Structural steel and methods for manufacturing structural steel

Abstract

A method for manufacturing structural steel according to one embodiment of the present invention includes the steps of: (a) reheating a steel material containing 0.17% by weight or less of carbon (C), 1.6% by weight or less of manganese (Mn), 0.10 to 0.35% by weight of chromium (Cr), 0.15% by weight or less of molybdenum (Mo), 0.05% by weight or less of niobium (Nb), 0.003% by weight or less of boron (B), 0.04% by weight or less of titanium (Ti), and the remaining iron (Fe) and other unavoidable impurities to 1200°C or higher; (b) hot rolling the steel material, controlling the rolling start temperature to 1050 to 1100°C and the rolling end temperature to 860 to 930°C; and (c) water cooling the steel material. This method embodies high-performance structural steel and a method for manufacturing structural steel that simultaneously possess seismic resistance and fire resistance.

Description

Technical Field

[0001] The present invention relates to a shaped steel and a method for manufacturing the shaped steel.

Background Art

[0002] The shaped steel generally means a steel material having a polygonal cross-sectional shape change. The shaped steel is applied as a structural steel material such as a column of a large building, and is also applied as a temporary civil engineering material such as a subway or a bridge, or a foundation pile. The shaped steel can be manufactured by hot rolling a slab such as a bloom, a billet, or a beam blank produced by continuous casting.

[0003] Recently, large earthquakes have occurred worldwide, causing great damage to human lives and property. In particular, in 2016 and 2017, strong earthquakes of magnitude 5.0 or more occurred successively in Gyeongju and Pohang, and the sense of uneasiness is increasing.

[0004] At the time of an earthquake, a fire that may occur secondarily along with the primary damage caused by the damage of a building can cause the softening of the reinforcing material that supports the structure, and can cause the collapse of the building together with the plastic deformation of the reinforcing material due to the earthquake. As a result, recently, building design standards have been strengthened to delay the collapse of buildings and minimize damage to human lives and property even in disaster situations such as earthquakes and fires in high-rise buildings.

[0005] In order to strengthen the safety of such buildings, it is essential to improve the seismic and fire resistance of building structural materials used in the production of structures, together with the seismic design of building designs and the installation of protective facilities such as sprinklers.

[0006] For this purpose, seismic steel that ensures seismic performance resistant to earthquakes by controlling the yield ratio and fire-resistant steel that can withstand fires by improving the high-temperature strength have been developed and used respectively.

[0007] However, as mentioned earlier, there is a risk of a series of fires breaking out due to building damage during an earthquake, so there is a growing demand for fire-resistant and earthquake-resistant steel that possesses both seismic and fire-resistant properties to prepare for such a situation. [Overview of the project] [Problems that the invention aims to solve]

[0008] In order to solve the problems of the conventional technology described above, the purpose of the structural steel and structural steel manufacturing method according to the present invention is to provide high-performance structural steel and structural steel manufacturing method having fire-resistant and seismic-resistant properties.

[0009] The problems addressed by the present invention are not limited to those mentioned above, and any other problems not mentioned will be clearly understood by those skilled in the art from the following description. [Means for solving the problem]

[0010] A method for manufacturing structural steel according to one embodiment of the present invention includes the steps of: (a) reheating a steel material containing 0.17% by weight or less of carbon (C), 1.6% by weight or less of manganese (Mn), 0.10 to 0.35% by weight of chromium (Cr), 0.15% by weight or less of molybdenum (Mo), 0.05% by weight or less of niobium (Nb), 0.003% by weight or less of boron (B), 0.04% by weight or less of titanium (Ti), and the remaining iron (Fe) and other unavoidable impurities to 1200°C or higher; (b) hot rolling the steel material, controlling the rolling start temperature to 1050 to 1100°C and the rolling end temperature to 860 to 930°C; and water cooling the steel material.

[0011] Furthermore, step (c) above can control the water cooling termination temperature to 680-880°C.

[0012] Furthermore, the steel material subjected to step (c) above can satisfy the following requirements: yield strength (YS) of 355 MPa or higher at room temperature, impact absorption energy (CVN) of 27 J or higher at 0°C, and elongation (EL) of 21% or higher.

[0013] Furthermore, the steel material that has undergone step (c) above may have a high-temperature yield strength (YS) of 238 MPa or higher at 600°C.

[0014] Furthermore, the steel material obtained by step (c) above may contain bainite in its final microstructure.

[0015] Furthermore, the steel material may contain 0.08-0.15% by weight of carbon (C), 0.5-1.6% by weight of manganese (Mn), 0.1-0.3% by weight of chromium (Cr), 0.10-0.15% by weight of molybdenum (Mo), 0.02-0.05% by weight of niobium (Nb), 0.03% by weight or less of titanium (Ti), and 0.001-0.003% by weight of boron (B).

[0016] Furthermore, the steel material may further contain 0.1 to 0.4% by weight of silicon (Si), 0.6% by weight or less of copper (Cu), 0.015% by weight or less of nitrogen (N), 0.01% by weight or less of sulfur (S), and 0.02% by weight or less of phosphorus (P).

