EP4610379A1

EP4610379A1 - Steel section and method for manufacturing same

Info

Publication number: EP4610379A1
Application number: EP23883158.0A
Authority: EP
Inventors: Young Hoon Lim; Chul Won Lee; Jae Young Kim; Beom Joon JEON; Dong Hyun Lim; Ho Yong Yang; Seong Woong Joo
Original assignee: Hyundai Steel Co
Current assignee: Hyundai Steel Co
Priority date: 2022-10-28
Filing date: 2023-10-27
Publication date: 2025-09-03
Also published as: KR20240059920A; WO2024091054A1; CN120153116A; JP2026510168A; KR102924263B1; AU2023367256A1

Abstract

A method for manufacturing a steel section, according to an embodiment of the present invention, comprises the steps of: (a) reheating steel to 1150-1300 °C, the steel comprising 0.04-0.14 wt% of carbon (C), 0.10-0.55 wt% of silicon (Si), 0.90-1.65 wt% of manganese (Mn), 0.020 wt% or less of phosphorus (P), 0.007 wt% or less of sulfur (S), 0.015-0.055 wt% of aluminum (Al), 0.010-0.080 wt% of vanadium (V), 0.005-0.025 wt% of titanium (Ti), 0.010-0.050 wt% of niobium (Nb), and the remainder of iron (Fe) and other inevitable impurities; and (b) rolling the steel, wherein the rolling start temperature is 900-1100 °C, the rolling medium temperature is 850-1000 °C, and the rolling end temperature is 800-900 °C. Accordingly, it is possible to realize a high-performance steel section which achieves uniform physical properties while securing low-temperature impact toughness and a method for manufacturing the steel section.

Description

[Technical Field]

The present invention relates to a steel section and a method of manufacturing the same.

[Background Art]

Steel sections generally refer to steel with polygonal cross-sectional shapes. Steel sections can be manufactured by hot rolling blooms, billets, and beam blanks that are produced through continuous casting. They are used as steel for structures such as columns of large buildings and as temporary construction materials for subways and bridges, as well as foundation piles.
Recently, with the advancement of the offshore plant industry, there is an increasing demand in steel sections that are light and have high-strength properties, particularly those that can withstand external impacts even at temperatures below -40 °C while securing low-temperature impact toughness in the field of marine structures.
The manufacturing method for steel that can ensure high-strength impact toughness even at low temperatures primarily involves the thermomechanical control process (TMCP) using quenching and self-tempering (QST) equipment.
However, when the TMCP is applied to hot rolling, it is possible to achieve a grain refinement effect, but issues arise due to temperature differences and deformation during cooling caused by the three-dimensional shape of the steel section and the thickness differences between the flange and the web.
In addition, in the process of manufacturing medium or small-sized steel sections, due to the relatively small size of the beam blank, when using the TMCP, cooling may proceed too quickly, and the cooling rate may not be precisely controlled.
Therefore, there is an increasing demand for a high-strength steel section that can secure low-temperature impact toughness while minimizing deformation and variations in physical properties, and its manufacturing method.

[Disclosure]

[Technical Problem]

To solve the above problems of the related art, the present invention aims to provide a high-performance steel section that secures low-temperature impact toughness while achieving uniform physical properties, and a method of manufacturing the same.
The objectives of the present invention are not limited to those described above, and other objectives not mentioned can be clearly understood by those skilled in the art from the description below.

[Technical Solution]

