Steel plate and its manufacturing method

Abstract

The present invention relates to materials used for automobile members, and more particularly to cold-rolled steel sheets and plated steel sheets with excellent bending properties, and methods for manufacturing these.

Description

[Technical Field]

[0001] The present invention relates to a material used for vehicle components, and more particularly to a cold-rolled steel sheet (and plated steel sheet) with excellent bending properties and a method for manufacturing the same. [Background technology]

[0002] Recently, there has been a demand for improved fuel efficiency in automobiles to protect the global environment, and in particular, there is a demand for higher levels of strength in automotive steel sheets in order to reduce the weight of vehicle bodies and ensure safety.

[0003] Various steel sheets that combine strength and formability have been devised and put into practical use. For example, dual-phase steel sheets, in which ferrite and low-temperature transformation phases such as martensite and bainite coexist, are used as high-strength steel sheets with excellent formability. Dual-phase steel sheets aim to simultaneously improve strength and formability by dispersing hard low-temperature transformation phases in soft ferrite.

[0004] On the other hand, when steel materials used as collision components are manufactured using cold forming techniques, there is a demand for the development of ultra-high strength steel with better processing characteristics, particularly excellent bendability, and active research is being conducted into methods for manufacturing ultra-high strength steel with a tensile strength of 1500 MPa or more using a single martensite phase.

[0005] Hot press forming (HPF) is a method developed to ensure the required strength by forming materials at high temperatures that facilitate easy forming and then water-cooling the material between the die and the material. While HPF is widely used in parts manufacturing because it can ensure high strength for a given thickness, excessive capital investment and increased process costs pose problems for application, necessitating the development of materials for cold stamping. Therefore, there is a need for the development of cold-rolled steel sheets with high strength, a high yield ratio, and excellent bending properties that are suitable for use as cold stamping materials and ensure crashworthiness.

[0006] A representative prior art example of such a method is Patent Document 1. Patent Document 1 uses a steel having a composition containing C: 0.25 to 0.4%, Si: 1.0% or less, Mn: 1.5 to 2.5%, P: 0.02% or less, S: 0.003% or less, Al: 0.01 to 0.1%, N: 0.005% or less, B: 0.0005 to 0.005%, Ti: 0.005 to 0.1%, and Nb: 0.005 to 0.1%, for a total of 0.005 to 0.1%, and the metal structure is a martensitic single-phase structure. This type of steel is used to manufacture steel sheets by heating and storing it in a temperature range from the Ae3 transformation point to 900°C, then rapidly cooling it from an average cooling rate of 300°C / s to 200°C or less, and then tempering it at 250°C or less.However, this has the disadvantage that the shape (flatness) deteriorates due to water cooling, resulting in defects during forming.

[0007] Patent Document 2 describes a technology for producing thin steel sheet with a steel microstructure containing 0.05% to 0.35% C, 0.01% to 2.0% Si, 0.8% to 3.0% Mn, 0.05% to 0.05% P, 0.005% to 0.10% S, 0.005% to 0.10% Al, and 0.0060% N, with a ferrite area fraction of 0% to 90% bainite area fraction of 5% or less (including 0%), martensite and tempered martensite area fractions of 10% or more (including 100%), and retained austenite area fraction of 2.0% or less (including 0%), a standard deviation of yield strength in the width direction of 30 MPa or less, and a maximum bending amount of 10 mm or less when sheared at a length of 1 m. However, this technology also suffers from the problem of shape defects due to rapid cooling after annealing, and has disadvantageous limitations from a practical standpoint. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-248565 [Patent Document 2] Japanese Patent Publication No. 2020-019992 Summary of the Invention [Problem to be solved by the invention]

[0009] One aspect of the present invention is to overcome the limitations of the conventional techniques described above, and aims to provide a steel sheet having an ultra-high strength of 1500 MPa or more while also having excellent shape and bending properties by optimizing the steel composition and manufacturing process.

[0010] The object of the present invention is not limited to the above-mentioned matters. Further object of the present invention is described in the entire contents of the specification, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional object of the present invention from the contents described in the specification of the present invention. [Means for solving the problem]

[0011] One embodiment of the present invention comprises: In weight percent, carbon (C): 0.1 to 0.3%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 1.3 to 2.5%, chromium (Cr): 0.2% or less (excluding 0%), molybdenum (Mo): 0.01 to 0.1%, boron (B): 0.0005 to 0.003%, phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.01 to 0.1%, niobium (Nb): 0.01 to 0.05%, titanium (Ti): 0.01 to 0.05%, and the balance being Fe and unavoidable impurities, Provided is a steel sheet in which the ratio (a / b×100) of the total content of C and Mn (a) in a region within 20 μm from the surface to the total content of C and Mn (b) at a point (1 / 4)×t (where t is the total thickness of the steel sheet) from the surface is 75% or more (excluding 100%).

[0012] The steel sheet may contain, as a microstructure in a region within 20 μm from the surface, 10% or less (excluding 0%) of one or more phases selected from the group consisting of ferrite and bainite, by area %.

[0013] The steel sheet may have a fine structure in a region within 20 μm from the surface, with the remainder being martensite.

[0014] The steel sheet may contain martensite in an area of ​​90 to 99% by area as a microstructure in a region within 20 μm from the surface.

