MOLD MANUFACTURING METHOD AND MOLD

Abstract

 A method for manufacturing a mold in accordance with an exemplary embodiment may include: solidifying molten steel in a first mold; detecting a solidification shrinkage amount (SD) occurring when the molten steel is solidified in the first mold for each height of the first mold; preparing a width for each height of a second mold to be manufactured by using the detected solidification shrinkage amount (SD) for each height; and preparing the second mold so that a width (W) for each height of the second mold is equal to the designed width for each height.
Thus, in accordance with exemplary embodiments, a compensation rate for solidified shell shrinkage may be improved. That is, the compensation rate for solidification shrinkage for each height may be improved by designing a width for each height of a mold according to different amounts of solidification shrinkage for each height to manufacture the mold. Thus, a defect caused by the solidified shell shrinkage may be suppressed or prevented from occurring on a surface of a cast piece.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method for manufacturing a mold and a mold, and more particularly, to a method for manufacturing a mold and a mold, which are capable of suppressing or preventing a defect from occurring.

BACKGROUND ART

[0002] A mold for casting a cast piece includes a pair of long side walls and a pair of short side walls. Also, molten steel is injected into an inner space partitioned by the pair of long side walls and the pair of short side walls and solidified in the mold to manufacture the cast piece.

[0003] When the molten steel is supplied into the mold, a solidified shell starts to be formed from a surface of the molten steel in the mold, and a thickness of the solidified shell gradually increases in a downward direction. Also, when the solidified shell is formed as the molten steel is solidified in the mold, solidification shrinkage occurs in the solidified shell. In particular, when the molten steel in a liquid phase is converted into a solid phase at an upper portion of the mold, large shrinkage of the solidified shell occurs. Also, an amount of the solidification shrinkage of the solidified shell is varied depending on a height of the mold. When the shrinkage of the solidified shell is not compensated by the mold, an air layer or a gap is generated between the mold and the solidified shell. When the gap is generated, a heat transfer performance between the mold and the solidified shell or the molten steel is reduced to generate break out and a damage in the cast piece.

[0004] In order to solve this limitation caused by the solidification shrinkage, a width of an inner wall surface of the mold gradually decreases in the downward direction. Here, an amount or a decrease rate in a width of the inner wall surface of the mold in the downward direction is constant. However, even in this case, since the shrinkage of the solidified shell may not be sufficiently compensated, the gap is still generated between the mold and the solidified shell. This is because, although the solidification shrinkage gradually decreases in a direction from an upper portion to a lower portion of the mold, the solidification shrinkage does not decrease at a constant rate. That is, the width of the inner wall gradually decreases at a constant rate in the downward direction, which may not sufficiently compensate for amounts of solidification shrinkage different for each height.

[PRIOR ART DOCUMENTS]

[0005] (Patent document 1) Korean Patent Registration No. 10-1060114.

DISCLOSURE OF THE INVENTION

TECHNICAL PROBLEM

[0006] The present disclosure provides a method for manufacturing a mold and a mold, which are capable of effectively compensating for solidification shrinkage of a solidified shell for each height.

[0007] The present disclosure also provides a method for manufacturing a mold and a mold, which are capable of improving a compensation rate for shrinkage of a solidified shell.

TECHNICAL SOLUTION

[0008] Exemplary embodiments provide a method for manufacturing a mold, including: solidifying molten steel in a first mold; detecting a solidification shrinkage amount SD occurring when the molten steel is solidified in the first mold for each height of the first mold; designing a width for each height of a second mold to be manufactured by using the detected solidification shrinkage amount SD for each height; and preparing the second mold so that a width W for each height of the second mold is equal to the designed width for each height.

[0009] The detecting of the solidification shrinkage amount SD for each height of the first mold may include: setting a plurality of design points DP having different heights on an inner wall surface of the first mold below a surface height P_M of the molten steel; detecting a solidification width SW by detecting a length between both ends in a width direction of a solidified shell formed by solidification of the molten steel in each of the plurality of design points DP set on the first mold; and calculating the solidification shrinkage amount SD in each of the plurality of designed points DP by subtracting the detected solidification width SW in each of the plurality of design points DP from a width W_M at the surface height P_M on the inner wall surface of the first mold.

[0010] The setting of the plurality of designed points DP on the first mold may be set so that distances between the plurality of design points DP gradually increase in a downward direction.

[0011] The setting of the plurality of designed points DP on the first mold may be set so that the distances between the plurality of design points DP gradually increase with a constant value.

[0012] The plurality of design points DP set on the first mold may include first to fourth design points DP₁ to DP₄ that are points sequentially away from the surface height P_M in the downward direction, and a distance G₁ between the surface height P_M and the first design point DP₁, a distance G₂ between the first design point DP₁ and the second design point DP₂, a distance G₃ between the second design point DP₂ and the third design point DP₃, a distance G₄ between the third design point DP₃ and the fourth design point DP₄ may gradually increase with a constant rate in a direction from the first distance G₁ to the fourth distance G₄.

[0013] The designing of the width for each height may include: setting a plurality of points P on the inner wall surface of the second mold at the same positions as the plurality of design points DP set on the first mold; subtracting (W_M - SD) the solidification shrinkage amount SD for each of the plurality of design points DP from a width W_M of the inner wall surface at the surface height P_M of the first mold; and determining a subtracted (W_M - SD) value at each of the plurality of design points DP as a width W at each of the plurality of points P set on the inner wall surface of the second mold.

[0014] When the second mold is prepared, a height, a width at an upper end, and a width at a preset surface height of the molten steel of the second mold may be designed to be the same as those of the first mold.

[0015] The method may further include preparing the first mold, and the preparing of the first mold may include preparing the first mold so that a width of an inner wall surface decreases in the downward direction and a decrease rate of a width of the inner wall surface is constant in a height direction.

[0016] The detecting of the solidification shrinkage amount SD at each of the plurality of points P may include: detecting a solidification shrinkage amount 1SD of first molten steel at the plurality of design points DP by supplying and solidifying the first molten steel into the first mold; detecting a solidification shrinkage amount 2SD of second molten steel at the plurality of design points DP by supplying and solidifying the second molten steel into the first mold; and calculating an average solidification shrinkage amount AS of the solidification shrinkage amount 1SD of the first molten steel and the solidification shrinkage amount 2SD of the second molten steel for each of the plurality of design points DP, wherein, in the subtracting (W_M - SD) of the solidification shrinkage amount SD for each of the plurality of design points DP from the width W_M of the inner wall surface at the surface height P_M of the first mold, the solidification shrinkage amount SD for each of the plurality of design points DP is the average solidification shrinkage amount AS or each of the plurality of design points DP.

[0017] The method may further include selecting the first and second molds, in which the selecting of the first and second molds may include: supplying and solidifying each of a plurality of molten steel into the first mold; detecting a solidification shrinkage amount occurring when each of the plurality of molten steel is solidified; and selecting molten steel having a largest solidification shrinkage amount among the detected solidification shrinkage amounts as first molten and molten steel having a smallest solidification shrinkage amount steel as second molten steel.

[0018] In accordance with another exemplary embodiment, a mold having an inner space into which molten steel is injected include a body having an inner space, in which a plurality of points having different heights are set on an inner wall surface of the body, a width at each of the plurality of points in the inner wall surface of the body gradually decreases in a downward direction, and a decrease rate by which the width decreases is varied at the plurality of points in a height direction.

[0019] Distances between the plurality of points in the height direction may be different.

[0020] The distances between the plurality of points in the height direction may gradually increase in the downward direction.

[0021] The distances between the plurality of points in the height direction may increase with a constant value.

[0022] The plurality of points may include first to fourth points that are sequentially spaced downward from a surface height of the molten steel supplied into the body, and a distance G₁ between the surface height and a first point, a distance G₂ between the first point and a second point, a distance G₃ between the second point and a third point, a distance G₄ between the third point and a fourth point may gradually increase with a constant rate in a direction from the first distance G₁ to the fourth distance G₄.

[0023] The inner wall surface of the body may be an inclined surface that is gradually spaced apart from an outer surface that is an opposite surface of the inner wall surface in the downward direction, and an inclination of the inner wall surface of the body may be varied by using the plurality of points as inflection points.

