Method for producing high chromium (CR) molten steel, method for producing casting, and method for producing press roll

Abstract

An embodiment of the present invention relates to a method for manufacturing a high chromium (Cr) molten steel having a chromium content (Cr) of 4.5 wt% to 5.5 wt%, and the method may comprise the operations of: inserting the molten steel into a steelmaking converter used in a process of manufacturing stainless steel; and feeding the chromium steel alloy containing chromium (Cr) into a steelmaking converter so that the content of chromium (Cr) in the molten steel reaches 4.5-5.5 wt%. Accordingly, according to an embodiment of the present invention, a high chromium (Cr) molten steel can be manufactured by using a steelmaking converter used in a steelmaking process of other steel types, without contaminating the steelmaking converter. In addition, when the temperature of the high chromium (Cr) molten steel is raised or degassing is performed to exhaust gas, the present invention makes it possible to effectively raise the temperature of the molten steel and improve degassing efficiency.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing high chromium (Cr) molten steel, a method for manufacturing slabs, and a method for manufacturing press rolls, and more specifically to a method for manufacturing high chromium (Cr) molten steel, a method for manufacturing slabs, and a method for manufacturing press rolls, which can efficiently manufacture high chromium (Cr) molten steel and can simply suppress or prevent the generation of segregation at low cost. Background Art

[0002] The press roller is made of steel containing a high content of chromium (Cr), and is manufactured by a process of solidifying molten steel to manufacture an ingot and then forging the ingot. In more detail, molten steel with a high content of chromium (Cr) is manufactured and then solidified to manufacture the ingot. In addition, the ingot is forged to be formed, thereby manufacturing the press roller. However, when the ingot is forged as it is, there are problems such as uneven structure of the press roller, cracks, and low hardness due to segregation inside the ingot. Therefore, before forging the ingot, an electroslag remelting (ESR) method is used to remove the segregation contained in the ingot. In order to illustrate the ESR method, first, an electrode rod is manufactured using the manufactured ingot. In addition, an arc is generated using the manufactured electrode rod to remelt the electrode rod. Here, the molten steel produced by remelting the electrode rod is dripped in the form of droplets and then solidified to manufacture the ingot again. Here, molten steel produced by remelting may be dropped in the form of droplets to produce an ingot with less segregation.

[0003] However, the ESR method requires a process of manufacturing the ingot manufactured as described above into an electrode rod, a process of remelting the electrode rod, and a process of dropping the remelted molten steel to resolidify the molten steel. That is, the ESR method must be performed through a multi-step process. Therefore, there is a significant problem in terms of time and cost consumed in the process of suppressing the occurrence of segregation.

[0004] (Prior Art Document) (Patent Document 1) Korean Patent Registration No. 10-1346636. Summary of the invention

[0005] Technical issues

[0006] The present invention provides a method for manufacturing high-chromium (Cr) molten steel. When a steelmaking converter is used to manufacture the high-chromium (Cr) molten steel, the method can manufacture the high-chromium (Cr) molten steel without polluting the steelmaking converter.

[0007] The present invention provides a method for manufacturing a slab capable of simplifying a process of suppressing or preventing the occurrence of segregation and reducing costs, and a method for manufacturing a press roller.

[0008] The present invention provides a method for manufacturing a slab capable of suppressing or preventing the occurrence of cracks, making the structure uniform and having high hardness, and a method for manufacturing a press roller.

[0009] Technical Solution

[0010] An embodiment of the present invention provides a method for producing high chromium (Cr) molten steel having a chromium (Cr) content of 4.5 wt % to 5.5 wt %, and the method may include the following processes: charging the molten steel into a steelmaking converter used in a process of manufacturing stainless steel; and inputting a chromium (Cr)-containing chromium steel alloy into the steelmaking converter so that the chromium (Cr) content in the molten steel reaches 4.5 wt % to 5.5 wt %.

[0011] The method for manufacturing high chromium (Cr) molten steel according to an embodiment of the present invention may also include: performing oxygen blowing by blowing oxygen into a steelmaking converter charged with chromium alloy steel to remove carbon (C) from the molten steel; removing oxygen (O) from the molten steel by inputting a deoxidizer into the steelmaking converter; and reducing chromium oxides contained in slag floating on the surface of the molten pool of the molten steel into chromium (Cr) by inputting a reducing agent into the steelmaking converter.

[0012] The method for manufacturing high chromium (Cr) molten steel according to an embodiment of the present invention may further include: discharging the molten steel from the steelmaking converter to a ladle; and controlling the temperature of the molten steel received from the ladle by using a ladle furnace (LF), which is a heating device used in a carbon steelmaking process.

[0013] When the temperature is controlled, the temperature of the molten steel can be controlled to be 1560°C to 1600°C.

[0014] The method for producing high chromium (Cr) molten steel according to an embodiment of the present invention may further include: using a Rheinstaal Huttenwerke und Heraus (RH) device to perform degassing to remove hydrogen (H2) and nitrogen (N2) from the molten steel received in the ladle, the Huttenwerke und Heraus (RH) device being a vacuum treatment device used in a carbon steelmaking process, wherein degassing may be performed after temperature control is completed.

[0015] When discharging molten steel from a steelmaking converter into a ladle, the molten steel inside the steelmaking converter can be discharged into the ladle in the carbon steelmaking process.

[0016] A method for manufacturing a slab according to an embodiment of the present invention includes: preparing molten steel containing 4.5 wt % to 5.5 wt % of chromium (Cr); performing a casting process, wherein the molten steel is supplied to a crystallizer of a casting device so that the molten steel solidifies inside the crystallizer, thereby manufacturing a slab containing unsolidified molten steel; drawing the slab manufactured in the casting process to the lower side of the crystallizer; and performing a solidification process, wherein a magnetic field is applied to the slab drawn to the lower side of the crystallizer so as to solidify the unsolidified molten steel contained in the slab while allowing the unsolidified molten steel contained in the slab to flow, thereby manufacturing the slab.

[0017] The solidification process may include heating the distal end of the slab that is drawn to the underside of the crystallizer.

[0018] The casting process may include applying a magnetic field to the mold to allow the molten steel to flow inside the mold.

[0019] When the slab is withdrawn to the lower side of the crystallizer, the withdrawal may be performed at a speed of 0.04 m / min or less.

[0020] The solidification process may include injecting cooling water onto the slab drawn to the lower side of the crystallizer, and when the cooling water is injected onto the slab, the injecting may be performed such that a surface temperature of the slab becomes 800 to 900°C.

[0021] In the withdrawal process, the slab may be withdrawn from the crystallizer in a direction perpendicular to the ground, and in the solidification process, the slab may be solidified in a state in which the slab is disposed in a direction perpendicular to the ground.

[0022] A method for manufacturing a press roller according to an embodiment of the present invention includes: heating a slab; forging the heated slab to mold the slab into the shape of a press roller; and heating the press roller manufactured in the molding to remove hydrogen (H2) from the press roller, wherein heating the slab includes heating the slab by increasing the temperature of the slab to a target temperature in multiple stages.

[0023] Heating the slab to a target temperature may include: heating the slab to a first temperature of 250°C to 350°C; heating the slab to a second temperature of 450°C to 550°C; heating the slab to a third temperature of 650°C to 750°C; and heating the slab to a target temperature of 1100°C to 1250°C.

[0024] When the slab is heated to the first temperature to the third temperature and the target temperature, the slab can be kept at the first temperature for 3 to 5 hours, the slab can be kept at the second temperature for 5 to 7 hours, the slab can be kept at the third temperature for 3 to 5 hours, and the slab can be kept at the target temperature for 14 to 18 hours.

[0025] When forging the slab, the slab heated to a temperature of 1100° C. to 1250° C. may be pressed down for forging such that the thickness of the slab decreases by 250 mm to 350 mm each time the slab is pressed down using a pressing device.

[0026] The method may further include heating the press roller to a temperature of 200° C. to 400° C. while removing the hydrogen (H 2 ) from the press roller.

[0027] When the press roller is heated to a temperature of 200° C. to 400° C., the press roller may be heated for 48 hours or more.

[0028] Beneficial Effects

[0029] According to an embodiment of the present invention, high chromium (Cr) molten steel can be manufactured by using a steelmaking converter used in a steelmaking process of other steel types without contaminating the steelmaking converter. In addition, when the temperature of high chromium (Cr) molten steel is increased or degassed to exhaust gas, the present invention can effectively increase the temperature of the molten steel and improve the degassing efficiency.

[0030] In addition, when compared with conventional methods, the present invention can suppress or prevent segregation inside the slab in a simple and low-cost manner. In addition, when a press roller is manufactured by forging a slab, a press roller having a uniform structure, high hardness and in which cracks are suppressed or prevented can be manufactured. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a flow chart illustrating a method for manufacturing a press roller by a method according to an embodiment of the present invention.

[0032] Figure 2 (a) to (g) are process diagrams sequentially illustrating a process of preparing molten steel according to an embodiment of the present invention.

[0033] Figure 3 (a) to (d) are views sequentially illustrating the operation of the casting apparatus according to the embodiment of the present invention.

[0034] Figure 4 (a) to (c) are process diagrams sequentially illustrating a method for manufacturing a press roller using a slab manufactured by the method according to an embodiment of the present invention.

[0035] Figure 5 (a) and (b) are views illustrating results obtained by confirming whether coarse segregation occurs or more coarse segregation occurs by cutting and etching the cross section of the press roller. DETAILED DESCRIPTION

[0036] Hereinafter, specific embodiments will be described in more detail with reference to the accompanying drawings. However, the present invention can be implemented in different forms and should not be construed as being limited to the embodiments set forth herein. More specifically, these embodiments are provided so that the present disclosure will be thorough and complete, and these embodiments will fully convey the scope of the present invention to those skilled in the art. In the accompanying drawings, for clarity of illustration, the sizes of layers and regions are exaggerated. The same reference numerals always refer to the same elements.

[0037] Figure 1 is a flow chart illustrating a method for manufacturing a press roller by a method according to an embodiment of the present invention.

[0038] Reference Figure 1 The method for manufacturing a press roller includes a process of preparing molten steel (S100), a process of solidifying the molten steel to manufacture a slab (S200), a process of heating the slab (S300), a process of forging the heated cast steel to manufacture a press roller (S400), and a process of heating the press roller (S500).

[0039] The press roller is made of alloy steel with a high content of chromium (Cr). More specifically, the press roller is made of high chromium (Cr) alloy steel containing a high content of chromium (Cr) of 4.5 to 5.5 wt % in the entire press roller. More specifically, the press roller is made of an alloy steel containing the following in its entirety: 4.5 wt % to 5.5 wt % chromium (Cr), 0.75 wt % to 0.95 wt % carbon (C), 0.2 wt % to 0.5 wt % silicon (Si), 0.2 wt % to 0.5 wt % manganese (Mn), 0.4 wt % to 0.65 wt % molybdenum (Mo), 0.5 wt % or less (0 wt % or more) nickel (Ni), 0.2 wt % or less (0 wt % or more) copper (Cu), 0.0025 wt % or less (0 wt % or more) phosphorus (P), 0.015 wt % or less (0 wt % or more) sulfur (S), and iron (Fe) as a residual content (91.16 wt % to 93.15 wt %).

[0040] For the entire press roller, the content of chromium (Cr) is controlled to be 4.5 wt % to 5.5 wt % and the content of carbon (C) is controlled to be 0.75 wt % to 0.95 wt % to ensure hardness and machinability. For example, if the content of chromium (Cr) in the press roller is less than 4.5 wt % or the content of carbon (C) is less than 0.75 wt %, the problem of low hardness of the press roller may arise. On the contrary, if the content of chromium (Cr) in the press roller exceeds 5.5 wt % or the content of carbon (C) exceeds 0.95 wt %, the machinability of the press roller is low, and it may be difficult to process the manufactured press roller as required.

[0041] In addition, the content of phosphorus (P) in the press roller is controlled to be 0.025 wt% or less, and the content of sulfur (S) is controlled to be 0.015 wt% or less, so as to allow the structure of the press roller to be uniform and suppress or prevent the occurrence of cracks. That is, if the content of phosphorus (P) in the press roller exceeds 0.025 wt% or the content of sulfur (S) exceeds 0.015 wt%, the structure may become uneven due to segregation inside the press roller, etc. Therefore, when an object to be rolled (hereinafter referred to as a rolled object) is pressed by using the press roller, a problem of cracks occurring in the press roller may occur. Therefore, the content of phosphorus (P) in the press roller is controlled to be 0.025 wt% or less, and the content of sulfur (S) is controlled to be 0.015 wt% or less.

[0042] In an embodiment of the present invention, the press roller to be manufactured may be a roller applied to a rolling device that applies force to a rolling object such as a slab or a steel plate to roll the object.