A method of manufacturing a steel section according to an embodiment of the present invention includes (a) reheating steel including 0.04 to 0.14 wt% of carbon (C), 0.10 to 0.55 wt% of silicon (Si), 0.90 to 1.65 wt% of manganese (Mn), 0.020 wt% or less of phosphorus (P), 0.007 wt% or less of sulfur (S), 0.015 to 0.055 wt% of aluminum (Al), 0.010 to 0.080 wt% of vanadium (V), 0.005 to 0.025 wt% of titanium (Ti), 0.010 to 0.050 wt% of niobium (Nb), and the remainder as iron (Fe) and other unavoidable impurities to a temperature of 1150 to 1300 °C; and (b) rolling the steel, wherein the rolling start temperature is 900 to 1100 °C, the rolling intermediate temperature is 850 to 1000 °C, and the rolling end temperature is 800 to 900 °C.
In step (b), the rolling intermediate temperature may be controlled by spraying cooling water from a selective cooling (S/C) device under the conditions of a waiting time of 0 to 120 seconds, a flow rate of 50 to 300 m³/hr, and a transport speed of 2.0 to 4.0 m/s.
The steel that has undergone step (b) may have a room-temperature microstructure at its center that includes ferrite and pearlite, and the ferrite grain size (F.G.S) may be 10 µm or less.
The steel that has undergone step (b) may have a yield strength (YS) of 420 MPa or more, a low-temperature impact toughness at -40 °C of 50 J or more, a yield ratio (YR) of 0.90 or less, and an elongation (EL) of 19% or more.
The steel that has undergone step (b) may be manufactured into H-section steel including a web and flanges, and the yield strength (YS) difference between the upper and lower parts of the flange based on the web may be 15 MPa or less.
The steel may include 0.1 to 0.2 wt% of silicon (Si), 1.57 to 1.65 wt% of manganese (Mn), 0.015 to 0.021 wt% of aluminum (Al), 0.040 to 0.045 wt% of vanadium (V), 0.005 to 0.008 wt% of titanium (Ti), and 0.040 to 0.045 wt% of niobium (Nb).
A steel section according to another embodiment of the present invention includes 0.04 to 0.14 wt% of carbon (C), 0.10 to 0.55 wt% of silicon (Si), 0.90 to 1.65 wt% of manganese (Mn), 0.020 wt% or less of phosphorus (P), 0.007 wt% or less of sulfur (S), 0.015 to 0.055 wt% of aluminum (Al), 0.010 to 0.080 wt% of vanadium (V), 0.005 to 0.025 wt% of titanium (Ti), 0.010 to 0.050 wt% of niobium (Nb), and the remainder as iron (Fe) and other unavoidable impurities, wherein the yield strength (YS) is 420 MPa or more.
The low-temperature impact toughness at -40 °C may be 50 J or more.
The yield ratio (YR) may be 0.90 or less.
The elongation (EL) may be 19% or more.
The steel section may have the shape of H-section steel including a web and flanges, and the yield strength (YS) difference between the upper and lower parts of the flange based on the web may be 15 MPa or less.
The room-temperature microstructure at the center may include ferrite and pearlite, and the ferrite grain size (F.G.S) may be 10 µm or less.
The steel may include 0.1 to 0.2 wt% of silicon (Si), 1.57 to 1.65 wt% of manganese (Mn), 0.015 to 0.021 wt% of aluminum (Al), 0.040 to 0.045 wt% of vanadium (V), 0.005 to 0.008 wt% of titanium (Ti), and 0.040 to 0.045 wt% of niobium (Nb).
A steel section according to still another embodiment of the present invention includes 0.04 to 0.14 wt% of carbon (C), 0.10 to 0.55 wt% of silicon (Si), 0.90 to 1.65 wt% of manganese (Mn), 0.020 wt% or less of phosphorus (P), 0.007 wt% or less of sulfur (S), 0.015 to 0.055 wt% of aluminum (Al), 0.010 to 0.080 wt% of vanadium (V), 0.005 to 0.025 wt% of titanium (Ti), 0.010 to 0.050 wt% of niobium (Nb), and the remainder as iron (Fe) and other unavoidable impurities, and the steel section may be manufactured by reheating to 1150 to 1300 °C and then rolling, with the rolling start temperature controlled to 900 to 1100 °C, the rolling intermediate temperature controlled to 850 to 1000 °C, and the rolling end temperature controlled to 800 to 900 °C.
The rolling intermediate temperature may be controlled by spraying cooling water from a selective cooling (S/C) device under the conditions of a waiting time of 0 to 120 seconds, a flow rate of 50 to 300 m³/hr, and a transport speed of 2.0 to 4.0 m/s.

[Advantageous Effects]

According to an embodiment of the present invention, it is possible to realize a high-performance steel section that secures low-temperature impact toughness while achieving uniform physical properties, and a method of manufacturing the same.
The effects of the present invention are not limited to those described above, and other effects not mentioned can be clearly understood by those skilled in the art from the description in the claims.

[Description of Drawings]

FIG. 1 is a flowchart of a steel section manufacturing method according to the present invention.
FIG. 2 is a diagram of a selective cooling (S/C) device and a steel section used in a steel section manufacturing method according to the present invention.
FIGS. 3A to 3C are microstructure photographs of specimens taken from the center of the flange of steel sections in comparative examples and an experimental example. Specifically, FIG. 3A is a set of microstructure photographs of Comparative Example 1, FIG. 3B is a set of microstructure photographs of Comparative Example 2, and FIG. 3C is a set of microstructure photographs of Experimental Example 1.

[Modes of the Invention]

In this specification, when a component (or region, layer, part, etc.) is described as being "on," "connected to," or "coupled to" another component, it may be directly disposed, connected, or coupled to the other component, or a third component may be disposed therebetween.
The same reference numbers refer to the same components. Additionally, in the drawings, the thicknesses, proportions, and dimensions of the components are exaggerated for the purpose of effectively explaining the technical content.
The term "and/or" includes all possible combinations of the associated components that can be defined.
Terms such as "first" and "second" may be used to describe various components, but these components should not be limited by these terms. These terms are used solely for the purpose of distinguishing one component from another. For example, without departing from the scope of the present invention, the first component may be referred to as the second component, and similarly, the second component may be referred to as the first component. Singular expressions include plural expressions unless the context clearly indicates otherwise.
In addition, terms such as "below," "lower," "above," and "upper" are used to describe the relationship between the components depicted in the drawings. These terms are relative concepts and are explained based on the directions indicated in the drawings.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Additionally, terms that are defined in commonly used dictionaries should be interpreted as having meanings consistent with the context of the relevant technology, and are explicitly defined herein unless they are interpreted in an idealized or overly formal sense.
Terms such as "include" and "have" are intended to indicate the presence of features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, and do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

