Section steel and section steel manufacturing method

Abstract

Section steel according to one embodiment of the present invention comprises 0.11-0.18 wt% of carbon (C), 1.0-1.65 wt% of manganese (Mn), 0.01-0.11 wt% of vanadium (V), 40-200 ppm of nitrogen (N), and the balance of iron (Fe) and other inevitable impurities, wherein the content ratio of vanadium (V) and nitrogen (N) satisfies relation 1 such that high strength and high impact toughness can be exhibited.

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] With the recent trend toward larger, taller, and underground buildings, the need for higher-strength structural steel is growing. Structural steel used in construction is typically H-beams and thick plates, and the demand for technologies to enhance their strength is also increasing.

[0003] Typically, the TMCP (Thermo-Mechanical Control Process) process, utilizing Quenching and Self-Tempering (QST), is used to enhance the strength of structural steel and ensure high impact toughness. However, shaped steel, a steel material with a diverse cross-sectional shape, requires complex rolling methods to achieve these shapes, making the TMCP process difficult to apply.

[0004] For this reason, conventionally, the method of increasing the strength of shaped steel by adding alloying elements such as manganese (Mn), vanadium (V), and niobium (Nb) for solid solution strengthening and precipitation strengthening is mainly used. However, the addition of the above alloying elements can increase the carbon equivalent (Ceq). In addition, if the addition of alloying elements is excessive, it may be difficult to satisfy the upper limit of carbon equivalent standards restricted by KS and EN standards, and the range of strength increase is limited by designing alloying elements at a level that satisfies the carbon equivalent standard.

[0005] Therefore, to solve the above-mentioned problem, a technology is needed to improve the strength of shaped steel without increasing the carbon equivalent and without the TMCP process.

[0006] The present invention has been devised to solve the above problems, and the object of the present invention is to provide a high-strength steel and a method for manufacturing the steel through alloy composition design.

[0007] 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.

[0008] According to one embodiment of the present invention, a shaped steel includes 0.11 to 0.18 wt% of carbon (C), 1.0 to 1.65 wt% of manganese (Mn), 0.01 to 0.11 wt% of vanadium (V), 40 to 200 ppm of nitrogen (N), and the remainder iron (Fe) and other inevitable impurities, and the content ratio of the vanadium (V) and the nitrogen (N) satisfies Equation 1.

[0009] [Formula 1]

[0010] 3.0 ≤ [V] / [N] ≤ 4.5

[0011] (Wherein, [V] and [N] represent the weight % of V and N, respectively.)

[0012] Additionally, the yield strength (YS) may be 460 MPa or more, the tensile strength (TS) may be 570 to 720 MPa, and the elongation (EL) may be 17% or more.

[0013] Additionally, the Charpy impact absorption energy (CVN) at 0℃ can be 130J or more.

[0014] Additionally, the carbon equivalent (Ceq) can be 0.38 to 0.45%.

[0015] In addition, it may further include 0.10 to 0.55 wt% of silicon (Si), 0.02 wt% or less of phosphorus (P) (excluding 0 wt%), 0.007 wt% or less of sulfur (S) (excluding 0 wt%), and 0.015 to 0.055 wt% of aluminum (Al).

[0016] A method for manufacturing a shaped steel according to one embodiment of the present invention comprises a first step of preparing a steel material containing 0.11 to 0.18 wt% of carbon (C), 1.0 to 1.65 wt% of manganese (Mn), 0.01 to 0.11 wt% of vanadium (V), 40 to 200 ppm of nitrogen (N), and the remainder iron (Fe) and other unavoidable impurities, a second step of reheating the steel material, a third step of hot-rolling the steel material subjected to the second step to form a hot-rolled steel material, and a fourth step of cooling the hot-rolled steel material.

[0017] Additionally, the reheating temperature can be 1150 to 1300°C.

[0018] Additionally, the rolling start temperature may be 1000 to 1100°C, and the rolling end temperature may be 890 to 930°C.

[0019] In addition, the steel may further include 0.10 to 0.55 wt% of silicon (Si), 0.02 wt% or less of phosphorus (P) (excluding 0 wt%), 0.007 wt% or less of sulfur (S) (excluding 0 wt%), and 0.015 to 0.055 wt% of aluminum (Al).

[0020] According to a steel section and a method for manufacturing steel sections according to one embodiment of the present invention, steel sections having high strength and high impact toughness can be manufactured by designing alloy components that do not increase carbon equivalent and controlling the content ratio of vanadium (V) and nitrogen (N).

[0021] 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.

[0022] FIG. 1 is a flowchart schematically illustrating a method for manufacturing a steel beam according to one embodiment of the present invention.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] Also, unless otherwise stated, 1 ppm is 0.0001 wt%.