[0043] In the process of preparing molten steel (S100), molten steel for manufacturing a press roll is prepared. That is, in the process of preparing molten steel (S100), molten steel having a component content required for a press roll is prepared. That is, high chromium (Cr) molten steel having a chromium (Cr) content of 4.5 wt % to 5.5 wt % is prepared. More specifically, when preparing molten steel, molten steel having the following contents is manufactured: 4.5 to 5.5 wt% of chromium (Cr), 0.75 to 0.95 wt% of carbon (C), 0.2 to 0.5 wt% of silicon (Si), 0.2 to 0.5 wt% of manganese (Mn), 0.4 to 0.65 wt% of molybdenum (Mo), 0.5 wt% or less (0 wt% or more) of nickel (Ni), 0.2 wt% or less (0 wt% or more) of copper (Cu), 0.0025 wt% or less (0 wt% or more) of phosphorus (P), 0.015 wt% or less (0 wt% or more) of sulfur (S), and iron (Fe) as a residual content (91.16 to 93.15 wt%). This can be achieved by sequentially performing each process of preparing molten steel described below.

[0044] Figure 2 (a) to (g) are process diagrams sequentially illustrating a process of preparing molten steel according to an embodiment of the present invention.

[0045] Reference Figure 2 The process of preparing molten steel M (S100) may include a process of treating the molten steel M using a steelmaking converter 11 (S110) (hereinafter referred to as a steelmaking converter treatment process), a process of discharging the molten steel M inside the steelmaking converter 11 into a ladle 20 (S120), a temperature control process of controlling the temperature of the molten steel M discharged into the ladle 20 using a temperature control device 30 (S130), and a degassing process of removing gas from the molten steel M using a vacuum treatment device 40 (S140).

[0046] The steelmaking converter process (S110) is a process of controlling the composition of the molten steel M while charging the molten steel M into the steelmaking converter 11. The steelmaking converter 11 used in the steelmaking converter process (S110) is a steelmaking converter used in a steelmaking process (hereinafter referred to as a stainless steelmaking process) for preparing molten steel for manufacturing stainless steel (hereinafter referred to as molten steel for manufacturing stainless steel). That is, when preparing the molten steel according to the embodiment, the steelmaking converter 11 used in the stainless steelmaking process is used.

[0047] For a more specific description, the steelmaking process using a steelmaking converter during the stainless steelmaking process can be briefly described. The stainless steelmaking process includes a process of charging molten steel into the steelmaking converter, a process of inputting alloy steel containing chromium (Cr) and alloy steel containing nickel (Ni) into the steelmaking converter, and an oxygen annealing process of injecting oxygen into the steelmaking converter by using a lance to remove carbon (C) and phosphorus (P). When iron containing chromium (Cr) is added to the steelmaking converter, the content of chromium (Cr) in the molten steel is input as 10.5 wt % to 11 wt %. Therefore, the molten steel prepared in the steelmaking process using the steelmaking converter during the stainless steelmaking process contains 10.5 wt % to 11 wt % of chromium.

[0048] For comparison, a steelmaking process using a steelmaking converter in a steelmaking process (hereinafter referred to as "carbon steel steelmaking process") for providing molten steel for manufacturing carbon steel (hereinafter referred to as "molten steel for manufacturing carbon steel") is briefly described. The carbon steelmaking process includes a process of charging the molten steel into the steelmaking converter, and an oxygen annealing process of injecting oxygen into the steelmaking converter by using a lance to remove carbon (C) and phosphorus (P). Carbon steel is steel with a relatively low chromium (Cr) content. Therefore, in the carbon steelmaking process, chromium (Cr) is not input into the steelmaking converter, and the content of chromium (Cr) in the molten steel is controlled to be relatively low, at 0.1 wt% to 1.0 wt%.

[0049] As described above, in the steelmaking converter used in the stainless steelmaking process, molten steel is prepared with a high chromium (Cr) content of 10.5 to 11 wt%. In other words, the interior of the steelmaking converter used in the stainless steelmaking process contains molten steel with a high chromium (Cr) content of 10.5 to 11 wt%. However, the interior of the steelmaking converter used in the carbon steelmaking process contains molten steel with a low chromium (Cr) content of 0.1 to 1.0 wt%.

[0050] Therefore, when preparing molten steel for manufacturing a press roll with a high chromium (Cr) content of 4.5% to 5.5% by weight, if a steelmaking converter used in a carbon steel steelmaking process is used, the steelmaking converter used to manufacture carbon steel may be contaminated. That is, if molten steel with a high chromium (Cr) content of 4.5% to 5.5% by weight is placed in a steelmaking converter used to manufacture carbon steel, even if the molten steel is cast, a large amount of chromium (Cr) may remain inside the steelmaking converter used to manufacture carbon steel. For example, a large amount of chromium (Cr) may be attached or adhered to the inner wall of the steelmaking converter used to manufacture carbon steel. Therefore, the steelmaking converter used to manufacture carbon steel is contaminated by chromium (Cr). In addition, when a steelmaking converter used to manufacture carbon steel containing a large amount of residual chromium (Cr) is used in a carbon steel steelmaking process, the molten steel will contain a large amount of chromium (Cr), and chromium (Cr) will act as an impurity to deteriorate the quality of the carbon steel.

[0051] However, when a high chromium (Cr) steel having a high chromium (Cr) content of 4.5 to 5.5 wt% is prepared using a steelmaking converter used in a stainless steelmaking process, the steelmaking converter used in the stainless steelmaking process will not be contaminated. That is, even if molten steel having a high chromium (Cr) content of 4.5 to 5.5 wt% is put into the steelmaking converter 11 for manufacturing stainless steel and then discharged and a large amount of chromium (Cr) remains inside the steelmaking converter for manufacturing stainless steel, the chromium remaining inside the steelmaking converter will not act as an impurity. This is because the steelmaking converter for manufacturing stainless steel is already a device that contains molten steel having a high chromium content of 10.5 to 11 wt%. In addition, this is because the chromium content of the molten steel put into the steelmaking converter for manufacturing stainless steel is higher than the chromium content of the molten steel to be manufactured in the embodiment. Therefore, even if the molten steel for manufacturing a press roll is placed in the steelmaking converter for manufacturing stainless steel, the steelmaking converter for manufacturing stainless steel will not be contaminated. Therefore, when molten steel for stainless steel is again manufactured using the steelmaking converter for manufacturing stainless steel in which molten steel according to the embodiment is accommodated, a problem does not occur due to chromium (Cr) in the previously accommodated molten steel.

[0052] Therefore, in an embodiment, when the molten steel is treated using the steelmaking converter 11 (S110), the steelmaking converter 11 for the stainless steelmaking process as described above is used. Therefore, the steelmaking converter 11 can be used without causing contamination of the steelmaking converter 11 due to chromium (Cr). In addition, the steelmaking converter for preparing the molten steel containing 4.5 wt% to 5.5 wt% of chromium (Cr) according to the embodiment can be used to prepare the molten steel for manufacturing stainless steel. That is, the steelmaking converter for manufacturing stainless steel can be used in an operation for preparing the molten steel for manufacturing stainless steel and in an operation for preparing the molten steel containing 4.5 wt% to 5.5 wt% of chromium (Cr) according to the embodiment in an interchangeable manner.

[0053] The temperature control device 30 for the temperature control process and the vacuum treatment device 40 for the degassing process use a ladle furnace (LF) and a Rheinstaal Huttenwerke und Heraus (RH) device used in a steelmaking process (hereinafter referred to as "carbon steelmaking process") for preparing molten steel for manufacturing carbon steel (hereinafter referred to as "molten steel for manufacturing carbon steel"). The reason for using the ladle furnace and the RH device in the carbon steelmaking process will be described later.

[0054] In the following, reference will be made to Figure 2 A process for preparing molten steel according to an embodiment of the present invention is described in more detail.

[0055] As described above, the molten steel preparation process (S100) includes a process of treating the molten steel M using the steelmaking converter 11 (S110), a process of discharging the molten steel M inside the steelmaking converter 11 into a ladle (S120), a process of controlling the temperature of the molten steel M using a temperature control device 30 (S130), and a degassing process of removing gas from the molten steel M using a vacuum treatment device 40 (S140).

[0056] Reference Figure 2 (a) to (d), the process (S110) of treating the molten steel M using the steelmaking converter 11 may include a process (S111) of inputting alloy steel containing chromium (Cr) (hereinafter referred to as chromium (Cr) alloy steel) into the steelmaking converter 11 ( Figure 2 (a)), oxygen annealing process (S112) of injecting oxygen into the molten steel inside the steelmaking converter 11 ( Figure 2 (b)), a deoxidation process (S113) of removing oxygen (O) contained in the molten steel M ( Figure 2 (c)), and a process (S114) of reducing chromium oxide contained in the slag floating on the surface of the molten steel M to chromium (Cr) ( Figure 2 (d)).

[0057] In addition, although not shown, the molten steel preparation process may also include a preliminary refining process for removing sulfur (S), phosphorus (P) and silicon (Si) contained in the molten steel before charging the molten steel into the steelmaking converter. First, the preliminary refining process will be briefly described. The preliminary refining process may include a first preliminary refining process for removing sulfur (S) from the molten steel, and a second preliminary refining process for removing phosphorus (P) and silicon (Si) from the molten steel. In the first preliminary refining process, the sulfur (S) content in the molten steel is controlled to be 0.04 weight % or less. In addition, in the second preliminary refining process, the content of silicon (Si) in the molten steel is controlled to be 0.05 weight % or more, and the content of phosphorus (P) is controlled to be 0.03 weight % or less. When performing the first preliminary refining process and the second preliminary refining process, quicklime (CaO) and fluorite (CaF2) are input into the ladle containing the molten steel to remove sulfur (S), silicon (Si) and phosphorus (P) from the molten steel. In addition, the first preliminary refining process and the second preliminary refining process may be performed in a hot metal pretreatment station (HMPS).

[0058] After the first and second preliminary refining processes are completed, molten steel M is charged into the steelmaking converter 11. Here, the steelmaking converter 11 is used to charge the molten steel M, and as described above, the steelmaking converter 11 used in the stainless steelmaking process is used.

[0059] When the molten steel M is charged into the steelmaking converter 11, Figure 2 As shown in (a) of FIG. 1 , chromium (Cr) alloy steel is input into the steelmaking converter 11. Therefore, the chromium (Cr) alloy steel input into the steelmaking converter 11 is melted by the heat of the molten steel M. Here, the input amount of the chromium (Cr) alloy steel is controlled so that the content of chromium (Cr) in the entire molten steel M is 4.5 wt % to 5.5 wt %. Therefore, high molten steel M with a chromium (Cr) content of 4.5 wt % to 5.5 wt % is produced.

[0060] Next, if Figure 2As shown in (b), oxygen annealing is performed by blowing or injecting oxygen into the molten steel M inside the steelmaking converter 11. That is, after a part of the lance is inserted into the interior of the steelmaking converter 11, oxygen (O) is supplied to the lance 12. Therefore, oxygen (O) is blown or injected from the lance 12. The oxygen (O) blown into the interior of the steelmaking converter 11 using the lance 12 reacts with the carbon (O) contained in the molten steel M (C+O→CO(gas)). Here, the carbon (O) in the molten steel M reacts with the oxygen (O) to generate carbon monoxide (CO) gas, and the generated carbon monoxide (CO) gas is discharged to the outside of the steelmaking converter 11. Therefore, the content of carbon (C) contained in the molten steel M is reduced. That is, a decarburization phenomenon occurs in which the carbon (C) content in the molten steel M is reduced. Here, at least one of the oxygen blowing flow rate or the oxygen blowing time is controlled so that the carbon (C) content in the molten steel M becomes 0.75 wt % to 0.95 wt %.

[0061] As described above, when decarburization is performed by blowing oxygen into the steelmaking converter 11, in addition to carbon monoxide (CO) gas, reaction byproducts provided as metal oxides are generated. In addition, the generated reaction byproducts rise toward the top surface of the molten steel M, that is, the molten pool surface of the molten steel M, and float on the upper part of the molten pool surface of the molten steel M. The reaction byproduct floating on the molten pool surface of the molten steel is slag SL.

