Steel section and steel section manufacturing method

Abstract

A steel section manufacturing method, according to one embodiment of the present invention, comprises the steps of: (a) reheating steel at a temperature of 1200°C or higher, the steel comprising 0.17% by weight or less of carbon (C), 1.6% by weight or less of manganese (Mn), 0.10-0.35% 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 inevitable impurities; (b) hot rolling the steel and controlling the rolling start temperature to 1050-1100°C and the rolling end temperature to 860-930°C; and (c) water cooling the steel. Thereby, it is possible to implement a high-performance steel section and a steel section manufacturing method which simultaneously have earthquake-resistant performance and fire-resistant performance.

Description

Steel profiles and methods for manufacturing steel profiles

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

[0002] Section steel generally refers to steel with a diverse range of cross-sectional shapes. Section steel is used as structural steel, such as columns in large buildings, and also as temporary structures for civil engineering projects such as subways and bridges, as well as as foundation piles. Section steel can be manufactured by hot rolling cast steel, such as blooms, billets, and beam blanks, produced through continuous casting.

[0003] Recently, large-scale earthquakes have been occurring around the world, causing massive loss of life and property. In particular, the successive earthquakes of magnitude 5.0 or greater in Gyeongju and Pohang in 2016 and 2017 have heightened anxiety.

[0004] In the event of an earthquake, in addition to the primary damage caused by building collapse, the secondary fire that can occur can soften the reinforcing materials supporting the structure, accelerating building collapse along with the resulting plastic deformation of the reinforcing materials. Consequently, building design standards are being strengthened to minimize casualties and property damage by delaying building collapse in disaster situations such as earthquakes or high-rise building fires.

[0005] To enhance the safety of these buildings, it is essential to improve the earthquake-resistant design of the building, install protective facilities such as sprinklers, and improve the earthquake-resistant and fire-resistant performance of the structural materials used in the construction of the structure.

[0006] To this end, earthquake-resistant steel that has earthquake-resistant performance through yield ratio control and fire-resistant steel that can withstand fire through improved high-temperature strength are being developed and used.

[0007] However, as mentioned above, since a fire may occur due to building damage in the event of an earthquake, there is an increasing demand for fire-resistant and earthquake-resistant steel that has both earthquake-resistant and fire-resistant performance to prepare for such situations.

[0008] In order to solve the problems of the above-described prior art, the purpose of the present invention is to provide a high-performance steel and a method for manufacturing the steel having fire-resistant and earthquake-resistant performance.

[0009] The tasks of the present invention are not limited to the tasks mentioned above, and other tasks not mentioned will be clearly understood by those skilled in the art from the description below.

[0010] According to one embodiment of the present invention, a method for manufacturing a shaped steel includes the steps of: (a) reheating a steel material containing carbon (C) 0.17 wt% or less, manganese (Mn) 1.6 wt% or less, chromium (Cr) 0.10 to 0.35 wt%, molybdenum (Mo) 0.15 wt% or less, niobium (Nb) 0.05 wt% or less, boron (B) 0.003 wt% or less, titanium (Ti) 0.04 wt% or less, the remainder iron (Fe) and other unavoidable impurities at 1200°C or higher; (b) hot-rolling the steel material, while 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.

[0011] Additionally, the above step (c) can control the water cooling end temperature to 680 to 880 ℃.

[0012] In addition, the steel material that has undergone the above step (c) can satisfy a yield strength (YS) of 355 MPa or more at room temperature, a shock absorption energy (CVN) of 27 J or more at 0°C, and an elongation (EL) of 21% or more.

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

[0014] Additionally, the steel material that has undergone the above step (c) may include bainite in the final microstructure.

[0015] In addition, the above steel may contain 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).

[0016] In addition, 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).

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

[0018] Additionally, the shock absorption energy (CVN) at 0℃ can satisfy more than 27J.

[0019] Additionally, the high temperature yield strength at 600 ℃ can be 238 MPa or more.

[0020] Additionally, the elongation at break (EL) can be greater than 21%.

[0021] Additionally, the final microstructure may include bainite.

