water cooling device can be ensured even with conventional technology. The upper limit for the Mn content of the slab should be 4.0%. This is because if the Mn content exceeds 4.0%, the martensite phase or the martensite-austenite mixed phase increases, which may reduce the hole-expanding properties of the hot-rolled steel sheet.

[0019] The slab may have one or more other component compositions added to it. Examples of such component compositions include titanium, vanadium, niobium, chromium, boron, molybdenum, copper, nickel, antimony, and tin. In other words, the slab may have a component composition consisting of C, Si, Mn, P, S, N, and aluminum Al, with one or more selected from titanium Ti, vanadium V, niobium Nb, chromium Cr, boron B, molybdenum Mo, copper Cu, nickel Ni, antimony As, and tin Sn added, with the remainder being Fe and unavoidable impurities. In this case, the slab should be selected from a composition of components such as Ti ≤ 0.05%, V ≤ 1.0%, Nb ≤ 1.0%, Cr ≤ 0.5%, B ≤ 0.005%, Mo ≤ 2.0%, Cu ≤ 2.0%, Ni ≤ 2.0%, As ≤ 0.2%, and Sn ≤ 0.5%, for example, in mass percent.

[0020] The slab is transported in the transport direction D1. A heating furnace 10 is provided upstream of the transport direction D1. The heating furnace 10 has a burner (not shown) that is ignited when fuel is supplied. In the heating furnace 10, the slab is heated to a predetermined temperature.

[0021] The descaling unit 20 is located downstream of the heating furnace 10 in the transport direction D1. The descaling unit 20 includes a rough descaling unit 21 located upstream in the transport direction D1 and a finishing descaling unit 22 located downstream. The rough descaling unit 21 descales the slabs extracted from the heating furnace 10. The finishing descaling unit 22 descales the sheet bars that have undergone rough rolling of the slabs.

[0022] The rough descaling section 21 and the finishing descaling section 22 have nozzle headers (not shown) that spray water onto the sheet bar. Multiple nozzle headers are arranged, for example, along the conveying direction D1.

[0023] A width-pressing section 30 is provided downstream of the rough descaling section 21 in the transport direction D1. The width-pressing section 30 presses down the slab from the left and right directions when viewed from the transport direction D1.

[0024] A rolling section 40 is provided on the downstream side of the width-pressed lower section 30 in the conveying direction D1. The rolling section 40 has a rough rolling section 41 provided on the upstream side in the conveying direction D1 and a finish rolling section 42 provided on the downstream side. In the conveying direction D1, a finish descaling section 22 is provided between the rough rolling section 41 and the finish rolling section 42.

[0025] From the finishing descaling section 22 to the cooling section 60, an iron oxide forming section 50 is provided for forming iron oxides on the surface of the steel plate.

[0026] In the iron oxide forming section 50, iron oxides are formed on the surface of the steel sheet that has been rolled over the sheet bar that has been descaled in the finishing descaling section 22, as it comes into contact with the atmosphere. In the iron oxide forming section 50, the manner in which iron oxides are formed on the surface of the steel sheet is adjusted by adjusting the time it takes for the sheet bar that has been descaled in the finishing descaling section 22 to begin cooling in the cooling section 60 as a steel sheet. In other words, the manner in which iron oxides are formed on the surface of the steel sheet is adjusted by adjusting the time required for the steel sheet to pass from the finishing descaling section 22 to the cooling section 60.

[0027] The length of this time can be adjusted, for example, by the distance from the finishing descaling section 22 to the cooling section 60 and the rolling speed which is the conveyance speed of the steel sheet passing through the final stand of the finishing rolling section 42.

[0028] A cooling section 60 for cooling the steel sheet with cooling water is provided. The cooling section 60 is a so-called run-out table. The cooling section 60 has a first cooling section 61 provided on the upstream side in the conveyance direction D1 and a second cooling section 62 provided on the downstream side.

[0029] A coiler 70 for winding up the steel sheet is provided on the downstream side in the conveyance direction D1 of the cooling section 60. Incidentally, the steel sheet manufacturing apparatus 100 includes a control section 80 that controls the operations of these members including the descaling section 20 and the cooling section 60.

[0030] Also, in the conveyance direction D1, a temperature measurement section 71 for measuring the temperature of the surface of the steel sheet is provided between the finishing rolling section 42 and the first cooling section 61. Also, in the conveyance direction D1, a temperature measurement section 71 for measuring the temperature of the surface of the steel sheet is provided between the first cooling section 61 and the second cooling section 62. A temperature measurement section 71 for measuring the temperature of the surface of the steel sheet is provided between the second cooling section 62 and the coiler 70.

[0031] FIG. 2 shows an overview of the cooling section 60. As shown in FIG. 2, conveyance rolls 63 formed in a cylindrical shape along the conveyance direction D1 are arranged in the cooling section 60. The steel sheet S is placed on the conveyance rolls 63. When the conveyance rolls 63 rotate, the steel sheet S is conveyed in the conveyance direction D1.

[0032] The first cooling section 61 is provided with a discharge nozzle 61a and a discharge nozzle 61b for discharging cooling water. The discharge nozzle 61a is provided above the conveyance roll 63. The broken line in FIG. 2 shows the locus of the cooling water. The discharge nozzle 61a has discharge ports on the upstream side and the downstream side in the conveyance direction D1. A plurality of discharge nozzles 61a are provided along the conveyance direction D1. The discharge nozzle 61a discharges cooling water toward the surface of the steel sheet S.

