Section steel and section steel manufacturing method

Abstract

A section steel according to an embodiment of the present invention comprises, 0.04-0.14 wt% of carbon (C), 0.10-0.55 wt% of silicon (Si); 0.90-1.65 wt% of manganese (Mn), 0.02 wt % or less (excluding 0 wt%) of phosphorus (P), 0.007 wt% or less (excluding 0 wt%) of sulfur (S), 0.015-0.055 wt % of aluminum (Al), 0.03-0.2 wt% of molybdenum (Mo), 0.01-0.08 wt% of vanadium (V), 0.005-0.025 wt % of titanium (Ti), 0.01-0.05 wt% of niobium (Nb), and the balance of iron (Fe) and other inevitable impurities, and thus can ensure high strength and low-temperature impact toughness.

Description

Steel profiles and methods for manufacturing steel profiles

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

[0002] As the offshore plant industry develops, demand for steel used in offshore structures is increasing. H-beams, with guaranteed impact toughness at temperatures between -20 and -40°C and a YS rating of 355 MPa, are primarily used for offshore structures. However, with the recent trend toward lighter offshore plants, the need for higher-strength steels is emerging to reduce steel usage while maintaining the same design characteristics.

[0003] To improve the strength of structural steel and ensure high impact toughness, a Thermo-Mechanical Control Process (TMCP) using Quenching and Self-Tempering (QST) is required. However, while hot rolling using the TMCP process can achieve grain refinement, the cooling process can lead to problems such as reduced toughness due to the formation of low-temperature transformed structures, and excessive grain refinement that exceeds the yield ratio. Furthermore, the TMCP process can cause problems such as temperature deviations due to the three-dimensional shape of the section steel, differences in flange and web thickness, and deformation during cooling.

[0004] The present invention has been devised to solve the above problems, and the object of the present invention is to provide a steel profile and a method for manufacturing the steel profile having high strength characteristics and high low-temperature impact toughness by controlling alloy components and a manufacturing process.

[0005] The tasks of the present invention are not limited to the tasks mentioned above, and other tasks not mentioned will be clearly understood by those skilled in the art from the description below.

[0006] According to one embodiment of the present invention, a steel sheet contains carbon (C) 0.04 to 0.14 wt%, silicon (Si) 0.10 to 0.55 wt%, manganese (Mn) 0.90 to 1.65 wt%, phosphorus (P) 0.02 wt% or less (excluding 0 wt%), sulfur (S) 0.007 wt% or less (excluding 0 wt%), aluminum (Al) 0.015 to 0.055 wt%, molybdenum (Mo) 0.03 to 0.2 wt%, vanadium (V) 0.01 to 0.08 wt%, titanium (Ti) 0.005 to 0.025 wt%, niobium (Nb) 0.01 to 0.05 wt%, and the remainder iron (Fe) and other unavoidable impurities.

[0007] Additionally, the microstructure of the surface layer may include tempered martensite, bainite, and acicular ferrite, and the microstructure of the center may include ferrite, pearlite, and acicular ferrite.

[0008] In addition, the area fraction of tempered martensite, bainite and acicular ferrite constituting the microstructure in the surface layer may be 5 to 20%, and the area fraction including ferrite, pearlite and acicular ferrite constituting the microstructure in the center may be 70 to 95%.

[0009] Additionally, the microstructure grain size of the central portion may be 8.50 ㎛ or less.

[0010] Additionally, the yield strength (YS) may be 460 MPa or more and the tensile strength (TS) may be 520 to 700 MPa.

[0011] Additionally, the elongation (EL) may be 17% or more and the yield ratio (YR) may be 0.90 or less.

[0012] Also, Charpy impact absorption energy (CVN) at -40℃ -40 ) can be more than 50J.

[0013] A method for manufacturing a shaped steel according to one embodiment of the present invention comprises a first step of preparing a steel material including carbon (C) 0.04 to 0.14 wt%, silicon (Si) 0.10 to 0.55 wt%, manganese (Mn) 0.90 to 1.65 wt%, phosphorus (P) 0.02 wt% or less (excluding 0 wt%), sulfur (S) 0.007 wt% or less (excluding 0 wt%), aluminum (Al) 0.015 to 0.055 wt%, molybdenum (Mo) 0.03 to 0.2 wt%, vanadium (V) 0.01 to 0.08 wt%, titanium (Ti) 0.005 to 0.025 wt%, niobium (Nb) 0.01 to 0.05 wt%, and the remainder iron (Fe) and other unavoidable impurities, a second step of reheating the steel material, It includes a third step of hot-rolling the steel material that has undergone the second step to form hot-rolled steel material, and a fourth step of cooling the hot-rolled steel material.

