Section steel and section steel manufacturing method

Abstract

A method for manufacturing profile steel according to one embodiment of the present invention comprises: a step (a); a steel material containing 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 a remainder of iron (Fe) and other unavoidable impurities is reheated at a temperature of 1200 DEG C or higher; (b) hot-rolling the steel material, in which the rolling start temperature is controlled to 1050 DEG C to 1100 DEG C and the rolling end temperature is controlled to 860 DEG C to 930 DEG C; and (c) water-cooling the steel material, whereby it is possible to implement a high-performance profile steel having both shock resistance and fire resistance, and a method for manufacturing the profile steel.

Description

Technical Field

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

[0002] Section steel generally refers to steel materials with various different cross-sectional shapes. Section steel is used as structural steel such as columns of large buildings, and is also used as temporary civil components such as those for subways and bridges and foundation piles. Section steel can be manufactured by hot rolling billets such as blooms, billets, and beam blanks manufactured by continuous casting.

[0003] In recent years, many large-scale earthquakes have occurred globally, resulting in huge casualties and property losses. Especially in 2016 and 2017, strong earthquakes with magnitudes above 5.0 occurred successively in Gyeongju and Pohang, South Korea, causing social unrest.

[0004] When an earthquake occurs, in addition to the direct losses caused by building damage, secondary fires that may occur may cause the softening of the reinforcement materials for supporting structures, which may further accelerate the collapse of buildings in addition to the plastic deformation of the reinforcement materials caused by the earthquake. Therefore, recently, in order to delay the collapse of buildings in the event of disasters such as earthquakes or high-rise building fires, thereby minimizing casualties and property losses, the design standards of buildings are being continuously strengthened.

[0005] In order to improve the safety of buildings, in addition to the seismic design of buildings and the installation of protective facilities such as sprinklers, it is necessary to improve the seismic resistance and fire resistance of building structural materials used for manufacturing structures.

[0006] For this purpose, seismic steel that ensures seismic performance capable of withstanding earthquakes by controlling the yield ratio, and fire-resistant steel that has fire resistance by increasing high-temperature strength have been developed and used respectively.

[0007] However, as described above, fires may occur due to building damage during an earthquake, so the demand for fire-resistant and seismic section steel that simultaneously has seismic performance and fire resistance is increasing to cope with such situations. Summary of the Invention

[0008] Technical Problem

[0009] In order to solve the problems of the above-mentioned prior art, the purpose of the present invention is to provide a high-performance section steel with fire resistance and seismic performance and a method for manufacturing the section steel.

[0010] However, the technical problems of the present invention are not limited to the above-mentioned technical problems. Through the description in the appended claims, those skilled in the art can clearly understand other technical problems not mentioned.

[0011] Solution to the problem

[0012] The manufacturing method of the section steel according to an embodiment of the present invention includes: step (a), reheating the steel 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), the balance of iron (Fe) and other inevitable impurities at a temperature of 1200 °C or higher; step (b), hot rolling the above section steel, wherein the starting rolling temperature is controlled to be 1050 °C to 1100 °C, and the ending rolling temperature is controlled to be 860 °C to 930 °C; and step (c), water-cooling the above steel.

[0013] In addition, in the above step (c), the water-cooling ending temperature can be controlled to be 680 °C to 880 °C.

[0014] In addition, the steel after the above step (C) can meet the conditions that the yield strength (YS) at normal temperature is 355 MPa or higher, the impact absorption energy (CVN) at 0 °C is 27 J or higher, and the elongation (EL) is 21% or higher.

[0015] In addition, the high-temperature yield strength (YS) of the steel after the above step (C) at 600 °C can be 238 MPa or higher.

[0016] Furthermore, the final microstructure of the steel after the above step (C) can include bainite.

[0017] In addition, the above steel can include 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 0.05% 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).

[0018] In addition, the above steel can further include 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).

[0019] The section steel according to an embodiment of the present invention includes carbon (C) of 0.17 wt% or less, manganese (Mn) of 1.6 wt% or less, chromium (Cr) of 0.10 wt% to 0.35 wt%, molybdenum (Mo) of 0.15 wt% or less, niobium (Nb) of 0.05 wt% or less, boron (B) of 0.003 wt% or less, titanium (Ti) of 0.04 wt% or less, the balance of iron (Fe) and other inevitable impurities, and the yield strength (YS) of the above section steel at room temperature can satisfy 355 MPa or more.

[0020] In addition, the impact energy absorption (CVN) at 0 °C can satisfy 27 J or more.

[0021] In addition, the high temperature yield strength (YS) at 600 °C can be 238 MPa or more.

[0022] In addition, the elongation (EL) can be 21% or more.

[0023] In addition, the final microstructure can include bainite.

[0024] In addition, the above section steel can include carbon (C) of 0.08 wt% to 0.15 wt%, manganese (Mn) of 0.5 wt% to 1.6 wt%, chromium (Cr) of 0.1 wt% to 0.3 wt%, molybdenum (Mo) of 0.10 wt% to 0.15 wt%, niobium (Nb) of 0.02 wt% to 0.05 wt%, titanium (Ti) of 0.03 wt% or less, and boron (B) of 0.001 wt% to 0.003 wt%.

[0025] In addition, the above section steel can further include silicon (Si) of 0.1 wt% to 0.4 wt%, copper (Cu) of 0.6 wt% or less, nitrogen (N) of 0.015 wt% or less, sulfur (S) of 0.01 wt% or less, and phosphorus (P) of 0.02 wt% or less.

