STEEL SECTION AND STEEL SECTION MANUFACTURING METHOD

Description

TECHNICAL FIELD

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

BACKGROUND ART

Steel sections generally refer to steel with polygonal cross-sectional shapes. 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. Steel sections can be manufactured by hot rolling blooms, billets, and beam blanks that are produced through continuous casting.

Recently, large earthquakes have occurred worldwide, resulting in significant loss of life and property. In particular, earthquakes with a magnitude of over 5.0 occurred successively in Gyeongju-si and Pohang-si, South Korea, in 2016 and 2017, increasing anxiety.

During earthquakes, fires that occur as secondary damage after the primary damage caused by the destruction of buildings can soften the reinforcement material that supports the structure, accelerating building collapse along with the plastic deformation of the reinforcement material due to the earthquake. As a result, recent building design standards are being strengthened to delay building collapses and minimize damage to life and property during disaster situations such as earthquakes and high-rise building fires.

To enhance building safety, it is crucial to improve the seismic and fire resistance of building materials used in structural construction, as well as to enhance the seismic design of building structures and install protective equipment such as sprinklers.

To this end, seismic-resistant steel with controlled yield ratios and fire-resistant steel with enhanced high-temperature strength have been developed and utilized.

However, since fires can occur after the destruction of buildings during earthquakes as described above, there is a growing demand for seismic-resistant, fire-resistant steel sections that simultaneously exhibit seismic-resistant and fire-resistant capabilities.

DISCLOSURE

Technical Problem

To solve the above problems of the related art, the present invention aims to provide a high-performance steel section that exhibits fire-resistant and seismic-resistant capabilities, and a steel section manufacturing method.

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 descriptions below.

Technical Solution

A steel section manufacturing method according to one aspect of the present invention includes (a) reheating steel including 0.17 wt % or less of carbon (C), 1.6 wt % or less of manganese (Mn), 0.10 to 0.35 wt % of chromium (Cr), 0.15 wt % or less of molybdenum (Mo), 0.05 wt % or less of niobium (Nb), 0.003 wt % or less of boron (B), 0.04 wt % or less of titanium (Ti), and the remainder as iron (Fe) and other unavoidable impurities at a temperature of 1200° C. or higher; (b) hot rolling the steel with the rolling start temperature controlled to 1050 to 1100° C. and the rolling end temperature controlled to 860 to 930° C.; and (c) water cooling the steel.

In step (c), the water cooling end temperature may be controlled to 680 to 880° C.

The steel that has undergone step (c) may exhibit a yield strength (YS) of 355 MPa or more at room temperature, an impact absorption energy (Charpy V-notch (CVN) impact test) of 27 J or more at 0° C., and an elongation (EL) of 21% or more.

The steel that has undergone step (c) may exhibit a high-temperature yield strength (YS) of 238 MPa or more at 600° C.

The steel that has undergone step (c) may include bainite in its final microstructure.

The steel may include 0.08 to 0.15 wt % of carbon (C), 0.5 to 1.6 wt % of manganese (Mn), 0.1 to 0.3 wt % of chromium (Cr), 0.10 to 0.15 wt % of molybdenum (Mo), 0.02 to 0.05 wt % of niobium (Nb), 0.03 wt % or less of titanium (Ti), and 0.001 to 0.003 wt % of boron (B).

The steel may further include 0.1 to 0.4 wt % of silicon (Si), 0.6 wt % or less of copper (Cu), 0.015 wt % or less of nitrogen (N), 0.01 wt % or less of sulfur(S), and 0.02 wt % or less of phosphorus (P).

A steel section according to another aspect of the present invention includes 0.17 wt % or less of carbon (C), 1.6 wt % or less of manganese (Mn), 0.10 to 0.35 wt % of chromium (Cr), 0.15 wt % or less of molybdenum (Mo), 0.05 wt % or less of niobium (Nb), 0.003 wt % or less of boron (B), 0.04 wt % or less of titanium (Ti), and the remainder as iron (Fe) and other unavoidable impurities, wherein the yield strength (YS) at room temperature is 355 MPa or more.

The impact absorption energy (CVN impact test) at 0° C. may be 27 J or more.

The high-temperature yield strength at 600° C. may be 238 MPa or more.

The elongation (EL) may be 21% or more.

The final microstructure may include bainite.

