Structural steel and methods for manufacturing structural steel

Abstract

A method for manufacturing structural steel according to one embodiment of the present invention includes: (a) carbon (C) 0.04~0.14 wt%, silicon (Si) 0.10~0.55 wt%, manganese (Mn) 0.90~1.65 wt%, phosphorus (P) 0.020 wt% or less, sulfur 0.007 wt% or less, aluminum (Al) 0.015~0.055 wt%, vanadium (V) 0.010~0.080 wt%, titanium (Ti) 0.005~0.025 wt%, niobium (Nb) The method includes (b) reheating a steel material containing 0.010 to 0.050 wt% of the remaining iron (Fe) and other unavoidable impurities at 1150 to 1300°C, and (b) rolling the steel material, with the rolling starting temperature being 900 to 1100°C, the intermediate rolling temperature being 850 to 1000°C, and the rolling ending temperature being 800 to 900°C, thereby realizing a high-performance structural steel and a method for manufacturing structural steel that achieves homogenization of the physical properties of the structural steel while ensuring low-temperature impact toughness.

Description

Technical Field

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

Background Art

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

[0003] Recently, as the offshore plant industry has developed, there has been an increasing demand for shaped steel having light weight and high strength in the field of steel materials for offshore structures, and particularly guaranteed low-temperature impact toughness that can withstand external impacts even at -40°C or lower.

[0004] As a method for manufacturing a steel material capable of ensuring high-strength impact toughness even at low temperatures, a TMCP (thermo-mechanical control process) process using QST (Quenching and Self-Tempering), which is an accelerated cooling facility, is mainly used.

[0005] However, when hot rolling is performed using the TMCP process, although a grain refinement effect can be obtained, there are problems such as temperature deviation due to the three-dimensional shape of the shaped steel and the difference in thickness between the flange and the web, and deformation occurring during cooling.

[0006] Furthermore, in the process of manufacturing medium or small-sized shaped steel, since the size of the beam blank is relatively small, when the TMCP process is used, cooling can proceed rapidly, and there is a risk that the cooling rate cannot be precisely controlled.

[0007] Therefore, there is a growing demand for high-strength structural steel and its manufacturing methods that can minimize deformation and property deviations while ensuring low-temperature impact toughness. [Overview of the project] [Problems that the invention aims to solve]

[0008] In order to solve the problems of the conventional technology described above, the purpose of the structural steel and method for manufacturing structural steel according to the present invention is to provide high-performance structural steel and a method for manufacturing structural steel that achieve homogenization of the physical properties of structural steel while ensuring low-temperature impact toughness.

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

[0010] A method for manufacturing structural steel according to one embodiment of the present invention is as follows: (a) Carbon (C) 0.04~0.14 wt%, silicon (Si) 0.10~0.55 wt%, manganese (Mn) 0.90~1.65 wt%, phosphorus (P) 0.020 wt% or less, sulfur (S) 0.007 wt% or less, aluminum (Al) 0.015~0.055 wt%, vanadium (V) 0.010~0.080 wt%, (b) a steel material containing 0.005 to 0.025 wt% tung (Ti), 0.010 to 0.050 wt% niobium (Nb), the remaining iron (Fe) and other unavoidable impurities, is reheated to 1150 to 1300°C; and (b) the steel material is rolled, with the rolling starting temperature being 900 to 1100°C, the intermediate rolling temperature being 850 to 1000°C, and the rolling ending temperature being 800 to 900°C.

[0011] Furthermore, in step (b) above, cooling water is injected from the S / C (Selective Cooling) device to control the intermediate rolling temperature, with a waiting time of 0 to 120 seconds and a water volume of 50 to 300 ml. 3 The procedure may also be performed under conditions of a transfer speed of 2.0 to 4.0 m / s, with a transfer rate of 2.0 to 4.0 m / s.

[0012] Furthermore, the steel material subjected to step (b) above may have a room-temperature microstructure in the center containing ferrite and pearlite, but the FGS (Ferrite grain size) may be 10 μm or less.

[0013] Furthermore, the steel material subjected to step (b) above may have a yield strength (YS) of 420 MPa or more, a low-temperature impact toughness at -40°C of 50 J or more, a yield ratio (YR) of 0.90 or less, and an elongation ratio (EL) of 19% or more.

[0014] The steel material obtained by step (b) above is manufactured as an H-shaped steel including a web and a flange, but the yield strength (YS) deviation between the upper and lower parts of the flange with respect to the web may be 15 MPa or less.

[0015] Furthermore, the steel material contains 0.1-0.2% by weight of silicon (Si), 1.57-1.65% by weight of manganese (Mn), 0.015-0.021% by weight of aluminum (Al), 0.040-0.045% by weight of vanadium (V), and 0.005-0.008% by weight of titanium (Ti). Niobium (Nb) may be present in a quantity of 0.040 to 0.045% by weight.