[0026] The section steel according to an embodiment of the present invention includes carbon (C) of 0.17 wt% or less, manganese (Mn) of 1.6 wt% or less, chromium (Cr) of 0.10 wt% to 0.35 wt%, molybdenum (Mo) of 0.15 wt% or less, niobium (Nb) of 0.05 wt% or less, boron (B) of 0.003 wt% or less, titanium (Ti) of 0.04 wt% or less, the balance of iron (Fe) and other inevitable impurities, and the above section steel can be manufactured by reheating at a temperature of 1200 °C or more, controlling the starting rolling temperature to 1050 °C to 1100 °C, controlling the finishing rolling temperature to 860 °C to 930 °C, and then performing water cooling.

[0027] In addition, in the above water cooling method, the end temperature of water cooling can be controlled to be from 680°C to 880°C.

[0028] Effects of the Invention

[0029] According to an embodiment of the present invention, it is possible to implement a high-performance steel section with both seismic performance and fire resistance and a manufacturing method thereof.

[0030] The effects of the present invention are not limited to the above-mentioned effects. Through the description in the appended claims, those skilled in the art can clearly understand other effects not mentioned. Description of the Drawings

[0031] Figure 1 It is a photograph of the microstructure observation of the sample at the center of the flange of the steel section according to the present invention.

[0032] Figure 2 It is a flowchart of the manufacturing method of the steel section according to the present invention. Detailed Description of the Embodiment

[0033] In this specification, when it is mentioned that a certain component (or region, layer, part, etc.) is "located on", "connected" or "combined" with another component, it means that this component can be directly arranged / connected / combined on another component, or a third component can be arranged between them.

[0034] The same reference numerals denote the same components. In addition, in the drawings, for the purpose of effectively explaining the technical content, the thickness, ratio, and size of the components may be exaggerated.

[0035] "And / or" means including one or more combinations defined by the relevant components.

[0036] Terms such as "first" and "second" can be used to describe various components, but the above components should not be limited to the above terms. The above terms are used to distinguish one structural element from another. For example, without departing from the protection scope of the present invention, the "first structural element" can be referred to as the "second structural element", and similarly, the "second structural element" can also be referred to as the "first structural element". As long as it is not clearly distinguished in the text, the description indicating the singular should be understood to include the plural.

[0037] In addition, terms such as "below", "lower side", "above", "upper side", etc. are used to describe the relationship between the components shown in the drawings. The above terms are relative concepts and are described based on the directions shown in the drawings.

[0038] Unless otherwise clearly defined, all terms (including technical and scientific terms) used in this specification shall have the same meaning as commonly understood by those skilled in the art to which this invention pertains. In addition, terms identical to those defined in commonly used dictionaries shall be interpreted as having the same meaning in the context of the relevant art, unless interpreted as an ideal or overly formal meaning, which is clearly defined herein.

[0039] It should be understood that terms such as "comprising" or "having" are used to specify the presence of features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and it should be understood that they do not pre-exclude the presence or additional possibility of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

[0040] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0041] section steel

[0042] The section steel according to an embodiment of the present invention includes carbon (C) of 0.17 wt% or less, manganese (Mn) of 1.6 wt% or less, chromium (Cr) of 0.10 wt% to 0.35 wt%, molybdenum (Mo) of 0.15 wt% or less, niobium (Nb) of 0.05 wt% or less, boron (B) of 0.003 wt% or less, titanium (Ti) of 0.04 wt% or less, the balance of iron (Fe) and other inevitable impurities, and its yield strength (YS) at normal temperature is 355 MPa or more. In addition, the section steel according to an embodiment of the present invention may further include silicon (Si) of 0.1 wt% to 0.4 wt%, copper (Cu) of 0.6 wt% or less, nitrogen (N) of 0.015 wt% or less, sulfur (S) of 0.01 wt% or less, and phosphorus (P) of 0.02 wt% or less, and its yield strength (YS) at room temperature may be 355 MPa or more.

[0043] More preferably, the above section steel may include carbon (C) of 0.08 wt% to 0.15 wt%, manganese (Mn) of 0.5 wt% to 1.6 wt%, chromium (Cr) of 0.1 wt% to 0.3 wt%, molybdenum (Mo) of 0.10 wt% to 0.15 wt%, niobium (Nb) of 0.02 wt% to 0.05 wt%, titanium (Ti) of 0.03 wt% or less, and boron (B) of 0.001 wt% to 0.003 wt%.

[0044] The yield strength (YS) of the section steel with the above alloy composition at room temperature satisfies 355 MPa or more. Moreover, the impact energy absorption (CVN) at 0 °C can be 27 J or more, the high-temperature yield strength (YS) at 600 °C can be 238 MPa or more, and the elongation (EL) can be 21% or more. Specifically, the yield strength (YS) at room temperature can be 400 MPa or more, the impact energy absorption (CVN) at 0 °C can be 40 J or more, the high-temperature yield strength (YS) at 600 °C can be 250 MPa or more, and the elongation (EL) can be 25% or more.

[0045] In addition, the final microstructure of the section steel with the above alloy composition may contain bainite. That is to say, since the section steel according to the present invention realizes a bainite matrix structure, as described above, the high-temperature yield strength can be improved. In addition, fine carbides can be realized together with the bainite matrix structure.

[0046] Thus, the section steel according to an embodiment of the present invention can be a high-performance section steel with a yield strength (YS) of 355 MPa or more, which satisfies the alloy composition system standard of the Korean hot-rolled section steel for building structures, i.e., KSD 3866, and has both seismic performance and fire resistance.

