Steel section, manufacturing method therefor, and cross member for trailer manufactured thereby

Abstract

A steel section according to an embodiment of the present invention comprises 0.20-0.30 wt% of carbon (C), 0.10-0.30 wt% of silicon (Si), 1.00-1.50 wt% of manganese (Mn), 0.030 wt% or less (excluding 0) of phosphorus (P), 0.030 wt% or less (excluding 0) of sulfur (S), greater than or equal to 0.14 wt% and less than 0.30 wt% of copper (Cu), 0.040-0.130 wt% of vanadium (V), 0.005-0.020 wt% of nitrogen (N), and the remainder of iron (Fe) and inevitable impurities, the steel section having a yield strength (YS) of 550 MPa or more.

Description

Sectional steel, its manufacturing method and trailer cross member manufactured thereby

[0001] The present invention relates to a steel beam, a method for manufacturing the same, and a cross member for a trailer manufactured thereby.

[0002] Section steel generally refers to steel with a diverse range of cross-sectional shapes. Section steel is used in automotive structural applications, such as cross members in trailers and columns in large buildings, as well as in civil engineering structures like bridges and foundation piles. Section steel can be manufactured by hot rolling cast steel, such as blooms, billets, and beam blanks, which are produced through continuous casting.

[0003] Among these, the cross member of the trailer is a member that supports the weight of the cargo loaded on the trailer from the bottom of the trailer, and is mainly manufactured using I-beam or channel steel.

[0004] Although cross members are load-bearing members, they are often exposed to the outside, so corrosion resistance is often secured through zinc plating, etc. However, if exposed to the outside for a long period of time, the coating may peel off, so in addition to zinc plating, the steel for the cross members itself must have corrosion resistance.

[0005] Therefore, there is a need to develop a steel profile for trailer cross members with improved corrosion resistance and a method for manufacturing the same.

[0006] Additionally, the trailer is arranged with cross members intersecting the main beam and the top plate placed on top of it. It is common to perform a welding process to connect the main beam and the cross members.

[0007] Therefore, there is a need to develop a trailer cross member steel having a high strength of 550 MPa or more based on the yield strength and excellent weldability, and a manufacturing method thereof.

[0008] According to one embodiment of the present invention, the purpose is to provide a steel beam having improved corrosion resistance and zinc plating properties, a method for manufacturing the same, and a cross member for a trailer manufactured thereby.

[0009] 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.

[0010] According to one embodiment of the present invention, a steel sheet contains carbon (C) 0.20 to 0.30 wt%, silicon (Si) 0.10 to 0.30 wt%, manganese (Mn) 1.00 to 1.50 wt%, phosphorus (P) 0.030 wt% or less (excluding 0), sulfur (S) 0.030 wt% or less (excluding 0), copper (Cu) 0.14 wt% or more and less than 0.30 wt%, vanadium (V) 0.040 to 0.130 wt%, nitrogen (N) 0.005 to 0.020 wt%, and the remainder iron (Fe) and unavoidable impurities, and has a yield strength (YS) of 550 MPa or more.

[0011] According to one embodiment of the present invention, the tensile strength (TS) may be 620 MPa or more.

[0012] According to one embodiment of the present invention, the elongation (EL) may be 15% or more.

[0013] According to one embodiment of the present invention, the average diameter of the crystal grains may be 5.6 μm or less.

[0014] A cross member for a trailer according to one embodiment of the present invention is manufactured using the above-described steel.

[0015] A method for manufacturing a shaped steel according to one embodiment of the present invention comprises the steps of (S1) preparing a steel material, (S2) reheating the steel material, (S3) hot rolling the steel material to form a hot-rolled steel material, and (S4) cooling the hot-rolled steel material to form a shaped steel material as a final product, wherein the final product comprises 0.20 to 0.30 wt% of carbon (C), 0.10 to 0.30 wt% of silicon (Si), 1.00 to 1.50 wt% of manganese (Mn), 0.030 wt% or less of phosphorus (P) (excluding 0), 0.030 wt% or less of sulfur (S) (excluding 0), 0.14 wt% or more of copper (Cu) and less than 0.30 wt%, 0.040 to 0.130 wt% of vanadium (V), 0.005 to 0.020 wt% of nitrogen (N), and the remainder of iron (Fe). It contains unavoidable impurities and has a yield strength (YS) of 550 MPa or more.