[0017] A structural steel according to one embodiment of the present invention contains 0.17% by weight or less of carbon (C), 1.6% by weight or less of manganese (Mn), 0.10 to 0.35% by weight of chromium (Cr), 0.15% by weight or less of molybdenum (Mo), 0.05% by weight or less of niobium (Nb), 0.003% by weight or less of boron (B), 0.04% by weight or less of titanium (Ti), and the remaining iron (Fe) and other unavoidable impurities, and satisfies the requirement of a yield strength (YS) of 355 MPa or more at room temperature.

[0018] Furthermore, the shock absorption energy (CVN) at 0°C can be 27J or more.

[0019] Furthermore, the high-temperature yield strength at 600°C may be 238 MPa or higher.

[0020] Furthermore, the elongation ratio (EL) may be 21% or higher.

[0021] Furthermore, the final microstructure may contain bainite.

[0022] Alternatively, it may contain 0.08 to 0.15% by weight of carbon (C), 0.5 to 1.6% by weight of manganese (Mn), 0.1 to 0.3% by weight of chromium (Cr), 0.10 to 0.15% by weight of molybdenum (Mo), 0.02 to 0.5% by weight of niobium (Nb), 0.03% by weight or less of titanium (Ti), and 0.001 to 0.003% by weight of boron (B).

[0023] Furthermore, it may further contain 0.1 to 0.4% by weight of silicon (Si), 0.6% by weight or less of copper (Cu), 0.015% by weight or less of nitrogen (N), 0.01% by weight or less of sulfur (S), and 0.02% by weight or less of phosphorus (P).

[0024] The section steel according to an embodiment of the present invention contains 0.17% by weight or less of carbon (C), 1.6% by weight or less of manganese (Mn), 0.10 to 0.35% by weight of chromium (Cr), 0.15% by weight or less of molybdenum (Mo), 0.05% by weight or less of niobium (Nb), 0.003% by weight or less of boron (B), 0.04% by weight or less of titanium (Ti), the balance being iron (Fe) and other inevitable impurities, and after reheating at 1200°C or higher, the hot rolling starting temperature is controlled to 1050 to 1100°C, and the rolling end temperature is controlled to 860 to 930°C, and then it may be manufactured by a method of water cooling.

[0025] Also, in the method of water cooling, it may be manufactured by controlling the water cooling end temperature to 680 to 880°C.

Advantages of the Invention

[0026] According to an embodiment of the present invention, it is possible to embody a high-performance section steel having both earthquake resistance performance and fire resistance performance and a method for manufacturing the section steel.

[0027] The effects of the present invention are not limited to the effects mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art from the description of the claims.

Brief Description of the Drawings

[0028] [Figure 1] FIG. 1 is a photograph of the microstructure observation of a specimen at the center of the flange of the section steel according to the present invention. [Figure 2] Figure 2 is a flowchart showing the method for manufacturing structural steel according to the present invention. [Modes for carrying out the invention]

[0029] In this specification, when a component (or region, layer, part, etc.) is referred to as being "on top of," "connected to," or "joined" another component, it means that it can be directly placed on top of / connected to / joined to the other component, or that a third component can be placed between them.

[0030] The same drawing reference numeral indicates the same component. Furthermore, in drawings, the thickness, proportions, and dimensions of components are exaggerated for the sake of effectively illustrating the technical content.

[0031] "and / or" includes all combinations of one or more that are defined by the relevant configuration.

[0032] Terms such as "first," "second," etc., are used to describe a variety of components, but the components are not limited to those defined 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. A singular expression includes plural expressions unless the context clearly indicates otherwise.

[0033] Furthermore, terms such as "down," "on the lower side," "up," and "on the upper side" are used to describe the correlations of the configuration shown in the drawings. These terms are relative concepts and are described in relation to the directions shown in the drawings.

[0034] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as those generally understood by those skilled in the art to which the present invention pertains. Furthermore, terms such as those defined in commonly used dictionaries should be interpreted as having the meaning consistent with their meaning in the context of the relevant art, and are expressly defined herein unless interpreted in an ideal or overly formal sense.

[0035] Terms such as "includes" or "possesses" mean that the features, figures, steps, actions, components, parts, or combinations thereof described in the specification exist, and should be understood not to preclude the existence or possibility of adding one or more other features, figures, steps, actions, components, parts, or combinations thereof.

[0036] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0037] Shaped steel

[0038] A structural steel according to one embodiment of the present invention contains 0.17% by weight or less of carbon (C), 1.6% by weight or less of manganese (Mn), 0.10 to 0.35% by weight of chromium (Cr), 0.15% by weight or less of molybdenum (Mo), 0.05% by weight or less of niobium (Nb), 0.003% by weight or less of boron (B), 0.04% by weight or less of titanium (Ti), the remaining iron (Fe) and other unavoidable impurities, and satisfies a yield strength (YS) of 355 MPa or more at room temperature. Furthermore, a structural steel according to one embodiment of the present invention may further contain 0.1 to 0.4% by weight of silicon (Si), 0.6% by weight or less of copper (Cu), 0.015% by weight or less of nitrogen (N), 0.01% by weight or less of sulfur (S), and 0.02% by weight or less of phosphorus (P), and can satisfy a yield strength (YS) of 355 MPa or more at room temperature.

[0039] More preferably, the carbon (C) may be 0.08 to 0.15% by weight, the manganese (Mn) 0.5 to 1.6% by weight, the chromium (Cr) 0.1 to 0.3% by weight, the molybdenum (Mo) 0.10 to 0.15% by weight, the niobium (Nb) 0.02 to 0.05% by weight, the titanium (Ti) 0.03% by weight or less, and the boron (B) 0.001 to 0.003% by weight.