[0047] On the other hand, the alloy composition system of the above KSD 3866 needs to satisfy the following conditions: the carbon (C) content is 0.20 wt% or less, the silicon (Si) content is 0.40 wt% or less, the manganese (Mn) content is 1.00 wt% to 1.60 wt% or less, the phosphorus (P) content is 0.035 wt% or less, the sulfur (S) content is 0.030 wt% or less, the chromium (Cr) content is 0.35 wt% or less, the molybdenum (Mo) content is 0.15 wt% or less, the copper (Cu) content is 0.60 wt% or less, and the niobium (Nb) content is 0.05 wt% or less.

[0048] Hereinafter, the functions and contents of each alloy element contained in the section steel according to an embodiment of the present invention will be described in detail.

[0049] carbon (C)

[0050] Carbon reacts with Nb, Ti, etc., promotes the formation of fine carbides, thereby effectively improving the strength of the steel through precipitation strengthening. At the same time, by hindering the movement of dislocations at high temperatures, the high-temperature strength is improved, thereby effectively ensuring the fire resistance. On the other hand, if the carbon addition amount is too high, coarse carbides may be formed, which will not only reduce the impact characteristics but also may cause discontinuous yield behavior, thereby increasing the yield ratio and resulting in a decrease in seismic performance. For this reason, the carbon content in the section steel according to an embodiment of the present invention can be 0.17 wt% or less, preferably 0.08 wt% to 0.15 wt%.

[0051] That is, when the carbon content is less than 0.08% by weight of the total weight, it may be difficult to ensure sufficient strength. On the contrary, when the carbon content exceeds 0.17% by weight of the total weight, coarse carbides may be generated, which will not only reduce the impact properties but also may cause discontinuous yielding behavior, thereby increasing the yield ratio and resulting in a decrease in seismic performance.

[0052] manganese (Mn)

[0053] Manganese, as a solid-solution strengthening element, not only helps to increase the strength of steel but also can improve the hardenability of steel, thus effectively promoting the formation of bainite structure. On the other hand, if the addition amount of manganese is too high, it may combine with sulfur (S) to form MnS inclusions or cause central segregation in the ingot. Therefore, the content of manganese in the section steel according to an embodiment of the present invention can be 1.6% by weight or less, preferably 0.5% to 1.6% by weight, and more preferably 0.5% to 1.3% by weight.

[0054] That is, when the manganese content is less than 0.5% by weight of the total weight, the effect of solid-solution strengthening may not be fully exerted, and when the manganese content exceeds 1.6% by weight of the total weight, it may combine with sulfur to form MnS inclusions or cause central segregation in the ingot, resulting in a decrease in the ductility and corrosion resistance of the section steel.

[0055] chromium (Cr)

[0056] Chromium can improve the hardenability of steel, thus helping to obtain a bainite microstructure. However, if the content is too high, it may increase the production cost of steel and form coarse carbides at the grain boundaries, thereby reducing the ductility of steel. Therefore, the content of chromium in the section steel according to an embodiment of the present invention can be 0.10% to 0.35% by weight, preferably 0.1% to 0.3% by weight

[0057] That is to say, when the chromium content is less than 0.1% by weight of the total weight, the addition effect of chromium may not be fully exerted, and when the chromium content exceeds 0.35% by weight, it may increase the production cost of steel and form coarse carbides at the grain boundaries, thereby reducing the ductility of steel. In addition, if the chromium content is 0.35% by weight or less, the alloy composition system standard of the Korean hot-rolled section steel for building structures, namely KSD 3866, can be satisfied.

[0058] molybdenum (Mo)

[0059] Molybdenum can improve the hardenability of steel, thus helping to obtain a bainite microstructure and also being an element that effectively ensures high-temperature strength. However, if the content of molybdenum is too high, it may increase the production cost of steel and promote the formation of grain boundary carbides, thereby reducing the ductility of steel. Therefore, the content of molybdenum in the section steel according to an embodiment of the present invention can be 0.15 wt% or less, preferably 0.10 wt% to 0.15 wt%.

[0060] That is, when the content of molybdenum is less than 0.10 wt% of the total weight, the added effect of molybdenum may not be fully exerted; on the contrary, when the content of molybdenum exceeds 0.15 wt% and is added in large amounts, it may increase the production cost of steel and promote the formation of grain boundary carbides, thereby reducing the ductility of steel. In addition, if the content of molybdenum is 0.15 wt% or less, it can meet the alloy composition system standard of the Korean hot-rolled section steel specification for building structures, namely KSD 3866.

[0061] niobium (Nb)

[0062] When niobium is dissolved in the austenite structure, it can inhibit the growth of grains, thus keeping the grains in a fine size. In addition, by reacting with carbon, it can promote the formation of fine carbides, thereby effectively improving the strength of steel through precipitation strengthening, especially improving the high-temperature yield strength. In addition, it can improve the hardenability, thus making the matrix structure bainitic, and thereby effectively improving the high-temperature yield strength. However, if the addition amount of niobium is too much, it may reduce the impact energy absorption of steel. Therefore, the niobium content in the section steel according to an embodiment of the present invention can be 0.05 wt% or less, preferably 0.02 wt% to 0.05 wt%, and more preferably 0.04 wt% to 0.05 wt%.

[0063] That is, when the content of niobium is less than 0.02 wt% of the total weight, it is difficult to fully exert the above-mentioned added effect of niobium, and when the content of niobium exceeds 0.05 wt% and is added in large amounts, it may reduce the impact energy absorption of steel. On the other hand, when the content of niobium is 0.04 wt% to 0.05 wt%, the above-mentioned added effect of niobium can be maximized while minimizing the reduction of the impact energy absorption of steel.