[0016] According to one embodiment of the present invention, the reheating temperature of the step (S2) may be 1250 to 1350°C.

[0017] According to one embodiment of the present invention, the rolling start temperature of the step (S3) may be 1000 to 1100°C.

[0018] According to one embodiment of the present invention, the rolling end temperature of the step (S3) may be greater than 550°C and less than or equal to 650°C.

[0019] According to one embodiment of the present invention, the tensile strength (TS) of the final product may be 620 MPa or more.

[0020] According to one embodiment of the present invention, the elongation (EL) of the final product may be 15% or more.

[0021] A steel beam with improved corrosion resistance and zinc plating properties, a method for manufacturing the same, and a cross member for a trailer manufactured using the same can be provided.

[0022] 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.

[0023] Figure 1 is a flowchart showing a method for manufacturing a steel beam according to one embodiment of the present invention.

[0024] Figure 2 is a drawing showing the results of corrosion resistance evaluation of comparative examples and examples.

[0025] Figure 3 is a drawing showing the optical microscope observation results of comparative examples and examples.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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 also be applied to numerical value A.

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

[0033] Section steel

[0034] According to one embodiment of the present invention, a shaped steel is manufactured from a steel material containing 0.20 to 0.30 wt% of carbon (C), 0.10 to 0.30 wt% of silicon (Si), 1.00 to 1.50 wt% of manganese (Mn), 0.030 wt% or less of phosphorus (P) (excluding 0), 0.030 wt% or less of sulfur (S) (excluding 0), 0.14 wt% or more of copper (Cu) and less than 0.30 wt%, 0.040 to 0.130 wt% of vanadium (V), 0.005 to 0.020 wt% of nitrogen (N), and the remainder of iron (Fe) and unavoidable impurities, and the shaped steel, which is a final product, may contain the same alloy components.

[0035] Below, the role and content of each alloy element included in the steel according to one embodiment of the present invention will be described in detail.

[0036] carbon (C)

[0037] Carbon (C) is an essential element for achieving the required strength. For example, carbon can be added to secure the strength of the microstructure by acting as an interstitial solid solution. Carbon can also be added to maintain the balance of precipitates.

[0038] Insufficient carbon content can lead to difficulties in achieving sufficient strength due to the aforementioned effects. Conversely, excessive carbon content can lead to reduced elongation and toughness of the steel, and reduced weldability.

[0039] Therefore, the steel according to one embodiment of the present invention may contain 0.20 to 0.30 wt% of carbon (C), and preferably 0.21 to 0.30 wt%.

[0040] Silicon (Si)

[0041] Silicon (Si) is added as a deoxidizer to remove oxygen in steel during the steelmaking process together with aluminum, and as a ferrite stabilizing element with a solid solution strengthening effect, it can induce ferrite formation and improve the hardenability and softening resistance of steel.

[0042] If the silicon content is insufficient, the aforementioned effects may be minimal. Conversely, if the silicon content is excessive, the steel's wettability and plating adhesion may be reduced, resulting in unplated areas during zinc plating.

[0043] Accordingly, the steel according to one embodiment of the present invention may contain 0.10 to 0.30 wt% of silicon (Si), preferably 0.10 to 0.20 wt%, and more preferably 0.17 to 0.18 wt%.

[0044] manganese (Mn)

[0045] Manganese (Mn) is a solid-solution strengthening element that not only contributes to strength but also enhances the hardenability of steel. Furthermore, as the manganese content increases, pearlite becomes finer and ferrite becomes solid-solution strengthened, thereby improving yield strength.

[0046] If manganese content is insufficient, the aforementioned effects may be minimal. Conversely, if manganese content is excessive, austenite may remain, reducing strength and toughness.