[0024] A width at each of the plurality of points may be designed by using a solidification shrinkage amount for each height obtained while the molten steel is solidified using a parent mold for designing the mold.

ADVANTAGEOUS EFFECTS

[0025] In accordance with the exemplary embodiments, the compensation rate for the solidified shell shrinkage may be improved. That is, the compensation rate for solidification shrinkage for each height may be improved by designing the width for each height of the mold according to the different solidification shrinkage amounts for each height to manufacture the mold. Thus, the defect caused by the solidified shell shrinkage may be suppressed or prevented from occurring on the surface of the cast piece.

[0026] Also, when the mold is manufactured, the solidification shrinkage rate may be detected by solidifying the plurality of kinds of molten steel, and the width for each height of the inner wall surface of the mold may be designed by reflecting the average value of the solidification shrinkage rate. Thus, the cast piece in which the solidification shrinkage rate is suppressed or prevented may be manufactured for the plurality of kinds of steel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] 

FIG. 1 is a view illustrating a casting apparatus in accordance with an exemplary embodiment.

FIG. 2 is a three-dimensional view illustrating the mold in accordance with an exemplary embodiment.

FIG. 3 is an exploded perspective view illustrating the mold in accordance with an exemplary embodiment.

(a) of FIG. 4 is a front view viewed from 'A' of FIG. 2 to explain a width for each height with respect to an inner wall surface of a first wall in the mold in accordance with an exemplary embodiment.

(b) of FIG. 4 is a front view viewed from 'B' of FIG. 2 to explain an inclination for each height with respect to the inner wall surface of the first wall in the mold in accordance with an exemplary embodiment, illustrating one of a pair of first walls.

(a) of FIG. 5 is a front view viewed from 'B' of FIG. 2 to explain a width for each height with respect to an inner wall surface of a second wall in the mold in accordance with an exemplary embodiment.

(b) of FIG. 5 is a front view viewed from 'A' of FIG. 2 to explain an inclination for each height with respect to the inner wall surface of the second wall in the mold in accordance with an exemplary embodiment, illustrating one of a pair of second walls.

FIG. 6 is an exploded perspective view illustrating a portion of a parent mold used to manufacture the mold in accordance with an exemplary embodiment.

FIG. 7 is a view for explaining a shrinkage amount of a solidified shell for each height when the molten steel is solidified in the parent mold.

FIG. 8 is a flowchart representing a method for manufacturing a mold in accordance with an exemplary embodiment.

FIG. 9 is a graph representing a solidification shrinkage amount and an average shrinkage amount in a first direction for each height of the mold when each of first molten steel and second molten steel is charged into the parent mold.

(a), (c), (e), (g), and (i) of FIG. 10 are photographs of a surface of a billet in accordance with first to fifth experimental examples.

(b), (d), (f), (h), and (j) of FIG. 10 are graphs representing roughness (mm), i.e., a height (mm), of the surface of the billet in the first to fifth experimental examples.

MODE FOR CARRYING OUT THE INVENTION

[0028] Hereinafter, exemplary embodiments will be described in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. In the figures, the dimensions of layers and regions are exaggerated for clarity of illustration. Like reference numerals refer to like elements throughout.

[0029] FIG. 1 is a view illustrating a casting apparatus including a mold in accordance with an exemplary embodiment.

[0030] Referring to FIG. 1, the casting apparatus includes a tundish 20 that receives molten steel from a ladle 10 and stores the received molten steel, a mold 3000 that receives the molten steel from the tundish 20 and initially solidifies the received molten steel into a certain shape, and a nozzle 22 that supplies the molten steel in the tundish 20 to the mold 3000.

[0031] Also, the casting apparatus includes a cooling unit 40 disposed below the mold 3000 and spraying cooling water to an unsolidified cast piece 1 drawn from the mold 3000 to completely solidify the cast piece 1. Here, the cooling unit 40 include a plurality of segments 41. Also, each of the plurality of segments 41 may include a plurality of rolls that are rotatable by movement force of the slab 1 and nozzles disposed between the plurality of rolls to spray cooling water to the cast piece 1.

[0032] Hereinafter, a mold in accordance with an exemplary embodiment will be described with reference to FIGS. 2 to 5. The mold in accordance with an exemplary embodiment may be for casting the billet. Also, the mold may be for casting a billet may of low-carbon steel, medium-carbon steel, or high-carbon steel.

[0033] Alternatively, the mold 3000 may manufacture all sorts of cast pieces in addition to the above-described billet made of low-carbon steel, medium-carbon steel, and high-carbon steel. Also, the mold may manufacture various cast pieces such as a slab and a bloom in addition to the billet.

[0034] FIG. 2 is a three-dimensional view illustrating the mold in accordance with an exemplary embodiment. FIG. 3 is an exploded perspective view illustrating the mold in accordance with an exemplary embodiment. (a) of FIG. 4 is a front view viewed from 'A' of FIG. 2 to explain a width for each height with respect to an inner wall surface of a first wall in the mold in accordance with an exemplary embodiment. (b) of FIG. 4 is a front view viewed from 'B' of FIG. 2 to explain an inclination for each height with respect to the inner wall surface of the first wall in the mold in accordance with an exemplary embodiment, illustrating one of a pair of first walls. (a) of FIG. 5 is a front view viewed from 'B' of FIG. 2 to explain a width for each height with respect to an inner wall surface of a second wall in the mold in accordance with an exemplary embodiment. (b) of FIG. 5 is a front view viewed from 'A' of FIG. 2 to explain an inclination for each height with respect to the inner wall surface of the second wall in the mold in accordance with an exemplary embodiment, illustrating one of a pair of second walls.

[0035] Referring to FIG. 2, the mold 3000 includes a body 3100 having an inner space IS. Also, the mold 3000 may include a coolant flow path (not shown) buried in the body 3100 so that a coolant circulates.

[0036] Referring to FIG. 3, the body 3100 includes an inner wall surface IF (IF_L and IF_S) facing the inner space IS and an outer wall surface OF (OF_L and OFs) that is an opposite surface of the inner wall surface IF (IF_L and IFs) and is exposed to the outside.

[0037] As illustrated in FIG. 3, in the body 3100, a width W (W_L and Ws) of the inner wall surfaces IF (IF_L and IF_S) decreases in a downward direction. Here, a plurality of four or more points, which are points of different heights below a top surface of the molten steel supplied into the body 3100, i.e., a surface height P_M of the molten steel, are set, and each of the plurality of points has a different width.

[0038] Here, the width W (W_L and Ws) of the inner wall surface IF (IF_L and IF_S) may represent a length in a horizontal direction. More specifically, the width W (W_L and Ws) of the inner wall surface IF (IF_L and IFs) may represent a length in a first direction (X-axis direction) and a length in a second direction (Y-axis direction).

[0039] Hereinafter, when the mold 3000 in accordance with an exemplary embodiment is described, the plurality of points are set to four as an example. Also, a first point P₁, a second point P₂, a third point P₃, and a fourth point P₄ are sequentially set in a downward direction from the surface height P_M of the molten steel surface.

[0040] When the first to fourth points P₁ to P₄ are set, a distance between the first to fourth points P₁ to P₄ is set to gradually decrease in an upward direction and gradually increase in the downward direction. That is, when the first through fourth points P₁ to P₄ are set, a distance from a point disposed directly above gradually decreases in the upward direction. In other words, the distance from the point directly above gradually increases in the downward direction. That is, a distance G₁ between the surface height P_M of the molten steel and the point 1 P₁ is a shortest distance, and a distance G₄ between the point 4 P₄ and the point 3 P₃ is a longest distance. In other words, a distance from the point directly above is in an order such that 'the distance G₁ between the first point P₁ and the surface height P_M of the molten steel < the distance G₂ between the second point P₂ and the first point P₁ < the distance G₃ between the third point P₃ and the second point P₂ < the distance G₄ between the fourth point P₄ and the third point P₃'.

[0041] Here, the width W (W_L and Ws) at the first to fourth points P₁ to P₄ set as described above is designed by a method in accordance with an exemplary embodiment that will be described later so that the width of the inner wall surface IF of the body 3100 decreases in the downward direction.