Steel section

A steel section according to an embodiment of the present invention includes 0.04 to 0.14 wt% of carbon (C), 0.10 to 0.55 wt% of silicon (Si), 0.90 to 1.65 wt% of manganese (Mn), 0.020 wt% or less of phosphorus (P), 0.007 wt% or less of sulfur (S), 0.015 to 0.055 wt% of aluminum (Al), 0.010 to 0.080 wt% of vanadium (V), 0.005 to 0.025 wt% of titanium (Ti), 0.010 to 0.050 wt% of niobium (Nb), and the remainder as iron (Fe) and other unavoidable impurities, wherein the yield strength (YS) is 420 MPa or more. The YS may be the yield strength at room temperature.
More preferably, 0.1 to 0.2 wt% of silicon (Si), 1.57 to 1.65 wt% of manganese (Mn), 0.015 to 0.021 wt% of aluminum (Al), 0.040 to 0.045 wt% of vanadium (V), 0.005 to 0.008 wt% of titanium (Ti), and 0.040 to 0.045 wt% of niobium (Nb) may be included.
The steel section with the above-described alloy composition may satisfy a low-temperature impact toughness at -40 °C of 50 J or more, a yield ratio (YR) of 0.90 or less, and an elongation (EL) of 19% or more.
Preferably, the YS at room temperature may be 435 MPa or more, the YR may be 0.85 or less, and the EL may be 21% or more. More preferably, the YS at room temperature may be 445 MPa or more, the low-temperature impact toughness at -40 °C may be 160 J or more, the YR may be 0.81 or less, and the EL may be 29.9% or more.
The steel section may have the shape of H-section steel including a web and flanges, and the yield strength (YS) difference between the upper and lower parts of the flange based on the web may be 15 MPa or less. Preferably, the YS difference between the upper and lower parts of the flange may be 12 MPa, and more preferably, it may be 8 MPa or less.
In addition, the steel section with the above-described alloy composition may have a room-temperature microstructure at its center that includes ferrite and pearlite, and the ferrite grain size (F.G.S) may be 10 µm or less. More specifically, the F.G.S may be 9.5 µm or less.
Thus, the present invention may realize a high-performance steel section that secures low-temperature impact toughness while achieving uniform physical properties, and a method of manufacturing the same.
In particular, through the addition of vanadium (V) and niobium (Nb) within the above composition range, it is possible to achieve precipitation strengthening, grain refinement, and sufficient cooling effects. In addition, the V-Nb composite design may secure high strength, stability, and mass production compared to conventional steel sections with a yield strength of 355 MPa.
Additionally, the addition of titanium (Ti) within the above composition range may delay the growth of austenite grains and improve weldability.
Hereinafter, the roles and contents of each alloy element included in the steel section according to an embodiment of the present invention are described in detail.

Carbon (C)

Carbon (C) is the most effective and important element for increasing the strength of steel, and as it promotes the formation of fine carbides by reacting with Nb, Ti, etc., carbon effectively enhances the strength through precipitation strengthening. Therefore, the steel section according to an embodiment of the present invention may include 0.04 to 0.14 wt% of carbon.
When the carbon content is less than 0.04 wt% of the total weight, it may be difficult to secure sufficient strength. On the other hand, when the carbon content exceeds 0.14 wt% of the total weight, coarse carbides may be formed, which may degrade impact properties and cause welding difficulties.

Silicon (Si)

Silicon (Si) is added as a deoxidizer in the steelmaking process, along with aluminum, to remove oxygen from the steel. In addition, silicon may have a solid solution strengthening effect.
Silicon may be added in an amount of 0.10 to 0.55 wt% of the total weight of the steel section according to an embodiment of the present invention. When the silicon content is less than 0.10 wt% of the total weight, the effect of silicon addition may not be fully exhibited. On the other hand, when the silicon is added in an excess of 0.55 wt% of the total weight, it may degrade the weldability of the steel and generate red scale during reheating and hot rolling, causing problems with surface quality.

Manganese (Mn)

Manganese (Mn) is a solid solution strengthening element and secures strength and improves the hardening ability of steel. Manganese reduces the acid resistance and oxidation resistance of steel, but refines pearlite and solid solution strengthens ferrite, thereby improving yield strength. Therefore, the steel section according to an embodiment of the present invention may include manganese in an amount of 0.90 to 1.65 wt%, preferably 1.57 to 1.65 wt%.
When the manganese content is less than 0.90 wt% of the total weight, the solid solution strengthening effect may not be fully exhibited. When the manganese content exceeds 1.65 wt%, manganese may combine with sulfur to form MnS inclusions or cause center segregation in the ingot, thereby reducing the ductility and corrosion resistance of the steel section.

Phosphorus (P)

Phosphorus (P) may serve to increase the strength of steel through solid solution strengthening and inhibit the formation of carbides. Phosphorus may be added in an amount of 0.020 wt% or less of the total weight of the steel section according to an embodiment of the present invention. When the phosphorus content exceeds 0.02 wt%, it may act as a tramp element, forming inclusions that reduce the ductility of the steel, and its precipitation behavior may decrease low-temperature impact toughness.

