Steel sheet and manufacturing method therefor

Abstract

The present invention can provide a steel sheet having high strength and excellent ductility and bendability, and a manufacturing method therefor.

Description

Steel plate and method of manufacturing the same

[0001] The present invention relates to a steel plate and a method for manufacturing the same.

[0002] Recently, as interest in carbon dioxide reduction and the associated regulatory standards have grown, automakers are making significant efforts to improve fuel efficiency through vehicle body lightweighting. To efficiently achieve this lightweighting, it is essential to adopt high-strength steel, which reduces body weight by decreasing steel plate thickness while simultaneously ensuring passenger safety.

[0003] Recently, there has been an increasing trend of applying high-strength steel to structural members such as members, seat rails, and A / B / C pillars to improve the impact resistance of the vehicle body. Automotive structural members have the characteristic that the higher the yield ratio (yield strength / tensile strength) relative to the yield strength, the more advantageous it is for absorbing impact energy.

[0004] However, since an increase in strength is accompanied by a decline in formability due to reduced ductility, there is a need to develop materials that possess high yield ratio characteristics while simultaneously improving formability. Therefore, in response to these technical requirements, it is necessary to develop automotive steel sheets that are suitable for automotive applications by possessing not only high strength but also excellent ductility and bendability.

[0005] Patent Document 1 discloses a technology for manufacturing a steel sheet having a martensite phase with an area fraction of 80% or more by using water cooling and overaging treatment during continuous annealing. When tempering is performed by immersing in water after cracking in the annealing process, a steel sheet having a tempered martensite structure can be manufactured. At this time, although the yield ratio increases due to the tempering effect of the martensite, there is a risk that workability will be reduced because the shape quality of the coil deteriorates due to temperature variations in the width and length directions of the steel sheet, or cracks occur during forming due to material variation issues.

[0006] Meanwhile, Patent Document 2 discloses a technology for manufacturing composite structure steel sheets with excellent workability by utilizing a retained austenite phase. Although this technology enables the simultaneous securing of strength and ductility required by automobile manufacturers by utilizing transformation-induced plasticity, it has the problem of difficulty in securing steelmaking and continuous casting quality due to the large amount of Si and Al added to create the retained austenite.

[0007] Accordingly, there is a need to develop a steel plate capable of simultaneously securing strength as well as ductility and bendability while solving the problems of the aforementioned existing technologies.

[0008] (Patent Document 1) Japanese Published Patent Application No. 1992-289120

[0009] (Patent Document 2) Japanese Published Patent Application No. 2015-113504

[0010] According to one embodiment of the present invention, a steel plate and a method for manufacturing the same are to be provided.

[0011] According to one embodiment of the present invention, the aim is to provide a high-strength cold-rolled steel sheet with excellent ductility and hole expansion properties and a method for manufacturing the same.

[0012] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0013] A steel plate according to one embodiment of the present invention is,

[0014] In weight percent, it comprises carbon (C): 0.1–0.35%, manganese (Mn): 1–3.6%, silicon (Si): 2.5% or less (excluding 0%), acid-soluble aluminum (Sol.Al): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), chromium (Cr): 1.0% or less (excluding 0%), and one or more selected from niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, and boron (B): 0.003% or less, and the remainder being iron (Fe) and unavoidable impurities, satisfying the following Equation 1, and

[0015] The microstructure contains 40% or more of tempered martensite and 4% or less of fresh martensite in area %, and the remainder comprises one or more of ferrite, bainite, and retained austenite, and

[0016] A first decarburization region having a thickness of 20㎛ to 40㎛ in the thickness direction of the steel plate on the surface of the steel plate; and a second decarburization region having a thickness of 10㎛ to 20㎛ in the thickness direction of the steel plate in the first decarburization region,

[0017] The above-mentioned first decarburization region contains 95% or more ferrite, and

[0018] The above second decarburized region may contain an area fraction of 4% or less of bainite and fresh martensite, and the remainder may contain ferrite.

[0019] [Relationship 1]

[0020] 5.0 ≤ 2[C] + 1.3[Si] + 1.5[Mn] + 1.2[Cr] + 1.2[B] + 1.8[Nb] ≤ 6.0

[0021] In Equation 1, [C], [Si], [Mn], [Cr], [B], and [Nb] are the weight percent of each element.

[0022] A plating layer may be formed on at least one surface of the above steel plate.

[0023] The above plating layer may be a zinc-based plating layer.

[0024] The above steel plate may have a yield strength of 950 MPa or more, a tensile strength of 980 MPa or more, and an elongation of 17% or more.

[0025] The above steel plate may have a yield ratio of 0.95 or higher.

[0026] The above steel plate may have a product of yield strength and elongation of 14,000 MPa·% or more, a product of tensile strength and elongation of 15,000 MPa·% or more, a 3-point bend of 110° or more, and a product of elongation and 3-point bend of 2,100%·° or more.

[0027]

[0028] A method for manufacturing a steel plate according to one embodiment of the present invention is,

[0029] A step of heating a steel slab satisfying the following equation 1, comprising, in weight percent, carbon (C): 0.1~0.35%, manganese (Mn): 1~3.6%, silicon (Si): 2.5% or less (excluding 0%), acid-soluble aluminum (Sol.Al): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), chromium (Cr): 1.0% or less (excluding 0%), and one or more selected from niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, boron (B): 0.003% or less, and the remainder being iron (Fe) and unavoidable impurities;

[0030] A step of hot rolling the above heated steel slab;

[0031] A step of cooling and coiling the above hot-rolled steel plate;

[0032] A step of cold rolling the above-mentioned wound steel plate;

[0033] A step of annealing the above cold-rolled steel plate at a temperature in the range of 800 to 870°C for 30 to 300 seconds;

[0034] A step of first cooling the above continuously annealed steel plate to a temperature range of 500 to 700℃ at an average cooling rate of 2 to 8℃ / s;

[0035] A step of secondary cooling at an average cooling rate of 56–65℃ / s to a temperature range of 100–300℃ after the above primary cooling; and

[0036] It may include a step of over-aging treatment in which, after the second cooling above, the temperature is reheated to a range of 325 to 425°C and maintained for a period of 100 to 700 seconds.

