Hot-rolled steel sheet, vehicle part, and method for manufacturing the same

Abstract

A method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention includes the steps of producing a molten metal using raw materials including molten pig iron produced in a blast furnace, direct reduced iron, and iron scrap, producing a semi-finished product, and producing a hot-rolled steel sheet.

Description

[Technical Field]

[0001] The present invention relates to a hot-rolled steel sheet, a vehicle part, and a method for manufacturing the same. [Background technology]

[0002] Generally, there are two methods for producing steel: the blast furnace-converter process and the electric furnace process.

[0003] The blast furnace-converter process includes a process in which iron ore and bituminous coal (e.g., coke) are charged into a blast furnace and melted with hot air to produce molten iron, and a process in which the molten iron tapped from the blast furnace is charged into a converter to remove impurities such as carbon and produce molten steel.

[0004] The electric arc furnace process involves melting steel scrap in an electric arc furnace (EAF) to produce molten steel.

[0005] The molten steel produced by the two methods mentioned above is manufactured into semi-finished products through the continuous casting process, and the semi-finished products are then manufactured into final products with dimensions suited to the needs of the customer through the subsequent rolling process.

[0006] Meanwhile, rapid climate change due to greenhouse gas emissions from various industrial sectors has recently become a hot topic internationally.

[0007] Carbon dioxide is a major greenhouse gas, and the steel industry is known to emit a large proportion of carbon dioxide compared to other industrial sectors.

[0008] For example, the blast furnace-converter process uses carbon monoxide (CO) produced by the combustion of coke as a reducing agent, resulting in the emission of large amounts of carbon dioxide.

[0009] As a result, major steel companies are currently focusing on developing electric arc furnace processes, which emit less carbon dioxide than the blast furnace-converter process.

[0010] However, electric furnace products that use scrap iron as the main raw material have the problem of a high content of tramp elements, which are impurity alloy elements that exist in trace amounts in steel products.

[0011] Specifically, tramp elements cannot easily dissolve in the base steel and tend to accumulate at the interface. If the tramp elements exceed the allowable limit, cracks may occur on the surface and inside the steel during rolling. Furthermore, tramp elements may act as a major factor in hot embrittlement, which significantly reduces ductility at high temperatures.

[0012] Therefore, in general, electric furnace products have limitations in their application to high-grade steel materials such as automotive steel sheets, which require high surface quality characteristics. Summary of the Invention [Problem to be solved by the invention]

[0013] In order to solve the above-mentioned problems, an object of the present invention is to provide a hot-rolled steel sheet, a vehicle part, and a method for manufacturing the same that can reduce carbon dioxide emissions generated in the blast furnace-converter process.

[0014] An object of the present invention is to provide a hot-rolled steel sheet and a vehicle part which have superior physical properties to those of conventional electric furnace products, and a method for manufacturing the same, using an electric furnace.

[0015] An object of the present invention is to provide a hot-rolled steel sheet, a vehicle part, and a method for manufacturing the same that minimizes the influence on the supply and demand of specific raw materials and allows flexibility in raw material selection.

[0016] The objects of the present invention are not limited to the objects mentioned above, and other objects not mentioned will be clearly understood by those skilled in the art from the following description. [Means for solving the problem]

[0017] A method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention includes the steps of producing a molten metal using raw materials including molten pig iron produced in a blast furnace, direct reduced iron, and iron scrap, producing a semi-finished product, and producing a hot-rolled steel sheet, wherein the raw materials include 20% to 60% by weight of the molten pig iron, 10% to 40% by weight of the direct reduced iron, and 20% to 60% by weight of the iron scrap.

[0018] According to an embodiment of the present invention, the step of producing the molten metal may include the steps of charging the raw materials into an electric furnace and melting the raw materials in the electric furnace.

[0019] According to an embodiment of the present invention, the copper content of the hot-rolled steel sheet may be 0.12 wt % or less (excluding 0).

[0020] According to one embodiment of the present invention, the copper content of the molten iron may be 0.03 wt % or less (excluding 0), the copper content of the direct reduced iron may be 0.02 wt % or less (excluding 0), and the copper content of the iron scrap may be 0.18 wt % or less (excluding 0).

[0021] According to an embodiment of the present invention, the tin content of the hot-rolled steel sheet may be 0.012 wt % or less (excluding 0).

[0022] According to one embodiment of the present invention, the tin content of the molten iron may be 0.003 wt % or less (excluding 0), the tin content of the direct reduced iron may be 0.002 wt % or less (excluding 0), and the tin content of the iron scrap may be 0.018 wt % or less (excluding 0).

[0023] According to an embodiment of the present invention, the step of manufacturing the hot-rolled steel sheet may include the steps of reheating at a reheating temperature in the range of 1150°C to 1350°C, hot rolling at a finish rolling temperature in the range of 880°C to 930°C, cooling at a cooling rate in the range of 60°C / sec to 110°C / sec, and coiling at a coiling temperature in the range of 380°C to 480°C.

[0024] A hot-rolled steel sheet according to an embodiment of the present invention contains, by weight, 0.07-0.12% carbon, 0.3-0.8% silicon, 1.5-2.0% manganese, 0.02% or less phosphorus, 0.005% or less sulfur, 0.12% or less (excluding 0) copper, 0.012% or less (excluding 0) tin, and the balance being iron (Fe) and unavoidable impurities. Here, the hot-rolled steel sheet is produced by melting 20% ​​to 60% by weight of blast furnace-based molten iron, 20% to 40% by weight of direct reduced iron, and 20% to 60% by weight of iron scrap.