The steel section may include 0.08 to 0.15 wt % of carbon (C), 0.5 to 1.6 wt % of manganese (Mn), 0.1 to 0.3 wt % of chromium (Cr), 0.10 to 0.15 wt % of molybdenum (Mo), 0.02 to 0.05 wt % of niobium (Nb), 0.03 wt % or less of titanium (Ti), and 0.001 to 0.003 wt % of boron (B).

The steel section may further include 0.1 to 0.4 wt % of silicon (Si), 0.6 wt % or less of copper (Cu), 0.015 wt % or less of nitrogen (N), 0.01 wt % or less of sulfur(S), and 0.02 wt % or less of phosphorus (P).

A steel section according to still another aspect of the present invention includes 0.17 wt % or less of carbon (C), 1.6 wt % or less of manganese (Mn), 0.10 to 0.35 wt % of chromium (Cr), 0.15 wt % or less of molybdenum (Mo), 0.05 wt % or less of niobium (Nb), 0.003 wt % or less of boron (B), 0.04 wt % or less of titanium (Ti), and the remainder as iron (Fe) and other unavoidable impurities, wherein the steel section is manufactured by reheating at 1200° C. or higher, controlling the hot rolling start temperature to 1050 to 1100° C. and the rolling end temperature to 860 to 930° C., and then water cooling.

In the water cooling, the water cooling end temperature may be controlled to 680 to 880° C.

Advantageous Effects

According to an embodiment of the present invention, it is possible to achieve a high-performance steel section that exhibits both seismic-resistant and fire-resistant capabilities, and a steel section manufacturing method.

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 descriptions in the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a microstructure observation photograph of a specimen taken from the center of the flange of a steel section according to the present invention.

FIG. 2 is a flowchart of a steel section manufacturing method according to the present invention.

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 0.17 wt % or less of carbon (C), 1.6 wt % or less of manganese (Mn), 0.10 to 0.35 wt % of chromium (Cr), 0.15 wt % or less of molybdenum (Mo), 0.05 wt % or less of niobium (Nb), 0.003 wt % or less of boron (B), 0.04 wt % or less of titanium (Ti), and the remainder as iron (Fe) and other unavoidable impurities, and exhibits a yield strength (YS) at room temperature of 355 MPa or more. The steel section according to an embodiment of the present invention may further include 0.1 to 0.4 wt % of silicon (Si), 0.6 wt % or less of copper (Cu), 0.015 wt % or less of nitrogen (N), 0.01 wt % or less of sulfur(S), and 0.02 wt % or less of phosphorus (P), and exhibit a yield strength (YS) at room temperature of 355 MPa or more.

More preferably, 0.08 to 0.15 wt % of carbon (C), 0.5 to 1.6 wt % of manganese (Mn), 0.1 to 0.3 wt % of chromium (Cr), 0.10 to 0.15 wt % of molybdenum (Mo), 0.02 to 0.05 wt % of niobium (Nb), 0.03 wt % or less of titanium (Ti), and 0.001 to 0.003 wt % of boron (B) may be included.

The steel section with the above-described alloy composition may exhibit a yield strength (YS) at room temperature of 355 MPa or more. The impact absorption energy (Charpy V-notch (CVN) impact test) at 0° C. may be 27 J or more, the high-temperature yield strength at 600° C. may be 238 MPa or more, and the elongation (EL) may be 21% or more. More preferably, the yield strength (YS) at room temperature may be 400 MPa or more, the impact absorption energy (CVN impact test) at 0° C. may be 40 J or more, the high-temperature yield strength at 600° C. may be 250 MPa or more, and the elongation (EL) may be 25% or more.

The steel section with the above-described alloy composition may include bainite in its final microstructure. Thus, since the steel section according to the present invention implements a bainitic matrix structure, high-temperature yield strength may be improved as described above. In addition, fine carbides may be formed together with the bainitic matrix structure.

Thus, the steel section according to an embodiment of the present invention may be a high-performance steel section with a yield strength (YS) of 355 MPa or more, which meets the alloy composition standards of the domestic hot-rolled steel section for building structures, KS D 3866, while simultaneously exhibiting seismic-resistant and fire-resistant capabilities.

Meanwhile, the alloy composition of KS D 3866 requires 0.20 wt % or less of carbon (C), 0.40 wt % or less of silicon (Si), 1.00 to 1.60 wt % or less of manganese (Mn), 0.035 wt % or less of phosphorus (P), 0.030 wt % or less of sulfur(S), 0.35 wt % or less of chromium (Cr), 0.15 wt % or less of molybdenum (Mo), 0.60 wt % or less of copper (Cu), and 0.05 wt % or less of niobium (Nb).