[0037] [Relationship 1]

[0038] 5.0 ≤ 2[C] + 1.3[Si] + 1.5[Mn] + 1.2[Cr] + 1.2[B] + 1.8[Nb] ≤ 6.0

[0039] In Equation 1, [C], [Si], [Mn], [Cr], [B], and [Nb] are the weight percent of each element.

[0040] The above annealing may be performed in an annealing furnace with an atmosphere of a dew point temperature of -10 to 20°C.

[0041] The above slab heating step is performed in a temperature range of 1000 to 1350℃, and

[0042] The above hot rolling step is performed at a finishing rolling temperature of Ar3 to Ar3+50℃, and

[0043] The above winding step is performed in a temperature range of 400 to 700℃, and

[0044] The above cold rolling step can be performed with a reduction rate of 30 to 70%.

[0045] After the above-mentioned winding step, a step of pickling the steel plate may be further included.

[0046] After the above over-aging treatment step, a step of temper rolling with a reduction rate of 0.1 to 1.0% may be further included.

[0047] After the above overaging treatment step, a step of hot-dip galvanizing treatment at a temperature range of 430 to 490°C may be further included.

[0048] After the above overaging treatment step, a step of alloying heat treatment at a temperature range of 480 to 600°C may be further included.

[0049] According to one embodiment of the present invention, a steel plate and a method for manufacturing the same can be provided.

[0050] According to one embodiment of the present invention, a high-strength cold-rolled steel sheet with excellent ductility and hole expansion properties and a method for manufacturing the same can be provided.

[0051] According to one embodiment of the present invention, a cold-rolled steel sheet and a method for manufacturing the same can be provided, which can be used as a material for automobiles, particularly for components requiring high formability, and which possesses excellent strength while simultaneously having excellent ductility and hole expansion properties.

[0052] Figure 1 is a micrograph of the microstructure of Invention Example 1 according to an embodiment of the present invention.

[0053] FIG. 2 is a graph showing the microhardness measured in the thickness direction of Invention Example 1 according to an embodiment of the present invention.

[0054] Preferred embodiments of the present invention are described below. The description will be made with reference to the drawings as necessary. However, embodiments of the present invention may be modified in various different forms, and the scope of the present invention is not limited to the embodiments described below.

[0055] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0056] In drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0057] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0058] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0059] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0060] In this specification, terms such as 'top', 'upper', 'upper surface', 'lower', 'lower surface', 'lower surface', and 'side surface' are based on the drawings and may actually vary depending on the direction in which the elements or components are arranged.

[0061] Additionally, throughout the specification, when it is said that one part is 'connected' to another part, this includes not only cases where they are 'directly connected,' but also cases where they are 'indirectly connected' with other elements in between.

[0062] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0063] The present invention will be described in detail below.

[0064] Below, the steel plate composition of the present invention will be described in detail.

[0065] Unless otherwise specifically stated in the present invention, the % indicating the content of each element is based on weight.

[0066] A steel sheet according to one embodiment of the present invention may comprise, in weight percent, carbon (C): 0.1 to 0.35%, manganese (Mn): 1 to 3.6%, silicon (Si): 2.5% or less (excluding 0%), acid-soluble aluminum (Sol.Al): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), chromium (Cr): 1.0% or less (excluding 0%), and one or more selected from niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, boron (B): 0.003% or less, and the remainder being iron (Fe) and unavoidable impurities.

[0067] Carbon (C): 0.1~0.35%

[0068] Carbon (C) is an element advantageous for securing strength by forming martensite within steel, and is an essential element for manufacturing high-strength steel. Generally, the higher the content of carbon (C), the easier the formation of martensite, which is advantageous for the formation of the composite structure required for manufacturing high-strength steel. Therefore, in the present invention, carbon (C) may be included in an amount of 0.1% or more to secure the target strength and form an appropriate level of martensite. In one embodiment of the present invention, it may be included in an amount of 0.12% or more. However, in order to simultaneously control the intended strength and elongation, it is necessary to control the content to an appropriate level. If the carbon (C) content exceeds 0.35%, there is a problem of inferior weldability and formability. According to one embodiment of the present invention, it may be included in an amount of 0.33% or less.

[0069] Manganese (Mn): 1~3.6%

[0070] Manganese (Mn) is an element that improves the hardenability of steel and plays a particularly important role in forming martensite. Furthermore, the manganese (Mn) contributes to increasing the strength of steel through solid solution strengthening effects and precipitates sulfur (S), which is inevitably added to steel, as MnS, thereby suppressing plate fracture and high-temperature brittleness caused by sulfur during hot rolling. To sufficiently obtain the aforementioned effects, the manganese (Mn) may be added in an amount of 1% or more. In one embodiment of the present invention, it may be included in an amount of 1.2% or more. However, if the content exceeds 3.6%, not only does weldability deteriorate, but excessive martensite formation leads to material instability, and the formation of band-shaped oxide strips increases the risk of processing cracks and plate fracture. Additionally, during annealing, manganese oxide may leach out onto the surface of the steel plate, potentially impairing plating properties. In one embodiment of the present invention, the manganese (Mn) may be included in an amount of 3.4% or less.

[0071] Silicon (Si): 2.5% or less (excluding 0%)

[0072] Silicon (Si) is a useful element that can secure strength without reducing the ductility of the steel sheet. This silicon (Si) promotes the formation of ferrite and is also advantageous for promoting the formation of martensite by encouraging C enrichment into untransformed austenite. However, if the silicon (Si) content exceeds 2.5%, there is a risk of causing hydrogen embrittlement and poor weldability; therefore, the present invention may include silicon (Si) at a level of 2.5% or less. However, 0% may be excluded considering the level that is inevitably added to the steel.

[0073] Aluminum for acidity (Sol.Al): 0.1% or less (excluding 0%)

[0074] Acid-soluble aluminum (Sol.Al) is an element added to steel for deoxidation and grain size refinement. However, if the content of Sol.Al is excessive and exceeds 0.1%, not only is castability reduced during continuous casting, but there is also a problem that the excessive formation of inclusions increases the likelihood of material defects in annealed materials and surface defects in plated materials.