[0014] Additionally, the third step may have a rolling start temperature of 900 to 1000°C and a rolling end temperature of 750 to 850°C.

[0015] In addition, the third step can control the rolling intermediate temperature to 870°C or lower.

[0016] Additionally, the fourth step can be performed by a QST (Quenching and Self-Tempering) process.

[0017] Additionally, the fourth step may spray a first quantity of cooling water on the outside of the steel beam, a second quantity of cooling water on the inner upper portion of the steel beam, and a third quantity of cooling water on the inner lower portion of the steel beam.

[0018] Additionally, the first quantity, the second quantity and the third quantity may be in a ratio of 2.5:1:1.3.

[0019] According to one embodiment of the present invention, a shaped steel and a method for manufacturing shaped steel can achieve high mechanical strength and excellent low-temperature impact toughness. More specifically, high strength and high low-temperature impact toughness can be guaranteed through alloy composition design and cooling process control.

[0020] 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.

[0021] FIG. 1 is a flowchart schematically illustrating a method for manufacturing a steel beam according to one embodiment of the present invention.

[0022] FIG. 2 is a drawing schematically illustrating a method of spraying cooling water to perform a cooling step in a method for manufacturing a shaped steel according to one embodiment of the present invention.

[0023] Figure 3a is a hardness graph and a microstructure photograph of the center measured at a point 1 / 6 of the upper part of a flange to which a cooling water injection method according to one embodiment of the present invention is applied.

[0024] Figure 3b is a hardness graph and a microstructure photograph of the center section measured at a point 1 / 6 of the lower part of a flange to which a cooling water injection method according to one embodiment of the present invention is applied.

[0025] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings so that those skilled in the art can easily implement the present invention. However, the present invention is not limited or restricted by the following embodiments.

[0026] Additionally, when a component (or region, layer, portion, etc.) is referred to as being "on," "connected to," or "coupled to" another component, it means that it can be directly placed / connected / coupled to the other component, or that a third component may be placed between them.

[0027] Terms such as "include" or "have" should be understood to specify the presence of a feature, number, step, operation, component, part, or combination thereof described in the specification, but not to exclude in advance the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0028] In order to clearly explain the present invention, a detailed description of a part that is irrelevant to the description or a related known technology that may unnecessarily obscure the gist of the present invention has been omitted, and when adding reference signs to components of each drawing in this specification, the same or similar reference signs are attached to the same or similar components throughout the specification.

[0029] In addition, terms and words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as meanings and concepts that conform to the technical idea of ​​the present invention based on the principle that the inventor can appropriately define the concept of the term to explain his or her own invention in the best way.

[0030] Unless otherwise specified, the notation 'A to B' for numerical values ​​A and B means 'A or more and B or less'. In such notation, if a unit is attached only to numerical value B, the unit shall be applied to numerical value A as well.

[0031] Also, unless otherwise stated, 1 ppm is 0.0001 wt%.

[0032] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0033]

[0034] Section steel

[0035] According to one embodiment of the present invention, a steel sheet contains carbon (C) 0.04 to 0.14 wt%, silicon (Si) 0.10 to 0.55 wt%, manganese (Mn) 0.90 to 1.65 wt%, phosphorus (P) 0.02 wt% or less (excluding 0 wt%), sulfur (S) 0.007 wt% or less (excluding 0 wt%), aluminum (Al) 0.015 to 0.055 wt%, molybdenum (Mo) 0.03 to 0.2 wt%, vanadium (V) 0.01 to 0.08 wt%, titanium (Ti) 0.005 to 0.025 wt%, niobium (Nb) 0.01 to 0.05 wt%, and the remainder iron (Fe) and other unavoidable impurities.

[0036] Hereinafter, the role and content of alloy elements included in the steel according to one embodiment of the present invention will be described in detail.

[0037]

[0038] Carbon (C)

[0039] Carbon (C) is an effective element for improving the strength of steel. It can be dissolved in austenite and form martensite during quenching. Furthermore, it combines with iron (Fe), molybdenum (Mo), niobium (Nb), and titanium (Ti) to promote the formation of fine carbides, enhancing the strength of steel through precipitation hardening.

[0040] If the carbon content is less than 0.04 wt%, it may be difficult to secure sufficient strength. Conversely, if the carbon content exceeds 0.14 wt%, coarse carbides may be formed, which may reduce impact toughness. Therefore, the steel section according to one embodiment of the present invention may contain 0.04 to 0.14 wt% carbon.

