Hot rolled steel and method of manufacturing the same

Abstract

The present invention comprises, in weight%, carbon (C): 0.07% to 0.1%, silicon (Si): 0.5% to 0.7%, manganese (Mn): 1.7% to 1.9%, aluminum (Al): 0.01% to 0.05%, niobium (Nb): 0.05% to 0.07%, titanium (Ti): 0.07% to 0.09%, chromium (Cr): 0.5% to 0.7%, phosphorus (P): greater than 0% to 0.02% or less, sulfur (S): greater than 0% to 0.005% or less, copper (Cu): 0.1% to 0.5%, tin (Sn): 0.001% to 0.05%, nickel (Ni): 0.01% to 0.5%, boron (B): 0.0015% to The present invention provides hot-rolled steel comprising 0.0025%, nitrogen (N): greater than 0% to less than or equal to 0.006%, and the remainder comprising iron (Fe) and unavoidable impurities, satisfying the formula: [Cu] + 10[Sn] - [Ni] ≤ 0.4.

Description

Technology Field

[0001] The technical concept of the present invention relates to steel materials and methods for manufacturing the same, and more specifically, to hot-rolled steel materials and methods for manufacturing the same. Background Technology

[0002] As environmental regulations are tightened across all industries, interest in environmental issues is intensifying in the automotive industry. Consequently, the application of ultra-high-strength steel for component lightweighting is being expanded as a countermeasure. Looking at the steel materials used in automobiles, cold-rolled soft steel sheets are used for exterior panels, while ultra-high-strength cold-rolled steel sheets are used for interior panels to reinforce the body. Meanwhile, thick hot-rolled steel sheets are applied to chassis components connected to the drivetrain to ensure durability. Automotive chassis components are located in the lower part of the vehicle body to connect and support the body, powertrain, steering, and drive components. Since they are subjected to repetitive impacts and loads, high fatigue durability is required. In particular, as chassis components are closely linked to vehicle body lightweighting and safety, high product strength is essential. However, since hole expandability is inversely proportional to strength, increasing product strength leads to a decrease in hole expandability. Therefore, the reduced hole expansion in chassis parts using high-strength steel plates increases the rate of cracking in the burring area, so it is necessary to develop steel plates that guarantee a higher level of elongation and hole expansion than existing ones. Research is continuing to reduce vehicle weight to improve fuel efficiency, and in terms of materials, research is being conducted to achieve lightweighting while securing physical properties suitable for the characteristics of the parts, and 1-giga-class materials require high strength and shear processability.

[0003] Meanwhile, various solutions are being attempted in the steel industry to respond to global carbon neutrality policies. For example, developing plate materials using electric furnaces can reduce the Carbon Footprint (CFP) to 50% of that of blast furnace materials. However, if scrap is used as a raw material along with blast furnace molten iron, the effect of reducing the carbon index may be lower. Therefore, when using scrap, it is necessary to classify the scrap grades (A, B, C) in advance before inputting them. Using Grade A scrap can increase steel production costs and lead to scrap supply issues. Conversely, if lower-grade scrap is used, controlling cyclic elements such as copper (Cu), tin (Sn), nickel (Ni), chromium (Cr), and nitrogen (N) is important; in particular, regarding copper (Cu), excessive enrichment on the surface can cause cracks, so countermeasures may be required. Prior art literature

[0004] Korean Patent Application No. 2009-0103205 The problem to be solved

[0005] The technical problem that the technical concept of the present invention aims to solve is to provide ultra-high strength hot-rolled steel and a method for manufacturing the same, and furthermore, to provide hot-rolled steel and a method for manufacturing the same that can respond to global carbon neutrality policies.

[0006] However, these tasks are exemplary, and the technical concept of the present invention is not limited thereto. means of solving the problem

[0007] According to one aspect of the present invention, hot-rolled steel and a method for manufacturing the same are provided.

[0008] According to one embodiment of the present invention, the hot-rolled steel comprises, in weight%, carbon (C): 0.07% to 0.1%, silicon (Si): 0.5% to 0.7%, manganese (Mn): 1.7% to 1.9%, aluminum (Al): 0.01% to 0.05%, niobium (Nb): 0.05% to 0.07%, titanium (Ti): 0.07% to 0.09%, chromium (Cr): 0.5% to 0.7%, phosphorus (P): greater than 0% to 0.02% or less, sulfur (S): greater than 0% to 0.005% or less, copper (Cu): 0.1% to 0.5%, tin (Sn): 0.001% to 0.05%, nickel (Ni): 0.01% to 0.5%, boron (B): 0.0015% ~ 0.0025%, nitrogen (N): greater than 0% ~ 0.006% or less and the remainder is iron (Fe) and unavoidable impurities, satisfying Formula 1 below.

[0009] Equation 1: [Cu] + 10[Sn] - [Ni] ≤ 0.4

[0010] (The above [Cu], [Sn], and [Ni] are the weight percentage values ​​of copper, tin, and nickel in the composition constituting the hot-rolled steel.)

[0011] The above hot-rolled steel can satisfy a yield strength (YS): 800 MPa or more, a tensile strength (TS): 980 MPa or more, an elongation (EL): 8% or more, and a hole expandability (HER): 25% or more.

[0012] The above hot-rolled steel may have a microstructure composed of bainite and tempered martensite. The area fraction of the bainite may be 80% or more to 95% or less, and the area fraction of the tempered martensite may be 5% or more to 20% or less.

[0013] The above hot-rolled steel may further include a precipitate comprising at least one of carbides, nitrides, and carbonitrides.

[0014] The above hot-rolled steel may have a Carbon Foot Print value of 1.95 or less among the Environmental Product Declaration (EPD) indicators required for UL (Underwriters' Laboratories) certification according to the ISO international standard (ISO 14040).

[0015] According to one embodiment of the present invention, the method for manufacturing the hot-rolled steel comprises (a) in weight%, carbon (C): 0.07% to 0.1%, silicon (Si): 0.5% to 0.7%, manganese (Mn): 1.7% to 1.9%, aluminum (Al): 0.01% to 0.05%, niobium (Nb): 0.05% to 0.07%, titanium (Ti): 0.07% to 0.09%, chromium (Cr): 0.5% to 0.7%, phosphorus (P): greater than 0% to 0.02% or less, sulfur (S): greater than 0% to 0.005% or less, copper (Cu): 0.1% to 0.5%, tin (Sn): 0.001% to 0.05%, nickel (Ni): 0.01% to 0.5%, boron (B): The method comprises: (b) preparing a steel material containing 0.0015% to 0.0025%, nitrogen (N): greater than 0% to 0.006% or less, and the remainder being iron (Fe) and unavoidable impurities, satisfying Formula 1 below; (b) reheating the steel material at a reheating temperature of 1180°C to 1220°C; (c) hot rolling the reheated steel material to end at a finishing rolling end temperature of 860°C to 900°C; (d) cooling the hot-rolled steel material; and (e) coiling the cooled steel material at a coiling temperature of 420°C to 480°C.