[0025] In the hot-rolled steel sheet according to an embodiment of the present invention, the copper content of the molten iron may be 0.03 wt % or less (excluding 0), the copper content of the direct reduced iron may be 0.02 wt % or less (excluding 0), and the copper content of the iron scrap may be 0.18 wt % or less (excluding 0).

[0026] According to one embodiment of the present invention, the tin content of the molten iron may be 0.003 wt % or less (excluding 0), the tin content of the direct reduced iron may be 0.002 wt % or less (excluding 0), and the tin content of the iron scrap may be 0.018 wt % or less (excluding 0).

[0027] According to one embodiment of the present invention, when the surface of the hot-rolled steel sheet is observed after the phosphate conversion treatment, the weight of the coating is 1.8 to 3.0 g / m 2 The crystal grains can be 2 to 10 μm.

[0028] According to an embodiment of the present invention, the hot-rolled steel sheet may further contain at least one element selected from the group consisting of chromium (Cr), niobium (Nb), titanium (Ti), and boron (B).

[0029] A vehicle part according to one embodiment of the present invention contains, by weight, 0.07 to 0.12% carbon, 0.3 to 0.8% silicon, 1.5 to 2.0% manganese, 0.02% or less phosphorus, 0.005% or less sulfur, 0.12% or less (excluding 0) copper, 0.012% or less (excluding 0) tin, and the balance being iron (Fe) and inevitable impurities. Here, the base material of the vehicle part is produced by melting 20% ​​to 60% by weight of blast furnace-based molten iron, 20% to 40% by weight of direct reduced iron, and 20% to 60% by weight of iron scrap.

[0030] According to one embodiment of the present invention, the copper content of the molten iron may be 0.03 wt % or less (excluding 0), the copper content of the direct reduced iron may be 0.02 wt % or less (excluding 0), and the copper content of the iron scrap may be 0.18 wt % or less (excluding 0).

[0031] According to one embodiment of the present invention, the tin content of the molten iron may be 0.003 wt % or less (excluding 0), the tin content of the direct reduced iron may be 0.002 wt % or less (excluding 0), and the tin content of the iron scrap may be 0.018 wt % or less (excluding 0).

[0032] According to one embodiment of the present invention, when observing the surface of a vehicle part after phosphate conversion coating, the weight of the coating is 1.8 to 3.0 g / m 2 The crystal grains can be 2 to 10 μm.

[0033] According to one embodiment of the present invention, the vehicle component may be a lower arm for a vehicle. [Effects of the Invention]

[0034] According to one embodiment of the present invention, the amount of molten iron used can be significantly reduced compared to the conventional blast furnace-converter process, and the amount of carbon dioxide generated during the production of steel products can be significantly reduced.

[0035] According to an embodiment of the present invention, the tramp element content can be controlled within an allowable range compared to conventional electric furnace products, thereby producing electric furnace products with excellent physical properties.

[0036] According to an embodiment of the present invention, the ratio of the added content of hot metal, direct reduced iron, and iron scrap can be adjusted within a preset range, and thus, even if a problem occurs in the supply and demand of a specific raw material, it can be flexibly dealt with. [Brief explanation of the drawings]

[0037] [Figure 1] FIG. 1 is a flowchart showing a method for producing a hot-rolled steel sheet according to an embodiment of the present invention. [Figure 2] FIG. 2 is a flowchart showing in detail the steps of producing the molten metal in the flowchart of FIG. [Figure 3] FIG. 3 is a photographic representation showing the results of the bending test evaluation of Comparative Example 3 and Example 6 shown in Table 2. [Figure 4] FIG. 4 is a photograph-substitute drawing showing the results of the pretreatment characteristic evaluation of Comparative Example 3 and Example 6 shown in Table 2. [Figure 5] FIG. 5 is a diagram for explaining the method for evaluating adhesion during the paintability evaluation described in Table 2. [Figure 6] FIG. 6 is a photographic representation showing the state of Example 6 shown in Table 2 after the impact resistance evaluation. [Figure 7] FIG. 7 is a photographic representation showing the state of Comparative Example 1, Comparative Example 3, and Example 6 shown in Table 2 after the weldability evaluation. [Figure 8] FIG. 8 is a photographic representation showing the state of Example 6 shown in Table 3 after the wire processability evaluation. DETAILED DESCRIPTION OF THE INVENTION

[0038] All terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

[0039] Terms, such as those defined in commonly used dictionaries, should be construed to have a meaning consistent with their meaning in the context of the relevant art, and are not to be construed in an idealized or overly formal sense unless expressly defined herein.

[0040] Terms such as first, second, and third may be used to describe various components, regions, portions, or layers, but are not limited to these terms, and may be used to distinguish one component, region, portion, or layer from another component, portion, or layer.

[0041] In this specification, when one part is said to be "on" or "above" another part, this is not limited to the case where the other part is located directly above the one part, but also includes the case where there is another part between the one part and the other part.

[0042] In this specification, the term "additionally containing an additional element" means that an additional element is contained in place of the remaining iron (Fe), and unless otherwise specified, "%" means "% by weight."

[0043] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0044] Composition of hot-rolled steel sheet according to one embodiment of the present invention The hot-rolled steel sheet according to one embodiment of the present invention contains iron (Fe), carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), copper (Cu), and tin (Sn).

[0045] The hot-rolled steel sheet according to an embodiment of the present invention may further include at least one of aluminum (Al), chromium (Cr), niobium (Nb), titanium (Ti), boron (B), and nitrogen (N).

