Steel sheet and method for manufacturing the same

Abstract

This invention relates to a steel sheet suitable for use as a structural component, reinforcement, etc. in automobiles, and more specifically, to a steel sheet with excellent hole expansion and surface hardness, and a method for manufacturing the same.

Description

Technical Field

[0001] This invention relates to a steel sheet suitable for use as a structural component, reinforcement, etc. in automobiles, and more specifically, to a steel sheet with excellent hole expansion and surface hardness, and a method for manufacturing the same. Background Technology

[0002] In recent years, with the goal of reducing global greenhouse gas emissions, the powertrain structure of automobiles has been shifting from internal combustion engines to electric motors. Competition in the electric vehicle market for lightweighting to increase driving range on a single charge is intensifying. As part of this, aluminum alloys, which have a lower density than steel, are being actively used to minimize the weight of battery pack casing components.

[0003] In addition, as automakers and parts manufacturers mass-produce electric vehicles to make them more accessible to the general public, they are also increasing their use of steel materials, which are relatively inexpensive, have low carbon emissions from a life cycle assessment perspective, and are highly resistant to thermal runaway, due to the decline in battery prices.

[0004] The following literature can be cited as relevant technologies.

[0005] Patent Document 1 illustrates the application of steel in the lower plate of the battery casing used to protect the battery cells. In this technology, from a collision stability perspective, ultra-high-strength steel with a tensile strength of 980-1800 MPa is used to protect the battery cells from impacts generated by the lower part of the vehicle body during driving. This ultra-high-strength steel not only provides collision stability but also supports the heavy battery pack. However, to ensure high strength, a large amount of iron alloy must be used, and the manufacturing cost is also relatively high due to the complex heat treatment process.

[0006] Patent Document 2 discloses a steel sheet for use as a structural component or reinforcement in automobile bodies, possessing a tensile strength of 490 MPa or higher, a tensile strength-to-elongation ratio (TS×E1) of 15,000 MPa% or higher, and excellent plating characteristics. In this technology, by adding Sb to the alloy composition, a uniform enriched layer is formed on the very surface of the steel sheet, suppressing the formation of oxide inclusions in the surface layer and thus ensuring plating performance. However, the range of Sb content required to form an Sb-rich layer on the steel sheet surface has limitations in ensuring impact resistance against external factors such as stone chipping that may occur during vehicle operation. Specifically, to ensure the aforementioned impact resistance, the surface hardness of the steel sheet must be uniform, but the solid solution strengthening and phase transformation strengthening effects required to achieve this may be insufficient. Furthermore, the addition of large amounts of Sb can not only worsen plating adhesion but, as an expensive element, also increases manufacturing costs, making it economically disadvantageous.

[0007] Furthermore, Patent Document 3 discloses a technique for manufacturing hot-dip galvanized steel sheets or alloyed hot-dip galvanized steel sheets with a tensile strength of 590 MPa, exhibiting excellent fatigue characteristics and impact resistance. In this technique, to suppress surface softening that typically occurs during steel sheet manufacturing, a leveling rolling process is performed after hot rolling to increase precipitation sites within the surface microstructure, thereby achieving additional precipitation strengthening and ensuring surface hardness. However, to achieve precipitation strengthening, expensive alloying elements such as Nb, Ti, Mo, and V must be added, and if these elements are not uniformly distributed in the surface, problems such as poor plating properties, like lack of plating, can occur. Moreover, relying solely on precipitation strengthening to ensure surface hardness of the steel sheet may lead to defects such as transverse cracks during component processing.

[0008] (Patent Document 1) Korean Patent Publication No. 2023-0032302 (Patent Document 2) Korean Patent No. 10-0711358 (Patent Document 3) Korean Patent No. 10-1313957 Summary of the Invention

[0009] (a) Technical problems to be solved One aspect of the present invention provides a steel plate and a method for manufacturing the steel plate, the steel plate being suitable as a material for structural components, reinforcements, etc. of automobiles, and having uniform surface hardness and excellent porosity in order to improve resistance to impacts that may occur during automobile use.

[0010] The technical problems of this invention are not limited to those described above. The technical problems of this invention can be understood from the entire contents of this specification, and those skilled in the art will have no difficulty understanding the additional technical problems of this invention.

[0011] (II) Technical Solution According to one aspect of the invention, a steel plate is provided, comprising, by weight percent: carbon (C): 0.050-0.150%, silicon (Si): 0.010-1.000%, manganese (Mn): 1.000-3.000%, aluminum (Sol.Al): 0.01-0.10%, phosphorus (P): 0.001-0.050%, sulfur (S): 0.001-0.020%, nitrogen (N): 0.0010-0.0100%, chromium (Cr): 0.020-0.300%, nickel (Ni): 0.010-0.100%, copper (Cu): 0.010-0.400%, with the balance being Fe and other unavoidable impurities.

[0012] According to one embodiment of the present invention, the alloy composition of the steel plate, Si, Cr and Ni, can satisfy the following relationship 1.