[0016] Equation 1: [Cu] + 10[Sn] - [Ni] ≤ 0.4

[0017] (The above [Cu], [Sn], and [Ni] are the weight percentage values ​​of copper, tin, and nickel in the composition constituting the hot-rolled steel.)

[0018] In the method for manufacturing the hot-rolled steel above, the step of cooling the hot-rolled steel may include: a first cooling step of cooling the steel to 640°C to 680°C at a cooling rate of 70°C / sec to 100°C / sec; a second cooling step of air cooling the steel at an air cooling start temperature of 640°C to 680°C for an air cooling time of 4 seconds to 6 seconds at a cooling rate of 1°C / sec to 10°C / sec; and a third cooling step of cooling the steel to 420°C to 480°C at a cooling rate of 70°C / sec to 100°C / sec.

[0019] In the above method for manufacturing hot-rolled steel, the step of preparing the steel includes: a step of providing a molten metal; and a step of manufacturing a semi-finished product from the molten metal; and the molten metal may be provided using 80% or less of blast furnace molten iron and 20% or more of electric furnace molten steel in terms of total weight%.

[0020] In the above method for manufacturing hot-rolled steel, at least a portion of the raw material may include iron scrap in the step of providing the molten metal.

[0021] In the above method for manufacturing hot-rolled steel, the step of providing the molten metal may be characterized by being prepared by adding Direct Reduced Iron (DRI) or Hot Briquetted Iron (HBI) as direct reduced iron.

[0022] In the above method for manufacturing hot-rolled steel, the semi-finished product may be any one of a slab, a billet, and a bloom.

[0023] In the method for manufacturing the above hot-rolled steel, the hot-rolled steel manufactured after performing the step of coiling the cooled steel can satisfy a yield strength (YS): 800 MPa or more, a tensile strength (TS): 980 MPa or more, an elongation (EL): 8% or more, and a hole expandability (HER): 25% or more.

[0024] In the method for manufacturing the above hot-rolled steel, the hot-rolled steel manufactured after performing the step of coiling the cooled steel has a microstructure composed of bainite and tempered martensite, wherein the area fraction of the bainite is 80% or more to 95% or less, and the area fraction of the tempered martensite is 5% or more to 20% or less. Effects of the invention

[0025] According to the technical concept of the present invention, it is possible to realize a hot-rolled steel that does not cause surface cracks while satisfying a yield strength (YS): 800 MPa or more, a tensile strength (TS): 980 MPa or more, an elongation (EL): 8% or more, and a hole expandability (HER): 25% or more, and furthermore, a hot-rolled steel and a method for manufacturing the same that can respond to global carbon neutrality policies can be realized.

[0026] The effects of the present invention described above are illustrative and the scope of the present invention is not limited by these effects. Brief explanation of the drawing

[0027] FIG. 1 is a process flowchart schematically showing a method for manufacturing hot-rolled steel according to an embodiment of the present invention. Figure 2 is a photograph showing whether cracks occurred on the surface of hot-rolled steel according to a comparative example among the experimental examples of the present invention. Figure 3 is a photograph showing whether cracks occurred on the surface of hot-rolled steel according to an embodiment of the experimental example of the present invention. Figure 4 is a graph showing the correlation between the composition and whether cracks occur in hot-rolled steel according to an experimental example of the present invention. Specific details for implementing the invention

[0028] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. The embodiments of the present invention are provided to more completely explain the technical concept of the present invention to those skilled in the art, and the following embodiments may be modified in various different forms, and the scope of the technical concept of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to make the present disclosure more faithful and complete and to fully convey the technical concept of the present invention to those skilled in the art. In this specification, the same reference numerals denote the same elements throughout. Furthermore, various elements and areas in the drawings are depicted schematically. Accordingly, the technical concept of the present invention is not limited by the relative sizes or spacing depicted in the attached drawings.

[0029] Hereinafter, a hot-rolled steel material, which is one aspect of the present invention, will be described.

[0030] Hot-rolled steel

[0031] A hot-rolled steel material, which is one aspect of the present invention, comprises, in weight%, carbon (C): 0.07% to 0.1%, silicon (Si): 0.5% to 0.7%, manganese (Mn): 1.7% to 1.9%, aluminum (Al): 0.01% to 0.05%, niobium (Nb): 0.05% to 0.07%, titanium (Ti): 0.07% to 0.09%, chromium (Cr): 0.5% to 0.7%, phosphorus (P): greater than 0% to 0.02% or less, sulfur (S): greater than 0% to 0.005% or less, copper (Cu): 0.1% to 0.5%, tin (Sn): 0.001% to 0.05%, nickel (Ni): 0.01% to 0.5%, boron (B): 0.0015% to 0.0025%, nitrogen (N): greater than 0% to less than or equal to 0.006%, and the remainder consists of iron (Fe) and unavoidable impurities. The steel sheet may be made of the composition and content of the elements described above.

[0032] The role and content of each component included in the hot-rolled steel according to the present invention are described below. In this case, the content of the component elements refers to weight percent.

[0033] Carbon (C)

[0034] Carbon can be added in an amount of 0.07% to 0.1% of the total weight of the hot-rolled steel. The carbon (C) is an element that contributes to increasing the strength of the steel. If the amount of carbon added is less than 0.07% by weight, the fraction of precipitates and the secondary phase is small, making it difficult to secure the target material and difficult to secure strength. If the carbon content exceeds 0.1% by weight, the fraction of the secondary phase increases, and pearlite is formed, which may lead to a decrease in material properties such as reduced strength. Additionally, as the fraction of the secondary phase with hard properties increases, a problem may arise where hole expansion is reduced, and the formation of carbides in the steel is promoted, which may lead to a decrease in formability and shear processing characteristics.

[0035] Silicon (Si)

[0036] Silicon can be added in an amount of 0.5% to 0.7% of the total weight of the hot-rolled steel. The silicon (Si) acts as a deoxidizer and is an element capable of increasing strength without reducing ductility. Furthermore, it can suppress the formation of carbides after the hot-rolled sheet of the present invention is cooled and coiled. Additionally, silicon is added for the purpose of ferrite solid solution strengthening and securing elongation. Silicon forms a ferrite structure and can diffuse carbon toward austenite at high temperatures. Furthermore, as a ferrite stabilizing element, silicon increases the degree of supercooling during ferrite transformation, thereby refining the grain size and suppressing the formation of harmful carbides. However, when added in large quantities, the rolling load may increase or segregation may occur, weldability may decrease, red scale may form during the hot rolling process, degrading surface quality, and plating properties may decrease after welding. If the silicon content is less than 0.5% by weight, it is difficult to suppress the formation of cementite during transformation, so the effect of improving elongation cannot be obtained. When the silicon content exceeds 0.7 wt%, the carbon content increases toward the austenite, so the separation of the two is good, but the difference in hardness with the ferrite matrix increases, the hole expansion ability decreases, and the yield strength also decreases.