[0042] When the width W (W_L and Ws) of the inner wall surface IF (IF_L and IF_S) gradually decreases in the downward direction, a decrease rate of the width is varied at the plurality of points P₁ to P₄ in a height direction. That is, the width W (W_L and Ws) of the inner wall surface IF (IF_L and IF_S) gradually decreases in the downward direction, and the decrease rate of the width W (W_L and Ws) of the inner wall surface IF (IF_L and IF_S) is varied in the height direction instead of being constant. Here, a width of a region from an upper end P_U to the first point P₁ has a constant decrease rate, a width of a region from the first point P₁ to the second point P₂ has a constant decrease rate, a width of a region from the second point P₂ to the third point P₃ has a constant decrease rate, and a width of a region from the third point P₃ to the fourth point P₄ has a constant decrease rate.

[0043] Also, since the width W (W_L and Ws) of the inner wall surface IF (IF_L and IF_S) decreases in the downward direction, a difference between a width of the upper end P_U and a width W_L1 and W_S1 of the first point P₁ is a largest difference. Here, since the width of the region from the upper end P_U to the first point P₁ has the constant decrease rate as described above, when differences between the first to fourth points P₁ to P₄ are compared, a difference between the width W_LM and W_SM of the surface height P_M of the molten steel surface and the width W_L1 and W_S1 of the first point P₁ is a largest difference. That is, the difference between the width W_LM and W_SM of the surface height P_M of the molten steel and the width W_L1 and W_S1 of the first point P₁ is greater than each of a difference between the width W_L1 and W_S1 of the first point P₁ and the width W_L2 and W_S2 of the second point P₂, a difference between the width W_L2 and W_S2 of the second point P₂ and the width W_L3 and W_S3 of the third point P₃, and a difference between the width W_L3 and W_S3 of the third point P₃ and the width W_L4 and W_S4 of the fourth point P₄.

[0044] Thus, a region in which a most significant change in width is generated below the surface height P_M of the molten steel is a region between the surface height P_M of the molten steel and the first point P₁. As described above, since the distance G₁ between the first point P₁ and the surface height P_M of the molten steel directly above the first point P₁ is the shortest distance, and the difference between the width W_LM and W_SM of the surface height P_M of the molten steel and the width W_L1 and W_S1 of the first point P₁ is the largest difference, the width from the surface height P_M of the molten steel to the first point P₁ is most dramatically varied.

[0045] As described above, the largest difference between the width W_LM and W_SM at the surface height P_M of the molten steel and the width W_L1 and W_S1 at the first point P₁ is because the shrinkage amount of the solidified shell increases in a direction close to the surface of the molten steel when the solidified shell is shrunk in the body 3100. That is, as the width W_LM and W_SM at the surface height P_M of the molten steel and the width W_L1 and W_S1 at the first point P₁ is dramatically changed, a solidification shrinkage compensation rate at an upper region adjacent to the surface of the molten steel in the body may be improved.

[0046] Also, as described above, the decrease rate by which the width W (W_L and Ws) of the inner wall surface IF (IF_L and IFs) decreases is different in the height direction instead of being constant. That is, the difference between the width W_M (W_LM and W_SM) at the surface height P_M and the width W_L1 and W_S1 at the first point P₁, the difference between the width W_L1 and W_S1 at the first point P₁ and the width W_L2 and W_S2 at the second point P₂, the difference between the width W_L2 and W_S2 at the second point P₂ and the width W_L3 and W_S3 at the third point P₃, and the difference between the width W_L3 and W_S3 at the third point P₃ and the width W_L4 and W_S4 at the fourth point P₄ may be different. This is because the shrinkage amount of the solidified shell may be different depending on a position for each height of the mold 3000 when the solidified shell is shrunk at the molten steel is solidified.

[0047] As described above, since the shrinkage amount of the solidified shell is different depending on the height of the mold 3000, in accordance with an exemplary embodiment, the width W (W_L and Ws) of the inner wall surface IF (IF_L and IF_S) of the body 3100 is designed by using the shrinkage amount of the solidified shell for each height of the mold in accordance with an exemplary embodiment to manufacture the mold. That is, a mold (hereinafter, referred to as a parent mold) for the design is separately prepared, and then at least four or more plurality of points are set below the surface height P_M of the parent mold. Then, the solidification shrinkage amount at each of the plurality of points is detected, and then the mold 3000 is manufactured by designing the width at each point according to the solidification shrinkage amount at each point.

[0048] A method for designing or determining the width W (W_L and Ws) of the inner wall surface IF (IF_L and IF_S) of the mold 3000, i.e., the body 3100, to be manufactured, according to the solidification shrinkage amount for each height in the parent mold will be described in detail below with reference to FIGS. 6 and 7.

[0049] Hereinafter, it will be described that the body 3100 includes a plurality of walls as described above.

[0050] The body 3100 includes a plurality of walls 3110 and 3120. That is, referring to FIG. 2, the body 3100 may include a pair of first walls 3110 each extending in the first direction (X-axis direction) and spaced apart from each other in the second direction (Y-axis direction) and a pair of second walls 3120 each extending in the second direction (Y-axis direction) that crosses an extension direction of the first walls 3110 and spaced apart from each other in the first direction (X-axis direction).

[0051] Also, the pair of first walls 3110 and the pair of second walls 3120 are connected to each other. For example, each of the pair of first walls 3110 has one end connected to one end and the other end of one of the pair of second walls 3120 and the other end connected to one end and the other end of the other of the pair of second walls 3120. Thus, the inner space IS surrounded by the pair of first walls 3110 and the pair of second walls 3120 is prepared.

[0052] Thus, it may be described that the above-described inner wall surface IF of the body 3100 includes an inner wall surface (hereinafter, referred to as a first inner wall surface IF_L) of each of the pair of first walls 3110 and an inner wall surface (hereinafter, referred to as a second inner wall surface IF_S) of each of the pair of second walls 3120. Also, it may be described that the outer wall surface OF the body 3100 includes an outer wall surface (hereinafter, referred to as a first outer wall surface OF_L) of each of the pair of first walls 3110 and an outer wall surface (hereinafter, referred to as a second outer wall surface OF_S) of each of the pair of second walls 3120.

[0053] Thus, as illustrated in FIG. 3 and (a) of FIG. 4, in the first inner wall surface IF_L of the first wall 3110, a length in the first direction (X-axis direction) is defined as a width W_L (W_LM, W_L1, W_L2, W_L3, and W_L4) of the first inner wall surface IF_L. Also, as illustrated in FIG. 3 and (a) of FIG. 5, in the second inner wall surface IF_S of the second wall 3120, a length in the second direction (Y-axis direction) is defined as a width Ws (W_SM, W_S1, W_S2, W_S3, and W_S4) of the second inner wall surface IF_S.

[0054] Also, in a horizontal direction of each of the first wall 3110 and the second wall 3120, a length that crosses a width direction is defined as a 'thickness'. Thus, a thickness T_L T_LM (T_L1, T_L2, T_L3, and T_L4) of the first wall 3110 is a length in the second direction (Y-axis direction) (refer to (b) in FIG. 4), and a thickness T_S (T_SM, T_S1, T_S2, T_S3, and T_S4) of the second wall 3120 is a length in the first direction (X-axis direction).

[0055] As descried above, although it is described that the body 3100 includes the pair of first walls 3110 and the pair of second walls 3120, the pair of first walls 3110 and the pair of second walls 3120 may be integrated with each other. That is, the pair of first walls 3110 and the pair of second walls 3120 may be integrated with each other by a method such as compression molding instead of being coupled by using a coupling unit. The mold including the above-described body 3100 is referred to as a "tube type mold".

[0056] Alternatively, the body 3100 may be provided by coupling the first wall 3110 and the second wall 3120 through a coupling unit.

[0057] As illustrated in FIG. 3, the body 3100 has a different width for each position or height in the height direction (Z-axis direction), and the width W (W_L and Ws) that gradually decreases in the downward direction. Regarding this, hereinafter, the first inner wall surface IF_L of the first wall 3110 and the second inner wall surface IF_S of the second wall 3120 will be separately described in more detail.