[0013] [Relation 1] 2.000≤17(Cr+Ni) / Si≤4.000 (In Equation 1, Cr, Ni, and Si represent the weight content of each element.) As described above, a steel sheet according to an embodiment of the present invention, which satisfies Equation 1, representing the relationship between alloy composition and the content of specific elements, can control the enrichment behavior of Cr and Ni in the surface layer. In particular, the relationship between the content of Cr and Ni and the enrichment value of Cr and Ni in the surface layer can satisfy the following Equation 2.

[0014] [Relationship 2] 0.150≤Δ(Cr+Ni)≤0.500 Δ(Cr+Ni)=[Cr (0.5μm) +Ni (0.5μm) ]-[Cr (1 / 4t) +Ni (1 / 4t) ] (In relation 2, Cr) (0.5μm) and Ni (0.5μm) This represents the maximum enrichment (by weight%) of Cr and Ni in the surface region extending from the surface of the steel plate to a depth of 500 nm along the thickness direction. (1 / 4t) and Ni (1 / 4t) This indicates the average weight content of Cr and Ni at position t / 4 along the thickness direction of the steel plate (where t represents the thickness of the steel plate (mm)). In one embodiment of the invention, the steel plate may further contain one or more of molybdenum (Mo), niobium (Nb) and antimony (Sb) in a total content of less than 1.0%, and may further contain boron (B) in a content of less than 0.0030%.

[0015] In one embodiment of the present invention, the microstructure may comprise: ferrite with an area fraction of 90% or more; and the balance of one or more of bainite, martensite and retained austenite.

[0016] This steel plate according to one embodiment of the invention has high strength, excellent hole expansion properties, and can uniformly ensure surface hardness.

[0017] As an example, the tensile strength (TS) of the steel plate is 590 MPa or higher, and the relationship between the tensile strength (TS) and the porosity (HER) can satisfy the following equation 3. Furthermore, the surface hardness (Vickers hardness) of the steel plate can be 170 or higher.

[0018] In one embodiment of the invention, the steel plate may contain a zinc-based coating on at least one side.

[0019] According to another aspect of the present invention, a method for manufacturing a steel plate includes the following steps: preparing a steel billet; heating the steel billet in a temperature range of 1000-1350°C; hot-finishing the heated steel billet in a temperature range of 800-1000°C to obtain a hot-rolled steel plate; coiling the hot-rolled steel plate in a temperature range of 400-650°C; cold-rolling the coiled hot-rolled steel plate with a reduction rate of 30% or more to obtain a cold-rolled steel plate; heating the cold-rolled steel plate to a temperature of Ac1 or higher and holding it for annealing; and cooling the annealed cold-rolled steel plate to a temperature range of 450-650°C at a cooling rate of 35°C / second or lower.

[0020] In one embodiment of the present invention, the steel billet may have the above-mentioned alloy composition and may simultaneously satisfy the aforementioned relation 1.

[0021] When a steel billet that satisfies the alloy composition and relation 1 according to one embodiment of the present invention is used, the manufactured steel plate can satisfy the above relation 2.

[0022] In one embodiment of the invention, the process may further include a step of hot-dip galvanizing the cooled cold-rolled steel sheet at a temperature range of 440-480°C to obtain a hot-dip galvanized steel sheet, and optionally may further include a step of alloying heat treatment of the hot-dip galvanized steel sheet.

[0023] (III) Beneficial Effects According to the present invention, a steel sheet with superior surface hardness uniformity and improved porosity can be provided compared with a high-strength steel sheet utilizing conventional phase transformation structures.

[0024] The steel sheet of this invention is easy to form into components, thus having the effect of being suitable for use in structural components or reinforcements of automobiles. Attached Figure Description

[0025] Figure 1 For a steel sheet according to one embodiment of the present invention, a portion of the results of measuring the enrichment values ​​of Cr and Ni from the surface to a depth of 1 μm in the thickness direction are shown using glow discharge spectrometer (GDS) analysis. Best practice

[0026] The inventors of this invention recognized the problems with ultra-high strength steel, which has been used in the past as a material for structural components and reinforcements in automobiles. Therefore, they conducted in-depth research on solutions that require ensuring resistance to continuous impacts in the automotive operating environment and that improve the perforation properties of the steel sheet for easy molding into parts, etc.

[0027] As a result, while optimizing the alloy composition and manufacturing conditions of the steel plate, the influence of the content relationship between specific elements on the surface hardness of the steel plate was confirmed, thus completing the present invention.

[0028] The present invention will now be described in detail.

[0029] The steel plate according to one aspect of the present invention comprises, by weight percent: carbon (C): 0.050-0.150%, silicon (Si): 0.010-1.000%, manganese (Mn): 1.000-3.000%, aluminum (Sol.Al): 0.01-0.10%, phosphorus (P): 0.001-0.050%, sulfur (S): 0.001-0.020%, nitrogen (N): 0.0010-0.0100%, chromium (Cr): 0.020-0.300%, nickel (Ni): 0.010-0.100%, and copper (Cu): 0.010-0.400%.