[0037] Manganese (Mn)

[0038] Manganese is an element that enhances the strength and workability of steel plates and can be added at a concentration of 1.7% to 1.9% of the total weight of hot-rolled steel. Manganese is a hardenable element added for the purpose of ensuring strength; however, excessive addition leads to problems such as the formation of segregation zones and band structures, as well as the formation and increase of manganese sulfide (MnS) inclusions. Manganese is an essential element for ensuring strength and, as an austenite-stabilizing element, is highly effective for solid solution strengthening and significantly influences the increase in hardenability. While strength, toughness, and yield ratio can be controlled depending on the manganese content, adding large amounts leads to the formation of manganese sulfide inclusions and center segregation during casting, thereby reducing the toughness of the steel. If the manganese content is less than 1.7 wt%, it may be difficult to ensure sufficient strength. If the manganese content exceeds 1.9 wt%, weldability deteriorates; although strength increases, segregation zones are prone to forming in the center of the steel, and the formation of martensite within these zones significantly hinders the formation of the desired structure. In particular, while the addition of manganese (Mn) is very effective for securing a tensile strength of 980 MPa or higher, it is desirable to add it in an amount of 1.9 weight% or less to simultaneously obtain high ductility.

[0039] Aluminum (Al)

[0040] Aluminum can be added in an amount of 0.01% to 0.05% of the total weight of hot-rolled steel. Aluminum acts as a deoxidizer to remove oxygen from the steel. Additionally, it can contribute to grain refinement through the formation of precipitates (e.g., AlN). If the aluminum content is less than 0.01%, the deoxidation effect is negligible and the effect of adding aluminum is insufficient. If the aluminum content exceeds 0.05%, ductility and toughness decrease, and nitrides (AlN) may precipitate by reacting with nitrogen during continuous casting, which can cause corner cracks or surface defects in the slab.

[0041] Niobium (Nb)

[0042] Niobium can be added in an amount of 0.05% to 0.07% of the total weight of the hot-rolled steel. Niobium is an element added to ensure strength and refine grain size through grain refinement via the formation of precipitates. At high temperatures, niobium combines with carbon and nitrogen contained in the steel to form carbides (NbC) or nitrides (NbN). These niobium-based carbides or nitrides inhibit recrystallization and grain growth during hot rolling, thereby refining the grains and improving both the strength and toughness of the hot-rolled steel. If the niobium content is less than 0.05 wt%, the effect of increasing strength is negligible. If the niobium content exceeds 0.07 wt%, a band structure develops, and the load increases during hot rolling. When rolling thin steel materials, the rolling load increases, making rolling difficult. To roll the above-mentioned steel thin material, it is desirable to add niobium in an amount of 0.05% to 0.07% to produce a grain refinement effect, and to additionally add titanium to compensate for the strength, thereby appropriately designing the composition.

[0043] Titanium (Ti)

[0044] Titanium can be added in an amount of 0.07% to 0.09% of the total weight of the steel as an element that improves the strength of steel through grain refinement and the formation of precipitates with excellent high-temperature stability. Titanium generates Ti(C, N) precipitates with high high-temperature stability, thereby refining the microstructure, which improves the toughness and strength of the steel, and also strengthens the ferrite matrix to improve hole expansion. If the titanium content is less than 0.07% by weight, it is difficult to exert the precipitation effect, and the strength may fall short of the target value. If the titanium content exceeds 0.09% by weight, coarse precipitates are formed, which may actually degrade the impact properties of the steel, and the slab quality deteriorates due to vulnerability to high-temperature ductility. Furthermore, there is a problem of increasing manufacturing costs as the reheating temperature for solid solution increases significantly.

[0045] Chrome (Cr)

[0046] In the present invention, chromium may be included in an amount of 0.5% to 0.7% of the total weight of the hot-rolled steel as an element that increases the temperature range in which a bainite microstructure is formed. Chromium is a hardenable element added for the purpose of securing strength, and it is an element that facilitates the formation of low-temperature phases by suppressing the formation of high-temperature ferrite. If the chromium content is less than 0.5% by weight, the effect of the addition is negligible. If the chromium content exceeds 0.7% by weight, the balance between strength and ductility of the hot-rolled steel sheet may be disrupted, the brittleness of the steel may increase, and toughness or workability may decrease.

[0047] Ph(P)

[0048] Phosphorus may be included in an amount greater than 0% to 0.02% by weight of the total weight of the hot-rolled steel. Phosphorus degrades weldability and reduces corrosion resistance by forming central and micro-segregations. Additionally, since it can degrade toughness by segregating at austenite grain boundaries, it is desirable to limit the amount to a range of 0.02% or less. Excessive addition of phosphorus reduces hole expansion properties through intergranular embrittlement.

[0049] Yellow (S)

[0050] Sulfur may be included in an amount exceeding 0% by weight to 0.005% by weight of the total weight of the hot-rolled steel. Sulfur is an element inevitably contained during the manufacture of steel along with phosphorus; as a low-melting-point element, it has a high potential for grain boundary segregation, which impairs toughness and weldability and increases MnS non-metallic inclusions, thereby reducing the corrosion resistance of the steel; therefore, it is desirable to limit the sulfur content to a range of 0.005% by weight or less. When sulfur is added in excess, MnS inclusions that combine with manganese form elongated inclusions during rolling, which reduces hole expansion properties.

[0051] Copper (Cu)

[0052] Copper (Cu) is dissolved up to 0.35 wt% at room temperature, and strength and hardness can be improved through the solid solution strengthening effect. In addition, when copper is added at 0.4 wt% or more, a precipitation hardening effect is exhibited due to the fine precipitation of copper, so hardness can be further improved. Furthermore, when copper is added, the infiltration of hydrogen (H2) into the steel is suppressed, thereby improving corrosion resistance.

[0053] Meanwhile, copper may be incorporated from iron scrap. Copper is an element that is hardly removed during the steelmaking process and corresponds to a tramp element. Accordingly, the hot-rolled steel according to one embodiment of the present invention may contain at least 0.1 weight percent of copper, and if the copper content is insufficient, it may be added separately.

[0054] Copper has a melting point of 1085°C; therefore, if hot working is performed at 1100°C or higher, the copper may liquefy and float to the surface of the steel. Consequently, a copper enrichment phase may form on the surface of the steel, forming a copper enrichment layer. If an excessive copper enrichment layer is formed on the surface of the steel, the copper enrichment phase may penetrate into the grains of the steel, causing brittleness in the steel. In one embodiment of the present invention, the excessive formation of the copper enrichment layer can be suppressed or prevented through Equation 1, which will be described later. That is, the value of [Cu] + 10[Sn] - [Ni] calculated in Equation 1 is the copper equivalent (Cu eq It can be understood as follows. In one embodiment of the present invention, copper may be included up to 0.5 weight%. If the copper content exceeds 0.5 weight%, an excessive copper enrichment layer may form on the surface of the steel, or the copper enrichment phase may penetrate into the grains of the steel, causing brittleness in the steel. In addition, if the copper content exceeds 0.5 weight%, a significant amount of nickel must be added to reduce the copper equivalent, which increases the manufacturing cost of the steel and may result in problems such as reduced weldability and toughness.