[0022] Additionally, it may contain 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).

[0023] Additionally, it 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).

[0024] According to one embodiment of the present invention, a shaped steel sheet contains carbon (C) 0.17 wt% or less, manganese (Mn) 1.6 wt% or less, chromium (Cr) 0.10 to 0.35 wt% or less, molybdenum (Mo) 0.15 wt% or less, niobium (Nb) 0.05 wt% or less, boron (B) 0.003 wt% or less, titanium (Ti) 0.04 wt% or less, the remainder iron (Fe) and other unavoidable impurities, and can be manufactured by a method of controlling the hot rolling start temperature to 1050 to 1100°C and the rolling end temperature to 860 to 930°C after reheating at 1200°C or higher, and then water cooling.

[0025] In addition, in the above water-cooling method, it can be manufactured by controlling the water-cooling end temperature to 680 to 880 ℃.

[0026] According to one embodiment of the present invention, it is possible to implement a high-performance steel and a method for manufacturing the steel having both seismic and fire-resistant performance.

[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.

[0028] Fig. 1 is a photograph of the structure of a central flange specimen of a steel member according to the present invention.

[0029] Figure 2 is a flowchart illustrating a method for manufacturing a shaped steel according to the present invention.

[0030] In this specification, when it is said that a component (or region, layer, portion, etc.) is “on,” “connected to,” or “coupled to” another component, it means that it can be directly disposed / connected / coupled to the other component, or a third component may be disposed between them.

[0031] Identical drawing numbers indicate identical components. Furthermore, in the drawings, the thicknesses, proportions, and dimensions of components are exaggerated for the purpose of effectively illustrating the technical content.

[0032] “And / or” includes any combination of one or more of the associated constructs that can be defined.

[0033] While terms such as "first" and "second" may be used to describe various components, these components should not be limited by these terms. These terms are used solely to distinguish one component from another. For example, without departing from the scope of the present invention, a first component may be referred to as a "second component," and similarly, a second component may also be referred to as a "first component." Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0034] Additionally, terms such as "below," "lower," "above," and "upper" are used to describe the relationships between components depicted in the drawings. These terms are relative concepts and are described based on the directions indicated in the drawings.

[0035] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art to which this invention pertains. Furthermore, terms defined in commonly used dictionaries should be interpreted to have a meaning consistent with their meaning in the relevant technical context, and unless interpreted in an idealized or overly formal sense, they are explicitly defined herein.

[0036] Terms such as "include" or "have" should be understood to specify the presence of a feature, number, step, operation, component, part, or combination thereof described in the specification, but not to exclude in advance the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0037] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0038]

[0039] Section steel

[0040] According to one embodiment of the present invention, a steel sheet contains carbon (C) 0.17 wt% or less, manganese (Mn) 1.6 wt% or less, chromium (Cr) 0.10 to 0.35 wt% or less, molybdenum (Mo) 0.15 wt% or less, niobium (Nb) 0.05 wt% or less, boron (B) 0.003 wt% or less, titanium (Ti) 0.04 wt% or less, the remainder iron (Fe) and other unavoidable impurities, and the yield strength (YS) at room temperature satisfies 355 MPa or more. In addition, the steel according to one 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 the yield strength (YS) at room temperature may satisfy 355 MPa or more.

[0041] More preferably, the carbon (C) may include 0.08 to 0.15 wt%, the manganese (Mn) may include 0.5 to 1.6 wt%, the chromium (Cr) may include 0.1 to 0.3 wt%, the molybdenum (Mo) may include 0.10 to 0.15 wt%, the niobium (Nb) may include 0.02 to 0.05 wt%, the titanium (Ti) may include 0.03 wt% or less, and the boron (B) may include 0.001 to 0.003 wt%.

[0042] The steel section having the above-described alloy composition satisfies 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.

[0043] In addition, the steel section having the above-described alloy composition may include bainite in its final microstructure. That is, the steel section according to the present invention may have improved high-temperature yield strength as described above by implementing a bainite matrix structure. Furthermore, microscopic carbides may be implemented along with the bainite matrix structure.