[0047] Accordingly, the steel according to one embodiment of the present invention may contain manganese (Mn) in an amount of 1.00 to 1.50 wt%, preferably 1.00 to 1.20 wt%, and more preferably 1.10 to 1.13 wt%.

[0048] Person (P)

[0049] Phosphorus (P) is an element that contributes to strength enhancement. However, excessive phosphorus content can deteriorate the ductility of steel and cause final material deviations due to billet center segregation. When phosphorus content exceeds 0.030 wt%, it can form center segregation and microsegregation, reducing the ductility of the steel. Furthermore, precipitation behavior can reduce impact strength.

[0050] Accordingly, the steel according to one embodiment of the present invention may contain phosphorus (P) in an amount of 0.030 wt% or less (excluding 0), preferably 0.005 to 0.025 wt%, and more preferably 0.011 to 0.016 wt%.

[0051] Yellow (S)

[0052] Sulfur (S) improves the machinability of steel when combined with manganese, zinc, titanium, molybdenum, etc., and can improve workability by forming fine precipitates (e.g., MnS) when combined with manganese. However, when the sulfur content exceeds 0.030 wt%, excessive MnS inclusions may form, which may deteriorate the surface quality during hot rolling.

[0053] Accordingly, the steel according to one embodiment of the present invention may contain sulfur (S) in an amount of 0.030 wt% or less, preferably 0.005 to 0.010 wt%, and more preferably 0.005 to 0.007 wt%.

[0054] copper (Cu)

[0055] Copper (Cu) can combine with oxygen in steel to form copper oxide (CuO). This formed copper oxide forms a thin film on the steel's surface, which can improve corrosion resistance.

[0056] If copper content is insufficient, the aforementioned effects may be minimal. Conversely, if copper content is excessive, a copper-concentrated layer may form on the steel's surface, causing surface defects such as red-hot embrittlement.

[0057] Accordingly, the steel according to one embodiment of the present invention may contain copper (Cu) in an amount of 0.14 wt% or more and less than 0.30 wt%, preferably 0.15 wt% or more and less than 0.30 wt%, and more preferably 0.17 wt% or more and less than 0.22 wt%.

[0058] Vanadium (V)

[0059] Vanadium (V) can combine with carbon and nitrogen in steel to form precipitates. Vanadium's carbide-forming ability is stronger than that of chromium (Cr), and because precipitates refine the steel's structure, they promote precipitation strengthening, contributing to increased strength. Furthermore, the addition of vanadium can enhance toughness.

[0060] If the vanadium content is insufficient, the aforementioned effects may be minimal. Conversely, if the vanadium content is excessive, the manufacturing cost of the steel increases, and oxides (e.g., V2O5) may evaporate at high temperatures.

[0061] Therefore, the steel according to one embodiment of the present invention may contain vanadium (V) in an amount of 0.040 to 0.130 wt%, and preferably 0.040 to 0.110 wt%.

[0062] Nitrogen (N)

[0063] Even trace amounts of nitrogen (N) can have a significant impact on the mechanical properties of steel.

[0064] Nitrogen can combine with vanadium and other metals to precipitate nitrides. These precipitates refine the steel's structure, resulting in precipitation strengthening, which helps improve strength. Furthermore, adding nitrogen can improve strength without compromising weldability.

[0065] If the nitrogen content is insufficient, the aforementioned effects may be minimal. Conversely, if the nitrogen content is excessive, the steel's impact toughness may be reduced.

[0066] Therefore, the steel according to one embodiment of the present invention may contain nitrogen (N) in an amount of 0.005 to 0.020 wt%, and preferably 0.005 to 0.017 wt%.

[0067] 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 iron (Fe).

[0068] 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 steel bar manufacturing process. Since these impurities are widely known in the field, a detailed description will be omitted.

[0069] The steel according to one embodiment of the present invention may have a yield strength (YS) of 550 MPa or more, preferably 630 MPa or more, and more preferably 636 MPa or more.

[0070] The steel according to one embodiment of the present invention may have a tensile strength (TS) of 620 MPa or more, preferably 690 MPa or more, and more preferably 693 MPa or more.