[0015] The above t can be 0.6 to 2.5 mm.

[0016] The steel sheet may include a zinc-based plating layer on the surface thereof.

[0017] The steel plate may have a tensile strength (TS) of 1500 MPa or more and a bendability (R / t) of 3.7 or less.

[0018] Yet another embodiment of the present invention is a step of reheating a steel slab containing, by weight, 0.1 to 0.3% carbon (C), 0.5% or less (excluding 0%) silicon (Si), 1.3 to 2.5% manganese (Mn), 0.2% or less (excluding 0%) chromium (Cr), 0.01 to 0.1% molybdenum (Mo), 0.0005 to 0.003% boron (B), 0.1% or less (excluding 0%) phosphorus (P), 0.01% or less (excluding 0%) sulfur (S), 0.01% or less (excluding 0%) nitrogen (N), 0.01 to 0.1% aluminum (Al), 0.01 to 0.05% niobium (Nb), 0.01 to 0.05% titanium (Ti), and the balance being Fe and unavoidable impurities, at a temperature of 1100 to 1300°C; hot rolling the reheated slabs; The hot-rolled steel sheet is coiled at 400-600°C; cold rolling the coiled steel sheet at a reduction ratio of 30 to 80%; Annealing the cold-rolled steel sheet by heat treating it at Ac3+10℃ to Ac3+80℃; The annealed steel sheet is subjected to primary cooling at an average cooling rate of 10°C / s or less to a primary cooling end temperature range of 680 to 749°C; and a step of secondarily cooling the primarily cooled steel sheet to a temperature of 100°C to Mf at an average cooling rate of 60 to 160°C / s.

[0019] The annealing step can be performed by heat treatment at Ac3+10°C to Ac3+80°C for 30 seconds or more.

[0020] The method may further include a step of reheating the second-cooled steel sheet to 150 to 240° C. and performing an overaging heat treatment.

[0021] The overaging heat treatment may be performed for 400 to 1000 seconds. [Effects of the Invention]

[0022] According to the present invention, it is possible to overcome the limitations of the prior art and provide a steel sheet that has an ultra-high strength of tensile strength of 1500 MPa or more while being excellent in shape and bending properties.

[0023] The various yet significant advantages and effects of the present invention are not limited to the above, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a photograph of a cross section in the thickness direction of a steel plate according to Example 1 of the present invention, taken using a scanning electron microscope (SEM). DETAILED DESCRIPTION OF THE INVENTION

[0025] The terms used herein are for the purpose of describing the invention and are not intended to limit the invention. Furthermore, as used herein, the singular forms "a," "an," and "the" include the plural forms unless the related definition clearly indicates a contrary meaning.

[0026] The term "comprises" as used herein embodies features and does not exclude the presence or addition of other features.

[0027] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Dictionary-defined terms are to be interpreted as having a meaning consistent with the relevant technical literature and the presently disclosed content.

[0028] Previously, research into steel sheets with high strength and high yield ratios to ensure crashworthiness has been ongoing, but conventional technology has had technical limitations, such as defects during forming and shape defects caused by rapid cooling after annealing during the process.

[0029] To solve the above-mentioned problems of the conventional technology, it was necessary to develop ultra-high strength cold-rolled and plated steel sheets with excellent shape and bending properties and a tensile strength of 1500 MPa or more, but until now, no technology has been developed to meet this demand.

[0030] Therefore, the present inventors have conducted extensive research to solve the above-mentioned problems of the conventional technology, and as a result, have confirmed through experiments that the desired physical properties can be secured when the chemical composition and operating conditions satisfy a specific relationship, which led to the completion of the present invention. As a result, the present invention overcomes the limitations of the conventional technology and makes it possible to provide a steel sheet having an ultra-high strength of 1500 MPa or more in tensile strength while also having excellent shape and bending properties by optimizing the steel composition and manufacturing process.

[0031] The present invention will be described in detail below. First, one embodiment of a steel sheet according to the present invention will be described in detail.

[0032] The alloy composition of the steel sheet according to the present invention is, in weight percent, 0.1 to 0.3% carbon (C), 0.5% or less (excluding 0%) silicon (Si), 1.3 to 2.5% manganese (Mn), 0.2% or less (excluding 0%) chromium (Cr), 0.01 to 0.1% molybdenum (Mo), 0.0005 to 0.003% boron (B), 0.1% or less (excluding 0%) phosphorus (P), 0.01% or less (excluding 0%) sulfur (S), 0.01% or less (excluding 0%) nitrogen (N), 0.01 to 0.1% aluminum (Al), 0.01 to 0.05% niobium (Nb), 0.01 to 0.05% titanium (Ti), and the balance being Fe and unavoidable impurities. Unless otherwise specified in the present invention, the content of each element is based on weight percent.

[0033] Carbon (C): 0.1~0.3% by weight Carbon is an interstitial solid solution element and is the most effective and important element for improving the strength of steel. It is an essential element for ensuring the strength of martensitic steel. To obtain ultra-high-strength steel satisfying the target yield ratio and tensile strength, carbon is preferably added in an amount of 0.1% or more, more preferably 0.15% or more. However, if the carbon content exceeds 0.3%, the increased hardenability can lead to excessive martensite formation during cooling, resulting in a rapid increase in strength and a deterioration in elongation. Furthermore, since an increased carbon content can impair weldability, the upper limit is preferably limited to 0.3%, more preferably 0.28% or less. Meanwhile, the lower limit of the carbon content can be 0.2%, or the upper limit of the carbon content can be 0.25%.