[0046] For example, the hot-rolled steel sheet according to the present embodiment may contain, on a weight percent basis, carbon (C): 0.07 to 0.12, silicon (Si): 0.3 to 0.8, manganese (Mn): 1.5 to 2.0, phosphorus (P): 0.02 or less, sulfur (S): 0.005 or less, aluminum (Al): 0.2 to 0.5, copper (Cu): less than 0.1 (excluding 0), tin (Sn): less than 0.01 (excluding 0), chromium (Cr): 0.5 to 1.1, niobium (Nb): 0.05 to 0.1, titanium (Ti): 0.06 to 1.1, boron (B): 0.001 to 0.0035, nitrogen (N): 0.006 or less, residual iron (Fe), and other unavoidably added impurities.

[0047] The components of the hot-rolled steel sheet and the ranges of each component will be described below.

[0048] [Carbon (C): 0.07% by weight or more and 0.12% by weight or less] Carbon (C) is an essential element for ensuring the necessary strength. For example, carbon (C) may be added to ensure the strength of the bainite structure. Carbon (C) may also be added to ensure the balance of precipitates.

[0049] However, if the amount of carbon (C) added is excessive, the workability and weldability of the steel sheet may be reduced, and conversely, if the amount of carbon (C) added is insufficient, the aforementioned effects of addition may be insufficient.

[0050] Therefore, in one embodiment of the present invention, the carbon (C) content may be 0.07% by weight to 0.12% by weight.

[0051] According to one embodiment of the present invention, the carbon (C) content may be about 0.07% to 0.12% by weight, preferably about 0.09% to 0.10% by weight.

[0052] [Silicon (Si): 0.3% by weight or more and 0.8% by weight or less] Silicon (Si) is a solid solution strengthening element. For example, silicon (Si) is a ferrite stabilizing element that dissolves in ferrite, and can increase strength without deteriorating ductility. Silicon (Si) can suppress the formation of carbides, thereby improving the elongation rate of steel sheets.

[0053] However, if the amount of silicon (Si) added is excessive, it may cause surface defects due to oxide scale on the surface of the hot-rolled steel sheet, which may reduce weldability. Conversely, if the amount of silicon (Si) added is insufficient, the aforementioned effects of addition may become insufficient.

[0054] Therefore, in one embodiment of the present invention, the silicon content may be 0.3 wt% to 0.8 wt%. According to one embodiment of the present invention, the silicon content may be about 0.3 wt% to 0.8 wt%.

[0055] [Manganese (Mn): 1.5% by weight or more and 2.0% by weight or less] Manganese improves the strength of steel by improving solution strengthening and hardenability. Manganese is an essential element for suppressing pearlite transformation and obtaining a bainite structure.

[0056] However, if the amount of manganese (Mn) added is excessive, it can deteriorate workability and hinder weldability, while if the amount of manganese (Mn) added is insufficient, the aforementioned effects of addition may become insufficient.

[0057] Therefore, in one embodiment of the present invention, the manganese (Mn) content may be 1.5 wt% to 2.0 wt%. According to one embodiment of the present invention, the manganese (Mn) content may be about 1.5 wt% to 2.0 wt%.

[0058] [Sulfur (S): 0.005% by weight or less] Sulfur (S) can impair toughness and weldability, and can increase non-metallic inclusions (sulfur compounds) during hot rolling, impairing the workability of steel. Therefore, in one embodiment of the present invention, the sulfur (S) content may be 0.005 wt.% or less. According to one embodiment of the present invention, the sulfur (S) content may be about 0.005 wt.% or less.

[0059] [Phosphorus (P): 0.02% by weight or less] Phosphorus (P) is an impurity element that promotes grain boundary segregation and high-temperature cracking. Therefore, in one embodiment of the present invention, the phosphorus (P) content may be 0.02 wt% or less. According to one embodiment of the present invention, the phosphorus (P) content may be about 0.02 wt% or less.

[0060] [Aluminum (Al): 0.02% by weight or more and 0.5% by weight or less] Aluminum (Al) is a typical deoxidizer. It is added to the converter before or during tapping to suppress the formation of pinholes and non-metallic inclusions. Aluminum (Al) combines with nitrogen in the steel to form nitride (AlN).

[0061] AlN, a nitride, precipitates at grain boundaries and is effective in refining the crystal grains of steel, thereby improving the toughness of the steel.

[0062] Furthermore, AlN effectively prevents high-temperature oxidation and can prevent cracks during slab production.

[0063] However, if the amount of aluminum (Al) added is excessive, defects may occur due to the formation of inclusions, while if the amount of aluminum (Al) added is insufficient, the aforementioned effects of addition may become insufficient.

[0064] Therefore, in one embodiment of the present invention, the aluminum (Al) content may be 0.02 wt% to 0.5 wt%. According to one embodiment of the present invention, the aluminum (Al) content may be about 0.02 wt% to 0.5 wt%.

[0065] [Copper (Cu): 0.12% by weight or less (excluding 0) / Tin (Sn): 0.012% by weight or less (excluding 0)] Copper (Cu) and tin (Sn) can be mixed in from steel scrap. Copper (Cu) and tin (Sn) are elements that are almost impossible to remove in the steelmaking process and are classified as tramp elements. Tramp elements can cause cracks on the surface and inside during rolling. Tramp elements can also act as a major cause of hot embrittlement, which significantly reduces ductility at high temperatures.

[0066] In particular, tin (Sn) inhibits the non-uniformity of the size of phosphates and the stability of the amount of phosphate deposited when a phosphate film is formed on a steel material.