[0071] The steel according to one embodiment of the present invention may have an elongation (EL) of 15% or more, preferably 16% or more, and more preferably 16.5% or more.

[0072] According to one embodiment of the present invention, the steel sheet may have excellent corrosion resistance as a result of a corrosion resistance evaluation.

[0073] According to one embodiment of the present invention, a trailer cross member can be manufactured using the aforementioned steel beam. The steel beam according to one embodiment of the present invention has excellent strength and corrosion resistance, making it suitable as a trailer cross member.

[0074] Hereinafter, a method for manufacturing a shaped steel according to one embodiment of the present invention will be described in detail.

[0075] Steel beam manufacturing method

[0076] Hereinafter, a method for manufacturing a steel beam according to one embodiment of the present invention will be described with reference to FIG. 1.

[0077] Figure 1 is a flowchart showing a method for manufacturing a steel beam according to one embodiment of the present invention.

[0078] A method for manufacturing a shaped steel according to one embodiment of the present invention includes a step (S1) of preparing a semi-finished steel product, a step (S2) of reheating the steel product, a step (S3) of hot-rolling the steel product to form a hot-rolled product, and a step (S4) of cooling the formed hot-rolled product.

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

[0080] According to one embodiment of the present invention, the step (S1) of preparing a semi-finished steel product is a step of preparing a steel product having the above-described alloy composition range in order to manufacture a shaped steel product as a final product.

[0081] Specifically, the step (S1) of preparing steel may be a step of designing alloy components within the aforementioned alloy composition range to manufacture a semi-finished product. The semi-finished product may be, but is not limited to, a beam blank, a billet, or a bloom. Furthermore, the manufacture of the semi-finished product may be performed using a process known in the relevant technical field, such as a steelmaking process or a casting process.

[0082] According to one embodiment of the present invention, a step (S2) of reheating the steel may be performed after the step (S1) of preparing the steel.

[0083] The step (S2) of reheating the steel is a pretreatment step of the rolling step, and may be a step of uniformly heating the steel by charging the steel into a heating furnace so that plastic deformation can be easily performed, thereby re-dissolving the components segregated during casting of the steel.

[0084] In the reheating step (S2), the steel may be reheated to 1250 to 1350°C. If the reheating temperature is below 1250°C, the rolling load may increase. Conversely, if the reheating temperature exceeds 1350°C, the austenite grains may coarsen, resulting in reduced strength. Furthermore, increased heating costs and time may lead to increased manufacturing costs and reduced productivity.

[0085] Therefore, in the present invention, the steel can be reheated at a temperature of 1250 to 1350°C.

[0086] According to one embodiment of the present invention, a step (S3) of hot rolling the steel to form a hot-rolled steel may be performed after a step (S2) of reheating the steel.

[0087] In the step of forming hot-rolled steel (S3), the temperature of the reheated steel is lowered due to transport or other reasons, so that hot rolling can begin at a temperature range of 1000 to 1100°C.

[0088] The above hot rolling may include rough rolling and finish rolling processes. Here, the rough rolling process may be to make the steel into a rolled material having an appropriate shape, thickness, and width, and the finish rolling process may be to adjust the steel to a specified size and roll it at a finishing temperature suitable for the intended use to obtain a good surface and shape.

[0089] In the step (S3) of forming a hot-rolled material, the finishing rolling temperature may be greater than 550°C and less than or equal to 650°C. When the finishing rolling temperature is less than or equal to 550°C, the grains become finer, improving strength, but reducing elongation.

[0090] Conversely, if the finishing rolling temperature exceeds 650℃, the strength may be rapidly reduced due to grain growth.

[0091] As a result, the method for manufacturing a shaped steel according to one embodiment of the present invention may have a finishing rolling temperature of more than 550°C and less than or equal to 650°C, and preferably more than or equal to 600°C and less than or equal to 650°C.

[0092] According to one embodiment of the present invention, after performing the step (S3) of forming a hot-rolled material, a step (S4) of cooling the formed hot-rolled material may be performed.

[0093] The step of cooling the hot rolled material (S4) can be performed through air cooling.