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) promotes the formation of fine carbides by reacting with Nb, Ti, etc., thereby effectively enhancing the strength through precipitation strengthening. Additionally, carbon improves high-temperature strength by hindering dislocation movement at high temperatures, which effectively secures fire resistance. However, excessive addition of carbon may lead to the formation of coarse carbides, which degrades impact properties and increases the yield ratio by causing discontinuous yield behavior, thereby reducing seismic-resistant capability. Therefore, the steel section according to an embodiment of the present invention may include carbon in an amount of 0.17 wt % or less, preferably 0.08 to 0.15 wt %.

When the carbon content is less than 0.08 wt % of the total weight, it may be difficult to secure sufficient strength. On the other hand, when the carbon content exceeds 0.17 wt % of the total weight, coarse carbides may be formed, which not only degrades impact properties, but also may increase the yield ratio by causing discontinuous yield behavior, thereby reducing seismic-resistant capability.

Manganese (Mn)

Manganese (Mn) is a solid solution strengthening element that secures strength and is effective in forming a bainite structure by improving the hardening ability of steel. However, excessive addition of manganese may lead to the formation of MnS inclusions by combining with sulfur(S) or cause center segregation in ingots. Therefore, the steel section according to an embodiment of the present invention may include manganese in an amount of 1.6 wt % or less, preferably 0.5 to 1.6 wt %, and more preferably 0.5 to 1.3 wt %.

When the manganese content is less than 0.5 wt % of the total weight, the solid solution strengthening effect may not be fully exhibited. When the manganese content exceeds 1.6 wt %, manganese may combine with sulfur to form MnS inclusions or cause center segregation in ingots, thereby reducing the ductility and corrosion resistance of the steel section.

Chromium (Cr)

Chromium (Cr) may secure a bainite microstructure by improving the hardening ability of steel. However, excessive addition may increase the manufacturing cost of steel and form coarse carbides at grain boundaries, thereby reducing the ductility of steel. Therefore, the steel section according to an embodiment of the present invention may include chromium in an amount of 0.10 to 0.35 wt %, preferably 0.1 to 0.3 wt %.

When the chromium content is less than 0.1 wt % of the total weight, the effect of chromium addition may not be fully exhibited. On the other hand, when chromium is added in excess of 0.35 wt %, it may increase the manufacturing cost of steel and form coarse carbides at grain boundaries, thereby reducing the ductility of steel. Additionally, when the chromium content is 0.35 wt % or less, the alloy composition standards of the domestic hot-rolled steel section for building structures, KS D 3866, may be satisfied.

Molybdenum (Mo)

Molybdenum (Mo) may secure a bainite microstructure by improving the hardening ability of steel and is highly effective in securing high-temperature strength. However, excessive addition of molybdenum may increase the manufacturing cost of steel and promote the formation of carbides at grain boundaries, thereby reducing the ductility of steel. Therefore, the steel section according to an embodiment of the present invention may include molybdenum in an amount of 0.15 wt % or less, preferably 0.10 to 0.15 wt %.

When the molybdenum content is less than 0.10 wt % of the total weight, the effect of molybdenum addition may not be fully exhibited. When the molybdenum is added in excess of 0.15 wt %, it may increase the manufacturing cost of steel and promote the formation of carbides at grain boundaries, thereby reducing the ductility of steel. Additionally, when the molybdenum content is 0.15 wt % or less, the alloy composition standards of the domestic hot-rolled steel section for building structures, KS D 3866, may be satisfied.

Niobium (Nb)

Niobium (Nb) is an element that inhibits grain growth and leads to finer grain sizes when dissolved in austenitic structures. In addition, niobium reacts with carbon to promote the formation of fine carbides, thereby effectively enhancing strength through precipitation strengthening and particularly enhancing high-temperature yield strength. In addition, niobium is effective in enhancing high-temperature yield strength by improving the hardening ability and bainitizing the matrix structure. However, excessive addition may reduce the impact absorption energy of steel. Therefore, the steel section according to an embodiment of the present invention may include niobium in an amount of 0.05 wt % or less, preferably 0.02 to 0.05 wt %, and more preferably 0.04 to 0.05 wt %.

When the niobium content is less than 0.02 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 steel. When the niobium content is 0.04 to 0.05 wt %, the above-described beneficial effects of niobium addition may be maximized while minimizing the reduction in the impact absorption energy of steel.