[0030] The reasons for limiting the alloy composition of the steel plate according to one aspect of the present invention will be explained in detail below. Furthermore, unless otherwise specified, the content of each element is based on weight, and the proportion of the microstructure is based on area.

[0031] Carbon (C): 0.050-0.150% Carbon (C) is the most economical and effective element for strengthening steel. The higher its content, the higher the fraction of hard phases (martensite, bainite, etc.) in the steel structure, and the higher the tensile strength.

[0032] In one embodiment of the invention, when the C content is less than 0.050%, the fraction of hard phase in the steel microstructure becomes insufficient, making it impossible to ensure the target strength level. On the other hand, when the C content exceeds 0.150%, coarse carbides or pearlite are easily formed in the steel, resulting in poor formability and reduced weldability.

[0033] Therefore, in one embodiment of the present invention, C may comprise 0.050-0.150%. According to another embodiment of the present invention, C may comprise 0.060% or more, or 0.070% or more. According to yet another embodiment of the present invention, C may be less than 0.145% or less, or less than 0.140%.

[0034] Silicon (Si): 0.010-1.000% Silicon (Si) is an element added to deoxidize molten steel, which can exert a solid solution strengthening effect.

[0035] In one embodiment of the present invention, the above-mentioned effects cannot be fully obtained when the Si content is less than 0.010%. On the other hand, when the Si content exceeds 1.000%, oxides will form on the steel surface, which will hinder the plating performance.

[0036] Therefore, in one embodiment of the present invention, the Si may be contained in amounts of 0.010-1.000%. According to another embodiment of the present invention, the Si may be contained in amounts of 0.020% or more, or 0.030% or more. According to yet another embodiment of the present invention, the Si may be contained in amounts of 0.900% or less, or 0.850% or less.

[0037] Manganese (Mn): 1.000-3.000% Manganese (Mn), like Si, can be added for solid solution strengthening. Furthermore, Mn improves the hardenability of steel, making it easier for hard phases to form in the steel's microstructure.

[0038] In one embodiment of the present invention, when the Mn content is less than 1.000%, the above-mentioned effects cannot be fully obtained. On the other hand, when the Mn content exceeds 3.000%, the hardenability of the steel increases significantly, the fraction of the hard phase increases excessively, and it may be difficult to ensure the expected ductility.

[0039] Therefore, in one embodiment of the present invention, Mn may contain 1.000-3.000%. According to another embodiment of the present invention, Mn may contain 1.100% or more, or 1.200% or more. According to yet another embodiment of the present invention, Mn may be less than 2.900% or less, or less than 2.850%.

[0040] Aluminum (Sol.Al): 0.01-0.10% Aluminum (Sol.Al) is an element added for the deoxidation of molten steel.

[0041] In one embodiment of the invention, the addition effect is insufficient when the content of acid-soluble aluminum (Sol.Al) is less than 0.01%. On the other hand, when the content of acid-soluble aluminum (Sol.Al) exceeds 0.10%, it combines with nitrogen (N) to form AlN, which easily causes corner cracks on the billet during continuous casting. Furthermore, there is a concern about the generation of defects due to the formation of Al-based inclusions.

[0042] Therefore, in one embodiment of the present invention, the acid-soluble aluminum (Sol.Al) may be contained in 0.01-0.10%.

[0043] Phosphorus (P): 0.001-0.050% Phosphorus (P) is an unavoidable element in steel. When the phosphorus (P) content is too high, it hinders the weldability of the steel and increases the risk of brittleness. In addition, controlling the P content to extremely low levels leads to a sharp increase in refining costs.

[0044] Therefore, in one embodiment of the present invention, the content of P can be limited to 0.001-0.050%.

[0045] Sulfur (S): 0.001-0.020% Sulfur (S) is an element that inevitably mixes into steel. When the sulfur (S) content is too high, it not only hinders the weldability of the steel but may also reduce its ductility. In addition, controlling the amount of S to extremely low levels leads to a sharp increase in refining costs.

[0046] Therefore, in one embodiment of the present invention, the content of S can be limited to 0.001-0.020%.

[0047] Nitrogen (N): 0.0010-0.0100% Nitrogen (N) is an unavoidable element in steel, and excessive nitrogen (N) content increases the risk of brittleness. Furthermore, excessive AlN precipitation due to its reaction with Al leads to reduced continuous casting quality. Additionally, controlling the N content to extremely low levels results in a sharp increase in refining costs.

[0048] Therefore, in one embodiment of the present invention, the content of N can be limited to 0.0010-0.0100%.

[0049] Chromium (Cr): 0.020-0.300% Chromium (Cr) is an element that delays the formation of ferrite and promotes the formation of hard phases during the cooling process in steel plate manufacturing.

[0050] In one embodiment of the present invention, when the Cr content is less than 0.020%, the hard phase cannot be sufficiently formed, and the target strength level cannot be ensured. On the other hand, when the Cr content exceeds 0.300%, an excessive martensitic phase will form in the steel microstructure, resulting in reduced ductility.