[0016] A structural steel according to one embodiment of the present invention contains 0.04-0.14 wt% carbon (C), 0.10-0.55 wt% silicon (Si), 0.90-1.65 wt% manganese (Mn), 0.020 wt% or less phosphorus (P), 0.007 wt% or less sulfur (S), 0.015-0.055 wt% aluminum (Al), 0.010-0.080 wt% vanadium (V), 0.005-0.025 wt% titanium (Ti), 0.010-0.050 wt% niobium (Nb), the remainder being iron (Fe) and other unavoidable impurities, and satisfies a yield strength (YS) of 420 MPa or more.

[0017] Furthermore, the low-temperature impact toughness at -40°C can meet the requirement of 50J or more.

[0018] Furthermore, the yield ratio (YR) can be 0.90 or less.

[0019] Also, the elongation (EL) may be 19% or more.

[0020] Also, it has the shape of an H-beam including a web and flanges, and the yield strength (YS) deviation between the upper and lower parts of the flanges based on the web may be 15 MPa or less.

[0021] Also, the room-temperature microstructure of the central part includes ferrite and pearlite, and the F.G.S (Ferrite grain size) may be 10 μm or less.

[0022] Also, the steel material may contain 0.1 to 0.2 wt% of silicon (Si), 1.57 to 1.65 wt% of manganese (Mn), 0.015 to 0.021 wt% of aluminum (Al), 0.040 to 0.045 wt% of vanadium (V), 0.005 to 0.008 wt% of titanium (Ti), and 0.040 to 0.045 wt% of niobium (Nb).

[0023] The shaped steel according to an embodiment of the present invention contains 0.04 to 0.14 wt% of carbon (C), 0.10 to 0.55 wt% of silicon (Si), 0.90 to 1.65 wt% of manganese (Mn), 0.020 wt% or less of phosphorus (P), 0.007 wt% or less of sulfur (S), 0.015 to 0.055 wt% of aluminum (Al), 0.010 to 0.080 wt% of vanadium (V), 0.005 to 0.025 wt% of titanium (Ti), 0.010 to 0.050 wt% of niobium (Nb), the remaining iron (Fe) and other inevitable impurities, and is reheated at 1150 to 1300 °C and then rolled, but the rolling start temperature is controlled at 900 to 1100 °C, the rolling intermediate temperature is controlled at 850 to 1000 °C, and the rolling end temperature is controlled at 800 to 900 °C, and can be manufactured by this method.

[0024] Also, in order to control the rolling intermediate temperature, cooling water is sprayed from an S / C (Selective Cooling) device, and the standby time is 0 to 120 seconds, the water volume is 50 to 300 m 3 / hr, and the transfer speed is controlled at 2.0 to 4.0 m / s, and can be manufactured by this method.

Advantages of the Invention

[0025] According to an embodiment of the present invention, it is possible to embody a high-performance section steel and a method for manufacturing the section steel that achieve physical property homogenization while ensuring low-temperature impact toughness.

[0026] The effects of the present invention are not limited to the effects mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art from the description of the claims.

Brief Description of the Drawings

[0027] [Figure 1] FIG. 1 is a flowchart showing a method for manufacturing a section steel according to the present invention. [Figure 2] FIG. 2 is a view showing an S / C device and a section steel used in the method for manufacturing a section steel according to the present invention. [Figure 3a] FIG. 3a is a photograph of the microstructure observation of a specimen at the center of the flange of a section steel according to a comparative example and an experimental example. Specifically, FIG. 3a is a microstructural photograph according to Comparative Example 1. [Figure 3b] FIG. 3b is a photograph of the microstructure observation of a specimen at the center of the flange of a section steel according to a comparative example and an experimental example. Specifically, FIG. 3b is a microstructural photograph according to Comparative Example 2. [Figure 3c] FIG. 3c is a photograph of the microstructure observation of a specimen at the center of the flange of a section steel according to a comparative example and an experimental example. Specifically, FIG. 3c is a microstructural photograph according to Experimental Example 1.

Modes for Carrying Out the Invention

[0028] In this specification, when a component (or region, layer, part, etc.) is referred to as being "on", "connected to", or "coupled to" another component, it means that it can be directly disposed / connected / coupled on the other component, or a third component can be disposed between them.

[0029] The same drawing symbol refers to the same component. Furthermore, in drawings, the thickness, proportions, and dimensions of components are exaggerated for the sake of effectively illustrating the technical content.

[0030] "and / or" includes all combinations of one or more that are defined by the relevant configuration.

[0031] Terms such as "first," "second," etc., are used to describe a variety of components, but the components are not limited to those defined by these terms. These terms are used solely for the purpose of distinguishing one component from another. For example, without departing from the scope of the present invention, the first component may be referred to as the second component, and similarly, the second component may be referred to as the first component. A singular expression includes plural expressions unless the context clearly indicates otherwise.

[0032] Furthermore, terms such as "down," "on the lower side," "up," and "on the upper side" are used to describe the correlations of the configuration shown in the drawings. These terms are relative concepts and are described in relation to the directions shown in the drawings.

[0033] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as those generally understood by those skilled in the art to which the present invention pertains. Furthermore, terms such as those defined in commonly used dictionaries should be interpreted as having the meaning consistent with their meaning in the context of the relevant art, and are expressly defined herein unless interpreted in an ideal or overly formal sense.