[0055] Equation 1: [Cu] + 10[Sn] - [Ni] ≤ 0.4

[0056] (The above [Cu], [Sn], and [Ni] are the weight percentage values ​​of copper, tin, and nickel in the composition constituting the hot-rolled steel.)

[0057] Comment (Sn)

[0058] Tin (Sn) impairs the non-uniformity of phosphate size and the stability of the phosphate adhesion amount when a phosphate film is formed on steel. Tin (Sn) can be incorporated from iron scrap. Like copper, tin is an element that is hardly removed during the steelmaking process and is classified as a tramp element. Accordingly, hot-rolled steel according to one embodiment of the present invention may contain at least 0.001 weight% of tin, and may be added separately if the tin content is insufficient. Although tin can improve high-temperature properties and corrosion resistance by existing in a solid solution state within the steel, since the melting point of tin is 231.9°C, it may form an enrichment phase together with copper during hot working, thereby causing brittleness in the steel. As the amount of added Cu increases, the surface enrichment phenomenon becomes more severe even with a slight increase in Sn content. If the tin content exceeds 0.05%, brittleness may be reduced due to the penetration of the enrichment phase. A hot-rolled steel according to one embodiment of the present invention may contain 0.001 to 0.05 weight percent of tin (Sn).

[0059] Nickel (Ni)

[0060] Nickel (Ni) may be incorporated from iron scrap. Nickel is an element that is hardly removed during the steelmaking process, similar to copper and tin, and is classified as a tramp element. Accordingly, the hot-rolled steel according to one embodiment of the present invention may contain at least 0.01 weight% of nickel, and if the nickel content is insufficient, it may be added separately. Nickel can improve the strength by enhancing the hardenability of the steel. In addition, nickel can react with copper or tin to form a complete solid solution, thereby suppressing or preventing the deterioration of hot workability caused by copper or tin. Specifically, when Ni is added in combination with Cu, the solid solution temperature of Cu increases, thereby suppressing surface enrichment. If the nickel content is insufficient, the aforementioned additive effect may be negligible. Conversely, if the nickel content is excessive, the manufacturing cost of the steel increases, and weldability and toughness may decrease. Therefore, the hot-rolled steel according to one embodiment of the present invention may contain 0.01 to 0.5 weight% of nickel.

[0061] Boron (B)

[0062] Boron (B) is included for the purpose of suppressing the formation of a ferrite microstructure in the hot-rolled steel sheet of the present invention. In one embodiment of the present invention, the boron is included in an amount greater than 0.0015% and less than or equal to 0.0025% by weight with respect to the total weight of the steel slab. If the boron is included in an amount less than 0.0015% by weight or in an amount greater than 0.0025% by weight, a ferrite structure may be formed, which may degrade the shear processing characteristics.

[0063] Nitrogen (N)

[0064] Even the presence of extremely small amounts of nitrogen (N) can have a significant effect on the mechanical properties of steel. For example, nitrogen (N) can increase tensile strength and yield strength while decreasing elongation. Additionally, free nitrogen can cause strain aging. In one embodiment of the present invention, the nitrogen (N) content may be 60 ppm or less.

[0065] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0066] By controlling the specific components of the alloy composition and the content range thereof as described above, the hot-rolled steel material realized through the method for manufacturing hot-rolled steel material described below can satisfy a yield strength (YS): 800 MPa or more, a tensile strength (TS): 980 MPa or more, an elongation (EL): 8% or more, and a hole expandability (HER): 25% or more. For example, the hot-rolled steel material can satisfy a yield strength (YS): 800 MPa to 850 MPa, a tensile strength (TS): 980 MPa to 1050 MPa, an elongation (EL): 8% to 15%, and a hole expandability (HER): 25% to 40%.

[0067] The above hot-rolled steel may have a microstructure composed of bainite and tempered martensite. The area fraction of the bainite may be 80% or more to 95% or less, and the area fraction of the tempered martensite may be 5% or more to 20% or less. The above area fraction refers to the area ratio derived from a microstructure photograph of the steel using an image analyzer.

[0068] The above hot-rolled steel may further include a precipitate comprising at least one of carbides, nitrides, and carbonitrides, such as titanium and niobium.

[0069] The above hot-rolled steel may have a Carbon Foot Print value of 1.95 or less among the Environmental Product Declaration (EPD) indicators required for UL (Underwriters' Laboratories) certification according to the ISO international standard (ISO 14040).

[0070] Another aspect of the present invention provides a method for manufacturing hot-rolled steel.

[0071] The method for manufacturing the above hot-rolled steel comprises, in weight%, carbon (C): 0.07% ~ 0.1%, silicon (Si): 0.5% ~ 0.7%, manganese (Mn): 1.7% ~ 1.9%, aluminum (Al): 0.01% ~ 0.05%, niobium (Nb): 0.05% ~ 0.07%, titanium (Ti): 0.07% ~ 0.09%, chromium (Cr): 0.5% ~ 0.7%, phosphorus (P): greater than 0% ~ 0.02% or less, sulfur (S): greater than 0% ~ 0.005% or less, copper (Cu): 0.1% ~ 0.5%, tin (Sn): 0.001% ~ 0.05%, nickel (Ni): 0.01% ~ 0.5%, boron (B): (a) a step of preparing steel (S100), wherein the steel contains 0.0015% to 0.0025%, nitrogen (N): greater than 0% to 0.006% or less, and the remainder is iron (Fe) and unavoidable impurities, satisfying Formula 1 below; (b) a step of reheating the steel at a reheating temperature of 1180℃ to 1220℃ (S110); (c) a step of hot rolling the reheated steel so as to be finished at a finishing rolling end temperature of 860℃ to 900℃ (S120); (d) a step of cooling the hot-rolled steel (S130); and (e) a step of coiling the cooled steel at a coiling temperature of 420℃ to 480℃ (S140).

[0072] The step of cooling the hot-rolled steel may include: a first cooling step of cooling the steel to 640°C to 680°C at a cooling rate of 70°C / sec to 100°C / sec; a second cooling step of air cooling the steel at an air cooling start temperature of 640°C to 680°C for an air cooling time of 4 seconds to 6 seconds at a cooling rate of 1°C / sec to 10°C / sec; and a third cooling step of cooling the steel to 420°C to 480°C at a cooling rate of 70°C / sec to 100°C / sec.

[0073] A method for manufacturing hot-rolled steel according to the present invention will be described below with reference to the attached drawings.

[0074] Generally, steel manufacturing methods include the blast furnace-converter process and the electric arc furnace process. The blast furnace-converter process involves charging iron ore and coke into a blast furnace (with optionally added iron scrap) and melting them with hot air to produce molten iron, as well as charging the molten iron discharged from the blast furnace into a converter to remove impurities such as carbon and produce molten steel. The electric arc furnace process involves melting steel scrap in an electric arc furnace (EAF) to produce molten steel. The molten steel produced by each of the aforementioned methods is processed into semi-finished products through a continuous casting process, and these semi-finished products are then produced into final products with dimensions suitable for customer needs through a subsequent rolling process.