[0033] The discharge nozzle 61b is provided below the conveying roll 63. A plurality of discharge nozzles 61b are provided along the conveying direction D1. The discharge nozzle 61b discharges cooling water toward the back surface of the steel plate S.

[0034] The discharge nozzles 61a and 61b are not particularly limited, but for example, a circular tube nozzle for laminar cooling, a circular tube nozzle for jet cooling, a slit nozzle, a spray nozzle for spray cooling, etc. can be used.

[0035] The second cooling unit 62 includes an upstream cooling unit 64 disposed on the upstream side in the conveying direction D1 and a downstream cooling unit 65 disposed on the downstream side in the conveying direction D1. The upstream cooling unit 64 is provided above the conveying roll 63 and discharges cooling water toward the front surface of the steel plate S. The downstream cooling unit 65 is provided above the conveying roll 63 and discharges cooling water toward the front surface of the steel plate S. The second cooling unit 62 has a back surface cooling unit 66 that discharges cooling water toward the back surface of the steel plate S. A plurality of back surface cooling units 66 are provided along the conveying direction D1.

[0036] The back surface cooling unit 66 discharges cooling water toward the back surface of the steel plate. Similar to the discharge nozzle 61b, for example, a circular tube nozzle for laminar cooling, a circular tube nozzle for jet cooling, a slit nozzle, a spray nozzle for spray cooling, etc. can be used.

[0037] FIG. 3 shows the arrangement modes of the upstream cooling unit 64 and the downstream cooling unit 65 of the second cooling unit 62. As shown in FIG. 3, in the upstream cooling unit, a plurality of upstream nozzles 64a are arranged along the conveying direction D1. In the example shown in FIG. 3, four upstream nozzles 64a are connected to one water supply pipe. Each upstream nozzle 64a has its mouth axis AX1 arranged toward the downstream side in the conveying direction D1.

[0038] In the downstream cooling unit 65, a plurality of downstream nozzles 65a are arranged along the conveying direction D1. In the example shown in FIG. 3, four downstream nozzles 65a are connected to one water supply pipe. Each downstream nozzle 65a has its mouth axis AX2 arranged toward the upstream side in the conveying direction D1.

[0039] In the example shown in Figure 3, the nozzle shaft AX1 of the upstream nozzle 64a and the nozzle shaft AX2 of the downstream nozzle 65a are arranged to intersect each other. More specifically, it is preferable that the nozzle shafts AX1 and AX2 of both nozzles intersect below the steel plate S. By providing the upstream cooling section 64 and the downstream cooling section 65 in this manner, cooling water can be applied directly to the steel plate S. Furthermore, the water on the steel plate S is stored between the upstream cooling section 64 and the downstream cooling section 65 and discharged in the left-right direction when viewed from the transport direction D1. This makes it possible to improve the cooling efficiency and drainage efficiency of the steel plate S.

[0040] Figure 4 shows the angles formed by the nozzle axis AX1 of the upstream nozzle 64a and the nozzle axis AX2 of the downstream nozzle 65a with respect to the transport direction D1. As shown in Figure 4, the angle θ1 formed by the nozzle axis AX1 of the upstream nozzle 64a with respect to the transport direction D1 is preferably 30° or more and 60° or less, preferably 35° or more and 55° or less, and more preferably 40° or more and 50° or less.

[0041] Furthermore, the angle θ2 that the mouth axis AX2 of the downstream nozzle 65a makes with respect to the transport direction D1 is preferably 30° or more and 60° or less, more preferably 35° or more and 55° or less, and more preferably 40° or more and 50° or less.

[0042] Because these angles θ1 and θ2 are within this range, the water flow from upstream to downstream in the conveying direction D1 and the water flow from downstream to upstream in the conveying direction D1 can be made to collide on the steel plate S. Furthermore, the smaller the angles θ1 and θ2, the better the drainage performance can be obtained, and the accumulation of cooling water on the steel plate S can be suppressed.

[0043] Figure 5 shows the nozzle shaft AX1 of the upstream nozzle 64a and the nozzle shaft AX2 of the downstream nozzle 65a, viewed from above. As shown in Figure 5, multiple upstream nozzles 64a and downstream nozzles 65a are arranged along the left-right direction when viewed from the transport direction D1.

[0044] The upstream nozzle 64a is positioned with its opening axis AX1 facing either the left direction Da or the right direction Db when viewed from the transport direction D1. In other words, the upstream nozzle 64a is provided such that its opening axis AX1 forms a V shape when viewed from above.

[0045] Similarly, the downstream nozzle 65a is positioned with its nozzle shaft AX2 facing either the left direction Da or the right direction Db when viewed from the transport direction D1. That is, the upstream nozzle 64a is provided such that the nozzle shaft AX2 of the downstream nozzle 65a forms a V shape when viewed from above.

[0046] The shaft AX1 of the upstream nozzle 64a and the shaft AX2 of the downstream nozzle 65a are arranged to intersect each other in the left direction Da and the right direction Db when viewed from the transport direction D1.

[0047] Figure 6 shows the manner in which cooling water is injected. In Figure 6, β is the angle between the cooling water injection line and the steel plate S (actual inclination angle), θ is the inclination angle with respect to the transport direction, and α is the angle directed outward in the width direction of the steel plate (outward angle).