[0040] The structural steel having the alloy composition described above satisfies the requirement of a yield strength (YS) of 355 MPa or more at room temperature. Furthermore, it may also have a shock absorption energy (CVN) of 27 J or more at 0°C, a high-temperature yield strength of 238 MPa or more at 600°C, and an elongation ratio (EL) of 21% or more. More specifically, it may also have a yield strength (YS) of 400 MPa or more at room temperature, a shock absorption energy (CVN) of 40 J or more at 0°C, a high-temperature yield strength of 250 MPa or more at 600°C, and an elongation ratio (EL) of 25% or more.

[0041] Furthermore, the structural steel having the alloy composition described above may contain bainite in its final microstructure. That is, the structural steel according to the present invention can improve its high-temperature yield strength as described above by embodying a bainitic matrix structure. In addition, fine carbides may be embodied along with the bainitic matrix structure.

[0042] As a result, a structural steel according to one embodiment of the present invention may satisfy the alloy composition system standards of KS D 3866, the standard for hot-rolled structural steel for building structures in South Korea, and may also be a high-performance structural steel with a yield strength (YS) of 355 MPa or higher, possessing both seismic resistance and fire resistance.

[0043] On the other hand, the alloy composition of KS D 3866 must satisfy the following: carbon (C) 0.20 wt% or less, silicon (Si) 0.40 wt% or less, manganese (Mn) 1.00 to 1.60 wt% or less, phosphorus (P) 0.035 wt% or less, sulfur (S) 0.030 wt% or less, chromium (Cr) 0.35 wt% or less, molybdenum (Mo) 0.15 wt% or less, copper (Cu) 0.60 wt% or less, and niobium (Nb) 0.05 wt% or less.

[0044] The roles and contents of each alloying element contained in structural steel according to one embodiment of the present invention will be described in detail below.

[0045] Carbon (C)

[0046] Carbon effectively contributes to improving strength through precipitation strengthening by promoting the formation of fine carbides through reaction with Nb, Ti, etc., while also improving high-temperature strength and ensuring fire resistance by inhibiting potential transfer at high temperatures. On the other hand, when added in excess, it can not only generate coarse carbides and reduce impact properties, but also increase the yield ratio and reduce seismic performance by causing discontinuous yielding behavior. For this reason, the structural steel according to one embodiment of the present invention may contain carbon at a concentration of 0.17% by weight or less, preferably 0.08 to 0.15% by weight.

[0047] In other words, if the carbon content is less than 0.08% by weight of the total weight, it may be difficult to ensure sufficient strength. Conversely, if the carbon content exceeds 0.17% by weight of the total weight, coarse carbides are formed, which not only reduces impact properties but also causes discontinuous yielding behavior, increasing the yield ratio and reducing seismic performance.

[0048] Manganese (Mn)

[0049] Manganese, as a solid solution strengthening element, not only contributes to ensuring strength but also improves the hardening ability of steel and is effective in forming a bainite structure. On the other hand, when added in excess, it can combine with sulfur to form MnS inclusions or cause central segregation in the ingot. For this reason, the structural steel according to one embodiment of the present invention may contain manganese in an amount of 1.6% by weight or less, preferably 0.5 to 1.6% by weight, and more preferably 0.5 to 1.3% by weight.

[0050] In other words, if the manganese content is less than 0.5% by weight of the total weight, the solid solution strengthening effect cannot be fully exerted, and if it exceeds 1.6% by weight, it can combine with sulfur to form MnS inclusions or cause central segregation in the ingot, which can reduce the ductility and corrosion resistance of the structural steel.

[0051] Chromium (Cr)

[0052] Chromium can improve the hardening ability of steel and contribute to the preservation of a bainite microstructure, but if added in excess, it can increase the manufacturing cost of steel and reduce the ductility of steel by forming coarse carbides at grain boundaries. Therefore, the structural steel according to one embodiment of the present invention may contain 0.10 to 0.35% by weight of chromium, preferably 0.1 to 0.3% by weight.

[0053] In other words, if the chromium content is less than 0.1% by weight of the total weight, the effect of adding chromium cannot be properly exerted. Conversely, if the chromium content exceeds 0.35% by weight, it can increase the unit cost of steel production and reduce the ductility of the steel by forming coarse carbides at the grain boundaries. Also, if the chromium content is 0.35% by weight or less, it can meet the alloy composition standards of KS D 3866, the standard for hot-rolled structural steel in South Korea.

[0054] Molybdenum (Mo)

[0055] Molybdenum can improve the hardening ability of steel and contribute to securing a bainite microstructure, making it a very effective element for ensuring high-temperature strength. However, if added in excess, it can increase the manufacturing cost of steel and promote the formation of grain boundary carbides, thereby reducing the ductility of the steel. Therefore, the structural steel according to one embodiment of the present invention may contain molybdenum in an amount of 0.15% by weight or less, and preferably in an amount of 0.10 to 0.15% by weight.

[0056] In other words, if the molybdenum content is less than 0.10% by weight of the total weight, the effect of adding molybdenum cannot be properly exerted, and if it exceeds 0.15% by weight, it can increase the unit cost of steel production and promote the formation of grain boundary carbides, thereby reducing the ductility of the steel. On the other hand, if the molybdenum content is 0.15% by weight or less, it can meet the alloy composition standards of KS D 3866, the standard for hot-rolled structural steel in South Korea.