[0067] Therefore, in one embodiment of the present invention, the copper (Cu) and tin (Sn) contents may be 0.12 wt.% or less (excluding 0) and 0.012 wt.% or less (excluding 0), respectively. According to one embodiment of the present invention, the copper (Cu) and tin (Sn) contents may be about 0.12 wt.% or less (excluding 0) and about 0.012 wt.% or less (excluding 0), respectively.

[0068] [Chromium (Cr): 0.5% by weight or more and 1.1% by weight or less] Chromium (Cr) is a solid solution strengthening element. Chromium (Cr) can delay the ferrite transformation and aid in the formation of martensite and bainite.

[0069] However, if the amount of chromium (Cr) added is excessive, it may cause the microstructure to become non-uniform, which may impair workability. Conversely, if the amount of chromium (Cr) added is insufficient, the aforementioned effects of addition may become insufficient.

[0070] Therefore, in one embodiment of the present invention, the chromium (Cr) content may be 0.5 wt% to 1.1 wt%. According to one embodiment of the present invention, the chromium (Cr) content may be about 0.5 wt% to 1.1 wt%.

[0071] [Nibium (Nb): 0.05% by weight or more and 0.1% by weight or less] Niobium (Nb) is effective in refining crystal grains and improving strength and impact toughness.

[0072] However, if the amount of niobium (Nb) added is excessive, it may excessively delay recrystallization, which may cause the precipitates to become coarse and reduce workability. Conversely, if the amount of niobium (Nb) added is insufficient, the aforementioned effects of addition may become insufficient.

[0073] Therefore, in one embodiment of the present invention, the niobium (Nb) content may be 0.05% to 0.1% by weight. According to one embodiment of the present invention, the niobium (Nb) content may be about 0.05% to 0.1% by weight.

[0074] [Titanium (Ti): 0.06% by weight or more and 1.1% by weight or less] Titanium (Ti) combines with nitrogen in steel to form nitrides (TiN), which inhibit grain growth and prevent the generation of free nitrogen. However, if the amount of titanium (Ti) added is excessive, excessive precipitates may form, which may reduce impact toughness. Conversely, if the amount of titanium (Ti) added is insufficient, the aforementioned effects of addition may become insufficient.

[0075] Therefore, in one embodiment of the present invention, the titanium (Ti) content may be 0.06 wt% to 1.1 wt%. According to one embodiment of the present invention, the titanium (Ti) content may be about 0.06 wt% to 1.1 wt%.

[0076] [Boron (B): 0.001% by weight or more and 0.0035% by weight or less] Boron (B) suppresses ferrite transformation and improves strength. However, if too much boron (B) is added, it can reduce weldability and workability. Conversely, if the amount of boron (B) added is insufficient, the aforementioned effects of addition may be insufficient.

[0077] Therefore, in one embodiment of the present invention, the content of boron (B) may be 0.001% to 0.0035% by weight. According to one embodiment of the present invention, the content of boron (B) may be about 0.001% to 0.0035% by weight.

[0078] [Nitrogen (N): 0.006% by weight or less] Nitrogen (N) can have a significant effect on the mechanical properties of steel even in very small amounts. For example, it increases tensile strength and yield strength, but can reduce elongation. Free nitrogen can also cause strain aging.

[0079] Therefore, in one embodiment of the present invention, the nitrogen (N) content may be 0.006 wt% or less. According to one embodiment of the present invention, the nitrogen (N) content may be 0.006 wt% or less.

[0080] The balance includes Fe and unavoidable impurities, which are impurities that are mixed in during the manufacturing process of the hot-rolled steel sheet, and are widely known in the art, so a detailed description thereof will be omitted.

[0081] In one embodiment of the present invention, elements other than the above-mentioned alloy components may be added, and various elements may be included within a range that does not impair the technical concept of the present invention. When an additional element is further included, it may be included to replace the remaining Fe.

[0082] As described above, the steel compositions and composition ranges of the steel plate according to one embodiment have been described. However, it is obvious to a person having ordinary skill in the art that the present invention can be embodied in other specific forms without departing from the spirit or category of the present invention, in addition to the above-described embodiment.

[0083] Method for manufacturing hot-rolled steel sheet FIG. 1 is a flow chart illustrating a method for producing steel according to one embodiment of the present invention.

[0084] Referring to FIG. 1, the method for producing steel includes a step S10 of producing a molten metal, a step S20 of producing a semi-finished product, and a step S30 of producing a hot-rolled steel sheet.

[0085] The step S10 of producing a molten metal refers to a step of melting raw materials to produce a molten metal (e.g., molten steel). The raw materials may include ore-based materials and scrap iron.

[0086] The ore-based material may include hot metal and direct reduced iron. Hot metal may be produced by charging iron ore and coke into a blast furnace and blowing hot air into it to reduce and melt the iron ore.

[0087] Direct reduced iron may be produced by reducing solid iron ore using a reducing gas (carbon monoxide, hydrogen, etc.). For example, the direct reduced iron may be DRI (direct reduction iron) or HBI (hot briquetted iron). The direct reduced iron may be processed into pellets.

[0088] However, the type and processing form of the direct reduced iron are not limited to those described above.

[0089] Steel scrap is obtained from the steelmaking process or from unusable steel products.

[0090] On the other hand, molten pig iron has a low impurity content and is suitable for producing high-quality steel products, but the problem is that the molten pig iron production process emits a large amount of carbon dioxide: for example, approximately 2 tons of carbon dioxide are emitted when producing 1 ton of molten pig iron.

[0091] Electric furnace molten steel is produced by melting scrap iron in an electric furnace. Electric furnace molten steel emits less carbon dioxide during the manufacturing process than the blast furnace process. For example, the carbon dioxide emissions from molten steel are one-quarter of those from molten pig iron.