Sulfur (S)

Sulfur (S) may improve workability by forming fine MnS precipitates. Sulfur may be added in an amount of 0.007 wt% or less of the total weight of the steel section according to an embodiment of the present invention. When the sulfur content exceeds 0.007 wt%, it may act as a tramp element, forming inclusions that reduce the ductility and weldability of the steel, and decrease low-temperature impact toughness.

Aluminum (Al)

Aluminum (Al) is added as a deoxidizer in the steelmaking process to remove oxygen from the steel. In addition, aluminum may contribute to grain refinement by precipitating as AlN in the steel. Aluminum may be added in an amount of 0.015 to 0.055 wt% of the total weight of the steel section according to an embodiment of the present invention, preferably 0.015 to 0.021 wt%. When the aluminum content is less than 0.015 wt%, the deoxidizing effect may be insufficient, and when the aluminum content exceeds 0.055 wt%, it may cause difficulties in continuous casting, reduce productivity, and form non-metallic inclusions, such as alumina (Al₂O₃), which may decrease ductility and toughness.

Vanadium (V)

Since vanadium (V) has a high carbide-forming ability, vanadium forms fine carbides to refine the steel structure and forms precipitates during rolling to increase strength. In particular, the amount of precipitate may be controlled based on the amount of nitrogen added. In addition, vanadium may act as a pinning agent at grain boundaries, contributing to strength enhancement.
Vanadium may be added in an amount of 0.010 to 0.080 wt% of the total weight of the steel section according to an embodiment of the present invention, preferably 0.040 to 0.045 wt%. When the vanadium content is less than 0.010 wt%, it may be difficult to fully achieve the desired effects. On the other hand, when the vanadium content exceeds 0.080 wt%, low-temperature impact toughness may be reduced.

Titanium (Ti)

Titanium (Ti) may form high-temperature stable Ti (C, N) precipitates. This may hinder the growth of austenite grains during welding, thereby refining the structure of the welded area, which improves the toughness and strength of the steel. Titanium may be added in an amount of 0.005 to 0.025 wt% of the total weight of the steel section according to an embodiment of the present invention, preferably 0.005 to 0.008 wt%. When the titanium content is less than 0.005 wt%, it may be difficult to fully achieve the desired effects. When the titanium content exceeds 0.025 wt%, it may form coarse precipitates, which may reduce the low-temperature impact toughness of the steel.

Niobium (Nb)

Niobium (Nb) is an element that inhibits grain growth and results in finer grain sizes when incorporated in austenitic structures. Specifically, niobium allows steel to quickly reach the non-recrystallization temperature (Tnr) or lower, thereby delaying recrystallization. In addition, niobium reacts with carbon to promote the formation of fine carbides, thereby effectively enhancing strength through precipitation strengthening. However, excessive addition may decrease the impact properties of the steel.
Therefore, the steel section according to an embodiment of the present invention may include niobium in an amount of 0.01 to 0.05 wt%, preferably 0.040 to 0.045 wt%. When the niobium content is less than 0.01 wt% of the total weight, the effect of niobium addition may not be fully exhibited, and when niobium is added in excess of 0.05 wt%, it may reduce the impact absorption energy of the steel. In addition, when the niobium content is 0.040 to 0.045 wt%, the above-described beneficial effects of niobium addition may be maximized while minimizing the reduction in impact absorption energy of the steel.

Nitrogen (N)

Nitrogen (N) significantly affects the mechanical properties of steel even in a very small amount, and may increase tensile strength and yield strength but reduce elongation. However, excessive addition of nitrogen may reduce weld toughness and impact strength. The steel section according to an embodiment of the present invention may include 110 to 120 ppm of nitrogen.
Meanwhile, the steel section according to an embodiment of the present invention includes 0.04 to 0.14 wt% of carbon (C), 0.10 to 0.55 wt% of silicon (Si), 0.90 to 1.65 wt% of manganese (Mn), 0.020 wt% or less of phosphorus (P), 0.007 wt% or less of sulfur (S), 0.015 to 0.055 wt% of aluminum (Al), 0.010 to 0.080 wt% of vanadium (V), 0.005 to 0.025 wt% of titanium (Ti), 0.010 to 0.050 wt% of niobium (Nb), and the remainder as iron (Fe) and other unavoidable impurities, and the steel section is manufactured by reheating to 1150 to 1300 °C and then rolling, with the rolling start temperature controlled to 900 to 1100 °C, the rolling intermediate temperature controlled to 850 to 1000 °C, and the rolling end temperature controlled to 800 to 900 °C.
The rolling intermediate temperature may be controlled by spraying cooling water from a selective cooling (S/C) device as shown in FIG. 2 under the conditions of a waiting time of 0 to 120 seconds, a flow rate of 50 to 300 m³/hr, and a transport speed of 2.0 to 4.0 m/s. The S/C device will be described in detail in the description of the steel section manufacturing method below.
The steel section having the above-described alloy composition and manufactured by the above-described method may satisfy a low-temperature impact toughness at -40 °C of 50 J or more, a yield ratio (YR) of 0.90 or less, and an elongation (EL) of 19% or more. Preferably, the YS at room temperature may be 435 MPa or more, the YR may be 0.85 or less, and the EL may be 21% or more. More preferably, the YS at room temperature may be 445 MPa or more, the low-temperature impact toughness at -40 °C may be 160 J or more, the YR may be 0.81 or less, and the EL may be 29.9% or more.
The steel section has the shape of H-section steel including a web and flanges, and the yield strength (YS) difference between the upper and lower parts of the flange based on the web may be 15 MPa or less. Preferably, the YS difference between the upper and lower parts of the flange may be 12 MPa, and more preferably, it may be 8 MPa or less.
In addition, the steel section having the above-described alloy composition and manufactured by the above-described method may have a room-temperature microstructure at its center that includes ferrite and pearlite (F+P), and the ferrite grain size (F.G.S) may be 10 µm or less. More specifically, the F.G.S may be 9.5 µm or less.
Thus, the steel section according to an embodiment of the present invention may achieve a high-performance steel section that secures low-temperature impact toughness while minimizing temperature differences and achieving uniform physical properties, and a method of manufacturing the same.
In particular, it is possible to achieve precipitation strengthening, grain refinement, and an accelerated cooling effect by adding vanadium (V) and niobium (Nb) within the above-described composition range and controlling the rolling intermediate temperature using a selective cooling (S/C) device. In addition, the V-Nb composite design may secure high strength, stability, and mass production compared to conventional steel sections with a yield strength of 355 MPa.
Additionally, the addition of titanium (Ti) within the above-described composition range may delay the growth of austenite grains and improve weldability.