[0075] Accordingly, the above Sol.Al may be included in an amount of 0.1% or less, provided that 0% may be excluded considering the level inevitably added in the steel. More advantageously, the above Sol.Al may be included in an amount of 0.01% or more.

[0076] Phosphorus (P): 0.05% or less (excluding 0%)

[0077] Phosphorus (P) is the most advantageous element for securing the strength of steel without significantly impairing formability, but when added in excess, it significantly increases the likelihood of brittle fracture, thereby increasing the possibility of plate breakage of slabs during hot rolling. In addition, P also has the problem of acting as an element that impairs the plating surface characteristics.

[0078] Considering this, the above P may be included in an amount of 0.05% or less, and 0% may be excluded considering the level inevitably added in the steel.

[0079] Sulfur (S): 0.01% or less (excluding 0%)

[0080] Sulfur (S) is an impurity element that is inevitably added to steel, and it is desirable to keep its content as low as possible. In particular, since sulfur (S) in steel can increase the likelihood of red-hot brittleness, the content can be limited to 0.01% or less in the present invention. However, 0% may be excluded considering the level that is inevitably added during steel manufacturing.

[0081] Nitrogen (N): 0.01% or less (excluding 0%)

[0082] Nitrogen (N) is an impurity element that is inevitably added to steel, and it is desirable to keep its content as low as possible. However, since this causes a sharp increase in steel refining costs, it can be controlled to 0.01% or less, which is within the range feasible under operating conditions. However, considering the level that is inevitably added during steel manufacturing, 0% can be excluded.

[0083] Chrome (Cr): 1.0% or less (excluding 0%)

[0084] Chromium (Cr) is an element added to steel to improve its hardenability and ensure high strength. This Cr is not only effective in forming martensite but also minimizes the decrease in elongation relative to the increase in strength, making it advantageous for manufacturing high-strength steel with high ductility.

[0085] However, if the above Cr content exceeds a certain level, it causes a problem in that it excessively increases the martensite formation rate and raises the fraction of coarse Cr-based carbides, leading to a decrease in elongation. In addition, there is a risk of hydrogen embrittlement and deterioration of weldability.

[0086] Accordingly, the addition of the above Cr can be limited to 1.0% or less, and more advantageously, to 0.8% or less. However, 0% may be excluded considering the level that is inevitably added during steel manufacturing.

[0087] In addition to the steel composed as described above, one or more of Nb, Ti, and B may be additionally included.

[0088] Niobium (Nb): 0.1% or less

[0089] Niobium (Nb) is an element that segregates at austenite grain boundaries and not only suppresses the coarsening of austenite grains during annealing but also contributes to the improvement of yield strength and tensile strength by forming fine carbides.

[0090] However, if the above Nb content is excessive, there is a risk that strength and elongation will decrease due to the precipitation of coarse carbides and the reduction of carbon content in the steel, and the manufacturing cost may increase, which may worsen economic feasibility.

[0091] Accordingly, the addition of the above Nb can be limited to 0.1% or less, and more advantageously, it can be included at 0.05% or less. However, considering the level that is inevitably added during steel manufacturing, 0% may be excluded.

[0092] Titanium (Ti): 0.1% or less

[0093] Titanium (Ti) contributes to securing yield strength and tensile strength by forming fine carbides, and can effectively reduce the risk of cracking during continuous casting by precipitating N in the steel as TiN and suppressing the precipitation of AlN. However, if the content is excessive, strength and elongation may decrease due to the precipitation of coarse carbides and a reduction in the carbon content in the steel, and nozzle clogging may also occur during continuous casting. Therefore, in the present invention, the upper limit of the addition of titanium (Ti) may be limited to 0.1%. In one embodiment of the present invention, the upper limit may be limited to 0.05%. However, 0% may be excluded considering the level that is inevitably added during steel manufacturing.

[0094] Boron (B): 0.003% or less

[0095] Boron (B) is effective in delaying the transformation of austenite into pearlite during the cooling process after continuous annealing. However, if the content of the boron (B) is excessive, the boron (B) may become concentrated on the surface of the steel sheet, potentially causing surface defects. Therefore, in the present invention, the upper limit of the boron (B) added may be limited to 0.003%. However, 0% may be excluded considering the level that is inevitably added during steel manufacturing.

[0096] The steel of the present invention may contain the remainder of iron (Fe) and unavoidable impurities in addition to the composition described above. Since unavoidable impurities may be unintentionally incorporated during the conventional manufacturing process, they cannot be excluded. Because such impurities are known to any person skilled in the field of ordinary steel manufacturing, all details thereof are not specifically mentioned in this specification.

[0097] The steel sheet of the present invention satisfying the alloy composition described above preferably satisfies the following relationship 1 for the content of C, Si, Mn, Cr, Nb, and B.

[0098] [Relationship 1]

[0099] 5.0 ≤ 2[C] + 1.3[Si] + 1.5[Mn] + 1.2[Cr] + 1.2[B] + 1.8[Nb] ≤ 6.0

[0100] (In Equation 1, [C], [Si], [Mn], [Cr], [B], and [Nb] are the weight percent of each element.)

[0101] The above relationship 1 is an empirical numerical relationship of the content of specific components to secure the basic material of the steel plate intended in the present invention.

[0102] If the R value defined in the above Equation 1 is less than 5.0, the hardenability of the steel decreases, and the phase fraction of the low-temperature transformation structure, such as martensite, decreases, resulting in a problem where the strength intended by the present invention cannot be secured. In one embodiment of the present invention, the R value may be 5.2 or higher. In one embodiment of the present invention, it may be 5.3 or higher. On the other hand, if the value exceeds 6.0, the hardenability of the steel becomes excessively high, making it impossible to obtain the formable material intended by the present invention. In one embodiment of the present invention, the upper limit of the R value may be 5.8.

[0103] Below, the microstructure of the steel plate of the present invention will be described in detail.

