METHOD FOR MANUFACTURING HOT-ROLLED COIL, AND METHOD FOR SHAPE-CORRECTION OF HOT-ROLLED COIL

Abstract

Embodiments include a method for manufacturing a hot-rolled coil and a method for correcting the shape of a hot-rolled coil. In one embodiment, the method for manufacturing the hot-rolled coil includes the steps of: reheating a steel slab comprising 0.18 to 0.56 wt % carbon (C), 0.1 to 0.5 wt % silicon (Si), 0.7 to 6.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.02 wt % sulfur (S), more than 0 wt % but not more than 0.3 wt % chromium (Cr), more than 0 wt % but not more than 0.004 wt % boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities; hot-rolling the steel slab at a finishing mill delivery temperature of 850° C. to 950° C., thereby forming a hot-rolled sheet; and cooling the hot-rolled sheet, followed by coiling at a coiling temperature of 700° C. or higher.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/KR2017/007870, filed Jul. 21, 2017, which claims the benefit of and priority to Korean Patent Application No. 10-2016-0093096 filed on Jul. 22, 2016, the entire content of each being incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a method for manufacturing a hot-rolled coil and a method for correcting the shape of a hot-rolled coil. More specifically, the present invention relates to a method for manufacturing a hot-rolled coil for preventing shape defects, which may prevent shape defects from being caused by self-weight during manufacturing of the hot-rolled coil, and to a method for correcting the shape of a hot-rolled coil.

BACKGROUND

In recent years, ensuring light weight has been considered to be an important factor in the development of automobile materials. This is intended to replace existing parts with high-strength materials, thereby ultimately improving fuel efficiency. To this end, materials that are structural materials for automobiles have been developed so as to improve performance by adding alloying elements, including manganese (Mn), nickel (Ni), chromium (Cr), molybdenum (Mo), titanium (Ti) and the like, and cold rolling and heat-treatment processes have been applied to ensure the strength of steel.

Background art related to the present invention is disclosed in Korean Patent Application Publication No. 1995-0016913 (published on Jul. 20, 1995; entitled “Telescopic correction apparatus for hot-rolled coil”).

SUMMARY

Technical Problem

One embodiment of the present invention is intended to provide a method for manufacturing a hot-rolled coil, which has an excellent effect of preventing the deformation of the hot-rolled coil.

Another embodiment of the present invention is intended to provide a method for correcting the shape of a hot-rolled coil, which may prevent deterioration in the material and physical properties of the hot-rolled coil.

Still another embodiment of the present invention is intended to provide a method for correcting the shape of a hot-rolled coil, which may prevent surface defects of the hot-rolled coil from occurring when correcting the shape by application of an external force.

Yet another embodiment of the present invention is intended to provide a method for correcting the shape of a hot-rolled coil, which has excellent economic efficiency.

Technical Solution

One aspect of the present invention is directed to a method for manufacturing a hot-rolled coil. In one embodiment, the method for manufacturing the hot-rolled coil includes the steps of: reheating a steel slab including 0.18 to 0.56 wt % carbon (C), 0.1 to 0.5 wt % silicon (Si), 0.7 to 6.5 wt % manganese (Mn), more than 0 but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.02 wt % sulfur (S), more than 0 wt % but not more than 0.3 wt % chromium (Cr), more than 0 wt % but not more than 0.004 wt % boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities; hot-rolling the steel slab at a finishing mill delivery temperature of 850° C. to 950° C., thereby forming a hot-rolled sheet; and cooling the hot-rolled sheet, followed by coiling at a coiling temperature of 700° C. or higher.

In one embodiment, the steel slab may include 0.21 to 0.37 wt % carbon (C), 0.1 to 0.4 wt % silicon (Si), 1.1 to 1.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.02 wt % sulfur (S), 0.1 to 0.3 wt % chromium (Cr), 0.001 to 0.004 wt % boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities.

In one embodiment, the steel slab may include 0.18 to 0.25 wt % carbon, 0.3 to 0.5 wt % silicon (Si), 2 to 6.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.01 wt % sulfur (S), more than 0 wt % but not more than 0.1 wt % chromium (Cr), more than 0 wt % but not more than 0.001 wt % boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities.