[0075] Meanwhile, rapid climate change caused by greenhouse gases emitted from various industrial sectors has recently become a major international issue. Carbon dioxide is a representative greenhouse gas. The steel industry is known to have a high proportion of carbon dioxide emissions among various industrial sectors. For example, the blast furnace-converter process emits a large amount of carbon dioxide because it utilizes carbon monoxide (CO), generated by the combustion of coke, as a reducing agent.

[0076] Consequently, major steel companies are currently focusing on developing electric furnace processes, which emit relatively less carbon dioxide compared to the blast furnace-converter process. However, electric furnace products that use steel scrap as their primary raw material present a problem regarding high levels of tramp elements. Tramp elements refer to impure alloying elements present in trace amounts in steel products. Specifically, tramp elements tend to accumulate at the interface rather than being easily dissolved in the base iron. Consequently, if tramp elements exceed permissible limits, they can cause cracks on the surface as well as the interior during rolling. Furthermore, tramp elements can act as a major factor in hot brittleness, which significantly reduces ductility at high temperatures.

[0077] Method for manufacturing hot-rolled steel

[0078] FIG. 1 is a process flowchart schematically showing a method for manufacturing hot-rolled steel according to an embodiment of the present invention.

[0079] The step of preparing the steel material (S100) includes: a step of providing a molten metal; and a step of manufacturing a semi-finished product from the molten metal.

[0080] The above molten metal may be provided using 80% or less of blast furnace molten iron and 20% or more of electric furnace molten steel in terms of total weight percent. For example, the above molten metal may be provided with a composition of 50% or more to 80% or less of blast furnace molten iron and 20% or more to 50% of electric furnace molten steel in terms of total weight percent.

[0081] In the step of providing the molten metal mentioned above, at least a portion of the raw materials may include iron scrap. When using scrap, it is necessary to classify the scrap grades (A, B, C) in advance and input them accordingly. Using Grade A scrap may increase steel production costs and lead to scrap supply issues. Conversely, if lower-grade scrap is used, controlling cyclic elements such as copper (Cu), tin (Sn), nickel (Ni), chromium (Cr), and nitrogen (N) is important; in particular, regarding copper (Cu), an excessive enrichment layer on the surface can cause cracks, so countermeasures may be required.

[0082] The step of providing the molten metal may include adding Direct Reduced Iron (DRI) or Hot Briquetted Iron (HBI) as direct reduced iron to improve the quality of the molten metal and reduce carbon emissions. DRI is iron produced by directly reducing iron ore using a reducing gas (mainly natural gas), and HBI is manufactured by compressing DRI at high temperatures; it is designed to enhance the physical stability of DRI and facilitate transportation and storage. While DRI and HBI are primarily used in the Electric Arc Furnace (EAF) process, they may also be partially added in place of iron ore used in blast furnaces.

[0083] In the method for manufacturing steel according to the present invention, the semi-finished product subject to the hot rolling process may be, for example, a slab, a billet, or a bloom. The steel in the semi-finished product state can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

[0084] The above steel comprises, in weight%, Carbon (C): 0.07% ~ 0.1%, Silicon (Si): 0.5% ~ 0.7%, Manganese (Mn): 1.7% ~ 1.9%, Aluminum (Al): 0.01% ~ 0.05%, Niobium (Nb): 0.05% ~ 0.07%, Titanium (Ti): 0.07% ~ 0.09%, Chromium (Cr): 0.5% ~ 0.7%, Phosphorus (P): greater than 0% ~ 0.02% or less, Sulfur (S): greater than 0% ~ 0.005% or less, Copper (Cu): 0.1% ~ 0.5%, Tin (Sn): 0.001% ~ 0.05%, Nickel (Ni): 0.01% ~ 0.5%, Boron (B): 0.0015% ~ 0.0025%, nitrogen (N): greater than 0% to less than or equal to 0.006% and the remainder may contain iron (Fe) and unavoidable impurities. For example, the steel sheet may be made of the composition and content of the elements described above.

[0085] Furthermore, the above steel satisfies the following Equation 1.

[0086] Equation 1: [Cu] + 10[Sn] - [Ni] ≤ 0.4

[0087] (The above [Cu], [Sn], and [Ni] are the weight percentage values ​​of copper, tin, and nickel in the composition constituting the hot-rolled steel.)

[0088] Referring to FIG. 1, a method for manufacturing hot-rolled steel according to an embodiment of the present invention includes, after preparing (S100) a steel having the composition described above, a reheating step (S110), a hot rolling step (S120), a cooling step (S130), and a coiling step (S140).

[0089] Reheating step (S110)

[0090] In the reheating step (S110), steel having the above composition, for example, a slab plate, is reheated at a slab reheating temperature (SRT) of 1180°C to 1220°C for about 60 minutes to 180 minutes. Through this reheating, the re-dissolution of components segregated during casting and the re-dissolution of precipitates containing at least one of carbides, nitrides, and carbonitrides, such as titanium or niobium, may occur.

[0091] If the reheating temperature is below 1180°C, segregated components and high-temperature precipitates formed from carbonitrides cannot be re-dissolved, and there is a problem in that the segregated components are not sufficiently evenly distributed during casting. If the reheating temperature exceeds 1220°C, the austenite grain size increases, causing the ferrite grain size to coarsen, which may result in a decrease in strength. In addition, it is necessary to adjust the holding time of the reheating temperature according to the thickness of the slab; the reheating time needs to be longer as the thickness increases, and shorter as the thickness decreases. If reheating is performed for less than 60 minutes, the re-dissolution of niobium carbides, etc., may be insufficient, and the degree of homogenization may be reduced, leading to quality degradation. If reheating is performed for more than 180 minutes, there are problems such as increased manufacturing costs and reduced productivity due to heating costs and the time required to extract to match the hot rolling temperature.

[0092] Hot rolling step (S120)

[0093] The above-mentioned heated steel is first subjected to hot rolling after heating to adjust its shape. The hot rolling can be performed sequentially in width rolling, rough rolling, and finish rolling. Through the hot rolling step, the steel can form a steel plate.

[0094] The above hot rolling may be terminated at a finish delivery temperature (FDT) of 860°C to 900°C. This temperature range may be selected to maintain the austenite phase. The above rolling delivery temperature may affect the ferrite grain size after transformation. If the above finishing delivery temperature is below 860°C, the reduction force during rolling becomes excessive, which has a negative effect on the rollability. If the above finishing delivery temperature exceeds 900°C, the microstructure becomes too coarse, and abnormal separation does not occur properly in the intermediate air cooling section, resulting in a problem of reduced strength and ductility.

[0095] Cooling step (S130)

[0096] The step of cooling the hot-rolled steel may include, for example, a step of cooling the hot-rolled steel. The cooling (step cooling) step may include: a step of first cooling the steel to 640°C to 680°C at a cooling rate of 70°C / sec to 100°C / sec on a run-out table (ROT); a step of second cooling the steel at an air cooling start temperature of 640°C to 680°C for an air cooling time of 4 seconds to 6 seconds at a cooling rate of 1°C / sec to 10°C / sec; and a step of third cooling the steel to 420°C to 480°C at a cooling rate of 70°C / sec to 100°C / sec.