[0041]

[0042] Silicon (Si)

[0043] Silicon (Si) is added as a deoxidizer to remove oxygen from steel during the steelmaking process and can also be added for solid solution strengthening. Silicon can inhibit carbide formation and prevent material degradation due to Fe3C formation. Furthermore, as a ferrite-stabilizing element, it can increase the ferrite fraction during cooling, thereby enhancing ductility.

[0044] If the silicon content is less than 0.1 wt%, the above-described effects cannot be properly achieved. On the other hand, if the silicon content exceeds 0.55 wt%, the toughness may be reduced and the plastic workability may be deteriorated. Therefore, the shaped steel according to one embodiment of the present invention may contain silicon in an amount of 0.1 to 0.55 wt%.

[0045]

[0046] manganese (Mn)

[0047] Manganese (Mn) is a solid-solution strengthening element that not only contributes to strength but also enhances the hardenability of steel. As the manganese content increases, pearlite becomes finer and ferrite is solid-solution strengthened, which can improve yield strength. Furthermore, some of the manganese can react with sulfur (S) dissolved in the steel to form MnS, which is elongated in the working direction during plastic working. Furthermore, the formation of MnS reduces the sulfur content in the steel, which can weaken grains and inhibit the formation of FeS, a low-melting-point compound.

[0048] If the manganese content is less than 0.9 wt%, the aforementioned effects may be minimal. Conversely, if the manganese content exceeds 1.65 wt%, austenite structures may remain, which may actually reduce strength and toughness. Therefore, the steel according to one embodiment of the present invention may contain manganese in an amount of 0.9 to 1.65 wt%.

[0049]

[0050] Person (P)

[0051] Phosphorus (P) is not a problem if it is uniformly distributed in the steel, but it usually forms undesirable compounds such as Fe3P. Fe3P is extremely brittle and segregates, so it does not become homogenized even after annealing, and can be elongated during processing such as rolling. In addition, if the phosphorus content exceeds 0.02 wt%, there may be a problem of low-temperature impact strength being reduced due to precipitation behavior, and therefore, in the present invention, it is necessary to control the content to be as low as possible. Therefore, the shaped steel according to one embodiment of the present invention may contain phosphorus at 0.02 wt% or less (excluding 0 wt%).

[0052]

[0053] Yellow (S)

[0054] Sulfur (S) improves the machinability of steel by combining with manganese (Mn), titanium (Ti), molybdenum (Mo), etc. In addition, it combines with manganese (Mn) to form MnS inclusions, and when the manganese content is insufficient, it combines with iron to form FeS. FeS is brittle and has a low melting point, so it can cause cracks during hot and cold working. In addition, when the sulfur content exceeds 0.007 wt%, it can deteriorate the toughness, weldability, and workability of steel, so it is desirable to control the content to the lowest possible. Therefore, the shaped steel according to one embodiment of the present invention may contain sulfur in an amount of 0.007 wt% or less (excluding 0 wt%).

[0055]

[0056] Aluminum (Al)

[0057] Aluminum (Al) is an element added as a deoxidizer to remove oxygen in steel. Aluminum primarily functions to strengthen solid solutions and suppress carbide formation. Furthermore, it can precipitate as AlN in steel, contributing to grain refinement. If the aluminum content is less than 0.015 wt%, the deoxidation effect is insufficient, and the aforementioned effects may be minimal. Conversely, if the aluminum content exceeds 0.055 wt%, non-metallic inclusions, Al2O3, may form, reducing ductility and toughness. Therefore, the section steel according to one embodiment of the present invention may contain 0.015 to 0.055 wt% of aluminum.

[0058]

[0059] molybdenum (Mo)

[0060] Molybdenum (Mo) is an element that is dissolved in steel and contributes to improving strength. According to an embodiment of the present invention, a small amount of molybdenum (Mo) is added to the steel to partially replace niobium in Nb(C,N) precipitates, thereby inducing the formation of (Nb, Mo)(C,N) precipitates. This increases the density and reduces the size of the precipitates, leading to grain refinement, which can improve strength. In addition, a bainite-type low-temperature transformation structure can be formed.

[0061] If the molybdenum content is less than 0.03 wt%, it may be difficult to properly achieve the aforementioned effects. On the other hand, if the molybdenum content exceeds 0.2 wt%, the room temperature strength may increase excessively, which may deteriorate weldability and toughness. Therefore, the steel section according to one embodiment of the present invention may contain molybdenum in an amount of 0.03 to 0.2 wt%.