In one embodiment, the steel slab may include 0.5 to 0.56 wt % carbon (C), 0.1 to 0.3 wt % silicon (Si), 0.7 to 1 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.01 wt % sulfur (S), 0.1 to 0.3 wt % chromium (Cr), more than 0 wt % but not more than 0.001 wt % boron (B), 0.01 to 0.02 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities.

Another aspect of the present invention is directed to a method for correcting the shape of a hot-rolled coil. In one embodiment, the method for correcting the shape of the hot-rolled coil includes the steps of: mounting the hot-rolled coil on a hanger forming the lower part of a C-hook; measuring the longest diameter of the hot-rolled coil using an outer diameter measuring means provided in the upper part of the C-hook; adjusting the longest diameter of the hot-rolled coil to be perpendicular to the C-hook by means of a driving roll provided on the hanger; and placing the C-hook, which has the hot-rolled coil mounted thereon, on a stand, followed by lifting, thereby correcting the shape of the hot-rolled coil by self-weight.

Still another aspect of the present invention is directed to a method for correcting the shape of a hot-rolled coil. In one embodiment, the method for correcting the shape of the hot-rolled coil includes the steps of: mounting the hot-rolled coil on a hanger forming the lower part of a C-hook; measuring the longest diameter of the hot-rolled coil using an outer diameter measuring means provided in the upper part of the C-hook; adjusting the longest diameter of the hot-rolled coil to be perpendicular to the C-hook by means of a driving roll provided on the lower hanger; and placing the C-hook, which has the hot-rolled coil mounted thereon, on a stand, followed by lifting, thereby correcting the shape of the hot-rolled coil by self-weight, wherein the hot-rolled coil is manufactured by a method including the steps of: reheating a steel slab including 0.18 to 0.56 wt % carbon (C), 0.1 to 0.5 wt % silicon (Si), 0.7 to 6.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.02 wt % sulfur (S), more than 0 wt % but not more than 0.3 wt % chromium (Cr), more than 0 wt % but not more than 0.004 wt % boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities; hot-rolling the steel slab at a finishing mill delivery temperature of 850° C. to 950° C., thereby forming a hot-rolled sheet; and cooling the hot-rolled sheet, followed by coiling at a coiling temperature of 700° C. or higher.

In one embodiment, the hot-rolled coil may include 0.21 to 0.37 wt % carbon (C), 0.1 to 0.4 wt % silicon (Si), 1.1 to 1.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.02 wt % sulfur (S), 0.1 to 0.3 wt % chromium (Cr), 0.001 to 0.004 wt % boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities.

In one embodiment, the hot-rolled sheet may be cooled and coiled at a coiling temperature of 700° C. to 900° C.

Advantageous Effects

When shape correction is performed for a hot-rolled coil manufactured by the method for manufacturing the hot-rolled coil according to the present invention, it may delay the phase transformation of the steel during cooling after hot rolling, thereby preventing deterioration in the material and physical properties of the hot-rolled coil while exhibiting an excellent effect of preventing deformation of the hot-rolled coil. In addition, the use of correction by self-weight and gravity makes it possible to prevent surface defects (such as scratches) of the hot-rolled coil, which occur when correction by an external force is used. In addition, it may reduce the correction cost and provide excellent economic efficiency.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a method for manufacturing a hot-rolled coil according to one embodiment of the present invention.

FIG. 2 shows a method for correcting the shape of a hot-rolled coil according to one embodiment of the present invention.

FIG. 3 schematically shows a method for correcting the shape of a hot-rolled coil according to one embodiment of the present invention.

FIG. 4(a) is a photograph showing a hot-rolled coil according to one example of the present invention immediately after coiling, and FIG. 4(b) is a photograph showing the hot-rolled coil after air cooling.

FIG. 5(a) is a photograph showing a hot-rolled coil according to another example of the present invention immediately after coiling, and FIG. 5(b) is a photograph showing the hot-rolled coil after air cooling.

FIG. 6(a) is a photograph showing a hot-rolled coil of a comparative example for the present invention immediately after coiling, and FIG. 6(b) is a photograph showing the hot-rolled coil after air cooling.

FIG. 7 is a graph comparing the phase transformation curves of hot-rolled coils with the passage of hot-rolled coil manufacturing time and shape correction time in an example of the present invention and a comparative example for the present invention.

MODE FOR INVENTION

Detailed Description

Hereinafter, the present invention will be described in detail. In the following description of the present invention, the detailed description of related known technologies or configurations will be omitted when it may unnecessarily obscure the subject matter of the present invention.