[0058] As illustrated in FIG. 3 and (a) of FIG. 4, the first inner wall surface IF_L of the first wall 3110 has a width that gradually decreases in the downward direction. Here, when widths at different positions in the height direction on the first inner wall surface IF_L are different, widths of at least four different points below the surface height P_M are different. For example, the widths W_L1, W_L2, W_L3, and W_L4 at the first to fourth points P₁ to P₄ that are four positions having different heights below the surface height P_M of the first inner wall surface IF_L are different. Here, each of the widths W_L1 to W_L4 at the first to fourth points P₁ to P₄ is less than a width P_LM of the surface height W_LM on the first inner wall surface IF_L.

[0059] Also, the second inner wall surface IF_S of the second wall 3110 also has the same shape as the above-described first inner wall surface IF_L. That is, when the second inner wall surface IF_S of the second wall 3110 has a width that gradually decreases in the downward direction, widths W_S1, W_S2, W_S3, and W_S4 at the same position as those in the first inner wall surface IF_L, i.e., the first to fourth points P₁ to P₄, are different. Here, each of the widths W_S1 to W_S4 at the first to fourth points P₁ to P₄ is less than a width W_SM of the surface height P_M on the second inner wall surface IFs.

[0060] The first to fourth points P₁ to P₄ on the first inner wall surface IF_L have the same heights as the first to fourth points P₁ to P₄ on the second inner wall surface IFs. Also, the first to fourth points P₁ to P₄ on each of the first and second inner wall surfaces IF_L and IFs are determined based on the surface height P_M of the molten steel charged into the body 3100.

[0061] The surface height P_M of the molten steel may be a height at a point spaced a predetermined distance downward from the upper end P_U. For example, when the molten steel is charged so that the surface of the molten steel is positioned at a point 100 mm downward from the upper end P_U of the body 3100, a distance from a lower end P_B of the body 3100 to the surface of the molten steel is referred to as a surface height. Here, when the surface height P_M of the molten steel is described when the upper end P_U of the body 3100 is positioned at 0 mm, the surface height P_M of the molten steel is positioned at a point of 100 mm.

[0062] Also, the first point P₁ of the first and second walls 3110 and 3120 is a point spaced a first distance S1 downward from the upper end P_U, and the second point P₂ is a point spaced a second distance S₂, which is greater than the first distance S₁, downward from the upper end P_U. Also, the third point P₃ is a point spaced a third distance S₃, which is greater than the second distance S₂, downward from the upper end P_U, and the fourth point P₃ is a point spaced a fourth distance S₄, which is greater than the third distance S₃, downward from the upper end P_U.

[0063] The region from the surface height P_M of the molten steel to the lower end P_B is divided into a plurality of regions in the height direction. Here, in the surface height P_M and the first to fourth points P₁ to P₄, which are vertically disposed, a distance between the adjacent points gradually increases in the downward direction. That is, in the first distance G₁ between the surface height P_M and the first point P₁, the second distance G₂ between the first point P₁ and the second point P₂, the third distance G₃ between the second point P₂ and the third point P₃, and the fourth distance G₄ between the third point P₃ and the fourth point P₄, the first distance G₁ is smallest, and the fourth distance G₄ is largest. In this case, the distance G₁ between the surface height P_M and the first point P₁ and the distance G₂ to G₄ between the first to fourth points P₁ to P₄ may increase with a constant value. Preferably, the first to fourth points P₁ to P₄ are set so that a ratio of the first to fourth distance (G₁ to G₄) is 1:2:3:4 (G₁:G₂:G₃:G₄ = 1:2:3:4).

[0064] For example, when the surface height P_M of the molten steel is a point 100 mm downward from the upper end P_U, a length from the surface height P_M (point of 100 mm) to the lower end P_B is divided so that the ratio of the first to fourth distance G₁ to G₄ is 1:2:3:4 (G₁:G₂:G₃:G₄ = 1:2:3:4).

[0065] Here, since the first point P₁ is a point spaced the first distance G₁ downward from the surface height P_M, the position of the first point P₁ may be described as a sum of the surface height P_M and the first distance G₁. Also, since the second point P₂ is separated from the first point P₁ by a second distance G₂ downward, a position of the second point P₂ may be described as a sum of the first point P₁ and the second distance G₂. Similarly, a position of the third point P₃ may be described as a sum of the second point P₂ and the third distance G₃, and a position of the fourth point P₄ may be described as a sum of the third point P₃ and the fourth distance G₄. In this case, the position of the fourth point P₄ may be the lower end P_B.

[0066] As the positions of the first through fourth points P₁ to P₄ are set in this manner, the distance from the directly above point gradually decreases in the upward direction. That is, "first distance G₁ < second distance G₂ < third distance G₃ < fourth distance G₄".

[0067] In each of the first and second inner wall surfaces IF_L and IF_S, the widths at the first to fourth points P₁ to P₄, i.e., the first to fourth width W_L1 to W_L4 and W_S1 to W_S4, are different. In this case, the width decreases from the first point P₁ to the fourth point P₄. That is, the second width W_L2 and W_S2 is smaller than the first width W_L1 and W_S1, the third width W_L3 and W_S3 is smaller than the second width W_L2 and W_S2, and the fourth width W_L4 and W_S4 is smaller than the third width W_L3 and W_S3.

[0068] As described above, the width of each of the first and second inner wall surfaces IF_L and IF_S may gradually decrease in the downward direction to compensate for the shrinkage of the solidification shell in the first direction (X-axis direction) and the shrinkage in the second direction (Y-axis direction). In other words, the width W_L1, W_L2, W_L3, and W_L4 of the first inner wall surface IF_L may gradually decreases in the downward direction to compensate for the shrinkage of the solidification shell in the first direction (X-axis direction). Also, the width W_S1, W_S2, W_S3, and W_S4 of the second inner wall surface IF_S may gradually decrease in the downward direction to compensate for the shrinkage of the solidification shell in the second direction (Y-axis direction).

[0069] Also, as the width of the first and second inner wall surfaces IF_L, and IF_S decreases in the downward direction, the decrease rate in the width of the inner wall surfaces IF_L and IF_S is varied in the height direction instead of being uniform or constant. Here, the decrease rate of the width is varied at the first to fourth points P₁ to P₄ in the height direction.

[0070] Here, the decrease rate in width may be calculated using a difference in width at each of the two adjacent points and the distance between the adjacent points (refer to Equation 1).

[0071] The decrease rate will be described with a more specific example. Here, a width decrease rate between the surface height P_M and the first point will be described as an example. To this end, a following assumption will be provided. A total height of the body 3100 or the first wall 3110 is 1000 mm, the distance G₁ between the first point P₁ and the surface height P_M is 210 mm, the width at the surface height P_M is 200 mm, and the width W_L1 at the first point P₁ is 199 mm. In this case, the width decrease rate of the width between the surface height P_M and the first point P₁ may be calculated as shown in calculation equation 1 below.

[0072] Also, a width decrease rate between the first point P₁ and the second point P₂, a width decrease rate between the second point P₂ and the third point P₃, and a width decrease rate between the third point P₃ and the fourth point P₄ may be calculated in the same manner as the method described above.

[0073] Also, a width decrease rate of the second inner wall surface IF_S of the second wall 3120 is calculated in the same manner as the width decrease rate of the first inner wall surface (IF_S) described above. Thus, a detailed description thereof will be omitted.

[0074] Thus, when the width of the first and second inner wall surfaces IF_L and IF_S gradually decreases in the downward direction, the width decrease rates of the inner wall surfaces IF_L and IF_S are different instead of being uniform or constant in the height direction.

[0075] Also, in the width difference from the point directly above, a difference between the width W_LM and W_SM of the surface height P_M and the width W_L1 and W_S1 of the first point P₁ is the largest, and the difference between the width W_L3 and W_S3 of the third point P₃ and the width W_L4 and W_S4 of the fourth point P₄ is the smallest. Accordingly, the width change is sharpest in the region between the surface height P_M and the first point P₁, and the width change is most gradual in the region between the third point P₃ and the fourth point P₄.

[0076] Thus, as the width change in the region between the surface height P_M and the first point P₁ is sharper than in other regions, it is possible to effectively compensate for the solidification shrinkage in an upper portion of the body 3100, in which the solidified shell shrinks the most. More specifically, the solidification shell shrinks the most in the upper portion of the body 3100 at a height adjacent to the surface, which may effectively compensate for solidification shrinkage in this region.