[0062] When the carbon (C) content in the molten steel M reaches 0.75 wt% to 0.95 wt% and decarburization is completed, or when the carbon (C) content in the molten steel M decreases to approximately 0.95 wt% and reaches the final decarburization stage, a dephosphorizing agent is input into the steelmaking converter 11. Here, oxygen is blown into the converter 11 using the lance 12 and the dephosphorizing agent is simultaneously input, and the dephosphorizing agent may be a material containing, for example, quicklime (CaO). Here, phosphorus (P) in the molten steel M reacts with the dephosphorizing agent and oxygen (O) (3CaO+2P+5O->3CaO·P2O5). In addition, 3CaO·P2O5 as a reaction byproduct rises and is absorbed into the slag SL floating on the surface of the molten pool of the molten steel M. That is, the phosphorus (P) separated from the molten steel M is absorbed into the slag SL floating on the surface of the molten pool of the molten steel M. Therefore, the content of phosphorus (P) contained in the molten steel M is reduced. That is, a dephosphorization phenomenon occurs in which the phosphorus (P) content in the molten steel M decreases. Here, at least one of the dephosphorization agent input amount, the oxygen blowing flow rate, or the upper sintering injection time is controlled so that the phosphorus (P) content in the molten steel M becomes 0.025 wt % or less.

[0063] After oxygen annealing is performed on the molten steel M for decarburization and dephosphorization, the oxygen (O) content in the molten steel M increases. The oxygen (O) in the molten steel M may cause pinholes in the slab. In addition, the oxygen (O) in the molten steel reacts with the metal contained in the molten steel and becomes a factor in generating metal oxide inclusions. The inclusions contained in the molten steel may cause defects such as cracks in the cast steel.

[0064] Therefore, after the oxygen annealing is completed, deoxidation is performed to remove oxygen (O) from the molten steel M. To this end, a deoxidizer containing at least one of silicon (Si) or aluminum (Al) is input into the interior of the steelmaking converter 11. The deoxidizer may use at least one of a silicon (Si) alloy or an aluminum (Al) alloy. Here, the silicon (Si) alloy may be an Fe-Si alloy containing silicon (Si) and iron (Fe), which contains 70% to 80% by weight of silicon (Si) and 20% to 30% by weight of iron (Fe). In addition, the aluminum (Al) alloy may be an alloy of nearly pure aluminum (Al) containing 90% to 95% by weight of aluminum (Al).

[0065] When the deoxidizer is input, oxygen (O) contained in the molten steel M reacts with at least one of silicon (Si) or aluminum (Al) contained in the deoxidizer. At least one of silicon oxides such as SiO2 or aluminum oxides such as Al2O3 is generated. In addition, at least one reaction byproduct of silicon oxide and aluminum oxide is absorbed into the slag SL floating on the surface of the molten pool of the molten steel M. Therefore, the content of oxygen (O) contained in the molten steel M is reduced (deoxidation). Here, the input amount of the deoxidizer is controlled so that the content of oxygen (O) in the molten steel M becomes 0.001 weight % or less.

[0066] As described above, at least one of a silicon (Si) alloy or an aluminum (Al) alloy is used as a deoxidizer. Here, when compared with using a silicon (Si) alloy as a deoxidizer or using an aluminum (Al) alloy as a deoxidizer, it is preferred that a silicon (Si) alloy and an aluminum (Al) alloy are used together as a deoxidizer.

[0067] When the content of oxygen (O) in the molten steel exceeds 0.001 wt %, pinhole defects may occur in the cast steel due to oxygen (O), and cracks may occur due to a large number of inclusions. Therefore, during the deoxidation process, the content of oxygen (O) in the molten steel M is controlled to 0.001 wt % or less.

[0068] As described above, when oxygen annealing is performed to remove carbon (C) and phosphorus (P) from the molten steel M, the chromium (Cr) input into the molten steel M (S110) is oxidized into chromium oxide. In addition, the generated chromium oxide is absorbed into the slag SL floating on the surface of the molten pool of the molten steel M. Therefore, the content of chromium (Cr) in the molten steel is reduced. Therefore, it is necessary to reduce the chromium oxide contained in the slag SL to chromium, and then supply the reduced chromium to the molten steel M again. To this end, a reducing agent is input into the steelmaking converter 11 where deoxidation is completed. More specifically, slag SL is input as a reducing agent. Here, the reducing agent may be a material containing silicon (Si), and more specifically, the reducing agent may be an alloy steel containing silicon (Si). As a more specific example, the reducing agent may contain 70% to 80% by weight of silicon (Si) and 20% to 30% by weight of iron (Fe).

[0069] When the reducing agent is input into the steelmaking converter 11, the chromium oxide contained in the slag SL and the silicon (Si) contained in the reducing agent react with each other. Therefore, the chromium oxide contained in the slag SL is reduced to chromium (Cr), and the generated chromium (Cr) is absorbed or supplied to the molten steel. Therefore, the content of chromium (Cr) in the molten steel increases. Here, the input amount of the reducing agent is controlled so that the content of chromium (Cr) in the molten steel is 4.5 wt% to 5.5 wt%.

[0070] After the chromium reduction is completed, the component content and temperature of the molten steel M are confirmed. For example, the molten steel M inside the steelmaking converter 11 is collected or sampled, and the component content and temperature of the collected molten steel are measured. In addition, it is confirmed whether the measured component content of the molten steel M is included in the target component content, and it is confirmed whether the measured temperature of the molten steel M is included in the target temperature. Here, the target temperature can be, for example, 1650°C to 1750°C.

[0071] To describe a more specific example, it is confirmed whether each of the contents of chromium (Cr), carbon (C), phosphorus (P), sulfur (S), and oxygen (O) is included in the target content. Here, if each of the contents of chromium (Cr), carbon (C), phosphorus (P), sulfur (S), and oxygen (O) is included in the target content and the temperature of the molten steel is included in the target temperature, the slag floating on the upper part of the molten pool surface of the molten steel is discharged (not shown). That is, the slag is removed from the molten pool surface of the molten steel. However, if each of the contents of chromium (Cr), carbon (C), phosphorus (P), sulfur (S), and oxygen (O) is not within the target content or the temperature of the molten steel is not within the target temperature, the process for controlling the component content or temperature is performed again. That is, if each of the contents of chromium (Cr), carbon (C), phosphorus (P), sulfur (S), and oxygen (O) contained in the molten steel is not within the target content, at least one of the following processes is performed: input of chromium (Cr) alloy steel, decarburization, dephosphorization, desulfurization, deoxidation, and chromium reduction. In addition, if the temperature of the molten steel M is lower than 1650° C., the molten steel M is heated so that the temperature of the molten steel is higher than 1650° C. On the contrary, if the temperature of the molten steel exceeds 1750° C., the temperature of the molten steel is lowered.

[0072] If the content of each component contained in the molten steel exceeds the target content, there may be a problem of quality deterioration of the slab S. That is, if at least one of the contents of chromium (Cr), carbon (C), phosphorus (P), sulfur (S), and oxygen (O) deviates from the target content, the hardness of the manufactured cast steel may be low, or there may be a problem of pinholes and cracks being generated in the surface or inside of the cast steel. This may reduce the hardness of the press roll and generate pinholes and cracks in the surface or inside of the cast steel.

[0073] In addition, if the temperature of the molten steel M is lower than 1650°C, it may be difficult to raise the temperature of the molten steel M to the target temperature (LF starting target temperature) during the subsequent temperature control process. Therefore, when the molten steel M is supplied through the tundish, the temperature of the molten steel M may not reach the casting target temperature. In this case, the nozzle that supplies the molten steel from the tundish to the crystallizer may be blocked. On the contrary, if the temperature of the molten steel M exceeds 1750°C, the refractory material constituting the steelmaking converter may be damaged due to erosion caused by high heat. Therefore, the temperature of the molten steel M in the steelmaking converter process is controlled to be between 1650°C and 1750°C.

[0074] When the content of each component in the molten steel M is within the target content and the temperature of the molten steel M is within the target temperature, slag floating on the top surface of the molten steel M is removed (not shown).

[0075] After the slag SL inside the steelmaking converter 11 is removed, the molten steel M inside the steelmaking converter 11 is discharged into the ladle 20. Here, it is desirable to use the ladle 20 used in the carbon steelmaking process to discharge the molten steel M. This is because the device for the process performed later uses the ladle 20 used in the carbon steelmaking process. That is, in the temperature control process, the ladle furnace LF is used as the temperature control device 30, and the Rheinstaal Huttenwerke und Heraus (RH) device is used as the vacuum treatment device 40 in the degassing process. Here, the ladle furnace LF and the RH device are devices used in the carbon steelmaking process, and are devices connected to the ladle used in the carbon steelmaking process.

[0076] In addition, the stainless steel steelmaking process does not use a ladle furnace LF. In addition, a vacuum treatment device is used in the stainless steel steelmaking process, but it is a vacuum tank degasser (VTD) device different from the RH device used in the carbon steel steelmaking process.

[0077] In the embodiment, when the molten steel inside the steelmaking converter 11 is discharged into the ladle, the molten steel is discharged into the ladle 20 used in the carbon steelmaking process. That is, in order to use the device used in the carbon steelmaking process in the subsequent temperature control process and degassing process, the molten steel is discharged into the ladle 20 used in the carbon steelmaking process.

[0078] As described above, in the embodiment, a ladle furnace is used as the temperature control device 30, and an RH device is used as the vacuum processing device 40. Hereinafter, for convenience of explanation, the temperature control device and the ladle furnace are denoted by the same reference numeral "30", and the vacuum processing device and the RH device are denoted by the same reference numeral "40".

[0079] When the discharge of the molten steel M into the ladle 20 is completed, Figure 2 As shown in (f) of FIG. 1 , the ladle 20 containing the molten steel M is moved to the temperature control device 30, and the ladle 20 is connected to the temperature control device 30. That is, the ladle 20 is moved to the ladle furnace 30 as the temperature control device 30 used in the carbon steelmaking process, and the ladle 20 is connected to the ladle furnace 30. Figure 2 As shown in (f) of FIG. 1 , the ladle furnace may include a cover 32 covering an upper opening of the ladle 20, and an electrode rod 31 inserted into the ladle 20 through the cover 32. In addition, in the ladle furnace 30, desulfurization is performed to remove sulfur (S) from molten steel and control the contents of carbon (C) and chromium (Cr), and then the temperature of the molten steel is controlled.

[0080] First, the desulfurization process using the ladle furnace 30 will be described. When the ladle 20 arrives at the ladle furnace 30, the cover 32 of the ladle furnace 30 is attached to the upper part of the ladle 20. Then, a desulfurizer is input into the ladle 20. Here, the desulfurizer can pass through an opening provided in the cover 32 and then be input into the ladle 20. Preferably, the desulfurizer uses at least one of a first material containing quicklime (CaO) and alumina (Al2O3) and a second material containing fluorite (CaF2). When the desulfurizer is input into the ladle 20, the desulfurizer reacts with sulfur (S) in the molten steel to produce a reaction by-product containing sulfur (S), and the produced reaction by-product is absorbed into the slag SL on the molten pool surface of the molten steel M. Therefore, the sulfur (S) content in the molten steel is reduced (desulfurization). Here, the content of sulfur (S) in the molten steel is set to 0.015 wt% or less, and this can be controlled by controlling the input amount of the desulfurizer.

[0081] When desulfurization is completed, the molten steel M contained in the ladle 20 is sampled to measure the contents of carbon (C) and chromium (Cr). In addition, the contents of carbon (C) and chromium (Cr) in the molten steel M are controlled according to the contents of carbon (C) and chromium (Cr). For example, when the carbon (C) content is as low as less than 0.75 weight %, a carbon material containing carbon (C) is input into the ladle 20. In addition, the input amount of carbon dioxide is controlled so that the carbon (C) content in the molten steel M is 0.75 weight % or more. On the contrary, when the measured carbon (C) content exceeds 0.95 weight %, solid oxygen such as iron ore such as FeO is input into the ladle 20. Here, the input amount of solid oxygen is controlled so that the carbon (C) content in the molten steel M is 0.95 weight % or less.

[0082] As another example, when the measured chromium (Cr) content is as low as less than 4.5 wt %, alloy steel containing chromium (Cr) is input into the ladle 20. In contrast, when the measured chromium (Cr) content exceeds 5.5 wt %, solid oxygen such as iron ore such as FeO is input into the ladle 20. Here, the input amount of solid oxygen is controlled so that the content of chromium (Cr) in the molten steel is 5.5 wt % or less.

[0083] In the above, it is explained that when the carbon (C) content exceeds 0.95 wt % or the chromium (Cr) content exceeds 5.5 wt %, solid oxygen is input into the ladle 20 connected to the ladle furnace 30. However, it is not limited thereto, and oxygen may be blown into the molten steel M in the vacuum processing device 40 implemented later to control the carbon (C) content to 0.95 wt % or less, or the chromium (Cr) content to 5.5 wt % or less.

[0084] When the molten steel M is discharged into the ladle 20, the ladle 20 is moved to the next process. However, the temperature of the molten steel M gradually decreases while the ladle 20 is moving. That is, even in the case where the temperature of the molten steel M is controlled to be 1650° C. to 1750° C. before the molten steel from the steelmaking converter 11 is discharged into the ladle 20, the temperature of the molten steel M decreases during the process of the ladle 20 moving to the next process.