In addition, the terms used in the following description are defined in consideration of their functions in the present invention and may vary depending on a user's or operator's intension or usual practice. Accordingly, the definition should be made based on the contents through the specification that describes the present invention.

Method for Manufacturing Hot-Rolled Coil

One aspect of the present invention is directed to a method for manufacturing a hot-rolled coil. FIG. 1 shows a method for manufacturing a hot-rolled coil according to one embodiment of the present invention. In one embodiment, the method for manufacturing the hot-rolled coil includes the steps of: (S10) reheating a steel slab; (S20) hot rolling; and (S30) coiling. More specifically, the method for manufacturing the hot-rolled coil includes the steps of: (S10) reheating a steel slab including 0.18 to 0.56 wt % carbon (C), 0.1 to 0.5 wt % silicon (Si), 0.7 to 6.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.02 wt % sulfur (S), more than 0 wt % but not more than 0.3 wt % chromium (Cr), more than 0 wt % but not more than 0.004 wt % boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities; (S20) hot-rolling the steel slab at a finishing mill delivery temperature of 850° C. to 950° C., thereby forming a hot-rolled sheet; and (S30) cooling the hot-rolled sheet, followed by coiling at a coiling temperature of 700° C. or higher.

Hereinafter, each step of the method for manufacturing the hot-rolled coil according to the present invention will be described in detail.

(S10) Steel Slab Reheating Step

This step is a step of reheating a steel slab including 0.18 to 0.56 wt % carbon (C), 0.1 to 0.5 wt % silicon (Si), 0.7 to 6.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.02 wt % sulfur (S), more than 0 wt % but not more than 0.3 wt % chromium (Cr), more than 0 wt % but not more than 0.004 wt % boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities.

Hereinafter, the components included in the steel slab will be described in detail.

Carbon (C)

Carbon (C) is added to ensure strength. Carbon is contained in an amount of 0.18 to 0.56 wt % based on the total weight of the steel slab. If the content of carbon is less than 0.18 wt %, it may be difficult to ensure sufficient strength. On the other hand, if the content of carbon is more than 0.56 wt %, toughness may be reduced.

Silicon (Si)

Silicon (Si) functions as a deoxidizer for removing oxygen from the steel and is added for solid solution strengthening. In one embodiment, silicon is contained in an amount of 0.1 to 0.5 wt % based on the total weight of the steel slab. If the content of silicon is less than 0.1 wt %, the effect of adding the same will be insufficient, and if the content of silicon is more than 0.5 wt %, it may reduce weldability and produce red scale during reheating and hot rolling, thus adversely affecting the surface quality. In addition, it may adversely affect the coating property after welding.

Manganese (Mn)

Manganese (Mn) is a solid solution strengthening element which is effective in ensuring strength by increasing the hardenability of the steel. In addition, manganese is an austenite stabilizing element which contributes to ferrite grain refinement by delaying ferrite and pearlite transformation.

In one embodiment, manganese is contained in 0.7 to 6.5 wt % based on the total weight of the steel slab. If the content of manganese is less than 0.7 wt %, the solid solution strengthening effect may be insufficient. On the other hand, the content of manganese is more than 6.5 wt %, weldability may be greatly reduced. In addition, a problem may arise in that the ductility of the steel sheet is greatly reduced due to the formation of an MnS inclusion and the occurrence of central segregation.

Phosphorus (P)

Phosphorus (P) is added to inhibit cementite formation and increase strength. However, phosphorus deteriorates weldability and causes a difference in final properties by slab center segregation. For this reason, in the present invention, the content of phosphorus (P) is limited to more than 0 wt % but not more than 0.02 wt % based on the total weight of the steel slab.

Sulfur

Sulfur (S) is an element that reduces the toughness and weldability of the steel and binds to manganese to form a non-metallic inclusion (MnS) that causes cracks during processing of the steel. For this reason, the content of sulfur (S) is limited to more than 0 wt % but not more than 0.02 wt % based on the total weight of the steel slab.

Chromium (Cr)

Chromium is added for the purpose of increasing the hardenability and strength of the steel. In one embodiment, chromium is contained in an amount of more than 0 wt % but not more than 0.3 wt % based on the total weight of the steel slab. If the content of chromium is more than 0.3 wt %, the toughness of the hot-rolled coil may be reduced.