[0044] Accordingly, the steel section according to one embodiment of the present invention can be a high-performance steel section having a yield strength (YS) of 355 MPa or higher that satisfies the alloy composition standard of KS D 3866, which is a domestic hot-rolled steel section standard for building structures, and at the same time has both seismic performance and fire resistance.

[0045] Meanwhile, the alloy composition of the above KS D 3866 must satisfy 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.

[0046] Below, the role and content of each alloy element included in the steel according to one embodiment of the present invention will be described in detail.

[0047]

[0048] carbon (C)

[0049] Carbon reacts with Nb, Ti, etc. to promote the formation of fine carbides, thereby effectively contributing to the enhancement of strength through precipitation strengthening. In addition, it is effective in securing fire resistance by improving high-temperature strength by hindering dislocation movement at high temperatures. On the other hand, if it is added excessively, it may generate coarse carbides, which not only reduces impact properties but also increases the yield ratio by causing discontinuous yield behavior, thereby reducing seismic performance. To this end, the section steel according to one embodiment of the present invention may contain carbon in an amount of 0.17 wt% or less, and preferably 0.08 to 0.15 wt%.

[0050] That is, if the carbon content is less than 0.08 wt% of the total weight, it may be difficult to secure sufficient strength. Conversely, if the carbon content exceeds 0.17 wt% of the total weight, coarse carbides are formed, which not only reduces impact properties but also causes discontinuous yield behavior, thereby increasing the yield ratio and lowering seismic performance.

[0051]

[0052] manganese (Mn)

[0053] Manganese, as a solid-solution strengthening element, not only contributes to securing strength but also improves the hardenability of steel, effectively contributing to the formation of bainite structure. However, if added excessively, it may combine with sulfur to form MnS inclusions or cause central segregation in the ingot. To this end, the steel section according to one embodiment of the present invention may contain 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%.

[0054] That is, when the manganese content is less than 0.5 wt% of the total weight, the effect of strengthening the solid solution cannot be fully exerted, and when it exceeds 1.6 wt%, it may combine with sulfur to form MnS inclusions or cause central segregation in the ingot, thereby lowering the ductility and corrosion resistance of the shaped steel.

[0055]

[0056] chromium (Cr)

[0057] Chromium can improve the hardenability of steel and contribute to securing a bainite microstructure, but if added excessively, it can increase the manufacturing cost of steel and reduce the ductility of steel by forming coarse carbides at grain boundaries. Therefore, the steel section according to one embodiment of the present invention may contain 0.10 to 0.35 wt% of chromium, and preferably 0.1 to 0.3 wt%.

[0058] That is, when the chromium content is less than 0.1 wt% of the total weight, the effect of chromium addition cannot be properly exerted, and conversely, when the chromium content exceeds 0.35 wt% and is added in large quantities, the manufacturing cost of steel increases and coarse carbides may be formed at grain boundaries, which may reduce the ductility of the steel. In addition, when the chromium content is 0.35 wt% or less, it can satisfy the alloy composition standard of KS D 3866, the domestic standard for hot-rolled section steel for architectural structures.

[0059]

[0060] molybdenum (Mo)

[0061] Molybdenum can improve the hardenability of steel, contributing to securing a bainite microstructure, and is a very effective element for securing high-temperature strength. However, if added excessively, it can increase the manufacturing cost of steel and promote the formation of grain boundary carbides, thereby reducing the ductility of steel. Therefore, the shape steel according to one embodiment of the present invention may contain molybdenum in an amount of 0.15 wt% or less, and preferably 0.10 to 0.15 wt%.

[0062] That is, when the content of molybdenum is less than 0.10 wt% of the total weight, the effect of adding molybdenum cannot be properly exerted, and when a large amount is added exceeding 0.15 wt%, it increases the manufacturing cost of steel and promotes the formation of grain boundary carbides, which can reduce the ductility of the steel. In addition, when the content of molybdenum is 0.15 wt% or less, it can satisfy the alloy composition standard of KS D 3866, the domestic standard for hot-rolled section steel for architectural structures.

