Steel section and method for manufacturing same

Abstract

The present application relates to a steel section and a method for manufacturing same. According to the steel section and the method for manufacturing same, of the present application, excellent seismic performance and fire resistance can be secured at the same time. The steel section of the present application comprises, by wt%, 0.07-0.17% of C, 0.10-0.40% of Si, 0.70-1.60% of Mn, greater than 0% and less than or equal to 0.020% of P, greater than 0% and less than or equal to 0.010% of S, greater than 0% and less than or equal to 0.50% of Cu, greater than 0% and less than or equal to 0.15% of Ni, 0.10-0.50% of Cr, 0.10-0.75% of Mo, 0.025-0.050% of Nb, 0.015-0.040% of Ti, 0.015-0.060% of V, 0.008-0.012% of N, 0.001-0.003% of B, and the remainder of Fe and other inevitable impurities, and includes nano-precipitates and nano-clusters therein.

Description

Steel profile and method of manufacturing the same

[0001] This application relates to a structural steel and a method for manufacturing the same.

[0002] Recently, large-scale earthquakes have been occurring both domestically and internationally, resulting in massive loss of life and property. In addition to the primary damage caused by building failure during an earthquake, secondary fires can cause the softening of reinforcing materials (columns, beams) supporting the building, which can lead to plastic deformation of the reinforcing materials and accelerate the collapse of the building.

[0003] Accordingly, countries such as Japan, the United States, Europe, and Australia are strengthening public and national safety by establishing building design standards designed to delay the collapse of buildings and minimize damage to life and property even in disaster situations such as earthquakes and fires. To enhance the safety of such buildings, seismic design, the installation of protective facilities such as sprinklers, and the improvement of the seismic or fire resistance performance of structural materials are essential.

[0004] To this end, there is a demand for structural steel and a method for manufacturing the same that can secure seismic performance capable of withstanding earthquakes through yield ratio control, and offers improved high-temperature strength and excellent high-temperature stability even in the event of a fire.

[0005] The objective of the present application is to provide a structural steel having excellent seismic and fire-resistant performance simultaneously, and a method for manufacturing the same.

[0006] To solve the above problem, the structural steel of the present application comprises, in weight percent, C: 0.07% or more and 0.17% or less, Si: 0.10% or more and 0.40% or less, Mn: 0.70% or more and 1.60% or less, P: greater than 0% and 0.020% or less, S: greater than 0% and 0.010% or less, Cu: greater than 0% and 0.50% or less, Ni: greater than 0% and 0.15% or less, Cr: 0.10% or more and 0.50% or less, Mo: 0.10% or more and 0.75% or less, Nb: 0.025% or more and 0.050% or less, Ti: 0.015% or more and 0.040% or less, V: 0.015% or more and 0.060% or less, N: 0.008% or more and 0.012% or less, and B: 0.001% It contains 0.003% or less, with the remainder being Fe and other unavoidable impurities, and contains nano precipitates and nano clusters inside.

[0007] In addition, the particle size of the above nano precipitates is greater than 15 nm and less than or equal to 55 nm.

[0008] In addition, the nano precipitates are at least one of (Ti, Mo, V)CN and (Nb, Ti, V)CN.

[0009] In addition, the size of the nano cluster is 1 nm or more and 11 nm or less.

[0010] In addition, the nano cluster is at least one of (Ti, Mo, V)C and (Ti, Mo, Nb)C.

[0011] In addition, the density of the above nanocluster is 2.5 × 10 17 pieces / cm 3 Above 5.0 x 10 17 pieces / cm 3 It is as follows.

[0012] In addition, the above-mentioned structural steel contains bainite and ferrite inside.

[0013] In addition, the above-mentioned structural steel further contains degenerated pearlite inside.

[0014] In addition, the above-mentioned structural steel has a Charpy impact absorption energy of 27J or more when measured at -5℃.

[0015] In addition, the above-mentioned structural steel has a yield strength of 238 MPa or higher when measured at 600℃.

[0016] The method for manufacturing the structural steel of the present application comprises, in weight percent, C: 0.07% or more and 0.17% or less, Si: 0.10% or more and 0.40% or less, Mn: 0.70% or more and 1.60% or less, P: greater than 0% and 0.020% or less, S: greater than 0% and 0.010% or less, Cu: greater than 0% and 0.50% or less, Ni: greater than 0% and 0.15% or less, Cr: 0.10% or more and 0.50% or less, Mo: 0.10% or more and 0.75% or less, Nb: 0.025% or more and 0.050% or less, Ti: 0.015% or more and 0.040% or less, V: 0.015% or more and 0.060% or less, N: 0.008% or more and 0.012% or less, and B: 0.001% or more. The method includes the steps of reheating steel containing 0.003% or less, the remainder being Fe and other unavoidable impurities, hot rolling the reheated steel, and cooling the rolled steel, wherein the cumulative reduction rate in the hot rolling step is 20% or more.

[0017] In addition, the structural steel produced by the above manufacturing method contains nano precipitates and nano clusters inside.

[0018] In addition, the particle size of the nano precipitate is greater than 15 nm and less than or equal to 55 nm, and the size of the nano cluster is greater than 1 nm and less than or equal to 11 nm.

[0019] In addition, the density of the above nanocluster is 2.5 × 10 17 pieces / cm 3 4.5 x 10 17 pieces / cm 3 It is as follows.

[0020] According to the structural steel and the method for manufacturing the same of the present application, it is possible to manufacture structural steel having excellent seismic performance and fire resistance performance simultaneously.