[0048] It is preferable that 0-35% of the velocity component of the cooling water injection direction be a velocity component directed outward in the width direction of the steel plate. Specifically, it is preferable that the ratio Lw / L (width direction velocity component ratio) of the length Lw corresponding to the velocity component in the width direction of the steel plate perpendicular to the transport direction to the actual length L of the cooling water injection is 0-35%.

[0049] For example, if the height of the upstream nozzle 64a is 1200 mm and the inclination angle θ with respect to the transport direction D1 is 45°, the component ratio will be 0-35% when the outward angle α is 0-25°. Also, if the inclination angle θ with respect to the transport direction D1 is 50°, the component ratio will be 0-35% when the outward angle α is 0-30°.

[0050] Figure 7 is a flowchart showing the manufacturing method of steel sheets. As shown in Figure 7, in the manufacturing method of steel sheets, a descaling process is performed to remove scale formed on the surface of the sheet bar, which has been hot-rolled from a slab, by spraying water onto it. In other words, the descaling process is a process to remove scale formed on the surface of the sheet bar that has been roughly rolled by the rough rolling section 41.

[0051] In the descaling process, first, the energy density is reduced to 0.05 J / mm². ２ A first descaling step is performed in which the above water is sprayed onto the seat bar (step S01). Next, the energy density is 0.15 J / mm ２ A second descaling step is performed in which the above water is sprayed onto the seat bar (step S02). In the first descaling step of step S01 and the second descaling step of step S02, the total energy density of the water sprayed onto the seat bar is 0.20 to 1.50 J / mm². ２ Therefore, by spraying water with this total energy density, it is possible to properly remove the scale formed on the surface of the sheet bar. The first descaling step in step S01 and the second descaling step in step S02 are performed in the finishing descaling section 22.

[0052] Next, a rolling process is performed on the sheet bar that has undergone the second descaling process in step S02 (step S03). The sheet bar that has undergone the rolling process in step S03 becomes a steel plate. In this embodiment, the rolling by the finishing rolling unit 42 is referred to as the rolling process.

[0053] Next, an iron oxide formation process is performed (step S04) to form iron oxides on the surface of the steel sheet that has undergone the rolling process in step S03.

[0054] Next, a cooling process is performed in which the steel plate is cooled with cooling water until the surface temperature is 500°C or lower. In the cooling process, first, a first cooling process is performed in which the cooling of the steel plate is completed when the temperature of the steel plate is higher than the transition boiling onset temperature (step S05). After the first cooling process in step S05 is performed, a second cooling process is performed in which the steel plate is cooled with cooling water with a water density that causes nucleation boiling (step S06).

[0055] Furthermore, the first descaling step in step S01 and the second descaling step in step S02 do not necessarily need to be performed together and can be carried out as appropriate depending on the implementation. Also, the first cooling step in step S05 and the second cooling step in step S06 do not necessarily need to be performed together and can be carried out as appropriate depending on the implementation. In addition, the rolling step in step S03 and the iron oxide formation step in step S04 may be carried out in any order.

[0056] Figure 8 shows a cross-section of the sheet bar 90 after rough rolling by the rough rolling section 41 and before the descaling process by the finishing descaling section 22. As shown in Figure 8, iron oxide 91 and alloy oxide 92 are formed on the surface of the sheet bar 90 before the descaling process.

[0057] In other words, alloy oxide 92 is formed on the surface of the slab heated in the heating furnace, and the alloy oxide 92 that was not removed in the rough descaling section 21 remains on the surface of the sheet bar 90. Then, as iron oxide 91 is newly generated during the rough rolling process in the rough rolling section 41, iron oxide 91 and alloy oxide 92 are formed on the surface of the sheet bar 90 before the descaling process is performed.

[0058] Figure 9 shows an aspect of the descaling process. In the descaling process, iron oxide 91 is removed from the sheet bar 90. Alloy oxide 92 remains on the sheet bar 90.

[0059] The first descaling process in step S01 of FIG. 7 can adjust the energy density by adjusting the model number of the nozzles attached to the nozzle header of the finishing descaling unit 22 and the conveyance speed of the sheet bar. The energy density is, for example, 0.05 J / mm ２ It is preferably set to 0.05 to 0.30 J / mm ２ preferably.

[0060] By setting the energy density of the first descaling process to 0.05 J / mm ２ or more, thermal stress can be generated due to thermal shrinkage of the sheet bar surface, and cracks can be generated in the scale thereby.

[0061] The second descaling process in step S02 of FIG. 7 can adjust the energy density by adjusting the model number of the nozzles attached to the nozzle header of the finishing descaling unit 22 and the conveyance speed of the sheet bar. The energy density is, for example, 0.15 J / mm ２ It is preferably set to 0.15 to 0.20 J / mm ２ preferably. By stopping the supply of descaling water, the energy density can also be set to 0.00 J / mm ２ as well.

[0062] By setting the energy density of the second descaling process to 0.15 J / mm ２ or more, high-energy water collides with the cracked scale, so the scale can be efficiently removed.