[0030] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0031]

[0032] Section steel

[0033] According to one embodiment of the present invention, the steel sheet contains 0.11 to 0.18 wt% of carbon (C), 1.0 to 1.65 wt% of manganese (Mn), 0.01 to 0.11 wt% of vanadium (V), 40 to 200 ppm of nitrogen (N), and the remainder of iron (Fe) and other unavoidable impurities.

[0034] 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.

[0035]

[0036] carbon (C)

[0037] 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.

[0038] If the carbon content is less than 0.11 wt%, it may be difficult to secure sufficient strength. Conversely, if the carbon content exceeds 0.18 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 0.11 to 0.18 wt% carbon.

[0039]

[0040] manganese (Mn)

[0041] 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.

[0042] If the manganese content is less than 1.0 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 section according to one embodiment of the present invention may contain manganese in an amount of 1.0 to 1.65 wt%.

[0043]

[0044] Vanadium (V)

[0045] Vanadium (V) can refine austenite grains by inhibiting the movement of austenite grain boundaries during reheating and hot rolling. Furthermore, it can enhance hardenability by inhibiting nucleation at austenite grain boundaries during phase transformation, and it can enhance strength by forming precipitates during phase transformation from austenite.

[0046] If the vanadium content is less than 0.01 wt%, the aforementioned effects may be difficult to achieve. On the other hand, if the vanadium content exceeds 0.11 wt%, workability may be reduced, causing cracks in the material during rolling. Therefore, the shaped steel according to one embodiment of the present invention may contain vanadium in an amount of 0.01 to 0.11 wt%, and more preferably, 0.05 to 0.11 wt%.

[0047]

[0048] Nitrogen (N)

[0049] Nitrogen (N) combines with other alloying elements such as vanadium (V), aluminum (Al), titanium (Ti), and niobium (Nb) to form nitride precipitates, thereby contributing to grain refinement. In addition, the precipitation strengthening effect can be enhanced by adding nitrogen (N) to steel to which vanadium (V) has been added. If the nitrogen content is less than 40 ppm, it is difficult to achieve the above-described effect. On the other hand, if the nitrogen content exceeds 200 ppm, the toughness of the weld joint may deteriorate and the impact value may decrease. Therefore, the steel section according to one embodiment of the present invention may contain nitrogen in an amount of 40 to 200 ppm.

[0050] In addition, the steel according to one embodiment of the present invention has a content ratio of vanadium (V) and nitrogen (N) that satisfies the following equation 1.

[0051] [Formula 1]

[0052] 3.0 ≤ [V] / [N] ≤ 4.5

[0053] However, the above [V] and [N] represent the content of vanadium (V) and the content of nitrogen (N) in weight%, respectively.

[0054] When the above [V] / [N] is less than 3.0, the impact toughness may be reduced due to the generation of free nitrogen. On the other hand, when the [V] / [N] exceeds 4.5, the strength improvement effect due to nitrogen addition is significantly reduced, and the impact toughness may be reduced due to coarsening of precipitates.

[0055] Thus, by increasing the precipitation driving force through the addition of a certain level of nitrogen (N), the precipitate fraction and precipitate microdispersion are increased, thereby improving strength and impact toughness. In addition, by adjusting the appropriate content ratio of vanadium (V) and nitrogen (N) according to the above formula 1, optimization of the addition amount for operational and cost efficiency can be achieved.

[0056]

[0057] According to one embodiment of the present invention, the steel sheet may further include 0.10 to 0.55 wt% of silicon (Si), 0.020 wt% or less of phosphorus (P), 0.007 wt% or less of sulfur (S), and 0.015 to 0.055 wt% of aluminum (Al).

[0058]

[0059] Silicon (Si)

[0060] Silicon (Si) is added as a deoxidizer during the steelmaking process to remove oxygen from steel. Silicon can also have a solid solution strengthening effect. It can suppress 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.

[0061] The above silicon (Si) may be included in an amount of 0.10 to 0.55 wt% of the total weight of the steel according to one embodiment of the present invention. If the silicon content is less than 0.10 wt%, the aforementioned silicon addition effect cannot be properly exerted. Conversely, if the silicon content exceeds 0.55 wt%, the weldability of the steel may be reduced and the plastic formability may be deteriorated.

[0062]

[0063] Person (P)

[0064] 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%).

[0065]

[0066] Yellow (S)

[0067] Sulfur (S) 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 crack during hot and cold working.

[0068] When the sulfur content exceeds 0.007 wt%, it can impair toughness and weldability and reduce low-temperature impact strength, so it is desirable to control the content to the lowest possible level. Therefore, the steel according to one embodiment of the present invention may contain sulfur in an amount of 0.007 wt% or less (excluding 0 wt%).

[0069]

[0070] Aluminum (Al)

[0071] 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.