[0034] Silicon (Si): 0.5% by weight or less (excluding 0% by weight) Silicon is known as a key element in TRIP (Transformation Induced Plasticity) steel, increasing the fraction of retained austenite and elongation. In the present invention, the addition of Si is an element that suppresses the occurrence of cracks during bending by suppressing the precipitation of cementite. Therefore, to achieve the above-mentioned effects, the Si content does not exceed 0 wt%. However, if the Si content exceeds 0.5%, not only the weldability but also the surface properties and galvanic properties of the steel sheet deteriorate. Therefore, the Si content is set to 0.5 wt%. To further improve the above-mentioned effects, the lower limit of the Si content may be 0.01%, and the upper limit of the Si content may be 0.3%.

[0035] Manganese (Mn): 1.3 to 2.5% by weight Manganese is an element added to ensure strength. If the Mn content is less than 1.3%, the hardenability is low. If the cooling rate after annealing is not fast enough, martensite does not form, making it difficult to achieve the required level of strength. On the other hand, if the Mn content exceeds 2.5%, the Ms temperature during cooling after annealing decreases. As the final cooling temperature decreases, the shape of the steel sheet becomes poor and it becomes difficult to maintain the initial martensite structure. Furthermore, during steelmaking / continuous casting operations, segregation bands form along the length of the Mn-based slab, which can actually deteriorate bendability. Therefore, an upper limit is set. That is, manganese segregation along the thickness direction can easily form manganese bands within the slab, which can cause problems such as continuous casting cracks and increased defect generation during the rolling process. Therefore, the Mn content is preferably 1.3 to 2.5%. On the other hand, to further improve the above-mentioned effects, the lower limit of the Mn content can be 1.5%, or the upper limit can be 2.1%.

[0036] Chromium (Cr): 0.2% by weight or less (excluding 0% by weight) Chromium is an alloying element that inhibits ferrite transformation, thereby facilitating the formation of a low-temperature transformation structure. When using a continuous annealing process with slow cooling, as in the present invention, chromium has the advantage of inhibiting ferrite formation. To achieve the above-mentioned effects, the Cr content does not exceed 0 wt%. However, if the Cr content exceeds 0.2%, delayed fracture resistance may be impaired. Furthermore, chromium forms carbides such as CrC, impairing hole expandability and bending workability, and excessive alloy loading may increase costs. Therefore, the Cr content is preferably 0.2% or less. To further improve the above-mentioned effects, the upper limit of the Cr content may be 0.15% or 0.1%. However, the lower limit of the Cr content does not need to be set specifically as long as the composition and operating conditions can be optimized for production, and may be 0.01%, for example.

[0037] Molybdenum (Mo): 0.01 to 0.1% by weight Molybdenum has the effect of improving the quenching properties of steel, the effect of forming fine carbides containing Mo that act as hydrogen trapping sites, and the effect of improving delayed fracture resistance by refining martensite. However, if the Mo content exceeds 0.1%, the effect is not significant compared to the increase in cost due to the addition of high-cost alloying elements, so it is preferable to set the upper limit to 0.1% or less. On the other hand, it has been experimentally confirmed that if the Mo content is less than 0.01%, the basic properties of Mo are not exhibited at all and there is no effect of improving delayed fracture, so the lower limit is set to 0.01% or more. On the other hand, to further improve the above-mentioned effects, the lower limit of the Mo content can be 0.012%, or more preferably, the upper limit of the Mo content is 0.08%.

[0038] Boron (B): 0.0005 to 0.003 wt% Boron is an element that inhibits ferrite formation, which has the advantage of inhibiting the formation of ferrite during cooling after annealing. However, if the B content exceeds 0.003%, ductility may be significantly reduced. On the other hand, if the B content is less than 0.0005%, the hardening effect is completely lost, and not only is the target strength not achieved, but ferrite tends to form in the surface layer, deteriorating bendability, leading to the need for a lower limit. On the other hand, to further improve the above-mentioned effects, the lower limit of the B content may be 0.0008%, or the upper limit of the B content may be 0.0022%.

[0039] Phosphorus (P): 0.1% by weight or less (excluding 0%) Phosphorus is an impurity element contained in steel, and the less phosphorus added to steel, the better. However, the content of 0% is excluded in consideration of the possibility that phosphorus may be unavoidably added during the manufacturing process. However, since a P content exceeding 0.1% may deteriorate weldability and cause steel embrittlement, the upper limit of the P content can be set to 0.1% or less. Meanwhile, to further improve the above-mentioned effects, the lower limit of the P content can be 0.0001%, or the upper limit of the P content can be 0.03%.