[0104] Unless otherwise specifically stated in the present invention, the % representing the fraction of the microstructure is based on area.

[0105] The steel sheet of the present invention having the alloy composition system described above comprises a martensite phase and a tempered martensite phase as the main phases, and may comprise one or more of ferrite, bainite, and retained austenite as the remainder structure. More specifically, the steel sheet of the present invention may comprise 40% or more of tempered martensite and 4% or less of fresh martensite in terms of area fraction. If the tempered martensite fraction is less than 40% or the fresh martensite exceeds 4%, it becomes impossible to secure a target level of strength. More advantageously, the tempered martensite phase may comprise 80% or more.

[0106] The steel plate of the present invention may include a decarburization region in the surface layer. As an example, the decarburization region may be divided into a first decarburization region having a thickness of 20 μm to 40 μm in the thickness direction of the steel plate from the surface of the steel plate, and a second decarburization region having a thickness of 10 μm to 20 μm in the thickness direction of the steel plate from the first decarburization region. As a preferred example, the total decarburization region of the first and second decarburization regions may be 50 μm or more from the surface.

[0107] The first decarburization region may include ferrite. Specifically, the first decarburization region may include 95% or more ferrite and may include tempered martensite, fresh martensite, bainite, and retained austenite as the remainder structure. By ensuring that the ferrite in the first decarburization region is 95% or more, the bendability can be improved.

[0108] The above second decarburization region may contain a sum of the area fractions of bainite and fresh martensite of 4% or less, and the remainder may contain ferrite. Meanwhile, in addition to ferrite, it may inevitably contain tempered martensite, retained austenite, etc. By securing the structure of the above second decarburization region, the effect of the above first decarburization region can be maximized.

[0109] Meanwhile, additionally, the steel plate of the present invention may include fine precipitates within the microstructure described above, and specifically, 10 of one or more fine precipitates selected from the group consisting of Ti-based and Nb-based materials with an average diameter (circular equivalent diameter) of 50 nm or less per unit area (m²). 12 It can include more than one.

[0110] The steel plate of the present invention has a yield strength of 950 MPa or more, a tensile strength of 980 MPa or more, and an elongation of 17% or more, so as to ensure excellent strength as well as ductility and bendability simultaneously.

[0111] In addition, the steel plate of the present invention has a yield ratio of 0.95 or higher, so it can simultaneously secure excellent strength as well as ductility and bendability.

[0112] Furthermore, the steel sheet of the present invention is characterized by having a product of yield strength and elongation of 16500 MPa·% or more, a product of tensile strength and elongation of 17300 MPa·% or more, a three-point bending angle of 135° or more, and a product of elongation and three-point bending angle of 2300%·° or more. In addition, the present invention can secure not only ductility but also a three-point bending angle of 135° or more by effectively reducing the difference in hardness between phases through martensitic tempering.

[0113] Generally, during phase transformations such as ferrite and bainite, the carbon concentration within the surrounding austenite increases, and consequently, the strength of the steel increases due to the martensite with increased dissolved carbon in the final structure. At this time, if the TRIP effect is accompanied by the stabilization of the retained austenite, an improvement in elongation can be expected simultaneously. However, according to the present invention, the high elongation despite the relatively low fraction of retained austenite can be attributed to the homogeneous structure of tempered martensite formed with an area fraction of 80% or more, along with a significant reduction in the hardness difference between phases. In other words, due to the high fraction of tempered martensite, high strength of 980 MPa or more can be achieved, along with flexibility through the homogeneity of the structure and the reduction in the hardness difference between phases.

[0114] Hereinafter, a method for manufacturing a high-strength cold-rolled steel sheet with excellent ductility and bendability provided by the present invention, which is another aspect of the present invention, will be described in detail.

[0115] A steel plate according to one embodiment of the present invention can be manufactured by heating a steel slab satisfying the alloy composition described above, hot rolling, coiling, cold rolling, annealing, primary cooling, secondary cooling, and overaging treatment.

[0116] heating

[0117] A steel slab satisfying the alloy composition of the present invention can be heated in a temperature range of 1000 to 1350°C.

[0118] If the slab heating temperature is less than 1000℃, there is a risk that hot rolling will be performed in a temperature range below the desired finishing rolling temperature. On the other hand, if the temperature exceeds 1350℃, there is a risk that it will reach the melting point of the steel and melt.

[0119] The steel slab used in the manufacturing method of the present invention may be refined and cast through a converter process or an electric furnace process.

[0120] In the converter process, molten iron supplied from a blast furnace is primarily used; however, depending on the supply and demand status of hot metal, some scrap or other iron sources may be added for refining to produce molten steel. In particular, when implementing low HMR operations that reduce the amount of molten iron used to meet requirements such as carbon neutrality, the amount of scrap used may increase, and as a result, elements not intended in this invention may be included in the molten steel within the allowable limits.

[0121] In the electric furnace process, molten steel can be obtained by primarily charging scrap, melting it using arc heat, and refining it. In some cases, molten iron may be added in addition to the scrap. As a result of including a large amount of scrap in this manner, elements not intended in this invention may be included in the molten steel within permissible limits.

[0122] Molten steel that has undergone the converter or electric furnace process may undergo an additional refining (secondary refining) process to adjust its composition and other properties.

[0123] Hot rolling

[0124] The above reheated steel slab can be hot-rolled at a finishing rolling temperature of Ar3 to Ar3+50℃.

[0125] If the finishing rolling temperature is below Ar3, there is a high probability that the resistance to hot deformation will increase sharply. On the other hand, if the temperature exceeds Ar3 + 50℃, not only will an excessively thick oxide scale form, but the grains of the hot-rolled steel sheet will also be formed coarsely, raising concerns that this may cause a deterioration in the physical properties of the final steel sheet. Here, the finishing rolling temperature may refer to the temperature at the exit side of the finishing rolling mill. For example, the above Ar3 can be derived using the following formula.