[0097] Winding stage (S140)

[0098] After the above third cooling is completed, the steel is coiled at a coiling temperature (CT) of 420°C to 480°C. If the coiling temperature is 420°C or lower, martensite is formed, which may reduce hole expansion and elongation due to increased strength. If the coiling temperature exceeds 480°C, coarsening of precipitates and formation of grain boundary precipitates may occur, which may result in material reduction.

[0099] By controlling the specific components of the alloy composition and the content range thereof as described above, the hot-rolled steel material realized through the method for manufacturing hot-rolled steel material described below can satisfy a yield strength (YS): 800 MPa or more, a tensile strength (TS): 980 MPa or more, an elongation (EL): 8% or more, and a hole expandability (HER): 25% or more. For example, the hot-rolled steel material can satisfy a yield strength (YS): 800 MPa to 950 MPa, a tensile strength (TS): 980 MPa to 1100 MPa, an elongation (EL): 8% to 15%, and a hole expandability (HER): 25% to 40%.

[0100] The hot-rolled steel may have a microstructure composed of bainite and tempered martensite. The area fraction of the bainite may be 80% or more to 95% or less, and the area fraction of the tempered martensite may be 5% or more to 20% or less. The hot-rolled steel may further include precipitates comprising at least one of carbides, nitrides, and carbonitrides, such as titanium and niobium.

[0101] The above hot-rolled steel may have a Carbon Foot Print value of 1.95 or less among the Environmental Product Declaration (EPD) indicators required for UL (Underwriters' Laboratories) certification according to the ISO international standard (ISO 14040).

[0102] The Environmental Product Declaration (EPD) is a system that quantitatively indicates the environmental impact of a product / service throughout its entire lifecycle, including raw material extraction, production, transportation / distribution, use, and disposal, in order to enhance the environmental performance of products and services. Table 1 shows the seven items that constitute the EPD indicators.

[0103] Indicator name explanation Resource footprint Global impact resulting from the development and consumption of minerals and fossil fuels carbon footprint The impact of greenhouse gases, such as carbon dioxide, released into the atmosphere on global climate change ozone layer impact The impact of ozone-depleting substances, such as Freon gas emitted into the atmosphere, on the stratospheric ozone layer acid rain The impact of atmospheric acidic substances dissolving in rainwater and falling to the surface on human activities and ecosystems Eutrophication Ecosystem impact resulting from excessive concentrations of organic substances, such as nitrogen and phosphorus, in the atmosphere, water systems, and soil Photochemical smog Impact on the human body and ecosystems caused by surface pollutants generated by the reaction of active substances produced by human activities with light water footprints The impact of human activities, such as agriculture and industry, on water resources, including water quality and quantity

[0104] Referring to Table 1, the carbon footprint is an indicator representing the impact of greenhouse gases, such as carbon dioxide released into the atmosphere, on global climate change. The formula for calculating the carbon footprint is as follows.

[0105] Carbon Footprint = ∑ (Activity × Emission Factor)

[0106] The above activity amount refers to various activity indicators such as energy consumption, production volume, and travel distance, and the above emission factor is a coefficient representing the amount of carbon dioxide emitted by the activity, which may include the amount of carbon dioxide (CO2) generated during the combustion of a specific fuel.

[0107] In the steel industry, the total carbon footprint can be calculated by multiplying emission factors by the volume of all activities occurring in the mining and transportation of raw materials and the steel production process. For example, it can be calculated by considering the amount of carbon dioxide (CO2) emitted per ton of steel produced.

[0108] Meanwhile, in the method for manufacturing hot-rolled steel according to the present invention, the step of preparing the steel before performing the hot-rolling process has been described primarily as a composite process of mixing blast furnace molten iron and electric furnace molten steel to produce a molten metal, but is not limited thereto. As a modified embodiment, it may be composed of a process of producing a molten metal using only electric furnace molten steel or a process of producing a molten metal using only blast furnace molten iron.

[0109] Experimental Example

[0110] The following describes experimental examples to aid in understanding the present invention. The following experimental examples are presented to aid in understanding the invention and are not limited to the following experimental examples of the present invention.

[0111] Hot-rolled steel was manufactured having the compositions presented in Tables 2 and 3 below. The remainder consists of iron (Fe) and impurities inevitably contained during the steelmaking process. The unit of content for each component is weight% or ppm as indicated in the table. In Table 2, a downward arrow (↓) indicates that the value is below the reference value.

[0112] C(wt%) Si(wt%) Mn(wt%) P(ppm) S(ppm) Al(wt%) Cr(wt%) Ti(wt%) Nb(wt%) B(ppm) Implementation 1 0.073 0.6 1.8 127 50↓ 0.04 0.55 0.075 0.065 25 Implementation 2 0.081 0.63 1.79 131 50↓ 0.03 0.6 0.08 0.06 15 Implementation 3 0.088 0.6 1.79 121 50↓ 0.05 0.65 0.085 0.055 20 Implementation 4 0.078 0.59 1.8 118 50↓ 0.02 0.55 0.075 0.065 25 Implementation 5 0.071 0.65 1.81 123 50↓ 0.01 0.6 0.08 0.06 15 Comparison 1 0.07 0.6 1.9 200↓ 50↓ 0.04 0.65 0.085 0.055 20 Comparison 2 0.1 0.5 1.8 200↓ 50↓ 0.02 0.65 0.085 0.055 20 Comparison 3 0.06 0.6 1.5 200↓ 50↓ 0.04 0.65 0.085 0.055 20 Comparison 4 0.13 0.7 2.1 200↓ 50↓ 0.04 0.65 0.085 0.055 20 Comparison 5 0.07 0.6 1.9 200↓ 50↓ 0.04 0.55 0.075 0.065 25 Comparison 6 0.09 0.6 1.8 200↓ 50↓ 0.04 0.55 0.075 0.065 25

[0113] Cu(wt%) Sn(wt%) Ni(wt%) [Cu] + 10[Sn] - [Ni] Implementation 1 0.1 0.001 0.01 0.1 Implementation 2 0.2 0.01 0.1 0.2 Implementation 3 0.3 0.02 0.2 0.3 Implementation 4 0.4 0.015 0.2 0.35 Implementation 5 0.5 0.01 0.3 0.3 Comparison 1 0.3 0.03 0.001 0.6 Comparison 2 0.5 0.01 0.001 0.6 Comparison 3 0.15 0.03 0.02 0.43 Comparison 4 0.3 0.01 0.02 0.38 Comparison 5 0.005 0.0001 0.001 0.01 Comparison 6 0.01 0.001 0.001 0.02

[0114] Referring to Tables 2 and 3, the composition of the hot-rolled steel sheets according to Examples 1 to 5 is, in weight%, Carbon (C): 0.07% to 0.1%, Silicon (Si): 0.5% to 0.7%, Manganese (Mn): 1.7% to 1.9%, Aluminum (Al): 0.01% to 0.05%, Niobium (Nb): 0.05% to 0.07%, Titanium (Ti): 0.07% to 0.09%, Chromium (Cr): 0.5% to 0.7%, Phosphorus (P): greater than 0% to 0.02% or less, Sulfur (S): greater than 0% to 0.005% or less, Copper (Cu): 0.1% to 0.5%, Tin (Sn): 0.001% to 0.05%, Nickel (Ni): 0.01% to 0.5%, The composition range satisfies Boron (B): 0.0015% ~ 0.0025%, Nitrogen (N): greater than 0% ~ 0.006% or less, and the remainder is Iron (Fe), and satisfies the following Formula 1.