Method of manufacturing steel section

The method of manufacturing a steel section according to an embodiment of the present invention includes (a) reheating and (b) rolling as shown in FIG. 1. Hereinafter, the steel section manufacturing method will be described in detail with reference to FIGS. 1 and 2.
The steel includes 0.04 to 0.14 wt% of carbon (C), 0.10 to 0.55 wt% of silicon (Si), 0.90 to 1.65 wt% of manganese (Mn), 0.020 wt% or less of phosphorus (P), 0.007 wt% or less of sulfur (S), 0.015 to 0.055 wt% of aluminum (Al), 0.010 to 0.080 wt% of vanadium (V), 0.005 to 0.025 wt% of titanium (Ti), 0.010 to 0.050 wt% of niobium (Nb), and the remainder as iron (Fe) and other unavoidable impurities, and the steel is reheated to a temperature of 1150 to 1300 °C. Afterward, the steel is rolled with a rolling start temperature of 900 to 1100 °C, a rolling intermediate temperature of 850 to 1000 °C, and a rolling end temperature of 800 to 900 °C.
Thus, the steel section 10 according to an embodiment of the present invention may achieve a high-performance steel section that secures low-temperature impact toughness while minimizing temperature differences and achieving uniform physical properties by securing the target microstructure and grain size, and a method of manufacturing the same.
In particular, through the addition of vanadium (V) and niobium (Nb) within the above-described composition range and the control of the rolling intermediate temperature, it is possible to achieve precipitation strengthening, grain refinement, and sufficient cooling effects. In addition, the V-Nb composite design may secure high strength, stability, and mass production compared to conventional steel sections with a yield strength of 355 MPa. Additionally, the addition of titanium (Ti) within the above-described composition range may delay the growth of austenite grains and improve weldability.
Hereinafter, the steel section manufacturing method according to an embodiment of the present invention will be described in detail.
In the reheating step, the steel with the above-described composition is reheated at a temperature of 1150 °C or higher. When the reheating temperature is lower than 1150 °C, the incorporation of various carbides may not be sufficient, and the components segregated during the continuous casting process may not be evenly distributed. In addition, the reheating temperature should not exceed 1300 °C. When the reheating temperature exceeds 1300 °C, coarse austenite grains may be formed, making it difficult to secure strength, and the increased heating costs and time may result in higher manufacturing costs and reduced productivity.
On the other hand, the steel may be manufactured by obtaining molten steel of a desired composition through a steelmaking process and then performing a continuous casting process. The steel may be, for example, a beam blank, but is not necessarily limited thereto.
Preferably, the composition of the steel may be 0.1 to 0.2 wt% of silicon (Si), 1.57 to 1.65 wt% of manganese (Mn), 0.015 to 0.021 wt% of aluminum (Al), 0.040 to 0.045 wt% of vanadium (V), 0.005 to 0.008 wt% of titanium (Ti), and 0.040 to 0.045 wt% of niobium (Nb). As a result, it is possible to further reduce the differences in room-temperature yield strength and low-temperature impact toughness between the upper and lower parts of the flange and secure a microstructure grain size of 10 µm or less.
In the rolling step (b), the rolling intermediate temperature may be controlled by spraying cooling water from a selective cooling (S/C) device 100 as shown in FIG. 2 under the conditions of a waiting time of 0 to 120 seconds, a flow rate of 50 to 300 m³/hr, and a transport speed of 2.0 to 4.0 m/s.
To ensure high-strength impact toughness, the TMCP is typically used. However, an embodiment of the present invention is applied to steel sections with medium or small-sized specifications without accelerated cooling equipment such as the QST equipment of the TMCP. In the manufacturing process of steel sections with medium or small-sized specifications, the S/C device 100 may be used together with a continuous mill (CM).
The QST equipment includes a side cooler, an upper cooling box, and a lower cooling nozzle, and since high-pressure cooling water is used, the cooling speed is fast, making it suitable for large-sized steel sections. On the other hand, the S/C device 100 consists of side coolers 110 and lower cooling nozzles 120, as shown in FIG. 2, which makes it easier to control the cooling speed and temperature.
In the case of the CM device, H-section steel products are continuously fed into the rolling mill, causing cooling water to accumulate on the upper part of the H-section steel and exposing it to the air. This leads to temperature differences during rolling and physical property differences between the upper part 12a and the lower part 12b of the flange 12 of the steel section 10. To overcome these problems, the S/C device 100 is used and its operating conditions are adjusted to intensively cool the lower part and precisely control the cooling temperature and speed, thereby resulting in reduced temperature differences, uniform physical properties, and some enhanced physical properties.
Thus, the steel processed through the rolling step under the above process conditions may have a room-temperature microstructure at its center that includes ferrite and pearlite, with a ferrite grain size (F.G.S) of 10 µm or less. As shown in FIG. 2, in the H-section steel 10 including a web 11 and flanges 12, the difference in F.G.S between the upper part 12a and the lower part 12b of the flange 12 based on the web 11 may be reduced. In this way, the steel section manufacturing method according to an embodiment of the present invention may secure the target microstructure and grain size and achieve a high-strength steel section.
Accordingly, the steel processed through the rolling step (b) under the above-described process conditions may have a yield strength (YS) of 420 MPa or more, a low-temperature impact toughness at -40 °C of 50 J or more, a yield ratio (YR) of 0.90 or less, and an elongation (EL) of 19% or more. In this way, the steel section manufacturing method according to an embodiment of the present invention may manufacture a high-performance steel section with secured low-temperature impact toughness.
In addition, the steel processed through the rolling step (b) under the above-described process conditions may be manufactured into H-section steel 10 including a web 11 and flanges 12, and the yield strength (YS) difference between the upper part 12a and the lower part 12b of the flange 12 based on the web 11 may be 15 MPa or less. In this way, the steel section manufacturing method according to an embodiment of the present invention may improve quality and mass production by securing uniform quality and stable physical properties compared to conventional methods.