[0040] Sulfur (S): 0.01% by weight or less (excluding 0%) Like P, sulfur is an element that is inevitably contained in steel and impairs the ductility and weldability of steel sheets. Therefore, it is preferable to control the sulfur content as low as possible. Therefore, in the present invention, the sulfur content is preferably limited to 0.01% or less. However, 0% is excluded in consideration of cases where sulfur is inevitably contained during the manufacturing process. Meanwhile, in order to further improve the above-mentioned effects, the lower limit of the S content may be 0.008% or 0.005%. Furthermore, in order to minimize MnS precipitates in the steel and further contribute to improving bendability, the upper limit of the S content may be 0.008% or 0.005%.

[0041] Nitrogen (N): 0.01% by weight or less (excluding 0%) Nitrogen is an impurity element, and if the N content exceeds 0.01%, the risk of cracking during continuous casting due to the formation of AlN, etc., is significantly increased. Therefore, it is preferable to limit the upper limit of the N content to 0.01%. However, 0% is excluded in consideration of cases where N is unavoidably included during the manufacturing process. Meanwhile, in order to further improve the above-mentioned effects, the lower limit of the N content may be 0.0001%, or the upper limit of the N content may be more preferably 0.008%, and even more preferably 0.006%.

[0042] Aluminum (Al): 0.01 to 0.1% by weight Aluminum can be added to remove oxygen from molten steel and, like Si, is an element that is effective in stabilizing retained austenite by inhibiting cementite precipitation during reheating and overaging. If the Al content is less than 0.01%, the steel is not sufficiently deoxidized, resulting in a loss of cleanliness. On the other hand, if the Al content exceeds 0.1%, not only will the castability of the slab deteriorate, but the temperature required for heating to the single-phase region during annealing will increase, potentially causing production and equipment problems. To further improve the above-mentioned effects, the lower limit of the Al content may be 0.02%, or the upper limit of the Al content may be 0.05%.

[0043] Niobium (Nb): 0.01 to 0.05% by weight Niobium is an element that segregates at austenite grain boundaries, inhibiting the growth of austenite grains during annealing, and contributes to increased strength through its precipitation strengthening effect. However, if the Nb content exceeds 0.05%, the precipitation of carbonitrides and the like increases, reducing the workability of the base material, and the cost increases as the alloying amount becomes excessive. On the other hand, if the Nb content is less than 0.01%, it does not contribute to increasing strength at all, so the lower limit is set to 0.01%. To further improve the above-mentioned effects, the lower limit of the Nb content can be 0.02%, or the upper limit of the Nb content can be 0.04%.

[0044] Titanium (Ti): 0.01 to 0.05% by weight Titanium is a nitride-forming element that scavenges N in steel by precipitating it as TiN. Without Ti, the formation of AlN can lead to cracks during continuous casting. However, if the Ti content exceeds 0.05%, in addition to removing solute N, the strength of martensite can be reduced by the precipitation of additional carbides, and the formation of carbonitrides such as TiC and TiN can impair hole expandability and bending workability. On the other hand, if the Ti content is less than 0.01%, it cannot contribute to strength enhancement, similar to Nb, so a lower limit is set. To further improve the above-mentioned effects, the lower limit of the Ti content can be 0.02%, or the upper limit can be 0.04%.

[0045] The remainder contains iron (Fe), and it is impossible to exclude unintended impurities that may be inevitably mixed in from raw materials or the surrounding environment during normal manufacturing processes. Since these impurities are obvious to anyone skilled in the art during manufacturing processes, the contents of all of them will not be specifically mentioned in this specification.

[0046] The features of the steel sheet according to the present invention will be described below.

[0047] The ratio (a / b) of the total content of C and Mn (a) in a region within 20 μm from the surface of the steel plate to the total content of C and Mn (b) at a point (1 / 4) × t (where t is the total thickness of the steel plate) from the surface is 75% or more (excluding 100%).

[0048] As a result of extensive research, the inventors discovered that bending properties tend to improve when the total content of C and Mn in a region within 20 μm from the surface of a steel sheet satisfies a specific ratio with respect to a point (¼)×t (where t is the total thickness of the steel sheet) from the surface (in the thickness direction of the steel sheet). Therefore, according to the present invention, if the ratio of the total content of C and Mn in a region within 20 μm from the surface of the steel sheet with respect to a point (¼)×t (where t is the total thickness of the steel sheet) from the surface is less than 75%, ferrite or bainite is excessively formed in the form of clusters in the surface layer, which can cause cracks to occur at the boundary between the ferrite and martensite phases, resulting in a problem of reduced bendability.

[0049] In order to further improve the above-mentioned effects, the lower limit of the ratio (a / b) may be 78%, or the upper limit of the ratio (a / b) may be 87%.

[0050] Furthermore, according to one embodiment of the present invention, the microstructure may contain, in area percent, 10% or less (excluding 0%) of one or more phases selected from the group consisting of ferrite and bainite in the region within 20 μm from the surface. If the proportion (A) of one or more phases selected from the group consisting of ferrite and bainite in the region within 20 μm from the surface exceeds 10%, soft ferrite or bainite may be excessively formed around the hard martensite phase, which may cause cracks to occur during bending. In this case, there is no particular lower limit for the proportion (A), but it is advantageous to keep it as low as possible within a range that is practically feasible for manufacturing.

[0051] At this time, according to one embodiment of the present invention, in the region within 20 μm from the surface, the remainder of the microstructure other than the above-mentioned ferrite and bainite may be martensite.