[0094] After the step of cooling the hot rolled material (S4) is performed, the final product, the shaped steel, can be formed.

[0095] The steel beam manufactured by the steel beam manufacturing method according to one embodiment of the present invention may be an I-beam.

[0096] The steel produced by the steel production method according to one embodiment of the present invention may have an average grain diameter of 5.6 ㎛ or less, preferably 4.8 ㎛ or less, and more preferably 4.77 ㎛ or less.

[0097] A steel beam manufactured by a method for manufacturing a steel beam according to one embodiment of the present invention can satisfy all of the above-mentioned yield strength (YS), tensile strength (TS), and elongation (EL) values.

[0098] A steel beam manufactured using a method for manufacturing a steel beam according to one embodiment of the present invention may have excellent corrosion resistance as a result of a corrosion resistance evaluation.

[0099] Comparative examples and examples

[0100] Below, preferred comparative examples and examples are presented to aid in understanding the present invention. However, the following comparative examples and examples are provided solely to aid in understanding the present invention, and the present invention is not limited to the examples below.

[0101] Table 1 shows the alloy element composition of comparative examples and examples, and Table 2 shows the finishing rolling temperature (hereinafter, FDT) and physical properties of comparative examples and examples.

[0102] The comparative examples and examples were each manufactured using semi-finished products having the alloy compositions listed in Table 1 below, and a reheating step was performed at a temperature of 1300°C. Thereafter, hot rolling was initiated at 1050°C and completed at the finish rolling temperature (FDT) listed in Table 2. Cooling was performed by air cooling.

[0103] Other than the conditions described above, the manufacturing process of the comparative examples and examples of the present invention was controlled under the same conditions within the range described in the method for manufacturing shaped steel according to one embodiment of the present invention described above, as a control variable.

[0104] Corrosion resistance was evaluated using the following method.

[0105] After tar plating and chipping were performed on the surface of the specimens of the comparative examples and examples, they were exposed at a temperature of -20℃ for 3 hours. Afterwards, 50 g of 2.5 to 5 mm sized tombstones were dissolved in salt water and sprayed at a 90° angle under a pressure of 5 bar, and the test was observed for 240 hours. After 240 hours, the surface was observed, and the degree of corrosion was recorded in Table 2 below, with an X indicating poor corrosion resistance and an O indicating good corrosion resistance.

[0106] For example, Fig. 2 is a photograph showing the results of corrosion resistance evaluation of comparative examples and examples.

[0107] Specifically, in Fig. 2, (2a) is a photograph of the surface observed after the corrosion resistance evaluation of Comparative Example 8, and (2b) is a photograph of the surface observed after the corrosion resistance evaluation of Example 3.

[0108] As shown in (2a) of Fig. 2, if the area where corrosion occurred on the surface of the specimen was 10% or more of the total surface area, the corrosion resistance was judged to be poor.

[0109] Conversely, as shown in (2b) of Fig. 2, if the area where corrosion occurred was less than 10% of the total surface area, the corrosion resistance was judged to be good.

[0110] In Table 1 below, the unit of composition of alloy elements is weight%, in Table 2 below, the unit of yield strength (YS) and tensile strength (TS) is MPa, the unit of elongation (EL) is %, and the unit of grain size is ㎛. Here, the grain size means the average diameter of the grains.

[0111]

[0112]

[0113] Referring to Table 1 and Table 2 above, Comparative Examples 1 and 2 are comparative examples in which the carbon (C) content is 0.15 wt% and 0.18 wt%, respectively, and the copper (Cu) content is 0.10 wt% and 0.12 wt%, respectively.

[0114] In the case of Comparative Examples 1 and 2, it can be confirmed that the content of carbon (C) does not satisfy the range of 0.20 wt% or more and 0.30 wt% or less according to one embodiment of the present invention described above.

[0115] Accordingly, it can be confirmed that the values ​​of yield strength (YS) and tensile strength (TS) of Comparative Examples 1 and 2 do not satisfy the ranges targeted by the present invention, which are 630 MPa or more and 690 MPa or more.