[0051] Therefore, in one embodiment of the invention, the Cr may be contained in the form of 0.020-0.300%. According to another embodiment of the invention, the Cr may be contained in the form of 0.025% or more, or 0.030% or more. According to yet another embodiment of the invention, the Cr may be less than 0.295% or less, or less than 0.290%.

[0052] Nickel (Ni): 0.010-0.100% Nickel (Ni), similar to Cr, is an element that delays the formation of ferrite and promotes the formation of hard phases during the cooling process in steel sheet manufacturing.

[0053] In one embodiment of the present invention, when the Ni content is less than 0.010%, the hard phase cannot be sufficiently formed, and the target strength level cannot be ensured. On the other hand, when the Ni content exceeds 0.100%, an excessive martensitic phase will form in the steel microstructure, resulting in reduced ductility.

[0054] Therefore, in one embodiment of the invention, the Ni may be contained in amounts of 0.010-0.100%. According to another embodiment of the invention, the Ni may be contained in amounts of 0.015% or more, or 0.020% or more. According to yet another embodiment of the invention, the Ni may be less than 0.098%.

[0055] Copper (Cu): 0.010-0.400% Copper (Cu) is an element that is useful in ensuring the weather resistance of steel and preventing embrittlement caused by hydrogen.

[0056] In one embodiment of the present invention, the above-mentioned effects cannot be fully obtained when the Cu content is less than 0.010%. On the other hand, when the Cu content exceeds 0.400%, a large number of scab (pit) defects may be generated on the surface of the steel billet due to grain boundary liquefaction.

[0057] Therefore, in one embodiment of the invention, the Cu content may be 0.010-0.400%. According to another embodiment of the invention, the Cu content may be 0.012% or more. According to yet another embodiment of the invention, the Cu content may be less than 0.380%.

[0058] In addition to the alloy composition described above, the steel plate according to one embodiment of the present invention may further contain one or more of molybdenum (Mo), niobium (Nb) and antimony (Sb) and / or less than 0.0030% boron (B).

[0059] Molybdenum (Mo), niobium (Nb), and boron (B) are elements that improve the hardenability of steel and contribute to the formation of the hard phase that forms the steel's microstructure. Antimony (Sb) is beneficial for ensuring plating quality by forming a uniform enriched layer on the surface of the steel sheet.

[0060] In one embodiment of the invention, when one or more of Mo, Nb, and Sb are added, a sharp increase in manufacturing costs occurs when their total content exceeds 1.0%. In particular, in the case of Sb, excessive surface enrichment may actually worsen the coating adhesion.

[0061] In one embodiment of the present invention, when the content of B exceeds 0.003%, the ductility of the steel may be reduced.

[0062] The remaining component of this invention is iron (Fe). However, in conventional manufacturing processes, unintended impurities inevitably enter from raw materials or the surrounding environment, and therefore cannot be eliminated. These impurities are known to those skilled in the art in conventional manufacturing processes, and therefore their contents are not specifically mentioned in this specification.

[0063] In one embodiment of the invention, in order to ensure both uniform surface hardness and ductility of the steel plate, the contents of Cr and Ni, which are hardenability elements, can be limited in relation to Si. Specifically, the Cr, Ni, and Si can satisfy the following relationship 1.

[0064] [Relation 1] 2.000≤17(Cr+Ni) / Si≤4.000 (In Equation 1, Cr, Ni, and Si represent the weight content of each element.) Si, present in steel, is an element that favors the formation of oxides on the steel surface. According to one embodiment of the invention, by increasing the content of Cr and Ni relative to Si, it is possible to increase the enrichment of Cr and Ni on the steel surface without forming Si-induced oxides.

[0065] In particular, according to one embodiment of the invention, by optimizing the relationship between alloy compositions as described above, while controlling the manufacturing conditions of the steel plate as detailed below, the enrichment values ​​of Cr and Ni in the steel surface layer can be ensured as follows.

[0066] [Relationship 2] 0.150≤Δ(Cr+Ni)≤0.500 Δ(Cr+Ni)=[Cr (0.5μm) +Ni (0.5μm) ]-[Cr (1 / 4t) +Ni (1 / 4t) ] (In relation 2, Cr) (0.5μm) and Ni (0.5μm) This represents the maximum enrichment (by weight%) of Cr and Ni in the surface region extending from the surface of the steel plate to a depth of 500 nm along the thickness direction. (1 / 4t) and Ni(1 / 4t) This indicates the average weight content of Cr and Ni at position t / 4 along the thickness direction of the steel plate (where t represents the thickness of the steel plate (mm)). That is, in the steel plate according to one embodiment of the present invention, Cr and Ni, respectively added in certain amounts, can be enriched primarily on the surface side of the steel plate rather than on the inner side, more specifically directly below the oxide layer formed on the surface of the steel plate. By enriching these hardenable elements on the surface side of the steel plate (directly below the oxide layer), solid solution and phase transformation strengthening effects can be introduced on the surface side of the steel plate. As a result, the effect of preventing the decrease in hardness of the surface layer of the steel plate can be obtained.