[0057] Niobium (Nb)

[0058] Niobium, when dissolved in an austenite structure, inhibits grain growth and results in fine grain size. Furthermore, by reacting with carbon to promote the formation of fine carbides, it is effective in improving strength through precipitation strengthening, particularly improving high-temperature yield strength. It also improves hardening ability and bainites the matrix structure, further enhancing high-temperature yield strength. However, when added in excess, it can reduce the impact absorption energy of the steel. Therefore, the structural steel according to one embodiment of the present invention may contain niobium in an amount of 0.05% by weight or less, preferably 0.02 to 0.05% by weight, and more preferably 0.04 to 0.05% by weight.

[0059] In other words, if the niobium content is less than 0.02% by weight of the total weight, the effect of adding niobium cannot be properly exerted, and if it is added in large quantities exceeding 0.05% by weight, the impact absorption energy of the steel can be reduced. On the other hand, if the niobium content is between 0.04% and 0.05% by weight, the effect of adding niobium described above can be maximized while minimizing the reduction in the impact absorption energy of the steel.

[0060] Boron (B)

[0061] Boron first segregates at austenite grain boundaries, suppressing the formation of ferrite, a soft structure, during cooling and thereby improving hardening ability. However, when added in excess, it can lead to grain boundary brittleness. For this reason, boron (B) may be included in amounts of 0.003% by weight or less, preferably in amounts of 0.001% to 0.003% by weight.

[0062] In other words, if the boron content is less than 0.001% by weight of the total weight, the austenite grain boundary segregation effect is insufficient, and if it exceeds 0.003% by weight, grain boundary brittleness problems may occur.

[0063] Titanium (Ti)

[0064] Titanium can form TiN with nitrogen. In this invention, the addition of boron is used to suppress the formation of protereminate ferrite in austenite grains as a method to improve the hardening ability of steel. However, if boron and nitrogen combine to form BN during the steelmaking process, the mechanism for improving hardening ability cannot be realized. Therefore, it is necessary to apply a VD (Vacuum Degassing) process in the steelmaking process to limit the content to 100 ppm or less, and titanium can be added to suppress the combination of residual nitrogen and boron, thereby first forming TiN, and consequently, it can play a role in improving the hardening ability of the steel. For this reason, in one embodiment of the present invention, titanium (Ti) may be included in an amount of 0.04% by weight or less, preferably 0.03% by weight or less, and more preferably 0.02 to 0.03% by weight.

[0065] Silicon (Si)

[0066] Silicon (Si), along with aluminum, is added as a deoxidizing agent in the steelmaking process to remove oxygen from the steel. Silicon may also have a solid solution strengthening effect.

[0067] Silicon may be added in a content ratio of 0.10 to 0.40% by weight of the total weight of the structural steel according to one embodiment of the present invention. If the silicon content is less than 0.10% by weight of the total weight, the silicon addition effect cannot be properly exhibited. Conversely, if the silicon content exceeds 0.40% by weight of the total weight, it can reduce the weldability of the steel and cause problems in surface quality by generating red scale during reheating and hot rolling.

[0068] Copper (Cu)

[0069] Copper (Cu) is an element that exhibits a solid solution strengthening effect by dissolving in ferrite. Furthermore, during the bainite transformation, supersaturated copper that does not precipitate remains dissolved in the structure at room temperature. When heated to 600°C, the operating temperature for refractory steel, the copper phase precipitates on the potential introduced by the bainite transformation, and its precipitation hardening increases the yield strength.

[0070] Copper may be added in a content ratio of 0.6% by weight or less of the total weight of the structural steel according to one embodiment of the present invention, and preferably in a content ratio of 0.5% by weight or less. When the copper content exceeds 0.6% by weight of the total weight and is added in large quantities, hot working becomes difficult, precipitation strengthening becomes saturated, toughness decreases, and problems such as red-hot brittleness occur.

[0071] Nitrogen (N)

[0072] Nitrogen (N) can contribute to grain refinement by forming nitride-based precipitates such as AlN, thereby contributing to ensuring high-temperature strength. The nitrogen may be added in a content ratio of 0.015% by weight or less of the total weight of the structural steel according to one embodiment of the present invention, and preferably in a content ratio of 0.012% by weight or less. If the nitrogen content exceeds 0.015% by weight, the toughness of the welded joint may decrease, and the impact value may decrease.

[0073] Sulfur (S)

[0074] Sulfur (S) can improve workability by forming fine MnS precipitates. The sulfur may be added in a content ratio of 0.01% by weight or less of the total weight of the structural steel according to one embodiment of the present invention. If the sulfur content exceeds 0.01% by weight, it may form inclusions as a tramp element, which may reduce the ductility of the steel and impede toughness and weldability.

[0075] Rin (P)

[0076] Phosphorus (P) can enhance the strength of steel through solid solution strengthening and suppress the formation of carbides. Phosphorus may be added in a content ratio of 0.02% by weight or less of the total weight of the structural steel according to one embodiment of the present invention. If the phosphorus content exceeds 0.02% by weight, it may form inclusions as a tramp element, reducing the ductility of the steel, and there is a problem that the impact value decreases due to precipitation behavior.