[0064] boron (B)

[0065] Boron can preferentially segregate at the austenite grain boundary, thus suppressing the formation of ferrite as a soft tissue during cooling, and then improving the hardenability. However, if the addition amount is too much, defects such as grain boundary brittleness may occur. For this reason, the content of boron (B) can be 0.003 wt% or less, preferably 0.001 wt% to 0.003 wt%.

[0066] That is, when the boron content is less than 0.001% by weight of the total weight, the austenite grain boundary segregation effect may be insufficient, and when the boron content exceeds 0.003% by weight, defects of grain boundary brittleness may occur.

[0067] titanium (Ti)

[0068] Titanium can form TiN together with nitrogen. In the present invention, as a method for improving the hardenability of steel, the formation of primary ferrite in austenite grains is suppressed by adding boron. At this time, if boron and nitrogen combine to form BN during the steelmaking process, the mechanism for improving hardenability cannot be achieved. Therefore, it is necessary to apply a Vacuum Degassing (VD) process in the steelmaking process to limit the nitrogen content to 100 ppm or less. In addition, in order to suppress the combination of the remaining nitrogen and boron, titanium (Ti) is added to preferentially form TiN, thereby ultimately playing a role in improving the hardenability of the steel. For this purpose, in an embodiment of the present invention, the content of titanium (Ti) can be 0.04% by weight or less, preferably 0.03% by weight or less, and more preferably 0.02% to 0.03% by weight.

[0069] silicon (Si)

[0070] Silicon (Si) and aluminum are added to steel as deoxidizers for removing oxygen in the steelmaking process. In addition, silicon can also have the effect of solid solution strengthening.

[0071] Silicon can be added in a content ratio of 0.10% to 0.40% by weight of the total weight of the section steel according to an embodiment of the present invention. When the silicon content is less than 0.10% by weight of the total weight, the addition effect of silicon may not be fully exerted. On the contrary, when the silicon content exceeds 0.40% by weight of the total weight and is added in large amounts, the welding performance of the steel may be reduced, and red scale may be generated during reheating and hot rolling, resulting in problems with the surface quality.

[0072] copper (Cu)

[0073] Copper (Cu) is an element that dissolves in ferrite and exhibits the effect of solid solution strengthening. In addition, during the bainite phase transformation, the supersaturated copper that has not precipitated is solid-solved in the structure at room temperature, and when heated at the service temperature of fire-resistant steel, i.e., 600 °C, the copper phase precipitates on the dislocations introduced by the bainite phase transformation, increasing the internal stress through this precipitation strengthening.

[0074] Copper can be added in a content ratio of 0.6 wt% or less, preferably 0.5 wt% or less, based on the total weight of the section steel according to an embodiment of the present invention. When the content of copper exceeds 0.6 wt% of the total weight and is added in a large amount, it may cause difficulties in hot working, the precipitation strengthening reaches saturation, the toughness decreases, and the problem of red hot embrittlement may occur.

[0075] nitrogen (N)

[0076] Nitrogen (N) can form nitride precipitates such as AlN, which helps to refine the grains and helps to ensure high-temperature strength. The above nitrogen can be added in a content ratio of 0.015 wt% or less, preferably 0.012 wt% or less, based on the total weight of the section steel according to an embodiment of the present invention. However, if the above nitrogen content exceeds 0.015 wt%, it may reduce the toughness of the welded part and reduce the impact value.

[0077] sulfur (S)

[0078] Sulfur (S) can improve the workability by forming fine MnS precipitates. The above sulfur can be added in a content ratio of 0.01 wt% or less, based on the total weight of the section steel according to an embodiment of the present invention. When the content of sulfur exceeds 0.01 wt%, it may generate inclusions, etc. as impurity elements, thereby reducing the ductility of the steel and may cause a decrease in toughness and weldability.

[0079] phosphorus (P)

[0080] Phosphorus (P) improves the strength of the steel by solid solution strengthening and can play a function of suppressing carbide formation. Phosphorus can be added in a content ratio of 0.02 wt% or less, based on the total weight of the section steel according to an embodiment of the present invention. When the content of phosphorus exceeds 0.020 wt%, it may generate inclusions, etc. as impurity elements, thereby reducing the ductility of the steel and causing a problem of a decrease in the impact value due to the precipitation behavior.

[0081] On the other hand, the section steel according to an embodiment of the present invention includes carbon (C) of 0.17 wt% or less, manganese (Mn) of 1.6 wt% or less, chromium (Cr) of 0.10 wt% to 0.35 wt%, molybdenum (Mo) of 0.15 wt% or less, niobium (Nb) of 0.05 wt% or less, boron (B) of 0.003 wt% or less, titanium (Ti) of 0.04 wt% or less, the balance of iron (Fe) and other inevitable impurities. After reheating at a temperature of 1200 °C or higher, the starting rolling temperature is controlled to 1050 °C to 1100 °C, the ending rolling temperature is controlled to 860 °C to 930 °C, and then water cooling is performed to manufacture the above section steel. Additionally, in the above water cooling method, the water cooling ending temperature can be controlled to 680 °C to 880 °C for manufacturing.