[0062]

[0063] Vanadium (V)

[0064] Vanadium (V) can refine austenite grains by inhibiting the movement of austenite grain boundaries during reheating and hot rolling. In addition, it can increase hardenability by inhibiting nucleation at austenite grain boundaries during phase transformation, and can increase strength by forming precipitates during phase transformation from austenite. If the vanadium content is less than 0.01 wt%, it may be difficult to achieve the above-described effects. On the other hand, if the vanadium content exceeds 0.08 wt%, it may reduce workability and cause cracks in the material during rolling. Therefore, the shaped steel according to one embodiment of the present invention may contain 0.01 to 0.08 wt% of vanadium.

[0065]

[0066] Titanium (Ti)

[0067] Titanium (Ti) is added to form Ti oxide, which serves as a nucleus for ferrite formation. In addition, by generating Ti(C, N) precipitates, it can hinder austenite grain growth during welding, thereby refining the structure of the weld zone and improving the toughness and strength of the steel. If the titanium content is less than 0.005 wt%, the above-described effect may not occur. Conversely, if the titanium content exceeds 0.025 wt%, coarse TiN or TiC may be generated, which may cause brittle fracture. Therefore, the steel section according to one embodiment of the present invention may contain 0.005 to 0.025 wt% of titanium.

[0068]

[0069] niobium (Nb)

[0070] Niobium (Nb) can increase hardness by combining with carbon to form precipitates such as NbC within grains. It is also an element that is advantageous in improving strength by refining crystal grains. When the niobium content is less than 0.01 wt%, the aforementioned effect does not appear, and when the niobium content exceeds 0.05 wt%, the precipitation strengthening effect is excessive, increasing strength but reducing ductility. Therefore, the section steel according to one embodiment of the present invention may contain niobium in an amount of 0.01 to 0.05 wt%.

[0071] In addition to the aforementioned steel components, the remainder may contain iron and unavoidable impurities. Unavoidable impurities are impurities introduced during the steelmaking process and the manufacturing process of non-oriented electrical steel sheets. Since these impurities are widely known in the field, a detailed description will be omitted.

[0072] In one embodiment of the present invention, the addition of elements other than the aforementioned alloy components is not excluded, and various elements may be included within a range that does not impair the technical spirit of the present invention. When additional elements are included, they may be included to replace the remaining Fe.

[0073]

[0074] The final microstructure of a steel section manufactured using the steel section manufacturing method described below with the alloy composition described above may be as follows. The microstructure of the surface layer may include tempered martensite, bainite, and acicular ferrite, and the microstructure of the central portion inside the surface layer may include ferrite, pearlite, and acicular ferrite.

[0075] The area fraction of tempered martensite, bainite, and acicular ferrite constituting the microstructure in the surface layer may be 5 to 20%, and the area fraction of ferrite, pearlite, and acicular ferrite constituting the microstructure in the central region may be 70 to 95%. In addition, in the case of the microstructure in the central region described above, the crystal grain size may be 8.5 μm or less.

[0076] When the fraction of tempered martensite, bainite, and acicular ferrite constituting the surface layer is less than 5%, the grain size in the center may exceed 8.5 ㎛. At this time, the strength may decrease due to a decrease in the fraction of low-temperature transformation structure and coarse grain size. Conversely, when the fraction of the microstructure constituting the surface layer exceeds 20%, the elongation (EL) may fall short and the yield ratio (YR) may exceed 0.90 due to excessive low-temperature transformation structure fraction and grain refinement. In addition, a decrease in impact toughness may occur.

[0077] The steel according to one embodiment of the present invention can guarantee excellent mechanical strength. More specifically, it can have a yield strength (YS) of 460 MPa or more, a tensile strength (TS) of 520 to 700 MPa, an elongation (EL) of 17% or more, and a yield ratio (YR) of 0.90 or less.

[0078] In addition, the steel according to the present invention can guarantee impact toughness at low temperatures. More specifically, the Charpy impact absorption energy (CVN) at -40°C -40 ) may be 50J or more, preferably 175J, and more preferably 188J or more.

[0079] Hereinafter, a method for manufacturing a shaped steel according to one embodiment of the present invention will be described in detail.

[0080]

[0081] Method for manufacturing shaped steel

[0082] Hereinafter, a method for manufacturing a steel beam according to one embodiment of the present invention will be described with reference to the drawings.

[0083] FIG. 1 is a flowchart schematically illustrating a method for manufacturing a steel beam according to one embodiment of the present invention.