[0115] Equation 1: [Cu] + 10[Sn] - [Ni] ≤ 0.4

[0116] (The above [Cu], [Sn], and [Ni] are the weight percentage values ​​of copper, tin, and nickel in the composition constituting the hot-rolled steel.)

[0117] In contrast, Comparative Example 1 and Comparative Example 2 do not satisfy the range of nickel (Ni): 0.01% to 0.5% and fall below it, and do not satisfy Equation 1 above.

[0118] Comparative Example 3 does not satisfy the range of carbon (C): 0.07% ~ 0.1% and manganese (Mn): 1.7% ~ 1.9% and falls below it, and does not satisfy Equation 1 above.

[0119] Comparative Example 4 does not satisfy the range of carbon (C): 0.07% ~ 0.1% and manganese (Mn): 1.7% ~ 1.9%, and exceeds it.

[0120] Comparative Example 5 does not satisfy the range of copper (Cu): 0.1% ~ 0.5%, tin (Sn): 0.001% ~ 0.05%, and nickel (Ni): 0.01% ~ 0.5%, and falls below that range, and Comparative Example 6 does not satisfy the range of copper (Cu): 0.1% ~ 0.5% and nickel (Ni): 0.01% ~ 0.5%, and falls below that range.

[0121] Meanwhile, in the experimental examples of the present invention, the process conditions disclosed in FIG. 1 were applied to specimens having the composition described above. In particular, in the step of preparing steel (S100), Examples 1 to 5 and Comparative Examples 1 to 4 correspond to a composite process of producing molten metal by mixing blast furnace molten iron and electric furnace molten steel, and iron scrap and direct reduced iron were added as raw materials. In contrast, Comparative Examples 5 to 6 applied a process of producing molten metal using only blast furnace molten iron. Furthermore, in performing the reheating step (S110), hot rolling step (S120), cooling step (S130), and coiling step (S140) in the experimental examples, a single process condition was applied in common while satisfying the process range according to the technical concept of the present invention described above. For example, conditions were applied such as reheat temperature: 1200℃, finish rolling end temperature: 880℃, first cooling end temperature: 660℃, first cooling rate: 80℃ / sec, second cooling time: 5 sec, third cooling end temperature: 450℃, third cooling rate: 80℃ / sec, coiling temperature: 450℃.

[0122] Table 4 shows the physical properties, carbon footprint (CFP), and whether surface cracks occur in hot-rolled steel according to the experimental example of the present invention.

[0123] TS(MPa) YS(MPa) El(%) HER(%) CFP crack Implementation 1 995 806 13 33 1.95 or less Non-occurrence Implementation 2 1048 807 11 37 1.95 or less Non-occurrence Implementation 3 1050 921 9 34 1.95 or less Non-occurrence Implementation 4 1043 949 8 33 1.95 or less Non-occurrence Implementation 5 1056 871 9 35 1.95 or less Non-occurrence Comparison 1 960 830 10 23 1.95 or less generation Comparison 2 1050 920 8 20 1.95 or less generation Comparison 3 978 791 10 19 1.95 or less generation Comparison 4 1260 1138 5 18 1.95 or less generation Comparison 5 1012 908 11.3 31 2.2~2.4 Non-occurrence Comparison 6 1025 910 10.6 35 2.2~2.4 Non-occurrence

[0124] Referring to Table 4, the hot-rolled steel sheets according to Examples 1 to 5 satisfy a yield strength (YS): 800 MPa or more, a tensile strength (TS): 980 MPa or more, an elongation (EL): 8% or more, and a hole expandability (HER): 25% or more.

[0125] Specifically, the hot-rolled steel sheets according to Examples 1 to 5 satisfy the ranges of yield strength (YS): 800 to 950 MPa, tensile strength (TS): 980 to 1100 MPa, elongation (EL): 8 to 15%, and hole expandability (HER): 25 to 40%.

[0126] In addition, it can be confirmed that the hot-rolled steel sheets according to Examples 1 to 5 do not have cracks on the surface layer of the hot-rolled steel, and satisfy the range of a Carbon Foot Print value of 1.95 or less among the Environmental Product Declaration (EPD) indicators required for UL (Underwriters' Laboratories) certification according to the ISO international standard (ISO 14040).

[0127] In contrast, Comparative Example 1 falls below the range of tensile strength (TS): 980 MPa or higher and hole expandability (HER): 25% or higher, and Comparative Example 2 falls below the range of hole expandability (HER): 25% or higher. It can be confirmed that cracks occur on the surface layer of hot-rolled steel in Comparative Examples 1 and 2.

[0128] When comprehensively examining Examples 3, 4, and 5 and Comparative Examples 1 and 2 disclosed in Tables 2 to 4, in the case of a composite process for producing molten metal by mixing blast furnace molten iron and electric furnace molten steel, even under conditions where a large amount of copper is included (0.3 wt% or more), the nickel content and copper equivalent (Cu eqIt can be confirmed that by controlling = [Cu] + 10[Sn] - [Ni]), sufficient strength is secured while simultaneously improving hole expandability (HER) and preventing cracks.

[0129] Meanwhile, looking at Comparative Examples 5 and 6, the content of copper, tin, and nickel is relatively very low, and the copper equivalent (Cu eq It is provided as a component system satisfying a value of 0.4 or less for = [Cu] + 10[Sn] - [Ni]), which offers excellent crack prevention and hole expansion capabilities, but it is confirmed that it is unsuitable for responding to global carbon neutrality policies as it does not satisfy the range of 1.95 or less for carbon footprint values ​​and exceeds it.

[0130] FIG. 2 is a photograph showing whether cracks occurred on the surface of a hot-rolled steel according to Comparative Example 1 among the experimental examples of the present invention, and FIG. 3 is a photograph showing whether cracks occurred on the surface of a hot-rolled steel according to Example 3 among the experimental examples of the present invention.

[0131] Referring to FIGS. 2 and FIGS. 3, it can be seen that while surface cracks occur on the surface of the hot-rolled steel according to Comparative Example 1 of the present invention, no surface cracks occur on the surface of the hot-rolled steel according to Example 3 of the present invention.