[0126] [ceremony]

[0127] Ar3 = 910-95*[C]-15.2*[Ni]+44.7*[Si]+104*[V]+31.5*[Mo]-(15*[Mn]+11*[Cr]+20*[Cu]-700*[P]-400*[Al]-400*[Ti])

[0128] In the above formula, [C], [Ni], [Si], [V], [Mo], [Mn], [Cr], [Cu], [P], [Al], and [Ti] are the weight percent of each element.

[0129] Kwon Chi

[0130] The above hot-rolled steel plate can be cooled and coiled in a temperature range of 400 to 700°C.

[0131] If the coiling temperature is less than 400℃, excessive martensite or bainite is generated, which may cause manufacturing problems such as shape defects due to load during the subsequent cold rolling process. On the other hand, if the temperature exceeds 700℃, there is a problem of deterioration in pickling performance due to an increase in surface scale. Meanwhile, the cooling conditions from hot rolling to the coiling temperature are not specifically limited, but may be cooling conditions applicable in the same technical field. In one embodiment of the present invention, air cooling may be used.

[0132] Cold rolling

[0133] The above-mentioned coiled steel plate can be cold-rolled with a reduction rate of 30 to 70 percent.

[0134] When cold rolling, if the reduction ratio is less than 30%, the recrystallization driving force is weakened, making it difficult to secure good recrystallized grains and making shape correction difficult. On the other hand, if the reduction ratio exceeds 70%, the likelihood of cracks occurring at the edge of the steel sheet increases, and there is a concern that the rolling load will increase rapidly.

[0135] According to one embodiment of the present invention, prior to cold rolling, a pickling process may be performed to remove scale formed on the surface of a steel sheet. This pickling process may be performed under ordinary conditions, and such conditions may not be particularly limited.

[0136] Annealing

[0137] The cold-rolled steel sheet manufactured according to the above can be subjected to annealing treatment, and through the annealing treatment, the basis for the microstructure intended in the present invention can be established.

[0138] Specifically, the annealing can be performed on the cold-rolled steel sheet at a temperature range of 800 to 870°C, and it is preferable to maintain it at that temperature for 30 to 300 seconds.

[0139] If the temperature during the annealing above is less than 800℃, the ferrite fraction will be present, which not only reduces strength but also increases the difference in hardness between phases, potentially deteriorating bendability. On the other hand, if the temperature exceeds 870℃, there is a risk that annealing oxides will form on the surface of the steel sheet. More preferably, the annealing can be performed at a temperature of 810℃ or higher and 850℃ or lower.

[0140] When a cold-rolled steel sheet is maintained in the above annealing temperature range, if the time is less than 30 seconds, recrystallization does not occur sufficiently, making it difficult to secure elongation, and increasing the likelihood of material variation in the length / width direction of the steel sheet. On the other hand, if the time exceeds 300 seconds, the annealing effect becomes saturated, and there is a problem of reduced productivity.

[0141] In this invention, the bendability of a steel sheet is improved by introducing a soft ferrite decarburization region on the surface of the steel sheet. When annealing the cold-rolled steel sheet, the process may be carried out in an annealing furnace with a dew point atmosphere of -10 to 20°C. When the steel sheet is annealed at the corresponding dew point temperature, a decarburization reaction occurs in which carbon present in the steel is discharged from the surface of the steel sheet into the annealing furnace atmosphere. Even if the annealing is performed at a temperature in the austenite single-phase region, if decarburization occurs, ferrite may be formed as an equilibrium phase. As the decarburization reaction increases on the surface of the steel sheet, the fraction of ferrite increases, and the grains may grow and become coarse at that temperature. However, if the dew point of the annealing furnace is below -10°C, the decarburization reaction is insufficient, making it difficult to secure the target thickness of the ferrite decarburization region, and thus the target bendability cannot be achieved. On the other hand, if the dew point temperature exceeds 25℃, an excessive amount of decarburization zones is generated, and the fraction of ferrite decarburization zones increases, which causes a problem of deteriorating the strength of the steel sheet.

[0142] Primary cooling

[0143] The above continuously annealed steel plate can be first cooled to a temperature range of 500 to 700°C at an average cooling rate of 2 to 8°C / s.

[0144] In the present invention, during the first cooling, slow cooling can be performed compared to the subsequent second cooling process, and plate shape degradation caused by a drop in temperature during the second cooling, which is a relatively rapid cooling section, can be suppressed.

[0145] When the first cooling is performed, if the cooling end temperature is less than 500°C or exceeds 700°C, it deviates from the appropriate temperature gradient range with the subsequent second cooling, making it difficult to secure stable cooling capacity.

[0146] When the first cooling is performed, if the average cooling rate exceeds 8℃ / s, there is a concern that sufficient enrichment of C and Mn may not occur in the austenite. In the present invention, the lower limit of the average cooling rate during the first cooling is not specifically limited, but cooling may be performed at a conventional slow cooling rate. In the present invention, the lower limit may be 2℃ / s.

[0147] Secondary cooling

[0148] After the above first cooling, a second cooling can be performed at an average cooling rate of 56 to 65°C / s to a temperature range of 100 to 300°C.

[0149] In the present invention, during secondary cooling, the cooling speed and the cooling end temperature are appropriately adjusted according to the width and thickness of the steel plate to be obtained, thereby securing an optimal plate shape.

[0150] When the above secondary cooling is performed, if the cooling end temperature is less than 100℃ or the average cooling rate exceeds 65℃ / s, it becomes a condition that is difficult to implement in a normal manufacturing process, which may result in a problem of reduced productivity.

[0151] On the other hand, if the temperature exceeds 300℃ or the average cooling rate is less than 56℃ / s, the fraction of martensite that undergoes transformation decreases during secondary cooling, which consequently leads to a decrease in the fraction of tempered martensite, and thus may cause a decrease in yield strength.

[0152] Statute of Limitations

[0153] After the above secondary cooling, an over-aging treatment can be performed by heating to a temperature range of 325 to 425°C and maintaining for 100 to 700 seconds.

[0154] When the overaging treatment is performed as described above, if the overaging temperature is less than 325°C, the amount of carbon (C) released from the transformed martensite during the overaging treatment decreases during the second cooling. Consequently, the degree of tempering decreases, which may result in a lower yield strength of the steel sheet. On the other hand, if the temperature exceeds 425°C, the amount of carbon (C) released from the transformed martensite during the overaging treatment increases excessively during the second cooling. Consequently, the degree of tempering deviates from the appropriate range, which may result in a decrease in the tensile strength of the steel sheet.