[0063]

[0064] niobium (Nb)

[0065] Niobium is an element that suppresses grain growth and ensures a fine grain size when incorporated into an austenite structure. It also reacts with carbon to promote the formation of fine carbides, effectively enhancing strength through precipitation strengthening, and particularly improves high-temperature yield strength. Furthermore, it effectively enhances high-temperature yield strength by improving hardenability and bainiticizing the matrix structure. However, if added excessively, it can reduce the impact absorption energy of the steel. Therefore, the section steel according to one embodiment of the present invention may contain 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%.

[0066] That is, when the content of niobium is less than 0.02 wt% of the total weight, the addition effect of niobium cannot be properly exerted, and when a large amount exceeds 0.05 wt%, the impact absorption energy of the steel can be reduced. On the other hand, when the content of niobium is 0.04 to 0.05 wt%, the addition effect of niobium described above can be maximized while minimizing the reduction in the impact absorption energy of the steel.

[0067]

[0068] Boron (B)

[0069] Boron can enhance hardenability by preferentially segregating at austenite grain boundaries and suppressing the formation of soft ferrite during cooling. However, excessive addition can lead to grain boundary embrittlement. To address this, boron (B) may be included in amounts of 0.003 wt% or less, preferably 0.001 wt% to 0.003 wt%.

[0070] That is, when the boron content is less than 0.001 wt% of the total weight, the austenite grain boundary segregation effect is insufficient, and when it exceeds 0.003 wt%, the problem of grain boundary embrittlement may occur.

[0071]

[0072] Titanium (Ti)

[0073] Titanium can form TiN together with nitrogen. In the present invention, the formation of proeutectoid ferrite in austenite grains is suppressed by adding boron as a method for improving the hardenability of steel. At this time, if boron and nitrogen combine to form BN during the steelmaking process, the hardenability improvement mechanism cannot be implemented. Therefore, a process is required in the steelmaking process to limit the nitrogen content to 100 ppm or less through the application of a VD (Vacuum Degassing) process, and titanium can be added to preferentially form TiN to suppress the combination of residual nitrogen and boron, which can ultimately play a role in improving the hardenability of steel. To this end, in one 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%.

[0074]

[0075] Silicon (Si)

[0076] Silicon (Si) is added to steelmaking processes along with aluminum as a deoxidizer to remove oxygen from the steel. Silicon can also have a solid solution strengthening effect.

[0077] Silicon may be added in an amount of 0.10 to 0.40 wt% based on the total weight of the steel according to one embodiment of the present invention. If the silicon content is less than 0.10 wt% based on the total weight, the effect of adding silicon cannot be properly achieved. Conversely, if the silicon content exceeds 0.40 wt% based on the total weight and is added in large quantities, the weldability of the steel may be reduced, and red scale may be generated during reheating and hot rolling, which may cause problems with the surface quality.

[0078]

[0079] copper (Cu)

[0080] Copper (Cu) is an element that exhibits a solid-solution strengthening effect when dissolved in ferrite. Furthermore, supersaturated copper, which does not precipitate during bainite transformation, remains dissolved in the structure at room temperature. When heated to a usable temperature of 600°C as refractory steel, copper precipitates on the dislocations introduced by bainite transformation, thereby increasing the strength through precipitation hardening.

[0081] Copper may be added in a content ratio of 0.6 wt% or less of the total weight of the shaped steel according to one embodiment of the present invention, and preferably, may be added in a content ratio of 0.5 wt% or less. When the content of copper exceeds 0.6 wt% of the total weight and is added in large quantities, problems arise in that hot working becomes difficult, precipitation strengthening becomes saturated, toughness is reduced, and red-hot embrittlement occurs.

[0082]

[0083] Nitrogen (N)

[0084] Nitrogen (N) can contribute to grain refinement by forming nitride precipitates such as AlN, and can contribute to securing high-temperature strength. The nitrogen may be added in a content ratio of 0.015 wt% or less of the total weight of the shaped steel according to one embodiment of the present invention, and preferably, it may be added in a content ratio of 0.012 wt% or less. If the nitrogen content exceeds 0.015 wt%, the toughness of the weld joint may deteriorate, and the impact value may deteriorate.