[0034] Terms such as "includes" or "possesses" mean that the features, figures, steps, actions, components, parts, or combinations thereof described in the specification exist, and should be understood not to preclude the existence or possibility of adding one or more other features, figures, steps, actions, components, parts, or combinations thereof.

[0035] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0036] Shaped steel A structural steel according to one embodiment of the present invention contains 0.04-0.14 wt% carbon (C), 0.10-0.55 wt% silicon (Si), 0.90-1.65 wt% manganese (Mn), 0.020 wt% or less phosphorus (P), 0.007 wt% or less sulfur (S), 0.015-0.055 wt% aluminum (Al), 0.010-0.080 wt% vanadium (V), 0.005-0.025 wt% titanium (Ti), 0.010-0.050 wt% niobium (Nb), the remainder being iron (Fe) and other unavoidable impurities, and satisfies a yield strength (YS) of 420 MPa or more. Here, yield strength (YS) may refer to the yield strength at room temperature.

[0037] More preferably, the silicon (Si) may be 0.1 to 0.2% by weight, the manganese (Mn) 1.57 to 1.65% by weight, the aluminum (Al) 0.015 to 0.021% by weight, the vanadium (V) 0.040 to 0.045% by weight, the titanium (Ti) 0.005 to 0.008% by weight, and the niobium (Nb) 0.040 to 0.045% by weight.

[0038] Structural steel having the alloy composition described above can satisfy the following requirements: low-temperature impact toughness of 50 J or more at -40°C, yield ratio (YR) of 0.90 or less, and elongation ratio (EL) of 19% or more.

[0039] Preferably, the yield strength (YS) at room temperature is 435 MPa or higher, the yield ratio (YR) is 0.85 or lower, and the elongation is 21% or higher. More preferably, the yield strength (YS) at room temperature is 445 MPa or higher, the low-temperature impact toughness at -40°C is 160 J or higher, the yield ratio (YR) is 0.81 or lower, and the elongation (EL) is 29.9% or higher.

[0040] Furthermore, although the structural steel has the shape of an H-beam including a web and a flange, the yield strength (YS) deviation between the upper and lower parts of the flange relative to the web may be 15 MPa or less. Preferably, the yield strength (YS) deviation between the upper and lower parts of the flange may be 12 MPa, and more preferably 8 MPa or less.

[0041] Furthermore, the structural steel having the alloy composition described above has a room-temperature microstructure in its core containing ferrite and pearlite, but the FGS (Ferrite grain size) may be 10 μm or less. More specifically, the FGS may be 9.5 μm or less.

[0042] This makes it possible to realize high-performance structural steel and a method for manufacturing structural steel that achieve homogenization of the physical properties of structural steel while ensuring low-temperature impact toughness according to one embodiment of the present invention.

[0043] In particular, the addition of vanadium (V) and niobium (Nb) within the aforementioned compositional range achieves precipitation strengthening and grain refinement, thereby obtaining a sufficient cooling effect. Furthermore, through the V-Nb composite design, higher strength, stability, and mass producibility can be ensured compared to conventional structural steel with a yield strength of 355 MPa.

[0044] Furthermore, the addition of titanium (Ti) within the aforementioned composition range can delay the growth of austenite crystal grains and improve welding performance.

[0045] The roles and contents of each alloying element contained in structural steel according to one embodiment of the present invention will be described in detail below.

[0046] Carbon (C) Carbon is the most effective and important element for increasing the strength of steel, and by reacting with Nb, Ti, etc., it promotes the formation of fine carbides, thereby effectively contributing to strength improvement through precipitation strengthening. Therefore, the structural steel according to one embodiment of the present invention may contain carbon in an amount of 0.04% to 0.14% by weight.

[0047] In other words, if the carbon content is less than 0.04% by weight of the total weight, it may be difficult to ensure sufficient strength. Conversely, if the carbon content exceeds 0.14% by weight of the total weight, coarse carbides may be formed, reducing impact properties and potentially leading to problems with reduced weldability.

[0048] Silicon (Si) Silicon (Si), along with aluminum, is added as a deoxidizing agent in the steelmaking process to remove oxygen from the steel. Silicon can also provide solid solution strengthening effects.

[0049] Silicon may be added in a content ratio of 0.10 to 0.55% by weight of the total weight of the structural steel according to one embodiment of the present invention. If the silicon content is less than 0.10% by weight of the total weight, the effect of silicon addition cannot be properly exerted. Conversely, if the silicon content exceeds 0.55% by weight of the total weight, it may reduce the weldability of the steel and cause problems with surface quality by generating red scale during reheating and hot rolling.

[0050] Manganese (Mn) Manganese not only contributes to ensuring strength as a solid solution strengthening element, but can also improve the hardening ability of steel. Although it inhibits the acid resistance and oxidation resistance of steel, it improves yield strength by refining pearlite and solid solution strengthening ferrite. Therefore, the structural steel according to one embodiment of the present invention may contain manganese in an amount of 0.90% to 1.65% by weight, preferably 1.57% to 1.65% by weight.