[0084] Referring to FIG. 1, a method for manufacturing a shaped steel according to an embodiment of the present invention comprises a first step (S1) of preparing a steel material including carbon (C) 0.04 to 0.14 wt%, silicon (Si) 0.1 to 0.55 wt%, manganese (Mn) 0.9 to 1.65 wt%, phosphorus (P) 0.02 wt% or less, sulfur (S) 0.007 wt% or less, aluminum (Al) 0.015 to 0.055 wt%, molybdenum (Mo) 0.03 to 0.2 wt%, vanadium (V) 0.01 to 0.08 wt%, titanium (Ti) 0.005 to 0.025 wt%, niobium (Nb) 0.01 to 0.05 wt% and the remainder iron (Fe) and other unavoidable impurities, a second step (S2) of reheating the steel material, It includes a third step (S3) of hot-rolling the steel material that has undergone the second step (S2) to form hot-rolled steel material, and a fourth step (S4) of cooling the hot-rolled steel material.

[0085] The alloying element content has been previously described, so further explanation will be omitted. Furthermore, since the alloying element content does not substantially change during the manufacturing process described below, the composition of the steel material and the alloy composition of the final product, the shaped steel, are substantially identical.

[0086] Hereinafter, each step of a method for manufacturing a shaped steel according to one embodiment of the present invention will be described in detail.

[0087] First, Step 1 (S1) involves preparing steel having the above-described alloy composition range. More specifically, this may involve designing alloy components within the above-described alloy composition range to manufacture semi-finished products. The semi-finished products may include, but are not limited to, beam blanks, blooms, billets, etc. Furthermore, the slabs may be manufactured using processes known in the relevant technical field, such as steelmaking and casting.

[0088] After the above first step (S1), a second step (S2) of reheating the steel may be performed. The reheating step may be performed prior to the hot rolling step described below, and may be a step for reheating the steel for subsequent processes. Specifically, it may be a step for uniformly heating the steel by loading the steel into a heating furnace to facilitate plastic deformation.

[0089] At this time, the steel is reheated, and the segregated components can be re-dissolved. If the reheating temperature is lower than 1150℃, the segregated components in the steel cannot be sufficiently dissolved, and the rolling load may increase, which may reduce the rollability. If the reheating temperature exceeds 1300℃, coarse austenite grains may be formed, making it difficult to secure strength, and heating costs and process times may increase, which may lead to problems such as increased manufacturing costs and decreased productivity. Therefore, the method for manufacturing a shaped steel according to one embodiment of the present invention can control the reheating temperature to 1150 to 1300℃.

[0090] The method for manufacturing a shaped steel according to the present invention may, after the second step (S2), perform a third step (S3) of hot-rolling the steel material that has undergone the second step (S2) to form a hot-rolled steel material. The hot rolling may include rough rolling and finish rolling processes. Here, the rough rolling process may mean making the steel material into a rolling material having an appropriate shape, thickness, and width. In addition, the finish rolling process may mean adjusting the steel material to a set thickness and width and rolling it at a finishing temperature suitable for the intended use to obtain a good surface and shape.

[0091] At this time, the rolling temperature can be controlled during hot rolling to ensure a homogeneous and fine microstructure to ensure low-temperature impact toughness. Furthermore, the cumulative rolling reduction ratio can be controlled to 40% or more based on the rolling temperature control.

[0092] More specifically, the rolling start temperature of the third step (S3) can be controlled to 900 to 1000°C, and the rolling end temperature can be controlled to 750 to 850°C. If the rolling end temperature is lower than 750°C, the rolling load may increase, and the yield ratio of the rolled product, the shaped steel, may increase. Furthermore, if the rolling end temperature exceeds 850°C, it may be difficult to secure the target strength and toughness.

[0093] In addition, the third step (S3) can control the rolling intermediate temperature to 870°C or lower. If the rolling intermediate temperature exceeds 870°C, the initial austenite structure growth may not be suppressed, so the grain refinement of the final microstructure may not be sufficient, and thus the target yield strength may not be achieved.

[0094] According to one embodiment of the present invention, a method for manufacturing a shaped steel may perform a fourth step (S4) of cooling the hot-rolled steel after the third step (S3). The fourth step (S4) may include a QST (Quenching and Self-Tempering) process using cooling water, and the fourth step (S4) may be performed using a QST facility. The QST facility is located at the rear end of a finishing rolling mill and accelerates cooling of the product after rolling to produce a high-quality product.

[0095] In the present invention, in order to secure a yield strength of 460 MPa, QST process conditions capable of inducing an appropriate surface hardening layer and grain refinement were established.