[0092] However, steel products produced as molten steel in an electric furnace contain a high content of impurities such as tramp elements, making it difficult to meet the physical properties required for high-quality steel products such as vehicle parts.

[0093] To solve the above-mentioned problems, in step S10 of producing molten iron according to one embodiment of the present invention, molten iron, which is an ore-based material, direct reduced iron, and iron scrap are charged into an electric furnace and melted to produce molten iron.

[0094] FIG. 2 is a flowchart showing in detail the steps of producing the molten metal in the flowchart of FIG.

[0095] Referring to FIG. 2, step S10 of producing a molten metal may include step S11 of preparing raw materials, step S12 of charging the raw materials, and step S13 of melting the raw materials.

[0096] In step S11 of preparing raw materials, molten iron, direct reduced iron, and iron scrap are prepared in accordance with a preset content ratio.

[0097] Specifically, the content ratio of molten iron is defined as X, the content ratio of direct reduced iron as Y, and the content ratio of iron scrap as Z. In addition, the maximum copper (Cu) content of the hot-rolled steel sheet produced in the hot-rolling step described below is defined as K1, and the maximum tin (Sn) content as K2. Each content ratio is based on weight percent. The sum of X + Y + Z may be 100.

[0098] According to one embodiment of the present invention, X, Y, Z, K1, and K2 satisfy the following formulas 1 to 4.

[0099]

number

[0100]

number

[0101]

number

[0102]

number

[0103] In the above formula 1, a1, a2, and a3 represent the copper content ratio of the molten iron, the copper content ratio of the direct reduced iron, and the copper content ratio of the iron scrap, respectively.

[0104] In the formula 3, b1, b2, and b3 represent the tin content ratio of the molten iron, the tin content ratio of the direct reduced iron, and the tin content ratio of the iron scrap, respectively.

[0105] The copper content ratio (a1) of the molten iron may be 0.03% by weight or less (excluding 0). The copper content ratio (a2) of the direct reduced iron may be 0.02% by weight or less (excluding 0). The copper content ratio (a3) ​​of the iron scrap may be 0.18% by weight or less (excluding 0).

[0106] According to one embodiment of the present invention, the copper content ratio (a1) of the hot metal may be about 0.03% by weight (excluding 0) or less, the copper content ratio (a2) of the direct reduced iron may be about 0.02% by weight (excluding 0) or less, and the copper content ratio (a3) ​​of the iron scrap may be about 0.18% by weight (excluding 0) or less.

[0107] The tin content ratio (b2) of the direct reduced iron may be 0.002% by weight (excluding 0) or less. The tin content ratio (b3) of the iron scrap may be 0.018% by weight (excluding 0) or less. The tin content ratio (b1) of the molten iron may be 0.003% by weight (excluding 0) or less.

[0108] According to one embodiment of the present invention, the tin content ratio (b2) of the direct reduced iron may be about 0.002% by weight (excluding 0) or less. The tin content ratio (b3) of the iron scrap may be about 0.018% by weight (excluding 0) or less. The tin content ratio (b1) of the hot metal may be about 0.003% by weight (excluding 0) or less.

[0109] However, the copper content ratios a1, a2, a3 and the tin content ratios b1, b2, b3 may increase or decrease within a predetermined range depending on the quality, processing degree, etc. of each raw material.

[0110] According to one embodiment of the present invention, if the content ratio (X) of the hot metal exceeds a predetermined range, the effect of reducing carbon dioxide emissions may be insufficient. Conversely, if the content ratio (X) of the hot metal is below the predetermined range, the melting of other raw materials (i.e., iron scrap and direct reduced iron) may not be performed properly due to a lack of heat source.

[0111] Therefore, in one embodiment of the present invention, the content ratio (X) of the hot metal may be 20% by weight to 60% by weight. According to one embodiment of the present invention, the content ratio (X) of the hot metal may be about 20% by weight to 60% by weight.

[0112] According to one embodiment of the present invention, when the content ratio (Y) of direct reduced iron exceeds a predetermined range, the amount of gangue in the molten iron increases, which requires the process temperature of the electric furnace to be maintained high, resulting in reduced energy efficiency, reduced refining capacity, and the need for a separate design to correct slag making during the dephosphorization process.

[0113] Conversely, if the direct reduced iron content (Y) is below a predetermined range, the content of hot metal or scrap iron must be increased to replace it. For example, if the hot metal content is increased instead of direct reduced iron, the effect of reducing carbon dioxide emissions may be insufficient. If the content of scrap iron is increased instead of direct reduced iron, the content of tramp elements (i.e., copper or tin) may become high.

[0114] Therefore, in one embodiment of the present invention, the content ratio (Y) of the direct reduced iron may be 10% by weight to 40% by weight. According to one embodiment of the present invention, the content ratio (Y) of the direct reduced iron may be about 10% by weight to 40% by weight.

[0115] According to one embodiment of the present invention, if the content ratio (Z) of the iron scrap exceeds a predetermined range, the content ratio of tramp elements becomes high, and the required physical properties of the final steel product cannot be achieved. Conversely, if the content ratio (Z) of the iron scrap is below the predetermined range, the effect of reducing carbon dioxide emissions may be insufficient.

[0116] Therefore, in one embodiment of the present invention, the content ratio of iron scrap may be 20 to 60 wt%. According to one embodiment of the present invention, the content ratio of iron scrap may be about 20 to 60 wt%.

[0117] In step S12, the raw materials are charged into the melting furnace. The raw materials may be charged sequentially or simultaneously.

[0118] Also, each raw material may be charged multiple times to prevent slag slopping or to ensure proper melting reaction.