[0072] 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.

[0073] 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.

[0074]

[0075] Steel having the above-described alloy composition can have high yield strength and excellent mechanical properties without increasing carbon equivalent (Ceq). More specifically, the shaped steel according to one embodiment of the present invention can have a carbon equivalent of 0.38 to 0.45%, and can be a shaped steel having excellent yield strength without exceeding the carbon equivalent standard for structural steel for construction.

[0076] A steel section manufactured using the alloy composition described above and the steel section manufacturing method described below can have excellent mechanical properties and can guarantee a yield strength of 460 MPa. More specifically, the yield strength (YS) can be 460 MPa or more, and preferably 486 MPa or more.

[0077] In addition, the steel according to the present invention can satisfy a tensile strength (TS) of 570 to 720 MPa, an elongation (EL) of 17% or more, and can guarantee impact toughness. More specifically, the Charpy impact absorption energy (CVN) at 0°C can be 130 J or more, and more preferably 145 J or more.

[0078] Hereinafter, a method for manufacturing a shaped steel according to one embodiment of the present invention will be described in detail.

[0079]

[0080] Method for manufacturing shaped steel

[0081] Hereinafter, a method for manufacturing a steel beam according to one embodiment of the present invention will be described with reference to the drawings.

[0082] FIG. 1 is a flowchart schematically illustrating a method for manufacturing a steel beam according to one embodiment of the present invention.

[0083] Referring to FIG. 1, a method for manufacturing a shaped steel according to an embodiment of the present invention includes a first step (S1) of preparing a steel material containing 0.11 to 0.18 wt% of carbon (C), 1.0 to 1.65 wt% of manganese (Mn), 0.01 to 0.11 wt% of vanadium (V), 40 to 200 ppm of nitrogen (N), and the remainder iron (Fe) and other unavoidable impurities, a second step (S2) of reheating the steel material, a third step (S3) of hot-rolling the steel material subjected to the second step (S2) to form a hot-rolled steel material, and a fourth step (S4) of cooling the hot-rolled steel material.

[0084] Additionally, the steel may further contain 0.10 to 0.55 wt% of silicon (Si), 0.020 wt% or less of phosphorus (P), 0.007 wt% or less of sulfur (S), and 0.015 to 0.055 wt% of aluminum (Al).

[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, the first step (S1) according to one embodiment of the present invention is a step of preparing a steel having the above-described alloy composition range. More specifically, this may be a step of designing alloy components within the above-described alloy composition range to manufacture a semi-finished product. The semi-finished product may be, but is not necessarily limited to, a beam blank, a bloom, a billet, etc. In addition, the manufacturing of the slab may be performed using a process known in the relevant technical field, such as a steelmaking process or a casting process.

[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 segregated components may be re-dissolved. If the reheating temperature is lower than 1150℃, the segregated components within the steel cannot be sufficiently dissolved, and the rolling load may increase, reducing rollability. If the reheating temperature exceeds 1300℃, coarse austenite grains may form, making it difficult to secure strength. In addition, heating costs and process times increase, leading to increased manufacturing costs and reduced productivity.

[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. More specifically, the rolling start temperature can be controlled to 1000 to 1100°C, and the rolling end temperature can be controlled to 890 to 930°C.

[0092] If the rolling start temperature is less than 1000℃, the load during rolling may increase, and if the rolling start temperature exceeds 1100℃, the rolling reduction in the unrecrystallized region may become excessive, which may deteriorate the low-temperature impact properties of the steel.

[0093] If the rolling finish temperature is lower than 890℃, coarse proeutectoid ferrite may be formed on the surface of the steel, which may reduce the strength of the steel. On the other hand, if the rolling finish temperature exceeds 930℃, high temperature-induced grain growth may occur, which may reduce the impact toughness. When the rolling finish temperature is controlled to 890 ~ ​​930℃, the deformation-induced precipitation effect due to rolling deformation occurs, increasing the precipitate fraction, and enhancing the grain refinement and precipitation strengthening effect.

[0094] A method for manufacturing a shaped steel according to one embodiment of the present invention may perform a fourth step (S4) of cooling the hot-rolled steel after the third step (S3). More specifically, the fourth step (S4) may be a step of air-cooling the hot-rolled steel.

[0095]

[0096] Comparative and experimental examples

[0097] 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.

[0098] Table 1 below shows the main alloy compositions and the content ratios of vanadium (V) and nitrogen (N) that make up the comparative and experimental examples of the present invention. In the case of alloy components not listed in Table 1 below, they were added in the same amount within the alloy composition range according to one embodiment of the present invention.