[0116] In addition, it can be confirmed that Comparative Examples 1 and 2 do not satisfy the range of 0.14 wt% or more and less than 0.30 wt% in the content of copper (Cu) according to one embodiment of the present invention described above.

[0117] Accordingly, it can be confirmed that the corrosion resistance of Comparative Examples 1 and 2 is poor as a result of the corrosion resistance evaluation.

[0118] Comparative Examples 3 and 4 are comparative examples in which the finishing rolling temperature (FDT) is 530°C and 550°C, respectively.

[0119] In the case of Comparative Examples 3 and 4, it can be confirmed that the finishing rolling temperature (FDT) does not satisfy the range of 550°C to 650°C, which is the range according to one embodiment of the present invention described above.

[0120] Accordingly, it can be confirmed that the elongation (EL) values ​​of Comparative Examples 3 and 4 do not satisfy the range targeted by the present invention, which is 15% or more.

[0121] Furthermore, in the case of Comparative Example 3, it can be confirmed that the corrosion resistance is poor as a result of the corrosion resistance evaluation.

[0122] Comparative Example 5 is a comparative example in which the finishing rolling temperature (FDT) is 680°C.

[0123] In the case of Comparative Example 5, it can be confirmed that the finishing rolling temperature (FDT) does not satisfy the range of 550°C to 650°C, which is the range according to one embodiment of the present invention described above.

[0124] Accordingly, in the case of Comparative Example 5, it can be confirmed that the values ​​of yield strength (YS) and tensile strength (TS) do not satisfy the ranges targeted by the present invention, which are 630 MPa or more and 690 MPa or more.

[0125] Comparative Examples 6 and 7 are comparative examples in which the copper (Cu) content is 0.30 wt% and 0.32 wt%, respectively.

[0126] In the case of Comparative Examples 6 and 7, it can be confirmed that the content of copper (Cu) does not satisfy the range of 0.14 wt% or more and less than 0.30 wt% according to one embodiment of the present invention described above.

[0127] Accordingly, in the case of Comparative Examples 6 and 7, cracks occurred during hot rolling due to red-hot embrittlement, and it was confirmed that the corrosion resistance was poor as a result of the corrosion resistance evaluation.

[0128] Furthermore, in the case of Comparative Example 7, it can be confirmed that the yield strength (YS) value does not satisfy the range targeted by the present invention, which is 630 MPa or more.

[0129] Comparative Example 8 is a comparative example in which the silicon (Si) content is 0.24 wt% and the copper (Cu) content is 0.13 wt%.

[0130] In the case of Comparative Example 8, it can be confirmed that the content of silicon (Si) does not satisfy the range of 0.10 wt% or more and 0.20 wt% or less according to one embodiment of the present invention described above.

[0131] Accordingly, in the case of Comparative Example 8, it can be confirmed that the elongation (EL) value does not satisfy the range targeted by the present invention, which is 630 MPa or more and 690 MPa or more.

[0132] In addition, it can be confirmed that Comparative Example 8 does not satisfy the range of 0.14 wt% or more and less than 0.30 wt% in the content of copper (Cu) according to one embodiment of the present invention described above.

[0133] Accordingly, in the case of Comparative Example 8, it can be confirmed that the corrosion resistance is poor as a result of the corrosion resistance evaluation.

[0134] On the other hand, in the case of Examples 1 to 5 according to one embodiment of the present invention, it can be confirmed that the alloy composition including carbon (C), silicon (Si), and copper (Cu) all satisfies the range targeted by the present invention, and the finish rolling temperature (FDT) all satisfies the range targeted by the present invention. Accordingly, it can be confirmed that the results of the evaluation of yield strength (YS), tensile strength (TS), elongation (EL), and corrosion resistance all satisfies the range targeted by the present invention.

[0135] In addition, in the case of Examples 1 to 5 according to one embodiment of the present invention, it can be confirmed that the crystal grain size is all satisfactorily 5.6 ㎛ or less.

[0136] For further details, see Figure 3.

[0137] Figure 3 is a photograph showing the optical microscope observation results of comparative examples and examples.