[0155] When undergoing over-aging treatment, if the holding time is excessive and exceeds 700 seconds, excessive bainite transformation may occur during the holding process, raising concerns that the fraction of bainite in the final microstructure will increase. This leads to a decrease in the fraction of martensite, raising concerns that the desired strength cannot be effectively achieved. On the other hand, if the time is less than 100 seconds, the bainite nose is avoided, which may result in the final microstructure being free of bainite, potentially causing a decrease in the ductility of the steel.

[0156] plating

[0157] Additionally, the cold-rolled steel sheet treated with over-aging according to the above may be immersed in a plating bath to perform plating. At this time, although not limited thereto, it may be hot-dip galvanizing or alloyed hot-dip galvanizing. For example, a hot-dip zinc-based plating bath may be used, and after such hot-dip galvanizing, an alloying heat treatment may be optionally performed.

[0158] In the present invention, the method of hot-dip galvanizing or alloyed hot-dip galvanizing is not particularly limited, and process conditions for hot-dip galvanizing or alloyed hot-dip galvanizing that are commonly applied may be applied. As a non-limiting example, the hot-dip galvanizing may be performed in a zinc-based plating bath at 430 to 490°C, and the alloying heat treatment may be performed in a temperature range of 480 to 600°C.

[0159] In addition, the composition of the plating bath during the above-mentioned hot-dip galvanizing or alloyed hot-dip galvanizing is not specifically limited, but may be a pure zinc plating bath or a zinc-based alloy plating bath containing Si, Al, Mg, etc.

[0160] In accordance with the above, the steel sheet, having completed the over-aging treatment, may be cooled to room temperature under normal conditions, and the cooling process is not specifically limited. However, it is obvious that it can be replaced with known cooling methods such as water cooling, oil cooling, or furnace cooling.

[0161] Tough rolling

[0162] Temper rolling can be further performed on the plated steel sheet according to the above, and at this time, it can be performed with a reduction rate of 0.1 to 1.0%.

[0163] Typically, when temper rolling is performed on steel sheets, the effect of increasing yield strength can be obtained without accompanying an increase in tensile strength. However, if the reduction rate of the above temper rolling is less than 0.1%, not only is the effect of increasing yield strength insufficient, but shape control is also difficult. On the other hand, if the reduction rate exceeds 1.0%, there is a risk that workability will be significantly inferior due to high elongation.

[0164] The steel sheet of the present invention, manufactured through the aforementioned series of processes, can simultaneously secure excellent strength as well as ductility and bendability by forming an intended microstructure, with a yield strength of 950 MPa or more, a tensile strength of 980 MPa or more, a yield ratio of 0.95 or more, and an elongation of 17% or more.

[0165] In addition, the steel plate of the present invention is characterized by having a product of yield strength and elongation of 16500 MPa·% or more, a product of tensile strength and elongation of 17300 MPa·% or more, a three-point bending angle of 135° or more, and a product of elongation and three-point bending angle of 2300%·° or more. Furthermore, the present invention can secure not only ductility but also a three-point bending angle of 135° or more by effectively reducing the difference in hardness between phases through martensitic tempering.

[0166] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0167] (Example)

[0168] A steel slab having the alloy composition listed in Table 1 below (the remainder being Fe and unavoidable impurities) was vacuum melted, heated in a temperature range of 1200°C, finished rolling was completed at an exit side temperature of 880–920°C, and coiled at 600°C. Afterward, the surface scale was removed by pickling, and then cold-rolled with a cold reduction rate of 50% to produce a cold-rolled steel sheet. Next, the cold-rolled steel sheet was subjected to continuous annealing, stepwise cooling, and overaging treatment in an annealing furnace under the conditions shown in Table 2 below.

[0169] The cold-rolled steel sheet, which had undergone the above-mentioned over-aging treatment, was temper-rolled at a reduction rate of 0.3% to produce a final steel sheet.

[0170] [Relationship 1]

[0171] 5.0 ≤ 2[C] + 1.3[Si] + 1.5[Mn] + 1.2[Cr] + 1.2[B] + 1.8[Nb] ≤ 6.0

[0172] In Equation 1, [C], [Si], [Mn], [Cr], [B], and [Nb] are the weight percent of each element.

[0173] Steel Grade Alloy Composition (Weight%) Relationship Formula 1 CS i Mn Cr BN b Ti Al PS NA 0.18 1.5 20.15 0.00 20.04 0.04 0.03 0.01 0.00 20.003 5.6 B 0.18 1.1 20.15 0.00 20.04 0.04 0.03 0.01 0.00 20.003 5.0 C 0.18 220.15 0.00 20.04 0.04 0.03 0.01 0.00 20.003 6.2 D 0.18 1.5 1.50.150.0020.040.040.030.010.0020.0034.8E0.181.53.50.150.0020.040.040.030.010.0020.0037.8 F0.181.5200.0020.040.040.030.010.0020.0035.4G0.181.520.70.0020.040.040.030.010.0020.0036.2

[0174] Steel Grade Continuous Annealing 1st Cooling 2nd Coolant Heating / Overaging Classification Temperature (°C) Holding Time (s) Ending Temperature (°C) Cooling Rate (°C / s) Ending Temperature (°C) Cooling Rate (°C / s) Temperature (°C) Holding Time (s) A790 100600 4.715060.3350500 Comparative Example 1 A850 100600 6.215060.3350500 Inventive Example 1 A880 100600 6.915060.3350500 Comparative Example 2 A850 100600 6.215060.3300500 Comparative Example 3 A850 100600 6.215060.3450500 Comparative Example 4A8501006006.220053.6350500Comparative Example 5A8501006006.225046.9350500Comparative Example 6A8501006006.230040.2350500Comparative Example 7A8501006006.235033.5350500Comparative Example 8A8501006006.240026.8470500Comparative Example 9A8501006006.245020.1470500Comparative Example 10A8501006006.250013.4470500Comparative Example 11A8501006006.25506.7470500Comparative Example 12B8501006006.215060.3350500Inventive Example 2C8501006006.215060.3350500Comparative Example 13D8501006006.215060.3350500Comparative Example 14E8501006006.215060.3350500Comparative Example 15F8501006006.215060.3350500Comparative Example 16G8501006006.215060.3350500Comparative Example 17

[0175] The microstructure of the steel plate manufactured in this way was observed and is shown in Table 3 below. Among the microstructures, martensite (M), tempered martensite (TM), and bainite (B) were observed through SEM after nital etching of the polished specimen cross-section, and retained austenite (R-γ) was measured through XRD analysis.