[0067] Therefore, when the steel plate according to one embodiment of the present invention is applied to structural components, reinforcements, etc. of automobiles, it has the advantage of enhancing resistance to continuous impacts generated in the actual use environment.

[0068] Additionally, it should be noted that the Cr and Ni measured at position t / 4 in the thickness direction of the steel plate in Equation 2 represent the matrix steel composition of each element added to the steel plate. To represent this, the content measured at position t / 4 is used as the baseline. In other words, Cr in Equation 2... (1 / 4t) and Ni (1 / 4t) These represent the contents of Cr and Ni in the base steel plate, respectively, and are set as representative values, measured at position t / 4 in the thickness direction.

[0069] According to one embodiment of the present invention, the steel sheet, as a fine microstructure, is dominated by ferrite, and as a reserve microstructure, may contain one or more of bainite, martensite, and retained austenite. The reserve microstructure is referred to as the second phase.

[0070] In one embodiment of the invention, ferrite, which forms the main phase, may contain an area fraction of 90% or more. When the ferrite fraction is less than 90%, the ductility of the steel sheet may be reduced.

[0071] Furthermore, the steel plate according to one embodiment of the present invention ensures the target strength by including a hard phase bainite (B) and a martensite (M) phase as a second phase. As an example, the hard phase (B+M) may be included in an area fraction of 3% or more, and not exceeding a maximum of 10%.

[0072] Furthermore, it may contain a retained austenite phase as the second phase, which is the remaining structure after the formation of the aforementioned ferrite and hard phase, even if it contains 0%. However, the retained austenite phase shall not exceed a maximum of 2%.

[0073] In addition, it is not excluded that the second phase may contain trace amounts of pearlite (P) phase. In addition to the hard phase, the second phase may also contain residual austenite phase and pearlite.

[0074] The steel sheet according to one embodiment of the present invention can have physical properties suitable for use as a material in automobiles, particularly as a structural component, reinforcement, etc. As an example, the steel sheet according to one embodiment of the present invention can have a tensile strength of 590 MPa or higher.

[0075] Furthermore, the steel plate according to one embodiment of the present invention can have the characteristics of uniform surface hardness and excellent hole expansion properties.

[0076] As an example, the relationship between the tensile strength (TS) and the porosity (HER) of the steel plate can satisfy the following equation 3, and it can have a surface hardness (Vickers hardness) of 170 or higher. Here, surface hardness refers to the hardness value measured with reference to a depth of 2 μm from the surface of the steel plate in the thickness direction.

[0077] [Relationship 3] TS×HER≥30000MPa% According to one embodiment of the invention, the steel sheet refers to a cold-rolled steel sheet, but may also include a coated steel sheet having a coating on at least one side of the cold-rolled steel sheet.

[0078] At this point, the coating can be a zinc-based coating with zinc (Zn) as the main component. It should be noted that the addition of Mg, Al, Si, etc., in addition to zinc is not excluded. That is, the coating can be a pure zinc coating or an alloy coating.

[0079] The following describes in detail a method for manufacturing a steel sheet according to another aspect of the present invention. It should be noted that the following manufacturing method is an example for manufacturing a steel sheet according to one embodiment of the present invention, particularly a cold-rolled steel sheet and a galvanized steel sheet utilizing the cold-rolled steel sheet.

[0080] According to one embodiment of the present invention, the steel plate can be manufactured from a prepared steel billet by a process of [heating-hot rolling-coiling-cold rolling-annealing-cooling], and each process step will be described in detail below.

[0081] [Bill Heating] First, a steel billet is prepared. In one embodiment of the invention, the steel billet can be manufactured using a converter or an electric furnace. Then, the prepared steel billet can be heated. The heating process of the steel billet is designed to facilitate the hot rolling process described later and to fully obtain the physical properties of the target steel plate.

[0082] As an example, the billet may have the same alloy composition and alloy composition formula (Formula 1) as the steel plate according to one embodiment of the present invention, with the description of each alloying element and the description of the composition formula replaced by the above content.

[0083] In one embodiment of the invention, the steel billet is heated within a temperature range of 1000-1350°C. When the heating temperature is below 1000°C, it is difficult to form an oxide scale layer through selective oxidation of Fe, and Cr and Ni cannot be sufficiently enriched directly beneath the oxide scale layer. On the other hand, when the heating temperature exceeds 1350°C, there is a problem of excessive oxide scale formation.

[0084] [Hot Rolled] The heated steel billet can be hot-rolled to obtain hot-rolled steel sheet.

[0085] In one embodiment of the invention, hot-rolled steel sheets can be manufactured by hot finishing rolling within a temperature range of 800-1000°C during the hot rolling process. In another embodiment, when the temperature during hot finishing rolling is below 800°C, there is a problem of a significant increase in rolling load. On the other hand, when the temperature exceeds 1000°C, there is a problem of a significant increase in thermal fatigue of the rolling rolls.