[0082] The above section steel may further include silicon (Si) of 0.1 wt% to 0.4 wt%, copper (Cu) of 0.6 wt% or less, nitrogen (N) of 0.015 wt% or less, sulfur (S) of 0.01 wt% or less, and phosphorus (P) of 0.02 wt% or less. More preferably, the above section steel may include carbon (C) of 0.08 wt% to 0.15 wt%, manganese (Mn) of 0.5 wt% to 1.6 wt%, chromium (Cr) of 0.1 wt% to 0.3 wt%, molybdenum (Mo) of 0.10 wt% to 0.15 wt%, niobium (Nb) of 0.02 wt% to 0.05 wt%, titanium (Ti) of 0.03 wt% or less, and boron (B) of 0.001 wt% to 0.003 wt%.

[0083] Thus, the section steel according to an embodiment of the present invention can be a high-performance section steel with a yield strength (YS) of 355 MPa or more, which has both seismic performance and fire resistance while meeting the alloy composition system standard of the Korean hot-rolled section steel specification for building structures, i.e., KSD 3866.

[0084] The yield strength (YS) of the section steel having the above alloy composition at normal temperature satisfies 355 MPa or more. And the impact energy absorption (CVN) at 0 °C can be 27 J or more, the high-temperature yield strength (YS) at 600 °C can be 238 MPa or more, and the elongation (EL) can be 21% or more. Specifically, the yield strength (YS) at normal temperature can be 400 MPa or more, the impact energy absorption (CVN) at 0 °C can be 40 J or more, the high-temperature yield strength (YS) at 600 °C can be 250 MPa or more, and the elongation (EL) can be 25% or more.

[0085] Figure 1 It is a microstructure observation photograph of a sample at the center of the flange of the section steel according to the present invention.

[0086] Refer toFigure 1 In the final microstructure of the section steel having the above alloy composition, bainite may be included. In addition, ferrite and fine carbides can be achieved together with the bainite matrix structure. As described above, the section steel according to the present invention can effectively improve the high-temperature yield strength by forming Cr, Mo, Nb-based carbides and ensuring the bainite matrix structure.

[0087] manufacturing method of section steel

[0088] Referring to Figure 2 First, a method for manufacturing a section steel according to an embodiment of the present invention includes: (a) a reheating step (S10), (b) a hot rolling step (S20), and (c) a water cooling step (S30) of the steel.

[0089] First, the steel contains carbon (C) of 0.17 wt% or less, manganese (Mn) of 1.6 wt% or less, chromium (Cr) of 0.10 wt% to 0.35 wt%, molybdenum (Mo) of 0.15 wt% or less, niobium (Nb) of 0.05 wt% or less, boron (B) of 0.003 wt% or less, titanium (Ti) of 0.04 wt% or less, the balance of iron (Fe) and other inevitable impurities, and step (a) of reheating the above steel at a temperature of 1200 °C or higher is performed. Next, step (b) of hot rolling the above steel is performed, wherein the starting rolling temperature is controlled to be 1050 °C to 1100 °C, and the ending rolling temperature is controlled to be 860 °C to 930 °C. Then, step (c) of water cooling the steel is included.

[0090] Thus, the method for manufacturing a section steel according to an embodiment of the present invention can manufacture a high-performance section steel that satisfies the alloy composition system standard of the Korean hot-rolled section steel specification for building structures, i.e., KSD 3866, and has earthquake resistance performance, fire resistance performance, and a yield strength (YS) of 355 MPa or higher.

[0091] Hereinafter, each step of the method for manufacturing a section steel will be described in detail.

[0092] First, in the reheating step, the steel having the above composition is reheated at a temperature of 1200 °C or higher. If the reheating temperature is lower than 1200 °C, various carbides may not be fully dissolved, and the components segregated in the continuous casting process may not be fully and uniformly distributed. In addition, the reheating temperature may not exceed 1250 °C. If the reheating temperature exceeds 1250 °C, coarse austenite grains may be formed, it is difficult to ensure strength, and due to the increase in heating cost and time, problems such as an increase in manufacturing cost and a decrease in productivity may occur.

[0093] On the other hand, after obtaining molten steel of a desired composition through a steelmaking process, the above-mentioned steel can be manufactured by a continuous casting process. The above-mentioned steel can be, for example, a beam blank, but is not limited thereto.

[0094] On the other hand, the composition of the steel may further include 0.1 wt% 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). Additionally, more preferably, in the above-mentioned steel, the content of the above-mentioned carbon (C) may be 0.08 wt% to 0.15 wt%, the content of the above-mentioned manganese (Mn) may be 0.5 wt% to 1.6 wt%, the content of the above-mentioned chromium (Cr) may be 0.1 wt% to 0.3 wt%, the content of the above-mentioned molybdenum (Mo) may be 0.10 wt% to 0.15 wt%, the content of the above-mentioned niobium (Nb) may be 0.02 wt% to 0.05 wt%, the content of the above-mentioned titanium (Ti) may be 0.03 wt% or less, and the content of the above-mentioned boron (B) may be 0.001 wt% to 0.003 wt%.

[0095] In the hot rolling step (b), the reheated above-mentioned steel is hot rolled. At this time, the starting rolling temperature is controlled at 1050°C to 1100°C, and the ending rolling temperature is controlled at 860°C to 930°C, so that even when the contents of chromium (Cr) and molybdenum (Mo) are relatively low, a bainite matrix structure can be ensured, and high-temperature yield strength can be ensured. In particular, when the above-mentioned ending rolling temperature is lower than 860°C, since rolling is performed in the non-recrystallization region, the rolling load may increase, resulting in an increase in the yield ratio of the section steel as the rolling product. In addition, when the above-mentioned ending rolling temperature exceeds 930°C, it may be difficult to ensure the target strength and toughness.