[0021] FIG. 1 is a TEM and TEM-EDS photograph showing nano precipitates of a structural steel according to one embodiment of the present application.

[0022] FIG. 2 is an APT photograph showing nano clusters of a structural steel according to one embodiment of the present application.

[0023] Figure 3 is an SEM image showing the microstructure of a structural steel according to Example 3 of the present application.

[0024] Figure 4 is an EBSD image showing the phase fraction of the microstructure of a structural steel according to Example 3 of the present application.

[0025] Figure 5 is an SEM image showing degenerated pearlite contained within a structural steel section according to Example 3 of the present application.

[0026] Embodiments of the present invention will be described in detail below. Furthermore, the scope of the present invention is not limited to the embodiments described below, and may be implemented with arbitrary modifications within the scope that does not deviate from the gist of the present invention.

[0027] In the numerical ranges described stepwise in this specification, an upper or lower limit value described in any numerical range may be substituted with an upper or lower limit value of another numerical range described stepwise, or may also be substituted with a value shown in the examples.

[0028]

[0029] This application relates to structural steel. Structural steel generally refers to steel materials having a multifaceted cross-sectional shape and is manufactured by hot rolling cast slabs, such as blooms, billets, and beam blanks produced by continuous casting. Such structural steel is applied as structural steel, such as columns in buildings, and is also used as temporary construction materials for civil engineering projects like subways and bridges, as well as for foundation piles.

[0030] The above structural steel comprises, in weight%, C: 0.07% or more and 0.17% or less, Si: 0.10% or more and 0.40% or less, Mn: 0.70% or more and 1.60% or less, P: greater than 0% and 0.020% or less, S: greater than 0% and 0.010% or less, Cu: greater than 0% and 0.50% or less, Ni: greater than 0% and 0.15% or less, Cr: 0.10% or more and 0.50% or less, Mo: 0.10% or more and 0.75% or less, Nb: 0.025% or more and 0.050% or less, Ti: 0.015% or more and 0.040% or less, V: 0.015% or more and 0.060% or less, N: 0.008% or more and 0.012% or less, and B: 0.001% or more and 0.003% The following contains the remainder of Fe and other unavoidable impurities, and contains nano precipitates and nano clusters internally. According to the structural steel of the present application, excellent seismic performance and fire resistance performance can be secured simultaneously.

[0031] The nano precipitates contained within the above-mentioned structural steel refer to metal carbonitrides having a particle size exceeding 15 nm. These nano precipitates act as a strengthening mechanism for the steel, and the strengthening mechanism of the steel basically hinders the movement of dislocations. One of the main factors hindering the movement of dislocations is the hindrance caused by the formation of nano precipitates. Since nano precipitates, which are metal carbonitrides with a particle size exceeding 15 nm, are large and hard, when a dislocation comes into contact with the precipitates during movement, the dislocation line is bent extensively. In other words, the precipitates are not deformed by the dislocations, and dislocation looping occurs, forming dislocation loops (Orowan looping), which leads to an increase in strength.

[0032] For example, the particle size of the nano precipitates contained within the above-mentioned structural steel may be greater than 15 nm and less than or equal to 55 nm. The particle size of the nano precipitates can be measured using TEM. The particle size of the nano precipitates refers to the longest diameter of the spherical cross-section of the nano precipitates in the TEM measurement results. If the particle size of the nano precipitates contained within the above-mentioned structural steel exceeds the upper limit of the aforementioned range, the precipitates may become coarse, which may reduce the precipitation strengthening effect; conversely, if it falls below the lower limit of the aforementioned range, it may be difficult to obtain a sufficient precipitation strengthening effect. Therefore, the particle size of the nano precipitates contained within the above-mentioned structural steel may be greater than 15 nm and less than or equal to 55 nm, and specifically, it may be greater than 17 nm and less than or equal to 53 nm, or greater than or equal to 20 nm and less than or equal to 50 nm. The particle size of the nano precipitates can be obtained by appropriately controlling the reheating temperature, hot rolling temperature, and cooling rate during the manufacture of the structural steel.

[0033] The nano precipitates contained within the above-mentioned structural steel may include at least one of (Ti, Mo, V)CN and (Nb, Ti, V)CN. The nano precipitates can be confirmed by measurement using TEM-EDS mode. The nano precipitates being (Ti, Mo, V)CN means that the precipitates are composed of metal carbonitrides consisting of TiCN, MoCN, and VCN. Additionally, the nano precipitates being (Nb, Ti, V)CN means that the precipitates are composed of metal carbonitrides consisting of NbCN, TiCN, and VCN.

[0034] The structural steel of the present application also contains nano-clusters. The nano-clusters refer to aggregates of multiple metal atoms that are clustered together and exhibit similar properties, and the nano-clusters contained within the structural steel have a size of 15 nm or less. The nano-clusters are one of the factors that critically influence the strengthening mechanism of steel, and they act to hinder dislocation movement through multi-junctions of dislocations formed by solid solution or clusters. That is, when multiple dislocations meet, a binary junction is formed, and when another dislocation is introduced under stress, they become entangled with each other, causing the multi-junction of dislocations to function as a dislocation proliferation mechanism (Frank-Read source) to achieve strengthening. The dislocation proliferation mechanism refers to the continuous generation of dislocations within the crystal.