[0077] On the other hand, a structural steel according to one embodiment of the present invention contains 0.17% by weight or less of carbon (C), 1.6% by weight or less of manganese (Mn), 0.10 to 0.35% by weight of chromium (Cr), 0.15% by weight or less of molybdenum (Mo), 0.05% by weight or less of niobium (Nb), 0.003% by weight or less of boron (B), 0.04% by weight or less of titanium (Ti), and the remaining iron (Fe) and other unavoidable impurities. It 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. Furthermore, in the water-cooling method, it can be manufactured by controlling the water-cooling end temperature to 680 to 880°C.

[0078] The shaped steel may further contain 0.1 to 0.4 wt% silicon (Si), 0.6 wt% or less copper (Cu), 0.015 wt% or less nitrogen (N), 0.01 wt% or less sulfur (S), and 0.02 wt% or less phosphorus (P). More preferably, the carbon (C) may contain 0.08 to 0.15 wt%, the manganese (Mn) may contain 0.5 to 1.6 wt%, the chromium (Cr) may contain 0.1 to 0.3 wt%, the molybdenum (Mo) may contain 0.10 to 0.15 wt%, the niobium (Nb) may contain 0.02 to 0.05 wt%, the titanium (Ti) may contain 0.03 wt% or less, and the boron (B) may contain 0.001 to 0.003 wt%.

[0079] As a result, a structural steel according to one embodiment of the present invention may satisfy the alloy composition system standards of KS D 3866, the standard for hot-rolled structural steel for building structures in South Korea, and may also be a high-performance structural steel with a yield strength (YS) of 355 MPa or higher, possessing both seismic resistance and fire resistance.

[0080] The structural steel having the alloy composition described above satisfies the requirement of a yield strength (YS) of 355 MPa or more at room temperature. Furthermore, it may also have a shock absorption energy (CVN) of 27 J or more at 0°C, a high-temperature yield strength of 238 MPa or more at 600°C, and an elongation ratio (EL) of 21% or more. More specifically, it may have a yield strength (YS) of 400 MPa or more at room temperature, a shock absorption energy (CVN) of 40 J or more at 0°C, a high-temperature yield strength of 250 MPa or more at 600°C, and an elongation ratio (EL) of 25% or more.

[0081] Figure 1 is a photograph showing the microstructure of a specimen of the central part of the flange of a structural steel according to the present invention.

[0082] Referring to Figure 1, the structural steel having the alloy composition described above may contain bainite in its final microstructure. Furthermore, ferrite and microcarbides can be realized along with the bainite matrix structure. Thus, the structural steel according to the present invention can effectively improve its high-temperature yield strength by ensuring the formation of Cr, Mo, and Nb-based carbides and the bainite matrix structure.

[0083] Manufacturing method for structural steel

[0084] Referring to Figure 2, first, a method for manufacturing structural steel according to one embodiment of the present invention includes the steps of (a) reheating the steel material (S10), (b) hot rolling it (S20), and (c) water cooling it (S30).

[0085] First, the steel material contains 0.17% by weight or less of carbon (C), 1.6% by weight or less of manganese (Mn), 0.10 to 0.35% by weight of chromium (Cr), 0.15% by weight or less of molybdenum (Mo), 0.05% by weight or less of niobium (Nb), 0.003% by weight or less of boron (B), 0.004% by weight or less of titanium (Ti), and the remaining iron (Fe) and other unavoidable impurities, and the steel material is (a) reheated to 1200°C or higher. Next, the steel material is (b) hot-rolled, with the rolling start temperature controlled to 1050 to 1100°C and the rolling end temperature to 860 to 930°C. After that, the steel material is (c) water-cooled.

[0086] As a result, the method for manufacturing structural steel according to one embodiment of the present invention satisfies the alloy composition system standards of KS D 3866, the standard for hot-rolled structural steel for building structures in South Korea, and can produce high-performance structural steel with a yield strength (YS) of 355 MPa or higher that simultaneously possesses seismic resistance and fire resistance.

[0087] The following describes in detail each step of the manufacturing method for structural steel.

[0088] First, in the reheating step, the steel material of the above composition is reheated to 1200°C or higher. If the reheating temperature is lower than 1200°C, the solid solution of various carbides may not be sufficient, and the components that segregated during the continuous casting process may not be distributed uniformly enough. Also, the reheating temperature may not exceed 1250°C. If the reheating temperature exceeds 1250°C, coarse austenite crystal grains may be formed, making it difficult to ensure strength, which may lead to increased manufacturing costs and decreased productivity due to increased heating costs and time.

[0089] On the other hand, the steel material may be manufactured through a continuous casting process after obtaining molten steel of a desired composition through a steelmaking process. The steel material may be, for example, a beam blank, but is not necessarily limited to this.

[0090] On the other hand, the composition of the steel material may further contain 0.1 to 0.4 wt% silicon (Si), 0.6 wt% or less copper (Cu), 0.015 wt% or less nitrogen (N), 0.01 wt% or less sulfur (S), and 0.02 wt% or less phosphorus (P). More preferably, the steel material may contain 0.08 to 0.15 wt% carbon (C), 0.5 to 1.6 wt% manganese (Mn), 0.1 to 0.3 wt% chromium (Cr), 0.10 to 0.15 wt% molybdenum (Mo), 0.02 to 0.05 wt% niobium (Nb), 0.03 wt% or less titanium (Ti), and 0.001 to 0.003 wt% boron (B).