[0085] Therefore, before the ladle 20 moves to the next process, that is, the vacuum processing device 40, it is necessary to heat the molten steel M inside the ladle 20. To this end, power is supplied to the electrode rod 31 to generate an arc and heat from the electrode rod 31, and the molten steel is heated by the generated arc and heat. Here, the molten steel M is heated so that the temperature becomes the initial target temperature, and the initial target temperature may be 1560°C to 1600°C. Here, the initial target temperature refers to the target temperature of the molten steel M contained in the ladle 20 when the ladle 20 separated from the ladle furnace 30 sets out for the next process. The temperature of the molten steel M may be controlled to the initial target temperature by controlling at least one of the intensity of the power supplied to the electrode rod 31 and the heating time of the molten steel using the electrode rod 31.

[0086] In the case where the temperature of the molten steel M (hereinafter referred to as the starting temperature) is lower than 1560°C or higher than 1600°C when the ladle 20 separated from the ladle furnace 30 starts the next process, when the ladle 20 reaches the tundish of the casting device, the temperature of the molten steel M may exceed the casting target temperature. Here, the casting target temperature refers to the target temperature of the molten steel received in the tundish, and the casting target temperature may be 1520°C to 1550°C. If the starting temperature is lower than 1560°C, the temperature of the molten steel M may be lower than 1520°C when the ladle reaches the tundish. In addition, if the starting temperature is lower than 1600°C, the temperature of the molten steel may exceed 1550°C when the ladle reaches the tundish.

[0087] When the ladle 20 reaches the tundish, if the temperature of the molten steel M is lower than 1520°C, the nozzle supplying the molten steel M from the tundish to the crystallizer may be clogged. In addition, in the case where the temperature of the molten steel M exceeds 1600°C when the ladle 20 reaches the tundish, when the molten steel solidifies in the casting device, the solidified shell will break and the molten steel in the solidified shell will be discharged. Therefore, the starting temperature of the molten steel M in the ladle furnace 30 is controlled to be 1560°C to 1600°C, so that when the ladle 20 reaches the tundish, the temperature of the molten steel is 1520°C to 1550°C.

[0088] The reason why the ladle furnace LF used in the carbon steelmaking process is used when performing the temperature control process (S130) is that this effectively increases the temperature without contaminating the molten steel M. That is, in the stainless steelmaking process, when the temperature of the molten steel is increased, a metal such as aluminum (Al) is input into the molten steel to cause an exothermic reaction and increase the temperature. That is, in the stainless steelmaking process, a separate heating device is not used to increase the temperature of the molten steel. Therefore, in an embodiment, the temperature of the molten steel M is controlled by using a ladle furnace 30 for increasing the temperature of the molten steel in the carbon steelmaking process. Therefore, the temperature of the molten steel M can be increased without inputting other materials into the molten steel M. That is, the temperature of the molten steel can be increased without changing the composition of the molten steel M.

[0089] When the control of the composition and temperature in the ladle furnace 30 is completed, the ladle 20 is separated from the ladle furnace 30. Next, the ladle 20 separated from the ladle furnace 30 is moved to the vacuum treatment device 40. In addition, degassing (S140) is performed using the vacuum treatment device 40 to remove nitrogen (N2) and hydrogen (H2) contained in the molten steel M.

[0090] The vacuum treatment device 40 is a Rheinstaal Huttenwerke und Heraus (RH) device 40, and is an RH device 40 used in a carbon steelmaking process. Figure 2 (g), the RH device 40 may include a container 41 having an internal space, a pump (not shown) connected to the container 41 to control the pressure inside the container 41, a pair of reflux pipes 42a and 42b provided in parallel on the left and right sides to be inserted into the ladle 20 and connected to the lower portion of the container 41, and a lance 43 inserted into the upper side of the container 41 to blow oxygen. Here, one of the pair of reflux pipes 42a and 42b is a riser for the molten steel inside the ladle 20 to rise, and the other reflux pipe 42b is a downcomer for the molten steel rising from the riser to descend toward the ladle 20.

[0091] The process of removing gas from molten steel M using the RH device 40 as described above is described. First, the ladle 20 is set on the lower side of a pair of reflux pipes 42a and 42b, and they are connected to each other for sealing. Then, the interior of the container 41 is depressurized to a vacuum pressure of, for example, 0.2 Torr or less, and then the ladle 20 is raised to immerse the pair of reflux pipes 42a and 42b into the molten steel inside the ladle 20. Thereafter, an inert gas such as argon (Ar) is blown to the pair of reflux pipes 42a and 42b to circulate the molten steel M inside the container 41. The circulating molten steel is exposed to the vacuum environment inside the container 41, and the nitrogen (N2) gas and hydrogen (H2) gas in the molten steel are discharged to the outside of the container 41. Therefore, the content of nitrogen (N) and hydrogen (H2) in the molten steel M is reduced. Here, the content of nitrogen (N2) is made 0.015 wt% or less, and the content of hydrogen (H2) is made 0.0005 wt% or less.

[0092] Hereinafter, the reason why the content of hydrogen (H 2 ) is controlled to be 0.0005 wt % or less in the degassing process ( S140 ) will be described.

[0093] When manufacturing the press roller, it is necessary to control the content of hydrogen (H2) contained in the press roller to 0.0002 weight % or less. This is because the hydrogen (H2) contained inside the press roller is easily accumulated in the inclusions and segregation contained in the press roller, and this may become a factor causing internal cracks in the press roller when the press roller is actually used. Here, the actual use of the press roller may refer to the situation where a rolling object such as a slab is rolled using the manufactured press roller. Therefore, the content of hydrogen (H2) contained in the press roller is controlled to 0.0002 weight % or less. Here, when the content of hydrogen (H2) contained in the press roller is 0.0002 weight % or less, cracks will not occur inside the press roller due to hydrogen (H2), or cracks will hardly exist. In addition, since it is actually difficult to completely remove hydrogen from the press roller, the hydrogen content is controlled to 0.0002 weight % or less.

[0094] As described above, in order to make the hydrogen (H2) content of the press roll 0.0002 wt% or less, it is desirable to reduce the hydrogen (H2) content in the molten steel to 0.0005 wt% or less in the degassing process (S140). That is, if the hydrogen (H2) content in the molten steel M is not controlled to 0.0005 wt% or less in the degassing process (S140), it is difficult to remove the hydrogen (H2) in the subsequent process of heating the press roll (S140). Figure 4 In (c)), the content of hydrogen (H2) is controlled to 0.0002 wt% or less. In other words, it is difficult to manufacture a press roll in which the content of hydrogen (H2) is controlled to 0.0002 wt% or less. Therefore, in the degassing process (S140), the content of hydrogen (H2) in the molten steel is controlled to 0.0005 wt% or less.

[0095] In addition, in the degassing process (S140), the content of nitrogen (N2) in the molten steel M is controlled to be 0.015 wt% or less. If the content of nitrogen (N2) in the molten steel M exceeds 0.015 wt%, precipitates due to nitrogen (N2) are generated when manufacturing cast steel, thereby causing a large number of cracks in the cast steel. Therefore, the content of nitrogen (N2) in the molten steel M is controlled to be 0.015 wt% or less through the degassing process (S130).

[0096] As described above, the content of hydrogen (H2) in the molten steel M can be controlled to be 0.0005 wt% or less and the content of nitrogen (N2) can be controlled to be 0.015 wt% or less by controlling the pressure and the treatment time of the container 41 of the RH device 40. That is, the pressure of the container 41 can be controlled to be 0.2 Torr or less and maintained at the pressure for 20 minutes or more to treat the molten steel, thereby producing molten steel having a hydrogen (H2) content of 0.0005 wt% or less and a nitrogen (N2) content of 0.015 wt% or less.

[0097] When the vacuum treatment device 40 is used for degassing, the reason for using the RH device 40 as the vacuum treatment device 40 is that the pressure control capability of other types of vacuum treatment devices is relatively low. That is, the RH device 40 used in the carbon steelmaking process can control the pressure inside the container 41 to 2 Torr or less, but the vacuum oxygen decarburization (VOD) device, that is, the vacuum treatment device used in the stainless steelmaking process, can only reduce the pressure to a maximum of 3 to 4 Torr. In addition, when removing hydrogen (H2) and nitrogen (N2) from molten steel, the lower the pressure, that is, the higher the vacuum degree, the more advantageous it is. Therefore, the RH device 40 for degassing in the carbon steelmaking process is used. Therefore, hydrogen (H2) and nitrogen (N2) can be easily removed from the molten steel M, and hydrogen (H2) and nitrogen (N2) can be removed more effectively than when other vacuum treatment devices such as VOD are used.

[0098] In the present embodiment, in the steelmaking converter treatment process (S110), the steelmaking converter 11 used in the stainless steelmaking process can be used. That is, in order to prepare molten steel for manufacturing a press roll with a high chromium (Cr) content, the steelmaking converter 11 used in the stainless steelmaking process is used. Therefore, molten steel for manufacturing a press roll with a high chromium (Cr) content can be prepared without contaminating a steelmaking converter for other types of steel. In addition, when the steelmaking converter for manufacturing stainless steel used to prepare molten steel for manufacturing press rolls is used to prepare molten steel for manufacturing stainless steel again, the chromium remaining in the steelmaking converter for manufacturing stainless steel does not serve as an impurity.

[0099] In addition, the ladle furnace 30 used in the carbon steelmaking process is used as a device for adjusting the composition of the molten steel M discharged from the steelmaking converter 11 and increasing the temperature of the molten steel. That is, the temperature of the molten steel M can be increased without inputting other materials into the molten steel. Therefore, the temperature of the molten steel can be increased without changing the composition of the molten steel.

[0100] In addition, when the vacuum treatment device 40 is used for degassing, the RH device 40 used in the carbon steelmaking process is used. That is, the RH device 40 capable of controlling the pressure of the container to 0.2 Torr or less is used for degassing. Therefore, nitrogen (N2) and hydrogen (H2) can be effectively removed from the molten steel.

[0101] In addition, when preparing the molten steel M for manufacturing the press roll, a new steelmaking converter, a heating device, and a vacuum treatment device are not separately prepared, but the stainless steelmaking converter 11, the carbon steelmaking ladle furnace 30, and the RH device 40 are used. Therefore, there is no need to prepare a separate device for preparing the molten steel for manufacturing the press roll, and therefore, there is an effect of reducing costs.

[0102] Figure 3 (a) to (d) are views sequentially illustrating the operation of the casting apparatus according to the embodiment of the present invention.

[0103] First, refer to Figure 3 (a) to (d) describe the casting device 100.

[0104] Reference Figure 3 In (a) to (d), the casting device 100 is a vertical casting device 100 that vertically draws out the slab S relative to the ground when the slab S is drawn out from the crystallizer 120. The casting apparatus according to the embodiment may be an apparatus capable of manufacturing a thick slab (i.e., a thick plate) having a thickness of about 700 mm, for example.

[0105] Reference Figure 3 (a) to (d), the casting device 100 includes a crystallizer 120 capable of solidifying molten steel M supplied thereto, a supporting portion 171 capable of being inserted into the crystallizer 120 to move up and down, a driving portion 172 connected to the supporting portion 171 to provide a moving force, a first magnetic field generating portion 150a arranged on the lateral outside of the crystallizer 120, a second magnetic field generating portion 150b arranged below the crystallizer 120 and on the lateral outside of the crystallizer 120, and a heating portion 160 arranged on the lateral outside of the crystallizer 120 and between the crystallizer 120 and the second magnetic field generating portion 150b.

[0106] In addition, the casting device 100 includes: a cooling part 140, which extends from the lower side of the crystallizer 120 in a direction perpendicular to the ground and injects cooling water into the slab S drawn to the lower side of the crystallizer 120 to solidify the slab S; a rotating part 190, which is arranged at the lower side of the second magnetic field generating part 150b to receive the slab S supported on the supporting part 171 and rotate it; and a moving part 180, which is arranged at the lower side of the second magnetic field generating part 150b and opposite to the rotating part 190 to push the slab S supported on the supporting part 171 toward the rotating part 190 and move it toward the rotating part 190.

[0107] In addition, the casting apparatus 100 may include a tundish 110 disposed at an upper side of the crystallizer 120 to supply the molten steel M to the crystallizer 120 , and a nozzle 130 connected to a lower portion of the tundish 110 to supply the molten steel M to the crystallizer 120 .

[0108] For ease of explanation, Figure 3 In the embodiment, the rotating part 190 is only Figure 3 As shown in (d) of Figure 3 However, even in the case of Figure 3 In the cases of (a) to (c), the rotating portion 190 is also disposed to face the moving portion 180 at the lower side of the cooling portion.