[0096] On the other hand, the water cooling step (c) is performed after hot rolling. In the manufacturing method of the section steel according to the present invention, the water cooling end temperature (or cooling and reheat temperature) can be controlled at 680°C to 880°C. The water cooling step can be carried out by a QST (Quenching and Self Tempering) device as a surface accelerated cooling device, and the above-mentioned QST device cools and self-tempers the section steel after hot rolling. The above-mentioned water cooling can adopt a quenching method (Quenching) of spraying cooling water onto the above-mentioned section steel, and by controlling the conveying speed of the above-mentioned section steel or the amount of the sprayed cooling water, the above-mentioned water cooling end temperature and self-tempering temperature can be controlled at 680°C to 880°C, and more preferably, it is carried out under a controlled state of 720°C to 760°C.

[0097] The yield strength (YS) at room temperature of the steel or section steel after the above step (c) satisfies 355 MPa or more. Moreover, the impact energy absorption (CVN) at 0 °C can be 27 J or more, the high-temperature yield strength (YS) at 600 °C can be 238 MPa or more, and the elongation (EL) can be 21% or more. Specifically, the yield strength (YS) at room temperature can be 400 MPa or more, the impact energy absorption (CVN) at 0 °C can be 40 J or more, the high-temperature yield strength (YS) at 600 °C can be 250 MPa or more, and the elongation (EL) can be 25% or more.

[0098] The steel or section steel after the above step (c) may include bainite in the final microstructure. That is to say, the section steel according to the present invention can improve the high-temperature yield strength as described above by realizing a bainite matrix structure. In addition, fine carbides can be realized together with the bainite matrix structure.

[0099] In the section steel and the manufacturing method of the section steel according to an embodiment of the present invention, niobium inhibits the growth of austenite grain boundaries, making it have a fine grain size, thereby improving the yield strength of the material at room temperature, improving the high-temperature yield strength by forming carbides, improving hardenability, and transforming the matrix structure into a bainite structure, thereby improving the high-temperature yield strength.

[0100] In addition, titanium preferentially combines with the residual nitrogen in the steel to form TiN, thereby inhibiting the combination and generation of BN, improving the hardenability of the steel through boron, obtaining a bainite matrix structure, and ensuring earthquake resistance and fire resistance.

[0101] Moreover, by reducing the contents of chromium and molybdenum, which are elements that effectively improve hardenability and transform the matrix structure into a bainite structure, the limit standards for alloy element addition in KSD 3866 are satisfied. And in order to ensure the bainite matrix structure insufficient due to the low contents of chromium and molybdenum, the finishing rolling temperature is controlled between 860 °C and 930 °C, and the water cooling end temperature is controlled between 680 °C and 880 °C.

[0102] Thus, the present invention can manufacture high-performance section steel with both earthquake resistance and fire resistance and a yield strength of 355 MPa or more grade, while satisfying the KSD 3866 standard.

[0103] comparative examples and experimental examples

[0104] Hereinafter, preferred comparative examples and experimental examples are presented to help understand the present invention. However, the following experimental examples are only examples for helping to understand the present invention, and the scope of the present invention is not limited to the following experimental examples.

[0105] Table 1 and Table 2 show the compositions of the main alloying elements in this experimental example and comparative examples (unit: wt%), Table 3 shows the manufacturing process conditions of the samples according to this experimental example and comparative examples, and Table 4 shows the measurement results of the mechanical properties of the samples made according to the process conditions in Table 3. After manufacturing a beam blank with the composition shown in Table 1 and Table 2 by an electric arc furnace, it is hot-rolled to make an H-shaped steel with a flange thickness of 15 mm.

[0106] Table 1

[0107]

[0108] Table 2

[0109]

[0110]

[0111] Table 3

[0112]

[0113] Table 4

[0114]

[0115]

[0116] Comparative Example 1 and Experimental Example 1

[0117] Compared with Experimental Example 1, there are differences in the composition system between Comparative Example 1 and Experimental Example 1, and there are also differences in the finish rolling temperature and the cooling reheat temperature.

[0118] First, referring to Table 1 to Table 3, the composition system 1 of Comparative Example 1 further includes 0.16 wt% of carbon (C), 0.62 wt% of chromium (Cr), 0.35 wt% of molybdenum (Mo), 0.017 wt% of niobium (Nb), 0.05 wt% of titanium (Ti), and other alloying elements. The finish rolling temperature is 910°C to 950°C, and the cooling reheat temperature is 765°C to 800°C.

[0119] The composition system 4 of Experimental Example 1 further includes 0.08 wt% of carbon (C), 0.21 wt% of chromium (Cr), 0.14 wt% of molybdenum (Mo), 0.044 wt% of niobium (Nb), 0.025 wt% of titanium (Ti), and other alloying elements. Compared with the composition system 1 of Comparative Example 1, the contents of carbon (C), chromium (Cr), molybdenum (Mo), and titanium (Ti) are lower, while the content of niobium (Nb) is higher. In addition, the finish rolling temperature of Experimental Example 1 is 860°C to 930°C, and the cooling reheat temperature is 720°C to 760°C, which are lower than the finish rolling temperature and the cooling reheat temperature of Comparative Example 1.

[0120] Referring to Table 4, the yield strength of Example 1 at room temperature was 415 MPa, the yield ratio was 71%, the elongation was 27.5%, the impact energy absorption at 0 °C was 43 J, and the high-temperature yield strength at 600 °C was 276 MPa to 278 MPa. It can be confirmed that the physical properties were improved compared with those of Comparative Example 1.

[0121] Comparative Example 2 and Experimental Example 1

[0122] There were differences in the composition systems between Comparative Example 2 and Example 1, but they were the same in terms of the finish rolling temperature and the cooling and reheating temperature.