[0063] FIG. 10 shows an aspect of the rolling process in step S03 of FIG. 7. As shown in FIG. 10, when the rolling process of step S03 is performed, the sheet bar 90 becomes a steel plate 90 with a reduced thickness to a predetermined thickness. The rolling process of step S03 preferably has a total reduction ratio of 85 to 96%. By having a total reduction ratio of 85 to 96%, it becomes possible to stretch the alloy oxide 92. That is, the alloy oxide 92 can be made thinner.

[0064] Furthermore, if the total reduction ratio is less than 85%, the expansion of the surface area of ​​the steel sheet during the rolling process may be insufficient, resulting in insufficient thinning of the alloy oxide 92. Also, insufficient expansion of the surface area of ​​the steel sheet may make it difficult to activate the surface of the steel sheet, resulting in insufficient formation of iron oxide 91. From the viewpoint of preventing excessive rolling load on the finishing rolling mill, it is preferable to set the total reduction ratio to 95% or less.

[0065] Figure 11 shows the iron oxide formation process and cooling process in step S04 of Figure 7. As shown in Figure 11, when the rolling process in step S03 is performed, the iron oxide 91 is formed again. In the iron oxide formation process in step S04, the iron oxide 91 is formed to cover the alloy oxide 92. This configuration can be confirmed by scanning electron microscope (SEM) on the cross-section of the steel sheet. Furthermore, this oxide structure is retained in the steel sheet even after the cooling process.

[0066] The iron oxide 91 is also formed while the rolling process of step S03 is being carried out. In other words, the iron oxide formation process of step 04 is carried out while the rolling process of step S03 is being carried out.

[0067] Specifically, in the rolling process, the steel sheet is exposed to the atmosphere during the time between multiple rolling passes and before being transported to the next stand. As the surface area of ​​the steel sheet increases in each rolling pass, the surface of the steel sheet is activated, and exposure to the atmosphere promotes the formation of iron oxides. In the rolling process, iron oxides are also formed when the steel sheet is exposed to the atmosphere from the finishing descaling section 22 to the finishing rolling section 42, and from the finishing rolling section 42 to the cooling section 60.

[0068] The iron oxide formation step in step S04 is performed between the completion of the second descaling step in step S02 and the start of the cooling step. The iron oxide formation step in step S04 is preferably performed for 7 to 25 seconds, more preferably for 10 to 22 seconds, and more preferably for 12 to 20 seconds. Performing the iron oxide formation step for such a time makes it possible to ensure a sufficient thickness of iron oxide 91.

[0069] Furthermore, the time for the iron oxide formation process may be adjusted according to the total reduction ratio of the rolling process in step S03. For example, if the total reduction ratio is 87 to 96%, the time for the iron oxide formation process is preferably 10 to 22 seconds. Also, if the total reduction ratio is 89 to 96%, the time for the iron oxide formation process is preferably 12 to 20 seconds. In this case, the time for the iron oxide formation process may be adjusted by temporarily holding the sheet bar upstream of the finishing rolling section 42 between the completion of the second descaling process in step S02 and the start of the rolling process in step S03.

[0070] In the cooling process, cooling water is applied using methods such as laminar cooling, spray cooling, jet cooling, and mist cooling. Laminar cooling is a cooling method in which a continuous laminar flow of liquid is sprayed from a circular or slit-shaped nozzle. Spray cooling is a cooling method in which liquid is sprayed as droplets by pressurizing the liquid and spraying it. Jet cooling is a cooling method in which a continuous turbulent flow of liquid is sprayed from a circular or slit-shaped nozzle. Mist cooling is a cooling method in which pressurized gas and liquid are mixed to form droplets for spraying.

[0071] In the cooling process, these cooling means are not particularly limited, but it is preferable to use laminar cooling or jet cooling, which have excellent straight-line flow and are continuous, to supply cooling water to the upper surface of the steel plate.

[0072] Figures 12 and 13 schematically show the relationship between the surface temperature of a steel plate and the heat flux when the steel plate is cooled by injecting cooling water. Specifically, Figure 12 shows the heat flux and boiling morphology in runout cooling. Figure 13 shows the change in heat flux and boiling morphology when the water density of the cooling water is increased compared to runout cooling conditions. Note that the heat flux is the amount of heat removed from the steel plate.

[0073] According to Figures 12 and 13, film boiling occurs in regions where the surface temperature of the steel plate is high, resulting in a low heat flux. Furthermore, in terms of heat transfer characteristics, the higher the water density of the cooling water, the higher the transition boiling initiation temperature and the nucleation boiling initiation temperature shift to the higher temperature side.

[0074] In other words, in the first cooling step of the cooling process, it is preferable to stop cooling at a steel plate temperature higher than the transition boiling initiation temperature. Furthermore, in the second cooling step, it is preferable to increase the water density of the cooling water and cool at a water density that causes nucleation boiling. By cooling the steel plate in this manner, it is possible to avoid the steel plate temperature passing through the transition boiling temperature region during the cooling process.

[0075] As shown in Figure 12, in normal runout cooling, transition boiling begins at approximately 500°C, and the heat flux increases as the steel plate temperature decreases. Therefore, the first cooling step is preferably performed until the steel plate temperature reaches approximately 500°C.

[0076] In the second cooling step of step S06, by increasing the water density of the cooling water and cooling entirely within the nucleation boiling temperature range, the occurrence of transition boiling can be suppressed during the cooling process, and the cooling completion temperature can be controlled with high precision.