[0091] (b) In the hot rolling step, the reheated steel material is hot-rolled. At this time, the rolling start temperature is controlled to 1050-1100°C and the rolling end temperature to 860-930°C, thereby ensuring a bainite matrix structure even with a slightly lower chromium (Cr) and molybdenum (Mo) content, and thus ensuring high-temperature yield strength. In particular, if the rolling end temperature is below 860°C, rolling proceeds in the unrecrystallized region, increasing the rolling load and potentially increasing the yield ratio of the resulting structural steel. Also, if the rolling end temperature exceeds 930°C, it may be difficult to ensure the target strength and toughness.

[0092] On the other hand, (c) the water cooling step is performed after hot rolling, and in the method for manufacturing structural steel according to the present invention, the water cooling completion temperature (or cooling and reheating temperature) can be controlled to 680 to 880°C. The water cooling step can be performed via a QST (Quenching and Self Tempering) facility, which is a surface acceleration cooling device, to cool and self-temper the hot-rolled structural steel. The water cooling can be performed by applying a quenching method in which cooling water is sprayed onto the structural steel, and by controlling the transfer speed of the structural steel or the amount of cooling water sprayed, the water cooling completion temperature and the self-tempering temperature can be controlled to 680 to 880°C, and more preferably, may be performed under controlled conditions of 720 to 760°C.

[0093] The steel material or structural steel that has undergone step (c) above satisfies the requirement of a yield strength (YS) of 355 MPa or more at room temperature. In addition, the impact absorption energy (CVN) 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) 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.

[0094] The steel material or structural steel obtained by step (c) above may contain bainite in its final microstructure. That is, the structural steel according to the present invention can improve its high-temperature yield strength as described above by embodying a bainite matrix structure. Furthermore, fine carbides may be embodied along with the bainite matrix structure.

[0095] In one embodiment of the present invention, structural steel and a method for manufacturing structural steel are provided by niobium, which suppresses the growth of austenite grain boundaries, resulting in finer grain sizes and improving the room-temperature yield strength of the material; forms carbides to improve the high-temperature yield strength; and improves hardening ability to bainite the matrix structure and further improve the high-temperature yield strength.

[0096] Furthermore, titanium first combines with nitrogen remaining in the steel to form TiN, and by suppressing the bonding and formation of BN, the hardening ability of the steel can be improved by boron, and a bainite matrix structure can be obtained to ensure earthquake-resistant and fire-resistant properties.

[0097] Furthermore, to meet the alloying element restriction standards within the KS D 3866 standard, the content of chromium and molybdenum, which are effective elements for improving hardening ability and bainite formation of the matrix structure, is reduced. In addition, to ensure the bainite matrix structure that is lacking due to the low content of chromium and molybdenum, the rolling end temperature is controlled to 860-930°C and the water cooling end temperature to 680-880°C.

[0098] As a result, the present invention enables the manufacture of high-performance structural steel with earthquake and fire resistance, yield strength of 355 MPa or higher, and satisfies the KS D 3866 standard.

[0099] Comparative Examples and Experimental Examples

[0100] The following are preferred comparative examples and experimental examples to aid in understanding the present invention. However, the experimental examples below are merely for the purpose of aiding in understanding the present invention, and the present invention is not limited to these experimental examples.

[0101] Tables 1 and 2 show the main alloying element compositions (in weight %) of the experimental example and comparative example. Table 3 shows the process conditions for manufacturing the specimens of the experimental example and comparative example. Table 4 shows the results of measuring the mechanical properties of the specimens produced under the process conditions in Table 3. After manufacturing beam blanks with the compositions shown in Tables 1 and 2 using an electric furnace, H-shaped steel with a flange thickness of 15 mm was produced by hot rolling.

[0102] [Table 1]

[0103] [Table 2]

[0104] [Table 3]

[0105] [Table 4]

[0106] Comparative Example 1 and Experimental Example 1

[0107] Comparative Example 1 differs from Experimental Example 1 in its composition, resulting in differences in the rolling completion temperature and cooling double-heat temperature.

[0108] First, referring to Tables 1 to 3, the composition system 1 of Comparative Example 1 further contains 0.16 wt% carbon, 0.62 wt% chromium, 0.35 wt% molybdenum, 0.017 wt% niobium, 0.05 wt% titanium, and other alloying elements, with a rolling completion temperature of 910 to 950°C and a cooling reheat temperature of 765 to 800°C.

[0109] Composition system 4 of Experimental Example 1 further contains 0.08 wt% carbon, 0.21 wt% chromium, 0.14 wt% molybdenum, 0.044 wt% niobium, 0.025 wt% titanium, and other alloying elements. Compared to composition system 1 of Comparative Example 1, the content of carbon, chromium, molybdenum, and titanium is lower, while the content of niobium is higher. Furthermore, the rolling completion temperature of Experimental Example 1 was 860-930°C, and the cooling and reheating temperature was 720-760°C, which are lower than the rolling completion temperature and cooling and reheating temperature of Comparative Example 1.

[0110] Referring to Table 4, the yield strength at room temperature for Experimental Example 1 was 415 MPa, the yield ratio was 71%, the elongation was 27.5%, the shock absorption energy at 0°C was 43 J, and the high-temperature yield strength at 600°C was 276-278 MPa. It can be confirmed that these properties were improved compared to those of Comparative Example 1.