[0077] The widths W_L1 to W_L4 and W_S1 to W_S4 at each of the first to fourth points P₁ to P₄ are designed based on the solidification shrinkage amounts of the solidified shells at the first to fourth points of the parent mold. In other words, the first width W_L1 and W_S1 at the first point P₁, the second width W_L2 and W_S2 at the second point P₂, the third width W_L3 and W_S3 at the third point P₃, and the fourth width W_L4 and W_S4 at the fourth point P₄ are each designed using the solidification shrinkage amount at each of the first to fourth points of the parent mold.

[0078] As described above, the solidified shell shrinkage for each height may be effectively compensated by the adjusted widths W_L1 to W_L4 and W_S1 to W_S4 of the inner wall surface IF_L and IF_S. That is, in the case of a conventional mold, the width of each of the first and second inner wall surfaces gradually decrease in the downward direction, but the width decrease rate is constant or the same. In other words, the width decreases with a constant rate regardless of the shrinkage amount at each position in the height direction. In this case, a tendency of the shrinkage to gradually decrease in the downward direction may be compensated for, but the tendency of the shrinkage to decrease in the downward direction did not decrease at a uniform rate, so the compensation for different shrinkage amounts at different heights is insufficient. Accordingly, a large number of surface defects due to the solidification shrinkage occur on the surface of the cast piece such as the billet.

[0079] However, in the embodiment, the solidified shell shrinkage may be effectively compensated by adjusting the width W_L1 to W_L4 and Wsi to W_S4 of the inner wall surface IF_L and IF_S for each height according to the different solidification shrinkage amounts for each height. Also, since the widths W_L1 to W_L4 and W_S1 to W_S4 of the inner wall faces IF_L and IF_S are designed to reflect the actual solidification shrinkage amount for each height, the solidification shrinkage for each height may be compensated more effectively. As a result, defects caused by solidification shrinkage on the surface of the cast piece may be minimized or prevented.

[0080] Also, each of the first and second inner wall surfaces IF_L and IF_S of the body 3100 has an inclined surface that is gradually inclined in a direction away from the outer wall surface OF_L and OF_S in the downward direction. In other words, the body 3100 has a thickness that gradually increases in the downward direction. More specifically, each of the first and second inner wall surfaces IF_L and IF_S may have four or more steps of inclinations, as illustrated in (b) of FIG. 4 and (b) of FIG. 5. That is, each of the first and second inner wall surfaces IF_L and IF_S is gradually inclined in a direction away from the outer wall surface OF_L and OF_S, in which the inclination is changed four or more times.

[0081] More specifically, each of the first and second inner wall surfaces IF_L and IF_S includes at least first and third inclined surfaces F_L1 to F_L4 and F_S1 to F_S4 that are changed in inclination at an inflection point P₁ to P₃. In other words, the first and second inner wall surfaces IF_L and IF_S are the first inclinations F_L1 and F_S1 that is inclined in a direction away from the outer wall surface OF_L and OFs from the upper end P_U to the first point P₁ as shown in (b) of FIG. 4 and (b) of FIG 5, the outer wall surface OF_L and OFs and the second inclination F_L2 and P_S2 from the second point P₂ to the third point P₃, the third inclination F_L3 and F_S3 from the third point P₃ to the third inclination OF_L and OFs, and the fourth inclination F_L4 and F_S4 from the fourth point P₄ to the fourth inclination OF_L and OFs.

[0082] Also, the first through fourth inclinations F_L1 to F_L4 and F_S1 to F_S4 are arranged to have different inclinations. That is, the first through fourth inclinations are different and have magnitudes in a following order: first inclination > second inclination > third inclination > fourth inclination.

[0083] Thus, by providing the first inner wall surface IF_L with an inclined surface that is gradually away from the first outer wall surface OF_L in the downward direction, it is possible to compensate for the solidification shell shrinkage in the second direction (Y-axis direction) that crosses the extension direction X-axis direction of the first inner wall surface IF_L. Also, by providing the second inner wall surface IFs with an inclined surface that is gradually away from the second outer wall surface OF_S in the downward direction, the solidification shell shrinkage may be compensated in the first direction (X-axis direction), i.e., the direction that crosses the extension direction (Y-axis direction) of the second inner wall surface IF_S.

[0084] Hereinafter, with reference to FIGS. 6 and 7, a method for manufacturing a mold in accordance with an exemplary embodiment will be described.

[0085] FIG. 6 is an exploded perspective view illustrating a portion of a parent mold used to manufacture a mold in accordance with an exemplary embodiment. FIG. 7 is a view illustrate a shrinkage amount of a solidified shell for each height when the molten steel is solidified in the parent mold.

[0086] Here, (a) of FIG. 7 is a top view to illustrate the surface height P_M of the parent mold, (b) of FIG. 7 is the first design point DP₁ of the parent mold, (c) of FIG. 7 is a plan view for explaining the second design point DP₂ of the parent mold, (d) of FIG. 7 is a plan view for explaining the third design point DP₃ of the parent mold, and (e) of FIG. 7 is a plan view for explaining a width of the solidified shell (solidification width) and a width of the inner space at each of the fourth design point DP₄ of the parent mold.

[0087] Hereinafter, the parent mold will be firstly described with reference to FIG. 6. For convenience of explanation, reference numerals of the first and second inner wall surfaces IF_L and IFs, the surface height P_M, and widths at the surface height W_LM and W_SM of the parent mold are described as the same as those of the mold in accordance with an exemplary embodiment.

[0088] Also, in the parent mold, four points below the surface height P_M are set, which are referred to as first to fourth design points DP₁ to DP₄ to be distinguished from the mold in accordance with an exemplary embodiment.

[0089] The parent mold 4000 may include a body having an inner space IS and a coolant flow path (not shown) buried in the body for circulation of coolant. In this case, the body of the parent mold 4000 is shaped so that the width W_L and Ws of the inner wall surface IF (IF_L and IF_S) facing the inner space IS gradually decreases in the downward direction. Here, the width W_L and Ws gradually decreases as the inner wall surface IF (IF_L and IF_S) gradually decreases in the downward direction, and a width decrease rate is constant.

[0090] More specifically, the body 4100 of the parent mold 4000 includes a pair of first walls 4110 each extending in the first direction (X-axis direction) and spaced apart from each other in the second direction (Y-axis direction), and a pair of second walls 4120 each extending in the second direction (Y-axis direction) and spaced apart from each other in the first direction (X-axis direction). Also, the width W_L of the first inner wall surface IF_L of the first wall body 4110 and the width Ws of the second inner wall surface IF_S of the second wall body 4120 each gradually decrease in the downward direction.

[0091] However, in the case of the parent mold 4000, the widths W_L and Ws of the first and second inner wall surfaces IF_L and IF_S gradually decrease in the downward direction, and a decreasing inclination or decreasing ratio is constant. Also, each of the first and second inner wall surfaces IF_L and IF_S is inclined in a direction away from the first and second outer wall surfaces OF_L and OF_S in the downward direction with a constant inclination from the upper end P_U to the lower end P_B as shown in FIG. 6.

[0092] This parent mold 4000 may be a conventional mold. More specifically, the mold may be a mold that has surface defects due to solidification shrinkage, and the surface defects cause a limitation in quality of the cast piece. Alternatively, the parent mold 4000 may be separately provided to have the above-described shape and structure.

[0093] Also, the parent mold 4000 may be referred to as a first mold, and a new mold manufactured by designing the width according to the solidification shrinkage of the parent mold 4000 may be referred to as a second mold 3000.

[0094] Hereinafter, a method for detecting a solidification shrinkage amount for each height in the above-described parent mold 4000 will be described.

[0095] When the parent mold 4000 is prepared, the molten steel is charged into the parent mold 4000. Here, the molten steel is charged so that a surface of the molten steel is at a predetermined height. For example, the molten steel is charged so that the surface is located at a point of 100 mm downward from the upper end P_U of the parent mold 4000.

[0096] The molten steel charged into the parent mold 4000 is cooled by coolant circulating in the parent mold 4000, and in this process, a solidified shell C is formed as shown in FIG. 7. Here, the solidified shell is formed from the surface height, and a thickness of the solidified shell C may gradually increase in the downward direction.

[0097] Also, when the molten steel is solidified to form the solidified shell C, solidification shrinkage occurs in the solidified shell C. Also, the shrinkage amount of the solidified shell C may be different in the height direction, and a larger shrinkage occurs at the top compared to the bottom.