[0109] The crystallizer 120 is a device for receiving liquid molten steel M from the tundish 110 to preliminarily solidify the molten steel M into a certain shape. The crystallizer 120 may have a cooling water pipeline (not shown) disposed inside, through which cooling water circulates. When the molten steel is supplied to the crystallizer 120 and preliminarily solidifies, the molten steel M becomes a reaction solid state in which a solidified region A and an unsolidified region B coexist.

[0110] Before the molten steel M is supplied to the crystallizer 120, the support portion 171 is inserted into the crystallizer 120 to close the lower opening of the crystallizer 120. In addition, when the molten steel is supplied to the crystallizer 120 with the support portion 171 closing the lower opening of the crystallizer 120, the molten steel M starts to solidify on the support portion 171. Therefore, the slab S in a semi-solidified state is supported on the support portion 171. In addition, when the driving portion 172 is operated to lower the support portion 171, the support portion 171 is lowered toward the lower side of the crystallizer 120 while supporting the slab S in a semi-solidified state. That is, by lowering the support portion 171, the slab S inside the crystallizer 120 is drawn out to the lower side of the crystallizer 120. The support portion may have a plate-like shape.

[0111] The driving part 172 may be a device for lifting the supporting part 171, and may be connected to the lower part of the supporting part 171. The driving part 172 may include a power source 172-1 that provides a driving force for lifting, and a driving part 172-2 that connects the power source 172-1 to the supporting part 171 so that lifting can be achieved by the driving force transmitted from the power source 172-1.

[0112] The power source 172-1 may be a device including a hydraulic piston. Alternatively, the power source 172-1 is not limited to the above example, and any device capable of raising and lowering the driving portion 172-2 may be used.

[0113] One end of the driving part 172-2 is connected to the power source 172-1, and the other end is connected to the support part 171. The driving part 172-2 may have a shape extending in the vertical direction, for example. In addition, the driving part 172-2 may be arranged so that the height of one end connected to the power source 172-1 is fixed, and the height of the other end connected to the support part 171 can be raised or lowered by the operation of the power source 172-1.

[0114] As described above, the driving part 172 is a device for lifting and lowering the supporting part 171, and can control the speed at which the supporting part 171 is lifted and lowered. In particular, the speed at which the supporting part 171 descends can be controlled to control the speed at which the slab S supported on the supporting part 171 is withdrawn downward from the crystallizer 120. Here, the speed at which the slab S is withdrawn to the lower side of the crystallizer 120 may refer to the casting speed. The driving part 172 controls the descending speed of the supporting part 171 by controlling its operation to descend at a speed of 0.04 m / min or less. More specifically, the movement is controlled to descend at a speed of 0.01 m / min to 0.04 m / min. In other words, the driving part 172 controls the descending speed of the supporting part so that the casting speed is 0.04 m / min or less, more specifically 0.01 m / min to 0.04 m / min.

[0115] As described above, controlling the casting speed to 0.04 m / min or less is intended to suppress or prevent segregation from occurring in the slab S and suppress or prevent productivity from being deteriorated. That is, if the casting speed exceeds 0.04 m / min, the unsolidified molten steel M inside the slab S may not be fully solidified, resulting in segregation. In addition, when the casting speed is less than 0.01 m / min, there is a problem of reduced productivity of the slab S. Therefore, it is desirable to control the casting speed to 0.01 m / min to 0.04 m / min.

[0116] The cooling part 140 includes: a plurality of rollers 141, which are arranged on the lower side of the crystallizer 120 along the direction in which the support part is lifted and lowered; and a nozzle (not shown) which is disposed between the plurality of rollers 141 to inject cooling water into the slab drawn toward the lower side of the crystallizer.

[0117] The plurality of rollers 141 may be arranged in a vertical direction relative to the ground. In addition, each of the plurality of rollers 141 is arranged so that the slab S is rotated by the force of the slab S descending downward by the support portion 171. Therefore, the slab S drawn downward from the crystallizer 120 descends in a direction perpendicular to the ground by the descent of the support portion 171 and the plurality of rollers 141.

[0118] Nozzles for injecting cooling water are provided between the plurality of rollers 141. Therefore, the slab S drawn to the lower side of the crystallizer 120 descends through the support portion 171, and is secondarily cooled by the cooling water injected from the nozzles.

[0119] In the process of secondary solidifying the slab S by injecting cooling water onto the slab S drawn to the lower side of the crystallizer 120, the injection flow rate of the cooling water is controlled so that the surface temperature of the slab S becomes 800° C. to 900° C. That is, the injection flow rate of the cooling water is controlled so that the surface of the entire slab S located on the lower side of the crystallizer 120 becomes 800° C. to 900° C. More specifically, the injection flow rate of the cooling water is controlled so that the surface temperature from the upper part to the lower part of the slab S is uniformly at 800° C. to 900° C. For this reason, among the plurality of nozzles arranged in the vertical direction, the injection amount of the cooling water may be controlled so that the nozzles disposed on the lower side are reduced.

[0120] The purpose of controlling the surface temperature of the slab S to 800° C. to 900° C. is to suppress or prevent the occurrence of surface cracks in the slab S and to prevent the occurrence of bulges. That is, when the temperature of the surface of the slab S is lower than 800° C., cracks may occur in the surface of the slab S due to overcooling. On the contrary, if the temperature of the casting surface exceeds 900° C., the strength of the solidified shell, that is, the surface of the slab S may be low, resulting in bulges caused by the expansion of the solidified shell. Therefore, the surface temperature of the slab S extracted from the crystallizer 120 is controlled to 800° C. to 900° C.

[0121] If the temperature of the molten steel M supplied to the crystallizer 120 is low, the molten steel M solidifies and stagnates, thereby becoming a factor causing segregation in the slab S. Therefore, in order to suppress the decrease in the temperature of the molten steel, mold powder as an insulating agent is applied to the surface of the molten pool of the molten steel supplied to the crystallizer 120. However, the mold powder alone is not sufficient to suppress the decrease in the temperature of the molten steel.

[0122] Therefore, in order to allow the molten steel M inside the crystallizer 120 to flow so as to suppress the temperature drop of the molten steel M, the first magnetic field generating part 150a is installed on the outside of the crystallizer 120. The first magnetic field generating part 150a is arranged on the lateral outside of the crystallizer 120 to generate a magnetic field. Here, the lateral outside of the crystallizer 120 may refer to the outside of the outer surface, which is the surface opposite to the inner surface of the crystallizer 120 that contacts the molten steel M. That is, the first magnetic field generating part 150a is installed on the outside of the crystallizer 120 in a manner facing the outer surface of the crystallizer 120. In other words, the first magnetic field generating part 150a may be installed to surround the crystallizer 120 from the lateral outside of the crystallizer 120. To this end, the first magnetic field generating part 150a may be provided in a hollow shape extending along the outer surface of the crystallizer 120. The first magnetic field generating part 150a may include a coil installed on and inside the body and generating a magnetic field by applying electric power.

[0123] When the magnetic field is generated in the first magnetic field generating part 150a, the generated magnetic field allows the molten steel M inside the crystallizer 120 to flow. Therefore, the molten steel M inside the crystallizer 120 flows by the magnetic field, and the temperature drop can be suppressed or prevented due to the flow of the molten steel M. When the molten steel M flows, the components contained in the molten steel M are uniformly or uniformly mixed. Therefore, the occurrence of segregation formed by a specific component accumulating or agglomerating on a certain area within the slab can be suppressed or prevented.

[0124] The magnetic field generated from the first magnetic field generating part 150a can vary according to the intensity of the current, which depends on the power or voltage applied to the first magnetic field generating part 150a. Therefore, the power or voltage supplied to the first magnetic field generating part 150a is controlled so that a current with a target intensity flows through the coil of the first magnetic field generating part 150a. As described above, the current can flow through the first magnetic field generating part 150a, and a magnetic field can be generated in the first magnetic field generating part 150a. In addition, the molten steel M inside the crystallizer 120 can flow through the magnetic field generated from the first magnetic field generating part 150a to suppress or prevent the temperature of the molten steel M from decreasing, and the crystallizer protection slag on the molten pool surface of the molten steel M can be suppressed or prevented from mixing into the molten steel.

[0125] If the current flowing in the first magnetic field generating part 150a is too small, the molten steel M inside the crystallizer 120 may not flow, and therefore, the temperature of the molten steel M may decrease and cause segregation in the slab S. On the contrary, if the current flowing in the first magnetic field generating part 150a is too large, the flow rate of the molten steel inside the crystallizer 120 may be very fast, causing the crystallizer protection slag to be mixed into the molten steel. The crystallizer protection slag mixed into the molten steel M may be an impurity that causes cracks in the cast steel. Therefore, by controlling the intensity of the power or voltage supplied to the first magnetic field generating part 150a, the intensity of the current flowing to the first magnetic field generating part 150a is controlled so that a magnetic field that allows the molten steel inside the crystallizer 120 to flow at an appropriate flow rate is generated.

[0126] When the molten steel M solidifies inside the crystallizer 120, not all of the molten steel M supplied into the inside of the crystallizer 120 solidifies at the same time. That is, the molten steel M solidifies sequentially from the edges in the width direction and the length direction of the crystallizer 120 toward the center area. Therefore, the edges in the width direction and the length direction of the inside of the crystallizer 120 solidify and the molten steel becomes a solidified shell or a solid state, but the center area in the width direction and the length direction of the crystallizer 120 is in an unsolidified state in which the molten steel M exists in a liquid state. In other words, the cast steel manufactured by partially solidifying the molten steel inside the crystallizer 120 is in a semi-solidified state in which a solidified region A and an unsolidified region B exist.

[0127] In addition, when the slab S in a semi-solidified state is drawn out to the lower side of the crystallizer 120, the slab S is secondarily cooled by the cooling water injected from the nozzle of the cooling part 140. Here, since the cooling part 140 is disposed in the lateral direction of the slab S, the molten steel solidifies sequentially from the edges in the width direction and the length direction of the slab S toward the central region. Therefore, the slab S drawn out to the lower portion of the crystallizer 120 may be in a semi-solidified state in which a solidified region A and an unsolidified region B exist. In addition, when the molten steel M on the unsolidified region B solidifies over time, a fully solidified cast steel without the unsolidified region B is manufactured.

[0128] In addition, in the slab S drawn out to the lower portion of the crystallizer 120, the cumulative solidification time of the upper portion near the crystallizer 120 is shorter, and the cumulative solidification time of the lower portion is longer. Therefore, in the slab S drawn out to the lower portion of the crystallizer 120, the surface area of ​​the unsolidified region B becomes wider as it approaches the upper portion near the crystallizer 120, and the surface area of ​​the unsolidified region B becomes smaller as it approaches the lower portion. In other words, in the slab S drawn out to the lower portion of the crystallizer 120, the surface area of ​​the solidified region A becomes smaller as it approaches the upper portion near the crystallizer 120, and the surface area of ​​the solidified region A becomes larger as it approaches the lower portion.

[0129] As described above, in the upper portion of the slab S drawn downward from the crystallizer 120, there is an unsolidified region B in which the molten steel M is not solidified. That is, the slab S contains the unsolidified molten steel M. In addition, when the slab S is drawn to the lower side of the crystallizer 120, the slab S is solidified by the cooling water injected from the cooling part 140. However, if the unsolidified molten steel M present in the slab S solidifies without stagnation, segregation may occur inside the slab S.

[0130] Therefore, the second magnetic field generating part 150b is installed at the lower side of the crystallizer 120 to allow the unsolidified molten steel M inside the slab S drawn to the lower side of the crystallizer 120 to flow. The second magnetic field generating part 150b is arranged at the lower side of the crystallizer 120 and is arranged at the lateral outer side of the crystallizer 120 to generate a magnetic field. More specifically, the second magnetic field generating part 150b can be arranged at the lower side of the crystallizer 120 to face the first magnetic field generating part 150a in the vertical direction. More specifically, the second magnetic field generating part 150b can be arranged at the lower side of the crystallizer 120 at the lateral outer side of the cooling part 140. To this end, the second magnetic field generating part 150b can be arranged in a hollow shape. In addition, the second magnetic field generating part 150b can be arranged to have the same configuration as the first magnetic field generating part 150a. That is, the second magnetic field generating part 150b can include a coil installed on and inside the body and generating a magnetic field by applying electric power.

[0131] When the magnetic field is generated in the second magnetic field generating part 150b, the generated magnetic field is applied to the slab. Therefore, the liquid steel inside the slab S flows due to the magnetic field. Therefore, the molten steel M inside the slab S can solidify while flowing to suppress or prevent segregation in the slab S.