[0138] Specifically, in Fig. 3, (3a) is a photograph observing the microstructure of Comparative Example 1, and (3b) is a photograph observing the microstructure of Example 3.

[0139] Referring to FIG. 3, Comparative Example 1 is a comparative example in which the carbon (C) content is 0.15 wt%, and the carbon (C) content does not satisfy the range of 0.20 wt% or more and 0.30 wt% or less according to the above-described embodiment of the present invention.

[0140] In other words, in the case of Comparative Example 1, the content of carbon (C) is lower than that of one embodiment of the present invention, and it can be confirmed that the size of the crystal grains is 7.88 μm.

[0141] As a result, it can be confirmed that Comparative Example 1 has coarsened grains and thus does not satisfy the yield strength (YS) and tensile strength (TS) of 630 MPa or more and 690 MPa or more, respectively, which are the ranges according to one embodiment of the present invention.

[0142] On the other hand, Example 3 can be confirmed to have an alloy composition including carbon (C) that satisfies all the ranges targeted by the present invention, and the grain size is 3.81 μm.

[0143] As a result, it can be confirmed that Example 3 has refined grains and satisfies both the yield strength (YS) and tensile strength (TS) of 630 MPa or more and 690 MPa or more, respectively, which are the ranges according to one embodiment of the present invention.

[0144] 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.

[0145] [Explanation of symbols]

[0146] S1: Step to prepare steel

[0147] S2: Reheating the steel

[0148] S3: Step of forming hot rolled steel

[0149] S4: Cooling the hot rolled steel

Claims

1. Contains carbon (C) 0.20 to 0.30 wt%, silicon (Si) 0.10 to 0.30 wt%, manganese (Mn) 1.00 to 1.50 wt%, phosphorus (P) 0.030 wt% or less (excluding 0), sulfur (S) 0.030 wt% or less (excluding 0), copper (Cu) 0.14 wt% or more and less than 0.30 wt%, vanadium (V) 0.040 to 0.130 wt%, nitrogen (N) 0.005 to 0.020 wt%, and the remainder iron (Fe) and inevitable impurities. The yield strength (YS) is 550 MPa or more. Steel beam.

2. In paragraph 1, Tensile strength (TS) is 620 MPa or more. Steel beam.

3. In paragraph 1, The elongation (EL) is 15% or more, Steel beam.

4. In paragraph 1, The average grain diameter is 5.6㎛ or less, Steel beam.

5. In paragraph 1, Manufactured using the above steel, Crossmember for trailer. 6.(S1) Step of preparing steel; (S2) A step of reheating the above steel material; (S3) a step of hot rolling the above steel to form a hot-rolled steel; and (S4) A step of cooling the hot-rolled material to form a final product, a shaped steel; Including, but not limited to, The above final product contains carbon (C) 0.20 to 0.30 wt%, silicon (Si) 0.10 to 0.30 wt%, manganese (Mn) 1.00 to 1.50 wt%, phosphorus (P) 0.030 wt% or less (excluding 0), sulfur (S) 0.030 wt% or less (excluding 0), copper (Cu) 0.14 wt% or more and less than 0.30 wt%, vanadium (V) 0.040 to 0.130 wt%, nitrogen (N) 0.005 to 0.020 wt%, and the remainder iron (Fe) and inevitable impurities. The yield strength (YS) is 550 MPa or more. Method for manufacturing shaped steel.

7. In paragraph 6, The reheating temperature of the above step (S2) is 1250 to 1350℃. Method for manufacturing shaped steel.

8. In paragraph 6, The rolling start temperature of the above (S3) step is 1000 to 1100℃. Method for manufacturing shaped steel.

9. In paragraph 6, The rolling end temperature of the above step (S3) is greater than 550℃ and less than or equal to 650℃. Method for manufacturing shaped steel.

10. In paragraph 6, The tensile strength (TS) of the above final product is 620 MPa or more. Method for manufacturing shaped steel.

11. In paragraph 6, The elongation (EL) of the above final product is 15% or more. Method for manufacturing shaped steel.

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