[0098] In an exemplary embodiment, the shrinkage amount of the solidification shell C for each height is detected, and the mold 3000 in accordance with an exemplary embodiment is manufactured using the detected shrinkage amount. To this end, firstly, the plurality of design points at different heights to detect the solidification shrinkage amount are set. Preferably, four or more design points are set to detect the solidification shrinkage amount. The embodiment describes an example of detecting solidification shrinkage amount at four design points DP₁ to DP₄ of different heights.

[0099] That is, on each of the first and second inner wall surfaces IF_L and IF_S, the first design point DP₁ is spaced a first distance G₁ downward from the surface height P_M, the second design point DP₂ is spaced a second distance G₂ from the first design point DP₁, the third design point DP₃ is spaced a third distance G₃ from the second design point DP₂, and a fourth design point DP₄ is spaced a fourth distance G₄ from the third design point DP₃.

[0100] In the first through fourth distance G₁ to G₄, the first to fourth design points DP₁ to DP₄ are set so that distances gradually increase in the downward direction. Here, the first to fourth design points DP₁ to DP₄ may be set in a ratio of the first distance G₁: second distance G₂: third distance G₃: fourth distance G₄ of 1:2:3:4 (G₁:G₂:G₃:G₄ = 1:2:3:4). In this case, when the surface height P_M of the molten steel is a point of 100 mm downwardly from the upper end P_U, a length from the surface height P_M of the molten steel (point of 100 mm) to the lower end P_B is divided so that a ratio of the first to fourth distance G₁ to G₄ is 1:2:3:4 (G₁:G₂:G₃:G₄ = 1:2:3:4).

[0101] Then, a position of the first point P₁ is set to a position of a value of the first distance G₁ added to the surface height P_M of the molten steel, and a position of the second point P₂ is set to a position of a value of the first distance P₁ added to the second distance G₂. Similarly, a position of the third point P₃ may be set to a position of the second point P₂ added to the third distance G₃, and a position of the fourth point P₄ may be set to a position of the third point P₃ added to the fourth distance G₄.

[0102] The solidification shrinkage amount SD (SD_L and SD_S) for each height is detected by using a length difference between two ends of the solidified shell C at each of the first to fourth points DP₁ to DP₄ and the width W_LM and W_SM of the inner space IS at the surface height P_M. Here, the length between the two ends of the solidification shell C may represent a length between the two ends of the solidification shell C in the first direction (X-axis direction) and a length between the two ends of the solidification shell C in the second direction (Y-axis direction).

[0103] Hereinafter, for convenience of explanation, the length between the two ends of the solidification shell C is referred to as a "solidification width SW (SW_L and SW_S)". Also, the solidification width SW_L (SW_L1 to SW_L4) in the first direction (X-axis direction) may be described as a length between the two ends of the first inner wall surface IF_L in an extension direction in the solidified shell C. Also, the solidification width SWs (SW_S1 to SW_S4) in the second direction (Y-axis direction) may be described as a length between the two ends in an extension direction of the second inner wall surface IF_S in the solidified shell (C).

[0104] Reflecting the above-described definitions, the method for calculating the solidification shrinkage amount for each height is explained again. The method may be calculated as a difference between the width W_LM and W_SM of the inner space IS at the surface height P_M of the molten steel and the solidification width SW_L1 to SW_L4 and SW_S1 to SW_S4 at each of the first to fourth design points DP₁ to DP₄.

[0105] That is, by calculating the difference between the solidification width SW_L1 to SW_L4 at the first to fourth design points DP₁ to DP₄ in the direction and the width W_LM of the inner space IS at the surface height P_M in the first direction, the amount of solidification shrinkage SD_L1 to SD_L4 at each of the first to fourth design points DP_L1 to DP_L4 in the first direction may be calculated. Also, by calculating the difference between the second directional solidification width SW_S1 to SW_S4 at the first to fourth design points DP₁ to DP₄ and the second directional width W_SM at the surface height P_M, the solidification shrinkage amount SD_S1 to SD_S4 at each of the first to fourth design points DP_S1 to DP_S4 in the second direction may be calculated.

[0106] Referring to FIG. 7, the method for calculating the solidification shrinkage amount for height in the first direction is described in more detail as follows. The solidification shrinkage amount SD_L1 at the first design point DP₁ is calculated by calculating a difference between the width W_LM in the first direction of the inner space IS at the surface height P_M of the molten steel and the solidification width SW_L1 at the first design point P₁ as shown in (b) of FIG. 7 (SD_L1 = W_LM-SW_L1). In addition, the solidification shrinkage amount SD_L2 at the second design point DP2 is calculated by calculating a difference between the width W_LM of the first direction of the inner space IS at the surface height P_M of the molten steel and the solidification width SW_L2 at the second design point P₂ as shown in (c) of FIG. 7 (SD_L2 = W_LM-SW_L2). The solidification shrinkage amount SD_L3 at the third design point P₃ and the solidification shrinkage amount SD_L4 at the fourth design point P₄ is calculated by the same method as described above (SD_L3 = W_LM-SW_L3, SD_L4 = W_LM-SW_L4).

[0107] Also, the method for calculating the solidification shrinkage amount for each height in the second direction is the same as the method in the first direction described above. That is, the solidification shrinkage amount SD_S1 at the first design point DP₁ is calculated by calculating a difference between the width W_SM of the second direction of the inner space IS at the surface height P_M of the molten steel and the solidification width SW_S1 at the first design point DP₁ as shown in (b) of FIG. 7 (SD_S1 = W_SM-SW_S1). Also, the solidification shrinkage amount SD_S2 at the second design point DP₂ is calculated by calculating a difference between the width W_SM of the inner space IS in the second direction at the surface height P_M of the molten steel and the solidification width SW_S2 at the second design point DP₂ as shown in (c) of FIG. 7 (SD_S2 = W_SM-SW_S2). Also, the solidification shrinkage amount SD_S3 at the third design point DP₃ and the solidification shrinkage amount SD_S4 at the fourth design point DP4 are calculated by the same method as described above (SD_S3 = W_SM-SW_S3, SD_S4 = W_SM-SW_S4).

[0108] As described above, the feature of charging the molten steel into the parent mold 4000, solidifying the same, and detecting the solidification shrinkage amount SD_L1 to SD_L4 and SD_S1 to SD_S4 at each of the first to fourth design points DP₁ to DP₄ in the first direction (X-axis direction) and the second direction (Y-axis direction) may be performed for a plurality of types of molten steel.

[0109] More specifically, among the plurality kinds of molten steel, the molten steel of a first steel kind with a largest solidification shrinkage amount (first steel) and the molten steel of a second steel kind with a smallest solidification shrinkage amount (second steel) are used. Here, each of the plurality kinds of molten steel is charged into the parent mold 4000 to detect the solidification shrinkage amount, and the solidification shrinkage amount at the same height is compared, and the molten steel with the largest solidification shrinkage amount is selected as the first molten steel and the molten steel with the smallest solidification shrinkage amount is selected as the second molten steel.

[0110] Here, the plurality kinds of molten steel may be manufactured from billet cast piece. Also, a content of main components in the first and second molten steel may be, for example, as shown in Table 1.

[Table 1]

	C(wt.%)	Si(wt.%)	Mn(wt%)
First molten steel	0.10	0.20	0.451
Second molten steel	0.82	0.20	0.449

[0111] Hereinafter, a method for manufacturing a mold in accordance with an exemplary embodiment using the first molten steel and the second molten steel will be described with reference to FIGS. 8 and 9.

[0112] FIG. 8 is a flowchart representing a method for manufacturing the mold in accordance with an exemplary embodiment. FIG. 9 is a graph showing a solidification shrinkage amount and an average shrinkage amount in the first direction for each height of the parent mold when each of the first molten steel and the second molten steel is charged and solidified in the parent mold.

[0113] When the first and second molten steel are prepared, each of the first and second molten steel is charged and solidified in the parent mold 4000 in operation S110 and S120. When the first and second molten steel, for example, the first molten steel, is firstly charged and solidified in the parent mold in operation S110. When solidification of the first molten steel begins, the solidification shrinkage amount 1SD_L1 to 1SD_L4 at each of the first to fourth design points DP₁ to DP₄ in the first direction (X-axis direction) and the solidification shrinkage amount 1SD_S1 to 1SD_S4 at each of the first to fourth design points DP1 to DP4 in the second direction are detected in operation S211 and S212.