[0132] The magnetic field generated from the second magnetic field generating part 150b can vary according to the strength of the root current, which depends on the power or voltage applied to the second magnetic field generating part 150b. Therefore, the power or voltage supplied to the second magnetic field generating part 150b is controlled so that a current with a target strength flows through the coil of the second magnetic field generating part 150b. As described above, a current can flow through the second magnetic field generating part 150b, and a magnetic field can be generated in the second magnetic field generating part 150b. In addition, the molten steel inside the slab S can be made to flow by the magnetic field generated from the second magnetic field generating part 150b, and the crystallizer protection slag can be suppressed or prevented from mixing into the molten steel M.

[0133] If the current flowing to the second magnetic field generating part 150b is too small, the molten steel M inside the slab S may not flow or may not flow sufficiently to cause segregation inside the slab S. On the contrary, if the current flowing to the first magnetic field generating part 150a is too large, the flow rate of the molten steel M may be very fast to cause the mold powder to be mixed into the molten steel. The mold powder mixed into the molten steel M may cause cracks in the slab S as impurities. Therefore, when the second magnetic field generating part 150b is used to generate a magnetic field, the current flowing in the second magnetic field generating part 150b is controlled by controlling the intensity of the power or voltage supplied to the second magnetic field generating part 150b so as to generate a magnetic field that enables the molten steel M inside the slab S to flow at an appropriate speed.

[0134] In addition, if the time taken to generate the magnetic field by operating the second magnetic field generating part 150b is too short, the slab S may not be fully solidified. That is, the casting may be completed in a state where unsolidified molten steel remains inside the slab S. On the contrary, if the time taken to generate the magnetic field by operating the second magnetic field generating part 150b is too long, the magnetic field may be applied to an area on which there is no unsolidified molten steel, that is, up to the lower part of the slab. That is, the magnetic field may be applied to an area where the magnetic field does not need to be applied. Therefore, when the second magnetic field generating part 150b is used to generate the magnetic field, the time taken to apply the magnetic field to the slab is controlled so that the unsolidified molten steel present inside the slab S is fully solidified, and the magnetic field is not unnecessarily applied to the solidification area.

[0135] As described above, the unsolidified region of the slab drawn out to the lower side of the crystallizer 120 has a larger surface area as it approaches the upper portion. Here, the lowermost end portion of the slab S is the region drawn out first from the crystallizer 120, and the uppermost end portion of the slab S is the region drawn out last from the crystallizer 120. Therefore, in the slab drawn out to the lower side of the crystallizer 120, the uppermost end portion of the slab S refers to the distal end portion of the slab S.

[0136] When the slab S drawn out to the lower side of the crystallizer 120 solidifies, the unsolidified molten steel at the distal end of the slab solidifies first compared to the molten steel M at the lower side of the distal end of the slab. As described above, the molten steel M at the distal end of the slab solidifies first compared to the molten steel M at the lower end of the slab, resulting in a pipe defect caused by solidification and shrinkage at the distal end of the slab. In addition, since the distal end of the casting where the pipe defect occurs may not be used as a product, the distal end of the casting where the pipe defect occurs is cut off after the casting is completed. Therefore, there is a problem of reducing the casting rate due to the length of the cut distal end, and since the cut distal end is discarded, there is a cost problem because a material equivalent to the cutting length is consumed in each operation.

[0137] Therefore, in order to suppress or prevent the solidification and shrinkage of the distal end of the slab, a heating portion 160 capable of heating the distal end of the slab is installed on the lower side of the crystallizer 120. The heating portion 160 is arranged on the lateral outer side of the crystallizer 120 and is arranged between the crystallizer 120 and the second magnetic field generating portion 150b. More specifically, the heating portion 160 can be arranged to face the first magnetic field generating portion 150a and the second magnetic field generating portion 150b in the vertical direction on the lower side of the crystallizer 120. In addition, the heating portion 160 can be arranged on the lateral outer side of the cooling portion 140. To this end, the heating portion 160 can be set to a hollow shape. The heating component 160 can be installed on and inside the body, and can include a heating element that generates heat by applying electric power.

[0138] When the heating part 160 is operated and heat is generated, the heat heats or induction heats the molten steel M inside the distal end of the slab. This can delay the solidification of the molten steel at the distal end of the slab. That is, the molten steel at the distal end of the slab can be controlled to solidify later than the molten steel at the lower side of the slab. Therefore, the occurrence of a pipe defect at the distal end of the slab can be suppressed or prevented.

[0139] The heat generated in the heating portion 160 may vary according to the power or voltage applied to the heating portion 160. Therefore, the power applied to the heating portion 160 is controlled so that heat of a target temperature is generated in the heating portion 160. As described above, power may be applied to the heating portion 160 to generate heat, and the distal end portion of the slab may be heated. Therefore, solidification and shrinkage of the distal end portion of the slab may be suppressed or prevented.

[0140] If the power supplied to the heating part 160 is too small, the temperature of the heat generated from the heating part 160 is low, so that the molten steel M at the distal end of the slab cannot be sufficiently heated, and therefore, solidification and shrinkage may occur at the distal end of the slab, resulting in pipe defects. On the contrary, if the power supplied to the heating part 160 is too large, the temperature of the heat generated from the heating part 160 is too high, so that in addition to heating the distal end of the slab, there is also a problem of remelting the lower part of the distal end due to heating. Therefore, when the distal end of the slab is heated using the heating part 160, the power supplied to the heating part 160 is controlled so that the distal end of the slab is sufficiently heated, and therefore, the power is controlled to prevent the lower part of the distal end of the slab from being heated and remelted.

[0141] In addition, if the time spent using the heating part 160 to heat the distal end of the slab is too short, the distal end of the slab may not be sufficiently heated. Therefore, compared with the molten steel M at the lower part of the slab, the molten steel M at the distal end of the slab may solidify first to cause solidification and shrinkage at the distal end of the slab, thereby causing pipeline defects. On the contrary, if the time spent using the heating part 160 to heat the distal end of the slab is too long, it is not necessary to use the heating part to heat the distal end of the slab because the molten steel at the distal end of the slab has been completely solidified. Therefore, when the distal end of the slab is heated using the heating part 160, the heating time of the distal end of the slab is controlled so that the distal end of the slab is fully heated, while the already completely solidified distal end of the slab is not heated.

[0142] In the present embodiment, when solidifying the molten steel inside the crystallizer 120, the first magnetic field generating part 150a is used to flow the molten steel inside the crystallizer 120. Therefore, the temperature of the molten steel M inside the crystallizer 120 can be suppressed or prevented from decreasing, and thus, the occurrence of segregation in the slab S can be suppressed or prevented.

[0143] In addition, when the slab drawn to the lower side of the crystallizer 120 solidifies, the unsolidified molten steel inside the slab S is made to flow using the second magnetic field generating part 150b provided at the lower side of the crystallizer 120. Therefore, when the cast steel solidifies outside the crystallizer 120, the unsolidified molten steel inside the slab S can solidify without flowing stagnantly. Therefore, segregation in the slab S caused by stagnation of the unsolidified molten steel can be suppressed or prevented.

[0144] In order to prevent segregation in the ingot used to manufacture the pressure roller, an electroslag remelting (ESR) method is used. In the following, the electroslag remelting (ESR) method is briefly described. First, the molten steel whose upper part is controlled to be used to manufacture the pressure roller is solidified, so that it is manufactured into an electrode rod. Then, an arc is generated using the manufactured electrode rod to re-melt the electrode rod, and the molten steel is solidified by dripping the re-melted molten steel in the shape of droplets to re-manufacture the ingot. Although this process suppresses or prevents segregation in the ingot, it requires a complex process of manufacturing the ingot into the electrode rod, re-melting the electrode rod, and re-solidifying the re-melted molten steel as described above. Therefore, there are significant problems in terms of the time and cost consumed in the process of suppressing segregation.

[0145] On the other hand, in the present embodiment, the first magnetic field generating part and the second magnetic field generating part M provided in the casting device 100 are used to flow the molten steel in the crystallizer 120 and the unsolidified molten steel M in the slab S, thereby suppressing or preventing the occurrence of segregation inside the slab S. Therefore, compared with the conventional electroslag remelting (ESR) method for manufacturing an ingot, when the method according to the present embodiment is used to manufacture a slab, there is an effect of simplifying the process and reducing time and cost. In addition, compared with the power intensity applied to generate an arc in the electrode rod in the conventional electroslag remelting (ESR) method, the power intensity applied in the embodiment to operate the first magnetic field generating part and the second magnetic field generating part is smaller. Therefore, compared with the conventional electroslag remelting (ESR) method for manufacturing an ingot, when the method according to the embodiment is used to manufacture a slab, there is an effect of reducing the amount of electric energy consumed.

[0146] In addition, in an embodiment, the distal end of the slab can be heated using the heating portion 160, and thus, the solidification of the molten steel M at the end of the casting can be delayed. Therefore, solidification and shrinkage at the distal end of the slab can be suppressed or prevented, and thus, the occurrence of a pipe defect at the distal end of the slab can be suppressed.

[0147] Back to Figure 3 , the moving part 180 and the rotating part 190 will be described.

[0148] The moving part 180 is a device for pushing the slab S drawn out to the lower side of the crystallizer 120 from one side to move the slab S toward the rotating part 190. That is, the moving part 180 is drawn out downward from the crystallizer 120, vertically supported on the supporting part 171, and pushes the solidified slab S to transfer the slab S to the rotating part 190. The moving part 180 may be, for example, a device driven horizontally relative to the ground, and, for example, the moving part may include a hydraulic cylinder or a pneumatic cylinder.

[0149] The rotating part 190 receives the slab S separated from the supporting part 171 by the moving part 180 to rotate the received slab S. That is, the rotating part 190 receives the slab supported on the supporting part 171 in a direction perpendicular to the ground to rotate the received slab parallel to the ground.

[0150] The rotating part 190 is arranged to face the moving part 180 in a horizontal direction. The rotating part 190 may include a rotating table 191 and a rotating member 192, the rotating table 191 receiving and supporting the slab S separated from the supporting part 171 to rotate, and the rotating member 192 is connected to the rotating table 191 so that the rotating table 191 rotates.

[0151] The rotating table 191 includes a first table 191 - 1 extending in one direction, and a second table 191 - 2 extending in a direction intersecting the extending direction of the first table 191 - 1 and having one end connected to the first table 191 - 1 .

[0152] In the slab S vertically supported on the supporting portion 171, the first workbench 191-1 is a device for supporting the side surface of the slab S, and the second workbench 191-2 is a device for supporting the bottom surface of the slab S. The first workbench 191-1 may be provided to have a longer extension length than the second workbench 191-2. In addition, it is preferred that the second workbench 191-2 is provided to have a surface area equal to or greater than the surface area of ​​the bottom surface of the slab S. A plurality of rotatable rollers 191-3 may be mounted on the first workbench 191-1, and the plurality of rollers 191-3 are arranged along the direction in which the first workbench 191-1 extends.

[0153] One end of the first workbench 191-1 and one end of the second workbench 191-2 may be connected to each other through the rotating member 192. Figure 3 As shown in (d), the rotating table 191 can be rotated or tilted by the rotating member 192. That is, when the slab S is vertically supported on the support part 171, the first table 191-1 of the rotating part is arranged to be perpendicular to the ground, and the second table 191-2 is arranged to be parallel to the ground. In addition, when the slab S supported on the support part 171 is transferred to the rotating part 190, the rotating part 190 is rotated or tilted. That is, the first table 191-1 of the rotating part 190 is rotated to be parallel to the ground, and the second table 191-2 is rotated to be perpendicular to the ground.

[0154] In the above description, the manufacturing of the slab using the vertical casting device is described. However, the present invention is not limited thereto, and the slab S may be manufactured using a casting device having various forms, the casting device including the first magnetic field generating part 150a installed laterally outside the crystallizer 120, the second magnetic field generating part 150b installed at the lower side of the crystallizer 120, and the heating part 160.

[0155] Figure 4 (a) to (c) are process diagrams sequentially illustrating a method for manufacturing a press roller using a slab manufactured by the method according to an embodiment of the present invention.

[0156] When the slab S is manufactured, the manufactured slab S is heated and softened (S300). To this end, the slab S is loaded into the first heating device 200. The first heating device 200 may include a steelmaking converter having an internal space and a heater for heating the steelmaking converter. Here, the heater is, for example, installed on the inside or outside of the wall forming the steelmaking converter, and may include a heating element capable of generating heat by applying electric power. As another example, the heater may include a burner that burns fuel to generate heat.

[0157] When the slab S is heated in the first heating device 200, the slab is heated to a temperature of 1100° C. to 1250° C., preferably 1140° C. to 1240° C. Here, instead of charging the slab into a steelmaking converter heated to 1100° C. to 1250° C., the slab is charged into a steelmaking converter controlled to a lower temperature, and then the temperature inside the steelmaking converter is gradually raised to 1140° C. to 1240° C.