Boron (B)

Boron (B) may improve the hardening ability by preferentially segregating at the austenite grain boundaries and inhibiting the formation of soft ferrite during cooling. However, excessive addition may lead to grain boundary embrittlement. Therefore, boron may be included in an amount of 0.003 wt % or less, preferably 0.001 to 0.003 wt %.

When the boron content is less than 0.001 wt % of the total weight, the effect of austenite grain boundary segregation is insufficient, and when the boron content exceeds 0.003 wt %, it may cause grain boundary embrittlement.

Titanium (Ti)

Titanium (Ti) may form TiN when combined with nitrogen. In the present invention, as a method of enhancing the hardening ability of steel, boron is added to inhibit the formation of pro-eutectoid ferrite in austenite grains. However, when boron and nitrogen combine to form BN during the steelmaking process, the mechanism for enhancing the hardening ability cannot be realized. Thus, it is necessary to apply a vacuum degassing (VD) process to limit the nitrogen content to 100 ppm or less during the steelmaking process. Additionally, titanium may be added to preferentially form TiN, which inhibits the binding of residual nitrogen and boron. This may ultimately enhance the hardening ability of steel. Thus, in an embodiment of the present invention, titanium (Ti) may be included in an amount of 0.04 wt % or less, preferably 0.03 wt % or less, and more preferably 0.02 to 0.03 wt %.

Silicon (Si)

Silicon (Si) is added as a deoxidizer along with aluminum to remove oxygen from the steel in the steelmaking process. In addition, silicon may have a solid solution strengthening effect.

Silicon may be added in an amount of 0.10 to 0.40 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 content is added in excess of 0.40 wt % of the total weight, it may degrade the weldability of steel and generate red scale during reheating and hot rolling, which may cause problems with surface quality.

Copper (Cu)

Copper (Cu) is an element that exhibits a solid solution strengthening effect by dissolving in ferrite. Additionally, during bainitic transformation, copper remains supersaturated and dissolved in the microstructure at room temperature without precipitating. When heated to 600° C. for use as fire-resistant steel, copper precipitates on the dislocations introduced by the bainitic transformation, thereby increasing strength through precipitation strengthening.

The steel section according to an embodiment of the present invention may include copper in an amount of 0.6 wt % or less of the total weight, preferably 0.5 wt % or less. When copper is added in excess of 0.6 wt % of the total weight, hot working becomes difficult, precipitation strengthening becomes saturated, toughness decreases, and problems arise that cause hot shortness.

Nitrogen (N)

Nitrogen (N) may promote grain refinement by forming nitride precipitates such as AlN and contribute to high-temperature strength. The steel section according to an embodiment of the present invention may include nitrogen in an amount of 0.015 wt % or less of the total weight, preferably 0.012 wt % or less. When the nitrogen content exceeds 0.015 wt %, it may reduce weld toughness and impact toughness.

Sulfur(S)

Sulfur(S) may improve workability by forming fine MnS precipitates. Sulfur may be added in an amount of 0.01 wt % or less of the total weight of the steel section according to an embodiment of the present invention. When the sulfur content exceeds 0.01 wt %, it may act as a tramp element, forming inclusions that may reduce the ductility of steel and degrade the toughness and weldability of steel.

Phosphorus (P)

Phosphorus (P) may increase the strength of steel through solid solution strengthening and inhibit the formation of carbides. Phosphorus may be added in an amount of 0.02 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 steel, and its precipitation behavior may decrease impact toughness.

Meanwhile, a steel section according to an embodiment of the present invention includes 0.17 wt % or less of carbon (C), 1.6 wt % or less of manganese (Mn), 0.10 to 0.35 wt % of chromium (Cr), 0.15 wt % or less of molybdenum (Mo), 0.05 wt % or less of niobium (Nb), 0.003 wt % or less of boron (B), 0.04 wt % or less of titanium (Ti), and the remainder as iron (Fe) and other unavoidable impurities, and is manufactured by reheating at 1200° C. or higher, controlling the hot rolling start temperature to 1050 to 1100° C. and the rolling end temperature to 860 to 930° C., and then water cooling. In the water cooling, the water cooling end temperature may be controlled to 680 to 880° C.