[0132] Meanwhile, referring to Tables 2 to 4, when the copper (Cu) content in the hot-rolled steel according to the embodiments of the present invention is relatively high at 0.3% to 0.5% (Examples 3, 4, and 5), there is a possibility that cracks may occur due to an excessive enrichment layer on the surface; however, by designing the nickel (Ni) content to be relatively high within the range of 0.01% to 0.5% (Ni: 0.2% or more), the copper equivalent (Cu eqIt can be confirmed that surface cracking can be suppressed by implementing = [Cu] + 10[Sn] - [Ni]) to 0.4 or less. This is because, even when a large amount of copper (Cu) is added, the solid solution temperature of copper (Cu) is increased when nickel (Ni) is added in combination, thereby suppressing surface enrichment.

[0133] Meanwhile, in the case where the copper (Cu) content in the hot-rolled steel according to the embodiments of the present invention is relatively low at 0.1% (Example 1), the possibility of cracks occurring due to an excessive copper enrichment layer on the surface is low, so it can be confirmed that surface cracks can be suppressed even without designing the nickel (Ni) content to be relatively high within the range of 0.01% to 0.5% (Ni: 0.01%).

[0134] Figure 4 is a graph showing the correlation between the composition and the occurrence of cracks in hot-rolled steel according to an experimental example of the present invention. The graph in Figure 4 can be understood as a graph showing crack sensitivity according to alloy components.

[0135] Referring to Fig. 4, it can be seen that the interaction with the added content of the tramp element affects whether cracks occur. Specifically, it can be seen that surface cracks do not occur on the surface of the hot-rolled steel when Equation 1 below is satisfied. Conversely, it can be seen that surface cracks occur on the surface of the hot-rolled steel when Equation 1 below is not satisfied.

[0136] Equation 1: [Cu] + 10[Sn] - [Ni] ≤ 0.4

[0137] (The above [Cu], [Sn], and [Ni] are the weight percentage values ​​of copper, tin, and nickel in the composition constituting the hot-rolled steel.)

[0138] It will be obvious to those skilled in the art that the technical concept of the present invention described above is not limited to the aforementioned embodiments and attached drawings, and that various substitutions, modifications, and changes are possible within the scope of the technical concept of the present invention.

Claims

Claim 1 In wt%, Carbon (C): 0.07% ~ 0.1%, Silicon (Si): 0.5% ~ 0.7%, Manganese (Mn): 1.7% ~ 1.9%, Aluminum (Al): 0.01% ~ 0.05%, Niobium (Nb): 0.05% ~ 0.07%, Titanium (Ti): 0.07% ~ 0.09%, Chromium (Cr): 0.5% ~ 0.7%, Phosphorus (P): >0% ~ 0.02% or less, Sulfur (S): >0% ~ 0.005% or less, Copper (Cu): 0.1% ~ 0.5%, Tin (Sn): 0.001% ~ 0.05%, Nickel (Ni): 0.01% ~ 0.5%, Boron (B): 0.0015% ~ 0.0025%, Hot-rolled steel comprising nitrogen (N): greater than 0% and less than or equal to 0.006%, with the remainder being iron (Fe) and unavoidable impurities, satisfying the following Formula 1, and having a microstructure composed of bainite and tempered martensite. Formula 1: [Cu] + 10[Sn] - [Ni] ≤ 0.4 (wherein [Cu], [Sn], and [Ni] are the weight% values ​​of copper, tin, and nickel in the composition constituting the hot-rolled steel). Claim 2 In claim 1, the hot-rolled steel satisfies yield strength (YS): 800 MPa or more, tensile strength (TS): 980 MPa or more, elongation (EL): 8% or more, and hole expandability (HER): 25% or more. Claim 3 delete Claim 4 A hot-rolled steel according to claim 1, wherein the area fraction of the bainite is 80% or more to 95% or less, and the area fraction of the tempered martensite is 5% or more to 20% or less. Claim 5 In claim 1, the hot-rolled steel further comprises a precipitate comprising at least one of carbides, nitrides, and carbonitrides. Claim 6 The hot-rolled steel according to claim 1 is characterized by having a Carbon Foot Print value of 1.95 or less among the Environmental Product Declaration (EPD) indicators required for UL (Underwriters' Laboratories) certification according to the ISO international standard (ISO 14040). Claim 7 (a) In wt%, Carbon (C): 0.07% ~ 0.1%, Silicon (Si): 0.5% ~ 0.7%, Manganese (Mn): 1.7% ~ 1.9%, Aluminum (Al): 0.01% ~ 0.05%, Niobium (Nb): 0.05% ~ 0.07%, Titanium (Ti): 0.07% ~ 0.09%, Chromium (Cr): 0.5% ~ 0.7%, Phosphorus (P): >0% ~ 0.02% or less, Sulfur (S): >0% ~ 0.005% or less, Copper (Cu): 0.1% ~ 0.5%, Tin (Sn): 0.001% ~ 0.05%, Nickel (Ni): 0.01% ~ 0.5%, Boron (B): 0.0015% ~ A step of preparing steel, comprising 0.0025%, nitrogen (N): greater than 0% to less than or equal to 0.006%, and the remainder being iron (Fe) and unavoidable impurities, satisfying Formula 1 below; (b) a step of reheating the steel at a reheating temperature of 1180°C to 1220°C; (c) a step of hot rolling the reheated steel so as to be completed at a finishing rolling end temperature of 860°C to 900°C; (d) a step of cooling the hot-rolled steel; and (e) a step of coiling the cooled steel at a coiling temperature of 420°C to 480°C; wherein the step of cooling the hot-rolled steel comprises a step of first cooling the steel to 640°C to 680°C at a cooling rate of 70°C / sec to 100°C / sec; A method for manufacturing hot-rolled steel, comprising: a second cooling step of air-cooling the steel at an air-cooling start temperature of 640°C to 680°C for an air-cooling time of 4 to 6 seconds at a cooling rate of 1°C / sec to 10°C / sec; and a third cooling step of cooling the steel to 420°C to 480°C at a cooling rate of 70°C / sec to 100°C / sec; wherein the hot-rolled steel implemented by the method is characterized by having a microstructure composed of bainite and tempered martensite. Formula 1: [Cu] + 10[Sn] - [Ni] ≤ 0.4 (wherein [Cu], [Sn] and [Ni] are weight% values ​​of copper, tin, and nickel in the composition constituting the hot-rolled steel) Claim 8 delete Claim 9 A method for manufacturing hot-rolled steel according to claim 7, wherein the step of preparing the steel comprises: a step of providing a molten metal; and a step of manufacturing a semi-finished product, which is one of a slab, a billet, and a bloom, from the molten metal, wherein the molten metal is provided using 80% or less of blast furnace molten iron and 20% or more of electric furnace molten steel in terms of total weight percent. Claim 10 A method for manufacturing hot-rolled steel according to claim 9, characterized in that at least a portion of the raw material in the step of providing the molten metal includes iron scrap. Claim 11 A method for manufacturing hot-rolled steel according to claim 9, wherein the step of providing the molten metal is prepared by adding Direct Reduced Iron (DRI) or Hot Briquetted Iron (HBI) as direct reduced iron. Claim 12 delete

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