[0051] If the manganese content is less than 0.90% by weight of the total weight, the solid solution strengthening effect cannot be fully realized. If it exceeds 1.65% by weight, it can combine with sulfur to form MnS inclusions or cause central segregation in the ingot, which can reduce the ductility and corrosion resistance of the structural steel.

[0052] Rin (P) Phosphorus (P) can enhance the strength of steel through solid solution strengthening and suppress the formation of carbides. The phosphorus may be added in a content ratio of 0.020% by weight or less of the total weight of the structural steel according to one embodiment of the present invention. If the phosphorus content exceeds 0.020% by weight, it may form inclusions as a tramp element, reducing the ductility of the steel, and there is a problem that the low-temperature impact value decreases due to precipitation behavior.

[0053] Sulfur (S) Sulfur (S) can improve workability by forming fine MnS precipitates. The sulfur may be added in a content ratio of 0.007% by weight or less of the total weight of the structural steel according to one embodiment of the present invention. If the sulfur content exceeds 0.007% by weight, it can form inclusions as a tramp element, inhibiting the ductility, toughness, and weldability of the steel, and lowering the low-temperature impact value.

[0054] Aluminum (Al) Aluminum (Al) is added to the steelmaking process as a deoxidizing agent to remove oxygen from the steel. It can also precipitate in the steel as AlN and contribute to grain refinement. The aluminum may be added in a content ratio of 0.015 to 0.055% by weight of the structural steel according to one embodiment of the present invention, and preferably in a content ratio of 0.015 to 0.021% by weight. If the aluminum content is less than 0.015% by weight, the deoxidizing effect is insufficient, and if it exceeds 0.055% by weight, continuous casting becomes difficult, productivity decreases, and problems may arise such as the formation of alumina (Al2O3), a nonmetallic inclusion, which reduces ductility and toughness.

[0055] Vanadium (V) Vanadium (V) has a high ability to form carbides, which refines the steel microstructure by creating fine carbides and increases strength by forming precipitates during rolling. In particular, the amount of precipitate can be controlled depending on the amount of nitrogen added. Vanadium can also contribute to strength improvement by acting as a pinning agent at grain boundaries.

[0056] The vanadium may be added in a content ratio of 0.010 to 0.080% by weight of the structural steel according to one embodiment of the present invention, and preferably in a content ratio of 0.040 to 0.045% by weight. If the vanadium content is less than 0.010% by weight, it is difficult to sufficiently secure the above effect. On the other hand, if the vanadium content exceeds 0.080% by weight, there is a problem that the low-temperature impact toughness will decrease.

[0057] Titanium (Ti) Titanium (Ti) can generate Ti(C,N) precipitates with high high-temperature stability. This inhibits the growth of austenite grains during welding, refines the microstructure of the weld, and improves the toughness and strength of the steel. The titanium may be added in a content ratio of 0.005 to 0.025% by weight of the structural steel weight according to one embodiment of the present invention, preferably in a content ratio of 0.005 to 0.008% by weight. If the titanium content is less than 0.005% by weight, it is difficult to ensure the above effect sufficiently. On the other hand, if the titanium content exceeds 0.025% by weight, it can reduce the low-temperature impact toughness of the steel by generating coarse precipitates.

[0058] Niobium (Nb) Niobium, when dissolved in an austenite structure, inhibits grain growth and results in finer grain sizes. In particular, it can delay recrystallization by rapidly causing steel to reach below its non-recrystallization temperature (Tnr). Furthermore, by reacting with carbon to promote the formation of fine carbides, it is effective in improving strength through precipitation strengthening. However, excessive addition can reduce the impact properties of steel.

[0059] Therefore, the structural steel according to one embodiment of the present invention may contain 0.01 to 0.05% by weight of niobium, preferably 0.040 to 0.045% by weight. That is, if the amount of niobium is less than 0.01% by weight of the total weight, the effect of adding niobium cannot be properly exerted, and if it is added in large amounts exceeding 0.05% by weight, the impact absorption energy of the steel can be reduced. On the other hand, if the niobium content is 0.040 to 0.045% by weight, the effect of adding niobium described above can be maximized, and the reduction in the impact absorption energy of the steel can be minimized.

[0060] Nitrogen (N) Even in extremely small amounts, nitrogen significantly affects the mechanical properties of steel, increasing tensile strength and yield strength, but reducing elongation. However, excessive nitrogen addition can reduce the toughness of the weld and decrease the impact strength. A structural steel according to one embodiment of the present invention may contain 110 to 120 ppm of nitrogen.

[0061] On the other hand, a structural steel according to one embodiment of the present invention contains 0.04-0.14 wt% carbon (C), 0.10-0.55 wt% silicon (Si), 0.90-1.65 wt% manganese (Mn), 0.020 wt% or less phosphorus (P), 0.007 wt% or less sulfur (S), 0.015-0.055 wt% aluminum (Al), 0.010-0.080 wt% vanadium (V), 0.005-0.025 wt% titanium (Ti), 0.010-0.050 wt% niobium (Nb), the remaining iron (Fe) and other unavoidable impurities, and is rolled after being reheated at 1150-1300°C, but is manufactured by controlling the rolling start temperature to 900-1100°C, the rolling intermediate temperature to 850-1000°C, and the rolling end temperature to 800-900°C.