[0085]

[0086] Yellow (S)

[0087] Sulfur (S) can improve workability by forming fine MnS precipitates. The sulfur may be added in an amount of 0.01 wt% or less based on the total weight of the steel according to one embodiment of the present invention. If the sulfur content exceeds 0.01 wt%, it may form inclusions as a tramp element, thereby reducing the ductility of the steel and hindering the toughness and weldability.

[0088]

[0089] Person (P)

[0090] Phosphorus (P) can enhance the strength of steel by solidification and suppress the formation of carbides. Phosphorus can be added in an amount of 0.02 wt% or less based on the total weight of the steel according to one embodiment of the present invention. If the phosphorus content exceeds 0.02 wt%, it can reduce the ductility of the steel by generating inclusions, etc. as a tramp element, and there is a problem of a decrease in the impact value due to precipitation behavior.

[0091]

[0092] Meanwhile, the section steel according to one embodiment of the present invention contains carbon (C) 0.17 wt% or less, manganese (Mn) 1.6 wt% or less, chromium (Cr) 0.10 to 0.35 wt% or less, molybdenum (Mo) 0.15 wt% or less, niobium (Nb) 0.05% or less, boron (B) 0.003 wt% or less, titanium (Ti) 0.04 wt% or less, the remainder iron (Fe) and other unavoidable impurities, and is manufactured by a method of controlling the hot rolling start temperature to 1050 to 1100°C and the rolling end temperature to 860 to 930°C after reheating at 1200°C or higher, and then cooling with water. In addition, the section steel can be manufactured by controlling the water cooling end temperature to 680 to 880°C in the water cooling method.

[0093] The above-described steel may additionally contain 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). In addition, 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%.

[0094] Accordingly, the steel section according to one embodiment of the present invention can be a high-performance steel section having a yield strength (YS) of 355 MPa or higher that satisfies the alloy composition standard of KS D 3866, which is a domestic hot-rolled steel section standard for building structures, and at the same time has both seismic performance and fire resistance.

[0095] The steel section having the above-described alloy composition satisfies 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.

[0096] Fig. 1 is a photograph of the structure of a central flange specimen of a steel member according to the present invention.

[0097] Referring to Fig. 1, the steel section having the above-described alloy composition may include bainite in its final microstructure. Furthermore, ferrite and micro-carbides may be realized alongside the bainite matrix structure. Thus, the steel section according to the present invention can effectively enhance high-temperature yield strength by forming Cr, Mo, and Nb-based carbides and securing the bainite matrix structure.

[0098]

[0099] Method for manufacturing shaped steel

[0100] Referring to FIG. 2, first, a method for manufacturing a shaped steel according to an embodiment of the present invention includes (a) a step of reheating (S10), (b) a step of hot rolling (S20), and (c) a step of water cooling (S30).

[0101] First, the steel contains carbon (C) 0.17 wt% or less, manganese (Mn) 1.6 wt% or less, chromium (Cr) 0.10 to 0.35 wt% or less, molybdenum (Mo) 0.15 wt% or less, niobium (Nb) 0.05 wt% or less, boron (B) 0.003 wt% or less, titanium (Ti) 0.04 wt% or less, the remainder iron (Fe) and other unavoidable impurities, and the step of (a) reheating the steel is performed at 1200 ℃ or higher. Next, the step of (b) hot rolling the steel is performed by controlling the rolling start temperature to 1050 to 1100 ℃ and the rolling end temperature to 860 to 930 ℃. Thereafter, the step of (c) water cooling the steel is included.

[0102] Accordingly, a method for manufacturing a shaped steel according to one embodiment of the present invention can manufacture a high-performance shaped steel having a yield strength (YS) of 355 MPa or higher that satisfies the alloy composition standard of KS D 3866, a domestic hot-rolled shaped steel standard for building structures, while simultaneously having both seismic performance and fire resistance.

[0103] Below, each step of the steel manufacturing method is described in detail.