[0158] To explain more specifically, first, the inside of the steelmaking converter is heated to 250°C to 350°C (first temperature), and maintained at the first temperature for 3 hours to 5 hours, preferably 3 hours 30 minutes to 4 hours 30 minutes. Thereafter, the inside of the steelmaking converter is heated to 450°C to 550°C (second temperature), and maintained at the second temperature for 5 hours to 7 hours, preferably 5 hours 30 minutes to 6 hours 30 minutes. Next, the inside of the steelmaking converter is heated to 650°C to 750°C (third temperature), and maintained at the third temperature for 3 hours to 5 hours, preferably 3 hours 30 minutes to 4 hours 30 minutes. Next, the inside of the steelmaking converter is heated to 1100°C to 1250°C (fourth temperature), and maintained at the fourth temperature for 14 hours to 18 hours, preferably 15 hours 30 minutes to 17 hours 30 minutes. In addition, each of the time taken to heat the interior of the steelmaking converter from the first temperature to the second temperature (first heating time) and the time taken to heat the interior of the steelmaking converter from the second temperature to the third temperature is preferably 3 hours to 4 hours, and the time taken to heat the interior of the steelmaking converter from the third temperature to the fourth temperature is preferably 9 hours to 10 hours. As described above, heating the interior of the steelmaking converter to the first temperature to the fourth temperature and maintaining the interior of the steelmaking converter at the first temperature to the fourth temperature means heating the temperature of the slab charged into the interior of the steelmaking converter to the first temperature to the fourth temperature and maintaining the temperature of the slab at the first temperature to the fourth temperature.

[0159] The reason why the temperature of the slab S is gradually raised to 1100° C. to 1250° C. as described above is that the slab S containing a high chromium amount of 4.5 wt % or more has a high hardness and is therefore prone to cracking during heating and cooling. Therefore, when the temperature of the slab S is raised to 1100° C. to 1250° C. in the above manner, cracking in the slab S can be suppressed or prevented.

[0160] When the process of heating or heat treating the slab S in the first heating device 200 is completed, the slab S is forged using the forging device 300 to manufacture a press roller. The forging device 300 may include, for example, an upper pressing portion and a lower pressing portion spaced apart from each other in the vertical direction, and a driving portion connected to the upper pressing portion and the lower pressing portion to apply a predetermined force. Here, the driving portion may be a hydraulically driven or pneumatically driven cylinder.

[0161] Hereinafter, a method for forging a slab S using a forging device 300 is described. First, a slab is disposed between an upper pressing portion and a lower pressing portion of the forging device 300. In addition, a driving portion is operated to allow the upper pressing portion to descend and the lower pressing portion to ascend, thereby reducing the distance between the upper pressing portion and the lower pressing portion. Next, the driving portion is operated to apply a greater downward force to the upper pressing portion and a greater upward force to the lower pressing portion. Here, after the upper pressing portion and the lower pressing portion contact the slab S, the force of the upper pressing portion and the lower pressing portion pressing the slab increases over time. Therefore, the slab S disposed between the upper pressing portion and the lower pressing portion is pressed to reduce the thickness of the slab. Then, when the thickness of the slab is reduced to a target thickness, the upper pressing portion and the lower pressing portion are separated from the slab. Here, the target thickness may be 250 mm to 350 mm. In addition, the process is repeated multiple times.

[0162] As described above, force may be applied by bringing the upper and lower pressing parts into contact with the slab S, and when the thickness of the slab S is reduced to a target thickness, the upper and lower pressing parts are separated from the slab S. This series of processes is defined as "one time". Here, in the embodiment, the target thickness is set to 250 mm to 350 mm. In addition, the reduced thickness of the slab S is controlled to be 250 mm to 350 mm each time. In addition, the above one time is repeated multiple times to manufacture a press roller.

[0163] If the thickness of the slab S is reduced by less than 250 mm each time, force may not be sufficiently applied to the central region in the thickness direction of the slab S, and therefore, pores in the central region of the slab S may not be removed. In addition, if pores are not removed at the center in the thickness direction of the slab S, it may become a factor that reduces the hardness of the press roll. On the contrary, if the thickness of the slab S is reduced by more than 350 mm each time, cracks may occur in the surface of the slab to cause defects in the press roll.

[0164] When the forging is completed and the pressing roller 141 is manufactured, the pressing roller 141 is heated to remove hydrogen (H2). To this end, the pressing roller 141 is loaded into the second heating device 400, and the pressing roller 141 is heated using the second heating device 400. Here, the second heating device 400 may be the same as or different from the first heating device 200 described above.

[0165] When the pressure roller 141 is heated using the second heating device 400, the pressure roller 141 is heated to 200° C. to 400° C., more preferably, to 250° C. to 350° C. In addition, the temperature of the pressure roller 141 is maintained at a temperature of 200° C. to 400° C. for 48 hours or more, more preferably, for 48 hours or more and 55 hours or less.

[0166] As described above, the pressure roller 141 is heated to 200° C. to 400° C., more preferably, to 250° C. to 350° C., and maintained at this temperature for 48 hours or more, while the hydrogen (H2) contained in or remaining in the pressure roller 141 is diffused to become hydrogen gas and the hydrogen gas is discharged to the outside. Therefore, the hydrogen content in the pressure roller 141 is reduced. Here, it is desirable to make the hydrogen content in the pressure roller 141 at 0.0002 wt % or less (0 wt % or more), and this can be controlled by controlling at least one of the temperature at which the pressure roller 141 is heated or the heat treatment time.

[0167] Hydrogen contained inside the press roll 141 is easily accumulated in inclusions and segregation contained in the press roll 141, and when the press roll 141 is actually used, this may become a factor causing internal cracks in the press roll 141. Here, the actual use of the press roll may refer to a case where a rolling object such as a slab S is rolled using the manufactured press roll 141.

[0168] Therefore, the manufactured press roller 141 is heated at 200° C. to 400° C. for 48 hours or more to remove hydrogen so that the content of hydrogen contained in the press roller 141 becomes 0.0002 wt % or less. Here, when the content of hydrogen contained in the press roller 141 is 0.0002 wt % or less, cracks do not occur inside the press roller 141 due to hydrogen. In addition, since it is actually difficult to completely remove hydrogen from the press roller 141, the hydrogen content is set to 0.0002 wt % or less.

[0169] In the following, reference will be made to Figures 1 to 4 A method for manufacturing a press roller according to an embodiment of the present invention is collectively described. Here, any contents overlapping with the above contents will be omitted or briefly described.

[0170] First, molten steel M for manufacturing a press roll is prepared (S100). To this end, first, the molten steel M is charged into a stainless steelmaking converter 11. Figure 2 As shown in (a) of FIG. 1 , chromium (Cr) alloy steel is fed into the steelmaking converter 11. Here, the amount of chromium (Cr) alloy steel fed is controlled so that the content of chromium (Cr) in the molten steel in the steelmaking converter 11 is 4.5 wt % to 5.5 wt %.

[0171] Next, if Figure 2 As shown in (b) of FIG. 1 , oxygen is blown into the interior of the steelmaking converter 11 using the lance 12. That is, oxygen is blown into the molten steel M in the steelmaking converter 11 to remove carbon (C) and phosphorus (P). Here, the content of carbon (C) in the molten steel M is set to 0.75 wt % to 0.95 wt %, and the content of phosphorus (P) is set to 0.025 wt % or less.

[0172] Next, if Figure 2 As shown in (c) of FIG. 1 , a deoxidizer is input into the steelmaking converter 11 to remove oxygen (O) from the molten steel M. Here, at least one of a silicon (Si) alloy or an aluminum (Al) alloy can be used as the deoxidizer. In addition, deoxidation is performed so that the content of oxygen (O) in the molten steel M becomes 0.001 wt % or less.

[0173] When deoxygenation is complete, Figure 2 As shown in (d) of FIG. 1 , a reducing agent, that is, an alloy steel containing silicon (Si), is input into the steelmaking converter 11. Therefore, the chromium oxide contained in the slag SL and the silicon (Si) contained in the reducing agent react with each other. Therefore, the chromium oxide contained in the slag SL is reduced to chromium (Cr), and the generated chromium (Cr) is absorbed or supplied to the molten steel M. Therefore, the content of chromium (Cr) in the molten steel M increases. Here, the input amount of the reducing agent is controlled so that the content of chromium (Cr) in the molten steel M is 4.5 wt % to 5.5 wt %.

[0174] When chromium reduction is complete, Figure 2 As shown in (e) of FIG. 1 , the molten steel M in the steelmaking converter 11 is discharged into the ladle 20. Here, the molten steel M is discharged into the ladle 20 for producing carbon steel.

[0175] Then, the ladle 20 is moved to the temperature control device 30, that is, the ladle furnace 30, and the ladle 20 is connected to the cover 32 ( Figure 2 (f)). Next, a desulfurizing agent is input into the ladle 20 to remove sulfur (S) from the molten steel. Here, the content of sulfur (S) in the molten steel is set to 0.015 wt% or less.

[0176] When desulfurization is completed, the molten steel M contained in the ladle 20 is sampled to measure the contents of carbon (C) and chromium (Cr). In addition, the carbon (C) content and the chromium (Cr) content in the molten steel M are controlled according to the measured carbon (C) content and the chromium (Cr) content. Here, according to the measured carbon (C) content and the chromium (Cr) content, at least one of a carburizing agent or an alloy steel containing chromium (Cr) is input, or solid oxygen such as iron ore is input, so that the measured carbon (C) content in the molten steel becomes 0.75 wt% to 0.95 wt%, and the chromium (Cr) content becomes 4.45 wt% to 5.5 wt%.

[0177] After the control of the carbon (C) component and the chromium (Cr) component is completed, the molten steel M is heated to increase the temperature of the molten steel M. That is, arc and heat are generated from the electrode rod 31 to heat the molten steel contained in the ladle 20. Here, the temperature of the molten steel must be between 1560°C and 1600°C.

[0178] When the temperature of the molten steel M reaches 1560°C to 1600°C, the ladle 20 is separated from the ladle furnace 30. Figure 2As shown in (g), the ladle 20 is moved to the vacuum treatment device, i.e., the RH device 40. In addition, the ladle 20 is arranged below a pair of reflux pipes 42a and 42b, and the pair of reflux pipes 42a and 42b are connected to each other for sealing. Next, the pump is operated to reduce the pressure inside the container 41 to a vacuum pressure of, for example, 0.2 Torr or less, and then the ladle 20 is raised so that the pair of reflux pipes 42a and 42b are immersed in the molten steel inside the ladle 20. Then, argon (Ar) gas is blown into the pair of reflux pipes 42a and 42b so that the molten steel circulates inside the container 41. The circulating molten steel is exposed to the vacuum environment inside the container 41, and the nitrogen (N2) gas and hydrogen (H2) gas in the molten steel are discharged to the outside of the container 41. Therefore, the content of nitrogen (N) and hydrogen (H2) in the molten steel is reduced. Here, the content of nitrogen (N2) is made 0.015 wt% or less, and the content of hydrogen (H2) is made 0.0005 wt% or less.

[0179] When the above Figure 2 During the processes (a) to (g), molten steel M for manufacturing press rolls is prepared.

[0180] When the molten steel M is prepared, the ladle 20 is moved to the tundish 110 of the casting device 100, and the molten steel is supplied to the tundish 110 to start casting. To this end, first, the lower opening of the crystallizer 120 is closed using the support part 171 of the casting device 100. In addition, as Figure 3 As shown in (a) of FIG. 2 , the molten steel M in the tundish 110 is supplied to the crystallizer 120 (S210). Therefore, the molten steel M supplied to the crystallizer 120 is solidified, that is, solidification is started from the upper part of the support part 171 (casting process) (S210). In addition, while the molten steel M from the tundish 110 is continuously supplied to the crystallizer 120, the driving part 172 is operated to allow the support part 171 to descend. Therefore, as shown in FIG. Figure 3 As shown in (b), the slab S inside the crystallizer 120 is gradually pulled downward to the lower side of the crystallizer (S220).

[0181] As described above, while the molten steel M is continuously supplied to the crystallizer 120, the first magnetic field generating part 150a provided at the outer side of the crystallizer 120 is operated to generate a magnetic field. Therefore, the magnetic field generated from the first magnetic field generating part 150a is applied to the inside of the crystallizer 120, and thus, the molten steel M inside the crystallizer 120 flows due to the magnetic field. Thus, a temperature decrease in the molten steel M inside the crystallizer 120 can be suppressed or prevented, and the mold powder on the molten pool surface of the molten steel M can be suppressed or prevented from mixing into the molten steel.