[0111] Comparative Example 2 and Experimental Example 1

[0112] Comparative Example 2 differs from Experimental Example 1 in its composition, but the rolling completion temperature and cooling double heat temperature are the same.

[0113] Referring to Tables 1 to 3, the composition system 2 of Comparative Example 2 further contains 0.08 wt% carbon, 0.61 wt% chromium, 0.35 wt% molybdenum, 0.018 wt% niobium, 0.04 wt% titanium, and other alloying elements, with a rolling completion temperature of 860 to 930°C and a cooling reheat temperature of 720 to 760°C.

[0114] Composition system 4 of Experimental Example 1 further contains 0.08 wt% carbon, 0.21 wt% chromium, 0.14 wt% molybdenum, 0.044 wt% niobium, 0.025 wt% titanium, and other alloying elements. Compared to composition system 2 of Comparative Example 2, the content of chromium, molybdenum, and titanium is lower, and the content of niobium is higher. In addition, the rolling completion temperature of Experimental Example 1 was 860-930°C and the cooling and reheating temperature was 720-760°C, which are the same as the rolling completion temperature and cooling and reheating temperature of Comparative Example 2.

[0115] Referring to Table 4, the yield strength at room temperature for Experimental Example 1 was 415 MPa, the yield ratio was 71%, the elongation was 27.5%, the shock absorption energy at 0°C was 43 J, and the high-temperature yield strength at 600°C was 276-278 MPa. It can be confirmed that these properties were improved compared to those of Comparative Example 2.

[0116] Comparative Example 2 and Experimental Example 2

[0117] Comparative Example 2 differs from Experimental Example 2 in its composition, but the rolling completion temperature and cooling double heat temperature are the same.

[0118] Referring to Tables 1 to 3, the composition system 2 of Comparative Example 2 contains 0.08 wt% carbon, 0.61 wt% chromium, 0.35 wt% molybdenum, 0.018 wt% niobium, 0.04 wt% titanium, and other alloying elements, with a rolling completion temperature of 860 to 930°C and a cooling reheat temperature of 720 to 760°C.

[0119] Composition system 5 of Experimental Example 2 further contains 0.08 wt% carbon, 0.34 wt% chromium, 0.15 wt% molybdenum, 0.020 wt% niobium, 0.03 wt% titanium, and other alloying elements. Compared to composition system 2 of Comparative Example 2, the content of chromium, molybdenum, and titanium is lower, and the content of niobium is higher. Furthermore, the rolling completion temperature of Experimental Example 2 was 860-930°C, and the cooling and reheating temperature was 720-760°C, which are the same as the rolling completion temperature and cooling and reheating temperature of Comparative Example 2.

[0120] Referring to Table 4, it can be confirmed that the yield strength at room temperature, yield ratio, elongation, 0°C impact absorption energy, and 600°C high-temperature yield strength of Experimental Example 2 were improved compared to the physical properties of Comparative Example 2.

[0121] Comparative Example 3 and Experimental Example 2

[0122] Comparative Example 3 differs from Experimental Example 2 in its composition, but the rolling completion temperature and cooling double heat temperature are the same.

[0123] Referring to Tables 1 to 3, the composition system 3 of Comparative Example 3 further contains 0.08 wt% carbon, 0.37 wt% chromium, 0.17 wt% molybdenum, 0.019 wt% niobium, 0.032 wt% titanium, and other alloying elements, with a rolling completion temperature of 860 to 930°C and a cooling reheat temperature of 720 to 760°C.

[0124] Composition system 5 of Experimental Example 2 further contains 0.08 wt% carbon, 0.34 wt% chromium, 0.15 wt% molybdenum, 0.020 wt% niobium, 0.03 wt% titanium, and other alloying elements. Compared to composition system 3 of Comparative Example 3, the content of chromium, molybdenum, and titanium is lower, and the content of niobium is higher. Furthermore, the rolling completion temperature of Experimental Example 2 was 860-930°C, and the cooling and reheating temperature was 720-760°C, which are the same as the rolling completion temperature and cooling and reheating temperature of Comparative Example 3.

[0125] Referring to Table 4, it can be confirmed that the yield strength at room temperature, yield ratio, elongation, 0°C impact absorption energy, and 600°C high-temperature yield strength of Experimental Example 2 were improved compared to the physical properties of Comparative Example 3.

[0126] Comparative Example 4 and Experimental Example 2

[0127] Comparative Example 4 has the same composition as Experimental Example 2, but there are differences in the rolling completion temperature and the cooling double-heat temperature.

[0128] Referring to Tables 1 to 3, the composition system 5 of Comparative Example 4 further contains 0.08 wt% carbon, 0.34 wt% chromium, 0.15 wt% molybdenum, 0.020 wt% niobium, 0.03 wt% titanium, and other alloying elements, with a rolling completion temperature of 910 to 950°C and a cooling reheat temperature of 765 to 800°C.

[0129] Composition system 5 of Experimental Example 2 further contains 0.08 wt% carbon, 0.34 wt% chromium, 0.15 wt% molybdenum, 0.020 wt% niobium, 0.03 wt% titanium, and other alloying elements, and is the same as the composition system of Comparative Example 4. In addition, the rolling completion temperature of Experimental Example 2 was 860-930°C and the cooling and reheating temperature was 720-760°C, which are lower than the rolling completion temperature and cooling and reheating temperature of Comparative Example 4.