[0104] First, in the reheating step, the steel of the above composition is reheated at 1200℃ or higher. If the reheating temperature is lower than 1200℃, the solid solution of various carbides may not be sufficient, and the segregated components during the continuous casting process may not be distributed evenly. In addition, the reheating temperature may not exceed 1250℃. If the reheating temperature exceeds 1250℃, coarse austenite grains may be formed, making it difficult to secure strength. In addition, the increased heating cost and time may lead to increased manufacturing costs and decreased productivity.

[0105] Meanwhile, the above steel material can be manufactured through a continuous casting process after obtaining molten steel of a desired composition through a steelmaking process. The above steel material may be, for example, a beam blank, but is not necessarily limited thereto.

[0106] Meanwhile, the composition of the steel may additionally 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). Also, more preferably, the steel may contain 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).

[0107] (b) In the hot rolling step, 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. As a result, even if the contents of chromium (Cr) and molybdenum (Mo) are somewhat low, a bainite matrix structure can be secured, and high-temperature yield strength can be secured. In particular, if the rolling end temperature is less than 860°C, rolling may proceed in an unrecrystallized region, which may increase the rolling addition, and the yield ratio of the rolled product, the shaped steel, may increase. In addition, if the rolling end temperature exceeds 930°C, it may be difficult to secure the target strength and toughness.

[0108] Meanwhile, the (c) water-cooling step is performed after hot rolling, and in the method for manufacturing shaped steel according to the present invention, the water-cooling end temperature (or cooling recovery temperature) can be controlled to 680 to 880°C. The water-cooling step can be performed using a QST (Quenching and Self Tempering) facility, which is a surface accelerated cooling device, and cools and self-tempers the hot-rolled shaped steel. The water-cooling can apply a quenching method of spraying cooling water on the shaped steel, and by controlling the conveying speed of the shaped steel or the amount of sprayed cooling water, the water-cooling end temperature and self-tempering temperature can be controlled to 680 to 880°C, and more preferably, it can be performed in a controlled state at 720 to 760°C.

[0109] The steel or section steel that has undergone the above step (c) satisfies 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.

[0110] The steel or section steel that has undergone the above step (c) may include bainite in its final microstructure. That is, the section steel according to the present invention may have improved high-temperature yield strength as described above by implementing a bainite matrix structure. In addition, micro-carbides may be implemented along with the bainite matrix structure.

[0111] According to one embodiment of the present invention, the steel and the method for manufacturing the steel improve the yield strength at room temperature by having a fine grain size by suppressing the growth of grain boundaries of austenite using niobium, improve the yield strength at high temperatures by forming carbides, and improve the hardenability by turning the matrix structure into bainitic to improve the yield strength at high temperatures.

[0112] In addition, titanium is preferentially combined with nitrogen remaining in the steel to form TiN, and by suppressing the combination and formation of BN, the hardenability of the steel is improved by boron, thereby obtaining a bainite matrix structure and securing earthquake-resistant and fire-resistant properties.

[0113] In addition, by lowering the content of chromium and molybdenum, which are effective elements in improving hardenability and converting the matrix structure into bainitic, the alloy element addition limit standard in the KS D 3866 standard is satisfied, and in order to secure the bainitic matrix structure that is insufficient 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.

[0114] Accordingly, the present invention can manufacture high-performance shaped steel having both earthquake-resistant and fire-resistant performance and a yield strength of 355 MPa or higher, and at the same time can satisfy the KS D 3866 standard.

[0115]

[0116] Comparative and experimental examples

[0117] To aid understanding of the present invention, preferred comparative and experimental examples are presented below. However, the following experimental examples are provided solely to aid understanding of the present invention, and the present invention is not limited to the following experimental examples.

[0118] Tables 1 and 2 show the main alloy element compositions (unit: weight%) of the experimental examples and comparative examples, Table 3 shows the process conditions for manufacturing the specimens of the experimental examples and comparative examples, and Table 4 shows the results of measuring the mechanical properties of the specimens implemented according to the process conditions of Table 3. Beam blanks having the compositions of Tables 1 and 2 were manufactured using an electric furnace, and then hot rolled to manufacture H-shaped steel with a flange thickness of 15 mm.