Comparative Examples and Experimental Example

Hereinafter, preferred comparative and experimental examples are presented to help understanding of the present invention. However, the experimental example is only for illustrative purposes and does not limit the scope of the present invention.
Table 1 below shows the main alloy compositions (units: wt%) of the experimental and comparative examples, Table 2 shows the process conditions (temperature units: °C, time units: sec, S/C flow rate units: m³/hr, speed units: m/s) for manufacturing the specimens of the experimental and comparative examples, and Table 3 shows the measured physical properties of the specimens under the process conditions shown in Table 2. The beam blank with the composition shown in Table 1 was manufactured using an electric arc furnace and then subjected to hot rolling to produce H-section steel with a flange thickness of 15 mm.

The process conditions included a reheating temperature of 1150 to 1300 °C, a waiting time of 0 to 120 seconds, a rolling start temperature of 900 to 1100 °C, a rolling intermediate temperature of 850 to 1000 °C, a rolling end temperature of 800 to 900 °C, an S/C flow rate of 50 to 300 m³/hr, and a transport speed of 2.0 to 4.0 m/s. Table 2 listed the actual process conditions for which experimental data was obtained. The target physical properties in the experimental example were a tensile strength (TS) of 500 to 660 MPa, a yield strength (YS) at room temperature of 420 MPa or more, an elongation (EL) of 19% or more, a yield ratio (YR) of 90% or less, and a low-temperature impact toughness at -40 °C of 50 J or more based on the flange. In addition, for the microstructure, the goal was to have a grain size of 10 µm or less with a ferrite-pearlite (F+P) composite structure in the center. Additionally, another goal was to reduce the difference in room-temperature yield strength (YS) between the upper and lower parts of the flange to 15 MPa, and preferably 12 MPa or less.

[Table 1]

Component	C	Si	Mn	P	S	Al	V	Ti	Nb
Comparative Example 1	0.1	0.21	1.51	0.01	0.002	0.031	0.036	0.016	0.035
Comparative Example 2	0.11	0.22	1.5	0.009	0.001	0.022	0.037	0.014	0.036
Experimental Example 1	0.1	0.2	1.58	0.013	0.002	0.017	0.04	0.007	0.044

[Table 2]

Classification	Device	Reheating temperature	Waitin g time	Rolling start tempera ture	Rolling interme diate tempera ture	Rolling end tempera ture	S/C flow rate	Transpo rt speed
Comparative Example 1	Not used	1150-1300	120	1040	931	823	0	3
Comparative Example 2	S/C	1150-1300	120	1028	933	819	50-300	3
Experimental Example 1	S/C	1150-1300	120	1030	930	820	50-300	3

[Table 3]