The steel section may further include 0.1 to 0.4 wt % of silicon (Si), 0.6 wt % or less of copper (Cu), 0.015 wt % or less of nitrogen (N), 0.01 wt % or less of sulfur(S), and 0.02 wt % or less of phosphorus (P). More preferably, 0.08 to 0.15 wt % of carbon (C), 0.5 to 1.6 wt % of manganese (Mn), 0.1 to 0.3 wt % of chromium (Cr), 0.10 to 0.15 wt % of molybdenum (Mo), 0.02 to 0.05 wt % of niobium (Nb), 0.03 wt % or less of titanium (Ti), and 0.001 to 0.003 wt % of boron (B) may be included.

Thus, the steel section according to an embodiment of the present invention may be a high-performance steel section with a yield strength (YS) of 355 MPa or more, which meets the alloy composition standards of the domestic hot-rolled steel section for building structures, KS D 3866, while simultaneously exhibiting seismic-resistant and fire-resistant capabilities.

The steel section with the above-described alloy composition may have a yield strength (YS) at room temperature of 355 MPa or more. In addition, the impact absorption energy (CVN impact test) at 0° C. may be 27 J or more, the high-temperature yield strength at 600° C. may be 238 MPa or more, and the elongation (EL) may be 21% or more. More specifically, the yield strength (YS) at room temperature may be 400 MPa or more, the impact absorption energy (CVN impact test) at 0° C. may be 40 J or more, the high-temperature yield strength at 600° C. may be 250 MPa or more, and the elongation (EL) may be 25% or more.

FIG. 1 is a microstructure observation photograph of a specimen taken from the center of the flange of a steel section according to the present invention.

Referring to FIG. 1, the steel section with the above-described alloy composition may include bainite in its final microstructure. In addition, ferrite and fine carbides may be formed along with the bainitic matrix structure. Thus, the steel section according to the present invention may effectively improve high-temperature yield strength by forming Cr-, Mo-, and Nb-based carbides and securing a bainitic matrix structure.

Steel Section Manufacturing Method

Referring to FIG. 2, a steel section manufacturing method according to an embodiment of the present invention includes (a) reheating steel (S10), (b) hot rolling the steel (S20), and (c) water cooling the steel (S30).

The steel includes 0.17 wt % or less of carbon (C), 1.6 wt % or less of manganese (Mn), 0.10 to 0.35 wt % of chromium (Cr), 0.15 wt % or less of molybdenum (Mo), 0.05 wt % or less of niobium (Nb), 0.003 wt % or less of boron (B), 0.04 wt % or less of titanium (Ti), and the remainder as iron (Fe) and other unavoidable impurities, and in step (a), the steel is reheated at a temperature of 1200° C. or higher. Next, in step (b), the steel is hot-rolled with the rolling start temperature controlled to 1050 to 1100° C. and the rolling end temperature controlled to 860 to 930° C. Afterward, the steel is water-cooled in step (c).

Thus, the steel section manufacturing method according to an embodiment of the present invention may manufacture a high-performance steel section with a yield strength (YS) of 355 MPa or more, which meets the alloy composition standards of the domestic hot-rolled steel section for building structures, KS D 3866, while exhibiting both seismic-resistant and fire-resistant capabilities.

Hereinafter, each step of the steel section manufacturing method is described in detail.

In the reheating step, the steel with the above-described composition is reheated at a temperature of 1200° C. or higher. When the reheating temperature is lower than 1200° C., the dissolution 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 may not exceed 1250° C. When the reheating temperature exceeds 1250° 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.

Meanwhile, the steel may be manufactured by obtaining molten steel with the 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 limited thereto.

The steel may further include 0.1 to 0.4 wt % of silicon (Si), 0.6 wt % or less of copper (Cu), 0.015 wt % or less of nitrogen (N), 0.01 wt % or less of sulfur(S), and 0.02 wt % or less of phosphorus (P). More preferably, the steel may include 0.08 to 0.15 wt % of carbon (C), 0.5 to 1.6 wt % of manganese (Mn), 0.1 to 0.3 wt % of chromium (Cr), 0.10 to 0.15 wt % of molybdenum (Mo), 0.02 to 0.05 wt % of niobium (Nb), 0.03 wt % or less of titanium (Ti), and 0.001 to 0.003 wt % of boron (B).

In the hot rolling step (b), the reheated steel is hot-rolled. At this time, the rolling start temperature is controlled to 1050 to 1100° C., and the rolling end temperature is controlled to 860 to 930° C. This allows the steel to secure a bainitic matrix structure and high-temperature yield strength even with relatively low contents of chromium (Cr) and molybdenum (Mo). Particularly, when the rolling end temperature is below 860° C., rolling proceeds in the non-recrystallization region, which may increase the rolling load and raise the yield ratio of the resulting steel section. Additionally, when the rolling end temperature exceeds 930° C., it may be difficult to secure the desired strength and toughness.