[0119]

[0120]

[0121]

[0122]

[0123] Comparative Example 1 and Experimental Example 1

[0124] Comparative Example 1 has a difference in composition compared to Experimental Example 1, and there are differences in the rolling end temperature and cooling recovery temperature.

[0125] First, referring to Tables 1 to 3, Composition 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 alloy elements, and the rolling end temperature is 910-950°C, and the cooling recovery temperature is 765-800°C.

[0126] 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, and thus has lower contents of carbon, chromium, molybdenum, and titanium, and higher contents of niobium, compared to Composition system 1 of Comparative Example 1. In addition, the rolling end temperature of Experimental Example 1 is 860-930°C, and the cooling and reheating temperature is 720-760°C, which are lower than the rolling end temperature and cooling and reheating temperature of Comparative Example 1.

[0127] Referring to Table 4, the room temperature yield strength of the experimental example 1 was 415 MPa, the yield ratio was 71%, the elongation was 27.5%, the impact absorption energy at 0°C was 43 J, and the high temperature yield strength at 600°C was 276-278 MPa, which can be confirmed to be improved compared to the properties of comparative example 1.

[0128]

[0129] Comparative Example 2 and Experimental Example 1

[0130] Comparative Example 2 has a different composition from Experimental Example 1, but the rolling end temperature and cooling recovery temperature are the same.

[0131] Referring to Tables 1 to 3, Composition 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 alloy elements, and the rolling end temperature is 860-930°C, and the cooling recovery temperature is 720-760°C.

[0132] 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, and thus has lower contents of chromium, molybdenum, and titanium and higher contents of niobium than Composition system 2 of Comparative Example 2. In addition, the rolling end temperature of Experimental Example 1 is 860-930°C, and the cooling and reheating temperature is 720-760°C, which are the same as the rolling end temperature and cooling and reheating temperature of Comparative Example 2.

[0133] Referring to Table 4, the room temperature yield strength of the experimental example 1 was 415 MPa, the yield ratio was 71%, the elongation was 27.5%, the impact absorption energy at 0°C was 43 J, and the high temperature yield strength at 600°C was 276-278 MPa, which can be confirmed to be improved compared to the properties of comparative example 2.

[0134]

[0135] Comparative Example 2 and Experimental Example 2

[0136] Comparative Example 2 differs from Experimental Example 2 in composition, but the rolling end temperature and cooling recovery temperature are the same.

[0137] Referring to Tables 1 to 3, Composition 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 alloy elements, and the rolling end temperature is 860-930°C, and the cooling recovery temperature is 720-760°C.

[0138] 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 thus has lower contents of chromium, molybdenum, and titanium and higher contents of niobium than Composition system 2 of Comparative Example 2. In addition, the rolling completion temperature of Experimental Example 2 is 860-930°C, and the cooling recovery temperature is 720-760°C, which are the same as the rolling completion temperature and cooling recovery temperature of Comparative Example 2.

[0139] Referring to Table 4, it can be confirmed that the room temperature yield strength, yield ratio, elongation, 0℃ impact absorption energy, and 600℃ high temperature yield strength of Experimental Example 2 are improved compared to the properties of Comparative Example 2.

[0140]

[0141] Comparative Example 3 and Experimental Example 2

[0142] Comparative Example 3 has a different composition from Experimental Example 2, but the rolling end temperature and cooling recovery temperature are the same.

[0143] Referring to Tables 1 to 3, Composition 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 alloy elements, and the rolling end temperature is 860-930°C, and the cooling recovery temperature is 720-760°C.

[0144] 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 thus has lower contents of chromium, molybdenum, and titanium and higher contents of niobium than Composition system 3 of Comparative Example 3. In addition, the rolling end temperature of Experimental Example 2 is 860-930°C, and the cooling and reheating temperature is 720-760°C, which are the same as the rolling end temperature and cooling and reheating temperature of Comparative Example 3.