Boron (B)

Boron (B) is added for the purpose of compensating for hardenability by replacing the expensive hardening element molybdenum, and has the effect of refining grains by increasing the austenite grain growth temperature.

In one embodiment, boron is contained in an amount of more than 0 wt % but not more than 0.004 wt % based on the total weight of the steel slab. If boron is contained in an amount of more than 0.004 wt %, the risk of reducing elongation may increase.

Titanium (Ti)

Titanium (Ti) is added for the purpose of enhancing hardenability and improving properties by precipitate formation. In addition, it effectively contributes to austenite grain refinement by forming precipitate phases such as Ti(C,N) at high temperature.

In one embodiment, titanium is contained in an amount of 0.01 to 0.04 wt % based on the total weight of the steel slab. If titanium is contained in an amount of less than 0.01 wt %, the effect of adding the same may be insufficient, and if titanium is contained in an amount of more than 0.04 wt %, continuous casting defects may occur, it may be difficult to ensure the physical properties of the hot-rolled coil, and cracks on the surface of the hot-rolled coil may occur.

The remainder other than the above-described components is substantially composed of iron (Fe). As used herein, the expression “remainder is substantially composed of iron (Fe)” means that one containing other trace elements, including inevitable impurities, may be included in the present invention, as long as it does not impair the effect of the present invention.

In one embodiment, the steel slab may be applied to a medium-carbon hot-rolled coil. For example, the steel slab may include 0.21 to 0.37 wt % carbon (C), 0.1 to 0.4 wt % silicon (Si), 1.1 to 1.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.02 wt % sulfur (S), 0.1 to 0.3 wt % chromium (Cr), 0.001 to 0.004 wt % boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities.

In another embodiment, the steel slab may be applied to a high-manganese hot-rolled coil. For example, the steel slab may include 0.18 to 0.25 wt % carbon, 0.3 to 0.5 wt % silicon (Si), 2 to 6.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.01 wt % sulfur (S), more than 0 wt % but not more than 0.1 wt % chromium (Cr), more than 0 wt % but not more than 0.001 wt % boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities.

In still another embodiment, the steel slab may be applied to a high-carbon hot-rolled coil. For example, the steel slab may include 0.5 to 0.56 wt % carbon (C), 0.1 to 0.3 wt % silicon (Si), 0.7 to 1 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.01 wt % sulfur (S), 0.1 to 0.3 wt % chromium (Cr), more than 0 wt % but not more than 0.001 wt % boron (B), 0.01 to 0.02 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities.

In one embodiment, the steel slab may be heated at a slab reheating temperature (SRT) of 1,150° C. to 1,250° C. At this slab reheating temperature, the effect of homogenizing the alloying elements may be excellent.

(S20) Hot-Rolling Step

This step is a step of hot-rolling the steel slab at a finishing mill delivery temperature of 850° C. to 950° C., thereby forming a hot-rolled sheet. When hot rolling is performed at this finishing mill delivery temperature, the hot-rolled coil may have both excellent rigidity and excellent moldability and is excellent in terms of coiling workability, and the effect of preventing deformation of the hot-rolled coil may be excellent.

(S30) Coiling Step

This step is a step of cooling the hot-rolled sheet, followed by coiling at a coiling temperature of 700° C. or higher. In one embodiment, the hot-rolled sheet may be cooled to the coiling temperature and coiled at that temperature. In one embodiment, the cooling may be performed by air cooling without using cooling water. When the cooling is performed under the above-described conditions, the occurrence of bulging defects on the hot-rolled coil may be effectively reduced. As used herein, the “bulging defects” may refer to shape distortion defects of the hot-rolled coil. Specifically, the “bulging defects” may refer to shape distortion defects caused by the change of the inner and outer diameters of the hot-rolled coil to an ellipse rather than a circle, due to the distortion of the hot-rolled coil in the direction of gravity, among shape defects that occur on the hot-rolled coil.

After the sheet including the alloying components of the present invention is hot-rolled, the control of cooling may be performed such that the coiling is completed at a temperature equal to or higher than a temperature at which phase transformation begins. When coiling is performed at the above-described coiling temperature, ferrite phase transformation begins after a certain time after the coiling, and for this reason, the time taken for phase transformation to be completed may increase rapidly due to slow cooling (air cooling) of the coil after the coiling, thereby advantageously preventing shape deformation. Namely, one embodiment of the present invention may provide process conditions that delay the time point of occurrence of phase transformation after coiling as much as possible.