[0035] For example, the size of the nano clusters contained within the above-mentioned structural steel may be 1 nm or more and 11 nm or less. The size of the nano clusters can be measured through APT (Atom Probe Tomography). By satisfying the aforementioned range for the size of the nano clusters contained within the above-mentioned structural steel, sufficient reinforcing effects can be obtained for the steel, thereby ensuring excellent seismic performance. Specifically, the size of the nano clusters may be 3 nm or more and 11 nm or 3 nm or more and 9 nm or less. The size of the nano clusters can be controlled through the reheating temperature, hot rolling temperature, cold rolling speed, etc., during the manufacture of the structural steel.

[0036] The above nano cluster may include at least one of (Ti, Mo, V)C and (Ti, Mo, Nb)C. The nano cluster being (Ti, Mo, V)C means that the nano cluster is composed of TiC, MoC, and VC. Additionally, the nano cluster being (Ti, Mo, Nb)C means that it is composed of TiC, MoC, and NbC.

[0037] The density of the above nanocluster is 2.5 × 10 17 pieces / cm 3 Above 5.0 x 10 17 pieces / cm 3 It may be less than or equal to 2.7 x 10 17 pieces / cm 3 Above 4.7 x 10 17 pieces / cm 3 Less than or equal to 3.0 x 10 17 pieces / cm 3 4.5 x 10 17 pieces / cm 3 It may be less than or equal to the above. Excellent seismic and fire-resistant performance can be secured as the density of the above nano cluster satisfies the aforementioned range.

[0038] The number of clusters within a unit volume of the above nano clusters may be 80 (30nm x 30nm x 300nm) or more and 120 (30nm x 30nm x 300nm) or less. The number of clusters within a unit volume of the above nano clusters was measured by determining the number of nano clusters contained within a rectangular column area of ​​30nm x 30nm x 300nm. Excellent seismic and fire-resistant performance can be secured as the number of clusters within a unit volume of the above nano clusters satisfies the aforementioned range. Specifically, the number of clusters within a unit volume of the nano cluster may be 90 (30nm x 30nm x 300nm) or more and 110 (30nm x 30nm x 300nm) or 95 (30nm x 30nm x 300nm) or more and 105 (30nm x 30nm x 300nm) or less.

[0039] In addition, the above-mentioned structural steel may contain bainite and ferrite internally. Specifically, the area fraction of the bainite structure contained within the above-mentioned structural steel may be 30% or more and 55% or less, or 35% or more and 50% or less. In addition, specifically, the area fraction of the ferrite structure contained within the above-mentioned structural steel may be 40% or more and 65% or less, or 45% or more and 60% or less. By satisfying the aforementioned ranges for the area fractions of the bainite and ferrite structures contained within the above-mentioned structural steel, excellent seismic performance and fire resistance performance can be secured simultaneously. Furthermore, the above-mentioned structural steel may additionally contain less than 5% of degenerated pearlite internally.

[0040] In one example, the above-mentioned structural steel may have a Charpy impact absorption energy (CVN) of 25 J or more when measured at -5°C. Charpy impact absorption energy can be measured through a Charpy impact test, which serves as an important criterion for evaluating the stability and reliability of structural steel when exposed to unexpected impacts. In the Charpy impact test, a V-notch specimen is prepared based on the KS B 0809 standard at a position 1 / 4 of the width and 1 / 4 of the thickness of the structural steel flange. The specimen is placed in a liquid or steam bath maintained at -5°C ± 1°C for 5 to 30 minutes, after which both ends of the specimen are supported and the side opposite the notched surface is struck with a hammer. After measuring the energy absorbed when the specimen collides with the hammer, the average value of the measured results is taken as the Charpy impact absorption energy at -5°C. Specifically, the Charpy shock absorption energy at -5℃ may be 27J or more, 30J or more, 40J or more, or 50J or more. By satisfying the aforementioned ranges for Charpy shock absorption energy at -5℃, excellent seismic performance can be secured.

[0041] In one example, the above-mentioned structural steel may have a yield strength of 230 MPa or more at 600°C. A specimen is prepared based on the KS D 0026 standard at 1 / 4 of the width of the structural steel flange and at the full thickness position, and the yield strength is measured by heating the prepared specimen to 600°C ± 4°C and maintaining it for 10 minutes. The average value of the measured yield strengths is taken as the yield strength at 600°C. Specifically, the yield strength at 600°C may be 238 MPa or more, 250 MPa or more, 260 MPa or more, or 270 MPa or more. In addition, the upper limit of the average yield strength of the above-mentioned structural steel at a high temperature of 600°C may be 360 ​​MPa or less, and specifically, may be 355 MPa or less, or 350 MPa or less. The above-mentioned structural steel can secure excellent fire resistance performance by satisfying the aforementioned range of yield strength at 600°C.

[0042] In one example, the above-mentioned structural steel may have an elongation of 15% or more at room temperature. At this time, the room temperature may be between 15°C and 25°C. Furthermore, the elongation at room temperature is measured by fabricating a specimen based on KS B 0801 at a position 1 / 4 of the width and 1 / 4 of the thickness of the structural steel flange, and the average value of the measured elongations is taken as the elongation. Specifically, the above-mentioned structural steel may have an elongation of 17% or more at room temperature. Additionally, the upper limit of the elongation at room temperature of the above-mentioned structural steel may be 30% or less, and specifically, may be 28% or less or 25% or less. By satisfying the aforementioned range for the elongation at room temperature, the above-mentioned structural steel can simultaneously secure excellent seismic resistance and fire resistance.