[0052] According to one embodiment of the present invention, the microstructure in the region within 20 μm from the surface can contain 90 to 99% by area of ​​martensite. Alternatively, as a more preferred range, the lower limit of the area percentage of martensite in the region within 20 μm from the surface can be 95%, or the upper limit of the area percentage of martensite in the region within 20 μm from the surface can be 98%.

[0053] According to one embodiment of the present invention, the t may be 0.6 to 2.5 mm.

[0054] Meanwhile, the steel sheet of the present invention may further include a coating layer. The coating layer is not particularly limited, and the type may be a zinc-based coating, an aluminum-based coating, or the coating method may be hot-dip coating, electroplating, or the like. In other words, any coating used in the technical field to which the present invention pertains is sufficient. However, in a preferred example of the present invention, the coating layer may be a zinc-based coating layer.

[0055] According to one embodiment of the present invention, it is possible to provide an ultra-high strength steel sheet having excellent bending properties, such as a tensile strength (TS) of 1500 MPa or more and a bendability (R / t) of 3.7 or less.

[0056] Furthermore, according to one embodiment of the present invention, it is possible to provide a steel sheet having a tensile strength (TS) of 1500 MPa or more, an elongation (El) of 3% or more (or, more preferably, an elongation range of 5% or more, and in particular, the higher the upper limit, the more advantageous it is, so it is not calculated), and a bendability (R / t) of 3.7 or less.

[0057] Next, a preferred method for producing a steel sheet according to the present invention will be described.

[0058] First, a steel slab having the above-mentioned chemical composition is reheated at a temperature of 1100 to 1300°C. This process is performed to facilitate the subsequent hot rolling process and to fully obtain the desired physical properties of the steel sheet. If the reheating temperature is less than 1100°C, a problem occurs in that the hot rolling load increases rapidly. On the other hand, if the reheating temperature exceeds 1300°C, the amount of surface scale increases, reducing the material yield, so the reheating temperature is limited.

[0059] The reheated slab is hot rolled. At this time, hot rolling can be performed at Ar3 to 1000°C. The finish hot rolling temperature of the reheated slab is limited to Ar3 or higher (the temperature at which ferrite begins to appear when austenite is cooled). This is because, below Ar3, rolling occurs in the two-phase region of ferrite + austenite or in the ferrite region, creating a duplex structure, which can lead to concerns about malfunction due to fluctuations in the hot rolling load.

[0060] Next, the hot-rolled steel sheet is coiled at a temperature in the range of 400 to 600°C. If the coiling temperature exceeds 600°C, an excessive oxide film may be formed on the surface of the steel sheet, which may cause defects and deteriorate the surface characteristics of the plated product, and therefore the upper limit of the coiling temperature is set. Furthermore, it is preferable to maintain a low coiling temperature in order to ensure material uniformity across the entire length and width by forming the hot-rolled sheet into a single-phase structure rather than a composite structure as much as possible. However, as the coiling temperature decreases, the strength of the hot-rolled steel sheet increases, which increases the rolling load in the subsequent cold rolling process, potentially making actual production impossible. Therefore, the lower limit of the coiling temperature is set to 400°C or higher. More preferably, the lower limit of the coiling temperature may be 420°C, or the upper limit may be 520°C, and water cooling may be performed after coiling.

[0061] Next, the oxide layer formed on the surface of the hot-rolled steel sheet coiled after the hot rolling is removed in a pickling process, and then the hot-rolled steel sheet is cold-rolled at a reduction of 30 to 80%. If the reduction in the cold rolling is less than 30%, not only is it difficult to achieve the target thickness, but the remaining hot-rolled crystal grains may affect austenite formation and final physical properties during annealing. Furthermore, if the reduction in the cold rolling exceeds 80%, there is a problem that the work hardening that occurs during cold rolling causes unevenness in the reduction in the length and width directions, resulting in variations in the quality of the final steel sheet, and it may be difficult to achieve the target thickness due to the rolling load.

[0062] After cold rolling, the steel is heat-treated for 30 seconds or more at an annealing temperature range of Ac3+10°C to Ac3+80°C. The Ac3 temperature varies depending on the steel composition and is therefore determined by the following formula 1. If the annealing temperature is less than Ac3+10°C, a duplex grain structure may be formed throughout the entire length of the coil due to annealing in a two-phase region rather than a single-phase region, which may adversely affect the material properties. Therefore, the lower limit is defined as Ac3+10°C. On the other hand, if the annealing temperature exceeds Ac3+80°C, equipment problems may occur due to overloading of the annealing furnace, so the upper limit is set to Ac3+80°C. On the other hand, more preferably, the lower limit of the annealing temperature may be 823°C, or the upper limit of the annealing temperature may be 916°C. [Formula 1] Ac3=910-203√[C]-15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W] (In the above formula 1, [C], [Ni], [Si], [V], [Mo], and [W] represent the weight percent content of each element in parentheses.)

[0063] According to one embodiment of the present invention, the heat treatment during annealing may be performed (maintained) for 30 seconds or more in a temperature range of Ac3+10°C to Ac3+80°C. If the heat treatment time at Ac3+10°C to Ac3+80°C is less than 30 seconds, the structure may not be sufficiently heat-treated as a single phase, making it difficult to obtain a martensitic structure.