[0114] When the solidification of the first molten steel is finished, a solidified material obtained by solidifying the first molten steel is removed from the parent mold 4000. Thereafter, the second molten steel is charged and solidified in the parent mold 4000 in operation S120. When solidification of the second molten steel begins, a solidification shrinkage amount 2SD_L1 to 2SD_L4 at each of the first to fourth design points DP₁ to DP₄ in the first direction (X-axis direction) and a solidification shrinkage amount 2SD_S1 to 2SD_S4 at each of the first to fourth design points DP₁ to DP₄ in the second direction are detected in operation S221 and S222.

[0115] Thereafter, an average shrinkage amount AS_L1, AS_L2, AS_L3, and AS_L4 between the first directional solidification shrinkage amount 1SD_L1 to 1SD_L4 detected at each of the first to fourth design points DP₁ to DP₄ during the solidification of the first molten steel and the second directional solidification shrinkage amount 2SD_L1 to 2SD_L4 detected at each of the first to fourth design points DP₁ to DP₄ during the solidification of the second molten steel is calculated in operation S310. That is, the first to fourth average shrinkage amounts in the first direction AS_L1 to AS_L4 may be obtained by calculating the average of the first to fourth solidification shrinkage amounts in the first direction 1SD_L1 to 1SD_L4 detected using the first molten steel and the first to fourth solidification shrinkage amounts in the first direction 2SD_L1 to 2SD_L4 detected using the second molten steel.

[0116] This is explained more specifically as follows. The first average shrinkage amount AS_L1 is calculated by averaging the first solidification shrinkage amount 1SD_L1 at the first design point DP₁ during the solidification of the first steel and the second solidification shrinkage amount 2SD_L1 at the first design point DP₁ during the solidification of the second molten steel. Also, the second average shrinkage amount AS_L2 is calculated by averaging the second shrinkage amount 1SD_L2 at the second design point DP₂ and the second shrinkage amount 2SD_L2 at the second design point DP₂ during the solidification of the first molten steel. Then, a third average shrinkage amount AS_L3 and a fourth average shrinkage AS_L4 are calculated in the same manner.

[0117] Referring to FIG. 9 and Table 2, in the first direction, the first average shrinkage amount AS_L1 may be 0.4 mm, the second average shrinkage amount AS_L2 may be 0.74 mm, the third average shrinkage amount AS_L3 may be 1.09 mm, and the fourth average shrinkage amount AS_L4 may be 1.36 mm.

[Table 2]

First shrinkage (ASL1)	average amount	Second shrinkage (ASL2)	average amount	Third shrinkage (ASL3)	average amount	Fourth shrinkage (ASL4)	average amount
0.4 mm		0.74 mm		1.08 mm		1.36 mm

[0118] Also, the average shrinkage amount AS_S1, AS_S2, AS_S3, and AS_S4 of the second directional shrinkage amount 1SD_S1 to 1SD_S4 detected at each of the first to fourth design points DP₁ to DP₄ during the solidification of the first molten steel and the second direction shrinkage amount 2SD_S1 to 2SD_S4 detected at each of the first to fourth design points DP₁ to DP₄ during the solidification of the second molten steel is calculated in operation S320. Since this is the same as the method for calculating the average shrinkage amount in the first direction described above, a detailed description and specific values will be omitted.

[0119] The first to fourth average shrinkage amounts AS_L1, AS_L2, AS_L3, and AS_L4 in the first direction and the first to fourth average shrinkage amounts AS_S1, AS_S2, AS_S3, and AS_S4 in the second direction obtained in the same manner are used to manufacture the mold. That is, using the first to fourth average shrinkage amounts AS_L1 to AS_L4 in the first direction, the first to fourth widths W_L1 to W_L4 of the first inner wall surface IF_L of the mold to be manufactured are designed in operation S410. Also, the first to fourth average shrinkage amounts AS_S1 to AS_S4 in the second direction are used to design the first to fourth widths W_S1 to W_S4 of the second inner wall surface IF_S in operation S420.

[0120] First, a method S410 for designing the first to fourth widths W_L1 to W_L4 of the first inner wall surface IF_L will be described in more detail as follows. The width at the first to fourth points P₁ to P₄ of the first inner wall surface IF_L is designed using the first to fourth average shrinkage amounts AS_L1 to AS_L4 in the first direction and the width W_LM in the first direction at the surface height P_M. That is, the first width W_L1 at the first point P₁ of the first inner wall surface IF_L is designed by subtracting the first average shrinkage AS_L1 from the width W_LM of the first direction at the surface height P_M of the molten steel (W_L1 =W_LM - AS_L1). Also, the second width W_L2 at the second point P₂ is designed by subtracting the second average shrinkage amount AS_L2 from the width W_LM of the first direction at the surface height P_M of the molten steel (W_L2 =W_LM - AS_L2). In the same manner, the third width W_L3 and the fourth width W_L4 are designed by subtracting the third average shrinkage AS_L3 and the fourth average shrinkage AS_L4, respectively, from the width W_LM of the first direction at the surface height P_M of the molten steel (W_L3 =W_LM-AS_L3, W_L4 =W_LM - AS_L4).

[0121] More specifically, an example in which a width W_LM at the surface height P_M of the first inner wall surface IF_L is 200 mm will be described. Since the first width W_L1 of the first point P₁ on the first inner wall surface IF_L is designed by subtracting the first average shrinkage amount AS_L1 (0.4 mm) from the width W_LM (200 mm) at the surface height P_M of the molten steel, the first width W_L1 is designed as 199.6 mm (200 mm-0.4 mm). Also, the second to fourth widths W_L2 to W_L4 may be designed by subtracting the second to fourth average shrinkage amounts AS_L2 to AS_L4 from the width W_LM at the surface height P_M of the molten steel in the same manner.

[0122] Also, the first to fourth widths W_S1, W_S2, W_S3, and W_S4 of the second inner wall surface IF_S are designed in the same manner as the first inner wall surface IF_L in operation S420. That is, the first width W_S1 of the first point P₁ on the second inner wall surface IF_S is designed by subtracting the first average shrinkage AS_S1 (mm) from the width W_LM at the surface height P_M. Also, each of the remaining second to fourth widths W_S2 through W_S4 is designed by subtracting the first, second, and third average shrinkage amounts AS_S2, AS_S3, and AS_S4 from the width W_LM at the surface height P_M of the molten steel, respectively.

[0123] Here, the width W_L4 and W_S4 of the fourth point P₄ at each of the first and second inner wall surfaces IF_L and IF_S may be designed by subtracting the fourth average shrinkage AS_L4 and AS_S4 minus 0.1 mm more from the width W_LM at the surface height P_M of the molten steel. This is to reduce friction between the first and second inner wall surfaces IF_L and IF_S and the cast piece at the bottom of the mold, thereby further extending a life of the mold.

[0124] In the same manner, when a design S410 of the first to fourth widths W_L1 to W_L4 of the first inner wall surface IF_L and a design S420 of the first to fourth widths W_S1 to W_S4 of the second inner wall surface IF_S are completed, a mold is manufactured by using the same in operation S500. That is, the body 3100 of the mold 3000 is manufactured so that the widths at the first to fourth points P₁ to P₄ of the first inner wall surface IF_L are the designed first to fourth widths W_L1 to W_L4, and the widths at the first to fourth points P₁ to P₄ of the second inner wall surface IFs are the designed first to fourth widths W_S1 to W_S4.

[0125] (a), (c), (e), (g), and (i) of FIG. 10 are photographs of the surface of the billet in accordance with the first to fifth experimental examples. (b), (d), (f), (h), and (j) of FIG. 10 are graphs showing roughness (mm) of the surface of the billet in the first to fifth experiments, i.e., a height (mm) of the surface.

[0126] In measuring the roughness, a tip of a roughness meter contacts the surface of the billet and measure a surface height while moving in a width direction of the billet. Also, in measuring the surface height while the tip moves in the width direction of the billet, a plurality of positions in the direction crossing the width direction, i.e., longitudinal direction, are measured. Also, an average height measured at the plurality of positions is shown in (b), (d), (f), (h), and (j) of FIG. 10.