[0062] Furthermore, in order to control the intermediate rolling temperature, cooling water is injected from the S / C (Selective Cooling) device shown in Figure 2, with a waiting time of 0 to 120 seconds and a water flow rate of 50 to 300 ml. 3 The steel can be manufactured by controlling the transfer speed to 2.0-4.0 m / s at a rate of / hr. The S / C device will be described in detail in the following section on the manufacturing method of structural steel.

[0063] A structural steel containing the alloy composition described above and manufactured by the method described above can satisfy the following requirements: a low-temperature impact toughness of 50 J or more at -40°C, a yield ratio (YR) of 0.90 or less, and an elongation ratio (EL) of 19% or more. Preferably, the yield strength (YS) at room temperature may be 435 MPa or more, the yield ratio (YR) may be 0.85 or less, and the elongation ratio may be 21% or more. More preferably, the yield strength (YS) at room temperature may be 445 MPa or more, the low-temperature impact toughness at -40°C may be 160 J or more, the yield ratio (YR) may be 0.81 or less, and the elongation ratio (EL) may be 29.9% or more.

[0064] Furthermore, although the structural steel has the shape of an H-beam including a web and a flange, the yield strength (YS) deviation between the upper and lower parts of the flange with respect to the web may be 15 MPa or less. Preferably, the yield strength (YS) deviation between the upper and lower parts of the flange may be 12 MPa, and more preferably 8 MPa or less.

[0065] Furthermore, the structural steel having the alloy composition described above and manufactured by the method described above has a central room-temperature microstructure containing ferrite and pearlite (F+P), but the FGS (Ferrite grain size) may be 10 μm or less. More specifically, the FGS may be 9.5 μm or less.

[0066] As a result, a structural steel according to one embodiment of the present invention can embody a high-performance structural steel and a method for manufacturing structural steel that ensures low-temperature impact toughness, minimizes temperature deviation, and achieves homogenization of physical properties.

[0067] In particular, by adding vanadium (V) and niobium (Nb) within the aforementioned composition range and controlling the intermediate rolling temperature using an S / C (Selective Cooling) device, precipitation strengthening and grain refinement can be achieved, and an accelerated cooling effect can be obtained. Furthermore, through the V-Nb composite design, higher strength, stability, and mass productivity can be ensured compared to conventional structural steel with a yield strength of 355 MPa.

[0068] Furthermore, the addition of titanium (Ti) within the aforementioned compositional range can delay the growth of austenite crystal grains and improve welding performance.

[0069] Manufacturing method for structural steel A method for manufacturing structural steel according to one embodiment of the present invention includes (a) a reheating step and (b) a rolling step, as shown in Figure 1. The method for manufacturing structural steel will be described in detail below with reference to Figures 1 and 2.

[0070] First, the steel material contains 0.04-0.14 wt% carbon (C), 0.10-0.55 wt% silicon (Si), 0.90-1.65 wt% manganese (Mn), 0.020 wt% or less phosphorus (P), 0.007 wt% or less sulfur (S), 0.015-0.055 wt% aluminum (Al), 0.010-0.080 wt% vanadium (V), 0.005-0.025 wt% titanium (Ti), 0.010-0.050 wt% niobium (Nb), the remaining iron (Fe) and other unavoidable impurities, and the steel material is reheated at 1150-1300°C. Next, the steel material is rolled, with the rolling starting temperature set at 900-1100°C, the intermediate rolling temperature at 850-1000°C, and the rolling ending temperature at 800-900°C.

[0071] As a result, the structural steel 10 according to one embodiment of the present invention can realize a high-performance structural steel and structural steel manufacturing method that ensures low-temperature impact toughness, minimizes temperature deviation, and achieves homogenization of physical properties by securing the target microstructure and grain size.

[0072] In particular, precipitation strengthening and grain refinement can be achieved and an appropriate cooling effect obtained by adding vanadium (V) and niobium (Nb) within the aforementioned composition range and controlling the intermediate rolling temperature. Furthermore, the V-Nb composite design ensures higher strength, stability, and mass productivity compared to conventional structural steel with a yield strength of 355 MPa. In addition, the addition of titanium (Ti) within the aforementioned composition range can delay the growth of austenite grains and improve weldability.

[0073] The following describes in detail a method for manufacturing structural steel according to one embodiment of the present invention.

[0074] First, in the reheating step, the steel material of the above composition is reheated to 1150°C or higher. If the reheating temperature is lower than 1150°C, the solid solution of various carbides may not be sufficient, and the components that segregated during the continuous casting process may not be distributed uniformly enough. Also, the reheating temperature may not exceed 1300°C. If the reheating temperature exceeds 1300°C, coarse austenite crystal grains may form, making it difficult to ensure strength, which may lead to increased manufacturing costs and decreased productivity due to increased heating costs and time.