If the hot-rolled sheet is coiled at a coiling temperature lower than 700° C., phase transformation of the hot-rolled sheet may proceed in the cooling process, and additional phase transformation may occur after formation of the hot-rolled coil, resulting in an increase in the coil volume, and then the hot-rolled coil may shrink with lowering temperature and the shape thereof is deformed to an elliptical shape by self-weight, thus causing bulging defects. In one embodiment, the hot-rolled sheet may be cooled and coiled at a coiling temperature of 700° C. to 900° C. For example, the coiling may be performed at a coiling temperature of 730° C. to 820° C. The manufactured hot-rolled coil may include ferrite and bainite microstructures.

Method for Correcting Shape of Hot-Rolled Coil

Another aspect of the present invention is directed to a method for correcting the shape of a hot-rolled coil. FIG. 2 shows a method for correcting the shape of a hot-rolled coil according to one embodiment of the present invention. Referring to FIG. 2, the method for correcting the shape of the hot-rolled coil includes the steps of: (S101) mounting the hot-rolled coil; (S102) measuring the longest diameter of the hot-rolled coil; (S103) adjusting the position of the hot-rolled coil; and (S104) lifting.

FIG. 3 schematically shows a method for correcting the shape of a hot-rolled coil according to one embodiment of the present invention. Referring to FIG. 3, the method for correcting the shape of the hot-rolled coil includes the steps of: (S101) mounting the hot-rolled coil on a hanger forming the lower part of a C-hook; (S102) measuring the longest diameter of the hot-rolled coil using an outer diameter measuring means provided in the upper part of the C-hook; (S103) adjusting the longest diameter of the hot-rolled coil to be perpendicular to the C-hook by means of a driving roll provided on the lower hanger; and (S104) placing the C-hook, which has the hot-rolled coil mounted thereon, on a stand, followed by lifting, thereby correcting the shape of the hot-rolled coil by self-weight.

For example, as shown in FIG. 3(a), a hot-rolled coil 100 is mounted on a hanger 201 forming the lower part of a C-hook 200. As shown in FIG. 3(b), the longest diameter of the hot-rolled coil 100 is measured using an outer diameter measuring means 210 provided in the upper part 202 of the C-hook. Next, as shown in FIG. 3(c), using a driving roll 220 provided on the hanger 201 forming the lower part, the longest diameter of the hot-rolled coil 100 is adjusted perpendicular to the C-hook. As shown in FIG. 3(e), the C-hook 200 having the hot-rolled coil mounted thereon is placed on a stand 300 and lifted, and thus as shown in FIG. 3(f), the hot-rolled coil shape distorted into an elliptical shape may be corrected into a circular shape by self-weight.

The hot-rolled coil is manufactured by a method including the steps of: reheating a steel slab including 0.18 to 0.56 wt % carbon (C), 0.1 to 0.5 wt % silicon (Si), 0.7 to 6.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.02 wt % sulfur (S), more than 0 wt % but not more than 0.3 wt % chromium (Cr), more than 0 wt % but not more than 0.004 wt % boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities; hot-rolling the steel slab at a finishing mill delivery temperature of 850° C. to 950° C., thereby forming a hot-rolled sheet; and cooling the hot-rolled sheet, followed by coiling at a coiling temperature of 700° C. or higher. In one embodiment, the hot-rolled coil may be manufactured by cooling the hot-rolled sheet, followed by coiling at a coiling temperature of 700° C. to 900° C. The manufactured hot-rolled coil may include ferrite and bainite microstructures.

The method for manufacturing the hot-rolled coil may be performed using the same steel slab as used in the above-described method for manufacturing the hot-rolled coil, and thus the detailed description thereof is omitted.

In one embodiment, the hot-rolled coil may be a medium-carbon hot-rolled material. It may include 0.21 to 0.37 wt % carbon (C), 0.1 to 0.4 wt % silicon (Si), 1.1 to 1.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.02 wt % sulfur (S), 0.1 to 0.3 wt % chromium (Cr), 0.001 to 0.004 wt % boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities.