[0176] Meanwhile, in Table 3, the first decarburization zone was measured at a point 15㎛ deep from the surface of each steel plate, and the second decarburization zone was measured at a point 45㎛ deep.

[0177] In addition, physical properties were measured for each specimen, and the results are described in Table 4 below. Yield strength (YS), tensile strength (TS), and elongation (El) were evaluated through tensile testing, and each mechanical property was measured by evaluating specimens taken according to JIS-5 standard based on a 90° direction relative to the rolling direction.

[0178] Classification Microstructure Microstructure within Decarburization Region 1 Decarburization Region 2 Decarburization Region FBTMFMR-γ FBTMFMR-γ FBTMFMR-γ Comparative Example 1 1 2758 498 20008 411050 Inventive Example 13 288 3498 200096 3010 Comparative Example 20 2895497 300096 2020 Comparative Example 3 22875496 400096 3010 Comparative Example 4 22858 396 400095 3020 Comparative Example 5 23839396 400095 2030 Comparative Example 6 238012397 3000943030 Comparative Example 7 237716 297 3000943030 Comparative Example 82376172964000933040Comparative Example 921070162973000933040Comparative Example 102866222964000905050Comparative Example 112760292964000895060Comparative Example 122554372955000895060Invention Example 2338824991000963010Comparative Example 13228646991000926020Comparative Example 14228754982000944020Comparative Example 15228844973000952030Comparative Example 16228754982000944020Comparative Example 17228844973000952030

[0179] Classification Mechanical Properties YS(MPa)TS(MPa)El(%)YRYSХEl(MPa·%)TSХEl(MPa·%)3-Point Bending( o )ElХ3-point bend(%· oComparative Example 1917101917.50.9016048178331101925 Inventive Example 1966101117.20.9616615173891372356 Comparative Example 2960101314.70.9514112148911351985 Comparative Example 3951102515.10.9314360154781332008 Comparative Example 4965103316.20.9315633167351221976 Comparative Example 5980103816.40.9416072170231211984 Comparative Example 6949105115.80.9014994166061151817Comparative Example 7936105516.30.8915257171971091777Comparative Example 8924106414.80.8713675157471051554Comparative Example 99381070170.8815946181901021734Comparative Example 10902108116.70.831506318053971620Comparative Example 11895108816.90.821512618387921555Comparative Example 128701091160.801392017456891424 Invention Example 2954100617.30.9516504174041382387 Comparative Example 13943102217.70.9216691180891252213 Comparative Example 14931101516.80.9215641170521262117 Comparative Example 15920103016.20.8914904166861181912 Comparative Example 16935100215.40.9314399154311121725 Comparative Example 17917102416.40.9015039167941262066

[0180] As shown in Tables 1 and 2, Invention Examples 1 and 2, which satisfy all the alloy composition systems and manufacturing conditions proposed in the present invention, have a martensite phase (+ tempered martensite phase) formed at an area of ​​90% or more as intended, and bainite and retained austenite phases appropriately formed as other microstructures. In addition, as fine precipitates are also formed as intended, not only high strength of 980 MPa or more but also an elongation of 17% or more and a three-point bending angle of 135° or more are secured. That is, it can be seen that the steel sheet produced by the present invention has improved strength, ductility, and bendability simultaneously.

[0181] On the other hand, the alloy composition systems of Comparative Examples 1 to 12 satisfy the proposal of the present invention, but the manufacturing conditions deviate from the present invention.

[0182] Comparative Example 1 satisfies the composition of the steel, but the annealing temperature is too low during annealing, so the fraction of the soft phase ferrite increases, and therefore it can be seen that the yield strength, yield ratio, and bendability targeted by the present invention are not secured.

[0183] In the case of Comparative Examples 5 to 8, the secondary cooling process was outside the scope proposed in the present invention, and as the required steel sheet microstructure was not secured, it was found that the required elongation, strength, or bendability was not secured. In Comparative Example 2, it was confirmed that the annealing temperature was too high during annealing, resulting in a high martensite fraction of the steel sheet and making it difficult to secure elongation and bendability. In Comparative Examples 3 and 4, the overaging temperature was low or high, and it was confirmed that the microstructure and physical properties intended by the present invention were not secured.

[0184] It can be seen that Comparative Examples 9 to 12 have high secondary cooling end temperatures and overaging temperatures, resulting in excessive fresh martensite, low yield strength and yield ratio, and insufficient bendability.

[0185] Comparative Examples 13 to 17 are cases where the manufacturing conditions satisfy the present invention, but the alloy composition system deviates from the proposal of the present invention.

[0186] Comparative Examples 13 to 15 and 17 are cases where the value of Equation 1 falls outside the range presented in the present invention, and it can be seen that the yield strength, yield ratio, ductility, and bendability targeted by the present invention are not secured.

[0187] Comparative Example 16 is a case in which Cr is not added, and thus the target level of strength and flexurality cannot be secured. In particular, although the value of Equation 1 for Comparative Example 16 was obtained as 5.4, the hardenability of the steel was reduced, and the strength intended by the present invention could not be secured. Fig. 1 is a micrograph of the microstructure of Invention Example 1 according to an embodiment of the present invention. Referring to Fig. 2, Invention Example 1 according to an embodiment of the present invention can be defined by dividing it into three layers in the thickness direction as follows.