[0043] In another example, the above-mentioned structural steel may have a tensile strength of 490 MPa or more and 750 MPa or less at room temperature, a yield strength of 340 MPa or more and 550 MPa or less, and a yield ratio of 0.85 or less. Additionally, the tensile strength and yield strength at room temperature are measured after preparing a specimen based on KS B 0801 at a position 1 / 4 of the width and 1 / 4 of the thickness of the structural steel flange. The average values ​​of the measured tensile strength and yield strength are respectively defined as the tensile strength and yield strength. Specifically, the above-mentioned structural steel may have an average tensile strength of 500 MPa or more and 740 MPa or 550 MPa or more and 730 MPa or less at room temperature, and a yield strength of 350 MPa or more and 730 MPa or 355 MPa or more and 725 MPa or less. At this time, the lower limit of the yield ratio of the structural steel at room temperature is not specifically limited, but, for example, it may be 0.55 or higher, and specifically, 0.60 or higher or 0.65 or higher. The structural steel can secure excellent seismic performance by satisfying the aforementioned ranges for tensile strength, yield strength, and yield ratio at room temperature.

[0044]

[0045] The alloy composition of the above-mentioned structural steel is explained below.

[0046] C: 0.07 wt% or more, 0.17 wt% or less

[0047] Carbon (C) is an element that effectively contributes to strength improvement through precipitation strengthening by reacting with niobium (Nb), vanadium (V), titanium (Ti), etc., to promote the formation of fine carbides. In addition, carbon is effective in securing fire resistance performance by improving high-temperature strength through hindering dislocation movement at high temperatures. If the carbon is included in the structural steel in an amount exceeding the upper limit of the aforementioned range, coarse carbides are formed, which not only degrades impact properties but also causes discontinuous yielding behavior, thereby increasing the yield ratio and potentially degrading seismic performance. Furthermore, if the carbon is included in the structural steel in an amount less than the lower limit of the aforementioned range, it may be difficult to secure sufficient strength. Therefore, the carbon may be included in the structural steel in an amount of 0.07 wt% or more and 0.17 wt% or less, and specifically, in an amount of 0.08 wt% or more and 0.15 wt% or less.

[0048] Si: 0.10 wt% or more, 0.40 wt% or less

[0049] Silicon (Si) is an element added along with aluminum as a deoxidizer to remove oxygen from steel during the steelmaking process. Additionally, silicon can also have a solid solution strengthening effect. If the silicon is included in the structural steel below the lower limit of the aforementioned range, the silicon addition effect cannot be properly exerted. Furthermore, if the silicon is included in the structural steel above the upper limit of the aforementioned range, it can impair the weldability of the steel and adversely affect surface quality by generating red scale during reheating and hot rolling. Therefore, the silicon may be included in the structural steel in an amount of 0.10 wt% or more and 0.40 wt% or less, specifically, in an amount of 0.13 wt% or more and 0.37 wt% or less, or in an amount of 0.15 wt% or more and 0.35 wt% or less.

[0050] Mn: 0.70 wt% or more, 1.60 wt% or less

[0051] Manganese (Mn) is a solid solution strengthening element that not only contributes to securing strength but also improves the hardenability of steel, making it an effective element for forming a bainite structure. If the manganese is included in the structural steel below the lower limit of the aforementioned range, it cannot fully exert the solid solution strengthening effect. Furthermore, if the manganese is included in the structural steel above the upper limit of the aforementioned range, it may combine with sulfur (S) to form MnS inclusions or cause central segregation in the ingot. Therefore, the manganese may be included in the structural steel in an amount of 0.70 wt% or more and 1.60 wt% or less, and specifically, in an amount of 0.80 wt% or more and 1.50 wt% or less, or in an amount of 0.90 wt% or more and 1.45 wt% or less.

[0052] P: Greater than 0 wt% and less than or equal to 0.020 wt%

[0053] Phosphorus (P) is an element that performs the function of increasing strength through solid solution strengthening and suppressing the formation of carbides. If the phosphorus is included in the above-mentioned structural steel in an amount exceeding the upper limit of the aforementioned range, it may reduce the ductility of the steel by generating inclusions, etc., as a tramp element. Therefore, the phosphorus may be included in the above-mentioned structural steel in an amount greater than 0 weight% and less than or equal to 0.020 weight%, and specifically, in an amount greater than or equal to 0.001 weight% and less than or equal to 0.015 weight%.

[0054] S: Greater than 0 wt% and less than or equal to 0.010 wt%

[0055] Sulfur (S) is an element that improves workability by forming fine MnS precipitates. If the sulfur is included in the structural steel in an amount exceeding the upper limit of the aforementioned range, it may reduce the ductility of the steel by forming inclusions, etc., as a tramp element. Accordingly, the sulfur may be included in the structural steel in an amount greater than 0 wt% and less than or equal to 0.010 wt%, and specifically, in an amount greater than or equal to 0.001 wt% and less than or equal to 0.009 wt% or greater than or equal to 0.003 wt% and less than or equal to 0.007 wt%.

[0056] Cu: Greater than 0 wt% and less than or equal to 0.50 wt%

[0057] Copper (Cu) is an element that is dissolved in ferrite and exhibits a solid solution strengthening effect. Furthermore, the copper does not precipitate during the bainite transformation, and supersaturated copper is dissolved in the microstructure at room temperature. When heated to 600°C, the operating temperature for refractory steel, copper phases precipitate on dislocations introduced by the bainite transformation, and the strength of the base material is increased through this precipitation hardening. If the copper is included in the structural steel below the lower limit of the aforementioned range, the effect of copper addition cannot be properly exerted. In addition, if the copper is included in the structural steel above the upper limit of the aforementioned range, problems may arise where hot working is difficult, the precipitation strengthening becomes saturated, reducing toughness, and causing red hot brittleness. Therefore, the copper may be included in the structural steel in an amount greater than 0 wt% and less than or equal to 0.50 wt%, specifically in an amount greater than or equal to 0.001 wt% and less than or equal to 0.40 wt%, or greater than or equal to 0.05 wt% and less than or equal to 0.30 wt%.