[0123] Referring to Tables 1 to 3, the composition system 2 of Comparative Example 2 included 0.08 wt% of carbon, 0.61 wt% of chromium, 0.35 wt% of molybdenum, 0.018 wt% of niobium, 0.04 wt% of titanium, and other alloying elements. The finish rolling temperature was 860 °C to 930 °C, and the cooling and reheating temperature was 720 °C to 760 °C.

[0124] The composition system 4 of Example 1 further included 0.08 wt% of carbon, 0.21 wt% of chromium, 0.14 wt% of molybdenum, 0.044 wt% of niobium, 0.025 wt% of titanium, and other alloying elements. Compared with the composition system 2 of Comparative Example 2, the contents of chromium, molybdenum, and titanium were lower, while the content of niobium was higher. In addition, the finish rolling temperature of Example 1 was 860 °C to 930 °C, and the cooling and reheating temperature was 720 °C to 760 °C, which was the same as the finish rolling temperature and the cooling and reheating temperature of Comparative Example 2.

[0125] Referring to Table 4, the room-temperature yield strength of Example 1 was 415 MPa, the yield ratio was 71%, the elongation was 27.5%, the impact energy absorption at 0 °C was 43 J, and the high-temperature yield strength at 600 °C was 276 - 278 MPa. It can be confirmed that the physical properties were improved compared with those of Comparative Example 2.

[0126] Comparative Example 2 and Experimental Example 2

[0127] There were differences in the composition systems between Comparative Example 2 and Example 2, but they were the same in terms of the finish rolling temperature and the cooling and reheating temperature.

[0128] Referring to Tables 1 to 3, the composition system 2 of Comparative Example 2 included 0.08 wt% of carbon, 0.61 wt% of chromium, 0.35 wt% of molybdenum, 0.018 wt% of niobium, 0.04 wt% of titanium, and other alloying elements. The finish rolling temperature was 860 °C to 930 °C, and the cooling and reheating temperature was 720 °C to 760 °C.

[0129] The composition system 5 of Experimental Example 2 also includes 0.08 wt% of carbon, 0.34 wt% of chromium, 0.15 wt% of molybdenum, 0.020 wt% of niobium, 0.03 wt% of titanium, and other alloy elements. Compared with the composition system 2 of Comparative Example 2, the contents of chromium, molybdenum, and titanium are lower, while the content of niobium is higher. In addition, the finishing rolling temperature of Experimental Example 2 is 860°C to 930°C, and the cooling and reheating temperature is 720°C to 760°C, which is the same as the finishing rolling temperature and cooling and reheating temperature of Comparative Example 2.

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

[0131] Comparative Example 3 and Experimental Example 2

[0132] There are differences in the composition system between Comparative Example 3 and Experimental Example 2, but they are the same in terms of the finishing rolling temperature and cooling and reheating temperature.

[0133] Referring to Tables 1 to 3, the composition system 3 of Comparative Example 3 includes 0.08 wt% of carbon, 0.37 wt% of chromium, 0.17 wt% of molybdenum, 0.019 wt% of niobium, 0.032 wt% of titanium, and other alloy elements. The finishing rolling temperature is 860°C to 930°C, and the cooling and reheating temperature is 720°C to 760°C.

[0134] The composition system 5 of Experimental Example 2 also includes 0.08 wt% of carbon, 0.34 wt% of chromium, 0.15 wt% of molybdenum, 0.020 wt% of niobium, 0.03 wt% of titanium, and other alloy elements. Compared with the composition system 3 of Comparative Example 3, the contents of chromium, molybdenum, and titanium are lower, while the content of niobium is higher. In addition, the finishing rolling temperature of Experimental Example 2 is 860°C to 930°C, and the cooling and reheating temperature is 720°C to 760°C, which is the same as the finishing rolling temperature and cooling and reheating temperature of Comparative Example 3.

[0135] Referring to Table 4, it can be confirmed that the room temperature yield strength, yield ratio, elongation, 0°C impact absorption energy, and 600°C high temperature yield strength of the above Experimental Example 2 have been improved compared with the physical properties of Comparative Example 3.

[0136] Comparative Example 4 and Experimental Example 2

[0137] There are no differences in the composition system between Comparative Example 4 and Experimental Example 2, but there are differences in the finishing rolling temperature and cooling and reheating temperature.

[0138] Referring to Tables 1 to 3, the composition system 5 of Comparative Example 4 further includes 0.08% by weight of carbon, 0.34% by weight of chromium, 0.15% by weight of molybdenum, 0.020% by weight of niobium, 0.03% by weight of titanium, and other alloying elements. The finish rolling temperature is 910°C to 950°C, and the cooling and reheating temperature is 765°C to 800°C.

[0139] The composition system 5 of Experimental Example 2 also includes 0.08% by weight of carbon, 0.34% by weight of chromium, 0.15% by weight of molybdenum, 0.020% by weight of niobium, 0.03% by weight of titanium, and other alloying elements, which is the same as that of Comparative Example 4. In addition, the finish rolling temperature of Experimental Example 2 is 860°C to 930°C, and the cooling and reheating temperature is 720°C to 760°C, which are lower than those of Comparative Example 4.

[0140] Referring to Table 4, it can be confirmed that the room temperature yield strength, yield ratio, elongation, 0°C impact absorption energy, and 600°C high temperature yield strength of Experimental Example 2 described above have been improved compared with the physical properties of Comparative Example 3.