[0119] Step S13 of melting the raw materials may be performed in an electric arc furnace (EAF). In this embodiment, the electric furnace may be an AC electric furnace. However, the present invention is not limited to this, and a DC electric furnace may also be used.

[0120] In step S20 of producing semi-finished products, the molten metal produced in step S10 is used to produce semi-finished products such as slabs, blooms, and billets.

[0121] For example, step S20 may be performed by a continuous casting process, in which molten metal is cast and rolled simultaneously while passing through a plurality of segments arranged in a line, to cast a semi-finished product having a predetermined width and thickness.

[0122] However, step S20 is not limited to the above. For example, step S20 may include a forging process following a continuous casting process. Specifically, after producing a bloom in the continuous casting process, it is also possible to produce a forged slab having predetermined dimensions through a forging process.

[0123] In step S30 of manufacturing a hot-rolled steel sheet, the semi-finished product is reheated and then rolled into a hot-rolled steel sheet having a predetermined thickness and width. For example, the thickness of the hot-rolled steel sheet may be 2 to 5 mm. However, the thickness of the hot-rolled steel sheet is not limited to the above.

[0124] Specifically, step S30 of producing a hot-rolled steel sheet includes step S31 of reheating, step S32 of hot rolling, step S33 of cooling, and step S34 of coiling.

[0125] In the reheating step S31, the semi-finished product is reheated within a preset temperature range, and the reheating step S31 can redissolve the components that segregated in step S20.

[0126] If the reheating temperature (RT) is lower than the preset temperature range, the efficiency of redissolution of segregated elements decreases, which may result in a deterioration in the bending workability of the final product. Conversely, if the reheating temperature is higher than the preset temperature range, the precipitates may become coarse, which may result in a deterioration in the surface quality of the hot-rolled steel.

[0127] Therefore, in the present invention, the reheating temperature is limited to a range of 1150°C to 1350°C. In one embodiment of the present invention, the reheating temperature is limited to a range of approximately 1150°C to 1350°C.

[0128] In step S32, the reheated semi-finished product is hot-rolled. If the finishing delivery temperature (FDT) in step S32 is lower than a preset range, problems such as the formation of a duplex structure due to rolling in the two-phase region may occur. Conversely, if the finishing delivery temperature exceeds the preset range, the crystal grains of the hot-rolled steel sheet may become coarse, which may cause a deterioration in the physical properties of the final product.

[0129] Therefore, in this embodiment, the finish rolling temperature is limited to a range of 880°C to 930°C. According to one embodiment of the present invention, the finish rolling temperature is limited to a range of approximately 880°C to 930°C.

[0130] In the cooling step S33, the hot-rolled steel sheet is cooled. If the cooling rate in step S33 is less than a predetermined range, a ferrite structure may be formed instead of a bainite structure. Conversely, if the cooling rate exceeds the predetermined range, the transformation of a martensite structure may be activated instead of a bainite structure, which may reduce workability.

[0131] Therefore, in this embodiment, the cooling rate is limited to a range of 60°C / sec to 110°C / sec. According to one embodiment of the present invention, the cooling rate is limited to a range of approximately 60°C / sec to 110°C / sec.

[0132] In the coiling step S34, the cooled hot-rolled steel sheet is coiled. If the coiling temperature in step S34 is below a preset range, coiling may not be easy. Conversely, if the coiling temperature exceeds the preset range, surface scale may form on the inside of the hot-rolled steel sheet.

[0133] Therefore, in this embodiment, the coiling temperature is limited to a range of 380°C to 480°C. According to one embodiment of the present invention, the coiling temperature is limited to a range of approximately 380°C to 480°C.

[0134] As a result, according to one embodiment of the present invention, the amount of molten iron used can be significantly reduced compared to the conventional blast furnace-converter process, and the amount of carbon dioxide generated during the production of steel products can be significantly reduced.

[0135] According to an embodiment of the present invention, the tramp element content can be controlled within an allowable range compared to conventional electric furnace products, thereby producing electric furnace products with superior physical properties.

[0136] According to an embodiment of the present invention, the ratio of the added content of hot metal, direct reduced iron, and iron scrap can be adjusted within a preset range, and thus, even if a problem occurs in the supply and demand of a specific raw material, it can be flexibly dealt with.

[0137] Properties of hot-rolled steel sheets according to the present invention Below, examples of the present invention and comparative examples are compared with each other through Tables 1 and 2. FIGS. 3 to 8 are diagrams for explaining the evaluation results of Table 2.

[0138] In the following Table 1, Examples 1 to 6 are experimental examples that satisfy the raw material content ratios according to the embodiments of the present invention, and Comparative Examples 1 to 4 are experimental examples that deviate from the raw material content ratios.

[0139] The examples and comparative examples differ only in the content ratios of the raw materials (hot metal, direct reduced iron, and iron scrap), and the remaining conditions (for example, process conditions) are all the same.

[0140] Specifically, the hot-rolled steel sheets according to the examples and comparative examples are manufactured by the following process.

[0141] First, raw materials are charged into an electric furnace according to the content ratios shown in Table 1 and then melted to produce molten steel. The molten steel is then produced into semi-finished products of predetermined specifications through a continuous casting process, and the semi-finished products are then produced into 3.2 mm thick hot-rolled steel sheets through hot rolling equipment.

[0142] [Table 1]

[0143] Referring to Table 1, the maximum copper content ratio K1 of the hot-rolled steel sheets according to Examples 1 to 6 is 0.12 or less, while the maximum copper content ratio K1 of the hot-rolled steel sheets according to Comparative Examples 1 to 4 is 0.12 or more.