[0064] Next, the annealed steel sheet is subjected to primary cooling at an average cooling rate of 10°C / s or less (greater than 0°C / s) to a primary cooling end temperature range of 680 to 749°C. Experimental results have shown that when the primary cooling end temperature is less than 680°C, the proportion of phases other than martensite (one or more phases selected from the group consisting of ferrite and bainite) in the surface layer within 20 μm from the surface exceeds 10% by area, and / or when comparing the total content of C and Mn (a) in the region within 20 μm from the surface with the total content of C and Mn (b) at a point (1 / 4) × t (where t is the total thickness of the steel sheet) from the surface, the ratio (a / b) becomes less than 75%, ultimately resulting in a bendability (R / t) evaluation value exceeding 3.7, which may result in poor formability. On the other hand, if the primary cooling end temperature exceeds 749°C, not only is it not possible to reproduce the temperature due to the equipment configuration, but the structure may become coarse and the strength may deteriorate, so the primary cooling end temperature is limited to 680 to 749°C. On the other hand, more preferably, the lower limit of the primary cooling end temperature can be 700°C, or the upper limit of the primary cooling end temperature can be 730°C.

[0065] Furthermore, if the average cooling rate for the primary cooling exceeds 10°C / s, problems may occur in the shape of the sheet, so the upper limit of the average cooling rate for the primary cooling is set to 10°C / s. Furthermore, the lower limit of the average cooling rate for the primary cooling can be set as low as possible, particularly in terms of equipment configuration, and no separate lower limit may be set, so the lower limit is set to be greater than 0°C / s (or 1°C / s or greater). On the other hand, more preferably, the lower limit of the average cooling rate for the primary cooling can be 4.3°C / s, or the upper limit of the average cooling rate for the primary cooling can be 7.7°C / s.

[0066] Next, the steel sheet that has undergone primary cooling is subjected to secondary cooling (rapid cooling) at an average cooling rate of 60 to 160°C / s to a temperature of 100°C to Mf, where Mf is the martensitic transformation finish temperature (Finish Temperature; Mf) and is measured using a dilatometer.

[0067] In order to ensure the required level of strength in the present invention, it is preferable to maintain rapid cooling conditions during the secondary cooling. Specifically, if the average cooling rate during the secondary cooling is less than 60°C / s, a bainite structure may be partially formed during cooling, making it difficult to achieve the target strength. On the other hand, if the average cooling rate during the secondary cooling exceeds 160°C / s, the rapid martensite transformation rate during the secondary cooling may cause problems such as deterioration of the steel sheet shape and variations in material properties across the width. On the other hand, to further improve the above-mentioned effects, the lower limit of the average cooling rate during the secondary cooling may be 80°C / s, or the upper limit of the average cooling rate during the secondary cooling may be 153°C / s.

[0068] Furthermore, the cooling end temperature of the secondary cooling is 100°C to the Mf temperature. If the cooling end temperature of the secondary cooling exceeds the Mf temperature, martensitic transformation does not occur sufficiently, making it difficult to obtain the microstructure desired by the present invention. On the other hand, if the cooling end temperature of the secondary cooling is less than 100°C, the cooling is performed at such a low temperature that it is disadvantageous in terms of shape and exceeds the manufacturing process range due to equipment limitations, so the lower limit is set to 100°C. On the other hand, to further improve the above-mentioned effects, the lower limit of the cooling end temperature of the secondary cooling can be 106°C, or the upper limit of the cooling end temperature of the secondary cooling can be 152°C.

[0069] Although not particularly limited, a method for manufacturing a steel sheet according to an embodiment of the present invention may satisfy the following relational expression 1. As a result of extensive research, the present inventors have confirmed that excellent properties such as strength and bendability can be ensured by satisfying a specific relationship such as the following relational expression 1 between the cooling end temperatures during primary and secondary cooling. [Equation 1] 4.5≦T1 / T2≦7 (In the above relational expression 1, T1 represents the cooling end temperature (°C) of the primary cooling, and T2 represents the cooling end temperature (°C) of the secondary cooling.)

[0070] Next, according to one embodiment of the present invention, the second-cooled steel sheet is reheated to 150 to 240°C and over-aged. The reheating and over-aging treatment transform the martensite obtained by the second-cooling process after quenching into tempered martensite, thereby increasing the yield strength. If the over-aging treatment temperature is less than 150°C, the tempering is insufficient, resulting in low yield strength and insufficient toughness. On the other hand, if the over-aging treatment temperature exceeds 240°C, the bending workability is poor due to the precipitation and coarsening of large amounts of carbides.

[0071] On the other hand, the lower the overaging heat treatment temperature, the more advantageous the bendability, but in consideration of equipment characteristics, the lower limit is preferably 150°C or higher, more preferably 170°C or higher. The upper limit of the overaging heat treatment temperature is preferably 200°C, more preferably 198°C.