In another embodiment, the hot-rolled coil may be a high-manganese hot-rolled material. It may include 0.18 to 0.25 wt % carbon, 0.3 to 0.5 wt % silicon (Si), 2 to 6.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.01 wt % sulfur (S), more than 0 wt % but not more than 0.1 wt % chromium (Cr), more than 0 wt % but not more than 0.001 wt % boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities.

In still another embodiment, the hot-rolled coil may be a high-carbon hot-rolled material. It may include 0.5 to 0.56 wt % carbon (C), 0.1 to 0.3 wt % silicon (Si), 0.7 to 1 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.01 wt % sulfur (S), 0.1 to 0.3 wt % chromium (Cr), more than 0 wt % but not more than 0.001 wt % boron (B), 0.01 to 0.02 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities.

When shape correction is performed for a hot-rolled coil manufactured by the method for manufacturing the hot-rolled coil according to the present invention, it may prevent the phase transformation of the steel during cooling after hot rolling, thereby preventing deterioration in the material and physical properties of the hot-rolled coil while exhibiting an excellent effect of preventing deformation of the hot-rolled coil. In addition, the use of correction by self-weight and gravity makes it possible to prevent surface defects (such as scratches) of the hot-rolled coil, which occur when correction by an external force is used. In addition, it may exclude an existing correction apparatus employing an external force, thus reducing the correction cost and providing excellent economic efficiency.

Hereinafter, the constitution and effects of the present invention will be described in more detail with reference to preferred examples. However, these examples are given merely as illustrative of the present invention and are not to be construed as limiting the scope of the present invention in any way.

EXAMPLES AND COMPARATIVE EXAMPLES

Example 1

As a medium-carbon material, a steel slab including 0.23 wt % carbon (C), 0.2 wt % silicon (Si), 1.2 wt % manganese (Mn), 0.015 wt % phosphorus (P), 0.01 wt % sulfur (S), 0.2 wt % chromium (Cr), 0.003 wt % boron (B), 0.02 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities, was reheated at 1200° C., and the steel slab was hot-rolled at a finishing mill delivery temperature of 880° C., thereby forming a hot-rolled sheet. Then, the hot-rolled sheet was cooled and coiled at a coiling temperature of 700° C., thereby manufacturing a hot-rolled coil.

Examples 2

As a high-manganese material, a steel slab including 0.2 wt % carbon (C), 0.4 wt % silicon (Si), 6 wt % manganese (Mn), 0.015 wt % phosphorus (P), 0.01 wt % sulfur (S), 0.05 wt % chromium (Cr), 0.001 wt % boron (B), 0.02 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities, was reheated at 1200° C., and the steel slab was hot-rolled at a finishing mill delivery temperature of 940° C., thereby forming a hot-rolled sheet. Then, the hot-rolled sheet was cooled and coiled at a coiling temperature of 700° C., thereby manufacturing a hot-rolled coil.

Example 3

As a high-carbon material, a steel slab including 0.55 wt % carbon (C), 0.2 wt % silicon (Si), 0.8 wt % manganese (Mn), 0.015 wt % phosphorus (P), 0.01 wt % sulfur (S), 0.2 wt % chromium (Cr), 0.001 wt % boron (B), 0.01 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities, was reheated at 1200° C., and the steel slab was hot-rolled at a finishing mill delivery temperature of 890° C., thereby forming a hot-rolled sheet. Then, the hot-rolled sheet was cooled and coiled at a coiling temperature of 730° C., thereby manufacturing a hot-rolled coil.

Comparative Example 1

A hot-rolled coil was manufactured in the same manner as described in Example 1, except that the hot-rolled sheet was coiled at a coiling temperature of 560° C.

Comparative Example 2

A hot-rolled coil was manufactured in the same manner as described in Example 1, except that the hot-rolled sheet was coiled at a coiling temperature of 600° C.

Comparative Example 3

A hot-rolled coil was manufactured in the same manner as described in Example 1, except that the hot-rolled sheet was coiled at a coiling temperature of 620° C.

Comparative Example 4

A hot-rolled coil was manufactured in the same manner as described in Example 1, except that the hot-rolled sheet was coiled at a coiling temperature of 650° C.