[0144] Furthermore, the maximum value of the tin content ratio K2 of the hot-rolled steel sheets according to Examples 1 to 6 is 0.012 or less, while the maximum value of the tin content ratio K2 of the hot-rolled steel sheets according to Comparative Examples 1 to 4 is 0.012 or more.

[0145] Table 2 below shows the product properties of the hot-rolled steel sheets according to Examples 1 to 6 and Comparative Examples 1 to 4 shown in Table 1.

[0146] In Table 2, the conditions for the bending test evaluation are R / T=0.94, 1.5, where R is the jig diameter (unit: mm) for the bending test, and T is the thickness (unit: mm) of the test steel plate.

[0147] In Table 2, the pretreatment characteristics were evaluated by evaluating the appearance, coating weight, and grain size after the surface of the hot-rolled steel sheet was treated with a phosphate (zinc phosphate) conversion coating. In the pretreatment characteristic evaluation, the coating weight was 1.8 to 3.0 g / m 2 is the appropriate range, and the appropriate range for the crystal grains is 2 to 10 μm.

[0148] In Table 2, the adhesion evaluation during the paintability evaluation was determined by the percentage of the painted area that could be removed by taping after a taping test on the painted surface of the hot-rolled steel sheet.

[0149] In Table 2, the impact resistance evaluation during the paintability evaluation was performed by dropping a 500 g weight 50 cm onto the painted surface of the hot-rolled steel sheet and evaluating the presence or absence of defects on the painted surface.

[0150] In Table 2, the weldability was evaluated by evaluating the deposited metal (i.e., beads) formed along the weld line at a feed rate of 6 to 11 m / min.

[0151] [Table 2]

[0152] [Bending test evaluation results] 3, the hot-rolled steel sheets according to Examples 1 to 6 and Comparative Examples 3 and 4 showed a "good" result in the bending test. In contrast, the hot-rolled steel sheets according to Comparative Examples 1 and 2 showed a "poor" result due to the occurrence of microcracks.

[0153] Referring to the bending test evaluation results, the bending properties decreased sharply when the maximum values ​​of K1 (copper content ratio) and K2 (tin content ratio) were 0.16 and 0.016 or more, respectively.

[0154] [Pretreatment characteristic evaluation results] 4, the hot-rolled steel sheets according to Examples 1 to 6 were evaluated as "good" in the pretreatment characteristic evaluation. In contrast, the hot-rolled steel sheets according to Comparative Examples 1 to 4 were evaluated as "poor" in the pretreatment characteristic evaluation.

[0155] The results of the pretreatment characteristic evaluation revealed that when the maximum copper content ratio and maximum tin content ratio were 0.13 and 0.013 or higher, respectively, the grain size of the coating formed on the surface of the hot-rolled steel sheet became non-uniform.

[0156] [Adhesion evaluation during paintability evaluation] 5, the hot-rolled steel sheets according to Examples 1 to 6 were evaluated as M=1 (good) in adhesion evaluation. In comparison, the hot-rolled steel sheets according to Comparative Examples 1 to 4 were evaluated as M=3 (poor) or M=2 (poor) in adhesion evaluation.

[0157] The results of the adhesion evaluation showed that the bond strength between the hot-rolled steel sheet and the painted surface decreased sharply when the minimum values ​​of K1 (copper content ratio) and K2 (tin content ratio) were 0.13 and 0.013 or higher, respectively.

[0158] [Impact resistance evaluation during paintability evaluation] Referring to Table 2 and FIG. 6, the hot-rolled steel sheets according to Examples 1 to 6 were evaluated as "good" in impact resistance, while the hot-rolled steel sheets according to Comparative Examples 1 to 4 were evaluated as "poor" in impact resistance.

[0159] The results of the adhesion evaluation confirmed that the impact resistance of the painted surface of the hot-rolled steel sheet rapidly decreased when the minimum values ​​of K1 (copper content ratio) and K2 (tin content ratio) were 0.13 and 0.013 or higher, respectively.

[0160] [Weldability evaluation] Referring to Table 2 and FIG. 7, the hot-rolled steel sheets according to Examples 1 to 6 were evaluated as "good", but the hot-rolled steel sheets according to Comparative Examples 1 to 4 either had insufficient deposition amounts or burn-through occurred.

[0161] The weldability evaluation results confirmed that the weldability of hot-rolled steel sheets rapidly deteriorates when the minimum values ​​of K1 (copper content ratio) and K2 (tin content ratio) are 0.16 and 0.016 or higher, respectively.

[0162] The conclusion was that the hot-rolled steel sheets according to the examples of the present invention showed excellent properties in all of the bending test evaluation, pretreatment characteristic evaluation, paintability evaluation, and weldability evaluation, but the hot-rolled steel sheets according to the comparative examples showed defects in at least one of the above evaluation items.

[0163] Vehicle component according to the present invention A vehicle part according to the present invention is manufactured using the hot-rolled steel sheet according to the above-described embodiment of the present invention. For example, the vehicle part may be a lower arm. However, the type of vehicle part is not limited thereto, and the hot-rolled steel sheet may also be used to manufacture commercial vehicle frames, special vehicle parts, passenger vehicle parts, etc.

[0164] Table 3 shows the evaluation of punch formability and wire formability of the lower arms manufactured by processing the hot-rolled steel sheets according to Examples 1 to 6 and Comparative Examples 1 to 4 described in Table 1. Fig. 8 is a diagram for explaining the evaluation results in Table 3.