[0058] Ni: Greater than 0 wt% and less than or equal to 0.15 wt%

[0059] Nickel (Ni) is an element that increases hardenability and improves toughness. Additionally, nickel (Ni) can increase the strength of the material and secure low-temperature impact value as a tramp element. Accordingly, the nickel may be included in the structural steel in an amount greater than 0 wt% and less than or equal to 0.15 wt%, specifically in an amount greater than or equal to 0.001 wt% and less than or equal to 0.13 wt%. If the nickel is included in the structural steel in an amount exceeding the upper limit of the aforementioned range, the room-temperature strength may become excessively high, and weldability and toughness may deteriorate.

[0060] Cr: 0.10 wt% or more, 0.50 wt% or less

[0061] Chromium (Cr) is an element that improves the hardenability of steel and contributes to securing a bainite microstructure. If the chromium is included in the structural steel below the lower limit of the aforementioned range, the effect of chromium addition cannot be properly exerted. Furthermore, if the chromium is included in the structural steel above the upper limit of the aforementioned range, it may increase the manufacturing cost of the steel and form coarse carbides at the grain boundaries, thereby reducing the ductility of the steel. Accordingly, the chromium may be included in the structural steel in an amount of 0.10 wt% or more and 0.50 wt% or less, and specifically, in an amount of 0.13 wt% or more and 0.45 wt% or less, or in an amount of 0.15 wt% or more and 0.30 wt% or less.

[0062] Mo: 0.10 wt% or more, 0.75 wt% or less

[0063] Molybdenum (Mo) is an element that can improve the hardenability of steel and contribute to securing a bainite microstructure, and is very effective for securing high-temperature strength. If the molybdenum is included in the structural steel below the lower limit of the aforementioned range, the effect of adding molybdenum cannot be properly exerted. Furthermore, if the molybdenum is included in the structural steel above the upper limit of the aforementioned range, it can increase the manufacturing cost of the steel and promote the formation of grain boundary carbides, thereby reducing the ductility of the steel. Therefore, the molybdenum may be included in the structural steel in an amount of 0.10 wt% or more and 0.75 wt% or less, and specifically, in an amount of 0.15 wt% or more and 0.70 wt% or less, or in an amount of 0.20 wt% or more and 0.65 wt% or less.

[0064] Nb: 0.025 wt% or more, 0.050 wt% or less

[0065] Niobium (Nb) is an element that inhibits grain growth and provides a fine grain size when dissolved in an austenite structure. Additionally, niobium is effective in improving strength through precipitation strengthening by reacting with carbon to promote the formation of fine carbides. If the niobium is included in the structural steel in an amount exceeding the upper limit of the aforementioned range, it can reduce the impact absorption energy of the steel. Therefore, the niobium may be included in the structural steel in an amount of 0.025 wt% or more and 0.050 wt% or less, and specifically, in an amount of 0.030 wt% or more and 0.045 wt% or less.

[0066] Ti: 0.015 wt% or more, 0.040 wt% or less

[0067] Titanium (Ti) is an element that improves the toughness and strength of steel by generating Ti(C, N) precipitates with high high-temperature stability, thereby hindering austenite grain growth during welding and refining the microstructure of the weldment. If the titanium is included in the structural steel below the lower limit of the aforementioned range, the titanium addition effect cannot be properly exerted. Furthermore, if the titanium is included in the structural steel above the upper limit of the aforementioned range, it generates coarse precipitates, thereby degrading the low-temperature impact properties of the steel and causing problems such as increased manufacturing costs without further addition effects. Therefore, the titanium may be included in the structural steel in an amount of 0.015 wt% or more and 0.040 wt% or less, specifically, in an amount of 0.020 wt% or more and 0.035 wt% or less.

[0068] V: 0.015 wt% or more, 0.060 wt% or less

[0069] Vanadium (V) forms precipitates in steel. These precipitates are carbides, nitrides, and complexes located at grain boundaries, which inhibit grain growth and form fine grains, thereby enabling high strength and hardness. If the vanadium is included in the structural steel in an amount less than the lower limit of the aforementioned range, strength cannot be secured. Furthermore, if the vanadium is included in the structural steel in an amount exceeding the upper limit of the aforementioned range, it creates coarse precipitates, thereby degrading the low-temperature impact properties of the steel and increasing manufacturing costs without any further additive effects. Therefore, the vanadium may be included in the structural steel in an amount of 0.015 wt% or more and 0.060 wt% or less, specifically, in an amount of 0.020 wt% or more and 0.050 wt% or less.

[0070] N: 0.008 wt% or more, 0.012 wt% or less

[0071] Nitrogen (N) is an element that contributes to grain refinement by forming nitride-based precipitates such as AlN and contributes to securing high-temperature strength. If the nitrogen is included in the structural steel in an amount exceeding the upper limit of the aforementioned range, it may form BN, thereby suppressing the effect of B and excessively promoting the formation of bainite, which may be disadvantageous for securing the elongation and impact value within the aforementioned range. Therefore, the nitrogen may be included in the structural steel in an amount of 0.008 wt% or more and 0.012 wt% or less, and specifically, in an amount of 0.009 wt% or more and 0.011 wt% or less.