[0096] More specifically, the hot-rolled steel was cooled and self-tempered using a QST facility. At this time, the cooling was performed using a quenching method that sprayed cooling water onto the hot-rolled steel. Furthermore, the self-tempering was performed to allow tempering to occur through reheating due to the internal heat of the hot-rolled steel after the quenching.

[0097] Below, the above QST process conditions will be described in more detail.

[0098] First, quenching during the QST process can cool the surface temperature of the hot-rolled steel to below the martensite transformation start temperature. For this purpose, the cooling end temperature can be controlled to 600 to 750°C, and the cooling water quantity can be 10 to 700 m 3 The hot rolled steel can be cooled while moving at a transport speed of 0.8 to 3.0 m / s along a cooling path maintained at / hr.

[0099] At this time, the cooling water quantity is 10m 3 If the cooling water quantity is less than 700 m / hr, cooling may be insufficient, making it difficult to achieve the target strength and impact toughness. Conversely, if the cooling water quantity is less than 700 m / hr,3 If it exceeds / hr, a problem may occur in which the elongation rate rapidly decreases due to supercooling.

[0100] Furthermore, the method for manufacturing a shaped steel according to one embodiment of the present invention can control the amount of cooling water sprayed on the top, bottom, and sides of the shaped steel. This allows for even cooling of the top, bottom, and sides of shaped steel having various shapes. More specifically, by controlling the amount of cooling water sprayed on the bottom where heat is trapped and the top where the air contact area is large depending on the shape of the shaped steel, the top, bottom, and sides of the shaped steel can be evenly cooled.

[0101] Hereinafter, this will be described in detail with reference to FIGS. 2, 3a and 3b.

[0102] FIG. 2 is a drawing schematically illustrating a method of spraying cooling water to perform a cooling step in a method for manufacturing a shaped steel according to one embodiment of the present invention.

[0103] Referring to Fig. 2, the cooling water supplied in a first quantity from the first cooling water supply device (10) is sprayed on the outside of the steel beam, and the cooling water supplied from the second cooling water supply device (20) is sprayed on the upper inner part of the steel beam. In addition, the cooling water supplied from the third cooling water supply device (30) is sprayed on the lower inner part of the steel beam.

[0104] Specifically, it may mean spraying a first quantity of cooling water on the outside of the flange (f) of the H-beam (1), spraying a second quantity of cooling water on the upper part of the web (w) of the beam and the inner part of the upper part of the flange (f), and spraying a third quantity of cooling water on the lower part of the web (w) of the beam and the inner part of the lower part of the flange (f). Here, the web (w) may mean a vertical portion of the central axis of the H-beam (1). In addition, the flange (f) here may mean a portion connected to the end of the web (w) and extending in a direction perpendicular to the web (w).

[0105] At this time, the injection amount of the coolant can be controlled at the following ratio, and the first quantity, the second quantity, and the third quantity can be controlled at a ratio of 2.5:1:1.3.

[0106] According to an embodiment of the present invention, a method for manufacturing a shaped steel can control the microstructure of the shaped steel and the fraction of the hardened layer on the outside of the flange (f) by using the cooling water injection method described above. Specifically, the shaped steel according to an embodiment of the present invention may have a fraction of the hardened layer of 5 to 20%. Here, the hardened layer may mean a boundary of the hardened layer, where the Vickers hardness is measured in a straight line along the entire thickness from the outside to the inside of the flange (f) of the shaped steel and the highest hardness value and the average hardness value of the ferrite and pearlite regions are measured.

[0107] Hereinafter, this will be described in detail with reference to FIGS. 3a and 3b.

[0108] FIG. 3a is a hardness graph and a microstructure photograph of the center portion measured at a point 1 / 6 of the upper portion of a flange (f) to which a cooling water injection method according to one embodiment of the present invention is applied, and FIG. 3b is a hardness graph and a microstructure photograph of the center portion measured at a point 1 / 6 of the lower portion.

[0109] After the QST cooling performed by the above-described method is completed, the hot-rolled steel can form a composite structure including ferrite, pearlite, and acicular ferrite in the center. Referring to FIGS. 3a and 3b, it can be confirmed that the microstructure of the inner and central portions of the flange (f) includes ferrite and pearlite, and the outer portion includes bainite and acicular ferrite. At this time, it can be confirmed that the hardness decreases from the outer portion to the center of the flange (f), and the hardness of the inner portion is confirmed to be higher than that of the center. In addition, it can be confirmed that the thickness of the hardened layer is 7% at the upper portion of the flange (f) and 17% at the lower portion.

[0110] At this time, in the case of the steel according to the cooling water injection method described above, it can be confirmed that the hardness of the outer side of the steel is high, and it can be confirmed that an appropriate hardened layer fraction for high strength and low-temperature toughness is secured.