[0075] On the other hand, the steel material may be manufactured through a continuous casting process after obtaining molten steel of a desired composition through a steelmaking process. The steel material may be, for example, a beam blank, but is not necessarily limited to this.

[0076] On the other hand, the composition range of the steel material may more preferably be 0.1-0.2 wt% silicon (Si), 1.57-1.65 wt% manganese (Mn), 0.015-0.021 wt% aluminum (Al), 0.040-0.045 wt% vanadium (V), 0.005-0.008 wt% titanium (Ti), and 0.040-0.045 wt% niobium (Nb). This further reduces the deviation of room-temperature yield strength and low-temperature impact toughness at the upper and lower ends of the flange, and ensures that the microstructure grain size is 10 μm or less.

[0077] (b) In the rolling step, in order to control the intermediate rolling temperature as described above, cooling water is injected from the S / C (Selective Cooling) device 100 shown in Figure 2, with a waiting time of 0 to 120 seconds and a water volume of 50 to 300 ml. 3 The process can be carried out under the conditions of a transfer speed of 2.0 to 4.0 m / s at a rate of / hr.

[0078] While the TMCP process is primarily used to guarantee high-strength impact toughness, one embodiment of the present invention is used for medium or small-sized products that lack accelerated cooling equipment such as QST equipment in the TMCP process. In the manufacturing process for medium or small-sized products, the S / C (Selective Cooling) device 100 can be used together with the CM (Continuous Mill).

[0079] QST equipment is characterized by having not only side coolers but also an upper cooling box and lower cooling nozzles, and because it uses high-pressure cooling water, it has a fast cooling rate and is used for large-sized structural steel. On the other hand, S / C equipment 100, as shown in Figure 2, consists only of side coolers 110 and lower cooling nozzles 120, and has the advantage of easier cooling rate and temperature control.

[0080] In the case of a CM (Compression Molding) device, since H-shaped steel products are continuously fed into the rolling mill, cooling water accumulates on the upper part of the steel shape and is exposed to the air, causing a rolling temperature deviation and a deviation in the physical properties of the upper part 12a and lower part 12b of the flange 12 of the steel shape 10. To improve this, the lower part is concentratedly cooled through the S / C (Scaling Cooling) device 100 and its operating conditions, and the cooling temperature and speed are precisely controlled to reduce the temperature deviation, homogenize the physical properties, and achieve some improvement in physical properties.

[0081] In this way, the steel material rolled using the above-described process conditions has a central room-temperature microstructure containing ferrite and pearlite, but the FGS (Ferrite grain size) may be 10 μm or less. As shown in Figure 2, in an H-shaped steel 10 including a web 11 and a flange 12, the FGS grain size deviation between the upper part 12a and the lower part 12b of the flange 12 can be reduced with respect to the web 11. Thus, by using the steel shaping method according to one embodiment of the present invention, the target microstructure and grain size can be secured, and high-strength steel shaping can be manufactured.

[0082] Furthermore, the steel material subjected to step (b) rolling under the above process conditions may have a yield strength (YS) of 420 MPa or more, a low-temperature impact toughness at -40°C of 50 J or more, a yield ratio (YR) of 0.90 or less, and an elongation ratio EL of 19% or more. Thus, by using the method for manufacturing structural steel according to one embodiment of the present invention, high-performance structural steel with impact toughness at low temperatures can be manufactured.

[0083] Furthermore, the steel material that has undergone the rolling step (b) according to the above process conditions is manufactured from an H-shaped steel 10 including a web 11 and a flange 12, but the yield strength deviation between the upper part 12a and the lower part 12b of the flange 12 with respect to the web 11 may be 15 MPa or less. In this way, the method for manufacturing shaped steel according to one embodiment of the present invention ensures more uniform quality and stable physical properties compared to conventional methods, and improves quality and mass productivity.

[0084] Comparative Examples and Experimental Examples The following are preferred comparative examples and experimental examples to aid in understanding the present invention. However, the following experimental examples are merely for the purpose of aiding in understanding the present invention, and the present invention is not limited to the following experimental examples.

[0085] Table 1 below shows the main alloying elemental composition (unit: weight %) of the experimental example and comparative example, and Table 2 shows the process conditions (temperature unit: °C, time unit: sec, S / C water unit: m) for manufacturing the specimens of the experimental example and comparative example. 3 Table 3 shows the results of measuring the mechanical properties of the specimens produced under the process conditions shown in Table 2 (speed unit: m / s). A beam blank having the composition shown in Table 1 was manufactured using an electric furnace, and then H-shaped steel with a flange thickness of 15 mm was produced by hot rolling.

[0086] The process conditions are: reheating temperature 1150-1300°C, waiting time 0-120 seconds, rolling start temperature 900-1100°C, rolling intermediate temperature 850-1000°C, rolling end temperature 800-900°C, and S / C water content 50-300 m 3The process was carried out at a transfer speed of 2.0 to 4.0 m / s, and Table 2 lists the process conditions under which actual experimental data were obtained. The target physical properties in the experimental examples were a tensile strength (TS) of 500 to 660 MPa, a yield strength (YS) of 420 MPa or higher, an elongation (EL) of 19% or higher, a yield ratio (YR) of 90% or lower, and a low-temperature impact toughness of 50 J or higher at -40°C based on the flange portion. For microstructures, the goal was to observe a composite structure of ferrite and pearlite with a grain size of 10 μm or less in the deeper regions. Furthermore, the goal was to reduce the deviation of the yield strength (YS) between the upper and lower ends of the flange to 15 MPa, preferably 12 MPa or less.