[0099] In addition, Table 2 below shows the results of measuring the mechanical properties of comparative examples and experimental examples manufactured using a method for manufacturing shaped steel according to an embodiment of the present invention, using steel having the alloy components described in Table 1. At this time, the manufacturing process conditions of the comparative examples and experimental examples were controlled to be the same as the control variables.

[0100] 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.

[0101] In the table below, 'V / N ratio' refers to the content ratio of vanadium (V) to nitrogen (N) in weight percent. 'TS' and 'YS' refer to tensile strength and yield strength, respectively, in MPa. In addition, 'EL' refers to elongation, in %. 'CVN' refers to Charpy impact absorption energy measured at 0℃, in J.

[0102]

[0103] ClassificationMajor chemical componentsV / NratioC (%)Mn (%)V (%)N (ppm)Comparative example 10.111.420.1031606.44Comparative example 20.111.440.0931625.74Experimental example 10.111.490.0681584.31Experimental example 20.111.460.0561663.38Comparative example 30.111.420.0231611.43Comparative example 40.111.40.0051500.33

[0104]

[0105] Classification Mechanical properties Impact toughness TS (MPa) YS (MPa) EL (%) CVN (J) Comparative example 16 18 5 20 4 8 8 8 Comparative example 26 16 5 12 4 7 12 0 Experimental example 16 25 5 10 4 6 15 3 Experimental example 26 0 5 4 8 6 4 5 14 5 Comparative example 3 5 6 9 4 9 4 5 7 6 Comparative example 4 5 5 5 4 2 6 4 3 2 2

[0106]

[0107] Referring to Tables 1 and 2, it can be confirmed that Comparative Examples 1 and 2 do not satisfy the content ratio of vanadium (V) and nitrogen (N) according to Equation 1. In addition, it can be confirmed that Comparative Examples 3 and 4 do not satisfy the vanadium (V) content according to one embodiment of the present invention, and the content ratio of vanadium (V) and nitrogen (N) does not satisfy Equation 1.

[0108] At this time, it can be confirmed that the Charpy impact absorption energy at 0℃ of Comparative Examples 1 to 4 is lower than that of Experimental Examples 1 and 2 according to an embodiment of the present invention. In addition, it can be confirmed that Comparative Examples 3 and 4 do not satisfy the mechanical properties targeted by the present invention, namely, a tensile strength of 570 to 720 MPa, a yield strength of 460 MPa or more, and a Charpy impact absorption energy at 0℃ of 130 J or more.

[0109] On the other hand, in the case of Experimental Examples 1 and 2, it can be confirmed that the mechanical strength targeted in the present invention is all satisfied, and the Charpy impact absorption energy at 0℃ is higher than that of Comparative Examples 1 to 4.

[0110] 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.

Claims

1. Contains 0.11 to 0.18 wt% of carbon (C), 1.0 to 1.65 wt% of manganese (Mn), 0.01 to 0.11 wt% of vanadium (V), 40 to 200 ppm of nitrogen (N), and the remainder of iron (Fe) and other inevitable impurities. A steel sheet having a content ratio of vanadium (V) and nitrogen (N) satisfying Equation 1. [Formula 1] 3.0 ≤ [V] / [N] ≤ 4.5 (Wherein, [V] and [N] represent the weight% of V and N, respectively.) 2. In paragraph 1, Steel with a yield strength (YS) of 460 MPa or more, a tensile strength (TS) of 570 to 720 MPa, and an elongation (EL) of 17% or more.

3. In paragraph 1, Steel having a Charpy impact absorption energy (CVN) of 130J or more at 0℃.

4. In paragraph 1, Steel with a carbon equivalent (Ceq) of 0.38 to 0.45%.

5. In paragraph 1, A steel sheet further containing 0.10 to 0.55 wt% of silicon (Si), 0.020 wt% or less of phosphorus (P) (excluding 0 wt%), 0.007 wt% or less of sulfur (S) (excluding 0 wt%), and 0.015 to 0.055 wt% of aluminum (Al).

6. The first step of preparing steel containing 0.11 to 0.18 wt% carbon (C), 1.0 to 1.65 wt% manganese (Mn), 0.01 to 0.11 wt% vanadium (V), 40 to 200 ppm nitrogen (N), 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.

7. In paragraph 6, A method for manufacturing shaped steel having a reheating temperature of 1150 to 1300℃.

8. In paragraph 6, A method for manufacturing shaped steel having a rolling start temperature of 1000 to 1100℃ and a rolling end temperature of 890 to 930℃.

9. In paragraph 6, The above steel material, A method for manufacturing a shaped steel further comprising 0.10 to 0.55 wt% of silicon (Si), 0.020 wt% or less of phosphorus (P) (excluding 0 wt%), 0.007 wt% or less of sulfur (S) (excluding 0 wt%), and 0.015 to 0.055 wt% of aluminum (Al).

Images