[0182] While the molten steel is continuously supplied into the crystallizer 120, the driving part 172 is operated to allow the support part 171 to gradually descend toward the lower side of the crystallizer 120. Here, the speed at which the support part 171 descends downward is controlled to be 0.04 m / min or less, and more specifically, to be controlled to be 0.01 m / min to 0.04 m / min. That is, the casting speed is controlled to be 0.04 m / min or less, and more specifically, to be controlled to be 0.01 m / min to 0.04 m / min.

[0183] When Figure 3 When the slab is drawn out to the lower side of the crystallizer 120 as shown in (b), the drawn out slab S is secondarily solidified (solidification process) by cooling water injected from the nozzle of the cooling part 140 (S220). In addition, the tundish 110 continuously supplies molten steel M to the crystallizer, and the supporting part 171 continuously descends, and therefore, the length of the slab drawn out to the lower side of the crystallizer 120 gradually increases. When the slab of the target length is drawn out to the lower side of the crystallizer 120, the supply of molten steel to the crystallizer 120 is stopped. In addition, as Figure 3 As shown in (c) of FIG. 120 , the uppermost end portion, i.e., the distal end portion, of the slab is pulled downward from the crystallizer (S230). In addition, when the distal end portion of the slab is pulled out to the lower side of the crystallizer, the second magnetic field generating portion 150b is operated to generate a magnetic field. Therefore, the unsolidified molten steel present in the slab S pulled out to the lower side of the crystallizer 120 flows due to the magnetic field generated from the second magnetic field generating portion 150b. This can suppress or prevent the mold powder from being mixed into the molten steel.

[0184] In the above, it is explained that when the distal end of the slab S is drawn out to the lower side of the crystallizer, the second magnetic field generating part 150b is operated to generate a magnetic field. However, it is not limited thereto, and when the slab begins to be drawn out toward the lower side of the crystallizer 120, the second magnetic field generating part 150b may be operated to generate a magnetic field.

[0185] When the distal end of the slab S is drawn out toward the lower side of the crystallizer, the heating part 160 is operated to heat the distal end of the slab. Therefore, solidification of the unsolidified molten steel M at the distal end of the slab S is delayed. This can suppress or prevent solidification and shrinkage at the distal end of the slab, and can prevent the region of the slab other than the distal end from being re-melted.

[0186] When solidification is completed to the distal end of the slab S, Figure 3(d) as shown in the figure, the moving part 180 is operated to push the slab S vertically placed on the supporting part 171 toward the rotating part 190. Therefore, the slab S of the supporting part 171 is transferred to the rotating part 190. Here, the side of the slab S is supported on the first workbench 191-1 of the rotating part 190, and the bottom surface of the slab is supported on the second workbench 191-2. Next, the rotating part 190 rotates, i.e. tilts (S240). More specifically, the rotating part 190 rotates so that the first workbench 191-1 is parallel to the ground and the second workbench 191-2 is horizontal relative to the ground. Therefore, the slab S is rotated to be placed horizontally relative to the ground. The slab placed horizontally on the rotating part 190 is transferred to the next process.

[0187] When manufacturing the slab S, the slab is heated using the first heating device 200 (S300). Here, the temperature inside the steelmaking converter loaded with the slab S is gradually increased to 1140°C to 1240°C (target temperature) to heat the slab. To explain more specifically, first, the slab S is heated to 250°C to 350°C (first temperature) and maintained for 3 to 5 hours. Thereafter, the slab S is heated to 450°C to 550°C (second temperature) and maintained for 5 to 7 hours. Next, the slab S is heated to 650°C to 750°C (third temperature) and maintained at the third temperature for 3 to 5 hours. Next, the slab S is heated to a target temperature of 1100°C to 1250°C (fourth temperature) and maintained for 14 to 18 hours. As described above, gradually increasing the temperature from the first temperature to the fourth temperature while heating the slab S to 1100°C to 1250°C (fourth temperature) can suppress or prevent cracks from occurring in cast steel containing a large amount of chromium (Cr) of 4.5 wt % or more due to heat.

[0188] When the process of heating the slab S in the first heating device 200 is completed, the slab is forged using the forging device 300 to manufacture a press roller (S400). Here, the thickness of the slab S is reduced by 250 to 350 mm each time, and this is repeated multiple times to manufacture the press roller 141.

[0189] When manufacturing the pressing roller R, the pressing roller 141 is heated using the second heating device 400 to remove hydrogen (H2). Here, the pressing roller 141 is heated at a temperature of 200° C. to 400° C. for 48 hours or more. Therefore, the hydrogen content in the pressing roller 141 can be controlled to 0.0002 wt % or less (0 wt % or more).

[0190] Table 1 is an evaluation table about the press roller manufactured by the method according to the embodiment of the present invention. In order to evaluate the quality, a part of the press roller manufactured by the method according to the embodiment was cut and used as a test sample.

[0191] Ultrasonic testing (UT) quality evaluation is a quality evaluation method for detecting internal defects such as pores and cracks. That is, ultrasonic testing quality evaluation is an evaluation method that detects the presence and appearance of pores and cracks inside a sample by transmitting ultrasonic waves into the sample and using the energy of the ultrasonic waves reflected from the discontinuities inside the sample and the propagation time of the ultrasonic waves.

[0192] Macroscopic quality evaluation is intended to determine the presence and amount of rough structures. After the samples for evaluation were etched using corrosion, the internal structure of the samples was observed using an optical microscope.

[0193] The purpose of inclusion quality evaluation is to determine the number and size of inclusions present in the sample. After polishing a portion of the sample for evaluation, the size and number of inclusions are examined using an optical microscope.

[0194] Hardness was measured using a Brinell hardness tester as the hardness of the specimen.

[0195] [Table 1]

[0196]

[0197] Referring to Table 1, the samples manufactured by the method according to the embodiment have acceptable UT quality, macro quality, inclusion quality and hardness quality. That is, the press roller manufactured by the method according to the embodiment passes the evaluation results for UT quality, macro quality, inclusion quality and hardness quality. Therefore, it can be seen that by manufacturing a press roller using the method according to the embodiment, a press roller with a small number of pores, cracks and inclusions, a uniform structure and high hardness can be manufactured.

[0198] Figure 5 (a) and (b) are diagrams illustrating results obtained by confirming whether coarse segregation occurs or more coarse segregation occurs by cutting and etching the cross section of the press roll.

[0199] Here, Figure 5 (a) is a cross-section of a press roller manufactured by the method of an embodiment, and Figure 5 (a) is a cross section of a press roller manufactured by the method according to the comparative example. In addition, the press roller manufactured by the method according to the comparative example is a press roller manufactured by an electroslag remelting (ESR) method.

[0200] When Figure 5When (a) and (b) are compared, no coarse segregation occurs in either the press roll according to the embodiment or the press roll according to the comparative example. That is, the press roll according to the embodiment has a uniform structure equivalent to that of the press roll according to the comparative example. Therefore, it can be seen that even in the case where the press roll is manufactured using a method similar to that according to the embodiment without using the conventional electroslag remelting (ESR) method, the occurrence of segregation is sufficiently suppressed. That is, even in the case where the slab is cast by using the first magnetic field generating part and the second magnetic field generating part to allow molten steel to flow during the casting process as in the embodiment, it can be seen that the occurrence of segregation is suppressed to the same level as the conventional electroslag remelting (ESR) method.

[0201] In addition, compared with the conventional electroslag remelting (ESR) method, the process of suppressing segregation is simplified and takes a shorter time by using the first magnetic field generating part and the second magnetic field generating part during the casting process as in the embodiment. That is, in the case of the embodiment, while suppressing the occurrence of segregation to a level comparable to that of the prior art, there is also the advantage of a shorter process required for the occurrence of segregation and a shorter process time.

[0202] Industrial Applicability

[0203] According to an embodiment of the present invention, high chromium (Cr) molten steel can be manufactured by using a steelmaking converter used in a steelmaking process of other steel types without contaminating the steelmaking converter. In addition, when the temperature of high chromium (Cr) molten steel is increased or degassing is performed to exhaust gas, the present invention can effectively increase the temperature of the molten steel and improve the degassing efficiency.

Claims

1. A method for producing a high chromium (Cr) molten steel having a chromium content (Cr) of 4.5 wt % to 5.5 wt %, the method comprising: The molten steel is charged into the steelmaking converter used in the process of making stainless steel; as well as A chromium alloy steel containing chromium (Cr) is input into the steelmaking converter so that the content of chromium (Cr) in the molten steel reaches 4.5 wt % to 5.5 wt %.

2. The method according to claim 1, further comprising: Blowing oxygen by blowing oxygen into the steelmaking converter charged with the chromium alloy steel to remove carbon (C) from the molten steel; removing oxygen (O) from the molten steel by introducing a deoxidizer into the steelmaking converter; as well as Chromium oxides contained in slag floating on the surface of a molten pool of the molten steel are reduced to chromium (Cr) by inputting a reducing agent into the steelmaking converter.

3. The method according to claim 1, further comprising: Discharging the molten steel from the steelmaking converter into a ladle; as well as The temperature for heating the molten steel received from the ladle is controlled by using a ladle furnace (LF), which is a heating device used in a carbon steelmaking process.

4. The method according to claim 3, wherein: When temperature control is performed, the temperature of the molten steel is controlled to be 1560°C to 1600°C.

5. The method according to claim 3, further comprising: Degassing is performed using a RheinstaalHuttenwerke und Heraus (RH) device, which is a vacuum treatment device used in the carbon steelmaking process, to remove hydrogen (H2) and nitrogen (N2) from the molten steel received in the ladle, Wherein, the degassing is performed after the temperature control is completed.

6. The method according to any one of claims 3 to 5, wherein: When the molten steel is discharged from the steelmaking converter into the ladle, the molten steel inside the steelmaking converter is discharged into the ladle in the carbon steelmaking process.

7. A method for manufacturing a slab, the method comprising: preparing molten steel containing 4.5 wt % to 5.5 wt % of chromium (Cr); The following casting process is performed: the molten steel is supplied to a crystallizer of a casting device so that the molten steel is solidified inside the crystallizer, thereby manufacturing a slab including unsolidified molten steel; extracting the slab produced in the casting process to the lower side of the crystallizer; as well as The following solidification process is performed: a magnetic field is applied to the slab drawn to the lower side of the crystallizer to solidify the unsolidified molten steel contained in the slab while allowing the unsolidified molten steel contained in the slab to flow, thereby manufacturing the slab.

8. The method according to claim 7, wherein: The solidification process includes heating the distal end of the slab that is drawn to the lower side of the crystallizer.

9. The method according to claim 7, wherein: The casting process includes applying a magnetic field to the crystallizer to allow the molten steel inside the crystallizer to flow.

10. The method according to claim 7, wherein: When the slab is withdrawn to the lower side of the crystallizer, the withdrawal is performed at a speed of 0.04 m / min or less.

11. The method according to claim 7, wherein: The solidification process includes injecting cooling water onto the slab being pumped to the lower side of the crystallizer, and When the cooling water is injected onto the slab, the injection is performed so that the surface temperature of the slab becomes 800°C to 900°C.

12. The method according to any one of claims 7 to 11, wherein: During the extraction process, the slab is extracted from the crystallizer in a direction perpendicular to the ground, and In the solidification process, the slab is solidified in a state where the slab is arranged in a direction perpendicular to the ground.

13. A method for manufacturing a press roller, the method comprising: heating the slab manufactured by the method for manufacturing a slab according to any one of claims 7 to 11; Forging the heated slab to mold the slab into a shape of a press roll; as well as heating the pressing roller manufactured in the molding to remove hydrogen (H2) from the pressing roller, The heating of the slab includes heating the slab by increasing the temperature of the slab to a target temperature in a plurality of stages.

14. The method according to claim 13, wherein: Heating the slab to the target temperature comprises: heating the slab to a first temperature of 250° C. to 350° C.; heating the slab to a second temperature of 450°C to 550°C; heating the slab to a third temperature of 650°C to 750°C; and The slab is heated to the target temperature of 1100°C to 1250°C.

15. The method according to claim 14, wherein: When the slab is heated to the first temperature to the third temperature and the target temperature, Maintaining the slab at the first temperature for 3 to 5 hours, maintaining the slab at the second temperature for 5 to 7 hours, The slab is maintained at the third temperature for 3 to 5 hours, and The slab is maintained at the target temperature for 14 to 18 hours.

16. The method according to claim 15, wherein: When forging the slab, the slab heated to a temperature of 1100° C. to 1250° C. is pressed downward for forging, and The slab is pressed downwardly so that each time the slab is pressed downwardly by the pressing device, the thickness of the slab is reduced by 250 mm to 350 mm.

17. The method according to claim 13, further comprising: While removing hydrogen (H2) from the press roll, the press roll is heated to a temperature of 200°C to 400°C.

18. The method according to claim 13, wherein: When the press roller is heated to a temperature of 200° C. to 400° C., the press roller is heated for 48 hours or more.

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