FIG. 4(a) is a photograph showing the hot-rolled coil of Example 1 according to the present invention immediately after coiling, and FIG. 4(b) is a photograph showing the hot-rolled coil after air cooling. FIG. 5(a) is a photograph showing the hot-rolled coil of Example 1 according to the present invention immediately after coiling, and FIG. 5(b) is a photograph showing the hot-rolled coil after air cooling. FIG. 6(a) is a photograph showing a hot-rolled coil of a comparative example for the present invention immediately after coiling, and FIG. 6(b) is a photograph showing the hot-rolled coil after air cooling. Referring to FIGS. 4(a) and 4(b), in Example 1, bulging defects were not observed immediately after coiling of the hot-rolled coil, but bulging defects were observed after air cooling. However, it could be seen that the degree of bulging defects was smaller than that in the Comparative Example. Referring to FIGS. 5(a) and 5(b), in Example 2, bulging defects were not observed immediately after coiling of the hot-rolled coil and after air cooling. Referring to FIGS. 6(a) and 6(b), in the Comparative Example, bulging defects were observed immediately after coiling of the hot-rolled coil, and it could be seen that the degree of the bulging defects become more severe as air cooling proceeded.

Correction of Shape of Hot-Rolled Coil

For the hot-rolled coils of Examples 1 to 3 and Comparative Examples 1 to 4, shape correction was performed. Each of the hot-rolled coil was mounted on a hanger forming the lower part of a C-hook, and the longest diameter of the hot-rolled coil was measured using an outer diameter measuring means provided on the upper part of the C-hook. Thereafter, using a driving roll provided on the hanger, the longest diameter of the hot-rolled coil was adjusted to be perpendicular to the C-hook. The C-hook having the hot-rolled coil mounted thereon was placed on a stand and lifted, thereby correcting the shape of the hot-rolled coil by self-weight.

For Examples 1 to 3 and Comparative Examples 1 to 4, the inner diameter of the coils and whether bulging defects would be corrected after shape correction were observed, and the results of the observation are shown in Table 1 below.

	TABLE 1
			Whether bulging
	Coiling	Coil inner	defects would be
	temperature	diameter	corrected by shape
	(° C.)	(mm)	correction
Example 1	700	740	Corrected
Example 2	730	760	Corrected
Example 3	730	740	Corrected
Comparative	560	700	Not corrected
Example 1
Comparative	600	710	Not corrected
Example 2
Comparative	620	680	Not corrected
Example 3
Comparative	650	720	Not corrected
Example 4

Referring to Table 1 above, it could be seen that, in the case of Examples 1 to 3, bulging defects did not appear after correction, but in the case of Comparative Examples 1 to 4 which were out of the coiling temperature of the present invention, bulging defects were not properly corrected even after correction.

FIG. 7 is a graph comparing the phase transformation curves of hot-rolled coils with the passage of hot-rolled coil manufacturing time and shape correction time in Example 1 and Comparative Example 1. Referring to FIG. 7, in the case of Example 1 of the present invention, in which a specific alloying element system was applied and coiling was performed at a temperature (700° C.) equal to or higher than the phase transformation temperature, and thus phase transformation to ferrite proceeded after a certain time after manufacturing of the hot-rolled coil, it could be seen that the time taken for phase transformation to be completed increased rapidly due to slow cooling (air cooling) of the coil after coiling, indicating that Example 1 was advantageous for shape correction. However, in the case of Comparative Example 1 in which coiling was performed at a temperature lower than the phase transformation temperature of the hot-rolled sheet, it could be seen that phase transformation to ferrite occurred earlier than in Example 1, making it difficult to ensure the time of start of the phase transformation of the present invention, indicating that Comparative Example 1 was disadvantageous for shape correction.

In addition, in accordance with the methods for manufacturing the hot-rolled coil and correcting the shape of the hot-rolled coil according to the present invention, the occurrence of bulging of the hot-rolled coil could be reduced, thereby reducing additional operations caused by breakage of the inner coil part, delayed operation time, facility breakage, etc., which would occur due to the bulging coil in a subsequent correction process, thereby providing effects, including increased work efficiency, increased material quality, reduced rate of occurrence of defective products disposed of as scrap, etc.

Simple modifications or alterations of the present invention may be easily made by those skilled in the art, and such modifications or alterations may be considered to be all included within the scope of the present invention.