[0077] Here, we will describe the results of a laboratory investigation into the relationship between the specific cooling water density and the transition boiling onset temperature and nucleation boiling onset temperature. In the laboratory, jet cooling was performed using multiple circular pipe nozzles arranged in the width and length directions of a steel plate, and the cooling water density was varied during this process. The transition boiling onset temperature and nucleation boiling onset temperature were then investigated from the cooling temperature history. The results are shown in Figure 14. Note that the cooling water density is the amount of cooling water injected per unit area.

[0078] As shown in Figure 14, the transition boiling initiation temperature and nucleation boiling initiation temperature increase as the water density of the cooling water increases. Furthermore, in order to make the nucleation boiling initiation temperature 500°C or higher, the water density of the cooling water must be 2000 L / min × m ２ It is clear that the above is sufficient. Therefore, in the second cooling step of step S06, the water density of the cooling water should be 2000 L / min × m ２ The above is sufficient, 2500 L / min × m ２ It is preferable to keep the above in place.

[0079] Furthermore, the typical runout cooling water density is 350-1200 L / min × m ２In this region, it can be seen that the transition boiling onset temperature is approximately 500°C or lower. In this embodiment, although alloy oxides remain on a portion of the steel plate surface, cooling is performed with the iron oxides formed thicker than the alloy oxides, so the influence of the alloy oxides on the relationship between the water density of the cooling water and the transition boiling onset temperature and the nucleation boiling onset temperature can be reduced. For this reason, the water density of the cooling water can be determined according to the relationship shown in Figure 14.

[0080] By performing the second cooling step with this cooling water density, the cooling water can penetrate the liquid film formed on the surface of the steel plate and be supplied to the steel plate, thereby suppressing the occurrence of transition boiling. ２ In the above case, the thickness of the liquid film will be approximately 50 mm.

[0081] The injection velocity of the cooling water is preferably 7 m / s or higher. An injection velocity of 7 m / s or higher allows for stable penetration of the liquid film. The injection velocity of the cooling water can be obtained as the flow velocity of the cooling water at the nozzle opening.

[0082] From this perspective, it is preferable that the cooling water be supplied by laminar cooling or jet cooling, which has high linearity and continuity. The cooling water supply nozzle is not particularly limited, but examples include cylindrical nozzles and slit nozzles.

[0083] The hole diameter of a cylindrical nozzle and the gap of a slit nozzle should be between 3 and 25 mm. By setting the nozzle hole diameter and gap to 3 mm or more, clogging by foreign matter can be suppressed. By setting the nozzle hole diameter and gap to 25 mm or less, sufficient cooling can be ensured while suppressing the need for larger equipment, even at injection speeds of 7 m / s or more.

[0084] As described above, according to the steel sheet manufacturing method of the present invention, a steel sheet is manufactured using a slab containing, by mass%, Si: 0.2 to 3.0% and Mn: 1.0 to 4.0%. The steel sheet manufacturing method involves a descaling step in which water is sprayed onto a sheet bar that has been hot-rolled from a slab to remove scale formed on the surface of the sheet bar. A rolling step is then performed on the sheet bar that has undergone the descaling step. Furthermore, an iron oxide formation step is performed in which iron oxide is formed on the surface of the steel sheet that has been rolled from the sheet bar. As a result, the alloy oxide formed on the surface of the steel sheet is stretched and thinned on the steel sheet, and then covered with iron oxide. Therefore, since the alloy oxide is not exposed, it is possible to suppress the occurrence of temperature unevenness due to alloy oxide and to stably manufacture a steel sheet with less variation in material properties.

[0085] <Example 1> Steel plates were manufactured using slabs heated in a heating furnace. In manufacturing the steel plates, the slab composition, the conditions of the descaling process, the conditions of the rolling process, and the conditions of the cooling process were changed to produce Comparative Examples 1 to 3 and Invention Examples 1 to 33. The manufacturing conditions for each steel plate for Comparative Examples 1 to 3 and Invention Examples 1 to 33 are shown in Table 1.

[0086]

[0087] The slab's composition, in addition to Si and Mn, includes, by mass%, C: 0.04-0.2%, P: 0.1% or less, S: 0.02% or less, Al: 1.0% or less, N: 0.01% or less, and Ti: 0.3% or less, with the remainder being Fe and unavoidable impurities. The content of elements other than Si and Mn in the slab's composition was adjusted according to the Si and Mn content so that the tensile strength (TS) of the steel plate was within the range of 780-910 MPa.

[0088] The composition of the slab was varied by mass percent, with Si being 0.9 to 1.6% and Mn 2.5 to 3.1%, to obtain Comparative Examples 1 to 3 and Invention Examples 1 to 33, as shown in Table 1. The slab had a thickness of 240 mm.

[0089] The total energy density of the descaling water injected during the descaling process is 0.15 to 0.40 J / mm². ２ The adjustment was made within the specified range. Specifically, the injection pressure in the first descaling step by the finishing descaling unit was set to 20 MPa. In each comparative example and the inventive example, the total energy density of the descaling water injected from the finishing descaling unit was set to 0.05 to 0.40 J / mm². ２ The first descaling process was performed after making changes within the specified range.

[0090] The energy density of the descaling water in the first descaling process was changed by altering the flow rate of the descaling water and the conveying speed of the sheet bar. The flow rate of the descaling water was changed by altering the nozzle model number attached to the nozzle header of the finishing descaling section.