Meanwhile, the water cooling step (c) is performed after hot rolling. In the steel section manufacturing method according to the present invention, the water cooling end temperature (or cooling and reheating temperature) may be controlled to 680 to 880° C. The water cooling step may be carried out using a surface accelerated cooling system, such as a quenching and self tempering (QST) system, which cools and self-tempers the hot-rolled steel section. In the water cooling, a quenching method where cooling water is sprayed onto the steel section may be applied, and by controlling the transport speed of the steel section or the amount of cooling water sprayed, the water cooling end temperature and self-tempering temperature may be controlled to 680 to 880° C., more preferably 720 to 760° C.

The steel or steel section that has undergone the step (c) exhibits a yield strength (YS) at room temperature of 355 MPa or more. The impact absorption energy (CVN impact test) at 0° C. may be 27 J or more, the high-temperature yield strength at 600° C. may be 238 MPa or more, and the elongation (EL) may be 21% or more. More specifically, the yield strength (YS) at room temperature may be 400 MPa or more, the impact absorption energy (CVN impact test) at 0° C. may be 40 J or more, the high-temperature yield strength at 600° C. may be 250 MPa or more, and the elongation (EL) may be 25% or more.

The steel or steel section that has undergone step (c) may include bainite in its final microstructure. Thus, since the steel section according to the present invention implements a bainitic matrix structure, high-temperature yield strength may be improved as described above. In addition, fine carbides may be formed along with the bainitic matrix structure.

According to an embodiment of the present invention, the steel section and the steel section manufacturing method may improve the room-temperature yield strength by allowing niobium to inhibit the grain growth of austenite, resulting in finer grain sizes, and enhance the high-temperature yield strength by forming carbides and by improving the hardening ability and bainitizing the matrix structure.

In addition, titanium preferentially combines with residual nitrogen in the steel to form TiN, thereby inhibiting the bonding and formation of BN. This allows boron to enhance the hardening ability of the steel, resulting in obtaining a bainitic matrix structure and securing seismic-resistant and fire-resistant properties.

Additionally, the content of chromium and molybdenum, which are effective in improving the hardening ability and bainitizing the matrix structure, is kept low to meet the alloy addition limit criteria within the KS D 3866 standard. At the same time, to compensate for the insufficient bainitic matrix structure due to the low content of chromium and molybdenum, the rolling end temperature is controlled to 860 to 930° C. and the water cooling end temperature is controlled to 680 to 880° C.

Thus, the present invention may manufacture a high-performance steel section with a yield strength of 355 MPa or more, which exhibits both seismic-resistant and fire-resistant capabilities, while meeting the KS D 3866 standard.

Comparative Examples and Experimental Examples

Hereinafter, preferred comparative and experimental examples are presented to help understand the present invention. However, the experimental examples below are only for illustrative purposes and do not limit the scope of the present invention.

Tables 1 and 2 show the main alloy compositions (units: wt %) of the experimental and comparative examples. Table 3 shows the process conditions used to manufacture the specimens of the experimental and comparative examples. Table 4 shows the measured mechanical properties of the specimens prepared under the process conditions described in Table 3. Beam blanks with the compositions specified in Tables 1 and 2 were manufactured using an electric arc furnace and then subjected to hot rolling to produce H-section steel with a flange thickness of 15 mm.

TABLE 1
Component	C	Mn	Cr	Mo	Nb	Ti	B

KS D 3866	≤0.20	1.0-1.6	≤0.35	≤0.15	≤0.05	—	—
Composition 1	0.16	1.38	0.62	0.35	0.017	0.05	0.002
Composition 2	0.08	1.2	0.61	0.35	0.018	0.04	0.002
Composition 3	0.08	1.21	0.37	0.17	0.019	0.032	0.002
Composition 4	0.08	1.2	0.21	0.14	0.044	0.025	0.0022
Composition 5	0.08	1.2	0.34	0.15	0.02	0.03	0.002

TABLE 2
Component	Si	P	S	Cu	N

KS D 3866	≤0.40	≤0.035	≤0.030	≤0.60	—
Composition 1	0.2	0.019	0.009	0.2	0.01
Composition 2	0.22	0.01	0.005	0.21	0.01
Composition 3	0.21	0.01	0.003	0.19	0.01
Composition 4	0.22	0.009	0.001	0.13	0.01
Composition 5	0.21	0.008	0.002	0.18	0.01