[0086] [Collection] The hot-rolled steel sheet described above can be coiled up.

[0087] In one embodiment of the invention, the coiling process can be carried out within a temperature range of 400-650°C. When the coiling temperature is below 400°C, excessive bainite and martensite will form in the hot-rolled steel sheet, leading to an increase in rolling load during subsequent cold rolling. On the other hand, when the coiling temperature exceeds 650°C, excessive oxide scale may form on the surface of the hot-rolled steel sheet, which hinders plating.

[0088] [Cold Rolled] The hot-rolled steel sheet that has been coiled can be cold-rolled to produce a cold-rolled steel sheet. The cold rolling is a process performed to control the shape and thickness of the steel sheet.

[0089] In one embodiment of the present invention, the reduction rate during cold rolling is not particularly limited. However, in order to suppress the formation of coarse ferrite during recrystallization in subsequent annealing, cold rolling can be performed with a reduction rate of 30% or more. Furthermore, there is no particular upper limit to the reduction rate; it should be noted that those skilled in the art can choose an appropriate reduction rate based on the target thickness.

[0090] In addition, before cold rolling, the hot-rolled steel sheet can be pickled to remove the oxide layer formed on the surface. The pickling process can be carried out under conventional conditions, and there are no particular limitations on these conditions.

[0091] [Annealing and Cooling] After annealing, the manufactured cold-rolled steel sheet can be cooled.

[0092] In one embodiment of the invention, annealing can be performed by heating the cold-rolled steel sheet to above Ac1. When the annealing temperature is below Ac1, the austenitic phase cannot be sufficiently formed during the annealing process, resulting in the final fine microstructure not forming as expected, which may lead to decreased strength and increased anisotropy of physical properties.

[0093] In one embodiment of the invention, after heating the cold-rolled steel sheet, it can be held for a certain period of time. During this time, the holding process can last for more than 10 seconds to induce sufficient recrystallization.

[0094] After the annealing and holding process, the cold-rolled steel sheet can be cooled. In one embodiment of the invention, cooling can be carried out at a cooling rate of less than 35°C / second (except 0°C / second) to a temperature range of 450-650°C.

[0095] When the cooling rate exceeds 35°C / second, the inhomogeneity of the ferrite phase transformation increases, potentially leading to poor steel plate shape, wave-like phenomena, and even plate breakage due to misalignment. Furthermore, when the cooling termination temperature is below 450°C, excessive bainite and martensite phases form, resulting in poor porosity of the steel plate. Conversely, when the temperature exceeds 650°C, excessive pearlite formation also potentially worsens the porosity of the steel plate.

[0096] In addition, there is no particular limitation on the lower limit of the cooling rate during the cooling process, but from the perspective of ensuring the expected microstructure and physical properties, it can be carried out at a rate of 3°C or higher.

[0097] Furthermore, during annealing and cooling as described above, the conveying speed of the steel sheet, i.e., the linear speed (m / min (mpm)), can be 80-200 m / min. As an example, the annealing of cold-rolled steel sheets can be carried out by loading the cold-rolled steel sheet into an annealing furnace. In this case, if the cold-rolled steel sheet passes through the furnace for too long, excessive oxide formation may occur on the surface of the steel sheet during annealing. Therefore, considering this, the linear speed can be set to 80 m / min or higher. However, when the speed exceeds 200 m / min, sufficient recrystallization may not occur during annealing. According to another embodiment of the invention, the linear speed of the steel sheet during annealing can be 100 m / min or higher, and can also be 180 m / min or lower.

[0098] According to one embodiment of the present invention, by controlling the alloy composition system and manufacturing conditions, a steel sheet (cold-rolled steel sheet) having soft ferrite and a certain fraction of hard phase can be obtained. In particular, by increasing the enrichment of Cr and Ni on the surface side of the steel sheet, uniform surface hardness can be ensured, and excellent porosity can be achieved.

[0099] In one embodiment of the present invention, cold-rolled steel sheets can be coated to obtain coated steel sheets. As an example, cold-rolled steel sheets can be hot-dip galvanized.

[0100] Hot-dip galvanizing A cold-rolled steel sheet according to one embodiment of the present invention can be immersed in a hot-dip galvanizing bath to produce a hot-dip galvanized steel sheet.

[0101] In one embodiment of the present invention, the hot-dip galvanizing process can be carried out under conventional conditions, but as an example, it can be carried out in a temperature range of 440-480°C. Furthermore, the composition of the hot-dip zinc bath used in the hot-dip galvanizing process is not particularly limited; it can be a pure zinc bath or a zinc alloy bath containing Si, Al, Mg, etc.

[0102] [Alloying Heat Treatment] Furthermore, if necessary, alloyed hot-dip galvanized steel sheets can be obtained by performing alloying heat treatment on hot-dip galvanized steel sheets manufactured according to one embodiment of the present invention.