[0072] Although not particularly limited, a method for manufacturing a steel sheet according to an embodiment of the present invention may satisfy the following relational expression 2. As a result of extensive research, the present inventors have further discovered that a steel sheet having more excellent properties such as strength and bendability can be obtained by satisfying a specific relationship such as the following relational expression 2 between the cooling end temperature of the secondary cooling and the overaging heat treatment temperature. [Equation 2] 1.2≦T3 / T2≦1.8 (In the above relational expression 2, T2 represents the cooling end temperature (°C) of the secondary cooling, and T3 represents the temperature (°C) of the overaging heat treatment.)

[0073] According to an embodiment of the present invention, the overaging heat treatment may be performed for 400 seconds or more, where the overaging heat treatment time refers to the time during which the material is maintained in the overaging heat treatment temperature range.

[0074] If the over-aging heat treatment time is less than 400 seconds, tempering may be insufficient, resulting in a low yield strength. While there is no particular upper limit to the over-aging heat treatment time, it is difficult to exceed 1,000 seconds due to the characteristics of continuous annealing equipment. Therefore, the over-aging heat treatment time may be 400 to 1,000 seconds. To further enhance the above-mentioned effects, the lower limit of the over-aging heat treatment time may be 428 seconds, or the upper limit of the over-aging heat treatment time may be 600 seconds.

[0075] If necessary, the plate may then go through temper rolling or tension leveling to improve the shape.

[0076] If necessary, the method may further include forming a plating layer on the surface of the steel sheet. The plating may be performed by a hot-dip plating method in which a plating bath is provided and the steel sheet is dipped in a hot-dip plating solution, or by electroplating using an electrolyte after annealing. The plating conditions are not particularly limited as long as they are generally known in the art to which the present invention pertains.

[0077] According to the above-described manufacturing method, it is possible to effectively obtain an ultra-high strength steel sheet that has an ultra-high strength of tensile strength of 1500 MPa class or more, and yet has excellent shape and bendability with a bendability (R / t) of 3.7 or less. [Example]

[0078] Examples of the present invention will now be described. Of course, those skilled in the art will appreciate that various modifications of the following examples are possible without departing from the scope of the present invention. The following examples are provided for the purpose of understanding the present invention, and the scope of the present invention should not be limited to the following examples, but should be defined by the claims below as well as equivalents thereof.

[0079] (Example) Molten steel having the alloy composition shown in Table 1 below was cast into an ingot and then sized and rolled to produce a steel slab. The steel slab was heated to 1200°C, maintained for 1 hour, and then finish hot rolled at 900°C. The slab was then charged into heating furnaces set under various conditions, maintained for 1 hour, and then cooled in the furnace to simulate hot rolling and coiling. The hot-rolled steel sheet was pickled and then cold-rolled at a 50% cold reduction. Following this, the cold-rolled steel sheet was subjected to annealing, primary cooling (slow cooling), secondary cooling (rapid cooling), reheating, and overaging treatment under the conditions shown in Table 2 below, and then electrogalvanized under conventional conditions.

[0080] The microstructure of the cold-rolled steel sheets manufactured in this way was evaluated using optical microscopes and SEM observations. In particular, the microstructure of the surface layer, which corresponds to the region within 20 μm from the surface, was observed using a 3000x SEM microscope. The area fraction of each phase was analyzed through image analysis of each phase, and the average value of three analyses was used as the representative value.

[0081] In addition, quantitative analysis was performed using a microanalyzer (FE-EPMA) with a TEM device to measure the ratio of the total content of C and Mn at the (1 / 4) × t point from the surface and in the region within 20 μm from the surface.

[0082] Specifically, the local C and Mn concentrations were determined by analyzing the quantitative concentration ratios (%) of C and Mn at five points within a diameter of approximately 20 μm, and then calculating the arithmetic mean to obtain a representative value. To compare the overall C and Mn concentration ratios, a line profile technique was used for relative comparison, allowing for an accurate evaluation of the ratio of the average total content of C and Mn in the region within 20 μm from the surface to the (1 / 4) × t point from the surface.

[0083] The tensile strength (TS) and yield strength (YS) were measured by taking JIS No. 5 size tensile test pieces in the direction perpendicular to the rolling direction and then conducting a tensile test at a strain rate of 0.01 / s.

[0084] For R / t (bending properties), cold-rolled steel sheets were processed into test pieces measuring 100 mm wide x 30 mm long, and then subjected to a 90° bending test at a test speed of 100 mm / min. The reliability of the results was improved by checking for cracks in the bent area using a microscope.

[0085] The flatness was measured by scanning the shape of the entire width of the 200 mm cut sheet using a 3D scanner in the longitudinal direction, and then measuring each section to evaluate the flatness. In general, in the present invention, a value of 3 mm or less was considered to be a satisfactory level.

[0086] Table 1 below shows the ranges of components used to manufacture the steels of the present invention and the comparative steels, and Table 2 summarizes the operating conditions for the steels of the present invention and the comparative steels. Operating conditions outside the ranges of the present invention for the steels of the present invention are marked with *, and operating conditions outside the ranges of the present invention for the comparative steels are also marked with *.