TABLE 3
					Cooling and
		Reheating	Rolling start	Rolling end	reheating
Classification	Composition	temperature	temperature	temperature	temperature
Comparative	Composition 1	1200-1250	1050-1100	910-950	765-800
Example 1
Comparative	Composition 2	1200-1250	1050-1100	860-930	720-760
Example 2
Comparative	Composition 3	1200-1250	1050-1100	860-930	720-760
Example 3
Comparative	Composition 5	1200-1250	1050-1100	910-950	765-800
Example 4
Experimental	Composition 4	1200-1250	1050-1100	860-930	720-760
Example 1
Experimental	Composition 5	1200-1250	1050-1100	860-930	720-760
Example 2

	TABLE 4
		High-
		temperature
	Room-temperature physical properties	physical

				Yield		CVN	properties
Classi-		TS	YS	ratio	EL	(J, at	at 600° C.
fication	Composition	(MPa)	(MPa)	(%)	(%)	0° C.)	YS (MPa)

KS D 3866	—	490-610	355-475	≤80	≥21	≥27	≥238
Comparative	Composi-	579	383	66	23	28	243-246
Example 1	tion 1
Comparative	Composi-	577	370	64	23	29	243-245
Example 2	tion 2
Comparative	Composi-	578	395	68	24.8	38	247-249
Example 3	tion 3
Comparative	Composi-	575	380	66	23	27	241-243
Example 4	tion 5
Experimental	Composi-	576	415	71	27.5	43	276-278
Example 1	tion 4
Experimental	Composi-	575	408	71	27	40	268-271
Example 2	tion 5

Comparative Example 1 and Experimental Example 1

Comparative Example 1 differs from Experimental Example 1 in terms of the composition, the rolling end temperature, and the cooling and reheating temperature.

Referring to Tables 1 to 3, the composition 1 of Comparative Example 1 includes 0.16 wt % of carbon, 0.62 wt % of chromium, 0.35 wt % of molybdenum, 0.017 wt % of niobium, 0.05 wt % of titanium, and other alloy elements, and the rolling end temperature is 910 to 950° C., and the cooling and reheating temperature is 765 to 800° C.

The composition 4 of Experimental Example 1 includes 0.08 wt % of carbon, 0.21 wt % of chromium, 0.14 wt % of molybdenum, 0.044 wt % of niobium, 0.025 wt % of titanium, and other alloy elements. Compared to the composition 1 of Comparative Example 1, Experimental Example 1 has lower contents of carbon, chromium, molybdenum, and titanium, but a higher content of niobium. Additionally, the rolling end temperature of Experimental Example 1 is 860 to 930° C. and the cooling and reheating temperature of Experimental Example 1 is 720 to 760° C., both of which are lower than those of Comparative Example 1.

Referring to Table 4, Experimental Example 1 exhibits a room-temperature yield strength of 415 MPa, a yield ratio of 71%, an elongation of 27.5%, an impact absorption energy at 0° C. of 43 J, and a high-temperature yield strength at 600° C. of 276 to 278 MPa. It can be seen that these physical properties are improved compared to those of Comparative Example 1.

Comparative Example 2 and Experimental Example 1

Comparative Example 2 differs from Experimental Example 1 in terms of the composition but has the same rolling end temperature and cooling and reheating temperature.

Referring to Tables 1 to 3, the composition 2 of Comparative Example 2 includes 0.08 wt % of carbon, 0.61 wt % of chromium, 0.35 wt % of molybdenum, 0.018 wt % of niobium, 0.04 wt % of titanium, and other alloy elements, and the rolling end temperature is 860 to 930° C., and the cooling and reheating temperature is 720 to 760° C.

The composition 4 of Experimental Example 1 includes 0.08 wt % of carbon, 0.21 wt % of chromium, 0.14 wt % of molybdenum, 0.044 wt % of niobium, 0.025 wt % of titanium, and other alloy elements. Compared to the composition 2 of Comparative Example 2, Experimental Example 1 has lower contents of chromium, molybdenum, and titanium, but a higher content of niobium. Additionally, the rolling end temperature and the cooling and reheating temperature of Experimental Example 1 are the same as those of Comparative Example 2, at 860 to 930° C. and 720 to 760° C., respectively.