[0103] In one embodiment of the present invention, there are no particular limitations on the alloying heat treatment process conditions; any conventional conditions are acceptable. As an example, the alloying heat treatment process can be carried out in a temperature range of 480-600°C. Detailed Implementation

[0104] The present invention will now be described in more detail through embodiments. However, it should be noted that the following embodiments are merely illustrative of the invention for more detailed explanation and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters recorded in the claims and those reasonably inferred therefrom.

[0105] (Example) After preparing steel billets with the alloy compositions shown in Table 1 below, each billet is heated in a temperature range of 1000-1350°C. Then, it undergoes a series of processes under the conditions shown in Table 2 below to manufacture cold-rolled steel sheets.

[0106] The microstructure and mechanical properties of the manufactured cold-rolled steel sheets were measured, and the results are shown in Table 3 below.

[0107] First, the enrichment values ​​of Cr and Ni in each cold-rolled steel sheet were measured from the surface to a depth of 1 μm along the thickness direction using glow discharge analysis (GDS). The (Cr+Ni) content at a depth of 500 nm from the surface was derived from the results. Furthermore, the (Cr+Ni) content at position t / 4 along the thickness direction of each cold-rolled steel sheet (where t represents the thickness (mm) of the cold-rolled steel sheet) was calculated using SEM-EDS and the average value was derived. The calculated Δ(Cr+Ni) value is shown in Table 3 below.

[0108] The types and fractions of microstructures in each cold-rolled steel sheet were determined by etching with nitric acid alcohol (Nital) and then observed and measured at magnification of 1000-5000x using a scanning electron microscope (SEM) and an image analyzer.

[0109] The tensile strength (TS) and elongation at break (E1) of the mechanical properties of each cold-rolled steel sheet were measured after collecting ASTM standard specimens in a direction parallel to the rolling direction. Furthermore, hardness was measured on the surface using a micro Vickers hardness tester. A load of 500g was applied, and with the analysis area set to 20mm × 10mm (transverse × longitudinal), 50 points were measured at 2mm intervals, and the minimum hardness value was then expressed.

[0110] In addition, the hole expansion ratio (HER) of each cold-rolled steel sheet was measured according to the JSF T1001-1996 standard for 90mm×120mm (transverse×longitudinal) samples.

[0111] [Table 1] [Table 2] [Table 3] As shown in Tables 1 to 3 above, in the inventive steels 1 to 5 that meet the alloy composition system and manufacturing conditions according to one embodiment of the present invention, the fine microstructure is mainly formed by ferrite, with a suitable formation of hard phases. As a result, while possessing high strength, excellent porosity can be ensured. Furthermore, since the enrichment behavior of Cr and Ni on the surface of the steel plate is controlled, the surface hardness value is 170 or higher, exhibiting excellent performance.

[0112] On the other hand, the expected physical properties cannot be ensured in comparative steels 1 to 4 that do not meet at least one of the alloy composition system and manufacturing conditions according to an embodiment of the present invention.

[0113] Among them, comparative steel 1 has excessive enrichment of Cr and Ni in its surface layer, failing to satisfy relation 2. As a result, the ferrite phase cannot form sufficiently, and the excess microstructure forms, leading to poor porosity. Comparative steel 2, due to insufficient enrichment of Cr and Ni in its surface layer, exhibits significantly reduced surface hardness. Comparative steels 3 and 4 represent cases where the annealing conditions were not met during manufacturing. Comparative steel 3, due to excessively low termination temperature in the quenching section, has an excessive fraction of hard phase, resulting in poor porosity. Comparative steel 4, due to excessively high termination temperature in the quenching section, has an excessive fraction of pearlite within its microstructure, leading to poor porosity and low surface hardness.

[0114] Figure 1 The results are the GDS curve measurements of the invention steel 5, the comparative steel 1, and the comparative steel 2.

[0115] That is, based on the (Cr+Ni) content measured from the surface of the steel plate in the thickness direction, with a thickness of about 0.1 μm as a benchmark, the (Cr+Ni) content of the comparative steel 1 is too high, the (Cr+Ni) content of the comparative steel 2 is quite low, and the Cr and Ni of the inventive steel 5 are present at appropriate levels, which confirms this point.