[0072] B: 0.001 wt% or more, 0.003 wt% or less

[0073] Boron (B) is an element that improves hardenability by preferentially segregating at austenite grain boundaries and suppressing the formation of ferrite, a soft structure, upon cooling. If the boron is included in the structural steel in an amount less than the lower limit of the aforementioned range, the austenite grain boundary segregation effect may be insufficient. Furthermore, if the boron is included in the structural steel in an amount exceeding the upper limit of the aforementioned range, there is a problem of exhibiting grain boundary brittleness. Therefore, the boron may be included in the structural steel in an amount of 0.001 wt% or more and 0.003 wt% or less, and specifically, in an amount of 0.002 wt% or more and 0.0027 wt% or less.

[0074] Remaining Fe and other unavoidable impurities

[0075] The aforementioned unavoidable impurities are impurities unintentionally incorporated from raw materials or the surrounding environment during the ordinary manufacturing process; as this is widely known in the art, a detailed description is omitted. In one embodiment of this application, the addition of elements other than the components of the structural steel described above is not excluded, and various elements may be included within a scope that does not impair the technical concept of this application. If additional elements are included, they may be included to replace the remainder of iron (Fe).

[0076]

[0077] This application also relates to a method for manufacturing a structural steel section. The method for manufacturing the structural steel section relates to a method for manufacturing the structural steel section, and since specific details regarding the structural steel section described below can be applied in the same way as those described for the structural steel section, they will be omitted.

[0078] The above method for manufacturing a structural steel includes the steps of reheating the steel, hot rolling the reheated steel, and cooling the rolled steel, wherein the hot rolling step has a cumulative reduction rate of 20% or more. According to the above method for manufacturing a structural steel, by including the aforementioned nano precipitates and nano clusters inside the structural steel, a structural steel having excellent seismic performance and fire resistance performance can be manufactured.

[0079] The above reheating step is a step of reheating the semi-finished steel material, wherein, in weight%, C: 0.07% or more and 0.17% or less, Si: 0.10% or more and 0.40% or less, Mn: 0.70% or more and 1.60% or less, P: greater than 0% and 0.020% or less, S: greater than 0% and 0.010% or less, Cu: greater than 0% and 0.50% or less, Ni: greater than 0% and 0.15% or less, Cr: 0.10% or more and 0.50% or less, Mo: 0.10% or more and 0.75% or less, Nb: 0.025% or more and 0.050% or less, Ti: 0.015% or more and 0.040% or less, V: 0.015% or more and 0.060% or less, N: 0.008% or more and 0.012% or less, and B: The steel containing 0.001% or more and 0.003% or less, with the remainder being Fe and other unavoidable impurities, is reheated in a furnace. Since the specific description of the alloy composition of the semi-finished product is the same as that described in the structural steel above, it will be omitted.

[0080] In one example, the reheating may be performed at a temperature of 1180°C or higher and 1250°C or lower. Specifically, the reheating may be performed at a temperature of 1180°C or higher and 1250°C or lower for a period of 100 minutes or more and 200 minutes or less. If the reheating temperature is below the lower limit or above the upper limit of the aforementioned range, coarse grains are formed, which excessively promotes the formation of bainite, and thus may be disadvantageous for securing the elongation and impact value within the aforementioned range. Therefore, the reheating may be performed at the aforementioned temperature for the aforementioned period.

[0081] The above hot rolling step is a step of hot rolling the reheated steel, and is performed by hot rolling the reheated steel in a hot rolling mill. Specifically, the cumulative reduction rate of the above hot rolling step may be 20% or more. Specifically, the cumulative reduction rate during the above hot rolling may be 20% or more and 43% or less, 25% or more and 40% or less, or 30% or more and 38% or less. When the above cumulative reduction rate satisfies the aforementioned range, a structural steel that simultaneously secures excellent seismic performance and fire resistance performance can be manufactured. On the other hand, if the above cumulative reduction rate during hot rolling is below the lower limit of the aforementioned range, bainite may be excessively formed, which may reduce elongation and impact resistance. Furthermore, if the above cumulative reduction rate during hot rolling exceeds the upper limit of the aforementioned range, bainite formation is suppressed, which may reduce high-temperature strength.

[0082] In addition, the hot rolling may be performed at an intermediate rolling temperature of 870°C or higher and 940°C or lower, and a rolling end temperature of 750°C or higher and 880°C or lower. Specifically, the hot rolling may be performed at an intermediate rolling temperature of 875°C or higher and 935°C or lower, and a rolling end temperature of 760°C or higher and 850°C or lower. By performing the hot rolling at the intermediate rolling temperature and rolling end temperature within the aforementioned ranges, a bainite structure can be secured and high-temperature yield strength can be secured even if the content of chromium (Cr) and molybdenum (Mo) is somewhat low. In particular, if the rolling end temperature is below the lower limit of the aforementioned range, the amount of rolling in the unrecrystallized region increases, which leads to an increased rolling load, potentially causing quality problems such as equipment load and dimensional instability, and may result in a decrease in high-temperature strength. In addition, if the above rolling end temperature exceeds the upper limit of the aforementioned range, it may be difficult to secure the target strength and toughness.

[0083] The cooling step described above is a step of manufacturing structural steel by cooling after hot rolling. For example, the cooling step may be performed using a conventional cooling method. By performing the cooling step through air cooling or water cooling, a bainite structure with a desired area fraction can be secured even without applying accelerated cooling, thereby enabling the manufacture of structural steel that simultaneously secures excellent seismic and fire resistance performance.

[0084] In one example, the structural steel manufactured by the above manufacturing method may contain nano precipitates and nano clusters inside. Since a specific description of the nano precipitates and nano clusters contained inside the structural steel is the same as that described in the structural steel, it will be omitted.