Referring to Table 4, Experimental Example 1 exhibits a room-temperature yield strength of 415 MPa, a yield ratio of 71%, an elongation of 27.5%, an impact absorption energy at 0° C. of 43 J, and a high-temperature yield strength at 600° C. of 276 to 278 MPa. It can be seen that these physical properties are improved compared to those of Comparative Example 2.

Comparative Example 2 and Experimental Example 2

Comparative Example 2 differs from Experimental Example 2 in terms of the composition but has the same rolling end temperature and cooling and reheating temperature.

Referring to Tables 1 to 3, the composition 2 of Comparative Example 2 includes 0.08 wt % of carbon, 0.61 wt % of chromium, 0.35 wt % of molybdenum, 0.018 wt % of niobium, 0.04 wt % of titanium, and other alloy elements, and the rolling end temperature is 860 to 930° C., and the cooling and reheating temperature is 720 to 760° C.

The composition 5 of Experimental Example 2 includes 0.08 wt % of carbon, 0.34 wt % of chromium, 0.15 wt % of molybdenum, 0.020 wt % of niobium, 0.03 wt % of titanium, and other alloy elements. Compared to the composition 2 of Comparative Example 2, Experimental Example 2 has lower contents of chromium, molybdenum, and titanium, but a higher content of niobium. Additionally, the rolling end temperature and the cooling and reheating temperature of Experimental Example 2 are the same as those of Comparative Example 2, at 860 to 930° C. and 720 to 760° C., respectively.

Referring to Table 4, Experimental Example 2 exhibits improved physical properties compared to Comparative Example 2, including the room-temperature yield strength, the yield ratio, the elongation, the impact absorption energy at 0° C., and the high-temperature yield strength at 600° C.

Comparative Example 3 and Experimental Example 2

Comparative Example 3 differs from Experimental Example 2 in terms of the composition but has the same rolling end temperature and cooling and reheating temperature.

Referring to Tables 1 to 3, the composition 3 of Comparative Example 3 includes 0.08 wt % of carbon, 0.37 wt % of chromium, 0.17 wt % of molybdenum, 0.019 wt % of niobium, 0.032 wt % of titanium, and other alloy elements, and the rolling end temperature is 860 to 930° C., and the cooling and reheating temperature is 720 to 760° C.

The composition 5 of Experimental Example 2 includes 0.08 wt % of carbon, 0.34 wt % of chromium, 0.15 wt % of molybdenum, 0.020 wt % of niobium, 0.03 wt % of titanium, and other alloy elements. Compared to the composition 3 of Comparative Example 3, Experimental Example 2 has lower contents of chromium, molybdenum, and titanium, but a higher content of niobium. Additionally, the rolling end temperature and the cooling and reheating temperature of Experimental Example 2 are the same as those of Comparative Example 3, at 860 to 930° C. and 720 to 760° C., respectively.

Referring to Table 4, it can be seen that Experimental Example 2 exhibits improved physical properties compared to Comparative Example 3, including the room-temperature yield strength, the yield ratio, the elongation, the impact absorption energy at 0° C., and the high-temperature yield strength at 600° C.

Comparative Example 4 and Experimental Example 2

Comparative Example 4 has the same composition as Experimental Example 2 but differs from it in terms of the rolling end temperature and the cooling and reheating temperature.

Referring to Tables 1 to 3, the composition 5 of Comparative Example 4 includes 0.08 wt % of carbon, 0.34 wt % of chromium, 0.15 wt % of molybdenum, 0.020 wt % of niobium, 0.03 wt % of titanium, and other alloy elements, and the rolling end temperature is 910 to 950° C., and the cooling and reheating temperature is 765 to 800° C.

The composition 5 of Experimental Example 2 includes 0.08 wt % of carbon, 0.34 wt % of chromium, 0.15 wt % of molybdenum, 0.020 wt % of niobium, 0.03 wt % of titanium, and other alloy elements, which is the same as the composition of Comparative Example 4. Additionally, the rolling end temperature of Experimental Example 2 is 860 to 930° C., and the cooling and reheating temperature of Experimental Example 2 is 720 to 760° C., both of which are lower than those of Comparative Example 4.

Referring to Table 4, it can be seen that Experimental Example 2 exhibits improved physical properties compared to Comparative Example 4, including the room-temperature yield strength, the yield ratio, the elongation, the impact absorption energy at 0° C., and the high-temperature yield strength at 600° C.

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.

LIST OF REFERENCE NUMERALS

• • S10: Reheating
  • S20: Hot rolling
  • S30: Water cooling