[0091] The injection pressure in the second descaling step by the finishing descaling unit was set to 15 MPa. In each comparative example and the inventive example, the energy density of the descaling water injected from the finishing descaling unit was 0.00 to 0.20 J / mm². ２ The second descaling process was performed after making changes within the specified range.

[0092] In the second descaling process, the energy density of the descaling water was changed by altering the flow rate of the descaling water and the conveying speed of the sheet bar. The flow rate of the descaling water was changed by altering the nozzle model number attached to the nozzle header of the finishing descaling section.

[0093] The slab was hot-rolled to a sheet bar with a thickness of 30 mm in the rough rolling section. The sheet bar was hot-rolled to a steel plate with a thickness of 3.9 to 4.8 mm in the finish rolling section. A 7-stand continuous rolling mill was used in the finish rolling section. The total reduction ratio in the finish rolling section was set to 84 to 89%.

[0094] The time for the iron oxide formation process was set to 7.5 to 12 seconds. The time for the iron oxide formation process was adjusted by changing the rolling speed, which is the conveying speed of the steel sheet as it passes through the final stand of the finishing rolling section.

[0095] The cooling process consisted of a first cooling process and a second cooling process, which was carried out to ensure that the temperature of the steel sheet when it was wound onto the coiler was between 250 and 450°C. Specifically, in the first cooling process, except for Invention Example 31 in Table 1, the cooling rate was 1000 L / min × m ２ Cooling water was injected at a water density of [value missing]. The injection velocity of the cooling water on the upper surface of the steel plate was set to 4 m / second. The first cooling process was terminated when the temperature of the steel plate reached 490-560°C.

[0096] In the first cooling process, a combination of a ring nozzle for laminar cooling and a spray nozzle for spray cooling was used as the discharge nozzle for discharging the cooling water. In the first cooling process, the water volume density of the cooling water was adjusted by controlling the valve opening for adjusting the water pressure of the spray nozzle.

[0097] Furthermore, when the steel plate is cooled with 30°C cooling water in the first cooling process, the water density is 1000 L / min × m ２ At approximately 500°C, the water density is 2000 L / min × m ２ It was confirmed that transition boiling begins at approximately 600°C for each of the solutions.

[0098] In the second cooling process, the cooling water injection velocity was set to 7 m / s. The angle between the upstream nozzle's nozzle axis and the conveying direction was set to 50°. Similarly, the angle between the downstream nozzle's nozzle axis and the conveying direction was set to 50°. The nozzles in the upstream and downstream header groups were arranged so that 50% of the nozzles were directed outwards in one direction in the width direction of the steel plate. 12% of the cooling water injection velocity component was directed outwards in the width direction of the steel plate.

[0099] Temperature variations during steel sheet winding contribute to variations in the steel sheet's material properties. Therefore, we evaluated the temperature variations during steel sheet winding and the variations in the tensile strength (TS) of the steel sheet. The results are shown in Table 2.

[0100]

[0101] The variation in the coiling temperature of the steel sheet was determined by finding the maximum and minimum surface temperatures in the width direction of the steel sheet as measured by the temperature measurement unit, and averaging the difference between these values ​​over the length direction of the steel sheet. The difference between the average of the maximum values ​​and the average of the minimum values ​​was calculated as the variation value.

[0102] The variation in the winding temperature of the steel sheet was evaluated as follows: (1) if it exceeded 55°C, (2) if it was between 40°C and 55°C, (3) if it was between 25°C and 40°C, and (4) if it was 25°C or less. A rating of "2" or higher was considered a pass.

[0103] The variation in the tensile strength (TS) of the steel plate was evaluated by the difference between the maximum and minimum tensile strength (TS) values ​​obtained from five test specimens at different locations in the width direction of the steel plate.

[0104] The tensile strength (TS) of the steel sheet was determined by taking a sample from the tail end after it had been coiled and cooled to room temperature, and then performing tensile tests on five test pieces at different positions in the width direction of the steel sheet, and calculating the average tensile strength.

[0105] Furthermore, regarding the variation in tensile strength, cases where the variation in tensile strength exceeds 60 MPa were evaluated as (1), cases where it exceeds 45 MPa but is 60 MPa or less were evaluated as (2), cases where it exceeds 25 MPa but is 45 MPa or less were evaluated as (3), and cases where it is 25 MPa or less were evaluated as (4). In the evaluation, the lower evaluation from the evaluation of the variation in steel plate winding temperature and the evaluation of the variation in tensile strength was adopted.

[0106] The tensile strength of the steel plates in the inventive examples and comparative examples shown in Tables 1 and 2 was 780 MPa or higher in all cases, indicating the production of so-called high-strength steel plates. As shown in Table 2, in inventive examples 1-8, 10, 18, 20, and 26-27, both the variation in winding temperature and the variation in tensile strength (TS) were rated as (3). In addition, in inventive examples 30, 31, and 33, both the variation in winding temperature and the variation in tensile strength (TS) were rated as (2). In other words, the temperature distribution of the steel plates in the cooling equipment was made uniform, and steel plates with suppressed variations in tensile strength were produced.

[0107] Furthermore, inventive examples 11-17, 19, and 25 all received a rating of (4) for both the variation in winding temperature and the variation in tensile strength. In other words, the temperature distribution of the steel sheet in the cooling equipment was made more uniform, and steel sheets with reduced variation in tensile strength were manufactured.