Classification	Location	Tensile properties					Impact toughness	Microstructure
Classification	Location	TS (MPa )	YS (MPa )	YS differ ence	EI (%)	YR (%)	J, at -40 °C (Flange)	Struct ure	Grain size (µm)
Comparative Example 1	Upper part of flange	565	450	23	30.7	0.8	161	F+P	9.5
Comparative Example 1	Lower part of flange	571	427	23	29.3	0.75	64	F+P	10.5
Comparative Example 2	Upper part of flange	563	432	13	28.3	0.77	122	F+P	10
Comparative Example 2	Lower part of flange	566	419	13	25.7	0.74	60	F+P	10.6
Experimental Example 1	Upper part of flange	568	447	8	31.6	0.79	166	F+P	9.5
Experimental Example 1	Lower part of flange	560	455	8	29.9	0.81	187	F+P	9.3

Comparative Example 1 and Experimental Example 1

Referring to Tables 1 to 3, Comparative Example 1 differs from Experimental Example 1 in terms of composition, and there is a difference in whether the S/C device 100 shown in FIG. 2 is used for controlling the rolling intermediate temperature or cooling. FIG. 3A is a set of microstructure photographs of Comparative Example 1, and FIG. 3C is a set of microstructure photographs of Experimental Example 1.
Referring to Table 1, in Comparative Example 1, the composition range of vanadium and niobium was intended to be 0.035 to 0.039 wt%, and the actual experimental data included vanadium at 0.036 wt% and niobium at 0.035 wt%. The temperature was controlled without any additional device.
In Experimental Example 1, the composition range of vanadium and niobium was intended to be 0.040 to 0.045 wt%, and the actual experimental data included vanadium at 0.040 wt% and niobium at 0.044 wt%. The S/C device was used to control the rolling intermediate temperature under the process conditions shown in Table 2.
Referring to Table 3 and FIG. 3, it can be seen that compared to Comparative Example 1, Experimental Example 1 has significantly smaller differences in yield strength (YS) and impact toughness between the upper and lower parts of the flange, with respective differences of 8 MPa and 21 J. Additionally, the grain size of the microstructure has a smaller difference, with both the upper and lower parts of the flange having grain sizes within 10 µm, indicating improved property uniformity.

Comparative Example 2 and Experimental Example 1

Referring to Tables 1 to 3, Comparative Example 1 differs from Experimental Example 1 in terms of composition, but the use of the S/C device 100 shown in FIG. 2 for controlling the rolling intermediate temperature or cooling is the same. FIG. 3B is a set of microstructure photographs of Comparative Example 2, and FIG. 3C is a set of microstructure photographs of Experimental Example 1.
Referring to Table 1, in Comparative Example 2, the composition range of vanadium and niobium was intended to be 0.035 to 0.039 wt%, and the actual experimental data included vanadium at 0.037 wt% and niobium at 0.036 wt%. The S/C device was used to control the rolling intermediate temperature under the process conditions shown in Table 2.
In Experimental Example 1, the composition range of vanadium and niobium was intended to be 0.040 to 0.045 wt%, and the actual experimental data included vanadium at 0.040 wt% and niobium at 0.044 wt%. The S/C device was used to control the rolling intermediate temperature under the process conditions shown in Table 2.
Referring to Table 3, it can be seen that Comparative Example 2 has a yield strength (YS) of 432 MPa (at the upper part of the flange) and 419 MPa (at the lower part of the flange), which is lower than an YS of 447 MPa (at the upper part of the flange) and 455MPa (at the lower part of the flange) in Experimental Example 1. Specifically, the YS at the lower part of the flange in Comparative Example 2 does not reach 420 MPa. Additionally, it can be seen that the difference in yield strength (YS) between the upper and lower parts of the flange in Comparative Example 2 is 13 MPa, and the difference in impact toughness is 62 J in Comparative Example 2, which are significantly larger than those of Experimental Example 1, where the differences are 8 MPa for YS and 21 J for impact toughness. Therefore, it is confirmed that Experimental Example 1 has better yield strength (YS) and impact toughness properties compared to Comparative Example 2, with smaller variations.
Meanwhile, regarding the grain size of the microstructure, Comparative Example 2 has grain sizes of 10.0 µm at the upper part and 10.6 µm at the lower part of the flange, exceeding 10 µm, and the variation is 0.6 µm, which is larger than that of Experimental Example 1.
Thus, the steel section and the method of manufacturing the same according to an embodiment of the present invention can achieve a high-performance offshore special steel section with excellent quality by securing the desired high strength, low-temperature impact toughness, microstructure, and grain size, as confirmed by the data from Experimental Example 1, while minimizing property differences between the upper and lower parts of the flange.
As described above, preferred embodiments of the present invention have been described, and it is apparent to those skilled in the art that the present invention may be embodied in other specific forms without departing from the spirit or scope of the present invention, in addition to the embodiments described above. In other words, the above-described embodiments should be considered as illustrative rather than restrictive, and accordingly, the present invention is not limited to the above description and may be modified within the scope of the appended claims and their equivalents.