Claims

1. A steel plate, by weight percent, comprising: carbon (C): 0.050-0.150%, silicon (Si): 0.010-1.000%, manganese (Mn): 1.000-3.000%, aluminum (Sol.Al): 0.01-0.10%, phosphorus (P): 0.001-0.050%, sulfur (S): 0.001-0.020%, nitrogen (N): 0.0010-0.0100%, chromium (Cr): 0.020-0.300%, nickel (Ni): 0.010-0.100%, copper (Cu): 0.010-0.400%, with the balance being Fe and other unavoidable impurities. The Si, Cr, and Ni satisfy the following relationship 1. The relationship between the Cr and Ni content and the enrichment values ​​of Cr and Ni in the surface layer satisfies the following Equation 2. [Relation 1] 2.000≤17(Cr+Ni) / Si≤4.000 In Equation 1, Cr, Ni, and Si represent the weight content of each element, respectively. [Relation 2] 0.150≤Δ(Cr+Ni)≤0.500 Δ(Cr+Ni)=[Cr (0.5μm) +Ni (0.5μm) ]-[Cr (1 / 4t) +Ni (1 / 4t) ] In relation 2, Cr (0.5μm) and Ni (0.5μm) This represents the maximum enrichment values ​​of Cr and Ni in the surface region extending from the surface of the steel plate to a depth of 500 nm along the thickness direction, expressed in weight %. (1 / 4t) and Ni (1 / 4t) This represents the average weight content of Cr and Ni at position t / 4 along the thickness direction of the steel plate. t represents the thickness of the steel plate, and its unit is mm.

2. The steel plate according to claim 1, wherein, The steel plate further comprises one or more elements selected from molybdenum (Mo), niobium (Nb), and antimony (Sb) in a total content of less than 1.0%.

3. The steel plate according to claim 1, wherein, The steel plate further contains less than 0.0030% boron (B).

4. The steel plate according to claim 1, wherein, The steel plate has a fine microstructure comprising: ferrite with an area fraction of 90% or more; and the balance of one or more of bainite, martensite and retained austenite.

5. The steel plate according to claim 1, wherein, The tensile strength (TS) of the steel plate is above 590 MPa, and the relationship between the tensile strength (TS) and the porosity (HER) satisfies the following equation 3. [Relationship 3] TS×HER≥30000MPa%.

6. The steel plate according to claim 1, wherein, The surface hardness of the steel plate, i.e., the Vickers hardness, is above 170. Surface hardness refers to the hardness value measured with reference to a depth of 2 μm from the surface of the steel plate in the thickness direction.

7. The steel plate according to any one of claims 1 to 6, wherein, The steel plate has a zinc-based coating on at least one side.

8. A method for manufacturing a steel plate, comprising the following steps: Prepare a steel billet, which, by weight percent, contains: carbon (C): 0.050-0.150%, silicon (Si): 0.010-1.000%, Manganese (Mn): 1.000-3.000%, Aluminum (Sol.Al): 0.01-0.10%, Phosphorus (P): 0.001-0.050%, Sulfur (S): 0.001-0.020%, Nitrogen (N): 0.0010-0.0100%, Chromium (Cr): 0.020-0.300%, Nickel (Ni): 0.010-0.100%, Copper (Cu): 0.010-0.400%, with the balance being Fe and other unavoidable impurities, and the Si, Cr and Ni satisfy the following relationship 1; The steel billet is heated within a temperature range of 1000-1350℃; Hot finishing rolling is performed on the heated steel billet within a temperature range of 800-1000℃ to obtain hot-rolled steel plate; The hot-rolled steel sheet is coiled within a temperature range of 400-650℃; Cold-rolled steel sheet is obtained by cold rolling the hot-rolled steel sheet that has been coiled with a reduction rate of 30% or more. The cold-rolled steel sheet is heated to a temperature above Ac1 and held for annealing; and The annealed cold-rolled steel sheet is cooled to a temperature range of 450-650°C at a cooling rate of less than 35°C / second. [Relation 1] 2.000≤17(Cr+Ni) / Si≤4.000 In Equation 1, Cr, Ni, and Si represent the weight content of each element.

9. The method for manufacturing a steel plate according to claim 8, wherein, The relationship between the Cr and Ni content of the steel plate and the enrichment values ​​of Cr and Ni in the surface layer satisfies the following equation 2. [Relation 2] 0.150≤Δ(Cr+Ni)≤0.500 Δ(Cr+Ni)=[Cr (0.5μm) +Ni (0.5μm) ]-[Cr (1 / 4t) +Ni (1 / 4t) ] In relation 2, Cr (0.5μm) and Ni (0.5μm) This represents the maximum enrichment values ​​of Cr and Ni in the surface region extending from the surface of the steel plate to a depth of 500 nm along the thickness direction, expressed in weight %. (1 / 4t) and Ni (1 / 4t) This represents the average weight content of Cr and Ni at position t / 4 along the thickness direction of the steel plate, where t represents the thickness of the steel plate in mm.

10. The method for manufacturing a steel plate according to claim 8, wherein, The manufacturing method further includes the step of hot-dip galvanizing the cooled cold-rolled steel sheet at a temperature range of 440-480°C to obtain a hot-dip galvanized steel sheet.

11. The method for manufacturing a steel plate according to claim 10, wherein, The manufacturing method further includes the step of performing an alloying heat treatment on the hot-dip galvanized steel sheet.

12. The method for manufacturing a steel plate according to claim 8, wherein, The steel billet further comprises one or more elements selected from molybdenum (Mo), niobium (Nb), and antimony (Sb) in a total content of less than 1.0%.

13. The method for manufacturing a steel plate according to claim 8, wherein, The billet further contains less than 0.0030% boron (B).

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