[0108] These examples of inventions involve a total energy density of 0.20 J / mm² between the first and second descaling steps. ２ The conditions described above were that the total reduction rate applied in the rolling process was 85% or more, and the time of the iron oxide formation process was 7 seconds or more. Therefore, the iron oxide was temporarily removed from the surface of the sheet bar by the descaling process, and the remaining alloy oxide was thinned by the rolling process. Along with this thinning of the alloy oxide, new iron oxide was formed to cover the alloy oxide by the iron oxide formation process, which is thought to have resulted in more uniform cooling during the cooling process.

[0109] In contrast, Comparative Examples 1 to 3 received a rating of (1) for either the variation in winding temperature or the variation in tensile strength.

[0110] <Example 2> Steel plates were manufactured using slabs heated in a heating furnace. In manufacturing the steel plates, the composition of the slab, the conditions of the descaling process, the conditions of the rolling process, and the conditions of the cooling process were changed to produce Comparative Example 4 and Invention Examples 41 to 59. The manufacturing conditions for each steel plate for Comparative Example 4 and Invention Examples 41 to 59 are shown in Table 3.

[0111]

[0112] The slab used had a composition of Si: 0.9%, Mn: 2.5%, and Cu: 0.1% by mass. In addition to Si, Mn, and Cu, the slab composition used also included C: 0.04-0.2%, P: 0.1% or less, S: 0.02% or less by mass, Al: 1.0% or less, N: 0.01% or less, and Ti: 0.3% or less. In addition to these compositional compositions, the slab was supplied with one or more elements optionally selected from Ti ≤ 0.05%, V ≤ 1.0%, Nb ≤ 1.0%, Cr ≤ 0.5%, B ≤ 0.005%, Mo ≤ 2.0%, Cu ≤ 2.0%, Ni ≤ 2.0%, As ≤ 0.2%, and Sn ≤ 0.5%, with the remainder being Fe and unavoidable impurities. In the examples, slabs with such component compositions were randomly assigned to Comparative Example 4 and Invention Examples 41 to 59 to manufacture steel plates.

[0113] The total energy density of the descaling water injected during the descaling process is 0.15–0.35 J / mm². ２ The adjustment was made within the specified range. The injection pressure in the first descaling step by the finishing descaling unit was set to 20 MPa. The total energy density of the descaling water in the first descaling step was 0.05 to 0.20 J / mm². ２ The changes were made within this range. On the other hand, the injection pressure in the second descaling step by the finishing descaling unit was set to 15 MPa. The total energy density of the descaling water in the second descaling step was 0.00 to 0.30 J / mm². ２ Changes were made within this range.

[0114] The energy density of the descaling water in the first and second descaling processes was changed by altering the flow rate of the descaling water and the conveying speed of the sheet bar. The flow rate of the descaling water was changed by altering the nozzle model number attached to the nozzle header of the finishing descaling section.

[0115] A 240 mm thick slab, heated to 1180-1250°C in a heating furnace, was rolled to a 30 mm thick sheet bar in the rough rolling section. The sheet bar was then rolled into a steel plate with a thickness of 2.7-4.8 mm in the finish rolling section. A 7-stand continuous rolling mill was used in the finish rolling section. The total reduction ratio in the finish rolling section was set to 84-91%.

[0116] The time for the iron oxide formation process was set to 6.5 to 20 seconds. The time for the iron oxide formation process was adjusted by changing the rolling speed, which is the conveying speed of the steel sheet passing through the final stand of the finishing rolling section.

[0117] The cooling process consisted of a first cooling process and a second cooling process, which was carried out to ensure that the temperature of the steel sheet when it was wound onto the coiler reached 300°C. Specifically, in the first cooling process, 500 to 1200 L / min × m ２ Cooling water was injected at a water density of [value missing]. In the first cooling process, a combination of a ring nozzle for laminar cooling and a spray nozzle for spray cooling was used as the discharge nozzle for discharging the cooling water. In the first cooling process, the water density of the cooling water was adjusted by controlling the valve opening for adjusting the water pressure of the spray nozzle. The spray velocity of the cooling water on the upper surface of the steel plate was set to 4 m / second. The first cooling process ended when the temperature of the steel plate reached 500-560°C.

[0118] In the second cooling process, 2000-3000 L / min × m ２ Cooling water was injected at a water density of . For the second cooling process, a second cooling section was used, which had an upstream cooling section and a downstream cooling section, as shown in Figure 3. In the upstream cooling section of the second cooling section, multiple upstream nozzles were arranged along the conveying direction of the steel plate, with their nozzle shafts facing downstream in the conveying direction. In the downstream cooling section, multiple downstream nozzles were arranged along the conveying direction, with their nozzle shafts facing upstream in the conveying direction. The upstream and downstream cooling sections were each connected to a water supply pipe, and the water density of the cooling water was adjusted by changing the number of nozzles that sprayed the cooling water and the rolling speed. The spraying speed of the cooling water in the second cooling process was set to 8 m / second. The second cooling process ended when the temperature of the steel plate reached 300°C, and the sheet was immediately wound up by a coiler.

[0119] The variation in winding temperature when winding steel sheets using a coiler, and the tensile strength (TS) of the steel sheets and its variation were evaluated. The results are shown in Table 4.