[0111] Meanwhile, after quenching, heat may diffuse from the inside of the hot-rolled steel to the surface, and reheating may occur. At this time, the surface of the hot-rolled steel may undergo self-tempering due to the reheating effect caused by the temperature difference between the inside and the surface, and a tempered martensite structure may be formed on the surface due to the increase in surface temperature. While the reheating is in progress, the hot-rolled steel may be maintained in an air-cooled state.

[0112]

[0113] Comparative and experimental examples

[0114] Below, preferred comparative examples and experimental examples are presented to aid understanding of the present invention. However, the following comparative examples and experimental examples are provided solely to aid understanding of the present invention, and the present invention is not limited to the following experimental examples.

[0115] Table 1 below shows the main alloy compositions that make up the comparative examples and experimental examples of the present invention. In addition, Table 2 below shows the manufacturing process conditions of the comparative examples and experimental examples of the present invention, and the process conditions not listed in Table 2 below were controlled as control variables and were controlled identically in both the comparative examples and experimental examples. Table 3 below shows the results of measuring the physical properties of shaped steel manufactured using steel materials having the alloy components listed in Table 1 through the process conditions listed in Table 2.

[0116] The mechanical properties of the comparative and experimental examples were measured according to EN ISO 6892-1 tensile test standard, EN ISO 2566-1 conversion of elongation values, and EN ISO 148-1 Metallic material-Charpy pendulum impact test.

[0117] In the table below, 'TS' and 'YS' can mean tensile strength and yield strength, respectively, and the unit is MPa. In addition, 'YR' means elongation, and the unit is %, and 'YR' can mean yield ratio. 'CVN' represents the Charpy impact absorption energy of the steel flange measured at -40℃. In the microstructure, 'F' means ferrite, and 'P' means pearlite. 'F top' and 'F bottom' can mean the top and bottom of the flange of the steel flange, respectively, based on the web of the steel flange.

[0118] Classification Chemical composition (weight%) CSiMnPSAlMoVTiNbN Comparative example 10.09 0.18 1.44 0.01 10.00 40.02 80.01 0.05 40.01 0.02 310 4 Comparative example 20.08 0.19 1.55 0.01 10.00 30.02 70.04 0.07 60.00 90.02 69 9 Comparative example 30.09 0.18 1.55 0.01 20.00 50.02 70.05 0.07 60.00 80.02 69 9 Experimental example 10.08 0.18 1.55 0.01 20.00 40.02 70.05 0.07 60.00 80.02 69 5

[0119] Process conditions: Rolling start temperature (℃), Rolling middle temperature (℃), Rolling end temperature (℃), Cooling water quantity (m) 3 / hr)Transport speed (m / s)Cooling end temperature (℃)Comparative example 197392079610 ~ 5003.4717Comparative example 2969-87403802Comparative example 397489083110 ~ 7003718Experimental example 19748707962.5704

[0120] ClassificationMechanical propertiesMicrostructureLocationTS(MPa)YS(MPa)EL(MPa)YR(MPa)CVN(J)Central structureGrain size(㎛)Curing sufficient rate(%)Comparative example 1FTop54045233.20.84220F+P8.721FBottom53445133.60.84174F+P8.61Comparative example 2FTop516433280.84227F+P10. 10F bottom 51744126.50.85208F+P9.830 Comparative example 3F top 52445627.50.87217F+P8.320F bottom 53847627.50.88213F+P8.825 Experimental example 1F top 55749926.50.9188F+P7.667F bottom 59152920.50.9202F+P7.0617

[0121] Referring to Tables 1 to 3, Comparative Example 1 is a comparative example in which the content of molybdenum (Mo) is insufficient, and it can be confirmed that the intermediate rolling temperature and the feed rate during cooling are not satisfied. At this time, it can be confirmed that the yield strength (YS) of the upper and lower parts of the flange did not satisfy the target yield strength (YS) of 460 MPa or more, and the crystal grain size of the central structure and the hardened layer fraction on the outside of the flange were also not satisfied.

[0122] In addition, Comparative Example 2 is a comparative example that did not perform the QST (Quenching and Self-Tempering) process, and thus the rolling intermediate and rolling end temperatures could not be controlled. In addition, since the QST process was not performed, the QST quantity was 0, and the cooling end temperature of the present invention was exceeded. At this time, it can be confirmed that the tensile strength (TS), yield strength (YS), grain size of the central structure, and hardened layer fraction all did not satisfy the values ​​targeted by the present invention.