[0085] In one example, the structural steel produced by the above manufacturing method may have a Charpy impact absorption energy of 27 J or more at -5°C and a yield strength of 238 MPa or more at 600°C. Since the specific description of the physical properties of the structural steel at -5°C and 600°C is the same as that described in the above structural steel, it will be omitted.

[0086] In one example, the structural steel produced by the above manufacturing method may have an elongation of 17% or more at room temperature, an average tensile strength of 490 MPa or more and 720 MPa or less at room temperature, an average yield strength of 355 MPa or more and 470 MPa or less, and an average yield ratio of 0.85 or less. Since the specific description of the physical properties of the above structural steel at room temperature is the same as that described in the above structural steel, it will be omitted.

[0087]

[0088] The present application will be described in more detail below through embodiments according to the present application and comparative examples not according to the present application, but the scope of the present application is not limited by the embodiments presented below.

[0089]

[0090] Preparation of semi-finished steel

[0091] Semi-finished steel was prepared by continuously casting molten steel containing the components shown in Table 1 below, the remainder of Fe, and other unavoidable impurities to produce a beam blank.

[0092] Composition of semi-finished product (weight%) CSI Mn PSCu Ni Cr Mo Nb Ti V NB Example 1 0.07 0.33 1.00 0.01 10.00 80.17 0.10 0.15 0.15 0.025 0.021 0.028 0.01 00.0025 Example 20.08 0.25 1.00 0.01 20.00 90.12 0.11 0.15 0.30 0.04 0.02 0.03 0.01 00.0027 Example 30.10 0.24 1.2 0.01 20.00 80.15 0.10 0.30 0.30 0.04 0.023 0.03 0.01 10.0013 Example 40.140.181.400.0110.0100.120.120.300.300.0350.0210.0200.0100.0012 Comparative Example 10.110.241.430.0120.0100.160.100.150.150.0300.0190.0020.0100.0028

[0093]

[0094] Manufacturing of structural steel

[0095] The semi-finished product prepared above was reheated and hot-rolled under the conditions shown in Table 2 below, and then cooled by water cooling using a cooling device called QST (Quenching & Self-Tempering) to manufacture H-shaped steel.

[0096] Reheating Temperature (°C) Intermediate Rolling Temperature (°C) Cumulative Reduction Rate (%) Rolling End Temperature (°C) Double Heat Temperature (°C) Example 1 1 2 5 0 9 3 0 2 6 -- Example 2 1 2 5 0 8 9 0 2 6 7 6 5 6 5 2 Example 3 1 1 9 0 8 8 0 3 4 7 5 2 6 7 1 Example 4 1 2 0 8 7 0 3 1 7 8 0 6 5 7 Comparative Example 1 1 1 9 0 9 4 0 8 8 0 7 5 0

[0097]

[0098] Evaluation Example 1. Microstructure Evaluation

[0099] The microstructure of the H-shaped steel flanges manufactured in each of the examples and comparative examples was observed using an optical microscope image analyzer and a SEM at the 1 / 4 width and 1 / 4 thickness positions, and the results are shown in Table 3 below. In Table 3 below, the symbol B represents bainite, F represents ferrite, DP represents degenerate pearlite, and P represents pearlite.

[0100]

[0101] Evaluation Example 2. Evaluation of nano precipitates

[0102] The interior of the H-shaped steel prepared in each of the examples and comparative examples was observed via TEM to measure the size of the nano precipitates, and the composition of the nano precipitates was confirmed via TEM-EDS mode.

[0103] The size of the nano precipitates was determined by observing the position at 1 / 4 the width and 1 / 4 the thickness of the flange portion of the structural steel using TEM, measuring the longest diameter of the spherical cross-section of the nano precipitates, and calculating the average value to define the particle size of the precipitates. The results are shown in Table 3 below.

[0104]

[0105] Evaluation Example 3. Evaluation of Nano Clusters

[0106] The interior of the H-shaped steel prepared in each of the examples and comparative examples was observed using APT to measure the size of the nano clusters and identify the composition of the nano clusters.

[0107] The size of the nano cluster was determined by measuring the longest diameter of the spherical cross-section of the nano cluster observed with APT, setting the smallest measured diameter as the lower limit of the nano cluster size and the largest as the upper limit. The results are shown in Table 3 below.

[0108] Nano precipitates, nano clusters, microstructure components, particle size (nm), component size (nm), density (pieces / cm³) 3)Number of clusters per unit volume (pieces, 30nm x 30nm x 300nm) Example 1 (Ti, Mo, V)CN50(Ti, Mo, V)C8~113.6 x 10 17 90B+F Example 2 (Nb, Ti, V)CN32(Ti, Mo, Nb)C5~104.2×10 17 97B+F Example 3 (Ti, Mo, V)CN20(Ti, Mo, V)C3~93.2 × 10 17 110B+FExample 4(Nb, Ti, V)CN26(Ti, Mo, Nb)C4~114.0×10 17 103B+F+DP Comparative Example 1 (Nb, Ti, V)CN 60~80 (Ti, Mo, Nb)C 11~15 2.1 × 10 17 75B+F+P

[0109]

[0110] Evaluation Example 4. Evaluation of Charpy shock absorption energy

[0111] In each of the examples and comparative examples, a rectangular test specimen with a notch at the 1 / 4 width and 1 / 4 thickness position of the flange portion of the H-shaped steel manufactured according to the KS B 0809 standard was prepared, and then placed in a liquid bath maintained at -5°C for 15 minutes. After placing the specimen in the liquid bath maintained at -5°C for 15 minutes, both ends of the specimen were supported and the opposite side of the notch was struck to measure the Charpy impact absorption energy (CVN) at -5°C, and the average of these values ​​was calculated and the results are shown in Table 4 below.