[0120]

[0121] The calculation and evaluation of the variation in the winding temperature of the steel sheet was performed using the same method as in Example 1. The calculation and evaluation of the tensile strength (TS) of the steel sheet were also performed in the same manner.

[0122] The tensile strength of the steel plates produced by the inventive examples and comparative examples shown in Tables 3 and 4 was 780 MPa or higher in all cases, indicating the production of so-called high-strength steel plates. As shown in Table 4, in inventive examples 41-50 and 52-59, both the variation in winding temperature and the variation in tensile strength (TS) were rated as (4). In inventive example 51, both the variation in winding temperature and the variation in tensile strength (TS) were rated as (2). In other words, the temperature distribution of the steel plates in the cooling equipment was made uniform, and steel plates with suppressed variations in tensile strength were produced.

[0123] Furthermore, as shown in Invention Examples 41 to 46, in the descaling process, the greater the total energy density of the water injected onto the sheet bar, the better the variation in winding temperature and tensile strength (TS). Also, as shown in Invention Example 43 and Invention Examples 52 to 55, the longer the time of the iron oxide formation process, the better the variation in winding temperature and tensile strength (TS). This is thought to be because the iron oxide formation process promotes the formation of iron oxides covering the alloy oxides, resulting in more uniform cooling in the cooling process.

[0124] In contrast, Comparative Example 4 received a rating of (1) for both the variation in winding temperature and the variation in tensile strength (TS).

[0125] 100 Steel plate manufacturing apparatus 20 Descaling section 40 Rolling section 41 Rough rolling section 42 Finish rolling section 50 Iron oxide forming section 60 Cooling section 61 First cooling section 62 Second cooling section 64 Upstream cooling section 64a Upstream nozzle 65 Downstream cooling section 65a Downstream nozzle AX1 Shaft of upstream nozzle AX2 Shaft of downstream nozzle D1 Conveying direction

Claims

1. A method for manufacturing a steel sheet using a slab containing, by mass%, Si: 0.2 to 3.0% and Mn: 1.0 to 4.0%, wherein the total energy density of a sheet bar obtained by hot rolling the slab is 0.20 to 1.50 J / mm². ２ A method for manufacturing a steel sheet, comprising: a descaling step of removing scale formed on the surface of the sheet bar by spraying water; a rolling step of rolling the sheet bar after the descaling step has been performed; a cooling step of cooling the steel sheet with cooling water until the surface temperature of the steel sheet is 500°C or less; and an iron oxide forming step of forming iron oxide on the surface of the steel sheet that has been rolled on the sheet bar between the end of the descaling step and the start of the cooling step.

2. The method for manufacturing a steel sheet according to claim 1, wherein the cooling step comprises: a first cooling step in which the cooling of the steel sheet is completed when the temperature of the steel sheet is higher than the transition boiling initiation temperature; and a second cooling step in which, after the first cooling step has been performed, the steel sheet is cooled with cooling water having a water density that causes nucleation boiling.

3. The descaling process is performed when the energy density is 0.05 J / mm ２ A first descaling step involves spraying the above-mentioned water onto the seat bar, and an energy density of 0.15 J / mm². ２ A method for manufacturing a steel plate according to claim 1 or 2, comprising a second descaling step of spraying the above-mentioned water onto the sheet bar.

4. The method for manufacturing a steel sheet according to any one of claims 1 to 3, wherein the iron oxide formation step is performed for 7 to 25 seconds.

5. The method for manufacturing a steel sheet according to any one of claims 1 to 4, wherein the rolling step is performed with a total reduction rate of 85 to 96%.

6. The method for manufacturing a steel sheet according to any one of claims 1 to 5, wherein the cooling step comprises a first cooling step in which the cooling of the steel sheet is completed when the temperature of the steel sheet is higher than the transition boiling initiation temperature, and a second cooling step in which the steel sheet is cooled with cooling water having a water density that causes nucleation boiling after the first cooling step has been performed, and in the second cooling step, the cooling water is sprayed at a rate of 7 m / s or more.

7. A steel plate manufacturing apparatus comprising: a descaling unit that sprays water onto a sheet bar that has been hot-rolled from a slab; a rolling unit that performs rolling on the sheet bar; an iron oxide forming unit that forms iron oxide on the surface of the steel plate that has been rolled onto the sheet bar; and a cooling unit that cools the steel plate with cooling water, wherein the cooling unit comprises a first cooling unit and a second cooling unit that discharge the cooling water, arranged along the conveying direction of the steel plate; the second cooling unit comprises an upstream cooling unit arranged on the upstream side in the conveying direction and a downstream cooling unit arranged on the downstream side in the conveying direction; the upstream nozzle of the upstream cooling unit that discharges the cooling water is provided facing downstream in the conveying direction of the steel plate; and the downstream nozzle of the downstream cooling unit that discharges the cooling water is provided facing upstream in the conveying direction of the steel plate.

8. The steel plate manufacturing apparatus according to claim 7, wherein the nozzle axis of the upstream nozzle and the nozzle axis of the downstream nozzle are at an angle of 30° to 60° with respect to the conveying direction.

9. The steel plate manufacturing apparatus according to claim 7 or 8, wherein the upstream nozzle and the downstream nozzle are arranged in a plurality along the left-right direction when viewed from the conveying direction, and the nozzle shafts of the upstream nozzle and the downstream nozzle are arranged to intersect each other.

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