[0123] Comparative Example 3 is a comparative example that does not satisfy the intermediate rolling temperature. At this time, it can be confirmed that the yield strength and hardened layer fraction at the top of the flange do not satisfy the values ​​targeted by the present invention. Furthermore, it can be confirmed that the grain size of the central structure targeted by the present invention is not satisfied at the bottom of the flange.

[0124] On the other hand, it can be confirmed that experimental example 1 according to one embodiment of the present invention satisfies both the mechanical properties and microstructural characteristics targeted by the present invention.

[0125] As described above, preferred embodiments of the present invention have been described. It will be apparent to those skilled in the art that the present invention may be embodied in other specific forms, in addition to the embodiments described above, without departing from the spirit or scope thereof. Therefore, the above-described embodiments should be considered illustrative rather than restrictive, and accordingly, the present invention is not limited to the above description, but may be modified within the scope of the appended claims and their equivalents.

[0126] [Explanation of symbols]

[0127] 1: Section steel

[0128] 10: First cooling water supply device

[0129] 20: Second cooling water supply device

[0130] 30: Third cooling water supply unit

[0131] w: web

[0132] f: flange

Claims

1. A steel section containing carbon (C) 0.04 to 0.14 wt%, silicon (Si) 0.10 to 0.55 wt%, manganese (Mn) 0.90 to 1.65 wt%, phosphorus (P) 0.02 wt% or less (excluding 0 wt%), sulfur (S) 0.007 wt% or less (excluding 0 wt%), aluminum (Al) 0.015 to 0.055 wt%, molybdenum (Mo) 0.03 to 0.2 wt%, vanadium (V) 0.01 to 0.08 wt%, titanium (Ti) 0.005 to 0.025 wt%, niobium (Nb) 0.01 to 0.05 wt%, and the remainder iron (Fe) and other unavoidable impurities.

2. In paragraph 1, The microstructure of the surface layer includes tempered martensite, bainite and acicular ferrite. The microstructure of the core is a steel containing ferrite, pearlite and acicular ferrite.

3. In paragraph 2, The area fraction of tempered martensite, bainite and needle-shaped ferrite constituting the microstructure in the above surface layer is 5 to 20%, A steel having an area fraction of 70 to 95% including ferrite, pearlite and needle-shaped ferrite constituting the microstructure in the center.

4. In paragraph 3, A steel sheet having a microstructure grain size of 8.50㎛ or less in the center.

5. In paragraph 1, Steel with a yield strength (YS) of 460 MPa or more and a tensile strength (TS) of 520 to 700 MPa.

6. In paragraph 1, Steel having an elongation (EL) of 17% or more and a yield ratio (YR) of 0.90 or less.

7. In paragraph 1, Charpy impact absorption energy (CVN) at -40℃ -40 ) Steel beam with a strength of 50J or more.

8. A first step of preparing a steel material including carbon (C) 0.04 to 0.14 wt%, silicon (Si) 0.10 to 0.55 wt%, manganese (Mn) 0.90 to 1.65 wt%, phosphorus (P) 0.02 wt% or less (excluding 0 wt%), sulfur (S) 0.007 wt% or less (excluding 0 wt%), aluminum (Al) 0.015 to 0.055 wt%, molybdenum (Mo) 0.03 to 0.2 wt%, vanadium (V) 0.01 to 0.08 wt%, titanium (Ti) 0.005 to 0.025 wt%, niobium (Nb) 0.01 to 0.05 wt% and the remainder iron (Fe) and other unavoidable impurities; A second step of reheating the above steel; A third step of hot rolling the steel material that has undergone the second step to form hot rolled steel material; and A method for manufacturing shaped steel, comprising a fourth step of cooling the hot-rolled steel.

9. In paragraph 8, The third step above is, A method for manufacturing shaped steel having a rolling start temperature of 900 to 1000°C and a rolling end temperature of 750 to 850°C.

10. In paragraph 8, The third step above is, A method for manufacturing shaped steel by controlling the rolling intermediate temperature to 870℃ or lower.

11. In paragraph 8, The fourth step above is, A method for manufacturing shaped steel using the QST (Quenching and Self-Tempering) process.

12. In paragraph 11, The fourth step above is, A method for manufacturing a steel section, comprising: spraying a first quantity of cooling water on the outer side of the steel section; spraying a second quantity of cooling water on the inner upper side of the steel section; and spraying a third quantity of cooling water on the inner lower side of the steel section.

13. In paragraph 12, A method for manufacturing a shaped steel, wherein the first quantity, the second quantity and the third quantity are in a ratio of 2.5:1:1.3.

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