[0087] [Table 1]

[0088] [Table 2]

[0089] [Table 3] Comparative Example 1 and Experimental Example 1 Referring to Tables 1 to 3, Comparative Example 1 differs from Experimental Example 1 in its compositional system and in the control or cooling of the intermediate rolling temperature using the S / C apparatus 100 shown in Figure 2. Figure 3a is a microstructural photograph of Comparative Example 1, and Figure 3c is a microstructural photograph of Experimental Example 1.

[0090] First, referring to Table 1, in Comparative Example 1, the composition range of vanadium and niobium was set to 0.035-0.039 wt%, and the actual experimental data values ​​included 0.036 wt% vanadium and 0.035 wt% niobium, with temperature control performed without the use of separate equipment.

[0091] On the other hand, in Experimental Example 1, the target composition range for vanadium and niobium was 0.040-0.045 wt%, and the actual experimental data values ​​included 0.040 wt% vanadium and 0.044 wt% niobium. The intermediate rolling temperature was controlled using an S / C apparatus under the process conditions shown in Table 2.

[0092] Referring to Table 3 and Figure 3, it can be seen that in Experimental Example 1, the yield strength (YS) deviation between the upper and lower ends of the flange was 8 MPa and the impact toughness deviation was 21 J, which are significantly smaller than in Comparative Example 1. Furthermore, the deviation in the crystal grains of the microstructure is also small, with both the upper and lower ends of the flange being within 10 μm, thus confirming that the physical property deviations have been improved.

[0093] Comparative Example 2 and Experimental Example 1 Referring to Tables 1 to 3, Comparative Example 1 differs from Experimental Example 1 in its composition, but the presence or absence of intermediate rolling temperature control or cooling using the S / C apparatus 100 shown in Figure 2 is the same. Figure 3b is a microstructure photograph of Comparative Example 2, and Figure 3c is a microstructure photograph of Experimental Example 1.

[0094] First, referring to Table 1, in Comparative Example 2, the composition range of vanadium and niobium was set to 0.035 to 0.039 wt%, and the actual experimental data values ​​included 0.037 wt% vanadium and 0.036 wt% niobium. The intermediate rolling temperature was controlled using an S / C device under the process conditions shown in Table 2.

[0095] On the other hand, in Experimental Example 1, the target composition range for vanadium and niobium was 0.040-0.045 wt%, and the actual experimental data values ​​included 0.040 wt% vanadium and 0.044 wt% niobium. The intermediate rolling temperature was controlled using an S / C apparatus under the process conditions shown in Table 2.

[0096] Referring to Table 3, Comparative Example 2 has a room-temperature yield strength (YS) of 432 MPa (upper flange) and 419 MPa (lower flange), which is lower than the room-temperature yield strength (YS) of Experimental Example 1 (447 MPa (upper flange) and 455 MPa (lower flange)). In particular, it can be confirmed that the yield strength at the lower flange of Comparative Example 2 does not reach 420 MPa. Furthermore, the yield strength (YS) deviation between the upper and lower flanges of Comparative Example 2 is 13 MPa, and the impact toughness deviation is 62 J, which is significantly larger than the yield strength (YS) deviation of 8 MPa and impact toughness deviation of 21 J of Experimental Example 1. Therefore, in the case of Experimental Example 1, compared to Comparative Example 2, it can be confirmed that the material properties of yield strength (YS) and impact toughness are not only superior, but the deviations are also smaller.

[0097] On the other hand, in the case of the grain size of the microstructure, Comparative Example 2 not only exceeded 10 μm at the upper and lower ends of the flange (10.0 μm and 10.6 μm respectively), but the deviation was also 0.6 μm, which is larger than that of Experimental Example 1.

[0098] Therefore, in the case of structural steel and a method for manufacturing structural steel according to one embodiment of the present invention, as confirmed from the data of Experimental Example 1, by ensuring the target high strength, low-temperature impact toughness, microstructure, and grain size, it is possible to realize high-performance special structural steel for deep-sea use with excellent quality and uniform quality with little deviation in physical properties between the upper and lower parts of the flange.