[0087] [Table 1]

[0088] [Table 2]

[0089] [Table 3]

[0090] In Table 3 above, (1) to (3) represent the following values. (1) The area ratio (%) of one or more phases selected from the group consisting of ferrite and bainite in the region within 20 μm from the surface (2) The ratio (a) of the total content of C and Mn in the region within 20 μm from the surface to the total content (b) of C and Mn at a point (1 / 4) × t (where t is the total thickness of the steel plate) from the surface (a / b × 100)

[0091] As can be seen from Table 1 above, the inventive examples of the present invention had a tensile strength of 1500 MPa or more, a bendability (R / t) of 3.7 or less, and a flatness of 3 mm or less, demonstrating that they had excellent shape and bendability while possessing ultra-high strength.

[0092] In particular, FIG. 1 shows a photograph of a cross section in the thickness direction of a steel sheet according to Example 1 of the present invention, taken with a scanning electron microscope (SEM).

[0093] On the other hand, it was confirmed that the comparative examples of the present invention did not meet the requirements of the present invention, and one or more of the above-mentioned strength, shape, and bendability were deteriorated.

[0094] Specifically, in Comparative Examples 1 and 2, when the primary and secondary cooling end temperatures and cooling rates deviated from the ranges required by the present invention, the flatness or bendability fell outside the ranges required by the present invention. This indicates that the proportion of at least one type of duplex grain structure selected from the group consisting of ferrite and bainite in addition to the martensite structure within the surface 20 μm exceeds 10%, or the total content of C and Mn falls outside the range required by the present invention, resulting in a deterioration in bendability.

[0095] It was also confirmed that comparative examples 3 and 4 were produced under conditions where the coiling temperature and annealing temperature were outside the ranges of the present invention, and that bendability deteriorated. It was also found that comparative examples 5 to 8 had steel components outside the target ranges, and that bendability also deteriorated.

[0096] That is, by controlling the components and operating conditions required in the present invention, it is possible to produce a 1500 MPa-class annealed and electrogalvanized steel sheet having the desired shape (flatness) and excellent bendability, as in the case of the invention examples.

Claims

1. The alloy contains, by weight, 0.1 to 0.3% carbon (C), 0.5% or less (excluding 0%) silicon (Si), 1.3 to 2.5% manganese (Mn), 0.2% or less (excluding 0%) chromium (Cr), 0.01 to 0.1% molybdenum (Mo), 0.0005 to 0.003% boron (B), 0.1% or less (excluding 0%) phosphorus (P), 0.01% or less (excluding 0%) sulfur (S), 0.01% or less (excluding 0%) nitrogen (N), 0.01 to 0.1% aluminum (Al), 0.01 to 0.05% niobium (Nb), 0.01 to 0.05% titanium (Ti), and the balance being Fe and inevitable impurities; A steel sheet in which the ratio (a / b×100) of the total content of C and Mn (a) in a region within 20 μm from the surface to the total content of C and Mn (b) at a point (¼)×t (where t is the total thickness of the steel sheet) from the surface is 75% or more (excluding 100%).

2. 2. The steel plate according to claim 1, wherein the microstructure contains, in a region within 20 μm from the surface, 10% or less (excluding 0%) of one or more phases selected from the group consisting of ferrite and bainite, in terms of area %.

3. The steel sheet according to claim 2, wherein the microstructure in the region within 20 μm from the surface is martensite with the remainder being martensite.

4. The steel plate according to claim 1, wherein the microstructure in the region within 20 μm from the surface contains 90 to 99% by area of ​​martensite.

5. The steel plate according to claim 1, wherein t is 0.6 to 2.5 mm.

6. The steel sheet according to claim 1 , further comprising a zinc-based plating layer on a surface of the steel sheet.

7. The steel sheet according to claim 1, having a tensile strength (TS) of 1500 MPa or more and a bendability (R / t) of 3.7 or less.

8. reheating a steel slab containing, by weight, 0.1 to 0.3% carbon (C), 0.5% or less (excluding 0%) silicon (Si), 1.3 to 2.5% manganese (Mn), 0.2% or less (excluding 0%) chromium (Cr), 0.01 to 0.1% molybdenum (Mo), 0.0005 to 0.003% boron (B), 0.1% or less (excluding 0%) phosphorus (P), 0.01% or less (excluding 0%) sulfur (S), 0.01% or less (excluding 0%) nitrogen (N), 0.01 to 0.1% aluminum (Al), 0.01 to 0.05% niobium (Nb), 0.01 to 0.05% titanium (Ti), and the balance Fe and unavoidable impurities at a temperature of 1100 to 1300°C; hot rolling the reheated slab; Coiling the hot-rolled steel sheet at 400 to 600°C; cold rolling the coiled steel sheet at a reduction ratio of 30 to 80%; Annealing the cold-rolled steel sheet by heat treating it at Ac3+10°C to Ac3+80°C; Primary cooling of the annealed steel sheet to a primary cooling end temperature range of 680 to 749°C at an average cooling rate of 10°C / s or less; and secondarily cooling the primarily cooled steel sheet to a temperature of 100°C to Mf at an average cooling rate of 60 to 160°C / s.

9. The method for manufacturing a steel sheet according to claim 8, wherein the annealing step comprises heat treatment at Ac3+10°C to Ac3+80°C for 30 seconds or more.

10. The method of claim 8, further comprising the step of reheating the second-cooled steel sheet to 150 to 240°C and performing an overaging heat treatment.

11. The method of claim 10, wherein the overaging heat treatment is performed for 400 to 1000 seconds.

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