[0127] Here, the billet in accordance with a first example is made from the parent mold. Also, the billet in accordance with second to fifth experimental examples is made from a mold in accordance with an embodiment. Also, the billet in accordance with any of the second to fifth experimental examples is cast by using the molten steel in which contents of at least carbon C and manganese Mn are different.

[Table 3]

	Carbon (C) wt%	Manganese (Mn) wt%
First experimental example	0.0981	0.451
Second experimental example	0.0929	0.46
Third experimental example	0.0935	0.447
Fourth experimental example	0.0863	0.449
Fifth experimental example	0.0897	0.425

[0128] Referring to (b) of FIG. 10, a large defect that depressed approximately 1.89 mm inward from a surface is detected in the billet manufactured from the parent mold. It may be known from (a) and (b) of FIG. 10 that the defect occurring at a point spaced approximately 120 mm from the one end (0 mm) in the width direction of the billet.

[0129] On the other hand, referring to (d), (f), (h), and (j) of FIG. 10, in the case of billets in accordance with examples 2 to 5, a size of a deepest recessed defect is less than 1 mm, which is significantly smaller than that of the first example. That is, the deepest defect is 0.25 mm in the second example, 0.71 mm in the third example, 0.94 mm in the fourth example, and 0.25 mm in the fifth example, all of which are smaller than the first example.

[0130] From this, it may be known that surface defects may be reduced when the cast piece is manufactured by using the mold in accordance with an exemplary embodiment. This is because, in the case of the mold in accordance with an exemplary embodiment, the widths W_L1 to W_L4 and W_S1 to W_S4 for each height of the inner wall surfaces IF_L and IF_S are designed and prepared by reflecting the different solidification shrinkage amount for each height, so that the solidification shrinkage amount for each height may be effectively compensated. Also, as the width change in the region between the surface height P_M of the molten steel and the first point P₁ is generated more than other regions, it is possible to effectively compensate for the solidification shrinkage in the upper portion of the body 3100 in which the solidified shell is shrunk the most.

[0131] Also, when the mold is manufactured, the solidification shrinkage rate is detected by solidifying the plurality kinds of steel, i.e., molten steel, and the width of the inner wall surface for each height is designed by reflecting the average value of the solidification shrinkage rate. Thus, in the case of the mold in accordance with an exemplary embodiment, the cast piece having a suppressed or prevented solidification shrinkage rate for the plurality kinds of steel may be manufactured.

INDUSTRIAL APPLICABILITY

[0132] In accordance with an exemplary embodiment, the compensation rate for solidified shell shrinkage may be improved. That is, as the mold is manufactured by designing the width for each height of the mold according to the different solidification shrinkage amounts for each height, the compensation rate for solidification shrinkage for each height is improved. Thus, the defect caused by the solidified shell shrinkage may be suppressed or prevented from occurring on the surface of the cast piece.

Claims

1. A method for manufacturing a mold, comprising:

solidifying molten steel in a first mold;

detecting a solidification shrinkage amount (SD) occurring when the molten steel is solidified in the first mold for each height of the first mold;

designing a width for each height of a second mold to be manufactured by using the detected solidification shrinkage amount (SD) for each height; and

preparing the second mold so that a width (W) for each height of the second mold is equal to the designed width for each height.

2. The method of claim 1, wherein the detecting of the solidification shrinkage amount (SD) for each height of the first mold comprises:

setting a plurality of design points (DP) having different heights on an inner wall surface of the first mold below a surface height (P_M) of the molten steel;

detecting a solidification width (SW) by detecting a length between both ends in a width direction of a solidified shell formed by solidification of the molten steel at each of the plurality of design points (DP) set on the first mold; and

calculating the solidification shrinkage amount (SD) at each of the plurality of designed points (DP) by subtracting the detected solidification width (SW) at each of the plurality of design points (DP) from a width (W_M) at the surface height (P_M) on the inner wall surface of the first mold.

3. The method of claim 2, wherein the setting of the plurality of designed points (DP) on the first mold is set so that distances between the plurality of design points (DP) gradually increase in a downward direction.

4. The method of claim 3, wherein the setting of the plurality of designed points (DP) on the first mold is set so that the distances between the plurality of design points (DP) gradually increase with a constant value.

5. The method of claim 4, wherein the plurality of design points (DP) set on the first mold comprise first to fourth design points (DPi to DP₄) that are sequentially and gradually spaced apart from the surface height (P_M) in the downward direction, and
a distance (G₁) between the surface height (P_M) and the first design point (DP₁), a distance (G₂) between the first design point (DP₁) and the second design point (DP₂), a distance (G₃) between the second design point (DP₂) and the third design point (DP₃), a distance (G₄) between the third design point (DP₃) and the fourth design point (DP₄) gradually increase with a constant rate in a direction from the first distance (G₁) to the fourth distance (G₄).

6. The method of claim 2, wherein the designing of the width for each height comprises:

setting a plurality of points (P) on the inner wall surface of the second mold at the same positions as the plurality of design points (DP) set on the first mold;

subtracting (W_M - SD) the solidification shrinkage amount (SD) for each of the plurality of design points (DP) from a width (W_M) of the inner wall surface at the surface height (P_M) of the first mold; and

determining a subtracted (W_M - SD) value at each of the plurality of design points (DP) as a width (W) at each of the plurality of points (P) set on the inner wall surface of the second mold.

7. The method of claim 6, wherein the detecting of the solidification shrinkage amount (SD) at each of the plurality of points (P) comprises:

detecting a solidification shrinkage amount (1 SD) of first molten steel at the plurality of design points (DP) by supplying and solidifying the first molten steel into the first mold;

detecting a solidification shrinkage amount (2SD) of second molten steel at the plurality of design points (DP) by supplying and solidifying the second molten steel into the first mold; and

calculating an average solidification shrinkage amount (AS) of the solidification shrinkage amount (1SD) of the first molten steel and the solidification shrinkage amount (2SD) of the second molten steel for each of the plurality of design points (DP),

wherein, in the subtracting (W_M - SD) of the solidification shrinkage amount (SD) for each of the plurality of design points (DP) from the width (W_M) of the inner wall surface at the surface height (P_M) of the first mold,

the solidification shrinkage amount (SD) for each of the plurality of design points (DP) is an average solidification shrinkage amount (AS) of each of the plurality of design points (DP).

8. The method of claim 7, further comprising selecting the first and second molds,
wherein the selecting of the first and second molds comprises:

supplying and solidifying each of a plurality of molten steel into the first mold;

detecting a solidification shrinkage amount occurring when each of the plurality of molten steel is solidified; and

selecting molten steel having a largest solidification shrinkage amount among the detected solidification shrinkage amounts as first molten steel and molten steel having a smallest solidification shrinkage amount steel as second molten steel.

9. A mold having an inner space into which molten steel is injected, comprising a body having an inner space,

wherein a plurality of points having different heights are set on an inner wall surface of the body,

a width at each of the plurality of points in the inner wall surface of the body gradually decreases in a downward direction, and

a decrease rate by which the width decreases is varied at the plurality of points in a height direction.

10. The mold of claim 9, wherein distances between the plurality of points in the height direction are different.

11. The mold of claim 10, wherein the distances between the plurality of points in the height direction gradually increase in the downward direction.

12. The mold of claim 11, wherein the distances between the plurality of points in the height direction increase with a constant value.

13. The mold of claim 12, wherein the plurality of points comprise first to fourth points that are sequentially spaced downward from a surface height of the molten steel supplied into the body, and
a distance (G₁) between the surface height and a first point, a distance (G₂) between the first point and a second point, a distance (G₃) between the second point and a third point, a distance (G₄) between the third point and a fourth point gradually increase with a constant rate in a direction from the first distance (G₁) to the fourth distance (G₄).

14. The mold of claim 9, wherein the inner wall surface of the body is an inclined surface that is gradually spaced apart from an outer surface that is an opposite surface of the inner wall surface in the downward direction, and
an inclination of the inner wall surface of the body is varied by using the plurality of points as inflection points.

15. The mold of claim 9, wherein a width at each of the plurality of points is designed by using a solidification shrinkage amount for each height obtained while the molten steel is solidified using a parent mold for designing the mold.

Drawing