S10: Reheating
S20: Rolling
10: Steel section
11: Web
12: Flange
12a: Upper part of flange
12b: Lower part of flange
100: S/C device
110: Side cooler
120: Lower cooling nozzle

Claims

A method of manufacturing a steel section, comprising:
(a) reheating steel including 0.04 to 0.14 wt% of carbon (C), 0.10 to 0.55 wt% of silicon (Si), 0.90 to 1.65 wt% of manganese (Mn), 0.020 wt% or less of phosphorus (P), 0.007 wt% or less of sulfur (S), 0.015 to 0.055 wt% of aluminum (Al), 0.010 to 0.080 wt% of vanadium (V), 0.005 to 0.025 wt% of titanium (Ti), 0.010 to 0.050 wt% of niobium (Nb), and the remainder as iron (Fe) and other unavoidable impurities to a temperature of 1150 to 1300 °C; and

(b) rolling the steel, wherein the rolling start temperature is 900 to 1100 °C, the rolling intermediate temperature is 850 to 1000 °C, and the rolling end temperature is 800 to 900 °C.
The method of claim 1, wherein in step (b), the rolling intermediate temperature is controlled by spraying cooling water from a selective cooling (S/C) device under the conditions of a waiting time of 0 to 120 seconds, a flow rate of 50 to 300 m³/hr, and a transport speed of 2.0 to 4.0 m/s.
The method of claim 1, wherein the steel that has undergone step (b) has a room-temperature microstructure at its center that includes ferrite and pearlite, and the ferrite grain size (F.G.S) is 10 µm or less.
The method of claim 1, wherein the steel that has undergone step (b) has a yield strength (YS) of 420 MPa or more, a low-temperature impact toughness at - 40 °C of 50 J or more, a yield ratio (YR) of 0.90 or less, and an elongation (EL) of 19% or more.
The method of claim 1, wherein the steel that has undergone step (b) is manufactured into H-section steel including a web and flanges, and the yield strength (YS) difference between the upper and lower parts of the flange based on the web is 15 MPa or less.
The method of claim 1, wherein the steel includes 0.1 to 0.2 wt% of silicon (Si), 1.57 to 1.65 wt% of manganese (Mn), 0.015 to 0.021 wt% of aluminum (Al), 0.040 to 0.045 wt% of vanadium (V), 0.005 to 0.008 wt% of titanium (Ti), and 0.040 to 0.045 wt% of niobium (Nb).
A steel section comprising 0.04 to 0.14 wt% of carbon (C), 0.10 to 0.55 wt% of silicon (Si), 0.90 to 1.65 wt% of manganese (Mn), 0.020 wt% or less of phosphorus (P), 0.007 wt% or less of sulfur (S), 0.015 to 0.055 wt% of aluminum (Al), 0.010 to 0.080 wt% of vanadium (V), 0.005 to 0.025 wt% of titanium (Ti), 0.010 to 0.050 wt% of niobium (Nb), and the remainder as iron (Fe) and other unavoidable impurities,
wherein the yield strength (YS) is 420 MPa or more.
The steel section of claim 7, wherein the low-temperature impact toughness at -40 °C is 50 J or more.
The steel section of claim 7, wherein the yield ratio (YR) is 0.90 or less.
The steel section of claim 7, wherein the elongation (EL) is 19% or more.
The steel section of claim 7, wherein the steel section has the shape of H-section steel including a web and flanges, and the yield strength (YS) difference between the upper and lower parts of the flange based on the web is 15 MPa or less.
The steel section of claim 7, wherein the room-temperature microstructure at its center includes ferrite and pearlite, and the ferrite grain size (F.G.S) is 10 µm or less.
The steel section of claim 7, wherein the steel includes 0.1 to 0.2 wt% of silicon (Si), 1.57 to 1.65 wt% of manganese (Mn), 0.015 to 0.021 wt% of aluminum (Al), 0.040 to 0.045 wt% of vanadium (V), 0.005 to 0.008 wt% of titanium (Ti), and 0.040 to 0.045 wt% of niobium (Nb).
A steel section comprising 0.04 to 0.14 wt% of carbon (C), 0.10 to 0.55 wt% of silicon (Si), 0.90 to 1.65 wt% of manganese (Mn), 0.020 wt% or less of phosphorus (P), 0.007 wt% or less of sulfur (S), 0.015 to 0.055 wt% of aluminum (Al), 0.010 to 0.080 wt% of vanadium (V), 0.005 to 0.025 wt% of titanium (Ti), 0.010 to 0.050 wt% of niobium (Nb), and the remainder as iron (Fe) and other unavoidable impurities,
wherein the steel section is manufactured by reheating to 1150 to 1300 °C and then rolling, with the rolling start temperature controlled to 900 to 1100 °C, the rolling intermediate temperature controlled to 850 to 1000 °C, and the rolling end temperature controlled to 800 to 900 °C.
The steel section of claim 14, wherein the rolling intermediate temperature is controlled by spraying cooling water from a selective cooling (S/C) device under the conditions of a waiting time of 0 to 120 seconds, a flow rate of 50 to 300 m³/hr, and a transport speed of 2.0 to 4.0 m/s.