[0165] [Table 3]

[0166] 8, the lower arms according to Examples 1 to 6 and Comparative Examples 3 and 4 were evaluated as "good" in the punch processability and wire processability evaluations. However, the lower arms according to Comparative Examples 1 and 2 were evaluated as "poor" in the processability and wire processability evaluations.

[0167] The reason for this is thought to be that the copper and tin content ratios contained in the hot-rolled steel sheet exceeded 0.13 and 0.013, respectively, resulting in a decrease in part formability.

[0168] While preferred embodiments of the present invention have been described above, it will be obvious to those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or scope of the present invention.

[0169] That is, the above-described embodiments should be considered as illustrative rather than restrictive, and therefore the present invention is not limited to the above description, but may be modified within the scope of the appended claims and their equivalents.

Claims

1. producing molten iron using raw materials including molten iron produced in a blast furnace, direct reduced iron, and iron scrap; producing a semi-finished product; and producing a hot rolled steel sheet, The raw materials comprise 20% by weight to 60% by weight of the molten iron, 10% by weight to 40% by weight of the direct reduced iron, and 20% by weight to 60% by weight of iron scrap.

2. The step of producing the molten metal includes: charging the raw material into an electric furnace; The method for manufacturing a hot-rolled steel sheet according to claim 1 , further comprising the step of: melting the raw materials in an electric furnace.

3. The method for producing a hot-rolled steel sheet according to claim 1, wherein the copper content of the hot-rolled steel sheet is 0.12 wt% or less (excluding 0).

4. The copper content of the molten iron is 0.03 wt% or less (excluding 0), The copper content of the direct reduced iron is 0.02 wt% or less (excluding 0), 4. The method for producing a hot-rolled steel sheet according to claim 3, wherein the copper content of the iron scrap is 0.18% by weight or less (excluding 0).

5. The method for producing a hot-rolled steel sheet according to claim 1, wherein the tin content of the hot-rolled steel sheet is 0.012 wt% or less (excluding 0).

6. The tin content of the hot metal is 0.003 wt% or less (excluding 0), The direct reduced iron has a tin content of 0.002 wt% or less (excluding 0), 6. The method for producing a hot-rolled steel sheet according to claim 5, wherein the tin content of the iron scrap is 0.018% by weight or less (excluding 0).

7. The step of manufacturing the hot-rolled steel sheet includes: Reheating at a reheating temperature in the range of 1150°C to 1350°C; A step of hot rolling at a finish rolling temperature in the range of 880°C to 930°C; cooling at a cooling rate in the range of 60°C / sec to 110°C / sec; and coiling at a coiling temperature in the range of 380°C to 480°C.

8. A hot-rolled steel sheet containing, on a weight percent basis, carbon: 0.07 to 0.12%, silicon: 0.3 to 0.8%, manganese: 1.5 to 2.0%, phosphorus: 0.02% or less, sulfur: 0.005% or less, copper: 0.12% or less (excluding 0), tin: 0.012% or less (excluding 0), the balance being iron (Fe) and inevitable impurities, The hot-rolled steel sheet is produced by melting 20% ​​to 60% by weight of blast furnace-based molten iron, 20% to 40% by weight of direct reduced iron, and 20% to 60% by weight of iron scrap.

9. The copper content of the molten iron is 0.03 wt% or less (excluding 0), The copper content of the direct reduced iron is 0.02 wt% or less (excluding 0), The hot-rolled steel sheet according to claim 8, wherein the copper content of the iron scrap is 0.18 wt% or less (excluding 0).

10. The tin content of the hot metal is 0.003 wt% or less (excluding 0), The direct reduced iron has a tin content of 0.002 wt% or less (excluding 0), The hot-rolled steel sheet according to claim 8, wherein the tin content of the iron scrap is 0.018 wt% or less (excluding 0).

11. When the surface of the hot-rolled steel sheet was observed after the phosphate conversion coating treatment, The weight of the coating is 1.8 to 3.0 g / m ２ The hot-rolled steel sheet according to claim 8, wherein the crystal grains satisfy 2 to 10 μm.

12. The hot-rolled steel sheet according to claim 8, further comprising at least one element selected from the group consisting of chromium (Cr), niobium (Nb), titanium (Ti), and boron (B).

13. A vehicle part containing, on a weight percent basis, carbon: 0.07 to 0.12%, silicon: 0.3 to 0.8%, manganese: 1.5 to 2.0%, phosphorus: 0.02% or less, sulfur: 0.005% or less, copper: 0.12% or less (excluding 0), tin: 0.012% or less (excluding 0), the balance being iron (Fe) and inevitable impurities, The base material of the vehicle part is produced by melting 20% ​​by weight to 60% by weight of blast furnace-based molten iron, 20% by weight to 40% by weight of direct reduced iron, and 20% by weight to 60% by weight of iron scrap.

14. The copper content of the molten iron is 0.03 wt% or less (excluding 0), The copper content of the direct reduced iron is 0.02 wt% or less (excluding 0), 14. The vehicle part according to claim 13, wherein the copper content of the iron scrap is 0.18% by weight or less (excluding 0).

15. The tin content of the hot metal is 0.003 wt% or less (excluding 0), The direct reduced iron has a tin content of 0.002 wt% or less (excluding 0), The vehicle part according to claim 13, wherein the tin content of the iron scrap is 0.018% by weight or less (excluding 0).

16. When observing the surface of vehicle parts after phosphate conversion coating, The weight of the coating is 1.8 to 3.0 g / m ２ The vehicle part according to claim 13, wherein the crystal grains satisfy the following: 2 to 10 μm.

17. The vehicle part according to claim 13, wherein the vehicle part is a lower arm.

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