[0112]

[0113] Evaluation Example 5. Evaluation of High-Temperature Properties

[0114] After preparing specimens according to KS D 0026 at the 1 / 4 width and 1 / 4 thickness position of the flange portion of the H-shaped steel manufactured in each of the examples and comparative examples, the specimens were heated to 600°C and maintained for 10 minutes. Tensile tests were performed using the specimens heated to 600°C and maintained for 10 minutes to measure the yield strength, and the average of these values ​​was calculated and the results are shown in Table 4 below.

[0115]

[0116] Evaluation Example 6. Evaluation of physical properties at room temperature

[0117] Specimens were prepared according to KS B 0801 standards at the 1 / 4 width and 1 / 4 thickness position of the flange portion of the H-shaped steel manufactured in each of the examples and comparative examples, and tensile tests were performed at room temperature (20℃) to measure tensile strength, yield strength, elongation, and yield ratio, and the average of these values ​​was calculated and the results are shown in Table 4 below.

[0118] Tensile Strength (MPa) Yield Strength (MPa) Elongation (%) Yield Ratio CVN (J, @ -5℃) Yield Strength (MPa, @ 600℃) Example 1 60 34 35 220.7 285 292 Example 2 64 34 26 230.6 67 8290 Example 3 67 14 90 200.7 38 2343 Example 4 71 34 74 180.6 65 6312 Comparative Example 15 60 39 6 220.7 12 2224

[0119]

[0120] As shown in Table 3 above, unlike the H-shaped steel produced in Comparative Example 1, the H-shaped steel produced in Examples 1 to 4 has a particle size of nano precipitates of 20 nm to 50 nm, a nano cluster size of 3 nm to 11 nm, a Charpy impact absorption energy (CVN) of 56 J or more at -5°C, and a yield strength of 290 MPa or more at 600°C.

Claims

1. In wt%, C: 0.07% or more and 0.17% or less, Si: 0.10% or more and 0.40% or less, Mn: 0.70% or more and 1.60% or less, P: greater than 0% and 0.020% or less, S: greater than 0% and 0.010% or less, Cu: greater than 0% and 0.50% or less, Ni: greater than 0% and 0.15% or less, Cr: 0.10% or more and 0.50% or less, Mo: 0.10% or more and 0.75% or less, Nb: 0.025% or more and 0.050% or less, Ti: 0.015% or more and 0.040% or less, V: 0.015% or more and 0.060% or less, N: 0.008% or more and 0.012% or less, and B: 0.001% or more and 0.003% The following includes the remaining Fe and other unavoidable impurities, A structural steel containing nano precipitates and nano clusters inside.

2. In Paragraph 1, A structural steel having a particle size of the above nano precipitates greater than 15 nm and less than or equal to 55 nm.

3. In Paragraph 1, The above nano-precipitates are a structural steel comprising at least one of (Ti, Mo, V)CN and (Nb, Ti, V)CN.

4. In Paragraph 1, A structural steel having a nano cluster size of 1 nm or more and 11 nm or less.

5. In Paragraph 1, The above nano cluster is a structural steel comprising at least one of (Ti, Mo, V)C and (Ti, Mo, Nb)C.

6. In Paragraph 1, The density of the above nanocluster is 2.5 × 10 17 Pieces / cm 3 Above 5.0 x 10 17 Pieces / cm 3 Lee Ha-in structural steel.

7. In Paragraph 1, A structural steel containing bainite and ferrite inside.

8. In Paragraph 7, Additionally, a structural steel containing degenerated pearlite inside.

9. In Paragraph 1, A structural steel having a Charpy impact absorption energy of 27J or more measured at -5℃.

10. In Paragraph 1, A structural steel having a yield strength of 238 MPa or higher measured at 600℃.

11. In wt%, C: 0.07% or more and 0.17% or less, Si: 0.10% or more and 0.40% or less, Mn: 0.70% or more and 1.60% or less, P: greater than 0% and 0.020% or less, S: greater than 0% and 0.010% or less, Cu: greater than 0% and 0.50% or less, Ni: greater than 0% and 0.15% or less, Cr: 0.10% or more and 0.50% or less, Mo: 0.10% or more and 0.75% or less, Nb: 0.025% or more and 0.050% or less, Ti: 0.015% or more and 0.040% or less, V: 0.015% or more and 0.060% or less, N: 0.008% or more and 0.012% or less, and B: 0.001% or more and 0.003% Below, a step of reheating steel containing the remaining Fe and other unavoidable impurities; A step of hot rolling the reheated steel; and It includes a step of cooling rolled steel, The above hot rolling step is a method for manufacturing a structural steel, wherein the cumulative reduction rate is 20% or more.

12. In Paragraph 11, Method for manufacturing a structural steel containing nano precipitates and nano clusters inside.

13. In Paragraph 11, A method for manufacturing a structural steel, wherein the particle size of the nano precipitate is greater than 15 nm and less than or equal to 55 nm, and the size of the nano cluster is greater than or equal to 1 nm and less than or equal to 11 nm.

14. In Paragraph 11, The density of the above nanocluster is 2.5 × 10 17 Pieces / cm 3 Above 5.0 x 10 17 Pieces / cm 3 Method for manufacturing structural steel of the following length.

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