Claims

1. A method for manufacturing a hot-rolled coil, the method comprising the steps of:

reheating a steel slab to form a reheated steel slab, the steel slab comprising 0.18 wt % to 0.56 wt % carbon (C), 0.1 wt % to 0.5 wt % silicon (Si), 0.7 wt % to 6.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.02 wt % sulfur (S), more than 0 wt % but not more than 0.3 wt % chromium (Cr), more than 0 wt % but not more than 0.004 wt % boron (B), 0.01 wt % to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities;

hot-rolling the reheated steel slab at a finishing mill delivery temperature of 850° C. to 950° C., thereby forming a hot-rolled sheet; and

cooling the hot-rolled sheet, followed by coiling at a coiling temperature of 700° C. or higher.

2. The method of claim 1, wherein the steel slab comprises 0.21 wt % to 0.37 wt % carbon (C), 0.1 wt % to 0.4 wt % silicon (Si), 1.1 wt % to 1.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.02 wt % sulfur (S), 0.1 wt % to 0.3 wt % chromium (Cr), 0.001 wt % to 0.004 wt % boron (B), 0.01 wt % to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities.

3. The method of claim 1, wherein the steel slab comprises 0.18 wt % to 0.25 wt % carbon, 0.3 wt % to 0.5 wt % silicon (Si), 2 wt % to 6.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.01 wt % sulfur (S), more than 0 wt % but not more than 0.1 wt % chromium (Cr), more than 0 wt % but not more than 0.001 wt % boron (B), 0.01 to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities.

4. The method of claim 1, wherein the steel slab comprises 0.5 wt % to 0.56 wt % carbon (C), 0.1 wt % to 0.3 wt % silicon (Si), 0.7 wt % to 1 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.01 wt % sulfur (S), 0.1 wt % to 0.3 wt % chromium (Cr), more than 0 wt % but not more than 0.001 wt % boron (B), 0.01 to 0.02 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities.

5. The method of claim 1, wherein the hot-rolled sheet is cooled and coiled at a coiling temperature of 700° C. to 900° C.

6. A method for correcting a shape of a hot-rolled coil, the method comprising the steps of:

mounting the hot-rolled coil on a hanger forming a lower part of a C-hook;

measuring a longest diameter of the hot-rolled coil using an outer diameter measuring means provided in an upper part of the C-hook;

adjusting the longest diameter of the hot-rolled coil to be perpendicular to the C-hook by means of a driving roll provided on the hanger; and

placing the C-hook, which has the hot-rolled coil mounted thereon, on a stand, followed by lifting, thereby correcting the shape of the hot-rolled coil by self-weight.

7. A method for correcting a shape of a hot-rolled coil, the method comprising the steps of:

mounting the hot-rolled coil on a hanger forming a lower part of a C-hook;

measuring a longest diameter of the hot-rolled coil using an outer diameter measuring means provided in an upper part of the C-hook;

adjusting the longest diameter of the hot-rolled coil to be perpendicular to the C-hook by means of a driving roll provided on the hanger; and

placing the C-hook, which has the hot-rolled coil mounted thereon, on a stand, followed by lifting, thereby correcting the shape of the hot-rolled coil by self-weight,

wherein the hot-rolled coil is manufactured by a method comprising the steps of:

reheating a steel slab comprising 0.18 wt % to 0.56 wt % carbon (C), 0.1 wt % to 0.5 wt % silicon (Si), 0.7 wt % to 6.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.02 wt % sulfur (S), more than 0 wt % but not more than 0.3 wt % chromium (Cr), more than 0 wt % but not more than 0.004 wt % boron (B), 0.01 wt % to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities;

hot-rolling the steel slab at a finishing mill delivery temperature of 850° C. to 950° C., thereby forming a hot-rolled sheet; and

cooling the hot-rolled sheet, followed by coiling at a coiling temperature of 700° C. or higher.

8. The method of claim 7, wherein the hot-rolled coil comprises 0.21 wt % to 0.37 wt % carbon (C), 0.1 wt % to 0.4 wt % silicon (Si), 1.1 wt % to 1.5 wt % manganese (Mn), more than 0 wt % but not more than 0.02 wt % phosphorus (P), more than 0 wt % but not more than 0.02 wt % sulfur (S), 0.1 wt % to 0.3 wt % chromium (Cr), 0.001 wt % to 0.004 wt % boron (B), 0.01 wt % to 0.04 wt % titanium (Ti), and the remainder being iron (Fe) and other inevitable impurities.

9. The method of claim 7, wherein the hot-rolled sheet is cooled and coiled at a coiling temperature of 700° C. to 900° C.