[0099] As described above, preferred embodiments, experimental examples, and comparative examples of the present invention have been examined. However, it is obvious to those ordinary skill in the art that the present invention can be embodied in other specific forms without departing from its spirit or category. Therefore, the embodiments described above should be considered illustrative rather than restrictive, and thus the present invention is not limited to the above description and may be modified within the scope of the appended claims and their equivalents. [Explanation of Symbols]

[0100] 100 Reheating Steps S20 Rolling step 10 Section steel 11 Web 12 flanges 12a Upper part of flange 12b Lower part of flange 100 S / C device 110 Side Cooler 120 Lower cooling nozzle

Claims

1. (a) A step of reheating a steel material containing 0.04-0.14 wt% carbon (C), 0.10-0.55 wt% silicon (Si), 0.90-1.65 wt% manganese (Mn), 0.020 wt% or less phosphorus (P), 0.007 wt% or less sulfur (S), 0.015-0.055 wt% aluminum (Al), 0.010-0.080 wt% vanadium (V), 0.005-0.025 wt% titanium (Ti), 0.010-0.050 wt% niobium (Nb), the remainder iron (Fe) and other unavoidable impurities at 1150-1300°C, (b) A method for manufacturing structural steel, comprising the step of rolling the steel material, wherein the rolling is started at a temperature of 900 to 1100°C, the intermediate rolling temperature is 850 to 1000°C, and the final rolling temperature is 800 to 900°C.

2. In step (b) above, To control the intermediate rolling temperature, cooling water is injected from an S / C (Selective Cooling) device, with a waiting time of 0 to 120 seconds and a water flow rate of 50 to 300 ml. ３ The method for manufacturing structural steel according to claim 1, wherein the process is carried out under the conditions of a transfer speed of 2.0 to 4.0 m / s.

3. The steel material subjected to step (b) above has a central room-temperature microstructure containing ferrite and pearlite, The method for manufacturing a shaped steel according to claim 1, wherein F.G.S (Ferrite grain size) is 10 μm or less.

4. The steel material after step (b) above is A method for manufacturing a structural steel according to claim 1, wherein the yield strength (YS) is 420 MPa or more, the low-temperature impact toughness at -40°C is 50 J or more, the yield ratio (YR) is 0.90 or less, and the elongation (EL) is 19% or more.

5. The steel material after step (b) above is A method for manufacturing a structural steel according to claim 1, wherein the structural steel is manufactured using an H-shaped steel including a web and a flange, and the yield strength (YS) deviation between the upper and lower parts of the flange with respect to the web is 15 MPa or less.

6. The aforementioned steel material is A method for manufacturing shaped steel according to claim 1, wherein the composition is 0.1 to 0.2% by weight of silicon (Si), 1.57 to 1.65% by weight of manganese (Mn), 0.015 to 0.021% by weight of aluminum (Al), 0.040 to 0.045% by weight of vanadium (V), 0.005 to 0.008% by weight of titanium (Ti), and 0.040 to 0.045% by weight of niobium (Nb).

7. Structural steel containing 0.04-0.14 wt% carbon (C), 0.10-0.55 wt% silicon (Si), 0.90-1.65 wt% manganese (Mn), 0.020 wt% or less phosphorus (P), 0.007 wt% or less sulfur (S), 0.015-0.055 wt% aluminum (Al), 0.010-0.080 wt% vanadium (V), 0.005-0.025 wt% titanium (Ti), 0.010-0.050 wt% niobium (Nb), the remainder being iron (Fe) and other unavoidable impurities, and satisfying a yield strength (YS) of 420 MPa or more.

8. The structural steel according to claim 7, wherein the low-temperature impact toughness at -40°C is 50 J or more.

9. The structural steel according to claim 7, wherein the yield ratio (YR) is 0.90 or less.

10. The structural steel according to claim 7, wherein the elongation ratio (EL) is 19% or more.

11. The structural steel according to claim 7, having the shape of an H-shaped steel including a web and a flange, wherein the yield strength (YS) deviation between the upper and lower parts of the flange with respect to the web is 15 MPa or less.

12. The structural steel according to claim 7, wherein the room-temperature microstructure of the central part contains ferrite and pearlite, but the F.G.S (Ferrite grain size) is 10 μm or less.

13. The structural steel according to claim 7, wherein the composition is 0.1 to 0.2% by weight of silicon (Si), 1.57 to 1.65% by weight of manganese (Mn), 0.015 to 0.021% by weight of aluminum (Al), 0.040 to 0.045% by weight of vanadium (V), 0.005 to 0.008% by weight of titanium (Ti), and 0.040 to 0.045% by weight of niobium (Nb).

14. Contains 0.04–0.14 wt% carbon (C), 0.10–0.55 wt% silicon (Si), 0.90–1.65 wt% manganese (Mn), 0.020 wt% or less phosphorus (P), 0.007 wt% or less sulfur (S), 0.015–0.055 wt% aluminum (Al), 0.010–0.080 wt% vanadium (V), 0.005–0.025 wt% titanium (Ti), 0.010–0.050 wt% niobium (Nb), and the remainder being iron (Fe) and other unavoidable impurities. Structural steel manufactured by reheating at 1150-1300°C before rolling, but with the rolling start temperature controlled to 900-1100°C, the intermediate rolling temperature to 850-1000°C, and the rolling end temperature to 800-900°C.

15. To control the intermediate rolling temperature, cooling water is injected from an S / C (Selective Cooling) device, with a waiting time of 0 to 120 seconds and a water flow rate of 50 to 300 ml. ３ The structural steel according to claim 14, manufactured by a method in which the transfer speed is controlled to 2.0 to 4.0 m / s, at a rate of 1 / hr.

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