[0145] Referring to Table 4, it can be confirmed that the room temperature yield strength, yield ratio, elongation, 0℃ impact absorption energy, and 600℃ high temperature yield strength of Experimental Example 2 are improved compared to the properties of Comparative Example 3.

[0146]

[0147] Comparative Example 4 and Experimental Example 2

[0148] Comparative Example 4 has no difference in composition from Experimental Example 2, but there is a difference in the rolling end temperature and cooling recovery temperature.

[0149] Referring to Tables 1 to 3, 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 alloy elements, and the rolling end temperature is 910-950°C, and the cooling recovery temperature is 765-800°C.

[0150] Composition 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, which is the same as the composition of Comparative Example 4. In addition, the rolling end temperature of Experimental Example 2 is 860-930°C, and the cooling and reheating temperature is 720-760°C, which are lower than the rolling end temperature and cooling and reheating temperature of Comparative Example 4.

[0151] Referring to Table 4, it can be confirmed that the room temperature yield strength, yield ratio, elongation, 0℃ impact absorption energy, and 600℃ high temperature yield strength of Experimental Example 2 are improved compared to the properties of Comparative Example 3.

[0152] As described above, preferred embodiments of the present invention have been described. It will be apparent to those skilled in the art that the present invention can be embodied in other specific forms, in addition to the embodiments described above, without departing from the spirit or scope thereof. Therefore, the above-described embodiments should be considered illustrative rather than restrictive, and accordingly, the present invention is not limited to the above description, but may be modified within the scope of the appended claims and their equivalents.

[0153] [Explanation of symbols]

[0154] S10: Reheating phase

[0155] S20: Hot rolling stage

[0156] S30: Water cooling stage

Claims

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

2. In paragraph 1, Step (c) above, A method for manufacturing shaped steel in which the water cooling end temperature is controlled to 680 to 880 ℃.

3. In paragraph 1, The steel material that has undergone the above step (c) is A method for manufacturing a steel profile that satisfies a yield strength (YS) of 355 MPa or more at room temperature, a shock absorption energy (CVN) of 27 J or more at 0°C, and an elongation (EL) of 21% or more.

4. In paragraph 1, The steel material that has undergone the above step (c) is A method for manufacturing a steel beam having a high temperature yield strength (YS) of 238 MPa or more at 600 ℃.

5. In paragraph 1, The steel material that has undergone the above step (c) is A method for manufacturing a steel section containing bainite in the final microstructure.

6. In paragraph 1, The above steel is, A method for manufacturing a shaped steel having carbon (C) 0.08 to 0.15 wt%, manganese (Mn) 0.5 to 1.6 wt%, chromium (Cr) 0.1 to 0.3 wt%, molybdenum (Mo) 0.10 to 0.15 wt%, niobium (Nb) 0.02 to 0.05 wt%, titanium (Ti) 0.03 wt% or less, and boron (B) 0.001 to 0.003 wt%.

7. In paragraph 1, The above steel is, A method for manufacturing a shaped steel, further comprising 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).

8. Contains carbon (C) 0.17 wt% or less, manganese (Mn) 1.6 wt% or less, chromium (Cr) 0.10 to 0.35 wt%, molybdenum (Mo) 0.15 wt% or less, niobium (Nb) 0.05 wt% or less, boron (B) 0.003 wt% or less, titanium (Ti) 0.04 wt% or less, the remainder iron (Fe) and other unavoidable impurities. Steel that satisfies the yield strength (YS) of 355 MPa or more at room temperature.

9. In paragraph 8, Steel that satisfies the impact absorption energy (CVN) of 27J or more at 0℃.

10. In paragraph 8, Steel with a high temperature yield strength of 238 MPa or more at 600 ℃.

11. In paragraph 8, Steel with an elongation (EL) of 21% or more.

12. In paragraph 8, Steel sections containing bainite in the final microstructure.

13. In paragraph 8, A steel sheet containing 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).

14. In paragraph 8, A steel sheet further containing 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).

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

16. In paragraph 15, In the above water cooling method, Steel products manufactured by controlling the water cooling end temperature to 680 to 880 ℃.

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