[0130] Referring to Table 4, it can be confirmed that the yield strength at room temperature, yield ratio, elongation, 0°C impact absorption energy, and 600°C high-temperature yield strength of Experimental Example 2 were improved compared to the physical properties of Comparative Example 3.

[0131] As described above, preferred embodiments of the present invention have been examined. However, it is obvious to those ordinary skill in the art that the present invention can be embodied in other specific forms without departing from its spirit or category. Therefore, the embodiments described above should be considered illustrative rather than restrictive, and thus the present invention is not limited to the above description and may be modified within the scope of the appended claims and their equivalents. [Explanation of Symbols]

[0132] S10 Reheating step S20 Hot rolling step S30 Water-cooled Steps

Claims

1. (a) A step of reheating a steel material containing 0.17% by weight or less of carbon (C), 1.6% by weight or less of manganese (Mn), 0.10 to 0.35% by weight of chromium (Cr), 0.15% by weight or less of molybdenum (Mo), 0.05% by weight or less of niobium (Nb), 0.003% by weight or less of boron (B), 0.04% by weight or less of titanium (Ti), and the remaining iron (Fe) and other unavoidable impurities to 1200°C or higher, (b) Hot-rolling the steel material, the step of controlling the rolling start temperature to 1050 to 1100°C and the rolling end temperature to 860 to 930°C, A method for manufacturing shaped steel, comprising the step of water-cooling the steel material.

2. Step (c) above is, A method for manufacturing shaped steel according to claim 1, wherein the water cooling completion temperature is controlled to 680 to 880°C.

3. The steel material after step (c) above is A method for manufacturing a structural steel according to claim 1, which satisfies the following conditions: yield strength (YS) of 355 MPa or more at room temperature, impact absorption energy (CVN) of 27 J or more at 0°C, and elongation (EL) of 21% or more.

4. The steel material after step (c) above is A method for manufacturing a structural steel according to claim 1, wherein the high-temperature yield strength (YS) at 600°C is 238 MPa or more.

5. The steel material after step (c) above is A method for manufacturing a structural steel according to claim 1, wherein the final microstructure contains bainite.

6. The aforementioned steel material is A method for manufacturing shaped steel according to claim 1, wherein the composition is 0.08 to 0.15 wt% carbon (C), 0.5 to 1.6 wt% manganese (Mn), 0.1 to 0.3 wt% chromium (Cr), 0.10 to 0.15 wt% molybdenum (Mo), 0.02 to 0.05 wt% niobium (Nb), 0.03 wt% or less titanium (Ti), and 0.001 to 0.003 wt% boron (B).

7. The aforementioned steel material is A method for producing shaped steel according to claim 1, further comprising 0.1 to 0.4% by weight of silicon (Si), 0.6% by weight or less of copper (Cu), 0.015% by weight or less of nitrogen (N), 0.01% by weight or less of sulfur (S), and 0.02% by weight or less of phosphorus (P).

8. Contains 0.17% by weight or less of carbon (C), 1.6% by weight or less of manganese (Mn), 0.10 to 0.35% by weight of chromium (Cr), 0.15% by weight or less of molybdenum (Mo), 0.05% by weight or less of niobium (Nb), 0.003% by weight or less of boron (B), 0.04% by weight or less of titanium (Ti), and the remainder being iron (Fe) and other unavoidable impurities. Structural steel that meets the requirement of a yield strength (YS) of 355 MPa or higher at room temperature.

9. The structural steel according to claim 8, wherein the impact absorption energy (CVN) at 0°C is 27 J or more.

10. The structural steel according to claim 8, wherein the high-temperature yield strength at 600°C is 238 MPa or more.

11. The structural steel according to claim 8, wherein the elongation ratio (EL) is 21% or more.

12. The structural steel according to claim 8, wherein the final microstructure contains bainite.

13. The structural steel according to claim 8, wherein the composition is 0.08 to 0.15 wt% carbon (C), 0.5 to 1.6 wt% manganese (Mn), 0.1 to 0.3 wt% chromium (Cr), 0.10 to 0.15 wt% molybdenum (Mo), 0.02 to 0.5 wt% niobium (Nb), 0.03 wt% or less titanium (Ti), and 0.001 to 0.003 wt% boron (B).

14. The structural steel according to claim 8, further comprising 0.1 to 0.4% by weight of silicon (Si), 0.6% by weight or less of copper (Cu), 0.015% by weight or less of nitrogen (N), 0.01% by weight or less of sulfur (S), and 0.02% by weight or less of phosphorus (P).

15. Contains 0.17% by weight or less of carbon (C), 1.6% by weight or less of manganese (Mn), 0.10 to 0.35% by weight of chromium (Cr), 0.15% by weight or less of molybdenum (Mo), 0.05% by weight or less of niobium (Nb), 0.003% by weight or less of boron (B), 0.04% by weight or less of titanium (Ti), and the remainder being iron (Fe) and other unavoidable impurities. Structural steel manufactured by reheating at over 1200°C, controlling the hot rolling start temperature to 1050-1100°C and the rolling end temperature to 860-930°C, and then water-cooling.

16. In the aforementioned water cooling method, The structural steel according to claim 15, manufactured by controlling the water cooling completion temperature to 680 to 880°C.

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