[0141] Although the preferred embodiments of the present invention have been described above, it is obvious that for those skilled in the art, in addition to the foregoing embodiments, the present invention can be implemented in other specific forms without departing from the purpose and scope of the present invention. Therefore, the foregoing embodiments should be used for illustration only and not for limiting the present invention. Therefore, the present invention is not limited to the foregoing description and can be modified within the scope of the appended claims and their equivalents.

[0142] Description of reference numerals

[0143] S10: Reheating step

[0144] S20: Hot rolling step

[0145] S30: Water cooling step

Claims

1. A manufacturing method of a section steel, characterized in that, comprising: Step (a), reheating a steel material containing carbon (C) of 0.17 wt% or less, manganese (Mn) of 1.6 wt% or less, chromium (Cr) of 0.10 wt% to 0.35 wt%, molybdenum (Mo) of 0.15 wt% or less, niobium (Nb) of 0.05 wt% or less, boron (B) of 0.003 wt% or less, titanium (Ti) of 0.04 wt% or less, the balance of iron (Fe) and other inevitable impurities at a temperature of 1200 °C or higher; Step (b), hot rolling the above steel material, wherein the starting rolling temperature is controlled to be 1050 °C to 1100 °C, and the ending rolling temperature is controlled to be 860 °C to 930 °C; and Step (c), water-cooling the above steel material.

2. The manufacturing method of a section steel according to claim 1, characterized in that, in the above step (c), the water-cooling ending temperature is controlled to be 680 °C to 880 °C.

3. The manufacturing method of a section steel according to claim 1, characterized in that, the steel material after the above step (c) satisfies the conditions that the yield strength (YS) at normal temperature is 355 MPa or more, the impact energy absorption (CVN) at 0 °C is 27 J or more, and the elongation (EL) is 21% or more.

4. The manufacturing method of a section steel according to claim 1, characterized in that, the steel material after the above step (c) has a high-temperature yield strength (YS) of 238 MPa or more at 600 °C.

5. The manufacturing method of a section steel according to claim 1, characterized in that, the final microstructure of the steel material after the above step (c) contains bainite.

6. The manufacturing method of a section steel according to claim 1, characterized in that, the above steel material includes carbon (C) of 0.08 wt% to 0.15 wt%, manganese (Mn) of 0.5 wt% to 1.6 wt%, chromium (Cr) of 0.1 wt% to 0.3 wt%, molybdenum (Mo) of 0.10 wt% to 0.15 wt%, niobium (Nb) of 0.02 wt% to 0.05 wt%, titanium (Ti) of 0.03 wt% or less, and boron (B) of 0.001 wt% to 0.003 wt%.

7. The manufacturing method of a section steel according to claim 1, characterized in that, the above steel material further includes silicon (Si) of 0.1 wt% to 0.4 wt%, copper (Cu) of 0.6 wt% or less, nitrogen (N) of 0.015 wt% or less, sulfur (S) of 0.01 wt% or less, and phosphorus (P) of 0.02 wt% or less.

8. A section steel, characterized in that, it includes carbon (C) of 0.17 wt% or less, manganese (Mn) of 1.6 wt% or less, chromium (Cr) of 0.10 wt% to 0.35 wt%, molybdenum (Mo) of 0.15 wt% or less, niobium (Nb) of 0.05 wt% or less, boron (B) of 0.003 wt% or less, titanium (Ti) of 0.04 wt% or less, the balance of iron (Fe) and other inevitable impurities, The above steel section satisfies the condition that the yield strength (YS) at normal temperature is 355 MPa or more.

9. The steel section according to claim 8, characterized in that the above steel section satisfies the condition that the impact energy absorption (CVN) at 0 °C is 27 J or more.

10. The steel section according to claim 8, characterized in that the high-temperature yield strength (YS) at 600 °C is 238 MPa or more.

11. The steel section according to claim 8, characterized in that the elongation rate (EL) is 21% or more.

12. The steel section according to claim 8, characterized in that the final microstructure contains bainite.

13. The steel section according to claim 8, characterized in that it includes 0.08 wt% to 0.15 wt% of carbon (C), 0.5 wt% to 1.6 wt% of manganese (Mn), 0.1 wt% to 0.3 wt% of chromium (Cr), 0.10 wt% to 0.15 wt% of molybdenum (Mo), 0.02 wt% to 0.05 wt% of niobium (Nb), titanium (Ti) of 0.03 wt% or less, and boron (B) of 0.001 wt% to 0.003 wt%.

14. The steel section according to claim 8, characterized in that it further includes 0.1 wt% to 0.4 wt% of silicon (Si), copper (Cu) of 0.6 wt% or less, nitrogen (N) of 0.015 wt% or less, sulfur (S) of 0.01 wt% or less, and phosphorus (P) of 0.02 wt% or less.

15. A steel section, characterized in that it includes carbon (C) of 0.17 wt% or less, manganese (Mn) of 1.6 wt% or less, chromium (Cr) of 0.10 wt% to 0.35 wt%, molybdenum (Mo) of 0.15 wt% or less, niobium (Nb) of 0.05 wt% or less, boron (B) of 0.003 wt% or less, titanium (Ti) of 0.04 wt% or less, the balance of iron (Fe) and other inevitable impurities, and the above steel section is manufactured by reheating at a temperature of 1200 °C or more, controlling the starting rolling temperature to 1050 °C to 1100 °C, controlling the ending rolling temperature to 860 °C to 930 °C, and then performing water cooling.

16. The steel section according to claim 15, characterized in that in the above water cooling method, the water cooling ending temperature is controlled to 680 °C to 880 °C.

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