[0188] The first decarburization region appears to have undergone significant softening, that is, hard phase ferrite formation due to decarburization, and it can be confirmed that it has a thickness of about 20㎛ to 40㎛ in the thickness direction of the steel plate from the surface of the steel plate. Accordingly, in the embodiment of the present invention, the outermost region where deformation stress due to bending is concentrated can withstand higher stress, thereby improving bendability.

[0189] It can be confirmed that the second decarburization region contains a sum of the area fractions of bainite and fresh martensite of 5% or less and has a thickness of approximately 10㎛ to 20㎛ in the thickness direction of the steel plate. That is, it can be seen that the second decarburization region undergoes relatively less softening compared to the first decarburization region. Accordingly, the second decarburization region of the embodiment of the present invention can function as a buffer region capable of securing bendability while maintaining high yield strength. Furthermore, it can improve problems such as corrosion that may occur as surface hardness decreases when a soft region is excessively introduced.

[0190] And, it can be confirmed that it includes a base material free from the effects of decarburization.

[0191] FIG. 2 is a graph showing the microhardness measured in the thickness direction of Invention Example 1 according to an embodiment of the present invention. Referring to FIG. 2, it can be seen that the surface layer of Invention Example 1 according to an embodiment of the present invention has the lowest hardness value because ferrite formation due to decarburization has mostly progressed.

Claims

1. In wt%, comprising carbon (C): 0.1–0.35%, manganese (Mn): 1–3.6%, silicon (Si): 2.5% or less (excluding 0%), acid-soluble aluminum (Sol.Al): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), chromium (Cr): 1.0% or less (excluding 0%), and one or more selected from niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, boron (B): 0.003% or less, and the remainder being iron (Fe) and unavoidable impurities, satisfying the following Equation 1, and The microstructure contains 40% or more of tempered martensite and 4% or less of fresh martensite in area %, and the remainder comprises one or more of ferrite, bainite, and retained austenite, and A first decarburization region having a thickness of 20㎛ to 40㎛ in the thickness direction of the steel plate on the surface of the steel plate; and a second decarburization region having a thickness of 10㎛ to 20㎛ in the thickness direction of the steel plate in the first decarburization region, The above-mentioned first decarburization region contains 95% or more ferrite, and The above second decarburized region comprises a steel plate in which the sum of the area fractions of bainite and fresh martensite is 4% or less, and the remainder comprises ferrite. [Relationship 1] 5.0 ≤ 2[C] + 1.3[Si] + 1.5[Mn] + 1.2[Cr] + 1.2[B] + 1.8[Nb] ≤ 6.0 In Equation 1, [C], [Si], [Mn], [Cr], [B], and [Nb] are the weight percent of each element.

2. In Paragraph 1, A steel plate having a plating layer formed on at least one surface of the steel plate.

3. In Paragraph 2, The above plating layer is a zinc-based plating layer, steel plate.

4. In Paragraph 1, The above steel plate is a steel plate having a yield strength of 950 MPa or more, a tensile strength of 980 MPa or more, and an elongation of 17% or more.

5. In Paragraph 1, The above steel plate is a steel plate having a yield ratio of 0.95 or higher.

6. In Paragraph 1, The above steel plate is a steel plate having a product of yield strength and elongation of 14,000 MPa·% or more, a product of tensile strength and elongation of 15,000 MPa·% or more, a 3-point bend of 110° or more, and a product of elongation and 3-point bend of 2,100%·° or more.

7. A step of heating a steel slab satisfying the following equation 1, comprising, in weight percent, carbon (C): 0.1~0.35%, manganese (Mn): 1~3.6%, silicon (Si): 2.5% or less (excluding 0%), acid-soluble aluminum (Sol.Al): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), chromium (Cr): 1.0% or less (excluding 0%), and one or more selected from niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, boron (B): 0.003% or less, and the remainder being iron (Fe) and unavoidable impurities; A step of hot rolling the above heated steel slab; A step of cooling and coiling the above hot-rolled steel plate; A step of cold rolling the above-mentioned wound steel plate; A step of annealing the above cold-rolled steel plate at a temperature in the range of 800 to 870℃ for 30 to 300 seconds; A step of first cooling the above continuously annealed steel plate to a temperature range of 500 to 700℃ at an average cooling rate of 2 to 8℃ / s; A step of secondary cooling at an average cooling rate of 56–65℃ / s to a temperature range of 100–300℃ after the above primary cooling; and A method for manufacturing a steel plate comprising the step of overaging treatment, wherein the steel plate is reheated to a temperature in the range of 325 to 425°C after the above secondary cooling and maintained for a time of 100 to 700 seconds. [Relationship 1] 5.0 ≤ 2[C] + 1.3[Si] + 1.5[Mn] + 1.2[Cr] + 1.2[B] + 1.8[Nb] ≤ 6.0 In Equation 1, [C], [Si], [Mn], [Cr], [B], and [Nb] are the weight percent of each element.

8. In Paragraph 7, A method for manufacturing a steel sheet in which the above annealing is performed in an annealing furnace with an atmosphere of a dew point temperature of -10 to 20℃.

9. In Paragraph 7, The above slab heating step is performed in a temperature range of 1000 to 1350℃, and The above hot rolling step is performed at a finishing rolling temperature of Ar3 to Ar3+50℃, and The above winding step is performed in a temperature range of 400 to 700℃, and The above cold rolling step is a method for manufacturing a steel sheet, performed at a reduction rate of 30 to 70%.

10. In Paragraph 7, A method for manufacturing a steel plate that further includes a step of pickling the steel plate after the above-mentioned winding step.

11. In Paragraph 7, A method for manufacturing a steel sheet, further comprising a step of temper rolling with a reduction rate of 0.1 to 1.0% after the above-mentioned overaging treatment step.

12. In Paragraph 7, A method for manufacturing a steel sheet, further comprising a step of hot-dip galvanizing at a temperature range of 430 to 490°C after the above-mentioned overaging treatment step.

13. In Paragraph 7, A method for manufacturing a steel plate, further comprising a step of alloying heat treatment in a temperature range of 480 to 600°C after the above-mentioned overaging treatment step.

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